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Sample records for lowest singlet excitation

  1. Stretching of hydrogen-bonded OH in the lowest singlet excited electronic state of water dimer.

    PubMed

    Chipman, Daniel M

    2006-01-28

    The lowest singlet excited electronic state of water monomer in the gas phase is strictly dissociative along a OH stretch coordinate but changes its nature when the stretched OH moiety is hydrogen bonded to a neighboring water molecule. This work extends previous exploration of the water dimer excited singlet potential-energy surface, using computational methods that are reliable even at geometries well removed from the ground-state equilibrium. First, the hydrogen-bonded OH moiety is stretched far enough to establish the existence of a barrier that is sufficient to support a quasibound vibrational state of the OH oscillator near the Franck-Condon region. Second, the constraint of an icelike structure is relaxed, and it is found that a substantial fraction of liquidlike structures also supports a quasibound vibrational state. These potential-energy explorations on stretching of the hydrogen-bonded OH moiety in a water dimer are discussed as a model for understanding the initial dynamics upon excitation into the lowest excited singlet state of condensed water. The possibility is raised that the excited-state lifetime may be long enough to allow for exciton migration, which would provide a mechanism for energy transport in condensed water phases. PMID:16460160

  2. The Ground and Two Lowest-lying Singlet Excited Electronic States of Copper Hydroxide (CuOH)

    SciTech Connect

    Wang, Suyun; Paul, Ankan; DeYonker, Nathan John; Yamaguchi, Yukio; Schaefer, Henry F

    2005-07-12

    The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Various ab initio methods, including self-consistent field (SCF), configuration interaction, coupled cluster (CC), and complete-active-space SCF (CASSCF), have been employed to study the electronic structure of copper hydroxide (CuOH). Geometries, total energies, dipole moments, harmonic vibrational frequencies, and zero-point vibrational energies are reported for the linear 1Σ+ and 1Π stationary points, and for the bent ground-state X˜ 1A', and excited-states 2 1A" and 1 1A". Six different basis sets have been used in the study, Wachters/DZP being the smallest and QZVPP being the largest. The ground- and excited-state bending modes present imaginary frequencies for the linear stationary points, indicating that bent structures are more favorable. The effects of relativity for CuOH are important and have been considered using the Douglas–Kroll approach with cc-pVTZ/cc-pVTZ_DK and cc-pVQZ/cc-pVQZ_DK basis sets. The bent ground and two lowest-lying singlet excited states of the CuOH molecule are indeed energetically more stable than the corresponding linear structures. The optimized geometrical parameters for the X˜ 1A' and 1 1A" states agree fairly well with available experimental values. However, the 2 1A' structure and rotational constants are in poor agreement with experiment, and we suggest that the latter are in error. The predicted adiabatic excitation energies are also inconsistent with the experimental values of 45.5 kcal mol-1 for the 2 1A' state and 52.6 kcal mol-1 for the 1 1A" state. The theoretical CC and CASSCF methods show lower

  3. Experimental and theoretical dipole moments of purines in their ground and lowest excited singlet states

    NASA Astrophysics Data System (ADS)

    Aaron, Jean-Jacques; Diabou Gaye, Mame; Párkányi, Cyril; Cho, Nam Sook; Von Szentpály, László

    1987-01-01

    The ground-state dipole moments of seven biologically important purines (purine, 6-chloropurine, 6-mercaptopurine, hypoxanthine, theobromine, theophylline and caffeine) were determined at 25°C in acetic acid (all the above compounds with the exception of purine) and in ethyl acetate (purine, theophylline and caffeine). Because of its low solubility, it was not possible to measure the dipole moment of uric acid. The first excited singlet-state dipole moments were obtained on the basis of the Bakhshiev and Chamma—Viallet equations using the variation of the Stokes shift with the solvent dielectric constant-refractive index term. The theoretical dipole moments for all the purines listed above and including uric acid were calculated by combining the use of the PPP (π-LCI-SCF-MO) method for the π-contribution to the overall dipole moment with the σ-contribution obtained as a vector sum of the σbond moments and group moments. The experimental and theoretical values were compared with the data available in the literature for some of the purines under study. For several purines, the calculations were carried out for different tautomeric forms. Excited singlet-state dipole moments are smaller than the ground-state values by 0.8 to 2.2 Debye units for all purines under study with the exception of 6-chloropurine. The effects of the structure upon the ground- and excited-state dipole moments of the purines are discussed.

  4. Mechanism of nonphotochemical quenching in green plants: energies of the lowest excited singlet states of violaxanthin and zeaxanthin.

    PubMed

    Frank, H A; Bautista, J A; Josue, J S; Young, A J

    2000-03-21

    The xanthophyll cycle is an enzymatic, reversible process through which the carotenoids violaxanthin, antheraxanthin, and zeaxanthin are interconverted in response to the need to balance light absorption with the capacity to use the energy to drive the reactions of photosynthesis. The cycle is thought to be one of the main avenues for safely dissipating excitation energy absorbed by plants in excess of that needed for photosynthesis. One of the key factors needed to elucidate the molecular mechanism by which the potentially damaging excess energy is dissipated is the energy of the lowest excited singlet (S(1)) state of the xanthophyll pigments. Absorption from the ground state (S(0)) to S(1) is forbidden by symmetry, making a determination of the S(1) state energies of these molecules by absorption spectroscopy very difficult. Fluorescence spectroscopy is potentially the most direct method for obtaining the S(1) state energies. However, because of problems with sample purity, low emission quantum yields, and detection sensitivity, fluorescence spectra from these molecules, until now, have never been reported. In this work these technical obstacles have been overcome, and S(1) --> S(0) fluorescence spectra of violaxanthin and zeaxanthin are presented. The energies of the S(1) states deduced from the fluorescence spectra are 14 880 +/- 90 cm(-)(1) for violaxanthin and 14 550 +/- 90 cm(-)(1) for zeaxanthin. The results provide important insights into the mechanism of nonphotochemical dissipation of excess energy in plants. PMID:10715102

  5. Lowest excitation energy of 9Be.

    PubMed

    Stanke, Monika; Kedziera, Dariusz; Bubin, Sergiy; Adamowicz, Ludwik

    2007-07-27

    Variational calculations employing explicitly correlated Gaussian functions and explicitly including the nuclear motion [i.e., without assuming the Born-Oppenheimer (BO) approximation] have been performed to determine the lowest singlet transition energy in the 9Be atom. The non-BO wave functions were used to calculate the alpha2 relativistic corrections (alpha=1/137.035,999,679). With those corrections and with the alpha3 and alpha4 QED corrections determined previously by others, we obtained 54,677.35 cm(-1) for the 3(1)S-->2(1)S transition energy. This result falls within the error bracket for the experimental transition of 54,677.26(10) cm(-1). This is the first time an electronic transition of Be has been calculated from first principles with the experimental accuracy. PMID:17678358

  6. Charge-transfer excitations steer the Davydov splitting and mediate singlet exciton fission in pentacene.

    PubMed

    Beljonne, D; Yamagata, H; Brédas, J L; Spano, F C; Olivier, Y

    2013-05-31

    Quantum-chemical calculations are combined to a model Frenkel-Holstein Hamiltonian to assess the nature of the lowest electronic excitations in the pentacene crystal. We show that an admixture of charge-transfer excitations into the lowest singlet excited states form the origin of the Davydov splitting and mediate instantaneous singlet exciton fission by direct optical excitation of coherently coupled single and double exciton states, in agreement with recent experiments. PMID:23767738

  7. Charge-Transfer Excitations Steer the Davydov Splitting and Mediate Singlet Exciton Fission in Pentacene

    NASA Astrophysics Data System (ADS)

    Beljonne, D.; Yamagata, H.; Brédas, J. L.; Spano, F. C.; Olivier, Y.

    2013-05-01

    Quantum-chemical calculations are combined to a model Frenkel-Holstein Hamiltonian to assess the nature of the lowest electronic excitations in the pentacene crystal. We show that an admixture of charge-transfer excitations into the lowest singlet excited states form the origin of the Davydov splitting and mediate instantaneous singlet exciton fission by direct optical excitation of coherently coupled single and double exciton states, in agreement with recent experiments.

  8. Ab Initio Calculations of Singlet and Triplet Excited States of Chlorine Nitrate and Nitric Acid

    NASA Technical Reports Server (NTRS)

    Grana, Ana M.; Lee, Timothy J.; Head-Gordon, Martin; Langhoff, Stephen R. (Technical Monitor)

    1994-01-01

    Ab initio calculations of vertical excitations to singlet and triplet excited states of chlorine nitrate and nitric acid are reported. The nature of the electronic transitions are examined by decomposing the difference density into the sum of detachment and attachment densities. Counterparts for the three lowest singlet excited states of nitric acid survive relatively unperturbed in chlorine nitrate, while other low-lying singlet states of chlorine nitrate appear to be directly dissociative in the ClO chromophore. These results suggest an assignment of the two main peaks in the experimental chlorine nitrate absorption spectrum. In addition, triplet vertical excitations and the lowest optimized triplet geometries of both molecules are studied.

  9. Location of the lowest exciton in C 60 single crystal by two-photon excitation spectroscopy

    NASA Astrophysics Data System (ADS)

    Muccini, M.; Danieli, R.; Zamboni, R.; Taliani, C.; Mohn, H.; Müller, W.; ter Meer, H. U.

    1995-10-01

    Two-photon excitation of C 60 single crystal at 4 K shows a sharp band at 1.846 eV which is assigned to the lowest forbidden Frenkel singlet exciton of T 1g symmetry. This assignment is supported by the analysis of Herzberg-Teller induced photoluminescence.

  10. Testing quantum electrodynamics in the lowest singlet state of neutral beryllium-9

    NASA Astrophysics Data System (ADS)

    Williams, Will; Cao, Melody; Kaplan, Emily

    2016-05-01

    We present high precision spectroscopic results on the 2s2p J = 1 singlet state in neutral beryllium-9. Combined with theoretical predictions this measurement serves as a test of quantum electrodynamics and various theoretical methods for predicting the energy of this state. Our experimental setup consists of an oven at 1200C that produces a beam of beryllium atoms. The singlet state is probed transverse to the atomic beam with 235nm light from a frequency quadrupled titanium sapphire laser, where the frequency doubled light at 470nm is stabilized to an ultra low expansion cavity. We also present our progress on spectroscopy on the lowest triplet states and the ionization threshold.

  11. Direct optical excitation of singlet oxygen in organic solvents

    NASA Astrophysics Data System (ADS)

    Bagrov, I. V.; Kiselev, V. M.; Kislyakov, I. M.; Sosnov, E. N.

    2014-04-01

    Efficient excitation of singlet oxygen is demonstrated for several organic solvents (CS2, CCl4, and C6F14) that are irradiated using LED in the visible spectral range in the absorption bands of the O2-O2 collision complexes at the corresponding cooperative transitions. It is shown that the two-photon interaction of the pumping radiation in the Herzberg I band of molecular oxygen with its excitation to the 3Σ{/u +} state and the subsequent collisional relaxation to the 1Σ g and 1Δ g singlet states contributes to the excitation of singlet oxygen.

  12. Singlet and triplet excited states and intersystem crossing in free-base porphyrin: TDDFT and DFT/MRCI study.

    PubMed

    Perun, Serhiy; Tatchen, Jörg; Marian, Christel M

    2008-02-01

    Extensive time-dependent DFT (TDDFT) and DFT/multireference configuration interaction (MRCI) calculations are performed on the singlet and triplet excited states of free-base porphyrin, with emphasis on intersystem crossing processes. The equilibrium geometries, as well as the vertical and adiabatic excitation energies of the lowest singlet and triplet excited states are determined. Single and double proton-transfer reactions in the first excited singlet state are explored. Harmonic vibrational frequencies are calculated at the equilibrium geometries of the ground state and of the lowest singlet and triplet excited states. Furthermore, spin-orbit coupling matrix elements of the lowest singlet and triplet states and their numerical derivatives with respect to nuclear displacements are computed. It is shown that opening of an unprotonated pyrrole ring as well as excited-state single and double proton transfer inside the porphyrin cavity lead to crossings of the potential energy curves of the lowest singlet and triplet excited states. It is also found that displacements along out-of-plane normal modes of the first excited singlet state cause a significant increase of the , , and spin-orbit coupling matrix elements. These phenomena lead to efficient radiationless deactivation of the lowest excited states of free-base porphyrin via intercombination conversion. In particular, the S1-->T1 population transfer is found to proceed at a rate of approximately 10(7) s(-1) in the isolated molecule. PMID:18189251

  13. Conical intersections and diabatic potential energy surfaces for the three lowest electronic singlet states of H{sub 3}{sup +}

    SciTech Connect

    Mukherjee, Saikat; Adhikari, Satrajit; Mukhopadhyay, Debasis

    2014-11-28

    We calculate the adiabatic Potential Energy Surfaces (PESs) and the Non-Adiabatic Coupling Terms (NACTs) for the three lowest singlet states of H{sub 3}{sup +} in hyperspherical coordinates as functions of hyperangles (θ and ϕ) for a grid of fixed values of hyperradius (1.5 ⩽ ρ ⩽ 20 bohrs) using the MRCI level of methodology employing ab initio quantum chemistry package (MOLPRO). The NACT between the ground and the first excited state translates along the seams on the θ − ϕ space, i.e., there are six Conical Intersections (CIs) at each θ (60° ⩽ θ ⩽ 90°) within the domain, 0 ⩽ ϕ ⩽ 2π. While transforming the adiabatic PESs to the diabatic ones, such surfaces show up six crossings along those seams. Our beyond Born-Oppenheimer approach could incorporate the effect of NACTs accurately and construct single-valued, continuous, smooth, and symmetric diabatic PESs. Since the location of CIs and the spatial amplitudes of NACTs are most prominent around ρ = 10 bohrs, generally only those results are depicted.

  14. Conical intersections and diabatic potential energy surfaces for the three lowest electronic singlet states of H_3^+

    NASA Astrophysics Data System (ADS)

    Mukherjee, Saikat; Mukhopadhyay, Debasis; Adhikari, Satrajit

    2014-11-01

    We calculate the adiabatic Potential Energy Surfaces (PESs) and the Non-Adiabatic Coupling Terms (NACTs) for the three lowest singlet states of H_3^+ in hyperspherical coordinates as functions of hyperangles (θ and ϕ) for a grid of fixed values of hyperradius (1.5 ⩽ ρ ⩽ 20 bohrs) using the MRCI level of methodology employing ab initio quantum chemistry package (MOLPRO). The NACT between the ground and the first excited state translates along the seams on the θ - ϕ space, i.e., there are six Conical Intersections (CIs) at each θ (60° ⩽ θ ⩽ 90°) within the domain, 0 ⩽ ϕ ⩽ 2π. While transforming the adiabatic PESs to the diabatic ones, such surfaces show up six crossings along those seams. Our beyond Born-Oppenheimer approach could incorporate the effect of NACTs accurately and construct single-valued, continuous, smooth, and symmetric diabatic PESs. Since the location of CIs and the spatial amplitudes of NACTs are most prominent around ρ = 10 bohrs, generally only those results are depicted.

  15. Low-lying electronic excited states of pentacene oligomers: a comparative electronic structure study in the context of singlet fission.

    PubMed

    Coto, Pedro B; Sharifzadeh, Sahar; Neaton, Jeffrey B; Thoss, Michael

    2015-01-13

    The lowest-lying electronic excited states of pentacene and its oligomers are investigated using accurate multireference wave function methods (CASPT2/CASSCF) and the many-body Greens's function approach (GW/BSE). The results obtained for dimers and trimers of different geometry reveal a complex electronic structure, which includes locally excited, charge transfer, and multiexciton states. For singlets of single-excitation character, both approaches yield excitation energies that are in good overall quantitative agreement. While the multiexciton states are located relatively high in energy in all systems investigated, charge transfer states exist in close proximity to the lowest-lying absorbing states. The implications of the results for the mechanisms of singlet fission in pentacene are discussed. PMID:26574213

  16. The Protolysis of Singlet Excited B-Naphtol.

    ERIC Educational Resources Information Center

    van Stam, Jan; Lofroth, Jan-Erik

    1986-01-01

    Presents a two-day experiment to estimate the pK for the protolysis of beta-naphtol in its ground state and the first singlet excited state. Results are compared to results obtained from the integrated rate equations in which values of the rate constants were taken from a time-resolved study. (JN)

  17. Electron delocalization and electron density of small polycyclic aromatic hydrocarbons in singlet excited states.

    PubMed

    Estévez-Fregoso, Mar; Hernández-Trujillo, Jesús

    2016-04-28

    The four lowest singlet electronic states of benzene, the acenes from naphthalene to pentacene, phenanthrene and pyrene were studied by means of theoretical methods. Their vertical excitation energies from the ground electronic states were computed at the CASPT2 approximation. As an attempt to explain the trends observed in the excitation energies, several descriptors based on the electron density were used and the similarity of these molecules with their ground state counterparts was analyzed. It was found that the changes of the topological properties at the C-C bond critical points do not explain the decreasing trends for the excitation energies with the increase of the number of rings, in part because the small changes that take place in the electron density occur above and below the molecular plane. A similarity index based on electron delocalization between quantum topological atoms was defined to compare a molecule in two different electronic states. It was found that, mainly for the acenes, this index goes in line with the excitation energies to the first excited state. Implications of the changes in electron delocalization on the aromatic character of these molecules are also discussed. In general, local aromaticity decreases upon excitation. PMID:26795361

  18. Photo-excitation of carotenoids causes cytotoxicity via singlet oxygen production.

    PubMed

    Yoshii, Hiroshi; Yoshii, Yukie; Asai, Tatsuya; Furukawa, Takako; Takaichi, Shinichi; Fujibayashi, Yasuhisa

    2012-01-01

    Carotenoids, natural pigments widely distributed in algae and plants, have a conjugated double bond system. Their excitation energies are correlated with conjugation length. We hypothesized that carotenoids whose energy states are above the singlet excited state of oxygen (singlet oxygen) would possess photosensitizing properties. Here, we demonstrated that human skin melanoma (A375) cells are damaged through the photo-excitation of several carotenoids (neoxanthin, fucoxanthin and siphonaxanthin). In contrast, photo-excitation of carotenoids that possess energy states below that of singlet oxygen, such as β-carotene, lutein, loroxanthin and violaxanthin, did not enhance cell death. Production of reactive oxygen species (ROS) by photo-excited fucoxanthin or neoxanthin was confirmed using a reporter assay for ROS production with HeLa Hyper cells, which express a fluorescent indicator protein for intracellular ROS. Fucoxanthin and neoxanthin also showed high cellular penetration and retention. Electron spin resonance spectra using 2,2,6,6-tetramethil-4-piperidone as a singlet oxygen trapping agent demonstrated that singlet oxygen was produced via energy transfer from photo-excited fucoxanthin to oxygen molecules. These results suggest that carotenoids such as fucoxanthin, which are capable of singlet oxygen production through photo-excitation and show good penetration and retention in target cells, are useful as photosensitizers in photodynamic therapy for skin disease. PMID:22185691

  19. Calculation of the lowest electronic excitations of the alkaline earth metals using the relativistic polarization propagator

    NASA Astrophysics Data System (ADS)

    Brandt, Sven; Pernpointner, Markus

    2015-07-01

    In this work we use the recently implemented four-component polarization propagator for accurate single excitation calculations of alkaline earth metals and compare our results to experimental data. Various approximations to the Dirac-Coulomb Hamiltonian are additionally tested. In Ca spin-orbit coupling already leads to noticeable zero field splitting, which gradually increases for the heavier homologs finally invalidating the singlet and triplet state characterizations. For all systems we observe a very good agreement with experimental transition energies in the considered energy range. For Sr, Ba and Ra non-relativistic approaches already exhibit unacceptable deviations in the reproduction of transition energies and spectral structure. The obtained excited final states are analyzed in terms of atomic donor and acceptor orbital contributions. Our results stress the necessity to use relativistic implementations of the polarization propagator for an accurate description of both electron correlation and relativistic effects contributing to excitation spectra of heavy systems.

  20. Photo-excitation of carotenoids causes cytotoxicity via singlet oxygen production

    SciTech Connect

    Yoshii, Hiroshi; Yoshii, Yukie; Asai, Tatsuya; Furukawa, Takako; Takaichi, Shinichi; Fujibayashi, Yasuhisa

    2012-01-06

    Highlights: Black-Right-Pointing-Pointer Some photo-excited carotenoids have photosensitizing ability. Black-Right-Pointing-Pointer They are able to produce ROS. Black-Right-Pointing-Pointer Photo-excited fucoxanthin can produce singlet oxygen through energy transfer. -- Abstract: Carotenoids, natural pigments widely distributed in algae and plants, have a conjugated double bond system. Their excitation energies are correlated with conjugation length. We hypothesized that carotenoids whose energy states are above the singlet excited state of oxygen (singlet oxygen) would possess photosensitizing properties. Here, we demonstrated that human skin melanoma (A375) cells are damaged through the photo-excitation of several carotenoids (neoxanthin, fucoxanthin and siphonaxanthin). In contrast, photo-excitation of carotenoids that possess energy states below that of singlet oxygen, such as {beta}-carotene, lutein, loroxanthin and violaxanthin, did not enhance cell death. Production of reactive oxygen species (ROS) by photo-excited fucoxanthin or neoxanthin was confirmed using a reporter assay for ROS production with HeLa Hyper cells, which express a fluorescent indicator protein for intracellular ROS. Fucoxanthin and neoxanthin also showed high cellular penetration and retention. Electron spin resonance spectra using 2,2,6,6-tetramethil-4-piperidone as a singlet oxygen trapping agent demonstrated that singlet oxygen was produced via energy transfer from photo-excited fucoxanthin to oxygen molecules. These results suggest that carotenoids such as fucoxanthin, which are capable of singlet oxygen production through photo-excitation and show good penetration and retention in target cells, are useful as photosensitizers in photodynamic therapy for skin disease.

  1. Time-resolved electron transfer from the excited singlet state of anchored perylene into Ag(1 1 0)

    NASA Astrophysics Data System (ADS)

    Gundlach, Lars; Willig, Frank

    2007-11-01

    Perylene chromophores were adsorbed on the Ag(1 1 0) surface via surface bond formation of the -CH 2-SH group that was covalently attached to the perylene moieties. The lowest excited singlet state of perylene was populated with a laser pulse of 440 nm wavelength. The decay of this donor state due to ultrafast electron transfer to the Ag electrode was probed with a second, delayed laser pulse centered at 280 nm that gave rise to a photoemission signal detected in ultra-high-vacuum by a spectrometer. The fit to the measured data revealed a time constant of 15 fs for the electron injection reaction.

  2. The influence of excitation radiation parameters on photosensitized generation of singlet oxygen in water

    NASA Astrophysics Data System (ADS)

    Il'ina, A. D.; Glazov, A. L.; Semenova, I. V.; Vasyutinskii, O. S.

    2016-06-01

    Photosensitized generation of singlet oxygen with the aid of Radahlorin® photosensitizer has been investigated. The dependences of the intensity of singlet oxygen phosphorescence and photosensitizer fluorescence on the excitation radiation wavelength in the range of 350-440 nm and on the irradiation dose have been obtained. The dependence of the ratio of the sensitizer fluorescence intensity at about 670 nm to the singlet oxygen phosphorescence intensity at a wavelength of 1270 nm on the excitation radiation wavelength is found to be nonmonotonic and have a minimum near the center of the absorption band on its red wing. The results obtained can be used to monitor the singlet oxygen concentration in solutions.

  3. Accurate ab initio potential energy curves and spectroscopic properties of the four lowest singlet states of C2

    SciTech Connect

    Boschen, Jeffery S.; Theis, Daniel; Ruedenberg, Klaus; Windus, Theresa L.

    2013-12-07

    The diatomic carbon molecule has a complex electronic structure with a large number of low-lying electronic excited states. In this work, the potential energy curves (PECs) of the four lowest lying singlet states (X-1 Sigma(+)(g), A(1)Pi(u), B-1 Delta(g), and B'(1)Sigma(+)(g)) were obtained by high-level ab initio calculations. Valence electron correlation was accounted for by the correlation energy extrapolation by intrinsic scaling (CEEIS) method. Additional corrections to the PECs included core-valence correlation and relativistic effects. Spin-orbit corrections were found to be insignificant. The impact of using dynamically weighted reference wave functions in conjunction with CEEIS was examined and found to give indistinguishable results from the even weighted method. The PECs showed multiple curve crossings due to the B-1 Delta(g) state as well as an avoided crossing between the two (1)Sigma(+)(g) states. Vibrational energy levels were computed for each of the four electronic states, as well as rotational constants and spectroscopic parameters. Comparison between the theoretical and experimental results showed excellent agreement overall. Equilibrium bond distances are reproduced to within 0.05 %. The dissociation energies of the states agree with experiment to within similar to 0.5 kcal/mol, achieving "chemical accuracy." Vibrational energy levels show average deviations of similar to 20 cm(-1) or less. The B-1 Delta(g) state shows the best agreement with a mean absolute deviation of 2.41 cm(-1). Calculated rotational constants exhibit very good agreement with experiment, as do the spectroscopic constants.

  4. Shape vibration and quasiparticle excitations in the lowest 0+ excited state in erbium isotopes

    NASA Astrophysics Data System (ADS)

    Chen, Fang-Qi; Egido, J. Luis

    2016-06-01

    The ground and first excited 0+ states of the -172Er156 isotopes are analyzed in the framework of the generator coordinate method. The shape parameter β is used to generate wave functions with different deformations which together with the two-quasiparticle states built on them provide a set of states. An angular momentum and particle number projection of the latter spawn the basis states of the generator coordinate method. With this ansatz and using the separable pairing plus quadrupole interaction we obtain a good agreement with the experimental spectra and E 2 transition rates up to moderate spin values. The structure of the wave functions suggests that the first excited 0+ states in the soft Er isotopes are dominated by shape fluctuations, while in the well deformed Er isotopes the two-quasiparticle states are more relevant. In between, both degrees of freedom are necessary.

  5. Singlet-to-Triplet Excitations in the Unconventional Spin-Peierls System TiOBr

    SciTech Connect

    Clancy, James P; Gaulin, Bruce D.; Adams, Carl P; Granroth, Garrett E; Kolesnikov, Alexander I; Sherline, Todd E; Chou, F. C.

    2011-01-01

    We have performed time-of-flight neutron scattering measurements on powder samples of the unconventional spin-Peierls compound TiOBr using the fine-resolution Fermi chopper spectrometer (SEQUOIA) at the SNS. These measurements reveal two branches of magnetic excitations within the commensurate and incommensurate spin-Peierls phases, which we associate with n = 1 and n = 2 triplet excitations out of the singlet ground state. These measurements represent the first direct measure of the singlet-triplet energy gap in TiOBr, which is found to have a value of Eg 21 meV.

  6. Singlet-Triplet Excitations in the Unconventional Spin-Peierls TiOBr Compound

    NASA Astrophysics Data System (ADS)

    Clancy, J. P.; Gaulin, B. D.; Adams, C. P.; Granroth, G. E.; Kolesnikov, A. I.; Sherline, T. E.; Chou, F. C.

    2011-03-01

    We have performed time-of-flight neutron scattering measurements on powder samples of the unconventional spin-Peierls compound TiOBr using the fine-resolution Fermi chopper spectrometer (SEQUOIA) at the Spallation Neutron Source at Oak Ridge National Laboratory. These measurements reveal two branches of magnetic excitations within the commensurate and incommensurate spin-Peierls phases, which we associate with n=1 and n=2 triplet excitations out of the singlet ground state. These results represent the first direct measurement of the singlet-triplet energy gap in TiOBr, which has a value of Eg=21.2±1.0meV.

  7. Zethrene biradicals: How pro-aromaticity is expressed in the ground electronic state and in the lowest energy singlet, triplet, and ionic states

    SciTech Connect

    Zafra, José Luis; González Cano, Rafael C.; Ruiz Delgado, M. Carmen; López Navarrete, Juan T.; Casado, Juan

    2014-02-07

    A analysis of the electronic and molecular structures of new molecular materials based on zethrene is presented with particular attention to those systems having a central benzo-quinoidal core able to generate Kekulé biradicals whose stability is provided by the aromaticity recovery in this central unit. These Kekulé biradicals display singlet ground electronic states thanks to double spin polarization and have low-energy lying triplet excited states also featured by the aromaticity gain. Pro-aromatization is also the driving force for the stabilization of the ionized species. Moreover, the low energy lying singlet excited states also display a profound biradical fingerprint allowing to singlet exciton fission. These properties are discussed in the context of the size of the zethrene core and of its substitution. The work encompasses all known long zethrenes and makes use of a variety of experimental techniques, such as Raman, UV-Vis-NIR absorption, transient absorption, in situ spectroelectrochemistry and quantum chemical calculations. This study reveals how the insertion of suitable molecular modules (i.e., quinoidal) opens the door to new intriguing molecular properties exploitable in organic electronics.

  8. Zethrene biradicals: How pro-aromaticity is expressed in the ground electronic state and in the lowest energy singlet, triplet, and ionic states

    NASA Astrophysics Data System (ADS)

    Zafra, José Luis; González Cano, Rafael C.; Ruiz Delgado, M. Carmen; Sun, Zhe; Li, Yuan; López Navarrete, Juan T.; Wu, Jishan; Casado, Juan

    2014-02-01

    A analysis of the electronic and molecular structures of new molecular materials based on zethrene is presented with particular attention to those systems having a central benzo-quinoidal core able to generate Kekulé biradicals whose stability is provided by the aromaticity recovery in this central unit. These Kekulé biradicals display singlet ground electronic states thanks to double spin polarization and have low-energy lying triplet excited states also featured by the aromaticity gain. Pro-aromatization is also the driving force for the stabilization of the ionized species. Moreover, the low energy lying singlet excited states also display a profound biradical fingerprint allowing to singlet exciton fission. These properties are discussed in the context of the size of the zethrene core and of its substitution. The work encompasses all known long zethrenes and makes use of a variety of experimental techniques, such as Raman, UV-Vis-NIR absorption, transient absorption, in situ spectroelectrochemistry and quantum chemical calculations. This study reveals how the insertion of suitable molecular modules (i.e., quinoidal) opens the door to new intriguing molecular properties exploitable in organic electronics.

  9. High resolution study of the six lowest doubly excited vibrational states of PH 2D

    NASA Astrophysics Data System (ADS)

    Leroy, C.; Ulenikov, O. N.; Bekhtereva, E. S.; Onopenko, G. A.; Chudinova, T. D.

    2005-12-01

    The five lowest doubly excited deformational vibrational bands ν4 + ν6, 2 ν6, ν3 + ν4, ν3 + ν6, and 2 ν3 of PH 2D have been recorded for the first time using a Bruker 120 HR interferometer with a resolution 0.0033 cm -1 and analysed. Some transitions belonging to a very weak band 2 ν4 have been also assigned. From the fit 24 and 86, respectively, diagonal and resonance interaction parameters were obtained which reproduce 1089 upper energy levels obtained from more than 4600 assigned transitions with the rms deviation of 0.00059 cm -1.

  10. Selective vibronic excitation of singlet oxygen--furan reactions in cryogenic matrices

    SciTech Connect

    Frei, H.; Pimentel, G.C.

    1983-10-01

    The reactions of 2,5-dimethylfuran (DMF), 2-methylfuran (MF), and furan with molecular oxygen to form endoperoxides have been induced in Ar and O/sub 2/ matrices at 12 K with selective vibronic excitation of O/sub 2/ using near infrared light. Reaction was induced through excitation of the /sup 1/..delta../sub g/(v' = 0, v' = 1) and /sup 1/..sigma../sub g//sup +/(v' = 0, v' = 1) O/sub 2/ states near 8000 and 13 100 cm/sup -1/, respectively, as well as by the (/sup 1/..delta../sub g/, /sup 1/..delta../sub g/) simultaneous transitions of (O/sub 2/)/sub 2/ at 15 900 and 17 300 cm/sup -1/. The /sup 1/..delta../sub g/ reverse arrow /sup 3/..sigma../sub g//sup -/ vibronic progression in solid O/sub 2/ was recorded by FTIR spectroscopy, whereas members of the /sup 1/..sigma../sub g//sup +/ reverse arrow /sup 3/..sigma../sub g//sup -/ and (/sup 1/..delta../sub g/, /sup 1/..delta../sub g/) reverse arrow (/sup 3/..sigma../sub g//sup -/, /sup 3/..sigma../sub g//sup -/) progressions in Ar matrices were located by ''reaction excitation'' spectroscopy. The DMF+O/sub 2/ reaction is a single photon process, apparently with unit quantum yield, for all vibronic levels excited. For MF+O/sub 2/(/sup 1/..delta../sub g/, v' = 0) and furan +O/sub 2/(/sup 1/..delta../sub g/, v' = 0), quantum yields were high, approx.0.6 and 0.4, respectively, but the reservoirs of reactive pairs were 10--20 times smaller than for DMF+O/sub 2/. The furan+O/sub 2/ reaction rate showed an /sup 18/O/sub 2/ isotope effect 0.78 +- 0.15, which can be interpreted in terms of quantum mechanical tunneling on the lowest singlet hypersurface.

  11. Failures of TDDFT in describing the lowest intramolecular charge-transfer excitation in para-nitroaniline

    NASA Astrophysics Data System (ADS)

    Eriksen, Janus J.; Sauer, Stephan P. A.; Mikkelsen, Kurt V.; Christiansen, Ove; Jensen, Hans Jørgen Aa.; Kongsted, Jacob

    2013-07-01

    We investigate the failure of time-dependent density functional theory (TDDFT) with the CAM-B3LYP exchange-correlation (xc) functional coupled to the polarisable embedding (PE) scheme (PE-CAM-B3LYP) in reproducing the solvatochromic shift of the lowest intense charge-transfer excitation in para-nitroaniline (pNA) in water by comparing with results obtained with the coupled cluster singles and doubles (CCSD) model also coupled to the polarisable embedding scheme (PE-CCSD). We determine the amount of charge separation in the ground and excited charge-transfer state with both methods by calculating the electric dipole moments in the gas phase and for 100 solvent configurations. We find that CAM-B3LYP overestimates the amount of charge separation inherent in the ground state and TDDFT/CAM-B3LYP drastically underestimates this amount in the excited charge-transfer state. As the errors in the solvatochromatic shift are found to be inverse proportional to the change in dipole moment upon excitation, we conclude that the flaws in the description of the solvatochromic shift of this excitation are related to TDDFT itself and how it responds to the solvent effects modelled by the PE scheme. We recommend therefore to benchmark results of TDDFT calculations with CAM-B3LYP for intramolecular charge-transfer excitations in molecular systems similar to pNA against higher level ab initio wave function methods, like, e.g. CCSD, prior to their use. Using the calculated change in dipole moment upon excitation as a measure for charge-transfer character, we furthermore confirm that the difference between excitation energies calculated with TDDFT and with the Tamm-Dancoff approximation (TDA) to TDDFT is indeed correlated with the charge-transfer character of a given electronic transition both in vacuo and in solution. This is supported by a corresponding correlation between the change in dipole moment and the size of the Λ index diagnostic for the investigated CT excitation.

  12. Low-energy singlet excitations in spin-1/2 Heisenberg antiferromagnet on square lattice

    NASA Astrophysics Data System (ADS)

    Aktersky, A. Yu.; Syromyatnikov, A. V.

    2016-05-01

    We present an approach based on a dimer expansion which describes low-energy singlet excitations (singlons) in spin-1/2 Heisenberg antiferromagnet on simple square lattice. An operator ("effective Hamiltonian") is constructed whose eigenvalues give the singlon spectrum. The "effective Hamiltonian" looks like a Hamiltonian of a spin-1/2 magnet in strong external magnetic field and it has a gapped spectrum. It is found that singlet states lie above triplet ones (magnons) in the whole Brillouin zone except in the vicinity of the point (π , 0), where their energies are slightly smaller. Based on this finding, we suggest that a magnon decay is possible near (π , 0) into another magnon and a singlon which may contribute to the dip of the magnon spectrum near (π , 0) and reduce the magnon lifetime. It is pointed out that the singlon-magnon continuum may contribute to the continuum of excitations observed recently near (π , 0).

  13. Benchmark calculations on the lowest-energy singlet, triplet, and quintet states of the four-electron harmonium atom

    SciTech Connect

    Cioslowski, Jerzy; Strasburger, Krzysztof; Matito, Eduard

    2014-07-28

    For a wide range of confinement strengths ω, explicitly-correlated calculations afford approximate energies E(ω) of the ground and low-lying excited states of the four-electron harmonium atom that are within few μhartree of the exact values, the errors in the respective energy components being only slightly higher. This level of accuracy constitutes an improvement of several orders of magnitude over the previously published data, establishing a set of benchmarks for stringent calibration and testing of approximate electronic structure methods. Its usefulness is further enhanced by the construction of differentiable approximants that allow for accurate computation of E(ω) and its components for arbitrary values of ω. The diversity of the electronic states in question, which involve both single- and multideterminantal first-order wavefunctions, and the availability of the relevant natural spinorbitals and their occupation numbers make the present results particularly useful in research on approximate density-matrix functionals. The four-electron harmonium atom is found to possess the {sup 3}P{sub +} triplet ground state at strong confinements and the {sup 5}S{sub −} quintet ground state at the weak ones, the energy crossing occurring at ω ≈ 0.0240919.

  14. Benchmark calculations on the lowest-energy singlet, triplet, and quintet states of the four-electron harmonium atom.

    PubMed

    Cioslowski, Jerzy; Strasburger, Krzysztof; Matito, Eduard

    2014-07-28

    For a wide range of confinement strengths ω, explicitly-correlated calculations afford approximate energies E(ω) of the ground and low-lying excited states of the four-electron harmonium atom that are within few μhartree of the exact values, the errors in the respective energy components being only slightly higher. This level of accuracy constitutes an improvement of several orders of magnitude over the previously published data, establishing a set of benchmarks for stringent calibration and testing of approximate electronic structure methods. Its usefulness is further enhanced by the construction of differentiable approximants that allow for accurate computation of E(ω) and its components for arbitrary values of ω. The diversity of the electronic states in question, which involve both single- and multideterminantal first-order wavefunctions, and the availability of the relevant natural spinorbitals and their occupation numbers make the present results particularly useful in research on approximate density-matrix functionals. The four-electron harmonium atom is found to possess the (3)P+ triplet ground state at strong confinements and the (5)S- quintet ground state at the weak ones, the energy crossing occurring at ω ≈ 0.0240919. PMID:25084902

  15. Proton collisional excitation in the lowest lying 3P terms of ions in the Be and Mg isoelectronic sequences

    NASA Technical Reports Server (NTRS)

    Landman, D. A.; Brown, T.

    1979-01-01

    Proton collisional excitation cross sections and rate constants are presented for transitions between the 3P(J) fine-structure levels of the lowest-lying sp configurations in a number of astrophysically important ions belonging to the Be and Mg isoelectronic sequences. The calculations were made by direct integration of the Schroedinger equation resulting from semiclassical Coulomb excitation theory. The cross sections and rate constants for the 3P(J) transitions in the lowest-lying P(2) configurations are expected to be similar to those for the corresponding sp configuration transitions, and this is illustrated for C III. For the high-temperature ion Ca XVII alpha particle excitation is shown to be unimportant for situations involving ordinary values of the He/H abundance ratio. A simple, but apparently accurate method for determining certain radial integrals for low-lying excited configurations is proposed.

  16. Singlet-Triplet Excitations in the Unconventional Spin-Peierls System TiOBr

    NASA Astrophysics Data System (ADS)

    Clancy, J. P.; Gaulin, B. D.; Adams, C. P.; Granroth, G. E.; Kolesnikov, A. I.; Sherline, T. E.; Chou, F. C.

    2011-03-01

    TiOBr belongs to a select group of quasi-one-dimensional materials which undergo a spin-Peierls (SP) phase transition and develop a dimerized singlet ground state at low temperatures. However, unlike conventional SP systems, TiOBr exhibits not one, but two successive phase transitions upon cooling: a continuous transition into an incommensurate SP state at TC 2 ~ 48 K, followed by a discontinuous transition into a commensurate SP state at TC 1 ~ 27 K. We have performed time-of-flight neutron scattering measurements on powder samples of TiOBr using the fine-resolution Fermi chopper spectrometer (SEQUOIA) at the Spallation Neutron Source. These measurements reveal two branches of magnetic excitations within the commensurate and incommensurate SP phases, which we associate with n = 1 and n = 2 triplet excitations out of the singlet ground state. This study represents the first direct measure of the singlet-triplet energy gap in TiOBr, which we have determined to be Eg = 21.2 +/- 1.0 meV.

  17. Singlet and triplet excitation management in a bichromophoric near-infrared-phosphorescent BODIPY-benzoporphyrin platinum complex

    SciTech Connect

    Whited, M. T.; Djurovich, P. I.; Roberts, Sean T.; Durrell, A. C.; Schlenker, C. W.; Bradforth, Stephen E.; Thompson, Mark E.

    2011-01-12

    Multichromophoric arrays provide one strategy for assembling molecules with intense absorptions across the visible spectrum but are generally focused on systems that efficiently produce and manipulate singlet excitations and therefore are burdened by the restrictions of (a) unidirectional energy transfer and (b) limited tunability of the lowest molecular excited state. In contrast, we present here a multichromophoric array based on four boron dipyrrins (BODIPY) bound to a platinum benzoporphyrin scaffold that exhibits intense panchromatic absorption and efficiently generates triplets. The spectral complementarity of the BODIPY and porphryin units allows the direct observation of fast bidirectional singlet and triplet energy transfer processes (kST(1BDP→1Por) = 7.8 × 1011 s-1, kTT(3Por→3BDP) = 1.0 × 1010 s-1, kTT(3BDP→3Por) = 1.6 × 1010 s-1), leading to a long-lived equilibrated [3BDP][Por]⇌[BDP][3Por] state. This equilibrated state contains approximately isoenergetic porphyrin and BODIPY triplets and exhibits efficient near-infrared phosphorescence (λem = 772 nm, Φ = 0.26). Taken together, these studies show that appropriately designed triplet-utilizing arrays may overcome fundamental limitations typically associated with core-shell chromophores by tunable redistribution of energy from the core back onto the antennae.

  18. Twisting the Phenyls in Aryl Diphosphenes (Ar-P=P-Ar). Significant Impact upon Lowest Energy Excited States

    PubMed Central

    Peng, Huo-Lei; Payton, John L.; Protasiewicz, John D.

    2009-01-01

    Aryl diphosphenes (Ar-P=P-Ar) possess features that may make them useful in photonic devices, including the possibility for photochemical E-Z isomerization. Development of good models guided by computations is hampered by poor correspondence between predicted and experimental UV/vis absorption spectra. An hypothesis that the phenyl twist angle (i.e. PPCC torsion) accounts for this discrepancy is explored, with positive findings. DFT and TDDFT (B3LYP) were applied to the phenyl-P=P-phenyl (Ph-P=P-Ph) model compound over a range of phenyl twist angles, and to the Ph-P=P-Ph cores of two crystallographically characterized diphosphenes: bis-(2,4,6-tBu3C6H2)-diphosphene (Mes*-P=P-Mes*) and bis-(2,6-Mes2C6H3)-diphosphene (Dmp-P=P-Dmp). A shallow PES is observed: the full range of phenyl twist angles is accessible for under 5 kcal/mol. The Kohn-Sham orbitals (KS-MOs) exhibit stabilization and mixing of the two highest energy frontier orbitals – the n+ and π localized primarily on the – P=P– unit. A simple, single-configuration model based upon this symmetry-breaking is shown to be consistent with the major features of the measured UV/vis spectra of several diphosphenes. Detailed evaluation of singlet excitations, transition energies and oscillator strengths with TDDFT showed that the lowest energy transition (S1 ← S0) does not always correspond to the LUMO ← HOMO configuration. Coupling between the phenyl rings and central –P=P– destabilizes the π-π* dominated state. Hence, the S1 is always n+-π* in nature, even with a π-type HOMO. This coupling of the ring and –P=P– π systems engenders complexity in the UV/vis absorption region, and may be the origin of the variety of photobehaviours observed in diphosphenes. PMID:19496568

  19. Towards elucidating the energy of the first excited singlet state of xanthophyll cycle pigments by X-ray absorption spectroscopy.

    PubMed

    Gruszecki, W I; Stiel, H; Niedzwiedzki, D; Beck, M; Milanowska, J; Lokstein, H; Leupold, D

    2005-06-01

    The first excited singlet state (S(1)) of carotenoids (also termed 2A(g)(-)) plays a key role in photosynthetic excitation energy transfer due to its close proximity to the S(1) (Q(y)) level of chlorophylls. The determination of carotenoid 2A(g)(-) energies by optical techniques is difficult; transitions from the ground state (S(0), 1A(g)(-)) to the 2A(g)(-) state are forbidden ("optically dark") due to parity (g <-- //--> g) as well as pseudo-parity selection rules (- <-- //--> -). Of particular interest are S(1) energies of the so-called xanthophyll-cycle pigments (violaxanthin, antheraxanthin and zeaxanthin) due to their involvement in photoprotection in plants. Previous determinations of S(1) energies of violaxanthin and zeaxanthin by different spectroscopic techniques vary considerably. Here we present an alternative approach towards elucidation of the optically dark states of xanthophylls by near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The indication of at least one pi* energy level (about 0.5 eV below the lowest 1B(u)(+) vibronic sublevel) has been found for zeaxanthin. Present limitations and future improvements of NEXAFS to study optically dark states of carotenoids are discussed. NEXAFS combined with simultaneous optical pumping will further aid the investigation of these otherwise hardly accessible states. PMID:15949988

  20. The holographic recording in photopolymer by excitation forbidden singlet-triplet transitions

    NASA Astrophysics Data System (ADS)

    Shelkovnikov, V. V.; Pen, E. F.; Russkich, V. V.; Vasiliev, E. V.; Kovalevsky, V. I.

    2006-05-01

    The possibility and features of the holographic recording by excitation of the forbidden singlet-triplet transitions are considered in the report. The experimental demonstration of the hologram recording on forbidden transition is carried out in thick photopolymer material sensitized by Erithrozine dye. The single hologram with diffraction efficiency DE=50% and 16 angle multiplexing hologram were recorded by irradiation of the low intensity He-Ne laser (632 nm) at high concentration of the sensitizing dye and at high optical density in allowed absorption band of dye. The growth of DE of transition hologram depending on the Kr+(647 nm) laser irradiation intensity of was studied. The observed linear dependence of the maximal rate of DE growth on the intensity of recording irradiation was explained by two steps-two photon excitation (T I<--S 0, T II<--T I) of the dye in the photopolymer samples.

  1. The electronic origin and vibrational levels of the first excited singlet state of isocyanic acid (HNCO)

    SciTech Connect

    Berghout, H. Laine; Crim, F. Fleming; Zyrianov, Mikhail; Reisler, Hanna

    2000-04-15

    The combination of vibrationally mediated photofragment yield spectroscopy, which excites molecules prepared in single vibrational states, and multiphoton fluorescence spectroscopy, which excites molecules cooled in a supersonic expansion, provides detailed information on the energetics and vibrational structure of the first excited singlet state (S{sub 1}) of isocyanic acid (HNCO). Dissociation of molecules prepared in individual vibrational states by stimulated Raman excitation probes vibrational levels near the origin of the electronically excited state. Detection of fluorescence from dissociation products formed by multiphoton excitation through S{sub 1} of molecules cooled in a supersonic expansion reveals the vibrational structure at higher energies. Both types of spectra show long, prominent progressions in the N-C-O bending vibration built on states with different amounts of N-C stretching excitation and H-N-C bending excitation. Analyzing the spectra locates the origin of the S{sub 1} state at 32 449{+-}20 cm{sup -1} and determines the harmonic vibrational frequencies of the N-C stretch ({omega}{sub 3}=1034{+-}20 cm{sup -1}), the H-N-C bend ({omega}{sub 4}=1192{+-}19 cm{sup -1}), and the N-C-O bend ({omega}{sub 5}=599{+-}7 cm{sup -1}), values that are consistent with several ab initio calculations. The assigned spectra strongly suggest that the N-C stretching vibration is a promoting mode for internal conversion from S{sub 1} to S{sub 0}. (c) 2000 American Institute of Physics.

  2. Long-Lived Triplet Excited States of Bent-Shaped Pentacene Dimers by Intramolecular Singlet Fission.

    PubMed

    Sakuma, Takao; Sakai, Hayato; Araki, Yasuyuki; Mori, Tadashi; Wada, Takehiko; Tkachenko, Nikolai V; Hasobe, Taku

    2016-03-24

    Intramolecular singlet fission (ISF) is a promising photophysical process to construct more efficient light energy conversion systems as one excited singlet state converts into two excited triplet states. Herein we synthesized and evaluated bent-shaped pentacene dimers as a prototype of ISF to reveal intrinsic characters of triplet states (e.g., lifetimes of triplet excited states). In this study, meta-phenylene-bridged TIPS-pentacene dimer (PcD-3Ph) and 2,2'-bipheynyl bridged TIPS-pentacene dimer (PcD-Biph) were newly synthesized as bent-shaped dimers. In the steady-state spectroscopy, absorption and emission bands of these dimers were fully characterized, suggesting the appropriate degree of electronic coupling between pentacene moieties in these dimers. In addition, the electrochemical measurements were also performed to check the electronic interaction between two pentacene moieties. Whereas the successive two oxidation peaks owing to the delocalization were observed in a directly linked-pentacene dimer (PcD) by a single bond, the cyclic voltammograms in PcD-Biph and PcD-3Ph implied the weaker interaction compared to that of p-phenylene-bridged TIPS-pentacene dimer (PcD-4Ph) and PcD. The femtosecond and nanosecond transient absorption spectra clearly revealed the slower ISF process in bent-shaped pentacene dimers (PcD-Biph and PcD-3Ph), more notably, the slower relaxation of the excited triplet states in PcD-Biph and PcD-3Ph. Namely, the quantum yields of triplet states (ΦT) by ISF approximately remain constant (ca. 180-200%) in all dimer systems, whereas the lifetimes of the triplet excited states became much longer (up to 360 ns) in PcD-Biph as compared to PcD-4Ph (15 ns). Additionally, the lifetimes of the corresponding triplet states in PcD-Biph and PcD-3Ph were sufficiently affected by solvent viscosity. In particular, the lifetimes of PcD-Biph triplet state in THF/paraffin (1.0 μs) increased up to approximately three times as compared to that in THF

  3. Vibrational dynamics of aniline (N2)1 clusters in their first excited singlet state

    NASA Astrophysics Data System (ADS)

    Hineman, M. F.; Kim, S. K.; Bernstein, E. R.; Kelley, D. F.

    1992-04-01

    The first excited singlet state S1 vibrational dynamics of aniline(N2)1 clusters are studied and compared to previous results on aniline(CH4)1 and aniline(Ar)1. Intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP) rates fall between the two extremes of the CH4 (fast IVR, slow VP) and Ar (slow IVR, fast VP) cluster results as is predicted by a serial IVR/VP model using Fermi's golden rule to describe IVR processes and a restricted Rice-Ramsperger-Kassel-Marcus (RRKM) theory to describe unimolecular VP rates. The density of states is the most important factor determining the rates. Two product states, 00 and 10b1, of bare aniline and one intermediate state ˜(00) in the overall IVR/VP process are observed and time resolved measurements are obtained for the 000 and ˜(000) transitions. The results are modeled with the serial mechanism described above.

  4. Singlet and triplet excited state properties of natural chlorophylls and bacteriochlorophylls

    SciTech Connect

    Niedzwiedzki, Dariusz; Blankenship, R. E.

    2010-11-18

    Ten naturally occurring chlorophylls (a, b, c{sub 2}, d) and bacteriochlorophylls (a, b, c, d, e, g) were purified and studied using the optical spectroscopic techniques of both steady state and time-resolved absorption and fluorescence. The studies were carried out at room temperature in nucleophilic solvents in which the central Mg is hexacoordinated. The comprehensive studies of singlet excited state lifetimes show a clear dependency on the structural features of the macrocycle and terminal substituents. The wide-ranging studies of triplet state lifetime demonstrate the existence of an energy gap law for these molecules. The knowledge of the dynamics and the energies of the triplet state that were obtained in other studies allowed us to construct an energy gap law expression that can be used to estimate the triplet state energies of any (B)chlorophyll molecule from its triplet lifetime obtained in a liquid environment.

  5. Excited singlet molecular O2 (1Δg) is generated enzymatically from excited carbonyls in the dark

    PubMed Central

    Mano, Camila M.; Prado, Fernanda M.; Massari, Júlio; Ronsein, Graziella E.; Martinez, Glaucia R.; Miyamoto, Sayuri; Cadet, Jean; Sies, Helmut; Medeiros, Marisa H. G.; Bechara, Etelvino J. H.; Di Mascio, Paolo

    2014-01-01

    In mammalian tissues, ultraweak chemiluminescence arising from biomolecule oxidation has been attributed to the radiative deactivation of singlet molecular oxygen [O2 (1Δg)] and electronically excited triplet carbonyl products involving dioxetane intermediates. Herein, we describe evidence of the generation of O2 (1Δg) in aqueous solution via energy transfer from excited triplet acetone. This involves thermolysis of 3,3,4,4-tetramethyl-1,2-dioxetane, a chemical source, and horseradish peroxidase-catalyzed oxidation of 2-methylpropanal, as an enzymatic source. Both sources of excited carbonyls showed characteristic light emission at 1,270 nm, directly indicative of the monomolecular decay of O2 (1Δg). Indirect analysis of O2 (1Δg) by electron paramagnetic resonance using the chemical trap 2,2,6,6-tetramethylpiperidine showed the formation of 2,2,6,6-tetramethylpiperidine-1-oxyl. Using [18O]-labeled triplet, ground state molecular oxygen [18O2 (3Σg-)], chemical trapping of 18O2 (1Δg) with disodium salt of anthracene-9,10-diyldiethane-2,1-diyl disulfate yielding the corresponding double-[18O]-labeled 9,10-endoperoxide, was detected through mass spectrometry. This corroborates formation of O2 (1Δg). Altogether, photoemission and chemical trapping studies clearly demonstrate that chemically and enzymatically nascent excited carbonyl generates 18O2 (1Δg) by triplet-triplet energy transfer to ground state oxygen O2 (3Σg−), and supports the long formulated hypothesis of O2 (1Δg) involvement in physiological and pathophysiological events that might take place in tissues in the absence of light. PMID:25087485

  6. Aromaticity Reversal in the Lowest Excited Triplet State of Archetypical Möbius Heteroannulenic Systems.

    PubMed

    Oh, Juwon; Sung, Young Mo; Kim, Woojae; Mori, Shigeki; Osuka, Atsuhiro; Kim, Dongho

    2016-05-23

    The aromaticity reversal in the lowest triplet state (T1 ) of a comparable set of Hückel/Möbius aromatic metalated expanded porphyrins was explored by optical spectroscopy and quantum calculations. In the absorption spectra, the T1 states of the Möbius aromatic species showed broad, weak, and ill-defined spectral features with small extinction coefficients, which is in line with typical antiaromatic expanded porphyrins. In combination with quantum calculations, these results indicate that the Möbius aromatic nature of the S0 state is reversed to Möbius antiaromaticity in the T1 state. This is the first experimental observation of aromaticity reversal in the T1 state of Möbius aromatic molecules. PMID:27079620

  7. Paramagnetic excitations in singlet ground state PrNi{sub 2}Si{sub 2}

    SciTech Connect

    Blanco, J.A.; Nicklow, R.M.; Schmitt, D.

    1997-11-01

    The dispersion curves for magnetic excitations along the principal directions of the Brillouin zone of the singlet ground state compound PrNi{sub 2}Si{sub 2} have been measured at 30 K by inelastic neutron-scattering experiments in the paramagnetic phase. From these data we have extracted information about the main exchange interaction coupling parameters J{sub ij} that are responsible for the appearance of an amplitude modulated magnetic order in this compound below T{sub N}=20K. The results are consistent with the long-range character of Ruderman-Kittel-Kasuya-Yosida interactions because in order to describe the dispersion curves observed at 30 K it is necessary to consider the influence of the interaction to the eighth nearest-neighbor Pr{sup 3+} ion along the c direction. In addition, we have studied the thermal dependence of the excitations at several representative points in the Brillouin zone that show an important softening as the temperature is lowered to T{sub N}. {copyright} {ital 1997} {ital The American Physical Society}

  8. High-Yield Excited Triplet States in Pentacene Self-Assembled Monolayers on Gold Nanoparticles through Singlet Exciton Fission.

    PubMed

    Kato, Daiki; Sakai, Hayato; Tkachenko, Nikolai V; Hasobe, Taku

    2016-04-18

    One of the major drawbacks of organic-dye-modified self-assembled monolayers on metal nanoparticles when employed for efficient use of light energy is the fact that singlet excited states on dye molecules can be easily deactivated by means of energy transfer to the metal surface. In this study, a series of 6,13-bis(triisopropylsilylethynyl)pentacene-alkanethiolate monolayer protected gold nanoparticles with different particle sizes and alkane chain lengths were successfully synthesized and were employed for the efficient generation of excited triplet states of the pentacene derivatives by singlet fission. Time-resolved transient absorption measurements revealed the formation of excited triplet states in high yield (172±26 %) by suppressing energy transfer to the gold surface. PMID:26997657

  9. Theoretical description of electronically excited vinylidene up to 10 eV: first high level ab initio study of singlet valence and Rydberg states.

    PubMed

    Boyé-Péronne, Séverine; Gauyacq, Dolores; Liévin, Jacques

    2014-11-01

    The first quantitative description of the Rydberg and valence singlet electronic states of vinylidene lying in the 0-10 eV region is performed by using large scale ab initio calculations. A deep analysis of Rydberg-valence interactions has been achieved thanks to the comprehensive information contained in the accurate Multi-Reference Configuration Interaction wavefunctions and an original population analysis highlighting the respective role played by orbital and state mixing in such interactions. The present theoretical approach is thus adequate for dealing with larger than diatomic Rydberg systems. The nine lowest singlet valence states have been optimized. Among them, some are involved in strong Rydberg-valence interactions in the region of the Rydberg state equilibrium geometry. The Rydberg states of vinylidene present a great similarity with the acetylene isomer, concerning their quantum defects and Rydberg molecular orbital character. As in acetylene, strong s-d mixing is revealed in the n = 3 s-d supercomplex. Nevertheless, unlike in acetylene, the close-energy of the two vinylidene ionic cores (2)A1 and (2)B1 results into two overlapped Rydberg series. These Rydberg series exhibit local perturbations when an accidental degeneracy occurs between them and results in avoided crossings. In addition, some Δl = 1 (s-p and p-d) mixings arise for some Rydberg states and are rationalized in term of electrostatic interaction from the electric dipole moment of the ionic core. The strongest dipole moment of the (2)B1 cationic state also stabilizes the lowest members of the n = 3 Rydberg series converging to this excited state, as compared to the adjacent series converging toward the (2)A1 ionic ground state. The overall energies of vinylidene Rydberg states lie above their acetylene counterpart. Finally, predictions for optical transitions in singlet vinylidene are suggested for further experimental spectroscopic characterization of vinylidene. PMID:25381524

  10. Theoretical description of electronically excited vinylidene up to 10 eV: First high level ab initio study of singlet valence and Rydberg states

    NASA Astrophysics Data System (ADS)

    Boyé-Péronne, Séverine; Gauyacq, Dolores; Liévin, Jacques

    2014-11-01

    The first quantitative description of the Rydberg and valence singlet electronic states of vinylidene lying in the 0-10 eV region is performed by using large scale ab initio calculations. A deep analysis of Rydberg-valence interactions has been achieved thanks to the comprehensive information contained in the accurate Multi-Reference Configuration Interaction wavefunctions and an original population analysis highlighting the respective role played by orbital and state mixing in such interactions. The present theoretical approach is thus adequate for dealing with larger than diatomic Rydberg systems. The nine lowest singlet valence states have been optimized. Among them, some are involved in strong Rydberg-valence interactions in the region of the Rydberg state equilibrium geometry. The Rydberg states of vinylidene present a great similarity with the acetylene isomer, concerning their quantum defects and Rydberg molecular orbital character. As in acetylene, strong s-d mixing is revealed in the n = 3 s-d supercomplex. Nevertheless, unlike in acetylene, the close-energy of the two vinylidene ionic cores 2A1 and 2B1 results into two overlapped Rydberg series. These Rydberg series exhibit local perturbations when an accidental degeneracy occurs between them and results in avoided crossings. In addition, some Δl = 1 (s-p and p-d) mixings arise for some Rydberg states and are rationalized in term of electrostatic interaction from the electric dipole moment of the ionic core. The strongest dipole moment of the 2B1 cationic state also stabilizes the lowest members of the n = 3 Rydberg series converging to this excited state, as compared to the adjacent series converging toward the 2A1 ionic ground state. The overall energies of vinylidene Rydberg states lie above their acetylene counterpart. Finally, predictions for optical transitions in singlet vinylidene are suggested for further experimental spectroscopic characterization of vinylidene.

  11. Theoretical description of electronically excited vinylidene up to 10 eV: First high level ab initio study of singlet valence and Rydberg states

    SciTech Connect

    Boyé-Péronne, Séverine; Gauyacq, Dolores; Liévin, Jacques

    2014-11-07

    The first quantitative description of the Rydberg and valence singlet electronic states of vinylidene lying in the 0–10 eV region is performed by using large scale ab initio calculations. A deep analysis of Rydberg-valence interactions has been achieved thanks to the comprehensive information contained in the accurate Multi-Reference Configuration Interaction wavefunctions and an original population analysis highlighting the respective role played by orbital and state mixing in such interactions. The present theoretical approach is thus adequate for dealing with larger than diatomic Rydberg systems. The nine lowest singlet valence states have been optimized. Among them, some are involved in strong Rydberg-valence interactions in the region of the Rydberg state equilibrium geometry. The Rydberg states of vinylidene present a great similarity with the acetylene isomer, concerning their quantum defects and Rydberg molecular orbital character. As in acetylene, strong s-d mixing is revealed in the n = 3 s-d supercomplex. Nevertheless, unlike in acetylene, the close-energy of the two vinylidene ionic cores {sup 2}A{sub 1} and {sup 2}B{sub 1} results into two overlapped Rydberg series. These Rydberg series exhibit local perturbations when an accidental degeneracy occurs between them and results in avoided crossings. In addition, some Δl = 1 (s-p and p-d) mixings arise for some Rydberg states and are rationalized in term of electrostatic interaction from the electric dipole moment of the ionic core. The strongest dipole moment of the {sup 2}B{sub 1} cationic state also stabilizes the lowest members of the n = 3 Rydberg series converging to this excited state, as compared to the adjacent series converging toward the {sup 2}A{sub 1} ionic ground state. The overall energies of vinylidene Rydberg states lie above their acetylene counterpart. Finally, predictions for optical transitions in singlet vinylidene are suggested for further experimental spectroscopic

  12. NpNn dependence of empirical formula for the lowest excitation energy of the 2+ states in even even nuclei

    NASA Astrophysics Data System (ADS)

    Jin, Guanghao; Yoon, Jin-Hee; Cha, Dongwoo

    2008-03-01

    We examine the effects of the additional term of the type {\\sim} e^{- \\lambda^{\\prime} N_pN_n} on the recently proposed empirical formula for the lowest excitation energy of the 2+ states in even-even nuclei. This study is motivated by the fact that this term carries the favorable dependence of the valence nucleon numbers dictated by the NpNn scheme. We show explicitly that there is not any improvement in reproducing Ex(2+1) by including the extra NpNn term. However, our study also reveals that the excitation energies Ex(2+1), when calculated by the NpNn term alone (with the mass number A dependent term), are quite comparable to those calculated by the original empirical formula.

  13. Does electron-transfer theory explain large rate differences in singlet and triplet excited state electron-transfer reactions?

    SciTech Connect

    Zusman, L.D.; Kurnikov, I.V.; Beratan, D.N.

    1995-12-31

    Gray and coworkers have shown that intramolecular electron-transfer rates from singlet and triplet excited states in iridium(spacer)pyridinium complexes can be vastly different (>5 orders of magnitude). We have analyzed the possible sources of these differences, including effects that may arise from reorganization energies, free energies, and tunneling matrix elements. When distance dependent reorganization energies and energy dependent tunneling matrix elements are included, a systematic framework emerges to describe these electron-transfer reactions.

  14. Generating Light from Upper Excited Triplet States: A Contribution to the Indirect Singlet Yield of a Polymer OLED, Helping to Exceed the 25% Singlet Exciton Limit

    PubMed Central

    Jankus, Vygintas; Aydemir, Murat; Dias, Fernando B.

    2016-01-01

    The mechanisms by which light is generated in an organic light emitting diode have slowly been elucidated over the last ten years. The role of triplet annihilation has demonstrated how the “spin statistical limit” can be surpassed, but it cannot account for all light produced in the most efficient devices. Here, a further mechanism is demonstrated by which upper excited triplet states can also contribute to indirect singlet production and delayed fluorescence. Since in a device the population of these TN states is large, this indirect radiative decay channel can contribute a sizeable fraction of the total emission measured from a device. The role of intra‐ and interchain charge transfer states is critical in underpinning this mechanism. PMID:27610333

  15. Bright fission: singlet fission into a pair of emitting states.

    PubMed

    Casanova, David

    2015-06-01

    This paper reintroduces and explores the generation of two bright states from a single photon via a singlet fission mechanism in organic materials. This particular photophysical process is labeled here as bright fission (BF). The central part of the study is devoted to set the theoretical foundations of BF by discussing possible electronic mechanisms, the role of different excited states with various physical nature, the presence of competing deactivation channels, and the possible requirements for the BF viability. In a second part, some of the properties related to BF are computationally explored in anthracene. The analysis of computed high-lying excited states identifies several optical transitions as good candidates to trigger BF in anthracene. The approximation of excitonic couplings of these high energy levels to other electronic states within the same energy range suggests possible paths to populate electronic configurations potentially able to split in two independent spin singlets, i.e. singlet-singlet states. The study also explores the electronic structure of the energetically lowest singlet-singlet states in anthracene dimers and discusses the presence of charge transfer configurations and their relation to the singlet-singlet manifold. The computational results suggest fast relaxation to the lowest singlet-singlet state, from which the excitonic fission may occur. All in all, the present work aims at motivating to pursue further efforts in the study of the BF process in organic materials. PMID:26575561

  16. Double excitations and state-to-state transition dipoles in π-π∗ excited singlet states of linear polyenes: Time-dependent density-functional theory versus multiconfigurational methods

    NASA Astrophysics Data System (ADS)

    Mikhailov, Ivan A.; Tafur, Sergio; Masunov, Artëm E.

    2008-01-01

    The effect of static and dynamic electron correlation on the nature of excited states and state-to-state transition dipole moments is studied with a multideterminant wave function approach on the example of all-trans linear polyenes ( C4H6 , C6H8 , and C8H10 ). Symmetry-forbidden singlet nAg states were found to separate into three groups: purely single, mostly single, and mostly double excitations. The excited-state absorption spectrum is dominated by two bright transitions: 1Bu-2Ag and 1Bu-mAg , where mAg is the state, corresponding to two-electron excitation from the highest occupied to lowest unoccupied molecular orbital. The richness of the excited-state absorption spectra and strong mixing of the doubly excited determinants into lower- nAg states, reported previously at the complete active space self-consistent field level of theory, were found to be an artifact of the smaller active space, limited to π orbitals. When dynamic σ-π correlation is taken into account, single- and double-excited states become relatively well separated at least at the equilibrium geometry of the ground state. This electronic structure is closely reproduced within time-dependent density-functional theory (TD DFT), where double excitations appear in a second-order coupled electronic oscillator formalism and do not mix with the single excitations obtained within the linear response. An extension of TD DFT is proposed, where the Tamm-Dancoff approximation (TDA) is invoked after the linear response equations are solved (a posteriori TDA). The numerical performance of this extension is validated against multideterminant-wave-function and quadratic-response TD DFT results. It is recommended for use with a sum-over-states approach to predict the nonlinear optical properties of conjugated molecules.

  17. Saturable absorption dynamics in the triplet system and triplet excitation induced singlet fluorescence of some organic molecules

    NASA Astrophysics Data System (ADS)

    Gratz, H.; Penzkofer, A.

    2001-01-01

    The triplet saturable absorption behaviour of the xanthene dyes eosin Y, erythrosin B, and rose bengal and of the fullerene molecule C 70 is studied. The molecules are excited to the S 1-state by intense picosecond pulses (wavelength λP=527 nm). They relax dominantly to the triplet system by intersystem crossing. The triplet-triplet saturable absorption is investigated with time-delayed intense picosecond pulses (wavelength λL=1054 nm) in the transparency region of the molecules in the singlet ground state. Higher excited-state triplet absorption cross-sections and higher excited-state triplet relaxation times are determined by numerical simulation of the experimental results. Time-resolved fluorescence measurements reveal higher excited-state triplet to singlet back-intersystem-crossing and multi-step triplet photoionization. Additionally the two-photon absorption cross-sections at λL=1054 nm are determined by measurement of the fundamental pulse two-photon induced fluorescence relative to the second-harmonic pulse single-photon induced fluorescence.

  18. Relativistic corrections to the non-Born-Oppenheimer energies of the lowest singlet Rydberg states of 3He and 4He.

    PubMed

    Stanke, Monika; Kedziera, Dariusz; Bubin, Sergiy; Adamowicz, Ludwik

    2007-05-21

    In this work the authors present an approach to calculate the leading-order relativistic corrections for ground and excited states of helium isotopomers. In the calculations they used variational wave functions expanded in terms of explicitly correlated Gaussians obtained without assuming the Born-Oppenheimer approximation. PMID:17523809

  19. The effect of gold nanoparticles on exchange processes in collision complexes of triplet and singlet oxygen molecules with excited eosin molecules

    NASA Astrophysics Data System (ADS)

    Bryukhanov, V. V.; Minaev, B. M.; Tsibul'nikova, A. V.; Slezhkin, V. A.

    2015-07-01

    We have studied exchange processes in contact complexes of triplet eosin molecules with oxygen molecules in the triplet (3Σ{/g -}) and singlet (1Δ g ) states in thin polyvinylbutyral films in the presence of gold nanoparticles. Upon resonant excitation of surface plasmons in gold nanoparticles into the absorption band of eosin molecules-singlet oxygen sensitizers-we have obtained an increase in the intensity of the delayed fluorescence and an increase in the lifetime of the dye with simultaneous quenching of the luminescence of singlet oxygen. The kinetics of the delayed fluorescence of the dye as a result of singlet-triplet annihilation of triplet eosin molecules with singlet oxygen molecules has been investigated. To compare theoretical and experimental data, we have numerically simulated energy transfer processes. Rate constants of energy transfer and of singlet-triplet annihilation, as well as quenching constants of triplet states of the dye by molecular oxygen, have been calculated. Luminescence quantum yield 1Δ g of polyvinylbutyral has been estimated. We have analyzed quantum-chemically electronic mechanisms of singlet-triplet annihilation of oxygen and eosin.

  20. Analysis of the Lowest In-Plane Bend and First Excited Torsional State of CH_3CH_2CN

    NASA Astrophysics Data System (ADS)

    Brauer, Carolyn S.; Pearson, John C.; Drouin, Brian J.; Yu, Shanshan

    2009-06-01

    Propionitrile (CH_3CH_2CN) is observed with large column densities in a number of high-mass star-forming cores, where core temperatures exceed 200 K. It is a near-prolate (κ=0.96) asymmetric top with appreciable dipole moment components on both the a- and b-axes (μ_a = 3.84 D, μ_b = 1.23 D). This, combined with the presence of four fundamental modes as well as four overtones and combination bands all occurring below 600 cm^{-1}, results in a very rich spectrum. It is known to be a major contributor to spectral line confusion in ground-based observations and is expected to complicate observations by Herschel, SOFIA and ALMA, making it imperative to fully characterize the entire spectrum. The lowest in-plane bend, ν_{13}, is 206.9(0.5) cm^{-1}, and the first excited torsional state, ν_{21}, which is just 186 GHz above, have been detected in hot cores with antenna temperatures of a few Kelvin. The close proximity of ν_{13} and ν_{21}, as well as their low-lying nature, offers a unique opportunity to study the vibration-torsion-rotation coupling problem in the case of two nearly degenerate vibrational states. As expected from C_s symmetry and their A^' and A^'' nature, these states exhibit strong a- and b-symmetry Coriolis interactions, as well as interactions resulting from different sets of Eckhart-Sayvetz conditions being required in ν_{13} and ν_{21}. In the present work, the ν_{13} and ν_{21} states of propionitrile have been analyzed to high frequency and angular momentum quantum number. The spectrum, molecular constants,and insights into the vibration-torsion-rotation problem will be discussed. H. M. Heise, H. Lutz & H. Dreizler,Z.Nat.,29a,1345 (1974). H. M. Heise, F. Winther & H. Lutz,J. Mol. Spectrosc.,90,531 (1981). D. M. Mehringer, J. C. Pearson, J. Keene & T. G. Phillips,Ap.J.,608,306 (2004).

  1. Photophysics of Threaded sp-Carbon Chains: The Polyyne is a Sink for Singlet and Triplet Excitation

    PubMed Central

    2014-01-01

    We have used single-crystal X-ray diffraction and time-resolved UV–NIR–IR absorption spectroscopy to gain insights into the structures and excited-state dynamics of a rotaxane consisting of a hexayne chain threaded through a phenanthroline macrocycle and a family of related compounds, including the rhenium(I) chlorocarbonyl complex of this rotaxane. The hexayne unit in the rhenium-rotaxane is severely nonlinear; it is bent into an arc with an angle of 155.6(1)° between the terminal C1 and C12 atoms and the centroid of the central C–C bond, with the most acute distortion at the point where the polyyne chain pushes against the Re(CO)3Cl unit. There are strong through-space excited-state interactions between the components of the rotaxanes. In the metal-free rotaxane, there is rapid singlet excitation energy transfer (EET) from the macrocycle to the hexayne (τ = 3.0 ps), whereas in the rhenium-rotaxane there is triplet EET, from the macrocycle complex 3MLCT state to the hexayne (τ = 1.5 ns). This study revealed detailed information on the short-lived higher excited state of the hexayne (lifetime ∼1 ps) and on structural reorganization and cooling of hot polyyne chains, following internal conversion (over ∼5 ps). Comparison of the observed IR bands of the excited states of the hexayne with results from time-dependent density functional calculations (TD DFT) shows that these excited states have high cumulenic character (low bond length alternation) around the central region of the chain. These findings shed light on the complex interactions between the components of this supramolecular rotaxane and are important for the development of materials for the emerging molecular and nanoscale electronics. PMID:25474628

  2. Photophysics of threaded sp-carbon chains: the polyyne is a sink for singlet and triplet excitation.

    PubMed

    Movsisyan, Levon D; Peeks, Martin D; Greetham, Gregory M; Towrie, Michael; Thompson, Amber L; Parker, Anthony W; Anderson, Harry L

    2014-12-31

    We have used single-crystal X-ray diffraction and time-resolved UV-NIR-IR absorption spectroscopy to gain insights into the structures and excited-state dynamics of a rotaxane consisting of a hexayne chain threaded through a phenanthroline macrocycle and a family of related compounds, including the rhenium(I) chlorocarbonyl complex of this rotaxane. The hexayne unit in the rhenium-rotaxane is severely nonlinear; it is bent into an arc with an angle of 155.6(1)° between the terminal C1 and C12 atoms and the centroid of the central C-C bond, with the most acute distortion at the point where the polyyne chain pushes against the Re(CO)3Cl unit. There are strong through-space excited-state interactions between the components of the rotaxanes. In the metal-free rotaxane, there is rapid singlet excitation energy transfer (EET) from the macrocycle to the hexayne (τ = 3.0 ps), whereas in the rhenium-rotaxane there is triplet EET, from the macrocycle complex (3)MLCT state to the hexayne (τ = 1.5 ns). This study revealed detailed information on the short-lived higher excited state of the hexayne (lifetime ∼1 ps) and on structural reorganization and cooling of hot polyyne chains, following internal conversion (over ∼5 ps). Comparison of the observed IR bands of the excited states of the hexayne with results from time-dependent density functional calculations (TD DFT) shows that these excited states have high cumulenic character (low bond length alternation) around the central region of the chain. These findings shed light on the complex interactions between the components of this supramolecular rotaxane and are important for the development of materials for the emerging molecular and nanoscale electronics. PMID:25474628

  3. Photosensitization of singlet oxygen formation by pterins and flavins. Time-resolved studies of oxygen phosphorescence under laser excitation.

    PubMed

    Egorov, S Y; Krasnovsky, A A; Bashtanov, M Y; Mironov, E A; Ludnikova, T A; Kritsky, M S

    1999-10-01

    To elucidate the biochemical roles of singlet molecular oxygen (1(O2)) in the light-dependent reactions photosensitized by biological blue-light photoreceptors, time-resolved measurements of photosensitized 1O2 phosphorescence (1270 nm) were performed in air-saturated aqueous ((D2)O) solutions of pterins (2-amino-4-hydroxy-6,7-dimethylpteridine (DMP) and 2-amino-4-hydroxy-6-tetrahydroxybutyl-(D-arabo)pteridine (TOP)) and flavins (riboflavin and flavin mononucleotide (FMN)) under excitation with nitrogen laser (337.1 nm) pulses. The 1(O2) quantum yields were found to be 0.16, 0.20, 0.50, and 0.50 for DMP, TOP, riboflavin, and FMN, respectively. The data indicate that pterins and flavins are rather efficient photosensitizers of 1(O2) production that might be important for their photobiological functions. PMID:10561557

  4. Thermally activated delayed fluorescence as a cycling process between excited singlet and triplet states: Application to the fullerenes

    NASA Astrophysics Data System (ADS)

    Baleizão, Carlos; Berberan-Santos, Mário N.

    2007-05-01

    In efficient thermally activated delayed fluorescence (TADF) the excited chromophore alternates randomly between the singlet and triplet manifolds a large number of times before emission occurs. In this work, the average number of cycles n¯ is obtained and is shown to have a simple experimental meaning: n¯+1 is the intensification factor of the prompt fluorescence intensity, owing to the occurrence of TADF. A new method of data analysis for the determination of the quantum yield of triplet formation, combining steady-state and time-resolved data in a single plot, is also presented. Application of the theoretical results to the TADF of [70]fullerenes shows a general good agreement between different methods of fluorescence analysis and allows the determination of several photophysical parameters.

  5. Spin-Free CC2 Implementation of Induced Transitions between Singlet Ground and Triplet Excited States.

    PubMed

    Helmich-Paris, Benjamin; Hättig, Christof; van Wüllen, Christoph

    2016-04-12

    In most organic molecules, phosphorescence has its origin in transitions from triplet exited states to the singlet ground state, which are spin-forbidden in nonrelativistic quantum mechanics. A sufficiently accurate description of phosphorescence lifetimes for molecules that contain only light elements can be achieved by treating the spin-orbit coupling (SOC) with perturbation theory (PT). We present an efficient implementation of this approach for the approximate coupled cluster singles and doubles model CC2 in combination with the resolution-of-the-identity approximation for the electron repulsion integrals. The induced oscillator strengths and phosphorescence lifetimes from SOC-PT are computed within the response theory framework. In contrast to previous work, we employ an explicitly spin-coupled basis for singlet and triplet operators. Thereby, a spin-orbital treatment can be entirely avoided for closed-shell molecules. For compounds containing only light elements, the phosphorescence lifetimes obtained with SOC-PT-CC2 are in good agreement with those of exact two-component (X2C) CC2, whereas the calculations are roughly 12 times faster than with X2C. Phosphorescence lifetimes computed for two thioketones with the SOC-PT-CC2 approach agree very well with reference results from experiment and are similar to those obtained with multireference spin-orbit configuration interaction and with X2C-CC2. An application to phosphorescent emitters for metal-free organic light-emitting diodes (OLEDs) with almost 60 atoms and more than 1800 basis functions demonstrates how the approach extends the applicability of coupled cluster methods for studying phosphorescence. The results indicate that other decay channels like vibrational relaxation may become important in such systems if lifetimes are large. PMID:26881830

  6. Difluorodiazirine (CF2N2): A comparative quantum mechanical study of the first triplet and first singlet excited states

    NASA Astrophysics Data System (ADS)

    Terrabuio, Luiz Alberto; Haiduke, Roberto Luiz Andrade; Matta, Chérif F.

    2016-07-01

    3,3‧-Difluorodiazirine is a precursor of difluorocarbene radical (:CF2) which is used in organic synthesis and photo affinity labelling. This molecule possesses no dipole moment in the ground electronic state (S0) but has a significant dipole moment (of magnitude ~0.97 D) in both its first (triplet, T1) and second (singlet S1) excited states. These equal dipole moments are shown to originate from widely differing atomic polarization and inter-atomic charge transfer terms (defined by the Quantum Theory of Atoms in Molecules (QTAIM)). The calculated vertical/adiabatic excitation energies for the T1 and S1 states are 2.81/2.63 and 3.99/3.78 eV, respectively. Geometries, vibrational frequencies, atomic charges and spin populations, and the localization-delocalization matrices (LDMs) (Matta, J. Comput. Chem. 35 (2014) 1165) of the excited states are compared with those of the ground state. All calculations have been conducted at the (U)QCISD/aug-cc-pVTZ level of theory.

  7. Surfactant-enhanced singlet energy transfer from the charge-transfer excited state of tris(2,2-bipyridine) ruthenium(II)

    NASA Astrophysics Data System (ADS)

    Mandal, Krishnagopal; Demas, J. N.

    1981-12-01

    Very efficient (45-75%) sodium lauryl sulfate (NaLS) enhanced singlet enengy transfer has been demonstrated from the spin-orbit charge-transfer excited state of [Ru(bpy) 3] 2+ (bpy = 2,2'-bipyridine) to the xxx violet, oxazine 1, and rhodamine 101 at concentrations of 10 -5 M, Energy transfer occurs in xxx.

  8. Vertical Singlet Excitations on Adenine Dimer: A Time Dependent Density Functional Study

    NASA Astrophysics Data System (ADS)

    Crespo-Hernández, Carlos E.; Marai, Christopher N. J.

    2007-12-01

    The condense phase, excited state dynamics of the adenylyl(3'→5')adenine (ApA) dinucleotide has been previously studied using transient absorption spectroscopy with femtosecond time resolution (Crespo-Hernández et al. Chem. Rev. 104, 1977-2019 (2004)). An ultrafast and a long-lived component were observed with time constants of <1 ps and 60±16 ps, respectively. Comparison of the time constants measured for the dinucleotide with that for the adenine nucleotide suggested that the fast component observed in ApA could be assigned to monomer dynamics. The long-lived component observed in ApA was assigned to an excimer state that originates from a fraction of base stacked conformations present at the time of excitation. In this contribution, supermolecule calculations using the time dependent implementation of density functional theory is used to provide more insights on the origin of the initial Franck-Condon excitations. Monomer-like, localized excitations are observed for conformations having negligible base stacking interactions, whereas delocalized excitations are predicted for conformations with significant vertical base-base overlap.

  9. Photosensitized Singlet Oxygen Production upon Two-Photon Excitation of Single-Walled Carbon Nanotubes and Their Functionalized Analogs

    PubMed Central

    Gandra, Naveen; Chiu, Pui Lam; Li, Wenbing; Anderson, Yolanda R.; Mitra, Somenath; He, Huixin; Gao, Ruomei

    2009-01-01

    Single-walled carbon nanotubes (SWNTs) functionalized with -COOH (along with some sulphonation and nitration), and/or modified with chitosan were prepared and tested for their singlet oxygen (1O2) production. The emission from 1O2 observed upon SWNT irradiation at 532 nm was due to a two-photon process, while 1O2 production via excitation at 355 nm occurred through a conventional one-photon pathway. The relative quantum yield of 1O2 production at excitation wavelength of 532 nm was found to be 0.00, 0.07-0.13 and 0.24-0.54 for highly-functionalized, partially-functionalized and non-functionalized SWNT samples respectively. The nanotube-mediated generation of 1O2 may find applications in both targeted destruction of tumor cells and selective degradation of drug molecules. Our research provides a practical approach to modulate the production of reactive oxygen species from SWNTs via surface functionalization/modification. PMID:20046942

  10. Polymer characterization using the time-resolved phosphorescence of singlet oxygen as a spectroscopic probe

    SciTech Connect

    Ogilby, P.R.; Kristiansen, M.; Dillon, M.P. . Dept. of Chemistry); Taylor, V.L.; Clough, R.L. )

    1990-01-01

    The lowest excited electronic state of molecular oxygen, singlet oxygen ({sup 1}{Delta}{sub g}0{sub 2}), can be produced in solid organic polymers by a variety of different methods. Once produced, singlet oxygen will return to the ground triplet state by two pathways, radiative (phosphorescence) and non-radiative decay. Although the quantum efficiency of phosphorescence is small ({minus}10{sup {minus}5}), singlet oxygen can be detected by its emission at 1270 mn in both steady-state and time-resolved experiments. The phosphorescence of singlet oxygen can be used to characterize many properties of a solid organic polymer. 2 refs., 5 figs.

  11. Effects of singlet and triplet excitation, oxidation and axial coordination on the bond orders in the macrocycle of bacteriochlorophyll a as revealed by resonance Raman spectroscopy

    NASA Astrophysics Data System (ADS)

    Koyama, Yasushi; Limantara, Leenawaty

    1998-08-01

    Application of resonance-Raman spectroscopy to bacteriochlorophyll a (BChl a) to detect the effects of excitation to the lowest triplet (T 1) and singlet (S 1) excited states, of one-electron oxidation to the cation radical (D 0) state, and of the state of axial co-ordination on the bond orders in the macrocycle is reviewed; reference is made also to the case of bacteriopheophytin a (BPhe a) to show the effect of the central magnesium. The Raman lines of BChl a and BPhe a in the S 0, T 1 and S 1 states have been assigned empirically based on the isotope shifts upon full 15N substitution. Shifts of the characterized Raman lines upon excitation to the T 1 and S 1 states are correlated to changes in the bond orders: In BChl a, systematic changes in the bond orders, i.e. decrease in the bond orders of the C a'-C m and C a-C b bonds and increase in the bond order of the C a-N bonds, are found in the order, the S 0, T 1 and then S 1 state. In BPhe a, the bond order of the C a'-C m bonds decreases almost equally upon triplet and singlet excitation. The penta- and hexa-coordinated states (with one and two axial ligands, respectively) have been identified in T 1 BChl a and D 0 BChl a+rad by two different regions of the ring-breathing (C a'-C m stretching) frequencies. The classification of solvents into the penta- and hexa-coordinating ones agree among three different electronic states, i.e. S 0 BChl a, T 1 BChl a and D 0 BChl a+rad (except for the case of 2-propanol in which transformation from the penta- to the hexa-coordinated state takes place upon oxidation). The ring-breathing frequency is always lower in the hexa-coordinated state than in the penta-coordinated state, suggesting a decrease in the bond order of the C a'-C m bonds in the former. The key feature of the hexa-coordinated state is the expansion of the macrocycle core to accommodate in its plane the magnesium atom to enable the equivalent axial ligation of two solvent molecules, whereas the key feature of

  12. Time-resolved photoelectron spectroscopy of a dinuclear Pt(II) complex: Tunneling autodetachment from both singlet and triplet excited states of a molecular dianion.

    PubMed

    Winghart, Marc-Oliver; Yang, Ji-Ping; Vonderach, Matthias; Unterreiner, Andreas-Neil; Huang, Dao-Ling; Wang, Lai-Sheng; Kruppa, Sebastian; Riehn, Christoph; Kappes, Manfred M

    2016-02-01

    Time-resolved pump-probe photoelectron spectroscopy has been used to study the relaxation dynamics of gaseous [Pt2(μ-P2O5H2)4 + 2H](2-) after population of its first singlet excited state by 388 nm femtosecond laser irradiation. In contrast to the fluorescence and phosphorescence observed in condensed phase, a significant fraction of the photoexcited isolated dianions decays by electron loss to form the corresponding monoanions. Our transient photoelectron data reveal an ultrafast decay of the initially excited singlet (1)A2u state and concomitant rise in population of the triplet (3)A2u state, via sub-picosecond intersystem crossing (ISC). We find that both of the electronically excited states are metastably bound behind a repulsive Coulomb barrier and can decay via delayed autodetachment to yield electrons with characteristic kinetic energies. While excited state tunneling detachment (ESETD) from the singlet (1)A2u state takes only a few picoseconds, ESETD from the triplet (3)A2u state is much slower and proceeds on a time scale of hundreds of nanoseconds. The ISC rate in the gas phase is significantly higher than in solution, which can be rationalized in terms of changes to the energy dissipation mechanism in the absence of solvent molecules. [Pt2(μ-P2O5H2)4 + 2H](2-) is the first example of a photoexcited multianion for which ESETD has been observed following ISC. PMID:26851919

  13. Time-resolved photoelectron spectroscopy of a dinuclear Pt(II) complex: Tunneling autodetachment from both singlet and triplet excited states of a molecular dianion

    NASA Astrophysics Data System (ADS)

    Winghart, Marc-Oliver; Yang, Ji-Ping; Vonderach, Matthias; Unterreiner, Andreas-Neil; Huang, Dao-Ling; Wang, Lai-Sheng; Kruppa, Sebastian; Riehn, Christoph; Kappes, Manfred M.

    2016-02-01

    Time-resolved pump-probe photoelectron spectroscopy has been used to study the relaxation dynamics of gaseous [Pt2(μ-P2O5H2)4 + 2H]2- after population of its first singlet excited state by 388 nm femtosecond laser irradiation. In contrast to the fluorescence and phosphorescence observed in condensed phase, a significant fraction of the photoexcited isolated dianions decays by electron loss to form the corresponding monoanions. Our transient photoelectron data reveal an ultrafast decay of the initially excited singlet 1A2u state and concomitant rise in population of the triplet 3A2u state, via sub-picosecond intersystem crossing (ISC). We find that both of the electronically excited states are metastably bound behind a repulsive Coulomb barrier and can decay via delayed autodetachment to yield electrons with characteristic kinetic energies. While excited state tunneling detachment (ESETD) from the singlet 1A2u state takes only a few picoseconds, ESETD from the triplet 3A2u state is much slower and proceeds on a time scale of hundreds of nanoseconds. The ISC rate in the gas phase is significantly higher than in solution, which can be rationalized in terms of changes to the energy dissipation mechanism in the absence of solvent molecules. [Pt2(μ-P2O5H2)4 + 2H]2- is the first example of a photoexcited multianion for which ESETD has been observed following ISC.

  14. Multiple Decay Mechanisms and 2D-UV Spectroscopic Fingerprints of Singlet Excited Solvated Adenine-Uracil Monophosphate.

    PubMed

    Li, Quansong; Giussani, Angelo; Segarra-Martí, Javier; Nenov, Artur; Rivalta, Ivan; Voityuk, Alexander A; Mukamel, Shaul; Roca-Sanjuán, Daniel; Garavelli, Marco; Blancafort, Lluís

    2016-05-23

    The decay channels of singlet excited adenine uracil monophosphate (ApU) in water are studied with CASPT2//CASSCF:MM potential energy calculations and simulation of the 2D-UV spectroscopic fingerprints with the aim of elucidating the role of the different electronic states of the stacked conformer in the excited state dynamics. The adenine (1) La state can decay without a barrier to a conical intersection with the ground state. In contrast, the adenine (1) Lb and uracil S(U) states have minima that are separated from the intersections by sizeable barriers. Depending on the backbone conformation, the CT state can undergo inter-base hydrogen transfer and decay to the ground state through a conical intersection, or it can yield a long-lived minimum stabilized by a hydrogen bond between the two ribose rings. This suggests that the (1) Lb , S(U) and CT states of the stacked conformer may all contribute to the experimental lifetimes of 18 and 240 ps. We have also simulated the time evolution of the 2D-UV spectra and provide the specific fingerprint of each species in a recommended probe window between 25 000 and 38 000 cm(-1) in which decongested, clearly distinguishable spectra can be obtained. This is expected to allow the mechanistic scenarios to be discerned in the near future with the help of the corresponding experiments. Our results reveal the complexity of the photophysics of the relatively small ApU system, and the potential of 2D-UV spectroscopy to disentangle the photophysics of multichromophoric systems. PMID:27113273

  15. Delayed fluorescence of n-methylcarbazole in heptane and in toluene; energy transfer from upper excited singlet states of n-methylcarbazole to toluene

    NASA Astrophysics Data System (ADS)

    Nickel, Bernhard; Roden, Gabriele

    1980-11-01

    Triplet—triplet annihilation of N-methylcarbazole (NMC) leads to a primary population of upper excited singlet states S j(NMC), j > 1, which can be detected by measurement of the delayed fluorescence (DF) S j(NMC) → S 0(NMC). With NMC in toleuen (Tol), a DF S t(Tol) → S 0(Tol) is observed which is caused by energy transfer from S j(NMC) to toluene.

  16. Transient resonance Raman spectra of benzophenone and its four isotopic analogues in the lowest excited triplet state

    SciTech Connect

    Tahara, T.; Hamaguchi, H.; Tasumi, M.

    1987-11-05

    Transient resonance Raman spectra of T/sub 1/ benzophenone (T/sub 1/BP) and its four isotopic analogues in carbon tetrachloride solutions were measured. Vibrational assignments of eight T/sub 1/ bands have been made on the basis of the observed isotopic frequency shifts. The assignments clarified the following three points concerning the structure of T/sub 1/ BP in solution. (1) The CO bond order in T/sub 1/ BP is much lower than that in the ground-state benzophenone (S/sub 0/ BP). The CO stretching frequency in T/sub 1/ is found to be 1222 cm/sup -1/, whereas the corresponding value in S/sub 0/ is 1665 cm/sup -1/. The former frequency indicates a single-bond-like character of the CO bonding in the T/sub 1/ state. (2) Vibrational frequencies of several ring modes show marked downshifts in going from S/sub 0/ to T/sub 1/. This suggests the delocalization of the ..pi..* electron into the ring part. (3) The assignment (1302 cm/sup -1/) of the symmetric C-phenyl stretch mode in the T/sub 1/ withdraws S/sub 0/ absorption spectrum is questioned. According to the present assignment, the frequency of this mode (approx. 1100 cm/sup -1/) is slightly lower than that in the ground state (1150 cm/sup -1/). The simple quantum chemical picture of T/sub 1/ BP, which predicted the increase of the C-phenyl bond order with the ..pi..* withdraws n excitation, should therefore be reconsidered.

  17. Toward singlet fission for excitonic solar cells

    NASA Astrophysics Data System (ADS)

    Michl, Josef; Nozik, Arthur J.; Chen, Xudong; Johnson, Justin C.; Rana, Geeta; Akdag, Akin; Schwerin, Andrew F.

    2007-09-01

    Sensitizer dyes capable of producing two triplet excited states from a singlet excited state produced by the absorption of a single photon would allow an increase of the efficiency of photovoltaic cells by up to a factor of 1.5, provided that each triplet injects an electron into a semiconductor such as TiO II. Although singlet fission in certain crystals and polymers was reported long ago, little is known about its efficiency in dyes suitable for use as sensitizers of photoinduced charge separation on semiconductors surfaces. Biradicaloids and large alternant hydrocarbons are desirable parent structures likely to meet the requirement E(T II), E(S I) > 2E(T I) for the excitation energies of the lowest excited singlet (S I) and the two triplet (T I, T II) states. We report results for 1,3-diphenylisobenzofuran, a model compound of the biradicaloid type. Its energy levels satisfy the desired relation, and in solution it shows no triplet formation by intersystem crossing. In the neat solid state, it forms triplets efficiently, and indirect evidence suggests that this is due to singlet fission. This appears to be the first compound displaying SF by design. When two such chromophores were combined into dimers, triplet formation yields of up to 9% were observed in polar solvents, possibly due to singlet fission, but possibly due to intersystem crossing. The triplet formation occurs in two steps, via an intermediate assigned as an intramolecular charge-transfer state and responsible for most of the observed excitation loss.

  18. Secondary organic aerosol from aqueous reactions of green leaf volatiles with organic triplet excited states and singlet molecular oxygen.

    PubMed

    Richards-Henderson, Nicole K; Pham, Andrew T; Kirk, Benjamin B; Anastasio, Cort

    2015-01-01

    Vegetation emits a class of oxygenated hydrocarbons--the green leaf volatiles (GLVs)--under stress or damage. Under foggy conditions GLVs might be a source of secondary organic aerosol (SOA) via aqueous reactions with hydroxyl radical (OH), singlet oxygen ((1)O2*), and excited triplet states ((3)C*). To examine this, we determined the aqueous kinetics and SOA mass yields for reactions of (3)C* and (1)O2* with five GLVs: methyl jasmonate (MeJa), methyl salicylate (MeSa), cis-3-hexenyl acetate (HxAc), cis-3-hexen-1-ol (HxO), and 2-methyl-3-butene-2-ol (MBO). Second-order rate constants with (3)C* and (1)O2* range from (0.13-22) × 10(8) M(-1) s(-1) and (8.2-60) × 10(5) M(-1) s(-1) at 298 K, respectively. Rate constants with (3)C* are independent of temperature, while values with (1)O2* show significant temperature dependence (Ea = 20-96 kJ mol(-1)). Aqueous SOA mass yields for oxidation by (3)C* are (84 ± 7)%, (80 ± 9)%, and (38 ± 18)%, for MeJa, MeSa, and HxAc, respectively; we did not measure yields for other conditions because of slow kinetics. The aqueous production of SOA from GLVs is dominated by (3)C* and OH reactions, which form low volatility products at a rate that is approximately half that from the parallel gas-phase reactions of GLVs. PMID:25426693

  19. Tuning of the stability and energy levels of singlet exciton fission relevant excited states of pentacenes by site-specific substitution.

    PubMed

    Shen, Li; Chen, Yuhan; Li, Xiyou; Li, Chungang

    2016-05-01

    Pentacene crystals or oligomers undergo efficient singlet exciton fission (SF) after photo-excitation, which is expected to be useful in overcoming the Shockley-Queisser theoretical limit of solar cells. However, pentacenes are extremely unstable in air due to oxidation by oxygen. In this work, we designed a group of pentacene compounds with different substituents at different positions. The energy levels of HOMO and LUMO, which are believed to be closely related to the stability of pentacene, were calculated. The relationship between the molecular structure and the stability was discussed. The driving force for SF was estimated from the difference between the energy of first singlet excited state (E(S1)) and the energy of two triplet excited state (2×E(T1)) following equation E(S1)-2E(T1). Strong electron-withdrawing groups can stabilize pentacene compounds significantly, but induce a decrease on the driving force of SF slightly. Electron-donating groups destabilize the pentacene compounds dramatically and hence the introduction of electron donating groups to pentacene is not recommended. TIPS is an ideal group to improve the stability of pentacene compounds. TIPS does not change the driving force of SF significantly. Sulfur containing groups are also efficient on stabilizing pentacene compounds. These groups increase the driving force of SF at ɑ position, and recued the driving force of SF at other positions. The results of this work provide a theoretical ground for rational design of new SF molecules based on pentacenes. PMID:27131283

  20. Application of recent double-hybrid density functionals to low-lying singlet-singlet excitation energies of large organic compounds

    NASA Astrophysics Data System (ADS)

    Meo, F. Di; Trouillas, P.; Adamo, C.; Sancho-García, J. C.

    2013-10-01

    The present work assesses some recently developed double-hybrid density functionals (B2π-PLYP, PBE0-DH, and PBE0-2) using linear-response Tamm-Dancoff Time-Dependent Density Functional Theory. This assessment is achieved against experimentally derived low-lying excitation energies of large organic dyes of recent interest, including some excitations dominated by charge-transfer transitions. Comparisons are made with some of the best-performing methods established from the literature, such as PBE0 or B3LYP hybrid or the recently proposed B2-PLYP and B2GP-PLYP double-hybrid models, to ascertain their quality and robustness on equal footing. The accuracy of parameter-free or empirical forms of double-hybrid functionals is also briefly discussed. Generally speaking, it turns out that double-hybrid expressions always provide more accurate estimates than corresponding hybrid methods. Double-hybrid functionals actually reach averaged accuracies of 0.2 eV, that can be admittedly considered close to any intended accuracy limit within the present theoretical framework.

  1. Fine-Tuning of β-Substitution to Modulate the Lowest Triplet Excited States: A Bioinspired Approach to Design Phosphorescent Metalloporphyrinoids.

    PubMed

    Ke, Xian-Sheng; Zhao, Hongmei; Zou, Xiaoran; Ning, Yingying; Cheng, Xin; Su, Hongmei; Zhang, Jun-Long

    2015-08-26

    Learning nature's approach to modulate photophysical properties of NIR porphyrinoids by fine-tuning β-substituents including the number and position, in a manner similar to naturally occurring chlorophylls, has the potential to circumvent the disadvantages of traditional "extended π-conjugation" strategy such as stability, molecular size, solubility, and undesirable π-π stacking. Here we show that such subtle structural changes in Pt(II) or Pd(II) cis/trans-porphodilactones (termed by cis/trans-Pt/Pd) influence photophysical properties of the lowest triplet excited states including phosphorescence, Stokes shifts, and even photosensitization ability in triplet-triplet annihilation reactions with rubrene. Prominently, the overall upconversion capability (η, η = ε·Φ(UC)) of Pd or Pt trans-complex is 10(4) times higher than that of cis-analogue. Nanosecond time-resolved infrared (TR-IR) spectroscopy experiments showed larger frequency shift of ν(C═O) bands (ca. 10 cm(-1)) of cis-complexes than those of trans-complexes in the triplet excited states. These spectral features, combining with TD-DFT calculations, suggest the strong electronic coupling between the lactone moieties and the main porphyrin chromophores and thus the importance of precisely positioning β-substituents by mimicking chlorophylls, as an alternative to "extended π-conjugation", in designing NIR active porphyrinoids. PMID:26247480

  2. Density functional theory calculations on rhodamine B and pinacyanol chloride. Optimized ground state, dipole moment, vertical ionization potential, adiabatic electron affinity and lowest excited triplet state.

    PubMed

    Delgado, Juan C; Selsby, Ronald G

    2013-01-01

    The ground state configuration of the gas phase cationic dyes pinacyanol chloride and rhodamine B are optimized with HF/6-311 + G(2d,2p) method and basis set. B3PW91/6-311 + G(2df,2p) functional and basis set is used to calculate the Mulliken atom charge distribution, total molecular energy, the dipole moment, the vertical ionization potential, the adiabatic electron affinity and the lowest excited triplet state, the last three as an energy difference between separately calculated open shell and ground states. The triplet and extra electron states are optimized to find the relaxation energy. In the ground state optimization of both dyes the chloride anion migrates to a position near the center of the chromophore. For rhodamine B the benzoidal group turns perpendicular to the chromophore plane. For both dyes, the LUMO is mostly of π character associated with the aromatic part of the molecule containing the chromophore. The highest occupied MOs consist of three almost degenerate eigenvectors involving the chloride anion coordinated with σ electrons in the molecular framework. The fourth highest MO is of π character. For both molecules in the gas phase ionization process the chloride anion loses the significant fraction of electric charge. In electron capture, the excess charge goes mainly on the dye cation. PMID:22891949

  3. SINGLET OXYGEN IN NATURAL WATERS

    EPA Science Inventory

    Singlet oxygen is a reactive, electronically excited form of molecular oxygen that rapidly oxidizes a wide variety of organic substances, such as the polycyclic aromatics in petroleum hydrocarbon and the amino acids, histidine, tryptophan, and methionine. Studies of water samples...

  4. Analytical energy gradient of the symmetry-adapted-cluster configuration-interaction general-R method for singlet to septet ground and excited states.

    PubMed

    Ishida, Mayumi; Toyota, Kazuo; Ehara, Masahiro; Frisch, Michael J; Nakatsuji, Hiroshi

    2004-02-01

    A method of calculating analytical energy gradients of the singlet and triplet excited states, ionized states, electron-attached states, and high-spin states from quartet to septet states by the symmetry-adapted-cluster configuration-interaction general-R method is developed and implemented. This method is a powerful tool in the studies of geometries, dynamics, and properties of the states of molecules in which not only one-electron processes but also two- and multielectron processes are involved. The performance of the present method was confirmed by calculating the geometries and the spectroscopic constants of the diatomic and polyatomic molecules in various electronic states involving the ground state and the one- to three-electron excited states. The accurate descriptions were obtained for the equilibrium geometries, vibrational frequencies, and adiabatic excitation energies, which show the potential usefulness of the present method. The particularly interesting applications were to the C' 1Ag state of acetylene, the A 2Deltau and B 2Sigmau+ states of CNC and the 4B1 and a 4Piu states of N3 radical. PMID:15268403

  5. Electron-impact excitation of the singlet states of N2. I - The Birge-Hopfield system /b 1 pi u - X 1 Sigma g +/. [in auroral zones

    NASA Technical Reports Server (NTRS)

    Zipf, E. C.; Gorman, M. R.

    1980-01-01

    Results of a study of the electron-impact excitation of the b 1 pi u state of N2, one of the singlet states thought to be excited by precipitating electrons in the auroral zones, and of its predissociation and radiative relaxation through the emission of the Birge-Hopfield band system (b 1 pi u - X 1 Sigma g +) are presented. A collimated electron beam was passed through N2 gas producing a variety of atomic states through dissociative excitation, and the radiation resulting from relaxation of these states was observed by VUV and visible-IR monochromators. Absolute emission cross sections for 11 Birge-Hopfield bands are obtained for energies from threshold to 500 eV, and used to calculate the absolute transition probabilities for BH(1, v-prime) bands and the variation of the electric dipole moment with internuclear distance. With the exception of the v-prime equals 1, 5 and 6 vibrational levels, all b 1 pi u levels are found to predissociate with a specific predissociation branching ratio greater than 0.99, representing a major source of nitrogen atoms.

  6. Prediction of Excited-State Energies and Singlet-Triplet Gaps of Charge-Transfer States Using a Restricted Open-Shell Kohn-Sham Approach.

    PubMed

    Hait, Diptarka; Zhu, Tianyu; McMahon, David P; Van Voorhis, Troy

    2016-07-12

    Organic molecules with charge-transfer (CT) excited states are widely used in industry and are especially attractive as candidates for fabrication of energy efficient OLEDs, as they can harvest energy from nonradiative triplets by means of thermally activated delayed fluorescence (TADF). It is therefore useful to have computational protocols for accurate estimation of their electronic spectra in order to screen candidate molecules for OLED applications. However, it is difficult to predict the photophysical properties of TADF molecules with LR-TDDFT, as semilocal LR-TDDFT is incapable of accurately modeling CT states. Herein, we study absorption energies, emission energies, zero-zero transition energies, and singlet-triplet gaps of TADF molecules using a restricted open-shell Kohn-Sham (ROKS) approach instead and discover that ROKS calculations with semilocal hybrid functionals are in good agreement with experiments-unlike TDDFT, which significantly underestimates energy gaps. We also propose a cheap computational protocol for studying excited states with large CT character that is found to give good agreement with experimental results without having to perform any excited-state geometry optimizations. PMID:27267803

  7. LASERS IN MEDICINE: Quantum efficiency of the laser-excited singlet-oxygen-sensitised delayed fluorescence of the zinc complex of tetra(4-tert-butyl)phthalocyanine

    NASA Astrophysics Data System (ADS)

    Bashtanov, M. E.; Drozdova, N. N.; Krasnovskii, A. A.

    1999-12-01

    An investigation was made of the ratios of the intensity Idf of the singlet-oxygen(1O2)-sensitised delayed fluorescence of the zinc complex of tetra(4-tert-butyl)phthalocyanine (ZnTBPc), with the maximum at λ = 685 nm, to the intensity I1270 of the photosensitised phosphorescence of 1O2 with the maximum at λ = 1270 nm in deuterated benzene when excited with λ = 337 nm nitrogen-laser pulses. Depending on the energy density of the laser radiation (0.25 — 0.7 mJ cm-2) and on the concentration of ZnTBPc (0.06 — 3.4 μM), the ratio of the zero-time intensities of the delayed fluorescence of ZnTBPc and of the singlet-oxygen phosphorescence Idf0/I12700 varied from 0.01 to 0.2 in air-saturated solutions of ZnTBPc. The intensity Idf0 decreased fivefold as a result of saturation with oxygen of air-saturated solutions. The quantum efficiency of the delayed fluorescence was represented by the coefficient α =(Idf0/I12700)kr/(γf[1O2]0[ZnTBPc]), where [1O2]0 is the zero-time concentration of 1O2 after a laser shot; kr is the rate constant of radiative deactivation of 1O2 in the investigated solvent; γf is the quantum yield of the ZnTBPc fluorescence. It was established that in the case of air-saturated solutions of ZnTBPc this coefficient was approximately 200 times less than for metal-free tetra(4-tert-butyl)phthalocyanine and its absolute value was ~2 × 1011 M-2 s-1.

  8. Two-Photon Study on the Electronic Interactions between the First Excited Singlet States in Carotenoid-Tetrapyrrole Dyads

    SciTech Connect

    Liao, Pen-Nan; Pillai, Smitha; Gust, Devens; Moore, Thomas A.; Moore, Ana L.; Walla, Peter J.

    2011-03-22

    Electronic interactions between the first excited states (S1) of carotenoids (Car) of different conjugation lengths (8-11 double bonds) and phthalocyanines (Pc) in different Car-Pc dyad molecules were investigated by two-photon spectroscopy and compared with Car S1-chlorophyll (Chl) interactions in photosynthetic light harvesting complexes (LHCs). The observation of Chl/Pc fluorescence after selective two-photon excitation of the Car S1 state allowed sensitive monitoring of the flow of energy between Car S1 and Pc or Chl. It is found that two-photon excitation excites to about 80% to 100% exclusively the carotenoid state Car S1 and that only a small fraction of direct tetrapyrrole two-photon excitation occurs. Amide-linked Car-Pc dyads in tetrahydrofuran demonstrate a molecular gear shift mechanism in that effective Car S1 → Pc energy transfer is observed in a dyad with 9 double bonds in the carotenoid, whereas in similar dyads with 11 double bonds in the carotenoid, the Pc fluorescence is strongly quenched by Pc → Car S1 energy transfer. In phenylamino-linked Car-Pc dyads in toluene extremely large electronic interactions between the Car S1 state and Pc were observed, particularly in the case of a dyad in which the carotenoid contained 10 double bonds. This observation together with previous findings in the same system provides strong evidence for excitonic Car S1-Pc Qy interactions. Very similar results were observed with photosynthetic LHC II complexes in the past, supporting an important role of such interactions in photosynthetic down-regulation.

  9. Electronic Structure Study of Singlet Fission in Tetracene Derivatives.

    PubMed

    Casanova, David

    2014-01-14

    A detailed theoretical study of the singlet fission process in tetracene and two of its derivatives, that is 5,12-diphenyltetracene (DPT) and rubrene, is presented. This work aims to unravel the intricacies and the differences of their singlet fission mechanism by means of electronic structure calculations using molecular and cluster models and a variety of computational tools. Although the electronic structure at the molecular level is very similar for the three compounds, their different crystal packing has important consequences in their ability to produce two triplet states from a single exciton. The results obtained indicate that the lowest singlet is found to delocalize at least over seven molecules. Computed relative energies rule out the presence of charge transfer (CT) states as intermediates in a two-step mechanism in all cases. On the other hand, CT states do play a role as mediators, specially in tetracene. They decisively participate in the coupling between single and multiexcitonic states through second-order contributions. Finally, the present study pinpoints that the transition from the optically allowed exciton to the dark multiexciton state might be facilitated by intramolecular motion toward the lowest excited singlet geometry. PMID:26579913

  10. Excitation Localization/Delocalization Isomerism in a Strongly Coupled Covalent Dimer of 1,3-Diphenylisobenzofuran.

    PubMed

    Schrauben, Joel N; Akdag, Akin; Wen, Jin; Havlas, Zdenek; Ryerson, Joseph L; Smith, Millie B; Michl, Josef; Johnson, Justin C

    2016-05-26

    Two isomers of both the lowest excited singlet (S1) and triplet (T1) states of the directly para, para'-connected covalent dimer of the singlet-fission chromophore 1,3-diphenylisobenzofuran have been observed. In one isomer, excitation is delocalized over both halves of the dimer, and in the other, it is localized on one or the other half. For a covalent dimer in solution, such "excitation isomerism" is extremely rare. The vibrationally relaxed isomers do not interconvert, and their photophysical properties, including singlet fission, differ significantly. PMID:27158903

  11. Simultaneous Two-Photon Absorption to Gerade Excited Singlet States of Diphenylacetylene and Diphenylbutadiyne Using Optical-Probing Photoacoustic Spectroscopy.

    PubMed

    Isozaki, Tasuku; Oba, Hikari; Ikoma, Tadaaki; Suzuki, Tadashi

    2016-08-11

    Simultaneous two-photon absorption to one-photon forbidden electronically excited states of diphenylacetylene (DPA) and diphenylbutadiyne (DPB) was investigated by means of highly sensitive optical-probing photoacoustic spectroscopy. The incident laser power dependencies on photoacoustic signal intensity indicate that the signals are dominated by the two-photon absorption regime. Two-photon absorption is responsible for transitions to gerade excited states based on the selection rule. The two-photon absorption bands observed in the heat action spectra were assigned with the aid of quantum chemical calculations. The relative magnitude of the two-photon absorption cross sections of DPA and DPB was estimated, and the larger two-photon absorption cross section of DPB was related to the resonance effect with the red-shifted one-photon allowed 1(1)B1u ← 1(1)Ag transition of DPB. PMID:27410388

  12. π-Conjugated Organometallic Isoindigo Oligomer and Polymer Chromophores: Singlet and Triplet Excited State Dynamics and Application in Polymer Solar Cells.

    PubMed

    Goswami, Subhadip; Gish, Melissa K; Wang, Jiliang; Winkel, Russell W; Papanikolas, John M; Schanze, Kirk S

    2015-12-01

    An isoindigo based π-conjugated oligomer and polymer that contain cyclometalated platinum(II) "auxochrome" units were subjected to photophysical characterization, and application of the polymer in bulk heterojunction polymer solar cells with PCBM acceptor was examined. The objective of the study was to explore the effect of the heavy metal centers on the excited state properties, in particular, intersystem crossing to a triplet (exciton) state, and further how this would influence the performance of the organometallic polymer in solar cells. The materials were characterized by electrochemistry, ground state absorption, emission, and picosecond-nanosecond transient absorption spectroscopy. Electrochemical measurements indicate that the cyclometalated units have a significant impact on the HOMO energy level of the chromophores, but little effect on the LUMO, which is consistent with localization of the LUMO on the isoindigo acceptor unit. Picosecond-nanosecond transient absorption spectroscopy reveals a transient with ∼100 ns lifetime that is assigned to a triplet excited state that is produced by intersystem crossing from a singlet state on a time scale of ∼130 ps. This is the first time that a triplet state has been observed for isoindigo π-conjugated chromophores. The performance of the polymer in bulk heterojunction solar cells was explored with PC61BM as an acceptor. The performance of the cells was optimum at a relatively high PCBM loading (1:6, polymer:PCBM), but the overall efficiency was relatively low with power conversion efficiency (PCE) of 0.22%. Atomic force microscopy of blend films reveals that the length scale of the phase separation decreases with increasing PCBM content, suggesting a reason for the increase in PCE with acceptor loading. Energetic considerations show that the triplet state in the polymer is too low in energy to undergo charge separation with PCBM. Further, due to the relatively low LUMO energy of the polymer, charge transfer

  13. Singlet exciton fission in solution

    NASA Astrophysics Data System (ADS)

    Walker, Brian J.; Musser, Andrew J.; Beljonne, David; Friend, Richard H.

    2013-12-01

    Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight.

  14. Singlet exciton fission in solution.

    PubMed

    Walker, Brian J; Musser, Andrew J; Beljonne, David; Friend, Richard H

    2013-12-01

    Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight. PMID:24256865

  15. Water-soluble naphthalene diimides as singlet oxygen sensitizers.

    PubMed

    Doria, Filippo; Manet, Ilse; Grande, Vincenzo; Monti, Sandra; Freccero, Mauro

    2013-08-16

    Bromo- and/or alkylamino-substituted and hydrosoluble naphthalene diimides (NDIs) were synthesized to study their multimodal photophysical and photochemical properties. Bromine-containing NDIs (i.e., 11) behaved as both singlet oxygen ((1)O2) photosensitizers and fluorescent molecules upon irradiation at 532 nm. Among the NDIs not containing Br, only 12 exhibited photophysical properties similar to those of Br-NDIs, by irradiation above 610 nm, suggesting that for these NDIs both singlet and triplet excited-state properties are strongly affected by length, structure of the solubilizing moieties, and pH of the solution. Laser flash photolysis confirmed that the NDI lowest triplet excited state was efficiently populated, upon excitation at both 355 and 532 nm, and that free amine moieties quenched both the singlet and triplet excited states by intramolecular electron transfer, with generation of detectable radical anions. Time-resolved experiments, monitoring the 1270 nm (1)O2 phosphorescence decay generated upon laser irradiation at 532 nm, allowed a ranking of the NDIs as sensitizers, based on their (1)O2 quantum yields (ΦΔ). The tetrafunctionalized 12, exhibiting a long-lived triplet state (τ ~ 32 μs) and the most promising absorptivity for photodynamic therapy application, was tested as efficient photosensitizers in the photo-oxidations of 1,5-dihydroxynaphthalene and 9,10-anthracenedipropionic acid in acetonitrile and water. PMID:23869544

  16. Search for Singlet Fission Chromophores

    SciTech Connect

    Havlas, Z.; Akdag, A.; Smith, M. B.; Dron, P.; Johnson, J. C.; Nozik, A. J.; Michl, J.

    2012-01-01

    Singlet fission, in which a singlet excited chromophore shares its energy with a ground-state neighbor and both end up in their triplet states, is of potential interest for solar cells. Only a handful of compounds, mostly alternant hydrocarbons, are known to perform efficiently. In view of the large number of conditions that a successful candidate for a practical cell has to meet, it appears desirable to extend the present list of high performers to additional classes of compounds. We have (i) identified design rules for new singlet fission chromophores and for their coupling to covalent dimers, (ii) synthesized them, and (iii) evaluated their performance as neat solids or covalent dimers.

  17. Lowest eigenvalues of random Hamiltonians

    SciTech Connect

    Shen, J. J.; Zhao, Y. M.; Arima, A.; Yoshinaga, N.

    2008-05-15

    In this article we study the lowest eigenvalues of random Hamiltonians for both fermion and boson systems. We show that an empirical formula of evaluating the lowest eigenvalues of random Hamiltonians in terms of energy centroids and widths of eigenvalues is applicable to many different systems. We improve the accuracy of the formula by considering the third central moment. We show that these formulas are applicable not only to the evaluation of the lowest energy but also to the evaluation of excited energies of systems under random two-body interactions.

  18. Degradation of organic pollutants in/on snow and ice by singlet molecular oxygen (¹O₂*) and an organic triplet excited state.

    PubMed

    Bower, Jonathan P; Anastasio, Cort

    2014-04-01

    Singlet molecular oxygen (¹O₂*) can be a significant sink for a variety of electron-rich pollutants in surface waters and atmospheric drops. We recently found that ¹O₂* concentrations are enhanced by up to a factor of 10(4) on illuminated ice compared to in the equivalent liquid solution, suggesting that ¹O₂* could be an important oxidant for pollutants in snow. To examine this, here we study the degradation of three model organic pollutants: furfuryl alcohol (to represent furans), tryptophan (for aromatic amino acids), and bisphenol A (for phenols). Each compound was studied in illuminated aqueous solution and ice containing Rose Bengal (RB, a sensitizer for ¹O₂*) and sodium chloride (to adjust the concentration of total solutes). The RB-mediated loss of each organic compound is enhanced on illuminated ice compared to in solution, by factors of 6400 for furfuryl alcohol, 8300 for tryptophan, and 50 for bisphenol A for ice containing 0.065 mM total solutes. Rates of loss of furfuryl alcohol and tryptophan decrease at a higher total solute concentration, in qualitative agreement with predictions from freezing-point depression. In contrast, the loss of bisphenol A on ice is independent of total solute concentration. Relative to liquid tests, the enhanced loss of tryptophan on ice during control experiments made with deoxygenated solutions and solutions in D₂O show that the triplet excited state of Rose Bengal may also contribute to loss of pollutants on ice. PMID:24487942

  19. Roles of singlet oxygen and triplet excited state of dissolved organic matter formed by different organic matters in bacteriophage MS2 inactivation.

    PubMed

    Rosado-Lausell, Sahid L; Wang, Hanting; Gutiérrez, Leonardo; Romero-Maraccini, Ofelia C; Niu, Xi-Zhi; Gin, Karina Y H; Croué, Jean-Philippe; Nguyen, Thanh H

    2013-09-15

    Inactivation of bacteriophage MS2 by reactive oxygen species (ROS) and triplet excited state of dissolved organic matter ((3)DOM*) produced by irradiation of natural and synthetic sensitizers with simulated sunlight of wavelengths greater than 320 nm was investigated. Natural sensitizers included purified DOM isolates obtained from wastewater and river waters, and water samples collected from Singapore River, Stamford Canal, and Marina Bay Reservoir in Singapore. Linear correlations were found between MS2 inactivation rate constants (kobs) and the photo-induced reaction rate constants of 2,4,6-trimethylphenol (TMP), a probe compound shown to react mainly with (3)DOM*. Linear correlations between MS2 kobs and singlet oxygen ((1)O2) concentrations were also found for both purified DOM isolates and natural water samples. These correlations, along with data from quenching experiments and experiments with synthetic sensitizers, Rose Bengal (RB), 3'-methoxyacetophenone (3'-MAP), and nitrite [Formula: see text] , suggest that (1)O2, (3)DOM*, and hydroxyl radicals ((•)OH) could inactivate bacteriophage MS2. Linear correlations between MS2 kobs and Specific Ultraviolet Absorption determined at 254 nm (SUVA254) were also found for both purified DOM isolates and natural samples. These results suggest the potential use of TMP as a chemical probe and SUVA254 as an indicator for virus inactivation in natural and purified DOM water samples. PMID:23866126

  20. The effect of caffeine on the reactions of the excited singlet state of pyrene in micellar sodium lauryl sulfate

    NASA Astrophysics Data System (ADS)

    Hashimoto, Shuichi; Thomas, J. Kerry

    1984-08-01

    The effect of caffeine on a few photo-induced reactions of pyrene in micellar sodium lauryl sulfate (NaLS) has been studied. In these systems caffeine complexes with the pyrene (K asso = 85 ± 10 M -1 and also with the other reactants, e.g. Cu 2+ or TI +. The efficiencies of reactions which involve contact, i.e. pyrene excimer formation, and quenching by TI + ions to give the triplet state of pyrene, are significantly reduced in the presence of caffeine, due to geometric inhibitions formed by the complexation processes. The kinetics of photo-induced electron transfer, e.g. between excited pyrene and Cu 2+, are not affected. However, the subsequent reactions of the products are modified and the yield of ionic products is markedly increased.

  1. Evidence for static localization in the lowest optically excited states of ruthenium(II) diimine complexes: a solvent- and time-dependent photoselection study at 77 K

    SciTech Connect

    Myrick, M.L.; Blakley, R.L.; DeArmond, M.K.; Arthur, M.L.

    1988-03-02

    The 77 K absorption, emission, steady-state excitation photoselection (SSExP), and time-resolved excitation photoselection (TRExP) results are reported for 11 (Ru(L)/sub 3/)(PF/sub 6/)/sub 2/ complexes with 2,2'-diimine ligands (L). The ligands are 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), 2,2'-bipyrazine (bpz), 2-(2-pyridyl)quinoline (pq), and 2,2'-biquinoline (biq). Complexes of the form (Ru(L)/sub 3/)/sup 2 +/ (L = bpy, phen, bpz, pq, and biq) were examined as well as mixed-ligand complexes of the form (Ru(bpy)/sub n/(L)/sub 3-n/)/sup 2 +/ (n = 1, 2) (L = phen, bpz, and pq). Data are also presented for the monomeric model complex, (Ru(bpy)(py)/sub 4/)/sup 2 +/ (py = pyridine). Data indicate that optical excitation leads to static localization of the optically excited electron. A model is developed that rationalizes the maximum value obtainable in the SSExP data. Solvent effects observed in the TRExP data are explained in terms of relative rates of spin-lattice relaxation associated with different solvents.

  2. Singlet and triplet instability theorems

    SciTech Connect

    Yamada, Tomonori; Hirata, So

    2015-09-21

    A useful definition of orbital degeneracy—form-degeneracy—is introduced, which is distinct from the usual energy-degeneracy: Two canonical spatial orbitals are form-degenerate when the energy expectation value in the restricted Hartree–Fock (RHF) wave function is unaltered upon a two-electron excitation from one of these orbitals to the other. Form-degenerate orbitals tend to have isomorphic electron densities and occur in the highest-occupied and lowest-unoccupied molecular orbitals (HOMOs and LUMOs) of strongly correlated systems. Here, we present a mathematical proof of the existence of a triplet instability in a real or complex RHF wave function of a finite system in the space of real or complex unrestricted Hartree–Fock wave functions when HOMO and LUMO are energy- or form-degenerate. We also show that a singlet instability always exists in a real RHF wave function of a finite system in the space of complex RHF wave functions, when HOMO and LUMO are form-degenerate, but have nonidentical electron densities, or are energy-degenerate. These theorems provide Hartree–Fock-theory-based explanations of Hund’s rule, a singlet instability in Jahn–Teller systems, biradicaloid electronic structures, and a triplet instability during some covalent bond breaking. They also suggest (but not guarantee) the spontaneous formation of a spin density wave (SDW) in a metallic solid. The stability theory underlying these theorems extended to a continuous orbital-energy spectrum proves the existence of an oscillating (nonspiral) SDW instability in one- and three-dimensional homogeneous electron gases, but only at low densities or for strong interactions.

  3. Singlet and triplet instability theorems

    NASA Astrophysics Data System (ADS)

    Yamada, Tomonori; Hirata, So

    2015-09-01

    A useful definition of orbital degeneracy—form-degeneracy—is introduced, which is distinct from the usual energy-degeneracy: Two canonical spatial orbitals are form-degenerate when the energy expectation value in the restricted Hartree-Fock (RHF) wave function is unaltered upon a two-electron excitation from one of these orbitals to the other. Form-degenerate orbitals tend to have isomorphic electron densities and occur in the highest-occupied and lowest-unoccupied molecular orbitals (HOMOs and LUMOs) of strongly correlated systems. Here, we present a mathematical proof of the existence of a triplet instability in a real or complex RHF wave function of a finite system in the space of real or complex unrestricted Hartree-Fock wave functions when HOMO and LUMO are energy- or form-degenerate. We also show that a singlet instability always exists in a real RHF wave function of a finite system in the space of complex RHF wave functions, when HOMO and LUMO are form-degenerate, but have nonidentical electron densities, or are energy-degenerate. These theorems provide Hartree-Fock-theory-based explanations of Hund's rule, a singlet instability in Jahn-Teller systems, biradicaloid electronic structures, and a triplet instability during some covalent bond breaking. They also suggest (but not guarantee) the spontaneous formation of a spin density wave (SDW) in a metallic solid. The stability theory underlying these theorems extended to a continuous orbital-energy spectrum proves the existence of an oscillating (nonspiral) SDW instability in one- and three-dimensional homogeneous electron gases, but only at low densities or for strong interactions.

  4. Singlet and triplet instability theorems.

    PubMed

    Yamada, Tomonori; Hirata, So

    2015-09-21

    A useful definition of orbital degeneracy—form-degeneracy—is introduced, which is distinct from the usual energy-degeneracy: Two canonical spatial orbitals are form-degenerate when the energy expectation value in the restricted Hartree-Fock (RHF) wave function is unaltered upon a two-electron excitation from one of these orbitals to the other. Form-degenerate orbitals tend to have isomorphic electron densities and occur in the highest-occupied and lowest-unoccupied molecular orbitals (HOMOs and LUMOs) of strongly correlated systems. Here, we present a mathematical proof of the existence of a triplet instability in a real or complex RHF wave function of a finite system in the space of real or complex unrestricted Hartree-Fock wave functions when HOMO and LUMO are energy- or form-degenerate. We also show that a singlet instability always exists in a real RHF wave function of a finite system in the space of complex RHF wave functions, when HOMO and LUMO are form-degenerate, but have nonidentical electron densities, or are energy-degenerate. These theorems provide Hartree-Fock-theory-based explanations of Hund's rule, a singlet instability in Jahn-Teller systems, biradicaloid electronic structures, and a triplet instability during some covalent bond breaking. They also suggest (but not guarantee) the spontaneous formation of a spin density wave (SDW) in a metallic solid. The stability theory underlying these theorems extended to a continuous orbital-energy spectrum proves the existence of an oscillating (nonspiral) SDW instability in one- and three-dimensional homogeneous electron gases, but only at low densities or for strong interactions. PMID:26395692

  5. Pigment-pigment interactions in thylakoids and LHCII of chlorophyll a/ c containing alga Pleurochloris meiringensis: analysis of fluorescence-excitation and triplet-minus-singlet spectra

    NASA Astrophysics Data System (ADS)

    Büchel, C.; Razi Naqvi, K.; Melø, T. B.

    1998-05-01

    Time-resolved triplet-minus-singlet (TmS) difference spectra, Δ A( λ; t), fluorescence excitation spectra, X( λ), and absorption spectra, A( λ), are used for probing pigment-pigment interactions in the thylakoids (Chl a/ c-Thyl) and isolated light-harvesting complexes associated with photosystem II (Chl a/ c-LHCII) of the alga Pleurochloris meiringensis, whose chromophores comprise chlorophyll a (Chl a), chlorophyll c (Chl c), and several carotenoids. The data provide information about interactions between Car*-and-Chl a0, Chl a†-and-Car 0, Car †-and-Chl a0 (where the abbreviation Car stands for carotenoid, an asterisk and a dagger denote singlet and triplet excitation, respectively, and the superscript 0 denotes a molecule in the ground state). In Chl a/c-Thyl, the efficiency of Car*→Chl a* transfer ( φLH), determined by comparing A( λ) and X( λ), is slightly less than unity (ca. 0.85), whereas the efficiency of Chl a†→Car † transfer of triplet energy ( φTT) must be much closer to unity, since no long-lived Chl a† could be detected; an interaction between Car † and Chl a0, already familiar from investigations concerning the TmS spectra of the trimers and aggregates of Chl a/ b-LHCII (the light-harvesting complex associated with the photosystem II of higher plants), which manifests itself through a depletion signal (in the Qy region of Chl a) decaying at the same rate as the Car TmS signal, is observed, and explained likewise. In Chl a/ c-LHCII, both efficiencies are found to be much lower; the drastic reduction in the two yields is attributed to the perturbation of the native molecular architecture of the complex by the detergent used in the isolation procedure. The overall TmS signal from Chl a/ c-LHCII can be decomposed into two contributions, Δ A( λ; t)=Δ 1A( λ; t)+Δ 2A( λ; t), where Δ 1A( λ; t) with a lifetime of about 8 μs; Δ 2A( λ; t), which persists for several hundred microseconds, is contributed by those Chl a

  6. Theoretical investigation on properties of the ground and lowest excited states of a red emitter with donor-π-acceptor structure

    NASA Astrophysics Data System (ADS)

    Liu, Xiaojun; Zhang, Xiao; Hou, Yanbing; Teng, Feng; Lou, Zhidong

    2011-03-01

    The ground and excited state properties of DCDPC, particularly designed as a red emitter for organic light emitting diodes applications have been studied by means of density functional theory (DFT) and time-dependent (TD)DFT. The electronic and geometrical structures of DCDPC in acetone, tetrahydrofuran and benzene solvents are reported for the first time. The experimental absorption and fluorescence spectra are reproduced by calculations. By comparison with experimental data, insight on the performance of 10 exchange correlation functionals is also given. M06 in the frame of DFT and TDDFT with a polarizable continuum model and a medium sized basis set emerges as the most effective strategy. Beside the good agreement between the calculational and experimental spectra proving the accuracy of the strategy, the calculations allow further insights into the electronic structure for the family of isophorone-based light emitting materials with D-π-A structure, especially the electronic and geometrical structures for the excited states.

  7. Reliable Prediction with Tuned Range-Separated Functionals of the Singlet-Triplet Gap in Organic Emitters for Thermally Activated Delayed Fluorescence.

    PubMed

    Sun, Haitao; Zhong, Cheng; Brédas, Jean-Luc

    2015-08-11

    The thermally activated delayed fluorescence (TADF) mechanism has recently attracted significant interest in the field of organic light-emitting diodes (OLEDs). TADF relies on the presence of a very small energy gap between the lowest singlet and triplet excited states. Here, we demonstrate that time-dependent density functional theory in the Tamm-Dancoff approximation can be very successful in calculations of the lowest singlet and triplet excitation energies and the corresponding singlet-triplet gap when using nonempirically tuned range-separated functionals. Such functionals provide very good estimates in a series of 17 molecules used in TADF-based OLED devices with mean absolute deviations of 0.15 eV for the vertical singlet excitation energies and 0.09 eV [0.07 eV] for the adiabatic [vertical] singlet-triplet energy gaps as well as low relative errors and high correlation coefficients compared to the corresponding experimental values. They significantly outperform conventional functionals, a feature which is rationalized on the basis of the amount of exact-exchange included and the delocalization error. The present work provides a reliable theoretical tool for the prediction and development of novel TADF-based materials with low singlet-triplet energetic splittings. PMID:26574466

  8. A theoretical study of the ground state and lowest excited states of PuO0/+/+2 and PuO20/+/+2

    SciTech Connect

    Gibson, John K.; La Macchia, Giovanni; Infante, Ivan; Gagliardi, Laura; Raab, Juraj

    2008-12-08

    The ground and excited states of neutral and cationic PuO and PuO2 have been studied with multiconfigurational quantum chemical methods followed by second order perturbation theory, the CASSCF/CASPT2 method. Scalar relativistic effects and spin-orbit coupling have been included in the treatment. As literature values for the ionization energy of PuO2 are in the wide range of ~;;6.6 eV to ~;;10.1 eV, a central goal of the computations was to resolve these discrepancies; the theoretical results indicate that the ionization energy is near the lower end of this range. The calculated ionization energies for PuO, PuO+ and PuO2+ are in good agreement with the experimental values.

  9. Low-lying excited states in armchair polyacene within Pariser-Parr-Pople model: a density matrix renormalization group study.

    PubMed

    Das, Mousumi

    2014-03-28

    We studied the nature of the ground state and low-lying excited states of armchair polyacene oligomers (Polyphenanthrene) within long-range Pariser-Parr-Pople model Hamiltonian with up to 14 monomers using symmetrized density matrix renormalization group technique. The ground state of all armchair polyacenes studied is found to be singlet. The results show that lowest singlet dipole allowed excited state has higher energy for armchair polyacenes as compared to linear fused polyacenes. Moreover, unlike linear fused polyacenes, the lowest singlet excited state of these oligomers is always found to lie below the lowest dipole forbidden two-photon state indicating that these armchair polyacene oligomers strongly fluoresce. The calculations of low-lying excitations on singly and triply electron doped armchair polyacene oligomers show a low energy band with strong transition dipole moment that coupled to charge conductivity. This implies armchair polyacene posses novel field-effect transistor properties. PMID:24697451

  10. Low-lying excited states in armchair polyacene within Pariser-Parr-Pople model: A density matrix renormalization group study

    SciTech Connect

    Das, Mousumi

    2014-03-28

    We studied the nature of the ground state and low-lying excited states of armchair polyacene oligomers (Polyphenanthrene) within long-range Pariser-Parr-Pople model Hamiltonian with up to 14 monomers using symmetrized density matrix renormalization group technique. The ground state of all armchair polyacenes studied is found to be singlet. The results show that lowest singlet dipole allowed excited state has higher energy for armchair polyacenes as compared to linear fused polyacenes. Moreover, unlike linear fused polyacenes, the lowest singlet excited state of these oligomers is always found to lie below the lowest dipole forbidden two-photon state indicating that these armchair polyacene oligomers strongly fluoresce. The calculations of low-lying excitations on singly and triply electron doped armchair polyacene oligomers show a low energy band with strong transition dipole moment that coupled to charge conductivity. This implies armchair polyacene posses novel field-effect transistor properties.

  11. Quantum Relaxation in Singlet Fission

    NASA Astrophysics Data System (ADS)

    Teichen, Paul; Eaves, Joel

    2013-03-01

    Singlet fission is a multielectron process in organic chromophores, where an initially excited singlet state decays into two independent triplets. First observed in organic semiconductors almost 40 years ago, the phenomenon may be a promising route for increasing yields in next-generation photovoltaics. Early theories that ignored quantum coherence between excited states were capable of explaining the fission process on nanosecond timescales, but recent observations of fission on sub picosecond timescales call several tenants of those theories into question. We present a theory of optical dephasing and decoherence in singlet fission, drawing on ideas from quantum information theory to establish conditions for decoherence and disentanglement between the relevant quantum states on the picosecond timescale.

  12. Singlet oxygen O2(a1Δg) formation via UV-excitation of isoprene-oxygen C5H8-O2 encounter complexes in gas phase

    NASA Astrophysics Data System (ADS)

    Pyryaeva, Alexandra P.; Goldort, Veniamin G.; Kochubei, Sergei A.; Baklanov, Alexey V.

    2014-08-01

    Photogeneration of singlet oxygen O2(a1Δg) resulted from photoexcitation at 248 and 266 nm of isoprene-oxygen mixtures at elevated oxygen pressure has been investigated. Pulse energy and oxygen pressure dependence of the singlet oxygen yield indicate one-quantum photoexcitation of the encounter complex isoprene-oxygen C5H8-O2 as a source of O2(a1Δg). Suggested mechanism of O2(a1Δg) photogeneration involves direct excitation of complex with simultaneous change of both partner spins like that observed in van der Waals complex C5H8-O2 [Vidma et al., J. Chem. Phys. 137 (2012) 054305]. Other supposed pathways involve collision-induced photogeneration of Herzberg III state O2(A'3Δg) or triplet isoprene as the intermediates.

  13. Singlet fission in reduced dimensions of crystals

    NASA Astrophysics Data System (ADS)

    Teichen, Paul; Eaves, Joel

    2014-03-01

    In some molecular systems the decay of an initially excited singlet into two independent triplets, a process called singlet fission, is highly efficient. Organic crystals are among the most promising candidates for increasing yields in next-generation photovoltaics. Although excitons are known to exist in reduced dimensions of crystals the role of dimensionality in the entanglement of two triplets born out of singlet fission remains unclear. We develop a quantum lattice model for singlet fission to examine the role of quantum entanglement and exciton delocalization.

  14. Singlet-Triplet Splittings in the Luminescent Excited States of Colloidal Cu(+):CdSe, Cu(+):InP, and CuInS2 Nanocrystals: Charge-Transfer Configurations and Self-Trapped Excitons.

    PubMed

    Knowles, Kathryn E; Nelson, Heidi D; Kilburn, Troy B; Gamelin, Daniel R

    2015-10-14

    The electronic and magnetic properties of the luminescent excited states of colloidal Cu(+):CdSe, Cu(+):InP, and CuInS2 nanocrystals were investigated using variable-temperature photoluminescence (PL) and magnetic circularly polarized luminescence (MCPL) spectroscopies. The nanocrystal electronic structures were also investigated by absorption and magnetic circular dichroism (MCD) spectroscopies. By every spectroscopic measure, the luminescent excited states of all three materials are essentially indistinguishable. All three materials show very similar broad PL line widths and large Stokes shifts. All three materials also show similar temperature dependence of their PL lifetimes and MCPL polarization ratios. Analysis shows that this temperature dependence reflects Boltzmann population distributions between luminescent singlet and triplet excited states with average singlet-triplet splittings of ∼1 meV in each material. These similarities lead to the conclusion that the PL mechanism in CuInS2 NCs is fundamentally different from that of bulk CuInS2 and instead is the same as that in Cu(+)-doped NCs, which are known to luminesce via charge-transfer recombination of conduction-band electrons with copper-localized holes. The luminescence of CuInS2 nanocrystals is explained well by invoking exciton self-trapping, in which delocalized photogenerated holes contract in response to strong vibronic coupling at lattice copper sites to form a luminescent excited state that is essentially identical to that of the Cu(+)-doped semiconductor nanocrystals. PMID:26389577

  15. The Non-Adiabatic dynamics of Singlet Fission in Polyacenes

    NASA Astrophysics Data System (ADS)

    Bradforth, Stephen

    2015-03-01

    Singlet fission involves the splitting of a single excitation into two coupled triplet excitations and is manifested in an increasing range of aromatic crystals and amorphous thin films. If the energy of the lowest triplet state is one half (or less) of the first singlet excited state, as it is for tetracene or pentacene and their derivatives, singlet fission may occur between two adjacent chromophores. Since there is no change in the overall spin state of the system, singlet fission can be exceptionally fast, occuring on the fs - ps range. If the triplets can diffuse away from the fission site they are available for harvesting as a dissociated carriers with up to two charge carrier pairs per absorbed photon. The possibility of recovering excess energy above the material band gap (in this case determined by the triplet energy) when a higher energy photon is absorbed has led to great recent interest in exploiting this process for increased efficiency solar energy harvesting. The nature of the electronic couplings between the chromophores, intermediate electronic configurations, and the role of entropy in the spin-allowed primary fission event have all come under great scrutiny. Results from a series of femtosecond spectroscopy experiments on a variety of amorphous thin films, nanoparticles and isolated acene dimer compounds will be presented that shed light on the electronic intermediate states key to the efficiency and speed of this process. Work supported as part of the Center for Energy Nanoscience, an Energy Frontier Research Center funded by the U.S. Department of Energy (DE-SC0001013).

  16. Charge transfer-mediated singlet fission.

    PubMed

    Monahan, N; Zhu, X-Y

    2015-04-01

    Singlet fission, the splitting of a singlet exciton into two triplet excitons in molecular materials, is interesting not only as a model many-electron problem, but also as a process with potential applications in solar energy conversion. Here we discuss limitations of the conventional four-electron and molecular dimer model in describing singlet fission in crystalline organic semiconductors, such as pentacene and tetracene. We emphasize the need to consider electronic delocalization, which is responsible for the decisive role played by the Mott-Wannier exciton, also called the charge transfer (CT) exciton, in mediating singlet fission. At the strong electronic coupling limit, the initial excitation creates a quantum superposition of singlet, CT, and triplet-pair states, and we present experimental evidence for this interpretation. We also discuss the most recent attempts at translating this mechanistic understanding into design principles for CT state-mediated intramolecular singlet fission in oligomers and polymers. PMID:25648486

  17. Singlet Fission: From Coherences to Kinetics.

    PubMed

    Piland, Geoffrey B; Burdett, Jonathan J; Dillon, Robert J; Bardeen, Christopher J

    2014-07-01

    Singlet fission, in which an initially excited singlet state spontaneously splits into a pair of triplet excitons, is a process that can potentially boost the efficiency of solar energy conversion. The separate electronic bands in organic semiconductors make them especially useful for dividing a high-energy singlet exciton into a pair of lower-energy triplet excitons. Recent experiments illustrate the role of spin coherence in fission, while kinetic models are used to describe how triplet and singlet states interact on longer time scales. Despite insights gained from recent experiments, the detailed structure and dynamics of the electronic states involved in the initial step of singlet fission remain active areas of investigation. On longer time scales, finding ways to efficiently harvest the triplet excitons will be an important challenge for making devices based on this phenomenon. A full understanding of singlet fission requires consideration of a sequence of photophysical events (decoherence, relaxation, and diffusion) occurring on different time scales. PMID:26279552

  18. Charge Transfer-Mediated Singlet Fission

    NASA Astrophysics Data System (ADS)

    Monahan, N.; Zhu, X.-Y.

    2015-04-01

    Singlet fission, the splitting of a singlet exciton into two triplet excitons in molecular materials, is interesting not only as a model many-electron problem, but also as a process with potential applications in solar energy conversion. Here we discuss limitations of the conventional four-electron and molecular dimer model in describing singlet fission in crystalline organic semiconductors, such as pentacene and tetracene. We emphasize the need to consider electronic delocalization, which is responsible for the decisive role played by the Mott-Wannier exciton, also called the charge transfer (CT) exciton, in mediating singlet fission. At the strong electronic coupling limit, the initial excitation creates a quantum superposition of singlet, CT, and triplet-pair states, and we present experimental evidence for this interpretation. We also discuss the most recent attempts at translating this mechanistic understanding into design principles for CT state-mediated intramolecular singlet fission in oligomers and polymers.

  19. 7-azaindole and its clusters with Ar, CH sub 4 , H sub 2 O, NH sub 3 , and alcohols: Molecular geometry, cluster geometry, and nature of the first excited singlet electronic state

    SciTech Connect

    Kim, S.K.; Bernstein, E.R. )

    1990-05-03

    Mass-resolved excitation vibronic spectra of jet-cooled 7-azaindole and its clusters with Ar, CH{sub 4}, NH{sub 3}, H{sub 2}O, D{sub 2}O, CH{sub 3}OH, and C{sub 2}H{sub 5}OH are reported and analyzed with regard to molecular and cluster geometry and the nature of the first excited singlet state. Large changes in the various spectra are observed upon clustering and upon deuteration of 7-azaindole. The observed vibronic spectra of both 7-azaindole and its clusters can be rationalized with two general assumptions: (1) the hydrogen attached to the pyrrole nitrogen of 7-azaindole is out of the molecular plane in the first excited singlet state; and (2) the observed spectra are characterized by strong n {pi}*-{pi}{pi}* mixing not completely removed by the clustering. MOPAC5 calculations of molecular geometry suggest that the S{sub 1} state is nonplanar.

  20. An Analysis of the Torsion-Rotation-Vibration Rotational Spectrum of the Lowest In-Plane Bend and First Excited Torsional State of the C(3V) Internal Rotor C2H5CN

    NASA Technical Reports Server (NTRS)

    Pearson, J. C.; Pickett, Herbert M.; Sastry, K. V. L. N.

    2000-01-01

    C2H5CN (Propionitrile or ethyl cyanide) is a well known interstellar species abundantly observed in hot cores during the onset of star formation. The onset of star formation generally results in elevated temperature, which thermally populates may low lying vibrational states such as the 206/cm in-plane bend and the 212/cm first excited torsional state in C2H5CN. Unfortunately, these two states are strongly coupled through a complex series of torsion-vibration-rotation interactions, which dominate the spectrum. In order to understand the details of these interactions and develop models capable of predicting unmeasured transitions for astronomical observations in C2H5CN and similar molecules, several thousand rotational transitions in the lowest excited in-plane bend and first excited torsional state have been recorded, assigned and analyzed. The analysis reveals very strong a- and b-type Coriolis interactions and a number of other smaller interactions and has a number of important implications for other C3V torsion-rotation-vibration systems. The relative importance and the physical origins of the coupling among the rotational, vibrational and torsional motions will be presented along with a full spectroscopic analysis and supporting astronomical observations.

  1. Triplet-singlet conversion in ultracold Cs{sub 2} and production of ground-state molecules

    SciTech Connect

    Bouloufa, Nadia; Aymar, Mireille; Dulieu, Olivier; Pichler, Marin

    2011-02-15

    We propose a process to convert ultracold metastable Cs{sub 2} molecules in their lowest triplet state into (singlet) ground-state molecules in their lowest vibrational levels. Molecules are first pumped into an excited triplet state, and the triplet-singlet conversion is facilitated by a two-step spontaneous decay through the coupled A {sup 1{Sigma}}{sub u}{sup +}-b {sup 3{Pi}}{sub u} states. Using spectroscopic data and accurate quantum chemistry calculations for Cs{sub 2} potential curves and transition dipole moments, we show that this process competes favorably with the single-photon decay back to the lowest triplet state. In addition, we demonstrate that this conversion process represents a loss channel for vibrational cooling of metastable triplet molecules, preventing an efficient optical pumping cycle down to low vibrational levels.

  2. The transition from the open minimum to the ring minimum on the ground state and on the lowest excited state of like symmetry in ozone: A configuration interaction study.

    PubMed

    Theis, Daniel; Ivanic, Joseph; Windus, Theresa L; Ruedenberg, Klaus

    2016-03-14

    The metastable ring structure of the ozone 1(1)A1 ground state, which theoretical calculations have shown to exist, has so far eluded experimental detection. An accurate prediction for the energy difference between this isomer and the lower open structure is therefore of interest, as is a prediction for the isomerization barrier between them, which results from interactions between the lowest two (1)A1 states. In the present work, valence correlated energies of the 1(1)A1 state and the 2(1)A1 state were calculated at the 1(1)A1 open minimum, the 1(1)A1 ring minimum, the transition state between these two minima, the minimum of the 2(1)A1 state, and the conical intersection between the two states. The geometries were determined at the full-valence multi-configuration self-consistent-field level. Configuration interaction (CI) expansions up to quadruple excitations were calculated with triple-zeta atomic basis sets. The CI expansions based on eight different reference configuration spaces were explored. To obtain some of the quadruple excitation energies, the method of Correlation Energy Extrapolation by Intrinsic Scaling was generalized to the simultaneous extrapolation for two states. This extrapolation method was shown to be very accurate. On the other hand, none of the CI expansions were found to have converged to millihartree (mh) accuracy at the quadruple excitation level. The data suggest that convergence to mh accuracy is probably attained at the sextuple excitation level. On the 1(1)A1 state, the present calculations yield the estimates of (ring minimum-open minimum) ∼45-50 mh and (transition state-open minimum) ∼85-90 mh. For the (2(1)A1-(1)A1) excitation energy, the estimate of ∼130-170 mh is found at the open minimum and 270-310 mh at the ring minimum. At the transition state, the difference (2(1)A1-(1)A1) is found to be between 1 and 10 mh. The geometry of the transition state on the 1(1)A1 surface and that of the minimum on the 2(1)A1 surface

  3. The transition from the open minimum to the ring minimum on the ground state and on the lowest excited state of like symmetry in ozone: A configuration interaction study

    NASA Astrophysics Data System (ADS)

    Theis, Daniel; Ivanic, Joseph; Windus, Theresa L.; Ruedenberg, Klaus

    2016-03-01

    The metastable ring structure of the ozone 11A1 ground state, which theoretical calculations have shown to exist, has so far eluded experimental detection. An accurate prediction for the energy difference between this isomer and the lower open structure is therefore of interest, as is a prediction for the isomerization barrier between them, which results from interactions between the lowest two 1A1 states. In the present work, valence correlated energies of the 11A1 state and the 21A1 state were calculated at the 11A1 open minimum, the 11A1 ring minimum, the transition state between these two minima, the minimum of the 21A1 state, and the conical intersection between the two states. The geometries were determined at the full-valence multi-configuration self-consistent-field level. Configuration interaction (CI) expansions up to quadruple excitations were calculated with triple-zeta atomic basis sets. The CI expansions based on eight different reference configuration spaces were explored. To obtain some of the quadruple excitation energies, the method of Correlation Energy Extrapolation by Intrinsic Scaling was generalized to the simultaneous extrapolation for two states. This extrapolation method was shown to be very accurate. On the other hand, none of the CI expansions were found to have converged to millihartree (mh) accuracy at the quadruple excitation level. The data suggest that convergence to mh accuracy is probably attained at the sextuple excitation level. On the 11A1 state, the present calculations yield the estimates of (ring minimum—open minimum) ˜45-50 mh and (transition state—open minimum) ˜85-90 mh. For the (21A1-1A1) excitation energy, the estimate of ˜130-170 mh is found at the open minimum and 270-310 mh at the ring minimum. At the transition state, the difference (21A1-1A1) is found to be between 1 and 10 mh. The geometry of the transition state on the 11A1 surface and that of the minimum on the 21A1 surface nearly coincide. More accurate

  4. The transition from the open minimum to the ring minimum on the ground state and on the lowest excited state of like symmetry in ozone: A configuration interaction study

    DOE PAGESBeta

    Theis, Daniel; Ivanic, Joseph; Windus, Theresa L.; Ruedenberg, Klaus

    2016-03-10

    The metastable ring structure of the ozone 11A1 ground state, which theoretical calculations have shown to exist, has so far eluded experimental detection. An accurate prediction for the energy difference between this isomer and the lower open structure is therefore of interest, as is a prediction for the isomerization barrier between them, which results from interactions between the lowest two 1A1 states. In the present work, valence correlated energies of the 11A1 state and the 21A1 state were calculated at the 11A1 open minimum, the 11A1 ring minimum, the transition state between these two minima, the minimum of the 21A1more » state, and the conical intersection between the two states. The geometries were determined at the full-valence multi-configuration self-consistent-field level. Configuration interaction (CI) expansions up to quadruple excitations were calculated with triple-zeta atomic basis sets. The CI expansions based on eight different reference configuration spaces were explored. To obtain some of the quadruple excitation energies, the method of CorrelationEnergy Extrapolation by Intrinsic Scaling was generalized to the simultaneous extrapolation for two states. This extrapolation method was shown to be very accurate. On the other hand, none of the CI expansions were found to have converged to millihartree (mh) accuracy at the quadruple excitation level. The data suggest that convergence to mh accuracy is probably attained at the sextuple excitation level. On the 11A1 state, the present calculations yield the estimates of (ring minimum—open minimum) ~45–50 mh and (transition state—open minimum) ~85–90 mh. For the (21A1–1A1) excitation energy, the estimate of ~130–170 mh is found at the open minimum and 270–310 mh at the ring minimum. At the transition state, the difference (21A1–1A1) is found to be between 1 and 10 mh. The geometry of the transition state on the 11A1 surface and that of the minimum on the 21A1 surface nearly coincide

  5. Recent Advances in Singlet Fission

    NASA Astrophysics Data System (ADS)

    Smith, Millicent B.; Michl, Josef

    2013-04-01

    A survey is provided of recent progress in the understanding of singlet fission, a spin-allowed process in which a singlet excited molecule shares its energy with a ground-state neighbor to produce two triplet excited molecules. It has been observed to occur in single-crystal, polycrystalline, and amorphous solids, on timescales from 80 fs to 25 ps, producing triplet yields as high as 200%. Photovoltaic devices using the effect have shown external quantum efficiencies in excess of 100%. Almost all the efficient materials are alternant hydrocarbons of the acene series or their simple derivatives, and it is argued that a wider structural variety would be desirable. The current state of the development of molecular structure design rules, based on first-principles theoretical considerations, is described along with initial examples of implementation.

  6. Singlet oxygen in photosensitization.

    PubMed

    Moan, Johan; Juzenas, Petras

    2006-01-01

    Oxygen is a ubiquitous element and a vitally important substance for life on the Earth, and especially for human life. Living organisms need oxygen for most, if not all, of their cellular functions. On the other hand, oxygen can produce metabolites that are toxic and potentially lethal to the same cells. Being reactive and chemically unstable reactive oxygen species (ROS) are the most important metabolites that initiate reduction and oxidation (redox) reactions under physiological conditions. Oxygen in its excited singlet state (1O2) is probably the most important intermediate in such reactions. Since the discovery of oxygen by Joseph Priestley in 1775 it has been recognized that oxygen can be both beneficial and harmful to life. PMID:16566709

  7. Lowest Π-Π* electronic transitions in linear and two-dimensional polycyclic aromatic hydrocarbons: enhanced electron density edge effect

    NASA Astrophysics Data System (ADS)

    Yadav, Amarjeet; Mishra, P. C.

    2014-04-01

    Polycyclic aromatic hydrocarbons (PAHs) form an important class of molecules as they are ubiquitous, pollute air and cause severe health problems. Lowest vertical π-π* singlet-singlet or triplet-triplet excitation energies and corresponding oscillator strengths were studied for several linear and two-dimensional PAHs employing time-dependent density functional theory. Excited-state electron density, molecular electrostatic potential (MEP) and spin density distributions in the PAHs, along with ground-state chemical hardness, were also studied. It has been found that, generally, excitation energies and oscillator strengths decrease with increase in PAH size, and excitation energies and chemical hardness are strongly linearly correlated. Enhanced electron density edge effect, which was found to occur in the ground states of the molecules, continues to hold in their excited states also. A strong similarity between the ground and π-π* excited-state MEP maps suggests that σ electrons are the main contributors to the enhanced electron density at the edges. Due to their strong electronic absorption transitions in the visible and infrared regions, the PAHs can be used for harnessing solar energy efficiently.

  8. Adsorbate-induced absorption redshift in an organic-inorganic cluster conjugate: Electronic effects of surfactants and organic adsorbates on the lowest excited states of a methanethiol-CdSe conjugate

    NASA Astrophysics Data System (ADS)

    Liu, Christopher; Chung, Sang-Yoon; Lee, Sungyul; Weiss, Shimon; Neuhauser, Daniel

    2009-11-01

    Bioconjugated CdSe quantum dots are promising reagents for bioimaging applications. Experimentally, the binding of a short peptide has been found to redshift the optical absorption of nanoclusters [J. Tsay et al., J. Phys. Chem. B 109, 1669 (2005)]. This study examines this issue by performing density functional theory (DFT) and time-dependent-DFT calculations to study the ground state and low-lying excited states of (CdSe)6[SCH3]-, a transition metal complex built by binding methanethiolate to a CdSe molecular cluster. Natural bond orbital results show that the redshift is caused by ligand-inorganic cluster orbital interaction. The highest occupied molecular orbital (HOMO) of (CdSe)6 is dominated by selenium 4p orbitals; in contrast, the HOMO of (CdSe)6[SCH3]- is dominated by sulfur 3p orbitals. This difference shows that [SCH3]- binding effectively introduces filled sulfur orbitals above the selenium 4p orbitals of (CdSe)6. The resulting smaller HOMO-LUMO gap of (CdSe)6[SCH3]- indeed leads to redshifts in its excitation energies compared to (CdSe)6. In contrast, binding of multiple NH3 destabilizes cadmium 5p orbitals, which contribute significantly to the lowest unoccupied molecular orbital (LUMO) of (CdSe)6, while leaving the selenium 4p orbitals near the HOMO relatively unaffected. This has the effect of widening the HOMO-LUMO gap of (CdSe)6ṡ6NH3 compared to (CdSe)6. As expected, the excitation energies of the passivated (CdSe)6ṡ6NH3 are also blueshifted compared to (CdSe)6. As far as NH3 is a faithful representation of a surfactant, the results clearly illustrate the differences between the electronic effects of an alkylthiolate versus those of surfactant molecules. Surface passivation of (CdSe)6[SCH3]- is then simulated by coating it with multiple NH3 molecules. The results suggest that the [SCH3]- adsorption induces a redshift in the excitation energies in a surfactant environment.

  9. Microscopic theory of singlet exciton fission. I. General formulation

    NASA Astrophysics Data System (ADS)

    Berkelbach, Timothy C.; Hybertsen, Mark S.; Reichman, David R.

    2013-03-01

    Singlet fission, a spin-allowed energy transfer process generating two triplet excitons from one singlet exciton, has the potential to dramatically increase the efficiency of organic solar cells. However, the dynamical mechanism of this phenomenon is not fully understood and a complete, microscopic theory of singlet fission is lacking. In this work, we assemble the components of a comprehensive microscopic theory of singlet fission that connects excited state quantum chemistry calculations with finite-temperature quantum relaxation theory. We elaborate on the distinction between localized diabatic and delocalized exciton bases for the interpretation of singlet fission experiments in both the time and frequency domains. We discuss various approximations to the exact density matrix dynamics and propose Redfield theory as an ideal compromise between speed and accuracy for the detailed investigation of singlet fission in dimers, clusters, and crystals. Investigations of small model systems based on parameters typical of singlet fission demonstrate the numerical accuracy and practical utility of this approach.

  10. Lowest Vibrational States of Acrylonitrile

    NASA Astrophysics Data System (ADS)

    Kisiel, Zbigniew; Martin-Drumel, Marie-Aline; Pirali, Olivier

    2015-06-01

    Recent studies of the broadband rotational spectrum of acrylonitrile, H_2C=CHC≡N, revealed the presence of multiple resonances between rotational levels in different vibrational states. The resonances affect even the ground state transitions and their analysis allowed determination of vibrational term values for the first three excited states above the ground state and of vibrational energy differences in several polyads above these states. At that time there was no infrared data of sufficient resolution to assess the reliability of the resonance based vibrational energy determinations. We presently report results based on a 40-700 cm-1 high-resolution spectrum of acrylonitrile recorded at the AILES beamline of the SOLEIL synchrotron. This spectrum was reduced by using the AABS packagea, and allowed assignment of vibration-rotation transitions in four fundamentals, five hot bands, and one overtone band. The infrared data and previous measurements made with microwave techniques have been combined into a single global fit encompassing over 31000 measured transitions. Precise vibrational term values have been determined for the eight lowest excited vibrational states. The new results validate the previous estimates from rotational perturbations and are also compared with results of ab~initio anharmonic force field calculations. Z. Kisiel, et al., J. Mol. Spectrosc. 280 134 (2012). A. López, et al., Astron. & Astrophys. 572, A44 Z. Kisiel, et al., J. Mol. Spectrosc. 233 231 (2005).

  11. Phosphorescence study of chlorophyll d photophysics. Determination of the energy and lifetime of the photo-excited triplet state. Evidence of singlet oxygen photosensitization.

    PubMed

    Neverov, Konstantin V; Santabarbara, Stefano; Krasnovsky, Alexander A

    2011-09-01

    Chlorophyll d (Chl d) is the major pigment in both photosystems (PSI and II) of the cyanobacterium Acaryochloris marina, whose pigment composition represents an interesting alternative in oxygenic photosynthesis. While abundant information is available relative to photophysical properties of Chl a , the understanding of Chl d photophysics is still incomplete. In this paper, we present for the first time a characterization of Chl d phosphorescence, which accompanies radiative deactivation of the photoexcited triplet state of this pigment. Reliable information was obtained on the energy and lifetime of the Chl d triplet state in frozen solutions at 77 K using diethyl ether and aqueous dispersions of Triton X100 as solvents. It is shown that triplet Chl d is effectively populated upon photoexcitation of pigment molecules and efficiently sensitizes singlet oxygen phosphorescence in aerobic solutions under ambient conditions. The data obtained are compared with the previous results of the phosphorescence studies of Chl a and Pheo a, and their possible biological implications are discussed. PMID:21573948

  12. Theoretical rationalization of the singlet-triplet gap in OLEDs materials: impact of charge-transfer character.

    PubMed

    Moral, M; Muccioli, L; Son, W-J; Olivier, Y; Sancho-García, J C

    2015-01-13

    New materials for OLED applications with low singlet-triplet energy splitting have been recently synthesized in order to allow for the conversion of triplet into singlet excitons (emitting light) via a Thermally Activated Delayed Fluorescence (TADF) process, which involves excited-states with a non-negligible amount of Charge-Transfer (CT). The accurate modeling of these states with Time-Dependent Density Functional Theory (TD-DFT), the most used method so far because of the favorable trade-off between accuracy and computational cost, is however particularly challenging. We carefully address this issue here by considering materials with small (high) singlet-triplet gap acting as emitter (host) in OLEDs and by comparing the accuracy of TD-DFT and the corresponding Tamm-Dancoff Approximation (TDA), which is found to greatly reduce error bars with respect to experiments thanks to better estimates for the lowest singlet-triplet transition. Finally, we quantitatively correlate the singlet-triplet splitting values with the extent of CT, using for it a simple metric extracted from calculations with double-hybrid functionals, that might be applied in further molecular engineering studies. PMID:26574215

  13. Singlet oxygen production in Chlamydomonas reinhardtii under heat stress.

    PubMed

    Prasad, Ankush; Ferretti, Ursula; Sedlářová, Michaela; Pospíšil, Pavel

    2016-01-01

    In the current study, singlet oxygen formation by lipid peroxidation induced by heat stress (40 °C) was studied in vivo in unicellular green alga Chlamydomonas reinhardtii. Primary and secondary oxidation products of lipid peroxidation, hydroperoxide and malondialdehyde, were generated under heat stress as detected using swallow-tailed perylene derivative fluorescence monitored by confocal laser scanning microscopy and high performance liquid chromatography, respectively. Lipid peroxidation was initiated by enzymatic reaction as inhibition of lipoxygenase by catechol and caffeic acid prevented hydroperoxide formation. Ultra-weak photon emission showed formation of electronically excited species such as triplet excited carbonyl, which, upon transfer of excitation energy, leads to the formation of either singlet excited chlorophyll or singlet oxygen. Alternatively, singlet oxygen is formed by direct decomposition of hydroperoxide via Russell mechanisms. Formation of singlet oxygen was evidenced by the nitroxyl radical 2,2,6,6-tetramethylpiperidine-1-oxyl detected by electron paramagnetic resonance spin-trapping spectroscopy and the imaging of green fluorescence of singlet oxygen sensor green detected by confocal laser scanning microscopy. Suppression of singlet oxygen formation by lipoxygenase inhibitors indicates that singlet oxygen may be formed via enzymatic lipid peroxidation initiated by lipoxygenase. PMID:26831215

  14. Singlet oxygen production in Chlamydomonas reinhardtii under heat stress

    PubMed Central

    Prasad, Ankush; Ferretti, Ursula; Sedlářová, Michaela; Pospíšil, Pavel

    2016-01-01

    In the current study, singlet oxygen formation by lipid peroxidation induced by heat stress (40 °C) was studied in vivo in unicellular green alga Chlamydomonas reinhardtii. Primary and secondary oxidation products of lipid peroxidation, hydroperoxide and malondialdehyde, were generated under heat stress as detected using swallow-tailed perylene derivative fluorescence monitored by confocal laser scanning microscopy and high performance liquid chromatography, respectively. Lipid peroxidation was initiated by enzymatic reaction as inhibition of lipoxygenase by catechol and caffeic acid prevented hydroperoxide formation. Ultra-weak photon emission showed formation of electronically excited species such as triplet excited carbonyl, which, upon transfer of excitation energy, leads to the formation of either singlet excited chlorophyll or singlet oxygen. Alternatively, singlet oxygen is formed by direct decomposition of hydroperoxide via Russell mechanisms. Formation of singlet oxygen was evidenced by the nitroxyl radical 2,2,6,6-tetramethylpiperidine-1-oxyl detected by electron paramagnetic resonance spin-trapping spectroscopy and the imaging of green fluorescence of singlet oxygen sensor green detected by confocal laser scanning microscopy. Suppression of singlet oxygen formation by lipoxygenase inhibitors indicates that singlet oxygen may be formed via enzymatic lipid peroxidation initiated by lipoxygenase. PMID:26831215

  15. Singlet fission of hot excitons in π-conjugated polymers.

    PubMed

    Zhai, Yaxin; Sheng, Chuanxiang; Vardeny, Z Valy

    2015-06-28

    We used steady-state photoinduced absorption (PA), excitation dependence (EXPA(ω)) spectrum of the triplet exciton PA band, and its magneto-PA (MPA(B)) response to investigate singlet fission (SF) of hot excitons into two separated triplet excitons, in two luminescent and non-luminescent π-conjugated polymers. From the high energy step in the triplet EXPA(ω) spectrum of the luminescent polymer poly(dioctyloxy)phenylenevinylene (DOO-PPV) films, we identified a hot-exciton SF (HE-SF) process having threshold energy at E≈2E(T) (=2.8 eV, where ET is the energy of the lowest lying triplet exciton), which is about 0.8 eV above the lowest singlet exciton energy. The HE-SF process was confirmed by the triplet MPA(B) response for excitation at E>2E(T), which shows typical SF response. This process is missing in DOO-PPV solution, showing that it is predominantly interchain in nature. By contrast, the triplet EXPA(ω) spectrum in the non-luminescent polymer polydiacetylene (PDA) is flat with an onset at E=E(g) (≈2.25 eV). From this, we infer that intrachain SF that involves a triplet-triplet pair state, also known as the 'dark' 2A(g) exciton, dominates the triplet photogeneration in PDA polymer as E(g)>2E(T). The intrachain SF process was also identified from the MPA(B) response of the triplet PA band in PDA. Our work shows that the SF process in π-conjugated polymers is a much more general process than thought previously. PMID:25987576

  16. Singlet fission of hot excitons in π-conjugated polymers

    PubMed Central

    Zhai, Yaxin; Sheng, Chuanxiang; Vardeny, Z. Valy

    2015-01-01

    We used steady-state photoinduced absorption (PA), excitation dependence (EXPA(ω)) spectrum of the triplet exciton PA band, and its magneto-PA (MPA(B)) response to investigate singlet fission (SF) of hot excitons into two separated triplet excitons, in two luminescent and non-luminescent π-conjugated polymers. From the high energy step in the triplet EXPA(ω) spectrum of the luminescent polymer poly(dioctyloxy)phenylenevinylene (DOO-PPV) films, we identified a hot-exciton SF (HE-SF) process having threshold energy at E≈2ET (=2.8 eV, where ET is the energy of the lowest lying triplet exciton), which is about 0.8 eV above the lowest singlet exciton energy. The HE-SF process was confirmed by the triplet MPA(B) response for excitation at E>2ET, which shows typical SF response. This process is missing in DOO-PPV solution, showing that it is predominantly interchain in nature. By contrast, the triplet EXPA(ω) spectrum in the non-luminescent polymer polydiacetylene (PDA) is flat with an onset at E=Eg (≈2.25 eV). From this, we infer that intrachain SF that involves a triplet–triplet pair state, also known as the ‘dark’ 2Ag exciton, dominates the triplet photogeneration in PDA polymer as Eg>2ET. The intrachain SF process was also identified from the MPA(B) response of the triplet PA band in PDA. Our work shows that the SF process in π-conjugated polymers is a much more general process than thought previously. PMID:25987576

  17. On the Electronically Excited States of Uracil

    SciTech Connect

    Epifanovsky, Evgeny; Kowalski, Karol; Fan, Peng-Dong; Valiev, Marat; Matsika, Spiridoula; Krylov, Anna

    2008-10-09

    Vertical excitation energies in uracil in the gas phase and in water solution are investigated by the equation-of-motion coupled-cluster and multi-reference configuration interaction methods. Basis set effects are found to be important for converged results. The analysis of electronic wave functions reveals that the lowest singlet states are predominantly of a singly excited character and are therefore well described by single-reference equation-of-motion methods augmented by a perturbative triples correction to account for dynamical correlation. Our best estimates for the vertical excitation energies for the lowest singlet n and are 5.0±0.1 eV and 5.3±0.1 eV, respectively. The solvent effects for these states are estimated to be +0.5 eV and ±0.1 eV, respectively. We attribute the difference between the computed vertical excitations and the maximum of the experimental absorption to strong vibronic interaction between the lowest A00 and A0 states leading to intensity borrowing by the forbidden transition.

  18. 1,1-Dilithioethylene: Toward Spectroscopic Identification of the Definitive Singlet Ground Electronic State of a Peculiar Structure.

    PubMed

    Liu, Yameng; Wang, Xiao; Liu, Yongdong; Zhong, Rugang; Xie, Yaoming; Schaefer, Henry F

    2016-06-01

    1,1-Dilithioethylene is a prototypical carbon-lithium compound that is not known experimentally. All low-lying singlet and triplet structures of interest were investigated by using high-level theoretical methods with correlation-consistent basis sets up to pentuple ζ. The coupled cluster methods adopted included up to full triple excitations and perturbative quadruples. In contrast to earlier studies that predicted the twisted C2v triplet to be the ground state, we found a peculiar planar Cs singlet ground state in the present research. The lowest excited electronic state of 1,1-dilithioethylene, the twisted Cs triplet, was found to lie 9.0 kcal mol(-1) above the ground state by using energy extrapolation to the complete basis set limit. For the planar Cs singlet and twisted Cs triplet states of 1,1-dilithioethylene, anharmonic vibrational frequencies were reported on the basis of second-order vibrational perturbation theory. The remarkably low (2050 cm(-1) ) C-H stretching fundamental (the C-H bond near the bridging lithium) of the singlet state was found to have very strong infrared intensity. These highly reliable theoretical findings may assist in the long-sought experimental identification of 1,1-dilithioethylene. Using natural bond orbital analysis, we found that lithium bridging structures were strongly influenced by electrostatic effects. All carbon-carbon linkages corresponded to conventional double bonds. PMID:27038425

  19. Multiphonon relaxation slows singlet fission in crystalline hexacene.

    PubMed

    Busby, Erik; Berkelbach, Timothy C; Kumar, Bharat; Chernikov, Alexey; Zhong, Yu; Hlaing, Htay; Zhu, X-Y; Heinz, Tony F; Hybertsen, Mark S; Sfeir, Matthew Y; Reichman, David R; Nuckolls, Colin; Yaffe, Omer

    2014-07-30

    Singlet fission, the conversion of a singlet excitation into two triplet excitations, is a viable route to improved solar-cell efficiency. Despite active efforts to understand the singlet fission mechanism, which would aid in the rational design of new materials, a comprehensive understanding of mechanistic principles is still lacking. Here, we present the first study of singlet fission in crystalline hexacene which, together with tetracene and pentacene, enables the elucidation of mechanistic trends. We characterize the static and transient optical absorption and combine our findings with a theoretical analysis of the relevant electronic couplings and rates. We find a singlet fission time scale of 530 fs, which is orders of magnitude faster than tetracene (10-100 ps) but significantly slower than pentacene (80-110 fs). We interpret this increased time scale as a multiphonon relaxation effect originating from a large exothermicity and present a microscopic theory that quantitatively reproduces the rates in the acene family. PMID:24983697

  20. Is Nitrate Anion Photodissociation Mediated by Singlet-Triplet Absorption?

    PubMed

    Svoboda, Ondřej; Slavíček, Petr

    2014-06-01

    Photolysis of the nitrate anion is involved in the oxidation processes in the hydrosphere, cryosphere, and stratosphere. While it is known that the nitrate photolysis in the long-wavelength region proceeds with a very low quantum yield, the mechanism of the photodissociation remains elusive. Here, we present the quantitative modeling of singlet-singlet and singlet-triplet absorption spectra in the atmospherically relevant region around 300 nm, and we argue that a spin-forbidden transition between the singlet ground state and the first triplet state contributes non-negligibly to the nitrate anion photolysis. We further propose that the nitrate anion excited into the first singlet excited state relaxes nonradiatively into its ground state. The full understanding of the nitrate anion photolysis can improve modeling of the asymmetric solvation in the atmospheric processes, e.g., photolysis on the surfaces of ice or snow. PMID:26273880

  1. Generation of singlet oxygen on the surface of metal oxides

    NASA Astrophysics Data System (ADS)

    Kiselev, V. M.; Kislyakov, I. M.; Burchinov, A. N.

    2016-04-01

    Generation of singlet oxygen on the surface of metal oxides is studied. It is shown that, under conditions of heterogeneous photo-catalysis, along with the conventional mechanism of singlet oxygen formation due to the formation of electron-hole pairs in the oxide structure, there is an additional and more efficient mechanism involving direct optical excitation of molecular oxygen adsorbed on the oxide surface. The excited adsorbate molecule then interacts with the surface or with other adsorbate molecules. It is shown that, with respect to singlet oxygen generation, yttrium oxide is more than an order of magnitude more efficient than other oxides, including titanium dioxide.

  2. Four-electron model for singlet and triplet excitation energy transfers with inclusion of coherence memory, inelastic tunneling and nuclear quantum effects

    NASA Astrophysics Data System (ADS)

    Suzuki, Yosuke; Ebina, Kuniyoshi; Tanaka, Shigenori

    2016-08-01

    A computational scheme to describe the coherent dynamics of excitation energy transfer (EET) in molecular systems is proposed on the basis of generalized master equations with memory kernels. This formalism takes into account those physical effects in electron-bath coupling system such as the spin symmetry of excitons, the inelastic electron tunneling and the quantum features of nuclear motions, thus providing a theoretical framework to perform an ab initio description of EET through molecular simulations for evaluating the spectral density and the temporal correlation function of electronic coupling. Some test calculations have then been carried out to investigate the dependence of exciton population dynamics on coherence memory, inelastic tunneling correlation time, magnitude of electronic coupling, quantum correction to temporal correlation function, reorganization energy and energy gap.

  3. Impact of ground- and excited-state aromaticity on cyclopentadiene and silole excitation energies and excited-state polarities.

    PubMed

    Jorner, Kjell; Emanuelsson, Rikard; Dahlstrand, Christian; Tong, Hui; Denisova, Aleksandra V; Ottosson, Henrik

    2014-07-21

    A new qualitative model for estimating the properties of substituted cyclopentadienes and siloles in their lowest ππ* excited states is introduced and confirmed through quantum chemical calculations, and then applied to explain earlier reported experimental excitation energies. According to our model, which is based on excited-state aromaticity and antiaromaticity, siloles and cyclopentadienes are cross-hyperconjugated "aromatic chameleons" that adapt their electronic structures to conform to the various aromaticity rules in different electronic states (Hückel's rule in the π(2) electronic ground state (S0) and Baird's rule in the lowest ππ* excited singlet and triplet states (S1 and T1)). By using pen-and-paper arguments, one can explain polarity changes upon excitation of substituted cyclopentadienes and siloles, and one can tune their lowest excitation energies by combined considerations of ground- and excited-state aromaticity/antiaromaticity effects. Finally, the "aromatic chameleon" model can be extended to other monocyclic compound classes of potential use in organic electronics, thereby providing a unified view of the S0, T1, and S1 states of a range of different cyclic cross-π-conjugated and cross-hyperconjugated compound classes. PMID:25043523

  4. Time-resolved infrared spectroscopy of the lowest triplet state of thymine and thymidine

    NASA Astrophysics Data System (ADS)

    Hare, Patrick M.; Middleton, Chris T.; Mertel, Kristin I.; Herbert, John M.; Kohler, Bern

    2008-05-01

    Vibrational spectra of the lowest energy triplet states of thymine and its 2'-deoxyribonucleoside, thymidine, are reported for the first time. Time-resolved infrared (TRIR) difference spectra were recorded over seven decades of time from 300 fs to 3 μs using femtosecond and nanosecond pump-probe techniques. The carbonyl stretch bands in the triplet state are seen at 1603 and ˜1700 cm -1 in room-temperature acetonitrile- d3 solution. These bands and additional ones observed between 1300 and 1450 cm -1 are quenched by dissolved oxygen on a nanosecond time scale. Density-functional calculations accurately predict the difference spectrum between triplet and singlet IR absorption cross sections, confirming the peak assignments and elucidating the nature of the vibrational modes. In the triplet state, the C4 dbnd O carbonyl exhibits substantial single-bond character, explaining the large (˜70 cm -1) red shift in this vibration, relative to the singlet ground state. Femtosecond TRIR measurements unambiguously demonstrate that the triplet state is fully formed within the first 10 ps after excitation, ruling out a relaxed 1nπ ∗ state as the triplet precursor.

  5. LHC phenomenology of lowest massive Regge recurrences in the Randall-Sundrum orbifold

    SciTech Connect

    Anchordoqui, Luis A.; Huang Xing; Goldberg, Haim; Taylor, Tomasz R.

    2010-11-15

    We consider string realizations of the Randall-Sundrum effective theory for electroweak symmetry breaking and explore the search for the lowest massive Regge excitation of the gluon and of the extra (color singlet) gauge boson inherent in D-brane constructions. In these curved backgrounds, the higher-spin Regge recurrences of standard model fields localized near the IR brane are warped down to close to the TeV range and hence can be produced at collider experiments. Assuming that the theory is weakly coupled, we make use of four gauge boson amplitudes evaluated near the first Regge pole to determine the discovery potential of LHC. We study the inclusive dijet mass spectrum in the central rapidity region |y{sub jet}|<1.0 for dijet masses M{>=}2.5 TeV. We find that with an integrated luminosity of 100 fb{sup -1}, the 5{sigma} discovery reach can be as high as 4.7 TeV. Observations of resonant structures in pp{yields}direct{gamma}+jet can provide interesting corroboration for string physics up to 3.0 TeV. We also study the ratio of dijet mass spectra at small and large scattering angles. We show that with the first fb{sup -1} such a ratio can probe lowest-lying Regge states for masses {approx}2.5 TeV.

  6. Singlet exciton fission photovoltaics.

    PubMed

    Lee, Jiye; Jadhav, Priya; Reusswig, Philip D; Yost, Shane R; Thompson, Nicholas J; Congreve, Daniel N; Hontz, Eric; Van Voorhis, Troy; Baldo, Marc A

    2013-06-18

    Singlet exciton fission, a process that generates two excitons from a single photon, is perhaps the most efficient of the various multiexciton-generation processes studied to date, offering the potential to increase the efficiency of solar devices. But its unique characteristic, splitting a photogenerated singlet exciton into two dark triplet states, means that the empty absorption region between the singlet and triplet excitons must be filled by adding another material that captures low-energy photons. This has required the development of specialized device architectures. In this Account, we review work to develop devices that harness the theoretical benefits of singlet exciton fission. First, we discuss singlet fission in the archetypal material, pentacene. Pentacene-based photovoltaic devices typically show high external and internal quantum efficiencies. They have enabled researchers to characterize fission, including yield and the impact of competing loss processes, within functional devices. We review in situ probes of singlet fission that modulate the photocurrent using a magnetic field. We also summarize studies of the dissociation of triplet excitons into charge at the pentacene-buckyball (C60) donor-acceptor interface. Multiple independent measurements confirm that pentacene triplet excitons can dissociate at the C60 interface despite their relatively low energy. Because triplet excitons produced by singlet fission each have no more than half the energy of the original photoexcitation, they limit the potential open circuit voltage within a solar cell. Thus, if singlet fission is to increase the overall efficiency of a solar cell and not just double the photocurrent at the cost of halving the voltage, it is necessary to also harvest photons in the absorption gap between the singlet and triplet energies of the singlet fission material. We review two device architectures that attempt this using long-wavelength materials: a three-layer structure that uses

  7. Singlets and triplets in hybrid nanodevices

    NASA Astrophysics Data System (ADS)

    Agranovich, V. M.; Dubovskii, O. A.; La Rocca, G. C.

    2014-05-01

    An organic material thin layer can be used to resonantly absorb light and nonradiatively transfer excitation to an adjacent inorganic quantum well the optical nonlinearities of which can in this way be turned on more efficiently than by direct optical pumping. We theoretically consider this process in a hybrid structure based on crystalline tetracene in which the singlet exciton energy is close to twice the one of a triplet exciton and thermally activated singlet exciton fission into two triplets can be efficient. We investigate how the temperature dependence of the singlet exciton diffusion length affects the functional properties of such hybrid organic-inorganic nanostructures based on tetracene. We show how temperature activated fission opens a new possibility to turn on and off the indirect pumping due to energy transfer from the organic into the inorganic subsystem.

  8. Biradical vs singlet oxygen photogeneration in suprofen-cholesterol systems.

    PubMed

    Palumbo, Fabrizio; Bosca, Francisco; Morera, Isabel Maria; Andreu, Inmaculada; Miranda, Miguel A

    2016-01-01

    Cholesterol (Ch) is an important lipidic building block and a target for oxidative degradation, which can be induced via free radicals or singlet oxygen ((1)O2). Suprofen (SP) is a nonsteroidal anti-inflammatory drug that contains the 2-benzoylthiophene (BZT) chromophore and has a π,π* lowest triplet excited state. In the present work, dyads (S)- and (R)-SP-α-Ch (1 and 2), as well as (S)-SP-β-Ch (3) have been prepared from β- or α-Ch and SP to investigate the possible competition between photogeneration of biradicals and (1)O2, the key mechanistic steps in Ch photooxidation. Steady-state irradiation of 1 and 2 was performed in dichloromethane, under nitrogen, through Pyrex, using a 400 W medium pressure mercury lamp. The spectral analysis of the separated fractions revealed formation of two photoproducts 4 and 5, respectively. By contrast, under the same conditions, 3 did not give rise to any isolable Ch-derived product. These results point to an intramolecular hydrogen abstraction in 1 and 2 from the C7 position of Ch and subsequent C-C coupling of the generated biradicals. Interestingly, 2 was significantly more photoreactive than 1 indicating a clear stereodifferentiation in the photochemical behavior. Transient absorption spectra obtained for 1-3 were very similar and matched that described for the SP triplet excited state (typical bands with maxima at ca. 350 nm and 600 nm). Direct kinetic analysis of the decay traces at 620 nm led to determination of triplet lifetimes that were ca. 4.1 μs for 1 and 2 and 5.8 μs for 3. From these data, the intramolecular quenching rate constants in 1 and 2 were determined as 0.78 × 10(5) s(-1). The capability of dyads 1-3 to photosensitize the production of singlet oxygen was assessed by time-resolved near infrared emission studies in dichloromethane using perinaphthenone as standard. The quantum yields (ΦΔ) were 0.52 for 1 and 2 and 0.56 for 3. In conclusion, SP-α-Ch dyads are unique in the sense that they can be

  9. Biradical vs singlet oxygen photogeneration in suprofen–cholesterol systems

    PubMed Central

    Palumbo, Fabrizio; Bosca, Francisco; Morera, Isabel Maria

    2016-01-01

    Summary Cholesterol (Ch) is an important lipidic building block and a target for oxidative degradation, which can be induced via free radicals or singlet oxygen (1O2). Suprofen (SP) is a nonsteroidal anti-inflammatory drug that contains the 2-benzoylthiophene (BZT) chromophore and has a π,π* lowest triplet excited state. In the present work, dyads (S)- and (R)-SP-α-Ch (1 and 2), as well as (S)-SP-β-Ch (3) have been prepared from β- or α-Ch and SP to investigate the possible competition between photogeneration of biradicals and 1O2, the key mechanistic steps in Ch photooxidation. Steady-state irradiation of 1 and 2 was performed in dichloromethane, under nitrogen, through Pyrex, using a 400 W medium pressure mercury lamp. The spectral analysis of the separated fractions revealed formation of two photoproducts 4 and 5, respectively. By contrast, under the same conditions, 3 did not give rise to any isolable Ch-derived product. These results point to an intramolecular hydrogen abstraction in 1 and 2 from the C7 position of Ch and subsequent C–C coupling of the generated biradicals. Interestingly, 2 was significantly more photoreactive than 1 indicating a clear stereodifferentiation in the photochemical behavior. Transient absorption spectra obtained for 1–3 were very similar and matched that described for the SP triplet excited state (typical bands with maxima at ca. 350 nm and 600 nm). Direct kinetic analysis of the decay traces at 620 nm led to determination of triplet lifetimes that were ca. 4.1 μs for 1 and 2 and 5.8 μs for 3. From these data, the intramolecular quenching rate constants in 1 and 2 were determined as 0.78 × 105 s−1. The capability of dyads 1–3 to photosensitize the production of singlet oxygen was assessed by time-resolved near infrared emission studies in dichloromethane using perinaphthenone as standard. The quantum yields (ΦΔ) were 0.52 for 1 and 2 and 0.56 for 3. In conclusion, SP-α-Ch dyads are unique in the sense that they

  10. Singlet excited state dipole moments of dual fluorescent N-phenylpyrroles and 4-(dimethylamino)benzonitrile from solvatochromic and thermochromic spectral shifts.

    PubMed

    Yoshihara, Toshitada; Galievsky, Victor A; Druzhinin, Sergey I; Saha, Satyen; Zachariasse, Klaas A

    2003-03-01

    The excited state dipole moments mue(ICT) and mue(LE) of the dual fluorescent molecules N-phenylpyrrole (PP), N-(4-cyanophenyl)pyrrole (PP4C) and N-(3-cyanophenyl)pyrrole (PP3C) are determined from solvatochromic and thermochromic measurements. It is shown that the best results are obtained when the solvatochromic as well as the thermochromic analysis of the spectral shifts is made relative to 4-(dimethylamino)benzonitrile (DMABN) as the model compound. Direct thermochromic experiments with PP4C, PP3C and DMABN in diethyl ether lead to reasonable results, but unrealistically large dipole moments mue(ICT) are found for PP, PP4C, PP3C and DMABN in acetonitrile, ethyl cyanide and n-propyl cyanide. The mue(ICT) values obtained for the N-phenylpyrroles from the thermochromic analysis in these solvents relative to DMABN (17 D) do not depend on solvent polarity: 13 D for PP, 15 D for PP4C and PP3C. The spectral shifts for the LE emission of the N-phenylpyrroles and aminobenzonitriles are much smaller than those for the ICT fluorescence, resulting in relatively small values for mue(LE). With PP and N-(4-methylphenyl)pyrrole (PP4M) the problem arises that one of the two values calculated by solving the quadratic equation for mue(LE) in the solvatochromic and thermochromic analysis cannot be discarded on photophysical or molecular grounds, as is the case for the other molecules. The experimental data for mue(ICT) of PP and PP4C are compared with theoretical values calculated for coplanar (PICT) and perpendicular (TICT) conformations of the pyrrole and phenyl or cyanophenyl groups. The experimental ICT dipole moment of PP4C has a value in between the theoretical results for mue(PICT) and mue(TICT), whereas the data for PP tend to favour the TICT configuration. It appears that in the LE state of PP and PP4M a negative charge remains on the pyrrole moiety, whereas a charge reversal takes place for the LE state of PP3C and the ICT state of PP, PP4C and PP3C. PMID:12713236

  11. Correlated Pair States Formed by Singlet Fission and Exciton-Exciton Annihilation.

    PubMed

    Scholes, Gregory D

    2015-12-24

    Singlet fission to form a pair of triplet excitations on two neighboring molecules and the reverse process, triplet-triplet annihilation to upconvert excitation, have been extensively studied. Comparatively little work has sought to examine the properties of the intermediate state in both of these processes-the bimolecular pair state. Here, the eigenstates constituting the manifold of 16 bimolecular pair excitations and their relative energies in the weak-coupling regime are reported. The lowest-energy states obtained from the branching diagram method are the triplet pairs with overall singlet spin |X1⟩ ≈ (1)[TT] and quintet spin |Q⟩ ≈ (5)[TT]. It is shown that triplet pair states can be separated by a triplet-triplet energy-transfer mechanism to give a separated, yet entangled triplet pair (1)[T···T]. Independent triplets are produced by decoherence of the separated triplet pair. Recombination of independent triplets by exciton-exciton annihilation to form the correlated triplet pair (i.e., nongeminate recombination) happens with 1/3 of the rate of either triplet migration or recombination of the separated correlated triplet pair (geminate recombination). PMID:26595530

  12. Using a Spreadsheet to Solve the Schro¨dinger Equations for the Energies of the Ground Electronic State and the Two Lowest Excited States of H[subscript2

    ERIC Educational Resources Information Center

    Ge, Yingbin; Rittenhouse, Robert C.; Buchanan, Jacob C.; Livingston, Benjamin

    2014-01-01

    We have designed an exercise suitable for a lab or project in an undergraduate physical chemistry course that creates a Microsoft Excel spreadsheet to calculate the energy of the S[subscript 0] ground electronic state and the S[subscript 1] and T[subscript 1] excited states of H[subscript 2]. The spreadsheet calculations circumvent the…

  13. The nature of singlet excitons in oligoacene molecular crystals

    SciTech Connect

    Yamagata, H.; Norton, J.; Hontz, E.; Olivier, Y.; Beljonne, D.; Bredas, J. L.; Silbey, R. J.; Spano, F. C.

    2011-01-01

    A theory for polarized absorption in crystalline oligoacenes is presented, which includes Frenkel exciton coupling, the coupling between Frenkel and charge-transfer (CT) excitons, and the coupling of all neutral and ionic excited states to the dominant ring-breathing vibrational mode. For tetracene, spectra calculated using all Frenkel couplings among the five lowest energy molecular singlet states predict a Davydov splitting (DS) of the lowest energy (0–0) vibronic band of only -32 cm-1, far smaller than the measured value of 631 cm-1 and of the wrong sign--a negative sign indicating that the polarizations of the lower and upper Davydov components are reversed from experiment. Inclusion of Frenkel-CT coupling dramatically improves the agreement with experiment, yielding a 0–0 DS of 601 cm-1 and a nearly quantitative reproduction of the relative spectral intensities of the 0–n vibronic components. Our analysis also shows that CT mixing increases with the size of the oligoacenes. We discuss the implications of these results on exciton dissociation and transport.

  14. Optical and magnetic probes of hot singlet exciton fission in π-conjugated polymers for organic photovoltaic applications

    NASA Astrophysics Data System (ADS)

    Zhai, Yaxin; Olejnik, Ella; Vardeny, Z. Valy

    2015-09-01

    We used steady state and picosecond transient photoinduced absorption (PA), excitation dependence (EXPA(ω)) spectrum of the triplet exciton PA band, and its magneto-PA (MPA(B)) response to investigate singlet fission (SF) of hot-excitons into two separated triplet excitons in luminescent π-conjugated polymers. From the high energy step in the triplet EXPA(ω) spectrum of poly(dioctyloxy)-phenylenevinylene (DOO-PPV) films, we identified a hot-exciton SF (HE-SF) process having threshold energy at E≍2ET (=2.8 eV, where ET is the energy of the lowest lying triplet exciton), which is about 0.8 eV above the lowest singlet exciton energy. The picosecond transient PA with 3.1 eV pump excitation shows that in DOO-PPV film a triplet exciton is generated at time, t<500 ps. However the ultrafast triplet generation is missing in DOO-PPV solution, indicating that the HE-SF is predominantly interchain in nature. The HE-SF process was confirmed by the triplet MPA(B) response for excitation at E>2ET, which shows a typical SF response. Our work shows that the SF process in π-conjugated polymers is a much more general process than thought previously.

  15. Microscopic theory of singlet exciton fission. III. Crystalline pentacene

    NASA Astrophysics Data System (ADS)

    Berkelbach, Timothy C.; Hybertsen, Mark S.; Reichman, David R.

    2014-08-01

    We extend our previous work on singlet exciton fission in isolated dimers to the case of crystalline materials, focusing on pentacene as a canonical and concrete example. We discuss the proper interpretation of the character of low-lying excited states of relevance to singlet fission. In particular, we consider a variety of metrics for measuring charge-transfer character, conclusively demonstrating significant charge-transfer character in the low-lying excited states. The impact of this electronic structure on the subsequent singlet fission dynamics is assessed by performing real-time master-equation calculations involving hundreds of quantum states. We make direct comparisons with experimental absorption spectra and singlet fission rates, finding good quantitative agreement in both cases, and we discuss the mechanistic distinctions that exist between small isolated aggregates and bulk systems.

  16. Microscopic theory of singlet exciton fission. III. Crystalline pentacene.

    PubMed

    Berkelbach, Timothy C; Hybertsen, Mark S; Reichman, David R

    2014-08-21

    We extend our previous work on singlet exciton fission in isolated dimers to the case of crystalline materials, focusing on pentacene as a canonical and concrete example. We discuss the proper interpretation of the character of low-lying excited states of relevance to singlet fission. In particular, we consider a variety of metrics for measuring charge-transfer character, conclusively demonstrating significant charge-transfer character in the low-lying excited states. The impact of this electronic structure on the subsequent singlet fission dynamics is assessed by performing real-time master-equation calculations involving hundreds of quantum states. We make direct comparisons with experimental absorption spectra and singlet fission rates, finding good quantitative agreement in both cases, and we discuss the mechanistic distinctions that exist between small isolated aggregates and bulk systems. PMID:25149804

  17. Microscopic theory of singlet exciton fission. III. Crystalline pentacene

    SciTech Connect

    Berkelbach, Timothy C. Reichman, David R.; Hybertsen, Mark S.

    2014-08-21

    We extend our previous work on singlet exciton fission in isolated dimers to the case of crystalline materials, focusing on pentacene as a canonical and concrete example. We discuss the proper interpretation of the character of low-lying excited states of relevance to singlet fission. In particular, we consider a variety of metrics for measuring charge-transfer character, conclusively demonstrating significant charge-transfer character in the low-lying excited states. The impact of this electronic structure on the subsequent singlet fission dynamics is assessed by performing real-time master-equation calculations involving hundreds of quantum states. We make direct comparisons with experimental absorption spectra and singlet fission rates, finding good quantitative agreement in both cases, and we discuss the mechanistic distinctions that exist between small isolated aggregates and bulk systems.

  18. Singlet biradical{yields}singlet zwitterion optical transition in a twisted olefin

    SciTech Connect

    Piotrowiak, P.; Strati, G.; Smirnov, S.N.; Warman, J.M.; Schuddeboom, W.

    1996-09-18

    We report the first direct observation of the singlet biradical $YLD singlet zwitterion transition in a twisted olefin, biphenanthrenylidene. Biphenanthrenylidene (full name, bi-4H-cyclopenta[def]phenanthren-4-ylidene, abbreviated as BPH) is an analogue of tetraphenylethylene (TPE) and stilbene, both of which are the favorite models of photoisomerization reactions. The investigations have been focusing on the dynamics of the twisting motion of the double bond which leads to decoupling of the two halves of the olefin and results in the formation of the D{sub 2d} 90{degree}-twisted zwitterionic or biradical excited state. 13 refs., 2 figs., 1 tab.

  19. LiH potential energy curves for ground and excited states with the free complement local Schrödinger equation method

    NASA Astrophysics Data System (ADS)

    Bande, Annika; Nakashima, Hiroyuki; Nakatsuji, Hiroshi

    2010-08-01

    The two lowest singlet and triplet Σ + potential energy curves of LiH were calculated using the free complement (FC) local Schrödinger equation (LSE) method. The overall potential curves and the properties calculated therefrom, like equilibrium bond length, dissociation energy, adiabatic and vertical excitation energies, zero point energy, vibrational spacings, etc., demonstrated the high accuracy of the FC LSE method for both the ground and excited states in comparison to the reference calculations and experiments.

  20. Hybrid Coupled Cluster and Molecular Dynamics Approach: Application to the Excitation Spectrum of Cytosine in the Native DNA Environment

    SciTech Connect

    Valiev, Marat; Kowalski, Karol

    2006-12-07

    Evolution of the excited state energies of cytosine base in the native DNA environment was investigated using hybrid coupled cluster and classical molecular dynamics approach. The time averaged excitation energies obtained with the variant of the completely renormalized equation-of-motion with singles, doubles, and non-iterative triples approach that includes a bulk of the correlation effects for excited states, are compared with the analogous calculations in the gas phase. Significant blue shifts for the two lowest singlet excitation energies can be observed as a result of interaction of the quantum system with surrounding environment.

  1. Generation of singlet oxygen in fullerene-containing media: 1. Photodesorption of singlet oxygen from fullerene-containing surfaces

    SciTech Connect

    Belousova, I M; Belousov, V P; Danilov, O B; Ermakov, A V; Kiselev, V M; Kislyakov, I M; Sosnov, E N

    2008-03-31

    It is shown that upon irradiation of fullerene-containing surfaces by laser or flashlamp pulses, oxygen adsorbed by these surfaces efficiently escapes to the gas phase. The observation of luminescence pulses in the spectral region of 762 and 1268 nm confirms the presence of oxygen molecules in the excited singlet state in the desorbed oxygen. The conditions for optimisation of the efficiency of singlet-oxygen production are studied. It is shown that singlet oxygen at the concentration sufficient for obtaining operation of a fullerene-oxygen-iodine laser can be produced in this way. (laser applications and other topics in quantum electronics)

  2. Theoretical Study of Tautomerization Reactions for the Ground and First Excited Electronic States of Adenine

    NASA Technical Reports Server (NTRS)

    Salter, Latasha M.; Chaban, Galina M.; Kwak, Dochan (Technical Monitor)

    2002-01-01

    Geometrical structures and energetic properties for different tautomers of adenine are calculated in this study, using multi-configurational wave functions. Both the ground and the lowest singlet excited state potential energy surfaces are studied. Four tautomeric forms are considered, and their energetic order is found to be different on the ground and the excited state potential energy surfaces. Minimum energy reaction paths are obtained for hydrogen atom transfer (tautomerization) reactions in the ground and the lowest excited electronic states. It is found that the barrier heights and the shapes of the reaction paths are different for the ground and the excited electronic states, suggesting that the probability of such tautomerization reaction is higher on the excited state potential energy surface. This tautomerization process should become possible in the presence of water or other polar solvent molecules and should play an important role in the photochemistry of adenine.

  3. Aroylnitrenes with singlet ground states: photochemistry of acetyl-substituted aroyl and aryloxycarbonyl azides

    SciTech Connect

    Sigman, M.E.; Autrey, T.; Schuster, G.B.

    1988-06-22

    The photochemistry of 4-acetylbenzoyl azide (ABA), 4-acetyl-4'-biphenoyl azide (ADA), and 4-acetylphenoxycarbonyl azide (APA) shows unusual wavelength and structural effects. Irradiation of ABA or ADA into their ..pi..-..pi..* bands with deep-UV light leads to formation of 4-acetylbenzoylnitrene (ABN) and 4-acetyl-4'-biphenoylnitrene (ADN), respectively, in competition with photo-Curtius rearrangement to form isocyanates. Irradiation of these azides into their n-..pi..* bands with near-UV light gives only the aroylnitrenes. The triplet excited states of the azides were detected chemically and by transient spectroscopic techniques. Nitrogen loss following near-UV irradiation occurs exclusively from the excited triplet azides. However, the chemical properties of ABN and ADN are consistent only with reactions originating from their singlet states. An ESR spectrum is observed at 8 K for ((4-acetylphenoxy)carbonyl) nitrene (APN) but not for 4-acetylbenzoylnitrene (ABN) or 4-acetyl-4'-biphenylcarbonylnitrene (ADN). The chemical properties of APN in tert-butyl alcohol show that its triplet is no more than 5 kcal/mol below its lowest single state. In contrast, the chemical properties of ABN and ADN indicate that these nitrenes have singlet ground states.

  4. Mechanism for singlet fission in pentacene and tetracene: from single exciton to two triplets.

    PubMed

    Zimmerman, Paul M; Bell, Franziska; Casanova, David; Head-Gordon, Martin

    2011-12-14

    Singlet fission (SF) could dramatically increase the efficiency of organic solar cells by producing two triplet excitons from each absorbed photon. While this process has been known for decades, most descriptions have assumed the necessity of a charge-transfer intermediate. This ab initio study characterizes the low-lying excited states in acene molecular crystals in order to describe how SF occurs in a realistic crystal environment. Intermolecular interactions are shown to localize the initially delocalized bright state onto a pair of monomers. From this localized state, nonadiabatic coupling mediated by intermolecular motion between the optically allowed exciton and a dark multi-exciton state facilitates SF without the need for a nearby low-lying charge-transfer intermediate. An estimate of the crossing rate shows that this direct quantum mechanical process occurs in well under 1 ps in pentacene. In tetracene, the dark multi-exciton state is uphill from the lowest singlet excited state, resulting in a dynamic interplay between SF and triplet-triplet annihilation. PMID:22084927

  5. The effect of excitonic interactions on singlet fission dynamics in crystalline tetracene

    NASA Astrophysics Data System (ADS)

    Zhang, Chunfeng; Zhang, Bo; Wang, Rui; Xiao, Min

    2015-03-01

    Singlet fission in organic semiconductors is interesting for its potential application in boosting the efficiency of solar conversion. Singlet-singlet annihilation induced by excitonic interaction has been regarded as a process that competes against singlet fission in high-density regime. In this work, we conduct a systematic transient optical study to investigate the density-dependent singlet fission dynamics in crystalline tetracene. Surprisingly, the transient absorption data indicate the rate of singlet fission is actually increased with increasing the excitation density, which is further supported by probing the quantum beating between the manifold states of triplet pairs following Burdett's approach. Our result suggests it is necessary to re-examine the role of excitonic interactions to uncover the physical mechanism underlying singlet fission in crystalline tetracene. This work is supported by National Science Foundation of China and ``973'' project.

  6. Excited State Dynamics of 7-AZAINDOLE Homodimer in Frozen Nitrogen Matrix

    NASA Astrophysics Data System (ADS)

    Mukherjee, Moitrayee; Bandyopadhyay, Biman; Karmakar, Shreetama; Chakraborty, Tapas

    2011-06-01

    In a fluid medium (liquid or gas), the doubly hydrogen bonded dimer of 7-azaindole (7AI) undergoes tautomerization via simultaneous exchange of two H-atoms/protons between the two moieties upon UV excitation to lowest excited singlet state. The excited dimer emits exclusively visible fluorescence from tautomeric configuration, and no UV fluorescence is detected from the locally excited state. We show here for the first time that this generic excited state dynamics of 7AI dimer is totally altered if the species is synthesized and confined in frozen nitrogen at 8 K. The dimer has been found to emit only from the locally excited state, and the photophysical channel leading to excited state tautomerization is completely blocked. The formation of the centrosymmetric dimer in nitrogen matrix is ensured by recording the FTIR spectrum of the dimer before initiating the photophysical measurements. The details of our findings and interpretation of the measured data will be presented in the talk.

  7. Two-photon-induced singlet fission in rubrene single crystal

    NASA Astrophysics Data System (ADS)

    Ma, Lin; Galstyan, Gegham; Zhang, Keke; Kloc, Christian; Sun, Handong; Soci, Cesare; Michel-Beyerle, Maria E.; Gurzadyan, Gagik G.

    2013-05-01

    The two-photon-induced singlet fission was observed in rubrene single crystal and studied by use of femtosecond pump-probe spectroscopy. The location of two-photon excited states was obtained from the nondegenerate two-photon absorption (TPA) spectrum. Time evolution of the two-photon-induced transient absorption spectra reveals the direct singlet fission from the two-photon excited states. The TPA absorption coefficient of rubrene single crystal is 52 cm/GW at 740 nm, as obtained from Z-scan measurements. Quantum chemical calculations based on time-dependent density functional theory support our experimental data.

  8. UV-vis spectra of singlet state cationic polycyclic aromatic hydrocarbons: Time-dependent density functional theory study

    SciTech Connect

    Dominikowska, Justyna Domagala, Malgorzata; Palusiak, Marcin

    2014-01-28

    A theoretical study of singlet state cations of polycyclic aromatic hydrocarbons is performed. Appropriate symmetry suitable for further calculations is chosen for each of the systems studied. The excitation states of such species are obtained by the time dependent density functional theory (TD-DFT) method. The computations are performed using both Pople and electronic response properties basis sets. The results obtained with the use of different basis sets are compared. The electronic transitions are described and the relationships for the lowest-lying transitions states of different species are found. The properties of in-plane and out-of-plane transitions are also delineated. The TD-DFT results are compared with the experimental data available.

  9. Singlet Oxygen Generation by Laser Irradiation of Gold Nanoparticles

    PubMed Central

    2016-01-01

    The formation of singlet oxygen by irradiation of gold nanoparticles in their plasmon resonance band with continuous or pulsed laser light has been investigated. Citrate-stabilized nanoparticles were found to facilitate the photogeneration of singlet oxygen, albeit with low quantum yield. The reaction caused by pulsed laser irradiation makes use of the equilibrated hot electrons that can reach temperatures of several thousand degrees during the laser pulse. Although less efficient, continuous irradiation, which acts via the short-lived directly excited primary “hot” electrons only, can produce enough singlet oxygen for photodynamic cancer therapy and has significant advantages for practical applications. However, careful design of the nanoparticles is needed, since even a moderately thick capping layer can completely inhibit singlet oxygen formation. Moreover, the efficiency of the process also depends on the nanoparticle size. PMID:27239247

  10. Large optical nonlinearity induced by singlet fission in pentacene films.

    PubMed

    Liu, Yunlong; Zhang, Chunfeng; Wang, Rui; Zhang, Bo; Tan, Zhanao; Wang, Xiaoyong; Xiao, Min

    2015-05-18

    By creating two triplet excitons from one photo-excited singlet exciton, singlet fission in organic semiconductors has drawn tremendous attention for its potential applications in boosting the efficiency of solar conversion. Here, we show that this carrier-multiplication effect can also be used to dramatically improve the nonlinear optical response in organic materials. We have observed large optical nonlinearity with a magnitude of χ((3)) up to 10(-9) esu in pentacene films, which is further shown to be a result of singlet fission by monitoring the temporal dynamics. The potential application of such efficient nonlinear optical response has been demonstrated with a singlet-fission-induced polarization rotation. PMID:25845461

  11. Ultrafast singlet and triplet dynamics in microcrystalline pentacene films

    NASA Astrophysics Data System (ADS)

    Marciniak, Henning; Pugliesi, Igor; Nickel, Bert; Lochbrunner, Stefan

    2009-06-01

    The exciton dynamics of microcrystalline pentacene films is investigated by femtosecond pump-probe experiments. Measurements are performed with ultrashort laser pulses applied at normal incidence and at an angle of incidence of 65° to disentangle singlet and triplet contributions by exploiting the different orientations of the molecular transition dipoles. The results indicate that the initial 70 fs fast relaxation step transforms the optically excited excitons in a reasonable mobile species with strongly reduced radiative transition strength. Fission into triplet excitons takes place on the picosecond time scale as a secondary, thermally activated process and with a small total yield of approximately 2%. Evidence is provided that the dominant species are singlet excitons with excimer character. To the subsequent dynamics contribute diffusion driven exciton-exciton annihilation and trapping of singlet and triplet excitons. Values for diffusion constants and trap densities are extracted by modeling the measurements with rate equations which include singlet and triplet dynamics.

  12. Nature of ground and electronic excited states of higher acenes.

    PubMed

    Yang, Yang; Davidson, Ernest R; Yang, Weitao

    2016-08-30

    Higher acenes have drawn much attention as promising organic semiconductors with versatile electronic properties. However, the nature of their ground state and electronic excited states is still not fully clear. Their unusual chemical reactivity and instability are the main obstacles for experimental studies, and the potentially prominent diradical character, which might require a multireference description in such large systems, hinders theoretical investigations. Here, we provide a detailed answer with the particle-particle random-phase approximation calculation. The (1)Ag ground states of acenes up to decacene are on the closed-shell side of the diradical continuum, whereas the ground state of undecacene and dodecacene tilts more to the open-shell side with a growing polyradical character. The ground state of all acenes has covalent nature with respect to both short and long axes. The lowest triplet state (3)B2u is always above the singlet ground state even though the energy gap could be vanishingly small in the polyacene limit. The bright singlet excited state (1)B2u is a zwitterionic state to the short axis. The excited (1)Ag state gradually switches from a double-excitation state to another zwitterionic state to the short axis, but always keeps its covalent nature to the long axis. An energy crossing between the (1)B2u and excited (1)Ag states happens between hexacene and heptacene. Further energetic consideration suggests that higher acenes are likely to undergo singlet fission with a low photovoltaic efficiency; however, the efficiency might be improved if a singlet fission into multiple triplets could be achieved. PMID:27528690

  13. Mechanism of singlet fission in thin films of 1,3-diphenylisobenzofuran.

    PubMed

    Schrauben, Joel N; Ryerson, Joseph L; Michl, Josef; Johnson, Justin C

    2014-05-21

    In order to elucidate the mechanism of singlet fission in thin films of 1,3-diphenylisobenzofuran (1) we have performed ultrafast transient absorption spectroscopy as a function of sample temperature and excitation fluence on polycrystalline thin films composed of two polymorphs. Our earlier investigations revealed that films enriched in a particular polymorph of 1 displayed near 200% efficiency for triplet formation at 77 K, while films composed primarily of a second polymorph had a very low triplet quantum yield. Present data confirm the triplet yield disparities in the two polymorphs and demonstrate the distinct fates of the initially prepared singlets in films of different structure. Singlet fission is inhibited in the more stable polymorph due to rapid excimer formation and trapping. The less stable polymorph undergoes highly efficient singlet fission with a dominant time constant of 10-30 ps and without strong thermal activation. Transient absorption measurements with varying excitation fluence indicate that singlet-singlet annihilation is a primary competitor of singlet fission at higher fluence and that fission from higher-lying states can also contribute to the triplet formation process. Measurements employing different excitation energies and sample temperatures reveal the role that trapping processes play in attenuating the triplet quantum yield to produce the complex temperature dependence of the singlet fission yield. The rate constants for singlet fission itself are essentially temperature independent. PMID:24735403

  14. Singlet oxygen kinetics in a double microwave discharge

    NASA Astrophysics Data System (ADS)

    Pitz, Greg A.; Lange, Matthew A.; Perram, Glen P.

    2004-09-01

    Chemical lasers offer the highest powers necessary for many weapons applications, but require significant logistical support in the delivery of specialized fuels to the battlefield. In the Chemical Oxygen-Iodine Laser (COIL), which is the weapon aboard the Airborne Laser (ABL), gaseous chlorine and liquid basic hydrogen peroxide are used to generate the singlet oxygen energy reservoir. The goal of the current multi-university research program is to demonstrate an oxygen-iodine laser with electrical discharge production of singlet oxygen. Typically, oxygen discharges are limited to about 15% yield for singlet oxygen. The electron excitation cross-sections as a function of E/N are well established. However, the kinetics for electron and singlet oxygen interactions is considerably more difficult to study. Optical diagnostics for O2(a, b), and O, have been applied to a double microwave discharge flow tube. By examining the difference in singlet oxygen kinetics between the two discharges in series, considerable information regarding the excited-state, excited-state interactions is obtained. Under certain discharge conditions, the O2(a) concentration significantly increases outside of the discharge, even after thermal effects are accounted.

  15. Singlet Fission and Multi-Exciton Generation in Organic Systems

    NASA Astrophysics Data System (ADS)

    Musgrave, Charles

    2012-02-01

    Multi-exciton generation (MEG) has been observed in a variety of materials and might be exploited in solar-cells to dramatically increase efficiency. In tetracene and pentacene MEG has been attributed to singlet fission (SF), however a fundamental mechanism for SF has not been previously described. Here, we use sophisticated ab initio calculations to show that MEG in pentacene proceeds by transition of the lowest optically allowed excited state S1 to a dark state (D) of multi-exciton character, which subsequently undergoes SF to generate two triplets (2xT0). D satisfies the energy requirement for SF (ED>2ET0) and lies just below S1 in pentacene, but above S1 in tetracene, consistent with the observed thermally activated SF process in tetracene, but no thermal activation in pentacene. While S1 exhibits single exciton character, D shows multi-exciton character comprising two separated electron-hole pairs. Dimer simulations predict S1 excimer formation and that fission of D into triplets proceeds through the excimer. The predicted energetics, wavefunctions and excimer interaction support the proposed mechanism, which accounts for the observed rapid, unactivated SF in pentacene. Results for SF in polyacenes, grapheme nanoribbons, rubrene and carbon nanotubes will be presented.

  16. Theoretical investigations of BBS (singlet)→BSB (triplet) transformation on a potential energy surface obtained from neural network fitting

    NASA Astrophysics Data System (ADS)

    Nguyen-Truong, Hieu T.; Thi, Cao Minh; Le, Hung M.

    2013-11-01

    B2S, the simplest BnSm cluster, has been shown to exhibit an interesting ground-state structure. B3LYP/6-311G (d,p) calculations suggest that its most stable configuration is singlet linear B-B-S. When promoted to the excited triplet state, B2S adopts the B-S-B configuration (C2v point group). To characterize its structural transformation, the lowest energy at each configuration is selected, and the neural network surface is developed with symmetry exchange incorporated. The triplet potential energy is found to be 0.48 eV above the ground state. Subsequently, the nudged-elastic-band method is employed to locate the BBS→BSB transition state. It is found that the barrier height is 1.35 eV above the equilibrium singlet BBS energy (0.88 eV for the reverse reaction). In addition, quasiclassical molecular dynamics with different vibrational excitations shows that the reaction is accelerated when the bending vibrational mode of BBS is excited, while the activation of BBS stretching modes causes a negative effect.

  17. Harvesting singlet fission for solar energy conversion: one versus two-electron transfer electron transfer from the quantum superposition state

    NASA Astrophysics Data System (ADS)

    Chan, Wai-Lun; Tritsch, John; Zhu, Xiaoyang

    2013-03-01

    Singlet fission (SF) is being explored to increase the efficiency of organic photovoltaics. A key question is how to effectively extract multiple electron-hole pairs from multiple excitons with the presence of other competing channels such as electron transfer from the singlet state. Recent experiments on the pentacene and tetracene show that a quantum superposition of the singlet (S1) and multiexciton (ME) state is formed during SF. However, little is known about the kinetics of electron transfer from this quantum superposition. Here, we apply time-resolved photoemission spectroscopy to the tetracene/C60 interface to probe one and two electron transfer from S1 and ME states, respectively. Because of the relatively slow (7 ps) SF in tetracene, both one- and two-electron transfer are allowed. We show evidence for the formation of two distinct charge transfer states due to electron transfer from photo-excited tetracene to the lowest unoccupied molecular orbital (LUMO) and the LUMO+1 levels in C60. Kinetic analysis shows that 60% of the quantum superposition transfers one electron through the S1 state to C60 while 40% undergoes two-electron transfer through the ME state.

  18. Combined phosphorescence-holographic approach for singlet oxygen detection in biological media

    NASA Astrophysics Data System (ADS)

    Semenova, I. V.; Belashov, A. V.; Beltukova, D. M.; Petrov, N. V.; Vasyutinskii, O. S.

    2015-06-01

    The paper presents a novel combined approach aimed to detect and monitor singlet oxygen molecules in biological specimens by means of the simultaneous recording and monitoring of their deactivation dynamics in the two complementary channels: radiative and nonradiative. The approach involves both the direct registration of phosphorescence at the wavelength of about 1270 nm caused by radiative relaxation of excited singlet oxygen molecules and holographic recording of thermal disturbances in the medium produced by their nonradiative relaxation. The data provides a complete set of information on singlet oxygen location and dynamics in the medium. The approach was validated in the case study of photosensitized generation of singlet oxygen in onion cell structures.

  19. The role of chromophore coupling in singlet fission.

    PubMed

    Johnson, Justin C; Nozik, Arthur J; Michl, Josef

    2013-06-18

    Certain organic materials can generate more than one electron-hole pair per absorbed photon, a property that could revolutionize the prospects for solar energy. This process, called singlet fission, is one possible "exciton multiplication" scheme that could be useful in a variety of photovoltaic device designs from dye-sensitized solar cells to solar cell bilayers to bulk heterojunctions. For such applications to be possible, however, singlet fission must occur with near perfect efficiency in compounds that also have other requisite properties such as strong visible light absorption and photostability. Many recent investigations of singlet fission have focused on crystalline polyacenes, which have been known for some time to undergo singlet fission. While these materials have promise, limitations in stability, cost, and performance may hinder practical application of polyacene solar cells, while their complex photophysics may limit our fundamental understanding of singlet fission in crystalline polyacenes. In this Account, we describe rationally designed singlet fission chromophores whose excited state dynamics should be fairly simple and whose coupling can be well controlled through the formation of covalent dimers, aggregates, or polycrystalline films. In principle, investigations of these chromophores should provide the clearest connection to theoretical concepts explaining how an excited state evolves from a singlet (S1) into two triplets (TT). Realizing the promise of efficient singlet fission rests with two tasks: (i) producing an ideal molecular energy level structure and (ii) inducing the correct type and strength of chromophore coupling. In this Account, we offer theoretical guidance for achieving (i) and consider more extensively recent results aimed at (ii). For (i), theoretical guidance suggests that, in addition to alternant hydrocarbons like tetracene and pentacene, biradicals (i.e., molecules with two independent radical centers) may also be used as

  20. Understanding Singlet and Triplet Excitons in Acene Crystals from First Principles

    NASA Astrophysics Data System (ADS)

    Rangel Gordillo, Tonatiuh; Sharifzadeh, Sahar; Kronik, Leeor; Neaton, Jeffrey

    2014-03-01

    Singlet fission, a process in which two triplet excitons are formed from a singlet exciton, has the potential to increase the solar cell efficiencies above 100%. Efficient singlet fission has been reported in larger acene crystals, such as tetracene and pentacene, in part attributable to their low-lying triplet energies. In this work, we use many-body perturbation theory within the GW approximation and the Bethe-Salpeter equation approach to compute quasiparticle gaps, low-lying singlet and and triplet excitations, and optical absorption spectra across the entire acene family of crystals, from benzene to hexacene. We closely examine the degree of localization and charge-transfer character of the low-lying singlets and triplets, and their sensitivity to crystal environment, and discuss implications for the efficiency of singlet fission in this systems. This work supported by DOE and computational resources provided by NERSC.

  1. Density Analysis of Intra- and Intermolecular Vibronic Couplings toward Bath Engineering for Singlet Fission.

    PubMed

    Ito, Soichi; Nagami, Takanori; Nakano, Masayoshi

    2015-12-17

    Vibronic coupling plays a crucial role in singlet fission whereby a singlet exciton splits into two triplet excitons. In order to reveal the physicochemical origin of the vibronic coupling associated with singlet fission as well as to clarify its relationship with chemical structure, we evaluate relevant vibronic couplings from the viewpoint of their spatial contributions described by vibronic coupling density. From the analysis using a model tetracene dimer, a typical singlet fission system, the frequency dependence of vibronic couplings in each electronic state is found to be significantly different from that of another depending on the nature of the electronic structure (intra/intermolecular excitation) and the related vibrational motion. These findings contribute not only to the fundamental understanding of the singlet fission mechanism from the viewpoint of vibronic couplings but also to opening a new path to designing highly efficient singlet fission materials through phonon-bath engineering. PMID:26673196

  2. Gaugino mass without singlets

    SciTech Connect

    Giudice, Gian F.; Luty, Markus A.; Murayama, Hitoshi; Rattazzi, Riccardo

    1998-12-21

    In models with dynamical supersymmetry breaking in the hidden sector, the gaugino masses in the observable sector have been believed to be extremely suppressed (below 1 keV), unless there is a gauge singlet in the hidden sector with specific couplings to the observable sector gauge multiplets. We point out that there is a pure supergravity contribution to gaugino masses at the quantum level arising from the superconformal anomaly. Our results are valid to all orders in perturbation theory and are related to the ''exact'' beta functions for soft terms. There is also an anomaly contribution to the A terms proportional to the beta function of the corresponding Yukawa coupling. The gaugino masses are proportional to the corresponding gauge beta functions, and so do not satisfy the usual GUT relations.

  3. Photodissociation of ozone in the Hartley band: Potential energy surfaces, nonadiabatic couplings, and singlet/triplet branching ratio

    NASA Astrophysics Data System (ADS)

    Schinke, R.; McBane, G. C.

    2010-01-01

    The lowest five A1' states of ozone, involved in the photodissociation with UV light, are analyzed on the basis of multireference configuration interaction electronic structure calculations with emphasis on the various avoided crossings in different regions of coordinate space. Global diabatic potential energy surfaces are constructed for the lowest four states termed X, A, B, and R. In addition, the off-diagonal potentials that couple the initially excited state B with states R and A are constructed to reflect results from additional electronic structure calculations, including the calculation of nonadiabatic coupling matrix elements. The A/X and A/R couplings are also considered, although in a less ambitious manner. The photodissociation dynamics are studied by means of trajectory surface hopping (TSH) calculations with the branching ratio between the singlet, O(D1)+O2(Δ1g), and triplet, O(P3)+O2(Σ3g-), channels being the main focus. The semiclassical branching ratio agrees well with quantum mechanical results except for wavelengths close to the threshold of the singlet channel. The calculated O(D1) quantum yield is approximately 0.90-0.95 across the main part of the Hartley band, in good agreement with experimental data. TSH calculations including all four states show that transitions B→A are relatively unimportant and subsequent transitions A→X/R to the triplet channel are negligible.

  4. Observation of Two Triplet-Pair Intermediates in Singlet Exciton Fission.

    PubMed

    Pensack, Ryan D; Ostroumov, Evgeny E; Tilley, Andrew J; Mazza, Samuel; Grieco, Christopher; Thorley, Karl J; Asbury, John B; Seferos, Dwight S; Anthony, John E; Scholes, Gregory D

    2016-07-01

    Singlet fission is an excitation multiplication process in molecular systems that can circumvent energy losses and significantly boost solar cell efficiencies; however, the nature of a critical intermediate that enables singlet fission and details of its evolution into multiple product excitations remain obscure. We resolve the initial sequence of events comprising the fission of a singlet exciton in solids of pentacene derivatives using femtosecond transient absorption spectroscopy. We propose a three-step model of singlet fission that includes two triplet-pair intermediates and show how transient spectroscopy can distinguish initially interacting triplet pairs from those that are spatially separated and noninteracting. We find that the interconversion of these two triplet-pair intermediates is limited by the rate of triplet transfer. These results clearly highlight the classical kinetic model of singlet fission and expose subtle details that promise to aid in resolving problems associated with triplet extraction. PMID:27281713

  5. Singlet-triplet annihilation in single LHCII complexes.

    PubMed

    Gruber, J Michael; Chmeliov, Jevgenij; Krüger, Tjaart P J; Valkunas, Leonas; van Grondelle, Rienk

    2015-08-14

    In light harvesting complex II (LHCII) of higher plants and green algae, carotenoids (Cars) have an important function to quench chlorophyll (Chl) triplet states and therefore avoid the production of harmful singlet oxygen. The resulting Car triplet states lead to a non-linear self-quenching mechanism called singlet-triplet (S-T) annihilation that strongly depends on the excitation density. In this work we investigated the fluorescence decay kinetics of single immobilized LHCIIs at room temperature and found a two-exponential decay with a slow (3.5 ns) and a fast (35 ps) component. The relative amplitude fraction of the fast component increases with increasing excitation intensity, and the resulting decrease in the fluorescence quantum yield suggests annihilation effects. Modulation of the excitation pattern by means of an acousto-optic modulator (AOM) furthermore allowed us to resolve the time-dependent accumulation and decay rate (∼7 μs) of the quenching species. Inspired by singlet-singlet (S-S) annihilation studies, we developed a stochastic model and then successfully applied it to describe and explain all the experimentally observed steady-state and time-dependent kinetics. That allowed us to distinctively identify the quenching mechanism as S-T annihilation. Quantitative fitting resulted in a conclusive set of parameters validating our interpretation of the experimental results. The obtained stochastic model can be generalized to describe S-T annihilation in small molecular aggregates where the equilibration time of excitations is much faster than the annihilation-free singlet excited state lifetime. PMID:26156159

  6. Understanding the Control of Singlet-Triplet Splitting for Organic Exciton Manipulating: A Combined Theoretical and Experimental Approach

    PubMed Central

    Chen, Ting; Zheng, Lei; Yuan, Jie; An, Zhongfu; Chen, Runfeng; Tao, Ye; Li, Huanhuan; Xie, Xiaoji; Huang, Wei

    2015-01-01

    Developing organic optoelectronic materials with desired photophysical properties has always been at the forefront of organic electronics. The variation of singlet-triplet splitting (ΔEST) can provide useful means in modulating organic excitons for diversified photophysical phenomena, but controlling ΔEST in a desired manner within a large tuning scope remains a daunting challenge. Here, we demonstrate a convenient and quantitative approach to relate ΔEST to the frontier orbital overlap and separation distance via a set of newly developed parameters using natural transition orbital analysis to consider whole pictures of electron transitions for both the lowest singlet (S1) and triplet (T1) excited states. These critical parameters revealed that both separated S1 and T1 states leads to ultralow ΔEST; separated S1 and overlapped T1 states results in small ΔEST; and both overlapped S1 and T1 states induces large ΔEST. Importantly, we realized a widely-tuned ΔEST in a range from ultralow (0.0003 eV) to extra-large (1.47 eV) via a subtle symmetric control of triazine molecules, based on time-dependent density functional theory calculations combined with experimental explorations. These findings provide keen insights into ΔEST control for feasible excited state tuning, offering valuable guidelines for the construction of molecules with desired optoelectronic properties. PMID:26161684

  7. Singlet molecular oxygen in photobiochemical systems: IR phosphorescence studies.

    PubMed

    Krasnovsky, A A

    1998-01-01

    Singlet molecular oxygen (1O2) is one of the most active intermediates involved in photosensitized oxygenation reactions in chemical and biological systems. Deactivation of singlet oxygen is accompanied by infrared phosphorescence (1270 nm) which is widely employed for 1O2 detection and study. This review considers techniques for phosphorescence detection, phosphorescence spectra, quantum yields and kinetics under laser excitation, the radiative and real 1O2 lifetimes in organic solvents and water, 1O2 quenching by biomolecules, and estimation of singlet oxygen lifetimes, diffusion lengths and phosphorescence quantum yields in blood plasma, cell cytoplasm, erythrocyte ghosts, retinal rod outer segments and chloroplast thylakoids. The experiments devoted to 1O2 phosphorescence detection in photosensitizer-containing living cells are discussed in detail. Information reviewed is important for understanding the mechanisms of photodestruction in biological systems and various applied problems of photobiology and photomedicine. PMID:10379647

  8. Cell death induced by direct laser activation of singlet oxygen at 1270 nm

    NASA Astrophysics Data System (ADS)

    Anquez, F.; El Yazidi Belkoura, I.; Suret, P.; Randoux, S.; Courtade, E.

    2013-02-01

    Singlet oxygen plays a major role in many chemical and biological photo-oxidation processes. It has a high chemical reactivity, which is commonly harnessed for therapeutic issues. Indeed, singlet oxygen is recognized as the major cytotoxic agent in photodynamic therapy. In this treatment of cancer, singlet oxygen is created, among other reactive species, by an indirect transfer of energy from light to molecular oxygen via excitation of a photosensitizer. In this paper, we show that the conventional singlet oxygen production scheme can be simplified. Production of singlet oxygen is achieved in living cells from photosensitizer-free 1270 nm laser excitation of the electronic ground state of molecular oxygen. The quantity of singlet oxygen produced in this way is sufficient to induce an oxidative stress leading to cell death. Other effects such as thermal stress are discriminated, and we conclude that cell death is only due to singlet oxygen creation. This new simplified scheme of singlet oxygen activation can be seen as a breakthrough for phototherapies of malignant diseases and/or as a non-invasive possibility to generate reactive oxygen species in a tightly controlled manner.

  9. Molecular packing determines singlet exciton fission in organic semiconductors.

    PubMed

    Kolata, Kolja; Breuer, Tobias; Witte, Gregor; Chatterjee, Sangam

    2014-07-22

    Carrier multiplication by singlet exciton fission enhances photovoltaic conversion efficiencies in organic solids. This decay of one singlet exciton into two triplet states allows the extraction of up to two electrons per harvested photon and, hence, promises to overcome the Shockley–Queisser limit. However, the microscopic mechanism of singlet exciton fission, especially the relation between molecular packing and electronic response, remains unclear, which therefore hampers the systematic improvement of organic photovoltaic devices. For the model system perfluoropentacene, we experimentally show that singlet exciton fission is greatly enhanced for a slip-stacked molecular arrangement by addressing different crystal axes featuring different packing schemes. This reveals that the fission process strongly depends on the intermolecular coupling: slip-stacking favors delocalization of excitations and allows for efficient exciton fission, while face-to-edge molecular orientations commonly found in the prevailing herringbone molecular stacking patterns even suppress it. Furthermore, we clarify the controversially debated role of excimer states as intermediary rather than competitive or precursory. Our detailed findings serve as a guideline for the design of next-generation molecular materials for application in future organic light-harvesting devices exploiting singlet exciton fission. PMID:24957197

  10. Singlet fission in pentacene through multi-exciton quantum states.

    PubMed

    Zimmerman, Paul M; Zhang, Zhiyong; Musgrave, Charles B

    2010-08-01

    Multi-exciton generation-the creation of multiple charge carrier pairs from a single photon-has been reported for several materials and may dramatically increase solar cell efficiency. Singlet fission, its molecular analogue, may govern multi-exciton generation in a variety of materials, but a fundamental mechanism for singlet fission has yet to be described. Here, we use sophisticated ab initio calculations to show that singlet fission in pentacene proceeds through rapid internal conversion of the photoexcited state into a dark state of multi-exciton character that efficiently splits into two triplets. We show that singlet fission to produce a pair of triplet excitons must involve an intermediate state that (i) has a multi-exciton character, (ii) is energetically accessible from the optically allowed excited state, and (iii) efficiently dissociates into multiple electron-hole pairs. The rational design of photovoltaic materials that make use of singlet fission will require similar ab initio analysis of multi-exciton states such as the dark state studied here. PMID:20651727

  11. Theoretical Studies of Singlet and Triplet Formation in Polymer LEDs.

    NASA Astrophysics Data System (ADS)

    Mazumdar, Sumit

    2002-03-01

    Spin statistics for electron-hole recombination in polymer LEDs suggest that the singlet excited state is formed with a probability of only 0.25. Recent experiments by several groups, however, indicate, that the probability of formation of the singlet is considerably larger than this. We present a comprehensive theory of the formation cross-sections of the singlet and triplet excitons in conjugated polymers. Within our model, electron correlations influence the relative yields of the singlet and triplet excitons in a profound manner. We have carried out the dynamics of the electron-hole recombination in pair of simple conjugated organic molecules, both with noninteracting electrons and within the Pariser-Parr-Pople model with interacting electrons. The formation cross-section for the singlet exciton is considerably larger than the triplet for the correlated electron case. Numerical results as well as simple physical interpretations of theoretical and experimental results will be presented. This work was supported in part by NSF DMR-0101659 and CSIR, India.

  12. A correlated electron view of singlet fission.

    PubMed

    Zimmerman, Paul M; Musgrave, Charles B; Head-Gordon, Martin

    2013-06-18

    Singlet fission occurs when a single exciton splits into multiple electron-hole pairs, and could dramatically increase the efficiency of organic solar cells by converting high energy photons into multiple charge carriers. Scientists might exploit singlet fission to its full potential by first understanding the underlying mechanism of this quantum mechanical process. The pursuit of this fundamental mechanism has recently benefited from the development and application of new correlated wave function methods. These methods-called restricted active space spin flip-can capture the most important electron interactions in molecular materials, such as acene crystals, at low computational cost. It is unrealistic to use previous wave function methods due to the excessive computational cost involved in simulating realistic molecular structures at a meaningful level of electron correlation. In this Account, we describe how we use these techniques to compute single exciton and multiple exciton excited states in tetracene and pentacene crystals in order to understand how a single exciton generated from photon absorption undergoes fission to generate two triplets. Our studies indicate that an adiabatic charge transfer intermediate is unlikely to contribute significantly to the fission process because it lies too high in energy. Instead, we propose a new mechanism that involves the direct coupling of an optically allowed single exciton to an optically dark multiexciton. This coupling is facilitated by intermolecular motion of two acene monomers that drives nonadiabatic population transfer between the two states. This transfer occurs in the limit of near degeneracies between adiabatic states where the Born-Oppenheimer approximation of fixed nuclei is no longer valid. Existing theories for singlet fission have not considered this type of coupling between states and, therefore, cannot describe this mechanism. The direct mechanism through intermolecular motion describes many

  13. The excited state antiaromatic benzene ring: a molecular Mr Hyde?

    PubMed

    Papadakis, Raffaello; Ottosson, Henrik

    2015-09-21

    The antiaromatic character of benzene in its first ππ* excited triplet state (T1) was deduced more than four decades ago by Baird using perturbation molecular orbital (PMO) theory [J. Am. Chem. Soc. 1972, 94, 4941], and since then it has been confirmed through a range of high-level quantum chemical calculations. With focus on benzene we now first review theoretical and computational studies that examine and confirm Baird's rule on reversal in the electron count for aromaticity and antiaromaticity of annulenes in their lowest triplet states as compared to Hückel's rule for the ground state (S0). We also note that the rule according to quantum chemical calculations can be extended to the lowest singlet excited state (S1) of benzene. Importantly, Baird, as well as Aihara [Bull. Chem. Soc. Jpn. 1978, 51, 1788], early put forth that the destabilization and excited state antiaromaticity of the benzene ring should be reflected in its photochemical reactivity, yet, today these conclusions are often overlooked. Thus, in the second part of the article we review photochemical reactions of a series of benzene derivatives that to various extents should stem from the excited state antiaromatic character of the benzene ring. We argue that benzene can be viewed as a molecular "Dr Jekyll and Mr Hyde" with its largely unknown excited state antiaromaticity representing its "Mr Hyde" character. The recognition of the "Jekyll and Hyde" split personality feature of the benzene ring can likely be useful in a range of different areas. PMID:25960203

  14. Low-Lying ππ* States of Heteroaromatic Molecules: A Challenge for Excited State Methods.

    PubMed

    Prlj, Antonio; Sandoval-Salinas, María Eugenia; Casanova, David; Jacquemin, Denis; Corminboeuf, Clémence

    2016-06-14

    The description of low-lying ππ* states of linear acenes by standard electronic structure methods is known to be challenging. Here, we broaden the framework of this problem by considering a set of fused heteroaromatic rings and demonstrate that standard electronic structure methods do not provide a balanced description of the two (typically) lowest singlet state (La and Lb) excitations. While the Lb state is highly sensitive to correlation effects, La suffers from the same drawbacks as charge transfer excitations. We show that the comparison between CIS/CIS(D) can serve as a diagnostic for detecting the two problematic excited states. Standard TD-DFT and even its spin-flip variant lead to inaccurate excitation energies and interstate gaps, with only a double hybrid functional performing somewhat better. The complication inherent to a balanced description of these states is so important that even CC2 and ADC(2) do not necessarily match the ADC(3) reference. PMID:27144975

  15. Photoexcited singlet and triplet states of a UV absorber ethylhexyl methoxycrylene.

    PubMed

    Kikuchi, Azusa; Hata, Yuki; Kumasaka, Ryo; Nanbu, Yuichi; Yagi, Mikio

    2013-01-01

    The excited states of UV absorber, ethylhexyl methoxycrylene (EHMCR) have been studied through measurements of UV absorption, fluorescence, phosphorescence and electron paramagnetic resonance (EPR) spectra in ethanol. The energy levels of the lowest excited singlet (S1) and triplet (T1) states of EHMCR were determined. The energy levels of the S1 and T1 states of EHMCR are much lower than those of photolabile 4-tert-butyl-4'-methoxydibenzoylmethane. The energy levels of the S1 and T1 states of EHMCR are lower than those of octyl methoxycinnamate. The weak phosphorescence and EPR B(min) signals were observed and the lifetime was estimated to be 93 ms. These facts suggest that the significant proportion of the S1 molecules undergoes intersystem crossing to the T1 state, and the deactivation process from the T1 state is predominantly radiationless. The photostability of EHMCR arises from the (3)ππ* character in the T1 state. The zero-field splitting (ZFS) parameter in the T1 state is D** = 0.113 cm(-1). PMID:23136952

  16. Competition between polaron pair formation and singlet fission observed in amorphous rubrene films

    NASA Astrophysics Data System (ADS)

    Jankus, Vygintas; Snedden, Edward W.; Bright, Daniel W.; Arac, Erhan; Dai, DeChang; Monkman, Andrew P.

    2013-06-01

    In this paper, we investigate excited state dynamics in amorphous rubrene vacuum sublimed films. We report the direct observation of singlet fission in amorphous rubrene films. We have determined the fission rate to be >2.5×1012 s-1. Simultaneously, we observe strong polaron pair absorption and propose that polaron pair formation could be competing with singlet fission. Another possible conclusion from our experiments could be that two triplets from singlet fission might arise via polaron pairs. In either case, polaron pairs play an important role in singlet fission in an amorphous rubrene film. We also observe that triplets created by singlet fission fuse to regenerate a singlet, giving delayed fluorescence (DF) scaling linearly with initial laser energy (i.e., one singlet gives two triplets and two triplets give back one singlet). This is a strong evidence of S1n→2T1. We did not observe substantial temperature dependence of DF decay curve shape, indicating that triplet migration in amorphous rubrene films is not hopping limited and that triplets undergo fusion before their migration.

  17. Effect of substitution on the ultrafast deactivation of the excited state of benzo[b]thiophene-arylamines.

    PubMed

    Pina, J; Queiroz, M-J R P; Seixas de Melo, J

    2016-08-01

    A complete and systematic study of the spectroscopic and photophysical properties of five novel diarylamines in the benzo[b]thiophene series (oligoanilines) was performed in solution at room (293 K) and low (77 K) temperature. The title compounds resulting from the link between one aniline unit with a benzo[b]thiophene unit (with two different methyl and methoxy substitution) were characterized using steady-state absorption, fluorescence and phosphorescence spectroscopy, as well as femto- to nano-second time resolved spectroscopies. The study involved the determination of the absorption, emission and triplet-triplet absorption together with all relevant quantum yields (fluorescence, phosphorescence, intersystem crossing, internal conversion and singlet oxygen yields), excited state lifetimes and the overall set of deactivation rate constants (kF, kIC and kISC). This study was further complemented with theoretical calculations, namely with the determination of the optimized ground-state molecular geometries for the diarylamines together with the prediction of the lowest vertical one-electron excitation energy and the relevant molecular orbital contours using DFT calculations. The DFT results were found to corroborate the observed charge-transfer character of the singlet excited state. The experimental results showed that the radiationless decay processes (internal conversion and intersystem-crossing) constitute the main excited state deactivation pathways and that substitution with methyl and methoxy groups induces significant changes in the spectroscopic and photophysical behaviour of these compounds. This was also corroborated by the femtosecond transient absorption study, where it was found that the ultrafast dynamics of the diarylamines was best described by a sequential model featuring fast solvent relaxation followed by conformational relaxation to a more planar excited state, from where singlet excited state deactivation occurs through internal conversion and

  18. Laser-induced luminescence of singlet molecular oxygen: generation by drugs and pigments of biological importance

    NASA Astrophysics Data System (ADS)

    Egorov, Sergei Y.; Krasnovsky, Alexander A., Jr.

    1991-05-01

    The photon counting technique and flashlaser excitation were applied to the timeresolved measurement of photosensitized singlet oxygen luminescence in organic and aqueous media. The quantum yields for singlet oxygen generation have been measured in solutions of photosynthetic pigments synthetic and natural porphyrins porphyrins conjugated with monoclonal antibodies furocoumarins flavins fluorescein tetracycline and endogenous photosensitizers of human lens. The data obtained indicate that the measurement of the singlet oxygen luminescence is a reliable tool to study the photosensitizing activity of drugs and to elucidate primary mechanisms of photodynamic destruction. 1.

  19. Controlling energy transfer in ytterbium complexes: oxygen dependent lanthanide luminescence and singlet oxygen formation.

    PubMed

    Watkis, Andrew; Hueting, Rebekka; Sørensen, Thomas Just; Tropiano, Manuel; Faulkner, Stephen

    2015-11-01

    Pyrene-appended ytterbium complexes have been prepared using Ugi reactions to vary the chromophore-lanthanide separation. Formation of the ytterbium(iii) excited state is sensitised via both the singlet and triplet excited states of the chromophore. Energy transfer from the latter is relatively slow, and gives rise to oxygen-dependent luminescence. PMID:26346499

  20. Singlet harvesting with brightly emitting Cu(I) and metal-free organic compounds

    NASA Astrophysics Data System (ADS)

    Yersin, Hartmut; Czerwieniec, Rafal; Hupfer, Alexander

    2012-06-01

    In an electroluminescent excitation, singlet and triplet excitons are generated. In this contribution it is proposed to harvest all excitons in an efficiently emitting singlet state by use of molecules which exhibit distinct thermally activated delayed fluorescence at T = 300K. Highly attractive examples, comprising Cu(I) complexes and the metal-free acridine orange, are presented and discussed with respect to their photophysical properties.

  1. Singlet Fission via an Excimer-Like Intermediate in 3,6-Bis(thiophen-2-yl)diketopyrrolopyrrole Derivatives.

    PubMed

    Mauck, Catherine M; Hartnett, Patrick E; Margulies, Eric A; Ma, Lin; Miller, Claire E; Schatz, George C; Marks, Tobin J; Wasielewski, Michael R

    2016-09-14

    Singlet fission (SF) in polycrystalline thin films of four 3,6-bis(thiophen-2-yl)diketopyrrolopyrrole (TDPP) chromophores with methyl (Me), n-hexyl (C6), triethylene glycol (TEG), and 2-ethylhexyl (EH) substituents at the 2,5-positions is found to involve an intermediate excimer-like state. The four different substituents yield four distinct intermolecular packing geometries, resulting in variable intermolecular charge transfer (CT) interactions in the solid. SF from the excimer state of Me, C6, TEG, and EH takes place in τSF = 22, 336, 195, and 1200 ps, respectively, to give triplet yields of 200%, 110%, 110%, and 70%, respectively. The transient spectra of the excimer-like state and its energetic proximity to the lowest excited singlet state in these derivatives suggests that this state may be the multiexciton (1)(T1T1) state that precedes formation of the uncorrelated triplet excitons. The excimer decay rates correlate well with the SF efficiencies and the degree of intermolecular donor-acceptor interactions resulting from π-stacking of the thiophene donor of one molecule with the DPP core acceptor in another molecule as observed in the crystal structures. Such interactions are found to also increase with the SF coupling energies, as calculated for each derivative. These structural and spectroscopic studies afford a better understanding of the electronic interactions that enhance SF in chromophores having strong intra- and intermolecular CT character. PMID:27547986

  2. New ab initio adiabatic potential energy surfaces and bound state calculations for the singlet ground X˜ 1A1 and excited C˜ 1B2(21A') states of SO2

    NASA Astrophysics Data System (ADS)

    Kłos, Jacek; Alexander, Millard H.; Kumar, Praveen; Poirier, Bill; Jiang, Bin; Guo, Hua

    2016-05-01

    We report new and more accurate adiabatic potential energy surfaces (PESs) for the ground X˜ 1A1 and electronically excited C˜ 1B2(21A') states of the SO2 molecule. Ab initio points are calculated using the explicitly correlated internally contracted multi-reference configuration interaction (icMRCI-F12) method. A second less accurate PES for the ground X ˜ state is also calculated using an explicitly correlated single-reference coupled-cluster method with single, double, and non-iterative triple excitations [CCSD(T)-F12]. With these new three-dimensional PESs, we determine energies of the vibrational bound states and compare these values to existing literature data and experiment.

  3. Nonadiabatic effects in the photodissociation of vibrationally excited HNCO: The branching between singlet (athinsp{sup 1}{Delta}) and triplet (Xthinsp{sup 3}{Sigma}{sup {minus}}) NH

    SciTech Connect

    Berghout, H.L.; Brown, S.S.; Delgado, R.; Fleming Crim, F.

    1998-08-01

    Initial vibrational excitation of a state containing three quanta of N{endash}H stretch (3{nu}{sub 1}) decreases the fractional photolysis yield of NH (athinsp{sup 1}{Delta}) relative to NH (Xthinsp{sup 3}{Sigma}{sup {minus}}) by a factor of approximately two compared to the isoenergetic photodissociation of a 300 K thermal sample of HNCO. At a total energy of 43thinsp480thinspcm{sup {minus}1}, NH (athinsp{sup 1}{Delta}) accounts for 24{percent} of the total NH yield in the direct photolysis but only 10{percent} in the photodissociation of 3{nu}{sub 1}. At 44thinsp440thinspcm{sup {minus}1}, the NH (athinsp{sup 1}{Delta}) yields are 65{percent} and 32{percent} in the single photon and two-step photodissociations, respectively. The variation in branching ratio may arise from dynamical behavior that is closely related to the preferential production of NCO in the photolysis of vibrationally excited HNCO. The initial vibrational excitation has no influence on the rotational and vibrational distributions of NH (Xthinsp{sup 3}{Sigma}{sup {minus}}), but it significantly increases the amount of energy in rotation of NH (athinsp{sup 1}{Delta}). These results, along with several recent experimental and theoretical studies, suggest the participation of at least three different potential energy surfaces in the photodissociation of isocyanic acid. {copyright} {ital 1998 American Institute of Physics.}

  4. The quantum coherent mechanism for singlet fission: experiment and theory.

    PubMed

    Chan, Wai-Lun; Berkelbach, Timothy C; Provorse, Makenzie R; Monahan, Nicholas R; Tritsch, John R; Hybertsen, Mark S; Reichman, David R; Gao, Jiali; Zhu, X-Y

    2013-06-18

    The absorption of one photon by a semiconductor material usually creates one electron-hole pair. However, this general rule breaks down in a few organic semiconductors, such as pentacene and tetracene, where one photon absorption may result in two electron-hole pairs. This process, where a singlet exciton transforms to two triplet excitons, can have quantum yields as high as 200%. Singlet fission may be useful to solar cell technologies to increase the power conversion efficiency beyond the so-called Shockley-Queisser limit. Through time-resolved two-photon photoemission (TR-2PPE) spectroscopy in crystalline pentacene and tetracene, our lab has recently provided the first spectroscopic signatures in singlet fission of a critical intermediate known as the multiexciton state (also called a correlated triplet pair). More importantly, we found that population of the multiexciton state rises at the same time as the singlet state on the ultrafast time scale upon photoexcitation. This observation does not fit with the traditional view of singlet fission involving the incoherent conversion of a singlet to a triplet pair. However, it provides an experimental foundation for a quantum coherent mechanism in which the electronic coupling creates a quantum superposition of the singlet and the multiexciton state immediately after optical excitation. In this Account, we review key experimental findings from TR-2PPE experiments and present a theoretical analysis of the quantum coherent mechanism based on electronic structural and density matrix calculations for crystalline tetracene lattices. Using multistate density functional theory, we find that the direct electronic coupling between singlet and multiexciton states is too weak to explain the experimental observation. Instead, indirect coupling via charge transfer intermediate states is two orders of magnitude stronger, and dominates the dynamics for ultrafast multiexciton formation. Density matrix calculation for the crystalline

  5. Excitation Dependence of Photoinduced Absorption (PA) in Π-Conjugated Polymers

    NASA Astrophysics Data System (ADS)

    Zhai, Yaxin; Basel, Tek; Vardeny, Z. Valy

    2014-03-01

    In order to study the process of singlet fission (SF), where a singlet exciton decomposes into a pair of triplets S0 +S1 -->T1 +T1 , we have investigated the excitation dependence of the photoinduced absorption band of triplet exciton (EXPA) and photoluminescence (EXPL) in various luminescent and non-luminescent π-conjugated polymers. We found that the EXPA spectrum of luminescent polymers is composed of two steps, showing that two different channels are operative for triplet photogeneration. One process starts at the optical gap and has flat response similar to that of the EXPL spectrum. We therefore identify this process as due to intersystem crossing from the lowest lying singlet exciton. Whereas the second process with an onset at E ~ 2ET , where ET is the triplet energy is due to singlet fission of hot excitons. We also found that the EXPA spectrum of some non luminescent polymers is different from that of the luminescent polymers. Supported in part by the NSF-MRSEC program at the University of Utah.

  6. Large Optical Nonlinearity Induced by Singlet Fission in Pentacene Films

    NASA Astrophysics Data System (ADS)

    Liu, Yunlong; Zhang, Chunfeng; Xiao, Min

    2015-03-01

    By creating two triplet excitons from one photo-excited singlet exciton, singlet fission in organic semiconductors has drawn tremendous attention for its potential application in boosting the efficiency of solar conversion. Here, we show that this carrier-multiplication effect can be used to dramatically improve the nonlinear optical response in organic materials. With the technique of dual-wavelength optical Kerr effect (OKE), we have observed large optical nonlinearity with a magnitude of χ (3) up to 10-9 esu in pentacene films, which is further shown to be a result of singlet fission as demonstrated by the detailed temporal dynamics and wavelength dependence experiment. Through the use of optical heterodyne detected OKE experiment, we have determined both the sign and value of Reχ (3) of the pentacene film. Such efficient third order nonlinear optical response has been successfully applied to demonstrate the all-optical switching. The results observed in this work indicate that the singlet fission could be served as an effective strategy to promote the optical nonlinearity in organic molecule systems. This work is supported by the National Basic Research Program of China (2013CB932903 and 2012CB921801, MOST), the National Science Foundation of China (91233103, 61108001, 11227406 and 11321063).

  7. Collective aspects of singlet fission in molecular crystals

    NASA Astrophysics Data System (ADS)

    Teichen, Paul E.; Eaves, Joel D.

    2015-07-01

    We present a model to describe collective features of singlet fission in molecular crystals and analyze it using many-body theory. The model we develop allows excitonic states to delocalize over several chromophores which is consistent with the character of the excited states in many molecular crystals, such as the acenes, where singlet fission occurs. As singlet states become more delocalized and triplet states more localized, the rate of singlet fission increases. We also determine the conditions under which the two triplets resulting from fission are correlated. Using the Bethe Ansatz and an entanglement measure for indistinguishable bipartite systems, we calculate the triplet-triplet entanglement as a function of the biexciton interaction strength. The biexciton interaction can produce bound biexciton states and provides a source of entanglement between the two triplets even when the triplets are spatially well separated. Significant entanglement between the triplet pair occurs well below the threshold for bound pair formation. Our results paint a dynamical picture that helps to explain why fission has been observed to be more efficient in molecular crystals than in their covalent dimer analogues and have consequences for photovoltaic efficiency models that assume that the two triplets can be extracted independently.

  8. Collective aspects of singlet fission in molecular crystals.

    PubMed

    Teichen, Paul E; Eaves, Joel D

    2015-07-28

    We present a model to describe collective features of singlet fission in molecular crystals and analyze it using many-body theory. The model we develop allows excitonic states to delocalize over several chromophores which is consistent with the character of the excited states in many molecular crystals, such as the acenes, where singlet fission occurs. As singlet states become more delocalized and triplet states more localized, the rate of singlet fission increases. We also determine the conditions under which the two triplets resulting from fission are correlated. Using the Bethe Ansatz and an entanglement measure for indistinguishable bipartite systems, we calculate the triplet-triplet entanglement as a function of the biexciton interaction strength. The biexciton interaction can produce bound biexciton states and provides a source of entanglement between the two triplets even when the triplets are spatially well separated. Significant entanglement between the triplet pair occurs well below the threshold for bound pair formation. Our results paint a dynamical picture that helps to explain why fission has been observed to be more efficient in molecular crystals than in their covalent dimer analogues and have consequences for photovoltaic efficiency models that assume that the two triplets can be extracted independently. PMID:26233118

  9. Collective aspects of singlet fission in molecular crystals

    SciTech Connect

    Teichen, Paul E.; Eaves, Joel D.

    2015-07-28

    We present a model to describe collective features of singlet fission in molecular crystals and analyze it using many-body theory. The model we develop allows excitonic states to delocalize over several chromophores which is consistent with the character of the excited states in many molecular crystals, such as the acenes, where singlet fission occurs. As singlet states become more delocalized and triplet states more localized, the rate of singlet fission increases. We also determine the conditions under which the two triplets resulting from fission are correlated. Using the Bethe Ansatz and an entanglement measure for indistinguishable bipartite systems, we calculate the triplet-triplet entanglement as a function of the biexciton interaction strength. The biexciton interaction can produce bound biexciton states and provides a source of entanglement between the two triplets even when the triplets are spatially well separated. Significant entanglement between the triplet pair occurs well below the threshold for bound pair formation. Our results paint a dynamical picture that helps to explain why fission has been observed to be more efficient in molecular crystals than in their covalent dimer analogues and have consequences for photovoltaic efficiency models that assume that the two triplets can be extracted independently.

  10. Interaction of triplet sensitizers with chlorophyll: Formation of singlet chlorophyll

    SciTech Connect

    Bohne, C.; Scaiano, J.C. )

    1989-03-29

    The interaction of several triplet sensitizers with chlorophyll a (Chla) has been examined using laser techniques. For the carbonyl sensitizers (with triplet energies > 53 kcal/mol) it was possible to measure the quenching rate constants; these were systematically {>=} 10{sup 10} M{sup {minus}1} s{sup {minus}1}. In the cases of acetone, benzophenone, and p-methoxyacetophenone the quenching process leads to the formation of the fluorescent singlet state of Chla. For benzophenone (k{sub q} = 2.4 {times} 10{sup 10} M{sup {minus}1} s{sup {minus}1}) approximately 3% of the quenching events lead to the formation of excited Chla. Several sensitizers (decafluorobenzophenone, benzil, and fluorenone) do not induce Chla fluorescence (or do it very inefficiently) in spite of having triplet energies above the S{sub 1} level of Chla. In light of their results the most probable mechanism involves energy transfer from the triplet sensitizer to an upper triple state of Chla ({sup 3}Chla**) which can undergo reverse intersystem crossing to the singlet manifold of Chla and thus induce fluorescence. The inefficient sensitizers are those where electron transfer between the excited singlet of Chla or {sup 3}Chla** and ground-state sensitizers is energetically favorable, leading to rapid in-cage quenching of the initially formed excited states of Chla. Formation of radical-ion pair between the triplet sensitizer and Chla followed by the generation of singlet Chla in the recombination of the radical ions could not be completely discarded.

  11. Ultrafast Singlet Fission in a Push-Pull Low-Bandgap Polymer Film.

    PubMed

    Kasai, Yukitomo; Tamai, Yasunari; Ohkita, Hideo; Benten, Hiroaki; Ito, Shinzaburo

    2015-12-30

    Excited-state dynamics in poly[4,6-(dodecyl-thieno[3,4-b]thiophene-2-carboxylate)-alt-2,6-(4,8-dioctoxylbenzo[1,2-b:4,5-b]dithiophene)] (PTB1) was studied by transient absorption spectroscopy. Upon photoexcitation at 400 nm, an additional transient species is promptly generated along with singlet excitons and survives up to nanoseconds, while singlet excitons disappear completely. In order to assign the long-lived species, we measured transient absorption spectra over the wide spectral range from 900 to 2500 nm. As a result, we found that the long-lived species is ascribed not to polarons but to triplet excitons, which is formed through the ultrafast singlet fission (SF). We discuss the ultrafast SF mechanism in push-pull low-bandgap polymer PTB1 films on the basis of the excited-state dynamics under various excitation wavelengths and intensities. PMID:26654295

  12. Excited States and Photodebromination of Selected Polybrominated Diphenyl Ethers: Computational and Quantitative Structure—Property Relationship Studies

    PubMed Central

    Luo, Jin; Hu, Jiwei; Wei, Xionghui; Li, Lingyun; Huang, Xianfei

    2015-01-01

    This paper presents a density functional theory (DFT)/time-dependent DFT (TD-DFT) study on the lowest lying singlet and triplet excited states of 20 selected polybrominateddiphenyl ether (PBDE) congeners, with the solvation effect included in the calculations using the polarized continuum model (PCM). The results obtained showed that for most of the brominated diphenyl ether (BDE) congeners, the lowest singlet excited state was initiated by the electron transfer from HOMO to LUMO, involving a π–σ* excitation. In triplet excited states, structure of the BDE congeners differed notably from that of the BDE ground states with one of the specific C–Br bonds bending off the aromatic plane. In addition, the partial least squares regression (PLSR), principal component analysis-multiple linear regression analysis (PCA-MLR), and back propagation artificial neural network (BP-ANN) approaches were employed for a quantitative structure-property relationship (QSPR) study. Based on the previously reported kinetic data for the debromination by ultraviolet (UV) and sunlight, obtained QSPR models exhibited a reasonable evaluation of the photodebromination reactivity even when the BDE congeners had same degree of bromination, albeit different patterns of bromination. PMID:25569092

  13. Explorative computational study of the singlet fission process

    NASA Astrophysics Data System (ADS)

    Havenith, Remco W. A.; de Gier, Hilde D.; Broer, Ria

    2012-10-01

    Different ab initio methods, namely multi-reference and nonorthogonal configuration interaction techniques, are explored for their applicability in studying the singlet fission problem. It has been shown for 2-methyl-1,5-hexadiene that the 1TT state can be identified using multi-reference techniques. The geometrical and vibrational properties of the 1TT state are such that they can be approximated with those of the 5TT state. A proof of principle is given for the calculation of the singlet fission pathway driven by nuclear motion: efficient singlet fission can take place if the 1TT and S1 states are close in energy with a large non-adiabatic coupling matrix element at the S1 geometry, and the energy of the S0 state is well below that of the 1TT state at the 1TT geometry. The nonorthogonal configuration interaction method was used to treat a tetracene trimer. It has been shown that the first excited states can be interpreted as delocalised states; interaction with charge-transfer base states plays an important role. The 1TT states are localised on one pair of molecules. The electronic coupling between the diabatic S[n] and 1TT[m] states is in the meV range, confirming previous estimates. The charge-transfer base states enhance the coupling between the S[1]/S[2] and 1TT[2] excited states.

  14. The energy barrier in singlet fission can be overcome through coherent coupling and entropic gain

    NASA Astrophysics Data System (ADS)

    Chan, Wai-Lun; Ligges, Manuel; Zhu, X.-Y.

    2012-10-01

    One strategy to improve solar-cell efficiency is to generate two excited electrons from just one photon through singlet fission, which is the conversion of a singlet (S1) into two triplet (T1) excitons. For efficient singlet fission it is believed that the cumulative energy of the triplet states should be no more than that of S1. However, molecular analogues that satisfy this energetic requirement do not show appreciable singlet fission, whereas crystalline tetracene displays endothermic singlet fission with near-unity quantum yield. Here we probe singlet fission in tetracene by directly following the intermediate multiexciton (ME) state. The ME state is isoenergetic with 2 × T1, but fission is not activated thermally. Rather, an S1 ⇔ ME superposition formed through a quantum-coherent process allows access to the higher-energy ME. We attribute entropic gain in crystalline tetracene as the driving force for the subsequent decay of S1 ⇔ ME into 2 × T1, which leads to a high singlet-fission yield.

  15. Magnetic dipolar interaction between correlated triplets created by singlet fission in tetracene crystals

    NASA Astrophysics Data System (ADS)

    Wang, Rui; Zhang, Chunfeng; Zhang, Bo; Liu, Yunlong; Wang, Xiaoyong; Xiao, Min

    2015-10-01

    Singlet fission can potentially break the Shockley-Queisser efficiency limit in single-junction solar cells by splitting one photoexcited singlet exciton (S1) into two triplets (2T1) in organic semiconductors. A dark multiexciton state has been proposed as the intermediate connecting S1 to 2T1. However, the exact nature of this multiexciton state, especially how the doubly excited triplets interact, remains elusive. Here we report a quantitative study on the magnetic dipolar interaction between singlet-fission-induced correlated triplets in tetracene crystals by monitoring quantum beats relevant to the multiexciton sublevels at room temperature. The resonances of multiexciton sublevels approached by tuning an external magnetic field are observed to be avoided, which agrees well with the theoretical predictions considering a magnetic dipolar interaction of ~0.008 GHz. Our work quantifies the magnetic dipolar interaction in certain organic materials and marks an important step towards understanding the underlying physics of the multiexciton state in singlet fission.

  16. External quantum efficiency above 100% in a singlet-exciton-fission-based organic photovoltaic cell.

    PubMed

    Congreve, Daniel N; Lee, Jiye; Thompson, Nicholas J; Hontz, Eric; Yost, Shane R; Reusswig, Philip D; Bahlke, Matthias E; Reineke, Sebastian; Van Voorhis, Troy; Baldo, Marc A

    2013-04-19

    Singlet exciton fission transforms a molecular singlet excited state into two triplet states, each with half the energy of the original singlet. In solar cells, it could potentially double the photocurrent from high-energy photons. We demonstrate organic solar cells that exploit singlet exciton fission in pentacene to generate more than one electron per incident photon in a portion of the visible spectrum. Using a fullerene acceptor, a poly(3-hexylthiophene) exciton confinement layer, and a conventional optical trapping scheme, we show a peak external quantum efficiency of (109 ± 1)% at wavelength λ = 670 nanometers for a 15-nanometer-thick pentacene film. The corresponding internal quantum efficiency is (160 ± 10)%. Analysis of the magnetic field effect on photocurrent suggests that the triplet yield approaches 200% for pentacene films thicker than 5 nanometers. PMID:23599489

  17. Spin-singlet quantum Hall states and Jack polynomials with a prescribed symmetry

    NASA Astrophysics Data System (ADS)

    Estienne, Benoit; Bernevig, B. Andrei

    2012-04-01

    We show that a large class of bosonic spin-singlet Fractional Quantum Hall model wavefunctions and their quasihole excitations can be written in terms of Jack polynomials with a prescribed symmetry. Our approach describes new spin-singlet quantum Hall states at filling fraction ν=2k/2r-1 and generalizes the (k,r) spin-polarized Jack polynomial states. The NASS and Halperin spin-singlet states emerge as specific cases of our construction. The polynomials express many-body states which contain configurations obtained from a root partition through a generalized squeezing procedure involving spin and orbital degrees of freedom. The corresponding generalized Pauli principle for root partitions is obtained, allowing for counting of the quasihole states. We also extract the central charge and quasihole scaling dimension, and propose a conjecture for the underlying CFT of the (k,r) spin-singlet Jack states.

  18. Orbital rotation in the lowest triplet state of benzophenone

    SciTech Connect

    Dinse, K.P.; Pratt, D.W.

    1982-04-07

    Optically detected magnetic resonance (ODMR) and electron-nuclear double resonance (ODENDOR) spectroscopy at both zero and high magnetic fields were used to examine single crystals of 4,4'-dibromodiphenyl ether (DOE) containing 0.1 mol % of an enriched perdeuterio (/sup 17/O) benzophenone (/sup 17/O-BP-d/sub 10/) guest. Representative spectra are presented and explanations are offered for the electron spin transitions. The summarized results indicate the currently used atomic orbital functions do not provide an accurate description of the lowest excited states of many aromatic carbonyls. (BLM)

  19. Solution-Phase Singlet Fission in a Structurally Well-Defined Norbornyl-Bridged Tetracene Dimer.

    PubMed

    Cook, Jasper D; Carey, Thomas J; Damrauer, Niels H

    2016-07-01

    The photophysics of a norbornyl-bridged covalent tetracene (Tc) dimer BT1 and a monomer analogue Tc-e were studied in room-temperature nonpolar solvents. Notably in BT1, a Davydov-split band is observed in UV absorption, heralding interchromophore electronic interactions. Emission spectra indicate an acene-like vibronic progression mirroring the lowest-energy visible absorption. For BT1, this argues against excited-state excimer formation. Evidence of intramolecular singlet fission (SF) comes from a comparison of time-resolved emission decay signals collected for BT1 versus Tc-e in toluene. In BT1, the multiexcitonic (1)TT state is produced in 70 ns in 6% yield. A ratio of fission versus fusion rate constants provides an experimental measure of the SF reaction free energy at 52 meV in good agreement with previous calculations. The low SF yield corroborates our expectations that orbital symmetry effects on diabatic coupling for SF are important for dimers that cannot rely on more favorable thermodynamics. PMID:27291516

  20. Measurements Of Singlet Oxygen In Photodynamic Therapy

    NASA Astrophysics Data System (ADS)

    Profio, A. E.; Shu, Kuang-Hsien

    1989-06-01

    Photochemical reactions are used in photodynamic therapy of cancer and other disease. The cytotoxic agent in photochemotherapy is usually singlet oxygen. Thus measurements of singlet oxygen production or concentration may allow prediction of the biological response. The decrease in fluorescence of L-tryptophan because of reaction with singlet oxygen, the decrease in absorbance of a dye such as RNO subject to secondary oxidation by singlet oxygen, and the decrease in fluorescence of the most common photosensitizer, dihematoporphyrin ether/ester (DHE) because of photobleaching, have been investigated in solutions in vitro. The most promising method for dosimetry and prediction of biological response appears to be the photobleaching of DHE.

  1. SBI allocation between heavy and singlet missiles

    SciTech Connect

    Canavan, G.H.

    1991-04-01

    The optimal allocation of space-based interceptors (SBIs) between fixed, heavy missiles and mobile singlets can be derived from approximate expressions for the boost-phase penetration of each. Singlets can cluster before launch and have shorter burn times, which reduce their availability to SBIs by an order of magnitude. Singlet penetration decreased slowly with the number of SBIs allocated to them; heavy missile penetration falls rapidly. The allocation to the heavy missiles falls linearly with their number. The penetration of heavy and singlet missiles is proportional to their numbers and inversely proportional to their availability. 8 refs., 2 figs.

  2. Standard Model with a real singlet scalar and inflation

    SciTech Connect

    Enqvist, Kari; Nurmi, Sami; Tenkanen, Tommi; Tuominen, Kimmo E-mail: sami.nurmi@helsinki.fi E-mail: kimmo.i.tuominen@helsinki.fi

    2014-08-01

    We study the post-inflationary dynamics of the Standard Model Higgs and a real singlet scalar s, coupled together through a renormalizable coupling λ{sub sh}h{sup 2}s{sup 2}, in a Z{sub 2} symmetric model that may explain the observed dark matter abundance and/or the origin of baryon asymmetry. The initial values for the Higgs and s condensates are given by inflationary fluctuations, and we follow their dissipation and relaxation to the low energy vacua. We find that both the lowest order perturbative and the non-perturbative decays are blocked by thermal effects and large background fields and that the condensates decay by two-loop thermal effects. Assuming instant reheating at T=10{sup 16} GeV, the characteristic temperature for the Higgs condensate thermalization is found to be T{sub h} ∼ 10{sup 14} GeV, whereas s thermalizes typically around T{sub s} ∼ 10{sup 6} GeV. By that time, the amplitude of the singlet is driven very close to the vacuum value by the expansion of the universe, unless the portal coupling takes a value λ{sub sh}∼< 10{sup -7} and the singlet s never thermalizes. With these values of the coupling, it is possible to slowly produce a sizeable fraction of the observed dark matter abundance via singlet condensate fragmentation and thermal Higgs scattering. Physics also below the electroweak scale can therefore be affected by the non-vacuum initial conditions generated by inflation.

  3. Photosensitized generation of singlet oxygen in porous silicon studied by simultaneous measurements of luminescence of nanocrystals and oxygen molecules

    SciTech Connect

    Gongalsky, M. B.; Kharin, A. Yu.; Zagorodskikh, S. A.; Osminkina, L. A.; Timoshenko, V. Yu.

    2011-07-01

    Photosensitization of singlet oxygen generation in porous silicon (PSi) was investigated by simultaneous measurements of the photoluminescence (PL) of silicon nanocrystals (nc-Si) and the infrared emission of the {sup 1}{Delta}-state of oxygen molecules at 1270 nm (0.98 eV) at room temperature. Photodegradation of the nc-Si PL properties was found to correlate with the efficiency of singlet oxygen generation. The quantum efficiency of singlet oxygen generation in PSi was estimated to be about 1%, while the lifetime of singlet oxygen was about fifteen ms. The kinetics of nc-Si PL intensity under cw excitation undergoes a power law dependence with the exponent dependent on the photon energy of luminescence. The experimental results are explained with a model of photodegradation controlled by the diffusion of singlet oxygen molecules in a disordered structure of porous silicon.

  4. Sigma meson and lowest possible glueball candidate in an extended linear {sigma} model

    SciTech Connect

    Mukherjee, Tamal K.; Huang Mei; Yan Qishu

    2012-10-23

    We formulate an extended linear {sigma} model of a quarkonia nonet and a tetraquark nonet as well as a complex iso-singlet (glueball) field to study the low-lying scalar meson. Chiral symmetry and U{sub A}(1) symmetry and their breaking play important role to shape the scalar meson spectrum in our work. Based on our study we will comment on what may be the mass of the lowest possible scalar and pseudoscalar glueball states. We will also discuss on what may be the nature of the sigma or f{sub 0}(600) meson.

  5. Hydrodynamics on the lowest Landau level

    NASA Astrophysics Data System (ADS)

    Geracie, Michael; Son, Dam Thanh

    2015-06-01

    Using the recently developed approach to quantum Hall physics based on Newton-Cartan geometry, we consider the hydrodynamics of an interacting system on the lowest Landau level. We rephrase the non-relativistic fluid equations of motion in a manner that manifests the spacetime diffeomorphism invariance of the underlying theory. In the massless (or lowest Landau level) limit, the fluid obeys a force-free constraint which fixes the charge current. An entropy current analysis further constrains the energy response, determining four transverse response functions in terms of only two: an energy magnetization and a thermal Hall conductivity. Kubo formulas are presented for all transport coefficients and constraints from Weyl invariance derived. We also present a number of Středa-type formulas for the equilibrium response to external electric, magnetic and gravitational fields.

  6. Photo-assisted intersystem crossing: The predominant triplet formation mechanism in some isolated polycyclic aromatic molecules excited with pulsed lasers

    DOE PAGESBeta

    Johnson, Philip M.; Sears, Trevor J.

    2015-07-28

    Naphthalene, anthracene, and phenanthrene are shown to have very long-lived triplet lifetimes when the isolated molecules are excited with nanosecond pulsed lasers resonant with the lowest singlet state. For naphthalene, triplet state populations are created only during the laser pulse, excluding the possibility of normal intersystem crossing at the one photon level, and all molecules have triplet lifetimes greater than hundreds of microseconds, similar to the behavior previously reported for phenylacetylene. Although containing 7–12 thousand cm⁻¹ of vibrational energy, the triplet molecules have ionization thresholds appropriate to vibrationless T₁ states. The laser power dependences (slopes of log-log power plots) ofmore » the excited singlet and triplet populations are about 0.7 for naphthalene and about 0.5 for anthracene. Kinetic modeling of the power dependences successfully reproduces the experimental results and suggests that the triplet formation mechanism involves an enhanced spin orbit coupling caused by sigma character in states at the 2-photon level. Symmetry Adapted Cluster-Configuration Interaction calculations produced excited state absorption spectra to provide guidance for estimating kinetic rates and the sigma character present in higher electronic states. It is concluded that higher excited state populations are significant when larger molecules are excited with pulsed lasers and need to be taken into account whenever discussing the molecular photodynamics.« less

  7. Photo-assisted intersystem crossing: The predominant triplet formation mechanism in some isolated polycyclic aromatic molecules excited with pulsed lasers.

    PubMed

    Johnson, Philip M; Sears, Trevor J

    2015-07-28

    Naphthalene, anthracene, and phenanthrene are shown to have very long-lived triplet lifetimes when the isolated molecules are excited with nanosecond pulsed lasers resonant with the lowest singlet state. For naphthalene, triplet state populations are created only during the laser pulse, excluding the possibility of normal intersystem crossing at the one photon level, and all molecules have triplet lifetimes greater than hundreds of microseconds, similar to the behavior previously reported for phenylacetylene. Although containing 7-12 thousand cm(-1) of vibrational energy, the triplet molecules have ionization thresholds appropriate to vibrationless T1 states. The laser power dependences (slopes of log-log power plots) of the excited singlet and triplet populations are about 0.7 for naphthalene and about 0.5 for anthracene. Kinetic modeling of the power dependences successfully reproduces the experimental results and suggests that the triplet formation mechanism involves an enhanced spin orbit coupling caused by sigma character in states at the 2-photon level. Symmetry adapted cluster-configuration interaction calculations produced excited state absorption spectra to provide guidance for estimating kinetic rates and the sigma character present in higher electronic states. It is concluded that higher excited state populations are significant when larger molecules are excited with pulsed lasers and need to be taken into account whenever discussing the molecular photodynamics. PMID:26233127

  8. Photo-assisted intersystem crossing: The predominant triplet formation mechanism in some isolated polycyclic aromatic molecules excited with pulsed lasers

    SciTech Connect

    Johnson, Philip M.; Sears, Trevor J.

    2015-07-28

    Naphthalene, anthracene, and phenanthrene are shown to have very long-lived triplet lifetimes when the isolated molecules are excited with nanosecond pulsed lasers resonant with the lowest singlet state. For naphthalene, triplet state populations are created only during the laser pulse, excluding the possibility of normal intersystem crossing at the one photon level, and all molecules have triplet lifetimes greater than hundreds of microseconds, similar to the behavior previously reported for phenylacetylene. Although containing 7–12 thousand cm{sup −1} of vibrational energy, the triplet molecules have ionization thresholds appropriate to vibrationless T{sub 1} states. The laser power dependences (slopes of log-log power plots) of the excited singlet and triplet populations are about 0.7 for naphthalene and about 0.5 for anthracene. Kinetic modeling of the power dependences successfully reproduces the experimental results and suggests that the triplet formation mechanism involves an enhanced spin orbit coupling caused by sigma character in states at the 2-photon level. Symmetry adapted cluster-configuration interaction calculations produced excited state absorption spectra to provide guidance for estimating kinetic rates and the sigma character present in higher electronic states. It is concluded that higher excited state populations are significant when larger molecules are excited with pulsed lasers and need to be taken into account whenever discussing the molecular photodynamics.

  9. Photo-assisted intersystem crossing: The predominant triplet formation mechanism in some isolated polycyclic aromatic molecules excited with pulsed lasers

    NASA Astrophysics Data System (ADS)

    Johnson, Philip M.; Sears, Trevor J.

    2015-07-01

    Naphthalene, anthracene, and phenanthrene are shown to have very long-lived triplet lifetimes when the isolated molecules are excited with nanosecond pulsed lasers resonant with the lowest singlet state. For naphthalene, triplet state populations are created only during the laser pulse, excluding the possibility of normal intersystem crossing at the one photon level, and all molecules have triplet lifetimes greater than hundreds of microseconds, similar to the behavior previously reported for phenylacetylene. Although containing 7-12 thousand cm-1 of vibrational energy, the triplet molecules have ionization thresholds appropriate to vibrationless T1 states. The laser power dependences (slopes of log-log power plots) of the excited singlet and triplet populations are about 0.7 for naphthalene and about 0.5 for anthracene. Kinetic modeling of the power dependences successfully reproduces the experimental results and suggests that the triplet formation mechanism involves an enhanced spin orbit coupling caused by sigma character in states at the 2-photon level. Symmetry adapted cluster-configuration interaction calculations produced excited state absorption spectra to provide guidance for estimating kinetic rates and the sigma character present in higher electronic states. It is concluded that higher excited state populations are significant when larger molecules are excited with pulsed lasers and need to be taken into account whenever discussing the molecular photodynamics.

  10. Photo-assisted intersystem crossing: The predominant triplet formation mechanism in some isolated polycyclic aromatic molecules excited with pulsed lasers

    SciTech Connect

    Johnson, Philip M.; Sears, Trevor J.

    2015-07-28

    Naphthalene, anthracene, and phenanthrene are shown to have very long-lived triplet lifetimes when the isolated molecules are excited with nanosecond pulsed lasers resonant with the lowest singlet state. For naphthalene, triplet state populations are created only during the laser pulse, excluding the possibility of normal intersystem crossing at the one photon level, and all molecules have triplet lifetimes greater than hundreds of microseconds, similar to the behavior previously reported for phenylacetylene. Although containing 7–12 thousand cm⁻¹ of vibrational energy, the triplet molecules have ionization thresholds appropriate to vibrationless T₁ states. The laser power dependences (slopes of log-log power plots) of the excited singlet and triplet populations are about 0.7 for naphthalene and about 0.5 for anthracene. Kinetic modeling of the power dependences successfully reproduces the experimental results and suggests that the triplet formation mechanism involves an enhanced spin orbit coupling caused by sigma character in states at the 2-photon level. Symmetry Adapted Cluster-Configuration Interaction calculations produced excited state absorption spectra to provide guidance for estimating kinetic rates and the sigma character present in higher electronic states. It is concluded that higher excited state populations are significant when larger molecules are excited with pulsed lasers and need to be taken into account whenever discussing the molecular photodynamics.

  11. Singlet-paired coupled cluster theory for open shells.

    PubMed

    Gomez, John A; Henderson, Thomas M; Scuseria, Gustavo E

    2016-06-28

    Restricted single-reference coupled cluster theory truncated to single and double excitations accurately describes weakly correlated systems, but often breaks down in the presence of static or strong correlation. Good coupled cluster energies in the presence of degeneracies can be obtained by using a symmetry-broken reference, such as unrestricted Hartree-Fock, but at the cost of good quantum numbers. A large body of work has shown that modifying the coupled cluster ansatz allows for the treatment of strong correlation within a single-reference, symmetry-adapted framework. The recently introduced singlet-paired coupled cluster doubles (CCD0) method is one such model, which recovers correct behavior for strong correlation without requiring symmetry breaking in the reference. Here, we extend singlet-paired coupled cluster for application to open shells via restricted open-shell singlet-paired coupled cluster singles and doubles (ROCCSD0). The ROCCSD0 approach retains the benefits of standard coupled cluster theory and recovers correct behavior for strongly correlated, open-shell systems using a spin-preserving ROHF reference. PMID:27369507

  12. Quantifying singlet fission in novel organic materials using nonlinear optics

    NASA Astrophysics Data System (ADS)

    Busby, Erik; Xia, Jianlong; Yaffe, Omer; Kumar, Bharat; Berkelbach, Timothy; Wu, Qin; Miller, John; Nuckolls, Colin; Zhu, Xiaoyang; Reichman, David; Campos, Luis; Sfeir, Matthew Y.

    2014-10-01

    Singlet fission is a form of multiple exciton generation in which two triplet excitons are produced from the decay of a photoexcited singlet exciton. In a small number of organic materials, most notably pentacene, this conversion process has been shown to occur with unity quantum yield on sub-ps timescales. However, a poorly understood mechanism for fission along with strict energy and geometry requirements have so far limited the observation of this process to a few classes of organic materials, with only a subset of these (most notably the polyacenes) showing both efficient fission and long-lived triplets. Here, we utilize novel organic materials to investigate how the efficiency of the fission process depends on the coupling and the energetic driving force between chromophores in both intra- and intermolecular singlet fission materials. We demonstrate how the triplet yield can be accurately quantified using a combination of traditional transient spectroscopies and recently developed excited state saturable absorption techniques. These results allow us to gain mechanistic insight into the fission process and suggest general strategies for generating new materials that can undergo efficient fission.

  13. Self-Consistent Optimization of Excited States within Density-Functional Tight-Binding.

    PubMed

    Kowalczyk, Tim; Le, Khoa; Irle, Stephan

    2016-01-12

    We present an implementation of energies and gradients for the ΔDFTB method, an analogue of Δ-self-consistent-field density functional theory (ΔSCF) within density-functional tight-binding, for the lowest singlet excited state of closed-shell molecules. Benchmarks of ΔDFTB excitation energies, optimized geometries, Stokes shifts, and vibrational frequencies reveal that ΔDFTB provides a qualitatively correct description of changes in molecular geometries and vibrational frequencies due to excited-state relaxation. The accuracy of ΔDFTB Stokes shifts is comparable to that of ΔSCF-DFT, and ΔDFTB performs similarly to ΔSCF with the PBE functional for vertical excitation energies of larger chromophores where the need for efficient excited-state methods is most urgent. We provide some justification for the use of an excited-state reference density in the DFTB expansion of the electronic energy and demonstrate that ΔDFTB preserves many of the properties of its parent ΔSCF approach. This implementation fills an important gap in the extended framework of DFTB, where access to excited states has been limited to the time-dependent linear-response approach, and affords access to rapid exploration of a valuable class of excited-state potential energy surfaces. PMID:26587877

  14. First-Principles Quantum Dynamics of Singlet Fission: Coherent versus Thermally Activated Mechanisms Governed by Molecular π Stacking.

    PubMed

    Tamura, Hiroyuki; Huix-Rotllant, Miquel; Burghardt, Irene; Olivier, Yoann; Beljonne, David

    2015-09-01

    Singlet excitons in π-stacked molecular crystals can split into two triplet excitons in a process called singlet fission that opens a route to carrier multiplication in photovoltaics. To resolve controversies about the mechanism of singlet fission, we have developed a first principles nonadiabatic quantum dynamical model that reveals the critical role of molecular stacking symmetry and provides a unified picture of coherent versus thermally activated singlet fission mechanisms in different acenes. The slip-stacked equilibrium packing structure of pentacene derivatives is found to enhance ultrafast singlet fission mediated by a coherent superexchange mechanism via higher-lying charge transfer states. By contrast, the electronic couplings for singlet fission strictly vanish at the C(2h) symmetric equilibrium π stacking of rubrene. In this case, singlet fission is driven by excitations of symmetry-breaking intermolecular vibrations, rationalizing the experimentally observed temperature dependence. Design rules for optimal singlet fission materials therefore need to account for the interplay of molecular π-stacking symmetry and phonon-induced coherent or thermally activated mechanisms. PMID:26382701

  15. First-Principles Quantum Dynamics of Singlet Fission: Coherent versus Thermally Activated Mechanisms Governed by Molecular π Stacking

    NASA Astrophysics Data System (ADS)

    Tamura, Hiroyuki; Huix-Rotllant, Miquel; Burghardt, Irene; Olivier, Yoann; Beljonne, David

    2015-09-01

    Singlet excitons in π -stacked molecular crystals can split into two triplet excitons in a process called singlet fission that opens a route to carrier multiplication in photovoltaics. To resolve controversies about the mechanism of singlet fission, we have developed a first principles nonadiabatic quantum dynamical model that reveals the critical role of molecular stacking symmetry and provides a unified picture of coherent versus thermally activated singlet fission mechanisms in different acenes. The slip-stacked equilibrium packing structure of pentacene derivatives is found to enhance ultrafast singlet fission mediated by a coherent superexchange mechanism via higher-lying charge transfer states. By contrast, the electronic couplings for singlet fission strictly vanish at the C2 h symmetric equilibrium π stacking of rubrene. In this case, singlet fission is driven by excitations of symmetry-breaking intermolecular vibrations, rationalizing the experimentally observed temperature dependence. Design rules for optimal singlet fission materials therefore need to account for the interplay of molecular π -stacking symmetry and phonon-induced coherent or thermally activated mechanisms.

  16. Exposure of vitamins to UVB and UVA radiation generates singlet oxygen.

    PubMed

    Knak, Alena; Regensburger, Johannes; Maisch, Tim; Bäumler, Wolfgang

    2014-05-01

    Deleterious effects of UV radiation in tissue are usually attributed to different mechanisms. Absorption of UVB radiation in cell constituents like DNA causes photochemical reactions. Absorption of UVA radiation in endogenous photosensitizers like vitamins generates singlet oxygen via photosensitized reactions. We investigated two further mechanisms that might be involved in UV mediated cell tissue damage. Firstly, UVB radiation and vitamins also generate singlet oxygen. Secondly, UVB radiation may change the chemical structure of vitamins that may change the role of such endogenous photosensitizers in UVA mediated mechanisms. Vitamins were irradiated in solution using monochromatic UVB (308 nm) or UVA (330, 355, or 370 nm) radiation. Singlet oxygen was directly detected and quantified by its luminescence at 1270 nm. All investigated molecules generated singlet oxygen with a quantum yield ranging from 0.007 (vitamin D3) to 0.64 (nicotinamide) independent of the excitation wavelength. Moreover, pre-irradiation of vitamins with UVB changed their absorption in the UVB and UVA spectral range. Subsequently, molecules such as vitamin E and vitamin K1, which normally exhibit no singlet oxygen generation in the UVA, now produce singlet oxygen when exposed to UVA at 355 nm. This interplay of different UV sources is inevitable when applying serial or parallel irradiation with UVA and UVB in experiments in vitro. These results should be of particular importance for parallel irradiation with UVA and UVB in vivo, e.g. when exposing the skin to solar radiation. PMID:24691875

  17. Singlet Fission in a Covalently Linked Cofacial Alkynyltetracene Dimer.

    PubMed

    Korovina, Nadezhda V; Das, Saptaparna; Nett, Zachary; Feng, Xintian; Joy, Jimmy; Haiges, Ralf; Krylov, Anna I; Bradforth, Stephen E; Thompson, Mark E

    2016-01-20

    Singlet fission is a process in which a singlet exciton converts into two triplet excitons. To investigate this phenomenon, we synthesized two covalently linked 5-ethynyl-tetracene (ET) dimers with differing degrees of intertetracene overlap: BET-X, with large, cofacial overlap of tetracene π-orbitals, and BET-B, with twisted arrangement between tetracenes exhibits less overlap between the tetracene π-orbitals. The two compounds were crystallographically characterized and studied by absorption and emission spectroscopy in solution, in PMMA and neat thin films. The results show that singlet fission occurs within 1 ps in an amorphous thin film of BET-B with high efficiency (triplet yield: 154%). In solution and the PMMA matrix the S1 of BET-B relaxes to a correlated triplet pair (1)(T1T1) on a time scale of 2 ps, which decays to the ground state without forming separated triplets, suggesting that triplet energy transfer from (1)(T1T1) to a nearby chromophore is essential for producing free triplets. In support of this hypothesis, selective excitation of BET-B doped into a thin film of diphenyltetracene (DPT) leads to formation of the (1)(T1T1) state of BET-B, followed by generation of both DPT and BET-B triplets. For the structurally cofacial BET-X, an intermediate forms in <180 fs and returns to the ground state more rapidly than BET-B. First-principles calculations predict a 2 orders of magnitude faster rate of singlet fission to the (1)(T1T1) state in BET-B relative to that of crystalline tetracene, attributing the rate increase to greater coupling between the S1 and (1)(T1T1) states and favorable energetics for formation of the separated triplets. PMID:26693957

  18. The lowest ionization potentials of Al2

    NASA Technical Reports Server (NTRS)

    Bauschlicher, Charles W., Jr.; Barnes, Leslie A.; Taylor, Peter R.

    1988-01-01

    Potential curves for the lowest two electronic states (X 2 sigma g + and A 2 pi u) of Al2(+) were computed using complete active space SCF/multireference CI wave functions and large Gaussian basis sets. The lowest observable vertical ionization potential (to Al2(+) X 2 sigma g +) of the Al2 X 3 pi u ground state is calculated to occur around 6.1 eV, in excellent agreement with the experimental range of 6.0 to 6.42 eV obtained in recent cluster ionization studies by Cox and co-workers. The second vertical ionization potential (to Al2(+) A 2 pi u) occurs near 6.4 eV, also within the experimental range. The adiabatic IP of 5.90 eV is in good agreement with the value of 5.8 to 6.1 eV deduced by Hanley and co-workers from the difference in thresholds between collision induced dissociation processes of Al3(+). The computed IP values are somewhat larger than those deduced from branching ratios in cluster fragmentation experiments by Jarrold and co-workers. The observation of an ionization threshold below 6.42 eV is shown to be incompatible with an Al2 ground electronic state assignment of 3 sigma g -, but the separation between the two lowest states of Al2 is so small that it is likely that both are populated in the experiments, so that this does not provide unambiguous support for the recent theoretical assignment of the ground state as 3 pi u.

  19. Towards building artificial light harvesting complexes: enhanced singlet-singlet energy transfer between donor and acceptor pairs bound to albumins.

    PubMed

    Kumar, Challa V; Duff, Michael R

    2008-12-01

    Specific donor and acceptor pairs have been assembled in bovine serum albumin (BSA), at neutral pH and room temperature, and these dye-protein complexes indicated efficient donor to acceptor singlet-singlet energy transfer. For example, pyrene-1-butyric acid served as the donor and Coumarin 540A served as the acceptor. Both the donor and the acceptor bind to BSA with affinity constants in excess of 2x10(5) M(-1), as measured in absorption and circular dichroism (CD) spectral titrations. Simultaneous binding of both the donor and the acceptor chromophores was supported by CD spectra and one chromophore did not displace the other from the protein host, even when limited concentrations of the host were used. For example, a 1:1:1 complex between the donor, acceptor and the host can be readily formed, and spectral data clearly show that the binding sites are mutually exclusive. The ternary complexes (two different ligands bound to the same protein molecule) provided opportunities to examine singlet-singlet energy transfer between the protein-bound chromophores. Donor emission was quenched by the addition of the acceptor, in the presence of limited amounts of BSA, while no energy transfer was observed in the absence of the protein host, under the same conditions. The excitation spectra of the donor-acceptor-host complexes clearly show the sensitization of acceptor emission by the donor. Protein denaturation, as induced by the addition of urea or increasing the temperature to 360 K, inhibited energy transfer, which indicate that protein structure plays an important role. Sensitization also proceeded at low temperature (77 K) and diffusion of the donor or the acceptor is not required for energy transfer. Stern-Volmer quenching plots show that the quenching constant is (3.1+/-0.2)x10(4) M(-1), at low acceptor concentrations (<35 microM). Other albumins such as human and porcine proteins also served as good hosts for the above experiments. For the first time, non

  20. Potential energy curves for the 48 lowest-lying molecular states of LiH +

    NASA Astrophysics Data System (ADS)

    Alikacem, A.; Aubert-Frécon, M.

    1985-05-01

    The electronic energies of the 20 lowest lying 2Σ + states, the 14 lowest lying 2Π states, and the 14 lowest lying 2Δ states of LiH + have been calculated in the range 2 ≤ R ≤ 20 a.u. from a model potential approach and using truncated diatomic orbitals (TDO) as basis set. Results in very good agreement with the more recent literature were obtained for the spectroscopic constants, Re and De, for the ground state X2Σ +.

  1. Singlet oxygen induced advanced glycation end-product photobleaching of in vivo human fingertip autofluorescence

    NASA Astrophysics Data System (ADS)

    Deng, Bin; Simental, Anabel; Lutz, Patrick; Shaheen, George; Chaiken, Joseph

    2012-01-01

    Nonenzymatic glycation and oxidation of ubiquitous proteins in vivo leads to irreversible formation of advanced glycation end products (AGEs). Due to their relatively long half life and low clearance rate AGEs tend to accumulate within static tissues and the circulatory system. Spectra obtained using 830 nm near-infrared (NIR) excitation suggest that the so-called "autofluorescence" from all tissues has a finite number of sources but the fact that senior and diabetic subjects produce more than other members of the general population suggests that a significant portion of the total autofluorescence from all sources originates from AGEs. Using pentosidine generated in a reaction mixture as described by Monnier as representative, an in vitro study unveiled very similar fluorescence and photobleaching pattern as observed for autofluorescence in vivo. A series of oxygen, air and argon purging experiments on the pentosidine-generating reaction mixture suggests that pentosidine is a singlet oxygen sensitizer and secondary reactions between the pentosidine itself and/or other fluorophores and the photosensitized singlet oxygen explain the observed photobleaching. Ab initio Gaussian calculations on pentosidine reveal the existence of low-lying triplet excited states required for the sensitization of ground state oxygen. A commercially available product known as singlet oxygen sensor green (SOSG) that specifically serves as a singlet oxygen detection reagent confirms the generation of singlet oxygen from NIR irradiated pentosidine trimixture. This study provides one definite chemical mechanism for understanding in vivo human skin autofluorescence and photobleaching.

  2. Singlet-Doublet Dark Matter

    SciTech Connect

    Cohen, Timothy; Kearney, John; Pierce, Aaron; Tucker-Smith, David; /Williams Coll.

    2012-02-15

    In light of recent data from direct detection experiments and the Large Hadron Collider, we explore models of dark matter in which an SU(2){sub L} doublet is mixed with a Standard Model singlet. We impose a thermal history. If the new particles are fermions, this model is already constrained due to null results from XENON100. We comment on remaining regions of parameter space and assess prospects for future discovery. We do the same for the model where the new particles are scalars, which at present is less constrained. Much of the remaining parameter space for both models will be probed by the next generation of direct detection experiments. For the fermion model, DeepCore may also play an important role.

  3. Singlet fission in pentacene dimers.

    PubMed

    Zirzlmeier, Johannes; Lehnherr, Dan; Coto, Pedro B; Chernick, Erin T; Casillas, Rubén; Basel, Bettina S; Thoss, Michael; Tykwinski, Rik R; Guldi, Dirk M

    2015-04-28

    Singlet fission (SF) has the potential to supersede the traditional solar energy conversion scheme by means of boosting the photon-to-current conversion efficiencies beyond the 30% Shockley-Queisser limit. Here, we show unambiguous and compelling evidence for unprecedented intramolecular SF within regioisomeric pentacene dimers in room-temperature solutions, with observed triplet quantum yields reaching as high as 156 ± 5%. Whereas previous studies have shown that the collision of a photoexcited chromophore with a ground-state chromophore can give rise to SF, here we demonstrate that the proximity and sufficient coupling through bond or space in pentacene dimers is enough to induce intramolecular SF where two triplets are generated on one molecule. PMID:25858954

  4. Singlet fission in pentacene dimers

    PubMed Central

    Zirzlmeier, Johannes; Lehnherr, Dan; Coto, Pedro B.; Chernick, Erin T.; Casillas, Rubén; Basel, Bettina S.; Thoss, Michael; Tykwinski, Rik R.; Guldi, Dirk M.

    2015-01-01

    Singlet fission (SF) has the potential to supersede the traditional solar energy conversion scheme by means of boosting the photon-to-current conversion efficiencies beyond the 30% Shockley–Queisser limit. Here, we show unambiguous and compelling evidence for unprecedented intramolecular SF within regioisomeric pentacene dimers in room-temperature solutions, with observed triplet quantum yields reaching as high as 156 ± 5%. Whereas previous studies have shown that the collision of a photoexcited chromophore with a ground-state chromophore can give rise to SF, here we demonstrate that the proximity and sufficient coupling through bond or space in pentacene dimers is enough to induce intramolecular SF where two triplets are generated on one molecule. PMID:25858954

  5. Vortex distribution in the lowest Landau level

    NASA Astrophysics Data System (ADS)

    Aftalion, Amandine; Blanc, Xavier; Nier, Francis

    2006-01-01

    We study the vortex distribution of the wave functions minimizing the Gross-Pitaevskii energy for a fast rotating condensate in the lowest Landau level (LLL): we prove that the minimizer cannot have a finite number of zeroes, thus the lattice is infinite, but not uniform. This uses the explicit expression of the projector onto the LLL. We also show that any slow varying envelope function can be approximated in the LLL by distorting the lattice. This is used in particular to approximate the inverted parabola and understand the role of “invisible” vortices: the distortion of the lattice is very small in the Thomas-Fermi region but quite large outside, where the “invisible” vortices lie.

  6. Application of equation-of-motion coupled-cluster methods to low-lying singlet and triplet electronic states of HBO and BOH

    NASA Astrophysics Data System (ADS)

    DeYonker, Nathan J.; Li, Se; Yamaguchi, Yukio; Schaefer, Henry F.; Crawford, T. Daniel; King, Rollin A.; Peterson, Kirk A.

    2005-06-01

    The equilibrium structures and physical properties of the X˜Σ+1 linear electronic states, linear excited singlet and triplet electronic states of hydroboron monoxide (HBO) (ÃΣ-1, B˜Δ1, ãΣ+3, and b˜Δ3) and boron hydroxide (BOH) (ÃΣ+1, B˜Π1, and b˜Π3), and their bent counterparts (HBO ãA'3, b˜A″3, ÃA″1, B˜A'1 and BOH X˜A'1, b˜A'3, c˜A″3, ÃA'1, B˜A'1, C˜A″1) are investigated using excited electronic state ab initio equation-of-motion coupled-cluster (EOM-CC) methods. A new implementation of open-shell EOM-CC including iterative partial triple excitations (EOM-CC3) was tested. Coupled-cluster wave functions with single and double excitations (CCSD), single, double, and iterative partial triple excitations (CC3), and single, double, and full triple excitations (CCSDT) are employed with the correlation-consistent quadruple and quintuple zeta basis sets. The linear HBO X˜Σ+1 state is predicted to lie 48.3kcalmol-1 (2.09eV) lower in energy than the BOH X˜Σ+1 linear stationary point at the CCSDT level of theory. The CCSDT BOH barrier to linearity is predicted to lie 3.7kcalmol-1 (0.16eV). With a harmonic zero-point vibrational energy correction, the HBO X˜Σ+1-BOHX˜A'1 energy difference is 45.2kcalmol-1 (1.96eV). The lowest triplet excited electronic state of HBO, ãA'3, has a predicted excitation energy (Te) of 115kcalmol-1 (4.97eV) from the HBO ground state minimum, while the lowest-bound BOH excited electronic state, b˜A'3, has a Te of 70.2kcalmol-1 (3.04eV) with respect to BOH X˜A'1. The Te values predicted for the lowest singlet excited states are ÃA″1←X˜Σ+1=139kcalmol-1 (6.01eV) for HBO and ÃA'1←X˜A'1=102kcalmol-1 (4.42eV) for BOH. Also for BOH, the triplet vertical transition energies are b˜A'3←X˜A'1=71.4kcalmol-1 (3.10eV) and c˜A″3←X˜A'1=87.2kcalmol-1 (3.78eV).

  7. Phonon coupling in optical transitions for singlet-triplet pairs of bound excitons in semiconductors

    NASA Astrophysics Data System (ADS)

    Pistol, M. E.; Monemar, B.

    1986-05-01

    A model is presented for the observed strong difference in selection rules for coupling of phonons in the one-phonon sideband of optical spectra related to bound excitons in semiconductors. The present treatment is specialized to the case of a closely spaced pair of singlet-triplet character as the lowest electronic states, as is common for bound excitons associated with neutral complexes in materials like GaP and Si. The optical transition for the singlet bound-exciton state is found to couple strongly only to symmetric A1 modes. The triplet state has a similar coupling strength to A1 modes, but in addition strong contributions are found for replicas corresponding to high-density-of-states phonons TAX, LAX, and TOX. This can be explained by a treatment of particle-phonon coupling beyond the ordinary adiabatic approximation. A weak mixing between the singlet and triplet states is mediated by the phonon coupling, as described in first-order perturbation theory. The model derived in this work, for such phonon-induced mixing of closely spaced electronic states, is shown to explain the observed phonon coupling for several bound-exciton systems of singlet-triplet character in GaP. In addition, the observed oscillator strength of the forbidden triplet state may be explained as partly derived from phonon-induced mixing with the singlet state, which has a much larger oscillator strength.

  8. Photosensitized reactions initiated by 6-carboxypterin: singlet and triplet reactivity.

    PubMed

    Tinel, L; Rossignol, S; Ciuraru, R; Dumas, S; George, C

    2016-06-22

    Pterins, derivatives of 2-aminopteridin-4(3H)-one, are natural photosensitizers, common to many biological systems. Indications that these photosensitizers are also present in the sea-surface microlayer motivated the study of the photophysical and photochemical properties of 6-carboxypterin (CPT), which was chosen as a model for this group of photoactive compounds. The kinetics of excited CPT in the singlet and triplet state in the presence of halides and organics were studied in aqueous solutions at neutral pH by means of steady-state fluorescence and laser-flash photolysis. The fluorescence of CPT was efficiently quenched by two halides (iodide and bromide) and by four carboxylic acids (lactic, malonic, propionic and citric acid) with reaction rates close to the diffusion-controlled limit. In the triplet state, the triplet absorption spectrum was measured and its pH dependence was studied. The triplet state of CPT showed relatively high reactivity towards iodide, but no reaction with bromide or chloride could be observed. No singlet or triplet state quenching in the presence of limonene could be measured. A reaction mechanism is proposed, initiated by electron transfer from the quencher to the excited photosensitizer. This type of photo-induced reaction in the sea-surface microlayer has the potential to trigger the production of many oxidized species, including halogen atoms, in the bulk and gaseous phases. PMID:27296228

  9. Spin-singlet Bose-Einstein condensation of two-electron atoms.

    PubMed

    Takasu, Yosuke; Maki, Kenichi; Komori, Kaduki; Takano, Tetsushi; Honda, Kazuhito; Kumakura, Mitsutaka; Yabuzaki, Tsutomu; Takahashi, Yoshiro

    2003-07-25

    We report the observation of a Bose-Einstein condensation of ytterbium atoms by evaporative cooling in a novel crossed optical trap. Unlike the previously observed condensates, a ytterbium condensate is a two-electron system in a singlet state and has distinct features such as the extremely narrow intercombination transitions which are ideal for future optical frequency standard and the insensitivity to external magnetic field which is important for precision coherent atom optics, and the existence of the novel metastable triplet states generated by optical excitation from the singlet state. PMID:12906649

  10. TDDFT study on the excited-state proton transfer of 8-hydroxyquinoline: Key role of the excited-state hydrogen-bond strengthening

    NASA Astrophysics Data System (ADS)

    Lan, Sheng-Cheng; Liu, Yu-Hui

    2015-03-01

    Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been employed to study the excited-state intramolecular proton transfer (ESIPT) reaction of 8-hydroxyquinoline (8HQ). Infrared spectra of 8HQ in both the ground and the lowest singlet excited states have been calculated, revealing a red-shift of the hydroxyl group (-OH) stretching band in the excited state. Hence, the intramolecular hydrogen bond (O-H···N) in 8HQ would be significantly strengthened upon photo-excitation to the S1 state. As the intramolecular proton-transfer reaction occurs through hydrogen bonding, the ESIPT reaction of 8HQ is effectively facilitated by strengthening of the electronic excited-state hydrogen bond (O-H···N). As a result, the intramolecular proton-transfer reaction would occur on an ultrafast timescale with a negligible barrier in the calculated potential energy curve for the ESIPT reaction. Therefore, although the intramolecular proton-transfer reaction is not favorable in the ground state, the ESIPT process is feasible in the excited state. Finally, we have identified that radiationless deactivation via internal conversion (IC) becomes the main dissipative channel for 8HQ by analyzing the energy gaps between the S1 and S0 states for the enol and keto forms.

  11. Intramolecular Singlet Fission in Oligoacene Heterodimers.

    PubMed

    Sanders, Samuel N; Kumarasamy, Elango; Pun, Andrew B; Steigerwald, Michael L; Sfeir, Matthew Y; Campos, Luis M

    2016-03-01

    We investigate singlet fission (SF) in heterodimers comprising a pentacene unit covalently bonded to another acene as we systematically vary the singlet and triplet pair energies. We find that these energies control the SF process, where dimers undergo SF provided that the resulting triplet pair energy is similar or lower in energy than the singlet state. In these systems the singlet energy is determined by the lower-energy chromophore, and the rate of SF is found to be relatively independent of the driving force. However, triplet pair recombination in these heterodimers follows the energy gap law. The ability to tune the energies of these materials provides a key strategy to study and design new SF materials-an important process for third-generation photovoltaics. PMID:26836223

  12. Solution-processable singlet fission photovoltaic devices.

    PubMed

    Yang, Le; Tabachnyk, Maxim; Bayliss, Sam L; Böhm, Marcus L; Broch, Katharina; Greenham, Neil C; Friend, Richard H; Ehrler, Bruno

    2015-01-14

    We demonstrate the successful incorporation of a solution-processable singlet fission material, 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), into photovoltaic devices. TIPS-pentacene rapidly converts high-energy singlet excitons into pairs of triplet excitons via singlet fission, potentially doubling the photocurrent from high-energy photons. Low-energy photons are captured by small-bandgap electron-accepting lead chalcogenide nanocrystals. This is the first solution-processable singlet fission system that performs with substantial efficiency with maximum power conversion efficiencies exceeding 4.8%, and external quantum efficiencies of up to 60% in the TIPS-pentacene absorption range. With PbSe nanocrystal of suitable bandgap, its internal quantum efficiency reaches 170 ± 30%. PMID:25517654

  13. Lowest instrumented vertebra selection in AIS.

    PubMed

    Erickson, Mark A; Baulesh, David M

    2011-01-01

    Appropriate selection of the lowest instrumented vertebra (LIV) is crucial to ensure positive outcomes after surgical management of patients with adolescent idiopathic scoliosis. Failure to do so can lead to curve decompensation and "adding on" of additional vertebrae to the deformity. Correct identification of the stable, end, and neutral vertebra, whether the curve(s) is structural or nonstructural, and classifying the type of curve are essential aspects of preoperative planning. Evaluating curve flexibility using fulcrum, side bending, push-prone, and traction can be used to predict the amount of observed postoperative correction for both fused and unfused curves. In addition, these measures can be used to foresee potential residual LIV-tilt and disc wedging postoperatively. Intraoperative techniques such as fine tuning, derotation, wide release, and in situ contouring and instrumentation type used all influence the LIV selection and therefore, must be taken into account preoperatively. Surgical goals when treating adolescent idiopathic scoliosis include achieving a well-balanced spine in all planes while working to preserve segments and therefore, maintain mobility. PMID:21173622

  14. Multiple exciton generation and singlet fission: the role of collective effects

    NASA Astrophysics Data System (ADS)

    Eaves, Joel; Sweeney, Mark; Teichen, Paul; Strong, Steven

    2014-03-01

    Materials that relax to multiply excited states present exciting possibilities for overcoming the Shockley-Queisser limit in photovoltaic devices. For an excited electronic state to relax in this manner, electron-electron couplings must be strong relative to electron-phonon couplings. In this talk, I will discuss some ways in which these criteria may be met. I will discuss our work on multiple exciton generation in single-walled carbon nanotubes using Luttinger liquid theory and on singlet fission in molecular crystals using quantum exciton models. The Luttinger liquid theory focuses on the long wavelength part of the Coulomb interaction between electrons, and therefore allows us to assess the suspected role that dimensionality plays in enhancing electron-electron interactions. I will also discuss our results for quantum lattice models of singlet fission in molecular crystals that allows us to examine collective dynamical effects that contribute to efficient fission.

  15. Transient photocurrent in molecular junctions: singlet switching on and triplet blocking.

    PubMed

    Petrov, E G; Leonov, V O; Snitsarev, V

    2013-05-14

    The kinetic approach adapted to describe charge transmission in molecular junctions, is used for the analysis of the photocurrent under conditions of moderate light intensity of the photochromic molecule. In the framework of the HOMO-LUMO model for the single electron molecular states, the analytic expressions describing the temporary behavior of the transient and steady state sequential (hopping) as well as direct (tunnel) current components have been derived. The conditions at which the current components achieve their maximal values are indicated. It is shown that if the rates of charge transmission in the unbiased molecular diode are much lower than the intramolecular singlet-singlet excitation/de-excitation rate, and the threefold degenerated triplet excited state of the molecule behaves like a trap blocking the charge transmission, a possibility of a large peak-like transient switch-on photocurrent arises. PMID:23676066

  16. Optical and time-resolved electron paramagnetic resonance studies of the excited states of a UV-B absorber (4-methylbenzylidene)camphor.

    PubMed

    Kikuchi, Azusa; Shibata, Kenji; Kumasaka, Ryo; Yagi, Mikio

    2013-02-21

    The excited states of UV-B absorber (4-methylbenzylidene)camphor (MBC) have been studied through measurements of UV absorption, phosphorescence, triplet-triplet (T-T) absorption, and steady-state and time-resolved electron paramagnetic resonance spectra in ethanol. The energy level and lifetime of the lowest excited triplet (T(1)) state of MBC were determined. The energy level of the T(1) state of MBC is much lower than that of photolabile 4-tert-butyl-4'-methoxydibenzoylmethane. The weak phosphorescence and strong time-resolved EPR signals, and T-T absorption band of MBC were observed. These facts suggest that the significant proportion of the lowest excited singlet (S(1)) molecules undergoes intersystem crossing to the T(1) state and the deactivation process from the T(1) state is predominantly radiationless. The quantum yields of singlet oxygen production by MBC determined by time-resolved near-IR luminescence measurements are 0.05 ± 0.01 and 0.06 ± 0.01 in ethanol and in acetonitrile, respectively. The photostability of MBC arises from the (3)ππ* character in the T(1) state. The zero-field splitting parameters in the T(1) state are D = 0.0901 cm(-1) and E = -0.0498 cm(-1). The sublevel preferentially populated by intersystem crossing is T(y) (y close to in-plane short axis and to the C═O direction). PMID:23320917

  17. Effects of heteroatoms of tetracene and pentacene derivatives on their stability and singlet fission.

    PubMed

    Chen, Yuhan; Shen, Li; Li, Xiyou

    2014-07-31

    The effects of the introduction of an sp(2)-hybridized nitrogen atom (═N-) and thiophene ring on the structure geometries, frontier molecular orbital energies, and excited state energies related to singlet fission (SF) for some tetracene and pentacene derivatives were theoretically investigated by quantum chemical methods. The introduction of a nitrogen atom significantly decreases the energies of frontier molecular orbitals and hence improves their stabilities in air and light illumination. More importantly, it is helpful for reducing the energy loss of the exothermic singlet fission of pentacene derivatives. For fused benzene-thiophene structures, the (α, β) connection pattern could stabilize the frontier molecular orbitals, while the (β, β) connection pattern can promote the thermodynamic driving force of singlet fission. These facts provide a theoretical ground for rational design of SF materials. PMID:25007000

  18. High Yield Ultrafast Intramolecular Singlet Exciton Fission in a Quinoidal Bithiophene.

    PubMed

    Varnavski, Oleg; Abeyasinghe, Neranga; Aragó, Juan; Serrano-Pérez, Juan J; Ortí, Enrique; López Navarrete, Juan T; Takimiya, Kazuo; Casanova, David; Casado, Juan; Goodson, Theodore

    2015-04-16

    We report the process of singlet exciton fission with high-yield upon photoexcitation of a quinoidal thiophene molecule. Efficient ultrafast triplet photogeneration and its yield are determined by photoinduced triplet-triplet absorption, flash photolysis triplet lifetime measurements, as well as by femtosecond time-resolved transient absorption and fluorescence methods. These experiments show that optically excited quinoidal bithiophene molecule undergoes ultrafast formation of the triplet-like state with the lifetime ∼57 μs. CASPT2 and RAS-SF calculations have been performed to support the experimental findings. To date, high singlet fission rates have been reported for crystalline and polycrystalline materials, whereas for covalently linked dimers and small oligomers it was found to be relatively small. In this contribution, we show an unprecedented quantum yield of intramolecular singlet exciton fission of ∼180% for a quinoidal bithiophene system. PMID:26263138

  19. Collisional energy transfer and quenching of electronic excitation

    PubMed Central

    Lin, S. H.; Eyring, H.

    1975-01-01

    The purpose of this paper has been to explore in a preliminary way the nature and mechanism of collisional energy transfer and quenching of electronic excitation. For this purpose, the Born approximation has been used, and the triplet-triplet and singlet-singlet transfer, and the triplet-triplet and singlet-singlet quenching have been studied. It has been shown theoretically that (i) the singlet-singlet transfer constants (or cross sections) are always larger than the triplet-triplet transfer constants (or cross sections) for the same system of donor and acceptor; (ii) for the singlet-singlet transfer, the observed cross section varies linearly with respect to the spectral overlap between the donor emission and the acceptor absorption; (iii) the reason that the quenching constants (or cross sections) are always smaller than the energy transfer constants (or cross sections) is due to the fact that for the quenching the vibration of the acceptor hardly participates in accepting the electronic excitation and for the energy transfer only part of the excited electron energy of the donor is converted into the energy of nuclear motion; and (iv) the polar acceptor molecules are better quenchers than nonpolar acceptor molecules. PMID:16592281

  20. Supramolecular nanoreactors for intracellular singlet-oxygen sensitization

    NASA Astrophysics Data System (ADS)

    Swaminathan, Subramani; Fowley, Colin; Thapaliya, Ek Raj; McCaughan, Bridgeen; Tang, Sicheng; Fraix, Aurore; Burjor, Captain; Sortino, Salvatore; Callan, John F.; Raymo, Françisco M.

    2015-08-01

    An amphiphilic polymer with multiple decyl and oligo(ethylene glycol) chains attached to a common poly(methacrylate) backbone assembles into nanoscaled particles in aqueous environments. Hydrophobic anthracene and borondipyrromethene (BODIPY) chromophores can be co-encapsulated within the self-assembling nanoparticles and transported across hydrophilic media. The reversible character of the noncovalent bonds, holding the supramolecular containers together, permits the exchange of their components with fast kinetics in aqueous solution. Incubation of cervical cancer (HeLA) cells with a mixture of two sets of nanoparticles, pre-loaded independently with anthracene or BODIPY chromophores, results in guest scrambling first and then transport of co-entrapped species to the intracellular space. Alternatively, incubation of cells with the two sets of nanocarriers in consecutive steps permits the sequential transport of the anthracene and BODIPY chromophores across the plasma membrane and only then allows their co-encapsulation within the same supramolecular containers. Both mechanisms position the two sets of chromophores with complementary spectral overlap in close proximity to enable the efficient transfer of energy intracellularly from the anthracene donors to the BODIPY acceptors. In the presence of iodine substituents on the BODIPY platform, intersystem crossing follows energy transfer. The resulting triplet state can transfer energy further to molecular oxygen with the concomitant production of singlet oxygen to induce cell mortality. Furthermore, the donor can be excited with two near-infrared photons simultaneously to permit the photoinduced generation of singlet oxygen intracellularly under illumination conditions compatible with applications in vivo. Thus, these supramolecular strategies to control the excitation dynamics of multichromophoric assemblies in the intracellular environment can evolve into valuable protocols for photodynamic therapy.An amphiphilic

  1. Role of the low-energy excited states in the radiolysis of aromatic liquids.

    PubMed

    Baidak, Aliaksandr; Badali, Matthew; LaVerne, Jay A

    2011-07-01

    The contribution of the low-energy excited states to the overall product formation in the radiolysis of simple aromatic liquids--benzene, pyridine, toluene, and aniline--has been examined by comparison of product yields obtained in UV-photolysis and in γ-radiolysis. In photolysis, these electronic excited states were selectively populated using UV-light excitation sources with various energies. Yields of molecular hydrogen and of "dimers" (biphenyl, bibenzyl, dipyridyl for benzene, toluene, pyridine, respectively, and of ammonia and diphenylamine for aniline) have been determined, since they are the most abundant radiolytic products. Negligibly small production of molecular hydrogen in the UV-photolysis of aromatic liquids with excitation to energies of 4.88, 5.41, 5.79, and 6.70 eV and the lack of a scavenger effect suggest that this product originates from short-lived high-energy singlet states. A significant reduction in "dimer" radiation-chemical yields in the presence of scavengers such as anthracene or naphthalene indicates that the triplet excited states are important precursors to these products. The results for toluene and aniline suggest that efficient dissociation from the lowest-energy excited triplet state leads to noticeable "dimer" production. For benzene and pyridine, the lowest-energy triplet excited states are not likely to fragment into radicals because of the relatively large energy gap between the excited state level and corresponding bond dissociation energy. The "dimer" formation in the radiolysis of benzene and pyridine is likely to involve short-lived high-energy triplet states. PMID:21634362

  2. The low-lying electronic states of pentacene and their roles in singlet fission.

    PubMed

    Zeng, Tao; Hoffmann, Roald; Ananth, Nandini

    2014-04-16

    We present a detailed study of pentacene monomer and dimer that serves to reconcile extant views of its singlet fission. We obtain the correct ordering of singlet excited-state energy levels in a pentacene molecule (E (S1) < E (D)) from multireference calculations with an appropriate active orbital space and dynamical correlation being incorporated. In order to understand the mechanism of singlet fission in pentacene, we use a well-developed diabatization scheme to characterize the six low-lying singlet states of a pentacene dimer that approximates the unit cell structure of crystalline pentacene. The local, single-excitonic diabats are not directly coupled with the important multiexcitonic state but rather mix through their mutual couplings with one of the charge-transfer configurations. We analyze the mixing of diabats as a function of monomer separation and pentacene rotation. By defining an oscillator strength measure of the coherent population of the multiexcitonic diabat, essential to singlet fission, we find this population can, in principle, be increased by small compression along a specific crystal direction. PMID:24697685

  3. Laser induced singlet-oxygen-sensitised delayed fluorescence of dyes in aqueous solutions

    SciTech Connect

    Krasnovskii, A A; Bashtanov, M E; Drozdova, N N; Yuzhakova, O A; Luk'yanets, Evgenii A

    2002-01-31

    It is shown that water-soluble derivatives of phthalocyanines - poly(diethoxyphosphinylmethyl)substituted aluminium phthalocyanines - emit intense singlet-oxygen-sensitised delayed fluorescence upon laser-induced formation of singlet oxygen in air-saturated aqueous (D{sub 2}O) solutions. The delayed fluorescence is emitted by the dye molecules which accepted energy from two molecules of singlet oxygen. The quantum efficiency of delayed fluorescence in aerated D{sub 2}O of the chloroaluminium complex of octa(diethoxyphosphinylmethyl) phthalocyanine corresponds to the rate constant of population of excited dye molecules which is equal to (5.5 {+-} 3) x 10{sup 12} mole{sup -2} L{sup 2} s{sup -1}. This value is only an order of magnitude smaller than that for tetra(4-tert.-butyl)phthalocyanine earlier studied in aerated organic solvents. It is shown that these phthalocyanine derivatives can be used as highly sensitive luminescence indicators of singlet oxygen produced in aqueous solutions of different compounds upon laser excitation. (laser applications and other topics in quantum electronics)

  4. Ab initio reaction pathways for photodissociation and isomerization of nitromethane on four singlet potential energy surfaces with three roaming paths

    NASA Astrophysics Data System (ADS)

    Isegawa, Miho; Liu, Fengyi; Maeda, Satoshi; Morokuma, Keiji

    2014-06-01

    Photodissociation pathways of nitromethane following π → π* electronic excitation are reported. The potential energy surfaces for four lowest singlet states are explored, and structures of many intermediates, dissociation limits, transition states, and minimum energy conical intersections were determined using the automated searching algorism called the global reaction route mapping strategy. Geometries are finally optimized at CASSCF(14e,11o) level and energies are computed at CAS(14o,11e)PT2 level. The calculated preferable pathways and important products qualitatively explain experimental observations. The major photodissociation product CH3 and NO2 (2B2) is formed by direct dissociation from the S1 state. Important pathways involving S1 and S0 states for production of various dissociation products CH3NO + O (1D), CH3O(X2E) + NO (X2Π), CH2NO + OH, and CH2O + HNO, as well as various isomerization pathways have been identified. Three roaming processes also have been identified: the O atom roaming in O dissociation from CH3NO2, the OH radical roaming in OH dissociation from CH2N(O)(OH), and the NO roaming in NO dissociation from CH3ONO.

  5. Ab initio reaction pathways for photodissociation and isomerization of nitromethane on four singlet potential energy surfaces with three roaming paths.

    PubMed

    Isegawa, Miho; Liu, Fengyi; Maeda, Satoshi; Morokuma, Keiji

    2014-06-28

    Photodissociation pathways of nitromethane following π → π(*) electronic excitation are reported. The potential energy surfaces for four lowest singlet states are explored, and structures of many intermediates, dissociation limits, transition states, and minimum energy conical intersections were determined using the automated searching algorism called the global reaction route mapping strategy. Geometries are finally optimized at CASSCF(14e,11o) level and energies are computed at CAS(14o,11e)PT2 level. The calculated preferable pathways and important products qualitatively explain experimental observations. The major photodissociation product CH3 and NO2 ((2)B2) is formed by direct dissociation from the S1 state. Important pathways involving S1 and S0 states for production of various dissociation products CH3NO + O ((1)D), CH3O(X(2)E) + NO (X(2)Π), CH2NO + OH, and CH2O + HNO, as well as various isomerization pathways have been identified. Three roaming processes also have been identified: the O atom roaming in O dissociation from CH3NO2, the OH radical roaming in OH dissociation from CH2N(O)(OH), and the NO roaming in NO dissociation from CH3ONO. PMID:24985641

  6. Ab initio reaction pathways for photodissociation and isomerization of nitromethane on four singlet potential energy surfaces with three roaming paths

    SciTech Connect

    Isegawa, Miho; Liu, Fengyi; Morokuma, Keiji; Maeda, Satoshi

    2014-06-28

    Photodissociation pathways of nitromethane following π → π{sup *} electronic excitation are reported. The potential energy surfaces for four lowest singlet states are explored, and structures of many intermediates, dissociation limits, transition states, and minimum energy conical intersections were determined using the automated searching algorism called the global reaction route mapping strategy. Geometries are finally optimized at CASSCF(14e,11o) level and energies are computed at CAS(14o,11e)PT2 level. The calculated preferable pathways and important products qualitatively explain experimental observations. The major photodissociation product CH{sub 3} and NO{sub 2} ({sup 2}B{sub 2}) is formed by direct dissociation from the S{sub 1} state. Important pathways involving S{sub 1} and S{sub 0} states for production of various dissociation products CH{sub 3}NO + O ({sup 1}D), CH{sub 3}O(X{sup 2}E) + NO (X{sup 2}Π), CH{sub 2}NO + OH, and CH{sub 2}O + HNO, as well as various isomerization pathways have been identified. Three roaming processes also have been identified: the O atom roaming in O dissociation from CH{sub 3}NO{sub 2}, the OH radical roaming in OH dissociation from CH{sub 2}N(O)(OH), and the NO roaming in NO dissociation from CH{sub 3}ONO.

  7. Far-red fluorescence probe for monitoring singlet oxygen during photodynamic therapy.

    PubMed

    Kim, Sooyeon; Tachikawa, Takashi; Fujitsuka, Mamoru; Majima, Tetsuro

    2014-08-20

    Singlet oxygen ((1)O2), molecular oxygen in the lowest excited state, has a critical role in the cell-killing mechanism of photodynamic therapy (PDT). Although (1)O2 phosphorescence measurement has been mainly used to monitor (1)O2 formation during PDT, its intensity is far insufficient to obtain two-dimensional images of intracellular (1)O2 with the subcellular spatial resolution using the currently available near-IR detector. Here, we propose a new far-red fluorescence probe of (1)O2, namely, Si-DMA, composed of silicon-containing rhodamine and anthracene moieties as a chromophore and a (1)O2 reactive site, respectively. In the presence of (1)O2, fluorescence of Si-DMA increases 17 times due to endoperoxide formation at the anthracene moiety. With the advantage of negligible self-oxidation by photoirradiation (ΦΔ < 0.02) and selective mitochondrial localization, Si-DMA is particularly suitable for imaging (1)O2 during PDT. Among three different intracellular photosensitizers (Sens), Si-DMA could selectively detect the (1)O2 that is generated by 5-aminolevulinic acid-derived protoporphyrin IX, colocalized with Si-DMA in mitochondria. On the other hand, mitochondria-targeted KillerRed and lysosomal porphyrins could not induce fluorescence change of Si-DMA. This surprising selectivity of Si-DMA response depending on the Sens localization and photosensitization mechanism is caused by a limited intracellular (1)O2 diffusion distance (∼300 nm) and negligible generation of (1)O2 by type-I Sens, respectively. For the first time, we successfully visualized (1)O2 generated during PDT with a spatial resolution of a single mitochondrial tubule. PMID:25075870

  8. Gadolinium(III) Porpholactones as Efficient and Robust Singlet Oxygen Photosensitizers.

    PubMed

    Ke, Xian-Sheng; Ning, Yingying; Tang, Juan; Hu, Ji-Yun; Yin, Hao-Yan; Wang, Gao-Xiang; Yang, Zi-Shu; Jie, Jialong; Liu, Kunhui; Meng, Zhao-Sha; Zhang, Zongyao; Su, Hongmei; Shu, Chunying; Zhang, Jun-Long

    2016-07-01

    Construction of Gd(III) photosensitizers is important for designing theranostic agents owing to the unique properties arising from seven unpaired f electrons of the Gd(3+) ion. Combining these with the advantages of porpholactones with tunable NIR absorption, we herein report the synthesis of Gd(III) complexes Gd-1-4 (1, porphyrin; 2, porpholactone; 3 and 4, cis- and trans-porphodilactone, respectively) and investigated their function as singlet oxygen ((1) O2 ) photosensitizers. These Gd complexes displayed (1) O2 quantum yields (ΦΔ s) from 0.64-0.99 with the order Gd-1lowest triplet states (T1 ) of the ligand and the energy level of the (1) Δg →(3) Σg transition of (1) O2 . In particular, Gd-4 is capable of excitation in the visible to NIR region (400-700 nm) with a quantum yield near unity. These Gd complexes were first demonstrated as efficient photosensitizers in photocatalysis such as oxidative C-H bond functionalization of secondary or tertiary amines, and the oxygenation of the natural product cholesterol. Finally, after glycosylation, these water-soluble Gd complexes showed potential applications in photodynamic therapy (PDT) in HeLa cells. This work revealed that Gd(III) complexes of "bioinspired" β-modified porpholactones are efficient NIR photosensitizers and form a chemical basis to construct appealing photocatalysts and theranostic agents based on lanthanides. PMID:27249665

  9. Exciton Correlations in Intramolecular Singlet Fission.

    PubMed

    Sanders, Samuel N; Kumarasamy, Elango; Pun, Andrew B; Appavoo, Kannatassen; Steigerwald, Michael L; Campos, Luis M; Sfeir, Matthew Y

    2016-06-15

    We have synthesized a series of asymmetric pentacene-tetracene heterodimers with a variable-length conjugated bridge that undergo fast and efficient intramolecular singlet fission (iSF). These compounds have distinct singlet and triplet energies, which allow us to study the spatial dynamics of excitons during the iSF process, including the significant role of exciton correlations in promoting triplet pair generation and recombination. We demonstrate that the primary photoexcitations in conjugated dimers are delocalized singlets that enable fast and efficient iSF. However, in these asymmetric dimers, the singlet becomes more localized on the lower energy unit as the length of the bridge is increased, slowing down iSF relative to analogous symmetric dimers. We resolve the recombination kinetics of the inequivalent triplets produced via iSF, and find that they primarily decay via concerted processes. By identifying different decay channels, including delayed fluorescence via triplet-triplet annihilation, we can separate transient species corresponding to both correlated triplet pairs and uncorrelated triplets. Recombination of the triplet pair proceeds rapidly despite our experimental and theoretical demonstration that individual triplets are highly localized and unable to be transported across the conjugated linker. In this class of compounds, the rate of formation and yield of uncorrelated triplets increases with bridge length. Overall, these constrained, asymmetric systems provide a unique platform to isolate and study transient species essential for singlet fission, which are otherwise difficult to observe in symmetric dimers or condensed phases. PMID:27183040

  10. Singlet fission in linear chains of molecules

    NASA Astrophysics Data System (ADS)

    Ambrosio, Francesco; Troisi, Alessandro

    2014-11-01

    We develop a model configuration interaction Hamiltonian to study the electronic structure of a chain of molecules undergoing singlet fission. We first consider models for dimer and trimer and then we use a matrix partitioning technique to build models of arbitrary size able to describe the relevant electronic structure for singlet fission in linear aggregates. We find that the multi-excitonic state (ME) is stabilized at short inter-monomer distance and the extent of this stabilization depends upon the size of orbital coupling between neighboring monomers. We also find that the coupling between ME states located on different molecules is extremely small leading to bandwidths in the order of ˜10 meV. This observation suggests that multi-exciton states are extremely localized by electron-phonon coupling and that singlet fission involves the transition between a relatively delocalized Frenkel exciton and a strongly localized multi-exciton state. We adopt the methodology commonly used to study non-radiative transitions to describe the singlet fission dynamics in these aggregates and we discuss the limit of validity of the approach. The results indicate that the phenomenology of singlet fission in molecular crystals is different in many important ways from what is observed in isolated dimers.

  11. Singlet fission in linear chains of molecules.

    PubMed

    Ambrosio, Francesco; Troisi, Alessandro

    2014-11-28

    We develop a model configuration interaction Hamiltonian to study the electronic structure of a chain of molecules undergoing singlet fission. We first consider models for dimer and trimer and then we use a matrix partitioning technique to build models of arbitrary size able to describe the relevant electronic structure for singlet fission in linear aggregates. We find that the multi-excitonic state (ME) is stabilized at short inter-monomer distance and the extent of this stabilization depends upon the size of orbital coupling between neighboring monomers. We also find that the coupling between ME states located on different molecules is extremely small leading to bandwidths in the order of ∼10 meV. This observation suggests that multi-exciton states are extremely localized by electron-phonon coupling and that singlet fission involves the transition between a relatively delocalized Frenkel exciton and a strongly localized multi-exciton state. We adopt the methodology commonly used to study non-radiative transitions to describe the singlet fission dynamics in these aggregates and we discuss the limit of validity of the approach. The results indicate that the phenomenology of singlet fission in molecular crystals is different in many important ways from what is observed in isolated dimers. PMID:25429953

  12. Singlet fission in linear chains of molecules

    SciTech Connect

    Ambrosio, Francesco E-mail: A.Troisi@warwick.ac.uk; Troisi, Alessandro E-mail: A.Troisi@warwick.ac.uk

    2014-11-28

    We develop a model configuration interaction Hamiltonian to study the electronic structure of a chain of molecules undergoing singlet fission. We first consider models for dimer and trimer and then we use a matrix partitioning technique to build models of arbitrary size able to describe the relevant electronic structure for singlet fission in linear aggregates. We find that the multi-excitonic state (ME) is stabilized at short inter-monomer distance and the extent of this stabilization depends upon the size of orbital coupling between neighboring monomers. We also find that the coupling between ME states located on different molecules is extremely small leading to bandwidths in the order of ∼10 meV. This observation suggests that multi-exciton states are extremely localized by electron-phonon coupling and that singlet fission involves the transition between a relatively delocalized Frenkel exciton and a strongly localized multi-exciton state. We adopt the methodology commonly used to study non-radiative transitions to describe the singlet fission dynamics in these aggregates and we discuss the limit of validity of the approach. The results indicate that the phenomenology of singlet fission in molecular crystals is different in many important ways from what is observed in isolated dimers.

  13. The effect of the structure and conformational dynamics on quenching of triplet states of porphyrins and their chemical dimers by molecular oxygen and on singlet oxygen generation

    NASA Astrophysics Data System (ADS)

    Ivashin, N. V.; Shchupak, E. E.; Sagun, E. I.

    2015-01-01

    Quantum-chemical calculations are performed to analyze the factors affecting rate constant k T of quenching of the lowest triplet state by molecular oxygen and quantum yield γΔ of singlet oxygen generation in chemical dimers of porphyrins bound by phenyl spacers at one of the meso positions (OEP)2-Ph, (TPP)2, and their Zn complexes. It is established that, for both types of dimers, the triplet excitation is localized on one of the macrocycles. The steric hindrance of macrocycles at the site of the phenyl ring of (OEP)2-Ph, (ZnOEP)2-Ph, and their monomeric analogues OEP-Ph and ZnOEP-Ph facilitates its rotation by 90° in the triplet state. The lowest triplet state energy in this ( U) conformation is lower than 7800 cm-1, which makes impossible electronic excitation energy transfer to molecular oxygen. The potential barrier of transformation to the U conformation is considerably lower for dimers than for monomers. Because of this, the rate of conformational transformations for dimers is higher and some of the (OEP)2-Ph and (ZnOEP)2-Ph molecules have time to transform into the new U conformation before diffusion collision with O2 molecules in solution. This leads to a noticeable decrease in γΔ in accordance with experimental data. It is shown that the behavior of k T in the series of the studied dimers, their monomeric analogues, and relative compounds corresponds to the model of dipole-dipole electronic excitation energy transfer 1(3M⋯3Σ{g/-}) → 1(1M0⋯1Δg) in collisional complexes.

  14. Tetra(1,1,4,4-tetramethyl-6,7-tetralino)porphyrazine as a novel luminescence sensor of laser-induced singlet oxygen generation in solutions

    SciTech Connect

    Krasnovskii, A A; Schweitzer, C; Leismann, H; Tanielian, C; Luk'yanets, Evgenii A

    2000-05-31

    Absorption and fluorescence spectra and quantum yields of fluorescence and singlet oxygen generation were studied in air-saturated solutions of a newly synthesised dye, tetra(1,1,4,4-tetramethyl-6,7-tetralino)porphyrazine (TMTP), in benzene. Photophysical properties of TMTP are similar to those of previously studied tetra(4-tert-butyl) phthalocyanine (TBPc). However, the TMTP absorption and fluorescence bands are shifted to the longer wavelengths compared with those of TBPc. The laser-induced generation of singlet molecular oxygen {sup 1}O{sub 2} in TMTP solutions caused strong singlet oxygen-sensitised delayed fluorescence of the dye. The efficiency of singlet-oxygen-sensitised formation of excited TMTP molecules is two times higher than that of TBPc. It is shown that TMTP can serve as an efficient luminescence sensor of singlet oxygen. (laser applications and other topics in quantum electronics)

  15. Antiferromagnetism in Pr3In: Singlet/triplet physics withfrustration

    SciTech Connect

    Christianson, A.D.; Lawrence, J.M.; Zarestky, J.L.; Suzuki, H.; Thompson, J.D.; Hundley, M.F.; Sarrao, J.L.; Booth, C.H.; Antonio, D.; Cornelius, A.L.

    2004-11-18

    We present neutron diffraction, magnetic susceptibility and specific heat data for a single-crystal sample of the cubic (Cu{sub 3}Au structure) compound Pr{sub 3}In. This compound is believed to have a singlet ({Lambda}{sub 1}) groundstate and a low-lying triplet ({Lambda}{sub 4}) excited state. In addition, nearest-neighbor antiferromagnetic interactions are frustrated in this structure. Antiferromagnetic order occurs below T{sub N} = 12K with propagation vector (0, 0, 0.5 {center_dot}{delta}) where {delta} {approx} 1/12. The neutron diffraction results can be approximated with the following model: ferromagnetic sheets from each of the three Pr sites alternate in sign along the propagation direction with a twelve-unit-cell square-wave modulation. The three moments of the unit cell of 1 {micro}{sub B} magnitude are aligned so as to sum to zero as expected for nearest-neighbor antiferromagnetic interactions on a triangle. The magnetic susceptibility indicates that in addition to the antiferromagnetic transition at 12K, there is a transition near 70K below which there is a small (0.005 {micro}{sub B}) ferromagnetic moment. There is considerable field and sample dependence to these transitions. The specific heat data show almost no anomaly at T{sub N} = 12K. This may be a consequence of the induced moment in the {Lambda}{sub 1} singlet, but may also be a sample-dependent effect.

  16. Direct Observation of Thermal Equilibrium of Excited Triplet States of 9,10-Phenanthrenequinone. A Time-Resolved Resonance Raman Study.

    PubMed

    Kumar, Venkatraman Ravi; Rajkumar, Nagappan; Ariese, Freek; Umapathy, Siva

    2015-10-01

    The photochemistry of aromatic ketones plays a key role in various physicochemical and biological processes, and solvent polarity can be used to tune their triplet state properties. Therefore, a comprehensive analysis of the conformational structure and the solvent polarity induced energy level reordering of the two lowest triplet states of 9,10-phenanthrenequinone (PQ) was carried out using nanosecond-time-resolved absorption (ns-TRA), time-resolved resonance Raman (TR(3)) spectroscopy, and time dependent-density functional theory (TD-DFT) studies. The ns-TRA of PQ in acetonitrile displays two bands in the visible range, and these two bands decay with similar lifetime at least at longer time scales (μs). Interestingly, TR(3) spectra of these two bands indicate that the kinetics are different at shorter time scales (ns), while at longer time scales they followed the kinetics of ns-TRA spectra. Therefore, we report a real-time observation of the thermal equilibrium between the two lowest triplet excited states of PQ, assigned to nπ* and ππ* of which the ππ* triplet state is formed first through intersystem crossing. Despite the fact that these two states are energetically close and have a similar conformational structure supported by TD-DFT studies, the slow internal conversion (∼2 ns) between the T(2)(1(3)nπ*) and T(1)(1(3)ππ*) triplet states indicates a barrier. Insights from the singlet excited states of PQ in protic solvents [ J. Chem. Phys. 2015 , 142 , 24305 ] suggest that the lowest nπ* and ππ* triplet states should undergo hydrogen bond weakening and strengthening, respectively, relative to the ground state, and these mechanisms are substantiated by TD-DFT calculations. We also hypothesize that the different hydrogen bonding mechanisms exhibited by the two lowest singlet and triplet excited states of PQ could influence its ISC mechanism. PMID:26381591

  17. Harvesting singlet fission for solar energy conversion: one- versus two-electron transfer from the quantum mechanical superposition.

    PubMed

    Chan, Wai-Lun; Tritsch, John R; Zhu, X-Y

    2012-11-01

    Singlet fission, the creation of two triplet excitons from one singlet exciton, is being explored to increase the efficiency of solar cells and photo detectors based on organic semiconductors, such as pentacene and tetracene. A key question is how to extract multiple electron-hole pairs from multiple excitons. Recent experiments in our laboratory on the pentacene/C(60) system (Chan, W.-L.; et al. Science 2011, 334, 1543-1547) provided preliminary evidence for the extraction of two electrons from the multiexciton (ME) state resulting from singlet fission. The efficiency of multielectron transfer is expected to depend critically on other dynamic processes available to the singlet (S(1)) and the ME, but little is known about these competing channels. Here we apply time-resolved photoemission spectroscopy to the tetracene/C(60) interface to probe one- and two-electron transfer from S(1) and ME states, respectively. Unlike ultrafast (~100 fs) singlet fission in pentacene where two-electron transfer from the multiexciton state resulting from singlet fission dominates, the relatively slow (~7 ps) singlet fission in tetracene allows both one- and two-electron transfer from the S(1) and the ME states that are in a quantum mechanical superposition. We show evidence for the formation of two distinct charge transfer states due to electron transfer from photoexcited tetracene to the lowest unoccupied molecular orbital (LUMO) and the LUMO+1 levels in C(60), respectively. Kinetic analysis shows that ~60% of the S(1) ⇔ ME quantum superposition transfers one electron through the S(1) state to C(60) while ~40% undergoes two-electron transfer through the ME state. We discuss design principles at donor/acceptor interfaces for optimal multiple carrier extraction from singlet fission for solar energy conversion. PMID:23066740

  18. The low-energy, charge-transfer excited states of 4-amino-4-prime-nitrodiphenyl sulfide

    NASA Technical Reports Server (NTRS)

    O'Connor, Donald B.; Scott, Gary W.; Tran, Kim; Coulter, Daniel R.; Miskowski, Vincent M.; Stiegman, Albert E.; Wnek, Gary E.

    1992-01-01

    Absorption and emission spectra of 4-amino-4-prime-nitrodiphenyl sulfide in polar and nonpolar solvents were used to characterize and assign the low-energy excited states of the molecule. Fluorescence-excitation anisotropy spectra and fluorescence quantum yields were also used to characterize the photophysics of these states. The lowest-energy fluorescent singlet state was determined to be an intramolecular charge transfer (ICT) state involving transfer of a full electron charge from the amino to the nitro group yielding a dipole moment of about 50 D. A low-energy, intense absorption band is assigned as a transition to a different ICT state involving a partial electron charge transfer from sulfur to the nitro group.

  19. Determination of the geometry change of benzimidazole upon electronic excitation from a combined Franck-Condon/rotational constants fit

    NASA Astrophysics Data System (ADS)

    Stuhlmann, Benjamin; Gmerek, Felix; Krügler, Daniel; Schmitt, Michael

    2014-08-01

    Single vibronic level fluorescence spectra of the electronic origin and of seven vibronic bands between 0,0 and 0,0 + 1265 cm-1 have been measured and analyzed by means of a combined Franck-Condon/rotational constants fit. The rotational constants in ground and lowest electronically excited singlet state of four different isotopologues have been taken from previous rotationally resolved measurements of Schmitt et al. (2006). The intensities of 182 vibronic emission bands and of 8 rotational constants have been used for a fit of the complete heavy atom geometry changes upon electronic excitation. Vibronic modes, about 1000 cm-1 above the electronic origin, show strong deviations from Franck-Condon behavior in emission. Herzberg-Teller coupling contributes to this effect. 1300 cm-1 above the origin, we observe the onset of intramolecular vibrational redistribution in the emission spectra.

  20. Antimatter signals of singlet scalar dark matter

    SciTech Connect

    Goudelis, A.; Mambrini, Y.; Yaguna, C. E-mail: yann.mambrini@th.u-psud.fr

    2009-12-01

    We consider the singlet scalar model of dark matter and study the expected antiproton and positron signals from dark matter annihilations. The regions of the viable parameter space of the model that are excluded by present data are determined, as well as those regions that will be probed by the forthcoming experiment AMS-02. In all cases, different propagation models are investigated, and the possible enhancement due to dark matter substructures is analyzed. We find that the antiproton signal is more easily detectable than the positron one over the whole parameter space. For a typical propagation model and without any boost factor, AMS-02 will be able to probe –via antiprotons– the singlet model of dark matter up to masses of 600 GeV. Antiprotons constitute, therefore, a promising signal to constraint or detect the singlet scalar model.

  1. Kinetics of oxygen species in an electrically driven singlet oxygen generator

    NASA Astrophysics Data System (ADS)

    Azyazov, V. N.; Torbin, A. P.; Pershin, A. A.; Mikheyev, P. A.; Heaven, M. C.

    2015-12-01

    The kinetics of oxygen species in the gaseous medium of a discharge singlet oxygen generator has been revisited. Vibrationally excited ozone O3(υ) formed in O + O2 recombination is thought to be a significant agent in the deactivation of singlet oxygen O2(a1Δ), oxygen atom removal and ozone formation. It is shown that the process O3(υ ⩾ 2) + O2(a1Δ) → 2O2 + O is the main O2(a1Δ) deactivation channel in the post-discharge zone. If no measures are taken to decrease the oxygen atom concentration, the contribution of this process to the overall O2(a1Δ) removal is significant, even in the discharge zone. A simplified model for the kinetics of vibrationally excited ozone is proposed. Calculations based on this model yield results that are in good agreement with the experimental data.

  2. Solution-based intramolecular singlet fission in cross-conjugated pentacene dimers.

    PubMed

    Zirzlmeier, Johannes; Casillas, Rubén; Reddy, S Rajagopala; Coto, Pedro B; Lehnherr, Dan; Chernick, Erin T; Papadopoulos, Ilias; Thoss, Michael; Tykwinski, Rik R; Guldi, Dirk M

    2016-05-21

    We show unambiguous and compelling evidence by means of pump-probe experiments, which are complemented by calculations using ab initio multireference perturbation theory, for intramolecular singlet fission (SF) within two synthetically tailored pentacene dimers with cross-conjugation, namely XC1 and XC2. The two pentacene dimers differ in terms of electronic interactions as evidenced by perturbation of the ground state absorption spectra stemming from stronger through-bond contributions in XC1 as confirmed by theory. Multiwavelength analysis, on one hand, and global analysis, on the other hand, confirm that the rapid singlet excited state decay and triplet excited state growth relate to SF. SF rate constants and quantum yields increase with solvent polarity. For example, XC2 reveals triplet quantum yields and rate constants as high as 162 ± 10% and (0.7 ± 0.1) × 10(12) s(-1), respectively, in room temperature solutions. PMID:27122097

  3. Effects of substituents on tetracene derivatives on their stabilities and singlet fission.

    PubMed

    Shen, Li; Chen, Yuhan; Li, Xiyou; Gao, Jun

    2014-06-01

    Tetracene is well known for its high singlet fission (SF) efficiency, which could be used to enhance the energy conversion efficiency in solar cells. However, its photoinstability toward oxygen must be improved before it can be used as a light-harvesting component. In this work, a series of substituted tetracenes were designed to identify tetracene compounds with not only good stability toward oxidation but also small SF activation energies. The stabilities were evaluated using the frontier molecular orbital energies, whereas the SF activation energies were deduced from the energy differences between the first excited singlet state and twice the first excited triplet state. Substitution strategies on tetracene are proposed for the purpose of simultaneously improving stability and reducing the SF activation energy. PMID:24863343

  4. Supramolecular nanoreactors for intracellular singlet-oxygen sensitization

    NASA Astrophysics Data System (ADS)

    Swaminathan, Subramani; Fowley, Colin; Thapaliya, Ek Raj; McCaughan, Bridgeen; Tang, Sicheng; Fraix, Aurore; Burjor, Captain; Sortino, Salvatore; Callan, John F.; Raymo, Françisco M.

    2015-08-01

    An amphiphilic polymer with multiple decyl and oligo(ethylene glycol) chains attached to a common poly(methacrylate) backbone assembles into nanoscaled particles in aqueous environments. Hydrophobic anthracene and borondipyrromethene (BODIPY) chromophores can be co-encapsulated within the self-assembling nanoparticles and transported across hydrophilic media. The reversible character of the noncovalent bonds, holding the supramolecular containers together, permits the exchange of their components with fast kinetics in aqueous solution. Incubation of cervical cancer (HeLA) cells with a mixture of two sets of nanoparticles, pre-loaded independently with anthracene or BODIPY chromophores, results in guest scrambling first and then transport of co-entrapped species to the intracellular space. Alternatively, incubation of cells with the two sets of nanocarriers in consecutive steps permits the sequential transport of the anthracene and BODIPY chromophores across the plasma membrane and only then allows their co-encapsulation within the same supramolecular containers. Both mechanisms position the two sets of chromophores with complementary spectral overlap in close proximity to enable the efficient transfer of energy intracellularly from the anthracene donors to the BODIPY acceptors. In the presence of iodine substituents on the BODIPY platform, intersystem crossing follows energy transfer. The resulting triplet state can transfer energy further to molecular oxygen with the concomitant production of singlet oxygen to induce cell mortality. Furthermore, the donor can be excited with two near-infrared photons simultaneously to permit the photoinduced generation of singlet oxygen intracellularly under illumination conditions compatible with applications in vivo. Thus, these supramolecular strategies to control the excitation dynamics of multichromophoric assemblies in the intracellular environment can evolve into valuable protocols for photodynamic therapy.An amphiphilic

  5. Production of Singlet Oxygen within a Flow Discharge

    NASA Astrophysics Data System (ADS)

    Lange, Matthew; Pitz, Greg; Perram, Glen

    2008-10-01

    The Airborne laser program is an Air Force sponsored program to place a laser on the battle field for use as a tactical weapon. The chemical oxygen iodine laser offers the powers necessary for this weapons application, but it requires significant logistical support. The goal of this current research program is to demonstrate an oxygen-iodine laser with electrical discharge production of singlet oxygen. Optical diagnostics have been applied to microwave and radio frequency discharges within a pure oxygen flow. The O2(a) emissions within a discharge are complicated by atomic oxygen emission requiring care in determining gas concentrations from optically measured emissions. Thermal effects also complicate optical emissions. The inclusion of vibrationally excited oxygen as a quencher of the O2(a) state appears to be the limiting rate for production of O2(a) within the electric discharge conditions studied in this research.

  6. On the low lying singlet states of BeO

    NASA Technical Reports Server (NTRS)

    Bauschlicher, C. W., Jr.; Lengsfield, B. H.; Yarkony, D. R.

    1980-01-01

    Calculations of the ground and low-lying singlet states of BeO are performed in order to gain an understanding of the techniques needed to treat the excited states of other, more complex, ionic molecules. The MCSCF and CI calculations are based on a Gaussian basis set of slightly better than double zeta plus polarization quality for single configuration descriptions of the states. The calculated X-A and X-B state separations are found to be in agreement with experimental measurements. The 1 Sigma - and 1 Delta states are predicted to lie approximately 40,000 kaysers above the ground state and are identified as the C and D states.The 2 1 Pi state is found to be approximately 15,000 kaysers and the 3 1 Sigma + state is found to be approximately 65,000 kaysers above the ground state.

  7. Reactions of singlet oxygen with pine pollen.

    NASA Technical Reports Server (NTRS)

    Dowty, B.; Laseter, J. L.; Griffin, G. W.; Politzer, I. R.; Walkinshaw, C. H.

    1973-01-01

    A study was initiated to determine whether viable atmospheric particles such as plant pollens and fungal spores containing unsaturated lipids can interact with singlet oxygen to give oxygenated products that are potentially toxic. The results obtained confirm that surface and near surface components of common viable particulate matter in the atmosphere may be subject to rapid oxidation by singlet oxygen, leading to products which are probably allylic hydroperoxides. In connection with increasing atmospheric pollution, it is important to note that materials toxic to mammalian lung tissue may be oxidatively produced on the surfaces of viable particulate matter.

  8. Supramolecular nanoreactors for intracellular singlet-oxygen sensitization.

    PubMed

    Swaminathan, Subramani; Fowley, Colin; Thapaliya, Ek Raj; McCaughan, Bridgeen; Tang, Sicheng; Fraix, Aurore; Captain, Burjor; Sortino, Salvatore; Callan, John F; Raymo, Françisco M

    2015-09-01

    An amphiphilic polymer with multiple decyl and oligo(ethylene glycol) chains attached to a common poly(methacrylate) backbone assembles into nanoscaled particles in aqueous environments. Hydrophobic anthracene and borondipyrromethene (BODIPY) chromophores can be co-encapsulated within the self-assembling nanoparticles and transported across hydrophilic media. The reversible character of the noncovalent bonds, holding the supramolecular containers together, permits the exchange of their components with fast kinetics in aqueous solution. Incubation of cervical cancer (HeLA) cells with a mixture of two sets of nanoparticles, pre-loaded independently with anthracene or BODIPY chromophores, results in guest scrambling first and then transport of co-entrapped species to the intracellular space. Alternatively, incubation of cells with the two sets of nanocarriers in consecutive steps permits the sequential transport of the anthracene and BODIPY chromophores across the plasma membrane and only then allows their co-encapsulation within the same supramolecular containers. Both mechanisms position the two sets of chromophores with complementary spectral overlap in close proximity to enable the efficient transfer of energy intracellularly from the anthracene donors to the BODIPY acceptors. In the presence of iodine substituents on the BODIPY platform, intersystem crossing follows energy transfer. The resulting triplet state can transfer energy further to molecular oxygen with the concomitant production of singlet oxygen to induce cell mortality. Furthermore, the donor can be excited with two near-infrared photons simultaneously to permit the photoinduced generation of singlet oxygen intracellularly under illumination conditions compatible with applications in vivo. Thus, these supramolecular strategies to control the excitation dynamics of multichromophoric assemblies in the intracellular environment can evolve into valuable protocols for photodynamic therapy. PMID:26238536

  9. Solution-based intramolecular singlet fission in cross-conjugated pentacene dimers

    NASA Astrophysics Data System (ADS)

    Zirzlmeier, Johannes; Casillas, Rubén; Reddy, S. Rajagopala; Coto, Pedro B.; Lehnherr, Dan; Chernick, Erin T.; Papadopoulos, Ilias; Thoss, Michael; Tykwinski, Rik R.; Guldi, Dirk M.

    2016-05-01

    We show unambiguous and compelling evidence by means of pump-probe experiments, which are complemented by calculations using ab initio multireference perturbation theory, for intramolecular singlet fission (SF) within two synthetically tailored pentacene dimers with cross-conjugation, namely XC1 and XC2. The two pentacene dimers differ in terms of electronic interactions as evidenced by perturbation of the ground state absorption spectra stemming from stronger through-bond contributions in XC1 as confirmed by theory. Multiwavelength analysis, on one hand, and global analysis, on the other hand, confirm that the rapid singlet excited state decay and triplet excited state growth relate to SF. SF rate constants and quantum yields increase with solvent polarity. For example, XC2 reveals triplet quantum yields and rate constants as high as 162 +/- 10% and (0.7 +/- 0.1) × 1012 s-1, respectively, in room temperature solutions.We show unambiguous and compelling evidence by means of pump-probe experiments, which are complemented by calculations using ab initio multireference perturbation theory, for intramolecular singlet fission (SF) within two synthetically tailored pentacene dimers with cross-conjugation, namely XC1 and XC2. The two pentacene dimers differ in terms of electronic interactions as evidenced by perturbation of the ground state absorption spectra stemming from stronger through-bond contributions in XC1 as confirmed by theory. Multiwavelength analysis, on one hand, and global analysis, on the other hand, confirm that the rapid singlet excited state decay and triplet excited state growth relate to SF. SF rate constants and quantum yields increase with solvent polarity. For example, XC2 reveals triplet quantum yields and rate constants as high as 162 +/- 10% and (0.7 +/- 0.1) × 1012 s-1, respectively, in room temperature solutions. Electronic supplementary information (ESI) available. See DOI: 10.1039/c6nr02493a

  10. Benzonitrile: Electron affinity, excited states, and anion solvation

    NASA Astrophysics Data System (ADS)

    Dixon, Andrew R.; Khuseynov, Dmitry; Sanov, Andrei

    2015-10-01

    We report a negative-ion photoelectron imaging study of benzonitrile and several of its hydrated, oxygenated, and homo-molecularly solvated cluster anions. The photodetachment from the unsolvated benzonitrile anion to the X ˜ 1 A 1 state of the neutral peaks at 58 ± 5 meV. This value is assigned as the vertical detachment energy (VDE) of the valence anion and the upper bound of adiabatic electron affinity (EA) of benzonitrile. The EA of the lowest excited electronic state of benzonitrile, a ˜ 3 A 1 , is determined as 3.41 ± 0.01 eV, corresponding to a 3.35 eV lower bound for the singlet-triplet splitting. The next excited state, the open-shell singlet A ˜ 1 A 1 , is found about an electron-volt above the triplet, with a VDE of 4.45 ± 0.01 eV. These results are in good agreement with ab initio calculations for neutral benzonitrile and its valence anion but do not preclude the existence of a dipole-bound state of similar energy and geometry. The step-wise and cumulative solvation energies of benzonitrile anions by several types of species were determined, including homo-molecular solvation by benzonitrile, hydration by 1-3 waters, oxygenation by 1-3 oxygen molecules, and mixed solvation by various combinations of O2, H2O, and benzonitrile. The plausible structures of the dimer anion of benzonitrile were examined using density functional theory and compared to the experimental observations. It is predicted that the dimer anion favors a stacked geometry capitalizing on the π-π interactions between the two partially charged benzonitrile moieties.

  11. Excited-state dynamics of pentacene derivatives with stable radical substituents.

    PubMed

    Ito, Akitaka; Shimizu, Akihiro; Kishida, Noriaki; Kawanaka, Yusuke; Kosumi, Daisuke; Hashimoto, Hideki; Teki, Yoshio

    2014-06-23

    The excited-state dynamics of pentacene derivatives with stable radical substituents were evaluated in detail through transient absorption measurements. The derivatives showed ultrafast formation of triplet excited state(s) in the pentacene moiety from a photoexcited singlet state through the contributions of enhanced intersystem crossing and singlet fission. Detailed kinetic analyses for the transient absorption data were conducted to quantify the excited-state characteristics of the derivatives. PMID:24788384

  12. Quantum dynamics study of singlet-triplet transitions in s-trans-1,3-butadiene

    NASA Astrophysics Data System (ADS)

    Nikoobakht, Behnam; Köppel, Horst

    2016-05-01

    The intersystem crossing dynamics of s-trans-1,3-butadiene in its lowest singlet and triplet states is studied theoretically, employing a fully quantal approach for the first time. The electronic states 21Ag, 11Bu, 13Bu and 13Ag, which interact vibronically and via the spin-orbit coupling are treated in the calculation, thus covering the lowest spin-forbidden electronic transitions. Up to five nuclear degrees of freedom, including out-of-plane dihedral angles are included in our investigation. The calculation of potential energy surfaces relies on the CASPT2 method, and the evaluation of spin-orbit coupling matrix elements using the full two-electron Breit-Pauli Hamiltonian is performed by utilizing the MRCI wavefunction. The latter dependence on the nuclear coordinates is included for the first time. An electronic population transfer on the sub-picosecond time scale due to intersystem crossing is obtained, a mechanism that can contribute to the singlet-triplet transitions in the electron energy loss spectrum of s-trans-1, 3-butadiene. It is found that the dependence of the spin-orbit coupling on the out-of-plane coordinates plays a dominant role in these singlet-triplet transitions. The amount of population transfer to the 13Ag and 13Bu states is roughly of the same order of magnitude.

  13. Time-Resolved Insight into the Photosensitized Generation of Singlet Oxygen in Endoperoxides

    PubMed Central

    2014-01-01

    A synergistic approach combining high-level multiconfigurational static calculations and full-dimensional ab initio surface hopping dynamics has been employed to gain insight into the photochemistry of endoperoxides. Electronic excitation of endoperoxides triggers two competing pathways, cycloreversion and O–O homolysis, that result in the generation of singlet oxygen and oxygen diradical rearrangement products. Our results reveal that cycloreversion or the rupture of the two C–O bonds occurs via an asynchronous mechanism that can lead to the population of a ground-state intermediate showing a single C–O bond. Furthermore, singlet oxygen is directly generated in its most stable excited electronic state 1Δg. The triplet states do not intervene in this mechanism, as opposed to the O–O homolysis where the exchange of population between the singlet and triplet manifolds is remarkable. In line with recent experiments performed on the larger anthracene-9,10-endoperoxide, upon excitation to the spectroscopic ππ* electronic states, the primary photoreactive pathway that governs deactivation of endoperoxides is O–O homolysis with a quantum yield of 65%. PMID:25688180

  14. Metal bacteriochlorins which act as dual singlet oxygen and superoxide generators.

    PubMed

    Fukuzumi, Shunichi; Ohkubo, Kei; Zheng, Xiang; Chen, Yihui; Pandey, Ravindra K; Zhan, Riqiang; Kadish, Karl M

    2008-03-01

    A series of stable free-base, Zn(II) and Pd(II) bacteriochlorins containing a fused six- or five-member diketo- or imide ring have been synthesized as good candidates for photodynamic therapy sensitizers, and their electrochemical, photophysical, and photochemical properties were examined. Photoexcitation of the palladium bacteriochlorin affords the triplet excited state without fluorescence emission, resulting in formation of singlet oxygen with a high quantum yield due to the heavy atom effect of palladium. Electrochemical studies revealed that the zinc bacteriochlorin has the smallest HOMO-LUMO gap of the investigated compounds, and this value is significantly lower than the triplet excited-state energy of the compound in benzonitrile. Such a small HOMO-LUMO gap of the zinc bacteriochlorin enables intermolecular photoinduced electron transfer from the triplet excited state to the ground state to produce both the radical cation and the radical anion. The radical anion thus produced can transfer an electron to molecular oxygen to produce superoxide anion which was detected by electron spin resonance. The same photosensitizer can also act as an efficient singlet oxygen generator. Thus, the same zinc bacteriochlorin can function as a sensitizer with a dual role in that it produces both singlet oxygen and superoxide anion in an aprotic solvent (benzonitrile). PMID:18254618

  15. The Predicted Spectrum and Singlet-Triplet Interaction of the Hypermetallic Molecule SrOSr

    NASA Astrophysics Data System (ADS)

    Ostojić, B.; Jensen, Per; Schwerdtfeger, P.; Bunker, P. R.

    2013-10-01

    In accordance with previous studies in our group on Be, Mg, and Ca hypermetallic oxides, we find that SrOSr has a linear X-1Σg+ ground electronic state and a very low lying first excited -3Σu+ triplet electronic state. No gas-phase spectrum of this molecule has been assigned yet, and to encourage and assist in its discovery we present a complete ab initio simulation, with absolute intensities, of the infrared absorption spectrum for both electronic states. The three-dimensional potential energy surfaces and the electric dipole moment surfaces of the X-1Σg+ and -3Σu+ electronic states are calculated using a multireference configuration interaction (MRCISD) approach in combination with internally contracted multireference perturbation theory (RS2C) based on complete active space self-consistent field (CASSCF) wave functions applying a Sadlej pVTZ basis set for both O and Sr and the Stuttgart relativistic small-core effective core potential for Sr. The infrared spectra are simulated using the MORBID program system. We also calculate vertical excitation energies and transition moments for several excited singlet and triplet electronic states in order to predict the positions and intensities of the most prominent singlet and triplet electronic absorption bands. Finally, for this heavy molecule, we calculate the singlet-triplet interaction matrix elements between close-lying vibronic levels of the X- and - electronic states and find them to be very small.

  16. Probing hard color singlet exchange at D Zero

    SciTech Connect

    Goussiou, A.

    1998-06-01

    We present latest preliminary results on hard color-singlet exchange in proton-antiproton collisions. The fraction of dijet events produced via color-singlet exchange is measured as a function of jet transverse energy, dijet pseudorapidity separation, and proton-antiproton center-of-mass energy. The results favour a color-singlet fraction that increases with increasing quark-initiated processes.

  17. Transient picosecond studies of singlet fission in PDTP-DFBT low band gap polymer

    NASA Astrophysics Data System (ADS)

    Huynh, Uyen; Vardeny, Valy; Li, Gang; Yang, Yang

    2014-03-01

    We measured picoseconds transient mid-IR photoinduced absorption (PA) spectra in PDTP-DFBT low band-gap polymer. With 800 nm pumping the PA spectrum at t =0 in pristine film and isolated polymer chain in polystyrene shows two prominent PA bands: PA1 at 0.4eV and Pa2 at 0.8eV. PA1 is assigned to absorption from singlet excitons (transition from 1Bu to mAg) , whereas PA2 is due to a state of triplet-pair, which is formed via singlet fission in the sub-ps time domain. We found that PA2 lifetime strongly depends on the excitation intensity, showing non linear recombination process in both pristine film and in polystyrene. We also found that the triplet-pair recombines with no trace of fusion back to the singlet exciton; we thus conclude that singlet fission is an exothermic process in this polymer. We therefore do not find any magnetic field effect on the transient dynamics of the triplet-pair within our experimental sensitivity (0.2%). Supported by DOE and NSF-MRSEC.

  18. Magnetic dipolar interaction between correlated triplets created by singlet fission in tetracene crystals.

    PubMed

    Wang, Rui; Zhang, Chunfeng; Zhang, Bo; Liu, Yunlong; Wang, Xiaoyong; Xiao, Min

    2015-01-01

    Singlet fission can potentially break the Shockley-Queisser efficiency limit in single-junction solar cells by splitting one photoexcited singlet exciton (S1) into two triplets (2T1) in organic semiconductors. A dark multiexciton state has been proposed as the intermediate connecting S1 to 2T1. However, the exact nature of this multiexciton state, especially how the doubly excited triplets interact, remains elusive. Here we report a quantitative study on the magnetic dipolar interaction between singlet-fission-induced correlated triplets in tetracene crystals by monitoring quantum beats relevant to the multiexciton sublevels at room temperature. The resonances of multiexciton sublevels approached by tuning an external magnetic field are observed to be avoided, which agrees well with the theoretical predictions considering a magnetic dipolar interaction of ∼ 0.008 GHz. Our work quantifies the magnetic dipolar interaction in certain organic materials and marks an important step towards understanding the underlying physics of the multiexciton state in singlet fission. PMID:26456368

  19. Generation of singlet oxygen in fullerene-containing media: 2. Fullerene-containing solutions

    SciTech Connect

    Bagrov, I V; Belousova, I M; Grenishin, A S; Danilov, O B; Ermakov, A V; Kiselev, V M; Kislyakov, I M; Murav'eva, T D; Sosnov, E N

    2008-03-31

    The generation of singlet oxygen in fullerene solutions is studied by luminescence methods upon excitation by pulsed, repetitively pulsed, and continuous radiation sources. The concentration of singlet oxygen in solutions is measured in stationary and pulsed irradiation regimes. The rate constants of quenching of O{sub 2}({sup 1}{delta}{sub g}) by fullerenes C{sub 70} and C{sub 60} in the CCl{sub 4} solution are measured to be (7.2{+-}0.1)x10{sup 7} L mol{sup -1} s{sup -1} and less than 6x10{sup 4} L mol{sup -1} s{sup -1}, respectively. The temperature and photolytic variations in the generation properties of the fullerene solution exposed to intense continuous radiation are studied by the methods of optical and EPR spectroscopy. Pulsed irradiation resulted in the production of singlet oxygen in suspensions of fullerene-like structures, in particular, astralenes. A liquid pulsed singlet-oxygen generator based on the fullerene solution in CCl{sub 4} is developed and studied, in which the yield of O{sub 2} ({sup 1}{delta}{sub g}) to the gas phase at concentrations up to 5x10{sup 16} cm{sup -3} is obtained. (laser applications and other topics in quantum electronics)

  20. Graphene oxide functionalized with methylene blue and its performance in singlet oxygen generation

    SciTech Connect

    Wojtoniszak, M.; Rogińska, D.; Machaliński, B.; Drozdzik, M.; Mijowska, E.

    2013-07-15

    Graphical abstract: - Highlights: • Adsorption of methylene blue (MB) on graphene oxide (GO). • Characterization of graphene oxide–methylene blue nanocomposite (MB–GO). • Examination of MB–GO efficiency in singlet oxygen generation (SOG). • MB–GO performs higher SOG efficiency than pristine MB. - Abstract: Due to unique electronic, mechanical, optical and structural properties, graphene has shown promising applications in many fields, including biomedicine. One of them is noninvasive anticancer therapy – photodynamic therapy (PDT), where singlet oxygen (SO), generated under the irradiation of light with appropriate wavelengths, kills cancer cells. In this study, authors report graphene oxide (GO) noncovalent functionalization with methylene blue (MB). MB molecules underwent adsorption on the surface of GO. Detailed characterization of the obtained material was carried out with UV–vis spectroscopy, Raman spectroscopy, FT-IR spectroscopy, and confocal laser scanning microscopy. Furthermore, its performance in singlet oxygen generation (SOG) under irradiation of laser with excitation wavelengths of 785 nm was investigated. Interestingly, GO functionalized with MB (MB–GO) showed enhanced efficiency in singlet oxygen generation compared to pristine MB. The efficiency in SOG was detected by photobleaching of 9,10-anthracenediyl-bis(methylene)dimalonic acid (ABMDMA). These results indicate the material is promising in PDT anticancer therapy and further in vitro and in vivo studies are required.

  1. Magnetic dipolar interaction between correlated triplets created by singlet fission in tetracene crystals

    PubMed Central

    Wang, Rui; Zhang, Chunfeng; Zhang, Bo; Liu, Yunlong; Wang, Xiaoyong; Xiao, Min

    2015-01-01

    Singlet fission can potentially break the Shockley–Queisser efficiency limit in single-junction solar cells by splitting one photoexcited singlet exciton (S1) into two triplets (2T1) in organic semiconductors. A dark multiexciton state has been proposed as the intermediate connecting S1 to 2T1. However, the exact nature of this multiexciton state, especially how the doubly excited triplets interact, remains elusive. Here we report a quantitative study on the magnetic dipolar interaction between singlet-fission-induced correlated triplets in tetracene crystals by monitoring quantum beats relevant to the multiexciton sublevels at room temperature. The resonances of multiexciton sublevels approached by tuning an external magnetic field are observed to be avoided, which agrees well with the theoretical predictions considering a magnetic dipolar interaction of ∼0.008 GHz. Our work quantifies the magnetic dipolar interaction in certain organic materials and marks an important step towards understanding the underlying physics of the multiexciton state in singlet fission. PMID:26456368

  2. Theoretical studies toward understanding the excited state dynamics of a bichromophoric molecule

    NASA Astrophysics Data System (ADS)

    Gao, Fang; Zhao, Yi; Liang, WanZhen

    2007-06-01

    By means of the time-dependent density functional theory, the authors study the torsional dynamics of the lowest singlet electronic excited state (S1) of a bichromophoric molecule, 2-(9-anthryl)-1H-imidazo [4,5-f]-phenanthroline (AIP). The intramolecular dynamical relaxation process, the S1 potential energy surface, and the vibrationally resolved electronic absorption and fluorescence spectra are estimated. The results reveal that the strong electron-nuclear coupling leads to a dynamic structural distortion in S1 state so that the mirror-image symmetry of absorption and fluorescence spectra of AIP breaks down. The torsional motion between the donor and acceptor moieties in AIP favors the intramolecular electronic energy transfer process. The transfer rate is dominated by the relaxation time along S1 low-frequency torsional motion.

  3. Excited-state intramolecular proton transfer: photoswitching in salicylidene methylamine derivatives.

    PubMed

    Jankowska, Joanna; Rode, Michał F; Sadlej, Joanna; Sobolewski, Andrzej L

    2014-06-01

    The effect of chemical substitutions on the photophysical properties of the salicylidene methylamine molecule (SMA) (J. Jankowska, M. F. Rode, J. Sadlej, A. L. Sobolewski, ChemPhysChem, 2012, 13, 4287-4294) is studied with the aid of ab initio electronic structure methods. It is shown that combining π-electron-donating and π-electron-withdrawing substituents results in an electron-density push-and-pull effect on the energetic landscape of the ground and the lowest excited ππ* and nπ* singlet states of the system. The presented search for the most appropriate SMA derivatives with respect to their photoswitching functionality offers an efficient prescreening tool for finding chemical structures before real synthetic realization. PMID:24782113

  4. Energy-dependent branching between fluorescence and singlet exciton dissociation in sexithienyl thin films

    NASA Astrophysics Data System (ADS)

    Dippel, O.; Brandl, V.; Bässler, H.; Danieli, R.; Zamboni, R.; Taliani, C.

    1993-12-01

    The fluorescence yield of thin films of sexithienyl drops rapidly above the S 1←S 0 absorption edge while the yield of photocarrier generation increases simultaneously. This unusual behavior of a molecular solid is interpreted in terms of an energy-dependent branching between fluorescence and dissociation of a singlet excitation into a weakly bound electron—hole pair. This is shown to be a characteristic feature of a disordered system in which the energy levels of both neutral and charged excitations are subject to inhomogeneous broadening. In T6 the latter arises from torsional displacement of the thienylene moities.

  5. Probing ground and low-lying excited states for HIO2 isomers

    NASA Astrophysics Data System (ADS)

    de Souza, Gabriel L. C.; Brown, Alex

    2014-12-01

    We present a computational study on HIO2 molecules. Ground state properties such as equilibrium structures, relative energetics, vibrational frequencies, and infrared intensities were obtained for all the isomers at the coupled-cluster with single and double excitations as well as perturbative inclusion of triples (CCSD(T)) level of theory with the aug-cc-pVTZ-PP basis set and ECP-28-PP effective core potential for iodine and the aug-cc-pVTZ basis set for hydrogen and oxygen atoms. The HOIO structure is confirmed as the lowest energy isomer. The relative energies are shown to be HOIO < HOOI < HI(O)O. The HO(O)I isomer is only stable at the density functional theory (DFT) level of theory. The transition states determined show interconversion of the isomers is possible. In order to facilitate future experimental identification, vibrational frequencies are also determined for all corresponding deuterated species. Vertical excitation energies for the three lowest-lying singlet and triplet excited states were determined using the configuration interaction singles, time-dependent density functional theory (TD-DFT)/B3LYP, TD-DFT/G96PW91, and equation of motion-CCSD approaches with the LANL2DZ basis set plus effective core potential for iodine and the aug-cc-pVTZ basis set for hydrogen and oxygen atoms. It is shown that HOIO and HOOI isomers have excited states accessible at solar wavelengths (<4.0 eV) but these states have very small oscillator strengths (<2 × 10-3).

  6. Probing ground and low-lying excited states for HIO2 isomers.

    PubMed

    de Souza, Gabriel L C; Brown, Alex

    2014-12-21

    We present a computational study on HIO2 molecules. Ground state properties such as equilibrium structures, relative energetics, vibrational frequencies, and infrared intensities were obtained for all the isomers at the coupled-cluster with single and double excitations as well as perturbative inclusion of triples (CCSD(T)) level of theory with the aug-cc-pVTZ-PP basis set and ECP-28-PP effective core potential for iodine and the aug-cc-pVTZ basis set for hydrogen and oxygen atoms. The HOIO structure is confirmed as the lowest energy isomer. The relative energies are shown to be HOIO < HOOI < HI(O)O. The HO(O)I isomer is only stable at the density functional theory (DFT) level of theory. The transition states determined show interconversion of the isomers is possible. In order to facilitate future experimental identification, vibrational frequencies are also determined for all corresponding deuterated species. Vertical excitation energies for the three lowest-lying singlet and triplet excited states were determined using the configuration interaction singles, time-dependent density functional theory (TD-DFT)/B3LYP, TD-DFT/G96PW91, and equation of motion-CCSD approaches with the LANL2DZ basis set plus effective core potential for iodine and the aug-cc-pVTZ basis set for hydrogen and oxygen atoms. It is shown that HOIO and HOOI isomers have excited states accessible at solar wavelengths (<4.0 eV) but these states have very small oscillator strengths (<2 × 10(-3)). PMID:25527931

  7. Probing ground and low-lying excited states for HIO{sub 2} isomers

    SciTech Connect

    Souza, Gabriel L. C. de; Brown, Alex

    2014-12-21

    We present a computational study on HIO{sub 2} molecules. Ground state properties such as equilibrium structures, relative energetics, vibrational frequencies, and infrared intensities were obtained for all the isomers at the coupled-cluster with single and double excitations as well as perturbative inclusion of triples (CCSD(T)) level of theory with the aug-cc-pVTZ-PP basis set and ECP-28-PP effective core potential for iodine and the aug-cc-pVTZ basis set for hydrogen and oxygen atoms. The HOIO structure is confirmed as the lowest energy isomer. The relative energies are shown to be HOIO < HOOI < HI(O)O. The HO(O)I isomer is only stable at the density functional theory (DFT) level of theory. The transition states determined show interconversion of the isomers is possible. In order to facilitate future experimental identification, vibrational frequencies are also determined for all corresponding deuterated species. Vertical excitation energies for the three lowest-lying singlet and triplet excited states were determined using the configuration interaction singles, time-dependent density functional theory (TD-DFT)/B3LYP, TD-DFT/G96PW91, and equation of motion-CCSD approaches with the LANL2DZ basis set plus effective core potential for iodine and the aug-cc-pVTZ basis set for hydrogen and oxygen atoms. It is shown that HOIO and HOOI isomers have excited states accessible at solar wavelengths (<4.0 eV) but these states have very small oscillator strengths (<2 × 10{sup −3})

  8. Hydroxy-plastochromanol and plastoquinone-C as singlet oxygen products during photo-oxidative stress in Arabidopsis.

    PubMed

    Szymańska, Renata; Nowicka, Beatrycze; Kruk, Jerzy

    2014-06-01

    In the present study, we have shown that hydroxy-plastochromanol and plastoquinone-C, the hydroxy derivatives of plastochromanol and plastoquinone-9, respectively, are specifically formed from the parent compounds upon action of singlet oxygen and can be regarded as stable, specific, natural products of singlet oxygen action during photo-oxidative stress in vivo. The presented data indicate that plastoquinone-C formation dominates mainly during relatively short periods of high light stress where efficient production of singlet oxygen takes place, whereas hydroxy-plastochromanol is rather formed under conditions of long-term, less pronounced generation of singlet oxygen. An interesting observation was that hydroxy-plastochromanol is formed even at very low light conditions (5-10 μmol photons m(-2) s(-1)), indicating that singlet oxygen is generated not only during high light stress but also its formation by photosystem II is inseparably connected with the functioning of this photosystem even at the lowest light intensities. PMID:24329808

  9. Efficient up-conversion of triplet excitons into a singlet state and its application for organic light emitting diodes

    NASA Astrophysics Data System (ADS)

    Endo, Ayataka; Sato, Keigo; Yoshimura, Kazuaki; Kai, Takahiro; Kawada, Atsushi; Miyazaki, Hiroshi; Adachi, Chihaya

    2011-02-01

    A material possessing a very small energy gap between its singlet and triplet excited states, ΔE1-3, which allows efficient up-conversion of triplet excitons into a singlet state and leads to efficient thermally activated delayed fluorescence (TADF), is reported. The compound, 2-biphenyl-4,6-bis(12-phenylindolo[2,3-a] carbazole-11-yl)-1,3,5-triazine, breaks the restriction of a large energy gap, with a ΔE1-3 of just 0.11 eV, while maintaining a high fluorescent radiative decay rate (kr˜107). The intense TADF provides a pathway for highly efficient electroluminescence.

  10. Captodatively stabilized biradicaloids as chromophores for singlet fission.

    PubMed

    Wen, Jin; Havlas, Zdenĕk; Michl, Josef

    2015-01-14

    Singlet fission offers an opportunity to improve solar cell efficiency, but its practical use is hindered by the limited number of known efficient materials. We look for chromophores that satisfy the desirable but rarely encountered adiabatic energy conditions, E(T2) - E(S0) > E(S1) - E(S0) ≈ 2[E(T1) - E(S0)], and are small enough to permit highly accurate calculations. We provide a rationale for the use of captodative biradicaloids, i.e., biradicals stabilized by direct interaction between their radical centers, which carry both an acceptor and a donor group. A computation of vertical excitation energies of 14 structures of this type by time-dependent density functional theory (TD-DFT) yielded 11 promising candidates. The vertical excitation energies from S0 and T1 were recalculated by complete-active-space second-order perturbation theory (CASPT2), and five of the compounds met the above energy criteria. Their adiabatic excitation energies from the S0 into the S1, S2, T1, and T2 excited states were subsequently calculated, and three of them look promising. For 2,3-diamino-1,4-benzoquinone, adiabatic E(T1) and E(S1) energies were close to optimal (1.12 and 2.23 eV above the S0 ground state, respectively), and for its more practical N-peralkylated derivative they were even lower (0.63 and 1.06 eV above S0, respectively). PCM/CASPT2 results suggested that the relative energies can be further tuned by varying the polarity of the environment. PMID:25478747

  11. Nonlinear Density Dependence of Singlet Fission Rate in Tetracene Films.

    PubMed

    Zhang, Bo; Zhang, Chunfeng; Wang, Rui; Tan, Zhanao; Liu, Yunlong; Guo, Wei; Zhai, Xiaoling; Cao, Yi; Wang, Xiaoyong; Xiao, Min

    2014-10-16

    Singlet fission holds the potential to dramatically improve the efficiency of solar energy conversion by creating two triplet excitons from one photoexcited singlet exciton in organic semiconductors. It is generally assumed that the singlet-fission rate is linearly dependent on the exciton density. Here we experimentally show that the rate of singlet fission has a nonlinear dependence on the density of photoexcited singlet excitons in tetracene films with small crystalline grains. We disentangle the spectrotemporal features of singlet and triplet dynamics from ultrafast spectroscopic data with the algorithm of singular value decomposition. The correlation between their temporal dynamics indicates a superlinear dependence of fission rate on the density of singlet excitons, which may arise from excitonic interactions. PMID:26278594

  12. Singlet molecular oxygen generated in dark biological process.

    PubMed

    Di Mascio, Paolo; Medeiros, Marisa H G

    2014-10-01

    Ultraweak chemiluminescence arising from biomolecules oxidation has been attributed to the radiative deactivation of singlet molecular oxygen [(1)O2] and electronically excited triplet carbonyl products involving dioxetane intermediates. As examples, we will discuss the generation of (1)O2 from lipid hydroperoxides, which involves a cyclic mechanism from a linear tetraoxide intermediate. The generation of (1)O2 in aqueous solution via energy transfer from the excited triplet acetone arising from the thermodecomposition of dioxetane a chemical source, and horseradish peroxidase-catalyzed oxidation of 2-methylpropanal, as an enzymatic source, will also be discussed. The approach used to unequivocally demonstrate the generation of (1)O2 in these reactions is the use of (18)O-labeled hydroperoxide / triplet dioxygen ((18)[(3)O2]), the detection of labeled compounds by HPLC coupled to tandem mass spectrometry (HPLC-MS/MS) and the direct spectroscopic detection and characterization of (1)O2 light emission. Characteristic light emission at 1,270nm, corresponding to the singlet delta state monomolecular decay was observed. Using(18)[(3)O2], we observed the formation of (18)O-labeled (1)O2 ((18)[(1)O2]) by the chemical trapping of (18)[(1)O2]with the anthracene-9,10-diyldiethane-2,1-diyl disulfate disodium salt (EAS) and detected the corresponding (18)O-labeled EAS endoperoxide usingHPLC-MS/MS. The combined use of the thermolysis of a water-soluble naphthalene endoperoxide as a generator of (18)O labeled (1)O2 and the sensitivity of HPLC-MS/MS allowed the study of (1)O2reactivity toward biomolecules. Photoemission properties and chemical trapping clearly demonstrate that the production of hydroperoxide and excited carbonyls generates (18)[(1)O2], and points to the involvement of (1)O2 in physiological and pathophysiological mechanism. Supported by FAPESP (2012/12663-1), CAPES, INCT Redoxoma (FAPESP/CNPq/CAPES; 573530/2008-4), NAP Redoxoma (PRPUSP; 2011.1.9352.1.8), CEPID

  13. Quantitative Intramolecular Singlet Fission in Bipentacenes.

    PubMed

    Sanders, Samuel N; Kumarasamy, Elango; Pun, Andrew B; Trinh, M Tuan; Choi, Bonnie; Xia, Jianlong; Taffet, Elliot J; Low, Jonathan Z; Miller, John R; Roy, Xavier; Zhu, X-Y; Steigerwald, Michael L; Sfeir, Matthew Y; Campos, Luis M

    2015-07-22

    Singlet fission (SF) has the potential to significantly enhance the photocurrent in single-junction solar cells and thus raise the power conversion efficiency from the Shockley-Queisser limit of 33% to 44%. Until now, quantitative SF yield at room temperature has been observed only in crystalline solids or aggregates of oligoacenes. Here, we employ transient absorption spectroscopy, ultrafast photoluminescence spectroscopy, and triplet photosensitization to demonstrate intramolecular singlet fission (iSF) with triplet yields approaching 200% per absorbed photon in a series of bipentacenes. Crucially, in dilute solution of these systems, SF does not depend on intermolecular interactions. Instead, SF is an intrinsic property of the molecules, with both the fission rate and resulting triplet lifetime determined by the degree of electronic coupling between covalently linked pentacene molecules. We found that the triplet pair lifetime can be as short as 0.5 ns but can be extended up to 270 ns. PMID:26102432

  14. Singlet fission: Towards efficient solar cells

    NASA Astrophysics Data System (ADS)

    Havlas, Zdeněk; Wen, Jin; Michl, Josef

    2015-12-01

    Singlet fission (SF) offers an opportunity to improve solar cell efficiency, but its practical use is hindered by the limited number of known efficient materials, limited knowledge of SF mechanism, mainly the relation between the dimer structure and SF efficiency and diffusion of the triplet states allowing injection of electrons into the solar cell semiconductor band. Here we report on our attempt to design new classes of chromophores and to study the relation between the structure and SF efficiency.

  15. Singlet fission: Towards efficient solar cells

    SciTech Connect

    Havlas, Zdeněk; Wen, Jin; Michl, Josef

    2015-12-31

    Singlet fission (SF) offers an opportunity to improve solar cell efficiency, but its practical use is hindered by the limited number of known efficient materials, limited knowledge of SF mechanism, mainly the relation between the dimer structure and SF efficiency and diffusion of the triplet states allowing injection of electrons into the solar cell semiconductor band. Here we report on our attempt to design new classes of chromophores and to study the relation between the structure and SF efficiency.

  16. Stark effect of interfering electronic states: Localization of the nπ* excitations in toluquinone

    NASA Astrophysics Data System (ADS)

    Galaup, J. P.; Trommsdorff, H. P.

    1984-04-01

    High-precision Stark measurements on oriented single crystals of toluquinone at low temperatures have been performed and lead to an assessment of the electronic parenthood of the levels giving rise to the complex spectral region of interference between the two nearby nπ* excited states. The origin bands of the lowest excited singlet and triplet states are characterized by a measure of their factor-group splittings and an evaluation of the change in dipole moment and in polarizability upon excitation. The value of the change in dipole moment is shown to vary strongly between different vibrational levels of the lower state and an evaluation of the degree of localization of the electronic excitation on one CO group is made. The previous assignment of the second nπ* state is confirmed by the sign of the corresponding Stark shift. From measurements on crystals having been oriented in an electric field the absolute orientation of the polar crystal as well as the sign of the pyroelectric coefficient are proposed.

  17. Singlet-stabilized minimal gauge mediation

    SciTech Connect

    Curtin, David; Tsai, Yuhsin

    2011-04-01

    We propose singlet-stabilized minimal gauge mediation as a simple Intriligator, Seiberg and Shih-based model of direct gauge mediation which avoids both light gauginos and Landau poles. The hidden sector is a massive s-confining supersymmetric QCD that is distinguished by a minimal SU(5) flavor group. The uplifted vacuum is stabilized by coupling the meson to an additional singlet sector with its own U(1) gauge symmetry via nonrenormalizable interactions suppressed by a higher scale {Lambda}{sub UV} in the electric theory. This generates a nonzero vacuum expectation value for the singlet meson via the inverted hierarchy mechanism, but requires tuning to a precision {approx}({Lambda}/{Lambda}{sub UV}){sup 2}, which is {approx}10{sup -4}. In the course of this analysis we also outline some simple model-building rules for stabilizing uplifted-ISS models, which lead us to conclude that meson deformations are required (or at least heavily favored) to stabilize the adjoint component of the magnetic meson.

  18. Statistical equilibrium in cometary C2. III - Triplet-singlet, Phillips, Ballik-Ramsay, and Mulliken bands

    NASA Technical Reports Server (NTRS)

    Swamy, K. S. K.; Odell, C. R.

    1981-01-01

    A new series of vibrational bands is predicted which arises from transitions between the lowest electron states of the triplet and singlet states of homonuclear C2. The predictions are useful for C2 identifications and for disentangling congested low resolution spectra that characterize new observations. Predictions are also made for Mulliken, Phillips, and Ballik-Ramsay band sequences, and all calculations are made with the assumed molecular constants and computational methods of KSO-II.

  19. Acceleration of Singlet Fission in an Aza-Derivative of TIPS-Pentacene.

    PubMed

    Herz, Julia; Buckup, Tiago; Paulus, Fabian; Engelhart, Jens; Bunz, Uwe H F; Motzkus, Marcus

    2014-07-17

    The influence of the carbon to nitrogen substitution on the photoinduced dynamics of TIPS-pentacene was investigated by ultrafast transient absorption measurements on spin-coated thin films in the visible and in the near-infrared spectral region. A global target analysis was performed to provide a detailed picture of the excited-state dynamics. We found that the chemical modification has a high impact on the triplet formation and leads to shorter dynamics; hence it speeds up the singlet fission process. A faster relaxation from the singlet into the triplet manifold implies a higher efficiency because other relaxation channels are avoided. The air-stable aza-derivatives have the potential to exceed the energy conversion efficiency of TIPS-pentacene. PMID:26277810

  20. Excitonic singlet-triplet ratios in molecular and polymeric organic materials

    NASA Astrophysics Data System (ADS)

    Baldo, Marc; Agashe, Shashank; Forrest, Stephen

    2002-03-01

    A simple technique is described for the determination of the internal efficiency and excitonic singlet-triplet formation statistics of electroluminescent organic thin films. The internal efficiency is measured by optically exciting a luminescent film within an electroluminescent device under reverse bias. This gives minimum singlet fractions of (0.20+/-0.03) and (0.19+/-0.04) for tris(8-hydroxyquinoline) aluminum (Alq3) and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), respectively. These results are discussed in terms of the current understanding of exciton formation within organic materials. We also present measurements of the out-coupling fraction, or the fraction of photons emitted in the forward direction, as a function of the position of the emitting layer within a microcavity.

  1. Observing the multiexciton state in singlet fission and ensuing ultrafast multielectron transfer.

    PubMed

    Chan, Wai-Lun; Ligges, Manuel; Jailaubekov, Askat; Kaake, Loren; Miaja-Avila, Luis; Zhu, X-Y

    2011-12-16

    Multiple exciton generation (MEG) refers to the creation of two or more electron-hole pairs from the absorption of one photon. Although MEG holds great promise, it has proven challenging to implement, and questions remain about the underlying photo-physical dynamics in nanocrystalline as well as molecular media. Using the model system of pentacene/fullerene bilayers and femtosecond nonlinear spectroscopies, we directly observed the multiexciton (ME) state ensuing from singlet fission (a molecular manifestation of MEG) in pentacene. The data suggest that the state exists in coherent superposition with the singlet populated by optical excitation. We also found that multiple electron transfer from the ME state to the fullerene occurs on a subpicosecond time scale, which is one order of magnitude faster than that from the triplet exciton state. PMID:22174249

  2. Photophysical Properties and Singlet Oxygen Generation Efficiencies of Water-Soluble Fullerene Nanoparticles

    PubMed Central

    Stasheuski, Alexander S; Galievsky, Victor A; Stupak, Alexander P; Dzhagarov, Boris M; Choi, Mi Jin; Chung, Bong Hyun; Jeong, Jin Young

    2014-01-01

    As various fullerene derivatives have been developed, it is necessary to explore their photophysical properties for potential use in photoelectronics and medicine. Here, we address the photophysical properties of newly synthesized water-soluble fullerene-based nanoparticles and polyhydroxylated fullerene as a representative water-soluble fullerene derivative. They show broad emission band arising from a wide-range of excitation energies. It is attributed to the optical transitions from disorder-induced states, which decay in the nanosecond time range. We determine the kinetic properties of the singlet oxygen (1O2) luminescence generated by the fullerene nanoparticles and polyhydroxylated fullerene to consider the potential as photodynamic agents. Triplet state decay of the nanoparticles was longer than 1O2 lifetime in water. Singlet oxygen quantum yield of a series of the fullerene nanoparticles is comparably higher ranging from 0.15 to 0.2 than that of polyhydroxylated fullerene, which is about 0.06. PMID:24893622

  3. Effects of Ferromagnetic Nanowires on Singlet and Triplet Exciton Fractions in Fluorescent and Phosphorescent Organic Semiconductors

    SciTech Connect

    Hu, Bin; Wu, Yue; Zhang, Zongtao; Dai, Sheng; Shen, Jian

    2006-01-01

    We report a magnetic field-dependent electroluminescence (EL) induced by ferromagnetic Co{sub 53}Pt{sub 47} nanowires in fluorescent conjugated polymer poly[2-methoxy-5-(2{prime}-ethylhexyloxy)-1,4-phenylenevinylene] and phosphorescent iridium-complex Ir(ppy){sub 3} molecules. The photoluminescence and EL studies indicate that the dispersed CoPt nanowires increase the singlet-to-triplet exciton ratio in organic semiconductors, suggesting that the spin-polarized holes were injected into the organic molecules from the CoPt nanowires under electrical excitation. Therefore, the use of ferromagnetic nanomaterials demonstrates a pathway to tune the optoelectronic properties that are related to singlet and triplet states in organic semiconducting materials.

  4. Mechanism of singlet oxygen deactivation in an electric discharge oxygen – iodine laser

    SciTech Connect

    Azyazov, V N; Mikheyev, P A; Torbin, A P; Pershin, A A; Heaven, M C

    2014-12-31

    We have determined the influence of the reaction of molecular singlet oxygen with a vibrationally excited ozone molecule O{sub 2}(a {sup 1}Δ) + O{sub 3}(ν) → 2O{sub 2} + O on the removal rate of O{sub 2}(a {sup 1}Δ) in an electric-discharge-driven oxygen – iodine laser. This reaction has been shown to be a major channel of O{sub 2}(a {sup 1}Δ) loss at the output of an electric-discharge singlet oxygen generator. In addition, it can also contribute significantly to the loss of O{sub 2}(a {sup 1}Δ) in the discharge region of the generator. (lasers)

  5. Does interchain stacking morphology contribute to the singlet-triplet interconversion dynamics in polymer heterojunctions?

    NASA Astrophysics Data System (ADS)

    Bittner, Eric R.; Burghardt, Irene; Friend, Richard H.

    2009-02-01

    Time-dependent density functional theory (TD-DFT) is used to examine the effect of stacking in a model semiconducting polymer hetrojunction system consisting of two co-facially stacked oligomers. We find that the excited electronic states are highly sensitive to the alignment of the monomer units of the two chains. In the system we examined, the exchange energy is nearly identical to both the and band off-set at the heterojunction and to the exciton binding energy. Our results indicate that the triplet excitonic states are nearly degenerate with the singlet exciplex states opening the possibility for the interconversion of singlet and triplet electronic states at the heterojunction interface via spin-orbit coupling localized on the heteroatoms. Using Russell-Saunders theory, we estimate this interconversion rate to be approximately 700-800 ps, roughly a 5-10-fold increase compared to isolated organic polymer chains.

  6. Carotenoid Excited State Kinetics in Bacterial RCs with the Primary Electron Donor Oxidized

    NASA Astrophysics Data System (ADS)

    Lin, Su; Katilius, Evaldas; Woodbury, Neal W.

    Carotenoid singlet excited state kinetics in wild type reaction centers from Rhodobacter sphaeroides was investigated using ultrafast laser spectroscopy under conditions where the primary electron donor is either neutral or oxidized.

  7. Real-time observation of multiexcitonic states in ultrafast singlet fission using coherent 2D electronic spectroscopy.

    PubMed

    Bakulin, Artem A; Morgan, Sarah E; Kehoe, Tom B; Wilson, Mark W B; Chin, Alex W; Zigmantas, Donatas; Egorova, Dassia; Rao, Akshay

    2016-01-01

    Singlet fission is the spin-allowed conversion of a spin-singlet exciton into a pair of spin-triplet excitons residing on neighbouring molecules. To rationalize this phenomenon, a multiexcitonic spin-zero triplet-pair state has been hypothesized as an intermediate in singlet fission. However, the nature of the intermediate states and the underlying mechanism of ultrafast fission have not been elucidated experimentally. Here, we study a series of pentacene derivatives using ultrafast two-dimensional electronic spectroscopy and unravel the origin of the states involved in fission. Our data reveal the crucial role of vibrational degrees of freedom coupled to electronic excitations that facilitate the mixing of multiexcitonic states with singlet excitons. The resulting manifold of vibronic states drives sub-100 fs fission with unity efficiency. Our results provide a framework for understanding singlet fission and show how the formation of vibronic manifolds with a high density of states facilitates fast and efficient electronic processes in molecular systems. PMID:26673260

  8. Real-time observation of multiexcitonic states in ultrafast singlet fission using coherent 2D electronic spectroscopy

    NASA Astrophysics Data System (ADS)

    Bakulin, Artem A.; Morgan, Sarah E.; Kehoe, Tom B.; Wilson, Mark W. B.; Chin, Alex W.; Zigmantas, Donatas; Egorova, Dassia; Rao, Akshay

    2016-01-01

    Singlet fission is the spin-allowed conversion of a spin-singlet exciton into a pair of spin-triplet excitons residing on neighbouring molecules. To rationalize this phenomenon, a multiexcitonic spin-zero triplet-pair state has been hypothesized as an intermediate in singlet fission. However, the nature of the intermediate states and the underlying mechanism of ultrafast fission have not been elucidated experimentally. Here, we study a series of pentacene derivatives using ultrafast two-dimensional electronic spectroscopy and unravel the origin of the states involved in fission. Our data reveal the crucial role of vibrational degrees of freedom coupled to electronic excitations that facilitate the mixing of multiexcitonic states with singlet excitons. The resulting manifold of vibronic states drives sub-100 fs fission with unity efficiency. Our results provide a framework for understanding singlet fission and show how the formation of vibronic manifolds with a high density of states facilitates fast and efficient electronic processes in molecular systems.

  9. Intracellular singlet oxygen photosensitizers: on the road to solving the problems of sensitizer degradation, bleaching and relocalization.

    PubMed

    da Silva, Elsa F F; Pimenta, Frederico M; Pedersen, Brian W; Blaikie, Frances H; Bosio, Gabriela N; Breitenbach, Thomas; Westberg, Michael; Bregnhøj, Mikkel; Etzerodt, Michael; Arnaut, Luis G; Ogilby, Peter R

    2016-02-01

    Selected singlet oxygen photosensitizers have been examined from the perspective of obtaining a molecule that is sufficiently stable under conditions currently employed to study singlet oxygen behavior in single mammalian cells. Reasonable predictions about intracellular sensitizer stability can be made based on solution phase experiments that approximate the intracellular environment (e.g., solutions containing proteins). Nevertheless, attempts to construct a stable sensitizer based solely on the expected reactivity of a given functional group with singlet oxygen are generally not sufficient for experiments in cells; it is difficult to construct a suitable chromophore that is impervious to all of the secondary and/or competing degradative processes that are present in the intracellular environment. On the other hand, prospects are reasonably positive when one considers the use of a sensitizer encapsulated in a specific protein; the local environment of the chromophore is controlled, degradation as a consequence of bimolecular reactions can be mitigated, and genetic engineering can be used to localize the encapsulated sensitizer in a given cellular domain. Also, the option of directly exciting oxygen in sensitizer-free experiments provides a useful complementary tool. These latter systems bode well with respect to obtaining more accurate control of the "dose" of singlet oxygen used to perturb a cell; a parameter that currently limits mechanistic studies of singlet-oxygen-mediated cell signaling. PMID:26878203

  10. Decamethylytterbocene Complexes of Bipyridines and Diazabutadienes: Multiconfigurational Ground States and Open-Shell Singlet Formation

    SciTech Connect

    Booth, Corwin H.; Walter, Marc D.; Kazhdan, Daniel; Hu, Yung-Jin; Lukens, Wayne W.; Bauer, Eric D.; Maron, Laurent; Eisenstein, Odile; Andersen, Richard A.

    2009-04-22

    Partial ytterbium f-orbital occupancy (i.e., intermediate valence) and open-shell singlet formation are established for a variety of bipyridine and diazabutadiene adducts with decamethylytterbocene, (C5Me5)2Yb, abbreviated as Cp*2Yb. Data used to support this claim include ytterbium valence measurements using Yb LIII-edge X-ray absorption near-edge structure spectroscopy, magnetic susceptibility, and complete active space self-consistent field (CASSCF) multiconfigurational calculations, as well as structural measurements compared to density functional theory calculations. The CASSCF calculations indicate that the intermediate valence is the result of a multiconfigurational ground-state wave function that has both an open-shell singlet f13(?*)1, where pi* is the lowest unoccupied molecular orbital of the bipyridine or dpiazabutadiene ligands, and a closed-shell singlet f14 component. A number of other competing theories for the unusual magnetism in these materials are ruled out by the lack of temperature dependence of the measured intermediate valence. These results have implications for understanding chemical bonding not only in organolanthanide complexes but also for f-element chemistry in general, as well as understanding magnetic interactions in nanoparticles and devices.

  11. Probing color-singlet exchange at D0

    SciTech Connect

    Abbott, B.; Abolins, M.; Acharya, B.S.; D0 Collaboration

    1997-07-01

    We present latest preliminary results on hard color-singlet exchange in proton-antiproton collisions. The fraction of dijet events produced via color-singlet exchange is measured as a function of jet transverse energy, dijet pseudorapidity separation, and proton-antiproton center-of-mass energy. These results are qualitatively consistent with a color-singlet fraction that increases with increasing quark-initiated processes.

  12. Sub-50 fs excited state dynamics of 6-chloroguanine upon deep ultraviolet excitation.

    PubMed

    Mondal, Sayan; Puranik, Mrinalini

    2016-05-18

    The photophysical properties of natural nucleobases and their respective nucleotides are ascribed to the sub-picosecond lifetime of their first singlet states in the UV-B region (260-350 nm). Electronic transitions of the ππ* type, which are stronger than those in the UV-B region, lie at the red edge of the UV-C range (100-260 nm) in all isolated nucleobases. The lowest energetic excited states in the UV-B region of nucleobases have been investigated using a plethora of experimental and theoretical methods in gas and solution phases. The sub-picosecond lifetime of these molecules is not a general attribute of all nucleobases but specific to the five primary nucleobases and a few xanthine and methylated derivatives. To determine the overall UV photostability, we aim to understand the effect of more energetic photons lying in the UV-C region on nucleobases. To determine the UV-C initiated photophysics of a nucleobase system, we chose a halogen substituted purine, 6-chloroguanine (6-ClG), that we had investigated previously using resonance Raman spectroscopy. We have performed quantitative measurements of the resonance Raman cross-section across the Bb absorption band (210-230 nm) and constructed the Raman excitation profiles. We modeled the excitation profiles using Lee and Heller's time-dependent theory of resonance Raman intensities to extract the initial excited state dynamics of 6-ClG within 30-50 fs after photoexcitation. We found that imidazole and pyrimidine rings of 6-ClG undergo expansion and contraction, respectively, following photoexcitation to the Bb state. The amount of distortions of the excited state structure from that of the ground state structure is reflected by the total internal reorganization energy that is determined at 112 cm(-1). The contribution of the inertial component of the solvent response towards the total reorganization energy was obtained at 1220 cm(-1). In addition, our simulation also yields an instantaneous response of the first

  13. Distribution of O{sub 2} molecules over vibrational levels at the output of a singlet-oxygen generator

    SciTech Connect

    Azyazov, V N; Pichugin, S Yu; Safonov, V S; Ufimtsev, N I

    2001-09-30

    Simple formulas are obtained for determining the population of the vibrational levels of singlet oxygen generated chemically in a singlet-oxygen generator. The rate of decrease in the vibrational energy of oxygen is limited by the exchange between its first vibrational level and the bending mode of the water molecule. It is shown that the populations of singlet oxygen molecules at the second and third vibrational levels are comparable with the population of oxygen in the excited electronic state b{sup 1}{Sigma}{sub g}{sup +}. The possibility of formation of electronically excited iodine in the reaction O{sub 2}({alpha}{sup 1}{Delta}{sub g}, {nu}=2) +I{sub 2}(X) {yields} O{sub 2}(X {sup 3}{Sigma}{sub g}{sup -}) +O{sub 2}({Lambda} {sup 3}{Pi}{sub 1u}), which may be the intermediate state in the process of dissociation of iodine in singlet-oxygen medium, is substantiated. (active media. lasers)

  14. Triplet diffusion in singlet exciton fission sensitized pentacene solar cells

    NASA Astrophysics Data System (ADS)

    Tabachnyk, Maxim; Ehrler, Bruno; Bayliss, Sam; Friend, Richard H.; Greenham, Neil C.

    2013-10-01

    Singlet fission sensitized photovoltaics have the potential to surpass the Shockley-Queisser limit for a single-junction structure. We investigate the dynamics of triplet excitons resulting from singlet fission in pentacene and their ionization at a C60 heterojunction. We model the generation and diffusion of excitons to predict the spectral response. We find the triplet diffusion length in polycrystalline pentacene to be 40 nm. Poly(3-hexylthiophene) between the electrode and pentacene works both to confine triplet excitons and also to transfer photogenerated singlet excitons into pentacene with 30% efficiency. The lower bound for the singlet fission quantum efficiency in pentacene is 180 ± 15%.

  15. The Nature of Singlet Exciton Fission in Carotenoid Aggregates

    PubMed Central

    2015-01-01

    Singlet exciton fission allows the fast and efficient generation of two spin triplet states from one photoexcited singlet. It has the potential to improve organic photovoltaics, enabling efficient coupling to the blue to ultraviolet region of the solar spectrum to capture the energy generally lost as waste heat. However, many questions remain about the underlying fission mechanism. The relation between intermolecular geometry and singlet fission rate and yield is poorly understood and remains one of the most significant barriers to the design of new singlet fission sensitizers. Here we explore the structure–property relationship and examine the mechanism of singlet fission in aggregates of astaxanthin, a small polyene. We isolate five distinct supramolecular structures of astaxanthin generated through self-assembly in solution. Each is capable of undergoing intermolecular singlet fission, with rates of triplet generation and annihilation that can be correlated with intermolecular coupling strength. In contrast with the conventional model of singlet fission in linear molecules, we demonstrate that no intermediate states are involved in the triplet formation: instead, singlet fission occurs directly from the initial 1Bu photoexcited state on ultrafast time scales. This result demands a re-evaluation of current theories of polyene photophysics and highlights the robustness of carotenoid singlet fission. PMID:25825939

  16. The nature of singlet exciton fission in carotenoid aggregates.

    PubMed

    Musser, Andrew J; Maiuri, Margherita; Brida, Daniele; Cerullo, Giulio; Friend, Richard H; Clark, Jenny

    2015-04-22

    Singlet exciton fission allows the fast and efficient generation of two spin triplet states from one photoexcited singlet. It has the potential to improve organic photovoltaics, enabling efficient coupling to the blue to ultraviolet region of the solar spectrum to capture the energy generally lost as waste heat. However, many questions remain about the underlying fission mechanism. The relation between intermolecular geometry and singlet fission rate and yield is poorly understood and remains one of the most significant barriers to the design of new singlet fission sensitizers. Here we explore the structure-property relationship and examine the mechanism of singlet fission in aggregates of astaxanthin, a small polyene. We isolate five distinct supramolecular structures of astaxanthin generated through self-assembly in solution. Each is capable of undergoing intermolecular singlet fission, with rates of triplet generation and annihilation that can be correlated with intermolecular coupling strength. In contrast with the conventional model of singlet fission in linear molecules, we demonstrate that no intermediate states are involved in the triplet formation: instead, singlet fission occurs directly from the initial 1B(u) photoexcited state on ultrafast time scales. This result demands a re-evaluation of current theories of polyene photophysics and highlights the robustness of carotenoid singlet fission. PMID:25825939

  17. Excitations and benchmark ensemble density functional theory for two electrons

    SciTech Connect

    Pribram-Jones, Aurora; Burke, Kieron; Yang, Zeng-hui; Ullrich, Carsten A.; Trail, John R.; Needs, Richard J.

    2014-05-14

    A new method for extracting ensemble Kohn-Sham potentials from accurate excited state densities is applied to a variety of two-electron systems, exploring the behavior of exact ensemble density functional theory. The issue of separating the Hartree energy and the choice of degenerate eigenstates is explored. A new approximation, spin eigenstate Hartree-exchange, is derived. Exact conditions that are proven include the signs of the correlation energy components and the asymptotic behavior of the potential for small weights of the excited states. Many energy components are given as a function of the weights for two electrons in a one-dimensional flat box, in a box with a large barrier to create charge transfer excitations, in a three-dimensional harmonic well (Hooke's atom), and for the He atom singlet-triplet ensemble, singlet-triplet-singlet ensemble, and triplet bi-ensemble.

  18. Doublet-singlet model and unitarity

    NASA Astrophysics Data System (ADS)

    Cynolter, G.; Kovács, J.; Lendvai, E.

    2016-12-01

    We study the renormalizable singlet-doublet fermionic extension of the Standard Model (SM). In this model, the new vector-like fermions couple to the gauge bosons and to the Higgs via new Yukawa couplings that allow for nontrivial mixing in the new sector, providing a stable, neutral dark matter candidate. Approximate analytic formulae are given for the mass spectrum around the blind spots, where the dark matter candidate coupling to h or Z vanishes. We calculate the two particle scattering amplitudes in the model, impose the perturbative unitarity constraints and establish bounds on the Yukawa couplings.

  19. Singlet (Phosphino)phosphinidenes are Electrophilic.

    PubMed

    Hansmann, Max M; Jazzar, Rodolphe; Bertrand, Guy

    2016-07-13

    A room-temperature stable (phosphino)-phosphinidene reacts with carbon monoxide, stable singlet carbenes, including the poor π-accepting imidazol-2-ylidene, and phosphines giving rise to the corresponding phosphaketene, phosphinidene-carbene and phosphinidene-phosphine adducts, respectively. Whereas the electronic ground-state calculations indicate a PP multiple bond character in which the terminal phosphorus is negatively charged, the observed reactivity clearly indicates that (phosphino)phosphinidenes are electrophilic as expected for an electron-deficient species. This is further demonstrated by competition experiments as well as by the results of Fukui function calculations. PMID:27340902

  20. Intramolecular charge transfer of push-pull pyridinium salts in the singlet manifold.

    PubMed

    Carlotti, Benedetta; Consiglio, Giuseppe; Elisei, Fausto; Fortuna, Cosimo G; Mazzucato, Ugo; Spalletti, Anna

    2014-05-22

    The solvent effect on the photophysical and photochemical properties of the iodides of three trans (E) isomers of 2-D-vinyl,1-methylpyridinium, where D is a donor group (4-dimethylaminophenyl, 3,4,5-trimethoxyphenyl and 1-pyrenyl), was studied by stationary and transient absorption techniques. The results obtained allowed the negative solvatochromism and relaxation pathways of the excited states in the singlet manifold to be reasonably interpreted. Resorting to ultrafast absorption techniques and DFT calculations allowed information on the excited state dynamics and the role of the solvent-controlled intramolecular charge transfer (ICT) processes to be obtained. The structure-dependent excited state dynamics in nonpolar solvents, where the ICT is slower than solvent rearrangement, and in polar solvents, where an opposite situation is operative, was thus explained. The push-pull character of the three compounds, particularly the anilino-derivative, suggests their potential application in optoelectronics. PMID:24779555

  1. A global search for the lowest energy isomer of C26

    NASA Astrophysics Data System (ADS)

    An, Jie; Gan, Li-Hua; Zhao, Jian-Qiang; Li, Rui

    2010-04-01

    The complete set of 2333 isomers of C26 fullerene composed of square, pentagonal, hexagonal, and heptagonal faces together with some noncage structures is investigated at the Hartree-Fock and density functional theory (DFT) levels. For the singlet states, a nonclassical isomer C26-10-01 with a square embedded is predicted by the DFT method as the lowest energy isomer, followed by the sole classical isomer C26-00-01. Further explorations reveal that the electronic ground state of C26-10-01 is triplet state in Cs symmetry, while that of C26-00-01 corresponds to its quintet in D3h symmetry. Both the total energies and nucleus independent chemical shift values at DFT level favor the classical isomer. It is found that both C26-00-01 and C26-10-01 possess high vertical electron affinity. The addition of electron(s) to C26-10-01 increases its aromatic character and encapsulation of Li atom into this cage is highly exothermic, indicating that it may be captured in the form of derivatives. To clarify the relative stabilities at elevated temperatures, the entropy contributions are taken into account based on the Gibbs free energy at the B3LYP/6-311+G∗ level. C26-10-01 behaves thermodynamically more stable than the classical isomer over a wide range of temperatures related to fullerene formation. The IR spectra of these two lowest energy isomers are simulated to facilitate their experimental identification.

  2. Seeking small molecules for singlet fission: a heteroatom substitution strategy.

    PubMed

    Zeng, Tao; Ananth, Nandini; Hoffmann, Roald

    2014-09-10

    We design theoretically small molecule candidates for singlet fission chromophores, aiming to achieve a balance between sufficient diradical character and kinetic persistence. We develop a perturbation strategy based on the captodative effect to introduce diradical character into small π-systems. Specifically, this can be accomplished by replacing pairs of not necessarily adjacent C atoms with isoelectronic and isosteric pairs of B and N atoms. Three rules of thumb emerge from our studies to aid further design: (i) Lewis structures provide insight into likely diradical character; (ii) formal radical centers of the diradical must be well-separated; (iii) stabilization of radical centers by a donor (N) and an acceptor (B) is essential. Following the rules, we propose candidate molecules. Employing reliable multireference calculations for excited states, we identify three likely candidate molecules for SF chromophores. These include a benzene, a napthalene, and an azulene, where four C atoms are replaced by a pair of B and a pair of N atoms. PMID:25140824

  3. Polymorphism influences singlet fission rates in tetracene thin films

    DOE PAGESBeta

    Arias, Dylan H.; Ryerson, Joseph L.; Cook, Jasper D.; Damrauer, Niels H.; Johnson, Justin C.

    2015-11-06

    Here, we report the effect of crystal structure and crystallite grain size on singlet fission (SF) in polycrystalline tetracene, one of the most widely studied SF and organic semiconductor materials. SF has been comprehensively studied in one polymoprh (Tc I), but not in the other, less stable polymorph (Tc II). Using carefully controlled thermal evaporation deposition conditions and high sensitivity ultrafast transient absorption spectroscopy, we found that for large crystallite size samples, SF in nearly pure Tc II films is significantly faster than SF in Tc I films. We also discovered that crystallite size has a minimal impact on themore » SF rate in Tc II films, but a significant influence in Tc I films. Large crystallites exhibit SF times of 125 ps and 22 ps in Tc I and Tc II, respectively, whereas small crystallites have SF times of 31 ps and 33 ps. Our results demonstrate first, that attention must be paid to polymorphism in obtaining a self-consistent rate picture for SF in tetracene and second, that control of polymorphism can play a significant role towards achieving a mechanistic understanding of SF in polycrystalline systems. In this latter context we show that conventional theory based on non-covalent tetracene couplings is insufficient, thus highlighting the need for models that capture the delocalized and highly mobile nature of excited states in elucidating the full photophysical picture.« less

  4. Polymorphism influences singlet fission rates in tetracene thin films

    SciTech Connect

    Arias, Dylan H.; Ryerson, Joseph L.; Cook, Jasper D.; Damrauer, Niels H.; Johnson, Justin C.

    2015-11-06

    Here, we report the effect of crystal structure and crystallite grain size on singlet fission (SF) in polycrystalline tetracene, one of the most widely studied SF and organic semiconductor materials. SF has been comprehensively studied in one polymoprh (Tc I), but not in the other, less stable polymorph (Tc II). Using carefully controlled thermal evaporation deposition conditions and high sensitivity ultrafast transient absorption spectroscopy, we found that for large crystallite size samples, SF in nearly pure Tc II films is significantly faster than SF in Tc I films. We also discovered that crystallite size has a minimal impact on the SF rate in Tc II films, but a significant influence in Tc I films. Large crystallites exhibit SF times of 125 ps and 22 ps in Tc I and Tc II, respectively, whereas small crystallites have SF times of 31 ps and 33 ps. Our results demonstrate first, that attention must be paid to polymorphism in obtaining a self-consistent rate picture for SF in tetracene and second, that control of polymorphism can play a significant role towards achieving a mechanistic understanding of SF in polycrystalline systems. In this latter context we show that conventional theory based on non-covalent tetracene couplings is insufficient, thus highlighting the need for models that capture the delocalized and highly mobile nature of excited states in elucidating the full photophysical picture.

  5. Molecular dynamics of excited state intramolecular proton transfer: 3-hydroxyflavone in solution

    SciTech Connect

    Bellucci, Michael A.; Coker, David F.

    2012-05-21

    The ultrafast enol-keto photoisomerization in the lowest singlet excited state of 3-hydroxyflavone is investigated using classical molecular dynamics in conjunction with empirical valence bond (EVB) potentials for the description of intramolecular interactions, and a molecular mechanics and variable partial charge model, dependent on transferring proton position, for the description of solute-solvent interactions. A parallel multi-level genetic program was used to accurately fit the EVB potential energy surfaces to high level ab initio data. We have studied the excited state intramolecular proton transfer (ESIPT) reaction in three different solvent environments: methylcyclohexane, acetonitrile, and methanol. The effects of the environment on the proton transfer time and the underlying mechanisms responsible for the varied time scales of the ESIPT reaction rates are analyzed. We find that simulations with our EVB potential energy surfaces accurately reproduce experimentally determined reaction rates, fluorescence spectra, and vibrational frequency spectra in all three solvents. Furthermore, we find that the ultrafast ESIPT process results from a combination of ballistic transfer, and intramolecular vibrational redistribution, which leads to the excitation of a set of low frequency promoting vibrational modes. From this set of promoting modes, we find that an O-O in plane bend and a C-H out of plane bend are present in all three solvents, indicating that they are fundamental to the ultrafast proton transfer. Analysis of the slow proton transfer trajectories reveals a solvent mediated proton transfer mechanism, which is diffusion limited.

  6. Electron, Hole, Singlet, and Triplet Energy Transfer in Photoexcited Porphyrin-Naphthalenediimide Dyads.

    PubMed

    Yushchenko, Oleksandr; Hangarge, Rahul V; Mosquera-Vazquez, Sandra; Boshale, Sheshanath V; Vauthey, Eric

    2015-06-18

    The excited-state dynamics of two molecular dyads, consisting of zinc (1) and free-base (2) porphyrin connected via a peptide linker to a core-substituted naphthalenediimide (NDI) have been investigated using optical spectroscopy. These dyads exhibit rich photophysics because of the large number of electronic excited states below 3 eV. In the case of 1 in apolar solvents, excitation energy transfer from the vibrationally hot singlet excited porphyrin to the NDI takes place with a 500 fs time constant. Electronic energy ends up in the NDI-localized triplet state, which decays to the ground state on a microsecond timescale. In polar solvents, ground-state recovery is faster by 5 orders of magnitude because of the occurrence of charge separation followed by recombination. On the other hand, excitation energy transfer in 2 takes place in the opposite direction, namely from the NDI to the porphyrin, which then undergoes intersystem crossing to the triplet state, followed by triplet energy transfer back to the NDI. Therefore, four distinct local electronic excited states are consecutively populated after excitation of the NDI unit of 2, with the energy shuttling between the two ends of the dyad. PMID:25418961

  7. Sub-Picosecond Singlet Exciton Fission in Cyano-Substituted Diaryltetracenes.

    PubMed

    Margulies, Eric A; Wu, Yi-Lin; Gawel, Przemyslaw; Miller, Stephen A; Shoer, Leah E; Schaller, Richard D; Diederich, François; Wasielewski, Michael R

    2015-07-20

    Thin films of 5,11-dicyano-6,12-diphenyltetracene (TcCN) have been studied for their ability to undergo singlet exciton fission (SF). Functionalization of tetracene with cyano substituents yields a more stable chromophore with favorable energetics for exoergic SF (2E(T1)-E(S1)=-0.17 eV), where S1 and T1 are singlet and triplet excitons, respectively. As a result of tuning the triplet-state energy, SF is faster in TcCN relative to the corresponding endoergic process in tetracene. SF proceeds with two time constants in the film samples (τ=0.8±0.2 ps and τ=23±3 ps), which is attributed to structural disorder within the film giving rise to one population with a favorable interchromophore geometry, which undergoes rapid SF, and a second population in which the initially formed singlet exciton must diffuse to a site at which this favorable geometry exists. A triplet yield analysis using transient absorption spectra indicates the formation of 1.6±0.3 triplets per initial excited state. PMID:26097009

  8. A study of a singlet-oxygen generator with a twisted aerosol flow

    SciTech Connect

    Adamenkov, A A; Vyskubenko, B A; Il'in, S P; Krukovskii, I M

    2002-06-30

    The results of a study of a singlet-oxygen generator (SOG) with a twisted aerosol flow are presented. The output parameters of the generator obtained in experiments exceed the corresponding characteristics reported earlier in the literature. The maximum chemical efficiency of the reactor amounts to {approx} 70%. The flux density of the electron energy stored by the excited oxygen molecules in the reaction zone is above 1.5 kJ cm{sup -2} s{sup -1}. The measured singlet-oxygen yield is {approx} 60% at a pressure of 100 Torr. Adding a buffer gas (N{sub 2}) to chlorine ensures an increase in the outlet pressure up to 250 Torr without a decrease in the singlet-oxygen yield. The utilisation of chlorine under such conditions exceeds 95 %. The SOG design with a twisted flow ensures atmospheric pressure of the waste solution at the reactor outlet, thus significantly simplifying the development of a system for liquid recycling. (active media)

  9. In situ measurement of exciton energy in hybrid singlet-fission solar cells.

    PubMed

    Ehrler, Bruno; Walker, Brian J; Böhm, Marcus L; Wilson, Mark W B; Vaynzof, Yana; Friend, Richard H; Greenham, Neil C

    2012-01-01

    Singlet exciton fission-sensitized solar cells have the potential to exceed the Shockley-Queisser limit by generating additional photocurrent from high-energy photons. Pentacene is an organic semiconductor that undergoes efficient singlet fission--the conversion of singlet excitons into pairs of triplets. However, the pentacene triplet is non-emissive, and uncertainty regarding its energy has hindered device design. Here we present an in situ measurement of the pentacene triplet energy by fabricating a series of bilayer solar cells with infrared-absorbing nanocrystals of varying bandgaps. We show that the pentacene triplet energy is at least 0.85 eV and at most 1.00 eV in operating devices. Our devices generate photocurrent from triplets, and achieve external quantum efficiencies up to 80%, and power conversion efficiencies of 4.7%. This establishes the general use of nanocrystal size series to measure the energy of non-emissive excited states, and suggests that fission-sensitized solar cells are a favourable candidate for third-generation photovoltaics. PMID:22910365

  10. In situ measurement of exciton energy in hybrid singlet-fission solar cells

    NASA Astrophysics Data System (ADS)

    Ehrler, Bruno; Walker, Brian J.; Böhm, Marcus L.; Wilson, Mark W. B.; Vaynzof, Yana; Friend, Richard H.; Greenham, Neil C.

    2012-08-01

    Singlet exciton fission-sensitized solar cells have the potential to exceed the Shockley-Queisser limit by generating additional photocurrent from high-energy photons. Pentacene is an organic semiconductor that undergoes efficient singlet fission—the conversion of singlet excitons into pairs of triplets. However, the pentacene triplet is non-emissive, and uncertainty regarding its energy has hindered device design. Here we present an in situ measurement of the pentacene triplet energy by fabricating a series of bilayer solar cells with infrared-absorbing nanocrystals of varying bandgaps. We show that the pentacene triplet energy is at least 0.85 eV and at most 1.00 eV in operating devices. Our devices generate photocurrent from triplets, and achieve external quantum efficiencies up to 80%, and power conversion efficiencies of 4.7%. This establishes the general use of nanocrystal size series to measure the energy of non-emissive excited states, and suggests that fission-sensitized solar cells are a favourable candidate for third-generation photovoltaics.

  11. Tunable singlet-triplet splitting in a few-electron Si/SiGe quantum dot

    NASA Astrophysics Data System (ADS)

    Shi, Zhan; Simmons, Christie; Prance, Jonathan; Gamble, John; Friesen, Mark; Savage, Donald; Lagally, Max; Coppersmith, Susan; Eriksson, Mark

    2012-02-01

    The singlet-triplet energy splitting in a double quantum dot is an important parameter for singlet-triplet qubits, because it determines the energy gap for both initialization and readout. This splitting can also be used to perform gate operations in a newly proposed hybrid qubit [1]. We describe measurements in which we tune the singlet-triplet energy splitting by changing gate voltages on a Si/SiGe double quantum dot [2]. We argue that the energy is changed largely by lateral translation of the dot, which changes the local atomic structure that the electrons experience in the quantum dot, leading to variations in the valley-orbit coupling. We present calculations indicating the experimental results are consistent with the first excited state of the dot having non-zero valley-orbit coupling. [1] Z. Shi, et al., e-print: http://lanl.arxiv.org/abs/1110.6622. [2] Z. Shi, et al., e-print: http://lanl.arxiv.org/abs/1109.0511.

  12. Excited state properties of naphtho-homologated xxDNA bases and effect of methanol solution, deoxyribose, and base pairing.

    PubMed

    Zhang, Laibin; Ren, Tingqi; Tian, Jianxiang; Yang, Xiuqin; Zhou, Liuzhu; Li, Xiaoming

    2013-04-18

    Design and synthesis of fluorescent nucleobase analogues for studying structures and dynamics of nucleic acids have attracted much attention in recent years. In the present work, a comprehensive theoretical study of electronic transitions of naphtho-homologated base analogues, namely, xxC, xxT, xxA, and xxG, was performed. The nature of the low-lying excited states was discussed, and the results were compared with those of x-bases. Geometrical characteristics of the lowest excited singlet ππ* states were explored using the CIS method. The calculated excitation maxima are 423, 397, 383, and 357 nm for xxA, xxG, xxC, and xxT, respectively, and they are greatly red-shifted compared with x-bases and natural bases, allowing them to be selectively excited in the presence of the natural bases. In the gas phase, the fluorescence from them would be expected to occur around 497, 461, 457, and 417 nm, respectively. The effects of methanol solution, deoxyribose, and base paring with their complementary natural bases on the relevant absorption and emission spectra of these modified bases were also examined. PMID:23531077

  13. External quantum efficiency exceeding 100% in a singlet-exciton-fission-based solar cell

    NASA Astrophysics Data System (ADS)

    Baldo, Marc

    2013-03-01

    Singlet exciton fission can be used to split a molecular excited state in two. In solar cells, it promises to double the photocurrent from high energy photons, thereby breaking the single junction efficiency limit. We demonstrate organic solar cells that exploit singlet exciton fission in pentacene to generate more than one electron per incident photon in the visible spectrum. Using a fullerene acceptor, a poly(3-hexylthiophene) exciton confinement layer, and a conventional optical trapping scheme, the peak external quantum efficiency is (109 +/-1)% at λ = 670 nm for a 15-nm-thick pentacene film. The corresponding internal quantum efficiency is (160 +/-10)%. Independent confirmation of the high internal efficiency is obtained by analysis of the magnetic field effect on photocurrent, which determines that the triplet yield approaches 200% for pentacene films thicker than 5 nm. To our knowledge, this is the first solar cell to generate quantum efficiencies above 100% in the visible spectrum. Alternative multiple exciton generation approaches have been demonstrated previously in the ultraviolet, where there is relatively little sunlight. Singlet exciton fission differs from these other mechanisms because spin conservation disallows the usual dominant loss process: a thermal relaxation of the high-energy exciton into a single low-energy exciton. Consequently, pentacene is efficient in the visible spectrum at λ = 670 nm because only the collapse of the singlet exciton into twotriplets is spin-allowed. Supported as part of the Center for Excitonics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001088.

  14. Singlet scalar resonances and the diphoton excess

    NASA Astrophysics Data System (ADS)

    McDermott, Samuel D.; Meade, Patrick; Ramani, Harikrishnan

    2016-04-01

    ATLAS and CMS recently released the first results of searches for diphoton resonances in 13 TeV data, revealing a modest excess at an invariant mass of approximately 750 GeV. We find that it is generically possible that a singlet scalar resonance is the origin of the excess while avoiding all other constraints. We highlight some of the implications of this model and how compatible it is with certain features of the experimental results. In particular, we find that the very large total width of the excess is difficult to explain with loop-level decays alone, pointing to other interesting bounds and signals if this feature of the data persists. Finally we comment on the robust Zγ signature that will always accompany the model we investigate.

  15. Supersymmetric Higgs singlet effects on FCNC observables

    SciTech Connect

    Hodgkinson, Robert N.

    2008-11-23

    Higgs singlet superflelds, usually present in extensions of the Minimal Supersymmetric Standard Model (MSSM) which address the {mu}-problem, such as the Next-to-Minimal Supersymmetric Standard Model (NMSSM) and the Minimal Nonminimal Supersymmetric Standard Model (mnSSM), can have significant contributions to B-meson flavour-changing neutral current observables for large values of tan{beta} > or approx. 50. Illustrative results are presented including effects on the B{sub s} and on the rare decay B{sub s}{yields}{mu}{sup +}{mu}{sup -}. In particular, we find that in the NMSSM, the branching ratio for B{sub s}{yields}{mu}{sup +}{mu}{sup -} can be enhanced or even suppressed with respect to the Standard Model prediction by more than one order of magnitude.

  16. A singlet - triplet T+ based qubit

    NASA Astrophysics Data System (ADS)

    Ribeiro, Hugo; Petta, Jason; Burkard, Guido

    2010-03-01

    We theoretically show that the electronic two-spin states singlet and triplet T+ are promising candidates for the implementation of a qubit in GaAs double quantum dots (DQD). A coherent superposition of the two-spin states is obtained by finite time Landau-Zener-St"uckelberg interferometry [1] and the single qubit rotations are performed by means of an external magnetic field with a typical amplitude of about 100 mT. In such a system, the coherent manipulation of the qubit takes place in a time scale of about 1 ns. We also study the nuclear induced decoherence, mainly due to hyperfine contact coupling between the electronic and nuclear spins, and compute the decoherence time T2^* ˜10 ns. [4pt] [1] H. Ribeiro and G. Burkard, Phys. Rev. Lett. 102, 216802 (2009)

  17. 48 CFR 47.306-2 - Lowest overall transportation costs.

    Code of Federal Regulations, 2014 CFR

    2014-10-01

    ... transportation costs. 47.306-2 Section 47.306-2 Federal Acquisition Regulations System FEDERAL ACQUISITION REGULATION CONTRACT MANAGEMENT TRANSPORTATION Transportation in Supply Contracts 47.306-2 Lowest overall transportation costs. (a) For the evaluation of offers, the transportation officer shall give to the...

  18. 48 CFR 47.306-2 - Lowest overall transportation costs.

    Code of Federal Regulations, 2012 CFR

    2012-10-01

    ... transportation costs. 47.306-2 Section 47.306-2 Federal Acquisition Regulations System FEDERAL ACQUISITION REGULATION CONTRACT MANAGEMENT TRANSPORTATION Transportation in Supply Contracts 47.306-2 Lowest overall transportation costs. (a) For the evaluation of offers, the transportation officer shall give to the...

  19. 48 CFR 47.306-2 - Lowest overall transportation costs.

    Code of Federal Regulations, 2011 CFR

    2011-10-01

    ... transportation costs. 47.306-2 Section 47.306-2 Federal Acquisition Regulations System FEDERAL ACQUISITION REGULATION CONTRACT MANAGEMENT TRANSPORTATION Transportation in Supply Contracts 47.306-2 Lowest overall transportation costs. (a) For the evaluation of offers, the transportation officer shall give to the...

  20. 48 CFR 47.306-2 - Lowest overall transportation costs.

    Code of Federal Regulations, 2010 CFR

    2010-10-01

    ... transportation costs. 47.306-2 Section 47.306-2 Federal Acquisition Regulations System FEDERAL ACQUISITION REGULATION CONTRACT MANAGEMENT TRANSPORTATION Transportation in Supply Contracts 47.306-2 Lowest overall transportation costs. (a) For the evaluation of offers, the transportation officer shall give to the...

  1. 48 CFR 47.306-2 - Lowest overall transportation costs.

    Code of Federal Regulations, 2013 CFR

    2013-10-01

    ... transportation costs. 47.306-2 Section 47.306-2 Federal Acquisition Regulations System FEDERAL ACQUISITION REGULATION CONTRACT MANAGEMENT TRANSPORTATION Transportation in Supply Contracts 47.306-2 Lowest overall transportation costs. (a) For the evaluation of offers, the transportation officer shall give to the...

  2. Excited-State Dynamics in 6-THIOGUANOSINE from Femtosecond to Microsecond Time Scale

    NASA Astrophysics Data System (ADS)

    Guo, Cao; Reichardt, Christian; Crespo-Hernández, Carlos E.

    2011-06-01

    6-thioguanine is a widely used pro-drug in which the oxygen atom in the carbonyl group of guanine is replaced by a sulfur atom. Previous studies have shown that patients treated with 6-thioguanine can metabolize and incorporate it in DNA as 6-thioguanosine (6tGuo). These patients show a high incidence of skin cancer when they are exposed to extended periods of sunlight irradiation. In this work, the photodynamics of 6tGuo is investigated by broad band time resolved transient spectroscopy. Similar to previously studied 4-thiothymidine, our results show that excitation of 6tGuo with UVA light at 340 nm results in efficient and ultrafast intersystem crossing to the triplet manifold (τ = 0.31±0.05 ps) and a high triplet quantum yield (φ = 0.8±0.2). The triplet state has a lifetime of 720±10 ns in N2-saturated vs. 460±10 ns in air-saturated aqueous solution. In addition, a minor picosecond deactivation channel (80±15 ps) is observed, which is tentatively assigned to internal conversion from the lowest-energy excited singlet state to the ground state. Quantum chemical calculations support the proposed kinetic model. Based on the high triplet quantum yield measured, it is proposed that the phototoxicity of 6tGuo is due to its ability to photosensitized singlet oxygen, which can result in oxidative damage to DNA. P. O'Donovan, C. M. Perrett, X. Zhang, B. Montaner, Y.-Z. Xu, C. A. Harwood, J. M. McGregor, S. L. Walker, F. Hanaoka, P. Karran, Science 309, 1871 (2005). C. Reichardt, C. Guo, C. E. Crespo-Hernández, J. Phys. Chem. B. in press (2011). C. Reichardt, C. E. Crespo-Hernández, J. Phys. Chem. Lett. 1, 2239 (2010) C. Reichardt, C. E. Crespo-Hernández, Chem. Comm. 46, 5963 (2010).

  3. Singlet oxygen production by amphiphilic C60 derivatives and its correlation to cell cytotoxicity in vitro

    NASA Astrophysics Data System (ADS)

    So, Grace; Karotki, Aliaksandr; Verma, Sarika; Hauck, Tanya S.; Wilson, Brian; Pritzker, Kenneth P. H.; Chiang, Long

    2005-09-01

    Fullerene derivatives have appealing properties that can potentially be used in materials science and medical applications. In particular, fullerenes are known to produce reactive oxygen species upon their excitation with light. This makes them particularly attractive as photosensitizers for photodynamic therapy (PDT). Photodynamic therapy is a new modality of treatment of cancer as well as some non-cancerous conditions. It involves the combined actions of a drug (photosensitizer) and light to produce a cytotoxic effect. Water-soluble hexa(sulfo-n-butyl)[60]fullerenes (FC4S) was reported recently to generate singlet oxygen (1O2) and superoxide radical (O2-.) upon its excitation with light, making it a promising candidate for PDT treatments. Recently, we synthesized new amphiphilic fullerene derivatives, namely, [60]fullerene-diphenylaminofluorene-oligo(ethylene glycol) conjugates, C60(>DPAF-PEG600) and C60(>DPAF-PEG2000), as potential photosensitizers. In this paper we compare FC4S to PEG-based fullerenes in terms of their singlet oxygen photosensitization ability. We measured time-resolved kinetics of singlet oxygen luminescence photosensitized by excitation of fullerenes via a 10 ns pulsed laser at 523 nm. For FC4S we observed "normal" kinetics with a monoexponential decay profile giving a time constant 3.8 us in water. In contrast, for the case of C60(>DPAF-PEG600) and C60(>DPAF-PEG2000), a non-monoexponential decay profile with a long tail (~ 102 μs) in water was observed. We hypothesize that this is due to formation of vesicles by PEG fullerenes in aqueous solution. To investigate photodynamic activity of these fullerene derivatives in vitro, we used HeLa human adenocarcinoma and B16 mouse melanoma cell lines. FC4S showed clear photodynamic effects in both cell lines. The total fluence required to kill 50% of the cells at the drug concentration of 20 μM was 36 Jcm-2 for HeLa cells and 72 Jcm-2 for B16 cells. Neither PEG-based fullerene derivatives showed any

  4. Computational Evidence of Inversion of (1)La and (1)Lb-Derived Excited States in Naphthalene Excimer Formation from ab Initio Multireference Theory with Large Active Space: DMRG-CASPT2 Study.

    PubMed

    Shirai, Soichi; Kurashige, Yuki; Yanai, Takeshi

    2016-05-10

    The naphthalene molecule has two important lowest-lying singlet excited states, denoted (1)La and (1)Lb. Association of the excited and ground state monomers yields a metastable excited dimer (excimer), which emits characteristic fluorescence. Here, we report a first computational result based on ab initio theory to corroborate that the naphthalene excimer fluorescence is (1)La parentage, resulting from inversion of (1)La and (1)Lb-derived dimer states. This inversion was hypothesized by earlier experimental studies; however, it has not been confirmed rigorously. In this study, the advanced multireference (MR) theory based on the density matrix renormalization group that enables using unprecedented large-size active space for describing significant electron correlation effects is used to provide accurate potential energy curves (PECs) of the excited states. The results evidenced the inversion of the PECs and accurately predicted transition energies for excimer fluorescence and monomer absorption. Traditional MR calculations with smaller active spaces and single-reference theory calculations exhibit serious inconsistencies with experimental observations. PMID:27082241

  5. An ab initio investigation of the ground and low-lying singlet and triplet electronic states of XNO{sub 2} and XONO (X = Cl, Br, and I)

    SciTech Connect

    Peterson, Kirk A.; Francisco, Joseph S.

    2014-01-28

    A systematic ab initio treatment of the nitryl halides (XNO{sub 2}) and the cis- and trans- conformers of the halide nitrites (XONO), where X = Cl, Br, and I, have been carried out using highly correlated methods with sequences of correlation consistent basis sets. Equilibrium geometries and harmonic frequencies have been accurately calculated in all cases at the explicitly correlated CCSD(T)-F12b level of theory, including the effects of core-valence correlation for the former. Where experimental values are available for the equilibrium structures (ClNO{sub 2} and BrNO{sub 2}), the present calculations are in excellent agreement; however, the X-O distances are slightly too long by about 0.01 Å due to missing multireference effects. Accurate predictions for the iodine species are made for the first time. The vertical electronic excitation spectra have been calculated using equation-of-motion coupled cluster methods for the low-lying singlet states and multireference configuration interaction for both singlet and triplet states. The latter also included the effects of spin-orbit coupling to provide oscillator strengths for the ground state singlet to excited triplet transitions. While for ClNO{sub 2} the transitions to excited singlet states all occur at wavelengths shorter than 310 nm, there is one longer wavelength singlet transition in BrNO{sub 2} and two in the case of INO{sub 2}. The long wavelength tail in the XNO{sub 2} species is predicted to be dominated by transitions to triplet states. In addition to red-shifting from X = Cl to I, the triplet transitions also increase in oscillator strength, becoming comparable to many of the singlet transitions in the case of INO{sub 2}. Hence in particular, the latter species should be very photolabile. Similar trends are observed and reported for the halogen nitrites, many of which for the first time.

  6. Fluctuations in Electronic Energy Affecting Singlet Fission Dynamics and Mixing with Charge-Transfer State: Quantum Dynamics Study.

    PubMed

    Fujihashi, Yuta; Ishizaki, Akihito

    2016-02-01

    Singlet fission is a spin-allowed process by which a singlet excited state is converted to two triplet states. To understand mechanisms of the ultrafast fission via a charge transfer (CT) state, one has investigated the dynamics through quantum-dynamical calculations with the uncorrelated fluctuation model; however, the electronic states are expected to experience the same fluctuations induced by the surrounding molecules because the electronic structure of the triplet pair state is similar to that of the singlet state except for the spin configuration. Therefore, the fluctuations in the electronic energies could be correlated, and the 1D reaction coordinate model may adequately describe the fission dynamics. In this work we develop a model for describing the fission dynamics to explain the experimentally observed behaviors. We also explore impacts of fluctuations in the energy of the CT state on the fission dynamics and the mixing with the CT state. The overall behavior of the dynamics is insensitive to values of the reorganization energy associated with the transition from the singlet state to the CT state, although the coherent oscillation is affected by the fluctuations. This result indicates that the mixing with the CT state is rather robust under the fluctuations in the energy of the CT state as well as the high-lying CT state. PMID:26732701

  7. Singlet-triplet gaps in polyacenes: a delicate balance between dynamic and static correlations investigated by spin-flip methods.

    PubMed

    Ibeji, Collins U; Ghosh, Debashree

    2015-04-21

    Over the last few years people have been interested in the process of singlet fission, owing to its relevance to solar cell technology. The energetics of singlet fission is intimately related to singlet-triplet (ST) gaps and energies of singlet excited states. However, accurate calculations of ST gaps in polyacenes are complicated due to near degeneracies in the π orbitals, and therefore, have been quite challenging. The spin-flip equation-of-motion coupled-cluster (SF-EOM-CC) and its perturbative approximation have been shown to correctly treat situations involving electronic degeneracies and near degeneracies. In this work, we use various spin-flip methods to benchmark the ST gaps of small polyacenes and show that the error in the ST gaps with respect to the experiment is small and does not increase appreciably with the system size. The diradical and polyradical character of the polyacene ground states increase with the system size. However, for the small polyacenes the open-shell character of the ground state is still small enough to be treated using single reference methods. PMID:25779893

  8. Temporal profile of the singlet oxygen emission endogenously produced by photosystem II reaction centre in an aqueous buffer.

    PubMed

    Li, Heng; Melø, Thor Bernt; Arellano, Juan B; Razi Naqvi, K

    2012-04-01

    The temporal profile of the phosphorescence of singlet oxygen endogenously photosensitized by photosystem II (PSII) reaction centre (RC) in an aqueous buffer has been recorded using laser excitation and a near infrared photomultiplier tube. A weak emission signal was discernible, and could be fitted to the functional form a[exp(-t/τ(2) - exp(-t/τ(1)], with a > 0 and τ(2) > τ(1). The value of τ(2) decreased from 11.6 ± 0.5 μs under aerobic conditions to 4.1 ± 0.2 μs in oxygen-saturated samples, due to enhanced bimolecular quenching of the donor triplet by oxygen, whereas that of τ(1), identifiable with the lifetime of singlet oxygen, was close to 3 μs in both cases. Extrapolations based on the low amplitude of the emission signal of singlet oxygen formed by PSII RC in the aqueous buffer and the expected values of τ(1) and τ(2) in chloroplasts indicate that attempts to analyse the temporal profile of singlet oxygen in chloroplasts are unlikely to be rewarded with success without a significant advance in the sensitivity of the detection equipment. PMID:22481218

  9. Singlet Oxygen Generation by Cyclometalated Complexes and Applications†

    PubMed Central

    Ashen-Garry, David; Selke, Matthias

    2014-01-01

    While cyclometalated complexes have been extensively studied for optoelectronic applications, these compounds also represent a relatively new class of photosensitizers for the production of singlet oxygen. Thus far, singlet oxygen generation from cyclometalated Ir and Pt complexes has been studied in detail. In this review, photophysical data for singlet oxygen generation from these complexes is presented, and the mechanism of 1O2 generation is discussed, including evidence for singlet oxygen generation via an electron transfer mechanism for some of cyclometalated Ir complexes. The period from the first report of singlet oxygen generation by a cyclometalated Ir complex in 2002 through August 2013 is covered in this review. This new class of singlet oxygen photosensitizers may prove to be rather versatile due to the ease of substitution of ancillary ligands without loss of activity. Several cyclometalated complexes have been tethered to zeolites, polystyrene, or quantum dots. Applications for photooxygenation of organic molecules, including “traditional” singlet oxygen reactions (ene reaction, [4+2] and [2+2] cycloadditions) as well as oxidative coupling of amines are presented. Potential biomedical applications are also reviewed. PMID:24344628

  10. Triple-Singlet Mixing in Si_3: the 1^3A_{1}^{''} - {a}{^3}A{^{'}_2} Transition

    NASA Astrophysics Data System (ADS)

    Zhang, Ruohan; Steimle, Timothy C.

    2013-06-01

    The electronic spectrum of the triplet states of the D_{3h} isomer of Si_3 recorded using both mass selected REMPI and LIF spectroscopy was recently reported. In that same study the dispersed laser induced fluorescence (DLIF) spectra resulting from excitation of various bands in the visible range were recorded. The DLIF spectra exhibited a progression with a 505 cm^{-1} spacing, which was assign to the breathing mode of the D_{3h}, equilateral triangle, Si_{3} molecule. In addition, and quite unexpectedly, the DLIF spectra exhibited a progression having a spacing of 173 cm^{-1}. This progression was tentatively assigned to transition involving the bending mode of the ^1A_1 state of the C_{2v} isomer. A possible explanation for the observation of transitions in the singlet manifold is that upon laser excitation in the D_{3h} triplet manifold there is rapid intersystem crossing to the singlet manifold followed by fluorescence to the ground state of C_{2v} isomer. Here we address the issue of possible intersystem crossing by recording the excitation on DLIF spectra in the present of a static magnetic field. Magnetic fields are known to enhance the singlet-triple mixing. Si_{3} was produced using a supersonic pulsed discharge source (900 V, 20 μs, 6kΩ) with a 1% SiH_{4} in argon mixture. Magnetic fields of approximately 500 and 950 Gauss were applied. We will report the interpretation of the magnetic field induced changes to the LIF and DLIF spectra and the implications for the singlet-triple mixing process. N. J. Reilly, X. Zhuang, V. Gupta, R. Nagarajan, R. C. Fortenberry, J. P. Maier, T. C. Steimle, J. F. Stanton, M. C. McCarthy; {J. Chem. Phys., {136(19)}, 194307, (2004). V. I. Makarov, I. V. Khmelinskii; {Advances in Chemical Phisics, {Volume 118}, 45-98, (2001). thanks

  11. Singlet oxygen production in superoxide ion-halocarbon systems

    SciTech Connect

    Kanofsky, J.R.

    1986-05-28

    A search for singlet oxygen chemiluminescence at 1268 nm was made in a number of reactions of superoxide ion. Carbon tetrachloride and carbon tetrabromide reacted with suspended potassium superoxide to produce 1268-nm emission consistent with singlet oxygen. Chloroform was less reactive but produced 1268-nm emission when the concentration of superoxide ion in the halocarbon phase was increased with 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6). The dismutation of superoxide ion in deuterium oxide was not accompanied by 1268-nm chemiluminescence. Less than 0.02 mol of singlet oxygen was produced per mole of superoxide ion between p/sup 2/H 6 and 9.

  12. Singlet fission efficiency in tetracene-based organic solar cells

    NASA Astrophysics Data System (ADS)

    Wu, Tony C.; Thompson, Nicholas J.; Congreve, Daniel N.; Hontz, Eric; Yost, Shane R.; Van Voorhis, Troy; Baldo, Marc A.

    2014-05-01

    Singlet exciton fission splits one singlet exciton into two triplet excitons. Using a joint analysis of photocurrent and fluorescence modulation under a magnetic field, we determine that the triplet yield within optimized tetracene organic photovoltaic devices is 153% ± 5% for a tetracene film thickness of 20 nm. The corresponding internal quantum efficiency is 127% ± 18%. These results are used to prove the effectiveness of a simplified triplet yield measurement that relies only on the magnetic field modulation of fluorescence. Despite its relatively slow rate of singlet fission, the measured triplet yields confirm that tetracene is presently the best candidate for use with silicon solar cells.

  13. Spin-singlet state formation in the cluster Mott insulator GaNb4S8 studied by μSR and NMR spectroscopy

    NASA Astrophysics Data System (ADS)

    Waki, T.; Kajinami, Y.; Tabata, Y.; Nakamura, H.; Yoshida, M.; Takigawa, M.; Watanabe, I.

    2010-01-01

    Muon spin relaxation (μSR) and nuclear magnetic resonance experiments revealed that the spin-singlet state with an excitation gap of ˜200K is realized from S=1/2Nb4 tetrahedral clusters in a cluster Mott insulator GaNb4S8 . The intercluster cooperative phenomenon to the singlet state at TS=32k is triggered by intracluster Jahn-Teller type structural instability developed from ˜3TS . Referring to the lattice symmetry, the formation of Nb8 octamer ( Nb4-Nb4 bond) is suggested.

  14. Guanidine and guanidinium cation in the excited state—theoretical investigation

    SciTech Connect

    Antol, Ivana Glasovac, Zoran; Crespo-Otero, Rachel; Barbatti, Mario

    2014-08-21

    Diverse ab initio and density-functional-theory methods were used to investigate geometries, energies, and electronic absorption spectra of guanidine and its protonated form, as well as their photo-deactivation processes. It was shown that the guanidine is a weakly absorbing species with the excitation spectrum consisting mostly of transitions to the Rydberg excited states and one valence n-π{sub 4} state. The lowest energy band has a maximum at ca. 6.9 eV (∼180 nm). The protonation of guanidine affects its excitation spectrum substantially. A major shift of the Rydberg states to higher energies is clearly visible and strongly absorbing transitions from the ground state to the π{sub 3}-π{sub 4} and π{sub 2}-π{sub 4} states appears at 7.8 eV (∼160 nm). Three low-lying conical intersections (two for guanidine and one for protonated guanidine) between the ground state and the first excited singlet state were located. They are accessible from the Franck–Condon region through amino N–H stretching and out-of-plane deformations in guanidine and protonated guanidine, respectively. The relaxation of the π{sub 3}-3s Rydberg state via amino N–H bond stretching was hindered by a barrier. The nondissociated conical intersection in protonated guanidine mediates the radiationless deactivation of the compound after excitation into the π{sub 3}-π{sub 4} state. This fact is detrimental for the photostability of guanidine, since its conjugate acid is stable in aqueous solution over a wide pH range and in protein environment, where guanidinium moiety in arginine is expected to be in a protonated form.

  15. Effect of H2 binding on the nonadiabatic transition probability between singlet and triplet states of the [NiFe]-hydrogenase active site.

    PubMed

    Kaliakin, Danil S; Zaari, Ryan R; Varganov, Sergey A

    2015-02-12

    We investigate the effect of H2 binding on the spin-forbidden nonadiabatic transition probability between the lowest energy singlet and triplet electronic states of [NiFe]-hydrogenase active site model, using a velocity averaged Landau-Zener theory. Density functional and multireference perturbation theories were used to provide parameters for the Landau-Zener calculations. It was found that variation of the torsion angle between the terminal thiolate ligands around the Ni center induces an intersystem crossing between the lowest energy singlet and triplet electronic states in the bare active site and in the active site with bound H2. Potential energy curves between the singlet and triplet minima along the torsion angle and H2 binding energies to the two spin states were calculated. Upon H2 binding to the active site, there is a decrease in the torsion angle at the minimum energy crossing point between the singlet and triplet states. The probability of nonadiabatic transitions at temperatures between 270 and 370 K ranges from 35% to 32% for the active site with bound H2 and from 42% to 38% for the bare active site, thus indicating the importance of spin-forbidden nonadiabatic pathways for H2 binding on the [NiFe]-hydrogenase active site. PMID:25603170

  16. Cryogenic exciter

    SciTech Connect

    Bray, James William; Garces, Luis Jose

    2012-03-13

    The disclosed technology is a cryogenic static exciter. The cryogenic static exciter is connected to a synchronous electric machine that has a field winding. The synchronous electric machine is cooled via a refrigerator or cryogen like liquid nitrogen. The static exciter is in communication with the field winding and is operating at ambient temperature. The static exciter receives cooling from a refrigerator or cryogen source, which may also service the synchronous machine, to selected areas of the static exciter and the cooling selectively reduces the operating temperature of the selected areas of the static exciter.

  17. Quantum Chemical Benchmark Studies of the Electronic Properties of the Green Fluorescent Protein Chromophore. 1. Electronically Excited and Ionized States of the Anionic Chromophore in the Gas Phase.

    PubMed

    Epifanovsky, Evgeny; Polyakov, Igor; Grigorenko, Bella; Nemukhin, Alexander; Krylov, Anna I

    2009-07-14

    We present the results of quantum chemical calculations of the electronic properties of the anionic form of the green fluorescent protein chromophore in the gas phase. The vertical detachment energy of the chromophore is found to be 2.4-2.5 eV, which is below the strongly absorbing ππ* state at 2.6 eV. The vertical excitation of the lowest triplet state is around 1.9 eV, which is below the photodetachment continuum. Thus, the lowest bright singlet state is a resonance state embedded in the photodetachment continuum, whereas the lowest triplet state is a regular bound state. Based on our estimation of the vertical detachment energy, we attribute a minor feature in the action spectrum as due to the photodetachment transition. The benchmark results for the bright ππ* state demonstrated that the scaled opposite-spin method yields vertical excitation within 0.1 eV (20 nm) from the experimental maximum at 2.59 eV (479 nm). We also report estimations of the vertical excitation energy obtained with the equation-of-motion coupled cluster with the singles and doubles method, a multireference perturbation theory corrected approach MRMP2 as well as the time-dependent density functional theory with range-separated functionals. Expanding the basis set with diffuse functions lowers the ππ* vertical excitation energy by 0.1 eV at the same time revealing a continuum of "ionized" states, which embeds the bright ππ* transition. PMID:26610014

  18. Zethrenes, extended p-quinodimethanes, and periacenes with a singlet biradical ground state.

    PubMed

    Sun, Zhe; Zeng, Zebing; Wu, Jishan

    2014-08-19

    absorption spectroscopy, electrochemistry, and spectroelectrochemistry. These systematic studies revealed that aromaticity played a very important role in determining their singlet biradical character, which is critically related to both their physical properties and their chemical reactivity. In particular, we found that Clar's aromatic sextet rule, which is useful for the closed-shell PAHs, can also predict the relative biradical character of benzenoid PH-based singlet biradicaloids. Other factors, such as structural flexibility of the biradical and quinoid resonance forms and the participation of the substitution in the π-conjugation, also influence the biradical character. These molecular materials demonstrate a number of unique properties such as near-infrared absorption, redox amphotericity, large two-photon absorption cross section, short excited state lifetime, stimuli-responsive magnetic activity, and singlet fission, which suggests promise for future applications. PMID:25068503

  19. Thermally induced excited-state coherent raman spectra of solids

    NASA Astrophysics Data System (ADS)

    Andrews, J. R.; Hochstrasser, R. M.

    1981-09-01

    A difference frequency resonance has been observed for the 747 cm -1 vibration in the first excited singlet state of pentacene in benzoic acid. The resonance is absent at low temperature (4.5 K) and its appearance is exponentially activated with an activation energy of 13.8 cm -1. These observations are compared to theoretical expectations.

  20. Singlet Oxygen in Aqueous Solution: A Lecture Demonstration

    ERIC Educational Resources Information Center

    Shakhashiri, Bassam Z.; Williams, Lloyd G.

    1976-01-01

    Describes a demonstration that illustrates the red chemiluminescence due to singlet molecular oxygen that can be observed when aqueous solutions of hypochlorite ion and hydrogen peroxide are mixed. (MLH)

  1. A tandem mass spectrometric method for singlet oxygen measurement.

    PubMed

    Karonen, Maarit; Mattila, Heta; Huang, Ping; Mamedov, Fikret; Styring, Stenbjörn; Tyystjärvi, Esa

    2014-01-01

    Singlet oxygen, a harmful reactive oxygen species, can be quantified with the substance 2,2,6,6-tetramethylpiperidine (TEMP) that reacts with singlet oxygen, forming a stable nitroxyl radical (TEMPO). TEMPO has earlier been quantified with electron paramagnetic resonance (EPR) spectroscopy. In this study, we designed an ultra-high-performance liquid chromatographic-tandem mass spectrometric (UHPLC-ESI-MS/MS) quantification method for TEMPO and showed that the method based on multiple reaction monitoring (MRM) can be used for the measurements of singlet oxygen from both nonbiological and biological samples. Results obtained with both UHPLC-ESI-MS/MS and EPR methods suggest that plant thylakoid membranes produce 3.7 × 10(-7) molecules of singlet oxygen per chlorophyll molecule in a second when illuminated with the photosynthetic photon flux density of 2000 μmol m(-2 ) s(-1). PMID:24849296

  2. Singlet Exciton Fission in Nanostructured Organic Solar Cells

    SciTech Connect

    Jadhav, P. J.; Mohanty, A.; Sussman, J.; Baldo, Marc

    2011-04-13

    Singlet exciton fission is an efficient multiexciton generation process in organic molecules. But two concerns must be satisfied before it can be exploited in low-cost solution-processed organic solar cells. Fission must be combined with longer wavelength absorption in a structure that can potentially surpass the single junction limit, and its efficiency must be demonstrated in nanoscale domains within blended devices. Here, we report organic solar cells comprised of tetracene, copper phthalocyanine, and the buckyball C{sub 6}0. Short wavelength light generates singlet excitons in tetracene. These are subsequently split into two triplet excitons and transported through the phthalocyanine. In addition, the phthalocyanine absorbs photons below the singlet exciton energy of tetracene. To test tetracene in nanostructured blends, we fabricate coevaporated bulk heterojunctions and multilayer heterojunctions of tetracene and C{sub 60}. We measure a singlet fission efficiency of (71 ± 18)%, demonstrating that exciton fission can efficiently compete with exciton dissociation on the nanoscale.

  3. Collisional excitation of intersteller molecules: Ammonia

    NASA Technical Reports Server (NTRS)

    Green, S.

    1981-01-01

    Theoretical rate constants are presented for excitation of NH3 by collisions with He. The lowest 22 levels of ortho-NH3 and the lowest 16 levels of para-NH3 are considered at kinetic temperatures of 15 to 300 K.

  4. Maximizing Singlet Fission by Intermolecular Packing.

    PubMed

    Wang, Linjun; Olivier, Yoann; Prezhdo, Oleg V; Beljonne, David

    2014-10-01

    A novel nonadiabatic molecular dynamics scheme is applied to study the singlet fission (SF) process in pentacene dimers as a function of longitudinal and lateral displacements of the molecular backbones. Detailed two-dimensional mappings of both instantaneous and long-term triplet yields are obtained, characterizing the advantageous and unfavorable stacking arrangements, which can be achieved by chemical substitutions to the bare pentacene molecule. We show that the SF rate can be increased by more than an order of magnitude through tuning the intermolecular packing, most notably when going from cofacial to the slipped stacked arrangements encountered in some pentacene derivatives. The simulations indicate that the SF process is driven by thermal electron-phonon fluctuations at ambient and high temperatures, expected in solar cell applications. Although charge-transfer states are key to construct continuous channels for SF, a large charge-transfer character of the photoexcited state is found to be not essential for efficient SF. The reported time domain study mimics directly numerous laser experiments and provides novel guidelines for designing efficient photovoltaic systems exploiting the SF process with optimum intermolecular packing. PMID:26278443

  5. Oxygen pressure measurement using singlet oxygen emission

    SciTech Connect

    Khalil, Gamal E.; Chang, Alvin; Gouterman, Martin; Callis, James B.; Dalton, Larry R.; Turro, Nicholas J.; Jockusch, Steffen

    2005-05-15

    Pressure sensitive paint (PSP) provides a visualization of two-dimensional pressure distributions on airfoil and model automobile surfaces. One type of PSP utilizes platinum tetra(pentafluorophenyl)porphine (PtTFPP) dissolved in a fluoro-polymer film. Since the intense 650 nm triplet emission of PtTFPP is quenched by ground state oxygen, it is possible to measure two-dimensional oxygen concentration from the 650 nm emission intensity using a Stern-Volmer-type relationship. This article reports an alternative luminescence method to measure oxygen concentration based on the porphyrin-sensitized 1270 nm singlet oxygen emission, which can be imaged with an InGaAs near infrared camera. This direct measurement of oxygen emission complements and further validates the oxygen measurement based on PtTFPP phosphorescence quenching. Initial success at obtaining a negative correlation between the 650 nm PtTFPP emission and the 1270 nm O{sub 2} emission in solution led us to additional two-dimensional film studies using surfaces coated with PtTFPP, MgTFPP, and H{sub 2}TFPP in polymers in a pressure and temperature controlled chamber.

  6. Charge separated states and singlet oxygen generation of mono and bis adducts of C60 and C70

    NASA Astrophysics Data System (ADS)

    Dallas, Panagiotis; Rogers, Gregory; Reid, Ben; Taylor, Robert A.; Shinohara, Hisanori; Briggs, G. Andrew D.; Porfyrakis, Kyriakos

    2016-02-01

    We present a series of fullerene derivatives and a study on their photoluminescence properties, complete with their efficiency as singlet oxygen generation photosensitizers. We demonstrate the intramolecular charge transfer between pyrene donor and fullerene acceptor. The opposite effect in decay lifetime measurements is observed for the mono and bis adducts of C60 and C70 for the first time, indicating an interplay between charge-separation and locally excited states. A monoexponential decay was observed for the mono adduct of C60 and the bis adduct of C70, while a biexponential decay was observed for the bis adduct of C60 and the mono adduct of C70. The effect of these molecules as sensitizers of the singlet oxygen radical was tested using detailed 3D excitation photoluminescence maps. A quenching of the singlet oxygen for the C60-mono and C70-bis adducts was observed while a strong photosensitizing effect was observed for the C60-bis and C70-mono adducts.

  7. Nanostructured singlet fission photovoltaics subject to triplet-charge annihilation.

    PubMed

    Thompson, Nicholas J; Hontz, Eric; Congreve, Daniel N; Bahlke, Matthias E; Reineke, Sebastian; Van Voorhis, Troy; Baldo, Marc A

    2014-03-01

    Singlet exciton fission is an efficient multiple-exciton generation process that is vulnerable to a characteristic loss process: triplet-charge annihilation. This loss process is characterized in singlet-fission photovoltaics and losses as high as 40% are observed in poorly designed devices. Techniques are demonstrated to improve charge extraction and reduce triplet-charge annihilation to negligible levels at short-circuit conditions. PMID:24281738

  8. Accurate adiabatic singlet-triplet gaps in atoms and molecules employing the third-order spin-flip algebraic diagrammatic construction scheme for the polarization propagator.

    PubMed

    Lefrancois, Daniel; Rehn, Dirk R; Dreuw, Andreas

    2016-08-28

    For the calculation of adiabatic singlet-triplet gaps (STG) in diradicaloid systems the spin-flip (SF) variant of the algebraic diagrammatic construction (ADC) scheme for the polarization propagator in third order perturbation theory (SF-ADC(3)) has been applied. Due to the methodology of the SF approach the singlet and triplet states are treated on an equal footing since they are part of the same determinant subspace. This leads to a systematically more accurate description of, e.g., diradicaloid systems than with the corresponding non-SF single-reference methods. Furthermore, using analytical excited state gradients at ADC(3) level, geometry optimizations of the singlet and triplet states were performed leading to a fully consistent description of the systems, leading to only small errors in the calculated STGs ranging between 0.6 and 2.4 kcal/mol with respect to experimental references. PMID:27586899

  9. Conformational analysis of N-methylformamide in ground S0 and excited S1 and T1 electronic states

    NASA Astrophysics Data System (ADS)

    Tukachev, N. V.; Bataev, V. A.; Godunov, I. A.

    2016-07-01

    For conformers of the N-methylformamide (HCONHCH3) molecule, calculations of equilibrium geometry parameters, harmonic vibration frequencies, energy differences and potential barriers to conformational transitions were performed in the ground (S0) and lowest excited singlet (S1) and triplet (T1) electronic states. In the S0 state, the molecule exists in trans and cis stable conformations (having Cs symmetry). Our calculations show that the electronic excitations T1←S0 and S1←S0 cause changes in the structure of conformers: both HCON and HNCC fragments become pyramidal and rotate around the CN bond. As a result, in each excited electronic state under consideration, there are 12 minima forming six pairs of equivalent conformers separated by relatively small potential barriers. One- and two-dimensional potential energy surface sections corresponding to different intramolecular large-amplitude motions were calculated using the MP2/aug-cc-pVTZ (S0) and CASPT2/cc-pVTZ (S1 and T1) methods. Anharmonic vibrational problems for large-amplitude motions were solved, and the corresponding frequencies were estimated.

  10. Higher Order π-Conjugated Polycyclic Hydrocarbons with Open-Shell Singlet Ground State: Nonazethrene versus Nonacene.

    PubMed

    Huang, Rui; Phan, Hoa; Herng, Tun Seng; Hu, Pan; Zeng, Wangdong; Dong, Shao-Qiang; Das, Soumyajit; Shen, Yongjia; Ding, Jun; Casanova, David; Wu, Jishan

    2016-08-17

    Higher order acenes (i.e., acenes longer than pentacene) and extended zethrenes (i.e., zethrenes longer than zethrene) are theoretically predicted to have an open-shell singlet ground state, and the radical character is supposed to increase with extension of molecular size. The increasing radical character makes the synthesis of long zethrenes and acenes very challenging, and so far, the longest reported zethrene and acene derivatives are octazethrene and nonacene, respectively. In addition, there is a lack of fundamental understanding of the differences between these two closely related open-shell singlet systems. In this work, we report the first synthesis of a challenging nonazethrene derivative, HR-NZ, and its full structural and physical characterizations including variable temperature NMR, ESR, SQUID, UV-vis-NIR absorption and electrochemical measurements. Compound HR-NZ has an open-shell singlet ground state with a moderate diradical character (y0 = 0.48 based on UCAM-B3LYP calculation) and a small singlet-triplet gap (ΔES-T = -5.2 kcal/mol based on SQUID data), thus showing magnetic activity at room temperature. It also shows amphoteric redox behavior, with a small electrochemical energy gap (1.33 eV). Its electronic structure and physical properties are compared with those of Anthony's nonacene derivative JA-NA and other zethrene derivatives. A more general comparison between higher order acenes and extended zethrenes was also conducted on the basis of ab initio electronic structure calculations, and it was found that zethrenes and acenes have very different spatial localization of the unpaired electrons. As a result, a faster decrease of singlet-triplet energy gap and a faster increase of radical character with increase of the number of benzenoid rings were observed in zethrene series. Our studies reveal that spatial localization of the frontier molecular orbitals play a very important role on the nature of radical character as well as the excitation

  11. Singlet oxygen-sensitized delayed fluorescence of common water-soluble photosensitizers.

    PubMed

    Scholz, Marek; Dědic, Roman; Breitenbach, Thomas; Hála, Jan

    2013-10-01

    Six common water-soluble singlet oxygen ((1)O2) photosensitizers - 5,10,15,20-tetrakis(1-methyl-4-pyridinio) porphine (TMPyP), meso-tetrakis(4-sulfonathophenyl)porphine (TPPS4), Al(III) phthalocyanine chloride tetrasulfonic acid (AlPcS4), eosin Y, rose bengal, and methylene blue - were investigated in terms of their ability to produce delayed fluorescence (DF) in solutions at room temperature. All the photosensitizers dissolved in air-saturated phosphate buffered saline (PBS, pH 7.4) exhibit easily detectable DF, which can be nearly completely quenched by 10 mM NaN3, a specific (1)O2 quencher. The DF kinetics has a biexponential rise-decay character in a microsecond time domain. Therefore, we propose that singlet oxygen-sensitized delayed fluorescence (SOSDF), where the triplet state of a photosensitizer reacts with (1)O2 giving rise to an excited singlet state of the photosensitizer, is the prevailing mechanism. It was confirmed by additional evidence, such as a monoexponential decay of triplet-triplet transient absorption kinetics, dependence of SOSDF kinetics on oxygen concentration, absence of SOSDF in a nitrogen-saturated sample, or the effect of isotopic exchange H2O-D2O. Eosin Y and AlPcS4 show the largest SOSDF quantum yield among the selected photosensitizers, whereas rose bengal possesses the highest ratio of SOSDF intensity to prompt fluorescence intensity. The rate constant for the reaction of triplet state with (1)O2 giving rise to the excited singlet state of photosensitizer was estimated to be ~/>1 × 10(9) M(-1) s(-1). SOSDF kinetics contains information about both triplet and (1)O2 lifetimes and concentrations, which makes it a very useful alternative tool for monitoring photosensitizing and (1)O2 quenching processes, allowing its detection in the visible spectral region, utilizing the photosensitizer itself as a (1)O2 probe. Under our experimental conditions, SOSDF was up to three orders of magnitude more intense than the infrared (1)O2

  12. Multidirectional Angular Electronic Flux during Adiabatic Attosecond Charge Migration in Excited Benzene.

    PubMed

    Hermann, Gunter; Liu, ChunMei; Manz, Jörn; Paulus, Beate; Pérez-Torres, Jhon Fredy; Pohl, Vincent; Tremblay, Jean Christophe

    2016-07-14

    Recently, adiabatic attosecond charge migration (AACM) has been monitored and simulated for the first time, with application to the oriented iodoacetylene cation where AACM starts from the initial superposition of the ground state (φ0) and an electronic excited state (φ1). Here, we develop the theory for electronic fluxes during AACM in ring-shaped molecules, with application to oriented benzene prepared in the superposition of the ground and first excited singlet states. The initial state and its time evolution are analogous to coherent tunneling where φ0 and φ1 have different meanings; however, they denote the wave functions of the lowest tunneling doublet. This analogy suggests to transfer the theory of electronic fluxes during coherent tunneling to AACM, with suitable modifications which account for (i) the different time scales and (ii) the different electronic states, and which make use of (iii) the preparation of the initial state for AACM by a linearly polarized laser pulse. Application to benzene yields the multidirectional angular electronic flux with a pincer-motion type pattern during AACM: this unequivocal result confirms a previous working hypothesis. Moreover, the theory of AACM allows quantification of the electronic flux; that is, the maximum number of electrons (out of 42) which flow concertedly during AACM in benzene is 6 × 0.08 = 0.48. PMID:27046151

  13. Evidence for conical intersection dynamics mediating ultrafast singlet exciton fission

    NASA Astrophysics Data System (ADS)

    Musser, Andrew J.; Liebel, Matz; Schnedermann, Christoph; Wende, Torsten; Kehoe, Tom B.; Rao, Akshay; Kukura, Philipp

    2015-04-01

    Singlet exciton fission is the process in organic semiconductors through which a spin-singlet exciton converts into a pair of spin-triplet excitons residing on different chromophores, entangled in an overall spin-zero state. For some systems, singlet fission has been shown to occur on the 100 fs timescale and with a 200% quantum yield, but the mechanism of this process remains uncertain. Here we study a model singlet fission system, TIPS-pentacene, using ultrafast vibronic spectroscopy. We observe that vibrational coherence in the initially photogenerated singlet state is transferred to the triplet state and show that this behaviour is effectively identical to ultrafast internal conversion for polyenes in solution. This similarity in vibronic dynamics suggests that both multi-molecular singlet fission and single-molecular internal conversion are mediated by the same underlying relaxation processes, based on strong coupling between nuclear and electronic degrees of freedom. In its most efficient form this leads to a conical intersection between the coupled electronic states.

  14. Singlet fission in pentacene through multiple exciton quantum states

    NASA Astrophysics Data System (ADS)

    Zhang, Zhiyong; Zimmerman, Paul; Musgrave, Charles

    2010-03-01

    Multi-exciton generation (MEG) has been reported for several materials and may dramatically increase solar cell efficiency. Singlet fission is the molecular analogue of MEG and has been observed in various systems, including tetracene and pentacene, however, no fundamental mechanism for singlet fission has yet been described, although it may govern MEG processes in a variety of materials. Because photoexcited states have single-exciton character, singlet fission to produce a pair of triplet excitons must involve an intermediate state that: (1) exhibits multi-exciton (ME) character, (2) is accessible from S1 and satisfies the fission energy requirement, and (3) efficiently dissociates into multiple electron-hole pairs. Here, we use sophisticated ab initio calculations to show that singlet fission in pentacene proceeds through a dark state (D) of ME character that lies just below S1, satisfies the fission energy requirement (ED>2ET0), and splits into two triplets (2xT0). In tetracene, D lies just above S1, consistent with the observation that singlet fission is thermally activated in tetracene. Rational design of photovoltaic systems that exploit singlet fission will require ab initio analysis of ME states such as D.

  15. Extending the scope of singlet-state spectroscopy.

    PubMed

    Sarkar, Riddhiman; Ahuja, Puneet; Moskau, Detlef; Vasos, Paul R; Bodenhausen, Geoffrey

    2007-12-21

    Different decoupling sequences are tested-using various shaped radio-frequency (RF) pulses-to achieve the longest possible lifetimes of singlet-state populations over the widest possible bandwidths, that is, ranges of offsets and relative chemical shifts of the nuclei involved in the singlet states. The use of sinc or refocusing broadband universal rotation pulses (RE-BURP) for decoupling during the intervals where singlet-state populations are preserved allows one to extend the useful bandwidth with respect to prior state-of-the-art methods based on composite-pulse WALTZ decoupling. The improved sinc decoupling sequences afford a more reliable and sensitive measure of the lifetimes of singlet states in pairs of spins that have widely different chemical shifts, such as the two aromatic protons H(5) and H(6) in uracil. Similar advantages are expected for nucleotides in RNA and DNA. Alternative approaches, in particular frequency-modulated decoupling sequences, also appear to be effective in preserving singlet-state populations, even though the profiles of the apparent relaxation rate constants as a function of the offset are somewhat perturbed. The best decoupling sequences prove their utility in sustaining longer lifetimes of singlet states than previously achieved for the side-chain tyrosine protons in bovine pancreatic trypsin inhibitor (BPTI) at 600 MHz (14.1 T), where the differences of chemical shifts between coupled protons are a challenge. PMID:18061913

  16. Emergence of frustrated antiferromagnet in the lowest Landau level

    NASA Astrophysics Data System (ADS)

    Rhim, Jun Won; Archer, Alexander C.; Jain, Jainendra K.; Park, Kwon; Condensed Matter Theory Collaboration

    2014-03-01

    We investigate the spin structure of the triangular composite fermion crystals (CFCs) in the lowest Landau level (LLL). In contrast to the usual Hund's rule, our Monte-Carlo (MC) calculation finds the spin exchange energy to be antiferromagnetic in certain parameter regimes in the vicinity of ν = 1 / 5 . For further physical intuition, we develop an effective two-body potential between composite fermions in the crystal phase, which provides a reasonable account of the MC results. We discuss the experimental feasibility of this physics.

  17. Lowest order QED radiative corrections to longitudinally polarized Moeller scattering

    SciTech Connect

    Ilyichev, A.; Zykunov, V.

    2005-08-01

    The total lowest-order electromagnetic radiative corrections to the observables in Moeller scattering of longitudinally polarized electrons have been calculated. The final expressions obtained by the covariant method for the infrared divergency cancellation are free from any unphysical cut-off parameters. Since the calculation is carried out within the ultrarelativistic approximation our result has a compact form that is convenient for computing. Basing on these expressions the FORTRAN code MERA has been developed. Using this code the detailed numerical analysis performed under SLAC (E-158) and JLab kinematic conditions has shown that the radiative corrections are significant and rather sensitive to the value of the missing mass (inelasticity) cuts.

  18. Dynamics of momentum entanglement in lowest-order QED

    SciTech Connect

    Lamata, L.; Leon, J.; Solano, E.

    2006-01-15

    We study the dynamics of momentum entanglement generated in the lowest-order QED interaction between two massive spin-(1/2) charged particles, which grows in time as the two fermions exchange virtual photons. We observe that the degree of generated entanglement between interacting particles with initial well-defined momentum can be infinite. We explain this divergence in the context of entanglement theory for continuous variables, and show how to circumvent this apparent paradox. Finally, we discuss two different possibilities of transforming momentum into spin entanglement, through dynamical operations or through Lorentz boosts.

  19. Singlet oxygen generation in gas discharge for oxygen-iodine laser pumping

    NASA Astrophysics Data System (ADS)

    Lopaev, D. V.; Braginsky, O. V.; Klopovsky, K. S.; Kovalev, A. S.; Mankelevich, Yu. A.; Popov, N. A.; Rakhimov, A. T.; Rakhimova, T. V.; Vasilieva, A. N.

    2004-09-01

    The possibility of development of effective discharged singlet oxygen (SO) generator (DSOG) for oxygen-iodine laser (OIL) is studied in detail. Researches of kinetics of oxygen atoms and oxygen molecules in the lowest metastable singlet states have been carried out in the different discharges and its afterglow (DC discharges, E-beam controlled discharge and RF discharges) in both CW and pulsed mode in a wide range of conditions (pressures, gas mixtures, energy deposits etc.). The models developed for all the discharges have allowed us to analyze SO generation and loss mechanisms and to find out the key-parameters controlling the highest SO yield. It is shown that in addition to spatial plasma uniformity at low E/N and high specific energy deposit per oxygen molecule, DSOG must be oxygen atom free to avoid fast three-body quenching of SO by atomic oxygen with increasing pressure and thereby to provide pressure scaling (in tens Torrs) for applying to real OIL systems.

  20. Theoretical Studies of Possible Synthetic Routes for the High Energy Density Material Td N4: Excited Electronic States

    NASA Technical Reports Server (NTRS)

    Lee, Timothy J.; Dateo, Christopher E.

    2001-01-01

    Vertical electronic excitation energies for single states have been computed for the high energy density material (HEDM) Td N4 in order to assess possible synthetic routes that originate from excited electronic states of N2 molecules. Several ab initio theoretical approaches have been used, including complete active space self-consistent field (CASSCF), state averaged CASSCF (SA-CASSCF), singles configuration interaction (CIS), CIS with second-order and third-order correlation corrections [CIS(D)) and CIS(3)], and linear response singles and doubles coupled-cluster (LRCCSD), which is the highest level of theory employed. Standard double zeta polarized (DZP) and triple zeta double polarized (TZ2P) one-particle basis sets were used. The CASSCF calculations are found to overestimate the excitation energies, while the SA-CASSCF approach rectifies this error to some extent, but not completely. The accuracy of the CIS calculations varied depending on the particular state, while the CIS(D), CIS(3), and LRCCSD results are in generally good agreement. Based on the LRCCSD calculations, the lowest six excited singlet states are 9.35(l(sup)T1), 10.01(l(sup)T2), 10.04(1(sup)A2), 10.07(1(sup)E), 10.12(2(sup)T1), and 10.42(2(sup)T2) eV above the ground state, respectively. Comparison of these excited state energies with the energies of possible excited states of N2+N2 fragments, leads us to propose that the most likely synthetic route for Td N4 involving this mechanism arises from combination of two bound quintet states of N2.

  1. Enhancement of singlet oxygen production based on FRET between Coumarin tri-compound and CdSe/ZnS QDs

    NASA Astrophysics Data System (ADS)

    Duong, Hong Dinh; Lee, Jee Won; Rhee, Jong Il

    2014-08-01

    The compatibility between coumarin-derived dendrimer (CdD)-captured silica particles (SiCdDs) and watersoluble CdSe/ZnS quantum dots (QDs) in the FRET process improved the excited state of QDs in the reaction of singlet oxygen production under LED irradiation. Sol-gel GA was successfully used to improve the binding between SiCdDs and QDs. Singlet oxygen production using QDs coated with SiCdDs through sol-gel GA was enhanced by about 80 % compared to that achieved using QDs only. The single oxygen produced by the QDs, the QDs/GA-SiCdDs complexes and the SiCdDs/GA-QDs complexes in this study could be used in the treatment of HeLa cells.

  2. Rationality, Irrationality and Escalating Behavior in Lowest Unique Bid Auctions

    PubMed Central

    Radicchi, Filippo; Baronchelli, Andrea; Amaral, Luís A. N.

    2012-01-01

    Information technology has revolutionized the traditional structure of markets. The removal of geographical and time constraints has fostered the growth of online auction markets, which now include millions of economic agents worldwide and annual transaction volumes in the billions of dollars. Here, we analyze bid histories of a little studied type of online auctions – lowest unique bid auctions. Similarly to what has been reported for foraging animals searching for scarce food, we find that agents adopt Lévy flight search strategies in their exploration of “bid space”. The Lévy regime, which is characterized by a power-law decaying probability distribution of step lengths, holds over nearly three orders of magnitude. We develop a quantitative model for lowest unique bid online auctions that reveals that agents use nearly optimal bidding strategies. However, agents participating in these auctions do not optimize their financial gain. Indeed, as long as there are many auction participants, a rational profit optimizing agent would choose not to participate in these auction markets. PMID:22279553

  3. A new analytical potential energy surface for the singlet state of He2H+

    NASA Astrophysics Data System (ADS)

    Liang, Jing-Juan; Yang, Chuan-Lu; Wang, Li-Zhi; Zhang, Qing-Gang

    2012-03-01

    The analytic potential energy surface (APES) for the exchange reaction of HeH+ (X1Σ+) + He at the lowest singlet state 11A/ has been built. The APES is expressed as Aguado-Paniagua function based on the many-body expansion. Using the adaptive non-linear least-squares algorithm, the APES is fitted from 15 682 ab initio energy points calculated with the multireference configuration interaction calculation with a large d-aug-cc-pV5Z basis set. To testify the new APES, we calculate the integral cross sections for He + H+He (v = 0, 1, 2, j = 0) → HeH+ + He by means of quasi-classical trajectory and compare them with the previous result in literature.

  4. Lowest l=0 proton resonance in {sup 26}Si and implications for nucleosynthesis of {sup 26}Al

    SciTech Connect

    Peplowski, P. N.; Baby, L. T.; Wiedenhoever, I.; Diffenderfer, E.; Hoeflich, P.; Rojas, A.; Volya, A.; Dekat, S. E.; Gay, D. L.; Grubor-Urosevic, O.; Kaye, R. A.; Keeley, N.

    2009-03-15

    Using a beam of the radioactive isotope {sup 25}Al, produced with the new RESOLUT facility, we measured the direct (d,n) proton-transfer reaction leading to low-lying proton resonances in {sup 26}Si. We observed the lowest l=0 proton resonance, identified with the 3{sup +} state at 5.914-MeV excitation energy. This result eliminates the largest uncertainty in astrophysical reaction rates involved in the nucleosynthesis of {sup 26}Al.

  5. Collisionally-Mediated Singlet-Triplet Crossing in ˜{a}1A1 CH_2 Revisited: (010) Coupling

    NASA Astrophysics Data System (ADS)

    Le, Anh T.; Hall, Gregory; Sears, Trevor

    2014-06-01

    Methylene, CH2, possesses a ground ˜{X}3B1 ground electronic state and an excited ˜{a}1A1 state only 3150cm-1 higher in energy. The collision-induced singlet-triplet crossing in the gaseous mixtures is important in determining overall reaction rates and chemical behavior. Accidental near-degeneracies between rotational levels of the singlet state and the vibrationally excited triplet state result in a few gateway rotational levels that mediate collision-induced intersystem crossing. The mixed states can be recognized and quantified by deperturbation, knowing the zero-order singlet and triplet energy levels. Hyperfine structure can be used as alternative indicator of singlet-triplet mixing. Non-zero mixing will induce hyperfine splittings intermediate between the unresolved hyperfine structure of pure singlet and the resolvable (≈50MHz) splittings of pure triplet, arising from the (I\\cdotS) interaction in the ortho states, where nuclear spin I=1. Collision-induced intersystem crossing rates from the (010) state are comparable to those for (000), yet the identities and characters of the presumed gateway states are unknown. A new spectrometer is under construction to investigate triplet mixing rotational levels of ˜{a}1A1(010) by sub-Doppler measurements of perturbation-induced hyperfine splittings. Their observation will permit the identification of gateway states and quantification of the degree of triplet contamination of the singlet wavefunction. Progress in the measurements and the analysis of rotational energy transfer in (010) will be reported. Acknowledgments: Work at Brookhaven National Laboratory was carried out under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy and supported by its Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. C.-H. Chang, G. E. Hall, T. J. Sears, J. Chem. Phys 133, 144310(2010) G. E. Hall, A. V. Komissarov, and T. J. Sears, J. Phys. Chem. A 108 7922-7927 (2004)

  6. Biological hydroperoxides and singlet molecular oxygen generation.

    PubMed

    Miyamoto, Sayuri; Ronsein, Graziella E; Prado, Fernanda M; Uemi, Miriam; Corrêa, Thais C; Toma, Izaura N; Bertolucci, Agda; Oliveira, Mauricio C B; Motta, Flávia D; Medeiros, Marisa H G; Mascio, Paolo Di

    2007-01-01

    The decomposition of lipid hydroperoxides (LOOH) into peroxyl radicals is a potential source of singlet molecular oxygen ((1)O(2)) in biological systems. Recently, we have clearly demonstrated the generation of (1)O(2) in the reaction of lipid hydroperoxides with biologically important oxidants such as metal ions, peroxynitrite and hypochlorous acid. The approach used to unequivocally demonstrate the generation of (1)O(2) in these reactions was the use of an isotopic labeled hydroperoxide, the (18)O-labeled linoleic acid hydroperoxide, the detection of labeled compounds by HPLC coupled to tandem mass spectrometry (HPLC-MS/MS) and the direct spectroscopic detection and characterization of (1)O(2) light emission. Using this approach we have observed the formation of (18)O-labeled (1)O(2) by chemical trapping of (1)O(2) with anthracene derivatives and detection of the corresponding labeled endoperoxide by HPLC-MS/MS. The generation of (1)O(2) was also demonstrated by direct spectral characterization of (1)O(2) monomol light emission in the near-infrared region (lambda = 1270 nm). In summary, our studies demonstrated that LOOH can originate (1)O(2). The experimental evidences indicate that (1)O(2) is generated at a yield close to 10% by the Russell mechanism, where a linear tetraoxide intermediate is formed in the combination of two peroxyl radicals. In addition to LOOH, other biological hydroperoxides, including hydroperoxides formed in proteins and nucleic acids, may also participate in reactions leading to the generation (1)O(2). This hypothesis is currently being investigated in our laboratory. PMID:17505972

  7. Mechanism of the photochemical process of singlet oxygen production by phenalenone.

    PubMed

    Segado, Mireia; Reguero, Mar

    2011-03-01

    Phenalenone (PN) is a very efficient singlet oxygen sensitiser in a wide range of solvents. This work uses ab initio quantum chemical calculations (CASSCF/CASPT2 protocol) to study the mechanism for populating the triplet state of PN responsible for this reaction, the (3)(π-π*) state. To describe in detail this reaction path, the singlet and triplet low-lying excited states of PN have been studied, the critical points of the potential energy surfaces corresponding to these states located and the vertical and adiabatic energies calculated. Our results show that, after the initial population of the S(2) excited state of (π-π*) character, the system undergoes an internal conversion to the (1)(n-π*) state. After populating the dark S(1) state, the system relaxes to the (1)(n-π*) minimum, but rapidly populates the triplet manifold through a very efficient intersystem crossing to the (3)(π-π*) state. Although the population of the minimum of this triplet state is strongly favoured, a conical intersection with the (3)(n-π*) surface opens an internal conversion channel to this state, a path accessible only at high temperatures. Radiationless deactivation processes are ruled out on the basis of the high-energy barriers found for the crossings between the excited states and the ground state. Our computational results satisfactorily explain the experimental findings and are in very good agreement with the experimental data available. In the case of the frequency of fluorescence, this is the first time that these data have been theoretically predicted in good agreement with the experimental results. PMID:21225064

  8. Singlet-triplet energy gaps for diradicals from particle-particle random phase approximation.

    PubMed

    Yang, Yang; Peng, Degao; Davidson, Ernest R; Yang, Weitao

    2015-05-21

    The particle-particle random phase approximation (pp-RPA) for calculating excitation energies has been applied to diradical systems. With pp-RPA, the two nonbonding electrons are treated in a subspace configuration interaction fashion while the remaining part is described by density functional theory (DFT). The vertical or adiabatic singlet-triplet energy gaps for a variety of categories of diradicals, including diatomic diradicals, carbene-like diradicals, disjoint diradicals, four-π-electron diradicals, and benzynes are calculated. Except for some excitations in four-π-electron diradicals, where four-electron correlation may play an important role, the singlet-triplet gaps are generally well predicted by pp-RPA. With a relatively low O(r(4)) scaling, the pp-RPA with DFT references outperforms spin-flip configuration interaction singles. It is similar to or better than the (variational) fractional-spin method. For small diradicals such as diatomic and carbene-like ones, the error of pp-RPA is slightly larger than noncollinear spin-flip time-dependent density functional theory (NC-SF-TDDFT) with LDA or PBE functional. However, for disjoint diradicals and benzynes, the pp-RPA performs much better and is comparable to NC-SF-TDDFT with long-range corrected ωPBEh functional and spin-flip configuration interaction singles with perturbative doubles (SF-CIS(D)). In particular, with a correct asymptotic behavior and being almost free from static correlation error, the pp-RPA with DFT references can well describe the challenging ground state and charge transfer excitations of disjoint diradicals in which almost all other DFT-based methods fail. Therefore, the pp-RPA could be a promising theoretical method for general diradical problems. PMID:25891638

  9. The relativistic polarization propagator for the calculation of electronic excitations in heavy systems

    SciTech Connect

    Pernpointner, Markus

    2014-02-28

    In this work, we present a new four-component implementation of the polarization propagator for accurate calculations of excited states in heavy systems. Differences to existing nonrelativistic realizations are detailed and the energetically lowest final states of the ns{sup 2}np{sup 6} → ns{sup 2}np{sup 5}(n + 1)s{sup 1} and ns{sup 2}np{sup 6} → ns{sup 2}np{sup 5}(n + 1)p{sup 1} transitions in noble gases are calculated and compared with experimental data. Already for the light atoms Ne and Ar spin-orbit coupling leads to noticeable zero field splitting that gradually increases in the heavier homologues and eventually invalidates the LS-based description of singlet and triplet excited states. For all four noble gases Ne through Xe, we observe a very good agreement with experimental transition energies in the considered energetic range where the extended version of the propagator implementation in general yields better excitation energy differences than the strict variant. In the extended version, off-diagonal first-order contributions in the two-particle-two-hole block are included that are not present in the strict variant. In case of Kr and Xe, nonrelativistic approaches already exhibit unacceptable deviations in the reproduction of transition energies and the spectral structure. The obtained excited final states are analyzed in terms of atomic contributions to the donor and acceptor orbitals constituting the corresponding wave functions. The relativistic polarization propagator provides a consistent description of electron correlation and relativistic effects especially relevant for the heavier systems where these two contributions are no longer separable.

  10. Scintillation advantages of lowest order Bessel-Gaussian beams

    NASA Astrophysics Data System (ADS)

    Eyyuboğlu, H. T.; Baykal, Y.; Sermutlu, E.; Cai, Y.

    2008-08-01

    For a weak turbulence propagation environment, the scintillation index of the lowest order Bessel-Gaussian beams is formulated. Its triple and single integral versions are presented. Numerical evaluations show that at large source sizes and large width parameters, when compared at the same source size, Bessel-Gaussian beams tend to exhibit lower scintillations than the Gaussian beam scintillations. This advantage is lost however for excessively large width parameters and beyond certain propagation lengths. Large width parameters also cause rises and falls in the scintillation index of off-axis positions toward the edges of the received beam. Comparisons against the fundamental Gaussian beam are made on equal source size and equal power basis.

  11. Singlet and triplet excitons and charge polarons in cycloparaphenylenes. A density functional theory study

    SciTech Connect

    Liu, Jin; Adamska, Lyudmyla; Doorn, Stephen K.; Tretiak, Sergei

    2015-05-14

    Conformational structure and the electronic properties of various electronic excitations in cycloparaphenylenes (CPPs) are calculated using hybrid Density Functional Theory (DFT). The results demonstrate that wavefunctions of singlet and triplet excitons as well as the positive and negative polarons remain fully delocalized in CPPs. In contrast, these excitations in larger CPP molecules become localized on several phenyl rings, which are locally planarized, while the undeformed ground state geometry is preserved on the rest of the hoop. As evidenced by the measurements of bond-length alternation and dihedral angles, localized regions show stronger hybridization between neighboring bonds and thus enhanced electronic communication. This effect is even more significant in the smaller hoops, where phenyl rings have strong quinoid character in the ground state. Thus, upon excitation, electron–phonon coupling leads to the self-trapping of the electronic wavefunction and release of energy from fractions of an eV up to two eVs, depending on the type of excitation and the size of the hoop. The impact of such localization on electronic and optical properties of CPPs is systematically investigated and compared with the available experimental measurements.

  12. The Dark Singlet State as Doorway State of Intersystem Crossing in DNA Monomers.

    NASA Astrophysics Data System (ADS)

    Reichardt, Christian; Crespo-Hernández, Carlos E.

    2010-06-01

    The excited state dynamics of 9-methylpurine (9MP) were studied with broadband transient absorption spectroscopy in the time regime from femtoseconds to 3 nanoseconds. Excitation of 9MP in aqueous solutions at 266 nm results in ultrafast internal conversion from the initially excited S_2(ππ^ast) state to the S_1 state. Quantum chemical calculations that include bulk and explicit solvent interactions show that the S_1 state has significant nπ^ast character. Population of the S_1 state is followed by intersystem crossing (ISC) to the T_1(ππ^ast) state on a time scale of hundreds of picoseconds. Vanishingly small fluorescence yields were measured, supporting the dark character of the S_1 state as well as the high triplet yield in 9MP. Analogous experiments in acetonitrile show a decrease in the ISC lifetime by almost 50 % but an equally high triplet yield. The results presented in this work demonstrate the important role that the dark singlet state has in modulating the excited-state dynamics of DNA monomers in solution.

  13. Singlet and triplet energy transfer in the peridinin-chlorophyll a-protein from Amphidinium carterae

    SciTech Connect

    Bautista, J.A.; Frank, H.A.; Hiller, R.G.; Sharples, F.P.; Gosztola, D.; Wasielewski, M. |

    1999-04-08

    The spectroscopic properties of peridinin in solution, and the efficiency and dynamics of energy transfer from peridinin to chlorophyll a in the peridinin-chlorophyll-protein (PCP) from Amphidinium carterae, were studied by steady-state absorption, fluorescence, fluorescence excitation, and fast transient optical spectroscopy. Steady-state measurements of singlet energy transfer from peridinin to chlorophyll revealed an 88 {+-} 2% efficiency. Fast-transient absorption experiments showed that the excited S{sub 1} state of peridinin decayed in 13.4 {+-} 0.6 ps in methanol and 3.1 {+-} 0.4 ps in the PCP complex after direct excitation of the carotenoid. The onset of the bleaching of the chlorophyll absorption band at 672 nm, signifying the arrival of the excitation from the carotenoid, occurred in 3.2 {+-} 0.3 ps. These data show that the primary route of energy transfer from peridinin to chlorophyll in the PCP complex is through the S{sub 1} state of peridinin. Nanosecond time-resolved transient optical spectroscopy revealed that chlorophyll triplet states are efficiently quenched by peridinin whose triplet state subsequently decays with a lifetime of 10 {+-} 1 {micro}s in the PCP complex. Close association between the peridinins and chlorophyll, which is clearly evident in the 3-D structure of the PCP complex, along with proper alignment of pigments and energy state matching are responsible for the high efficiencies of the photochemical processes.

  14. Singlet and triplet excitons and charge polarons in cycloparaphenylenes. A density functional theory study

    DOE PAGESBeta

    Liu, Jin; Adamska, Lyudmyla; Doorn, Stephen K.; Tretiak, Sergei

    2015-05-14

    Conformational structure and the electronic properties of various electronic excitations in cycloparaphenylenes (CPPs) are calculated using hybrid Density Functional Theory (DFT). The results demonstrate that wavefunctions of singlet and triplet excitons as well as the positive and negative polarons remain fully delocalized in CPPs. In contrast, these excitations in larger CPP molecules become localized on several phenyl rings, which are locally planarized, while the undeformed ground state geometry is preserved on the rest of the hoop. As evidenced by the measurements of bond-length alternation and dihedral angles, localized regions show stronger hybridization between neighboring bonds and thus enhanced electronic communication.more » This effect is even more significant in the smaller hoops, where phenyl rings have strong quinoid character in the ground state. Thus, upon excitation, electron–phonon coupling leads to the self-trapping of the electronic wavefunction and release of energy from fractions of an eV up to two eVs, depending on the type of excitation and the size of the hoop. The impact of such localization on electronic and optical properties of CPPs is systematically investigated and compared with the available experimental measurements.« less

  15. The singlet-triplet absorption and photodissociation of the HOCl, HOBr, and HOI molecules calculated by the MCSCF quadratic response method

    SciTech Connect

    Minaev, B.F.

    1999-09-09

    The molecular absorption spectra of hypochlorous, hypobromous, and hypoiodous acids have been studied by multiconfiguration self-consistent field (MCSCF) calculations with linear and quadratic response techniques. The complete form of the spin-orbit coupling (SOC) operator is accounted. The singlet-triplet transition to the lowest triplet state {sup 3}A{double{underscore}prime} {l{underscore}arrow} X{sup 1}A{prime} is shown to be responsible for the weak long-wavelength tail absorption and photodissociation in these molecules. The transition is polarized along the O-X bond (X = Cl, Br, I) and has an oscillator strength equal 6 x 10{sup {minus}6}, 8 x 10{sup {minus}5}, and 2 x 10{sup {minus}4} for hypochlorous, hypobromous, and hypoiodous acids, respectively. The second singlet-triplet transition {sup 3}A{prime} {l{underscore}arrow} X{sup 1}A{prime} comes to the region of the first singlet-singlet {sup 1}A{double{underscore}prime} {l{underscore}arrow} X{sup 1}A{prime} absorption and contributes significantly to the total cross-section at wavelengths {lambda} {approx} 300--320 nm (X = Cl), {lambda} {approx} 340--360 nm (X = Br), and {lambda} {approx} 400 nm (X = I). In the last case the singlet-triplet transition {sup 3}A{prime} {l{underscore}arrow} X{sup 1}A{prime} produces predominant contribution to HOI absorption in the visible region. All states are dissociative, so the singlet-triplet absorption contributes to the yield of photolysis in the important near-UV and visible region close to the intense solar actinic flux. Contributions to the removal mechanisms for atmospheric HOCl, HOBr, and HOI species are shortly discussed. The minor loss process of ozone in troposphere because of the HOI reservoir sink is getting evident on the ground of this calculations. The importance of SOC accounting for atmospheric photochemistry problems is stressed.

  16. Triplet-triplet energy transfer and protection mechanisms against singlet oxygen in photosynthesis

    NASA Astrophysics Data System (ADS)

    Kihara, Shigeharu

    In photosynthesis, (bacterio)chlorophylls ((B)Chl) play a crucial role in light harvesting and electron transport. (B)Chls, however, are known to be potentially dangerous due to the formation of the triplet excited state which forms the singlet oxygen (1O2*) when exposed to the sunlight. Singlet oxygen is highly reactive and all modern organisms incorporate special protective mechanisms to minimize the oxidative damage. One of the conventional photoprotective mechanisms used by photosynthetic organisms is by the nearby carotenoids quenching the excess energy and releasing it by heat. In this dissertation, two major aspects of this process are studied. First, based on experimental data and model calculations, the oxygen content in a functioning oxygenic photosynthetic oxygen cell was determined. These organisms perform water splitting and as a result significant amount of oxygen can be formed within the organism itself. It was found, that contrary to some published estimates, the excess oxygen concentration generated within an individual cell is extremely low -- 0.025 ... 0.25 microM, i.e. about 103-104 times lower than the oxygen concentration in air saturated water. Such low concentrations imply that the first oxygenic photosynthetic cells that evolved in oxygen-free atmosphere of the Earth ~2.8 billion years ago might have invented the water splitting machinery (photosystem II) without the need for special oxygen-protective mechanisms, and the latter mechanisms could have evolved in the next 500 million years during slow rise of oxygen in the atmosphere. This result also suggests that proteins within photosynthetic membranes are not exposed to significant O2 levels and thus can be studied in vitro under the usual O2 levels. Second, the fate of triplet excited states in the Fenna Matthew Olson (FMO) pigment-protein complex is studied by means of time-resolved nanosecond spectroscopy and exciton model simulations. For the first time, the properties of several

  17. Toward Designed Singlet Fission: Electronic States and Photophysics of 1,3-Diphenylisobenzofuran

    SciTech Connect

    Schwerin, A.F.; Miller, J.; Johnson, J.C.; Smith, M.B.; Sreearunothai, P.; Popovic, D.; Cerny, J.; Havlas, Z.; Paci, I.; Akdag, A.; MacLeod, M.K.; Chen, X.; David, D.E.; Ratner, M.A.; Nozik, A.J.; Mich, J.

    2009-12-21

    Single crystal molecular structure and solution photophysical properties are reported for 1,3-diphenylisobenzofuran (1), of interest as a model compound in studies of singlet fission. For the ground state of 1 and of its radical cation (1{sup +{sm_bullet}}) and anion (1{sup -{sm_bullet}}), we report the UV-visible absorption spectra, and for neutral 1, also the magnetic circular dichroism (MCD) and the decomposition of the absorption spectrum into purely polarized components, deduced from fluorescence polarization. These results were used to identify a series of singlet excited states. For the first excited singlet and triplet states of 1, the transient visible absorption spectra, S{sub 1} {yields} S{sub x} and sensitized T{sub 1} {yields} T{sub x}, and single exponential lifetimes, {tau}{sub F} = {approx} 5.3 ns and {tau}{sub T} = {approx}200 {mu}s, are reported. The spectra and lifetimes of S{sub 1} {yields} S{sub 0} fluorescence and sensitized T{sub 1} {yields} T{sub x} absorption of 1 were obtained in a series of solvents, as was the fluorescence quantum yield, {Phi}{sub F} = 0.95-0.99. No phosphorescence has been detected. The first triplet excitation energy of solid 1 (11,400 cm{sup -1}) was obtained by electron energy loss spectroscopy, in agreement with previously reported solution values. The fluorescence excitation spectrum suggests an onset of a nonradiative channel at {approx} 37,000 cm{sup -1}. Excitation energies and relative transition intensities are in agreement with those of ab initio (CC2) calculations after an empirical 3000 cm{sup -1} adjustment of the initial state energy to correct differentially for a better quality description of the initial relative to the terminal state of an absorption transition. The interpretation of the MCD spectrum used the semiempirical PPP method, whose results for the S{sub 0} {yields} S{sub x} spectrum require no empirical adjustment and are otherwise nearly identical with the CC2 results in all respects

  18. Valence and Ionic Lowest-Lying Electronic States of Isobutyl Formate Studied by High-Resolution Vacuum Ultraviolet Photoabsorption, Photoelectron Spectroscopy, and Ab Initio Calculations.

    PubMed

    Śmiałek, M A; Łabuda, M; Guthmuller, J; Hoffmann, S V; Jones, N C; MacDonald, M A; Zuin, L; Mason, N J; Limão-Vieira, P

    2015-08-13

    The highest resolution vacuum ultraviolet photoabsorption spectrum of isobutyl formate, C5H10O2, yet reported is presented over the energy range 4.5-10.7 eV (275.5-118.0 nm) revealing several new spectral features. Valence and Rydberg transitions and their associated vibronic series observed in the photoabsorption spectrum have been assigned in accordance with new ab initio calculations of the vertical excitation energies and oscillator strengths. Calculations have also been carried out to determine the ionization energies and fine structure of the lowest ionic state of isobutyl formate and are compared with a newly recorded photoelectron spectrum (from 9.0 to 27.0 eV). The value of the first ionization energy was determined to be 10.508 eV (adiabatic) and 10.837 eV (vertical). New vibrational structure is observed in the first photoelectron band, predominantly resulting from C-O and C═O stretches of the molecule. The photoabsorption cross sections have been used to calculate the photolysis lifetime of isobutyl formate in the upper stratosphere (20-50 km), indicating that the hydroxyl radical processes will be the main loss process for isobutyl formate. PMID:26176891

  19. Singlet levels of the NV- centre in diamond

    NASA Astrophysics Data System (ADS)

    Rogers, L. J.; Doherty, M. W.; Barson, M. S. J.; Onoda, S.; Ohshima, T.; Manson, N. B.

    2015-01-01

    The characteristic transition of the N{{V}-} centre at 637 nm is between 3{{A}2} and 3E triplet states. There are also intermediate 1{{A}1} and 1E singlet states, and the infrared transition at 1042 nm between these singlets is studied here using uniaxial stress. The stress shift and splitting parameters are determined, and the physical interaction giving rise to the parameters is considered within the accepted electronic model of the centre. It is established that this interaction for the infrared transition is due to a modification of electron-electron Coulomb repulsion interaction. This is in contrast to the visible 637 nm transition where shifts and splittings arise from modification to the one-electron Coulomb interaction. It is also established that a dynamic Jahn-Teller interaction is associated with the singlet 1E state, which gives rise to a vibronic level 115 cm-1 above the 1E electronic state. Arguments associated with this level are used to provide experimental confirmation that the 1{{A}1} is the upper singlet level and 1E is the lower singlet level.

  20. Impact of Intermolecular Distance on Singlet Fission in a Series of TIPS Pentacene Compounds.

    PubMed

    Wu, Yishi; Liu, Ke; Liu, Huiying; Zhang, Yi; Zhang, Haoli; Yao, Jiannian; Fu, Hongbing

    2014-10-16

    Singlet fission has attracted considerable interest for its potential application in organic photovoltaics. However, the underlying microscopic mechanism is not well understood and the molecular parameters that govern SF efficiency remain unclear. We herein study the primary exciton photogeneration and evolution in the thin film of a series of pentacene derivatives (TIPS-Pn and ADPD-Pn) using femtosecond transient absorption spectroscopy. With a favorable "long-edge on" packing motif, the singlet-excited slip-stacked TIPS-Pn and ADPD-Pn molecules undergo ultrafast fission to produce triplet excitonic states with time constants of ∼0.3 ps. More importantly, the ADPD-Pn compound features a considerably higher triplet yield than TIPS-Pn (162 ± 10% vs 114 ± 15%). The enhanced electronic coupling as a result of closer interchromophore distance (3.33 Å for ADPD-Pn vs 3.40 Å for TIPS-Pn) is suggested to account for the much higher triplet yield for ADPD-Pn relative to that for TIPS-Pn, proving SF can be readily modulated by adjusting the intermolecular distance. PMID:26278592

  1. On The Geometric Nature of ``Singlet Fission'' in Certain Crystalline Conjugated Polymers

    NASA Astrophysics Data System (ADS)

    Rahman, Noah

    2013-03-01

    In recent years, the coherent fission of low-lying singlet electronic excitations in conjugated polymers has attracted interest as a possible way to exceed the Shockley-Queisser limit in organic photovoltaics. Femtosecond spectroscopic and fluorescence measurements of such singlets and the resulting triplets in crystalline anthracene, tetracene and naphthalene reveal curious phenomena associated with certain vibrational modes, such as ultrafast propagation on a timescale inconsistent with conventional intersystem crossing, long-lived electronic coherence, and triplet magnetic anisotropy whose structure is consistent across all three materials. This conflicts with NRG and quantum chemical simulations, which posit isotropic triplets. I explain this by a dynamical Rashba spin-orbit interaction that decays as R-6. This arises from a geometric SU(2) gauge potential generated by a nuclear-motion-induced parametric near-degeneracy of the molecular electronic states. The anisotropy is shown to follow from the work of Affleck and Oshikawa on spin one-half Heisenberg chains. Possible directions for future work are discussed, especially with regard to adiabatic pumping and topological insulators.

  2. Direct Oxygen Abundances for the Lowest Luminosity LVL Galaxies

    NASA Astrophysics Data System (ADS)

    Berg, Danielle; Skillman, E. D.; Marble, A. R.; van Zee, L.; Engelbracht, C. W.

    2012-01-01

    We present new MMT spectroscopic observations of HII regions in 42 of the lowest luminosity galaxies in the Spitzer Local Volume Legacy (LVL) survey. For 31 of the galaxies in our sample we were able to measure the [OIII] ? auroral line at a strength of 4σ or greater, and thus determine oxygen abundances using the direct method. Direct oxygen abundances were compared to B-band luminosity, 4.5 μm luminosity, and stellar mass to characterize the luminosity-metallicity (L-Z) and mass-metallicity (M-Z) relationships at low-luminosity. We examined a "Combined Select” sample composed of 38 objects, from the present sample and the literature, with direct oxygen abundances and reliable distance determinations (based on the tip of the red giant branch or Cepheid variables). The B-band and 4.5 μm L-Z relationships were found to be 12+log(O/H) = (6.19±0.07) + (-0.12±0.01)MB and 12+log(O/H) = (5.93±0.11) + (-0.11±0.01)M[4.5] with dispersions of σ = 0.17 and σ = 0.14 respectively. Since the slope of the L-Z relationship doesn't seem to vary from the optical to the near-IR, as has been observed in studies of more luminous galaxies, we propose that less extinction due to dust is created in the lowest luminosity galaxies. We subsequently derived a M-Z relationship of 12+log(O/H) = (5.49±0.23) + (0.31±0.03)log M*, with a dispersion of σ = 0.16. None of the relationships seem to hold an advantage with respect to dispersion, supporting the idea of minimized dust. Additionally, the trend of N/O abundance with respect to B-V color and oxygen abundance was examined. Similar to the conclusions of van Zee & Haynes (2006), we find a positive correlation between N/O ratio and B-V color: log(N/O) = 0.92 (B-V) - 1.83. Furthermore, there are no objects with high N/O ratio below 12+log(O/H)=7.9.

  3. Positioning configurations with the lowest GDOP and their classification

    NASA Astrophysics Data System (ADS)

    Xue, Shuqiang; Yang, Yuanxi

    2015-01-01

    The positioning configuration optimization is a basic problem in surveying, and the geometric dilution of precision (GDOP) is a key index to handle this problem. Simplex graphs as regular polygons and regular polyhedrons are the well-known configurations with the lowest GDOP. However, it has been proved that there are at most five kinds of regular polyhedrons. We analytically solve the GDOP minimization problem with arbitrary observational freedom to extend the current knowledge. The configuration optimization framework established is composed of the algebraic and geometric operators (including combination, reflection, collinear mapping, projection and three kinds of equivalence relations), basic properties to GDOP minimization (including continuity, combination invariant, reflection invariant, rotation invariant and collinear invariant) and the lowest GDOP configurations (including cones, regular polygons, regular polyhedrons, Descartes configuration, helical configuration and generalized Walker configuration, and their reflections and combinations). GDOP minimization criterion and D-maximization criterion both reduce to the same criterion matrices that the optimization becomes the problem for solving an underdetermined quadratic equation system. Making use of the concepts for solving underdetermined linear equation system, the concepts of base configuration (single classification) and general configuration (combined classification) are applied to the GDOP minimization to analytically solve the quadratic equation system. Firstly, the problems are divided into two subproblems by two kinds of GDOP to reveal the impact of the clock-offset on the configuration optimization, and it shows that the symmetry and uniformity play a key role in identifying the systematic errors. Then, the solution of the GDOP minimization is classified by the number of symmetry axes, that the base configurations with at least one symmetry axis and the general configurations without symmetry

  4. Reaction of C2H2+ (n.ν2, m.ν5) with NO2: Reaction on the singlet and triplet surfaces

    NASA Astrophysics Data System (ADS)

    Boyle, Jason M.; Bell, David M.; Anderson, Scott L.

    2011-01-01

    Integral cross sections and product recoil velocity distributions were measured for reaction of C2H2+ with NO2, in which the C2H2+ reactant was prepared in its ground state, and with mode-selective excitation in the cis-bend (2ν5) and CC stretch (n.ν2, n = 1, 2). Because both reactants have one unpaired electron, collisions can occur with either singlet or triplet coupling of these unpaired electrons, and the contributions are separated based on distinct recoil dynamics. For singlet coupling, reaction efficiency is near unity, with significant branching to charge transfer (NO2+), O- transfer (NO+), and O transfer (C2H2O+) products. For triplet coupling, reaction efficiency varies between 13% and 19%, depending on collision energy. The only significant triplet channel is NO+ + triplet ketene, generated predominantly by O- transfer, with a possible contribution from dissociative charge transfer at high collision energies. NO2+ formation (charge transfer) can only occur on the singlet surface, and appears to be mediated by a weakly bound complex at low energies. O transfer (C2H2O+) also appears to be dominated by reaction on the singlet surface, but is quite inefficient, suggesting a bottleneck limiting coupling to this product from the singlet reaction coordinate. The dominant channel is O- transfer, producing NO+, with roughly equal contributions from reaction on singlet and triplet surfaces. The effects of C2H2+ vibration are modest, but mode specific. For all three product channels (i.e., charge, O-, and O transfer), excitation of the CC stretch fundamental (ν2) has little effect, 2.ν2 excitation results in ˜50% reduction in reactivity, and excitation of the cis-bend overtone (2.ν5) results in ˜50% enhancement. The fact that all channels have similar mode dependence suggests that the rate-limiting step, where vibrational excitation has its effect, is early on the reaction coordinate, and branching to the individual product channels occurs later.

  5. Smarter than Others? Conjectures in Lowest Unique Bid Auctions

    PubMed Central

    Hu, Rui; Chen, Qinghua

    2015-01-01

    Research concerning various types of auctions, such as English auctions, Dutch auctions, highest-price sealed-bid auctions, and second-price sealed-bid auctions, is always a topic of considerable interest in interdisciplinary fields. The type of auction, known as a lowest unique bid auction (LUBA), has also attracted significant attention. Various models have been proposed, but they often fail to explain satisfactorily the real bid-distribution characteristics. This paper discusses LUBA bid-distribution characteristics, including the inverted-J shape and the exponential decrease in the upper region. The authors note that this type of distribution, which initially increases and later decreases, cannot be derived from the symmetric Nash equilibrium framework based on perfect information that has previously been used. A novel optimization model based on non-perfect information is presented. The kernel of this model is the premise that agents make decisions to achieve maximum profit based on imaginary information or assumptions regarding the behavior of others. PMID:25849631

  6. Lowest cost due to highest productivity and highest quality

    NASA Astrophysics Data System (ADS)

    Wenk, Daniel

    2003-03-01

    Since global purchasing in the automotive industry has been taken up all around the world there is one main key factor that makes a TB-supplier today successful: Producing highest quality at lowest cost. The fact that Tailored Blanks, which today may reach up to 1/3 of a car body weight, are purchased on the free market but from different steel suppliers, especially in Europe and NAFTA, the philosophy on OEM side has been changing gradually towards tough evaluation criteria. "No risk at the stamping side" calls for top quality Tailored- or Tubular Blank products. Outsourcing Tailored Blanks has been starting in Japan but up to now without any quality request from the OEM side like ISO 13919-1B (welding quality standard in Europe and USA). Increased competition will automatically push the quality level and the ongoing approach to combine high strength steel with Tailored- and Tubular Blanks will ask for even more reliable system concepts which enables to weld narrow seams at highest speed. Beside producing quality, which is the key to reduce one of the most important cost driver "material scrap," in-line quality systems with true and reliable evaluation is going to be a "must" on all weld systems. Traceability of all process related data submitted to interfaces according to customer request in combination with ghost-shift-operation of TB systems are tomorrow's state-of-the-art solutions of Tailored Blank-facilities.

  7. Reinvestigation of the triplet-minus-singlet spectrum of chloroplasts

    NASA Astrophysics Data System (ADS)

    Jávorfi, T.; Garab, G.; Razi Naqvi, K.

    2000-01-01

    A comparison of the triplet-minus-singlet (TmS) absorption spectrum of spinach chloroplasts, recorded some thirty years ago, with the more recently published TmS spectrum of isolated Chl a/ b LHCII (light-harvesting complexes associated with photosystem II of higher plants) shows that the two spectra are very similar, which is to be expected, since only the carotenoid pigments contribute to each spectrum. Be that as it may, the comparison also reveals a dissimilarity: photoexcitation of the sample does, or does not, affect the absorbance in the Qy region (650-700 nm), depending on whether the sample is a suspension of chloroplasts or of isolated LHCII. The Qy-signal in the TmS spectrum of LHCII decays, it should be noted, at the same rate as the rest of the difference spectrum, and its most prominent feature is a negative peak. As the carotenoids do not absorb in the Qy region, the presence of a signal in this region calls for an explanation: van der Vos, Carbonera and Hoff, the first to find as well as fathom the phenomenon, attributed the Qy-signal to a change, in the absorption spectrum of a chlorophyll a (Chl a) molecule, brought about by the presence of triplet excitation on a neighbouring carotenoid (Car). The difference in the behaviours of chloroplasts and LHCII, if reproducible, would imply that the Car triplets which give rise to the TmS spectrum of chloroplasts do not influence the absorption spectra of their Chl a neighbours. With a view to reaching a firm conclusion about this vexed issue, spinach chloroplasts and thylakoids have been examined with the aid of the same kinetic spectrometer as that used for investigating LHCII; the TmS spectra of both chloroplasts and thylakoids contain prominent bleaching signals centred at 680 nm, and the triplet decay time in each case is comparable to that of the Chl a/ b LHCII triplets. Results pertaining to other closely related systems are recalled, and it is concluded that, so far as the overall appearance of the

  8. Ground state of naphthyl cation: Singlet or triplet?

    SciTech Connect

    Dutta, Achintya Kumar; Vaval, Nayana Pal, Sourav; Manohar, Prashant U.

    2014-03-21

    We present a benchmark theoretical investigation on the electronic structure and singlet-triplet(S-T) gap of 1- and 2-naphthyl cations using the CCSD(T) method. Our calculations reveal that the ground states of both the naphthyl cations are singlet, contrary to the results obtained by DFT/B3LYP calculations reported in previous theoretical studies. However, the triplet states obtained in the two structural isomers of naphthyl cation are completely different. The triplet state in 1-naphthyl cation is (π,σ) type, whereas in 2-naphthyl cation it is (σ,σ{sup ′}) type. The S-T gaps in naphthyl cations and the relative stability ordering of the singlet and the triplet states are highly sensitive to the basis-set quality as well as level of correlation, and demand for inclusion of perturbative triples in the coupled-cluster ansatz.

  9. Understanding electronically non-adiabatic relaxation dynamics in singlet fission.

    PubMed

    Tao, Guohua

    2015-01-13

    Nonadiabatic relaxation of one singlet state into two triplet states is the key step in singlet fission dynamics, the understandings of which may help design next generation solar cells. In this work we perform the symmetrical quasi-classical (SQC) nonadiabatic molecular dynamics (MD) simulation [Cotton and Miller, J. Phys. Chem. A, 2013, 117, 7190; Meyer and Miller, J. Chem. Phys. 1979, 70, 3214] for a model system to study the real-time fission dynamics. The dependence of the nonadiabatic relaxation dynamics on energy levels, electronic couplings, and electronic-phonon couplings has been examined, in comparison with other analytical approximations, such as Förster theory and Marcus theory. Unlike many other methods, the SQC nondiabatic MD simulation approach is able to describe fission dynamics efficiently and accurately enough to provide microscopic insights into singlet fission. PMID:26574200

  10. Remarks on Models with Singlet Neutrino in Large Extra Dimensions

    NASA Astrophysics Data System (ADS)

    Agashe, Kaustubh

    2002-03-01

    In this talk, we discuss two non-minimal models with singlet neutrino in large extra dimensions: (1) singlet neutrino in a sub-space of the bulk and (2) Dirac mass for singlet with lepton number violating Yukawa couplings. The motivation is to obtain the mass scale relevant for atmospheric neutrino oscillations with a quantum gravity scale M* ~ a few TeV and with at most O(1) higher-dimensional couplings. In both non-minimal models, such a low M* can be consistent with the limit on BR(μ → eγ) (unlike in the minimal model). We also show that in this scenario, with 2 Higgs doublets, the decay of charged Higgs into left-handed τ can be significantly enhanced, with O(1) branching ratio, due to the large number of Kaluza-Klein states of the right-handed neutrino.

  11. Singlet fission efficiency in tetracene-based organic solar cells

    SciTech Connect

    Wu, Tony C. Thompson, Nicholas J.; Congreve, Daniel N.; Hontz, Eric; Yost, Shane R.; Van Voorhis, Troy; Baldo, Marc A.

    2014-05-12

    Singlet exciton fission splits one singlet exciton into two triplet excitons. Using a joint analysis of photocurrent and fluorescence modulation under a magnetic field, we determine that the triplet yield within optimized tetracene organic photovoltaic devices is 153% ± 5% for a tetracene film thickness of 20 nm. The corresponding internal quantum efficiency is 127% ± 18%. These results are used to prove the effectiveness of a simplified triplet yield measurement that relies only on the magnetic field modulation of fluorescence. Despite its relatively slow rate of singlet fission, the measured triplet yields confirm that tetracene is presently the best candidate for use with silicon solar cells.

  12. Probing scalar coupling differences via long-lived singlet states

    NASA Astrophysics Data System (ADS)

    DeVience, Stephen J.; Walsworth, Ronald L.; Rosen, Matthew S.

    2016-01-01

    We probe small scalar coupling differences via the coherent interactions between two nuclear spin singlet states in organic molecules. We show that the spin-lock induced crossing (SLIC) technique enables the coherent transfer of singlet order between one spin pair and another. The transfer is mediated by the difference in syn and anti vicinal or long-range J couplings among the spins. By measuring the transfer rate, we calculate a J coupling difference of 8 ± 2 mHz in phenylalanine-glycine-glycine and 2.57 ± 0.04 Hz in glutamate. We also characterize a coherence between two singlet states in glutamate, which may enable the creation of a long-lived quantum memory.

  13. Negative Ion Photoelectron Spectroscopy Confirms the Prediction that D-3h Carbontrioxide (CO3) Has a Singlet Ground State

    DOE PAGESBeta

    Hrovat, David; Hou, Gao-Lei; Chen, Bo; Wang, Xue B.; Borden, Weston

    2015-11-13

    The CO3 radical anion (CO3•–) has been formed by electrospraying carbonate dianion (CO32–) into the gas phase. The negative ion photoelectron (NIPE) spectrum of CO3•– shows that, unlike trimethylenemethane [C(CH2)3], carbontrioxide (CO3) has a singlet ground state. From the NIPE spectrum, the electron affinity of CO3 was determined to be EA = 4.06 ± 0.03 eV, and the singlet-triplet energy difference was found to be ΔEST = - 17.8 ± 0.9 kcal/mol. B3LYP, CCSD(T), and CASPT2 calculations all find that the two lowest triplet states of CO3 are very close in energy, a prediction that is confirmed by the relativemore » intensities of the bands in the NIPE spectrum of CO3•–. The 560 cm-1 vibrational progression, seen in the low energy region of the triplet band, enables the identification of the lowest, Jahn-Teller-distorted, triplet state as 3A1, in which both unpaired electrons reside in σ MOs, rather than 3A2, in which one unpaired electron occupies the b2 σ MO, and the other occupies the b1 π MO.« less

  14. Highly efficient spin-conversion effect leading to energy up-converted electroluminescence in singlet fission photovoltaics.

    PubMed

    Pandey, Ajay K

    2015-01-01

    Free charge generation in donor-acceptor (D-A) based organic photovoltaic diodes (OPV) progresses through formation of charge-transfer (CT) and charge-separated (CS) states and excitation decay to the triplet level is considered as a terminal loss. On the other hand a direct excitation decay to the triplet state is beneficial for multiexciton harvesting in singlet fission photovoltaics (SF-PV) and the formation of CT-state is considered as a limiting factor for multiple triplet harvesting. These two extremes when present in a D-A system are expected to provide important insights into the mechanism of free charge generation and spin-character of bimolecular recombination in OPVs. Herein, we present the complete cycle of events linked to spin conversion in the model OPV system of rubrene/C60. By tracking the spectral evolution of photocurrent generation at short-circuit and close to open-circuit conditions we are able to capture spectral changes to photocurrent that reveal the triplet character of CT-state. Furthermore, we unveil an energy up-conversion effect that sets in as a consequence of triplet population build-up where triplet-triplet annihilation (TTA) process effectively regenerates the singlet excitation. This detailed balance is shown to enable a rare event of photon emission just above the open-circuit voltage (V(OC)) in OPVs. PMID:25585937

  15. Highly efficient spin-conversion effect leading to energy up-converted electroluminescence in singlet fission photovoltaics

    NASA Astrophysics Data System (ADS)

    Pandey, Ajay K.

    2015-01-01

    Free charge generation in donor-acceptor (D-A) based organic photovoltaic diodes (OPV) progresses through formation of charge-transfer (CT) and charge-separated (CS) states and excitation decay to the triplet level is considered as a terminal loss. On the other hand a direct excitation decay to the triplet state is beneficial for multiexciton harvesting in singlet fission photovoltaics (SF-PV) and the formation of CT-state is considered as a limiting factor for multiple triplet harvesting. These two extremes when present in a D-A system are expected to provide important insights into the mechanism of free charge generation and spin-character of bimolecular recombination in OPVs. Herein, we present the complete cycle of events linked to spin conversion in the model OPV system of rubrene/C60. By tracking the spectral evolution of photocurrent generation at short-circuit and close to open-circuit conditions we are able to capture spectral changes to photocurrent that reveal the triplet character of CT-state. Furthermore, we unveil an energy up-conversion effect that sets in as a consequence of triplet population build-up where triplet-triplet annihilation (TTA) process effectively regenerates the singlet excitation. This detailed balance is shown to enable a rare event of photon emission just above the open-circuit voltage (VOC) in OPVs.

  16. Highly efficient spin-conversion effect leading to energy up-converted electroluminescence in singlet fission photovoltaics

    PubMed Central

    Pandey, Ajay K.

    2015-01-01

    Free charge generation in donor-acceptor (D-A) based organic photovoltaic diodes (OPV) progresses through formation of charge-transfer (CT) and charge-separated (CS) states and excitation decay to the triplet level is considered as a terminal loss. On the other hand a direct excitation decay to the triplet state is beneficial for multiexciton harvesting in singlet fission photovoltaics (SF-PV) and the formation of CT-state is considered as a limiting factor for multiple triplet harvesting. These two extremes when present in a D-A system are expected to provide important insights into the mechanism of free charge generation and spin-character of bimolecular recombination in OPVs. Herein, we present the complete cycle of events linked to spin conversion in the model OPV system of rubrene/C60. By tracking the spectral evolution of photocurrent generation at short-circuit and close to open-circuit conditions we are able to capture spectral changes to photocurrent that reveal the triplet character of CT-state. Furthermore, we unveil an energy up-conversion effect that sets in as a consequence of triplet population build-up where triplet-triplet annihilation (TTA) process effectively regenerates the singlet excitation. This detailed balance is shown to enable a rare event of photon emission just above the open-circuit voltage (VOC) in OPVs. PMID:25585937

  17. Generation of Three-Qutrit Singlet States with Two-Level Trapped Ions

    NASA Astrophysics Data System (ADS)

    Yang, Rong-Can; Ye, Li-Xiang; Lin, Xiu; Chen, Xiang

    2015-10-01

    We propose an adiabatic-passage scheme for the generation of three-qutrit singlet states with two-level trapped ions. Distinctly different from previous proposals, we encode qutrits in Dicke states with two-level ions and use the adiabatic-evolution techniques in order not to exactly control the laser pulses, making the realization of the scheme much easier. Furthermore, the phonon is only virtually excited in the procedure, so the effect of the phonon losses can be reduced. Supported by National Natural Science Foundation of China under Grant Nos. 61308012 and 61275215, by the Fujian Natural Science Foundation under Grant Nos. 2013J01008 and 2012J01004 and by the Fund from Fujian Educational Department under Grant Nos. JA14075, JB13021, and JB12014

  18. Nanoscopic mechanisms of singlet fission in amorphous molecular solid

    NASA Astrophysics Data System (ADS)

    Mou, Weiwei; Hattori, Shinnosuke; Rajak, Pankaj; Shimojo, Fuyuki; Nakano, Aiichiro

    2013-04-01

    Fission of a spin-singlet exciton into two triplet excitons, if realized in disordered organic solid, could revolutionize low-cost fabrication of efficient solar cells. Here, a divide-conquer-recombine approach involving nonadiabatic quantum molecular dynamics and kinetic Monte Carlo simulations identifies the key molecular geometry and exciton-flow-network topology for singlet-fission "hot spots" in amorphous diphenyl tetracene, where fission occurs preferentially. The simulation reveals the molecular origin of experimentally observed two time scales in exciton population dynamics and may pave a way to nanostructural design of efficient solar cells from first principles.

  19. Using singlet oxygen to synthesize polyoxygenated natural products from furans.

    PubMed

    Montagnon, Tamsyn; Tofi, Maria; Vassilikogiannakis, Georgios

    2008-08-01

    [Reaction: see text]. Singlet oxygen is a powerful tool in the armament of the synthetic organic chemist and possibly in that of nature itself. In this Account, we illustrate a small selection of the many ways singlet oxygen can be harnessed in the laboratory to aid in the construction of the complex molecular motifs found in natural products. A more philosophical question is also addressed: namely, how much do singlet oxygen oxidations influence the biogenesis of these natural products? All the synthetic examples surveyed in this Account can be characterized as belonging to the same class because they all involve the oxidation of a substituted furan nucleus by singlet oxygen. Readily accessible and relatively simple furans can be transformed into a host of complex motifs present in a diverse range of natural products by the action of singlet-oxygen-mediated reaction sequences. These reactions are highly advantageous because they frequently deliver a rapid and dramatic increase in molecular complexity in high yield. Furthermore, an unusually wide structural diversity is exhibited by the molecular motifs obtained from these reaction sequences. For example, relatively minor modifications to the starting substrate and to the reaction conditions may lead to products as variable as spiroketal lactones, 3-keto-tetrahydrofurans, various types of bis-spiroketals, 4-hydroxy cyclopentenones, or spiroperoxylactones. In addition, two more specialized examples are discussed in this Account. The core of the prunolide molecules and the chinensine family of natural products were rapidly synthesized using effective and short singlet oxygen mediated strategies; this adds weight to the assertion that singlet oxygen is a very effective moderator of complex cascade reaction sequences. We also show how our synthetic investigations have provided evidence that these same strategies might be used in the biogenesis of these molecules. In the cases of the chinensines and the

  20. An optical boiler generating singlet oxygen O{sub 2} (a{sup 1{Delta}}{sub g})

    SciTech Connect

    Lipatov, N I; Gulyamova, E S; Biryukov, A S

    2008-12-31

    An ecologically perfect generator of singlet oxygen O{sub 2} (a{sup 1{Delta}}{sub g}) is proposed which fundamentally differs from existing singlet-oxygen generators. Excited O{sub 2} (a{sup 1{Delta}}{sub g}) molecules are generated due to interaction of the O{sub 2} (X{sup 3}{Sigma}{sup -}{sub g}) molecules with a quasi-monochromatic field, which is supplied from an external source to a closed volume - an optical boiler containing oxygen. It is shown that, by pumping continuously the optical boiler by the light field of power {approx}3x10{sup 5} W, it is possible to accumulate up to 40% of singlet oxygen (O{sub 2}(b{sup 1}{Sigma}{sup +}{sub g})) + (O{sub 2} (a{sup 1}{Delta}{sub g})) in the boiler volume during {approx}10{sup -2} s. (laser applications and other topics in quantum electronics)

  1. Aromaticity effects on the profiles of the lowest triplet-state potential-energy surfaces for rotation about the C=C bonds of olefins with five-membered ring substituents: an example of the impact of Baird's rule.

    PubMed

    Zhu, Jun; Fogarty, Heather A; Möllerstedt, Helene; Brink, Maria; Ottosson, Henrik

    2013-08-01

    A density functional theory study on olefins with five-membered monocyclic 4n and 4n+2 π-electron substituents (C4H3X; X=CH(+), SiH(+), BH, AlH, CH2, SiH2, O, S, NH, and CH(-)) was performed to assess the connection between the degree of substituent (anti)aromaticity and the profile of the lowest triplet-state (T1) potential-energy surface (PES) for twisting about olefinic C=C bonds. It exploited both Hückel's rule on aromaticity in the closed-shell singlet ground state (S0) and Baird's rule on aromaticity in the lowest ππ* excited triplet state. The compounds CH2=CH(C4H3X) were categorized as set A and set B olefins depending on which carbon atom (C2 or C3) of the C4H3X ring is bonded to the olefin. The degree of substituent (anti)aromaticity goes from strongly S0 -antiaromatic/T1 -aromatic (C5H4 (+)) to strongly S0 -aromatic/T1- antiaromatic (C5H4(-)). Our hypothesis is that the shapes of the T1 PESs, as given by the energy differences between planar and perpendicularly twisted olefin structures in T1 [ΔE(T1)], smoothly follow the changes in substituent (anti)aromaticity. Indeed, correlations between ΔE(T1) and the (anti)aromaticity changes of the C4 H3 X groups, as measured by the zz-tensor component of the nucleus-independent chemical shift ΔNICS(T1;1)zz , are found both for sets A and B separately (linear fits; r(2) =0.949 and 0.851, respectively) and for the two sets combined (linear fit; r(2) =0.851). For sets A and B combined, strong correlations are also found between ΔE(T1) and the degree of S0 (anti)aromaticity as determined by NICS(S0,1)zz (sigmoidal fit; r(2) =0.963), as well as between the T1 energies of the planar olefins and NICS(S0,1)zz (linear fit; r(2) =0.939). Thus, careful tuning of substituent (anti)aromaticity allows for design of small olefins with T1 PESs suitable for adiabatic Z/E photoisomerization. PMID:23794153

  2. Production of Singlet Oxygen in a Non-Self-Sustained Discharge

    SciTech Connect

    Vasil'eva, A.N.; Klopovskii, K.S.; Kovalev, A.S.; Lopaev, D.V.; Mankelevich, Yu.A.; Popov, N.A.; Rakhimov, A.T.; Rakhimova, T.V.

    2005-04-15

    The production of O{sub 2}(a{sup 1}{delta}{sub g}) singlet oxygen in non-self-sustained discharges in pure oxygen and mixtures of oxygen with noble gases (Ar or He) was studied experimentally. It is shown that the energy efficiency of O{sub 2}(a{sup 1}{delta}{sub g}) production can be optimized with respect to the reduced electric field E/N. It is shown that the optimal E/N values correspond to electron temperatures of 1.2-1.4 eV. At these E/N values, a decrease in the oxygen percentage in the mixture leads to an increase in the excitation rate of singlet oxygen because of the increase in the specific energy deposition per O{sub 2} molecule. The onset of discharge instabilities not only greatly reduces the energy efficiency of singlet oxygen production but also makes it impossible to achieve high energy deposition in a non-self-sustained discharge. A model of a non-self-sustained discharge in pure oxygen is developed. It is shown that good agreement between the experimental and computed results for a discharge in oxygen over a wide range of reduced electric fields can be achieved only by taking into account the ion component of the discharge current. The cross section for the electron-impact excitation of O{sub 2}(a{sup 1}{delta}{sub g}) and the kinetic scheme of the discharge processes with the participation of singlet oxygen are verified by comparing the experimental and computed data on the energy efficiency of the production of O{sub 2}(a{sup 1}{delta}{sub g}) and the dynamics of its concentration. It is shown that, in the dynamics of O{sub 2}(a{sup 1}{delta}{sub g}) molecules in the discharge afterglow, an important role is played by their deexcitation in a three-body reaction with the participation of O({sup 3}P) atoms. At high energy depositions in a non-self-sustained discharge, this reaction can reduce the maximal attainable concentration of singlet oxygen. The effect of a hydrogen additive to an Ar : O{sub 2} mixture is analyzed based on the results

  3. Unveiling Singlet Fission Mediating States in TIPS-pentacene and its Aza Derivatives.

    PubMed

    Herz, Julia; Buckup, Tiago; Paulus, Fabian; Engelhart, Jens U; Bunz, Uwe H F; Motzkus, Marcus

    2015-06-25

    Femtosecond pump-depletion-probe experiments were carried out in order to shed light on the ultrafast excited-state dynamics of triisopropylsilylethynyl (TIPS)-pentacene and two nitrogen-containing derivatives, namely, diaza-TIPS-pentacene and tetraaza-TIPS-pentacene. Measurements performed in the visible and near-infrared spectral range in combination with rate model simulations reveal that singlet fission proceeds via the extremely short-lived intermediate (1)TT state, which absorbs in the near-infrared spectral region only. The T1 → T3 transition probed in the visible region shows a rise time that comprises two components according to a consecutive reaction (S1 → (1)TT → T1). The incorporation of nitrogen atoms into the acene structure leads to shorter dynamics, but the overall triplet formation follows the same kinetic model. This is of particular importance, since experiments on tetraaza-TIPS-pentacene allow for investigation of the triplet state in the visible range without an overlapping singlet contribution. In addition, the pump-depletion-probe experiments show that the triplet absorption in the visible (T1 → T3) and near-infrared (T1 → T2) regions occurs from the same initial state, which was questioned in previous studies. Furthermore, an additional ultrafast transfer between the excited triplet states (T3 → T2) is identified, which is also in agreement with the rate model simulation. By applying depletion pulses, which are resonant with higher vibrational levels, we gain insight into internal vibrational energy redistribution processes within the triplet manifold. This additional information is of great relevance regarding the study of loss channels within these materials. PMID:26001065

  4. Fear and loathing in the lowest Landau level

    SciTech Connect

    Arovas, D.P.

    1986-01-01

    The standard incompressible fluid theory of the fractional quantized Hall effect is reviewed. A new approach to the problem is then proposed, which is based on a loop-gas picture of exchange condensation in an interacting two-dimensional electron gas. This cooperative ring-exchange theory describes a transition from a low-density, sparse ring phase (a Wigner crystal) to a higher-density ring condensate (the fractional quantized Hall phase) at a critical value of the Landau level-filling fraction, v = nhc/eB. The phase transition is inferred from the behavior of the many-body partition function, which is expressed as a trace over ring-exchange events, each contribution being approximated in a semiclassical path integral formalism. Like the fluid, the dense ring condensate is incompressible and supports fractionally charged quasi-particle excitations. Also formulated is an effective Hamiltonian for the collective-mode problem, which predicts a stiffening of the Wigner-lattice magnetophonons in the presence of a dense exchange phase. This latter result is presently only qualitative, for the collective-mode gap in the fluid theory has not been reproduced. Though speculative at this stage, the cooperative ring-exchange picture, unlike the standard one, clearly identifies an instability that drives a transition into the condensed phase.

  5. Calculation of singlet oxygen formation from one photon absorbing photosensitizers used in PDT

    NASA Astrophysics Data System (ADS)

    Potasek, M.; Parilov, Evgueni; Beeson, K.

    2013-03-01

    Advances in biophotonic medicine require new information on photodynamic mechanisms. In photodynamic therapy (PDT), a photosensitizer (PS) is injected into the body and accumulates at higher concentrations in diseased tissue compared to normal tissue. The PS absorbs light from a light source and generates excited-state triplet states of the PS. The excited triplet states of the PS can then react with ground state molecular oxygen to form excited singlet - state oxygen or form other highly reactive species. The reactive species react with living cells, resulting in cel l death. This treatment is used in many forms of cancer including those in the prostrate, head and neck, lungs, bladder, esophagus and certain skin cancers. We developed a novel numerical method to model the photophysical and photochemical processes in the PS and the subsequent energy transfer to O2, improving the understanding of these processes at a molecular level. Our numerical method simulates light propagation and photo-physics in PS using methods that build on techniques previously developed for optical communications and nonlinear optics applications.

  6. Fast Triplet Formation via Singlet Exciton Fission in a Covalent Perylenediimide-β-apocarotene Dyad Aggregate.

    PubMed

    Mauck, Catherine M; Brown, Kristen E; Horwitz, Noah E; Wasielewski, Michael R

    2015-06-01

    A covalent dyad was synthesized in which perylene-3,4,:9:10-bis(dicarboximide) (PDI) is linked to β-apocarotene (Car) using a biphenyl spacer. The dyad is monomeric in toluene and forms a solution aggregate in methylcyclohexane (MCH). Using femtosecond transient absorption (fsTA) spectroscopy, the monomeric dyad and its aggregates were studied both in solution and in thin films. In toluene, photoexcitation at 530 nm preferentially excites PDI, and the dyad undergoes charge separation in τ = 1.7 ps and recombination in τ = 1.6 ns. In MCH and in thin solid films, 530 nm excitation of the PDI-Car aggregate also results in charge transfer that competes with energy transfer from (1)*PDI to Car and with an additional process, rapid Car triplet formation in <50 ps. Car triplet formation is only observed in the aggregated PDI-Car dyad and is attributed to singlet exciton fission (SF) within the aggregated PDI, followed by rapid triplet energy transfer from (3)*PDI to the carotenoid. SF from β-apocarotene aggregation is ruled out by direct excitation of Car films at 414 nm, where no triplet formation is observed. Time-resolved electron paramagnetic resonance measurements on aggregated PDI-Car show the formation of (3)*Car with a spin-polarization pattern that rules out radical-pair intersystem crossing as the mechanism of triplet formation as well. PMID:25961130

  7. Photogeneration of singlet oxygen by humic substances: comparison of humic substances of aquatic and terrestrial origin.

    PubMed

    Paul, Andrea; Hackbarth, Steffen; Vogt, Rolf D; Röder, Beate; Burnison, B Kent; Steinberg, Christian E W

    2004-03-01

    The singlet oxygen (1(O2)) luminescence of 27 isolated humic substances (HS), natural organic matter, ultrafiltrates, and the synthetic fulvic acid HS1500 has been investigated by time-resolved spectroscopy in buffered D(2)O. The samples include both reverse osmosis isolates from lakes in Scandinavia, Canada, and Germany, and IHSS fulvic and humic acids of aquatic and terrestrial origin. The quantum yields of 1(O2) formation (PhiDelta) obtained on laser excitation at 480 nm ranged between 0.06 (HS1500) and 2.7%(fulvic acid from soil, IHSS). In our study, a general trend towards higher PhiDelta in terrestrial HS was observed. The comparison of reverse osmosis isolates from surface waters collected during fall 1999 and spring 2000 from five Scandinavian sites yielded, in all cases, higher PhiDelta for the spring samples. For the aquatic sampling sites Hietajarvi and Birkenes, PhiDelta even exceeded values of 0.6%, which were found to be typical for terrestrial or soil water material. Investigation of the excitation wavelength dependence of PhiDelta in the spectral range 355-550 nm yielded different spectral shapes for aquatic HS and "non-aquatic" HS, respectively. On the basis of these excitation spectra, 1(O2) production rates were calculated for eight representative HS. PMID:14993944

  8. Cooperative Singlet and Triplet Exciton Transport in Tetracene Crystals Visualized by Ultrafast Microscopys

    SciTech Connect

    Wan, Yan; Guo, Zhi; Zhu, Tong; Yan, Suxia; Johnson, Justin; Huang, Libai

    2015-09-14

    Singlet fission presents an attractive solution to overcome the Shockley–Queisser limit by generating two triplet excitons from one singlet exciton. Although triplet excitons are long-lived, their transport occurs through a Dexter transfer, making them slower than singlet excitons, which travel by means of a Förster mechanism. A thorough understanding of the interplay between singlet fission and exciton transport is therefore necessary to assess the potential and challenges of singlet-fission utilization. We report a direct visualization of exciton transport in single tetracene crystals using transient absorption microscopy with 200 fs time resolution and 50 nm spatial precision. Moreover, these measurements reveal a new singlet-mediated transport mechanism for triplets, which leads to an enhancement in effective triplet exciton diffusion of more than one order of magnitude on picosecond to nanosecond timescales. These results establish that there are optimal energetics of singlet and triplet excitons that benefit both singlet fission and exciton diffusion.

  9. Cooperative singlet and triplet exciton transport in tetracene crystals visualized by ultrafast microscopy.

    PubMed

    Wan, Yan; Guo, Zhi; Zhu, Tong; Yan, Suxia; Johnson, Justin; Huang, Libai

    2015-10-01

    Singlet fission presents an attractive solution to overcome the Shockley-Queisser limit by generating two triplet excitons from one singlet exciton. However, although triplet excitons are long-lived, their transport occurs through a Dexter transfer, making them slower than singlet excitons, which travel by means of a Förster mechanism. A thorough understanding of the interplay between singlet fission and exciton transport is therefore necessary to assess the potential and challenges of singlet-fission utilization. Here, we report a direct visualization of exciton transport in single tetracene crystals using transient absorption microscopy with 200 fs time resolution and 50 nm spatial precision. These measurements reveal a new singlet-mediated transport mechanism for triplets, which leads to an enhancement in effective triplet exciton diffusion of more than one order of magnitude on picosecond to nanosecond timescales. These results establish that there are optimal energetics of singlet and triplet excitons that benefit both singlet fission and exciton diffusion. PMID:26391077

  10. Transient magneto-photoinduced absorption study of singlet fission in low band gap copolymers

    NASA Astrophysics Data System (ADS)

    Huynh, Uyen; Vardeny, Z. Valy

    2015-03-01

    We have observed the existence of singlet fission in thin films of low band gap (LBG) copolymers, PDTP-DFBT and PTB7, using the ultrafast optical pump/probe spectroscopy, probed at the energy range from IR to MIR. The singlet fission is the dissociation of a singlet exciton into two triplets through an intermediate triplet pair state (TT pair) in an overall singlet configuration; in the studied copolymers, it was observed to be very fast, in femtosecond time domain. The intermediate TT state, which dissociates into two separated triplets at later time, or recombines to the ground state appears instantaneously with the singlet exciton formation using our laser system that has ~ 150 fs time resolution. The interplay between the rate of singlet fission into sTT pairs, triplet fusion back to singlet excitons and relaxation between the TT spin sublevels explains the obtained opposite pattern of the transient magnetic field response on the dynamics of singlet excitons and TT pairs.

  11. Singlet-based photon upconversion in multichromophore organic thin films (Presentation Recording)

    NASA Astrophysics Data System (ADS)

    Weingarten, Daniel H.; LaCount, Michael; Rumbles, Garry; van de Lagemaat, Jao; Lusk, Mark T.; Shaheen, Sean E.

    2015-10-01

    Solid-state energy upconversion has many potential applications, from nonlinear photonics and biophotonics to expanding the spectrum available for solar energy harvest. In organic molecular systems, upconversion is frequently done in solution to mitigate aggregation-induced photoluminescence quenching or to facilitate the diffusion of triplet donors in Triplet-Triplet Annihilation (TTA) systems. Here we demonstrate an organic thin film upconversion system utilizing two-photon absorption (TPA) properties to improve upconversion efficiency. In blend films of Stilbene-420 and Rhodamine 6G we observe a tenfold increase in up-converted fluorescence compared to the fluorescence yield of TPA in pristine stilbene films. While TPA normally has quadratic dependence on excitation intensity, these blend films exhibit sub-quadratic intensity dependence, indicating a combination of linear and quadratic upconversion processes and dramatically improving upconversion efficiency at lower excitation intensities. This improvement in intensity dependence allows for relatively efficient upconversion upon excitation by a nanosecond laser pulse, in contrast to the more expensive femtosecond lasers generally required for excitation in TPA microscopy and similar systems. Time-resolved photoluminescence decay measurements reveal that all excited states involved in this upconversion process are singlets, which indicates the potential for reduced energy losses when compared to TTA upconversion systems and their inherent intersystem-crossing energy losses. We observe emission from both the Rhodamine 6G donor molecules and Stilbene-420 acceptor molecules, indicating the presence of prompt fluorescence from the donor as well as upconversion to the acceptor, and FRET losses from acceptor back to donor. By fitting to a kinetic model we extract rates for these competing processes.

  12. Capturing Transient Endoperoxide in the Singlet Oxygen Oxidation of Guanine.

    PubMed

    Lu, Wenchao; Liu, Jianbo

    2016-02-24

    The chemistry of singlet O2 toward the guanine base of DNA is highly relevant to DNA lesion, mutation, cell death, and pathological conditions. This oxidative damage is initiated by the formation of a transient endoperoxide through the Diels-Alder cycloaddition of singlet O2 to the guanine imidazole ring. However, no endoperoxide formation was directly detected in native guanine or guanosine, even at -100 °C. Herein, gas-phase ion-molecule scattering mass spectrometry was utilized to capture unstable endoperoxides in the collisions of hydrated guanine ions (protonated or deprotonated) with singlet O2 at ambient temperature. Corroborated by results from potential energy surface exploration, kinetic modeling, and dynamics simulations, various aspects of endoperoxide formation and transformation (including its dependence on guanine ionization and hydration states, as well as on collision energy) were determined. This work has pieced together reaction mechanisms, kinetics, and dynamics data concerning the early stage of singlet O2 induced guanine oxidation, which is missing from conventional condensed-phase studies. PMID:26813583

  13. Photoactivatable protein labeling by singlet oxygen mediated reactions.

    PubMed

    To, Tsz-Leung; Medzihradszky, Katalin F; Burlingame, Alma L; DeGrado, William F; Jo, Hyunil; Shu, Xiaokun

    2016-07-15

    Protein-protein interactions regulate many biological processes. Identification of interacting proteins is thus an important step toward molecular understanding of cell signaling. The aim of this study was to investigate the use of photo-generated singlet oxygen and a small molecule for proximity labeling of interacting proteins in cellular environment. The protein of interest (POI) was fused with a small singlet oxygen photosensitizer (miniSOG), which generates singlet oxygen ((1)O2) upon irradiation. The locally generated singlet oxygen then activated a biotin-conjugated thiol molecule to form a covalent bond with the proteins nearby. The labeled proteins can then be separated and subsequently identified by mass spectrometry. To demonstrate the applicability of this labeling technology, we fused the miniSOG to Skp2, an F-box protein of the SCF ubiquitin ligase, and expressed the fusion protein in mammalian cells and identified that the surface cysteine of its interacting partner Skp1 was labeled by the biotin-thiol molecule. This photoactivatable protein labeling method may find important applications including identification of weak and transient protein-protein interactions in the native cellular context, as well as spatial and temporal control of protein labeling. PMID:27220724

  14. First-Principle Characterization for Singlet Fission Couplings.

    PubMed

    Yang, Chou-Hsun; Hsu, Chao-Ping

    2015-05-21

    The electronic coupling for singlet fission, an important parameter for determining the rate, has been found to be too small unless charge-transfer (CT) components were introduced in the diabatic states, mostly through perturbation or a model Hamiltonian. In the present work, the fragment spin difference (FSD) scheme was generalized to calculate the singlet fission coupling. The largest coupling strength obtained was 14.8 meV for two pentacenes in a crystal structure, or 33.7 meV for a transition-state structure, which yielded a singlet fission lifetime of 239 or 37 fs, generally consistent with experimental results (80 fs). Test results with other polyacene molecules are similar. We found that the charge on one fragment in the S1 diabatic state correlates well with FSD coupling, indicating the importance of the CT component. The FSD approach is a useful first-principle method for singlet fission coupling, without the need to include the CT component explicitly. PMID:26263271

  15. How Morphology Affects Singlet Fission in Crystalline Tetracene.

    PubMed

    Piland, Geoffrey B; Bardeen, Christopher J

    2015-05-21

    The dependence of exciton dynamics on the crystalline morphology of tetracene is investigated using time-resolved photoluminescence. Single crystals exhibit relatively slow singlet decays with times that range from 130 to 300 ps depending on the sample. This decay has an activation energy of ∼450 cm(-1) over the temperature range of 200-400 K. Single-crystal samples also exhibit more pronounced quantum beats due to the triplet pair spin coherences. Polycrystalline thin films grown by thermal evaporation have singlet decay times on the order of 70-90 ps with a much weaker temperature dependence. Many thin-film samples also exhibit a red-shifted excimer-like emission. When a polycrystalline thin film is thermally annealed to produce larger crystal domains, single-crystal behavior is recovered. We hypothesize that the different dynamics arise from the ability of singlet excitons in the thin films to sample regions with defects or packing motifs that accelerate singlet fission. PMID:26263258

  16. Photosensitized generation of singlet oxygen by rhenium(I) complex

    NASA Astrophysics Data System (ADS)

    Burchinov, A. N.; Kiselev, V. M.; Penni, A. A.; Khistyaeva, V. V.

    2015-12-01

    The photosensitized generation of singlet oxygen in solutions of rhenium(I) complex fac-[Re(bipy)(CO)3NCCH3]+OTf-, where bipy=2,2'-bipyridine, in chloride methylene and carbon tetrachloride under continuous LED irradiation in the UV and visible ranges has been investigated.

  17. Temperature-independent singlet exciton fission in tetracene.

    PubMed

    Wilson, Mark W B; Rao, Akshay; Johnson, Kerr; Gélinas, Simon; di Pietro, Riccardo; Clark, Jenny; Friend, Richard H

    2013-11-01

    We use transient absorption spectroscopy to demonstrate that the dynamics of singlet exciton fission in tetracene are independent of temperature (10–270 K). Low-intensity, broad-band measurements allow the identification of spectral features while minimizing bimolecular recombination. Hence, by directly observing both species, we find that the time constant for the conversion of singlets to triplet pairs is ~90 ps. However, in contrast to pentacene, where fission is effectively unidirectional, we confirm that the emissive singlet in tetracene is readily regenerated from spin-correlated "geminate" triplets following fission, leading to equilibrium dynamics. Although free triplets are efficiently generated at room temperature, the interplay of superradiance and frustrated triplet diffusion contributes to a nearly 20-fold increase in the steady-state fluorescence as the sample is cooled. Together, these results require that singlets and triplet pairs in tetracene are effectively degenerate in energy, and begin to reconcile the temperature dependence of many macroscopic observables with a fission process which does not require thermal activation. PMID:24148017

  18. Electroweak interacting dark matter with a singlet scalar portal

    NASA Astrophysics Data System (ADS)

    Chiang, Cheng-Wei; Senaha, Eibun

    2015-11-01

    We investigate an electroweak interacting dark matter (DM) model in which the DM is the neutral component of the SU(2)L triplet fermion that couples to the standard model (SM) Higgs sector via an SM singlet Higgs boson. In this setup, the DM can have a CP-violating coupling to the singlet Higgs boson at the renormalizable level. As long as the nonzero Higgs portal coupling (singlet-doublet Higgs boson mixing) exists, we can probe CP violation of the DM via the electric dipole moment of the electron. Assuming the O (1) CP-violating phase in magnitude, we investigate the relationship between the electron EDM and the singlet-like Higgs boson mass and coupling. It is found that for moderate values of the Higgs portal couplings, current experimental EDM bound is not able to exclude the wide parameter space due to a cancellation mechanism at work. We also study the spin-independent cross section of the DM in this model. It is found that although a similar cancellation mechanism may diminish the leading-order correction, as often occurs in the ordinary Higgs portal DM scenarios, the residual higher-order effects leave an O (10-47) cm2 correction in the cancellation region. It is shown that our benchmark scenarios would be fully tested by combining all future experiments of the electron EDM, DM direct detection and Higgs physics.

  19. Singlet oxygen and organic light-emitting diodes

    SciTech Connect

    Jacobs, S.J.; Sinclair, M.B.; Valencia, V.S.; Kepler, R.G.; Clough, R.L.; Scurlock, R.D.; Ogilby, P.R.

    1995-07-01

    The preparation of light emitting diodes employing a new class of materials, 5,10-dihetera 5,10-dihydro-indeno[3,2b]indenes, as hole transport agents is described. These materials have been found to be more resistant to degradation by singlet oxygen than a poly(p-phenylene vinylene) (PPV) derivative.

  20. Effects of a real singlet scalar on Veltman condition

    NASA Astrophysics Data System (ADS)

    Karahan, Canan Nurhan; Korutlu, Beste

    2014-05-01

    We revisit the fine-tuning problem in the Standard Model (SM) and show the modification in the Veltman condition by virtue of a minimally-extended particle spectrum with one real SM gauge singlet scalar field. We demand the new scalar to interact with the SM fields through Higgs portal only, and the new singlet to acquire a vacuum expectation value, resulting in a mixing with the CP-even neutral component of the Higgs doublet in the SM. The experimental bounds on the mixing angle are determined by the observed best-fit signal strength σ/σ. While, the one-loop radiative corrections to the Higgs mass squared, computed with an ultraviolet cut-off scale Λ, come with a negative coefficient, the quantum corrections to the singlet mass squared acquire both positive and negative values depending on the parameter space chosen, which if positive might be eliminated by introducing singlet or doublet vector-like fermions. However, based upon the fact that there is mixing between the scalars, when transformed into the physical states, the tree-level coupling of the Higgs field to the vector-like fermions worsens the Higgs mass hierarchy problem. Therefore, the common attempt to introduce vector-like fermions to cancel the divergences in the new scalar mass might not be a solution, if there is mixing between the scalars.

  1. Singlet exciton fission in polycrystalline pentacene: from photophysics toward devices.

    PubMed

    Wilson, Mark W B; Rao, Akshay; Ehrler, Bruno; Friend, Richard H

    2013-06-18

    Singlet exciton fission is the process in conjugated organic molecules bywhich a photogenerated singlet exciton couples to a nearby chromophore in the ground state, creating a pair of triplet excitons. Researchers first reported this phenomenon in the 1960s, an event that sparked further studies in the following decade. These investigations used fluorescence spectroscopy to establish that exciton fission occurred in single crystals of several acenes. However, research interest has been recently rekindled by the possibility that singlet fission could be used as a carrier multiplication technique to enhance the efficiency of photovoltaic cells. The most successful architecture to-date involves sensitizing a red-absorbing photoactive layer with a blue-absorbing material that undergoes fission, thereby generating additional photocurrent from higher-energy photons. The quest for improved solar cells has spurred a drive to better understand the fission process, which has received timely aid from modern techniques for time-resolved spectroscopy, quantum chemistry, and small-molecule device fabrication. However, the consensus interpretation of the initial studies using ultrafast transient absorption spectroscopy was that exciton fission was suppressed in polycrystalline thin films of pentacene, a material that would be otherwise expected to be an ideal model system, as well as a viable candidate for fission-sensitized photovoltaic devices. In this Account, we review the results of our recent transient absorption and device-based studies of polycrystalline pentacene. We address the controversy surrounding the assignment of spectroscopic features in transient absorption data, and illustrate how a consistent interpretation is possible. This work underpins our conclusion that singlet fission in pentacene is extraordinarily rapid (∼80 fs) and is thus the dominant decay channel for the photoexcited singlet exciton. Further, we discuss our demonstration that triplet excitons

  2. Singlet Fission of Non-polycyclic Aromatic Molecules in Organic Photovoltaics.

    PubMed

    Kawata, So; Pu, Yong-Jin; Saito, Ayaka; Kurashige, Yuki; Beppu, Teruo; Katagiri, Hiroshi; Hada, Masaki; Kido, Junji

    2016-02-24

    Singlet fission of thienoquinoid compounds in organic photovoltaics is demonstrated. The escalation of the thienoquinoid length of the compounds realizes a suitable packing structure and energy levels for singlet fission. The magnetic-field dependence of the photocurrent and the external quantum efficiency of the devices reveal singlet fission of the compounds and dissociation of triplet excitons into charges. PMID:26663207

  3. Theoretical prediction of the potential curves for the lowest-lying states of the C2 + molecular ion

    NASA Astrophysics Data System (ADS)

    Petrongolo, Carlo; Bruna, Pablo J.; Peyerimhoff, Sigrid D.; Buenker, Robert J.

    1981-04-01

    Ab initio MRD-CI potential curves have been calculated for C2+ in its first 16 electronic states and vertical transition energies Tv have been computed for a number of higher-lying species, all of which correlate with the first dissociation limit C(3Pg)+C+(2Pu). The ground state of this molecular ion is found to be X 4Σg- while the first excited state is 1 2Πu, with a calculated Te value of 0.84 eV. On the basis of this work the C2 I.P. value known experimentally is ascribed to the a 3Πu→1 2Πu process while the transition involving both ground states appears to be difficult to detect experimentlly. Thus, the measured De value for C+2 should involve fragmentation of the 1 2Πu states as well. A comparison with previous calculations which attempt to estimate the correlation energies of the various C+2 states in a semiempirical manner shows very large discrepancies, both in the transition energies themselves and in the ordering of these states. Finally the assignment for the Meinel experimental band system at 4.98 eV as a 2Σ-g←2Πu transition in C+2 is not supported by the present theoretical study.

  4. Intramolecular charge transfer effects on the diradical character and second hyperpolarizabilities of open-shell singlet X-π-X (X = donor/acceptor) systems.

    PubMed

    Fukuda, Kotaro; Nakano, Masayoshi

    2014-05-15

    We investigate the effect of the quadrupole-type intramolecular charge transfer (ICT) in open-shell singlet donor-π-donor (D-π-D) molecules on the singlet open-shell (diradical) character and the longitudinal second hyperpolarizabilities γ (the third-order nonlinear optical (NLO) properties at the molecular scale). For this investigation we used the para-quinodimethane (PQM) with point charges (pc's) model calculated with the unrestricted coupled cluster method including single and double excitations with a perturbative treatment of the triple excitations (UCCSD(T)). In this model, the diradical character y and the amount of the ICT, that is, the D-π-D nature, can be varied primarily by changing the exocyclic carbon-carbon bond (C-C) lengths and the external pc's Q, respectively. It turns out that the increase in the D-π-D nature decreases the y values, moves the y values (ymax) giving the maximum γ (γmax) to the large y region, and enhances the γmax values, for example, the γmax of the singlet diradical PQM with Q = -2.8 au reaches twice that of the singlet diradical PQM without any pc's. This result indicates that open-shell singlet D-π-D systems with ICT are promising candidates for a new class of third-order NLO molecules, whose γ values are more enhanced than those of conventional closed-shell D-π-D systems and of symmetric open-shell singlet systems without the ICT. To confirm this tendency, we examine the boron-disubstituted PQM dianion model, which is found to exhibit further enhancement of γ as compared to the PQM model with intermediate diradical character due to the synergy effects of the intermediate open-shell singlet nature and the strong field-induced ICT nature in the dianionic state of the D-π-D system. Further investigation of the acceptor-π-acceptor (A-π-A) type ICT effect in the PQM-pc model shows that both D-π-D and A-π-A type symmetric ICTs give similar effects on the relationship between y and γ, though there are some

  5. Charge-Transfer Versus Charge-Transfer-Like Excitations Revisited

    SciTech Connect

    Moore, Barry; Sun, Haitao; Govind, Niranjan; Kowalski, Karol; Autschbach, Jochen

    2015-07-14

    Criteria to assess charge-transfer (CT) and `CT-like' character of electronic excitations are examined. Time-dependent density functional theory (TDDFT) with non-hybrid, hybrid, and tuned long-range corrected (LC) functionals is compared with with coupled-cluster (CC) benchmarks. The test set includes an organic CT complex, two `push-pull' donor-acceptor chromophores, a cyanine dye, and several polycyclic aromatic hydrocarbons. Proper CT is easily identified. Excitations with significant density changes upon excitation within regions of close spatial proximity can also be diagnosed. For such excitations, the use of LC functionals in TDDFT sometimes leads to dramatic improvements of the singlet energies, similar to proper CT, which has led to the concept of `CT-like' excitations. However, `CT-like' excitations are not like charge transfer, and the improvements are not obtained for the right reasons. The triplet excitation energies are underestimated for all systems, often severely. For the `CT-like' candidates, when going from a non-hybrid to an LC functional the error in the singlet-triplet (S/T) separation changes from negative to positive, providing error compensation. For the cyanine, the S/T separation is too large with all functionals, leading to the best error compensation for non-hybrid functionals.

  6. Pulsed electron-beam-sustained discharge in oxygen-containing gas mixtures: electrical characteristics, spectroscopy,and singlet oxygen yield

    SciTech Connect

    Vagin, Nikolai P; Ionin, Andrei A; Klimachev, Yu M; Kotkov, A A; Podmar'kov, Yu P; Seleznev, L V; Sinitsyn, D V; Frolov, M P; Yuryshev, Nikolai N; Kochetov, Igor' V; Napartovich, A P; Hager, G D

    2004-09-30

    The electrical and spectroscopic characteristics of electron-beam-sustained discharge (EBSD) in oxygen and oxygen-containing gas mixtures are studied experimentally under gas pressures up to 100 Torr in a large excitation volume ({approx}18 L). It is shown that the EBSD in pure oxygen and its mixtures with inert gases is unstable and is characterised by a small specific energy contribution. The addition of small amounts ({approx}1%-10%) of carbon monoxide or hydrogen to oxygen or its mixtures with inert gases considerably improves the stability of the discharge, while the specific energy contribution W increases by more then an order of magnitude, achieving {approx}6.5 kJ L{sup -1} atm{sup -1} per molecular component of the gas mixture. A part of the energy supplied to the EBSD is spent to excite vibrational levels of molecular additives. This was demonstrated experimentally by the initiation of a CO laser based on the O{sub 2} : Ar : CO = 1 : 1 : 0.1 mixture. Experimental results on spectroscopy of the excited electronic states O{sub 2}(a{sup 1{Delta}}{sub g}) and O{sub 2}(b{sup 1{Sigma}}{sub g}{sup +}), of oxygen formed in the EBSD are presented. A technique was worked out for measuring the concentration of singlet oxygen in the O{sub 2}(a{sup 1{Delta}}{sub g}) state in the afterglow of the pulsed EBSD by comparing with the radiation intensity of singlet oxygen of a given concentration produced in a chemical generator. Preliminary measurements of the singlet-oxygen yield in the EBSD show that its value {approx}3% for W {approx} 1.0 kJ L{sup -1} atm{sup -1} is in agreement with the theoretical estimate. Theoretical calculations performed for W {approx} 6.5 kJ L{sup -1} atm{sup -1} at a fixed temperature show that the singlet-oxygen yield may be {approx}20%, which is higher than the value required to achieve the lasing threshold in an oxygen-iodine laser at room temperature. (laser applications and other topics in quantum electronics)

  7. Deep two-photon microscopic imaging through brain tissue using the second singlet state from fluorescent agent chlorophyll α in spinach leaf

    NASA Astrophysics Data System (ADS)

    Shi, Lingyan; Rodríguez-Contreras, Adrián; Budansky, Yury; Pu, Yang; An Nguyen, Thien; Alfano, Robert R.

    2014-06-01

    Two-photon (2P) excitation of the second singlet (S) state was studied to achieve deep optical microscopic imaging in brain tissue when both the excitation (800 nm) and emission (685 nm) wavelengths lie in the "tissue optical window" (650 to 950 nm). S2 state technique was used to investigate chlorophyll α (Chl α) fluorescence inside a spinach leaf under a thick layer of freshly sliced rat brain tissue in combination with 2P microscopic imaging. Strong emission at the peak wavelength of 685 nm under the 2P S state of Chl α enabled the imaging depth up to 450 μm through rat brain tissue.

  8. Ultrafast electron transfer reactions initiated by excited CT states of push pull perylenes

    NASA Astrophysics Data System (ADS)

    Miller, Scott E.; Zhao, Yongyu; Schaller, Richard; Mulloni, Viviana; Just, Eric M.; Johnson, Robert C.; Wasielewski, Michael R.

    2002-01-01

    Two new chromophores that absorb in the visible spectrum, the 9-( N-pyrrolidinyl)- and 9-( N-piperidinyl)perylene-3,4-dicarboximides, 5PMI and 6PMI, respectively, were synthesized and shown to possess lowest excited singlet states with about 70% charge transfer (CT) character. Changing the ring size of the cyclic amine from 5 to 6 significantly changes the energies of the CT states, as well as the redox potentials of the chromophores. These chromophores were linked to pyromellitimide (PI) and 1,8:4,5-naphthalenediimide (NI) electron acceptors using a single N-N bond between their respective imides to yield the corresponding donor-acceptor dyads 5PMI-PI, 5PMI-NI, 6PMI-PI, and 6PMI-NI. The donors and acceptors in these molecules are positioned relative to one another in a rod-like arrangement at fixed distances and restricted orientations. The rates of charge separation and recombination were measured using transient absorption spectroscopy. These chromophores were also used to prepare rigid donor-acceptor triads 5PMI-PI-NI and 6PMI-PI-NI, which display one- or two-step electron transfer mechanisms that depend on solvent polarity. These compounds exhibit a broad range of structure and media driven changes in electron transfer mechanism.

  9. A systematic approach to vertically excited states of ethylene using configuration interaction and coupled cluster techniques

    SciTech Connect

    Feller, David Peterson, Kirk A.; Davidson, Ernest R.

    2014-09-14

    A systematic sequence of configuration interaction and coupled cluster calculations were used to describe selected low-lying singlet and triplet vertically excited states of ethylene with the goal of approaching the all electron, full configuration interaction/complete basis set limit. Included among these is the notoriously difficult, mixed valence/Rydberg {sup 1}B{sub 1u} V state. Techniques included complete active space and iterative natural orbital configuration interaction with large reference spaces which led to variational spaces of 1.8 × 10{sup 9} parameters. Care was taken to avoid unintentionally biasing the results due to the widely recognized sensitivity of the V state to the details of the calculation. The lowest vertical and adiabatic ionization potentials to the {sup 2}B{sub 3u} and {sup 2}B{sub 3} states were also determined. In addition, the heat of formation of twisted ethylene {sup 3}A{sub 1} was obtained from large basis set coupled cluster theory calculations including corrections for core/valence, scalar relativistic and higher order correlation recovery.

  10. Carbon dioxide photolysis from 150 to 210 nm: singlet and triplet channel dynamics, UV-spectrum, and isotope effects.

    PubMed

    Schmidt, Johan A; Johnson, Matthew S; Schinke, Reinhard

    2013-10-29

    We present a first principles study of the carbon dioxide (CO2) photodissociation process in the 150- to 210-nm wavelength range, with emphasis on photolysis below the carbon monoxide + singlet channel threshold at ~167 nm. The calculations reproduce experimental absorption cross-sections at a resolution of ~0.5 nm without scaling the intensity. The observed structure in the 150- to 210-nm range is caused by excitation of bending motion supported by the deep wells at bent geometries in the and potential energy surfaces. Predissociation below the singlet channel threshold occurs via spin-orbit coupling to nearby repulsive triplet states. Carbon monoxide vibrational and rotational state distributions in the singlet channel as well as the triplet channel for excitation at 157 nm satisfactorily reproduce experimental data. The cross-sections of individual CO2 isotopologues ((12)C(16)O2, (12)C(17)O(16)O, (12)C(18)O(16)O, (13)C(16)O2, and (13)C(18)O(16)O) are calculated, demonstrating that strong isotopic fractionation will occur as a function of wavelength. The calculations provide accurate, detailed insight into CO2 photoabsorption and dissociation dynamics, and greatly extend knowledge of the temperature dependence of the cross-section to cover the range from 0 to 400 K that is useful for calculations of propagation of stellar light in planetary atmospheres. The model is also relevant for the interpretation of laboratory experiments on mass-independent isotopic fractionation. Finally, the model shows that the mass-independent fractionation observed in a series of Hg lamp experiments is not a result of hyperfine interactions making predissociation of (17)O containing CO2 more efficient. PMID:23776249

  11. Carbon dioxide photolysis from 150 to 210 nm: Singlet and triplet channel dynamics, UV-spectrum, and isotope effects

    PubMed Central

    Schmidt, Johan A.; Johnson, Matthew S.; Schinke, Reinhard

    2013-01-01

    We present a first principles study of the carbon dioxide (CO2) photodissociation process in the 150- to 210-nm wavelength range, with emphasis on photolysis below the carbon monoxide + singlet channel threshold at ∼167 nm. The calculations reproduce experimental absorption cross-sections at a resolution of ∼0.5 nm without scaling the intensity. The observed structure in the 150- to 210-nm range is caused by excitation of bending motion supported by the deep wells at bent geometries in the and potential energy surfaces. Predissociation below the singlet channel threshold occurs via spin-orbit coupling to nearby repulsive triplet states. Carbon monoxide vibrational and rotational state distributions in the singlet channel as well as the triplet channel for excitation at 157 nm satisfactorily reproduce experimental data. The cross-sections of individual CO2 isotopologues (12C16O2, 12C17O16O, 12C18O16O, 13C16O2, and 13C18O16O) are calculated, demonstrating that strong isotopic fractionation will occur as a function of wavelength. The calculations provide accurate, detailed insight into CO2 photoabsorption and dissociation dynamics, and greatly extend knowledge of the temperature dependence of the cross-section to cover the range from 0 to 400 K that is useful for calculations of propagation of stellar light in planetary atmospheres. The model is also relevant for the interpretation of laboratory experiments on mass-independent isotopic fractionation. Finally, the model shows that the mass-independent fractionation observed in a series of Hg lamp experiments is not a result of hyperfine interactions making predissociation of 17O containing CO2 more efficient. PMID:23776249

  12. Carbon dioxide photolysis from 150 to 210 nm: Singlet and triplet channel dynamics, UV-spectrum, and isotope effects

    NASA Astrophysics Data System (ADS)

    Schmidt, Johan A.; Johnson, Matthew S.; Schinke, Reinhard

    2013-10-01

    We present a first principles study of the carbon dioxide (CO2) photodissociation process in the 150- to 210-nm wavelength range, with emphasis on photolysis below the carbon monoxide + singlet channel threshold at ∼167 nm. The calculations reproduce experimental absorption cross-sections at a resolution of ∼0.5 nm without scaling the intensity. The observed structure in the 150- to 210-nm range is caused by excitation of bending motion supported by the deep wells at bent geometries in the and potential energy surfaces. Predissociation below the singlet channel threshold occurs via spin-orbit coupling to nearby repulsive triplet states. Carbon monoxide vibrational and rotational state distributions in the singlet channel as well as the triplet channel for excitation at 157 nm satisfactorily reproduce experimental data. The cross-sections of individual CO2 isotopologues (12C16O2, 12C17O16O, 12C18O16O, 13C16O2, and 13C18O16O) are calculated, demonstrating that strong isotopic fractionation will occur as a function of wavelength. The calculations provide accurate, detailed insight into CO2 photoabsorption and dissociation dynamics, and greatly extend knowledge of the temperature dependence of the cross-section to cover the range from 0 to 400 K that is useful for calculations of propagation of stellar light in planetary atmospheres. The model is also relevant for the interpretation of laboratory experiments on mass-independent isotopic fractionation. Finally, the model shows that the mass-independent fractionation observed in a series of Hg lamp experiments is not a result of hyperfine interactions making predissociation of 17O containing CO2 more efficient.

  13. Up-converted fluorescence from photosynthetic light-harvesting complexes linearly dependent on excitation intensity.

    PubMed

    Leiger, Kristjan; Freiberg, Arvi

    2016-01-01

    Weak up-converted fluorescence related to bacteriochlorophyll a was recorded from various detergent-isolated and membrane-embedded light-harvesting pigment-protein complexes as well as from the functional membranes of photosynthetic purple bacteria under continuous-wave infrared laser excitation at 1064 nm, far outside the optically allowed singlet absorption bands of the chromophore. The fluorescence increases linearly with the excitation power, distinguishing it from the previously observed two-photon excited fluorescence upon femtosecond pulse excitation. Possible mechanisms of this excitation are discussed. PMID:25764015

  14. Direct excitation of the 'dark' b {sup 3{Pi}} state predicted by deperturbation analysis of the A {sup 1}{Sigma}{sup +}-b {sup 3{Pi}} complex in KCs

    SciTech Connect

    Tamanis, M.; Klincare, I.; Kruzins, A.; Nikolayeva, O.; Ferber, R.; Pazyuk, E. A.; Stolyarov, A. V.

    2010-09-15

    The diode-laser-induced fluorescence spectra b {sup 3}{Pi}{sub {Omega}=0}{sup +}{yields}X{sup 1}{Sigma}{sup +} originated from the rovibronic levels of the 'dark' triplet b {sup 3}{Pi}{sub {Omega}=0}{sup +} state of KCs dimers were recorded with a Fourier transform spectrometer with a resolution of 0.03 cm{sup -1}. Term values of 30 rovibronic levels (v{sub b0}*{element_of}[14,18];J{sup '}{element_of}[47,134]) below the minimum of perturbing A {sup 1}{Sigma}{sup +} state were determined with 0.003-0.01 cm{sup -1} uncertainty. The optimal excitation and detection lines for very weak spin-forbidden b {sup 3}{Pi}{sub {Omega}=0}{sup +}-X{sup 1}{Sigma}{sup +} transitions have been properly predicted in the framework of the coupled-channels deperturbation analysis recently performed for the singlet-triplet A {sup 1}{Sigma}{sup +}-b {sup 3}{Pi}{sub {Omega}=0,1,2}{sup +} complex [A. Kruzins et al., Phys. Rev. A 81, 042509 (2010)]. The lowest observed triplet level has only 8.6% of singlet admixture.

  15. Narrow Energy Gap between Triplet and Singlet Excited States of Sn2+ in Borate Glass

    PubMed Central

    Masai, Hirokazu; Yamada, Yasuhiro; Suzuki, Yuto; Teramura, Kentaro; Kanemitsu, Yoshihiko; Yoko, Toshinobu

    2013-01-01

    Transparent inorganic luminescent materials have attracted considerable scientific and industrial attention recently because of their high chemical durability and formability. However, photoluminescence dynamics of ns2-type ions in oxide glasses has not been well examined, even though they can exhibit high quantum efficiency. We report on the emission property of Sn2+-doped strontium borate glasses. Photoluminescence dynamics studies show that the peak energy of the emission spectrum changes with time because of site distribution of emission centre in glass. It is also found that the emission decay of the present glass consists of two processes: a faster S1-S0 transition and a slower T1-S0 relaxation, and also that the energy difference between T1 and S1 states was found to be much smaller than that of (Sn, Sr)B6O10 crystals. We emphasize that the narrow energy gap between the S1 and T1 states provides the glass phosphor a high quantum efficiency, comparable to commercial crystalline phosphors. PMID:24345869

  16. Narrow energy gap between triplet and singlet excited states of Sn2+ in borate glass.

    PubMed

    Masai, Hirokazu; Yamada, Yasuhiro; Suzuki, Yuto; Teramura, Kentaro; Kanemitsu, Yoshihiko; Yoko, Toshinobu

    2013-01-01

    Transparent inorganic luminescent materials have attracted considerable scientific and industrial attention recently because of their high chemical durability and formability. However, photoluminescence dynamics of ns(2)-type ions in oxide glasses has not been well examined, even though they can exhibit high quantum efficiency. We report on the emission property of Sn(2+)-doped strontium borate glasses. Photoluminescence dynamics studies show that the peak energy of the emission spectrum changes with time because of site distribution of emission centre in glass. It is also found that the emission decay of the present glass consists of two processes: a faster S1-S0 transition and a slower T1-S0 relaxation, and also that the energy difference between T1 and S1 states was found to be much smaller than that of (Sn, Sr)B6O10 crystals. We emphasize that the narrow energy gap between the S1 and T1 states provides the glass phosphor a high quantum efficiency, comparable to commercial crystalline phosphors. PMID:24345869

  17. Open-Shell Singlet Character of Stable Derivatives of Nonacene, Hexacene and Teranthene

    SciTech Connect

    Gao, Xingfa; Jiang, Deen; Zhang, Prof. Shengbai; Chen, Zhongfang; Miller, Glen; Hodgson, Jennifer

    2011-01-01

    The electronic ground states of the recently synthesized stable nonacene derivatives (J. Am. Chem. Soc.2010, 132, 1261) are open-shell singlets with a polyradical nature instead of closed-shell singlets as originally assumed, according to the unrestricted broken spin-symmetry density functional theory (UBS-DFT) computations (at B3LYP/6-31G*). It is the bulky protecting groups, not the transfer from the open-shell singlet to closed-shell singlet ground state, that stabilizes these longest characterized acenes. Similar analyses also confirmed the open-shell singlet character of the hexacene and teranthene derivatives.

  18. Multiphase reacting flow modeling of singlet oxygen generators for chemical oxygen iodine lasers.

    SciTech Connect

    Salinger, Andrew Gerhard; Pawlowski, Roger Patrick; Hewett, Kevin B.; Madden, Timothy J.; Musson, Lawrence Cale

    2008-08-01

    Singlet oxygen generators are multiphase flow chemical reactors used to generate energetic oxygen to be used as a fuel for chemical oxygen iodine lasers. In this paper, a theoretical model of the generator is presented along with its solutions over ranges of parameter space and oxygen maximizing optimizations. The singlet oxygen generator (SOG) is a low-pressure, multiphase flow chemical reactor that is used to produce molecular oxygen in an electronically excited state, i.e. singlet delta oxygen. The primary product of the reactor, the energetic oxygen, is used in a stage immediately succeeding the SOG to dissociate and energize iodine. The gas mixture including the iodine is accelerated to a supersonic speed and lased. Thus the SOG is the fuel generator for the chemical oxygen iodine laser (COIL). The COIL has important application for both military purposes--it was developed by the US Air Force in the 1970s--and, as the infrared beam is readily absorbed by metals, industrial cutting and drilling. The SOG appears in various configurations, but the one in focus here is a crossflow droplet generator SOG. A gas consisting of molecular chlorine and a diluent, usually helium, is pumped through a roughly rectangular channel. An aqueous solution of hydrogen peroxide and potassium hydroxide is pumped through small holes into the channel and perpendicular to the direction of the gas flow. So doing causes the solution to become aerosolized. Dissociation of the potassium hydroxide draws a proton from the hydrogen peroxide generating an HO{sub 2} radical in the liquid. Chlorine diffuses into the liquid and reacts with the HO{sub 2} ion producing the singlet delta oxygen; some of the oxygen diffuses back into the gas phase. The focus of this work is to generate a predictive multiphase flow model of the SOG in order to optimize its design. The equations solved are the so-called Eulerian-Eulerian form of the multiphase flow Navier-Stokes equations wherein one set of the

  19. The mechanism of excited state proton dissociation in microhydrated hydroxylamine clusters.

    PubMed

    Thisuwan, Jittima; Suwannakham, Parichart; Lao-ngam, Charoensak; Sagarik, Kritsana

    2016-02-21

    The dynamics and mechanism of excited-state proton dissociation and transfer in microhydrated hydroxylamine clusters are studied using NH2OH(H2O)n (n = 1-4) as model systems and the DFT/B3LYP/aug-cc-pVDZ and TD-DFT/B3LYP/aug-cc-pVDZ methods as model calculations. This investigation is based on the Förster acidity scheme and emphasizes the photoacid dissociation in the ground (S0) and lowest singlet-excited states (S1) and the interplay between the photo and thermal excitations. The quantum chemical results suggest that the intermediate complexes are formed only in the S1 state in a low local-dielectric environment (e.g., ε = 1) and that upon the S0→ S1 transition, the photon energy excites mostly NH2OH, which leads to a homolytic cleavage of the O-H bond and to dynamically stable charge-separated Rydberg-like H-bond complexes (e.g., NH2O˙-H3O(+)˙). The potential energy surfaces for proton displacement in the smallest Rydberg-like H-bond complex support the intersection of the S0 and S1 states in low local-dielectric environments, whereas in a high local-dielectric environment (e.g., ε = 78), these two states are completely separated. Based on the static results, a photoacid-dissociation mechanism that involves Rydberg-like H-bond complex formation, an H-bond chain extension and fluctuations in the local-dielectric environment is proposed. NVT-BOMD simulations confirm the static results and show that the dynamic behavior of the dissociating proton in the S1 state is not different from that of the protonated H-bond systems in the ground state, which consists of the oscillatory shuttling and structural diffusion motions. These findings allow our theoretical methods, which have been used successfully in protonated H-bond systems in the ground state, to be applied in the study of the photoacid-dissociation processes. The current theoretical study suggests effective steps as well as guidelines for the investigation of the dynamics of the photoacid-dissociation and

  20. Singlet-triplet electron scattering admixture due to fine- and hyper-fine interactions in Cs Rydberg molecules

    NASA Astrophysics Data System (ADS)

    Markson, Samuel; Rittenhouse, Seth; Sadeghpour, Hossein

    2016-05-01

    We will present the admixture of singlet electron scattering into the more dominant triplet scattering in the formation of ultracold Cs Rydberg molecules excited into non-zero electronic angular momentum states. This admixture comes about due to both spin-orbit (SO) coupling in the Rydberg atom as well as the hyperfine (HF) coupling in the ground state atom. In Cs, the Rydberg SO and ground HF interactions are on par. The interaction between the Rydberg electron and the ground state atom includes both s-wave and p-wave scattering components which can cause additional mixing of electronic Rydberg states in the bound molecules. We intend to apply the formalism to Rydberg excitation in Cs in p and d states and will give a progress report at the meeting.

  1. Electron-Phonon Coupling and CT-Character in the lowest Triplet Excited State of Anthracene EDA-Complex Crystals

    NASA Astrophysics Data System (ADS)

    Maier, S.; Port, H.

    1987-11-01

    Photoexcitation spectra of triplet (T1← S0) zero-phonon lines and phonon sidebands in different anthracene electron donor-acceptor (EDA) complex crystals (A-PMDA, A-TCNB, A-TCPA) have been analyzed between 1.3 K and 50 K at high spectral resolution. From the electron-phonon coupling strength at T = 0 K values of the charge-transfer (CT) character in the range between 6% and 10% are calculated. The differences in these values are found to be correlated with the energetic positions of the triplet state, which are explained within the framework of the Mulliken theory.

  2. Calculating helium atomic excited states in coordinate space

    NASA Astrophysics Data System (ADS)

    Hall, Shane; Siegel, P. B.

    2015-12-01

    Two coupled Schrödinger equations are used to calculate excited states of atomic helium. Using product state functions for the two-electron state, the shooting method is used to numerically determine the energies of the allowed singlet and triplet levels. The calculations agree well with the data, and the coordinate-space basis yields Schrödinger equations for helium that are familiar to students who have used similar methods for the hydrogen atom.

  3. Fast Singlet Exciton Decay in Push-Pull Molecules Containing Oxidized Thiophenes.

    PubMed

    Busby, Erik; Xia, Jianlong; Low, Jonathan Z; Wu, Qin; Hoy, Jessica; Campos, Luis M; Sfeir, Matthew Y

    2015-06-18

    A common synthetic strategy used to design low-bandgap organic semiconductors employs the use of "push-pull" building blocks, where electron -rich and electron-deficient monomers are alternated along the π-conjugated backbone of a molecule or polymer. Incorporating strong "pull" units with high electron affinity is a means to further decrease the optical gap for infrared optoelectronics or to develop n-type semiconducting materials. Here we show that the use of thiophene-1,1-dioxide as a strong acceptor in "push-pull" oligomers affects the electronic structure and carrier dynamics in unexpected ways. Critically, the overall excited-state lifetime is reduced by several orders of magnitude relative to unoxidized analogs due to the introduction of low-energy optically dark states and low-energy triplet states that allow for fast internal conversion and intramolecular singlet fission. We found that the electronic structure and excited-state lifetime are strongly dependent on the number of sequential thiophene-1,1-dioxide units. These results suggest that both the static and dynamical optical properties are highly tunable via small changes in chemical structure that have drastic effects on the optoelectronic properties, which can impact the types of applications that involve these materials. PMID:25654490

  4. Singlet fermion dark matter within left-right model

    NASA Astrophysics Data System (ADS)

    Patra, Sudhanwa; Rao, Soumya

    2016-08-01

    We discuss singlet fermion dark matter within a left-right symmetric model promoting baryon and lepton numbers as separate gauge symmetries. We add a simple Dirac fermionic dark matter singlet under SU(2) L , R with nonzero and equal baryon and lepton number which ensures electric charge neutrality. Such a dark matter candidate interacts with SM particles through the extra ZB,ℓ gauge bosons. This can give rise to a dark matter particle of a few hundred GeV that couples to ∼TeV scale gauge bosons to give the correct relic density. This model thus accommodates TeV scale ZB,ℓ gauge bosons and other low scale BSM particles, which can be easily probed at LHC.

  5. Phenomenology of a 750 GeV singlet

    NASA Astrophysics Data System (ADS)

    Falkowski, Adam; Slone, Oren; Volansky, Tomer

    2016-02-01

    We study the recently reported excess in the diphoton resonance search by ATLAS and CMS. We investigate the available parameter space in the combined run-1 and run-2 diphoton data and study its interpretation in terms of a singlet scalar field which possibly mixes with the Standard Model Higgs boson. We show that the mixing angle is already strongly constrained by high-mass Higgs searches in the diboson channel, and by Higgs coupling measurements. While a broad resonance is slightly favored, we argue that the signal is consistent with a narrow-width singlet which couples to colored and electromagnetically-charged vector-like fermions. Dijet signals are predicted and may be visible in upcoming analyses. Allowing for additional decay modes could explain a broader resonance, however, we show that monojet searches disfavor a large invisible width. Finally, we comment on the possible relation of this scenario to the naturalness problem.

  6. A transferable model for singlet-fission kinetics

    NASA Astrophysics Data System (ADS)

    Yost, Shane R.; Lee, Jiye; Wilson, Mark W. B.; Wu, Tony; McMahon, David P.; Parkhurst, Rebecca R.; Thompson, Nicholas J.; Congreve, Daniel N.; Rao, Akshay; Johnson, Kerr; Sfeir, Matthew Y.; Bawendi, Moungi G.; Swager, Timothy M.; Friend, Richard H.; Baldo, Marc A.; van Voorhis, Troy

    2014-06-01

    Exciton fission is a process that occurs in certain organic materials whereby one singlet exciton splits into two independent triplets. In photovoltaic devices these two triplet excitons can each generate an electron, producing quantum yields per photon of >100% and potentially enabling single-junction power efficiencies above 40%. Here, we measure fission dynamics using ultrafast photoinduced absorption and present a first-principles expression that successfully reproduces the fission rate in materials with vastly different structures. Fission is non-adiabatic and Marcus-like in weakly interacting systems, becoming adiabatic and coupling-independent at larger interaction strengths. In neat films, we demonstrate fission yields near unity even when monomers are separated by >5 Å. For efficient solar cells, however, we show that fission must outcompete charge generation from the singlet exciton. This work lays the foundation for tailoring molecular properties like solubility and energy level alignment while maintaining the high fission yield required for photovoltaic applications.

  7. Trap-induced photoconductivity in singlet fission pentacene diodes

    NASA Astrophysics Data System (ADS)

    Qiao, Xianfeng; Zhao, Chen; Chen, Bingbing; Luan, Lin

    2014-07-01

    This paper reports a trap-induced photoconductivity in ITO/pentacene/Al diodes by using current-voltage and magneto-conductance measurements. The comparison of photoconductivity between pentacene diodes with and without trap clearly shows that the traps play a critical role in generating photoconductivity. It shows that no observable photoconductivity is detected for trap-free pentacene diodes, while significant photoconductivity is observed in diodes with trap. This is because the initial photogenerated singlet excitons in pentacene can rapidly split into triplet excitons with higher binding energy prior to dissociating into free charge carriers. The generated triplet excitons react with trapped charges to release charge-carriers from traps, leading to a trap-induced photoconductivity in the single-layer pentacene diodes. Our studies elucidated the formation mechanisms of photoconductivity in pentacene diodes with extremely fast singlet fission rate.

  8. Singlet exciton fission-sensitized infrared quantum dot solar cells.

    PubMed

    Ehrler, Bruno; Wilson, Mark W B; Rao, Akshay; Friend, Richard H; Greenham, Neil C

    2012-02-01

    We demonstrate an organic/inorganic hybrid photovoltaic device architecture that uses singlet exciton fission to permit the collection of two electrons per absorbed high-energy photon while simultaneously harvesting low-energy photons. In this solar cell, infrared photons are absorbed using lead sulfide (PbS) nanocrystals. Visible photons are absorbed in pentacene to create singlet excitons, which undergo rapid exciton fission to produce pairs of triplets. Crucially, we identify that these triplet excitons can be ionized at an organic/inorganic heterointerface. We report internal quantum efficiencies exceeding 50% and power conversion efficiencies approaching 1%. These findings suggest an alternative route to circumvent the Shockley-Queisser limit on the power conversion efficiency of single-junction solar cells. PMID:22257168

  9. High efficiency organic multilayer photodetectors based on singlet exciton fission

    NASA Astrophysics Data System (ADS)

    Lee, J.; Jadhav, P.; Baldo, M. A.

    2009-07-01

    We employ an exciton fission process that converts one singlet exciton into two triplet excitons to increase the quantum efficiency of an organic multilayer photodetector beyond 100%. The photodetector incorporates ultrathin alternating donor-acceptor layers of pentacene and C60, respectively. By comparing the quantum efficiency after separate pentacene and C60 photoexcitation we find that singlet exciton fission in pentacene enhances the quantum efficiency by (45±7)%. In quantitative agreement with this result, we also observe that the photocurrent generated from pentacene excitons is decreased by (2.7±0.2)% under an applied magnetic field of H =0.4 T, while the C60 photocurrent is relatively unchanged.

  10. Probing the reactivity of singlet oxygen with purines

    PubMed Central

    Dumont, Elise; Grüber, Raymond; Bignon, Emmanuelle; Morell, Christophe; Moreau, Yohann; Monari, Antonio; Ravanat, Jean-Luc

    2016-01-01

    The reaction of singlet molecular oxygen with purine DNA bases is investigated by computational means. We support the formation of a transient endoperoxide for guanine and by classical molecular dynamics simulations we demonstrate that the formation of this adduct does not affect the B-helicity. We thus identify the guanine endoperoxide as a key intermediate, confirming a low-temperature nuclear magnetic resonance proof of its existence, and we delineate its degradation pathway, tracing back the preferential formation of 8-oxoguanine versus spiro-derivates in B-DNA. Finally, the latter oxidized 8-oxodGuo product exhibits an almost barrierless reaction profile, and hence is found, coherently with experience, to be much more reactive than guanine itself. On the contrary, in agreement with experimental observations, singlet-oxygen reactivity onto adenine is kinetically blocked by a higher energy transition state. PMID:26656495

  11. Conference on Singlet Molecular Oxygen (COSMO 84), program and abstracts

    NASA Astrophysics Data System (ADS)

    Stevens, B.

    1984-09-01

    The Conference objective was to promote interdisciplinary awareness and communication by assembling research workers in such diverse fields as photochemistry, photophysics, synthetic chemistry, photobiology, photomedicine, laser and atmospheric physics; and to present their recent findings with singlet molecular oxygen as the common theme. This was prompted by several recent developments: notably the direct observation of 02 superscript 1 micron g in emission at 1.27 microns which now provides solvent-dependent life-times, reaction or quenching rate constants, and sensitized yields directly; the trapping of zwitterionic intermediates and the role of catalysts in electron transfer peroxidation; the feasibility of a singlet oxygen-iodine chemical laser; direct observation of 02 microns g in enzymic processes; the use of endoperoxides as actinometers; the phototherapy of malignant tumors; prospects for 02 superscript 1 delta g as a solar energy storage intermediate.

  12. A transferable model for singlet-fission kinetics.

    PubMed

    Yost, Shane R; Lee, Jiye; Wilson, Mark W B; Wu, Tony; McMahon, David P; Parkhurst, Rebecca R; Thompson, Nicholas J; Congreve, Daniel N; Rao, Akshay; Johnson, Kerr; Sfeir, Matthew Y; Bawendi, Moungi G; Swager, Timothy M; Friend, Richard H; Baldo, Marc A; Van Voorhis, Troy

    2014-06-01

    Exciton fission is a process that occurs in certain organic materials whereby one singlet exciton splits into two independent triplets. In photovoltaic devices these two triplet excitons can each generate an electron, producing quantum yields per photon of >100% and potentially enabling single-junction power efficiencies above 40%. Here, we measure fission dynamics using ultrafast photoinduced absorption and present a first-principles expression that successfully reproduces the fission rate in materials with vastly different structures. Fission is non-adiabatic and Marcus-like in weakly interacting systems, becoming adiabatic and coupling-independent at larger interaction strengths. In neat films, we demonstrate fission yields near unity even when monomers are separated by >5 Å. For efficient solar cells, however, we show that fission must outcompete charge generation from the singlet exciton. This work lays the foundation for tailoring molecular properties like solubility and energy level alignment while maintaining the high fission yield required for photovoltaic applications. PMID:24848234

  13. A flippon related singlet at the LHC II

    NASA Astrophysics Data System (ADS)

    Li, Tianjun; Maxin, James A.; Mayes, Van E.; Nanopoulos, Dimitri V.

    2016-06-01

    We consider the 750 GeV diphoton resonance at the 13 TeV LHC in the ℱ-SU(5) model with a Standard Model (SM) singlet field which couples to TeV-scale vector-like particles, dubbed flippons. This singlet field assumes the role of the 750 GeV resonance, with production via gluon fusion and subsequent decay to a diphoton via the vector-like particle loops. We present a numerical analysis showing that the observed 8 TeV and 13 TeV diphoton production cross-sections can be generated in the model space with realistic electric charges and Yukawa couplings for light vector-like masses. We further discuss the experimental viability of light vector-like masses in a General No-Scale ℱ-SU(5) model, offering a few benchmark scenarios in this consistent GUT that can satisfy all experimental constraints imposed by the LHC and other essential experiments.

  14. Trap-induced photoconductivity in singlet fission pentacene diodes

    SciTech Connect

    Qiao, Xianfeng Zhao, Chen; Chen, Bingbing; Luan, Lin

    2014-07-21

    This paper reports a trap-induced photoconductivity in ITO/pentacene/Al diodes by using current-voltage and magneto-conductance measurements. The comparison of photoconductivity between pentacene diodes with and without trap clearly shows that the traps play a critical role in generating photoconductivity. It shows that no observable photoconductivity is detected for trap-free pentacene diodes, while significant photoconductivity is observed in diodes with trap. This is because the initial photogenerated singlet excitons in pentacene can rapidly split into triplet excitons with higher binding energy prior to dissociating into free charge carriers. The generated triplet excitons react with trapped charges to release charge-carriers from traps, leading to a trap-induced photoconductivity in the single-layer pentacene diodes. Our studies elucidated the formation mechanisms of photoconductivity in pentacene diodes with extremely fast singlet fission rate.

  15. Probing the reactivity of singlet oxygen with purines.

    PubMed

    Dumont, Elise; Grüber, Raymond; Bignon, Emmanuelle; Morell, Christophe; Moreau, Yohann; Monari, Antonio; Ravanat, Jean-Luc

    2016-01-01

    The reaction of singlet molecular oxygen with purine DNA bases is investigated by computational means. We support the formation of a transient endoperoxide for guanine and by classical molecular dynamics simulations we demonstrate that the formation of this adduct does not affect the B-helicity. We thus identify the guanine endoperoxide as a key intermediate, confirming a low-temperature nuclear magnetic resonance proof of its existence, and we delineate its degradation pathway, tracing back the preferential formation of 8-oxoguanine versus spiro-derivates in B-DNA. Finally, the latter oxidized 8-oxodGuo product exhibits an almost barrierless reaction profile, and hence is found, coherently with experience, to be much more reactive than guanine itself. On the contrary, in agreement with experimental observations, singlet-oxygen reactivity onto adenine is kinetically blocked by a higher energy transition state. PMID:26656495

  16. The convergence of complete active space self-consistent-field configuration interaction including all single and double excitation energies to the complete basis set limit.

    PubMed

    Petersson, George A; Malick, David K; Frisch, Michael J; Braunstein, Matthew

    2006-07-28

    Examination of the convergence of full valence complete active space self-consistent-field configuration interaction including all single and double excitation (CASSCF-CISD) energies with expansion of the one-electron basis set reveals a pattern very similar to the convergence of single determinant energies. Calculations on the lowest four singlet states and the lowest four triplet states of N(2) with the sequence of n-tuple-zeta augmented polarized (nZaP) basis sets (n=2, 3, 4, 5, and 6) are used to establish the complete basis set limits. Full configuration-interaction (CI) and core electron contributions must be included for very accurate potential energy surfaces. However, a simple extrapolation scheme that has no adjustable parameters and requires nothing more demanding than CAS(10e(-),8orb)-CISD/3ZaP calculations gives the R(e), omega(e), omega(e)X(e), T(e), and D(e) for these eight states with rms errors of 0.0006 Angstrom, 4.43 cm(-1), 0.35 cm(-1), 0.063 eV, and 0.018 eV, respectively. PMID:16942134

  17. Combining Ruthenium(II) Complexes with Metal-Organic Frameworks to Realize Effective Two-Photon Absorption for Singlet Oxygen Generation.

    PubMed

    Zhang, Wenxiang; Li, Bin; Ma, Heping; Zhang, Liming; Guan, Yunlong; Zhang, Yihe; Zhang, Xindan; Jing, Pengtao; Yue, Shumei

    2016-08-24

    Singlet oxygen ((1)O2), as a reactive oxygen species, has garnered serious attention in physical, chemical, and biological studies. In this paper, we designed and synthesized a new type of singlet-oxygen generation system by exchanging cationic ruthenium complexes (RCs) into anionic bio-MOF-1. The resulting bio-MOF-1&RCs can be used as effective photocatalysts for generation of singlet oxygen under both single-photon and two-photon excitation. Especially, the excellent two-photon absorption (TPA) behavior of bio-MOF-1&RCs aroused our interest greatly because their two-photon absorption band lies in the optical window of biological tissue. Here, we measured the ability of bio-MOF-1&RCs to generate (1)O2 by irradiation under both 490 and 800 nm wavelength light in DMF. 1,3-Diphenylisobenzofuran (DPBF) and 2',7'-dichlorofluorescein (DCFH) were used as typical (1)O2 traps to detect and evaluate the efficiency of generation of (1)O2 under single-photon and two-photon excitation, respectively. Results indicated that bio-MOF-1&[Ru(phen)3](2+) was able to effectively generate (1)O2 under both conditions. Our work creates a novel synergistic TPA system with the excellent photophysical properties of RCs and the unique microporous structure benefit of MOFs, which may open a new avenue for creation of a cancer treatment system with both photodynamic therapy and chemotherapy. PMID:27483010

  18. IDMS: inert dark matter model with a complex singlet

    NASA Astrophysics Data System (ADS)

    Bonilla, Cesar; Sokolowska, Dorota; Darvishi, Neda; Diaz-Cruz, J. Lorenzo; Krawczyk, Maria

    2016-06-01

    We study an extension of the inert doublet model (IDM) that includes an extra complex singlet of the scalars fields, which we call the IDMS. In this model there are three Higgs particles, among them a SM-like Higgs particle, and the lightest neutral scalar, from the inert sector, remains a viable dark matter (DM) candidate. We assume a non-zero complex vacuum expectation value for the singlet, so that the visible sector can introduce extra sources of CP violation. We construct the scalar potential of IDMS, assuming an exact Z 2 symmetry, with the new singlet being Z 2-even, as well as a softly broken U(1) symmetry, which allows a reduced number of free parameters in the potential. In this paper we explore the foundations of the model, in particular the masses and interactions of scalar particles for a few benchmark scenarios. Constraints from collider physics, in particular from the Higgs signal observed at the Large Hadron Collider with {M}h≈ 125 {{GeV}}, as well as constraints from the DM experiments, such as relic density measurements and direct detection limits, are included in the analysis. We observe significant differences with respect to the IDM in relic density values from additional annihilation channels, interference and resonance effects due to the extended Higgs sector.

  19. Remarks on models with singlet neutrino in large extra dimensions*

    NASA Astrophysics Data System (ADS)

    Agashe, K.; Wu, G.-H.

    2001-01-01

    Small Dirac masses for neutrinos are natural in models with singlet fermions in large extra dimensions with quantum gravity scale M*~1-100 TeV. We study two modifications of the minimal model in order to obtain the mass scale relevant for atmospheric neutrino oscillations with at most /O(1) higher-dimensional Yukawa couplings and with M*~ a few TeV. (1) In models with singlet fermions in smaller number of extra dimensions than gravity, we find that the effects on BR/(μ-->eγ) and on charged-current universality in π--->eν¯,μν¯ decays are suppressed as compared to that in the minimal model with neutrino and gravity in the same space. (2) If small Dirac masses for the singlets are added along with lepton number violating couplings, then the mass scales and mixing angles for neutrino oscillations can be different from those relevant for /μ-->eγ and π--->eν¯,μν¯. Thus, in both modified models the constraints on M* from BR/(μ-->eγ) and π--->eν¯,μν¯ decays can be significantly relaxed. Furthermore, constraints from supernova /1987a strongly disfavor oscillations of active neutrinos to sterile neutrinos in both the minimal and the modified models.

  20. Macroscopic singlet oxygen model incorporating photobleaching as an input parameter

    NASA Astrophysics Data System (ADS)

    Kim, Michele M.; Finlay, Jarod C.; Zhu, Timothy C.

    2015-03-01

    A macroscopic singlet oxygen model for photodynamic therapy (PDT) has been used extensively to calculate the reacted singlet oxygen concentration for various photosensitizers. The four photophysical parameters (ξ, σ, β, δ) and threshold singlet oxygen dose ([1O2]r,sh) can be found for various drugs and drug-light intervals using a fitting algorithm. The input parameters for this model include the fluence, photosensitizer concentration, optical properties, and necrosis radius. An additional input variable of photobleaching was implemented in this study to optimize the results. Photobleaching was measured by using the pre-PDT and post-PDT sensitizer concentrations. Using the RIF model of murine fibrosarcoma, mice were treated with a linear source with fluence rates from 12 - 150 mW/cm and total fluences from 24 - 135 J/cm. The two main drugs investigated were benzoporphyrin derivative monoacid ring A (BPD) and 2-[1-hexyloxyethyl]-2-devinyl pyropheophorbide-a (HPPH). Previously published photophysical parameters were fine-tuned and verified using photobleaching as the additional fitting parameter. Furthermore, photobleaching can be used as an indicator of the robustness of the model for the particular mouse experiment by comparing the experimental and model-calculated photobleaching ratio.