Sample records for oligophenylenes

  1. Synthesis of Bridged Oligophenylene Laser Dyes

    DTIC Science & Technology


    azaoligophenylenes were fluorescent. The naphthalene analog fluoresced at the shortest wavelength (480 nm In the blue green) while the longer...solvents and thus can provide potential laser dyes in the blue - green to the yellow (480-570 nm) regions of the visible spectrum. An additional desirable...Four compounds belong the the hydrocarbon oligophenylene class. Three of these are symmetrical sexiphenyls with one, two and three dipropyl- methylene

  2. Long-Range Ruthenium-Amine Electronic Communication through the para-Oligophenylene Wire

    PubMed Central

    Shen, Jun-Jian; Zhong, Yu-Wu


    The studies of long-range electronic communication are hampered by solubility and potential-splitting issues. A “hybridized redox-asymmetry” method using a combination of organic and inorganic redox species is proposed and exemplified to overcome these two issues. Complexes 1(PF6)–6(PF6) (from short to long in length) with the organic redox-active amine and inorganic cyclometalated ruthenium termini bridged by the para-oligophenylene wire have been prepared. Complex 6 has the longest Ru-amine geometrical distance of 27.85 Å. Complexes 3(PF6) and 4(PF6) show lamellar crystal packing on the basis of a head-to-tail anti-parallelly aligned dimeric structure. Two redox waves are observed for all complexes in the potential region between +0.2 and +0.9 V vs Ag/AgCl. The electrochemical potential splitting is 410, 220, 143, 112, 107, and 105 mV for 1(PF6) through 6(PF6), respectively. Ruthenium (+2) to aminium (N•+) charge transfer transitions have been identified for the odd-electron compounds 12+–62+ by spectroelectrochemical measurements. The electronic communication between amine and ruthenium decreases exponentially with a decay slope of −0.137 Å−1. DFT calculations have been performed to complement these experimental results. PMID:26344929

  3. Electronic and structural properties of oligophenylene ethynylenes on Au(111) surfaces

    NASA Astrophysics Data System (ADS)

    Miao, Ling; Seminario, Jorge M.


    The interaction of oligophenylene ethynylene (OPE) on the (111) surface of a gold slab resembling a self-assembled monolayer (SAM) is investigated using ab initio density functional theory calculations. The authors performed a full optimization including all atoms in the OPE and in the slab to better understand OPE adsorption on the surface. It is found that OPE has two energetically favorable adsorption sites on the Au surface with relatively different molecular geometries: the nontop site adsorption greatly modifies the (111) surface structure; however, the extensive electron interactions enable a delocalized electron density distribution, implying an improved conductivity between OPE and Au, and the top site which is 0.9eV higher in energy than the nontop and features weaker Au-S bonds. Interestingly the on top configuration shows a strong spin imbalance along the molecule and the nontop shows a small spin imbalance on the surface. This feature is of strong interest for the development of resonators for the detection of chemical and biological agents. They have also calculated the frequency spectrum of these SAMs, which yield deformations in the gold surface yielding peak frequency shifts specific to each absorption site.

  4. Spin-selective charge transport pathways through p-oligophenylene-linked donor-bridge-acceptor molecules.


    Scott, Amy M; Miura, Tomoaki; Ricks, Annie Butler; Dance, Zachary E X; Giacobbe, Emilie M; Colvin, Michael T; Wasielewski, Michael R


    A series of donor-bridge-acceptor (D-B-A) triads have been synthesized in which the donor, 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An), and the acceptor, naphthalene-1,8:4,5-bis(dicarboximide) (NI), are linked by p-oligophenylene (Ph(n)) bridging units (n = 1-5). Photoexcitation of DMJ-An produces DMJ(+*)-An(-*) quantitatively, so that An(-*) acts as a high potential electron donor, which rapidly transfers an electron to NI yielding a long-lived spin-coherent radical ion pair (DMJ(+*)-An-Ph(n)-NI(-*)). The charge transfer properties of 1-5 have been studied using transient absorption spectroscopy, magnetic field effects (MFEs) on radical pair and triplet yields, and time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The charge separation (CS) and recombination (CR) reactions exhibit exponential distance dependencies with damping coefficients of beta = 0.35 A(-1) and 0.34 A(-1), respectively. Based on these data, a change in mechanism from superexchange to hopping was not observed for either process in this system. However, the CR reaction is spin-selective and produces the singlet ground state and both (3*)An and (3*)NI. A kinetic analysis of the MFE data shows that superexchange dominates both pathways with beta = 0.48 A(-1) for the singlet CR pathway and beta = 0.35 A(-1) for the triplet CR pathway. MFEs and TREPR experiments were used to measure the spin-spin exchange interaction, 2J, which is directly related to the electronic coupling matrix element for CR, V(CR)(2). The magnitude of 2J also shows an exponential distance dependence with a damping coefficient alpha = 0.36 A(-1), which agrees with the beta values obtained from the distance dependence for triplet CR. These results were analyzed in terms of the bridge molecular orbitals that participate in the charge transport mechanism.

  5. Exceptionally Small Statistical Variations in the Transport Properties of Metal-Molecule-Metal Junctions Composed of 80 Oligophenylene Dithiol Molecules.


    Xie, Zuoti; Bâldea, Ioan; Demissie, Abel T; Smith, Christopher E; Wu, Yanfei; Haugstad, Greg; Frisbie, C Daniel


    Strong stochastic fluctuations witnessed as very broad resistance (R) histograms with widths comparable to or even larger than the most probable values characterize many measurements in the field of molecular electronics, particularly those measurements based on single molecule junctions at room temperature. Here we show that molecular junctions containing 80 oligophenylene dithiol molecules (OPDn, 1 ≤ n ≤ 4) connected in parallel display small relative statistical deviations-δR/R ≈ 25% after only ∼200 independent measurements-and we analyze the sources of these deviations quantitatively. The junctions are made by conducting probe atomic force microscopy (CP-AFM) in which an Au-coated tip contacts a self-assembled monolayer (SAM) of OPDs on Au. Using contact mechanics and direct measurements of the molecular surface coverage, the tip radius, tip-SAM adhesion force (F), and sample elastic modulus (E), we find that the tip-SAM contact area is approximately 25 nm(2), corresponding to about 80 molecules in the junction. Supplementing this information with I-V data and an analytic transport model, we are able to quantitatively describe the sources of deviations δR in R: namely, δN (deviations in the number of molecules in the junction), δε (deviations in energetic position of the dominant molecular orbital), and δΓ (deviations in molecule-electrode coupling). Our main results are (1) direct determination of N; (2) demonstration that δN/N for CP-AFM junctions is remarkably small (≤2%) and that the largest contributions to δR are δε and δΓ; (3) demonstration that δR/R after only ∼200 measurements is substantially smaller than most reports based on >1000 measurements for single molecule break junctions. Overall, these results highlight the excellent reproducibility of junctions composed of tens of parallel molecules, which may be important for continued efforts to build robust molecular devices.

  6. Topography and transport properties of oligo(phenylene ethynylene) molecular wires studied by scanning tunneling microscopy

    NASA Technical Reports Server (NTRS)

    Dholakia, Geetha R.; Fan, Wendy; Koehne, Jessica; Han, Jie; Meyyappan, M.


    Conjugated phenylene(ethynylene) molecular wires are of interest as potential candidates for molecular electronic devices. Scanning tunneling microscopic study of the topography and current-voltage (I-V) characteristics of self-assembled monolayers of two types of molecular wires are presented here. The study shows that the topography and I-Vs, for small scan voltages, of the two wires are quite similar and that the electronic and structural changes introduced by the substitution of an electronegative N atom in the central phenyl ring of these wires does not significantly alter the self-assembly or the transport properties.

  7. Synthesis and electrical characterization of oligo(phenylene ethynylene) molecular wires coordinated to transition metal complexes.


    Ng, Zhaoyue; Loh, Kian Ping; Li, Liqian; Ho, Peter; Bai, Ping; Yip, John H K


    Organometallic wires are interesting alternatives to conventional molecular wires based on a pure organic system because of the presence of d orbitals in the transition metal complex. However, synthetic problems, such as decreased stability of the compounds when labile metal complexes are present, often impede their isolation in a pure state and preclude a rapid development of such hybrid molecular wires. In this work, we show that preassembled self-assembled monolayers (SAM) based on pyridine-terminated 1-((4-acetylthiophenyl)ethynyl)-4-((4-pyridyl)ethynyl)benzene can act as a template for the architectural build up of a second layer of transition metal complexes to form an array of organometallic molecular wires on gold. Ru(II)(terpy)(bipy)(2+) (terpy = 2,2':6',2''-terpyridine and bipy = 2,2'-bipyridine) or cyclometalated Pt(II)(pbipy) (pbipy = 6-phenyl-2,2'-bipyridine) were axially coordinated onto the organic SAM via its terminal pyridinium moieties. Current-voltage studies show that the electronic coupling between the transition metal and organic wire produces a molecular wire that exhibits higher conductance than the original organic chain. The presence of the transition metal complexes in the hybrid molecular wire introduces distinctive negative differential resistance (NDR) effects.

  8. Synthesis of rigid homo- and heteroditopic nucleobase-terminated molecules incorporating adenine and/or thymine.


    Jacobsen, Mikkel F; Andersen, Casper S; Knudsen, Martin M; Gothelf, Kurt V


    A series of homo- and heteroditopic thymine- and/or adenine-terminated molecules incorporating rigid aryl or oligo(phenylene ethynylene) linkers has been efficiently synthesized. The key steps involved in the synthesis are the construction of the N-arylated nucleobases using the Chan-Lam-Evans-modified Ullman coupling and their further elaboration using the Sonogashira coupling. Furthermore, the synthesis of a rigid tripodal thymine derivative is reported.

  9. Methods of Attaching or Grafting Carbon Nanotubes to Silicon Surfaces and Composite Structures Derived Therefrom

    NASA Technical Reports Server (NTRS)

    Tour, James M. (Inventor); Chen, Bo (Inventor); Flatt, Austen K. (Inventor); Stewart, Michael P. (Inventor); Dyke, Christopher A. (Inventor); Maya, Francisco (Inventor)


    The present invention is directed toward methods of attaching or grafting carbon nanotubes (CNTs) to silicon surfaces. In some embodiments, such attaching or grafting occurs via functional groups on either or both of the CNTs and silicon surface. In some embodiments, the methods of the present invention include: (1) reacting a silicon surface with a functionalizing agent (such as oligo(phenylene ethynylene)) to form a functionalized silicon surface; (2) dispersing a quantity of CNTs in a solvent to form dispersed CNTs; and (3) reacting the functionalized silicon surface with the dispersed CNTs. The present invention is also directed to the novel compositions produced by such methods.

  10. Fabrication of carbon nanotube nanogap electrodes by helium ion sputtering for molecular contacts

    SciTech Connect

    Thiele, Cornelius; Vieker, Henning; Beyer, André; Gölzhäuser, Armin; Flavel, Benjamin S.; Hennrich, Frank; Muñoz Torres, David; Eaton, Thomas R.; Mayor, Marcel; Kappes, Manfred M.; Löhneysen, Hilbert v.; and others


    Carbon nanotube nanogaps have been used to contact individual organic molecules. However, the reliable fabrication of a truly nanometer-sized gap remains a challenge. We use helium ion beam lithography to sputter nanogaps of only (2.8 ± 0.6) nm size into single metallic carbon nanotubes embedded in a device geometry. The high reproducibility of the gap size formation provides a reliable nanogap electrode testbed for contacting small organic molecules. To demonstrate the functionality of these nanogap electrodes, we integrate oligo(phenylene ethynylene) molecular rods, and measure resistance before and after gap formation and with and without contacted molecules.

  11. Ultrafast photoinduced charge transfer in pi-conjugated electron systems: Effects of structure, delocalization, and energetics (Conference Presentation)

    NASA Astrophysics Data System (ADS)

    Schanze, Kirk S.; Jones, Austin; Gish, Melissa; Zeman, Charles J.; Alsam, Amani A.; Aly, Shawkat M.; Papanikolas, John M.; Mohammed, Omar F.


    Photoinduced charge transfer is a key step in the mechanism of charge generation in organic solar cells. Charge transfer typically occurs from a photoexcited conjugated polymer donor to an electron acceptor. In an effort to better understand the primary events in solar cells, we have investigated photoinduced charge transfer in model donor-acceptor systems consisting of pi-conjugated oligomer donors that are covalently linked to diimide electron acceptors. These studies utilized oligo(thiophene), oligo(phenylene ethynylene) and oligo(fluorene) pi-conjugated systems with lengths varying from 4 to 12 repeat units linked to naphthalene diimide electron acceptors. Excitation with 100 femtosecond pulses at wavelengths correspoinding to the conjugated oligomer absorption band(s) leads to rapid photoinduced charge transfer to produce a charge separated state, (oligomer+)-(NDI-), which subsequently decays on timescales ranging from 100 ps to 5 ns. The dynamics of the forward and reverse electron transfer reactions depend strongly on the structure and length of the pi-conjugated oligomers, with the fastest rates occurring for oligo(thiophene)s, and considerably slower rates for oligo(phenylene ethynylene)s. The talk will discuss the structure-property relationships and energetic correlations that control the dynamics of charge separation and recombination.

  12. Functional Si and CdSe quantum dots: synthesis, conjugate formation, and photoluminescence quenching by surface interactions.


    Sudeep, P K; Emrick, Todd


    Silicon quantum dots (QDs) were prepared with a corona of di-n-octyl phosphine oxides, by performing hydrosilylation chemistry on the surface of hydrogen-terminated Si QDs. These novel Si QDs proved well-suited to serve as "ligands" for other semiconductor QDs, such as CdSe, by interaction of the phosphine oxide corona with the CdSe surface. A pronounced photoluminescence quenching of CdSe quantum dots was observed upon introduction of the phosphine oxide functionalized Si QDs to a CdSe QD solution. Surface functionalization of the Si QDs proved critically important to observing these effects, as conventional (alkane-covered) Si QD samples gave no evidence of electronic interactions with TOPO-covered CdSe. In a comparative system, phosphine oxide terminated oligo(phenylene vinylene) molecules acting as CdSe QD ligands provide a similar fluorescence quenching, with exciton decay kinetics supporting the formation of an electronically interacting hybrid materials system.

  13. Coverage-mediated suppression of blinking in solid state quantum dot conjugated organic composite nanostructures.


    Hammer, Nathan I; Early, Kevin T; Sill, Kevin; Odoi, Michael Y; Emrick, Todd; Barnes, Michael D


    Size-correlated single-molecule fluorescence measurements on CdSe quantum dots functionalized with oligo(phenylene vinylene) (OPV) ligands exhibit modified fluorescence intermittency (blinking) statistics that are highly sensitive to the degree of ligand coverage on the quantum dot surface. As evidenced by a distinct surface height signature, fully covered CdSe-OPV nanostructures (approximately 25 ligands) show complete suppression of blinking in the solid state on an integration time scale of 1 s. Some access to dark states is observed on finer time scales (100 ms) with average persistence times significantly shorter than those from ZnS-capped CdSe quantum dots. This effect is interpreted as resulting from charge transport from photoexcited OPV into vacant trap sites on the quantum dot surface. These results suggest exciting new applications of composite quantum dot/organic systems in optoelectronic systems.

  14. Cationic phenylene ethynylene polymers and oligomers exhibit efficient antiviral activity.


    Wang, Ying; Canady, Taylor D; Zhou, Zhijun; Tang, Yanli; Price, Dominique N; Bear, David G; Chi, Eva Y; Schanze, Kirk S; Whitten, David G


    The antiviral activities of poly(phenylene ethynylene) (PPE)-based cationic conjugated polyelectrolytes (CPE) and oligo-phenylene ethynylenes (OPE) were investigated using two model viruses, the T4 and MS2 bacteriophages. Under UV/visible light irradiation, significant antiviral activity was observed for all of the CPEs and OPEs; without irradiation, most of these compounds exhibited high inactivation activity against the MS2 phage and moderate inactivation ability against the T4 phage. Transmission electron microscopy (TEM) and SDS polyacrylamide gel electrophoresis (SDS-PAGE) reveal that the CPEs and OPEs exert their antiviral activity by partial disassembly of the phage particle structure in the dark and photochemical damage of the phage capsid protein under UV/visible light irradiation.

  15. Insulator-protected mechanically controlled break junctions for measuring single-molecule conductance in aqueous environments

    NASA Astrophysics Data System (ADS)

    Muthusubramanian, N.; Galan, E.; Maity, C.; Eelkema, R.; Grozema, F. C.; van der Zant, H. S. J.


    We present a method to fabricate insulated gold mechanically controlled break junctions (MCBJ) by coating the metal with a thin layer of aluminum oxide using plasma enhanced atomic layer deposition. The Al2O3 thickness deposited on the MCBJ devices was varied from 2 to 15 nm to test the suppression of leakage currents in deionized water and phosphate buffered saline. Junctions coated with a 15 nm thick oxide layer yielded atomically sharp electrodes and negligible conductance counts in the range of 1 to 10-4 G0 (1 G0 = 77 μS), where single-molecule conductances are commonly observed. The insulated devices were used to measure the conductance of an amphiphilic oligophenylene ethynylene derivative in deionized water.

  16. Low temperature enantiotropic nematic phases from V-shaped, shape-persistent molecules.


    Lehmann, Matthias; Seltmann, Jens


    A series of V-shaped, shape-persistent thiadiazole nematogens, based on an oligo(phenylene ethynylene) scaffold with ester groups connected via alkyloxy spacers, was efficiently prepared by a two-step procedure. Phase engineering results in an optimum of the mesophase range and low melting temperature when the nematogens are desymmetrised with a butoxy and a heptyloxy spacer. The mesophases are enantiotropic and over the whole temperature range nematic. For the optimised mesogen structure, optical investigations by conoscopy monitored a uniaxial nematic phase upon cooling from the isotropic phase to room temperature (ΔT = 150° C). X-ray studies on magnetic-field-aligned samples of this mesogen family revealed a general pattern, indicating the alignment of two molecular axes along individual directors in the magnetic field. These observations may be rationalised with larger assemblies of V-shaped molecules isotropically distributed around the direction of the magnetic field.

  17. Supramolecular self-assembly of conjugated diblock copolymers.

    SciTech Connect

    Wang, H.; You, W.; Jiang, P.; Yu, L.; Wang, H. H.; Univ. of Chicago


    This paper describes the synthesis and characterization of a novel series of copolymers with different lengths of oligo(phenylene vinylene) (OPV) as the rod block, and poly(propylene oxide) as the coil block. Detailed characterization by means of transmission electron microscopy (TEM), atomic force microscopy (AFM), and small-angle neutron scattering (SANS) revealed the strong tendency of these copolymers to self-assemble into cylindrical micelles in solution and as-casted films on a nanometer scale. These micelles have a cylindrical OPV core surrounded by a poly(propylene glycol) (PPG) corona and readily align with each other to form parallel packed structures when mica is used as the substrate. A packing model has been proposed for these cylindrical micelles.

  18. Quinoidal/Aromatic Transformations in π-Conjugated Oligomers: Vibrational Raman studies on the Limits of Rupture for π-Bonds.


    Burrezo, Paula Mayorga; Zafra, José L; López Navarrete, Juan T; Casado, Juan


    The vibrational Raman spectra of several series of aromatic and quinoidal compounds have been analyzed considering the downshifts and upshifts of the frequencies of the relevant Raman bands as a function of the number of repeating units. Oligothiophenes, oligophenylene-vinylenes, and oligoperylenes (oligophenyls) derivatives are studied in a common context. These shifts are taken as spectroscopic fingerprints of the changes in π-conjugation. For a given family, aromatic and quinoidal oligomers have been studied together, and according to their Raman frequency shifts located in the two-well BLA-energy curve of their ground electronic state as a function of the bond-length-alternation pattern (BLA). The connection among BLA values, π-conjugation, and Raman frequencies is taken here as the basis of the study. These Raman shifts/BLA changes have been related to important electronic properties of these one-dimensional linear π-electron delocalized systems such as quinoidal (polyene) and aromatic characters.

  19. Organic scintillators with pulse shape discrimination for detection of radiation (Conference Presentation)

    NASA Astrophysics Data System (ADS)

    Mabe, Andrew; Carman, M. Leslie; Glenn, Andrew M.; Zaitseva, Natalia P.; Payne, Stephen A.


    The detection of neutrons in the presence of gamma-ray fields has important applications in the fields of nuclear physics, homeland security, and medical imaging. Organic scintillators provide several attractive qualities as neutron detection materials including low cost, fast response times, ease of scaling, and the ability to implement pulse shape discrimination (PSD) to discriminate between neutrons and gamma-rays. This talk will focus on amorphous organic scintillators both in plastic form and small-molecule organic glass form. The first section of this talk will describe recent advances and improvements in the performance of PSD-capable plastic scintillators. The primary advances described in regard to modification of the polymer matrix, evaluation of new scintillating dyes, improved fabrication conditions, and implementation of additives which impart superior performance and mechanical properties to PSD-capable plastics as compared to commercially-available plastics and performance comparable to PSD-capable liquids. The second section of this talk will focus on a class of small-molecule organic scintillators based on modified indoles and oligophenylenes which form amorphous glasses as PSD-capable neutron scintillation materials. Though indoles and oligophenylenes have been known for many decades, their PSD properties have not been investigated and their scintillation properties only scantily investigated. Well-developed synthetic methodologies have permitted the synthesis of a library of structural analogs of these compounds as well as the investigation of their scintillation properties. The emission wavelengths of many indoles are in the sensitive region of common photomultiplier tubes, making them appropriate to be used as scintillators in either pure or doped form. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. This work has been supported by the U

  20. Conductance of Conjugated Organic Compounds in Controlled Environments

    NASA Astrophysics Data System (ADS)

    Schonenberger, Christian


    We use the mechanical and the electromigration break junction technique, as well as nanoparticle arrays, to measure the electrical conductance of a range of conjugated organic molecules with different end functionalities at room temperature in a liquid cell. We first report on a comparison between oligo(phenylene vinylene) (OPV) oligo(phenylene ethynylene) (OPE). We find that OPV conducts slightly better than OPE. Solubilizing side groups do not prevent the molecules from being anchored within a break junction. With the aim to realize a functional switch, we show preliminary electrical conductance studies of a newly synthesized cruciform molecule. Using the nanoparticle platform we further demon-strate light and electrochemical-induced conductance switching of photochromic and redox molecules. We further discuss OPV and OPE molecules with different end groups, including asymmetric ones. To our surprise, molecules having an anchor group only on one side also gave rise to a pronounced mo-lecular signal. We attribute this effect to the interaction between neighboring molecules in the junction likely induced by π-π stacking. This remarkable property highlights the importance of intermolecu-lar interaction in molecular junctions, an often overlooked aspect. If time permits, a recent study on low- frequency fluctuations in molecular junctions will be mentioned as well. Collaborators are (alphabetic order): J. Agustsson, J. Brunner, M. Calame, T. Gonzalez, S. Grunder, V. Horhoiu, R. Huber, J. Liao, M. Mayor, M. Mangold, S. Oberholzer, M. Steinacher, S. Wu, Z.M. Wu, (all at the Swiss Nanoscience Institute at the Univ. of Basel) and M. R. Bryce (Durham University, UK).

  1. Growth of Thin, Anisotropic, π-Conjugated Molecular Films by Step-Wise `Click' Assembly of Molecular Building Blocks: Characterizing Reaction Yield, Surface Coverage, and Film Thickness vs. Addition Step Number

    NASA Astrophysics Data System (ADS)

    Demissie, Abel; Haugstad, Greg; Frisbie, C. Daniel


    Molecular electronics is an active field of nanotechnology that has gained much interest due to the advent of modern microscopy techniques, and thin film synthesis using click chemistry - an approach which has enabled scientists to achieve a sub-angstrom control of monolayer length. Among the major challenges to grow oriented, surface-confined wires by click chemistry is development of synthetic routes that yield monodisperse wires, and lack of systematic way to measure the surface coverage of molecules. In this work, we report a comprehensive characterization of π-conjugated oligophenylene imine (OPI) wires synthesized step-wise by imine condensation click chemistry. OPI wire synthesis began with a self-assembled monolayer (SAM) of 4-formylthiophenol or 4-aminothiophenol on Au, followed by alternate addition of terepthaldehyde or phenylenediamine. OPI wires were characterized after each monomer addition via Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, cyclic voltammetry, reflection-absorption infra-red spectroscopy, and nuclear reaction analysis. We have determined an average extent of reaction greater than 98% completion for each growth step using five different techniques. Overall, these nanoscale scale surface characterization techniques proved to be an extremely sufficient method for monitoring wire growth and surface coverage.

  2. Charge transport through molecular rods with reduced pi-conjugation.


    Lörtscher, Emanuel; Elbing, Mark; Tschudy, Meinrad; von Hänisch, Carsten; Weber, Heiko B; Mayor, Marcel; Riel, Heike


    A series of oligophenylene rods of increasing lengths is synthesized to investigate the charge-transport mechanisms. Methyl groups are attached to the phenyl rings to weaken the electronic overlap of the pi-subsystems along the molecular backbones. Out-of-plane rotation of the phenyl rings is confirmed in the solid state by means of X-ray analysis and in solution by using UV/Vis spectroscopy. The influence of the reduced pi-conjugation on the resonant charge transport is studied at the single-molecule level by using the mechanically controllable break-junction technique. Experiments are performed under ultra-high-vacuum conditions at low temperature (50 K). A linear increase of the conductance gap with increasing number of phenyl rings (from 260 meV for one ring to 580 meV for four rings) is revealed. In addition, the absolute conductance of the first resonant peaks does not depend on the length of the molecular wire. Resonant transport through the first molecular orbital is found to be dominated by charge-carrier injection into the molecule, rather than by the intrinsic resistance of the molecular wire length.

  3. Solvent-mediated conductance increase of dodecanethiol-stabilized gold nanoparticle monolayers

    PubMed Central

    Tisserant, Jean-Nicolas; Sánchez-Ferrer, Antoni; Mezzenga, Raffaele


    Gold nanoparticle monolayers provide convenient templates to study charge transport in organic molecules beyond single junction techniques. Conductance is reported to increase by several orders of magnitude following immersion of alkanethiol-stabilized gold nanoparticle monolayers in a solution containing conjugated thiol-functionalized molecules. Typically, this observation is attributed to molecular exchange. Less attention has been paid to the role of the solvent alone. Here, we report on an increase in conductance of dodecanethiol-stabilized gold nanoparticle monolayers on Si/SiO2 by an average factor of 36 and 22 after immersion in pure ethanol (EtOH) and tetrahydrofuran (THF), respectively. Analysis by scanning electron microscopy (SEM) and small-angle X-ray scattering (SAXS) reveals a solvent-induced decrease in lattice constant of close-packed monolayers. We compare the conductance of the monolayer after molecular exchange with two different oligophenylenes to shed light on the respective contribution of the solvent-induced structural change and the molecular exchange itself on the conductance increase. PMID:28144553

  4. Energy transfer processes along a supramolecular chain of π-conjugated molecules.


    Schmid, S A; Abbel, R; Schenning, A P H J; Meijer, E W; Herz, L M


    We have investigated the energy transfer dynamics in a supramolecular linear polymer chain comprising oligofluorene (OF) energy donor units linked by quadruple hydrogen-bonding groups, and oligophenylene (OPV) chain ends that act as energy acceptors. Using femtosecond spectroscopy, we followed the dynamics of energy transfer from the main chain of OF units to the OPV chain ends and simulated these data taking a Monte Carlo approach that included different extents of electronic wave function delocalization for the energy donor and acceptor. Best correlations between experimental and theoretical results were obtained for the assumption of electronic coupling occurring between a localized donor dipole moment and a delocalized acceptor moment. These findings emphasize that geometric relaxation following initial excitation of the donor needs to be taken into account, as it leads to a localization of the donor's excited state wave function prior to energy transfer. In addition, our simulations show that the energy transfer from the main chain to the ends is dominated by an interplay between slow and spatially limited exciton migration along the OF segments comprising the main chain and the comparatively faster hetero-transfer to the end-cap acceptors from directly adjoining OF segments. These results clearly support the description of host-guest energy transfer in linear polymer chains as a two-step mechanism with exciton diffusion in the host being a prerequisite to energy transfer to the guest.

  5. Star-shaped and linear nanosized molecules functionalized with hexa-peri-hexabenzocoronene: synthesis and optical properties.


    Cao, Xiao-Yu; Zi, Hong; Zhang, Wei; Lu, Hua; Pei, Jian


    [structure: see text] A synthetic strategy promising the establishment of a new star-shaped and linear polycyclic aromatic hydrocarbons (PAHs) family with distinct molecular topologies has been developed. The Sonogashira reaction between the iodide derivatives 2a-e and phenylacetylene catalyzed with Pd(0) affords 3a-e in high yields. The Diels-Alder and decarbonylation reactions between 3a-e and tetraphenylcyclophentadiene following the oxidation by FeCl(3) produce the star-shaped and linear PAHs 5a-e containing a five-membered ring. The structural analysis and the optical properties of all new compounds are performed by a combination of MALDI-TOF mass spectrometry, UV-vis, and fluorescence spectrometry. The electronic and photophysical properties are studied by orthogonal comparisons of the absorption and fluorescence spectra in THF solutions, which not only give insight into the interactions among aromatic submoieties in each molecule and the effects of meta-conjugation and para-conjugation on electronic delocalization, but also indicate effective conjugation length variations from oligophenylacetylenes 3a-e to oligophenylene dendrimers 4a-e and PAHs 5a-e. The star-shaped 5c exhibits the highest aggregation in excited states compared with the other four hexa-peri-hexabenzocoronene (HBC) derivatives.

  6. On the nucleation and initial film growth of rod-like organic molecules

    PubMed Central

    Winkler, Adolf


    In this article, some fundamental topics related to the initial steps of organic film growth are reviewed. General conclusions will be drawn based on experimental results obtained for the film formation of oligophenylene and pentacene molecules on gold and mica substrates. Thin films were prepared via physical vapor deposition under ultrahigh-vacuum conditions and characterized in-situ mainly by thermal desorption spectroscopy, and ex-situ by X-ray diffraction and atomic force microscopy. In this short review article the following topics will be discussed: What are the necessary conditions to form island-like films which are either composed of flat-lying or of standing molecules? Does a wetting layer exist below and in between the islands? What is the reason behind the occasionally observed bimodal island size distribution? Can one describe the nucleation process with the diffusion-limited aggregation model? Do the impinging molecules directly adsorb on the surface or rather via a hot-precursor state? Finally, it will be described how the critical island size can be determined by an independent measurement of the deposition rate dependence of the island density and the capture-zone distribution via a universal relationship. PMID:27482122

  7. Solvent-mediated conductance increase of dodecanethiol-stabilized gold nanoparticle monolayers.


    Reissner, Patrick A; Tisserant, Jean-Nicolas; Sánchez-Ferrer, Antoni; Mezzenga, Raffaele; Stemmer, Andreas


    Gold nanoparticle monolayers provide convenient templates to study charge transport in organic molecules beyond single junction techniques. Conductance is reported to increase by several orders of magnitude following immersion of alkanethiol-stabilized gold nanoparticle monolayers in a solution containing conjugated thiol-functionalized molecules. Typically, this observation is attributed to molecular exchange. Less attention has been paid to the role of the solvent alone. Here, we report on an increase in conductance of dodecanethiol-stabilized gold nanoparticle monolayers on Si/SiO2 by an average factor of 36 and 22 after immersion in pure ethanol (EtOH) and tetrahydrofuran (THF), respectively. Analysis by scanning electron microscopy (SEM) and small-angle X-ray scattering (SAXS) reveals a solvent-induced decrease in lattice constant of close-packed monolayers. We compare the conductance of the monolayer after molecular exchange with two different oligophenylenes to shed light on the respective contribution of the solvent-induced structural change and the molecular exchange itself on the conductance increase.

  8. Role of solvent environments in single molecule conductance used insulator-modified mechanically controlled break junctions

    NASA Astrophysics Data System (ADS)

    Muthusubramanian, Nandini; Maity, Chandan; Galan Garcia, Elena; Eelkema, Rienk; Grozema, Ferdinand; van der Zant, Herre; Kavli Institute of Nanoscience Collaboration; Department of Chemical Engineering Collaboration

    We present a method for studying the effects of polar solvents on charge transport through organic/biological single molecules by developing solvent-compatible mechanically controlled break junctions of gold coated with a thin layer of aluminium oxide using plasma enhanced atomic layer deposition (ALD). The optimal oxide thickness was experimentally determined to be 15 nm deposited at ALD operating temperature of 300°C which yielded atomically sharp electrodes and reproducible single-barrier tunnelling behaviour across a wide conductance range between 1 G0 and 10-7 G0. The insulator protected MCBJ devices were found to be effective in various solvents such as deionized water, phosphate buffered saline, methanol, acetonitrile and dichlorobenzene. The yield of molecular junctions using such insulated electrodes was tested by developing a chemical protocol for synthesizing an amphipathic form of oligo-phenylene ethynylene (OPE3-PEO) with thioacetate anchoring groups. This work has further applications in studying effects of solvation, dipole orientation and other thermodynamic interactions on charge transport. Eu Marie Curie Initial Training Network (ITN). MOLECULAR-SCALE ELECTRONICS: ``MOLESCO'' Project Number 606728.

  9. Effect of polymer chain length on membrane perturbation activity of cationic phenylene ethynylene oligomers and polymers.


    Wang, Ying; Jones, Emmalee M; Tang, Yanli; Ji, Eunkyung; Lopez, Gabriel P; Chi, Eva Y; Schanze, Kirk S; Whitten, David G


    The interactions of poly(phenylene ethynylene)- (PPE-) based cationic conjugated polyelectrolytes (CPEs) and oligo(phenylene ethynylene)s (OPEs) with different model lipid membrane systems were investigated to gain insight into the relationship between molecular structure and membrane perturbation ability. The CPE and OPE compounds exhibit broad-spectrum antimicrobial activity, and cell walls and membranes are believed to be their main targets. To better understand how the size, in terms of the number of repeat units, of the CPEs and OPEs affects their membrane disruption activities, a series of PPE-based CPEs and OPEs were synthesized and studied. A number of photophysical techniques were used to investigate the interactions of CPEs and OPEs with model membranes, including unilamellar vesicles and lipid monolayers at the air/water interface. CPE- or OPE-induced dye leakage from vesicles reveals that the CPEs and OPEs selectively perturb model bacterial membranes and that their membrane perturbation abilities are highly dependent on molecular size. Consistent with dye-leakage assay results, the CPEs and OPEs also exhibit chain-length-dependent ability to insert into 1,2-dipalmitoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DPPG) monolayers. Our results suggest that, for PPE-based CPE and OPE antimicrobials, chain length can be tuned to optimize their membrane perturbation ability.

  10. On the nucleation and initial film growth of rod-like organic molecules

    NASA Astrophysics Data System (ADS)

    Winkler, Adolf


    In this article, some fundamental topics related to the initial steps of organic film growth are reviewed. General conclusions will be drawn based on experimental results obtained for the film formation of oligophenylene and pentacene molecules on gold and mica substrates. Thin films were prepared via physical vapor deposition under ultrahigh-vacuum conditions and characterized in-situ mainly by thermal desorption spectroscopy, and ex-situ by X-ray diffraction and atomic force microscopy. In this short review article the following topics will be discussed: What are the necessary conditions to form island-like films which are either composed of flat-lying or of standing molecules? Does a wetting layer exist below and in between the islands? What is the reason behind the occasionally observed bimodal island size distribution? Can one describe the nucleation process with the diffusion-limited aggregation model? Do the impinging molecules directly adsorb on the surface or rather via a hot-precursor state? Finally, it will be described how the critical island size can be determined by an independent measurement of the deposition rate dependence of the island density and the capture-zone distribution via a universal relationship.

  11. Shape and size effects on the optical properties of piezochromic organic nanoparticles

    NASA Astrophysics Data System (ADS)

    Huo, Jianqiang; Yan, Shuai; Hou, Xueqing; Li, Yahong; Yin, Lingxia; Arulsamy, Navamoney


    Two oligo(phenylene vinylenes), namely, (E,E)-1,4-alkyloxy-2,5-bis[2-cyano-2-(4-(methylsulfanyl)phenyl)vinyl]benzene (1) and (E,E)-1,4-dioctyloxy-2,5-bis[2-(4-(methylsulfanyl)phenyl)vinyl]benzene (2) are synthesized. Both compounds are piezochromic fluorescent, and the alkyloxy substituents present in the compounds significantly influence their optical properties. Nanoparticles of various shapes ranging from spherical to clavate are prepared by the reprecipitation method. The fluorescent excitation and emission properties measured for the nanoparticles dispersed in water are compared with those measured for dilute solutions of the compound in THF. The nanoparticles exhibit a red shift in their fluorescence emission spectra with a decrease in efficiency as their size increases from 30 to 50 nm. In contrast, the molecular UV-Vis absorptions of the nanoparticles are blue-shifted in comparison to that of the solution spectra of the compound. The large n-octyloxy substituents at the phenylene core cause distortion of the aromatic rings from planarity and hinder π-stacking.

  12. Analyzing the molecular weight distribution in supramolecular polymers.


    Schmid, Stephan A; Abbel, Robert; Schenning, Albertus P H; Meijer, E W; Sijbesma, Rint P; Herz, Laura M


    We have investigated the formation process of supramolecular linear polymer chains and its influence on the resulting chain length distribution function. For this purpose, we explored the migration of excitation energy between oligofluorene units coupled together through quadruple hydrogen-bonding groups to form linear chains that are terminated by oligophenylene vinylene end-caps acting as energy traps. The energy transfer dynamics from the main chain to the chain end was monitored experimentally using time-resolved PL spectroscopy and compared to an equivalent Monte Carlo simulation incorporating information on the structure of the chains, the transition transfer rates, and various weight distribution trial functions. We find that the assumption of a Flory distribution of chain lengths leads to excellent agreement between experimental and simulated data for a wide range of end-cap concentrations. On the other hand, both a Poisson function and a simplified assumption of a monodisperse distribution significantly underestimate the presence of long chains in the ensemble. Our results therefore show that supramolecular polymerization is a steplike process equivalent to polycondensation reactions in linear covalent polymers. These findings emphasize that equal reactivity of the supramolecular building blocks leads to a dynamic growth process for the supramolecular chain involving all chain components at all times.

  13. Biradicaloid character of thiophene-based heterophenoquinones: the role of electron-phonon coupling.


    Fazzi, Daniele; Canesi, Eleonora V; Negri, Fabrizia; Bertarelli, Chiara; Castiglioni, Chiara


    The quinoidal versus biradicaloid character of the ground state of a series of thiophene-based heterophenoquinones is investigated with quantum-chemical calculations. The role of the ground-state electronic character on molecular structure and vibrational properties is emphasized. The vibrational activities are experimentally determined and their analysis is performed by taking advantage of the definition of a collective vibrational coordinate (the R coordinate) maximizing the electron-phonon coupling, and connecting the quinoid and the aromatic biradicaloid resonance structures. The combined experimental and computational investigation supports the biradicaloid nature of the longer oligomers. The modulation of Raman intensities and frequency dispersion, experimentally observed by increasing the length of the chromophore, is shown to be reproduced well by model calculations on a single chromophore as a function of geometry displacements along the R-mode. These results underline the role of electron-phonon coupling in governing the structure-property relationship of highly conjugated organic compounds, underscoring the similarity of thiophene heterophenoquinone systems with other, more classical, oligophenylene and oligothiophene derivatives.

  14. Synthesis of a new π-conjugated redox oligomer: Electrochemical and optical investigation

    NASA Astrophysics Data System (ADS)

    Blili, Saber; Zaâboub, Zouhour; Maaref, Hassen; Haj Said, Ayoub


    A new π-conjugated redox oligomer was prepared according a two-Step Synthesis. Firstly, an oligophenylene (OMPA) was obtained from the anodic oxidation of the (4-methoxyphenyl)acetonitrile. Then, the resulting material was chemically modified by the Knoevenagel condensation with the ferrocenecarboxaldehyde. This reaction led to a redox-conjugated oligomer the Fc-OMPA. The synthesized material was characterized using different spectroscopic techniques: NMR, FTIR, UV-vis and photoluminescence (PL) spectroscopy. The Fc-OMPA was used to modify a platinum electrode surface and the electrochemical response of the ferrocene redox-center was investigated by cyclic voltammetry. Moreover, the room temperature PL spectra of Fc-OMPA revealed that the ferrocene moiety, which acts as an electron donor, can effectively quench the oligomer luminescence. However, when ferrocene was oxidized to ferrocenium ion, the intramolecular charge transfer process was prevented which consequently enhanced the light emission. Thus, the oligomer light-emission can be, chemically or electrochemically tuned. The obtained results showed that the prepared material is a good candidate for the elaboration of electrochemical sensors and for the development of luminescent Redox-switchable devices.

  15. Efficient light emission from inorganic and organic semiconductor hybrid structures by energy-level tuning.


    Schlesinger, R; Bianchi, F; Blumstengel, S; Christodoulou, C; Ovsyannikov, R; Kobin, B; Moudgil, K; Barlow, S; Hecht, S; Marder, S R; Henneberger, F; Koch, N


    The fundamental limits of inorganic semiconductors for light emitting applications, such as holographic displays, biomedical imaging and ultrafast data processing and communication, might be overcome by hybridization with their organic counterparts, which feature enhanced frequency response and colour range. Innovative hybrid inorganic/organic structures exploit efficient electrical injection and high excitation density of inorganic semiconductors and subsequent energy transfer to the organic semiconductor, provided that the radiative emission yield is high. An inherent obstacle to that end is the unfavourable energy level offset at hybrid inorganic/organic structures, which rather facilitates charge transfer that quenches light emission. Here, we introduce a technologically relevant method to optimize the hybrid structure's energy levels, here comprising ZnO and a tailored ladder-type oligophenylene. The ZnO work function is substantially lowered with an organometallic donor monolayer, aligning the frontier levels of the inorganic and organic semiconductors. This increases the hybrid structure's radiative emission yield sevenfold, validating the relevance of our approach.

  16. Ambipolar Phosphine Derivatives to Attain True Blue OLEDs with 6.5% EQE.


    Kondrasenko, Ilya; Tsai, Zheng-Hua; Chung, Kun-You; Chen, Yi-Ting; Ershova, Yana Yu; Doménech-Carbó, Antonio; Hung, Wen-Yi; Chou, Pi-Tai; Karttunen, Antti J; Koshevoy, Igor O


    A family of new branched phosphine derivatives {Ph2N-(C6H4)n-}3P → E (E = O 1-3, n = 1-3; E = S 4-6, n = 1-3; E = Se 7-9, n = 1-3; E = AuC6F5 4-6, n = 1-3), which are the donor-acceptor type molecules, exhibit efficient deep blue room temperature fluorescence (λem = 403-483 nm in CH2Cl2 solution, λem = 400-469 nm in the solid state). Fine tuning the emission characteristics can be achieved varying the length of aromatic oligophenylene bridge -(C6H4)n-. The pyramidal geometry of central R3P → E fragment on the one hand disrupts π-conjugation between the branches to preserve blue luminescence and high triplet energy, while on the other hand provides amorphous materials to prevent excimer formation and fluorescence self-quenching. Hence, compounds 2, 3, 5, and 12 were used as emitters to fabricate nondoped and doped electroluminescent devices. The luminophore 2 (E = O, n = 2) demonstrates excellently balanced bipolar charge transport and good nondoped device performance with a maximum external quantum efficiency (EQEmax) of 3.3% at 250 cd/m(2) and Commission International de L'Eclairage (CIE) coordinates of (0.15, 0.08). The doped device of 3 (E = O, n = 3) shows higher efficiency (EQEmax of 6.5, 6.0 at 100 cd/m(2)) and high color purity with CIE (0.15, 0.06) that matches the HDTV standard blue. The time-resolved electroluminescence measurement indicates that high efficiency of the device can be attributed to the triplet-triplet annihilation to enhance generation of singlet excitons.

  17. Rigid lipid membranes and nanometer clefts: motifs for the creation of molecular landscapes.


    Li, Guangtao; Fudickar, Werner; Skupin, Marc; Klyszcz, Andreas; Draeger, Christian; Lauer, Matthias; Fuhrhop, Jürgen-Hinrich


    Amphiphilic lipids associate in water spontaneously to form micelles, vesicles, monolayers, or biological membranes. These aggregates are soft and their shape can be changed easily. They behave like complex fluids because they are merely held together by weak, nondirected forces. The most important characteristic of these monolayers is their ability to dissolve hydrophobic molecules in the form of freely movable monomers. The fluid molecular layers are not suitable to anchor the components of chain reactions. However, if the alkyl chains are replaced by rigid skeletons or if the head groups are connected through intermolecular interactions, the aggregates become rigid and their fluid solvent character is lost. The construction of chiral surfaces by synkinesis (synthesis of noncovalent compounds) and of enzyme-type surface clefts of defined size can now be carried out by using rigid lipid membranes. Monolayers and nanometer pores on solid substrates attain sharp edges, and upright nanometer columns on smooth surfaces no longer dissipate. Five examples illustrate the advantages of using rigid molecular assemblies: 1) Cationic domains of rigid edge amphiphiles in fluid membranes act as manipulable ion channels. 2) Spherical micelles, micellar helical fibers, and vesicular tubes can be dried and stored as stable material. Molecular landscapes form on smooth surfaces. 3) alpha,omega-Diamide bolaamphiphiles form rigid nanometer-thick walls on smooth surfaces and these barriers cannot be penetrated by amines. Around steroids and porphyrins, they form rigid nanometer clefts whose walls and water-filled centers can be functionalized. 4) The structure of rigid oligophenylene- and quinone monolayers on electrodes can be changed drastically and reversibly by changing the potential. 5) 10(10) Porphyrin cones on a 1-cm2 gold electrode can be controlled individually by AFM- and STM-tips and investigated by electrochemical, photochemical, and mechanical means. In summary, rigid

  18. Time resolved single molecule spectroscopy of semiconductor quantum dot/conjugated organic hybrid nanostructures

    NASA Astrophysics Data System (ADS)

    Odoi, Michael Yemoh

    Single molecule studies on CdSe quantum dots functionalized with oligo-phenylene vinylene ligands (CdSe-OPV) provide evidence of strong electronic communication that facilitate charge and energy transport between the OPV ligands and the CdSe quantum dot core. This electronic interaction greatly modify, the photoluminescence properties of both bulk and single CdSe-OPV nanostructure thin film samples. Size-correlated wide-field fluorescence imaging show that blinking suppression in single CdSe-OPV is linked to the degree of OPV coverage (inferred from AFM height scans) on the quantum dot surface. The effect of the complex electronic environment presented by photoexcited OPV ligands on the excited state property of CdSe-OPV is measured with single photon counting and photon-pair correlation spectroscopy techniques. Time-tagged-time-resolved (TTTR) single photon counting measurements from individual CdSe-OPV nanostructures, show excited state lifetimes an order of magnitude shorter relative to conventional ZnS/CdSe quantum dots. Second-order intensity correlation measurements g(2)(tau) from individual CdSe-OPV nanostructures point to a weak multi-excitonic character with a strong wavelength dependent modulation depth. By tuning in and out of the absorption of the OPV ligands we observe changes in modulation depth from g(2) (0) ≈ 0.2 to 0.05 under 405 and 514 nm excitation respectively. Defocused images and polarization anisotropy measurements also reveal a well-defined linear dipole emission pattern in single CdSe-OPV nanostructures. These results provide new insights into to the mechanism behind the electronic interactions in composite quantum dot/conjugated organic composite systems at the single molecule level. The observed intensity flickering , blinking suppression and associated lifetime/count rate and antibunching behaviour is well explained by a Stark interaction model. Charge transfer from photo-excitation of the OPV ligands to the surface of the Cd

  19. Molecular Rotors as Switches

    PubMed Central

    Xue, Mei; Wang, Kang L.


    The use of a functional molecular unit acting as a state variable provides an attractive alternative for the next generations of nanoscale electronics. It may help overcome the limits of conventional MOSFETd due to their potential scalability, low-cost, low variability, and highly integratable characteristics as well as the capability to exploit bottom-up self-assembly processes. This bottom-up construction and the operation of nanoscale machines/devices, in which the molecular motion can be controlled to perform functions, have been studied for their functionalities. Being triggered by external stimuli such as light, electricity or chemical reagents, these devices have shown various functions including those of diodes, rectifiers, memories, resonant tunnel junctions and single settable molecular switches that can be electronically configured for logic gates. Molecule-specific electronic switching has also been reported for several of these device structures, including nanopores containing oligo(phenylene ethynylene) monolayers, and planar junctions incorporating rotaxane and catenane monolayers for the construction and operation of complex molecular machines. A specific electrically driven surface mounted molecular rotor is described in detail in this review. The rotor is comprised of a monolayer of redox-active ligated copper compounds sandwiched between a gold electrode and a highly-doped P+ Si. This electrically driven sandwich-type monolayer molecular rotor device showed an on/off ratio of approximately 104, a read window of about 2.5 V, and a retention time of greater than 104 s. The rotation speed of this type of molecular rotor has been reported to be in the picosecond timescale, which provides a potential of high switching speed applications. Current-voltage spectroscopy (I-V) revealed a temperature-dependent negative differential resistance (NDR) associated with the device. The analysis of the device I–V characteristics suggests the source of the

  20. Supported Intrinsically Porous Oligomers as Hybrid Materials for Separations, Storage, and Sensing

    NASA Astrophysics Data System (ADS)

    Thompson, Anthony Boone

    Adsorption-desorption phenomena are often difficult to study at the molecular level because the surfaces on which they occur can be heterogeneous, giving a wide distribution of adsorption sites and associated energies. Considering that these phenomena underlie an incredibly wide variety of industrially important processes, a better understanding could aid in the development of more efficient methods. In this work, we describe an approach to designing materials with well-defined adsorption sites by covalently attaching intrinsically porous molecules to solid surfaces by a rigid multidentate linker. These cup-shaped molecules are intended to act as adsorption sites on the material, whereas the rigid attachment to the solid support serves to prevent movement and conformational changes of the sites, leading to better understanding of adsorption phenomena. As a proof-of-concept application, materials were used for adsorption of n-butanol biofuel and related compounds from dilute aqueous solution. The materials were thermally and hydrolytically stable, and adsorption phenomena were reversible. Adsorption sites containing more hydrophobic molecular area led to stronger adsorption, suggesting that it is driven by weak van der Waals forces. Likewise, adsorption sites that were strongly polarized performed poorly, possibly reflecting a greater energy penalty of removing water molecules from the cavity. Upon placing a Lewis acidic metal at the bottom of the cavity, an enhancement was seen only with the most acidic metal, which may indicate weak guest coordination. Observing that hydrophobic interactions dominate adsorption on these materials, efforts were made to develop hybrid materials with large hydrophobic area for adsorption. Glaser coupling of diethynylbenzene was used to grow oligo(phenylene butadiynylene)s from the surface of silica, resulting in materials that were more than 25% organic by weight. In addition to their potential use as adsorbents, these materials may

  1. Transport phenomena in molecular-scale devices

    NASA Astrophysics Data System (ADS)

    Keane, Zachary

    The physics of atomic-scale systems is a subject of considerable interest, from both a basic-science and an engineering standpoint. We discuss three sets of experiments, each designed to elucidate a particular aspect of nanoscale physics. The first of these aspects is spin-dependent transport in atomic-scale ferromagnetic wires. Early reports of large magnetoresistive effects in this type of device led to speculation about possible mechanisms for enhancing spin polarization in ferromagnetic constrictions, as well as excitement about the potential applications for such an effect. An experiment carefully designed to exclude other mechanisms for conductance changes, however, leads us to conclude that there is no evidence for a large magnetoresistive effect per se in constricted ferromagnetic wires. A second area of interest is hysteretic conductance switching in single-molecule transistors incorporating bipyridyl dinitro oligo(phenylene ethynylene) dithiol (BPDN-DT). An early hypothesis to explain the observed hysteresis involved strong electron-vibration coupling leading to shifts in molecular energy levels. A change in the charge state of the molecule could both lead to a change in the conductance across the molecule and tend to stabilize the charge on the molecule, leading to hysteretic switching behavior. To examine this hypothesis, we fabricated and measured three-terminal devices allowing us to control the charge on the molecule independent of the source-drain bias. We find that the evidence argues against a charge-transfer-based mechanism for the conductance switching; instead, it is more likely that a change in the molecule-electrode coupling is responsible for this behavior. The final area addressed in this dissertation is that of current-dependent electronic noise in single molecules. In many nanoscale devices, the discrete nature of the carriers of electric current leads to fluctuations about the average current; these fluctuations are known as shot noise

  2. Design and synthesis of polyphosphazenes: Hard tissue scaffolding biomaterials and physically crosslinked elastomers

    NASA Astrophysics Data System (ADS)

    Modzelewski, Tomasz

    oar's on adjacent polymer chains, and lock the chains in place, similar to the way in which the oars on one ship will interdigitate with the oars of another ship if they get too close. Chapter 6 expands the chemistry of the non-traditional elastomers described in Chapter 5. Specifically, the substituent groups on the cyclotriphosphazene groups are changed from 2,2,2- trifluoroethoxy to phenoxy, while the remaining chlorine atoms along the polymer backbone are still replaced with 2,2,2-trifluoroethoxide. The new polymers are shown to have better mechanical properties then the polymers described in Chapter 5. Chapter 7 describes a further extension of the ideas in Chapters 5 and 6. Specifically it involves the synthesis and mechanical testing of polyphosphazenes bearing oligo-p-phenylene groups co-substituted with 2,2,2-trifluoroethoxide. The oligo-phenylene groups are incorporated to act as variable length cross-linking moieties to further expand the new family of non-traditional polyphosphazene elastomers. The mechanical and physical properties of these polymers reveal a strong dependence on both the length and concentration of the oligo-phenylene minor co-substituent groups. (Abstract shortened by UMI.).

  3. Axially assembled photosynthetic reaction center mimics composed of tetrathiafulvalene, aluminum(iii) porphyrin and fullerene entities

    NASA Astrophysics Data System (ADS)

    Poddutoori, Prashanth K.; Lim, Gary N.; Sandanayaka, Atula S. D.; Karr, Paul A.; Ito, Osamu; D'Souza, Francis; Pilkington, Melanie; van der Est, Art


    The distance dependence of sequential electron transfer has been studied in six, vertical, linear supramolecular triads, (TTF-Phn-py --> AlPor-Phm-C60, n = 0, 1 and m = 1, 2, 3), constructed using tetrathiafulvalene (TTF), aluminum(iii) porphyrin (AlPor) and fullerene (C60) entities. The C60 and TTF units are bound to the Al center on opposite faces of the porphyrin; the C60 through a covalent axial bond using a benzoate spacer, and the TTF through a coordination bond via an appended pyridine. Time-resolved optical and EPR spectroscopic methods and computational studies are used to demonstrate that excitation of the porphyrin leads to step-wise, sequential electron transfer (ET) between TTF and C60, and to study the electron transfer rates and exchange coupling between the components of the triads as a function of the bridge lengths. Femtosecond transient absorption studies show that the rates of charge separation, kCS are in the range of 109-1011 s-1, depending on the length of the bridges. The lifetimes of the charge-separated state TTF&z.rad;+-C&z.rad;-60 obtained from transient absorbance experiments and the singlet lifetimes of the radical pairs obtained by time-resolved EPR are in good agreement with each other and range from 60-130 ns in the triads. The time-resolved EPR data also show that population of the triplet sublevels of the charge-separated state in the presence of a magnetic field leads to much longer lifetimes of >1 μs. The data show that a modest stabilization of the charge separation lifetime occurs in the triads. The attenuation factor β = 0.36 Å-1 obtained from the exchange coupling values between TTF&z.rad;+ and C&z.rad;-60 is consistent with values reported in the literature for oligophenylene bridged TTF-C60 conjugates. The singlet charge recombination lifetime shows a much weaker dependence on the distance between the donor and acceptor, suggesting that a simple superexchange model is not sufficient to describe the back reaction