Science.gov

Sample records for oligophenylenes

  1. Soluble Graphene Nanoribbons from Planarization of Oligophenylenes.

    PubMed

    Sen, Choong Ping; Valiyaveettil, Suresh

    2017-01-31

    A solution-based chemical synthesis of two graphene nanoribbons with armchair edges is reported. The precursor oligophenylene molecules are synthesized and subjected to oxidative cyclodehydrogenation to afford the target molecules, G-1 and G-2. These molecules have good solubility in organic solvents, and show a large redshift in their absorption edge (up to 185 nm) and emission maximum (up to 125 nm) after planarization. Fibrous structures are formed upon self-assembly of molecules through columnar π-π stacking. Such molecular assemblies may be useful for various applications. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

  2. Fluorescence and laser properties of D2-, C2- and D3 symmetry series oligophenylenes.

    PubMed

    Mabbs, R; Nijegorodov, N; Downey, W S

    2003-04-01

    The fluorescence and laser properties of ten aromatic compounds, specially chosen from the p-oligophenylenes (D(2) symmetry) or m-oligophenylenes (C(2) or D(3) symmetry) are studied experimentally (at 293 K) and quantum chemically. The quantum yields, gamma and the decay times, tau(f) of fluorescence are measured for deaerated and non-deaerated cyclohexane solutions. The oscillator strengths, f(e) of the S(0)-->S(p) (1A-->(1)L(a)) and S(0)-->S(alpha) (1A-->(1)L(b)) transitions, fluorescence, k(f) and intersystem crossing, k(ST), rate constants, and natural lifetimes, tau(0)(T) are calculated. The lowest 1L(b), 1L(a) and 3L(b) (77 K) levels are determined. It is found that all p-oligophenylenes from p-terphenyl onwards are excellent, photochemically stable laser dyes although the solubility in this series decreases dramatically. On the basis of trends observed in p-oligophenylenes (D(2)-series) and on the properties of the experimentally studied m-oligophenylenes of the C(2)- and D(3)-series, the fluorescence and laser properties of other compounds from these series are estimated/predicted. It is shown, for the first time, that m-oligophenylenes of the C(2)-series, from 1,3-di(p-terphenyl)benzene will acquire fluorescence of 1L(a)-->(1)A nature and could be extremely effective laser dyes. It is also shown that m-oligophenylenes of the D(3)-series, from 1,3,5-tri(p-quaterphenyl)benzene will also acquire 1L(a)-->(1)A nature fluorescence and laser ability, although this would not be as good as that of compounds in the C(2)-series. It is concluded that m-oligophenylenes can be used not only for passive mode locking but some may also be used as laser dyes and scintillators. The results obtained are important for various practical purposes and theoretical considerations.

  3. Understanding the unconventional effects of halogenation on the luminescent properties of oligo(phenylene vinylene) molecules.

    PubMed

    Sun, Chun-Lin; Li, Jun; Geng, Hong-Wei; Li, Hui; Ai, Yong; Wang, Qiang; Pan, Shan-Lin; Zhang, Hao-Li

    2013-12-01

    It is commonly known that halogenation tends to decrease the luminescence quantum yield of an organic dye, owing to the high electronegativity and heavy-atom effect of the halogen atom. However, based on an investigation of the effects of halogenation on the luminescence of the oligo(phenylene vinylene) (OPV) framework, we demonstrate that halogenation can have positive impact on the solid-state fluorescence and electrochemiluminescence (ECL) properties of OPV derivatives. The chlorinated OPV exhibits a very high solid-state fluorescence quantum yield (91%), whilst the brominated analogue gives the highest ECL emission intensity. Time-dependent density functional theory calculations, natural bond orbital analysis, and natural transition orbital analysis were performed to assist the understanding of the origin of these positive halogenation effects, which provide insight into the rational design of highly luminescent halogenated organic materials for solid-state devices and ECL applications. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  4. Energy-Level Engineering at ZnO/Oligophenylene Interfaces with Phosphonate-Based Self-Assembled Monolayers.

    PubMed

    Timpel, Melanie; Nardi, Marco V; Ligorio, Giovanni; Wegner, Berthold; Pätzel, Michael; Kobin, Björn; Hecht, Stefan; Koch, Norbert

    2015-06-10

    We used aromatic phosphonates with substituted phenyl rings with different molecular dipole moments to form self-assembled monolayers (SAMs) on the Zn-terminated ZnO(0001) surface in order to engineer the energy-level alignment at hybrid inorganic/organic semiconductor interfaces, with an oligophenylene as organic component. The work function of ZnO was tuned over a wide range of more than 1.7 eV by different SAMs. The difference in the morphology and polarity of the SAM-modified ZnO surfaces led to different oligophenylene orientation, which resulted in an orientation-dependent ionization energy that varied by 0.7 eV. The interplay of SAM-induced work function modification and oligophenylene orientation changes allowed tuning of the offsets between the molecular frontier energy levels and the semiconductor band edges over a wide range. Our results demonstrate the versatile use of appropriate SAMs to tune the energy levels of ZnO-based hybrid semiconductor heterojunctions, which is important to optimize its function, e.g., targeting either interfacial energy- or charge-transfer.

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

    NASA Astrophysics Data System (ADS)

    Miao, Ling; Seminario, Jorge M.

    2007-05-01

    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.

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

    NASA Astrophysics Data System (ADS)

    Shen, Jun-Jian; Zhong, Yu-Wu

    2015-09-01

    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.

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

    PubMed

    Shen, Jun-Jian; Zhong, Yu-Wu

    2015-09-07

    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 1(2+)-6(2+) 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.

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

    PubMed Central

    Shen, Jun-Jian; Zhong, Yu-Wu

    2015-01-01

    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

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

    PubMed

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

    2009-12-09

    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.

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

    PubMed

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

    2017-04-14

    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.

  11. Programmed Synthesis of Molecular Wires with Fixed Insulation and Defined Length Based on Oligo(phenylene ethynylene) and Permethylated α-Cyclodextrins.

    PubMed

    Masai, Hiroshi; Fujihara, Tetsuaki; Tsuji, Yasushi; Terao, Jun

    2017-06-03

    The development of new tuning methods for π-conjugated insulated molecular wires with strictly defined axle lengths as well as positions and degrees of macrocycle coverage would provide unprecedented insight into insulation effects in functionalized materials. Herein, iterative reactions of oligo(phenylene ethynylene) (OPE) linked with permethylated α-cyclodextrins were carried out to fabricate insulated molecular wires with a defined length and insulation in desired areas. Insulated OPEs were elongated in a stepwise manner by performing sequential coupling/deprotection reactions. The insulated areas on the OPE units in each expansion step were selectively controlled by means of programmed solvent conditions (high/low polarity). Moreover, a completely insulated OPE (up to a linked [11]rotaxane) with high structural regularity and high covering ratio was synthesized by appropriate tuning of the Pd catalyst and an extension unit bearing a traceless capping unit based on a tert-butyldimethylsilyl group. This strategy may guide the development of the selective synthesis of fully insulated, partially insulated, and uninsulated molecular wires with well-defined lengths and covered/uncovered areas. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

  12. Synthesis of Bridged Oligophenylene Laser Dyes

    DTIC Science & Technology

    1991-05-10

    recrystallization solvent were found. However, pure 5 could be quaternized slowly (48 hrs reflux required) in acetonitrile or quickly (᝿ mins.) in benzonitrile at...quaternization of 7 by propane sultone in hot benzonitrile gave the sulfoprc,yl zwitterion 7A which fluoresces yellow in alcohols and seems quite soluble...To a hot solution of 5 (1.5 g 6.38 mmol) in benzonitrile (5 mL) was added 1,3-propane sultone (1.2 g, 9.6 mmol, Aldrich P5,070-6). The solution was

  13. 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.

    2003-01-01

    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.

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

    PubMed

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

    2009-08-25

    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.

  15. Incorporation of spiroxanthene units in blue-emitting oligophenylene frameworks: a new molecular design for OLED applications.

    PubMed

    Poriel, Cyril; Cocherel, Nicolas; Rault-Berthelot, Joëlle; Vignau, Laurence; Jeannin, Olivier

    2011-11-04

    We report herein the incorporation of xanthenyl units into two extended π-conjugated phenylene systems, namely indenofluorene and pentaphenylene. Thus, dispiroxanthene-indenofluorene (DSX-IF) and dispiroxanthene-ladderpentaphenylene (DSX-LPP) have been designed and synthesized through short and efficient synthetic approaches. These two molecules possess a 3π-2-spiro architecture (3π-systems/2-spiro bridges), in which two xanthenyl cores are spirolinked to a π-conjugated backbone either indenofluorene for DSX-IF or pentaphenylene for DSX-LPP. The structural, electrochemical, and photophysical properties of these blue/violet emitters have been studied in detail and compared to those of their 'all carbon' analogues with spirofluorenyl cores instead of spiroxanthenyl cores, namely dispirofluorene-indenofluorene (DSF-IF) and dispirofluorene-ladderpentaphenylene (DSF-LPP), previously reported in the literature. Finally, the application of DSX-IF and DSX-LPP as new light-emitting materials in nondoped organic light emitting diodes is reported. A detailed optical study of the different electroluminescence spectra is notably presented, with an emphasis 1) on the origin of the low-energy emission band observed in the case of DSX-LPP and 2) on the unexpected optical contribution of the well-known hole-transporting-layer NPB (N,N'-di(naphtyl)-N,N'-diphenyl(1,1'-biphenyl)-4,4'-diamine). Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  16. Trimethylsilyl-terminated oligo(phenylene ethynylene)s: an approach to single-molecule junctions with covalent Au-C σ-bonds.

    PubMed

    Hong, Wenjing; Li, Hui; Liu, Shi-Xia; Fu, Yongchun; Li, Jianfeng; Kaliginedi, Veerabhadrarao; Decurtins, Silvio; Wandlowski, Thomas

    2012-11-28

    A new and efficient approach using cleaving of trimethylsilyl groups to create covalent Au-C anchoring sites has been developed for single-molecule junction conductance measurements. Employing the mechanically controllable break junction (MCBJ) technique in liquid, we demonstrate the formation of highly conducting single molecular junctions of several OPE derivatives. The created junctions are mechanically stable and exhibit conductances around one order of magnitude higher than those of their dithiol analogues. Extended assembly and reaction times lead to oligomerization. Combined STM imaging and gap-mode Raman experiments provide structure evidence to support the formation of covalent Au-C contacts and further oligomerization.

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

    PubMed

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

    2007-07-19

    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.

  18. Synthesis and complementary self-association of novel lipophilic π-conjugated nucleoside oligomers.

    PubMed

    Camacho-García, J; Montoro-García, C; López-Pérez, A M; Bilbao, N; Romero-Pérez, S; González-Rodríguez, D

    2015-04-21

    A series of lipophilic nucleosides comprising natural and non-natural bases that are π-conjugated to a short oligophenylene-ethynylene fragment has been synthesized. These bases comprise guanosine, isoguanosine, and 2-aminoadenosine as purine heterocycles, and cytidine, isocytosine and uridine as complementary pyrimidine bases. The hydrogen-bonding dimerization and association processes between complementary bases were also studied by (1)H NMR and absorption spectroscopy in order to obtain the relevant association constants.

  19. 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

    2014-03-10

    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.

  20. 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)

    2012-01-01

    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.

  1. 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.

    2016-09-01

    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.

  2. Charge transport through self-assembled monolayers of compounds of interest in molecular electronics.

    PubMed

    Fan, Fu-Ren F; Yang, Jiping; Cai, Lintao; Price, David W; Dirk, Shawn M; Kosynkin, Dmitry V; Yao, Yuxing; Rawlett, Adam M; Tour, James M; Bard, Allen J

    2002-05-15

    The electrical properties of self-assembled monolayers (SAMs) on metal surfaces have been explored for a series of molecules to address the relation between the behavior of a molecule and its structure. We probed interfacial electron transfer processes, particularly those involving unoccupied states, of SAMs of thiolates or arylates on Au by using shear force-based scanning probe microscopy (SPM) combined with current-voltage (i-V) and current-distance (i-d) measurements. The i-V curves of hexadecanethiol in the low bias regime were symmetric around 0 V and the current increased exponentially with V at high bias voltage. Different than hexadecanethiol, reversible peak-shaped i-V characteristics were obtained for most of the nitro-based oligo(phenylene ethynylene) SAMs studied here, indicating that part of the conduction mechanism of these junctions involved resonance tunneling. These reversible peaked i-V curves, often described as a negative differential resistance (NDR) effect of the junction, can be used to define a threshold tip bias, V(TH), for resonant conduction. We also found that for all of the SAMs studied here, the current decreased with increasing distance, d, between tip and substrate. The attenuation factor beta of hexadecanethiol was high, ranging from 1.3 to 1.4 A(-1), and was nearly independent of the tip bias. The beta-values for nitro-based molecules were low and depended strongly on the tip bias, ranging from 0.15 A(-1) for tetranitro oligo(phenylene ethynylene) thiol, VII, to 0.50 A(-1) for dinitro oligo(phenylene) thiol, VI, at a -3.0 V tip bias. Both the V(TH) and beta values of these nitro-based SAMs were also strongly dependent on the structures of the molecules, e.g. the number of electroactive substituent groups on the central benzene, the molecular wire backbone, the anchoring linkage, and the headgroup. We also observed charge storage on nitro-based molecules. For a SAM of the dintro compound, V, approximately 25% of charge collected in

  3. High surface coverage of a self-assembled monolayer by in situ synthesis of palladium nanodeposits.

    PubMed

    Herrer, Lucía; Sebastian, Victor; Martín, Santiago; González-Orive, Alejandro; Pérez-Murano, Francesc; Low, Paul J; Serrano, José Luis; Santamaría, Jesús; Cea, Pilar

    2017-09-14

    Nascent metal|monolayer|metal devices have been fabricated by depositing palladium, produced through a CO-confined growth method, onto a self-assembled monolayer of an amine-terminated oligo(phenylene ethynylene) derivative on a gold bottom electrode. The high surface area coverage (85%) of the organic monolayer by densely packed palladium particles was confirmed by X-ray photoemission spectroscopy (XPS) and atomic force microscopy (AFM). The electrical properties of these nascent Au|monolayer|Pd assemblies were determined from the I-V curves recorded with a conductive-AFM using the Peak Force Tunneling AFM (PF-TUNA™) mode. The I-V curves together with the electrochemical experiments performed rule out the formation of short-circuits due to palladium penetration through the monolayer, suggesting that the palladium deposition strategy is an effective method for the fabrication of molecular junctions without damaging the organic layer.

  4. 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.

    2016-07-01

    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.

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

    PubMed

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

    2006-07-27

    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.

  6. Nonsymmetric bent-core liquid crystals based on a 1,3,4-thiadiazole core unit and their nematic mesomorphism

    SciTech Connect

    Seltmann, Jens; Marini, Alberto; Mennucci, Benedetta; Dey, Sonal; Kumar, Satyendra; Lehmann, Matthias

    2012-09-06

    The synthesis and thermotropic properties of novel V-shaped molecules having a central 1,3,4-thiadiazole core with a bend-angle of 160 degrees are reported. The compounds consist of a shape-persistent oligo(phenylene ethynylene) scaffold with lateral alkyloxy substituents. One of the terminal aromatic units possesses an alkoxy chain capped by an ethyl ester group while the second terminus is a pyridyl group. They exhibit enantiotropic nematic phases and are characterized by polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction. Results from conoscopy indicate a biaxial nature of the nematic phase near room temperature. DFT calculations of dipole moments and molecular polarizabilities are used to substantiate the experimental findings.

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

    PubMed

    Lehmann, Matthias; Seltmann, Jens

    2009-12-04

    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.

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

    PubMed

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

    2011-07-01

    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.

  9. Direct imaging of covalent bond structure in single-molecule chemical reactions.

    PubMed

    de Oteyza, Dimas G; Gorman, Patrick; Chen, Yen-Chia; Wickenburg, Sebastian; Riss, Alexander; Mowbray, Duncan J; Etkin, Grisha; Pedramrazi, Zahra; Tsai, Hsin-Zon; Rubio, Angel; Crommie, Michael F; Fischer, Felix R

    2013-06-21

    Observing the intricate chemical transformation of an individual molecule as it undergoes a complex reaction is a long-standing challenge in molecular imaging. Advances in scanning probe microscopy now provide the tools to visualize not only the frontier orbitals of chemical reaction partners and products, but their internal covalent bond configurations as well. We used noncontact atomic force microscopy to investigate reaction-induced changes in the detailed internal bond structure of individual oligo-(phenylene-1,2-ethynylenes) on a (100) oriented silver surface as they underwent a series of cyclization processes. Our images reveal the complex surface reaction mechanisms underlying thermally induced cyclization cascades of enediynes. Calculations using ab initio density functional theory provide additional support for the proposed reaction pathways.

  10. En route to surface-bound electric field-driven molecular motors.

    PubMed

    Jian, Huahua; Tour, James M

    2003-06-27

    Four caltrop-shaped molecules that might be useful as surface-bound electric field-driven molecular motors have been synthesized. The caltrops are comprised of a pair of electron donor-acceptor arms and a tripod base. The molecular arms are based on a carbazole or oligo(phenylene ethynylene) core with a strong net dipole. The tripod base uses a silicon atom as its core. The legs of the tripod bear sulfur-tipped bonding units, as acetyl-protected benzylic thiols, for bonding to a gold surface. The geometry of the tripod base allows the caltrop to project upward from a metallic surface after self-assembly. Ellipsometric studies show that self-assembled monolayers of the caltrops are formed on Au surfaces with molecular thicknesses consistent with the desired upright-shaft arrangement. As a result, the zwitterionic molecular arms might be controllable when electric fields are applied around the caltrops, thereby constituting field-driven motors.

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

    SciTech Connect

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

    2016-07-04

    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 Al{sub 2}O{sub 3} 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{sup −4} G{sub 0} (1 G{sub 0} = 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.

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

    PubMed

    Sudeep, P K; Emrick, Todd

    2009-12-22

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

    PubMed

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

    2017-02-20

    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.

  14. Supramolecular self-assembly of conjugated diblock copolymers.

    SciTech Connect

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

    2004-02-20

    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.

  15. Conductance of Conjugated Organic Compounds in Controlled Environments

    NASA Astrophysics Data System (ADS)

    Schonenberger, Christian

    2009-03-01

    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).

  16. 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.

    2016-09-01

    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

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

    PubMed Central

    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.

    2015-01-01

    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. PMID:25872919

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

    PubMed

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

    2016-01-01

    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.

  19. Coordination-driven hierarchical organization of pi-conjugated systems: from molecular to supramolecular pi-stacked assemblies.

    PubMed

    Yao, Yishan; Shen, Wenting; Nohra, Brigitte; Lescop, Christophe; Réau, Régis

    2010-06-25

    The reaction of U-shaped, bimetallic, Cu(I) complexes, assembled from a heteroditopic pincer, with cyano-capped pi-conjugated linkers gives a straightforward access to pi-stacked metallocyclophanes in good yields. In these assemblies, the pi-walls have an almost face-to-face arrangement. The versatility of this rational supramolecular synthesis is demonstrated with the use of linkers that have nanoscale lengths (up to 27.7 A), different chemical compositions (oligo(para-phenylenevinylene)s OPVs, oligo(phenylene)s, oligo(phenylethynylene)s), and alternative geometries (linear, angular). Linkers that incorporate an internal pyridyne moiety can also be employed. X-ray diffraction studies revealed that the metallocyclophanes based on linear linkers self-organize into infinite pi-stacked columns in the solid state with intermolecular distances of about 3.6 A. This approach, based on coordination-driven self-assembly, provides a novel and rational strategy for the stacking of extended pi-systems in the solid state.

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

    PubMed Central

    Winkler, Adolf

    2016-01-01

    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

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

    PubMed

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

    2005-04-29

    [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.

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

    PubMed

    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

    2015-04-15

    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.

  3. Analyzing the molecular weight distribution in supramolecular polymers.

    PubMed

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

    2009-12-09

    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.

  4. 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

    2017-01-01

    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.

  5. Theory of magnetoresistance of organic molecular tunnel junctions with nonmagnetic electrodes

    NASA Astrophysics Data System (ADS)

    Shi, Sha; Xie, Zuoti; Liu, Feilong; Smith, Darryl L.; Frisbie, C. Daniel; Ruden, P. Paul

    2017-04-01

    Large room-temperature magnetoresistance observed for devices composed of self-assembled monolayers of different oligophenylene thiols sandwiched between gold contacts has recently been reported [Z. Xie, S. Shi, F. Liu, D. L. Smith, P. P. Ruden, and C. D. Frisbie, ACS Nano 10, 8571 (2016), 10.1021/acsnano.6b03853]. The transport mechanism through the organic molecules was determined to be nonresonant tunneling. To explain this kind of magnetoresistance, we develop an analytical model based on the interaction of the tunneling charge carrier with an unpaired charge carrier populating a contact-molecule interface state. The Coulomb interaction between carriers causes the transmission coefficients to depend on their relative spin orientation. Singlet and triplet pairing of the tunneling and the interface carriers thus correspond to separate conduction channels with different transmission probabilities. Spin relaxation enabling transitions between the different channels, and therefore tending to maximize the tunneling current for a given applied bias, can be suppressed by relatively small magnetic fields, leading to large magnetoresistance. Our model elucidates how the Coulomb interaction gives rise to transmission probabilities that depend on spin and how an applied magnetic field can inhibit transitions between different spin configurations.

  6. 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.

  7. 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

    2015-03-01

    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.

  8. Enhancing the conductivity of molecular electronic devices

    NASA Astrophysics Data System (ADS)

    Stuyver, Thijs; Fias, Stijn; De Proft, Frank; Geerlings, Paul; Tsuji, Yuta; Hoffmann, Roald

    2017-03-01

    We show in this work that conjugated π -electron molecular chains can, in quite specific and understood circumstances, become more conductive the longer they get, in contradiction to what would be expected intuitively. The analysis, done in the framework of the source and sink potential method, and supported by detailed transmission calculations, begins by defining "relative transmission," an inherent measure of molecular conduction. This, in turn, for conjugated hydrocarbons, is related to a simple molecular orbital expression—the ratio of secular determinants of a molecule and one where the electrode contacts are deleted—and a valence bond idea, since these secular determinants can alternatively be expressed in terms of Kekulé structures. A plausible argument is given for relating the relative transmission to the weight of the diradical resonance structures in the resonance hybrid for a molecule. Chemical intuition can then be used to tune the conductivity of molecules by "pushing" them towards more or less diradical character. The relationship between relative transmission (which can rise indefinitely) and molecular transmission is carefully analyzed—there is a sweet spot here for engineering molecular devices. These new insights enable the rationalization of a wide variety of experimental and theoretical results for π-conjugated alternant hydrocarbons, especially the striking difference between extended oligophenylenes and related quinoid chains. In this context, oligo-p-phenylene macrocycles emerge as a potential molecular switch.

  9. ZnO/organic interfaces: formation of hybrid excitations and relation to charge separation (Conference Presentation)

    NASA Astrophysics Data System (ADS)

    Blumstengel, Sylke; Eyer, Moritz; Hecht, Stefan; Kobin, Björn; Koch, Norbert

    2016-09-01

    ZnO is attracting significant interest as a candidate for hybrid photovoltaic and light-emitting devices. We studied electronic coupling at interfaces of ZnO with conjugated organic molecules like ladder-type oligo(phenylenes) (LOP) and NTCDA whose fundamental optical excitations are resonant to the ZnO band gap as well as with polymers employing a combination of time-resolved techniques as well as in situ differential reflectance and photoemission spectroscopy. Our studies provide evidence for the formation of hybrid charge transfer excitations (HCTE) across (Zn,Mg)O/organic interfaces. We show that by interfacial design the properties of these HCTE can be tuned and by that the charge separation process. The impact of the HCTE on photovoltaic parameters like the open circuit voltage and short circuit current is exemplarily demonstrated in (Zn,Mg)O/P3HT diodes. Furthermore, we show that by proper alignment of the frontier molecular orbitals with the semiconductor valence and conduction band edges, exciton dissociation at the interface can be switched off while exciton transfer efficiencies of up to 80 % are maintained. Thus, efficient conversion of ZnO excitons into highly emissive excitons of the organic (LOP) layer is achieved which is essential for the realization of hybrid light-emitting diodes.

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

    PubMed Central

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

    2016-01-01

    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

  11. Charge-Transfer Interactions in Organic Functional Materials

    PubMed Central

    Lin, Hsin-Chieh; Jin, Bih-Yaw

    2010-01-01

    Our goal in this review is three-fold. First, we provide an overview of a number of quantum-chemical methods that can abstract charge-transfer (CT) information on the excited-state species of organic conjugated materials, which can then be exploited for the understanding and design of organic photodiodes and solar cells at the molecular level. We stress that the Composite-Molecule (CM) model is useful for evaluating the electronic excited states and excitonic couplings of the organic molecules in the solid state. We start from a simple polyene dimer as an example to illustrate how interchain separation and chain size affect the intercahin interaction and the role of the charge transfer interaction in the excited state of the polyene dimers. With the basic knowledge from analysis of the polyene system, we then study more practical organic materials such as oligophenylenevinylenes (OPVn), oligothiophenes (OTn), and oligophenylenes (OPn). Finally, we apply this method to address the delocalization pathway (through-bond and/or through-space) in the lowest excited state for cyclophanes by combining the charge-transfer contributions calculated on the cyclophanes and the corresponding hypothetical molecules with tethers removed. This review represents a step forward in the understanding of the nature of the charge-transfer interactions in the excited state of organic functional materials. PMID:28883326

  12. Gold nanoparticles assembled with dithiocarbamate-anchored molecular wires

    PubMed Central

    Reeler, Nini E. A.; Lerstrup, Knud A.; Somerville, Walter; Speder, Jozsef; Petersen, Søren V.; Laursen, Bo W.; Arenz, Matthias; Qiu, Xiaohui; Vosch, Tom; Nørgaard, Kasper

    2015-01-01

    A protocol for the bottom-up self-assembly of nanogaps is developed through molecular linking of gold nanoparticles (AuNPs). Two π-conjugated oligo(phenylene ethynylene) molecules (OPE) with dithiocarbamate anchoring groups are used as ligands for the AuNPs. OPE-4S with a dithiocarbamate in each end of the molecule and a reference molecule OPE-2S with only a single dithiocarbamate end group. The linking mechanism of OPE-4S is investigated by using a combination of TEM, UV-Vis absorption and surface enhanced Raman spectroscopy (SERS) as well as studying the effect of varying the OPE-4S to AuNP concentration ratio. UV-Vis absorption confirms the formation of AuNP aggregates by the appearance of an extended plasmon band (EPB) for which the red shift and intensity depend on the OPE-4S:AuNP ratio. SERS confirms the presence of OPE-4S and shows a gradual increase of the signal intensity with increasing OPE-4S:AuNP ratios up to a ratio of about 4000, after which the SERS intensity does not increase significantly. For OPE-2S, no linking is observed below full coverage of the AuNPs indicating that the observed aggregate formation at high OPE-2S:AuNP ratios, above full AuNP coverage, is most likely of a physical nature (van der Waals forces or π-π interactions). PMID:26471461

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

    NASA Astrophysics Data System (ADS)

    Winkler, Adolf

    2016-10-01

    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.

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

    PubMed

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

    2008-10-24

    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.

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

    PubMed

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

    2012-08-13

    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.

  16. 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

    2015-11-01

    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.

  17. Rapid proton-coupled electron-transfer of hydroquinone through phenylenevinylene bridges.

    PubMed

    Trammell, Scott A; Seferos, Dwight S; Moore, Martin; Lowy, Daniel A; Bazan, Guillermo C; Kushmerick, James G; Lebedev, Nikolai

    2007-01-16

    We describe the synthesis of two oligo(phenylene vinylene)s (OPVs) with a hydroquinone moiety and a thiol anchor group: 4-(2',5'-dihydroxystyryl)benzyl thioacetate and 4-[4'-(2' ',5' '-dihydroxystyryl)styryl]benzyl thioacetate. Monolayers on gold of these molecules were examined by electrochemical techniques to determine the electron transfer kinetics of the hydroquinone functionality (H2Q) through these delocalized tethers ("molecular wires") as a function of pH. Between pH 4 and 9, rate constants were ca. 100-fold faster than for the same H2Q functionality confined to the surface via alkane tethers. Also, in this same pH range rate constants were independent of the length of the OPV bridge. These new electroactive molecules in which the hydroquinone functionality is wired to the gold surface by means of OPV tethers should be useful platforms for constructing bioelectronic devices such as biosensors, biofuel cells, and biophotovoltaic cells with a fast response time.

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

    PubMed

    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

    2016-05-04

    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.

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

    PubMed

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

    2002-06-03

    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

  20. Nanoscale molecular device fabrication via solution and vapor phase deposition

    NASA Astrophysics Data System (ADS)

    Gergel-Hackett, Nadine

    This work describes the fabrication of molecular electronic devices using solution phase and vapor phase assembly methods. The project was motivated by the existing limits of molecular electronics including: a lack of reproducible nanoscale molecular test devices, limited device fabrication techniques that resulted in low yields, and molecular devices that lacked potential for integration with traditional CMOS components. To address these issues, I first designed and fabricated a nanoscale molecular test device using traditional solution phase assembly methods. This test device was shown to be effective and reproducible by characterizing molecules with well-established electrical behaviors. I then used this test device to investigate the electrical behavior of an oligo(phenylene ethynylene) molecule with a nitro sidegroup, known as the nitro molecule. This molecule exhibited interesting electrical behavior with the potential for use in memory and logic devices. In order to better understand the behavioral variations observed from the nitro molecule, I investigated the effect that different molecular environments had on its electrical behavior. Next, molecular device fabrication procedures were improved by developing a method of vapor phase assembly. For this vapor phase deposition, I modified an existing ultra-high vacuum molecular beam epitaxy chamber and developed procedures for purifying the organic molecules prior to assembly. Vapor phase deposition was used to assemble single monolayers of various conducting molecules on gold substrates and the monolayers were characterized to confirm that they were chemisorbed, dense, uncontaminated, and ordered. Nanowell test devices that were fabricated via vapor phase deposition showed the expected electrical characteristics - verifying the effectiveness of vapor phase assembly for molecular electronic device fabrication. I also used vapor phase deposition to assemble conducting molecules on silicon substrates. This switch

  1. Long-range electron transfer in zinc-phthalocyanine-oligo(phenylene-ethynylene)-based donor-bridge-acceptor dyads.

    PubMed

    Göransson, Erik; Boixel, Julien; Fortage, Jérôme; Jacquemin, Denis; Becker, Hans-Christian; Blart, Errol; Hammarström, Leif; Odobel, Fabrice

    2012-11-05

    In the context of long-range electron transfer for solar energy conversion, we present the synthesis, photophysical, and computational characterization of two new zinc(II) phthalocyanine oligophenylene-ethynylene based donor-bride-acceptor dyads: ZnPc-OPE-AuP(+) and ZnPc-OPE-C(60). A gold(III) porphyrin and a fullerene has been used as electron accepting moieties, and the results have been compared to a previously reported dyad with a tin(IV) dichloride porphyrin as the electron acceptor (Fortage et al. Chem. Commun. 2007, 4629). The results for ZnPc-OPE-AuP(+) indicate a remarkably strong electronic coupling over a distance of more than 3 nm. The electronic coupling is manifested in both the absorption spectrum and an ultrafast rate for photoinduced electron transfer (k(PET) = 1.0 × 10(12) s(-1)). The charge-shifted state in ZnPc-OPE-AuP(+) recombines with a relatively low rate (k(BET) = 1.0 × 10(9) s(-1)). In contrast, the rate for charge transfer in the other dyad, ZnPc-OPE-C(60), is relatively slow (k(PET) = 1.1 × 10(9) s(-1)), while the recombination is very fast (k(BET) ≈ 5 × 10(10) s(-1)). TD-DFT calculations support the hypothesis that the long-lived charge-shifted state of ZnPc-OPE-AuP(+) is due to relaxation of the reduced gold porphyrin from a porphyrin ring based reduction to a gold centered reduction. This is in contrast to the faster recombination in the tin(IV) porphyrin based system (k(BET) = 1.2 × 10(10) s(-1)), where the excess electron is instead delocalized over the porphyrin ring.

  2. Important issues facing model-based approaches to tunneling transport in molecular junctions.

    PubMed

    Bâldea, Ioan

    2015-08-21

    Extensive studies on thin films indicated a generic cubic current-voltage I-V dependence as a salient feature of charge transport by tunneling. A quick glance at I-V data for molecular junctions suggests a qualitatively similar behavior. This would render model-based studies almost irrelevant, since, whatever the model, its parameters can always be adjusted to fit symmetric (asymmetric) I-V curves characterized by two (three) expansion coefficients. Here, we systematically examine popular models based on tunneling barriers or tight-binding pictures and demonstrate that, for a quantitative description at biases of interest (V slightly higher than the transition voltage Vt), cubic expansions do not suffice. A detailed collection of analytical formulae as well as their conditions of applicability is presented to facilitate experimentalist colleagues to process and interpret their experimental data obtained by measuring currents in molecular junctions. We discuss in detail the limits of applicability of the various models and emphasize that uncritically adjusting the model parameters to experiment may be unjustified because the values deduced in this way may fall in ranges rendering a specific model invalid or incompatible to ab initio estimates. We exemplify with the benchmark case of oligophenylene-based junctions, for which the results of ab initio quantum chemical calculations are also reported. As a specific issue, we address the impact of the spatial potential profile and show that it is not notable up to biases V ≳ Vt, unlike at higher biases, where it may be responsible for negative differential resistance effects.

  3. A core-shell strategy for constructing a single-molecule junction.

    PubMed

    Wang, Le-Jia; Zhou, Kai-Ge; Tan, Lin; Wang, Hong; Shi, Zi-Fa; Wu, Guo-Ping; Xu, Zhu-Guo; Cao, Xiao-Ping; He, Hui-Xin; Zhang, Hao-Li

    2011-07-18

    Understanding the effects of intermolecular interactions on the charge-transport properties of metal/molecule/metal junctions is an important step towards using individual molecules as building blocks for electronic devices. This work reports a systematic electron-transport investigation on a series of "core-shell"-structured oligo(phenylene ethynylene) (Gn-OPE) molecular wires. By using dendrimers of different generations as insulating "shells", the intermolecular π-π interactions between the OPE "cores" can be precisely controlled in single-component monolayers. Three techniques are used to evaluate the electron-transport properties of the Au/Gn-OPE/Au molecular junctions, including crossed-wire junction, scanning tunneling spectroscopy (STS), and scanning tunneling microscope (STM) break-junction techniques. The STM break-junction measurement reveals that the electron-transport pathways are strongly affected by the size of the side groups. When the side groups are small, electron transport could occur through three pathways, including through single-molecule junctions, double-molecule junctions, and molecular bridges between adjacent molecules formed by aromatic π-π coupling. The dendrimer shells effectively prohibit the π-π coupling effect, but at the same time, very large dendrimer side groups may hinder the formation of Au-S bonds. A first-generation dendrimer acts as an optimal shell that only allows electron transport through the single-molecule junction pathway, and forbids the other undesired pathways. It is demonstrated that the dendrimer-based core-shell strategy allows the single-molecule conductance to be probed in a homogenous monolayer without the influence of intermolecular π-π interactions. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  4. Hollow crescents, helices, and macrocycles from enforced folding and folding-assisted macrocyclization.

    PubMed

    Gong, Bing

    2008-10-01

    This Account reviews the progress made by us on creating porous molecular crescents, helices, and macrocycles based on aromatic oligoamides. Inspired by natural pore- or cavity-containing secondary structures, work described in this Account stemmed from the development of foldamers consisting of benzene rings linked by secondary amide groups. Highly stable, three-center intramolecular hydrogen bonds involving the amide linkages are incorporated into these aromatic oligoamides, which, along with meta-linked benzene units that introduce curvatures into the corresponding backbones, leads to tape-like, curved backbones. Depending on their chain lengths, aromatic oligoamides that fold into crescent and helical conformations have been obtained. Combining results from modeling and experimentally measured data indicates that the folding of these oligomers is readily predictable, determined by the localized intramolecular three-center H-bonds and is independent of side-chain substitution. As a result, a variety of reliably folded, modifiable scaffolds can now be constructed. The well-defined crescent or helical conformations contain noncollapsible internal cavities having multiple introverted amide oxygen atoms. Changing the backbone curvature by tuning the ratio of meta- to para-linked benzene units leads to crescents or helices with cavities of tunable sizes. For example, oligoamides consisting of meta-linked units contain cavities of approximately 9 A across, while those with alternating meta- and para-linked units have cavities of over 30 A across. The generality of such a folding and cavity-creating strategy has also been demonstrated by the enforced folding of other types of aromatic oligomers such as oligo(phenylene ethynylene)s, aromatic oligoureas, and aromatic oligosulfonamides. More recently, the folding of aromatic oligoamides was found to assist efficient macrocyclization reactions, which has provided a convenient method for preparing a new class of large shape

  5. Molecular Rotors as Switches

    PubMed Central

    Xue, Mei; Wang, Kang L.

    2012-01-01

    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

  6. 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

  7. 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

  8. 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

  9. Molecular self-assembly into one-dimensional nanostructures.

    PubMed

    Palmer, Liam C; Stupp, Samuel I

    2008-12-01

    bundles. Surprisingly, TEM of a PA substituted by a nitrobenzyl group revealed assembly into quadruple helical fibers with a braided morphology. Upon photocleavage of this the nitrobenzyl group, the helices transform into single cylindrical nanofibers. Finally, inspired by the tobacco mosaic virus, we used a dumbbell-shaped, oligo(phenylene ethynylene) template to control the length of a PA nanofiber self-assembly (<10 nm). AFM showed complete disappearance of long nanofibers in the presence of this rigid-rod template. Results from quick-freeze/deep-etch TEM and dynamic light scattering demonstrated the templating behavior in aqueous solution. This strategy could provide a general method to control size the length of nonspherical supramolecular nanostructures.

  10. 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.).

  11. 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

    2015-07-01

    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