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Electronic Coupling (electronic + coupling)

Distribution by Scientific Domains

Chemistry	75%

Selected Abstracts

Customized Electronic Coupling in Self-Assembled Donor,Acceptor Nanostructures

ADVANCED FUNCTIONAL MATERIALS, Issue 22 2009
Dimas G. de Oteyza
Abstract Charge transfer processes between donor,acceptor complexes and metallic electrodes are at the heart of novel organic optoelectronic devices such as solar cells. Here, a combined approach of surface-sensitive microscopy, synchrotron radiation spectroscopy, and state-of-the-art ab initio calculations is used to demonstrate the delicate balance that exists between intermolecular and molecule,substrate interactions, hybridization, and charge transfer in model donor,acceptor assemblies at metal-organic interfaces. It is shown that charge transfer and chemical properties of interfaces based on single component layers cannot be naively extrapolated to binary donor,acceptor assemblies. In particular, studying the self-assembly of supramolecular nanostructures on Cu(111), composed of fluorinated copper-phthalocyanines (F16CuPc) and diindenoperylene (DIP), it is found that, in reference to the associated single component layers, the donor (DIP) decouples electronically from the metal surface, while the acceptor (F16CuPc) suffers strong hybridization with the substrate. [source]

Metal,Metal Electronic Coupling in syn and anti Stereoisomers of Mixed-Valent (FeCp)2 -, (RhL2)2 -, and (FeCp)(RhL2)- as -Indacenediide Ions

CHEMISTRY - A EUROPEAN JOURNAL, Issue 28 2007
Saverio Santi Prof.
Abstract The extent of metal,metal electronic coupling was quantified for a series of syn and anti stereoisomers of (FeCp)2 -, (RhL2)2 - and (FeCp)(RhL2)- (L2=1,5-cyclooctadiene (cod), L=CO) as -indacenediide mixed-valent ions by spectroelectrochemical and DFT studies. The effect of the syn/anti orientation of the metal units with respect to the planar aromatic ligand indicates that electron transfer occurs through the bridge rather than through space. The nature of the metal was found to be crucial: while homobimetallic diiron species are localised valence-trapped ions (Class,II), the dirhodium analogues are almost delocalised mixed-valent ions (borderline and Class,III). Finally, despite their redox asymmetry, even in the heterobimetallic iron,rhodium as -indacenediide complexes, strong metal,metal coupling is present. In fact, oxidation of the iron centre is accompanied by electron transfer from rhodium to iron and formation of a reactive 17-electron rhodium site. syn and anti Fe,Rh as -indacenediide complexes are rare examples of heterobimetallic systems which can be classified as borderline Class,II/Class,III species. [source]

Kinetic, Thermodynamic, and Mechanistic Patterns for Free (Unbound) Cytochrome c at Au/SAM Junctions: Impact of Electronic Coupling, Hydrostatic Pressure, and Stabilizing/Denaturing Additives

CHEMISTRY - A EUROPEAN JOURNAL, Issue 27 2006
Dimitri E. Khoshtariya Prof. Dr.
Abstract Combined kinetic (electrochemical) and thermodynamic (calorimetric) investigations were performed for an unbound (intact native-like) cytochrome c (CytC) freely diffusing to and from gold electrodes modified by hydroxyl-terminated self-assembled monolayer films (SAMs), under a unique broad range of experimental conditions. Our approach included: 1) fine-tuning of the charge-transfer (CT) distance by using the extended set of Au-deposited hydroxyl-terminated alkanethiol SAMs [-S-(CH2)n -OH] of variable thickness (n=2, 3, 4, 6, 11); 2) application of a high-pressure (up to 150,MPa) kinetic strategy toward the representative Au/SAM/CytC assemblies (n=3, 4, 6); 3) complementary electrochemical and microcalorimetric studies on the impact of some stabilizing and denaturing additives. We report for the first time a mechanistic changeover detected for "free" CytC by three independent kinetic methods, manifested through 1) the abrupt change in the dependence of the shape of the electron exchange standard rate constant (ko) versus the SAM thickness (resulting in a variation of estimated actual CT range within ca. 15 to 25 Å including ca. 11 Å of an "effective" heme-to-,-hydroxyl distance). The corresponding values of the electronic coupling matrix element vary within the range from ca. 3 to 0.02 cm,1; 2) the change in activation volume from +6.7 (n=3), to ,0 (n=4), and ,5.5 (n=6) cm3,mol,1 (disclosing at n=3 a direct pressure effect on the protein's internal viscosity); 3) a "full" Kramers-type viscosity dependence for ko at n=2 and 3 (demonstrating control of an intraglobular friction through the external dynamic properties), and its gradual transformation to the viscosity independent (nonadiabatic) regime at n=6 and 11. Multilateral cross-testing of "free" CytC in a native-like, glucose-stabilized and urea-destabilized (molten-globule-like) states revealed novel intrinsic links between local/global structural and functional characteristics. Importantly, our results on the high-pressure and solution-viscosity effects, together with matching literature data, strongly support the concept of "dynamic slaving", which implies that fluctuations involving "small" solution components control the proteins' intrinsic dynamics and function in a highly cooperative manner as far as CT processes under adiabatic conditions are concerned. [source]

Mono- and Binuclear Arylnickel Complexes of the ,-Diimine Bridging Ligand 2,2,-Bipyrimidine (bpym)

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 6 2010
Axel Klein
Abstract The mono- and binuclear organometallic NiII complexes [(,-bpym){Ni(Mes)Br}n] (bpym = 2,2,-bipyrimidine; n = 1 or 2; Mes = mesityl = 2,4,6-trimethylphenyl) were prepared and characterised electrochemically and spectroscopically (NMR, UV/Vis/NIR) in detail. The long-wavelength absorptions for the binuclear complex reveal a marked electronic coupling of the two metal centres over the ligand bridge via their low-lying ,*-orbitals. While the mononuclear complex undergoes rapid dissociation of the bromido ligand after one-electron reduction the binuclear derivative exhibits reversible reductive electrochemistry and both of them yield stable radical anionic complexes with mainly bpym ligand centred spin density as shown by EPR spectroscopy of the free ligand bpym and the nickel complexes. The molecular structure of the binuclear bpym complex [(,-bpym){Ni(Mes)Br}2] was studied by EXAFS in comparison to the mononuclear analogue [(bpym)Ni(Mes)Br] revealing markedly increased Ni,C/N distance of the first coordination shell for the binuclear derivative suggesting an optimum overlap for the mononuclear complex, while two nickel complex fragments {Ni(Mes)Br} are seemingly too large to fit into the bis-chelate coordination site. [source]

Multicomponent Supramolecular Devices: Synthesis, Optical, and Electronic Properties of Bridged Bis-dirhodium and -diruthenium Complexes,

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 19 2006
Anne Petitjean
Abstract Four ruthenium- and rhodium-based metal,metal-bonded multicomponent systems have been synthesized, and their absorption, redox, spectroelectrochemical and structural properties have been studied. The absorption spectra of the four bis-dimetallic compounds M2LM2, where L is a bridging ligand and M is rhodium or ruthenium, exhibit very strong bands in the UV, visible and, for the diruthenium species, near-IR region. The low-energy absorption bands are assigned to charge-transfer transitions involving a metal,metal bonding orbital as the donor and an orbital centered on the bis-tetradentate aromatic ligands as the acceptor (metal,metal to ligand charge transfer, M2LCT). Each compound exhibits reversible bridging-ligand-centered reductions at mild potentials and several reversible oxidation processes. The oxidation signals of the two equivalent dimetallic centers of each bis-dimetallic compound are split, with the splitting , a measure of the electronic coupling , depending on both the metal and bridging ligand. The mixed-valence species of the dirhodium species was investigated, and the electronic coupling matrix element calculated from the experimental intervalence band parameters for one of them (86 cm,1) indicates a significant inter-component electronic interaction which compares well with good electron conducting anionic bridges such as cyanides. Although none of these compounds is luminescent, the M2LCT excited state of one of the bis-dirhodium complexes is relatively long-lived (about 6 ,s) in degassed acetonitrile at room temperature. The results presented here are promising for the development of linear poly-dimetallic complexes built on longer naphthyridine-based strands, with significant long-range electronic coupling and molecular-wire-like behavior. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source]

Molecular Design of Unsymmetrical Squaraine Dyes for High Efficiency Conversion of Low Energy Photons into Electrons Using TiO2 Nanocrystalline Films

ADVANCED FUNCTIONAL MATERIALS, Issue 17 2009
Thomas Geiger
Abstract An optimized unsymmetrical squaraine dye 5-carboxy-2-[[3-[(2,3-dihydro-1, 1-dimethyl-3-ethyl-1H -benzo[e]indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-3,3-dimethyl-1-octyl-3H -indolium (SQ02) with carboxylic acid as anchoring group is synthesized for dye-sensitized solar cells (DSCs). Although the , -framework of SQ02 is insignificantly extended compared to its antecessor squaraine dye SQ01, photophysical measurements show that the new sensitizer has a much higher overall conversion efficiency , of 5.40% which is improved by 20% when compared to SQ01. UV-vis spectroscopy, cyclic voltammetry and time dependent density functional theory calculations are accomplished to rationalize the higher conversion efficiency of SQ02. A smaller optical band gap including a higher molar absorption coefficient leads to improved light harvesting of the solar cell and a broadened photocurrent spectrum. Furthermore, all excited state orbitals relevant for the ,,,* transition in SQ02 are delocalized over the carboxylic acid anchoring group, ensuring a strong electronic coupling to the conduction band of TiO2 and hence a fast electron transfer. [source]

Quantum-Chemical Characterization of the Origin of Dipole Formation at Molecular Organic/Organic Interfaces

ADVANCED FUNCTIONAL MATERIALS, Issue 4 2009
Igor Avilov
Abstract Recent experiments have reported a vacuum level shift at the interface between organic materials due to the formation of an interface dipole layer. On the basis of quantum-chemical calculations, this paper sheds light on the factors contributing to the formation of an interface dipole between an electron donor and an electron acceptor, considering as model system a complex made of tetrathiafulvalene (TTF) as a donor and tetracyanoquinodimethane (TCNQ) as an acceptor. The results indicate that the interface dipole is governed both by charge-transfer and polarization effects and allow for disentangling of their respective contributions. Two regimes of charge transfer can be distinguished depending on the strength of the electronic coupling: a fractional charge transfer occurs in the strong coupling regime while only integer charges are transferred when the coupling is weak. The polarization contribution can be significant, even in the presence of a pronounced charge transfer between the donor and acceptor molecules. The values of ionization potential and electron affinity of the donor and acceptor molecules may experience shifts as large as several tenths of an eV at the interface with respect to the isolated compounds. [source]

Modeling Electron and Hole Transport in Fluoroarene-Oligothiopene Semiconductors: Investigation of Geometric and Electronic Structure Properties,

ADVANCED FUNCTIONAL MATERIALS, Issue 2 2008
E. Koh
Abstract A theoretical study using density functional theory is undertaken to gain insight into how the structural, electronic, and electron-transfer characteristics of three Fluoroarene-oligothiophene semiconductors influence the preferred transport of electrons versus holes in field-effect transistor applications. The intermolecular electronic coupling interactions are analyzed through both a simplified energy-splitting in dimer (ESID) model and as a function of the entire dimer Hamiltonian in order to understand the impact of site energy differences; our results indicate that these differences are generally negligible for the series and, hence, use of the ESID model is valid. In addition, we also investigate the reduction and oxidation processes to understand the magnitudes of the intramolecular reorganization energy for the charge-hopping process and expected barrier heights for electron and hole injection into these materials. From the electronic coupling and intramolecular reorganization energies, estimates of the nearest-neighbor electron-transfer hopping rate constant for electrons are obtained. The ionization energetics suggest favored electron injection for the system with perfluoroarene groups at the end of the thiophene core, in agreement with experiments. The combined analyses of the electron-transfer properties and ionization processes suggest possible ambipolar behavior for these materials under favorable device conditions. [source]

Length-dependence of electron transfer coupling matrix in polyene wires: Ab initio molecular orbital theory study,

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2009
Govind Mallick
Abstract The electron transfer (ET) properties of ,-electron conjugated quasi-one-dimensional molecular wires, consisting of polyene, [>CC<]n (n = 1,11), including ,-carotene, is investigated using ab initio molecular orbital theory within Koopmans theorem (KT) approach. The ET coupling matrix element, VDA, for 1,3- trans -butadiene molecule calculated with the KT approach shows excellent agreement with the corresponding results obtained with two-state model. The calculated values of VDA for the polyene oligomers exhibit exponential decrease in magnitude with increasing length of the wire. However, the decay curve exhibits three different regimes. The magnitude of the decay constant, ,, decreases with the increase in length of the wire. A highly delocalized ,-electron cloud in the polyene chain appears to facilitate retention of the electronic coupling at large separations between the donor and acceptor centers. Published 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source]

Fourier transform vibrational circular dichroism as a decisive tool for conformational studies of peptides containing tyrosyl residues

BIOPOLYMERS, Issue 1 2003
Attila Borics
Abstract Previous UV,circular dichroism (UV,CD) and NMR studies showed that Ac-AAAAAAAEAAKA-NH2 has an ,-helical structure in 50% (v/v) aqueous trifluoroethanol. Replacement of Ala1 to Ala6 with Tyr results in spectra that show an apparent loss of helicity in the same solvent. This apparent loss of helicity could be attributed to the coupling of the tyrosyl side chain chromophore with the backbone amide. However, such electronic coupling does not affect the vibrational CD (VCD) spectra. The VCD spectra of the peptides with tyrosyl residues were identical to that of the peptide containing no Tyr, which shows the same ,-helical structure. Because it is now clear that Tyr replacement does not change the backbone conformation of peptides, UV,CD measurements should be complemented by VCD to determine the secondary structure when electronic effects can disturb the UV,CD spectrum of the inherent structure. © 2002 Wiley Periodicals, Inc. Biopolymers (Biospectroscopy) 72: 21,24, 2003 [source]

Single-Step Electron Transfer on the Nanometer Scale: Ultra-Fast Charge Shift in Strongly Coupled Zinc Porphyrin,Gold Porphyrin Dyads

CHEMISTRY - A EUROPEAN JOURNAL, Issue 11 2008
Jérôme Fortage Dr.
Abstract The synthesis, electrochemical properties, and photoinduced electron transfer processes of a series of three novel zinc(II),gold(III) bisporphyrin dyads (ZnPSAuP+) are described. The systems studied consist of two trisaryl porphyrins connected directly in the meso position via an alkyne unit to tert -(phenylenethynylene) or penta(phenylenethynylene) spacers. In these dyads, the estimated center to center interporphyrin separation distance varies from 32 to 45,Å. The absorption, emission, and electrochemical data indicate that there are strong electronic interactions between the linked elements, thanks to the direct attachment of the spacer on the porphyrin ring through the alkyne unit. At room temperature in toluene, light excitation of the zinc porphyrin results in almost quantitative formation of the charge shifted state .+ZnPSAuP., whose lifetime is in the order of hundreds of picoseconds. In this solvent, the charge-separated state decays to the ground state through the intermediate population of the zinc porphyrin triplet excited state. Excitation of the gold porphyrin leads instead to rapid energy transfer to the triplet ZnP. In dichloromethane the charge shift reactions are even faster, with time constants down to 2,ps, and may be induced also by excitation of the gold porphyrin. In this latter solvent, the longest charge-shifted lifetime (,=2.3,ns) was obtained with the penta-(phenylenethynylene) spacer. The charge shift reactions are discussed in terms of bridge-mediated super-exchange mechanisms as electron or hole transfer. These new bis-porphyrin arrays, with strong electronic coupling, represent interesting molecular systems in which extremely fast and efficient long-range photoinduced charge shift occurs over a long distance. The rate constants are two to three orders of magnitude larger than for corresponding ZnPAuP+ dyads linked via meso -phenyl groups to oligo-phenyleneethynylene spacers. This study demonstrates the critical impact of the attachment position of the spacer on the porphyrin on the electron transfer rate, and this strategy can represent a useful approach to develop molecular photonic devices for long-range charge separations. [source]

Metal,Metal Electronic Coupling in syn and anti Stereoisomers of Mixed-Valent (FeCp)2 -, (RhL2)2 -, and (FeCp)(RhL2)- as -Indacenediide Ions

Solid- and Solution-State Studies of the Novel ,-Dicyanamide-Bridged Dinuclear Spin-Crossover System {[(Fe(bztpen)]2[,-N(CN)2]}(PF6)3,n,H2O,

CHEMISTRY - A EUROPEAN JOURNAL, Issue 19 2005
Norma Ortega-Villar
Abstract The mononuclear diamagnetic compound {Fe(bztpen)[N(CN)2]}(PF6)CH3OH (1) (bztpen = N -benzyl- N,N,,N,-tris(2-pyridylmethyl)ethylenediamine) has been synthesized and its crystal structure studied. Complex 1 can be considered to be the formal precursor of two new dinuclear, dicyanamide-bridged iron(II) complexes with the generic formula {[(Fe(bztpen)]2[,-N(CN)2]}(PF6)3,n,H2O (n = 1 (2) or 0 (3)), which have been characterized in the solid state and in solution. In all three complexes, the iron atoms have a distorted [FeN6] octahedral coordination defined by a bztpen ligand and a terminal (1) or a bridging dicyanamide ligand (2 and 3). In the solid state, 2 and 3 can be considered to be molecular isomers that differ by the relative position of the phenyl ring of the two {Fe(bztpen)[N(CN)2]}+ halves (cis and trans, respectively). Depending on the texture of the sample, 2 exhibits paramagnetic behavior or displays a very incomplete spin transition at atmospheric pressure. Complex 3 undergoes a gradual two-step spin transition with no observed hysteresis in the solid state. Both steps are approximately 100 K wide, centered at ,200 K and ,350 K, with a plateau of approximately 80 K separating the transitions. The crystal structure of 3 has been determined in steps of approximately 50 K between 400 K and 90 K, which provides a fascinating insight into the structural behavior of the complex and the nature of the spin transition. Order,disorder transitions occur in the dicyanamide bridge and the PF6, ions simultaneously, with the spin-crossover behavior suggesting that these transitions may trigger the two-step character. In solution, 2 and 3 display very similar continuous spin conversions. Electrochemical studies of 2 and 3 show that the voltammograms are typical of dimeric systems with electronic coupling of the metals through the dicyanamide ligand. [source]

Exciton Migration in Conjugated Dendrimers: A Joint Experimental and Theoretical Study

CHEMPHYSCHEM, Issue 18 2009
Muhammet E. Köse Prof.
Abstract We report a joint experimental and theoretical investigation of exciton diffusion in phenyl-cored thiophene dendrimers. Experimental exciton diffusion lengths of the dendrimers vary between 8 and 17 nm, increasing with the size of the dendrimer. A theoretical methodology is developed to estimate exciton diffusion lengths for conjugated small molecules in a simulated amorphous film. The theoretical approach exploits Fermi's Golden Rule to estimate the energy transfer rates for a large ensemble of bimolecular complexes in random relative orientations. Utilization of Poisson's equation in the evaluation of the Coulomb integral leads to very efficient calculation of excitonic couplings between the donor and the acceptor chromophores. Electronic coupling calculations with delocalized transition densities revealed efficient coupling pathways in the bulk of the material, but do not result in strong couplings between the chromophores which are calculated for more localized transition densities. The molecular structures of dendrimers seem to be playing a significant role in the magnitude of electronic coupling between chromophores. Simulated diffusion lengths correlate well with the experimental data. The chemical structure of the chromophore, the shape of the transition densities and the exciton lifetime are found to be the most important factors in determining the size of the exciton diffusion length in amorphous films of conjugated materials. [source]

A Colloidal Au Monolayer Modulates the Conformation and Orientation of a Protein at the Electrode/Solution Interface

CHEMPHYSCHEM, Issue 8 2005
Xiue Jiang
Abstract The orientation and conformation of adsorbed cytochrome c (cyt c) at the interface between an electrode modified with colloidal Au and a solution were studied by electrochemical, spectroscopic, and spectroelectrochemical techniques. The results indicate that the colloidal Au monolayer formed via preformation of an organic self-assembled monolayer (SAM) can increase the electronic coupling between the SAM and cyt c in the same manner as bifunctional molecular bridges, one functional group of which is bound to the electrode surface while the other interacts with the protein surface. The approach of cyt c to the modified electrode/solution interface can be assisted by strong interactions of the intrinsic charge of colloidal particles with cyt c, while the heme pocket remains almost unchanged due to the screening effect of the negatively charged field created by the intrinsic charge. The conformational changes of cyt c induced by its adsorption at a bare glassy carbon electrode/solution interface and the effect of the electric field on the ligation state of the heme can be avoided at the colloidal-Au-modified electrode/solution interface. Finally, a possible model for the adsorption orientation of cyt c at the colloidal-Au-modified electrode/solution interface is proposed. [source]