Electronic Coupling Parameter Research Articles

Influence of methoxy groups on the properties of 1,1-bis(4-aminophenyl)cyclohexane based arylamines: experimental and theoretical approach

Three new isomeric cyclohexylidene linked triphenylamines containing methoxy groups in different positions were synthesized via Ullmann coupling from 1,1-bis(4-aminophenyl)cyclohexane and respective aryl iodides. Thermal behaviour, optical and photoelectrical properties of the obtained materials were investigated. Calculations based on the density functional methods (DFT) were also carried out in order to better understand structure–property relationships. Methoxy-substituted derivatives of 1,1-bis(4-aminophenyl)cyclohexane show lower ionization potentials and higher hole drift mobilities than the corresponding derivative having no methoxy groups. The ionization potentials of the solid samples of the methoxy-substituted compounds established by the electron photoemission technique are in the range of 5.34–5.55 eV. Hole-drift mobility values of the amorphous layers of the methoxy-substituted materials established by the time-of-flight technique range from 4.0 × 10−4 to 1.2 × 10−3 cm2 V−1 s−1 at the electric field of 106 V cm−1. The highest hole mobilities were observed for the para-substituted derivative. Theoretical results suggest that the hole mobility in the bulk materials is driven by the electronic coupling parameter whilst the reorganization energy parameter predicts the wrong mobility trend. The role of the methoxy groups is found to be related to the well-known mesomeric (π-donor) effect and the possibility to establish C–H⋯π(Ph) and C–H⋯X (X = O, N) hydrogen bonds. The effect of these properties on the enhanced Stokes shifts of the para-methoxy-substituted compound, on the decrease of the ionization potentials for the methoxy substituted compounds as compared to the non-substituted one, and on the enhanced possibility to establish considerable electronic couplings between adjacent molecules is discussed in detail.

Charge Delocalization in a Cyclometalated Bisruthenium Complex Bridged by a Noninnocent 1,2,4,5-Tetra(2-pyridyl)benzene Ligand

Two ruthenium atoms are covalently connected to the para positions of a phenyl ring in 1,2,4,5-tetra(2-pyridyl)benzene (tpb) to form a linear Ru-tpb-Ru arrangement. This unique structure leads to appealing electronic properties for the biscyclometalated complex [(tpy)Ru(tpb)Ru(tpy)](2+), where tpy is 2,2';6',2″-terpyridine. It could be stepwise oxidized at substantially low potential (+0.12 and +0.55 V vs Ag/AgCl) and with a noticeably large comproportionation constant (1.94 × 10(7)). In addition to the routinely observed metal-to-ligand charge-transfer transitions, [(tpy)Ru(tpb)Ru(tpy)](2+) displays a separate and distinct absorption band at 805 nm with appreciable absorptivity (ε = 9000 M(-1) cm(-1)). This band is assigned to the charge transition from the Ru-tpb-Ru motif to the pyridine rings of tpb with the aide of density functional theory (DFT) and time-dependent DFT calculations. Complex [(tpy)Ru(tpb)Ru(tpy)](2+) was precisely titrated with 1 equiv of cerium ammonium nitrate to produce [(tpy)Ru(tpb)Ru(tpy)](3+), which shows intense multiple NIR transitions. The electronic coupling parameters H(ab) of individual NIR components are determined to be 5812, 4942, 4358, and 3560 cm(-1). DFT and TDDFT calculation were performed on [(tpy)Ru(tpb)Ru(tpy)](3+) to elucidate its electronic structure and spin density population and the nature of the observed NIR transitions. Electron paramagnetic resonance studies of [(tpy)Ru(tpb)Ru(tpy)](3+) exhibit a discernible rhombic signal with the isotropic g factor of ⟨g⟩ = 2.144. These results point to the strong orbital interaction of tpb with metal centers and that tpb behaves as a redox noninnocent bridging ligand in [(tpy)Ru(tpb)Ru(tpy)](2+). Complex [(tpy)Ru(tpb)Ru(tpy)](3+) is determined to be a Robin-Day class III system with full charge delocalization across the Ru-tpb-Ru motif.

Molecular ion–electron recombination in an expanding ultracold neutral plasma of NO+

Using state-selected double-resonant excitation, we create a Rydberg gas of NO molecules excited to the principal quantum number n = 50 of the f-series converging to the ion rotational level, N(+) = 2. This gas evolves to form an ultracold plasma, which expands under the thermal pressure of its electrons, and dissipates by electron-ion recombination. Under conditions chosen for this experiment, the observed rates of expansion vary with selected plasma density. Electron temperatures derived from these expansion rates vary from T(e) = 12 K for the highest density up to 16 K at four-fold lower density. Over this range, the apparent electron coupling parameter, defined as Γ(e) = e(2)/4πε(0)ak(B)T(e), falls from nearly three to about one. The decay of charged-particle density fits with a kinetic model that includes parallel paths of direct two-body and stepwise three-body dissociative recombination. The overall recombinative decay follows a second-order rate law, with an observed rate constant that fits with established scattering-theory estimates for elementary two-body dissociative recombination. A small residual increase in this rate constant with decreasing charged-particle density suggests a growing importance of the three-body recombination channel under conditions of decreasing electron correlation.

Local Field Corrections Effect on Equation of State in Dense Hydrogen Plasma: Plasma Phase Transition

AbstractWe present calculations of Hydrogen plasma pressure including local field correction (LFC) effects modelled in the Singwi Tosi Land and Sjölander model extended to arbitrary temperature (ATSTLS), neutrals contributions are not included. We show that LFC induces thermodynamic instabilities (phase transition) in the vicinity of the maximum electron coupling parameter which corresponds to maximum departure of the ATSTLS electron polarizability from the Random Phase Approximation one. Comparison of our results to Monte‐Carlo calculations induced us to think that free electron coupling plays a dominant role in plasma phase transition. Critical temperature is estimated and compared to existing values (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic pathway in reaction centers from Rhodobacter sphaeroides and Chloroflexus aurantiacus

The reaction centers (RC) of Chloroflexus aurantiacus and Rhodobacter sphaeroidesH(M182)L mutant were investigated. Prediction for electron transfer (ET) at very low temperatures was also performed. To describe the kinetics of the C. aurantiacus RCs, the incoherent model of electron transfer was used. It was shown that the asymmetry in electronic coupling parameters must be included to explain the experiments. For the description of R. sphaeroidesH(M182)L mutant RCs, the coherent and incoherent models of electron transfer were used. These two models are discussed with regard to the observed electron transfer kinetics. It seems likely that the electron transfer asymmetry in R. sphaeroides RCs is caused mainly by the asymmetry in the free energy levels of L- and M-side cofactors. In the case of C. aurantiacus RCs, the unidirectionality of the charge separation can be caused mainly by the difference in the electronic coupling parameters in two branches.

Molecular beam epitaxy of high mobility In0.75Ga0.25As for electron spin transport applications

The authors describe the molecular beam epitaxy of relaxed, nominally undoped In0.75Ga0.25As–In0.75Al0.25As quantum well structures grown on InP substrates. The maximum two-dimensional electron density is 2×1011cm−2, with a peak mobility of 2.2×105cm2V−1s−1 at 1.5K. In high magnetic field, the electron g-factor was shown to have a magnitude of 9.1±0.1 at Landau-level filling factor of 4. The Rashba coefficient, determined from the analysis of the magnetoresistance at high Landau-level filling factor (>12), is 1×10−11eVm. The mobility is sufficiently high in these two-dimensional electron gases that spin-orbit effects are observed up to 4.2K. The interface asymmetry, defined as the difference between the wavefunction penetration into the upper and lower In0.75Al0.25As quantum barriers, makes no contribution to the Rashba spin-orbit coupling parameter in this system. Quantum wires defined in these two-dimensional electron gases using insulated, split surface gates show clear quarter-integer quantized conductance plateaux at exactly 0.25(2e2∕h) and 0.75(2e2∕h) in nonequilibrium transport. In0.75Ga0.25As may have important application as an alternative field effect transistor channel to silicon, and the large electronic g-factor and Rashba spin-orbit coupling parameter make this material combination suitable for exploring spin related phenomena in one-dimensional systems.

Low-Energy Bands of Ferrocene−Ferrocenium Dimers: Bandshape Analysis with a Four-Level Two-Mode Vibronic Coupling Configuration Interaction (VCCI) Model Including Asymmetry

Ferrocene-ferrocenium dimers exhibit a double-peak intervalence charge-transfer (IVCT) band in the NIR/MIR region, which is analyzed in terms of a four-level, two-mode vibronic coupling configuration interaction (VCCI) scheme. Besides providing satisfactory fits of the measured spectra, the model also gives electronic and vibronic coupling parameters as well as CI mixing coefficients. A temperature-dependent asymmetry of the potential is introduced in order to describe the temperature dependence of the solid-state spectra and account for complementary Mossbauer data, which indicate a temperature-driven electron localization-delocalization transition in the singly bridged radical cations. The VCCI model is also successfully applied to the doubly bridged Fe(II)-Fe(III) bisfulvalenide dimer, which exhibits a double-peak IVCT transition as well. The VCCI analysis reveals that all dimers have a one-minimum potential in the ground state, leading in the absence of asymmetry to class III behavior (electron delocalization). If electron localization corresponding to class II characteristics occurs, it is due to an asymmetric (but still one-minimum) potential and not, as usual, to a double-minimum potential, explaining the class II-III borderline behavior observed for these systems.

Computer simulation and SERR detection of cytochrome c dynamics at SAM-coated electrodes

In this paper we present a combined experimental and theoretical study of the heterogeneous electron transfer reaction of cytochrome c electrostatically adsorbed on metal electrodes coated with monolayers of 6-mercaptohexanoic acid. Molecular dynamics simulations and pathways calculations show that adsorption of the protein leads to a broad distribution of orientations and, thus, to a correspondingly broad distribution of electron transfer rate constants due to the orientation-dependence of the electronic coupling parameter. The adsorbed protein exhibits significant mobility and, therefore, the measured reaction rate is predicted to be a convolution of protein dynamics and tunnelling probabilities for each orientation. This prediction is confirmed by time-resolved surface enhanced resonance Raman which allows for the direct monitoring of protein (re-)orientation and electron transfer of the immobilised cytochrome c. The results provide a consistent explanation for the non-exponential distance-independence of electron transfer rates usually observed for proteins immobilized on electrodes.

Local-field-correction effects on the electron response functions and on the electrical conductivity in a hydrogen plasma

In a one-component plasma constituted by electrons with a uniform positive background, the correlations are studied in the framework of the Singwi, Tosi, Land, and Sjölander (STLS) model. The local-field corrections (LFCs) are calculated with electron density ranging from 10;{19}to10;{26}cm;{-3} and with a temperature of 10;{4}K . Then, a dielectric formalism is used to deduce the potential energy of a positive test charge (a proton) imbedded in the electron medium. A significant departure of this potential from the random phase approximation (RPA) one has been found. In particular, as the density increases, the discrepancy between the two potentials grows up to reach a maximum correlated to the maximum of the electron coupling parameter. In addition, it is found that in both high and low density limits, the STLS and RPA approaches yield similar results. On the other hand, the effect of the LFC on the electrical conductivity is also estimated in hydrogen plasmas.

Direct Observation of a Photoinduced Radical Pair in a Cryptochrome Blue‐Light Photoreceptor

Compass component: Blue-light excitation of the photoreceptor cryptochrome generates a transient radical pair by electron transfer from a tryptophan (Trp) at the surface to the flavin cofactor in the center of the protein (see picture; FAD=flavin adenine dinucleotide). Simulated EPR spectra show that the electronic coupling parameters of these radical pairs are suitable for animal magnetoreception.

Ultrafast Bimolecular Electron Transfer Dynamics in Micellar Media

Ultrafast photoinduced bimolecular electron transfer (ET) dynamics between 7-aminocoumarin derivatives and N,N-dimethylaniline (DMAN) has been studied in neutral (TX100), cationic (DTAB) and anionic (SDS) micellar media. A very fast decay time constant (tau(fast)) shorter than approximately 10 ps has been observed for the coumarins in the presence of DMAN in all of the three micellar media. In this time scale, reactants in the micellar phase undergo ET interactions without involving diffusion or reorientation of the reactants and thus can be envisaged as equivalent to nondiffusive bimolecular ET reaction. The fastest ET rates estimated as the inverse of the shortest lifetime components of the fluorescence decay (k(et) congruent with tau(fast)(-1)) nicely follow the predicted Marcus inversion behavior with reaction exergonicity (-DeltaG degrees), irrespective of the nature of micelles considered. Onset of inversion in ET rates occur at approximately 0.61 eV lower exergonicity in SDS and TX100 micelles compared with that in DTAB micelle and are rationalized following two-dimensional ET (2DET) theory. These differences suggest the possibility of tuning Marcus inversion by proper selection of micelles. Interestingly, ET rates (k'(et)) obtained from the conventional Stern-Volmer analysis of the relatively longer time constants of the fluorescence decays also exhibit similar Marcus correlation with DeltaG degrees, showing clear inversion behavior. Fitting of Marcus correlation curves for k(et) and k'(et) indicate two largely different values for the electronic coupling parameters. In micellar media, as the interacting donor-acceptor molecules are on an average expected to be separated by an intervening surfactant chain and the reorientation rate of the reactants is quite slow, it is predicted that the ultrafast ET (k(et)) component arises because of the surfactant separated donor-acceptor pairs that are orientated perfectly to give the maximum electronic coupling. The slower ET (k'(et)) is predicted to arise because of those pairs where the donor-acceptor orientations are not very suitable but good enough to give a sizable electronic coupling.

Real-space pseudopotential calculations of spin-dependent electron transport in magnetic molecular junctions

A real-space pseudopotential approach is developed to calculate the spin-dependent transport in nanoscale junctions. Our method is based on self-consistent solution of the Kohn-Sham equation of density functional theory with asymptotic boundary conditions. This method is applied to a simple magnetic molecule, the Sc dimer, bridging nonmagnetic, planar jellium electrodes for a series of molecule-lead spacing. We find that the spin-dependent conductance within this formalism is rather robust over a wide range of electronic coupling parameters. The minority channel of parallel-aligned ${\mathrm{Sc}}_{2}$ produces a fairly stable conductance of roughly half of a quantum unit $({e}^{2}∕h)$. Other systems show sensitive dependence on the coupling strength. Atomic origins of the dependence are discussed.

Embedding effects on charge-transport parameters in molecular organic materials

We present a generalized version of the tight-binding approach to determine the electronic coupling parameter in charge (hole) transport phenomena in organic materials. The main novelty of this approach is that the "embedding effects" of the environment (either a solvent or a crystal packing) can be explicitly included in the calculation by considering an embedded dimer. One of the main features shown by the application of the method to both model systems and oligoacene crystals is that the routinely used "energy splitting in a dimer" approximation gives reasonable results even if the transfer units are not equivalent by symmetry but the embedding effects are properly taken into account.

Synthesis, Electrochemical, and Optical Properties of Linear Homo- and Heterometallocene Triads

The synthesis, electrochemical, and optical properties of homo- (5, 8, 9, and 12) and heterometallic (6, 7, 10, and 11) ferrocene-ruthenocene triads, are presented. Triferrocenyl derivatives 5 and 9 form the mixed-valence species 5*+ and 92+ by partial oxidation, which show intramolecular electro-transfer phenomena. Interestingly, spectroelectrochemical studies of compound 11, bearing two peripheral ferrocene units and one central ruthenocene moiety, revealed the presence of low-energy bands in the near-infrared (NIR) region, which indicate a rather unusual intramolecular charge-transfer between the ferrocene and ruthenocene units. The value of the electronic coupling parameter V(ab) = 150 cm(-1) calculated by deconvolution of the observed Fe(II)-Fe(III) IVCT transition in the mixed-valence compound 11*+, (d(Fe(II)-Fe(III)) = 18.617 A), indicates the ability of the ruthenocene system to promote a long distance intervalence electron-transfer. Moreover, the reported triads show selective cation sensing properties. Triads 5, 9, and 11 behave as dual redox and optical chemosensors for Zn(2+), Hg(2+), and Pb(2+). Their oxidation redox peaks are anodically shifted (up to 130 mV), and their low-energy (LE) bands of the absorption spectra are red-shifted (up to 115 nm) upon complexation with these metal cations. These changes in the absorption spectra are accompanied by dramatic color changes which allow the potential for "naked eye" detection.

Synthesis of Molecular Motors Incorporating para‐Phenylene‐Conjugated or Bicyclo[2.2.2]octane‐Insulated Electroactive Groups

The insulating role of the bicyclo[2.2.2]octane fragment has been theoretically evaluated by comparing the electronic coupling parameter (V(ab)) in 1,4-bis(ferrocenyl)benzene (1) and 1,4-bis(ferrocenyl)bicyclo[2.2.2]octane (2). The geometries were optimized by DFT and an extended Hückel calculation was performed to evaluate V(ab) by the dimer splitting method. The calculations showed a 12-fold decrease of the electronic coupling from 60 meV for 1 to 5 meV for 2. The second part describes the synthesis of two potential molecular motors with one incorporating the insulating bicyclo[2.2.2]octane fragment. These molecules are based on a ruthenium complex bearing a tripodal stator functionalized to be anchored onto surfaces. The ferrocenyl electroactive groups and the cyclopentadienyl (Cp) rotor are connected through a p-phenylene spacer (5) or through a spacer incorporating an insulating bicyclo[2.2.2]octane moiety (6).

Electronic and Magnetic Communication in Mixed-Valent and Homovalent Ruthenium Complexes Containing Phenylcyanamide Type Bridging Ligands

Four new phenylcyanamido-bridge dinuclear ruthenium complexes [{Ru(tpy)(thd)}2(mu-L)] with tpy = 2,2':6',2' '-terpyridine, thd = 2,2,6,6-tetramethyl-3,5-heptanedione and L = dcbp = 4,4'-dicyanamidobiphenyl; bcpa = bis(4-cyanamidophenyl)acetylene; bcpda = bis(4-cyanamidophenyl)diacetylene; bcpea = 9,10,-bis(4-cyanamidophenylethynyl)anthracene have been prepared and fully characterized. The mixed valent Ru(II)Ru(III) and homovalent paramagnetic Ru(III)Ru(III) forms of all the complexes were electrochemically generated and studied by UV-vis-NIR and EPR spectroscopy. Electronic communication was quantified by the electronic coupling parameter V(ab) extracted from intervalence measurements in the near IR area, and magnetic communication was quantified in terms of the exchange coupling constant J, accessible from the intensity of the EPR signal when varying the temperature. Exponential decays for both electronic and magnetic coupling versus intermetallic distance were obtained and discussed.

Studies of Electronic Coupling and Mixed Valency in Metal−Metal Quadruply Bonded Complexes Linked by Dicarboxylate and Closely Related Ligands

Complexes of the form [(tBuCO2)3M2]2(mu-O2C-X-CO2), where M is Mo or W and X is a pi conjugated organic group, are ideally suited for studies of electronic coupling between the two redox centers via M2 delta-bridge pi conjugation. The complexes have intense metal-to-bridge charge-transfer transitions in the visible or near-IR region of the spectrum and exhibit thermo-, solvato- and electrochromic behavior. Chemical oxidation results in the formation of mixed-valence species that are particularly well-suited for the study of the class II/III border. The extent of electronic coupling is determined by a variety of spectroscopic techniques and, in particular, by EPR and electronic absorption spectroscopy. The latter provides a direct measure of the electronic coupling parameter Hab in pairs (Mo and W) of otherwise identical complexes. Similarly, the substitution within the bridge of the CO2 group by COS or RNCO allows an evaluation of the mechanism of the electronic coupling in closely related complexes. Electronic structure calculations employing density functional theory complement frontier molecular orbital theory in the interpretation of the physicochemical properties of these complexes.

Observation and dynamics of “mixed-valence isomers” and a thermodynamic estimate of electronic coupling parameters

The thermodynamics of mixed-valence isomers – the two limiting charge distributions in a mixed-valence complex – are reviewed and used as a basis for calculating the electronic coupling ( H AB). Infrared spectroscopy is used to determine equilibrium constants between the two mixed-valence isomers in the strongly coupled adiabatic limit. Equilibrium constants in the diabatic ( H AB = 0) limit are independently estimated from electrochemical data for corresponding symmetric complexes. Analysis of these data provide direct measurements of Δ Δ G 0 = Δ G 0 0 - Δ G 1 0 , the difference in thermodynamic driving force for electron transfer in the diabatic and strongly coupled adiabatic limits which it is shown can be interpreted within the framework of appropriate potential energy surfaces for ET to estimate H AB.

Electron delocalization in mixed-valence butadienediyl-bridged diruthenium complexes

We report electrochemical and spectroelectrochemical investigations on the butadienediyl-bridged diruthenium complexes [{Ru(PPh3)2(CO)Cl}2(μ-C4H4)] (1), [{Ru(PEt3)3(CO)Cl}2(μ-C4H4)] (2), and [{Ru(PPh3)2(CO)Cl(NC5H4COOEt-4)}2(μ-C4H4)] (3). All these complexes are oxidized in two consecutive one-electron steps separated by 315 to 680 mV, depending on the co-ligands. The first oxidation is a chemically and electrochemically reversible process whereas the second varies from nearly reversible to irreversible at room temperature. We have generated and investigated the mixed-valence monocations and observed CO band shifts of ca 25 cm−1 and the appearance of new bands in the visible regime at ca 720 to 800 and 430 to 450 nm. The lower-energy band which tails into the near infrared has been assigned as a charge-resonance (or intervalence charge-transfer) absorption and used to estimate the electronic coupling parameter H AB. Our investigations point to valence delocalization for 2 + , and nearly delocalized behavior for 1 + and 3 + . Even the complex with the smallest potential splitting is, however, fully delocalized on the longer ESR timescale, as is evident from the coupling pattern of the solution spectrum. Overall IR band shifts on full oxidation and the hyperfine splittings for 1 + argue for charge and spin delocalization onto the bridging C4H4 ligand. This issue has also been addressed by quantum chemical calculations employing DFT methods. Geometry optimizations at each oxidation level reveal inversion of the C–C bond pattern from a short–long–short to a long–short–long alteration and a bis(carbenic) structure at the dication stage. All spectroscopic features such as IR band shifts, average g-values and g-tensor anisotropies are fully reproduced by the calculations.

Electron Transfer and Electron Exchange between [Cp*(dppe)Fe]n+(n= 0, 1) Building Blocks Mediated by the 9,10-Bis(ethynyl)anthracene Bridge

A novel bis(iron) alkynyl-bridged complex, Cp*(dppe)Fe(C⋮C-9,10-ant-C⋮C)Fe(dppe)Cp*, 10 (ant = anthracene), and its oxidized forms, 10·PF6, 10·TCNQ, and 10·2PF6, were synthesized and characterized by X-ray crystal structures. The cyclic voltammogram of 10 shows two well-reversible redox couples at −0.40 and −0.04 V (vs SCE) and a third redox process close to the solvent edge. Density-functional theory (DFT) calculations carried out on the substituted model complex (η5-C5H5)(η2-dpe)Fe−C⋮C-9,10-ant-C⋮C−Fe(η2-dpe)(η5-C5H5), 10-H (dpe = H2P−(CH2)2−PH2), suggest that the HOMO, which is depopulated upon oxidation, has a dominant anthracene character. The 1H NMR and the magnetic susceptibility measurements indicate that the complex 10·2PF6 is diamagnetic, in contrast with that of its congeners of the bis(iron) series, which do not contain the anthracene fragment in the bridge. Experimental measurements and DFT calculations reveal a large energy gap between the singlet ground state and the triplet excited state (ΔEST(exp) < −1200 cm-1). Mössbauer spectroscopy reveals that the two iron centers are spectroscopically equivalent for the three oxidation states. The parameters found for 10·2PF6 are not typical of an iron(III) center, but rather characteristic of iron(II). The spectrum of 10·TCNQ is in accord with a detrapped mixed-valence (MV) electronic structure with a large contribution of the delocalized unpaired electron on the carbon bridge. In addition, Mössbauer and UV−vis data and DFT calculations indicate also that the electronic structure of the MV complex is much closer to the dication 10·2PF6 than the parent complex 10. The analysis of the NIR absorption band allows the determination of a large electronic coupling parameter, as expected for a class III MV complex (Vab = 2180 cm-1). Taken as a whole, the experimental and theoretical data emphasize the specific role of the anthracene fragment inserted in the carbon bridge, which allows good electronic communication between the iron centers, but significantly contributes to the displacement of the spin density from the metal centers onto the α and β sp carbon atoms in the vicinity of the metal and the ipso carbon of the anthracene.