Self-exchange Electron Transfer Research Articles

Quantum Chemical Calculations of Reorganization Energy for Self-Exchange Electron and Hole Transfers of Aromatic Diamines as Electron-donor Molecules

ABSTRACTReorganization energy is one of the important factors to decide the rate of electron transfer according to the Marcus theory. Small reorganization energy is highly desirable in design of optoelectronic and electronic devices like as organic light emitting diode. For this reason, reorganization energy of aromatic diamine derivatives, N,N′-diphenyl-N,N′-bis(3-methylphenyl)- (1,1′-biphenyl)-4,4′-diamine (TPD) and 4,4′-diphenyl-N,N,N′,N′-tetraphenylbenzidine (DTPB) have been studied theoretically by self-exchange electron transfer theory. By executing the Gaussian 03 calculation we can easily figure out the optimization point which needed for calculation of the inner reorganization energy (λ) of self-exchange electron transfer reaction. Also ionization potential and electron affinities of these molecules can be calculated at the density functional theory level with basis set 6-31G** and 6-31G* using Gaussian 03 software on the basis of ab initio method. It gives possibility to develop a semi-empirical model for the observed absorption and photoluminescence spectrum.

Medium Effects on the Nucleation and Growth Mechanisms during the Redox Switching Dynamics of Conducting Polymers: Case of Poly(3,4-ethylenedioxythiophene)

The redox switching dynamics of poly(3,4-ethylenedioxythiophene) (PEDOT) in an acetonitrile solution and a room temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmiTFSI), are investigated by means of potential step experiments. Redox switching can be viewed as a phase transition in which the nucleation and growth processes occur. We have developed a phenomenological model allowing the determination of the kinetic parameters. Two limiting cases are shown as follows: (i) a progressive and (ii) an instantaneous nucleation. In all cases, the growth process is described in terms of a self-exchange electron transfer reaction. We show that the mechanisms depend upon the medium. In acetonitrile, progressive nucleation and growth occur during oxidation (p-doping), whereas nucleation is instantaneous in the reduction of the PEDOT film. On the other hand, instantaneous nucleation and growth mechanisms are observed for both oxidation and reduction in EmiTFSI. The difference in the mechanisms results from the ionic exchange process associated with electron transfer and the initial structure of the film (open or compact). The influence of the applied potential on the dynamics is analyzed for both media.

Molecular factor analysis in self-exchange electron transfer reactions in solution

We present a systematic method to access molecular factors and rate constants for self-exchange electron transfer reactions in solution using partial least squares and a number of chosen descriptors. In this work it is shown that the log response for the set of reactions considered can be mostly described in terms of the Intersecting State Model (ISM) when the nonadiabaticity of transfers involving electrons in σ*-orbitals is considered.

Nonequilibrium solvation energy by means of constrained equilibrium thermodynamics and its application to self-exchange electron transfer reactions

This work presents a self-consistent thermodynamic approach to nonequilibrium solvation energy. By imposing an extra electric field onto the nonequilibrium solvation system, a constrained equilibrium state is prepared. New expressions of nonequilibrium solvation energy and solvent reorganization energy have been formulated. The numerical algorithm combining the new formulation with the dielectric polarizable continuum model has been implemented. As an application, self-exchange electron transfer (ET) reactions between tetramethylhydrazine, tetraethylhydrazine, and tetrapropylhydrazine and their corresponding radical cations have been investigated. The inner and solvent reorganization energies are calculated by the "four-point" method and the new method for nonequilibrium solvation, respectively. Besides, we also calculated the electronic coupling matrix. The rate constants for the three self-exchange ET reactions correlate well with experimental results. We have shown that the inner reorganization energies of these self-exchange ET are not very sensitive to compound size while the compound size has some effect on the solvent reorganization energy in acetonitrile. The new method for nonequilibrium solvation energy based on continuum model provides a reasonable result for the solvent reorganization energy.

Three-Coordinate Copper(I) Amido and Aminyl Radical Complexes

A three-coordinate Cu-NR(2) system (R = p-tolyl) supported by the anionic bis(phosphino)borate ligand [Ph(2)B(CH(2)P(t)Bu(2))(2)](-) has been isolated and structurally characterized in both its anionic Cu(I) and neutral (formally) Cu(II) oxidation states. A large rate constant for the self-exchange electron-transfer reaction (k(S) >or= 10(7) M(-1) s(-1)) makes this system a functional model for the type-1 active sites in blue copper proteins. Multiedge X-ray absorption spectroscopy, multifrequency electron paramagnetic resonance, and density functional theory analyses collectively indicate that the oxidized form is best regarded as a Cu(I)-aminyl radical complex rather than a Cu(II)-amido species, with about 70% localization of the unpaired electron on the NR(2) unit. Hydrogen-atom transfer and C-C coupling reactions are presented as examples of chemical reactivity manifested by this unusual electronic structure.

Nonequilibrium Thermodynamics Formalism for Marcus Theory of Heterogeneous and Self-Exchange Electron-Transfer Rate Constants

The cross-exchange electron-transfer rate constant expression of Marcus is derived from the Flux-force formalism of non-equilibrium thermodynamics. The relationship governing the Onsager's phenomenological coefficients for cross-exchange and self-exchange electron-transfer processes is deduced. Onsager's phenomenological coefficient pertaining to the Butler-Volmer equation is derived and estimated from the experimental exchange current densities. The correlation between the heterogeneous and the homogeneous electron-transfer rate constants derived by Marcus is analyzed in terms of the corresponding phenomenological coefficients.

Quinones as Electron Acceptors. X-Ray Structures, Spectral (EPR, UV−vis) Characteristics and Electron-Transfer Reactivities of Their Reduced Anion Radicals as Separated vs Contact Ion Pairs

Successful isolation of a series of pure (crystalline) salts of labile quinone anion radicals suitable for X-ray crystallographic analysis allows for the first time their rigorous structural distinction as "separated" ion pairs (SIPs) vs "contact" ion pairs (CIPs). The quantitative evaluation of the precise changes in the geometries of these quinones (Q) upon one-electron reduction to afford the anion radical (Q-*) is viewed relative to the corresponding (two-electron) reduction to the hydroquinone (H2Q) via the Pauling bond-length/bond-order paradigm. Structural consequences between such separated and contact ion pairs as defined in the solid state with those extant in solution are explored in the context of their spectral (EPR, UV-vis) properties and isomerization of tightly bound CIPs. Moreover, the SIP/CIP dichotomy is also examined in intermolecular interactions for rapid (self-exchange) electron transfer between Q-* and Q with second-order rate constants of kET approximately equal to 10(8) M-1 s-1, together with the spectral observation of the paramagnetic intermediates [Q,Q-*]leading to 1:1 adducts, as established by X-ray crystallography.

Iron–sulfur clusters: Why iron?

This communication addresses a simple question by means of density functional calculations: Why is iron used as the metal in iron–sulfur clusters? While there may be several answers to this question, it is shown here that one feature – the well-defined inner-sphere reorganization energy of self-exchange electron transfer – is very much favored in iron–sulfur clusters as opposed to metal substituted analogues of Mn, Co, Ni, and Cu. Furthermore, the conclusion holds for both 1Fe and 2Fe type iron–sulfur clusters. The results show that only iron provides a small inner-sphere reorganization energy of 21 kJ/mol in 1Fe (rubredoxin) and 46 kJ/mol in 2Fe (ferredoxin) models, whereas other metal ions exhibit values in the range 57–135 kJ/mol (1Fe) and 94–140 kJ/mol (2Fe). This simple result provides an important, although partial, explanation why iron alone is used in this type of clusters. The results can be explained by simple orbital rules of electron transfer, which state that the occupation of anti-bonding orbitals should not change during the redox reactions. This rule immediately suggests good and poor electron carriers.

Self-exchange electron transfer in high oxidation state non-oxo metal complexes: amavadin

The electron transfer self-exchange rate constant between the oxidized and reduced forms of amavadin equals approximately 1 x 10(5) dm3 mol(-1) s(-1) at 25 degrees C and represents the first unambiguous example for a vanadium(IV/V) couple.

Homogeneous versus Heterogeneous Self-Exchange Electron Transfer Reactions of Metal Complexes: Insights from Pressure Effects

Homogeneous versus Heterogeneous Self-Exchange Electron Transfer Reactions of Metal Complexes: Insights from Pressure Effects

Aspects of Aqueous Iron and Manganese (II/III) Self-Exchange Electron Transfer Reactions

Ab initio methods were applied to the calculation of the reorganization energy λ and the electronic coupling matrix element VAB for the outer-sphere Fe(OH2)6II/III and Mn(OH2)6II/III self-exchange electron transfer (ET) reactions. For the Fe case, we find an appreciable effect on VAB depending on whether the minority spin electron occupies the dxy orbital or a mixture of dxz/dyz orbitals in the FeII ion. While these two possible nearly isoenergetic electron accepting states alter the magnitude and distance dependence of VAB, they do not affect the internal reorganization energy λI to any significant level. The magnitude and distance dependence of VAB are found to be strongly dependent on encounter orientation, as expected. VAB values for corner-to-corner encounter orientations are substantially larger at any given ET distance considered than those for face-to-face encounter orientations. Values of the decay parameter β are in good agreement with well-accepted values. The adiabaticity criterion is tied to orientation and distance dependence of VAB.

Self-Exchange Electron Transfer Kinetics and Reduction Potentials for Anthraquinone Disulfonate

An electron transfer model for self-exchange reactions of 9,10-anthraquinone-2,6-disulfonate (AQDS) in aqueous solution has been formulated by using a combination of density functional theory (DFT) calculations and Marcus theory. One-electron self-exchange reactions are predicted to be fast (log k ≈ 6−9 M-1 s-1) but not diffusion limited. The internal component of the reorganization energy makes a large contribution to the total reorganization energy and cannot be neglected. Analysis and theoretical extensions of crystal structure data led to predicted precursor complex structures that, in the end, yielded theoretical electron transfer rates in good agreement with experimental ones. Electron transfer distances in solution are predicted to be in the 7−9 Å range. Calculated values of the electronic coupling matrix element indicate that the distinction between adiabatic and nonadiabatic electron transfer in this system likely occurs in this distance range as well. A set of reduction potentials was also produced by combining the density functional theory calculations with equilibrium expressions and the known acidity constants in the AQDS system.

Reaction of hydroquinone with hematite: II. Calculated electron-transfer rates and comparison to the reductive dissolution rate

The rate of reaction of hematite with quinones and the quinone moieties of larger molecules may be an important factor in limiting the rate of reductive dissolution of hematite, especially by iron-reducing bacteria. It is possible that the rate of reductive dissolution of hematite in the presence of excess hydroquinone at pH 2.5 may be limited by the electron-transfer rate. Here, a reductive dissolution rate was measured and compared to electron-transfer rates calculated using Marcus theory. An experimental rate constant was measured at 9.5×10 −6 s −1 and the reaction order with respect to the hematite concentration was found to be 1.1. Both the dissolution rate and the reaction order of hematite concentration compare well with previous measurements. Of the Marcus theory calculations, the inner-sphere part of the reorganization energy and the electronic coupling matrix element for hydroquinone self-exchange electron transfer are calculated using ab initio methods. The second order self-exchange rate constant was calculated to be 1.3×10 7 M −1 s −1, which compares well with experimental measurements. Using previously published data calculated for hexaquairon(III)/(II), the calculated electron-transfer rate for the cross reaction with hydroquinone also compares well to experimental measurements. A hypothetical reductive dissolution rate is calculated using the first-order electron-transfer rate constant and the concentration of total adsorbed quinone. Three different models of the hematite surface are used as well as multiple estimates for the reduction potential, the surface charge, and the adsorption density of hydroquinone. No calculated dissolution rate is less than five orders of magnitude faster than the experimentally measured one.

π-Complex formation in electron-transfer reactions of porphyrins

π-Complex formation between porphyrins and their radical cations plays an important role in self-exchange electron transfer between neutral porphyrins and the radical cations, leading to negative activation enthalpies when the stabilization energy of the π-complex is larger than the activation energy for the intracomplex electron transfer in the π-complex. A number of porphyrin molecules are self-organized on three-dimensional gold nanoclusters to form monolayer-protected gold nanoclusters (MPCs) that act as an efficient photocatalyst for the uphill reduction of HV2+ by BNAH to produce 1-benzylnicotinamidinium ion (BNA+) and hexyl viologen radical cation (HV·+). Such three-dimensional architectures of porphyrin MPCs with large surface area allow supramolecular π-complexation between MPCs and HV2+, resulting in fast electron transfer from the singlet excited state of porphyrin to HV2+ on MPCs. The π-π interaction is exploited to develop efficient photovoltaic devices consisting of porphyrin and fullerene assemblies which have an enhanced light-harvesting efficiency throughout the solar spectrum together with a highly efficient conversion of the harvested light into electrical energy.

Redox photochemistry of thiouredopyrenetrisulfonate.

1-Thiouredopyrene-3,6,8-trisulfonate (TUPS) has recently been used as a photoinduced covalent redox label capable of reducing various cofactors of proteins. A new reaction of this dye, whereby its excited triplet state oxidizes suitable electron donors, is now reported. The characteristic difference spectrum of the reduced radical of TUPS is determined. We also observe the self-exchange electron transfer between two TUPS molecules in their triplet excited states and determine the reaction scheme and the rate constants of the various pathways in the process of triplet depletion. The ability of photoexcited TUPS to withdraw an electron from reduced cytochrome-c is also observed. It is thus demonstrated that TUPS is an appropriate photoinduced covalent redox label for initiating both the oxidative and reductive phases of electron transfer processes in biological macromolecules.

Slow hydrogen atom self-exchange between Os(IV) anilide and Os(III) aniline complexes: relationships with electron and proton transfer self-exchange.

Hydrogen atom, proton and electron transfer self-exchange and cross-reaction rates have been determined for reactions of Os(IV) and Os(III) aniline and anilide complexes. Addition of an H-atom to the Os(IV) anilide TpOs(NHPh)Cl(2) (Os(IV)NHPh) gives the Os(III) aniline complex TpOs(NH(2)Ph)Cl(2) (Os(III)NH(2)Ph) with a new 66 kcal mol(-1) N-H bond. Concerted transfer of H* between Os(IV)NHPh and Os(III)NH(2)Ph is remarkably slow in MeCN-d(3), with k(ex)(H*) = (3 +/- 2) x 10(-3) M(-1) s(-1) at 298 K. This hydrogen atom transfer (HAT) reaction could also be termed proton-coupled electron transfer (PCET). Related to this HAT process are two proton transfer (PT) and two electron transfer (ET) self-exchange reactions, for instance, the ET reactions Os(IV)NHPh + Os(III)NHPh(-) and Os(IV)NH(2)Ph(+) + Os(III)NH(2)Ph. All four of these PT and ET reactions are much faster (k = 10(3)-10(5) M(-1) s(-1)) than HAT self-exchange. This is the first system where all five relevant self-exchange rates related to an HAT or PCET reaction have been measured. The slowness of concerted transfer of H* between Os(IV)NHPh and Os(III)NH(2)Ph is suggested to result not from a large intrinsic barrier but rather from a large work term for formation of the precursor complex to H* transfer and/or from significantly nonadiabatic reaction dynamics. The energetics for precursor complex formation is related to the strength of the hydrogen bond between reactants. To probe this effect further, HAT cross-reactions have been performed with sterically hindered aniline/anilide complexes and nitroxyl radical species. Positioning steric bulk near the active site retards both H* and H(+) transfer. Net H* transfer is catalyzed by trace acids and bases in both self-exchange and cross reactions, by stepwise mechanisms utilizing the fast ET and PT reactions.

Fast self-exchange electron transfer and delocalization of unpaired electron between zinc porphyrin radical cations and zinc porphyrins

Self-exchange electron transfer rates between π-radical cations of zinc porphyrins and the neutral metalloporphyrins have been determined from the line-width broadening in the ESR spectra in different solvents at various temperatures. Fine tuning of the substituent on the porphyrin ring and the proper choice of the solvent have enabled us to observe negative activation enthalpies for the self-exchange electron transfer reactions. The observation of negative activation enthalpies indicates that the self-exchange electron transfer occurs via the charge-transfer π-complexes formed between zinc porphyrin radical cations and the neutral zinc porphyrins. The complete delocalization of the unpaired electron over two porphyrin moieties is observed in the radical cation of a zinc porphyrin dimer, 5,5'-bis(10,20-bis(3,5-di-tert-butylphenyl)porphyrinatozinc(II)). This is regarded as the extreme limit of the rapid self-exchange electron transfer between zinc porphyrin radical cation and the neutral form.

Studies of the solvent effects on the internal reorganization energy for electron transfer of uracil and its anion with ONIOM

In this paper, the aqueous adiabatic electron affinity (AEA) of Uracil (U) and internal reorganization energy λ i of the self-exchange electron transfer (ET) reaction between Uracil and Uracil anion radical (U − ) in aqueous solution were studied. The effect of the solvation was studied with the recently developed hybrid quantum molecular chemical method, ONIOM. In all calculations, the geometrical optimization for U and U − was performed at B3LYP/6-31++G(d) level. As for the solvent surroundings, the seven water molecules as the first hydration shell were adopted and treated with B3LYP, PM3 and AMBER methods, namely, ONIOM (B3LYP:B3LYP), ONIOM (B3LYP:PM3) and ONIOM (B3LYP:Amber) methods, respectively. The values of AEA for Uracil, predicted by the above three methods, are small positive ones. The geometrical differences between neutral and anion radical molecules of U originate mainly from those of dihedral angles. According to the corresponding dipole moment values, the excess electron in U − should be trapped dominantly by dipole-bound way. The calculated λ i values by ONIOM (B3LYP:B3LYP) and ONIOM (B3LYP:PM3) are close to each other within 0.89%. The λ i value from ONIOM (B3LYP:Amber) is in agreement with the one from SCRF-CPCM very well. Finally, the calculation results of the detailed geometries and molecular interaction mode effect of U and U − and related water molecules in the hydration shell were discussed.

Internal Reorganization Energy Contributed by Torsional Motion in Electron Transfer Reaction between Biphenyl and Biphenyl Anion Radical

AbstractConcerning the theoretical estimation of internal reorganization energy contributed by the tortional motion between biphenyl and biphenyl anion radical, direct calculation of self‐exchange electron transfer reaction was investigated. With the introduction of a proper average bond length and angle parameters (bond Bp), a multiple step relaxation Nelson method was developed to deal with the torsional reorganization energy. Based on the above model, an estimation of pure torsional reorganization energy λt.p with an approximation of λt.1 was achieved. The results of 0.140 and 0.125 eV of torsional reorganization energy for a cross‐reaction at the levels of 4‐31G and HP/DZP, respectively, are in good agreement with the value of 0.13 eV obtained by Miller et al. from the rate measurements. This implies the efficiency and validity of our method to estimate the reorganization energy contributed by pure torsional motion of Bp.

High sensitivity of the fluorine NMR signals of difluorovinyl analogs of natural hemin: reconstituted heme proteins and self‐exchange electron transfer in model compounds

AbstractThe chemical shifts of fluorine in difluorovinyl deuteroporphyrin iron complexes were shown to be very sensitive to the spin state of the metal and the nature of the ligand(s). Reconstituted myoglobin was used as a model heme protein with an exchangeable heme. Large variations in the fluorine chemical shifts in both the ferric and ferrous states were observed. This strong sensitivity to the nature of the metal ligand and the structural resemblance to natural hemin make this fluorinated porphyrin a good probe for the study of heme proteins. The large variations of chemical shifts depending on the oxidation state also permitted the measurement of the electron self‐exchange rate constants of bis(1‐methylimidazole)iron complexes in various solvents by analysis of line broadening of the 19F NMR signals. The experimental rate constants were strongly affected by the nature of the solvent, varying from 3.9 × 107 to 24.1 × 108 mol l−1 s−1 for DMSO‐d6 and acetone‐d6, respectively. The solvent parameters were used to estimate the outer‐sphere reorganization energies. The experimental rate constants in chloroform and in DMSO are in good agreement with these calculated outer‐sphere reorganization energies. Copyright © 2001 John Wiley & Sons, Ltd.