Electric field reverse electron transfer

The net result of a photochemical redox reaction often gives very little information on the quantum yield of the primary electron transfer reaction since this is in many cases compensated by reverse electron transfer between the primary reaction products. This is equally so in homogeneous as well as in heterogeneous reactions. While the reverse process in homogeneous reactions can only by suppressed by consecutive irreversible chemical steps, one has a chance of preventing the reverse reaction in heterogeneous electron transfer processes by applying suitable electric fields. We shall see that this can best be done with semiconductor or insulator electrodes and that there it is possible to study photochemical primary processes with the help of such electrochemical techniques 5-G>7>. 

Shinkarev, V.P., Crofts, A.R., and Wraight, C.A. (2001) The electric field generated by photosynthetic reaction center induced rapid reversed electron transfer in the bcl complex, Biochemistry 40, 12584-12590. 

As = surface area of a semiconductor contact [A ] = concentration of the reduced form of a redox couple in solution [A] = concentration of the oxidized form of a redox couple in solution A" = effective Richardson constant (A/A ) = electrochemical potential of a solution cb = energy of the conduction band edge Ep = Fermi level EF,m = Fermi level of a metal f,sc = Fermi level of a semiconductor SjA/A") = redox potential of a solution ° (A/A ) = formal redox potential of a solution = electric field max = maximum electric field at a semiconductor interface e = number of electrons transferred per molecule oxidized or reduced F = Faraday constant / = current /o = exchange current k = Boltzmann constant = intrinsic rate constant for electron transfer at a semiconductor/liquid interface k = forward electron transfer rate constant = reverse electron transfer rate constant = concentration of donor atoms in an n-type semiconductor NHE = normal hydrogen electrode n = electron concentration b = electron concentration in the bulk of a semiconductor ... 

PULSED ELECTRIC FIELD INDUCED REVERSE ELECTRON TRANSFER FROM GROUND STATE BChl2 TO THE CYTOCHROME c HEMES IN Rps. viridis... 

In both cases the electron transfer process took place on a sub millisecond time scale. Removal of the voltage resulted in rapid return to the starting state. This is the first time that reverse electron transfer has been activated by voltages applied directly from an external electric source. This demonstration shows the feasibility of electric field induced reverse electron transfer, and holds promise as an unusually flexible alternative for pulse activation of ground state electron transfer. 

For collision frequencies large compared with the frequency of the electric field, the current remains in phase with the electric field in the reverse case, the current is 90° out of phase. The in-phase component of the current gives rise to an energy loss from the field (Joule heating loss) microscopically, this is seen to be due to the energy transferred from the electrons to the atoms upon collision. 

The electrochemical potential fif (Guggenheim, 1929) ofachargedparticlej inphaseaistheworkthatmust be done in reversible isothermal transfer of one particle (one mole of particles in some texts) from a field-free vacuum to the interior of the phase. It is the sum of a chemical and an electrical component jXf = -t, where pf is the chemical potential of i in phase a and Zi is the algebraic number of electronic charges q on i. For an electron, z, = -1. 

When a metal, M, is immersed in a solution containing its ions, M, several reactions may occur. The metal atoms may lose electrons (oxidation reaction) to become metaUic ions, or the metal ions in solution may gain electrons (reduction reaction) to become soHd metal atoms. The equihbrium conditions across the metal-solution interface controls which reaction, if any, will take place. When the metal is immersed in the electrolyte, electrons wiU be transferred across the interface until the electrochemical potentials or chemical potentials (Gibbs ffee-energies) on both sides of the interface are balanced, that is, Absolution electrode Until thermodynamic equihbrium is reached. The charge transfer rate at the electrode-electrolyte interface depends on the electric field across the interface and on the chemical potential gradient. At equihbrium, the net current is zero and the rates of the oxidation and reduction reactions become equal. The potential when the electrode is at equilibrium is known as the reversible half-ceU potential or equihbrium potential, Ceq. The net equivalent current that flows across the interface per unit surface area when there is no external current source is known as the exchange current density, f. 

An attractive approach to solving the mass transfer limitations of these investigations is to immobilize the electroactive species at the electrode surface within a monomolecular film. Clearly, if the electroactive species is immobilized at the electrode surface, diffusion of the species to the electrode does not need to occur prior to electron transfer. In addition, immobilization at an electrode surface can preconcentrate the species of interest, resulting in higher currents that are easier to detect. Electroactive adsorbed monolayers have been developed that exhibit close to ideal reversible electrochemical behavior under a wide variety of experimental conditions of timescale, temperature, solvent, and electrolyte. These studies have elucidated the effects of electron transfer distance, tunneling medium, molecular structure, electric fields, and ion pairing on heterogeneous electron transfer dynamics. 

Abstract Faradaic electron transfer in reverse microemulsions of water, AOT, and toluene is strongly influenced by cosurfactants such as primary amides. Cosurfactant concentration, as a field variable, drives redox electron transfer processes from a low-flux to a high-flux state. Thresholds in this electron-transport phenomenon correlate with percolation thresholds in electrical conductivity in the same microemulsions and are inversely proportional to the interfacial activity of the cosurfactants. The critical exponents derived from the scaling analyses of low-frequency conductivity and dielectric spectra suggest that this percolation is close to static percolation limits, implying that percolative transport is along the extended fractal clusters of swollen micellar droplets. and NMR spectra show that surfactant packing... 

Two features of this definition are worth noting. One is that EPH is defined as the heat of a reversible reaction, which essentially eliminates the various uncertainties arising from the irreversible factors such as overvoltage. Joule heat, thermal conductivity, concentration gradient and forced transfer of various particles like ions and electrons in electrical field, and makes the physical quantity more definite and comparable. This indicates that EPH is a characteristic measure of a cell reaction, because the term 8 (AG)/8T) p is an amoimt independent on reaction process, and only related to changes in the function of state. That is to say, EPH is determined only by the initial and the final states of the substances taking part in the reaction that occurs on the electrode-electrolyte interfaces, although other heats due to irreversible factors are accompanied. EPH is, unlike the heat of dissipation (Joule heat and the heats due to irreversibility of electrode processes and transfer processes), one of the fundamental characteristics of the electrode process. 

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