Driving force for electron transfer

Similarly, the reaction of photoexcited 9,10-dicyanoanthracene (DCA) with a benzylstannane yields the contact ion pair in which the cation radical undergoes rapid mesolytic cleavage of the C—Sn bond to afford benzyl radical and tributyltin cation (which then adds to DCA- )77 (Scheme 14). When such unimolecular processes are faster than the energy-wasting back electron transfer (/cbet) within the contact ion pair, the D/A reactions occur rapidly despite unfavorable driving forces for electron transfer. 

Figure 16. Relationship between the activation jree energy and the driving force for electron transfer for alkylmetals to TCNE (left) and IrCl6z (right) according to Marcus Equation 4.

Electrode reactions are inner-sphere reactions because they involve adsorption on electrode surfaces. The electrode can act as an electron source (cathode) or an electron sink (anode). A complete electrochemical cell consists of two electrode reactions. Reactants are oxidized at the anode and reduced at the cathode. Each individual reaction is called a half cell reaction. The driving force for electron transfer across an electrochemical cell is the Gibbs free energy difference between the two half cell reactions. The Gibbs free energy difference is defined below in terms of electrode potential,... 

The practical impact of such considerations is that the reversible potential of a mediated biocatalytic electrode is a mixed potential dominated by the mediator couple. By extension, the open-circuit potential of a biofuel cell comprising two such electrodes is primarily determined by the difference in redox potential of the two mediator couples. The difference in redox potential between the mediator and the consumed reactant represents a driving force for electron transfer and therefore must be nonzero. As... 

Energy barrier between the injection energy and the barrier height Driving force for electron transfer Electron transfer... 

The driving forces for electron transfer are a high-energy level of the highest occupied molecular orbital or a steric strain of the starting molecule. Complexation of oxygen by the electron-rich organic molecule has often been indicated as the first step of the mechanism. 

A higher driving force for electron transfer in the case of 254+ compared to 244+ could explain the predominance of the electron transfer reaction with respect to energy transfer paths in the latter case. 

No activation (energy) barrier separates the donor and the acceptor from the ET products (and vice versa). The electron transfer in Scheme 18 is not a kinetic process, but is dependent on the thermodynamics, whereby electron redistribution is concurrent with complex formation. Accordingly, the rate-limiting activation barrier is simply given by the sum of the energy gain from complex formation and the driving force for electron transfer, i.e. ... 

The overall driving forces for electron transfer [— AG = (E + is ,)] for ion-pair annihilation in THF between mo and feL+ (i.e., cations 1, 2, 3, and 4) are AG — 3.2, 3.9, 5.3, and 6.0 kcal/mol, respectively, based on the electrochemical measurements (115). Such driving forces all easily lie within the isoergonic bounds for the facile electron transfer between feL+ and mo-. Moreover, the differences in driving forces are not sufficient to strongly distinguish the cations 1 and 2 from their cyclic analogs 3 and 4 for the consideration of simultaneous nucleophilic addition and electron transfer, as presented in possibility (c) above. 

Application of Hush theory to the observed IPCT bands yielded information about the relationship between optical and thermal ET in these systems. The redox potentials of both the metal dithiolene donors and the viologen acceptors can be systematically varied, which, in turn, tunes the thermodynamic driving force for electron transfer. The researchers found that the IPCT band energy increases linearly with more positive free energy AG for ET, and that the reorganization energy (x) remains constant with variation in the metal or cation redox potentials (66, 67). 

Driving force for electron transfer) Term widely used to indicate the negative of the standard Gibbs energy change (AG / for (photoinduced) outer-sphere electron transfer. 

Light absorption modifies the driving force for electron transfer processes in all kinds of materials. As photoactivated species are always better oxidants and reductants than their ground state equivalents, an enhanced redox reactivity is usually observed in the excited state. Photoreactions are therefore ideally suited to trigger, study, and mimic bioinorganic electron transfer. 

Electron transfer as described by Eq. 1 may occur in three regimes. In the normal region, increasing thermodynamic driving force for electron transfer (as AG° becomes more negative) leads to more rapid electron transfer, as would be expected... 

A simple way to model this effect is to treat the solvent as a dielectric continuum. This treatment often fails quantitatively, but is a useful qualitative framework in which to consider solvent effects. For example, the effect of solvent on the driving force for electron transfer, AG°, is given by Eq. 3, which is taken from the work of Weller [7]. This treatment assumes electron transfer from the first excited singlet state of a donor D to an acceptor A and... 

A similar result was obtained by Osuka and coworkers with dyad 10 and related molecules in which the driving force for electron transfer was altered by quinone substitution [79, 80], No evidence for inverted behavior in photoinduced charge separation was observed up to a driving force of 1.54 eV. In the charge recombination reaction of Pzn" -Q j a decrease in rate with increasing driving force was observed, in accord with inverted behavior in Eq. 1. 

A group of researchers from Heidelberg and Munich have studied an extensive series of cyclophane systems related to 22 [77, 86, 92, 94, 95, 123, 129-133]. Dyads with relatively low driving force for electron transfer in polar solvents (comparable to 19) show relatively slow photoinduced electron transfer in nonpolar solvents, and significantly faster transfer in polar solvents. For a few molecules with large negative AG° values, photoinduced electron transfer occurs in about 1 ps in all solvents... 

In the case of conducting substrates, the electrode provides an infinitely tunable electrochemical driving force for electron transfer. 

In electrochemical experiments, the molecular assembly is deposited on a conductive electrode surface that acts as one of the redox partners . The other partner is a redox molecule either tethered to the molecular film or free in solution. In either case, the film ideally provides a well-defined separation between the electrode and the redox probe. The electrode potential is either swept or stepped to increase the driving force for electron transfer. The chief advantage of the electrochemical elec-... 

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