Electron transfer superexchange mechanism

The reader may desire an explanation of the low values of y derived by Barton and coworkers [30, 131, 137-140] from fluorescence quenching data for systems in which the dynamics of electron transfer have not been directly measured. In most cases, the absolute efficiency (quantum yield) of the quenching processes studied in these systems is rather low, and thus they may represent long-range electron transfer by mechanisms other than superexchange, such as to those described by Felts et al. [125], Davis and colleagues [126], and Okada et al. [127]. However, the author considers that it is highly unlikely that such processes occur with rate constants > 10 s . In view of the complex nature of these systems, the author is loath to offer a detailed interpretation, and refers the reader to commentaries by others who have been directly involved in this research [13, 15]. 

Fig. 2b. Both kcs and kcr are seen to decrease as the distance R between Sa and G C increases. In accord with a superexchange mechanism for photoin-duced electron transfer, the distance dependence can be described by Eq. (2) ... 

It is useful to introduce a nomenclature for distinguishing between ET occurring by the conduction and superexchange mechanisms. The term electron transport is used in the context of molecular wire behavior, while electron transfer is used in the context of the superexchange mechanism. 

Figure 15. A schematic illustrating the difference between the superexchange mechanism and molecular wire behavior in a D-B-A dyad. Superexchange the bridge states lie above the D level consequently the electron is transferred in one coherent jump and is never localized within the bridge. The distance dependence behavior is exponential decay.

In this last interpretation, B could promote the electron transfer through a superexchange mechanism involving the (P B H) charge-transfer state. A variant of this model involves an internal charge-transfer state of the Bchl dimer [172]. 

The general framework of the quantum mechanical rate expression for long-range electron transfer processes in the very weak or non-adiabatic regime will be presented in Sect. 2 with an emphasis on the inclusion of superexchange interactions. The relation between the simplest case of direct donor-acceptor interactions, on the one hand, and long-range electronic interactions important in proteins, on the other, is considered in terms of the elements of electron transfer theory. 

Electron transfer through a molecular bridge can occur by single or multiple-step mechanisms [5-7]. The multiple steps may involve real (hopping) or virtual (superexchange) bridging states. Single electron transfer steps... 

Superexchange is another mechanism of electron transfer over relatively large distances in which the solvent or matrix acts as a bridge between the donor molecule D and the acceptor molecule A. It differs from electron hopping in that the electron is at no time actually localized on a molecule of the medium there is an interaction between the orbitals of the molecules A, B and D which form a sort of very loose supermolecule over which the electron is delocalized (Figure 4.10). This mechanism seems plausible when the relevant orbitals of A, B and D are rather close in energy. This is similar to the requirement for the interaction of atomic orbitals to form a molecule. 

When the donors and acceptors lack spherical symmetry, there will also be an orientation dependence. In cases such as those to be discussed below, where the donor and acceptor moieties are linked by covalent bonds, there is considerable evidence that in certain situations the electron transfer occurs through the linkage bonds [22]. Although such linkages are not present in photosynthetic reaction centers, it has been proposed that the accessory Bchl or other intervening material may still take part in electron transfer through a superexchange mechanism [8, 26]. The distance dependence of photoinitiated electron transfer has recently been reviewed [13]. 

Fig. 3.3 Schematic representation of orbital (upper) and state (lower) energy diagrams of superexchange (left) and hopping (right) mechanisms of photoinduced electron transfer in DBA...

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