Dynamics of electron transfer for a nonsuperexchange coherent mechanism. 1

In addition to mechanisms such as superexchange and a chemical intermediate mechanism for electron transfer from a donor D* to an acceptor A via a molecular bridge B, a third possibility occurs when the BA electronic coupling is very strong and the D*B and D+B- states have energies moderately close to each other. This mechanism is discussed here. Like superexchange, it is a coherent one, in contrast to the chemical intermediate mechanism, where the transfer is sequential and incoherent. The dynamics of the mechanism are discussed, particularly the maximum population of "B-" and the question of whether an effective rate constant for its disappearance can be considerably larger than the maximum adiabatic rate constant. There are, as yet, no experimental data on the mechanism, though the synthesis of suitable D*BA's may permit its observation. In the treatment three collective nuclear coordinates are introduced, permitting independent reorganization energies for each reactive center. With certain approximations, namely, equal vibration frequencies and a nonadiabatic first step, the problem is reduced analytically to a one-coordinate one, which can be readily treated numerically. One rough but simple analytical result for the latter is also given