Electron transfer in three-center chemical systems

Electron transfer is examined in three redox center (donor-bridge-acceptor) systems. Here, electron transfer cannot be described as arising from one-d imensional diffusional motion along the reaction coordinate. Instead, two-d imensional diffusion over two statistically independent reaction coordinate s emerges. We show that electron transfer in three-center systems can be re duced to electron transfer between the pairs of adjacent centers. The inter play between sequential and superexchange mechanisms is examined. New expre ssions for the electron transfer rate, including effects of the two-dimensi onal reaction coordinate and of solvent dynamics, are derived. Using this a nalysis, new driving force effects are predicted, and rich behavior is reve aled. (C) 1999 American Institute of Physics. [S0021-9606(99)52020-5].