Electron transfer rates in bridged molecular systems: A phenomenological approach to relaxation

A model for bridge-assisted, long-range electron transfer in a molecule interacting with a dissipative external bath is presented. The effects of the system-bath interaction are included phenomenologically in the evolution of the system density matrix as energy dephasings on the bridge sites. When the bridge dephasings are small, the steady state ET rate in this model is found to be the sum of two competing terms; the first is a McConnell-type rate arising from direct tunneling from donor to acceptor, and the second is a dephasing-dependent, length-independent scattering channel through the bridge sites. In the limit of large dephasings, an incoherent channel dominates the dynamics and leads to ET rates that can become only weakly dependent (k_ET ∝ 1/N) on the number of bridge sites in the system, for multisite bridges.