Activated electron transfer at zero reorganization energy induced by a fluctuating donor–acceptor coupling

We study analytically and numerically a rate model for donor-to-acceptor charge transfer in the case of a fluctuating donor–acceptor coupling and of zero reorganization energy. The model applies to situations where the donor and acceptor reorganization energies are very low such that there is no polaron formation and the donor-to-acceptor transition arises purely from dynamic disorder in the coupling. We consider both quantum and classical limits for the reaction coordinate that modulates the coupling and describe the adiabaticity parameter for charge transfer induced by coupling fluctuations, analogous to the Landau–Zener parameter used in the case of transfer induced by energy-level fluctuations. Our purpose is to explore the magnitudes of the charge-transfer rate for realistic parameter values, the transition from non-adiabatic to adiabatic transfer, and the behavior of the rate as a function of the donor–acceptor energy gap and temperature. We find that the coupling-fluctuation mechanism can lead to fast rates. Furthermore, the energy-gap and temperature dependencies can differ from Marcus theory describing polaron transfer. Therefore, these dependencies may be used to identify experimentally whether charge transfer is induced by coupling fluctuations.