Variable-range hopping electron transfer through disordered bridge states: Application to DNA

A theory for electron transfer through a donor-bridge-acceptor system is described that involves tunneling and hopping-like transfers and an intermediate regime. The theory considers how a delocalization of electronic states and static and dynamic disorder in electronic energies influence the charge transfer rate and is used to study experiments on hole transfer through DNA. While an exponential distance dependence of the yield of hole trapping is observed experimentally for small bridges, the yield for long bridges is reported to be almost distance-independent. For long bridge lengths, for which thermally activating hopping dominates over tunneling, the model considers two competing channels, a hopping via localized states and a transfer through partly delocalized states. The variable-range hopping mechanism and the delocalized states aspect of the theory are used to interpret the flat rather than a slow decrease of yield with increasing distance reported in experiments with long bridges.