Anharmonic effects on photo-induced electron transfer: A Redfield approach

Abstract

Photo-induced electron transfer experiments examine intrinsically nonequilibrium processes. A theoretical description of photoinduced processes should take into account the fact that the approximations and assumptions made for equilibrium electron transfer need not be appropriate. Under nonequilibrium conditions, anharmonic distortions in the potential energy surfaces of nuclear motion coupled to the electron transfer may effect the dynamics. This work is a study of the effects of anharmonicity on photo-induced electron transfer reactions for condensed phase systems where one vibrational mode is strongly coupled to the electron transfer and a stochastic bath. For this vibrational mode, both harmonic and anharmonic potential energy surfaces for the excited states are considered and the electron transfer dynamics is monitored in a range of system–bath coupling regimes. The study focuses on two effects due to anharmonic distortions of the intramolecular modes: changes to the system Hamiltonian, and differences in the dephasing processes caused by the anharmonic distortions. These calculations show that for small differences in the donor and acceptor state energies, the effects of vibrational anharmonicity is very small. However, when this energy difference is large, the dynamics for anharmonic and harmonic modes is significant. The relative role played by the competing physical processes is easily understood by examining the vibronic state populations obtained using a multistate Redfield approach.