Thermal effects in the ultrafast photoinduced electron transfer from a molecular donor anchored to a semiconductor acceptor

A nonadiabatic molecular dynamics (MD) simulation of the photoin- duced electron transfer (ET) from a molecular electron donor to the TiO 2 semicon- ductor acceptor is carried out in a microcanonical ensemble with an average tem- perature of 350 K. The electronic structure of the dye-semiconductor system and the adiabatic dynamics are simulated by ab initio MD, while the nonadiabatic (NA) effects are incorporated by a quantum-classical mean-field approach. The ET dy- namics are driven by thermal fluctuations that dominate ionic motions at the simu- lated temperature. The ground and excited state ion dynamics are similar; therefore, the change in the quantum force due to the electronic photoexcitation can be neglected, and the final analysis is greatly simplified. The simulated ET occurs on a 5-fs timescale, in agreement with recent ultrafast experimental data. Vibrational motions of the chromophore ring carbons induce an oscillation of the photoexcited state energy, resulting in a bimodal distribution of the initial conditions for ET. At low energies the photoexcited state is localized primarily on the chromophore, while at high energies the photoexcited state is substantially delocalized into the first 3 surface layers of the TiO 2 surface. Thermally driven adiabatic transfer is the domi- nant ET mechanism. Compared to the earlier simulation at 50 K, the rate of NA transfer at 350 K remains almost unchanged, whereas the rate of adiabatic ET increases substantially.