Nonadiabatic molecular dynamics simulation of light-induced, electron transfer from an anchored molecular electron donor to a semiconductor acceptor

A nonadiabatic molecular dynamics (MD) simulation of the photoinduced electron transfer (ET) from a molecular electron donor to the TiO2 acceptor is reported. The system under study is typical of the dye sensitized semiconductor nanomaterials used in solar cell, photocatalysis, and photoelectrolysis applications. The electronic structure of the dye-semiconductor system and the adiabatic dynamics are simulated by ab initio MID, whereas the nonadiabatic effects are incorporated by the quantum-classical mean-field approach. A novel procedure separating the nonadiabatic and adiabatic ET pathways is proposed. The simulation provides a detailed picture of the ET process. For the specific system under study, ET occurs on a 30 A time scale, in agreement with the ultrafast experimental data. Both adiabatic and nonadiabatic pathways for the ET are observed. The nonadiabatic transfer entirely dominates at short times and can occur due to strong localized avoided crossing as well as extended regions of weaker nonadiabatic coupling. Although the adiabatic ET contribution accumulates more slowly, it approaches that of the nonadiabatic ET pathway asymptotically. It follows from the simulation that the nonadiabatic ET rate expressions, such as the Fermi golden rule, can be rigorously applied only for the fastest 30% of the ET process. The electron acceptor states are formed by the d orbitals of Ti atoms of the semiconductor and are localized within the first three to four layers of the surface. About 20% of the acceptor state density is localized on a single Ti atom of the first surface layer. The simulation predicts a complex non-single-exponential time dependence of the ET process.