Ab Initio Molecular Dynamics of Ultrafast Electron Injection from Molecular Donors to the TiO_2 Acceptor

摘要

The photoinduced electron transfer (ET) from a molecular electron donor to the TiO_2 semiconductor acceptor triggering Graetzel solar cells and other photochemical applications is investigated. The reported simulations reproduce the experimentally observed ET time-scale, establish the reaction mechanism, and provide a detailed picture of the ET process. The electronic structure of the chromophore-semiconductor system is simulated by density functional theory (DFT). Ab initio molecular dynamics (MD), including non-adiabatic (NA) transitions between electronic states, NAMD, is used to follow the ET reaction in real-time and at the molecular level. The simulation indicates that thermally driven adiabatic ET is dominant at room temperature. Vibrational motions of the chromophores induce oscillations of the photoexcited state energy that drives the photoexcited state in and out of the TiO_2 conduction band. Two distinct types of ET events are observed depending on the initial conditions. At low initial energies the photoexcited state is well localized on the chromophore, and an activation is required for ET, with comparable contributions from both the adiabatic and NA mechanisms. At high initial energies the photoexcited state is already substantially delocalized into the TiO_2 substrate. The remaining fraction of the ET process occurs rapidly and by the adiabatic mechanism.