Excited state dynamics with nonadiabatic transitions for model photoinduced proton-coupled electron transfer reactions

Photoinduced proton-coupled electron transfer is investigated for a minimal model consisting of three coupled degrees of freedom that represent an electron, a proton, and a collective solvent coordinate. Altering the parameters in this model generates a wide range of proton-coupled electron transfer (PCET) dynamics. Four different models are presented in this paper. Three of these models represent sequential mechanisms and one represents a concerted mechanism. The adiabatic potential energy curves as a function of solvent coordinate and the corresponding two-dimensional wave functions, which depend on both the proton and the electron coordinates, are calculated in order to study the possible mechanisms of photoinduced PCET. The surface hopping method "molecular dynamics with quantum transitions" (MDQT), which incorporates nonadiabatic transitions between adiabatic quantum states, is utilized to simulate the dynamics of photoinitiated PCET for two of these model systems. In this application of MDQT the proton and electron coordinates are treated quantum mechanically, and the solvent coordinate is treated classically. A relatively large number (e.g., 11) of mixed proton/electron adiabatic states are included in the MDQT simulations. The reaction is initiated on the electronically excited state, and many different dynamical pathways to lower energy stable states are observed. Nonadiabatic effects are shown to play an essential role in determining the rates and mechanisms of photoinduced PCET reactions. This paper differs from previous studies of PCET reactions in that it presents real-time nonadiabatic molecular dynamics simulations of model PCET reactions initiated on an electronically excited state.