Early-stage dynamics in coupled proton-electron transfer from the π-π* state of phenol to solvent ammonia clusters: a nonadiabatic electron dynamics study.

We reexmine the mechanism and interpretation of photochemical reaction of phenol molecule with small ammonia clusters, which is schematically written as Ph*OH···(NH(3))(n) → PhO(•)···[H(NH(3))(n)]*(•) with n ≤ 5. The low-lying excited states of this system in the adiabatic representation are densely quasi-degenerate due to the presence of the Rydberg-like diffused states in ammonia clusters. To treat the dynamics on such highly quasi-degenerate electronic states, we have carried out a large scale semiclassical Ehrenfest dynamics, nonadiabatic electron wavepacket dynamics in terms of very many configuration-state functions, to track the nonadiabatic electron and proton transfer dynamics in the time step of attosecond scale, integrating up to 300 fs. It turns out that the mechanism is more complicated than that referred to as excited-state hydrogen-atom transfer, which is widely accepted now. The pathways of jumping electron and shifting proton nucleus are identified to be mutually different, which necessarily results in charge separation in ammonia clusters after the transitions. The global feature of the present transfer dynamics is fully analyzed as one of the general prototypes of coupled electron-proton transfer in excited states.