Surface hopping investigation of benzophenone excited state dynamics

Abstract

We present a simulation of the photodynamics of benzophenone for the first 20 ps after n → π* excitation, performed by trajectory surface hopping calculations with on-the-fly semiempirical determination of potential energy surfaces and electronic wavefunctions. Both the dynamic and spin–orbit couplings are taken into account, and time-resolved fluorescence emission is also simulated. The computed decay time of the S₁ state is in agreement with experimental observations. The direct S₁ → T₁ intersystem crossing (ISC) accounts for about 2/3 of the S₁ decay rate. The remaining 1/3 goes through T₂ or higher triplets. The nonadiabatic transitions within the triplet manifold are much faster than ISC and keep the population of T₁ at about 3/4 of the total triplet population, and that of the other states (mainly T₂) at 1/4. Two internal coordinates are vibrationally active immediately after n → π* excitation: one is the CO stretching and the other one is a combination of the conrotatory torsion of phenyl rings and of bending involving the carbonyl C atom. The period of the torsion-bending mode coincides with oscillations in the time-resolved photoelectron spectra of Spighi et al. and substantially confirms their assignment.