Theoretical study of the photochemical generation of triplet acetophenone

Acetophenone has a rich photochemistry, which strongly depends on the absorbing state. For example, the excitation to the lowest singlet excited state (S-1) leads to a triplet population with a phosphorescence quantum yield of one, while the excitation to S-2 leads to photocleavage reactions. Here, we rationalize the photochemistry of acetophenone after being absorbed into the S-1, S-2 and S-3 states by performing a systematic study of all the singlet and triplet minimum energy structures and state crossings between the relevant electronic states. We calculate these structures at the complete-active space self-consistent field (CASSCF) level of theory and at the correlated extended second-order quasi-degenerate multi-reference perturbation theory (XMCQDPT2), emphasizing the importance of correlation effects in the determination of structures.