Short-time dynamics and decay mechanism of 2(1H)-pyridinone upon excitation to the light-absorbing S(2𝝅𝝅) state.

The excited-state structural dynamics and the decay mechanism of 2(1H)-pyridinone (NHP) after excitation to the S(2ππ) light-absorbing state were studied using resonance Raman spectroscopy and complete-active space self-consistent field (CASSCF) calculations. The B-band absorption cross-section and the corresponding absolute resonance Raman cross-sections were simulated using a simple model based on time-dependent wave-packet theory. The geometric structures of the singlet electronic excited states and their curve-crossing points were optimized at the CASSCF level of theory. The obtained short-time structural dynamics in easy-to-visualize internal coordinates were then compared with the CASSCF-predicted structural-parameter changes of S(2ππ)/S(2nπ)-MIN, S(2ππ)/S(1nπ)-MIN, and S(2ππ)-MIN. Our results indicate that the initial population of NHP in the S state bifurcates in or near the Franck-Condon region, leading to two predominant (SS-MIN and SS-MIN) internal conversion pathways. The lowest-lying S(1ππ) excited state is finally formed via subsequent internal conversions S(2nπ)/S(1ππ)-MIN and S(1nπ)/S(1ππ)-MIN. The enol-keto tautomeric mechanism does not seem to play a role. The decay mechanism in the singlet realm is proposed.