Short-time dynamics and decay mechanism of 2(1H)-pyridinone upon excitation to the light-absorbing S4(21𝝅𝝅*) state.

The excited-state structural dynamics and the decay mechanism of 2(1H)-pyridinone (NHP) after excitation to the S4(21ππ*) 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 S4(21ππ*)/S3(21nπ*)-MIN, S4(21ππ*)/S1(11nπ*)-MIN, and S4(21ππ*)-MIN. Our results indicate that the initial population of NHP in the S4 state bifurcates in or near the Franck-Condon region, leading to two predominant (S4S3-MIN and S4S1-MIN) internal conversion pathways. The lowest-lying S2(11ππ*) excited state is finally formed via subsequent internal conversions S3(21nπ*)/S2(11ππ*)-MIN and S1(11nπ*)/S2(11ππ*)-MIN. The enol-keto tautomeric mechanism does not seem to play a role. The decay mechanism in the singlet realm is proposed.