Time-resolved resonance Raman investigation and ab initio calculations of the T1-state structure of thiocoumarin

The T1 state of thiocoumarin has been investigated by transient absorption, resonance Raman scattering, and ab initio calculations. For comparison, the ground state molecule has also been examined by Raman spectroscopy and ab initio calculations. This analysis shows that the benzo moiety is only slightly modified in the T1 state compared to the ground state and keeps its aromatic character. In contrast, stronger distortions arise in the pyranthione moiety, resulting essentially from a lowering of the π electron density on the CC bond in position β to the CS bond, concomitant with an increase of the π density on the CC bond adjacent to the CS bond. However, the CS bond appears not significantly modified on going from S0 to T1, demonstrating definitely the ππ* nature of the T1 state. The planar conformation of the ground state is preserved in the triplet state.