A Static Picture of the Relaxation and Intersystem Crossing Mechanisms of Photoexcited 2-Thiouracil

Accurate excited-state quantum chemical calculations on 2-thiouracil, employing large active spaces and up to quadruple-zeta quality basis sets in multistate complete active space perturbation theory calculations, are reported. The results suggest that the main relaxation path for 2-thiouracil after photoexcitation should be S-2 -> S-1 -> T-2 -> T-1, and that this relaxation occurs on a subpicosecond time scale. There are two deactivation pathways from the initially excited bright S-2 state to S-1, one of which is nearly barrierless and should promote ultrafast internal conversion. After relaxation to the S-1 minimum, small singlet-triplet energy gaps and spin-orbit couplings of about 130 cm(-1) are expected to facilitate intersystem crossing to T-2, from where very fast internal conversion to T-1 occurs. An important finding is that 2-thiouracil shows strong pyramidalization at the carbon atom of the thiocarbonyl group in several excited states.