2-Thiouracil deactivation pathways and triplet states population

The photochemical reaction path approach and the CASPT2//CASSCF protocol have been employed to investigate the nonadiabatic photophysics and photochemistry of 2-thiouracil, with emphasis on the population of the lowest triplet state. Minimum energy structures, conical intersections, intersystem crossings, minimum energy paths, and spin–orbit couplings were discussed along the potential energy profiles, in order to investigate the efficiency of the nonadiabatic deactivation pathways from the initially populated state S2 1 ( n S π ∗ ) , which can be useful to rationalize the absence of fluorescence and the population of the lowest triplet state.