On the population of triplet states of 2-seleno-thymine

The population and depopulation mechanisms leading to the lowest-lying triplet states of 2-Se-Thymine were studied at the MS-CASPT2/cc-pVDZ level of theory. Several critical points on different potential energy hypersurfaces were optimized, including minima, conical intersections, and singlet-triplet crossings. The accessibility of all relevant regions on the potential energy hypersurfaces was investigated by means of minimum energy paths and linear interpolation in internal coordinates techniques. Our analysis indicates that, after the population of the bright S-2 state in the Franck-Condon region, the first photochemical event is a barrierless evolution towards one of its two minima. After that, three viable photophysical deactivation paths can take place. In one of them, the population in the S-2 state is transferred to the T-2 state via intersystem crossing and subsequently to the T-1 state by internal conversion. Alternatively, the S-1 state could be accessed by internal conversion through two distinct conical intersections with S-2 state followed by singlet-triplet crossing with the T-2 state. The absence of a second minimum on the T-1 state and a small energy barrier on pathway along the potential energy surface towards the ground state from the lowest triplet state are attributed as potential reasons to explain why the lifetime of the triplet state of 2-Se-Thymine might be reduced in comparison with its thio-analogue.