  Radiationless deactivation channels of 2-thiocytosine in aqueous environment
are revisited by means of quantum-chemical simulations of excited-state
absorption spectra, and investigations of potential energy surfaces of the
chromophore clustered with two water molecules using the algebraic diagrammatic
construction method to the second-order (ADC(2)), and multireference
configuration interaction with single and double excitations (MR-CISD) methods.
We argue that interactions of explicit water molecules with thiocarbonyl group
might enable water-chromophore electron transfer (WCET) which leads to
formation of intersystem crossing that was not considered previously. This is
the first example of a WCET process occurring in the triplet manifold of
electronic states. This phenomenon might explain nonradiative decay of the
triplet state population observed in thiopyrimidines in the absence of
molecular oxygen. According to our calculations this WCET process might also
entail a subsequent, virtually barrierless, electron-driven proton transfer
(EDPT) resulting in the formation of hydroxyl radical which could further
participate in photohydration or deamination reactions.
