A Unified Experimental/Theoretical Description of the Ultrafast Photophysics of Single and Double Thionated Uracils

Photoinduced processes in thiouracil derivatives have lately attracted considerable attention due to their suitability for innovative biological and pharmacological applications. Here, we combine sub-20 fs broadband transient absorption spectroscopy in the near-UV with CASPT2/MM decay path calculations to unravel the excited-state decay channels of water solvated 2-thio and 2,4-dithiouracil. These molecules feature linear absorption spectra with overlapping ??* bands, leading to parallel decay routes which we systematically track for the first time. The results reveal that different processes lead to the triplet states population, both directly from the ??* absorbing state and via the intermediate n?* dark state. Moreover, we show that the 2,4-dithiouracil decay pathways are strongly correlated either to those of 2- or 4-thiouracil, depending on the sulphur atom on which the electronic transition localizes.