Intersystem crossing dynamics in singly substituted thiouracil studied by time-resolved photoelectron spectroscopy: Micro-environmental effects due to sulfur position

The ultraviolet (UV) photophysics of the natural and modified nucleobases can be surprisingly different. In response to UV radiation, the natural pyrimidine nucleobases undergo ultrafast internal conversion back to the ground state, whereas their thiobase analogues, in which an oxygen has been replaced by sulfur, instead display efficient intersystem crossing to the triplet manifold. Here, the effect of the substituent position is investigated with time-resolved photoelectron spectroscopy on 4-thiouracil, which is contrasted to previous work on 2-thiouracil. Although the photophysical pathway of both structural isomers is similar, i.e., leading from the S2 (1ππ∗) state, via S1 (1nπ∗), to the triplet manifold and subsequently back to the ground state, the intersystem crossing dynamics are strongly influenced by the surrounding intramolecular environment of the sulfur atom.