Intersystem crossing in thiobases proceeds by a dark intermediate state

Thiobases are DNA or RNA nucleobases where an exocyclic carbonyl oxygen is replaced by a sulfur atom [1]. Thiation induces a red-shift in the absorption spectrum and causes also a dramatic change in the photophysics with respect to the canonical nucleobases: while in DNA/RNA monomers internal conversion from the photoexcited singlet 1ππ state to the ground state mediated by a conical intersection (CI) is the main deactivation channel, the major relaxation pathway in thiobases is via the population of triplet states through an ultrafast intersystem crossing (ISC). Thiobases have attracted much interest in recent years due to their photo-therapeutic applications [1]. In spite of that, their ultrafast deactivation mechanisms are still debated.