Sub-20-fs broadband transient transmission spectroscopy in the UV range used to track primary photoinduced processes in thiobases*

Thiobases are DNA/RNA nucleobases where an exocyclic carbonyl oxygen is replaced by a sulfur atom. 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 nucleobases internal conversion from the photoexcited singlet state to the ground state mediated by a conical intersection is the main deactivation channel, the major relaxation pathway in thiobases is via the population of triplet states through an ultrafast intersystem crossing.
We employ ultrafast transient transmission spectroscopy in the near-UV range together with CASPT2/MM decay path calculations to unravel the excited-state lifetimes and decay pathways of thiouracils solvated systems. Their linear absorption spectra show overlapping bright states bands leading to different/parallel bright states decay pathways which we systematically track. This comprehensive evaluation together shows with different processes lead to the triplet population, both directly from the bright absorbing state and via the dark intermediate state.