In this work, we investigate the relaxation processes of 2-thiouracil after UV photoexcitation to the S₂ state through the use of ultrafast, single-colour, pump-probe UV/UV spectroscopy. We place focus on investigating the appearance and subsequent decay signals of ionized fragments. We complement this with VUV-induced dissociative photoionisation studies collected at a synchrotron, allowing us to better understand and assign the ionisation channels involved in the appearance of the fragments. We find that all fragments appear when single photons with energy > 11 eV are used in the VUV experiments and hence appear through 3+ photon-order processes when 266 nm light is used. We also observe three major decays for the fragment ions: a sub-autocorrelation decay (i.e., sub-370 fs), a secondary ultrafast decay on the order of 300–400 fs, and a long decay on the order of 220 to 400 ps (all fragment dependent). These decays agree well with the previously established S₂ → S₁ → Triplet → Ground decay process. Results from the VUV study also suggest that some of the fragments may be created by dynamics occurring in the excited cationic state.

The photophysics of nucleobases has been the subject of both theoretical and experimental studies over the past decades due to the challenges posed by resolving the steps of their radiationless relaxation dynamics, which cannot be described in the framework of the Born-Oppenheimer approximation (BOA). In this context, the ultrafast dynamics of 2-thiouracil has been investigated with a time-resolved NEXAFS study at the Free Electron Laser FLASH. Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS) can be used to observe electronic transitions in ultrafast molecular relaxation. We performed time-resolved UV-pump/X-ray probe absorption measurements at the sulfur 2s (L1) and 2p (L2/3) edges. We are able to identify absorption features corresponding to the S2 (ππ*) and S1 (nπ*) electronic states. We observe a delay of 102 ± 11 fs in the population of the nπ* state with respect to the initial optical excitation and interpret the delay as the time scale for the S2 → S1 internal conversion. We furthermore identify oscillations in the absorption signal that match a similar observation in our previous X-ray photoelectron spectroscopy study on the same molecule.

Thionated nucleobases have drawn considerable attention due to their role in medical treatment and biology, which triggered studies of their photophysics and photochemistry with various experimental and theoretical techniques. In particular, vacuum ultraviolet (VUV) induced dissociative photoionization of 2-thiouracil (2-TU) has been recently studied using synchrotron radiation [Robinson et al., Molecules 2023, 28, 2354]. Here, using molecular dynamics simulations at the semiempirical OM2 level, we study fragmentation dynamics of the cations of 2-TU and its isomer, 4-thiouracil (4-TU). Considering various VUV photon energies, we calculate energy-resolved mass spectra and breakdown diagrams for 2-TU and 4-TU. Remarkably, we find that the major fragments are different among the two compounds — 69 amu (C3NH3O+) for 2-TU, and 85 amu (C3NH3S+) for 4-TU. Our simulations provide direct mechanistic insight into this observation and in fragmentation processes of the thionated uracils in general.