Internal conversion and intersystem crossing dynamics of doubly thionated Uracil upon photoexcitation at the first and second absorption band

2,4-dithiouracil (2,4DTU) has an absorption spectrum extending from the UV-C to UV-A and, upon photoexcitation, undergoes internal conversion (IC) and intersystem crossing (ISC), leading to a population of the triplet state with near-unity quantum yields. The first and second regions of 2,4DTU align with 4TU and 2TU, respectively, which raises the question of whether selective excitation of one of the 2,4 DTU bands results in photodynamics resembling those of the corresponding singly thionated uracil. We use time-resolved photoelectron spectroscopy and ab initio calculations to investigate this question. A 376nm pump wavelength excites the ¹??_S4??* which internally converts to the lower ¹n_S4??* state and then populates the ³??_S4??*. At 267nm, we observe a much faster process which, in conjunction with theory, reveals a different story. The photoelectron spectrum shows the initial involvement of two states, ¹??_S2??*’ and ¹??_S4??*’, which means two parallel decay routes contribute to the relaxation dynamics. However, both evolve to the ¹n_S2??*’ state and eventually the ³??_S2??*’. For 376 nm, we, therefore, conclude that the excited state pathway remains localized on S₄ (sulfur in position 4) orbitals and resembles 4TU. On the other hand, deactivation following 267nm excitation involves states with S₂ (sulfur in position 2) localized orbitals. Therefore, the faster IC and ISC observed at 267nm could be explained by a switch in the relaxation pathway rather than excess vibrational excitation.