Vertical excitation energies (VEEs) were calculated to predict the electronic relaxation pathways upon UVA excitation (see Tables and for data in PBS and ACN, respectively). For PBS, explicit solvation was modeled by incorporating three water molecules into the first solvation shell, complementing the implicit CPCM solvation model (Figures ). The X3LYP functional was selected for these computations due to its strong correlation with the experimental absorption spectra in both solvents. While the PBE0 functional also provided reasonable results, it consistently predicted blue-shifted absorption bands compared to experimental data in both solvents (Figures and). In both solvents, the S 1 state has predominantly of nπ* character with negligible oscillator strength, consistent with the lack of observed fluorescence (fluorescence quantum yield of dTAT1 is <0.001). In contrast, the S 2 state exhibits ππ* character and is directly excited at 390 nm (3.18 eV). From the S 2 state, the excited population can be predicted to undergo internal conversion to the S 1 (nπ*) state or intersystem crossing (ISC) to the triplet manifold. Three triplet states lie energetically close to or below the S 2 (ππ*) state in both solvents, allowing relaxation from the Franck-Condon (FC) region through multiple ISC pathways. Within the computational error margin of ±0.3 eV, the S 2 (ππ*) state is practically isoenergetic to the T 2 (nπ*) and T 3 (ππ*) states in both solvents. This energetic proximity suggests the possibility of strong singlet-triplet coupling in the FC region, potentially enabling efficient ISC to the triplet manifold, as observed in other thionated nucleobases. To support this hypothesis, spin-orbit coupling constants (SOCs) calculations between the two lowest-energy singlet excited states and the triplet states of dTAT1 were performed at three key points on the potential energy surfaces (PESs); the FC region, the S 1 minimum, and the S 2 minimum (see Tables and). Based on SOC values and El-Sayed's propensity rules, 33 S 2 → T 2 and S 1 → T 1 transitions are predicted to occur with high efficiency in both solvents. Notably, the participation of these ISC pathways is predicted from the three different regions of the PES regions and in both solvent environments. The percentage contributions of the single electron transitions and the Kohn-Sham orbitals in both solvents are illustrated in Tables and and Fig. and S5. Finally, the optimized minimum-energy structures of the S 1 , S 2 , and T 1 states in both solvents reveal planar geometries with minimal structural distortions (Figures and). Collectively, the electronic structure calculations predict that ISC to the triplet manifold in dTAT1 should be highly efficient and occur in ultrafast time scales, as has been observed in other organic and nucleobase derivatives upon oxygen-for-sulfur substitution of their carbonyl groups. To investigate the excited state dynamics of dTAT1, and confirm the computational predictions, femtosecond broadband transient absorption spectroscopy was performed. Figures and depict the collected transient absorption spectra in PBS and ACN, respectively. Upon excitation at 390 nm, a broad transient species with a maximum centered at ca. 570 nm is observed, alongside the emergence of a negative amplitude band around 360 nm. The latter band is due to ground-state depopulation, in agreement with the absorption spectrum of dTAT1 in Figure . Between the time delays of ca. 0.8 and 11 ps, the transient species at 570 nm grows in intensity and undergoes a slight blueshift, while the negative band at 360 nm diminishes in intensity, eventually transforming into a small positive band spanning 406-475 nm with a maximum at 425 nm. Notably, an apparent isosbestic point is distinctly observed at ~582 nm in PBS, which slightly redshifts to 590 nm in ACN. Finally, from ca. 50 ps to the end of the 3 ns probe window, the transient species with maxima at 425 nm and 360 nm begins to decay. A global analysis of the data using a three-component sequential kinetic model reveals three lifetimes (Table ). Building on prior theoretical and experimental insights, thiocarbonyl-containing nucleobases typically undergo ultrafast internal conversion from the S 2 (ππ*) state to the S 1 (nπ*) state, followed by intersystem crossing to the T 1 (ππ*) state, with a simultaneous competition of S 2 (ππ*)  T n (nπ*) ISC. Hence, the first lifetime of approximately 0.20 ± 0.04 ps in both solvents, and the corresponding black EADS1 (Figure ) are attributed to a combination of the S 2 (ππ*) → S 1 (nπ*) → T 1 (ππ*) and S 2 (ππ*) → T 2 (nπ*) → T 1 (ππ*) pathways. This assignment is supported by the substantial SOC values and the relatively small energy gaps between the S 1 → T 1 and S 2 → T 2 transitions. Specifically, the SOC values between S 1 /T 1 and S 2 /T 2 are 123 cm -1 and 131 cm -1 in PBS (Table ), and 129 cm -1 and 135 cm -1 in ACN (Table ).