The T1(ππ*) state was also optimized with DFT at the UPBE0/IEFPCM/6-311+G(d,p) level of theory in water, and its excited state absorption spectrum was calculated at the TD-PBE0/IEFPCM/6-311+G(d,p) level of theory. Importantly, as noted above, optimization of the T1(ππ*) state at the TD-PBE0/IEFPCM/6-311+G(d,p) level of theory yielded a planar minima (Figure ), whereas the optimization of the T1(ππ*) state at the UPBE0/IEFPCM/6-311+G(d,p) level of theory yielded an out-of-plane minimum (Figure ), in which the sulfur atom is out of the molecular plane. We note that previous multiconfigurational calculations have demonstrate the presence of two triplet minima in the T1(ππ*) potential energy surface of several the thiobases, which has been supported by experimentation. Collectively, the computational and experimental results suggest that the topology of the T1(ππ*) potential energy surface is characterized by the presence of at least two minima, which can play an important role in the generation of reactive oxygen species (see discussion section). Vertical excitation energies were also calculated for the neutral and deprotonated forms of Aza using the CAM-B3LYP, ωB97X, and the PBE0 functionals. The calculations show that the PBE0 and the CAM-B3LYP functionals underperformed relative to the ωB97X functional (Figure ). Therefore, the results obtained with the PBE0 and the CAM-B3LYP functionals are not discussed further. The VEEs for the two lowest-energy excited singlets states and five/six lowest-energy excited triplet states for the neutral/deprotonated form of Aza are reported in 0.9 1.1 a Oscillator strengths are shown in parentheses. In the case of deprotonated Aza, T6 (4.09, 86% ππ*, CT) is also lower in energy than the S2. ππ* + nπ* corresponds to transitions where the HOMO-X in the single electron transition has both n orbital (in the nitroimidazole) and π orbital (in the purine) characters. Table 4 reports the SOCs between relevant singlet and triplet states. The S1  T2 (43 cm - 1 ), S2  T2 (20 cm -1 ), S2  T1 (22 cm -1 ), and S2  T4 (21 cm -1 ) electronic transitions exhibit the largest SOCs for the neutral of Aza. For the deprotonated form, the S1  T2 (27 cm -1 ) and S2  T2 (37 cm -1 ) transitions exhibit the largest SOCs. As shown in Tables and, the magnitudes of SOCs are significantly reduced compared to 6MP, but they are still relatively large, i.e., in the order of tens of wavenumbers. Due to the molecular size of the neutral and deprotonated forms, we were unable to optimize the electronic excited states of Aza within the computational resources available to us.