Excitation of Aza at 334 or 316 nm leads to the observation of transient absorption spectra with maxima at ca. 350 and 690 nm in pH 5.5, 7.4, and in acetonitrile within the initial ca. 0.5 ps, while it leads to the observation of transient spectra with maxima at ca. 383 and 695 nm in pH 9.8. The transient spectra decay to populate a minor, long-lived transient species that last longer than 3 ns. Considering the VEEs reported in Table for Aza, the transient absorption species observed within 0.5 ps is assigned to the excited state absorption of the S1(*, CT) state for both the neutral and the deprotonated forms of Aza. It should be remarked that both S1(*, CT) and S2(*, CT) states are expected to be simultaneously populated upon excitation of Aza at 334 or 316 nm (see Figure for details) independent of pH (e.g., ~95/77% directly populates the S1 and ~5/23% the S2 state upon excitation of Aza with 334/316 nm at pH 5.5, while ~79/34% directly populates the S1 and ~21/66% populates the S2 state upon excitation with 334/316 nm at pH 9.8). Irrespective of the exact fraction of the population that reaches both excited singlet states, the population reaching the S2 state is expected to internally convert in an ultrafast time scale to the S1(*, CT) state (i.e., faster than the IRF of the spectrometer of ca. 250 fs). As shown in Figure , the S1 state has significant (>42%) CT character in both the neutral and the deprotonated forms. Thus, it is proposed that most of its population leads to ultrafast intramolecular charge separation, followed by charge recombination repopulating the ground state within 1. Indeed, rapid intramolecular charge recombination is often observed in molecules having intramolecular donor and acceptor moieties such as Aza, where the interchromophoric distance is small and the charges cannot separate for long periods across the donor-acceptor pair to stabilize the charge separation state. The small-amplitude residual signals observed after the primary charge recombination event is completed and that decays in longer than 3 ns independent of the pH of the solution in range from 5.5 to 9.8 (Figure ), is assigned to the T1 state of both the neutral (at pH 5.5) and the deprotonated (at pH 9.8) species. Relative yields of 15%, 7% and 3% are estimated from the transient absorption data for Aza at pH 5.5, 7.4 and 9.8, respectively (see Methods). The putative assignment of this transient species to photoproduct formation can be safely discarded because Aza did not degrade during the transient absorption experiments. The calculations also predict that singlet to triplet intersystem crossing events can effectively occur between the S1  T2, S2  T2, S2  T1, and S2  T4 states for the neutral form or between the S1  T2 and S2  T2 for the deprotonated form of Aza (Tables and). However, because intersystem crossing in the neutral and the deprotonated forms must compete with ultrafast charge separation and recombination in the S1(*, CT) state, the triplet yields are expected to be very small, as observed. The low triplet yield is supported by the low 1 O2 yield reported in Table , and by the observation of phosphorescence at 77 K 62 for Aza but not at room temperature. The possibility that the triplet state of Aza can be populated is also indirectly supported by the observation of triplet state population in S -methylthioinosine, which also features a C6-S single bond, in both aqueous and acetonitrile solutions. Furthermore, transient absorption experiments performed for Aza in acetonitrile and in PBS at pH 7.4 in back-to-back conditions demonstrated nearly identical spectral features but a 44% slowdown of the charge recombination lifetime and a 2.6-fold increase in the amplitude of the longer-lived species in acetonitrile compared to pH 7.4 (Figure ). The slowdown of the charge recombination lifetime together with the increased amplitude of the long-lived species demonstrates that the initial population in the singlet manifold bifurcates to populate an intramolecular charge-transfer state and a long-lived transient species. Solvent polarity is known to affect the intramolecular charge transfer state dynamics in such systems, with highly polar solvents having a stronger interaction with the intramolecular charge-transfer state, which can result in faster solvation and subsequent decay to lower energy levels. It has also been reported that certain push-pull systems with less CT character are less influenced by solvent polarity, which can result in a higher prevalence of decay channels other than charge recombination, for example, intersystem crossing to the triplet state. 65 Scheme 3. Proposed electronic relaxation mechanisms for a) 6MP and b) Aza prodrugs upon UVA excitation. The planar T1 minimum is lower in energy than the out-of-plane minimum. The planar T1 is expected to facilitate the production of more 1 O2 than the out-of-plane minimum, while the out-of-plane T1 is expected to facilitate the production of more superoxide than the planar minimum. Note that the calculations predict that upon excitation at 334 or 316 nm, and depending on the protonation status of Aza, the S2 state can also be populated, and a small fraction of its population may also intersystem cross to the triplet manifold, while the large majority is expected to internally convert to the S1 state (not shown in the scheme, see main text).