Scheme 3 summarizes the electronic relaxation mechanisms outlined in this study for the 6MP and Aza prodrugs. On the one hand, it is proposed that upon UVA photoactivation, 6MP populates the triplet manifold in ultrafast timescales (τ1) by two competitive pathways. The first pathway is direct intersystem crossing from the S2(ππ*) state to the T2(nπ*) state. The second pathway involves internal conversion from the S2(ππ*) state to the S1(nπ*) state, followed by intersystem crossing to the T1(ππ*) state. Both pathways agree with the El-Sayed's propensity rules. The population reaching the vibrational excited T1(ππ*) state, bifurcates and populate two T1(ππ*) minima within τ2, through internal conversion undertaking conformational relaxation and vibrational cooling dynamics in the T1(ππ*) potential energy surface. After the T1(ππ*) state reaches the out-of-plane and planar minima, it decays back to the ground-state with lifetimes of 3.5  0.2 s and 0.95  0.07 s for the neutral and the deprotonated species, respectively. The relatively long triplet decay lifetime in 6MP allows the diffusion-controlled sensitization of 1 O2 with a yield of 11% (deprotonated) to 19% (neutral) and the generation the superoxide radical.