The triplet state of 6MP decays with a single lifetime of 3.5  0.2 s at pH 5.5 (Figure ), whereas it decays with two lifetimes of 3.5  0.2 s and 0.95  0.07 s at pH 7.4. This supports the idea that the 3.5  0.2 s lifetime corresponds to the triplet decay lifetime of the neutral form, whereas the 0.95  0.07 s lifetime corresponds to the triplet decay of the deprotonated form of 6MP. In Figure , the triplet state was demonstrated to sensitize the generation of 1 O2 with an efficiency of 19% and 11% at pH 5.5 and 7.5, respectively. These values are low considering the near-unity triplet yield estimated for both the neutral and deprotonated forms of 6MP in this study. However, as described in the results section, and as has been previously proposed for 6tGua and 6tGuo, molecular oxygen can quench the triplet state of 6MP by forming reactive oxygen species other than singlet oxygen. Hence, to shed light on this possibility, the Gibbs free energies associated with the electron transfer process (ΔGET) from the 3 * state of both the neutral and deprotonated forms of 6MP and O2 were calculated for both the planar and out-ofplane T1 minima (Table ). According to Schweitzer et al., a charge transfer complex can dominate the interaction between a 3 * state and O2 when the ΔGET value is more negative than -30 kJ/mol. As shown in Table , ΔGET of -129 (-81.2) and -374 (-365.7) kJ/mol were estimated for the out-of-plane (planar) minima of the neutral and the deprotonated forms of 6MP, respectively. These Gibbs free energy estimates support the idea that a significant fraction of the triplet state could be quenched by electron transfer to generate superoxide radicals, with the out-of-plane T1 minima exhibiting higher probability for electron transfer to O2 than the planar minima.