Even though the magnitude of the triplet decay lifetime of the neutral and the deprotonated forms of 6MP is significantly different, the pathways leading to the reactive triplet state population are expected to be the same. This is supported by the observation that the spectral features in the transient data, the VEEs, and the SOCs are nearly the same regardless of whether the neutral or the deprotonated form is investigated. The 3.7-fold longer triplet decay lifetime observed for the neutral relative to the deprotonated species is proposed to be due to the larger energy gap between the triplet minima to access the intersystem crossing region leading to the repopulation of the ground state. The singlet oxygen yield is larger at pH 5.5 than at pH 7.4 showing that the neutral species generates singlet oxygen more efficiently than the deprotonated species. This is consistent with the longer triplet decay lifetime of the neutral species compared to the triplet decay lifetime of the deprotonated species. It also seems to be consistent with the larger driving force for electron transfer from the triplet state of the deprotonated species to molecular oxygen compared to the triplet state of the neutral species, as suggested by the calculations reported in Table .