Recently, Bai and Barbatti proposed that the topology of the T1(ππ*) potential energy surface of the thiobases is characterized by two minima, in agreement with our computational results and those by others. It was further proposed that depending on the energetic balance between the two minima, the rate determining step can be a slow transition between both minima that favors the sensitization of singlet oxygen when the out-of-plane minimum is higher in energy than the other minimum, which they labeled as ring distorted minimum, or faster intersystem crossing back to the ground-state when the distorted minimum is higher in energy than the out-of-plane minimum. According to our calculations, the energy gap between the outof-plane and planar minima is ~0.5 eV for the neutral 6MP, whereas the energy gap between both minima is ~0.08 eV for the deprotonated form of 6MP. In both cases, the planar minimum is lower in energy than the out-of-plane minimum, which should favor reaction with O2 over intersystem crossing to the ground state. Then, the generation of the superoxide radical can partially explain the relatively low yields of 1 O2 generation observed in this study. We also note that the larger the energy gap between both minima, the longer the triplet decay lifetime and the higher the singlet oxygen quantum yield are. Hence, the energy gap between both T1(ππ*) minima in the neutral and the deprotonated forms of 6MP, together with the favorable probability to form superoxide radicals (Table ), may explain the relatively low yields of 1 O2 generation and the reduction in 1 O2 quantum yield in going from the neutral form of 6MP to the deprotonated form.