In this contribution, we show that 6MP and Aza prodrugs exhibit remarkably different photochemical properties and electronic relaxation mechanisms. As observed for all other thiobases investigated to date, UVA excitation of 6MP leads to near-unity triplet yield and relatively-high singlet oxygen ( 1 O2) generation. Conversely, excitation of Aza leads to triplet yields in the range of 15 to 3% depending on the protonation status of Aza and less than 1% 1 O2 generation, which should lead to a significant reduction in photocytotoxic side effects before drug metabolization. (Sigma-Aldrich) and 0.3000 g sodium dihydrogen (Sigma-Aldrich) in 250 mL ultrapure water.