The central finding of this investigation is that Aza prodrug exhibits a remarkably different electronic relaxation mechanism and photochemistry than 6MP, as well as the other thiobases investigated to date. Excitation of 6MP with UVA radiation results in nearunity triplet yield, up to 30% 1 O2 yield in saturated O2 conditions, and the plausible formation of superoxide radicals. The experimental and computational results suggest that not only the metabolites of 6MP and Aza, 9 but 6MP itself can induce oxidatively generated damage within the intracellular pool. Hence, direct absorption of UVA radiation by 6MP could be another important contributing factor for the up to 250-fold increase in basal and squamous cell carcinomas in patients. Conversely, excitation of Aza with UVA radiation leads to less than 3-15% triplet yield depending on the pH and less than 1% 1 O2 yield. Nitroimidazole functionalization to form Aza destabilizes the 1 nπ* state in the Franck-Condon region and increases the charge transfer character of the S1(*) state. The efficient intramolecular charge recombination decreases the 1 O2 yield by 10-fold relative to 6MP. We remark that formation of a carbon-sulfur single bond is not a sufficient condition to reduce the population of the T1 state in Aza because S -methylthioinosine populates the T1 state in a high yield. Aza's poor photosensitization ability reveals the prospective use of interchromophoric interactions (i.e., the use of electron-withdrawing groups covalently attached to the thiocarbonyl sulfur atom) for the design of photostable prodrugs exhibiting a significant reduction in photocytotoxic side effects before drug metabolization. 5 the two lowest-energy absorption bands were fit with a center wavelength of 276 nm, FWHM of 36 nm and amplitude of ca. 13.3 mM -1 cm -1 , and a center wavelength of 358 nm, FWHM of 56 nm and amplitude of ca. 4.0 mM -1 cm -1 . For pH 9.8 the two lowest-energy absorption bands were fit with a center wavelength of 283 nm, FWHM of 43 nm and amplitude of ca. 8.3 mM -1 cm -1 , and a center wavelength of 346 nm, FWHM of 57 nm and amplitude of ca. 2.3 mM -1 cm -1 . The percent fraction of the population that reach each respective excited singlet state upon the different excitation wavelengths can be determined using equation S1, where X and Y represent the respective singlet states: