LAUSR

Sulfur‐substituted nucleobases (i.e., thiobases) are a prospective class of compounds for clinical and cosmetic topical phototherapies. Recent investigations of several thiobases have revealed the ultrafast and efficient population of reactive triplet states upon ultraviolet‐A (UVA) irradiation and the subsequent generation of singlet oxygen in high yield. In this contribution, we examine the photosensitizing activities of three of the most promising thiobase derivatives discovered to date: 2,4‐dithiothymine, 2,4‐dithiouracil, and 2,6‐dithiopurine. These derivatives are shown to decrease the proliferation of human epidermoid carcinoma cells by up to 63 % in vitro, only upon activation with a low dose of UVA radiation (5 J cm−2). The generation of reactive oxygen species plays a minor role in the mode of action, suggesting these dithiobases may be effective within oxygen‐deficient environments. Importantly, the photosensitized activity correlates with the magnitude of the triplet lifetime, which should guide the molecular design of next‐generation photodynamic agents.