Reaction kinetics between thiobases and singlet oxygen studied by direct detection of the ¹O₂ luminescence decay^†

Thiobase derivatives have received important investigations due to their wide usage as phototherapeutic agents and their potential carcinogenic side effects as immunosuppressants. The substitution of oxygen atom by the sulfur atom makes the ultraviolet absorption of thiobases redshifted and absorbs UVA light (>300 nm), resulting in unusual high quantum yield of triplet state to generate the singlet oxygen (¹O₂) through photosensitization. As a type of reactive oxygen species, ¹O₂ is highly reactive toward thiobases. Herein, we report the measurements of reaction rate constants between different thiobases and ¹O₂ in different solvents through the direct detection of ¹O₂ luminescence decay kinetics at 1270 nm. The rate constants of thiouracils with ¹O₂ are five times smaller than that of thioguanine with ¹O₂, which suggests that thiopurines are more reactive than thiopyrimidines and thus less suitable to be a photosensitive drug on the application of photodynamic therapy. Additionally, the rate constants of thiobases and ¹O₂ were found to be obviously influenced by the solvent polarity. With the increase of solvent polarity, the rate constants of thiobases and ¹O₂ decrease.