Photogeneration of singlet oxygen via excitation of forbidden transitions in thioxanthene dyes within the biological-tissue transparency window

Subject of study . The subject of this study is a series of thioxanthene dyes that exhibit a long-wavelength absorption band in the spectral range of 480–590 nm and can undergo direct singlet–triplet excitation under near-infrared radiation. Aim of study. The aim of this study is to determine the quantum yields of singlet oxygen generation by thioxanthene dyes and use these dyes as singlet oxygen sensitizers in photodynamic therapy (PDT). Method. The efficiency of singlet oxygen generation by dye sensitizers was evaluated based on the method proposed by Merkel and Kearns. This method involves spectroscopic monitoring of the kinetics of consumption of the singlet oxygen trap 1,3-diphenylisobenzofuran during the excitation of singlet oxygen generators. Main results. A high efficiency of singlet oxygen generation was demonstrated for several thio-derivatives of xanthene dyes, both within the main absorption band and within the shoulder corresponding to singlet–triplet absorption. A comparison of the singlet oxygen photogeneration efficiencies of erythrosine and thioerythrosine, within the biological-tissue transparency window, revealed that the heterocyclic sulfur atom significantly influenced the photoactivity of the triplet state when the dyes were exposed to near-infrared radiation. Practical significance. The approach proposed in this study will aid in the development of efficient singlet oxygen generators for application in PDT. Thioerythrosine can be used as an effective singlet oxygen sensitizer under near-infrared irradiation.