Thianthrenium Salts in Photochemistry

of electron-rich aromatic residues on biomacromolecules in aqueous media. While the C-Se bond in these selenonium salts remains stable under ambient conditions, it undergoes efficient homolytic cleavage upon irradiation due to their visible light absorption and low BDE of ∼70 kcal/mol for the C-Se bond. Therefore, photochemical late-stage modifications of peptides, proteins, and nucleic acids can be achieved under physiologically compatible conditions.This Account retraces the conceptual evolution of thianthrenium chemistry in our laboratory─from its origins in aromatic C-H functionalization to its diverse applications in photochemistry. We highlight conceptual and practical advances enabled by thianthrenium salts in photocatalysis, which are classified into two categories: photocatalytic SET (photoredox catalysis) and photocatalytic EnT. Within photoredox catalysis, three mechanistic modes are distinguished: (i) conventional photoredox catalysis; (ii) dual photoredox/transition-metal catalysis; and (iii) photoinduced transition-metal catalysis. In addition, we discuss the direct homolytic cleavage of thianthrenium and selenium salts under visible-light irradiations. By contrasting these strategies, we explain how thianthrenium chemistry provides practical and mechanistically distinct solutions to modern radical chemistry.