Thianthrenium Salts in Photochemistry

ConspectusThianthrenium (TT) salts have emerged as versatile reagents with utility across transition-metal (TM) catalysis, photochemistry, biocatalysis, electrochemistry, and polar transformations. Initially introduced as an aryl (pseudo)halide surrogate in traditional TM-catalyzed cross-coupling reactions, thianthrenium salts have since demonstrated conceptually distinct advantages over their (pseudo)halide analogues in single-electron mediated processes, particularly under visible-light irradiation.The positively charged thianthrenium group raises the substrate's reduction potential into the range accessible to common photocatalysts, and upon single-electron transfer, the exocyclic C-S_TT bond undergoes ultrafast mesolytic cleavage to generate aryl or alkyl radicals while avoiding the back-electron-transfer often observed with halide substrates. This combination of favorable redox properties and rapid bond fragmentation distinguishes thianthrenium salts as efficient radical precursors in photoredox catalysis.Beyond the electron transfer mechanism in photoredox catalysis, thianthrenium salts have distinct advantages in <i>triplet energy transfer (EnT)</i> catalysis. In contrast to simple aryl (pseudo)halides, which possess high triplet energies (<i>E</i>_T ≈ 78-82 kcal/mol), Ar-TT⁺ salts exhibit consistently lower triplet energies (<i>E</i>_T ≈ 60-66 kcal/mol), largely independent of the arene substitution pattern. This energy range allows for efficient <i>triplet-triplet energy transfer</i> from photosensitizers such as thioxanthone (TXO, <i>E</i>_T = 65.5 kcal/mol) for radical generation via EnT in high quantum yield.In addition to photocatalytic pathways, direct photolysis of thianthrenium salts has emerged as a third mode of activation. This reactivity was initially employed to homolyze the CF₃-S_TT bond of trifluoromethyl thianthrenium (CF₃-TT⁺) triflate that has a low bond dissociation energy (BDE), with blue LEDs. A more general, biocompatible approach emerged through the development of selenonium-based TT analogues. TT-like selenonium-based reagents have been designed to realize <i>site-selective selenylation</i> 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.