# Thianthrenium Salts in Photochemistry

## TL;DR

Thianthrenium salts are versatile chemical reagents that enable efficient radical generation in photochemical reactions, offering advantages over traditional halide-based methods.

## Contribution

This paper highlights the unique redox and photoredox properties of thianthrenium salts, enabling novel strategies for radical generation in photochemistry.

## Key findings

- Thianthrenium salts enable efficient single-electron transfer and rapid bond cleavage for radical generation.
- They exhibit lower triplet energies than aryl halides, enabling efficient triplet–triplet energy transfer for radical formation.
- Direct photolysis of thianthrenium and selenonium salts allows site-selective modifications of biomacromolecules under physiological conditions.

## Abstract

Thianthrenium (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–STT 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 triplet energy
transfer (EnT) catalysis. In contrast to simple aryl (pseudo)­halides,
which possess high triplet energies (E
T ≈ 78–82 kcal/mol), Ar–TT+ salts
exhibit consistently lower triplet energies (E
T ≈ 60–66 kcal/mol), largely independent of the
arene substitution pattern. This energy range allows for efficient triplet–triplet energy transfer from photosensitizers
such as thioxanthone (TXO, E
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–STT 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 site-selective selenylation 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.