Counterion Effects on the Properties and Reactivity of Charged Photoredox Catalysts

Electrostatic interactions represent the strongest intermolecular forces, in which the opposite charges attract, and the same charges repulse each other. Over the past decades, the incorporation of a charged unit to the photocatalyst's core has been widely employed as a strategy to modify their redox properties. In contrast, the choice of the accompanying counterion has attracted far less attention, despite the first reports of the counterion effects dating back to the 1980s. In recent years, interest in ion pairing in photochemistry has been reignited, leading to the identification of notable effects on the spectroscopic and photophysical properties of well‐known photocatalysts. In addition to mechanistic studies, the counterion effect has also been harnessed synthetically as a driving force for single‐electron transfer and as a chirality‐transfer reactant in asymmetric synthesis. Despite these advances, systematic screening for counterions remains absent from most of the recent literature, leaving a significant factor in performance optimization gravely underutilized.