Photoredox catalysis of acridinium and quinolinium ion derivatives

Photoredox catalysis has attracted increasing attention because of wide range of synthetic transformations and solar energy conversion applications. Reviews on photoredox catalysis have so far focused predominantly on the synthetic applications. This review highlights how organic photoredox catalysts were developed and how they function as efficient photocatalysts in mechanistic point of views. In particular, 9‐mesityl‐10‐methylactidinium (Acr⁺–Mes) has been highlighted as one of the best organic photoredox catalysts. Acr⁺–Mes was originally developed as a model compound of the photosynthetic reaction center to mimic the long lifetime of the charge‐separated state in which the energy is converted to chemical energy in photosynthesis. The reason why Acr⁺–Mes acts as one of the most efficient photoredox catalyst is clarified in terms of the one‐electron redox potentials and long lifetimes of the electron‐transfer state (Acr^•–Mes^•+) produced upon photoexcitation of Acr⁺–Mes in different solvents. The reason why the mesityl substituent at the 9‐position of the Acr⁺ moiety is essential for the efficient photoredox catalysis is discussed in comparison with acridinium ions with different substituents R (Acr⁺–R) including 10‐methylacridinium ion with no substituent (AcrH⁺). The mechanisms of photoredox catalysis of Acr⁺–Mes are discussed in various synthetic transformations and solar energy conversion reactions mimicking photosynthesis. Photoredox catalysis of quinolinium ion and its derivatives is also discussed in comparison with that of Acr⁺–Mes. Finally, immobilization of Acr⁺–Mes and quinolinium ions to form the composite catalysts with redox catalyst is discussed to improve the photoredox catalytic activity and stability.