From photons to reactions: key concepts in photoredox catalysis

Photoredox catalysis has emerged as a powerful tool for organic synthesis, enabling the generation of reactive intermediates under mild reaction conditions. In this review, a special emphasis is placed on key concepts essential to understand and subsequently optimize photoredox catalytic processes. These focus on the formation and deactivation of the excited state and its reactivity. Specifically, the importance of the excited-state lifetime for diffusional quenching, the formation of the encounter complex, and the crucial role of non-covalent interactions in facilitating pre-association between reagents are discussed. Furthermore, excited-state deactivation pathways induced by a quencher are considered, differentiating between energy-transfer and electron-transfer processes. Finally, we address the importance of cage escape yields for the case of electron-transfer quenching and the determination of reaction quantum yield by actinometry measurements. Comprehension of these parameters will enable the development of more efficient and selective photocatalytic processes and open technological locks.