Redox catalysis via photoinduced electron transfer

Abstract

This perspective article highlights redox catalysis of organic and inorganic molecules via photoinduced electron transfer, which is well exploited for a number of important photoredox reactions including hydrogen evolution, water oxidation and a number of synthetic applications. Organic and inorganic photoredox catalysis is also combined with thermal transition metal redox catalysis to achieve overall photocatalytic redox reactions, which would otherwise not be possible by using photoredox catalysis or thermal redox catalysis alone. Both thermodynamic and kinetic data are discussed to understand the photoinduced electron-transfer processes of organic and inorganic photoredox catalysts in the light of the Marcus theory of electron transfer, providing a comprehensive and valuable guide for employing organic and inorganic redox catalysts via photoinduced electron transfer. The excited states of electron donors including radicals and anions act as super-reductants in the photoinduced electron-transfer reactions, whereas the excited states of electron acceptors including cations act as super-oxidants in the photoinduced electron-transfer reactions. Photoexcitation of simple electron donor–acceptor linked molecules with small reorganization energies of electron transfer results in formation of long-lived electron-transfer states, which can oxidize and reduce substrates to make various chemical transformations possible with use of transition metal redox catalysis. Finally molecular model systems of photosystems I and II are combined to achieve water splitting to evolve H₂ and O₂.