Hydrogen Evolution by Molecular Photocatalysis

This chapter focuses on photocatalytic mechanisms of hydrogen (H₂) evolution to clarify how photons are converted to two electrons that are required for H₂ production from two protons. A two-electron-reduced metal complex is produced via disproportionation of the one-electron-reduced species of a metal complex produced via photoinduced electron transfer, leading to H₂ evolution. A one-photon two-electron process is made possible in photocatalytic H₂ evolution by combination of thermal and photoinduced electron transfer. Photoexcitation of 9-mesityl-10-methylacridinium ion (Acr⁺–Mes) with NADH that is a hydride (two-electrons and a proton) donor resulted in the reduction of two equivalents of Acr⁺–Mes to produce two equivalents of Acr^•-Mes that reduce protons to produce H₂ in the presence of an H₂ evolution catalyst. Acr⁺-Mes can also be applied to photocatalytic generation of H₂, accompanied by dehydrogenative oxygenation of an alkene and selective C(sp²)-H amination of arenes. A one-photon two-electron process is also made possible by a bimolecular reaction of the excited state of a metal-hydride complex with the ground state complex to produce H₂.