Hydrogen Evolution by Molecular Photocatalysis

Abstract

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.