Hydrogen and oxygen evolution with metalloporphyrins and analogs

Metalloporphyrins and analogs have merited special attention as good candidates for efficient photoredox and thermal redox catalysts in relation with the important roles of chlorophylls (porphyrin analogs) in photosynthesis and iron porphyrins in the reverse reaction, i.e., respiration, as well as cytochrome P450 enzymes in biological oxidation reactions. Diagnostic visible absorption bands and rich redox chemistry of metalloporphyrins and analogs provide valuable mechanistic insights into the reactive intermediates in the photocatalytic reactions. This review focuses on photoredox and thermal redox catalysis of metallopophyrins and analogs in photocatalytic H₂ and O₂ evolution. Metalloporphyrins and analogs act as photoredox catalysts undergoing electron transfer from the photoexcited states of metalloporphyrins to electron mediators in photocatalytic H₂ evolution using sacrificial electron donors and H₂ evolving redox catalysts. A diprotonated dodecaphenylporphyrin acts as an electron acceptor, undergoing photoinduced electron transfer from electron donors to the photoexcited state of diprotonated dodecaphenylporphyrin, leading to photocatalytic H₂ evolution. Metalloporphyrins and analogs also act as H₂ evolving catalysts as well as O₂ evolving catalysts. The catalytic mechanism of metalloporphyrins and analogs in H₂ and O₂ evolution reactions is discussed including the catalytic intermediates.