Photochemistry and photocatalysis of transition-metal porphyrin complexes and analogues

Although photoexcited state lifetimes of transition-metal porphyrin complexes are usually too short to exhibit any appreciable reactivity in intermolecular redox reactions with substrates, there are several cases to show enough lifetimes of the photoexcited states of transition-metal porphyrins and analogues to be involved in intermolecular photoredox reactions. This minireview focuses on such examples of photochemistry and photocatalysis of transition-metal porphyrins and analogues. For example, supramolecular complexes of transition-metal porphyrins with fullerene and its derivatives undergo fast charge separation to produce the charge-separated states upon photoexcitation, the lifetimes of which are longer than those of transition-metal porphyrins themselves. The photoexcited state of [CoIIITPP]+ (TPP = tetraphenylporphyrin dianion) acts as a good electron acceptor to form an exciplex with hexamethylbenzene, leading to the photocatalytic oxygenation of hexamethylbenzene with dioxygen. In contrast to [CoIIITPP]+, the photoexcited state of [MnIII(TBP8Cz): TBP8Cz = octakis(p-tert-butylphenyl)corrolazinato3-] acts as an electron donor to react with O2 (electron acceptor), producing the superoxo-Mn(IV) complex, by which hexamethylbenzene is oxidized to yield pentamethylbenzyl alcohol. A manganese porphyrin complex that is covalently linked to graphene oxide (GO) sheets is shown to act as a photoredox catalyst for photocatalytic asymmetric epoxidation of trans-stilbene with O2 and isobutyraldehyde in the presence of imidazole to show high enantioselectivity (ee 100%) under photoirradiation. The photoexcited states of photo-labile Mn-porphyrin chlorates or bromates undergo homolytic cleavage to produce active radical species which are responsible for photocatalysis in the photocatalytic oxidation with substrates.