Mechanistic Studies of Photoredoxcatalytic DeMayo [2+2] Cycloaddition Reaction
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A complete cycle of photocatalysis requires judicious alignments of excited states between a photocatalyst (PC) and a substrate. We investigated the mechanism of the photoredoxcatalytic DeMayo-type [2+2] cycloaddition. The catalysis involved indol-fused polycycloarenes as PCs activating the triplet state of a substrate. Our mechanistic investigations suggested that the triplet activation did not involve direct Dexter energy transfer. Quantum chemical calculations and transient spectroscopic analyses indicated that the substrate was activated through consecutive steps of initial photoinduced electron transfer from the photoexcited catalyst to a substrate, and charge recombination to populate the triplet state of a substrate. This unusual finding offers an understanding of the photocatalysis and provides valuable insights into substrate triplet state activation.