Charge-recombinative triplet sensitization of alkenes for DeMayo-type [2 + 2] cycloaddition

Synthetic photochemistry has undergone significant development, largely owing to the development of visible-light-absorbing photocatalysts (PCs). PCs have significantly improved the efficiency and precision of cycloaddition reactions, primarily through energy or electron transfer pathways. Recent research has identified photocatalysis that does not follow energy- or electron-transfer formalisms, indicating the existence of other, undiscovered photoactivation pathways. This study unveils an alternative route: a charge-neutral photocatalytic process called charge-recombinative triplet sensitization (CRTS), a mechanism with limited precedents in synthetic chemistry. Our investigations revealed CRTS occurrence in DeMayo-type [2 + 2] cycloaddition reactions catalyzed by indole-fused organoPCs. Our mechanistic investigations, including steady-state and transient spectroscopic analyses, electrochemical investigations, and quantum chemical calculations, suggest a mechanism involving substrate activation through photoinduced electron transfer, followed by charge recombination, leading to substrate triplet state formation. Our findings provide valuable insights into the underlying photocatalytic reaction mechanisms and pave the way for the systematic design and realization of innovative photochemical processes.