Beyond energy transfer: ground-state association-driven [2 + 2] cycloadditions with indole-fused organophotocatalysts

Abstract

A visible-light-driven [2 + 2] cycloaddition strategy with indole-fused organophotocatalysts (organoPCs) developed in our laboratory is presented, highlighting a sustainable approach with minimal solvent use and no sacrificial reagents. Mechanistic investigations, supported by spectroscopic analyses and density functional theory (DFT) calculations, suggest that this transformation proceeds via a ground-state association mechanism rather than the more commonly proposed energy transfer pathway. Specifically, noncovalent interactions between the organoPC and a cinnamate substrate enable the formation of a [PC⋯substrate] complex, which, upon photoexcitation, engages in an efficient route to the triplet state that drives [2 + 2] cycloaddition. Structural tuning of the organoPC framework proves critical to catalytic performance, as pentacyclic architectures featuring extended π-conjugation exhibit enhanced π–π interactions and superior reactivity. This design principle facilitates regioselective cycloadditions across a broad range of functionalized cinnamate derivatives, highlighting the versatility and atom economy achievable under visible-light irradiation.