Beyond Energy Transfer: Ground-State Association-Driven [2+2] Cycloadditions with Indole-Fused Organophotocatalysts

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 usage 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 display enhanced π–π interactions and the 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.