Counterintuitive Photochemistry of an Isolated Acridinyl Radical: ConPET via Preassembly Despite a Long-Lived Excited State

Photoredox chemistry has seen a dramatic rise in popularity in recent years, but mechanistic understanding has persistently lagged behind reaction development itself. This is particularly true for the emerging area of consecutive photoinduced electron transfer (conPET), which has attracted both great interest due to its ability to activate inert substrates selectively and under mild conditions, and continuing controversy over its mechanistic feasibility. We describe herein the isolation of the key radical intermediate state of an acridinium-based conPET catalyst and detailed investigations of its photochemistry by a suite of (photo)reactivity, photoluminescence, and transient absorption techniques, supported by computational studies. We observe strong wavelength and solvent dependencies in the reactivity profile, which correlate well with observations of a long-lived, fluorescent excited state that would be compatible with diffusion-limited reactivity. However, photoluminescence and transient absorption spectroscopies reveal that, counter-intuitively, this state does not actually participate in reactivity. Instead, we see changes occurring far faster than the diffusion limit, which provides strong, direct evidence for preassembly of the photocatalyst and substrate prior to photoexcitation, rather than any alternative hypotheses of conPET reactivity. Moreover, it demonstrates that such preassembly can be mechanistically important even for photocatalysts that possess a long-lived excited state.