Energy Transfer Catalysis Enabled by Multichannel Through-Space Charge Transfer TADF Photosensitizers: [2 + 2] Photocycloaddition to Cyclobutane-Fused Phosphindole 3-Oxides.

Triplet energy transfer (TEnT) photocatalysis has emerged as a powerful tool for enabling mild and selective bond-forming reactions. However, its generality remains limited by the intrinsic trade-offs in existing photosensitizers (PSs). Noble metal (e.g., Ir, Ru) complexes offer high efficiency yet suffer from cost and sustainability concerns, while organic carbonyl sensitizers suffer from weak visible-light absorption and short-lived triplet states. Herein, we report heavy-atom-free organic PSs featuring a multi-channel through-space charge transfer (TSCT) architecture within a thermally activated delayed fluorescence (TADF) framework. A highly twisted donor-acceptor topology enforces near-complete frontier orbital separation, leading to ultralow singlet-triplet energy gaps (ΔEST down to 0.013 eV) and thereby enabling efficient spin interconversion. Consequently, these PSs simultaneously achieve high intersystem crossing (ISC) quantum yields, microsecond-scale triplet lifetimes, and high triplet energies (ET up to 63.7 kcal mol-1), rivaling those of state-of-the-art noble-metal systems. Leveraging these features, we demonstrate the first visible-light-driven intermolecular [2 + 2] cycloaddition of 1,3-diphenylphosphindole 1-oxides with alkenes and alkynes, enabling rapid access to structurally complex cyclobutane-fused phosphindole 3-oxides that hold potential value in drug screening but are typically difficult to obtain. Mechanistic studies support a TEnT pathway, in which minimal ΔEST and efficient ISC are key to achieving high TEnT reactivity.