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

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ABSTRACT 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 (Δ E ST 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 ( E T 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 Δ E ST and efficient ISC are key to achieving high TEnT reactivity.

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