Twist to Activate: Structural Distortion–Enhanced Photocatalysis in Indenophenanthrenes

Abstract

Indenophenanthrenes have remained largely overlooked as photocatalysts because their intrinsically high exciton binding energies and low molar absorption coefficients restrict both light-harvesting efficiency and single-electron transfer (SET) capability. In this work, we introduce a twisted indenophenanthrene derivative that overcomes these long-standing limitations through intentional distortion of its π-conjugated framework. The resulting nonplanar geometry enhances light absorption and substantially decreases electron–hole pair binding energy, thereby enabling efficient photoinduced SET for the first time within this molecular family. This structurally engineered chromophore functions as a robust metal-free photoredox catalyst. Its catalytic performance was validated through a prototypical decarboxylative coupling reaction between α-amino acids and electron-deficient alkenes. Under mild visible-light irradiation, the catalyst delivers the desired C–C bond-forming products in high yields across a broad substrate scope. A wide range of natural and synthetic amino acids, as well as diverse alkene acceptors, are well tolerated, demonstrating the generality and versatility of this newly developed catalyst platform. Mechanistic studies comprising radical trapping, fluorescence quenching, Stern–Volmer analysis, and a series of control experiments collectively provide compelling evidence for an oxidative quenching pathway mediated by radical intermediates. Over-all, this study establishes a modular design principle for engineering the photophysical and electrochemical properties of purely organic photocatalysts through geometric twisting. By demonstrating that twisted indenophenanthrenes can mediate challenging redox transformations under mild, metal-free conditions, this work positions them as a promising new class of sustainable organic photoredox catalysts for advanced synthetic applications.

Keywords

Planar twist / π-Conjugation distortion / Indenophenanthrenes / Visible-light photoredox / Decarboxylative coupling / Radical intermediates / Photoinduced single-electron transfer / Organic photocatalyst

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