Charge-neutral organic photocatalysts with exceptional excited-state reducing power

Organic photocatalysts with exceptionally strong reducing power have emerged as powerful tools for enabling transformations that are inaccessible to conventional photoredox systems. However, developing robust, charge-neutral organic photocatalysts that combine extreme excited-state reducing ability with practical stability under visible-light irradiation remains a significant challenge. Herein, we report the rational design of carbazole-based organic photocatalysts featuring multiple sterically compact amino substituents installed at positions with large HOMO coefficients. Among the substituents evaluated, azetidinyl groups act as particularly effective electron donors, outperforming the commonly used dimethylamino group. This enhancement arises from their compact steric profile, which minimizes congestion around the nitrogen atom and promotes a more coplanar orientation of the amino substituent relative to the carbazole core, thereby enhancing conjugation between the nitrogen lone pair and the carbazole p-system. As a result, these charge-neutral photocatalysts exhibit extraordinarily deep negative excited-state oxidation potentials, exceeding those of elemental sodium and lithium, while maintaining sufficient air stability and photostability. Leveraging this exceptional reducing power, visible-light-driven C–H carboxylation of a broad range of (hetero)arenes with CO 2 proceeds smoothly under mild conditions, highlighting an advantage of charge-neutral photocatalysts over anionic systems, which can revert to their neutral forms under the acidic CO 2 atmosphere. In addition, the same photocatalyst platform enables challenging reductive hydrodefluorination of aryl fluorides. Collectively, this work defines molecular design principles for achieving extreme reducing power in charge-neutral organic photocatalysts and expands the range of photoreductive transformations accessible under visible light.