Enhancing Photosynthesis Efficiency of Hydrogen Peroxide by Modulating Side Chains to Facilitate Water Oxidation at Low‐Energy Barrier Sites

Abstract Hydrogen peroxide (H 2 O 2 ) is a crucial oxidant in advanced oxidation processes. In situ, photosynthesis of it in natural water holds the promise of practical application for water remediation. However, current photosynthesis of H 2 O 2 systems primarily relies on oxygen reduction, leading to limited performance in natural water with low dissolved oxygen or anaerobic conditions found in polluted water. Herein, a novel photocatalyst based on conjugated polymers with alternating electron donor–acceptor structures and electron‐withdrawing side chains on electron donors is introduced. Specifically, carbazole functions as the electron donor, triazine serves as the electron acceptor, and cyano acts as the electron‐withdrawing side chain. Notably, the photocatalyst exhibits a remarkable solar‐to‐chemical conversion of 0.64%, the highest reported in natural water. Furthermore, even in anaerobic conditions, it achieves an impressive H 2 O 2 photosynthetic efficiency of 1365 µmol g −1 h −1 , surpassing all the reported photosynthetic systems of H 2 O 2 . This remarkable improvement is attributed to the effective relocation of the water oxidation active site from a high‐energy carbazole to a low‐energy acetylene site mediated by the side chains, resulting in enhanced O 2 or H 2 O 2 generation from water. This breakthrough offers a new avenue for efficient water remediation using advanced oxidation technologies in oxygen‐limited environments, holding significant implications for environmental restoration.