Modulating Charge Dynamics in Hydrazone-Covalent Organic Frameworks through Side-Chain Modification for Enhanced H 2 O 2 Photosynthesis

Covalent organic frameworks (COFs) hold great promise for solar-driven H 2 O 2 production due to their designable donor–acceptor (D–A) structures. However, leveraging side-chain engineering to precisely control these D–A interactions for enhanced performance remains a key challenge. In this study, we introduced a side-chain engineering strategy to optimize D–A COFs for efficient photocatalytic H 2 O 2 production. The strategy involves systematically tuning the electron density of the acceptor units by introducing methoxy, hydrogen, or fluorine substituents into the molecular scaffolds, yielding FMP-COF, FPB-COF, and DFF-COF, respectively. The synthesized methoxy-functionalized FMP-COF exhibited significantly enhanced crystallinity, a narrowed bandgap, and optimized D–A interactions, leading to a notable H 2 O 2 production rate of 5384 μmol g –1 h –1 in pure water, surpassing its hydrogen- and fluorine-substituted counterparts (FPB-COF and DFF-COF, respectively). Comprehensive in situ characterization and theoretical simulations revealed that electron-donating methoxy groups not only facilitate charge carrier separation and migration but also promote a dual-path reaction mechanism. In this mechanism, polarized hydrazone linkages serve as active sites for the water oxidation half-reaction, whereas the optimized electronic structure directs electrons efficiently toward the oxygen reduction pathway. This study established a clear structure–activity relationship, demonstrating that side-chain electronic properties are pivotal in steering photocatalytic efficiency, thereby providing a molecular-level design principle for advanced COF photocatalysts.