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 H2O2 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 H2O2 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 H2O2 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.