Microdroplet Interfacial Electric Field Triggers Hydroxyl Radical Coupling Toward Efficient H₂O₂ Photosynthesis

The two-electron water oxidation reaction (2e^- WOR) via ·OH coupling offers a direct route for H₂O₂ photosynthesis, but conventional bulk systems remain rigidly limited by high thermodynamic barriers, short-lived ·OH, and disordered H-bond networks. Inspired by the potential of reaction microenvironments to drive thermodynamically unfavorable pathways, we developed a microdroplet photocatalytic platform using hydrophilic nitrogen-rich COFs (Tp-DAPd and Tp-DAPm), achieving H₂O₂ photosynthesis rates of 11.12 and 12.15 mmol·L^-1·g^-1, with 11.6- and 7.9-fold enhancements over their bulk counterparts, respectively. Product-labeling fluorescence imaging microscopy (PLFIM) at the single-microdroplet scale visualized size-dependent acceleration and inward expansion of the interfacial reactive zone. Multiscale mechanistic investigations reveal that the intrinsic microdroplet interfacial electric field actively regulates the 2e^- WOR process by restructuring the H-bond network into a low-entropy state, stabilizing partially solvated ·OH, and inducing a favorable trans-gauche configuration. These effects work together to create a unique microenvironment that promotes water dissociation and ·OH coupling, thereby lowering the thermodynamic barrier for 2e^- WOR under operational conditions. This work represents a microenvironment-oriented strategy to overcome intrinsic thermodynamic limitations and inspires new strategies for the rational design of microdroplet-assisted photosynthetic systems.