Light-Induced Dynamic Burstein-Moss-Type Modulation in Polar Hydrazone-Linked COFs for H₂O₂ Photosynthesis

Efficient photocatalytic H₂O₂ synthesis in covalent organic frameworks (COFs) requires photogenerated electrons that retain sufficient reducing power after exciton dissociation for O₂ activation and proton-coupled *OOH formation. However, separated electrons may undergo low-energy relaxation, localization, trapping, or recombination before reaching reactive sites, thereby weakening their effective reactivity. In photoexcited semiconductors, bandgap renormalization (BGR) commonly favors optical-gap narrowing and lower-energy transitions. Herein, we report BTH-BTDA-COF, a polar thienyl-regulated hydrazone-linked COF that exhibits light-induced dynamic Burstein-Moss-type modulation enabled by local charge inhomogeneity. Thienyl incorporation amplifies asymmetric charge polarization, promoting exciton dissociation and photoinduced electron enrichment. Excitation-density-dependent PL spectral redistribution, illumination-dependent transient absorption evolution, light-induced surface-potential changes, and prolonged charge-separated-state dynamics collectively support carrier-density-dependent state filling, which suppresses low-energy relaxation and helps preserve photogenerated electrons with sufficient reducing power for O₂ activation. Meanwhile, the hydrazone-linked microenvironment provides proton-accessible hydrogen-bonding sites that stabilize oxygenated intermediates and facilitate proton-assisted *OOH formation. Consequently, BTH-BTDA-COF achieves an H₂O₂ production rate of 7.5 mmol g^‒1 h^‒1, an apparent quantum yield (AQY) of 11.4% at 420 nm, and a solar-to-chemical conversion (SCC) efficiency of 1.04%. This work establishes dynamic Burstein-Moss-type modulation as a molecular strategy for regulating post-dissociation electron reactivity and promoting efficient H₂O₂ photosynthesis in COFs.