Attenuating Imine Bond Polarization in Covalent Organic Frameworks Accelerates Charge and Proton Transport for High-Efficiency Photocatalytic Hydrogen Evolution.

While the structural tunability and π-electron delocalization of covalent organic frameworks (COFs) make them promising candidates for photocatalytic hydrogen evolution, the intrinsic polarization of their imine bonds often impedes efficient charge separation and transport. To overcome this fundamental kinetic barrier, we report a molecular engineering strategy that attenuates imine bond polarization through the functionalization of COF pore walls with electron-donating methoxy (-OMe) motifs. This structural modulation significantly enhances π-conjugation and interlayer interactions, promoting efficient charge separation and extending charge carrier lifetimes. Furthermore, the methoxy groups facilitate optimal proton transport by establishing an ordered hydrogen-bond network. Ultrafast transient absorption spectroscopy and proton conductivity measurements conclusively verify the generation of long-lived polarons and rapid proton hopping within the framework. Consequently, the engineered methoxy-functionalized COF achieves an exceptional photocatalytic hydrogen evolution rate of ca. 100 mmol h g and an apparent quantum yield of 21.8 ± 0.80% at 450 nm, vastly outperforming its neutral and electron-withdrawing counterparts. These mechanistic insights provide a dependable blueprint for the rational design of high-performance imine-linked COF photocatalysts.