Induced Charge-Compensation Effect for Boosting Photocatalytic Water Splitting in Covalent Organic Frameworks

Imine-based covalent organic frameworks (COFs) are promising for photocatalytic water splitting, but their performance is often constrained by inefficient charge separation due to the high electron localization nature of polar imine bonds. In this study, we have optimized the electron delocalization across the imine linkage within a COF by implementing a charge compensation effect. This effect is achieved when a strong electron-donating thieno[3,2-b]thiophene linker is directly attached to the iminic carbon of a zinc-porphyrinic COF. This modification significantly reduces the electron binding effect within the imine bonds of the COF, facilitating both in-plane charge separation and out-plane charge transfer to the catalytic site. Conversely, the use of strong electron-withdrawing pyrizine linker aggravates the electron localization at the imine linkage in the ZnP-Pz variant. Consequently, ZnP-Tt shows a substantially improved photocatalytic water-splitting activity under visible light irradiation, with a hydrogen evolution of 44288±2280 μmol g^-1 in 4 h, which exceeds the ZnP-Pz counterpart by a factor of 10. These results offer fresh perspectives for the design of imine-based COFs to overcome their limitations in charge separation efficiency.