Changing Benzoxazole Ring into Nonring Imine Linkages on Covalent Organic Frameworks with Tuning HO Photosynthesis Performance.

Two π-conjugated covalent organic frameworks (COFs) with nonring imine or benzoxazole ring linkages were prepared by reacting 3,3'-dihydrooxybenzidine (BDOH) with 3,5-triformylbenzene (Tb) in the presence or absence of benzimidazole (BDOH-Tb- and BDOH-Tb-). Although two COFs indicated similar composition, crystalline structures, and morphologies, imine-based BDOH-Tb- exhibited a photocatalytic HO production rate of 2490 μmol·g·h in sacrificial reagent-free pure water, higher than that of benzoxazole-based BDOH-Tb- (1168 μmol·g·h). The higher photocatalytic activity of BDOH-Tb- was attributed to its more efficient photoinduced charge separation and utilization efficiency and different 2e ORR active sites over the two COFs. This study demonstrated an available ring effect to adjust photocatalytic performance between π-conjugated COFs.