Insights into Substituent Effects on the Fundamental Photocatalytic Processes of Covalent Organic Frameworks toward H2 Evolution and H2O2 Production Reactions

Covalent organic frameworks (COFs) have demonstrated enormous potential in photocatalysis. To construct more efficient COF-based photocatalysts, it is essential to delve into the relationship between molecular-level structure of the COF and the fundamental photocatalytic processes. COF is built by small molecular monomers, so the classic substitution effect on small molecules should be accumulated in the COF. However, to accurately investigate the substituent effect, the other structural parameters of the COF should be kept as unchanged as possible. This work designed and constructed COFs with identical skeleton but different substituents (-H, -CH3, and -OH) at the same position, which displayed very similar crystallinity, surface area, pore size distribution, and morphology, but completely different photocatalytic H-2 evolution or H2O2 production performances. Comparative analyses of the fundamental photocatalytic processes indicated that -OH-containing COF possessed broader visible light absorption, higher charge separation efficiency, and more rational surface properties for the reactions compared with -H and -Me-containing COFs, thus resulting in superior photocatalytic performances. This study reveals that a small change in the substituent will lead to big differences in the photocatalytic processes and thus the final photocatalytic performances, which have instructive significance for the structural design and performance evaluation of COF-based photocatalysts.