Channel surface engineering in pore microenvironments of covalent organic frameworks for photocatalytic hydrogen evolution

Engineering the pore microenvironment of covalent organic frameworks (COFs) is important to boost their photocatalytic activity. However, current strategies focus on the post‑functionalization of COFs and the bottom-up method using linkers with substituents directly attached to the backbone, but either approach often yields a non‑uniform distribution of functional groups or disturbs the framework’s electronic structure, obscuring clear structure-function correlations. Here we have synthesized a series of linear oligo(phenylenevinylene) (OPV)-based monomers, L-OPV-Ph-R (R = H, F, OMe), in which the side groups slightly impact the electronic properties of the OPV backbone, decoupling pore-microenvironment tuning from backbone electronics. The resulting isoreticular COFs-Ph-R demonstrate consistent structural features, as confirmed by crystallinity, specific surface area, pore size distribution, and light absorption measurements, establishing a clean platform for probing microenvironment effects. COF‑Ph‑OMe achieves a hydrogen‑evolution rate of 28.8 mmol g−1 h−1 under visible light, surpassing its fluoro‑ and unsubstituted counterparts. Finally, photophysical experiments, theoretical calculations, and hydrophilicity/hydrophobicity measurements confirm that the hydrophilic, electron‑donating –OMe group lowers the charge‑transfer barrier and enhances water access within the pores, whereas the hydrophobic –F group hinders both processes. These findings demonstrate that subtle channel surface engineering—performed without disturbing the electronic skeleton—offers an effective route to rationally optimize COF photocatalysts for hydrogen evolution.