Linkage-Locking Cyclization Enables Hydrolysis-Resistant Imine COFs for Photocatalytic Water Splitting

Imine-linked covalent organic frameworks (COFs) are promising metal-free photocatalysts for overall water splitting (OWS), but their practical use is limited by protonation-initiated hydrolysis at the imine linkage. Here, we use first-principles calculations to examine whether post-synthetic imine-linkage cyclization can reduce the susceptibility of the linkage to proton attack while retaining favorable photocatalytic function. Using the triazine-imine-triazine (TIT) COF as a model, we construct four cyclized derivatives with X = NH, O, S, and Se. Cyclization lowers the proton adsorption affinity at the linkage by ∼0.6 eV and shifts the thermodynamically preferred hydrogen evolution reaction (HER) site from the imine nitrogen to the triazine unit, making the hydrolysis-prone linkage less involved in proton-coupled reduction. The cyclized frameworks also show stronger interfragment polarization, reduced exciton binding energy, and more delocalized excited-state carriers. Electrons accumulate on the 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (TFPT)-derived triazine unit, whereas holes localize mainly on the 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT)-derived phenyl ring, shifting the reduction and oxidation propensities to different parts of the framework. Among the series, TIT-NH gives the highest predicted solar-to-hydrogen (STH) efficiency of 2.4%, compared with 2.2% for the TIT COF. These results identify imine-linkage cyclization as a computationally supported strategy for improving hydrolytic robustness while preserving, and in TIT-NH slightly improving, the photocatalytic water splitting performance in imine-based COFs.