Mechanistic Insights Into Protonation of Imine-Linked Covalent Organic Frameworks: The Correlations With...

Covalent organic frameworks (COFs), especially imine-linked ones, are promising due to facile synthesis, stability, and tunable electronics. In these COFs, imine N acts as a Lewis basic site, exhibiting variable protonation behavior depending on its interaction with protons. Ascorbic acid is commonly used as a sacrificial electron donor in imine-COF photocatalysis, and its protonation behavior affects hydrogen production. A full understanding of protonation factors and their impact on performance is lacking, hindering optimal design. In this work, we investigated the protonation mechanisms of imine-linked COFs using model photocatalysts with varying electron-donating abilities and steric hindrances. In photocatalytic sacrificial hydrogen evolution, we established correlations between protonation behavior and critical photocatalytic process, including light absorption, hydrophilicity, charge separation efficiency, reaction kinetics. Contrary to reports of strong D-A interactions, we found that weak D-A interaction and a planar imine COF (COF-BTT-H) with significant protonation and preserved planarity deliver the highest performance (50.08 mmol g^-1 h^-1). Guided by these insights, we redesigned a new imine-linked Pt-loaded COF catalyst, COF-Py-H, achieving 488.48 mmol g^-1 h^-1 H₂ evolution with ascorbic acid as sacrificial donor and proton source. This protonation-focused study broadens COF molecular diversity and highlights the importance of protonation in designing imine-linked COF catalysts.