Reversed linkage-oriented intermolecular electron-transfer in isomeric covalent organic frameworks for electrochemical CO reduction.

Designing specific electroactive sites and modulating their local microenvironments of covalent organic frameworks (COFs) toward electrochemical CO reduction (ECR) have received increasing attention. However, the underlying impact of the change in intramolecular electron-transfer capability, caused by the tuning in electronic state of active sites, on the redox-mediated catalytic process still remains inadequately understood. In this work, we constructed a metalloporphyrin-based COF as the isomer of the representative COF-367-Co, with an identical chemical composition but a reversed imine-linkage orientation via Schiff-base condensation reaction using [meso-tetrakis(4-formylphenyl)porphyrin]cobalt (CoTFPP) and Benzidine (BD) as the precursors, denoted as CoTFPP-BD-COF, to exclusively investigate the linkage orientation as an individual variable for enhanced electron transmission efficiency toward ECR. The CoTFPP-BD-COF exhibits impressively higher CO Faradaic efficiencies (FE) of over 90 % than the benchmark COF-367-Co (below 50 %) within a wide potential range of 600 mV. The experimental and computational results collectively suggest that as compared to the isomeric COF-367-Co, the reversal of imine-linkage orientation in CoTFPP-BD-COF not only enhances the CO adsorption capacity of active centers in cobalt porphyrin through tuning its electronic configuration, but also facilitates the intramolecular oriented electron-transfer by suppressing its electron donor effect, thereby synergistically facilitating ECR. This study exclusively demonstrates the linkage contribution in remote electronic tuning of COFs, and uncovers its associated mechanism toward electrosynthesis.