N-position isomerization to enhance the CO2 photoreduction performance of bipyridine-based covalent organic frameworks

Covalent organic frameworks (COFs) that incorporate heterocycles featuring multiple nitrogen atoms, such as triazine, bipyridine, and heptazine, are seen as promising heterogeneous photocatalysts for CO₂ photoreduction, while the substantial effect of N-atom relative locations in this transformation has rarely been systematically studied. Herein, a collection of isostructural porphyrin-based COF catalysts bearing bipheyl, 2–2′-bipyridine and 3–3′-bipyridine are judiciously designed for CO₂ photoreduction under simulated sunlight irradiation (1000 W/m²). As results, comparing with the frequently used biphenyl and 2–2′-bipyridine unit, the porphyrin COF incorporating 3–3′-bipyridine yields a significant enhancement in photoelectric properties, and the CO production rate of 3–3′-Bpy-COF is 2 and 10 times higher than that of 2–2′-Bpy-COF and Bp-COF. Insights on the origin of the improved photocatalytic activity and reaction mechanism are further investigated, revealing above tiny variation of N sites results in a larger dipole moment for the 3–3′-Bpy-COF, facilitating the separation and transfer of photogenerated excitons. Moreover, The X-ray Photoelectron Spectroscopy (XPS) and Density Functional Theory (DFT) calculations reveal that the 3–3′-bipyridine N-atoms can strategically adjust the electron density surrounding the M−N4 active center of porphyrins, which stabilize the *COOH intermediate and reduce the energy barrier of the rate-determining step. This work highlights the collaborative influence of porphyrin and the steric location of N atoms within COFs photocatalysts for CO₂ reduction reaction, offering a new insight to design COFs endowed with exceptional capabilities for photocatalysis.