Synergistic Light Harvesting and Catalysis in a Photoactive π-Scaffolded Covalent Organic Framework−Re Hybrid for CO ₂ -to-CO Photoreduction

Integrating strong light harvesting with spatially isolated yet electronically coupled catalytic sites is essential for advancing heterogeneous CO2 photoreduction. Here, we report TU-62, a 2D (6,2)-connected covalent organic framework (COF) synthesized by condensing a hexatopic D3h-symmetric 2,3,6,7,10,11-hexakis(4-formylphenyl)triphenylene (HFPTP) node with a 2,2′-bipyridine-5,5′-diamine (Bpy) linker. The HFPTP unit provides an extended π-donor scaffold that reinforces long-range conjugation and efficient exciton migration, while the bipyridine linker serves as a directional electron-accepting and metal-chelatable bridge. This combination yields a highly crystalline, permanently porous, and π-conjugated hxl-topology COF ideal for photoredox applications. Postsynthetic metalation with Re(CO)5Cl affords TU-62-Re, a framework-tethered analogue of [Re(bpy)(CO)3Cl], in which the COF backbone acts as a light-absorbing antenna and electron-delocalizing support, while the immobilized Re centers serve as well-defined catalytic sites. TU-62-Re exhibits strong visible-light absorption and significantly enhanced performance relative to its homogeneous counterpart, achieving a CO evolution rate of 4.42 mmol g−1 h−1 with 95.9% selectivity under photosensitizer- and cocatalyst-free conditions. This activity surpasses all previously reported rhenium-based photocatalysts, establishing TU-62-Re as the current benchmark for Re-mediated CO2 photoreduction. Notably, under only 3% CO2, it maintains a high rate of 3.74 mmol g−1 h−1 with 97.7% selectivity, underscoring its potential for practical carbon-capture and syngas-relevant CO generation from diluted CO2 streams. Density functional theory reveals that CO2 reduction proceeds most favorably via a ligand-exchange pathway, wherein CO2 replaces a coordinated ligand at Re, dramatically lowering the overpotential and suppressing competing hydrogen evolution; explicit solvent assistance further reduces the ligand-exchange barrier.