Synergistic Electronic and Steric Engineering of Thiophene Donors in Covalent Organic Frameworks for Efficient CO2 Photoconversion.

Covalent organic frameworks (COFs), featuring broad light absorption and permanent porosity, serve as excellent supports for metal catalytic sites in photocatalytic CO2 reduction, offering great promise for efficient and stable CO2 photoconversion. Herein, two rationally designed thiophene-rich monomers enable the effective coupling of thiophene and benzothiadiazole units within the same COF materials, yielding D-A type COFs with bex topological structures (TTB-COF and TTP-COF). By modulating the central unit, the intramolecular built-in electric field is enhanced, and the rotational freedom of thiophene units is also constrained, leading to a wavy structure in the monolayer COF, which significantly improves the material's photocatalytic performance. Using 2,2'-bipyridine as an additive, TTB-COF-Co achieves a high CO production rate of 28.0 mmol g-1 h-1 with a selectivity of 88.4% in the presence of the photosensitizer [Ru(bpy)3]Cl2·6H2O and the sacrificial agent triethanolamine (TEOA), being one of the best performances among COF-based catalysts. Analysis of the cumulative free energy change diagrams reveals that TTB-COF-Co lowers the energy barrier of the rate-determining step in CO2 photoreduction, thereby demonstrating superior photocatalytic performance. This work provides a referable design strategy for constructing high-performance COF-based photocatalysts for CO2 reduction.