Benzothiazole-Based Covalent Organic Frameworks with Different Symmetrical Combinations for Photocatalytic CO₂ Conversion

The physical and photochemical properties of covalent organic frameworks (COFs) can be tuned by their structural features such as the chemical composition and conjugation of building units. The combination of building units with different intrinsic properties can also influence their intrinsic electronic, adsorption, and optical properties. For the study on the symmetrical feature of COF and its physical/photochemical properties, we prepared benzothiazole-based COFs (TTzTp and BTzTp) with two different combinations, composed of tris-benzothiazole triamine (TTz) or bis-benzothiazole diamine (BTz) with triformylphloroglucinol (Tp), respectively. For their C₃+C₃ and C₃+C₂ symmetries, the COFs, synthesized from benzothiazole core units with approximately twice the difference in their pore dimensions, showed good affinity with CO₂ and light-harvesting properties depending on the conjugation state of building units. The COFs were then applied as photosensitizers in a photocatalytic CO₂ conversion system. The larger-pore-sized BTzTp showed a higher CO₂-to-CO conversion rate than TTzTp, which stemmed from enhanced photocatalytic driving force by suitable band alignment, wider band gap with longer separated charge lifetime, and higher charge carrier mobility with a higher crystalline fraction in the structure.