Band Gap Engineering in Solvochromic 2D Covalent Organic Framework Photocatalysts for Visible Light-Driven Enhanced Solar Fuel Production from Carbon Dioxide

Solar light-driven fuel production from carbon dioxide using organic photocatalysts is a promising technique for sustainable energy sources. Band gap engineering in sustainable organic photocatalysts for improving efficiency and fulfilling the requirements is highly anticipated. Here, we present a new strategy to engineer the band gap in covalent organic framework (COF) photocatalysts by varying the push–pull electronic effect. To implement this strategy, we have designed and synthesized four different COFs using a tripodal amine 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)tris(([1,1′-biphenyl]-4-amine)) [Ttba] with 1,3,5-triformylbenzene (COF-1), 2,4,6-triformylphloroglucinol (COF-2), 2,4,6-triformylphenol (COF-3), and 2,4,6-triformylresorcinol (COF-4). On varying the number of hydroxyl units in the aldehyde precursor, the resulting COFs allow the fine-tuning of their band gap and band edge positions and result in different morphologies with varying surface areas. The enhanced optical properties of COF-3 and COF-4 with very suitable band gaps of 2.02 and 1.95 eV, respectively, enable them to demonstrate a high-efficiency photobiocatalytic system for NADH photoregeneration and enhanced visible light-driven formic acid production at a rate of 226.3 μmol g–1 in 90 min. The triazine core enables efficient charge separation, while the hydroxyl groups induce an electronic push–pull effect, regulating their photocatalytic efficiency. The results demonstrated the morphology-guided enhanced surface area and dual keto–enol tautomerism-induced push–pull effect in asymmetrical charge distribution as key features in the fine-tuning of the photocatalysts.