Regulating energy band structures of triazine covalent organic frameworks with electron-donating/withdrawing substituents for visible-light-responsive photocatalytic tetracycline degradation and Cr(VI) reduction

Environmental contaminations have raised soaring concerns about human health worldwide. Developing metal-free photocatalysts as green agents to solve these problems is urgent. Covalent organic frameworks (COFs) are considered a promising platform for the molecule-level design of visible-light-responsive photocatalysts due to their tailored coordination/electronic structures and excellent charge carrier mobility. However, COFs without substituents (e.g., COFs-H) still suffer from broad bandgaps and low electron-hole separation efficiency. In this work, we introduced electron-donating/withdrawing substituents on COFs-H to fine-tune the bandgap and photocatalytic performance of COFs. Theoretical and experimental studies revealed that all substituents narrowed the bandgap of COFs and enhanced the electron-hole separation efficiency. Electron-withdrawing/donating substituents significantly alter the energy level of COFs-R, improving the redox capacities of photo-generated holes and electrons for tetracycline (TC) degradation and Cr(VI) reduction. The large difference in electrostatic potential between the two monomers in COFs-R enhances the charge carrier generation and intramolecular electron transfer intrinsically. This work unravels how substituents with different electronic effects regulate the energy band structures and photo-redox capacities of COFs. It further provides new insight into the precise regulation of COFs toward highly efficient visible-light-driven photocatalytic remediation of organic contaminants and heavy metal ions.