Methylation‐Induced Permanent Charge Polarization in Covalent Organic Frameworks for Visible Light‐Driven Water Decontamination and Disinfection

Abstract Covalent organic frameworks (COFs) show promise for photocatalytic environmental remediation and antibacterial applications; however, their efficiency is often constrained by strong excitonic effects that impede charge separation. Here, a targeted in situ methylation strategy is reported to engineer permanent cationic centers within a robust non‐substituted quinoline‐linked COF (NQ‐COF _S1 ). Methyl grafting at the nitrogen sites generates quaternary ammonium groups, inducing pronounced local charge polarization and a strong built‐in electric field. This modification drastically reduces the exciton binding energy from 41 to 33 meV, thereby promoting highly efficient charge separation. In synergy with electron‐rich thiophene units, the resulting NQ‐COF _S1 ‐Me exhibits outstanding photocatalytic activity, characterized by strong reactive oxygen species generation. It achieves > 95% inactivation of Gram‐positive, Gram‐negative, and drug‐resistant bacteria within 10 min, and 96.84% degradation of chloramphenicol—38.72 times faster than NQ‐COF _S1 . These findings demonstrate that methylation‐induced permanent charge polarization offers a powerful strategy for developing high‐performance photocatalytic COFs with broad potential in environmental and public health applications.