HBr-Regulated Smart 2D Copper-Organic Framework for Highly Efficient and Selective Photocatalytic Oxidation

Smart covalent organic frameworks capable of modulating photocatalytic properties responsive to external stimuli represent next generation of emerging catalysts sought by researchers, yet studies on these dynamic materials and structure-activity relationships remain scarce. Herein, we report a smart metal covalent organic framework (Cu₃-MCOF) incorporating copper cyclic trinuclear clusters (Cu₃) as redox-active and structurally adaptive nodes, which exhibits dual responsiveness to hydrogen bromide (HBr) stimulation, undergoing reversible imine protonation and Br^--mediated reconstruction of copper nodes. These transformations induce pronounced narrowing of optical gap from 2.50 eV to 2.02 eV, a redshift in absorption onset from 500 nm to 613 nm, and enhanced charge separation efficiency. Consequently, Cu₃-MCOF-HBr achieves 40-fold increase in photocatalytic activity for α-terpinene oxidation, with superoxide radical selectivity exceeding 99%, compared to 41% for Cu₃-MCOF. The same stimulus-triggered enhancement is demonstrated in the photocatalytic 1,2,4-trimethoxybenzene bromination. Spectroscopic studies and density functional theory (DFT) calculations reveal that synergistic imine protonation and cluster restructuring promote electron localization within Cu₃ units, facilitate spatial charge separation, and shift oxygen adsorption to imine sites, thereby selectively promoting superoxide radical generation. This work provides the smart MCOF photocatalyst based on dynamic metal-cluster nodes and offers fundamental insights into stimulus-driven regulation of photocatalytic pathways and selectivity.