Luminescence covalent organic frameworks for metal ions detection via turntable sites

In recent years, environmental issues have been caused by heavy metals, which have not been effectively resolved. Fluorescent materials are being used as promising detectors. Covalent organic frameworks (COFs) possess excellent porosity that provides extensive space for guest molecule. The turntable skeleton is also a crucial factor in designing the target site for capturing them. In this study, the hydrazone-based crystalline H–COF-1 and H–COF-2 were successfully synthesized through a solvothermal process. Their luminescence activities are controlled from blue to green for H–COF-1 and H–COF-2 due to the turntable conjugated sites on the walls. More specifically, the intramolecular hydrogen between the ortho-hydroxyl group and imine bond constructs the conjugations binding site of H–COF-2. H–COF-2 possesses more interacted sites for metal ions and demonstrates efficient fluorescence quenching for Cu2+ at 85 % in water, surpassing the performance of H–COF-1. The selectivity and detection limits were also significantly improved as a result. The turntable sites present a novel design approach for luminescent materials as chemical sensors.