Stable and emissive covalent organic frameworks as highly sensitive and selective sensors for hydrazine

Covalent organic frameworks (COFs) have attracted considerable interest owing to their lightweight properties, remarkable durability, and well-organized π-structures, positioning them as excellent candidates for emissive sensors. This research centers on the synthesis and characterization of two stable, emissive hydrazone-linked COFs (SEH-COFs) developed through solvothermal methods. The hydrazone linkages, together with the N-H single bond groups on the COF walls, are essential in reducing fluorescence quenching, which commonly occurs due to aggregation, thereby enhancing the emission activity of SEH-COFs. Furthermore, methoxy groups act as electron donors, delocalizing the electronic cloud from the vertex to the hydrazone linkage through p-π conjugation, which enhances the stability of SEH-COFs. The SEH-COFs obtain abundant interaction sites, primarily nitrogen and oxygen atoms, which facilitate efficient interactions with guest molecules. This feature contributes to the SEH-COFs' remarkable fluorescence quenching efficiency, reaching up to 82 %. Additionally, SEH-COFs exhibit high sensitivity and selectivity for hydrazine detection, with an exceptionally low detection limit of 0.78 nM in water, positioning them as one of the most effective fluorescent probes reported to date. This study emphasizes the importance of interaction sites in enhancing the performance of COF-based sensors and paves the way for developing high-performance emissive materials.