Emissive Covalent Organic Frameworks: Improved Fluorescence via Flexible Building Blocks and Selective Sensing of Nitroaromatic Explosives

2D covalent organic frameworks (COFs) are attractive for fluorescence sensing due to their lightweight, robust, and highly ordered porous structures. However, the highly conjugated structures between adjacent layers of covalent organic frameworks can often result in aggregation-caused quenching (ACQ) properties. Here, the study designs two flexible hydrazone-linked COFs to suppress ACQ effects, thereby enhancing their luminescent activities. Furthermore, the high density of nitrogen and oxygen atoms on these flexible walls serves as binding sites for hydrogen bonding interactions, indicating sensitivity and selectivity towards 2,4,6-trinitrophenol detection. Two flexible emissive hydrazone-linked COFs (HL-COFs) are synthesized. The incorporation of flexible units onto the walls effectively inhibits the fluorescence quenching process caused by ACQ, resulting in significantly enhanced fluorescence of the COFs. As a result, HL-COFs show high sensitivity and selectivity for 2,4,6-trinitrophenol sensing. image