Dual functional N,O,P containing covalent organic frameworks for adsorping iodine and fluorescence sensing to p-nitrophenol and iodine

Covalent-organic-frameworks (COFs) are emerging porous organic materials with fascinating structural features and potential applications in the fields of iodine capture and spectroscopy. However, no dual-functional COFs with both iodine adsorption and fluorescence sensing have been reported. We herein report the construction of the three dual-functional COFs via Schiff base polycondensations with flexible building blocks cyclophosphazene-based aldehydes (NOP-6-CHO) and benzidine (BD) with different substituent groups (-OCH3 and –OH). The resulting COFs show high thermal stability and exhibit different crystallinity. The specific surface area, pore volume, and crystallinity decrease due to the introduction of the substituent group into the linker. Hydrogen bonding interaction makes the significant improvement of microscopic order degree as well as crystallinity of the COFs. Remarkably, the as-prepared COFs exhibit excellent adsorptive performance for iodine molecules via vapor trapping adsorption, and adsorption capacity reach as high as 4.41, 3.78, and 4.47 g g−1 for HBD, HDASD, and HDOBD, respectively. When suspended in polar organic solvents, the three COFs have high fluorescence under UV light. They can detect p-nitrophenol (p-NP) and iodine with high sensitivity and selectivity.