The cyclotriphosphazene-based covalent organic frameworks constructed entirely by flexible building blocks for adsorping and fluorescence sensing iodine

Abstract

A series of flexible cyclotriphosphazene-based COFs, we call them fully flexible cyclotriphosphazene-based COFs (HDADE, HBAPB and HBPDA), were prepared by means of reversible Schiff base polycondensation reactions for the first time, which were constructed entirely by flexible building blocks, not only hexa(4-formyl-phenoxy)cyclotriphosphazene (NOP–6–CHO) as a flexible knot but also aromatic ether diamine as flexible linkers. The fully flexible cyclotriphosphazene-based COFs are crystalline porous materials. Their crystallinity and BET specific surface areas increase with the length of the linkers, and are higher than that of the corresponding COFs constructed by the rigid linkers. HDADE, HBAPB and HBPDA can absorb iodine in both gas or liquid phase. The adsorptive capacities are 5.22, 4.55, and 3.86 g g−1, respectively, which are higher than the corresponding COFs formed by rigid linkers. The fully flexible cyclotriphosphazene-based COFs can fluorescently sense iodine with high sensitivity. Their quenching constants (KSV) are 2.22 × 104, 7.07 × 104, and 1.04 × 104 L mol−1. Their LODs are 1.35 × 10−10, 4.24 × 10−11, and 2.88 × 10−10 mol L−1, respectively. The fluorescence quenching of iodine to the fully flexible cyclotriphosphazene-based COFs has both photo-induced charge transfer and resonant energy transfer processes.