Azine- and azo-based flexible covalent organic frameworks for fluorescence sensing nitro-aromatic compounds and iodine and adsorbing iodine

By Schiff base polycondensation reactions, hydrazine hydrate (HAZ) or 4,4′-Azodianiline (AZDA) with hexa(4-formyl-phenoxy)cyclotriphosphazene (NOP-6-CHO) under solvothermal conditions yield flexible crystalline covalent organic frameworks (HHAZ and HAZDA). The azine- and azo-based frameworks showcase permanent porosity, outstanding thermal and chemical stability. HHAZ and HAZDA are highly fluorescent dispersed in polar solvents and emit green and light blue fluorescence under UV light of 365 nm, respectively. HHAZ and HAZDA can detect 2,4-dinitrotoluene (DNT), trinitrophenol (TNP) and iodine with extremely high sensitivity and selectivity. The Stern–Volmer constants of HHAZ for DNT and HAZDA for TNP are 9.40 × 104 and 1.59 × 105 L mol−1. The Stern–Volmer constants of HHAZ and HAZDA for iodine are 2.76 × 104 and 1.56 × 104 L mol−1. Limits of detection are from 1.89 × 10−12 to 1.09 × 10−10 mol L−1. Moreover, pretty and reversible adsorption properties to volatile iodine which can up to 2.27 and 5.18 g g−1 are found in the prepared COFs. Polyiodide plays a momentous role in iodine adsorption, as we detected high contents of polyiodide (I3−and I5−) in adsorbents.