Selective encapsulation of azo compounds by tetracationic cyclophane in water and photo-controlled reversible release

Abstract

Many azo compounds and their intermediates are toxic and have been linked to various health issues, representing a growing global problem. Molecular engineering for selective encapsulation of azobenzene compounds is critical, given their significant use in smart materials and prevalence as environmental micropollutants released from the food and dye industries. However, the current host molecules catering to azobenzene compounds are mainly limited to cyclodextrins, pillar[n]arenes and cucurbit[n]urils, demonstrating a moderate affinity. This report describes that a novel 3,3′-bipyridinium-based cyclophane was capable of encapsulating anionic azobenzene compounds in water with high binding affinity and pH stability through electrostatic attraction-enhanced mechanism, surpassing the extensively reported supramolecular systems. 1D&2D NMR experiments, UV–vis spectrum, X-ray crystallography and computational modeling were carried out to understand the host-guest complexation. It's worth noting that the tetracationic cyclophane exhibited good selective and anti-interference encapsulation properties in binary, ternary and seawater systems. Furthermore, upon UV/white light irradiation, the reversible conversion between (E)-4,4′-azobisbenzoate and (Z)-4,4′-azobisbenzoate triggers the dissociation/recomplexation of the host-guest complex within 3 min. This reversible photo-switchable (E)-disodium 4,4′-azobisbenzoate-BPy-Box⁴⁺ supramolecular system holds promise for designing novel materials for extraction/release of azo compounds and other small smart materials.