Phenoxazine-based supramolecular metal-organic cage for picric acid detection

A phenoxazine-based metal-organic cage Zn2L3 (L = N-n-butyl-3, 7-dipyridine phenoxazine) was synthesized by the coordination-driven self-assembly, which could work as a fluorescent sensor for sensitive and selective detection of picric acid (PA) among nitro-explosives. The cage contains three V-shaped ligands L linked by zinc ions, and the fluorescence of the ligands is preserved and transferred to the cage. Different from the free phenoxazine-based ligand, Zn2L3 showed strong emission at 568 nm and exhibited the highest fluorescence quenching efficiency toward picric acid over other related nitroaromatic explosives. The cage structure with cavities exhibits the better sensing property than monomeric ligand. Electrospray ionization mass spectrometry (ESI-MS), NMR and fluorescence titration suggested that the formation of host-guest complex of the cage with the picric acid target, should be thought as the key factor for its nice recognition ability.