Self-assembly of tripyrazolate-linked [M₆L₂] cages for the selective sensing of HSO₃⁻ and gaseous SO₂ by turn-on fluorescence

Owing to their structural designability and tuneable properties, supramolecular metal-organic complexes have recently emerged as promising candidates for the sensing and detection of molecules and anions. Herein, we synthesised three tripyrazolate-linked [M₆L₂] metallocages with the formulas [(bpyPd)₆L₂](NO₃)₆ (1), [(dmbpyPd)₆L₂](NO₃)₆ (2), and [(phenPd)₆L₂](NO₃)₆ (3) (H₃L = tris(4-(5-(trifluoromethyl)-1H-pyrazol-3-yl)phenyl)amine, bpy = 2,2′-bipyridine, dmbpy = 4,4′-dimethylbipyridine, phen = 1,10-phenanthroline). Crystallography revealed that metal-directed coordination and the bidentate chelate behaviour of the ligand induced the self-assembly of supramolecular metal-organic cages. Notably, these cages were employed as turn-on fluorescence sensors for SO₂ and its derivative (HSO₃⁻) through a disassembly mechanism. Cages 1, 2, and 3 showed a highly selective and sensitive detection of HSO₃⁻ over other common anions in aqueous solutions and of SO₂ gas over other common gasses, with an excellent anti-interference ability. These metallocages were subsequently applied as sensors in environmental and biological samples. This study not only enriches the ongoing research on metal-organic supramolecular materials but also facilitates the future preparation of stimuli-responsive supramolecular coordination complexes.