A series of CuX (X = Br and I) units bearing Ba-MOFs: structures, fluorescence and sensing properties

As one of the alkaline earth metal ions with high coordination numbers, the barium ion (Ba²⁺) typically shows versatile coordination modes when coordinating with organic ligands to construct metal-organic frameworks (MOFs) featuring novel structural attributes. In this work, based on the diverse coordination modes between Ba²⁺ and isonicotinic acid (HINA) or its derivatives, three MOFs, namely, Ba₂(Cu₂Br₂)(3-F-INA)₄(H₂O) (1, 3-F-INAH = 3-fluoroisonicotinate acid), Ba₂(H₂O)₇Cu₄Br₃(INA)₅ (2) and Ba₂K₅(DMF)₃[Cu₇I₆(INA)₁₀](H₂O)_3.5 (3), with novel structural features are synthesized and characterized. In these compounds, CuX (X = Br and I) participates in the assembly reaction via coordination with the pyridine N atom of the isonicotinate ligand, featuring one-dimensional (1D) chain-like architectures that are incorporated into the resultant 3D MOFs. Significantly, by varying the substituent groups on HINA and altering the halogen species, the luminescent properties of the as-synthesized MOFs could be tuned. Notably, compound 3 exhibits a bright orange emission and demonstrates a selective fluorescence (FL) quenching response toward the sulfur-containing small molecule cysteine (Cys), highlighting its potential utility in FL-based biomolecular sensing. Furthermore, 3 displays sensitive FL sensing toward Fe³⁺ ions. This study represents the first report on Ba-CuX-INA frameworks exhibiting FL sensing properties, thereby providing a valuable foundation for the development of novel FL CuX-based MOFs for sensing applications.