A Stable Viologen-BasedMetal–Organic Frameworkfor Fluorescence Detection of Nitroaromatics and Nitrofuran Antibioticsin Water

The solvothermal reaction of In³⁺ and 1,1′-bis(4-carboxyphenyl)-(4,4′-bipyridinium) dichloride (H₂bcbpCl₂) in the presence of oxalate resulted in the formation of a 3D metal–organic framework (<b>1</b>) with a 6⁵.8 topology. Compound <b>1</b> features a unique oxalate-bridged honeycomb (hcb) layer [In₂(μ₂-C₂O₄)₃]_∞, pillared by bcbp ligands. To the best of our knowledge, this presents the first documented 3D metal–organic coordination polymer constructed by an oxalate-bridged hcb layer. Remarkably, compound <b>1</b> demonstrates selective detection of nitroaromatics and nitrofurans in water through a fluorescence quenching mechanism, exhibiting high quenching efficiencies (<i>K</i>_sv) and low limits of detection (LOD). Mechanistic studies reveal that this quenching phenomenon is attributed to photoinduced electron transfer (PET), inner filter effect (IFE) and fluorescence resonance energy transfer (FRET). Moreover, compound <b>1</b> displays photochromic properties, with the structural variations and electron transfer pathways elucidated through single-crystal X-ray diffraction (SC-XRD) analysis.