Highly sensitive and selective fluorescent probes for Hg²⁺ in Ag(i)/Cu(ii) 3D supramolecular architectures based on...

Three novel metal-organic assemblies (MOAs), namely, [Ag(2,4'-Hpdc)(4,4'-bpy)]_n (1), [Ag(2,2'-Hpdc)(4,4'-bpy)_0.5]_n (2), and [Cu(2,2'-Hpdc)₂(1,4-bib)]_n (3) [2,4'-H₂pdc = 2,4'-biphenyldicarboxylic acid; 2,2'-H₂pdc = 2,2'-biphenyldicarboxylic acid; 4,4'-bpy = 4,4'-bipyridine; 1,4-bib = 1,4-bis(1-imidazolyl)benzene] have been hydrothermally synthesized by using mixed ligands and characterized by single crystal X-ray diffraction, infrared (IR), elemental analysis and thermogravimetric analysis (TGA). The crystal structures of the three compounds indicate that the hydrogen bonding (C-HO and C-Hπ) and ππ stacking interactions play critical roles in the formation of an extended supramolecular array. The combination of C-Hπ and ππ stacking interactions endow 1 with a 3D network. 2 displays a rare 3D structure with a unique 1D structure based on an eight-membered {Ag₂C₂O₄} ring and compound 3 shows a 1D chain structure, which is propagated to form an extended 3D structure only by C-Hπ hydrogen bonding. The two Ag(i)-compounds display blue emissions in the solid state at 298 K and 77 K. More significantly, compound 1 shows excellent selectivity, fast detection time (<5 min), and high sensitivity (detection limit, 9.63 nM) for Hg²⁺ ions in aqueous solution due to a great enhancement of 1-luminescence, which can be attributed to Hg²⁺ cation binding by a non-coordinated carboxyl group efficiently. This is a rare example of Hg²⁺ detection in aqueous solution based on luminescent silver MOAs. In addition, adsorption spectra reveal the semiconductive nature (2.76 eV for 2, but not detected for 1), thus the role of the AgAg interaction in controlling the performance of the semiconductor properties is highlighted.