Three new luminescent Cd(ii)-MOFs by regulating the tetracarboxylate and auxiliary co-ligands, displaying high sensitivity for Fe3+ in aqueous solution

Abstract

Three new Cd(II)-MOFs, [Cd(L₁)_0.5(bpp)] (1), [Cd(L₁)_0.5(bipy)] (2), and [Cd(L₂)_0.5(bipy)] (3) [H₄L_n = (1,1′:4′,1′′-terphenyl)-2′,x,x′′,5′-tetracarboxylic acid (n = 1, x = 3, and n = 2, x = 4), bpp = 1,3-di(4-pyridyl)propane, and bipy = 2,2′-bipyridine], have been synthesized using Cd(II) ions and isomeric H₄L_n ligands in the presence of auxiliary ligands via solvothermal reaction, and have been fully characterized by elemental analysis, FT-IR spectroscopy, powder X-ray diffraction (PXRD), and thermogravimetric analysis (TGA). Complex 1 is a three-dimensional (3D) pillar-layered network built from L₁⁴⁻ and bridging bpp co-ligands with a binodal (4,8)-connected gsp2 topology and a symbol of (4⁴·6²)(4¹⁶·6¹²). 2 was successfully synthesized using bipy instead of the bpp ligand used to assemble 1 giving a 2D 4-connected sql net. 3 is a 3D reticular framework with [Cd₂(COO)₄] SBUs based on L₂⁴⁻ ligands, displaying a (4,4)-connected lvt (4²·8⁴) topology. X-ray diffraction analysis revealed that the structures of the MOFs can be mediated by the steric effect of the tetracarboxylate and secondary co-ligands. The solid-state luminescence properties of the MOFs were measured carefully by ultraviolet irradiation at room temperature. In particular, 3 exhibited a high sensitivity for Fe³⁺ in aqueous solutions of mixed metal ions, which makes it a promising crystalline material to be a luminescent probe for Fe³⁺. Importantly, 3 can keep its original framework and be reused in sensing experiments. The sensing mechanism has also been studied in detail.