A novel hydrolytically stable fluorescent Cd(II) coordination polymer showing solvent-dependent multi-responsive fluorescence sensing to pH and some metal ions

Abstract

Herein, the assembly between the multi-functional 2-amino-4-sulfonyl-benzoic acid (H2asba) and Cd(II) in the presence of the hydrophobic ligand 4-bis[(2-methyl-1H-imidazole-1-yl)methyl]benzene (2-mmzm) results in a novel hydrolytically sTable 1D Cd(II)-FCP, {[Cd(asba)(2-mmzm)(H2O)2]·3H2O}n (1). The water tetramer-based hydrogen-bonded ring R22 (8) bridges the chains into a 2D supramolecular layer. The novel discrete water decamer further links the layers into a 3D supramolecular network.1 shows extremely acidic and basic resistance in a wide pH range of 1.0–12.0. More importantly, 1 exhibits accurate sensing of pH of the artificial gastric juices. Interestingly, 1 demonstrates solvent-dependent multi-responsive fluorescence sensing behavior to metal ions. 1 can sense Fe3+/Cu2+/Tb3+ in water, Fe3+/Cu2+/Tb3+/Co2+ in MeOH and Fe3+/Cu2+/Tb3+/Co2+/Ba2+ in ethanol, respectively. The fluorescence sensing of 1 toward Fe3+/Tb3+/Ba2+ is resulted from the structural collapse in the corresponding solvents. However, 1 keeps integrity and shows higher chemical resistance to Cu2+ and Co2+ in the corresponding solvents. The fluorescence sensing of Cu2+ can be ascribed to the photo-induced electron transfer (PET) and fluorescence resonance energy transfer (FRET) mechanism. The sensing of Co2+ can be attributed to the PET mechanism. The solvents have great influence on the sensing sensitivities to Cu2+ and Co2+. 1 shows the highest sensing sensitivity to Cu2+ and Co2+ in ethanol.