1,3-Alternate Calix[4]arene Functionalized With Pyrazole and Triazole Ligands as a Highly Selective Fluorescent Sensor for Hg and Ag Ions.

We report here the synthesis of a 1,3-alternate calix[4]arene , with bis-pyrazolylmethylpyrenes on the one end and bis-triazolylmethylphenyls on the other end, as a homoditropic fluorescent sensor for both Hg and Ag ions. Calix[4]arene , with lower-rim bis-pyrazolylmethylpyrenes in cone conformation, was also synthesized as a control compound. UV-Vis and fluorescence spectra were used for metal ions screening, and we found that both ligands and showed strong excimer emission of pyrenes when they are as a free ligand in CHCl/MeOH (v/v, 3:1) solution; however, they both showed a high selectivity toward Hg and Ag ions with strong fluorescence quenching and yet with different binding ratios. The fluorescence of ligand was strongly quenched by Hg but was only partially quenched by Ag ions; however, the fluorescence of ligand was strongly quenched by Hg, Ag, and Cu ions. Job plot experiments showed that ligand formed a 1:2 complex with both Hg and Ag ions; ligand formed a 1:1 complex with Hg, but it formed a 2:3 complex with Ag. The binding constant of ligand with Hg and Ag ions was determined by the Benesi-Hildebrand plot of UV-vis titration experiments to be 2.99 × 10 and 3.83 × 10 M, respectively, while the association constant of ligand with Hg and Ag was determined by Hill plot to be 1.46 × 10 and 9.24 × 10 M, respectively. Ligand forms a strong complex with either two Hg or two Ag ions using both the upper and lower rims of the 1,3-alternate calix[4]arene as the binding pockets; hence, it represents one of the highly selective fluorescent sensors for the homoditropic sensing of Hg and Ag ions.