1,3-Alternate Calix[4]arene Functionalized With Pyrazole and Triazole Ligands as a Highly Selective Fluorescent Sensor for Hg2+ and Ag+ Ions

Abstract

We report here the synthesis of a 1,3-alternate calix[4]arene 8, with bis-pyrazolylmethylpyrenes on the one end and bis-triazolylmethylphenyls on the other end, as a homoditropic fluorescent sensor for both Hg²⁺ and Ag⁺ ions. Calix[4]arene 3, 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 8 and 3 showed strong excimer emission of pyrenes when they are as a free ligand in CHCl₃/MeOH (v/v, 3:1) solution; however, they both showed a high selectivity toward Hg²⁺ and Ag⁺ ions with strong fluorescence quenching and yet with different binding ratios. The fluorescence of ligand 8 was strongly quenched by Hg²⁺ but was only partially quenched by Ag⁺ ions; however, the fluorescence of ligand 3 was strongly quenched by Hg²⁺, Ag⁺, and Cu²⁺ ions. Job plot experiments showed that ligand 8 formed a 1:2 complex with both Hg²⁺ and Ag⁺ ions; ligand 3 formed a 1:1 complex with Hg²⁺, but it formed a 2:3 complex with Ag⁺. The binding constant of ligand 3 with Hg²⁺ and Ag⁺ ions was determined by the Benesi-Hildebrand plot of UV-vis titration experiments to be 2.99 × 10³ and 3.83 × 10³ M^-1, respectively, while the association constant of ligand 8 with Hg²⁺ and Ag⁺ was determined by Hill plot to be 1.46 × 10¹² and 9.24 × 10¹¹ M^-2, respectively. Ligand 8 forms a strong complex with either two Hg²⁺ 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 Hg²⁺ and Ag⁺ ions.

Keywords: 1; 3-alternate calix[4]arene; fluorescent sensor; homoditropic; mercury (II) sensor; pyrazole; silver (I) sensor; triazole.

Scheme 1

Syntheses of target molecule 8 and control compound 3.

Figure 1

(A) Fluorescence changes of ligand 3 (10 μM) by the addition of 10 equiv of various metal perchlorates (Ag⁺, Ba²⁺, Ca²⁺, Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, Hg²⁺, Li⁺, Na⁺, K⁺, Mg²⁺, Ni²⁺, Pb²⁺, and Zn²⁺) in CHCl₃/MeOH (v/v, 3:1). Extinction wavelength was 345 nm. (B) Percentage fluorescence intensity changes of ligand 3 by the addition of 10 equiv of various metal perchlorates in CHCl₃/MeOH (v/v, 3:1).

Figure 2

(A) Fluorescence changes of ligand 8 (10 μM) by the addition of 10 equiv of various metal perchlorates (Ag⁺, Ba²⁺, Ca²⁺, Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, Hg²⁺, Li⁺, Na⁺, K⁺, Mg²⁺, Ni²⁺, Pb²⁺, and Zn²⁺) in CHCl₃/MeOH (v/v, 3:1). Excitation wavelength was 347 nm. (B) Percentage fluorescence intensity changes of ligand 8 by the addition of 10 equiv of various metal perchlorates in CHCl₃/MeOH (v/v, 3:1).

Figure 3

¹H NMR titration spectra of ligand 8 (1.33 mM) in the presence of different amount of AgClO₄ in CDCl₃/CD₃OH (v/v, 3:1); where * denotes signals from residual CHCl₃ and # denotes signals from external CHCl₃.

Scheme 2

Two possible binding modes of ligands 3 and 8 with Ag⁺ and Hg²⁺ ions, where the chemical shift differences (Δδ = δ_complex – δ_ligand) are shown for Ag⁺ (data for Hg⁺² complexes are in parentheses).