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.
Highlights
Over the past two decades, the design and synthesis of fluorescent chemosensors for the selective sensing of heavy and transition-metal (HTM) ions have attracted much attention in the fields of supramolecular chemistry, biology, and medicinal chemistry (Czarnik, 1997; de Silva et al, 1997; Prodi et al, 2000; Valeur and Leray, 2000; Li et al, 2014)
We showed that a 1,3-alternate calix[4]arene, containing bistriazoles on the one rim and bis-enaminone groups on the other rim, functioned as a homoditropic fluorescent sensor for two Ag+ ions (Ho et al, 2011)
We report here the synthesis of 1,3-alternate calix[4]arene 8, which contains bis-pyrazolylmethyphenyls on the one rim and bis-triazolylmethylpyrenes on the other rim, as a highly selective fluorescent sensor for Hg2+ and Ag+ ions
Summary
Over the past two decades, the design and synthesis of fluorescent chemosensors for the selective sensing of heavy and transition-metal (HTM) ions have attracted much attention in the fields of supramolecular chemistry, biology, and medicinal chemistry (Czarnik, 1997; de Silva et al, 1997; Prodi et al, 2000; Valeur and Leray, 2000; Li et al, 2014). As part of our continuous interest in the design and synthesis of novel fluorescent chemosensors (Chang et al, 2007, 2008; Hung et al, 2009a,b; Senthilvelan et al, 2009; Ho et al, 2011; Wang et al, 2011, 2012), we utilize here the 1,3-alternate calix[4]arene platform to construct a ditopic fluorescent sensor 8, which contains bis-pyrazoles on the one rim and bis-triazoles on the other rim as metal ion binding sites and pyrene pendants as fluorophores (reporters). In 2008, Kim and Yang independently reported (Park et al, 2008; Zhu et al, 2008) the synthesis of a calix[4]arene with two lower-rim pyrene units linked with triazole groups, which showed ratiometric fluorescence response in acetonitrile solution toward Zn2+ and Cd2+ and selective fluorescence quenching toward HTM ions such as Cu2+, Hg2+, and Pb2+. Experiments were performed with 1.33 mM solutions of compound 3 or 8 in CDCl3/CD3OH (v/v, 3:1) co-solvent by adding various concentrations of AgClO4 or Hg(ClO4) at 25◦C
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