1,3-alternate calix[4]arene as a homobinuclear ditopic fluorescent chemosensor for Ag⁺ ions

A novel 1,3-alternate calix-[4]arene L, containing two different cationic binding sites of bis-triazoles and bis-enaminone groups, was synthesized and shown to be a homobinuclear ditopic fluorescent chemosensor for Ag⁺ ions. The fluorescence intensity of L was selectively enhanced by binding with Ag⁺ ions in methanol/chloroform (49:1, v/v) cosolvent. In the presence of most competing metal ions, L retains its selectivity toward Ag ⁺ ion. The binding constants K₁ and K₂ of the successive complexation of L with the first and second Ag⁺ ions were calculated to be 4.46×10³ and 9.20×10⁴M ^-1, respectively. The higher K₂ value revealed that a positive allosteric effect participated in the complexation of L with the second equivalent of Ag⁺. Based on ¹HNMR titration results, we inferred that the two distal bis-enaminone and bis-triazole groups on L cooperatively coordinated to Ag⁺ ions with the help of cation-π interactions from the phenoxy rings. Furthermore, the ESI-MS spectrometry clearly proved the formation of the homobinuclear complex L·(Ag ⁺)₂, because a base peak at m/z of 750.1701 was detected and its isotope pattern was in excellent agreement with the calculated one.