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 K(1) and K(2) of the successive complexation of L with the first and second Ag(+) ions were calculated to be 4.46×10(3) and 9.20×10(4) M(-1), respectively. The higher K(2) value revealed that a positive allosteric effect participated in the complexation of L with the second equivalent of Ag(+). Based on (1)H NMR 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.