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 x 10(3) and 9.20 x 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 H-1 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-pi interactions from the phenoxy rings. Furthermore, the ESI-MS spectrometry clearly proved the formation of the homobinuclear complex L center dot(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.