Synthesis, complexation and photophysics of 1,3-alternate calix[4]arene-crown-6 bearing fluorophoric units on the bridge

Three new calix[4]arene-crown-6 derivatives bearing a fluorophoric unit on the ether bridge were synthesized. All these compounds complex alkali metal picrates in chloroform, and the naphthocrown (2) shows the highest efficiency for cesium ion binding among the calix-crown-6 known so far. Cesium over sodium selectivity is remarkably higher in acetonitrile than in chloroform solution. H-1 NMR studies, carried out in CDCl3 on the cesium picrate complex of ligand 2, show that the anion is involved in a pi - pi stacking with the naphthyl unit. The lariat calix[4]-crown-6 (4) does not show any relevant change in absorption and fluorescence spectra upon cesium binding, thus indicating that the dansyl unit is not perturbed by metal ion complexation. On the contrary, for ligands 2 and 3 a luminescence intensity decrease is observed upon cation binding, which allows an easy detection of cesium even at very low concentrations (10(-7) M).