Selective sensing of caesium ions by novel calix[4]arene bis(dibenzocrown) ethers in an aqueous environment

Calix[4]arene bis(dibenzocrown) ethers 3 and 4 were successfully synthesized in the fixed 1,3-alternate conformation in over 70% yield via the reaction of calix[4]arene with dibenzo dimesylates 6 and 7, respectively, in acetonitrile in the presence of caesium carbonate as base. Exhibiting typical ¹H NMR spectrum patterns, 3 and 4 are in the 1,3-alternate conformation, which is confirmed by X-ray analysis. Among four synthetic compounds tested, 3 shows the best selectivity and efficiency for a caesium ion complexation over other alkali metal ions using the single flux method through a bulk liquid membrane system. Enhancement of the complexation ability is attributable to the flatness and lipophilicity of the ethereal linkages caused by introducing the dibenzo groups on the calix[4]arene moiety. Two phase extraction experiments using calix[4]arene bis(crown) ethers with caesium ions are found to give a 1:1 ligand-metal complexation ratio.