Calix[4]arenes featuring a direct lower rim attachment of dansyl groups. Synthesis, fluorescence properties and first report on crystal structures

Abstract

A series of new fluorescent calix[4]arenes (2–7; 5 has been isolated as two not yet described conformational isomers) featuring two directly lower rim attached dansyl moieties besides other lower or upper rim site substituents have been synthesized and investigated with reference to their fluorescence properties including potential sensing capability for metal ions. Except for the nitro substituted compound 7, which showed a moderate fluorescence quenching on the addition of Hg²⁺ in acetonitrile, the calixarenes 2–6 were found to selectively recognize Cu²⁺ in the 10⁻⁶ mol L⁻¹ concentration range detectable by the nearly total quenching of their intrinsic fluorescence. Using fluorescence titration experiments, the ideal complexation ratio [metal ion]/[ligand] for three exemplarily investigated calixarenes (3, 5a and 7) was determined to be 2:1. For 5a, a new red shifted signal was observed by the complexation of Cu²⁺ while the addition of Hg²⁺ only yields a moderate quenching of the parent signal, indicating a different binding mode for both metal ions. This finding enables the calixarene 5a to be used as a suitable chemosensor for the simultaneous determination of copper and mercury. In this paper we also present the first crystal structures of dansylated calixarenes having the dansyl groups directly attached to the lower rim site. They involve the unsolvated calixarene 5b and the two solvent inclusion compounds 2·CH₂(OH)CN and 3·3CH₃CN, each showing an extensive pattern of non-covalent interactions. In both solvates, the calixarenes are fixed in a cone conformation while the unsolvated calixarene 5b adopts the partial cone conformation. A solution ¹H NMR study in CDCl₃ reveals also the cone conformation for all dansylated calixarenes, except the more conformationally flexible upper rim unsubstituted calixarene 5 which showed cone and partial cone conformers 5a and 5b, respectively, in an approximate 2:3 ratio.