Selective Binding Affinity between Quaternary Ammonium Cations and Water-Soluble Calix[4]resorcinarene

An amphiphilic calix[4]resorcinarene bearing four hydrophilic sulfonate sites at the upper rim and four hydrophobic n-pentyl chains at the lower rim (SR4A5) was synthesized by sulfonation of tetramethoxyresorcinarene. The molecular binding behaviors of SR4A5 with different types of organic cations, i.e., singly and doubly charged aliphatic ammonium salts and singly and doubly charged pi-aromatic ammonium salts, were comprehensively investigated by means of H-1 NMR, fluorescence, and UV/vis spectroscopic titration experiments. The competitive binding titrations demonstrate that, superior to the reported p-sulfonatocalix[4]arene systems, the stability constants upon association with SR4A5 can reach up to 10(6) M-1 order of magnitude in water, ultimately leading to better binding affinity and molecular selectivity toward dicationic guests. Significantly, UV/vis spectroscopic experiments further revealed that the specific binding behaviors of SR4A5 with bispyridinium guests can be attributed to the charge transfer interaction between electron-rich and electron-deficient aromatics upon host-guest complexation. These obtained results provide an effective strategy to realize the highly selective molecular recognition process with multiply charged macrocyclic receptors and will definitely promote the development of the field of water-soluble resorcinarene-based supramolecular assemblies.