Spectroscopic study on anion recognition properties of calix[4]pyrroles: Effects of tetraalkylammonium cations

The solution binding properties of calix[4]pyrroles with anion (added as tetraalkylammonium salts) were investigated using UV–vis spectroscopic techniques. The obvious red-shift of absorption maximum band of calix[4]pyrrole in EtOH in the presence of the tetramethylammonium (TMA+) or tetraethylammonium (TEA+) salts were observed. These results displayed in electronic absorption spectra indicated calix[4]pyrrole receptors linking anionic species through multiple hydrogen bonding interactions are capable of using the periphery electron-rich “walls” for selectively binding electron-deficient tetraalkylammonium cation subunits by cation-π charge–transfer interaction. It was seen that the stability of the calix[4]pyrrole–anion complex depends strongly on the cation. The meso-alkyl groups of the calix[4]pyrrole, the affinity for the anion subunits and the structure of tetraalkylammonium cations have considerable effects on the formation of cation-π charge–transfer interaction.