A fluorogenic calix[4]pyrrole with a small rigid strap showing different fluorescent responses to anions

Abstract

<p>A strapped calix[4]pyrrole (<b>2</b>) with a fluorogenic pyrene unit linked to the rigid strap via the methylene bridge has been synthesised. It was established by ¹H NMR and fluorescence spectroscopic analyses and isothermal titration calorimetry (ITC) studies that receptor <b>2</b> is capable of binding the F⁻, Cl⁻, (ppi) and anions in organic media, with high selectivity for F⁻ being observed relative to the other anions. Upon exposure to the F⁻ and , the fluorescence of <b>2</b> is quenched, an effect ascribed to facilitated PET (photo-induced electron transfer) from the nitrogen atom to the pyrene moiety, which is induced by anion binding. In contrast, the fluorescence of <b>2</b> was enhanced upon treatment with the HSO₄⁻ anion as the result of the PET inhibition that results from nitrogen atom protonation. A single crystal X-ray structure of the chloride complex of <b>2</b> and the ¹H NMR spectral data revealed that the chloride anion is bound to the receptor via aliphatic C–H•••Cl⁻ hydrogen bonds, as well as N–H•••Cl⁻ interactions.</p