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

Abstract

A strapped calix[4]pyrrole (2) with a fluorogenic pyrene unit linked to the rigid strap via the methylene bridge has been synthesised. It was established by H-1 NMR and fluorescence spectroscopic analyses and isothermal titration calorimetry (ITC) studies that receptor 2 is capable of binding the F-, Cl-, anions in organic media, with high selectivity for F- being observed relative to the other anions. Upon exposure to the F- and HP2O73- the fluorescence of 2 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 2 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 2 and the H-1 NMR spectral data revealed that the chloride anion is bound to the receptor via aliphatic C-H center dot center dot center dot Cl- hydrogen bonds, as well as N-H center dot center dot center dot Cl- interactions. [GRAPHICS] .