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 1H NMR and fluorescence spectroscopic analyses and isothermal titration calorimetry (ITC) studies that receptor 2 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 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 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.
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