Lower rim isoxazole-calix[4]arene derivatives as fluorescence sensors for copper(II) ions

Chemosensors designed around a calix[4]arene scaffold, possessing distal isoxazole nuclei as potential recognition sites in conjugation with fluorogenic reporting moieties, have been synthesised and their molecular structures determined by single crystal X-ray analysis. PIC (pyrene-isoxazole-calix[4]arene) and AIC (anthracene-isoxazole-calix[4]arene) are found to be selective and sensitive for the recognition of copper(II) ions over a range of divalent metal ions in a spectrofluorometric study in acetonitrile. Following exposure to copper(II) ions the fluorescence emission intensity of the host was dramatically quenched, in contrast there were no significant shifts to the UV–vis spectra. 1H NMR spectroscopic studies could not provide unambiguous evidence for the mode of ion recognition. Hybrid Density Functional Theory (DFT) calculations for the PIC-copper(II) complex suggests that the metal ion adopts tetrahedral geometry in an acetonitrile solution, while a distorted square pyramidal coordination is also possible in the absence of solvent molecules. Both of these binding conformations cause the disruption of the pyrene–pyrene stacking, consistent with the observed fluorescence quenching