Cation-induced fluorescent excimer emission in calix[4]arene-chemosensors bearing quinoline as a fluorogenic unit: experimental, molecular modeling and crystallographic studies

A number of calix[4] arene-based fluorescent chemosensors containing amide as a binding site and quinoline as a fluorogenic unit have been synthesised and characterized by a single crystal X-ray diffraction study. These compounds have been designed with variations in conformation and steric crowding in the calix moiety to investigate its effect on ion selectivity and thereby on excimer emission. The ion-binding property of these fluoroionophores has been investigated with a large number of cations and anions and the ion-recognition event was monitored by luminescence, UV-Vis and H-1 NMR (for anions) spectral changes. Out of a large number of cations, Hg2+, Pb2+, Fe3+ and Cu2+ exhibited strong complexation with all of the ionophores, as evident from luminescence and UV-vis spectroscopy. In the case of anions, F- and HSO4- exhibited strong complexation with two of the ionophores, as shown by fluorescence and NMR spectroscopy. Interestingly, complexation with metal ions resulted in a new band at lower energy due to excimer emission, which was not observed for anions. The binding constants for all of the three fluoroionophores with strongly interacting ions have been determined from fluorescence, UV-vis and NMR titration data. The DFT calculations for all of the three compounds were performed and the results show that the modulation of frontier molecular orbital energies upon complexation of metal ions causes the fluorescent excimer emission.