Calix[4]arene based fluorescent chemosensor bearing coumarin as fluorogenic unit: Synthesis, characterization, ion-binding property and molecular modeling

摘要

A series of calix[4]arene-based fluorescent chemosensors in cone and 1,3-alternate conformations have been synthesised incorporating ethylene glycol units as binding sites and coumarin as fluorescent probe. These compounds have been designed with variation in substituents, conformations of the calixarene unit and steric crowding. The open chain ethylene glycol moieties are flexible so that they can orient themselves in space to make effective interactions with metal ions. Ion-binding study of these fluoroionophores has been carried out using a large number of metal ions and the ion recognition event is monitored by fluorescence and absorption spectroscopy. Substantial quenching in emission intensity in presence of Fe~(3+) and Cu~(2+) and enhancement of emission intensity in presence of Ca~(2+) suggests strong binding of these metal ions with the ionophores. Complexation with these metal ions is also evident from UV-Vis spectral changes. Composition of the complexes formed (1:1) has been confirmed by ESMS analysis. Binding constants with the strongly interacting metal ions have been evaluated from fluorescence titration data. DFT calculations have been carried out to find out the metal binding sites of the calix ionophores and frontier molecular orbital analyses have rationalized the fluorescence behaviors observed upon complexation with metal ions in these cases.