Calix[4]arene based molecular sensors with pyrene as fluoregenic unit: Effect of solvent in ion selectivity and colorimetric detection of fluoride

Fluorescent molecular sensors having excimer emission property have been designed and synthesised incorporating calix[4]arene derivatives in cone and 1,3-alternate conformation as ionophore and two pyrene moieties at close proximity as fluorophore. They exhibit strong excimer emission around 515 nm, which is used to monitor interaction of metal ions with the ionophores. Ion-binding study of these fluoroionophore has been investigated in acetonitrile–chloroform and THF–H2O with a wide range of cations and anions and the recognition process is monitored by luminescence, UV–Vis and 1H NMR (for F−) spectral changes. The present study demonstrated profound influence of solvent in ion selectivity, in acetonitrile–chloroform they formed complexes with Hg2+, Pb2+, Cu2+ and Ni2+, whereas in THF–H2O they exhibit selectivity only for Cu2+. In the case of anions, selectivity for only F− with color change is observed. Composition of the complexes formed was determined from mass spectrometry and the binding constants were determined from fluorescence titration data. The reasons for formation of excimer emission, quenching of it in presence of certain metal ions, role of solvent in selectivity and energy/electron transfer process involved in the ion-recognition event have been discussed on the basis of experimental data.

Graphical abstractFluorescent receptors having strong excimer emission have been synthesised incorporating calix[4]arene as ionophore and pyrene as fluorophore and study on their ion-binding ability in different solvents with a wide range of cations and anions revealed profound influence of solvent in ion-selectivity, the hydrated shell of the cation plays crucial role in selectivity.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Fluoroionophores containing calix[4]arene and pyrene exhibit strong excimer emission. ► Solvent plays crucial role in determining selectivity towards metal ions. ► Strong interaction with metal ion decreases intensity of the excimer emission. ► Complexation with metal ion breaks the π–π stacking of the pyrene units. ► Size of the hydrated shell of the metal ion determines selectivity in aqueous media.