Luminescent metalloreceptors with pendant macrocyclic ionophores with NS2O3 donor sites: Synthesis, characterization and ion-binding property

Three new luminescent metalloreceptors have been synthesized, incorporating the Ru(II)-bipyridine moiety as a fluorogenic unit and a six membered macrocyclic moiety containing NS2O3 donor atoms as ionophores. The sulfur donor atoms have been incorporated to enhance the complexing ability of the ionophore towards heavy metal ions, although for comparison, an ionophore having NO5 donor sites has also been used. The ion-binding property of these fluoroionophores has been investigated with a large number of cations and anions, and the recognition event is monitored by fluorescence and NMR spectroscopy. The luminescence intensity is strongly quenched in the presence of Cu2+and Fe3+ for 1; Cu2+, Fe3+, Hg2+ and Pb2+ for 2 and with Cu2+ for 3, indicating strong complexation of these metal ions with the ionophore. The fluoroionophore 2 also exhibited strong interactions with the anions F−, H2PO4− and I−. A 1H NMR study was carried out to get information about the binding site of the ions with the ionophores. Binding constants (Ks) and the composition of the complexes formed for all the three fluoroionophores with strongly interacting ions have been determined from emission titration data. The Ks values for the metal ions are in the range 3.19 × 102–8.86 × 103 M−1 and for anions they are in the range 8.89 × 102–5.33 × 103 M−1. The ion-selectivity and energy transfer involved in quenching process have been discussed.

Graphical abstractFluorescent molecular receptors containing Ru(II)-bipyridine as a fluorogenic unit and macrocyclic ionophores with NS2O3/NO5 donor atoms have been synthesized and their ion-binding properties with a large number of cations and anions have been investigated with the aid of fluorescence and NMR spectroscopy. The inclusion of a sulfur donor atom in the ionophore significantly enhanced the ion-binding ability towards Pb2+, Hg2+ and Fe3+. The anions, F− and H2PO4− exhibit interactions with certain aromatic protons.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Fluoroionophores containing macrocycles with NS2O3/NO5 donors were synthesized. ► The ion-binding properties of these ionophores were investigated with cations and anions. ► The recognition event is monitored by fluorescence and NMR spectroscopy. ► The soft/hardness of the donors and metal ions played a predominant role in selectivity. ► The positive charge on Ru(II) facilitated interactions with anions.