Fluorescent chemosensors containing ruthenium(II) bipyridine as fluorogenic unit and modified calix[4]arene as ionophore: Synthesis, characterization, electrochemistry and ion-binding property

Abstract

A series of metallo-receptors containing ruthenium(II) bipyridine moiety as signaling unit and calix[4]arene coupled with azacrown as ionophore have been synthesized and characterized. These molecules exhibit metal to ligand charge transfer (MLCT) absorption band at 456 nm and 3MLCT luminescence band at 614 nm. Electrochemical study revealed that the Ru(II) of these complexes oxidize in the potential range + 1.23 to + 1.30 V and the ligands exhibit three redox waves in the potential −1.36 to −1.89 V. Ion-binding property of these molecules has been studied using a large number of metal ions and anions with the aid of luminescence spectral change, which revealed that all of the three receptors bind Hg2+ and Pb2+ strongly, as evident from the enhancement in emission intensity with red shift of λmax. Among anions, H2PO4− and F− exhibited substantial quenching in emission intensity, suggesting their strong interaction with the receptors. To evaluate the binding constants (Ks), luminescence titration of the receptor molecules with the strongly interacting ions have been performed and the Ks values have been calculated from the titration data. The Ks values are in the range 4.77 × 104 to 1.20 × 103 M−1 with descending order of Hg2+ > Pb2+ > H2PO4− > F−. For anions, 1H NMR spectral change were recorded with incremental addition of anions, the results obtained are discussed in light of ion-binding property and mechanism of interaction.