Synthesis, characterization, electrochemistry and ion-binding studies of ruthenium(II) bipyridine receptor molecules containing calix[4]arene-azacrown as ionophore

A number of molecular receptors containing ruthenium(II) bipyridine moiety as fluorophore and calix[4]arene-azacrown hybrid molecule as ionophore have been synthesized and characterized. These receptors (1-4) exhibit strong (MLCT)-M-3 luminescence bands in the range 613-618 nm. The cation-binding property of these. uoroionophores have been investigated with the ions Na+, K+, Mg2+, Ca2+, Cs+, Zn2+, Cd2+, Hg2+ and Pb2+ and the recognition event monitored by luminescence, (HNMR)-H-1 spectroscopy, the oxidation potential of metal ion and UV/Vis absorption studies. The luminescence study suggests complexation of all four receptors with Zn2+, Cd2+ and Hg2+ and for 2 and 3 also with Pb2+. The binding constants (K-s, except 4) and stoichiometries of the complexes have been calculated from the luminescence titration data, with values of K-s ranging from 2.53 x 10(5) to 6.27 x 10(3) M-1 . The H-1 NMR spectroscopy study exhibits interactions of Na+ and K+ with 1 and 4, K+ and Cs+ with 2, and K+ with 3. Binding constants with these metal ions have been calculated from H-1 NMR titrations using computer programs or by a direct method depending on the type of change in the chemical shifts observed upon addition of metal ions. Stoichiometry of the complexes has been determined from Job's plot. Electrochemical properties of 1-4 have been studied in the absence and also in the presence of selected metal ions and the shift of the oxidation potentials of the Ru(II) in the presence of guest ions suggests an interaction of Na+, Hg2+ and Pb2+ with 1 and 2 but not with 3 and 4, indicating a selective electrochemical response towards certain metal ions. The results obtained are presented and discussed in light of ion-binding properties and methods of detection.