Thermodynamic and fluorescence emission properties of the Zn(II), Cd(II) and Pb(II) complexes with a fluorescent chelator bearing phenanthroline and naphthalene subunits

The new ligand L consisting of a 1,10-phenanthroline unit bearing in 2,9 positions two triamine branches with terminal naphthalene groups was synthesized and used for protonation and metal ion (Zn(II), Cd(II), Pb(II)) coordination studies. Protonation and metal ion coordination were analyzed by means of potentiometric measurements, affording speciation of the systems and determination of the relevant equilibrium constants, and by means of 1H NMR, UV–Vis and fluorescence emission spectroscopy. L forms stable complexes with the three metal ions showing very high selectivity for the binding of Zn(II) over Pb(II) (selectivity coefficient of 42,000 around pH 8.3), high selectivity for the binding of Cd(II) over Pb(II) (selectivity coefficient of 710 around pH 7.7) and modest selectivity for the bind of Zn(II) over Cd(II) (selectivity coefficient of about 70 around pH 9). ex emission in the visible region due to the interaction of the coordinated phenanthroline unit with a naphthalene group is observed for the Zn(II) and Cd(II) complexes, while the Pb(II) complex is completely quenched. Such thermodynamic and photochemical properties define the conditions for selective sensing of Zn(II) and Cd(II) in the presence of Pb(II).