Conformationally regulated fluorescent sensors. Study of the selectivity in Zn2+ versus Cd2+ sensing

The Zn2+ and Cd2+ complexing properties of four ligands containing a 4,4′-substituted biphenyl moiety are described. Ligands and , containing only one 1-aza-18-crown-6 cavity, lead to selective complexation of Cd2+ versus Zn2+. Ligand , with two crown cavities linked to a tetramethylbenzidine unit, is able to form 1:1 complexes with Zn2+ and Cd2+, showing a higher complexing constant with Zn2+ than with Cd2+, probably due to enthalpic factors. Several complementary experiments suggest that the 1:1 complexes formed by ligand involve both crown cavities acting together to give rise to clamp structures. The formation of this type of zinc complex gives rise to red shifted emission bands and distinct quenching of the fluorescence. A similar situation is observed with cadmium but the change is then less pronounced. When mixtures of both salts are used, ligand selectively responds to zinc. Finally, ligand , which also has two crown cavities but contains nitro rather than amino groups in the biphenyl moiety, shows no propensity to form clamp complexes and, for this reason, it complexes cadmium much more strongly than zinc and binds the former selectively when mixtures of both salts are used in complexing experiments.