Conformationally regulated fluorescent sensors. Study of the selectivity in Zn versus Cd sensing

The Zn²⁺ and Cd²⁺ complexing properties of four ligands containing a 4,4′-substituted biphenyl moiety are described. Ligands 1 and 3, containing only one 1-aza-18-crown-6 cavity, lead to selective complexation of Cd²⁺ versus Zn²⁺. Ligand 4, 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 4 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 4 selectively responds to zinc. Finally, ligand 2, 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.