Macrocyclic polyamines containing phenanthroline moieties -fluorescent chemosensors for H⁺ and Zn²⁺ ions

The macrocyclic ligands L2 and L3, containing a triethylenetetraamine and a tetraethylenepentaamine moiety linked to the methyl groups of 2,9- dimethyl-1,10-phenanthroline, bind H⁺ and Zn²⁺ ions giving rise to modulation of the fluorescence emission intensity. The equilibrium constants and the enthalpy changes for ligand protonation were determined by means of pH-metric and microcalorimetric methods in 0.1 M Me₄NCl solutions at 298.1±0.1 K. Also the stability constants of the Zn²⁺ complexes were determined under the same experimental conditions. L2 forms only mononuclear complexes, while L3 also forms dizinc(II) species. The phenanthroline group has fluorescence emission properties, but interaction with the lone pairs of benzylic nitrogen atoms produces an efficient quenching of the emission. Such a quenching effect can be avoided by deactivation of the benzylic nitrogen atoms by means of protonation or Zn²⁺ complexation. Hence, L2 and L3 behave as chemosensor for H⁺ and Zn²⁺, the photochemical properties of the ligands being modulated by the formation of different protonated and complexed species. In the case of L3, the fluorescence emission is also controlled by the metal to ligand molar ratio, because of the formation of an emissive binuclear complex.