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 th e fluorescence emission intensity. The equilibrium constants and the enthal py 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 sa me experimental conditions. L2 forms only mononuclear complexes, while L3 a lso forms dizinc(II) species. The phenanthroline group has fluorescence emi ssion properties, but interaction with the lone pairs of benzylic nitrogen atoms produces an efficient quenching of the emission. Such a quenching eff ect 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 modulat ed by the formation of different protonated and complexed species. In the c ase of L3, the fluorescence emission is also controlled by the metal to lig and molar ratio, because of the formation of an emissive binuclear complex.