Luminescent Lanthanide Helicates Self-Assembled from Ditopic Ligands Bearing Phosphonic Acid or Phosphoester Units
A series of hexadentate ditopic receptors incorporating benzimidazole moieties have been designed, which are fitted with phosphonic acid or phosphoethylester coordinating units. In addition, poly(oxyethylene) pendants have been introduced on the benzimidazole backbone of two ligands to increase water solubility. The ligands self-assemble with lanthanide ions under stoichiometric conditions, yielding triple-stranded homobimetallic helicates, as ascertained by mass spectrometry and UV-visible titrations. The helicates display large thermodynamic stability, for example, log β23 ≈ 21-24 for all the EuIII complexes. Photophysical measurements reveal sensitization of the metal-centered luminescence in the europium and terbium complexes, which is modulated by the nature of the ligand. Hydration numbers determined by the lifetime method are essentially zero. The Eu(5D0) lifetimes are long and reach values up to 3.2 ms, while quantum yields as high as 25% are obtained in water at pH 7.4. Back transfer limits the sensitization efficiency for TbIII luminescence, and both lifetimes and quantum yields are much smaller. The properties of the helicates are discussed with respect to those self-assembled from ligands bearing carboxylate coordinating units.