Intermolecular energy transfer between lanthanide complexes. 8. Tb(III) donor and Eu(III) acceptor complexes of citric acid

Intermolecular energy transfer between the citric acid complexes of Tb(III) and Eu(III) has been used to evaluate the conditions under which the formation of polynuclear species might take place. The luminescence was followed both as a function of pH (ranging between 2 and 12), and metal/ligand ratio (working at ratios of 1:1, 1:2, 1:3, 1:5, and 1:10). Consideration of the luminescence results permitted a conclusion that all equilibria could be explained if only Ln(CIT) and Ln(CIT)₂ complexes existed under the conditions used. At the 1:1 metal/ligand ratio, extensive complex association was noted upon ionization of two of the citrate carboxyl protons. Since little hydroxide ion exists at these pH values, it would appear that the citrate ligand prefers to bind multiple metal centers while still in the bidentate bonding mode. However, upon ionization of the final carboxyl group, the associated complexes broke apart into monomeric species. At all higher ratios, the complexes were found to be monomeric at all pH values, thus indicating that the binding of two citrate ligands by a lanthanide ion (regardless of the bonding mode of the ligand) is also capable of preventing the formation of polynuclear species.