Modified dipicolinic acid ligands for sensitization of europium(III) luminescence.

The effect of substitution at the 4 and 3,5 positions in the pyridine ring of europium(III) pyridine-2,6-dicarboxylate complexes has been investigated with particular emphasis on sensitization of the Eu3+ ion. Sensitization of the Eu3+ 615-nm emission was achieved through excitation of the ligands in which the 4 substituent was -H, -OH, and -Cl and the 3,5 position was -H. In these cases, the ligand-to-Eu3+ ratio was confirmed as being 3:1. The sensitization was found to increase following substitution of the 4 position in the order Cl > H > OH. This is attributed to energy transfer occurring from the ligands into different Eu3+ intra-atomic energy levels, with spin selection rules governing the efficiency of this process. The Eu3+ luminescence lifetime was measured and found to vary from 1.16 to 2.90 ms depending on the excitation energy, ligand, and solvent. For the case of the 3,5-dibromo-4-hydroxy derivative, no sensitization was observed and a ligand-to-Eu3+ ratio of 1:1 was found. The solubility of these complexes in water and their long emission lifetime make them attractive for use as probes in biological systems.