Study on luminescent ternary EuEDTA complexes with a set of substituted 4-phenylethynyl and 4-aryl pyridine-2,6-dicarboxylic acids

A set of pyridine-2,6-dicarboxylic acids (dipicolinic acids, DPAD, Lx) were prepared to study the photo-physical properties of their ternary complexes with Eu(III) and ethylenediaminetetraacetic acid (EDTA or Y). The stability constant of the formation of EuY-L1 (L1= dipicolinic acid, measured log Kter =5.38) was determined and found to be comparable to the formation constant of Eu(L1)3(logK3 =5.51). All the studied ternary complexes exhibit two predominant luminescence decay times. The origin of the shorter lifetime was shown to be due to the dissociation of carboxylate group(s) and their replacement by water molecules. The presence of the short-lived component decreases the quantum yields compared those reported for complexes of Eu(Lx)3. However, the use of ternary complexes of EuY-Lx offers a simple comparison method between different dipicolinic acids (Lx). Eight of the studied ligands Lx form brighter ternary complexes than the currently used ligand in the duplex PCR assay, with quantum yields over 20%, and the most luminescent one has three times higher signal level compared to the currently used ligand.