Luminescence studies of Eu(III) mixed ligand complexes

A series of europium(III) mixed ligand complexes, Ln/L/X based on pyridine carboxylic acid N-oxides, L, plus 2,2'-bipyridine (bipy) or 1,10-phenanthroline (phen), X, have been studied. The complexes were characterized using Eu luminescence (conventional and laser-induced) spectroscopy. A number of N-oxide ligand designs, including mono- and dicarboxylic acids and their derivatives (aryl-substituted), having various steric groups, have been tested with the main goal of optimizing the luminescence properties under various experimental conditions. Potentiometric titration of acid N-oxides have been carried out in aqueous solution in order to determine their pK(a) and the pH values, important for complexation conditions. Based on the Eu(III) luminescence lifetime measurements, hydration numbers of the Eu(III) complexes have been obtained, which evidenced the formation of binary EuL and EuL, complexes and mixed ligand EuL2X complexes. The results suggest that in ternary complex formation the steric requirement of the secondary ligand play an important role. The results of these studies lead us to achieve a better understanding of the factors, as aryl substituted groups (e.g. Br, NO2, NH2, CH3, OCH3) and a kind of solvent, influencing the luminescence intensity and lifetime of the Eu(III) complexes studied. (C) 2002 Elsevier Science B.V. All rights reserved.