Spectroscopic study of lanthanide(III) complexes with carboxylic acids

The complexation of trivalent lanthanides with aromatic (mellitic, pyromellitic, hemimellitic, trimellitic, trimesic, phthalic, isophthalic, terephthalic and benzoic) carboxylic acids was studied by luminescence and absorption spectroscopic methods and luminescence lifetime measurements. Stability constants with these carboxylate ligands were calculated from the F-7(0)-D-5(0) excitation spectra of Eu(III). The stability constant of the 1:1 complexes showed linear correlation with the basicity of the ligand. The luminescence decay constants indicate that the ligands with carboxylates in the ortho position are bidentate, replacing two water molecules from the metal upon complexation. Benzoate and terephthalate are unidentate. Isophthalate and trimesate replace approximately 1.5 water molecules, suggesting greater binding capability of these ligands due to the contribution from the non-binding carboxylate group(s). The oscillator strengths in the hypersensitive transition range of Nd(III) and Ho(III) complexed with these ligands were non-linearly proportional to the effective ligand basicity. Ligand polarizability, along with other factors, are proposed as responsible for the variation in oscillator strength in these complexes. (C) 1999 Elsevier Science S.A. All rights reserved.