Luminescence and structural properties of lanthanide complexes of Schiff bases derived from pyridoxal and amino acids

Abstract

Lanthanide complexes of Schiff bases (SBs) with 1 : 1 and 1 : 2 (M : Lig) stoichiometric ratios were prepared by condensation of pyridoxal (PL) and aspartic acid (Asp) or L-histidine (His), respectively, in the presence of the appropriate metal chloride as a templating agent. These complexes were studied by optical spectroscopy and single crystal X-ray diffraction techniques. Crystallographic studies of 1 : 1 ([Eu(PL-Asp)(H₂O)₄](H₂O)) and 1 : 2 ([Eu(PL-His)₂(H₂O)₂]Cl(H₂O)₄) complexes show that Eu^III is eight-coordinate in both structures, in a distorted square antiprism environment formed by the phenolic oxygen of PL, the nitrogen atom of carbon–nitrogen double bond, oxygen atoms of the carboxylate groups of His or Asp, and oxygen atoms of the water molecules. The main species formed in aqueous solutions containing these SBs have been determined by analysis of the luminescence spectra, lifetimes of Eu^III excited states and vibronic interaction as well as structural features of the Eu^III coordination sphere. Possible tetradentate coordination function of SBs in aqueous solutions with relatively high concentrations as well as the potential application of the lanthanide SB complexes as new luminescence materials are discussed.