Synthesis, structure and sensitized luminescence of Eu(III) and Tb(III) complexes with Schiff bases

Abstract

A new family of lanthanide/Schiff base complexes have been synthesized and fully characterized from the novel Schiff base ligand: L1, (E)-N′-(dipyridin-2-ylmethylene)acetohydrazide and L2, (E)-N′-(pyridin-2-ylmethylene)acetohydrazide. The single crystal X-ray structure and mass spectroscopy reveal the isostructural mononuclear complexes: [LnL2(NO3)2]NO3 and [LnL(HFAC)3]3 where Ln=La, Eu, Gd and Tb and HFAC=hexafluoroacetylacetonate. Upon excitation of the Ln(III) complexes in solution, their luminescence spectra display the broad ligand-centred (LC) emission centred at ∼435nm as well as the narrow f–f transitions. Generally, the ligands can efficiently sensitize the Tb(III) complexes because of the better triplet state energy match to the Tb(III)–5D0 excited states. The luminescence lifetimes of the Ln(III)-nitrate chelates are found shorter than those from the corresponding HFAC complexes. The nitrate chelates can quench the luminescence through energy transfer pathways involving π∗–n transitions. The intrinsic quantum yields for the Eu(III) complexes have also been investigated. The ϕEu value for the complex Eu1N is found the lowest, 2.9% due to deactivation from intramolecular H-bonding NH⋯N involving the pyridine and amine units in the ligand as proved by X-ray crystallography.