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

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 ∼435 nm 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 ϕ_Eu 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.