Investigation of europium(III) and gadolinium(III) complexes with naphthoyltrifluoroacetone and bidentate heterocyclic amines

Lewis base adducts of the type Ln(NTA)3·phen and the binuclear complexes [Ln(NTA)3]2·bpym [NTA=1-(2-naphthoyl)-3,3,3-trifluoroacetone, phen=1,10-phenanthroline, bpym=2,2′-bipyrimidine; Ln=Eu, Gd] have been prepared by replacement of the coordinated water molecules in the complexes Ln(NTA)3·2H2O. The compounds were characterised by elemental analysis, 1H NMR, X-ray diffraction (XRD), thermogravimetric analysis, FTIR and FT Raman spectroscopy, and photoluminescence (PL) spectroscopy. For both, new bands were observed in the Raman spectra in the low-wavenumber region (100-600 cm−1), assigned to Ln-N stretching vibrations (in agreement with ab initio calculations). The crystal structures of the EuIII complexes were determined by single crystal XRD. In both complexes the coordination polyhedra can be regarded as square antiprisms, distorted towards a bicapped trigonal prism for the mononuclear complex and towards a dodecahedron for the binuclear complex. A Eu-Eu distance of 6.856 Å was found for the latter. Powder XRD indicates that the corresponding gadolinium(III) compound has a similar structure. The room temperature PL spectra of the europium complexes in the solid state present a series of sharp lines assigned to 5D0→7F0-4 transitions. No emission from the organic part of the compounds is observed, indicating that energy transfer from the ligands to the Eu3+ ion is quite efficient. A number of parameters were determined from the PL spectra, including estimates for the overall crystal field strengths, and 5D0 lifetimes and quantum efficiencies. The PL spectra for ethanol solutions of the Eu complexes confirm that the NTA and N-ligands remain attached. Slight changes in the PL data upon solubilisation are attributed to an interaction of solvent molecules with the metal centres.