Structural and Spectroscopic Studies of Spontaneously Formed Crystalline Eu(iii)-aliphatic Dicarboxylates at Room Temperature

Complexation of actinides and lanthanides with carboxylic organic ligands is a critical issue affecting radionuclide migration from deep geological disposal systems of spent nuclear fuel. A series of Eu(iii)-aliphatic dicarboxylate compounds, as chemical analogs of radioactive Am(iii) species, Eu2(Ox)3(H2O)6, Eu2(Mal)3(H2O)6, and Eu2(Suc)3(H2O)2, were synthesized and characterized using X-ray crystallography and time-resolved laser fluorescence spectroscopy to examine the ligand-dependent binding modes and the corresponding changes in spectroscopic properties. Powder X-ray crystallography results confirmed that all of the compounds presented a crystalline polymer structure with a trigonal prism square-face tricapped polyhedron geometry centered on Eu(iii) in a nine-coordinate environment involving nine oxygen atoms. This study captures the transition of the coordination modes of aliphatic dicarboxylate ligands from side-on to end-on binding as the carbon chain length increases. This transition is illustrated in malonate bindings involving a combination of side-on and end-on modes. Strongly enhanced luminescence, especially for the hypersensitive peak, indicates a low site symmetry in the formation of solid compounds. The number of remaining bound water molecules was estimated from the resultant increased luminescence lifetimes, which were in good agreement with crystal structures. The excitation-emission matrix spectra of these crystalline polymers suggest that Ox ligands promote the sensitized luminescence of Eu(iii), especially in the UV region. In the case of Mal and Suc ligands, charge transfer occurs in the opposite direction from Eu(iii) to the ligands under UV excitation, resulting in weaker luminescence.