Structures and Properties of Luminescent Pentanitratoeuropate(III) Ionic Liquids

Abstract

A series of luminescent ionic liquids based on pentanitratoeuropate(III) anions combined with imidazolium cations, namely, 1,2,3-trimethylimidazolium {[MC1mim]2[Eu(NO3)5] (1)}, 1,3-dimethylimidazolium {[C1mim]2[Eu(NO3)5] (2)}, 1-ethyl-3-methylimidazolium {[C2mim]2[Eu(NO3)5] (3)}, 1-butyl-3-methylimdazolium {[C4mim]2[Eu(NO3)5] (4)}, 1-hexyl-3-methylimidazolium {[C6mim]2[Eu(NO3)5] (5)}, and 1-methyl-3-octylimidazolium {[C8mim]2[Eu(NO3)5] (6)}, were synthesized. These complexes were characterized by NMR and IR spectroscopy as well as elemental analysis. They exhibit high thermostability and wide liquidus ranges of more than 300 °C. Colorless plate crystals of 1 were isolated, and the crystal structure was determined by single-crystal X-ray diffraction [monoclinic system, C2/c space group with the cell parameters a = 21.6870(6) A, b = 9.9807(4) A, c = 15.0267(6) A, β = 131.140(4)°, Z = 4]. In the [Eu(NO3)5]2– anion, the 10-coordinate EuIII ion is coordinated by five bidentate nitrate ligands and has a highly symmetrical trigonal-bipyramidal geometry. Compounds 1–6 exhibit bright red luminescence of high colorimetric purity. Only two characteristic narrow monochromatic bands, arising from the 5D07FJ (J = 1 and 2) intraconfigurational f–f transitions, were clearly observed in their emission spectra at 298 K. The intense red photoluminescence is still stable at 150 °C. These 10-coordinated EuIII complexes are of interest as potential thermally stable luminescent soft materials.