Highly Luminescent Ionic Liquids Based on Complex Lanthanide Saccharinates

Four strongly luminescent ionic liquids with complex lanthanide saccharinate anions, [C(4)mim](3)[Eu-Sac)(6)(H2O)(2)] (1), {C(4)mpy}(5){[Ln-(Sac)(6)(H2O)(2)][Ln-(Sac)(5)(H2O)(3)]}{(H2O)(2)(CH3CN)(2)} (Ln = Sm for 2a; Ln = Eu for 2b), and [C(4)mpy](3)[Eu(Sac)(6)][CH3CN] (3) (C(4)mim = 1-butyl-3-methylimidazolium; C(4)mpy = N-butyl-4-methylpyridinium; Sac = saccharinate), have been obtained by reacting the ionic liquids 1-butyl-3-methylimidazolium saccharinate, [C(4)mim] [Sac], and N-butyl-4-methylpyridinium saccharinate, [C(4)mpy] [Sac], with the respective lanthanide saccharinates. Single-crystal X-ray diffraction analyses reveal the respective lanthanide center to be six- or eight-coordinated by five or six saccharinate anions and two or three aqua ligands in the cases of 1 and 2 when lanthanide saccharinated hydrate was employed as the starting material. Coordination of water to the lanthanide can be avoided when using the anhydrous lanthanide saccharinate as shown by the structure of 3. Using a co-solvent, acetonitrile, to facilitate the reaction led to incorporation of solvent molecules into the crystal structure of the final materials (2 and 3). Differential scanning calorimetry analyses reveal that 1 is an ionic liquid whereas 2 and 3 are low-temperature molten salts. The three europium(III)-containing compounds (1, 2b, and 3) all show characteristic intense red emissions of Eu(III) upon excitation into levels of the saccharinate ligands (321 nm) or Eu(III) (393 nm). At room temperature, the decay times of 1 and 2b are all similar to 0.5 ms, whereas the decay time of 3 amounts to 3.85 ms due to removal of aqua ligands in the first coordination sphere; accordingly, the quantum efficiencies of 1, 2b, and 3 were determined to be 15.9%, 22.95%, and 57.75%, respectively. The CIE chromaticity coordinates for all Eu compounds are in the red region and approach the NTSC standard CIE values when the temperature is increased. Sm(III)-containing compound 2a shows characteristic Sm(III) emission peaks. As expected, the CIE coordinates of samarium compound 2a fall in the orangered region.