Syntheses, Characterization and Fluorescent Properties of Lanthanide-Containing Ionic Liquids [Cnmim][Ln(NO3)4

Abstract

Five kinds of ionic liquids 1-alkyl-3-methylimidazolium nitrate (Cnmim)NO3 (n=2, 4, 6, 8, 10; mim= N-methylimidazolium) were synthesized from N-methylimidazolium, alkyl bromides and sodium nitrate. The structures of ionic liquids were characterized and analyzed by infrared spectra (IR), proton nuclear magnetic resonance ( 1 H NMR), mass spectrometry (MS) and elemental analyses (EA). Lanthanide-containing ionic liquids (Cnmim)(Ln(NO3)4) (Ln=Eu, Tb) were prepared by reactions of 1-alkyl-3-methylimidazolium nitrate and europium nitrate/terbium nitrate in 1∶1 molar ratio. (Cnmim)(Ln(NO3)4) were all sticky liquids at room temperature. The strength of nitrate absorption peaks of (Cnmim)(Ln(NO3)4) in the IR spectra was stronger than (Cnmim)NO3 which indicated that the coordination of nitrate anions to lanthanide ions enhanced the nitrate absorption peaks. The structures of (Cnmim)(Ln(NO3)4) contained 1-alkyl-3-methylimidazolium cations and (Ln(NO3)4) - anions in which each lanthanide ion was coordinated with eight oxy- gen atoms from four nitrate ions were further confirmed by ESI mass spectrometry. The thermal decomposition of (Cnmim)NO3 and (Cnmim)(Ln(NO3)4) were investigated by thermogravimetric analysis (TGA), which indicated that all the compounds represented high stability until ca. 250 ℃. Under UV lamp irradiated at 365 nm, (Cnmim)(Eu(NO3)4) and (Cnmim)(Tb(NO3)4) showed red and green fluorescence, respectively, which indicated that these lanthanide-containing ionic liquids could be used as good soft luminescent materials. The photoluminescence properties of the lanthanide-containing ionic liquids were studied at room temperature in water by measuring emission and excitation spectra. These experiments demonstrated that the solution of (Cnmim)(Ln(NO3)4) (Ln=Eu, Tb) showed strong characteristic fluorescence. The red fluo- rescence of (Cnmim)(Eu(NO3)4) was assigned to Eu 3 + 5 D0- 7 F2 transition at 617 nm, while the green fluorescence of (Cnmim)(Tb(NO3)4) was assigned to Tb 3