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

Five kinds of ionic liquids 1-alkyl-3-methylimidazolium nitrate [C(n)mim]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 (H-1 NMR), mass spectrometry (MS) and elemental analyses (EA). Lanthanide-containing ionic liquids [C(n)mim][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. [C(n)mim][Ln(NO3)(4)] were all sticky liquids at room temperature. The strength of nitrate absorption peaks of [C(n)mim][Ln(NO3)(4)] in the IR spectra was stronger than [C(n)mim]NO3 which indicated that the coordination of nitrate anions to lanthanide ions enhanced the nitrate absorption peaks. The structures of [C(n)mim][Ln(NO3)(4)] contained 1-alkyl-3-methylimidazolium cations and [Ln(NO3)(4)](-) anions in which each lanthanide ion was coordinated with eight oxygen atoms from four nitrate ions were further confirmed by ESI mass spectrometry. The thermal decomposition of [C(n)mim]NO3 and [C(n)mim][Ln(NO3)(4)] were investigated by thermogravimetric analysis (TGA), which indicated that all the compounds represented high stability until ca. 250 degrees C. Under UV lamp irradiated at 365 nm, [C(n)mim][Eu(NO3)(4)] and [C(n)mim][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 [C(n)mim][Ln(NO3)(4)] (Ln=Eu, Tb) showed strong characteristic fluorescence. The red fluorescence of [C(n)mim][Eu(NO3)(4)] was assigned to Eu3+ D-5(0)-F-7(2) transition at 617 nm, while the green fluorescence of [C(n)mim][Tb(NO3)(4)] was assigned to Tb3+ 5D4-F-7(5) transition at 545 nm. Temperature and concentration influenced the solution fluorescence intensity of [C(n)mim][Ln(NO3)(4)]. Due to the intermolecular collision intensified in solution with temperature increasing, the fluorescence intensity of [C(n)mim][Ln(NO3)(4)] decreased. Fluorescence intensity increased with the concentration increasing, while fluorescence quenching phenomenon appeared when exceeded a certain concentration.