Switchable Green and White Luminescence in Terbium-Based Ionic Liquid Crystals

[C(12)mim] Br (C(12)mim = 1-dodecyl-3-methyl) and [C(12)mpyr] Br (C(12)mpyr = N-dodecyl-N-methylpyrrolidinium), both doped with TbBr3, as well as the neat compounds [C(12)mim](3)[TbBr6] and [C(12)mpyr](3)[TbBr6] show mesomorphic behavior and are able to form smectic liquid crystalline phases. The thermal phase behavior of all compounds was investigated by hot-stage polarizing optical microscopy and differential scanning calorimetry. Whilst the doped compounds crystallize around room temperature, the neat compounds solidify as glasses around -5 degrees C and qualify also as ionic liquids. The crystal structure of the acetonitrile solvate of the imidazolium compound, [C(12)mim](3)[TbBr6]center dot 2CH(3)CN, could be determined by single X-ray diffraction. The structure features alternating double layers of [C(12)mim](+) cations and [TbBr6](3-) octahedra with hydrogen bonded acetonitrile. [C(12)mim](3)[TbBr6]center dot 2CH(3)CN can serve as a structure model for the smectic mesophase of the solvate-free [C(12)mim](3)[TbBr6]. All materials show strong green luminescence from the D-5(4)-level of Tb3+ after excitation into the 4f(8)-> 4f(7)5d(1) transition. In case of the imidazolium compounds, the color of this emission can be switched between green and blue-white depending on the excitation energy. After excitation with lambda(ex) = 254 nm, strong green emission mainly from the D-5(4)-level of Tb3+ is observed. With lambda(ex) = 366 nm, only the blue-white luminescence from the imidazolium cation itself is detected.