DOI: 10.1021/cg900463b
Dysprosium-Based Ionic Liquid Crystals: Thermal, Structural, Photo- and Magnetophysical Properties
2009, Vol. 9 4429–4437
Anna Getsis,† Benjamin Balke,‡ Claudia Felser,‡ and Anja-Verena Mudring*,† † Anorganische Chemie I, Ruhr-Universit€ at Bochum, Universit€ atsstraβe 150, D-44801 Bochum, at Mainz, D-55099 Mainz, Germany Germany, and ‡Anorganische Chemie, Gutenberg Universit€
Received April 26, 2009; Revised Manuscript Received July 5, 2009
ABSTRACT: [C12mim]3[DyBr6] (C12mim=1-dodecyl-3-methylimidazolium) represents a new material with interesting luminescent behavior as well as mesomorphic and magnetic properties. The compound was found to show thermotropic liquid crystalline behavior and forms smectic mesophases which were investigated by hot-stage polarizing optical microscopy and differential scanning calorimetry. The emission color of [C12mim]3[DyBr6] can be tuned from white to orange-yellow by the choice of the excitation wavelength. Sample excitation with λex=366 nm leads to the blue-whitish luminescence from the imidazolium cation itself. With λex= 254 nm the common Dy(III) emission is observed which mainly arises from the 4F9/2 f 6H13/2 transition and, in consequence, the sample appears orange. Magnetic measurements show for Dy(III) in [C12mim]3[DyBr6] an effective magnetic moment of μeff=9.6 μB at room temperature. The sample shows superparamagnetism and can be manipulated by an external magnetic field. In addition, the crystal structure of the corresponding acetonitrile solvate [C12mim]3[DyBr6] 3 2CH3CN (orthorhombic, Pbca, No. 62, Z=8, Pearson code oP1328, a=14.888(4) A˚, b=18.240(7) A˚, c=49.411(13) A˚, 5596 unique reflections with Io > 2σ(Io), R1=0.1047, wR2 0.2442, GOF=1.167, T=298(2) K) has been elucidated. It is characterized by alternating double layers of [C12mim]þ cations of opposite orientation and nearly ideal [DyBr6]3- octahedra with hydrogen bonded acetonitrile. The structure of [C12mim]3[DyBr6] 3 2CH3CN serves as a structure model for the solvate free [C12mim]3[DyBr6].
*To whom correspondence should be addressed. Fax: (49) 234 27408. E-mail:
[email protected]. Web: http://www.anjamudring.de.
cations used for both ILs and ILCs.6 It is known that certain 1alkyl-3-methylimidazolium salts with 12 and more carbon atoms in the side chain can form lamellar mesophases whose (mesophase) stability increases with increasing length of the alkyl chain.6c Obviously the research field of ILCs includes a multitude of conceivable compounds whose mesomorphic behavior is based on the amphiphilic nature one of the ions (either cation or anion). The unlimited capacities in ILC research correspond to the variety of ionic liquids as recent works show.7 Moreover, in the last few decades, metal-containing liquid crystals (metalomesogens) have been studied as a special class of ILCs. By choosing a suitable metal ion as the ILC consitutent additional functionalities can be introduced to the material. Thus, multifunctional materials can be received by the combination of the mesomorphic behavior with unique properties of metal ions (such as redox-activity, magnetism, or luminescence). Several reviews survey the intensive studies on metallomesogens.8a-8d In contrast to neutral, uncharged mesophorphic metal complexes their ionic analogues have been studied far less. Some recent reviews give an overview of the research activities in the field of d-,8c,9 and f-element mesogenes.10 The majority of investigated metallomesogens (including ILCs) comprises the metal ion in the anisotropic part of the compound. In contrast, fewer works report on ILCs where an anisotropically shaped cation is coupled with a complex metal anion of high symmetry such as tetrahedral or octahedral. So far, in this field most work has been undertaken on tetrahalometallates of d-elements. However, similar compounds with f-element cations are of potential interest because of their photoluminescent and magnetic properties. There is an immense interest in light emitting liquid crystals.11 By the variation of the emitting lanthanide center compounds emitting in the three basic colors (Tm3þ for blue, Eu3þ for red, and Tb3þ for green) can be made. Indeed, by doping ionic liquids crystals with lanthanide complexes highly luminescent
r 2009 American Chemical Society
Published on Web 08/18/2009
Introduction Ionic liquid crystal (ILC) research is located at the boundary of two fields of research: ionic liquids (ILs) and liquid crystals (LCs) which attract particular attention of scientists both in industry and academia.1,2 Ionic liquids have received substantial interest in recent years. Their inherent properties such as in most cases negligible vapor pressures, wide liquidus ranges, good thermal stabilities, considerable electric conductivities, and wide electrochemical windows have been shown to be advantageous for a large number of applications.3 Today they are predominately used in separation, various electrochemical applications, in organic synthesis and catalysis.3 As salts, ionic liquids are composed of distinct cations and anions which make them widely tunable. The variety of cation/anion-combinations leads to the description of ILs as “designer solvents”.2 Indeed, they may be designed for specific applications by choosing the appropriate cation and anion combination. At the same time the interdisciplinary and intensive development of liquid crystals is based on the variety of mesogenic phases and their diverse properties based on the anisotropic nature of this state of matter.4 Ionic compounds which contain one or two anisotropically shaped ions (rod-like or disk-like) are likely able to form mesophases. Such compounds can be addressed as ionic liquid crystals. ILCs feature some specific properties untypical for “traditional” LCs consisting of neutral molecules. Here, for example, ionic conductivity and uncommon ordering of the liquid-crystalline states (tetragonal smectic and nematic columnar phases) can be named as examples.5 At the present time derivatized N-methyl-imidazolium cations are by far the most popular
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materials could be obtained.12 Application in emissive liquid displays has been suggested here. Furthermore, aligned luminescent LCs can emit polarized light.13 A high magnetic anisotropy of the lanthanide ion is advantageous for the alignment of the mesophase in an external magnetic field. Tb3þ, Dy3þ, and Tm3þ are thought to be the best suited trivalent lanthanide ions for such magnetic alignment.14 Furthermore, aligned luminescent LCs can emit polarized light. Such compounds have been proposed to be of use for electric and magnetic switchable devices.15 Here we report on the synthesis as well as on the structural, thermal, photophysical, and magnetic properties of [C12mim]3[DyBr6] and the structure of the related acetonitrile solvate [C12mim]3[DyBr6] 3 2CH3CN. Experimental Details Synthesis and sample handling were carried out under standard Schlenk and Argon-glovebox techniques. All solvents were dried using standard procedures. Coulometric Karl Fischer titration shows no measurable water content in the samples under investigation. 1-Methyl-3-dodecylimidazolium Bromide, [C12mim]Br. 3.4 mL of 1-methylimidazole (3.50 g, 0.043 mol) (99%, Acros organics, Geel, B) is dissolved in 100 mL of dry acetonitrile. Then 10.0 mL (10.38 g, 0.042 mol) of dodecylbromide (99% Acros organics, Geel, B) are added dropwise and the reaction mixture is heated under reflux for 18 h. After the mixture is cooled to room temperature about 50 mL of the solvent is removed under vacuum. Upon adding the concentrated acetonitrile solution dropwise to cold (-30 °C) dry toluene [C12mim]Br precipitates as a white powder. The reaction product is filtered off and recrystallized two times from dry acetonitrile/ toluene. Before use the product is dried from any remaining solvent for 48 h under dynamic vacuum at 100 °C.
H NMR δH (298 K, 300 MHz, DMSO-d6): 0.868 [t, 3H, H-1]; 1.234 [br. S., 18H, H-2]; 1.766 [quint., 2H, H-3]; 3.848 [s, 3H, H-5]; 4.149 [t, 2H, H-4]; 7.708 [m,1H, H-6]; 7.777 [m, 1H, H-7]; 9.151 [br. S., 1H, H-8]. Elemental analysis (%) calcd for [C12mim]Br: C 57.63, H 9.74, N 8.42; found: C 58.00, H 9.74, N 8.45. Vibrational Spectroscopy. MIR (KBr pellet). νh/[cm-1] = 3226.5 (w), 3149.4 (m), 3083.8 (s), 3062.6 (s), 2950.7 (s), 2916.0 (s), 2869.7 (s), 2852.4 (s), 1772.4 (w), 1668.2 (w), 1629.7 (m), 1571.8 (m), 1473.4 (m), 1427.2 (w), 1384.7 (w), 1344.2 (w), 1317.2 (w), 1292.2 (w), 1178.4 (s), 1126.3 (w), 1089.7 (w), 1054.9 (w), 1043.4 (w), 1022.2 (w), 1004.8 (w), 943.1 (w), 887.2 (w, shoulder), 862.1 (m), 792.6 (m), 763.7 (w), 742.5 (w), 729.0 (w), 715.5 (m), 661.5 (m), 622.9 (m). FIR (PE pellet). νh/[cm-1]=562.2 (m, shoulder), 509.1 (w), 498.9 (w), 472.5 (w), 453.2 (m), 417.5 (m), 396.3 (m), 349.1 (w), 284.5 (w), 230.5 (w), 154.3 (m, shoulder), 93.5 (s), 64.6 (s, shoulder). Raman. νh/[cm-1]=3134.2 (w), 3082.9 (w), 2956.7 (w), 2933.6 (s), 2883.5 (s), 2848.7 (s), 2727.2 (w), 1560.4 (vw), 1452.4 (w, shoulder), 1432.1 (w), 1421.6 (w, shoulder). 1375.3 (vw), 1334.8 (vw), 1296.2 (w), 1128.4 (w), 1105.3 (vw), 1087.9 (vw), 1062.9 (w), 1014.6 (w), 887.4 (vw), 733.1 (vw), 692.6 (vw), 654.0 (vw), 605.8 (vw), 414.9 (vw), 303.0 (vw), 231.6 (vw), 181.5 (vw, shoulder), 150.6 (w, shoulder), 114.0 (w), 85.1 (w, shoulder). For a graphical representation of the spectra see Supporting Information. Dysprosium Tribromide, DyBr3. DyBr3 was synthesized according to a modified literature procedure.16 2.00 g (0.0054 mol) of dysprosium(III)oxide (99.9% Fluka Chemie AG, Buchs, Ch) and 6.40 g (0.0648 mol) of ammonium bromide (suprapur 99.995% 1
Getsis et al. Merck, Darmstadt, D) were dissolved in 48% conc. aqueous hydrobromic acid (Aldrich, Steinheim, D). After the solvent was removed, the formed (NH4)3DyBr6 3 yH2O is decomposed to DyBr3 at 900 °C. At this temperature, any excess ammonium bromide sublimes off. The crude DyBr3 is then purified by sublimation under vacuum (
