Synthesis and Mesophase Behaviour of Morpholinium Ionic Liquid Crystals

Chinese Journal of Chemistry, 2009, 27, 1471—1475 Full Paper

* E-mail: [email protected]; Tel. and Fax: 0086-027-87218534 Received December 15, 2008; revised February 15, 2009; accepted April 10, 2009. Project supported by China Postdoctoral Science Fundation (No. 20080431002).

© 2009 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Synthesis and Mesophase Behaviour of Morpholinium Ionic Liquid Crystals

YU, Weia(余巍) PENG, Huia(彭慧) ZHANG, Haiboa,b(张海波) ZHOU, Xiaohai*,a(周晓海)

a College of Chemstry, Wuhan University, Wuhan, Hubei 430072, China b Feixiang Chemicals (Zhangjiagang) Co. Ltd., Zhangjiagang, Jiangsu 215613, China

A novel series of morpholinium salts, N-alkyl-N-methylmorpholinium salts (Mor1,nX, n＝10, 12, 14, 16, X＝

4BF )− and Brönsted acidic N-hexadecyl morpholinium salts (Mor16X, X＝ 4 3 3BF , CH SO− − and 3 6 4 3-CH C H SO ) p − were synthesized. And their mesomorphic properties were characterized by differential scanning calorimetry and polarized optical microscopy. The single crystal X-ray diffraction and the detailed temperature dependent small an-gle X-ray diffraction were used to investigate the molecular structure of Mor1,16BF4. These results suggested that the

4BF− favor the formation and stabilization of mesogenic compounds. And hydrogen bonds between the 4BF− and morpholinium cations contributed to the conformational change of phase transition of Mor1,16BF4.

Keywords molecular structure, mesomorphic property, hydrogen bond, ionic liquid crystal

Introduction

Ionic liquid crystals (ILC),1 based on the structure of ionic liquids, are a class of liquid-crystalline compounds that contain anions and cations. These salts can be con-sidered as materials combining the properties of liquid crystals and ionic liquids. Just as ionic liquids, the properties of ionic liquid crystals can be tuned by an appropriate choice of anions and cations. These salts have been used in various applications such as ordered solvents,2-4 templates for the synthesis of mesoporous and zeolitic materials,5-7 optoelectronics,8,9 and dye-sen- sitized solar cells.10

Molecular liquid crystals have been extensively de-signed and studied,11-13 however, the type of ionic liquid crystals is still limited. Among the known ILC, imida-zolium or pyridinium salts are the mostly studied com-pounds.14,15 Besides, some functional groups, such as aromatic cores,16,17 alkyl-chains with different lengths and natures,18 lateral substitution groups,19,20 metal-con- taining groups,21 and counteranions,22,24 have been ex-tensively studied for the preparation of mesogenic compounds. Only a few thermotropic ionic liquid crys-tals based on other cations have been reported.25

Most papers have been focused on the properties and the phase transition of these new materials, but the rea-son causing conformational change in the phase transi-tion is rarely studied. Some non-classcical hydrogen bonds have been detected in the crystal structure of these salts,20,22 especially that between the Br anions and cations, however, their effect on the formation of mesophase has not been discussed.

In this work we studied morpholinium salts as ionic liquid crystals, which are different from morpholinium ionic liquids.26 Three salts in these compounds had me-somorphic behaviour. The effect of different cations on the ionic liquid crystals was investigated. Extensive hy-drogen bonding interactions between the 4BF− and morpholinium cations in Mor1,16BF4 was found, which has also been studied on the formation of mesogenic compounds.

Experimental

Materials and methods

All the solvents used were of reagent grade and used as received. Elemental analysis was performed using a VarioEL III Elemental analyzer. 1H NMR measurement was conducted on a Mercury VX-300 (Varian, 300 MHz) spectrometer. Polarized optical microscopy (POM) studies on the salts were carried out using an Olympus BX-51 polarized microscope under cross-polarized light at 100× magnification. Differential scanning calorimetry (DSC) measurements were col-lected on a Mettler Toledo DSC 822e calorimeter, at heating and cooling rates of 10 ℃•min－1. Data were only recorded after the sample had first been heated to its clearing point and then allowed to cool to its crystal-lization point. The X-ray diffraction spectrum (XRD) was obtained on a Bruker D8 advance X-ray diffracto-meter with Cu Kα line. Diffraction patterns were re-corded in θ/2θ geometry with scans from 2° to 10° in steps of 0.02° and controlled by the hot stage. The

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powder samples were made as lamellae with a diameter of 0.10 or 0.15 mm. Single crystal X-ray diffraction data of Mor16BF4 were collected on a Bruker Smart APEX II CCD diffractometer with Mo Kα radiation (λ＝0.71073 Å) at 294(2) K in the range of 1.58°＜h＜25.0°. The structure was solved by direct method (SHELXL-97) and refined against F2 in anisotropic approximation (SHELXL-97).

Preparation

All morpholinium salts (Figure 1) reported here were anhydrous and stable in air. N-Alkyl-N-methylmorpholinium bromide salts (n＝10, 12, 14, 16) were prepared in good yields (＞85%, based on 1-methylmorpholine) by the alkylation of 1-methylmorpholine with corre-sponding 1-bromoalkanes. Acetonitrile was used as solvent in an inert atmosphere. The desired products were produced by further metathesis with NH4BF4. N-Hexadecyl morpholinium salts were prepared in good yields (＞80%, based on morpholine) by the reaction of 1-bromohexadecane and morpholine with NaHCO3. Then the obtained N-hexadecyl morpholine was mixed with HBF4, CH3SO3H, or CH3C6H4SO3H, respectively, therefore, the corresponding products were obtained. Purification was achieved by recrystallization from acetonitrile. The only by-products, acids and halide salts, were removed by washing with water.

Figure 1 The molecular structures of morpholinium salts.

Synthesis of N-hexadecyl-N-methylmorpholinium tetrafluoroborate salt (Mor1,16BF4)

1-Methylmorpholine (10.12 g, 0.1 mol) and 1-bromohexadecane (30.5 g, 0.1 mol) were mixed in a 100 mL round bottomed flask with acetonitrile as solvent at 80 ℃ for 2 d in an inert atmosphere. After cooling, white solid was recrystallined from acetonitrile in a yield of 94%. Then N-hexadecyl-N-methylmorpholin-ium bromide (8.13 g, 0.02 mol) and tetrafluoroborate ammonium (2.1 g, 0.02 mol) were stirred at room tem-perature for 2 d with water as solvent. The crystalline product was collected by filtration and recrystallined from ethanol twice and dried under vacuum for 5 h. The yield was about 87%. 1H NMR (CDCl3, 300 MHz) δ: 0.82—0.92 (m, 3H), 1.18—1.36 (m, 26H), 1.57 (s, 2H), 3.27 (s, 3H), 3.34—3.57 (m, 6H), 4.01 (s, 4H). Anal. calcd for C21H44BF4NO: C 61.01, H 10.73, N 3.39; found C 61.19, H 10.68, N 3.42.

The other Mor1,nBF4 (n＝10, 12, 14) compounds were prepared as for n＝16.

N-decyl-N-methylmorpholinium tetrafluoroborate salt (Mor1,10BF4) White solid, yield 95%. 1H NMR (CDCl3, 300 MHz) δ: 0.81—0.94 (m, 3H), 1.12—

1.44 (m, 14H), 1.72 (s, 2H), 3.23 (s, 3H), 3.43 (s, 6H), 4.00 (s, 4H). Anal. calcd for C15H32BF4NO: C 54.72, H 9.80, N 4.25; found C 54.74, H 9.72, N 4.25.

N-dodecyl-N-methylmorpholinium tetrafluoroborate salt (Mor1,12BF4) White solid, yield 96%. 1H NMR (CDCl3, 300 MHz) δ: 0.80—0.93 (m, 3H), 1.12—1.44 (m, 18H), 1.74 (s, 2H), 3.20 (s, 3H), 3.28—3.58 (m, 6H), 3.98 (s, 4H). Anal. calcd for C17H36BF4NO: C 57.15, H 10.16, N 3.92; found C 57.11, H 10.19, N 3.91.

N-tetradecyl-N-methylmorpholinium tetrafluoroborate salt (Mor1,14BF4) White solid, yield 97%. 1H NMR (CDCl3, 300 MHz) δ: 0.82—0.94 (m, 3H), 1.12—1.44 (m, 22H), 1.75 (s, 2H), 3.24 (s, 3H), 3.28—3.58 (m, 6H), 4.00 (s, 4H). Anal. calcd for C19H40BF4NO: C 59.22, H 10.46, N 3.63; found C 59.38, H 10.51, N 3.62.

Synthesis of N-hexadecylmorpholinium tetrafluoroborate salt (Mor16BF4)

Morpholine (3 g, 0.03 mol) and 1-bromohexadecane (6.1 g, 0.02 mol) were mixed in a 100 mL round bot-tomed flask with ethanol as solvent with NaHCO3 at 80℃ for 8 h. After cooling, the material was washed with water twice and acetone twice. To this solution, white solid was obtained in a yield of 85%. Then N-hexadecyl morpholine (3.1 g, 0.01 mol) was mixed with HBF4 (1 g, 0.011 mol) with acetone as solvent to get the desired product, which was recrystallined from acetonitrile/ acetone and dried under vacuum for 5 h. The yield was over 90%. 1H NMR (CDCl3, 300 MHz) δ: 0.84—0.92 (m, 3H), 1.15—1.44 (m, 26H), 1.82 (s, 2H), 2.94—3.10 (m, 4H), 3.58 (d, J＝11.7 Hz, 2H), 4.04 (s, 4H) 8.25 (s, 1H). Anal. calcd for C20H42BF4NO: C 60.15, H 10.6, N 3.51; found C 60.18, H 10.03, N 3.47.

The other Mor16X 3 3(X CH SO , or －

＝ 3 6 4-CH C H -p 3SO )－ compounds were prepared as Mor16BF4. N-hexadecylmorpholinium methylsulfonic acid

salt (Mor16CH3SO3) White solid, yield 94%. 1H NMR (CDCl3, 300 MHz) δ: 0.68—0.84 (m, 3H), 1.04—1.38 (m, 26H), 1.63 (s, 3H), 1.78 (s, 2H), 2.77—2.98 (m, 4H), 3.43—3.93 (m, 4H), 4.10 (s, 2H), 11.34 (s, 1H). Anal. calcd for C21H45NO4S: C 61.87, H 11.13, N 3.44; found C 61.84, H 11.13, N 3.46.

N-hexadecylmorpholinium p-toluenesulfonic acid salt (Mor16CH3C6H4SO3) White solid, yield 87%. 1H NMR (300 MHz, CDCl3) δ: 0.82—0.94 (m, 3H), 1.12—1.38 (m, 26H), 1.81 (s, 2H), 2.36 (s, 3H), 2.82—3.04 (m, 4H), 3.53—4.0 (m, 4H), 4.18 (s, 2H), 7.18 (d, J＝8.1 Hz, 2H), 7.77 (d, J＝8.1 Hz, 2H), 11.35 (s, 1H). Anal. calcd for C27H49NO4S: C 67.04, H 10.21, N 2.90; found C 66.92, H 10.22, N 2.96.

Results and discussion

Liquid crystalline properties

The DSC data in Table 1 are the phase transition temperatures and enthalpies during the cooling and heating processes. The compounds of Mor1,nBr could not be determined because of decomposition below their

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clearing points. It should be attributed to the weak in-teraction between Br anions and morpholium cations. Compared with the Mor1,nBr, the Mor1,nBF4 series of compounds had a relatively low melting temperature and did not decompose until 300 ℃. Except Mor1,14BF4, Mor1,16BF4 and Mor16BF4, other salts in Table 1 showed distinct freezing and melting points. Different transition temperatures with similar transition enthalpies sug-gested that no stable mesophase be formed. Besides, the DSC curves of Mor1,14BF4, Mor1,16BF4 and Mor16BF4 were similar to those of ionic liquid crystals reported previously,14,15 they had two transition temperatures and corresponding transition enthalpies not only in cooling but aslo in heating, which revealed that the stable mesophase was formed with the 4BF− anion.

Table 1 Transition temperatures and enthalpies of Mor1,nBF4 and Mor16X

a

Compound Transition T/℃ heating [ΔH/(kJ•mol－

1)]

T/℃ cooling [ΔH/(kJ•mol－1)]

Freezing point

n 39.8 (－36.4)

Mor1,10BF4 Melting point 70.7 (35.3) n

Freezing point

n 63.3 (－43.3)

Mor1,12BF4 Melting point 81.8 (40.2) n

Cr—SmA 86.2 (48.3) 72.8 (－27.1) Mor1,14BF4 SmA—I 146.3 (3.9) 141 (－3.7)

Cr—SmA 95 (32.4) 82.3 (－37.7) Mor1,16BF4 SmA—I 183.7 (0.8) 182.5 (－0.9)

Freezing point

n 64.8 (－62.6)

Mor16CH3SO3 Melting point 105.8 (39.9) n

Freezing point

n 108 (－35.3)

Mor16CH3C6H4SO3 Melting point 123.7 (32) n

Cr—SmA 106.6 (67.2) 97 (－70.7) Mor16BF4 SmA—I 140.1 (14.3) 136.2 (－15.2) a Cr, SmA and I indicate crystalline state, smectic A phase and isotropic liquid, respectively. n indicates no transition can be observed.

The textures of mesomorphic salts are shown in Fig-ure 2, and as expected, the phase transition temperature was consistent with the results observed by DSC. Ani-sotropic texture could be seen at the transition point on heating and disappeared soon. On cooling, the textures were stabilized from the same temperature to the freez-ing point. In these stable mesophases, Mor1,16BF4 exhib-ited the fan-like texture, while the palm-like textures were observed in Mor1,14BF4 and Mor16BF4.

Figure 2 Textures of morpholinium salts under crossed polar-izers. Magnification ×100. (a) Mor1, 16BF4 at 170 ℃ , (b) Mor1,14BF4 at 134 ℃, and (c) Mor16BF4 at 145 ℃.

Considering both the amphiphilic structure of the mesogen and the optical texture, with further powder X-ray diffraction studies, the type of these mesophases could be defined as smectic A phase.27

Crystal structure and mesomorphism structure analysis of Mor1, 16BF4

Results here reported the mesomorphism structure analysis of Mor1,16BF4. According to the DSC data and POM textures, the formation of morpholinium ionic liquid crystals depends both on the alkyl length and the anion. Compared to the Br－ anion, the 4BF− anion showed stronger interaction with morpholinium cations, herein the crystal structure and XRD data of Mor1,16BF4 were used to investigate this interaction.

The structure of Mor1,16BF4 has been determined by single crystal X-ray diffraction (Figure 3). The ORTEP drawing of this compound is shown in Figure 3a. The morpholinium head cores and tetrafluoroborate anions form hydrophilic layers separated by hydrophobic layers of alkyl chains. The cations exhibit a monolayer of in-terdigitated lamellar structure along the b-axis (Figure 3b), and the direction of the rods tilts ca. 15° from the normal of the layer-plane (a—c plane). As shown in Figure 3c, the cations show a W-shaped interdigitated

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lamellar structure along axis a, and the direction of the rods tilts ca. 30° from the normal of the layer-plane (a—c plane). Furthermore, each tetrafluoroborate anion forms three kinds of hydrogen bonds with one mor-pholinium cation, which are H(4B)…F(1), H(4A)…F(3), and H(6B)…F(4) in Figure 4, respectively.

Figure 3 (a) ORTEP drawing of Mor1,16BF4. (b) Crystal pack-ing of Mor1,16BF4 showing the interdigitation and the tilted alkyl chains along the b-axis. (c) Crystal packing of Mor1,16BF4 viewed down the a-axis.

Figure 4 H-bonding interactions around morpholinium moiety (Å): H(4B)…F(1) 2.50, H(4A)…F(3) 2.50, and H(6B)…F(4) 2.55.

In order to investigate the influence of hydrogen bonds on the mesomorphism. XRD data were collected for Mor1,16BF4 as a function of temperature.

Figure 5 shows the heating and cooling cycles for Mor1,16BF4, and lamellar structures were found at dif-ferent temperatures, the layer spacing (d) was calculated by using Bragg’s law. The layer spacing in the mesophase increased with increasing the temperature (Figure 5a—5c). After cooling from the isotropic liquid

(Figure 5d), there were two obvious peaks formed at 170 ℃ (Figure 5e), and three peaks formed at 100 ℃ (Figure 5f).

Figure 5 XRD patterns for Mor1,16BF4 at (a) 23 ℃, (b) 100 ℃, (c) 150 ℃, (d) 190 ℃ on heating, (e) 170 ℃, and (f) 100 ℃ on cooling respectively.

According to the layer spacing d in the mesophase, when the temperature increased from 23 to 150 ℃, the layer spacing increased, which should be caused by the extended alkyl chain or the decreased angle of tilt with respect to the layer normal. The phenomenon was simi-lar with most reported ionic liquid crystals.14,19 In this period, the structure change of mesophase was mainly based on its crystal structure. However, when mesog-enic groups were self-assembled from isotropic liquid on cooling, the hydrogen bonds between the mor-pholinium cation and tetrafluoroborate anion could be the main reason for the structure change of the mesog-enic group. Cations could undergo rearrangement with the asymmetric effect of hydrogen bonds in Figure 4. Three morpholinium cations approached the 4BF− an-ion because of hydrogen bonds while the rest one left with the attraction of other 4BF− anions, which will cause different degrees of rearrangement and the struc-ture undergoes a conformational change. Following the process, more than two liquid crystalline layer spacings should be fromed, for example, d1 (34.67 Å) and d2 (33.68 Å) existed in the mesophase at 170 ℃ or d3 (35.02 Å), d4 (32.21 Å) and d5 (31.52 Å) existed in the mesophase at 100 ℃ (Figure 5e, 5f).

Conclusion

In this work, novel ionic liquid crystals based on morpholinium cations have been prepared. The thermal behaviour of these salts has been studied by DSC and POM. Among them, liquid crystalline properties can be seen on cooling from isotropic liquid in Mor1,14BF4, Mor1,16BF4 and Mor16BF4, palm-like texture and normal

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fan-like texture are found in these three salts, which are defined as SmA phase. The layer spacing of Mor1, 16BF4 in the crystal and mesophase was determined by virable temperature small angle XRD and the correlation be-tween the layer spacing and temperature was discussed. The introduction of the morpholinium headgroup should be the reason for the formation of palm-like texture, the effect of 4BF− anion on hydrogen-bond in Mor1,16BF4 will lead to the stabilization and conformational change of its mesophase.

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Synthesis and Mesophase Behaviour of Morpholinium Ionic Liquid Crystals