Synthesis and mesomorphic properties of novel

tolane-type liquid crystals

Dong Yu Zhao, Qing Yong Meng, Xiao Peng Cui, Huai Yang *

Department of Materials Physics and Chemistry, School of Materials Science and Engineering,

University of Science and Technology Beijing, Beijing 100083, China

Received 13 March 2009

Abstract

Two series of novel tolane-type liquid crystals (LCs) comprising of hydrogen-bonded organic acids were synthesized. The

formation of dimerized H-bond LCs was confirmed by IR spectroscopy, and mesomorphic properties of the LCs were investigated

by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). It was found that the end groups of the liquid

crystals as well as the unsaturated rigid core had effect on the mesomorphic properties.

# 2009 Huai Yang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Keywords: Tolane; Hydrogen-bonded liquid crystal; Mesomorphic properties; Synthesis

Design and synthesis of novel liquid crystalline molecules are a focus of research in the field of liquid crystal

displays (LCDs). LCs containing acetylene groups are useful components in liquid crystal mixtures owing to their high

birefringence and low viscosity. So far, several kinds of liquid crystals containing the phenylacetylene have been

developed. Sekine et al. developed series of new phenylacetylene-based liquid crystals with high birefringence (Dn)

which are highly conjugated alone the molecular long axis to retain a high optical anisotropy [1–5]. In addition, Wen

et al. prepared new fluorinated tolane-type liquid crystalline materials and investigated the mesomorphic properties.

They found that with fluorination in the mesogenic core, the melt and clearing points were reduced [6,7].

In the last decade, hydrogen bonding has attracted much attention because it offers the possibility in molecular

recognition and self-assembly. A number of novel supramolecular hydrogen-bonded liquid crystals have been

obtained by molecular assembly through intermolecular hydrogen bonding [8–10]. In the present study, we reported

two series of new tolane-type liquid crystals comprising of dimerized carboxylic acids, which contain unsaturated

groups in the linearly mesogenic core and alkyl end groups. The new thermotropic liquid crystals may have potential

applications, such as liquid crystal alignment [11] and liquid crystal composite materials [12,13].

The target two kinds of liquid crystalline compounds were prepared according to the procedures shown in Schemes

1 and 2, respectively. Some 4-n-alkylphenylacetylenes (nAPA) were prepared by reported methods [4], and the

compounds 4-(2(4-n-alkylphenyl)ethynyl)benzoid acid (nAPEBA) were then obtained by a Pd (a)-catalysed cross-

coupling reaction between nAPA and 4-iodocinnamic acid. The compounds 4-(2(4-n-alkylphenyl)ethynyl)cinnamic

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Chinese Chemical Letters 20 (2009) 1283–1286

* Corresponding author.

E-mail address: yanghuai@mater.ustb.edu.cn (H. Yang).

1001-8417/$ – see front matter # 2009 Huai Yang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

doi:10.1016/j.cclet.2009.04.038

acid (nAPECA) were prepared by a Knoevenagal reaction to give the intermediate, followed by a Pd (a)-catalysed

cross-coupling reaction. All the compounds were characterized by FT-IR and 1H NMR spectroscopy [14].

All of the nAPECBA and nAPECA existed as acid dimers in their solid state, as shown in Fig. 1. The formation of

the H-bonded dimerized compounds shown in Fig. 1(a) and (b) was confirmed by IR spectroscopy. Fig. 2 shows the IR

D.Y. Zhao et al. / Chinese Chemical Letters 20 (2009) 1283–12861284

Scheme 1. Synthetic route for nAPEBA liquid crystals.

Scheme 2. Syhthetic route for nAPECA liquid crystals.

Fig. 1. Supramolecular structures of (a) nAPEBA and (b) nAPECA.

Fig. 2. Infrared spectra of 3APEBA, 5APEBA, 3APECA and 5APEBA at room temperature.

spectra of nAPECBA and nAPECA at room temperature. The IR spectra showed the characteristic stretching bands

(from �2500 to �3200 cm�1) resulting from dimerized carboxylic acids through intermolecular hydrogen bonding.

Mesomorphic properties of nAPEBA and nAPECA compounds were measured by polarizing optical microscopy

(POM) in conjunction with a hot stage, and by differential scanning calorimeter (DSC). Results of DSC and POM

revealed that these compounds, containing alkyl end groups as well as unsaturated –C C– and –CBBC– bonds in their

rigid core, show enantiotropic liquid crystalline behaviour. Fig. 3 shows the polarizing optical micrographs of the

nematic phases of these compounds. Schlieren textures, typical of the nematic phase, were clearly observed. In

addition, the phase transition temperatures and associated enthalpy changes of nAPEBA and nAPECA obtained by

DSC measurements are listed in Table 1. For both series nAPEBA and nAPECA, with increasing the length of the

terminal alkyl (n), the temperatures of the melting (Cr–N) transition as well as the clear points (N–I) decrease while the

nematic temperature ranges increase, which can be attributed to the increase of flexibility of these compounds. When

comparing nAPEBA compounds with nAPECA compounds for the same length of the terminal alkyl (n), with

increasing the unsaturated –C C– groups in their rigid core, the temperatures of the melting transition as well as the

clear points increase, which is owing to the increase of rigidity of these compounds.

In conclusion, all the target tolane-containing compounds exhibit the enantiotropic nematic phase. For both series

of nAPEBA and nAPECA, temperatures of the melting transition and the clear points decreased while the nematic

temperature ranges increased with increasing length of the terminal alkyl (n). For the same length of the terminal alkyl

(n), compounds in nAPECA which containing the unsaturated double bonds in their rigid core, exhibited the higher

temperatures of the melting transition and the clear points than those of the nAPEBA compounds.

D.Y. Zhao et al. / Chinese Chemical Letters 20 (2009) 1283–1286 1285

Fig. 3. Optical textures of the nematic phases: (a) 3APEBA at 274 8C, (b) 5APEBA at 263 8C, (c) 3APECA at 276 8C, and (d) 5APECA at 266 8C.

Table 1

Phase transition temperatures (8C) of the hydrogen-bonded carboxylic acid compounds (Cr = crystal phase, N = nematic phase, I = isotropic liquid).

Compound Phase transition temperature (8C) and associated enthalpy changes (J/g)

3APEBA Cr 234.9 (88.42); N 282.5 (19.64); Iso

5APEBA Cr 211.7 (39.16); N 270.3 (16.51); Iso

3APECA Cr 242.1 (54.85); N 302.2 (15.52); Iso

5APECA Cr 222.5 (41.16); N 289.0 (16.70); Iso

Acknowledgments

This research was supported by National Natural Science foundation (No. 20674005), Program of National High

Technology 863 program of China (No. 2006AA03Z108), National Key Technology Program (No. 2007BAE31B00).

Key Program for Panel Display of 863 program of China (No. 2008AA03A318).

References

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[14] Analytical and spectroscopic data for nAPEBA. 3APEBA, yield 72.1%. FT-IR (KBr, cm�1): 2540–3079 (acidic�OH stretching), 1683 (C O

stretching), 2212 (CBC), 1602, 1500 (aromatic C C), 1450. 1H NMR (400 MHz, DMSO-d6, d ppm): 0.87 (t, 3H, –CH3), 1.55 (m, 2H, –

CH3CH2), 2.57 (t, 2H, –CH2), 7.26 (d, 2H, Ar–H), 7.49 (d, 2H, Ar–H), 7.64 (d, 2H, Ar–H), 7.96 (d, 2H, Ar–H), 13.14 (–OH). 5APEBA, yield

71.3%. FT-IR (KBr, cm�1): 2927, 2855 (–CH2–), 2538–3070 (acidic –OH stretching), 1682 (C O stretching), 2213 (CBC), 1601, 1506

(aromatic C C). 1H NMR (400 MHz, DMSO-d6, d ppm): 0.843 (t, 3H, –CH3), 1.224 (m, 4H, –CH3CH2CH2), 1.536 (m, 2H, –CH3CH2CH2),

2.583 (t, 2H, –CH3CH2CH2CH2), 7.250 (d, 2H, Ar–H), 7.487 (d, 2H, Ar–H), 7.641 (d, 2H, Ar–H), 7.964 (d, 2H, Ar–H), 13.149 (–OH).

Analytical and spectroscopic data for nAPECA. 3APECA, yield 54.7%. FT-IR (KBr, cm�1) 2956, 2859 (–CH2–), 2516–3070 (acidic –OH

stretching), 1685 (C O stretching), 2213 (CBC), 1601, 1506 (aromatic C C). 1H NMR (400 MHz, DMSO-d6, d ppm): 0.876 (t, 3H, –CH3),

1574 (m, 2H, –CH3CH2), 2.572 (t, 2H, (ArCH2–), 6.565 (d, 1H, Ar–CH ), 7.590 (d, 1H, CH–COOH), 7.251 (d, 2H, Ar–H), 7.474 (d, 2H,

Ar–H), 7.557 (d, 2H, Ar–H), 7.733 (d, 2H, Ar–H), 12.466 (–OH). 5APECA, yield 59.6%. FT-IR (KBr, cm�1): 2924, 2849 (–CH2–), 2537–3070

(acidic –OH stretching), 1685 (C O stretching), 2213 (CBC), 1601, 1511 (aromatic C C). 1H NMR (400 MHz, DMSO-d6, d ppm): 0.845 (t,

3H, –CH3), 1.221 (m, 4H, –CH3CH2CH2), 1.539 (m, 2H, –CH3CH2CH2CH2), 2.584 (ArCH2–), 6.566 (d, 1H, Ar–CH ), 7.589 (d, 1H, CH–

COOH), 7.248 (d, 2H, Ar–H), 7.468 (d, 2H, Ar–H), 7.555 (d, 2H, Ar–H), 7.732 (d, 2H, Ar–H), 12.477 (–OH).

D.Y. Zhao et al. / Chinese Chemical Letters 20 (2009) 1283–12861286