Supramolecular liquid crystals with hydrogen-bonding self-assembled T-shaped mesogens

Article abstract of DOI:10.1246/cl.2001.1156

A new class of supramolecular liquid crystals with T-shaped mesogens are obtained by doubly hydrogen-bonded self-assembly of 2,6-bis(4-hexyloxybenzoylamino)pyridine and benzoic acid derivatives.

Full text of DOI:10.1246/cl.2001.1156

1
156
Chemistry Letters 2001
Supramolecular Liquid Crystals with Hydrogen-Bonding Self-Assembled T-Shaped Mesogens
†
Dongzhong Chen,* Lei Wan, Jianglin Fang, and Xuehai Yu
Department of Polymer Science and Engineering, College of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, P. R. China
†
Center for Materials Analysis, Nanjing University, Nanjing 210093, P. R. China
(Received July 30, 2001; CL-010721)
A new class of supramolecular liquid crystals with T-
one sharp peak at 170.8 °C in the DSC heating run and at 160.7
1
shaped mesogens are obtained by doubly hydrogen-bonded
self-assembly of 2,6-bis(4-hexyloxybenzoylamino)pyridine and
benzoic acid derivatives.
°C upon cooling. H NMR for 1(CDCl , 25 °C, ppm): 8.25,
3
8.22, 8.00, 7.80, 6.88, 3.95, 0.92–1.89. Phase transitions of the
solid samples of molecular complexes were examined by DSC
and visual observation on polarized optical microscope.
DSC curves for the complex 1/2b in heating and cooling
runs are shown in Figure 1. It is noteworthy that the complex
behaves as a single species and no transition duo to either of 1
and 2b is observed for the mixture. The thermogram for 1/2c is
similar to that of 1/2b in form of transition peaks except with
different temperatures. Both of the complexes 1/2b and 1/2c
show the enantiotropic LC characters. While the complex 1/2a
melts sharply to an isotropic state at 133.3 °C on heating, and
upon cooling, two exothermic peaks which can be ascribed to
isotropic–mesophase and mesophase–crystalline transitions are
observed for the complex, which are consistent with the charac-
ters of monotropic LC behavior. The transition peak tempera-
tures and corresponding enthalpy changes of all the complexes
are summarized in Table 1.
In the conventional liquid crystals, the mesogenic units
usually have the shape of rod-like(calamitic) or disc-like(discot-
ic). The X-shaped or T-shaped mesogens were also reported in
1
,2
polymeric liquid crystals.
In supramolecular liquid crystals
self-assembled by non-convalent interactions, there have been
many examples in calamitic liquid crystals from hydrogen-
3
bonding extended mesogens, and discotic hydrogen-bonding
associated mesogens from triazines and carboxylic acids which
exhibited columnar liquid crystalline (LC) behavior were also
4
reported. It had been extensively demonstrated by Hamilton et
5
,6
al. that double hydrogen bonds were formed by complexation
between a 2-aminopyridyl and a carboxylic acid, and based on
this type of doubly hydrogen-bonding, Kato et al. have built a
7
–10
series of supramolecular liquid crystals.
The 1:1 complexes
of 2,6-bis(acylamino)pyridines with alkoxybenzoic acid
7
showed monotropic LC behavior, while the supramolecular
complexes of 2,6-bis(acylamino)pyridines and azo mesogenic
compounds containing carboxyl-terminated alkyl chains exhib-
8
ited thermally stable enantiotropic LC behavior. In all these
examples, the LC units were long-alkyl chains modified
calamitic mesogens. We report here a new class of supramolec-
ular liquid crystals with hydrogen-bonded T-shaped mesogens.
The aminopyridine-containing hydrogen-bonding component
2
,6-bis(4-hexyloxybenzoylamino)pyridine (1) has been selected
1
1
to form supramolecular complexes with benzoic acid deriva-
tives such as 4-methoxybenzoic acid (2a), 4-hexyloxybenzoic
1
2
acid (2b) and 4-(4-methoxybenzoyloxy)benzoic acid (2c) .
Compound 1 was prepared from 2,6-diaminopyridine and hexy-
loxybenzoyl chloride. It was a white crystalline solid with only
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
1157
Under cross-polarized optical microscope, complex 1/2a
was observed analogic focal conic texture between 128 °C and
10 °C only on cooling, while complex 1/2b showed a texture
similar to that of 1/2a both on heating and on cooling, the tem-
Financial support on "Design of Supramolecular Side-
chain Liquid Crystalline Polymers and Their Syntheses by Self-
Assembly"(No.20004005) from National Natural Science
Foundation of China is gratefully acknowledged.
1
perature intervals coincided with DSC results. These textures
1
3
were similar to those of 4-alkoxy-4'-stilbazoles and benzoic
References and Notes
14
acid/dipyridyl complexes, which were ascribed to smectic liq-
1
S. Berg, V. Krone, and H. Ringsdorf, Makromol. Chem.,
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uid crystals.1
3,14
Whereas a dendritic growth texture was
observed for the complex 1/2c both on heating and cooling,
Figure 2 shows the texture at 155 °C on cooling and it could
recover almost completely when the same temperature was
reached during heating cycle. Similar dendritic growth texture
was ever reported in the monotropic LC phase of hydrogen bond-
ing associated side-chain polyacrylate supramolecular liquid
crystals from benzoic acid and bis(acylamino)pyridine compo-
nents, which was ascribed to columnar phases. The determina-
tion of the exact LC types of the complexes herein remains to be
confirmed by the temperature varied X-ray diffraction analysis.
We attribute these results mentioned above to the forma-
tion of T-shaped supramolecular LC complexes shown in
Figure 3. The complexes are built by the doubly hydrogen-
bonded self-assembly of the benzoic acid derivatives and 2,6-
bis(4-hexyloxybenzoylamino)pyridine.
2
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1 The complexes were prepared by slow evaporation of the
1:1 molar ratio mixtures from 2wt% pyridine solution.
2 Compound 2a was commercially available from Shanghai
Chemical Reagents Company, recrystallized from ethanol,
mp 185 °C. Compound 2b was synthesized according to
literature (B. Jones, J. Chem. Soc., 1935, 1874), which
showed nematic liquid crystal between 106.2 °C and 154.8
°C on heating. Elemental analysis (EA) for 2b: C 70.32%
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by a Schotten–Baumann reaction between 4-methoxyben-
zoyl chloride and 4-hydroxybenzoic acid, and showed
nematic liquid crystal between 218.8 °C and 265.7 °C on
heating. EA for 2c: C 65.98% (calc. 66.17), H 4.44%
(4.44). The thermal transition temperatures of 2c are in
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By comparison of the three complexes, it could be con-
cluded that both the introduction of the longer alkoxy(1/2b) and
the extension of the rigid part of the benzoic acid
derivatives(1/2c) have enhanced the thermal stability of the
hydrogen-bonded supramolecular liquid crystals and resulted in
the transition from the monotropic(1/2a) to enantiotropic liquid
crystals.
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