DSC study on thermotropic liquid crystalline compounds with alkyl substituants

Article abstract of DOI:10.1080/15421400500376539

A series of new liquid crystalline epoxy compounds with alkyl substituents from 1 to 15 carbon atoms were characterized by differential scanning calorimetry (DSC). The result showed the length of lateral alkyl substituents of liquid crystalline compounds

Full text of DOI:10.1080/15421400500376539

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DSC Study on Thermotropic
Liquid Crystalline Compounds
with Alkyl Substituents
Yiquan Zheng a , Shaoping Ren b , Youdao Ling c &
Mangeng Lu d
a Key Laboratory of Polymer Materials for
Electronics , Guangdong, China
b Graduate School of the Chinese Academy of
Sciences , Beijing, China
c Guangzhou Institute of Chemistry, Chinese
Academy of Sciences , Guangzhou, China
d Key Laboratory of Polymer Materials for
Electronics , Guangdong, China and Guangzhou
Institute of Chemistry, Chinese Academy of
Sciences , Guangzhou, China
Published online: 31 Aug 2006.
To cite this article: Yiquan Zheng , Shaoping Ren , Youdao Ling & Mangeng Lu (2006)
DSC Study on Thermotropic Liquid Crystalline Compounds with Alkyl Substituents,
Molecular Crystals and Liquid Crystals, 452:1, 3-9, DOI: 10.1080/15421400500376539
To link to this article: http://dx.doi.org/10.1080/15421400500376539
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Mol. Cryst. Liq. Cryst., Vol. 452, pp. 3–9, 2006
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400500376539
DSC Study on Thermotropic Liquid Crystalline
Compounds with Alkyl Substituents
Yiquan Zheng
Key Laboratory of Polymer Materials for Electronics,
Guangdong, China
Shaoping Ren
Graduate School of the Chinese Academy of Sciences, Beijing, China
Youdao Ling
Guangzhou Institute of Chemistry, Chinese Academy of Sciences,
Guangzhou, China
Mangeng Lu
Key Laboratory of Polymer Materials for Electronics, Guangdong,
China and Guangzhou Institute of Chemistry, Chinese Academy
of Sciences, Guangzhou, China
A series of new liquid crystalline epoxy compounds with alkyl substituents from 1
to 15 carbon atoms were characterized by differential scanning calorimetry (DSC).
The result showed the length of lateral alkyl substituents of liquid crystalline
compounds was the dominant reason for the change of phase behavior. The result
implied that TN-I exhibited the odd–even effect when the length of lateral alkyl
chains from 5 to 8 was approximately half of the fully extended length of mesogenic
core with end polar epoxy groups.
Keywords: differential scanning calorimetry; lateral substituents; liquid crystalline
epoxides
INTRODUCTION
It was generally accepted that lateral substituents diminish the meso-
geneity of a compound, and the extent of the effect depends on their
molecular size. However, Weissflog and Demus found that compounds
Address correspondence to Mangeng Lu, Guangzhou Institute of Chemistry, Chinese
Academy of Sciences, Guangzhou, 510650, China. E-mail: mglu@gic.ac.cn
3

4
Y. Zheng et al.
with large flexible lateral substituents exhibited liquid crystalline
(LC) phases in the early 1980 s [1,2]. Compounds with large flexible
lateral substituents, assuming that the lateral chains are oriented
nearly parallel to the basic molecule, may be considered as variants
of the classical rod-lid-like molecules [3]. Much of their works focused
on relationship between the long and large lateral subsituents and the
phase behavior of the LC compounds [1–5]. Few studies were reported
about thermotropic LC compounds linked to end polar groups with
lateral alkyl substituents.
Thermotropic LC compounds with lateral alkyl substituents were
used for study of the LC thermosets [5] and LC elastomers [6], which
had shape memory properties. In this present article, we have synthe-
sized a series of LC epoxides with large and long substituents and inves-
tigated the effect of lengthening of flexible lateral sunstituents upon the
LC phase behavior. Even though similar works of these compounds with
lateral long-chain substituents have been done before [1–5], the study
has not been carried out in a systematic fashion by mesogens linked to
end polar groups like epoxy with substituent lengths from C1 to C15.
EXPERIMENTAL
Materials
2,5-Bihydroxy benzoic acid (gentistic acid) was purchased from Taizhou
Candorly Sea Biochemical and Health Products Co., purified by
1
recrystallization, and confirmed by H NMR and thin-layer chromato-
graphy (TLC). n-Alkanol, 3-chloroperoxybenzoic acid (MCPBA), and
4-dimethylamin-pyridine (DMAP) were purchased from Flucka and
used as received. All solvents were purified by standard procedures.
Synthesis
All synthetic procedures of liquid crystalline epoxides (LCEs) with dif-
ferent lateral substituents were similar (Scheme 1). A typical example
using C-8 LCE was given as follows: 2,5-Didydroxybenzoate octanol
ester was prepared by esterfication of gentistic acid with octanol cata-
lyzed by sulfuric acid. 4-(2-Propenyloxy) benzoic acid was prepared
according to the procedure proposed by Broer et al. [7].

Thermotropic Liquid Crystalline Compounds
5
SCHEME 1 Synthetic scheme of substituted liquid crystalline epoxides.
2,5-bis[4-(2,3-Diallypropoxy)-oxy]benzoic Acid Octanol Ester
Into a 250-ml round-bottomed flask equipped with a magnetic stir
bar, were 3.6 g [4-(2-propenyloxy)benzoic acid], 10 ml of benzene,
3 ml thoinyl choride placed. The flask was equipped with a condenser
and heated to reflux with an oil bath. After 2 h, the benzene and excess
thionyl chloride were removed by distillation under and reduced press-
ure. Under an N2 atmosphere, the product was placed into the flask
with an ice bath. A solution (0.1 g DMAP in 10 ml of THF and 10 ml
of triethyl amine) was then added over a period of 10 min. The reaction
was kept for 8 h, then at 25ꢀC for 2 days. The reaction mixture was
poured into 500 ml of acidified aqueous solution. The solid was filtered
and washed with 500 ml of water and 500 ml of ethanol twice in
sequence. The solid product was dried in a vacuum oven at 40ꢀC. Yield:
55%. 1H NMR [CDCl3=TMS, d (ppm)]: 0.84 (3H, t, CH3) 1.23 (10H, m,
CH2), 1.46 (2H, m, CH2), 4.13 (2H, t, ꢁCH2ꢁOOCꢁ), 4.62 (4H, s, CH2),
5.32 (2H, d,ꢁCH¼CH2), 5.43 (2H, d, ꢁCH¼CH2), 6.07 (2H, m,
ꢁCH¼CH2),7.00 (4H, dd, aromatic),7.23 (1H, s, aromatic), 7.43
(1H, d, aromatic), 7.87 (1H, s, aromatic), 8.15 (4H, dd, aromatic).
2,5-bis [4-(2,3-Epoxypropoxy)-oxy]benzoic Acid Octanol Ester
Diallyl mononer (2,5-bis[4-(2,3-diallypropoxy)-oxy]benzoic acid
octanol ester) was oxidized with MCPBA in dichloromethane at 25ꢀC
for 7 days. After filtration, the filtrate was washed successively with

6
Y. Zheng et al.
5% Na2SO3 solution, 5% Na2CO3 solution, and concentrated NaCl
solution and dried over MgSO4, and the solvent was evaporated. A
pure product was obtained after recrystallization from ethyl acetate.
1
Yield: 60%. H NMR [CDCl3=TMS, d (ppm)]: 0.84 (3H, t, CH3), 1.23
(10H, m, CH2), 1.46 (2H, m, CH2), 2.78 (2H, m, CH2 of epoxy), 2.93
(2H, dd, CH2 of epoxy), 3.38 (2H, t, CH of epoxy), 4.01 (2H, m,
ꢁOꢁCH2), 4.12 (2H, t, ꢁCH2ꢁOOCꢁ), 4.32 (2H, m, ꢁOꢁCH2), 7.00
(4H, dd, aromatic), 7.23 (1H, s, aromatic), 7.43 (1H, d, aromatic),
7.87 (1H, s, aromatic), 8.15 (4H, dd, aromatic).
RESULTS AND DISCUSSION
The phase transition of the C-n compounds have been studied by
differential scanning calorimetry (Perkin-Elmer DSC II, USA) at
heating=cooling rate of 10ꢀC minꢁ1 as given in Table 1. The changes
in entropies were calculated from the enthalpy changes obtained
during phase transitions. For comparison, a plot of the transition
temperatures against the number of methylene groups in the lateral
substituents is exhibited in Fig. 1 and shows a very large decrease
in TN-I values with an increasing number of methylene groups.
Remarkably, the difference in the TM and TN-I depends on whether
the flexible spacer contains an even or odd number of methylene
TABLE 1 Thermal Propertiess of LC Epoxides
DHM
DHN-I
DSN-I
TM (ꢀC), (kJ ꢂ molꢁ1),
DSM
TN-I (ꢀC), (kJ ꢂ molꢁ1), (J ꢂ molꢁ1ꢂKꢁ1),
first
first
heating
(J.molꢁ1 ꢂ Kꢁ1),
first heating
second
second
heating
second
heating
Samples heating
heating
C-1
C-2
C-3
C-4
C-5
C-6
C-7
C-8
125.4
134.2
117
29.46
33.81
27.75
25.45
19.43
26.67
22.39
36.95
34.66
21.86
21.28
34.51
39.20
41.43
28.91
73.9
83.0
71.1
68.8
54.5
74.1
67.2
105.4
100.8
60.2
155.0
142.2
124.7
118.4
84.8
99.3
76.5
87.6
86.8
74.9
63.5
61.3
63.6
69.0
48.4
1.60
1.52
1.45
1.16
0.72
1.29
0.88
1.27
1.11
0.91
0.79
0.98
0.79
0.72
0.55
3.7
3.6
3.6
2.9
2.0
3.4
2.5
3.5
3.1
2.6
2.3
2.9
2.3
2.1
1.7
96.6
83.3
86.9
59.8
77.4
70.8
89.9
56.8
58.4
69.7
52.8
52.6
C-9
C-10
C-11
C-12
C-13
C-14
C-15
64.5
104.1
114.3
127.1
88.7

Thermotropic Liquid Crystalline Compounds
7
FIGURE 1 Transition temperature in the homologous of the LC epoxides.
groups. The plot of TN-I versus the number of carbon atoms in the alkyl
chain exhibited the odd–even effect between the fourth member and
eighth member of the series.
The lateral alkyloxycarbonyl chains linked to mesogens acted as a
diluent, increasing the mean space between molecules; this in turn
prevented the regular stacking of layers and reduced the anisotropy
of the intermolecular forces, and so made the mesophase less stable.
It has been previously reported that the identity and position of ter-
minal substituent groups could strongly influence the LC properties
of resulting compounds. For phenylene alkoxybenzoate there was
mutual conjunction between the alkyl and carboxy groups; this should
increase the negativity of the carbonyl oxygen and so help to stabilize
the mesophase. However, the presence of polar groups at the ends of
molecule increased the intermolecular attractive forces and decreased
the usual tendency of rod-shaped molecules to form LC phases.
Because the epoxy groups in these LC eopxides were low, they had a
small negative effect on LC phase stability [8].
When the length of lateral alkyl chains was from 5 to 8, it was
approximately half of the fully extended length of mesogenic core with

8
Y. Zheng et al.
end polar epoxy groups. TNI increased when n was even, but decreased
when n was odd because of the increased size of the protruding group.
One possible reason was the changes in orientation of the lateral alkyl
group with the long axis of the basic molecule. When the number of
lateral methyl groups was odd, the terminal methyl group of lateral
substituents was off-axis and the interaction between the neighbor
molecules might be strong. This made it difficult for the molecules to
pack neatly in the parallel arrangement needed for an LC phase.
Consequently, this effect greatly decreased the TN-I value and
enthalpy (DHN-I). However, when n was even, the terminal methyl
group of lateral substituents was on-axis and the interaction between
the neighbor molecules might be weak. This effect greatly increased
the TN-I value and enthalpy (DHN-I).
An orientation of fully extended all-trans-conformation lateral ali-
phatic chain nearly parallel to the long axis of the basic molecule
(Fig. 2) is assumed. When the lateral chain length was shorter than
the half length of fully extended mesogenic core, its effects have been
rationalized in terms of a reduction in anisotropic polarisability and an
increased axial separation, thus giving a reduction in intermolecular
attraction forces. The changes of TM and TN-I and the increased
isotropic entropies in Table 1 resulted from that lateral groups
functioned as a bulge enhancing positional correlation and short range
order in nematic phase (by an interlocking effect) [9].
For the same lateral substituents linked to 1,4-phenylene bis
(4-n-octyloxybenzoates), the nematic–isotropic transition points have
simply decreased with increasing length of the lateral aliphatic substi-
tuents without end polar groups [5]. Even though the presence of polar
FIGURE 2 Model of the C-8 LC epoxides compounds with long alteral chains
through GaussView 2.1.

Thermotropic Liquid Crystalline Compounds
9
end groups increased the intermolecular attractive forces and
decreased the usual tendency of rod-shaped molecules to form LC
phases, the intermolecular attractive could make odd–even effects
through protruding the lateral alkyl groups turn up.
When the lateral chain length continued to increase to longer than
the half-length of fully extended mesogenic core with end polar epoxy
groups, the increase in length would lead to increase its geometrical
anisotropy. Lateral chains were oriented nearly parallel to the basic
molecule and might be considered as variants of the classical rod-like
molecules [3,4]. The effect of lateral alkyloxycarbonyl chains in the
middle of three-ring moiety acted as a diluent, and the intermolecular
attractive might continually be relatively weak. This combination of
effects could simply result in the decrease of TM and TN-I
.
The relationship between the length of lateral substituents and
length of three-ring mesogenic core indicated that the trans-
conformation lateral substituents nearly paralleled the long axis of
the basic molecule. The length of lateral alkyl substituents of LC
compounds was the dominant reason for the change of phase behavior.
TN-I decreased largely as the increasing number of lateral alkyl
groups. Even though the presence of polar groups at the ends of
molecule increased the intermolecular attractive forces and decreased
the usual tendency of rod-shaped molecules to form LC phases, the
intermolecular attractive could make odd–even effects through pro-
truding the lateral alkyl groups. When the length of lateral alkyl
chains was from 5 to 8, TNI increased when n was even but decreased
when n was odd.
ACKNOWLEDGMENT
We are indebted to CAS Century Program for financial support of
this work.
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[3] Demus, D., Goodby, J. W., Gray, G. W., Spiess, H. W. (1998). Handbook of liquid
crystals, Wiley-VCH: New York, Chapter XI, Vol. 2B, 835.
[4] Weissflog, W., Wiegeleben, A., & Haddawi, S. (1996). Mol. Cryst. Liq. Cryst., 281, 15.
[5] Lee, J. Y. & Jang, J. (1998). J. Polym. Sci Part A: Polym. Chem., 36, 911.
[6] Rousseau, I. A. & Mather, P. T. (2003). J. Am. Chem. Soc., 125, 15300.
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[9] Luckhurst, G. R. & Gray, G. W. (1979). The molecular physics of liquid crystals,
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