Synthesis and Mesogenic Properties of Liquid Crystals Based on Cholesterol and Cinnamic Acid Derivatives

Article abstract of DOI:10.1080/10691316.2018.1541843

A group of new mesomorphic compounds with cholesterol-based, which have derivative of the cinnamic acid, with different lengths of terminal alkoxy chains, are synthesized and characterized. The mesomorphic properties were investigated by DSC and POM. All target compounds showed a cholesteric phase. For 12-TC and 14-TC besides a cholesteric phase, also revealed a SmA* phase. The experimental results demonstrated that with the increase in the terminal chain lengths, the thermal stability of the smectic mesophases of these liquid crystals was increased, and these compounds shows brilliant color changes in the temperature ranges of cholesteric phase.

Full text of DOI:10.1080/10691316.2018.1541843

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Synthesis and Mesogenic Properties of Liquid
Crystals Based on Cholesterol and Cinnamic Acid
Derivatives
Xuenan Zong & Changcheng Wu
To cite this article: Xuenan Zong & Changcheng Wu (2018) Synthesis and Mesogenic Properties
of Liquid Crystals Based on Cholesterol and Cinnamic Acid Derivatives, Molecular Crystals and
Liquid Crystals, 666:1, 40-53, DOI: 10.1080/10691316.2018.1541843
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
2018, VOL. 666, NO. 1, 60–73
https://doi.org/10.1080/10691316.2018.1541843
Synthesis and Mesogenic Properties of Liquid Crystals
Based on Cholesterol and Cinnamic Acid Derivatives
Xuenan Zong and Changcheng Wu
School of Material Science & Engineering, Tianjin Polytechnic University, Tianjin 300387, PR, China
KEYWORDS
ABSTRACT
Liquid crystal; cholesterol-
based; cinnamic acid;
terminal alkoxy
A group of new mesomorphic compounds with cholesterol-based,
which have derivative of the cinnamic acid, with different lengths of
terminal alkoxy chains, are synthesized and characterized. The meso-
morphic properties were investigated by DSC and POM. All target
compounds showed a cholesteric phase. For 12-TC and 14-TC
chain; synthesis
ꢀ
besides a cholesteric phase, also revealed a SmA phase. The experi-
mental results demonstrated that with the increase in the terminal
chain lengths, the thermal stability of the smectic mesophases of
these liquid crystals was increased, and these compounds shows bril-
liant color changes in the temperature ranges of cholesteric phase.
1. Introduction
Because of its unique thermoelectric properties, liquid crystal compounds have been
widely used in many fields, such as photoelectric materials, information storage and bio-
medicine [1–11]. The double bonds of cinnamic acid been attracting attention in the
field of liquid crystal due to bond isomerization, and thus to switching the meso-
morphic properties, and to photodimerisation and cross-linking the molecules in the
liquid crystalline state [12]. The presence of double bonds in cinnamic acid results in
the formation of rod-like molecules and enhances the thermal stability of the meso-
phase, which has attracted research interest in the domain of liquid crystals [13–16]. In
recent years, cholesterol-based dimers have received extensive attention and research.
This can be attributed to their unique molecular structural features that not only result
in a stable mesophase, but also produce a variety of liquid crystal texture. In particular,
their phase behavior seems to depend heavily on the length of the flexible chain
[17–20]. The length of the alkyl chain has a considerable influence on many properties
such as phase transition temperature, phase structure, dielectric anisotropy, and optical
anisotropy. For these reasons, different liquid crystal properties can be obtained through
the change in spacer length and terminal chain length [21–32]. By changing the position
and length of the alkyl chain, the same series of liquid crystal compounds can exhibit
ꢀ
different liquid crystal mesophases such as nematic, Colh, smectic A and C phases
[33–44]. In this article, we report on the synthesis and the mesomorphic properties of a
new series of the cinnamic acid and cholesteryl derivatives that have at one side a
CONTACT Changcheng Wu
ccwu@tjpu.edu.cn
School of Material Science & Engineering, Tianjin Polytechnic
University, Tianjin, 300160, China.
ß 2018 Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
61
different number of alkyl chains with a different number of carbon atoms and, at the
other side, a cholesterol-based. This study indicated that the dependence of mesomor-
phism (mesophase type, stability and thermal range) and thermo-optical features on the
length of their terminal chain.
The molecular structures of the target compounds were characterized by elemental
1
analysis, Fourier transforms infrared spectroscopy (FT-IR) and H NMR techniques.
Their liquid crystal properties were investigated by differential scanning calorimetry
(DSC) and polarizing optical microscopy (POM).
2. Experimental
2.1. Material
All the chemicals and solvents were obtained from commercial sources. The crude sam-
ples were purified by column chromatography using silica gel (400 mesh). 1-bromobu-
tane, 1-bromohexane, 1-bromooctane, 1-bromodecane, 1-bromododecane, 1-
Bromotetradecane, anhydrous EtOH, 4-Benzyloxyphenol, Methanol and ptoluenesul-
fonic acid (PTSA) were GR grade reagents, Cholesterol succinic acid monoester was
prepared from our previous work [32]. Dichloromethane (DCM) and tetrahydrofuran
(THF) were dried over CaH2. All other solvents and reagents were purchased commer-
cially and used without any further purification.
2.2. Characterization
Chemical structures of the intermediates and dimesogenic compounds were character-
1
ized with FT-IR by using Bruker Tensor 307 spectrometer and H NMR spectrum by
using Bruker 400MHz with tetramethylsilane as an internal standard. Compositions of
the compounds were determined by vario El cube Elemental analyzer. The phase transi-
tions of the dimesogenic compounds were determined by means of the Olympus BX51
polarized optical microscope (POM) equipped with Linkam THMS600 hot stage. A dif-
ferential scanning calorimeter (DSC) (Netzsch DSC204) was used to measure the trans-
formation temperature and transformation enthalpy at the heating rate of 10ꢁC/min.
2.3. Synthesis
The synthetic route used in the preparation of target compounds series as seen in
Scheme 1, it involved three steps: (1) Synthesis of methyl p-hydroxycinnamate from p-
hydroxycinnamic acid and subsequent reaction with different bromoalkyl groups to
obtain p-alkoxycinnamate (m-A) and further reaction to produce p-alkoxycinnamic acid
(m-B) (2) Cholesterol succinic acid monoester reacts with 4-benzyloxyphenol to form
ester (C), which is then dehydrogenated to phenol (D) (3) Esterification of alkoxycin-
namic acids (B) and D produces the final product m-TC (Where m is the number of
carbon atoms in the terminal alkyl chain, m = 4, 6, 8, 10, 12 and 14). The synthetic
route used to prepare this series of target compounds is shown in Scheme 1.

62
X. ZONG AND C. WU
Scheme 1. The synthesis route of the final product m-TC (where m is the number of carbon atoms
in the terminal alkyl chain)
2.3.1. Methyl p-hydroxycinnamate
16 mmol (2.64 g) of p-hydroxycinnamic acid, 0.25 g of p-toluenesulfonic acid, dissolved
in 40 ml of anhydrous methanol, and heated under reflux for 8 hours. It was synthesized
following the similar method described in the literature [45]. It was purified by column
chromatography with silica gel using petroleum ether/ethyl acetate = 6:1. The resulting
product was white solid. Yield: 82%。

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
63
2.3.2. Methyl p-alkoxycinnamate (m-B)
3 mmol (0.543 g) of methyl p-hydroxycinnamate and 6 mmol (0.83 g) of K2CO3, 0.2 g of
KI were dissolved in 30 ml of butanone, 3.6 mmol (0.493 g) 1-bromobutane was dripped
into the vigorously stirring solution. After the reaction, the crude was washed with
water and then filtrated and dried under vacuum to afford the product as a white solid
(4-A). Yield: 79.3%. 3mmol 4-A and 9mmol KOH 0.504 g were dissolved in 10ml of
water, and heated to reflux for 4 hours. After completing the reaction, 100ml water was
added, neutralize to pH =3-4 with concentrated hydrochloric acid. The crude was
washed with water, then filtrated and dried under vacuum to afford product as a white
solid (4-B). Yield: 72.5%。1HNMR (CDCl3, 400M, ppm): d 7.74 (d, 1H), 7.50 (d, 2H),
6.91 (d, 2H), 6.31 (d, 1H), 4.00 (t, 2H), 1.83-0.98 (m, 7H). FTIR (KBr, cmꢂ1): 2938,
2868, 1666, 1596, 1510, 1432, 1170, 944, 822, 517.
Reactions with 1-bromohexane, 1-bromooctane, 1-bromodecane, 1-bromododecane
and 1-bromotetradecane to form m-B (m ¼ 6, 8, 10, 12 and 14), respectively, and the
synthesis method remains unchanged.
1
6-B: HNMR (CDCl3, 400M, ppm): d 7.74 (d, 1H), 7.50 (d, 2H), 6.91 (d, 2H), 6.31
(d, 1H), 3.99 (t, 2H), 1.84-0.87 (m, 11H). FTIR (KBr, cmꢂ1): 2938, 2868, 1666, 1596,
1510, 1432, 1300, 1214, 1170, 979, 822, 578, 517.
1
8-B: HNMR (CDCl3, 400M, ppm): d 7.73 (d, 1H), 7.49 (d, 2H), 7.02 (m, 2H), 6.31
(d, 1H), 3.95 (dt, 2H), 1.92-0.89 (m, 15H). FTIR (KBr, cmꢂ1): 2920, 2850, 1666, 1596,
1510, 1432, 1300, 1240, 1222, 1170, 979, 831, 726, 587, 517.
1
10-B: HNMR (CDCl3, 400M, ppm): d 7.74 (d, 1H), 7.49 (d, 2H), 6.91 (d, 2H), 6.31
(d, 1H), 3.99 (t, 2H), 1.84-0.88(m, 19H). FTIR (KBr, cmꢂ1): 2920, 2850, 1666, 1596,
1510, 1432, 1300, 1240, 1170, 979, 822, 717, 578, 517.
1
12-B: HNMR (CDCl3, 400M, ppm): d 7.73 (d, 1H), 7.49 (d, 2H), 6.91 (d, 2H), 6.31
(d, 1H), 3.99 (t, 2H), 1.79-0.88 (m, 23H). FTIR (KBr, cmꢂ1): 2920, 2850, 1666, 1596,
1510, 1432, 1309, 1240, 1214, 1170, 979, 822, 717, 587, 517.
1
14-B: HNMR (CDCl3, 400M, ppm): d 7.72 (d, 1H), 7.49 (d, 2H), 6.90 (d, 2H), 6.31
(d, 1H), 3.99 (t, 2H), 1.83-0.88(m, 27H). FTIR (KBr, cmꢂ1): 2920, 2850, 1666, 1596,
1510, 1432, 1300, 1240, 1170, 979, 831, 717, 517.
2.3.3. Cholesterol 4-hydroxy-4’-ethoxy carbonate
Cholesterol succinic acid monoester 5mmol (2.43g), 4-benzyloxyphenol 5mmol (1g) and
DMAP 1mmol (1g) were dissolved in anhydrous CH2Cl2 (30 mL), DCC 6mmol (1.236g)
was dissolved in 10mL of CH2Cl2 and dripped into the vigorously stirring solution.
After completing the reaction, the resulting solution was washed with distilled water
twice and drying over MgSO4. The product was purified by silica gel column chroma-
tography using petroleum ether/ethyl acetate 6:1 as the eluent. Then, palladium carbon,
which is one-tenth of the weight of the reactants, was added and reacted under a hydro-
gen atmosphere for 6 hours in THF solution. After the reaction was stopped, it was fil-
tered and recrystallized from petroleum ether/ethyl acetate. Yield:49.8%。 1H NMR
(CDCl3, 400M, ppm): d 6.95 (d, 2H), 6.84-6.76 (m, 2H), 5.37 (d, 1H), 4.65 (d, 1H), 2.85
(t, 2H), 2.71 (t, 2H), 2.33-0.83 (m, 44H). FTIR (KBr, cmꢂ1): 3416, 2859, 1719, 1510,
1187, 1127, 995, 840, 796, 517.

64
X. ZONG AND C. WU
2.3.4 The target compounds (m-TC)
(4-B) 0.5mmol (0.289g), Cholesterol 4-hydroxy-4’-ethoxy carbonate 0.5mmol (0.11g) and
DMAP 1mmol (1g) were dissolved in anhydrous CH2Cl2 (30mL), DCC 0.525mmol
(0.108g) was dissolved in 10mL of CH2Cl2 and dripped into the vigorously stirring solu-
tion. The reaction mixture was stirred for 24h at room temperature. After completing the
reaction, the resulting solution was washed with distilled water twice and drying over
MgSO4. The product was purified by silica gel column chromatography using petroleum
ether/ethyl acetate 6:1 as the eluent. Yield:54.3%。1H NMR (CDCl3, 400M, ppm): d 7.81
(d, 1H), 7.52-6.92 (m, 8H), 6.47 (d, 1H), 5.38 (m, 1H), 4.57 (m, 1H), 4.01 (t, 2H), 2.88-
0.68 (t, 54H). FTIR (KBr, cmꢂ1): 520, 824, 1129, 1175, 1259, 1499, 1600, 1730, 2867,
2931. Elemental analysis：Calculated: C 76.89%, H 8.78%. Found: C 76.66%, H 8.92%.
Five compounds 6-TC, 8-TC, 10-TC, 12-TC and 14-TC were prepared by the same
method. In the above, the synthesis of 4-TC will be described as an example.
1
6-TC: H NMR (CDCl3, 400M, ppm): d 7.81 (d, 1H), 7.52-6.92 (m, 8H), 6.47 (d,
1H), 5.38 (d, 1H), 4.65 (s, 1H), 4.00 (t, 2H), 2.80- 0.68 (m, 58H). FTIR (KBr, cmꢂ1):
520, 824, 1120, 1175, 1250, 1499, 1600, 1730, 2867, 2931. Elemental analysis：
Calculated: C 77.19%, H 8.97%. Found: C 77.29%, H 9.29%.
1
8-TC: H NMR (CDCl3, 400M, ppm): d 7.81 (d, 1H), 7.57-6.85 (m, 8H), 6.51 (d,
1H), 5.38 (d, 1H), 4.68(s, 1H), 4.00 (t, 2H), 2.81-0.65 (m, 62H). FTIR (KBr, cmꢂ1): 520,
822, 1127, 1500, 1510, 1605, 1728, 2857, 2931. Elemental analysis： Calculated: C
77.47%, H 9.15%. Found: C 77.95%, H 9.35%.
1
10-TC: H NMR (CDCl3, 400M, ppm): d 7.81 (d, 1H), 7.58-6.85 (m, 8H), 6.48 (d,
1H),5.38 (d, 1H), 4.67(s, 1H), 4.00 (t, 2H), 2.87-0.68 (m, 66H). FTIR (KBr, cmꢂ1): 520,
824, 1120, 1175, 1250, 1472, 1500, 1600, 1730, 2857, 2931. Elemental analysis：
Calculated: C 77.74%, H 9.32%. Found: C 77.45%, H 9.60%.
1
12-TC: H NMR (CDCl3, 400M, ppm): d 7.81 (d, 1H), 7.59-6.83(m, 8H), 6.48 (d,
1H), 5.38 (d, 1H), 4.65 (s, 1H), 4.00 (t, 2H), 2.96-0.68 (m, 70H) FTIR (KBr, cmꢂ1): 520,
824, 1129, 1175, 1250, 1499, 1600, 1730, 2857, 2922. Elemental analysis：Calculated: C
77.98%, H 9.48%. Found: C 78.03%, H 9.58%.
1
14-TC: H NMR (CDCl3, 400M, ppm): d 7.81 (d, 1H), 7.52-6.92 (m, 8H), 6.48 (d,
1H), 5.38 (d, 1H), 4.65 (s, 1H), 4.00 (t, 2H), 2.87-0.68 (m, 74 H). FTIR (KBr, cmꢂ1):520,
824, 1129, 1175, 1259, 1499, 1600, 1730, 2857, 2931. Elemental analysis：Calculated: C
78.22%, H 9.63%. Found: C 78.11%, H 9.93%.
3. Results and discussion
3.1. Molecular structural characterization
The target compounds m-TC (m ¼ 4, 6, 8, 10, 12 and 14) were synthesized to investi-
gate the influence of the length of terminal chain. Their chemical structures are con-
1
firmed by spectroscopic studies. Figure 1 shows the H NMR spectra of 10-TC. The
related functional groups and protons are noted in the spectra.
1H NMR spectrum of 10-TC revealed that two double peak signals at 7.81 ppm and
6.48 ppm of -CH ¼ CH- group were observed. Resonance signals around 5.37 and
4.67 ppm are usually taken to be confirmatory evidence for the presence of a single
hydrogen in cholesterol. A set of multiplets in the range of 7.81 ppm-6.92 ppm confirms

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
65
1
Figure 1. The 400MHz H NMR results of 10-TC in CDCl3.
Note: Below 2.5 ppm is the other 43H (except h and g) in cholesterol moiety and 19 H in the alkyl chain.
Figure 2. Fourier transform infrared spectrum of target product 10-TC.
the presence of aromatic protons. Methylene is shown by a signal of about 3.94 ppm.
Two double peaks of 2.87 ppm and 2.72 ppm, each having two hydrogens, correspond-
ing to hydrogen close to the benzene ring and methylene group. Below 2.5 ppm is the
other 43H (except h and g) in cholesterol moiety and 19 H in the alkyl chain. The num-
bers and chemical shift of hydrogen are in accordance with their molecular structure.
Figure 2 shows the Fourier transform infrared spectrum of 10-TC. The band appear-
ing at 2931 cmꢂ1 and 2857 cmꢂ1 can be ascribed to stretching of -CH3 and -CH2. The

66
X. ZONG AND C. WU
Figure 3. Optical polarizing micrographs of compound 4-TC (a) The oily-streak texture at 155ꢁC of
Ch phase during heating process (b) The focal-conic texture at 187 ꢁC of Ch phase during cool-
ing process.
band appearing at 1730 cmꢂ1 can be ascribed to the stretching of the C ¼ O bond
belonging to the ester group. The band appearing at 1600 cmꢂ1 can be ascribed to
stretching of C ¼ C str. vinyl group of cinnamate group of cinnamate. A strong absorp-
tion peak appears at 1500 and 1472 cmꢂ1, which is attributed to the C ¼ C stretching
vibration of the benzene ring. Formation of the aromatic ether bond (C-O-Ar) was con-
firmed based on the absorption band at 1250 cmꢂ1
With the best results of elemental analysis, spectral data obtained by FT-IR and H
NMR, also confirmed by the only one spot in the TLC (thin layer chromatography),
therefore, it is concluded that all target compounds were successfully synthesized.
.
1
3.2. Mesomorphic properties
The liquid crystal textures of target compounds m-TC were observed with POM upon
heating and cooling cycle.
All the homologues of this series were liquid crystalline; exhibiting mesophase under
the POM observation. The optical textures of the LC obtained were observed by POM
with a hot stage. The 4-TC exhibited typical cholesteric oily-streak textures when it was
heated at 131 ꢁC. By further heating, the birefringence disappeared at 223 ꢁC. During
cooling process, a focal-conic cholesteric texture (when sheared, it was changed to oily-
streak textures) appeared at 222 ꢁC and the liquid crystalline phase began to crystallize
at 39 ꢁC. Figure 3 is a polarized photomicrograph of the product of 4-TC upon heating
and cooling process. The LC behavior of 6-TC is very similar to that of 4-TC.
The 8-TC exhibited typical oily-streak textures at 137 ꢁC and a focal-conic texture at
147 ꢁC during heating process. The birefringence disappeared at 236 ꢁC. The 10-TC
exhibited typical oily-streak textures at 134 ꢁC and the birefringence disappeared
at 204 ꢁC.
ꢁ
ꢀ
In the POM observation of 12-TC, when it was heated at 121 C, typical SmA focal-
conic texture appeared. By further heating to 154 ꢁC, the liquid crystalline texture was
turned to cholesteric oily-streak texture, and the birefringence disappeared at 195 ꢁC.
ꢁ
ꢀ
During cooling process, a typical oily-streak textures appeared at 186 C and the SmA
focal-conic texture appeared at 147 ꢁC, finally the liquid crystalline phase began to

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
67
ꢁ
ꢀ
Figure 4. Optical polarizing micrographs of compound 12-TC (a) SmA , focal-conic texture at 136 C
(b) ChLC, oily-streak texture at 160 ꢁCꢁin heating process (c) ChLC, focal-conic texture at 172 ꢁC (d)
ꢀ
SmA , typical focal-conic texture at 95 C in cooling process.
crystallize at 89 ꢁC. The LC behavior of 14-TC is very similar to that of 12-TC. The LC
behavior of m-TC (8, 10, 12 and 14) is very similar to that of 12-TC during cooling
process. The optical texture of the 12-TC is shown in Figure 4.
The phase transition temperatures and corresponding enthalpy changes of com-
pounds were determined using DSC. The DSC traces for target compounds are pre-
sented in Figure 5. In the heating process of 4-TC, there are three peaks appeared at
90.2, 130.9 and 222.8 ꢁC. In the cooling processes, two peaks appeared at 222.1ꢁCand
39.4 ꢁC. Combined with the observations of POM, we can conclude that the corre-
sponding transitions in the heating process are the transition between crystals, the tran-
sition from crystal to cholesteric phase and the transition from cholesteric phase to
isotropic, respectively. During cooling two peaks appeared, one is the isotropic to cho-
lesteric phase transition at higher temperature, the other is the LC to crystal transition
at lower temperature.
The transition process of 6-TC is similar to that of 4-TC. Upon heating 6-TC also
exhibits three transitions, in which transitions from crystal to the crystal (82.7 ꢁC), the
crystal to cholesteric phase (144.3 ꢁC) and cholesteric phase to isotropic occur
(217.2 ꢁC). The transitions in the cooling process are: transition from I to ChLC
(216.2 ꢁC), transition from Ch phase to crystal (126.1 ꢁC), and transition between crys-
tals (63.9 ꢁC).
In the DSC curve of 12-TC, four peaks appeared at peak values of 80.5, 120.9, 154.3
and 194.8 ꢁC; accompanying POM observations showed that they represented crystal to
ꢀ
crystal transition, melting transition, SmA to Ch phase transition and cholesteric phase

68
X. ZONG AND C. WU
Figure 5. Differential scanning calorimetry traces for m-TC (m¼ 4, 6, 8, 10, 12 and 14) during heating
and cooling.
to isotropic (I), respectively. During cooling three peaks appeared, revealing I to Ch
ꢀ
ꢀ
phase transition at higher temperature, Ch to SmA phase transition, SmA to crystal
transition at lower temperature.
From these two representative analyses, it can be seen that m-TC (m ¼ 4, 6 and 10)
mainly undergoes transitions which (transitions between crystals) crystal to cholesteric
phases and cholesterol to isotropic transformation during the heating process. The m-
TC (m ¼ 8, 12 and 14) mainly undergoes these transitions, which are the transition
ꢀ
ꢀ
(transitions between crystals) from crystal to smectic A phase, the smectic A phase to
the cholesteric phase, and the cholesteric phase to isotropic.
The results determined by the DSC were consistent with that observed under POM.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
69
Table 1. Phase transition temperatures (ꢁC) and associated transition enthalpy values [DH, J gꢂ1
]
from the second heating and first cooling in DSC thermograms (temperature gradient: 10 ꢁC minꢂ1).
ꢂ1
Transition temperatures (ꢁC) and associated transition enthalpy values [DH, J g
]
m
2nd heating
4
6
8
10
12
14
Cr
Cr
–
–
Cr
–
90.2
82.7
Cr
Cr
Cr
Cr
Cr
Cr
–
–
–
–
SmA
–
SmA
130.9(18.88)
144.3(22.08)
146.8(1.38)
133.5(41.82)
154.3(1.33)
178.6(1.57)
ChLC
ChLC
ChLC
ChLC
ChLC
ChLC
222.8(7.68)
217.2(6.97)
236.0(2.61)
203.6(6.99)
194.8(4.78)
204.8(5.97)
I
I
I
I
I
I
ꢀ
–
–
137.0(8.51)
–
120.9(25.08)
123.6(30.81)
ꢀ
80.5
ꢀ
–
SmA
m
1st cooling
4
6
8
10
12
14
I
I
I
I
I
I
222.1(ꢂ6.85)
216.2(ꢂ7.48)
231.3(ꢂ8.17)
200.3(ꢂ6.14)
186.8(ꢂ1.1)
202.0(ꢂ7.30)
ChLC
ChLC
ChLC
ChLC
ChLC
ChLC
–
–
–
–
SmA
SmA
SmA
SmA
39.4(ꢂ8.77)
126.1(ꢂ22.42)
109.4(ꢂ34.63)
87.0(ꢂ32.51)
88.9(ꢂ22.14)
99.6(ꢂ29.51)
Cr
Cr
Cr
Cr
Cr
Cr
–
63.9
–
–
–
–
Cr
–
–
–
ꢀ
ꢀ
ꢀ
ꢀ
142.3(ꢂ2.06)
135.8(ꢂ1.36)
147.6(ꢂ2.44)
175.4(ꢂ1.31)
63.6
Cr
Therefore, from combination of the results of DSC studies and POM observation, we
also summarize the LC properties of the m-TC (m= 4, 6, 8, 10, 12 and 14) in Table 1.
The obtained transition temperatures as a function of carbon numbers (m) are plotted
for the aim of clarity and shown in Figure 6. As seen from the data listed in Table 1
and Figure 6, the length of the terminal chains had a considerable influence on the
phase types and phase transition temperatures of m-TC. The compounds of m-TC
(m ¼ 4, 6, 8, 10, 12 and 14) all have relatively broad cholesteric phases, with the increase
of the length of the terminal carbon chain, the phase temperature range gradually
decreases. For example, the compound 4-TC is as wide as 91.9 ꢁC, but the compound
14-TC is only 26.2 ꢁC. On the contrary, the temperature range of the smectic phase
broadens as the carbon chain increases, and the 14-TC smectic phase temperature range
reaches 55 ꢁC. This is mainly due to as the terminal alkyl chain length increases, the
aspect ratio of the molecule increases, the polarizability of the molecule decreases, the
ꢀ
wide smectic A phase is easily broadened, and the cholesteric phase is nar-
rowed [46–49]. Lim [50], Selvarasu and Kannan [51] also pointed out that as the length
of the alkyl chain at one end increases, the overall polarizability of the molecule
increases, which helps to show the smectic texture. In general, the lower alkyl chain
liquid crystals are nematic, while the middle chain alkyl chains exhibit nematic and
smectic phases. The length of the terminal alkoxy chain affects the melting and clearing
temperatures as well as the mesophase ranges. This is because the increase in the length
of the terminal alkyl chain makes the force perpendicular to the long axis of the mol-
ecule increase.
For compounds of 4-TC and 6-TC, enantiotropic cholesteric phase has emerged,
while for those of 8-TC, 10-TC, 12-TC and 14-TC with long terminal chains, smectic
ꢀ
A
phase and cholesteric phase were formed. With increasing the number of alkyl in
ꢀ
molecules, the smectic A phase not only appeared but also as m increases the smectic
ꢀ
A phase stability increases as would be anticipated. The changes of this series can be
explained in terms of greater molecular length and polarizability of molecule resulting
from -C ¼ C- units in the central linkage [51–53].
ꢀ
The result indicates from Figure 6 that the tendency toward smectic A mesomor-
phism increases and the temperature of the clearing point decreases, with increasing

70
X. ZONG AND C. WU
Figure 6. (Colour online) Transition temperature plots as a function of carbon numbers upon heating
(a) and cooling (b).
terminal alkoxy chain length. And as the length of the series of terminal alkoxy chains
ꢀ
increases, the thermal stability of the smectic A phase increases. Jin [54–57], Marcelis
and Yelamaggad et al [58, 59] have studied the phase behavior of non-symmetric
dimers, which contains a flexible alkyl spacer. The type and stability of liquid crystal
depend on the length of the terminal alkoxy chain changes. The compounds with
shorter end alkoxy chain showed nematic phase while those with longer showed smec-
tic phase.
3.3. Optical properties
The optical properties of cholesteric liquid crystals are attributed to their unusual helical
molecular structure and well-defined pitch [60, 61]. When the cholesteric liquid crystals

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
71
Figure 7. 8-TC shows different colors during heating (a) 170 ꢁC (b) 185 ꢁC (c) 197 ꢁC (d) 231 ꢁC.
are in a uniform parallel orientation, the periodic spiral structure exhibits unique select-
ive reflection optical properties [62–64]. Selective reflection means that the liquid crystal
of the cholesteric phase will selectively reflect light of certain wavelengths, thus causing
some cholesteric liquid crystals to show brilliant colors under the irradiation of white
light. The relationship between the wavelength and the pitch obeys the Bragg equa-
tion [65].
To study the change in color, we used a hot stage of a polarizing microscope to
observe the color of the sample at different temperatures. All samples disperse the
reflected white light and display the rainbow color with temperature increases. The select-
ive reflection wavelengths of cholesteric compounds m-TC decrease with increasing tem-
perature. Figure 7 shows the color change of 8-TC during the heating process. From the
figure 7, 8-TC shows the change of four colors with increasing temperature. The reflec-
tion color changed from red, orange, yellow to blue with increasing temperature and it
was in reverse direction as the temperature decreased in the cooling cycle. This unique
optical property is related to the helical molecular structure of cholesteric liquid crystals.
As the helical pitch is temperature dependent, the change of temperature will result in
the alteration of the pitch of the helical structure of the cholesteric phase. Therefore, the
reflection color changes at different temperatures. Yao DS [66], Luo CC [67] and
Cha SW [55] synthesize cholesteric liquid crystal compounds and change the pitch by
adjusting the temperature to present different colors. The change in temperature causes a
change of the pitch of the helical structure of the cholesteric phase. Wang JW [68] has
reported a series of new chiral side-chain cholesteric liquid crystalline exhibit different
colors at room temperature and the reflection wavelengths change sharply near the tem-
ꢀ
perature of the SmA -Ch phase transition. When the smectic to the cholesteric phase, the
molecules of layer structure are distorted into a cholesteric helix, resulting in the selective
reflection wavelength sharply varying [50]. In terms of application, the sharp change of
ꢀ
the reflection wavelength near the transition temperature of SmA phase to the choles-
teric phase can be used to determine the distribution of temperature [68–70].
This selective reflection optical characteristic is of great significance in applications
such as reflective display technology detection sensors, fiber optics, telecommunications
and anti-counterfeiting [71–78].
4. Conclusions
Six liquid crystals compounds m-TC (m ¼ 4, 6, 8, 10, 12 and 14) were prepared and
characterized. All the LC phases observed are confirmed by the optical textural

72
X. ZONG AND C. WU
observation and DSC thermogram studies. All of the obtained target compounds dis-
played very wide mesophase temperature ranges. The effect of terminal alkoxy plays a
crucial role in the formation of mesophase. The m-TC (m ¼ 4 and 6) showed cholesteric
ꢀ
phase. The m-TC (m ¼ 8, 10, 12, and 14) exhibited the SmA phase and the cholesteric
phase. With increasing the length of terminal alkyl chain in molecules, the aspect ratio
of the molecule increases, the polarizability of the molecule decreases, the wide smectic
ꢀ
A phase is easily broadened, and the cholesteric phase is narrowed. The nature of the
smectic A phase formed has been found to depend on the relative length of the ter-
ꢀ
minal chains. In the cholesteric phase, the peak wavelength of the reflected light of all
the samples moves toward the short wave direction with increasing temperature, and
the reflected white light is dispersed to present a rainbow color. The results of this study
provide theoretical and experimental basis for the synthesis and application of liquid
crystal compounds with wider mesophase and more stable thermal behavior.
Funding
This research did not receive any specific grant from funding agencies in the public,
commercial, or not-for-profit sectors.
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