Mesomorphic properties of chiral three-arm liquid crystals containing 1,2,4-butanetriol as core

Article abstract of DOI:10.1016/j.molstruc.2015.11.040

A series of symmetric and asymmetric three-arm liquid crystals (TALCs) of which the molecular structure with a central core of 1, 2, 4-butanetriol attached by three rod-like mesogenic moieties have successfully been synthesized. The rod-like mesogenic sid

Full text of DOI:10.1016/j.molstruc.2015.11.040

Accepted Manuscript
Mesomorphic properties of Chiral Three-arm Liquid Crystals Containing 1,2,4-
Butanetriol as Core
Mei Tian, Shuang-jie Wu, Xiao-Wei Tian, Dan-Shu Yao, Chong-Liang Li, Jian-She Hu,
Bao-Yan Zhang
PII:
S0022-2860(15)30444-0
10.1016/j.molstruc.2015.11.040
MOLSTR 21990
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 17 July 2015
Revised Date: 12 November 2015
Accepted Date: 18 November 2015
Please cite this article as: M. Tian, S.-j. Wu, X.-W. Tian, D.-S. Yao, C.-L. Li, J.-S. Hu, B.-Y. Zhang,
Mesomorphic properties of Chiral Three-arm Liquid Crystals Containing 1,2,4- Butanetriol as Core,
Journal of Molecular Structure (2015), doi: 10.1016/j.molstruc.2015.11.040.
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ACCEPTED MANUSCRIPT
Mesomorphic properties of Chiral Three-arm Liquid
Crystals Containing 1,2,4- Butanetriol as Core
1
1
2
1
1
Mei Tian , Shuang- jie Wu , Xiao - Wei Tian , Dan - Shu Yao ,Chong - Liang Li , Jian
1
1
-
She Hu , Bao - Yan Zhang
1
Centre for Molecular Science and Engineering, Northeastern University, Shen-yang
10004, People’s Republic of China.
1
2
Heilongjiang University, Harbin, 150080, People’s Republic of China.
Abstract:
A series of symmetric and asymmetric three-arm liquid crystals (TALCs) of which
the molecular structure with a central core of 1, 2, 4 - butanetriol attached by three
rod-like mesogenic moieties have successfully been synthesized. The rod-like
mesogenic side arms are 4’ - (4 - (trifluoromethyl) benzoyloxy) biphenyl- 4 -
carboxylic acid (TFBA) and 4’- (4 - (allyloxy) benzoyloxy) biphenyl - 4 - carboxylic
acid (AOBA), respectively. BTA0 and BTA3 are symmetric TALCs with three
TFBA or three AOBA as LC side arms, respectively. BTA1 is an asymmetric TALC
with one AOBA and two TFBA as side arms. BTA2 is an asymmetric TALC with
two AOBA and one TFBA as side arms. The chemical structures and LC properties of
1
the LC side arms and TALCs were characterised by FTIR, H NMR, elemental
analysis, DSC, TG, POM and X-ray diffractometer. TFBA displayed smectic B (S B)
m
phase, AOBA exhibited nematic (N) pahse. The TALCs all displayed chiral
*
mesophase properties. BTA0 displayed chiral smectic C (S C ) mesophase. BTA1,
m
BTA2 and BTA3 exhibited cholesteric (ch) mesophase. In addition, a chiral smectic A
1

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*
(
S A ) mesophase was observed for BTA1. The results indicated that the 1, 2, 4 -
m
butanetriol is vital in inducing chiral mesophase of the TALCs. The side arms also
played an important role in the mesophase type and mesogenic region. The TALCs
displayed cholesteric mesophase when nematic LC side arm AOBA was introduced
into the chiral core. The mesogenic region of the TALCs increased with the content
of the wide-mesophase-region LC side arm AOBA introduced into the TALCs
increasing. The melting temperature and the clear temperature of the TALCs were
lower than those of the LC side arms (TFBA and AOBA). The mesophase regions of
the TALCs were wider than those of the LC side arms.
Key words: chiral star-shaped liquid crystal, symmetric star-shaped, asymmetric
star-shaped, three-arm, cholesteric, nematic, smectic
1
. Introduction
Cholesteric liquid crystal (LC) materials have attracted considerable interest because
of their unique optical and electrical properties, which include selective reflection of
light, thermochromism and potential applications such as optical-electro materials
[
1-8]. As one kind of unconventional LCs, star-shaped LC usually has a core and
symmetric or asymmetric mesogens as the side arms. Currently, star-shaped LC has
aroused a great deal of interest among chemists because of its symmetric molecular
structure and interesting optical properties [9-18]. The common mesophase of
star-shaped liquid crystals is columnar [10,13, 19-21]. To the best of our knowledge,
cholesteric star-shaped LCs have been described rarely in the past. Yao [15], Zhang
2

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[
17] and Saezz [22] have introduced cholesteric mesomorphic arms into a core and
obtained cholesteric star-shaped LCs. In these cases, the cholesteric mesomorphic
phase of the star-shaped LC was mainly the result of cholesteric mesomorphic arms.
We have tried other strategies to obtain cholesteric star-shaped LCs that nematic
mesomorphic arms were introduced into chiral cores [23-25]. Glucose, maltose,
melitose and sorbitol were adopted as chiral cores. These studies indicate that chiral
core is prone to induce the cholesteric phase in the star-shaped LCs with nematic side
arms. In these studies, star-shaped LCs are symmetric structure.
Recently Hird [26] summarized the properties and applications of fluorinated
liquid crystals. The small size of the fluoro substituent enables its incorporation into
all types of liquid crystal, including calamitic, discotic, banana, lyotropic and
polymers, without destroying the liquid crystalline nature of the material. However,
the fluoro substituent is larger than the hydrogen atom, and hence causes a significant
steric effect, which combines with the high polarity, to confer many fascinating and
often remarkable modifications to melting point, mesophase morphology and
transition temperatures, as well as many other very important physical properties such
as dielectric anisotropy, optical anisotropy and visco-elastic properties.
The introduction of trifluoromethyl terminal substituents to calamitic LC is prone
to lead to smectic mesophase due to the stronger lateral force of the molecules [27-32],
while star-shaped LCs containing trifluoromethyl terminal substituents have been
seldom reported. The simultaneous introduction of nematic LC side-arm and smectic
LC side-arm containing trifluoromethyl terminal substituents to a chiral core leads to
3

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new chiral asymmetric functional materials. It is necessary to know the effect of the
chiral core and both LC side arms on behaviors of star-shaped LCs. In this article, we
synthesized a series of symmetric or asymmetric three-arm LCs bearing nematic side
arms or smectic side arms containing trifluoromethyl substituents or both the LC side
arms based on 1,2,4 - butanetriol.
2
. Experimental procedures
2
.1. Materials and methods
4
- (Trifluoromethyl) benzoic acid , 4'-hydroxy - 4 - biphenylcarboxylic acid are
obtained from Sigma-aldrich without any further purification. 1,2,4 - Butanetriol (BT),
is obtained from Chengdu Best-reagent Co. BT used in this manuscript is a mixture of
o
S and R isomer, and its specific rotation (in THF) is -2.9 . Ltd. Allyl bromide is
obtained from Yancheng Longgang chemical plant. N,N′ - dicyclohexyl carbodiimide
(DCC) and 4 - dimethylamino pyridine (DMAP) are obtained from Shanghai
Chemical Industry Company. Hydroxybenzoic acid is obtained from Beijing
Reagent Factory. Tetrahydrofuran (THF) is dried over sodium metal and distilled.
Pyridine is purified by distillation over KOH before using. Other analytical reagents
are used as received.
2
.2. Measurement
1
13
H NMR, C NMR and FTIR are measured by Varian WH - 90PFT NMR
Spectrometer (Varian Associates, Palo Alto, CA) and Perkinelmer instruments
Spectrum One Spectrometer (Perkinelmer, Foster City, CA), respectively.
Thermogravimetric analysis (TGA) and DSC measurements are carried out with a
4

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NETZSCH TGA 209C thermogravimetric analyzer, and a NETZSCH instruments
o
-1
DSC 204 (Netzsch, Wittelbacherstrasse, Germany) at a scanning rate of 10 C min
under a flow of dry nitrogen. XRD measurements is performed with nickel-filtered Cu
Kα (λ＝1.54 Ǻ) radiation with a D8 ADVANCE XRD (Bruker, Karlsruhe, Germany).
The X-ray measurements include wide-angle X-ray diffraction (WAXD) experiments
and small-angle X-ray scattering (SAXS) measurements. Measurement of optical
rotation (a) is carried out with a PerkinElmer instrument Model 341 Polarimeter at
room temperatures using sodium light source (λ＝589nm). The polarized optical
microscopy (POM) study is performed using a Leica DMRX (Leica,Wetzlar,
Germany) equipped with a Linkam THMSE - 600 (Linkam, Surrey, England) heating
stage.
2
.3 Synthesis
The synthetic routes to the mesogenic side arms, 4’- (4 - (trifluoromethyl) benzoyloxy)
biphenyl - 4 - carboxylic acid (TFBA) and 4’ - (4 - (allyloxy) benzoyloxy) biphenyl -
4
- carboxylic acid (AOBA) are shown in scheme 1.
2
.3.1. 4’- (4 - (trifluoromethyl) benzoyloxy)biphenyl - 4 - carboxylic acid (TFBA)
A solution of 4 - (trifluoromethyl) benzoic acid (40g, 0.210mol) and thionyl chloride
40mL) was placed in a round flask equipped with a absorption apparatus to absorb
(
hydrogen chloride. The mixture was stirred at room temperature for 3 h, and heated to
o
6
0 C for 10 h to allow the reaction to proceed to completion. The excess thionyl
chloride was distilled under reduced pressure. 4 - (trifluoromethyl)benzoyl chloride
o
(
TFB - C) was obtained by distillation under reduced pressure (b.p.105 C/38mmHg).
5

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Yield: 87%.
A sample of 4' - hydroxy - 4 - biphenylcarboxylic acid (38.5g, 0.18mol) was
dissolved in 80mL dry THF and 20mL dry pyridine. The TFB - C (37.4g, 0.18mol)
was dissolved in 20mL dry THF. The THF solution of TFB - C was added dropwise
into the solution of 4' - hydroxy - 4 - biphenylcarboxylic acid. The reaction mixture
was refluxed until the result of FTIR analysis showed that the characteristic
absorption bands of chloride and hydroxyl disappeared completely and then the
solvent was partially removed under reduced pressure. After cooling to room
temperature, the residue was poured into 600mL ice water and neutralized with dilute
hydrochloric acid. The crude product was obtained by filtration and washed with cold
ethanol. The white powder 4’ - (4 - (trifluoromethyl) benzoyloxy) biphenyl - 4 -
carboxylic acid (TFBA) was obtained after several re-crystallization from ethanol.
o
-1
Yield: 86%. m.p.: 253.5 C. IR (KBr,cm ): 3063-2560 (-OH in Carboxylic acid dimer),
1
1
7
2
739, 1674 (C = O), 1610,1515 (Ar). H NMR (CDCl , d, ppm): 7.186 (s, 2H, Ar - H);
3
.553-7.562 (d, J=5.4 Hz, 2H, Ar - H); 7.784-7.837 (m, 4H, Ar - H); 8.351-8.526 (m,
H, Ar - H); 8.708 (s, 2H, Ar - H);
2
.3.2. 4’ - (4 - (allyloxy) benzoyloxy) biphenyl - 4 - carboxylic acid (AOBA)
- (allyloxy)benzoyl cholride (ABA - C) was prepared by 4 - (allyloxy) benzoic acid
ABA was prepared according to ref. 33) and thionyl chloride. A solution of ABA (20
4
(
g, 0.11mol) and thionyl chloride (35mL) was placed into a round flask equipped with
a absorption apparatus to absorb hydrogen chloride. The mixture was stirred at room
o
temperature for 4 h, and heated to 60 C for 3 h. The excess thionyl chloride was
6

ACCEPTED MANUSCRIPT
distilled off under reduced pressure. ABA - C was obtained by distillation under
reduced pressure. Yield: 82%.
A sample of ABA - C (27.5g, 0.14mol) was dissolved in 100mL dry THF in a
5
00mL round flask. 4' - Hydroxy - 4 - biphenylcarboxylic acid (30.0g, 0.14mol) was
dissolved in 100mL dry THF and 20mL dry pyridine. The THF solution of ABA-C
was added dropwise into the solution of 4' - hydroxy - 4 - biphenylcarboxylic acid.
The reaction mixture was refluxed until the result of FTIR analysis showed that the
characteristic absorption bands of chloride and hydroxyl disappeared completely and
then the solvent was partially removed under reduced pressure. After cooling to room
temperature, the mixture was poured into 800mL ice water and neutralized with dilute
hydrochloric acid. The crude product was obtained by filtration and washed with cold
ethanol. The light yellow powder 4’- (4 - (allyloxy) benzoyloxy) biphenyl - 4 -
carboxylic acid (AOBA) was obtained after several re-crystallization from ethanol.
o
-1
Yield: 58%. m.p.: 198.7 C. IR (KBr, cm ): 3070-2544 (-OH in Carboxylic
1
acid dimer), 2997-2866 (-CH ), 1729, 1686 (C = O), 1605, 1496 (Ar). H NMR
2
(
CDCl , d, ppm): 4.405-4.472 (m, 2H, CH = CHCH O); 5.349-5.366 (d, J = 10.2
3 2 2
Hz,1H, CH = CHCH O); 5.449 - 5.480 (d, J = 18.6 Hz,1H, CH =
2
2
2
CHCH O);6.055-6.125 (m,1H, CH = CHCH O); 7.018-7.032 (d, J = 8.4 Hz, 2H, Ar -
2
2
2
H); 7.270-7.330 (m, 2H, Ar - H); 7.675-7.687 (d, J = 7.2 Hz, 4H, Ar - H);
8
.111-8.144 (d, J = 6.6 Hz, 2H, Ar - H); 8.175-8.215 (d, J = 8.0 Hz, 2H, Ar - H);
2
.3.2. Tree arms liquid crystals (BTA0-BTA3)
The synthetic routes to butane - 1,2,4 - triyl tris (4' - (4 - (trifluoromethyl) benzoyloxy)
7

ACCEPTED MANUSCRIPT
biphenyl - 4 - carboxylate) (BTA0), 2-(4'-(4 - (allyloxy) benzoyloxy)
biphenylcarbonyloxy) butane-1,4-diyl bis (4'-(4-(trifluoromethyl) benzoyloxy)
biphenyl-4-carboxylate)
(BTA1),
2-(4'-(4-(trifluoromethyl)benzoyloxy)
biphenylcarbonyloxy)butane-1,4-diyl bis(4'-(4-(allyloxy)benzoyloxy) biphenyl- 4
-
carboxylate) (BTA2) and butane-1,2,4-triyl tris(4'-(4-(allyloxy)benzoyloxy) biphenyl
4 - carboxylate) (BTA3) are shown in Scheme 2 and the details of the experiment
-
are summarized in Table 1. BTA0 and BTA3 are symmetric three-arm LCs (TALCs) .
BTA1 and BTA2 are asymmetric TALCs.
The symmetric three-arm LCs, BTA0 and BTA3, have been synthesised by the
one-step reaction between O-H groups of BT and aromatic acid of the LC side arms in
THF and pyridine toluene, using DCC and DMAP as catalyst. The synthesis of BTA0
is given as a representative example.
A solution of 1,2,4 - butanetriol (BT, 0.318g, 3mmol) and pyridine (15mL) were
placed into a 100mL round flask. A solution of N, N′-dicyclohexyl carbodiimide
(DCC, 1.86g, 9mmol), 4-dimethylamino pyridine (DMAP, 0.55g) and THF (20mL)
was put into the flask. A sample of TFBA (3.48g, 9mmol) was dissolved in 60mL
o
pyridine and then put them into the flask, too. The mixture was stirred at 40 C until
the result of FTIR analysis showed that the characteristic absorption band of C=N
-1
(
2117 cm ) disappeared completely. The 1,3 - dicyclohexylurea (DCU) was separated
from the reaction mixture by filtration for several times. The filtrate was concentrated
and then poured into water. The water solution was neutralized with dilute
hydrochloric acid. The crude product was obtained by filtration and washed with
8

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water. The white powder of BTA0 was obtained after several re-crystallization from
o
ethyl ether. Yield: 48%. M.p.: 118.0 C. Elemental analysis calculated for C H F O
6
7
43
9
12:
C 66.45; H, 3.58; F, 14.12; O, 15.85%. Found: C, 66.41; H, 3.61; F, 14.10; O,
,
1
5.88%.
-1
IR (KBr, cm ): 2997-2850 (C - H), 1738, 1711 (C = O), 1609,1496 (Ar), 1123
(C - F).
1
H NMR (CDCl , TMS, δ): 2.110-2.210 (m, 2H, – CH –), 4.190-4.455 (m, 5H, –
3
2
H
C
CH –，
), 7.330-7.340(m, 6H, Ar - H), 7.660-7.670(m, 6H, Ar – H), 7.690-7.710
2
(m, 6H, Ar – H), 7.820-7.834 (d, J = 8.4 Hz, 6H, Ar - H), 8.147-8.161(m, 6H, Ar - H),
8
.365-8.379 (d, J = 8.4 Hz, 6H, Ar - H).
For the synthesis of asymmetric TALCs, BTA1 and BTA2, regioselectivity should
be considered. The activity of the three hydroxyl groups of BT is different. The
activity of the two primary hydroxyl groups of BT is stronger than that of the
secondary hydroxyl groups. That the different side arms reacting with BT
simultaneously will result in the generation of the isomers. Their properties and
spectra are similar. It is difficult to separate them. So the key is to reduce the
generation of the isomers as much as possible. The measures of two-step method and
adding the solution of the LC side arm dropwise to the reactive system were adopted
for the synthesis of asymmetric TALCs. These measures can decrease the generation
of the isomers. The synthesis of BTA1 is given as a representative example of the
two-step method.
A solution of BT (0.318g, 3mmol) and 10mL pyridine and 15mL THF were placed
9

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into a 100mL round flask. A solution of DCC (1.86g, 9mmol）), DMAP (0.55g) and
THF (20mL) was put into the flask. A sample of TFBA (2.32g, 6mmol) was dissolved
in 50mL THF. The solution of TFBA was added dropwise into the BT solution for
o
about 12 h. The reaction mixture was stirred at 40 C until the result of FTIR analysis
showed that the characteristic absorption bands of carboxylic acid disappeared
completely. A sample of AOBA (1.12g, 3mmol) was dissolved in 30mL THF. The
solution of AOBA was added into the above reaction mixture. The reaction mixture
o
was stirred at 40 C until the result of FTIR analysis showed that the characteristic
-1
absorption bands of C=N (2117cm ) disappeared completely. The DCU was
separated from the reaction mixture by filtration for several times. The filtrate was
concentrated and then poured into water. The water solution was neutralized with
dilute hydrochloric acid. The crude product was obtained by filtration and washed
with water. The white powder of BTA1 was obtained after several re-crystallization
o
from ethyl ether. Yield: 40%. m.p.: 63.8 C. Elemental analysis. Calculated for
C H F O C 69.11; H, 4.03; F, 9.51; O, 17.35%. Found: C, 69.06; H, 4.11; F, 9.46;
69
48
6
13:
,
O, 17.37%.
-1
IR (KBr, cm ): 2955-2854 (C - H), 1736, 1713 (C = O), 1606,1492 (Ar), 1123
(C - F).
1
H NMR (CDCl , TMS, δ) 2.186-2.204 (m, 2H, – CH –), 4.126-4.137 (m, 2H,
3
2
H
C
–
CH O –), 4.314-4.371 (m, 1H,
), 4.404-4.474 (m, 2H, – CH O –), 4.650-4.687
2
2
(
m, 2H, – CH O), 5.347-5.364 (d, J = 10.2，1H, = CH ), 5.449-5.480 (d, J = 18.6, 1H,
2
2
=
CH ), 6.056-6.111 (m, 1H, - CH =), 7.239-7.241(d, J = 1.2 Hz, 2H, Ar - H), 7.339-
2
1
0

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7
7
8
.358 (dd, J = 2.7 Hz, J = 6.0 Hz, 6H, Ar - H), 7.677-7.691 (m, 6H, Ar - H),
1
2
.700-7.715 (d, J = 9.0 Hz, 4H, Ar - H), 7.808-7.821 (d, J = 7.8 Hz, 6H, Ar - H),
.121-8.148 (m, 8H, Ar-H), 8.352-8.366 (d, J = 8.4 Hz, 4H, Ar - H).
3
. Results and discussion
3
.1. Structural characterization
Chemical structures of TFBA, AOBA and the TALCs were characterized with FTIR,
1
1
elemental analysis and H NMR spectra. In the H NMR spectra of TFBA, AOBA and
TALCs, the peaks with chemical shifts 2.110-2.210 and 4.120-4.687 ppm correspond
to alkyl protons, and 5.349-6.115 and 7.018-8.366 ppm correspond to olefinic protons
and aromatic protons, respectively. The IR spectra of TFBA, AOBA and TALCs
-1
contained characteristic peaks near 3070-2544 cm associated with stretching
band of hydroxyl groups in carboxylic acid dimers, strong peaks near 1739 and 1729
-1
-1
cm corresponding to C=O in ester, and the strong peaks near 1685 cm
corresponding to C=O in carboxylic acid dimers. The results of FT-IR, elemental
1
analysis and H NMR spectroscopy of TFBA, AOBA and TALCs were in good
agreement with the anticipated structures.
1
,2,4 - Butanetriol (BT) is a kind of chiral molecule which has three hydroxyl
o
groups. The specific rotation of BT (in THF) is -2.9 . The TALCs (in THF) exhibited
optical active, too. The TALCs’ specific rotations were given in Table 1. It is obvious
that the chiral core played an important role in the TALCs’ optical active. An
interesting phenomenon is that the TALCs’ specific rotations are greater than their
chiral core. This may be attributed to that larger steric effect of the TALCs hindered
11

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the free rotation of single bond of C-C and C-O in the BT, which resulted in one of the
two optical isomers increasing.
3
.2. Liquid crystal properties of LC arms
The liquid crystalline behavior of the LC side arms TFBA and AOBA was studied
using DSC, POM and XRD. TFBA and AOBA exhibited thermotropic mesomorphic
behavior. The transition temperatures and associated enthalpy changes seen for TFBA
and AOBA are summarized in Table 2, and the DSC thermograms are displayed in
Fig.1, Fig.2, Fig.3, and Fig.4.
o
TFBA displayed liquid crystal phase at about 253.5 C on heating. The sample
displayed no LC - isotropic transition but solidification and carbonization when the
sample was heated further. Liquid crystal phase was not observed in the subsequent
o
o
cooling. There are two phase transition at 253.5 C and 291.2 C in the DSC heating
curve of TFBA (see Fig.1). There was not any phase transition in the DSC curve on
cooling. DSC and POM studies indicated that the melting temperature of TFBA is
o
2
53.5 C. The TGA result helped us to get a better understanding of the thermal
properties and mesomorphic properties of TFBA. TGA result (see Fig. 1) indicated
o
o
that 3.7% and 31% weight loss occurred at 291.2 C and 350 C, respectively. The
obvious decomposition may destroy the structure of TFBA. This may be the cause
that there was no phase transition and liquid crystal phase being observed in the
subsequent cooling process. While TFBA still displayed mesomorphic properties at
o
T2% (271.6 C, the temperature at which 2% mass loss occurred in TGA). So T2% was
adopted as the final temperature of the mesophase in the present study.
1
2

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o
TFBA was heated to 271.6 C and then cooled to study its thermal properties and
mesomorphic phase on cooling. In the subsequent cooling process, the liquid crystal
o
phase was observed by POM and the sample crystallized at about 203 C. The DSC
o
curve of TFBA shows crystallization at 203.9 C on cooling, too. (see Fig.2).
The results of DSC and POM indicated that TFBA was a thermotropic
o
enantiotropic LC. It shows a melting transition at 253.5 C, the temperature at which
o
2
1
% weight loss occurred is 271.6 C and the mesomorphic region (ꢀT ) is about
1
o
8.1 C on first heating (The mesomorphic region on heating cycle is to be calculated
o
in this way: 271.6-253.5=18.1 C). While on cooling, crystallization occurred at
o
o
2
03.9 C. The mesomorphic region (ꢀT ) is about 67.7 C. (The mesomorphic region
2
o
on cooling cycle is to be calculated in this way: 271.6-203.9=67.7 C).
o
Fig. 3 shows only one phase transition at 198.7 C on heating for AOBA and
there is not any phase transition in the DSC curve on cooling. When AOBA was
o
heated, the sample melt at about 198 C. The sample displayed no LC - isotropic
transition but solidification and carbonization when the sample was heated further.
Liquid crystal phase was not observed in the subsequent cooling. Also the TGA result
helped us to get a better understanding of the thermal properties and mesomorphic
o
properties of AOBA. Fig. 3 indicated that the sample lost 3% weight at 300.0 C. The
decomposition may destroy AOBA’s structure. This may be the cause that the LC
o
phase solidified and the sample carbonized after AOBA being heated to 300 C. It is
also the reason for no LC phase being observed in the subsequent cooling process.
o
T2% (291.0 C) was adopted as the final temperature of AOBA’s mesophase in the
1
3

ACCEPTED MANUSCRIPT
o
present study (AOBA still displayed mesomorphic properties at 291.0 C ).
o
AOBA was heated to not too high but only 260.0 C and then cooled to study
the cooling process' thermal properties and mesomorphic phase. The liquid crystal
o
phase was observed by POM and the sample crystallized at about 193 C in the
o
subsequent cooling process. Fig. 4 shows that AOBA crystallized at 193.7 C on
cooling.
The results of DSC and POM indicated that AOBA is a thermotropic
o
enantiotropic LC. It showed a melting endotherm at 198.7 C, the temperature at
o
which 2% weight loss occurred is 291.0 C and the mesomorphic region (ꢀT ) is about
1
o
o
9
2.3 C on heating, as well as LC–crystal transition is at 193.7 C and the mesomorphic
o
region (ꢀT ) is about 97.3 C on cooling (The calculating method of the mesomorphic
2
regions is similar with that of TFBA) .
TFBA exhibited mosaic texture on heating and cooling cycle (see Fig. 5a). A
variable-temperature X-ray diffraction study was carried out on TFBA to obtain more
detailed information on its LC phase structure. Fig. 6 shows the X-ray patterns at
o
o
different temperature (260 C and 270 C) within TFBA’s liquid crystal rang. The sharp
peaks at 2θ = 3.0 (d = 29.41 Å) seen at different temperature indicated that layer
spacing was independent of temperature. The molecular length L of TFBA is
estimated to be 15.6 Å, while that of the dimmer of TFBA is estimated to be 28.6 Å,
and approximately equals to the smectic layer spacing, suggesting that the molecular
arrangement is in the form of dimmers and the director n is perpendicular to the
smectic layer plane. The results of the X-ray and mosic texture of TFBA suggested
1
4

ACCEPTED MANUSCRIPT
that it is a smectic B (S B) LC. AOBA exhibited thread-like texture on heating and
m
cooling cycle. Thread-like texture is typical texture of nematic. The results of DSC
and POM indicated that AOBA is a nematic (N) LC.
The chemical structure of TFBA and AOBA is somewhat similar. The two LC
arms all have the structure of 4’ - (benzoyloxy) biphenyl - 4 - carboxylic acid. The
difference between them is the terminal group. The terminal group of AOBA is
allyloxy, while the terminal group of TFBA is trifluoromethyl. In general, fluorine’s
strong electronegativity results in a strongly polar C-F bond. The stronger polarity of
terminal trifluoromethyl group may increase the anisotropy of the molecule, and result
in an increase of the inter-molecular forces. This may be the cause that the melting
o
temperature of TFBA is 54.8 C higher than that of AOBA.
3
.2. Liquid crystal properties of TALCs
The thermal properties and phase behavior of TALCs (BTA0, BTA1, BTA2 and
BTA3) were investigated with DSC. The phase-transition temperatures and
corresponding enthalpy changes of the TALCs obtained on the first heating and
cooling cycle were summarized in Table 2, and the DSC curves are presented in Fig. 7,
Fig.8, Fig.9 and Fig.10. The exothermic peaks of crystallization of BTA0 - BTA3 are
not obvious and some of them even have not been observed. This may be due to their
dendritic structures. In general, the dendritic material is not easy to crystallize. In this
study, the temperatures of crystallization of some TALCs were obtained from POM.
o
-1
DSC curves of BTA0 show a melting endotherm at 118.0 C (ꢀH=27.24 J g )
*
o
and a chiral smectic C (S C ) - isotropic (I) phase transition at 201.0 C (ꢀH=2.28 J
m
1
5

ACCEPTED MANUSCRIPT
-1
*
o
-1
g ) on heating, as well as I - SmC transition at 181.0 C (ꢀH= -3.05 J g ) and
o
-1
crystallization at 95.2 C (ꢀH= -11.4 J g ) on cooling. DSC curves of BTA1 show a
o
-1
*
melting endotherm at 63.8 C (ꢀH=0.70 J g ), a chiral smectic A (S A ) - cholesteric
m
o
-1
o
(
ch) phase transition at 92.5 C (ꢀH=0.78 J g ) and ch - I phase transition at163.2 C
-1
o
-1
(
ꢀH=0.43 J g ) on heating, as well as I - ch transition at 156.1 C (ꢀH= -0.08 J g )
*
o
-1
and ch - S A transition at 142.5 C (ꢀH= -0.20 J g ). The exothermic peak of
m
crystallization of BTA1 has not been observed on DSC’s cooling curve. The transition
o
temperature of chiral smectic A to crystal phase (54.5 C) of BTA1 on Table 2 was
o
obtained by POM. DSC curves of BTA2 show a melting endotherm at 101.1 C
-1
o
-1
(
ꢀH=2.61 J g ) and ch - I phase transition at 222.6 C (ꢀH=0.48 J g ) on heating, as
o
-1
o
well as I - ch transition at 222.1 C (ꢀH= -0.18 J g ) and crystallization at 73.9 C
-1
(
ꢀH= -0.21 J g ) on cooling. DSC curves of BTA3 show a melting endotherm at
o
-1
o
-1
9
0.4 C (ꢀH=8.10 J g ) and ch - I phase transition at 217.6 C (ꢀH=3.20 J g ) on
o
-1
heating, as well as I - ch transition at 213.9 C (ꢀH= -0.25 J g ) on cooling. The
exothermic peak of crystallization of BTA3 has not been observed on cooling cycle on
o
DSC curve. The transition temperature of cholesteric to crystal phase(60.5 C) of
BTA3 on Table 2 was obtained from POM, too.
The melting temperatures of TALCs (BTA0 - BTA3) are lower than those of the
LC side arms. This may be due to their different structure. The structure of the LC
side arms (TFBA and AOBA) is aromatic acid. The hydrogen bond of aromatic acid
makes the LC side arms have higher melting temperature. In addition, the lower
melting temperature of TALCs may be attributed to their dendritic structures. The
1
6

ACCEPTED MANUSCRIPT
mesomorphic regions of TALCs (see Fig.11) are wider than those of the
corresponding LC side arms. This may be attributed to the lower melting temperatures
and the lower temperatures of crystallization of TALCs. Fig. 11 shows that the
mesomorphic regions increase gradually from BTA0 to BTA3 on heating and cooling.
This may be attributed to the increase of the content of AOBA, which has wider
mesomorphic region.
Fig. 12a - Fig. 12j show TALCs’ characteristic textures. BTA0 displayed thermal
*
enantiotropic chiral smectic C (S ) texture. When BTA0 was heated, the broken
C
fan-shaped texture appeared (see Fig. 12(a)). The texture did not disappear until the
sample became isotropic. The droplet texture separated from the isotropic melt at
o
about 181 C on cooling. As the sample was cooled further, the droplets gradually
aggregated to form a broken fan-shaped texture with striations due to the helical
superstructure (see Fig. 12(b)).The broken fan-shaped texture did not disappear until
*
the sample crystallized. BTA1 displayed thermal enantiotropic S A and cholesteric
m
o
textures on heating and cooling. When BTA1 was heated to about 63 C, focal-conic
texture appeared at first, and then there was a subtle change and broken fan-shaped
texture displayed. The texture did not disappear until the sample became isotropic.
The fan-shaped texture appeared at first on cooling (see Fig. 12 (c)). There was also a
subtle change as the sample was cooled further. The fan-shaped texture changed into
focal-conic texture (see Fig. 12(d)). The focal-conic texture did not disappear until the
sample crystallized. BTA2 displayed thermal enantiotropic cholesteric textures on
heating and cooling. When BTA2 was heated, the sample melted and catenoid
1
7

ACCEPTED MANUSCRIPT
birefringent region containing small fan-shaped texture appeared (see Fig. 12 (e)). As
the sample was heated further, fingerprint texture appeared (see Fig. 12 (f)) and did
not disappear until the sample became isotropic. The cholesteric droplet texture
separated from the isotropic melt on cooling (see Fig. 12 (g)). The cholesteric droplet
texture did not disappear until the sample crystallized. BTA3 displayed thermal
enantiotropic cholesteric textures on heating and cooling. When BTA3 was heated,
the sample melted and focal-conic texture appeared at first (see Fig. 12 (h)) and then
gradually changed into fingerprint texture (see Fig. 12 (i)). Fingerprint texture did not
disappear until the sample became isotropic. As the sample was cooled, the
o
cholesteric droplet texture separated from the isotropic melt at about 214 C and did
not disappear (see Fig. 12 (j)) until the sample crystallized.
The fan-shaped or focal conic textures are preferably observed in rather thin
sample preparations in S A phase. From a texture point of view, the S A and the
m
m
*
S A phases are equivalent in appearance. It is used to distinguish the S A and the
m
m
*
*
S A phases whether the LC displays optical active. An often observed S C
m
m
appearance is that of the so-called broken fan-shaped texture. This formation of a
*
*
helical superstructure in chiral bulk S C samples makes it easy to detect the S A
m
m
*
*
and S C . In S A , the backs of the fans of focal conic or fan-shaped textures appear
m
m
smooth, the occurrence of an arced equidistant line pattern indicates the mesophase is
*
S C .
m
Fingerprint texture is a typical texture of cholesteric phase, and fan-shaped
texture is often observed in cholesteric phase, too. But the orientation of the helix axis
1
8

ACCEPTED MANUSCRIPT
in them is different. For fingerprint texture, the twist axis lies in the plane of the
substrate, within the substrate plane there is no preferred direction, and thus the
direction of the twist axis is allowed to vary smoothly over macroscopic distances.
The varying director field leads to an equidistant pattern of dark lines, which is called
a fingerprint texture. Dark stripes appear, whenever the local director is oriented along
the direction of light propagation, i. e. at vanishing birefringence. While for
fan-shaped texture, the twist axis naturally deviates from the substrate normal.
Fan-shaped textures of cholesteric phase are very similar to those observed in S A
m
phases. The cholesteric fan-like textures are observed for strongly twisted materials,
while fan-shaped textures that exhibit focal conics are found for materials with a
slightly smaller twist. Especially the latter textures are very similar to that of the S A
m
phase, but with a somewhat smoother appearance of the fans. [34]
The variable-temperature X -ray diffraction study was carried out on BTA0 -
BTA3 to obtain more detailed information on its LC phase structure. Fig. 13(a) shows
the X-ray patterns at different temperature within BTA0’s liquid crystal rang. The
o
o
sharp peaks at 2θ = 3.3 (strong) and 2θ = 7.0 (weak), corresponding to d = 26.74 Å,
o
d = 12.61 Å, respectively, were observed at 135 C on heating. The sharp peaks at 2θ =
o
o
o
2
2
.3 (strong), 2θ = 3.9 (weak) and 7.6 (very weak), corresponding to d = 38.37 Å, d =
o
2.63 Å and d = 11.77 Å, respectively, were observed at 160 C on heating. The
o
diffuse peaks at about 2θ = 19.0 , corresponding to molecular lateral spacing, were
observed at the two temperatures.
Fig. 13(b) shows the X-ray patterns on different temperature within BTA1’s liquid
1
9

ACCEPTED MANUSCRIPT
o
crystal rang on cooling. Only the diffuse peak 2θ = 18.0 was observed for BTA1 at
o
*
o
1
=
=
50 C. Within the S A mesomorphic region, the sharp peaks at 2θ = 3.5 (strong), 2θ
m
o
o
7.1 (weak) and 10.7 (very weak), corresponding to d = 25.21 Å, d = 12.44 Å and d
o
o
8.26 Å, respectively, were observed at 120 C and 100 C. The diffuse peaks at about
o
o
2
θ = 18.0 - 19.0 , which corresponding to the molecular lateral spacing of BTA1,
o
o
were observed at 120 C and 100 C, too.
Fig. 13(c) and Fig. (d) show the X-ray patterns at different temperature on heating
within LC rang of BTA2 and BTA3, respectively. Only diffuse peak at about 2θ =
o
o
1
8.0 - 19.0 was observed for BTA2 and BTA3. These were consistent with their
optical textures, suggesting that BTA2 and BTA3 are cholesteric LCs.
That the location of the sharp peaks of BTA0 changed at different temperature
indicated that the layer space depended on temperature. For S C, the title angle θ
m
increases with decreasing temperature and thus the smectic layer spacing decreases
with decreasing temperature. The situation of BTA0 that the tilt θ increased with
decreasing temperature is consistent with that of S C. We also noticed that the
m
o
o
intensity of the sharp peaks at 160 C was much greater than that at 135 C, suggesting
that the degree of long-range ordering of BTA0 increased with increasing temperature.
The cause of this may be due to the fact that the molecular mobility of BTA0
increased with increasing temperature, providing help to the LC molecules in forming
an ordered lamellar arrangement.
The X-ray patterns of BTA0 and BTA1 displayed the sharp peaks at different
angle at a certain temperature within their LC rang, for example, BTA1 displayed
2
0

ACCEPTED MANUSCRIPT
o
o
o
sharp peaks at 2θ = 3.5 , 7.1 , 10.7 . The sharp peaks appeared at different angles
indicated the existence of layers of different thickness (d = 25.21 Å, d = 12.44 Å and
d = 8.26 Å, respectively). Star-shaped LC molecule is different from rod-like LC
molecule, whose side arms extend to space as far as possible. Fig. 14 is the 3D
molecular model of BTA1. Fig. 14 (A) is the front view and Fig. 14 (B) is the side
view. Fig. 14 displayed that the height, width and length of BTA1 molecule are
2
1.37Å, 8.57Å and 11.95Å, respectively. When these molecules arrange in layers, the
long range order with different thickness is formed (see Fig. 15). This may be the
cause for that different sharp peaks were observed. That the difference between
BTA1’s layer thickness measured by X-ray (25.21 Å, 12.44 Å and 8.26 Å) and its
height, width and length calculated (21.37Å, 8.57Å and 11.95Å) is not much, which
confirmed our conjecture in a certain degree. The intensity of the sharp peak at 2θ =
o
o
o
3
.5 is much stronger than that at 2θ=7.1 or 10.7 , indicating that long rang order in a
certain direction dominated.
o
o
*
BTA1’s XRD results at 120 C and 100 C (within the S A mesomorphic region)
m
indicated that the layer space was independent of temperature. This may be duo to
that the director n and the optic axis are perpendicular to the smectic layer plane in the
*
o
o
o
S A phase. The intensity of the sharp peaks at 2θ = 3.5 , 7.1 and 10.7 increased
m
o
o
with the temperature decreasing from 120 C to 100 C, suggesting that the lamellar
structure perfected with decreasing temperature. BTA1’s XRD results were consistent
*
with its optical textures, suggesting that BTA1 is S A LC at the lower temperature
m
mesomorphic region and cholesteric LC at the higher temperature mesomorphic
2
1

ACCEPTED MANUSCRIPT
region.
We noticed that chiral core and the mesomorphic type of the two LC side arms
played important effects on the mesomorphic type of the TALCs in this study. The
TALCs displayed chiral properties affected by the chiral core, but their mesomorphic
types were affected by the mesomorphic types of the LC side arms. The TALC BTA3
displayed cholesteric phase. The result is similar to the previous studies [23-25],
which chiral core is prone to induce the cholesteric phase in the multi-arm LCs with
nematic side arms. The only difference is that the nematic LC side arms were
introduced into the chiral core by flexible spacer in previous studies, while in this
study, the rigid nematic LC side arm was introduced into the chiral core directly,
*
without flexible spacer. The TALC BTA0, with three S B side arms, displayed S .
m
C
The result indicated that the chiral core made the TALC display chiral property but
the dendritic structure reduced the degree of order of the lamellar liquid crystal. BTA1
and BTA2 with both the nematic LC side arm and the smectic LC side arm in them
also displayed chiral properties. The introduction of the nematic LC side arm made
the TALCs (BTA1, BTA2 and BTA3) be prone to display cholesteric phase.
4
. Conclusion
A series of symmetric and asymmetric three-arm liquid crystals (TALCs) with 1, 2, 4
butanetriol as chiral core and 4’ - (4 - (trifluoromethyl) benzoyloxy) biphenyl- 4 -
-
carboxylic acid (TFBA) and 4’- (4 - (allyloxy) benzoyloxy) biphenyl - 4 - carboxylic
acid (AOBA) as LC side arms have been synthesised. The influence on TALCs
properties played by the chiral core and the nature of the LC side arms have been
2
2

ACCEPTED MANUSCRIPT
studied. TFBA is a smectic B LC, while AOBA is a nematic LC. Compared to TFBA,
the melting temperature of AOBA is lower and its mesophase temperature range is
wider. The TALC BTA0 exhibited chiral smectic C, while BTA1, BTA2 and BTA3
displayed cholesteric phase, in addition BTA1 displayed chiral smectic A at the lower
temperature of the mesophase temperature range. The introduction of chiral core into
TALCs makes both the symmetric and the asymmetric TALCs display chiral
properties. The absolute values of specific rotation of the TALCs are bigger than that
of the chiral core. Compared to the S B LC side arm (TFBA), the degrees of order of
m
the TALCs (BTA0 - BTA2, with TFBA in them) are reduced and this can be
attributed to their dendritic structure. The introduction of the nematic LC side arm
made the TALCs (BTA1, BTA2 and BTA3) be prone to display cholesteric phase.
The melting temperatures of TALCs are lower than those of the corresponding LC
side arms, while their mesophase temperature ranges are wider. The mesomorphic
regions increased gradually from BTA0 to BTA3 on heating and cooling.
Acknowledgements
This work was financially supported by the Fundamental Research Funds for the
Central Universities (N130205001), Science and Technology Bureau of Shenyang
(F14-231-1-05) and the National Natural Science Fund of China (51273035).
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[
2
[
2
6

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Table 1. Inventory ratio, Specific Rotation of the three-arm liquid crystals
sample
Feed/mmol
specific
o
n (BT )
n (TFBA)
n (AOBA)
Rotation ( )
+18.60
+7.78
BTA0
BTA1
BTA2
BTA3
3
3
3
3
9
6
3
0
0
3
6
9
-5.54
-8.30

ACCEPTED MANUSCRIPT
Table 2. Phase transitions, phase transition enthalpies and mesogenic range for LC side arms and the
three - arm LCs.
o
a
o
Sample Phase transitions, C (corresponding enthalpy changes, J/g)
△T,
C
Meso-
phase
b
TFBA heating: Cr 253.5 (10.60) S B 271.6 (-) I
18.1
m
S B
m
b
cooling: Cr 203.9 (-4.77) S B 271.6 (-) I
67.7
92.3
m
b
AOBA heating: Cr 198.7 (48.06) N 291.0 (-) I
N
b
9
7.3
cooling: Cr 193.7 (-5.29) N 291.0 (-) I
*
BTA0 heating: Cr 118.0 (27.24) S C 201.0 (2.28) I
83.0
85.8
m
*
S C
m
*
cooling: Cr 95.2 (-11.40) S C 181.0 (-3.05) I
m
*
BTA1 heating: Cr 63.8 (0.70) S A 92.5 (0.78) ch 163.2 (0.43) I
99.4
m
*
S A - ch
c
*
m
cooling: Cr 54.5 (-) S A 142.5 (-0.20) ch 156.1 (-0.08) I
101.6
121.5
148.2
127.2
153.4
m
BTA2 heating: Cr 101.1 (2.61) ch 222.6 (0.48) I
cooling: Cr 73.9 (-0.21) ch 222.1 (-0.18) I
ch
ch
BTA3 heating: Cr 90.4 (8.10) ch 217.6 (3.20) I
c
cooling: Cr 60.5 (-) ch 213.9 (-0.25) I
a
*
*
Cr, crystalline phase; S B, smectic B; S C , chiral smectic C; S A , chiral smectic A; ch, cholesteric;I, isotropic
m
m
m
phase.
b
the temperature at which 2% weight loss occurrs.
The transition temperature obtained from POM.
c

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