Structure and properties of novel three-armed star-shaped liquid crystals

Article abstract of DOI:10.1016/j.molstruc.2005.01.057

A series of novel three-armed star-shaped liquid crystals based on 1,3,5-trihydroxybenzene as a core and ω-[4-(p-alkoxybenzoloxy) phenoxycarbonyl]valeric acid as mesogenic arms were synthesized. Their chemical structures were confirmed by FTIR and ^1<

Full text of DOI:10.1016/j.molstruc.2005.01.057

Journal of Molecular Structure 741 (2005) 135–140
www.elsevier.com/locate/molstruc
Structure and properties of novel three-armed star-shaped liquid crystals
*
Bao-Yan Zhang , Dan-Shu Yao, Fan-Bao Meng, Yuan-Hao Li
The Research Centre for Molecular Science and Engineering, Northeastern University, Shenyang 110004, China
Received 23 October 2004; revised 16 January 2005; accepted 18 January 2005
Available online 11 March 2005
Abstract
A series of novel three-armed star-shaped liquid crystals based on 1,3,5-trihydroxybenzene as a core and u-[4-(p-alkoxybenzoloxy)
phenoxycarbonyl]valeric acid as mesogenic arms were synthesized. Their chemical structures were confirmed by FTIR and 1H NMR spectra,
their mesomorphic properties and phase behavior were investigated using differential scanning calorimetry (DSC), polarizing optical
microscopy (POM) and X-ray diffraction (XRD) measurements. The results show that the three-armed star-shaped liquid crystals exhibit a
broad range of liquid crystalline phases at moderate temperature, no crystallization but vitrifying to form glass-forming mesogens during
cooling from the isotropic melt. The formed crystals are glassy liquid crystals. The three-armed molecules are highly thermal stable and the
mesomorphic ranges (DT) become wider as the terminal alkoxy chains become longer. Threadlike texture, a typical nematic phase, can be
observed in the liquid crystalline state.
q 2005 Elsevier B.V. All rights reserved.
Keywords: Liquid crystals; Three-armed star-shaped; Vitrify; Nematic phase
1. Introduction
low molecular weight star-shaped liquid crystals hold the
lower viscosity and higher chemical purity and are readily
processed into macroscopically ordered solid films [11]. In
addition to their application, unconventional star-shaped
liquid crystals are especially important for the theoretical
understanding of the liquid-crystalline phenomenon.
Liquid crystals are fascinating materials with properties
intermediate between those of solids and liquids. Conven-
tional liquid crystals are composed of rod-like or discotic
molecules. With the development of liquid crystals science
and technology, there is a growing interest in the synthesis
and investigation of unconventional liquid crystals [1–6].
Star-shaped liquid crystal is one kind of them, which usually
has a small core and a few extended rigid mesogenic units as
liquid crystal arms. At a glance, star-shaped molecular
structure seems to be discotic, however, due to the flexible
spacers the compound may exhibit nematic phases rather
than columnar phase, which is preferred for the electro-
optical application. Some of the star-shaped liquid crystals
do not crystallize during cooling from the isotropic melt but
vitrify to form glass-forming mesogens, which attracts
much attention because glass-forming liquid crystal
materials possess many special optical, mechanical and
thermal stable properties [7–10]. Compared to polymers,
In order to successfully synthesize a new kind of star-
shaped liquid crystal and meet the quality requirements for
glass-forming liquid crystal materials, we use 1,3,5-trihy-
droxybenzene as a core and u-[4-(p-alkoxybenzoloxy)phe-
noxycarbonyl]valeric acid as mesogenic units. Their
1
chemical structures are confirmed by FTIR and H NMR
spectra. Their mesomorphic properties and phase behavior
are investigated by using DSC, POM and X-ray diffraction
measurements.
2. Experimental
2.1. Materials
p-Methyloxybenzoic acid, p-ethyloxybenzoic acid,
p-propyloxybenzoic acid, p-benzenediol, 1,3,5-trihydroxy-
benzene, hexanedioic acid were obtained from Beijing
* Corresponding author. Tel.: C86 02483687446; fax: C86
02423906300.
E-mail address: baoyanzhang@hotmail.com (B.-Y. Zhang).
0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2005.01.057

136
B.-Y. Zhang et al. / Journal of Molecular Structure 741 (2005) 135–140
Chemical Company (China) and used without any further
purification. All other solvents and reagents were purified by
standard methods.
a DMAX-3A Rigaku (Rigaku, Japan) powder
diffractometer.
2.3. Synthesis
2.2. Characterization
The synthetic routes of the mesogenic units 2a–2c and
star-shaped liquid crystals 3a–3c were shown in Scheme 1.
Their yields and structural characterization were summar-
ized in Table 1.
FTIR spectra were measured on a Nicolet 510 FTIR
spectrometer (Nicolet Instruments, Madison, WI). 1H NMR
spectra (300 MHz) were recorded on a Varian WH-90PFT
spectrometer (Varian Associates, Palo Alto, CA). Phase
transition temperatures and thermodynamic parameters
were determined by using a Netzsch DSC 204 (Netzsch,
Germany) equipped with a liquid nitrogen cooling system
under nitrogen atmosphere. The heating rates were
10 8C minK1, and cooling rates were 10 8C minK1. Phase
transition temperatures were collected during the second
heating and the first cooling scans. The thermal stability of
the three-armed star-shaped liquid crystals under atmos-
phere was measured with a Netzsch TGA 209C thermo-
grarimetric analyzer. A Leica DMRX (Leica, Germany)
polarizing optical microscope equipped with a Linkam
THMSE-600 (Linkam, England) hot stage was used to
observe phase transition temperatures and optical textures to
analyze liquid crystal properties of the mesogenic units and
three-armed star-shaped liquid crystals. XRD measurements
were performed using a nickel-filtered Cu Ka radiation with
2.3.1. u-[4-(p-alkoxybenzoloxy)phenoxy
carbonyl]valeric acid (2a–2c)
4-Hydroxy-40-[(p-alkoxy)benzoyloxy] phenyl (1) was
synthesized according to a published procedure [12].
u-[4-(p-Alkoxybenzoloxy)phenoxy carbonyl]valeric acid
2a–2c was synthesized from excess adipyl chloride with
compound 1. Typical procedure for compound 2b:
4-hydroxy-40-[(p-ethoxy)benzoyloxy] phenyl (5.16 g,
20 mmol), dissolved in 20 ml THF, was slowly added to a
solution containing 18.3 g (100 mmol) of adipyl chloride,
5 ml of pyridine, and 50 ml of THF. The reaction mixture
was stirred and boiled for 8 h in a dry atmosphere. After
cooling to room temperature, the mixture was poured into
cold water and acidified with hydrochloric acid. The product
was filtered and recrystallized from ethanol and dried in
O
COOH
CnH2n+1
SOCl2
THF
OH
HO
1
O
COO
THF/Pyridine
OH
CnH2n+1
ClCOC(CH2)4COCl
O
COO
SOCl2
OOC(CH2)4COOH
( 2a:n=1, 2b:n=2, 2c:n=3
CnH2n+1
2
)
OH
OH
HO
Pyridine
O
O
COO
O
C H
n
2n+1
O
O
O
O
2n+1
O
C H
COO
n
O
O
O
O
COO
O
C H
n
O
2n+1
3
O
( 3a: n=1, 3b: n=2, 3c: n=3 )
Scheme 1. The synthetic route of star-shaped liquid crystals.

B.-Y. Zhang et al. / Journal of Molecular Structure 741 (2005) 135–140
137
Table 1
Yields and characterization of mesogenic units and star-shaped liquid crystals
Compound
n
Yield%
55
IR (KBr)/cmK1
1H NMR chemical shifts (CDCl3, d/ppm)
2a
1
3310–2560 (–OH in –COOH); 1763, 1730, 1715
(CaO); 1604, 1509 (Ar)
1.88 (m, 4H); 2.65 (t, 4H); 3.97 (s, 3H); 6.98–8.08 (m, 8H);
10.80 (s, 1H);
2b
2c
3a
3b
3c
2
3
1
2
3
52
44
61
53
57
3250–2555 (–OH in –COOH); 1758, 1731, 1716
(CaO); 1604, 1510 (Ar)
1.45 (t, 3H); 1.90 (m, 4H); 2.67 (t, 4H); 4.12 (q, 2H); 6.96–8.03
(m, 8H); 11.20 (s, 1H);
3325–2550 (–OH in –COOH); 1755, 1728, 1714
(CaO); 1606, 1510 (Ar)
2933, 2871 (CH3,CH2); 1759, 1731 (CaO); 1610, 1.86 (m, 12H); 2.62 (t, 12H); 3.90 (s, 9H); 7.01–8.10 (m, 27H);
0.93 (t, 3H); 1.68 (m, 2H); 1.89 (m, 4H); 2.64 (t, 4H); 4. 05
(t, 2H); 6.99–8.08 (m, 8H); 10.70 (s, 1H);
1508 (Ar)
2946, 2840 (CH3,CH2); 1756, 1730, 1606, 1503
(Ar)
1.48 (t, 9H); 1.87 (m, 12H); 2.63 (t,12H); 4.12 (q, 6H);
6.98–8.06 (m, 27H);
2939, 2851 (CH3,CH2); 1753, 1726 (CaO); 1605, 0.93 (t, 9H); 1.67–1.83 (m, 18H); 2.65 (t, 12H); 4.10 (t, 6H);
1501 (Ar)
6.98–8.06 (m, 27H);
a vacuum oven to achieve a white solid of 2b with a yield of
52%.
three-armed star-shaped liquid crystals 3a–3c. Taking IR
spectra of 2b and 3b as an example, the absorption of –OH
stretching vibration in carboxylic acid groups in the range of
3250–2555 cmK1 was clearly identified in the spectrum
of mesogenic unit 2b, while the disappearance of the peaks
of the –OH stretching vibration in the spectrum of three-
armed star-shaped liquid crystal 3b was observed,
indicating a successful esterifying reaction. In addition, 2b
was apparently different from 3b that the former contained
1758, 1731, 1716 cmK1 three CaO stretching vibration
absorption bands representing Ar–O–CO–R, Ar–CO–O–R
and –COOH, respectively, but the latter only had 1756,
1730 cmK1 two CaO characteristic bands indicating a
stretch vibration in different ester modes, Ar–O–CO–R and
2.3.2. tri[u-4-(p-alkoxybenzoloxy)phenoxycarbonyl]
valeric acid phloroglucinol ester (3a–3c)
In a general procedure, appropriate u-[4-(p-alkoxyben-
zoloxy)phenoxy carbonyl]valeric acid 2a–2c (6.2 mmol, 3.1
equiv) was stirred in 25 ml of thionyl chloride and 0.5 ml of
DMF was added as a catalyst. The solution was heated at
reflux for 5 h whereby a clear solution was obtained. The
excess of thionyl chloride was removed by vacuum and the
acid chloride dried in vacuum for 1 h. Acid chloride was
dissolved in 10 ml of dry THF, and 25 ml dry pyridine
followed by adding 0.25 g (2 mmol, 1 equiv) of 1,3,5-
trihydroxy benzene to achieve esterification. The reaction
mixture was stirred at 70 8C for 12 h in a dry atmosphere.
After cooling to room temperature, the mixture was poured
into 150 ml cold water and acidified with 6 N hydrochloric
acid. The crude product obtained was collected by filtration
and repeatedly washed with distilled water, followed by
recrystallization from ethyl acetate/ethanol (1:1) to result in
compounds 3a–3c with a yield of 53–61%.
1
Ar–CO–O–R. Analysed with H NMR Spectra, 2b but not
3b showed a peak of hydrogen on carboxy group at
11.20 ppm. These results clearly indicated the existence of
1,3,5-trihydroxybenzene esters products.
3.2. Liquid crystalline behavior of mesogenic units
The liquid crystalline properties of the mesogenic units
2a–2c were characterized using differential scanning
calorimetry (DSC) and polarizing optical microscopy
(POM). The results were shown in Table 2. Fig. 1 displayed
the representative DSC thermograms. In heating trace, 2b
and 2c both exhibited two endothermic peaks. One peak
occurred at lower temperature (Tm) and it had large enthalpy
change corresponding to the transition from crystal to the
liquid crystalline phase. The other peak occurred at higher
3. Results and discussion
3.1. Spectroscopic analysis
The spectroscopic analysis confirmed the predicted
molecular structures of the mesogenic units 2a–2c and
Table 2
Phase transition temperatures of mesogenic units
a
b
Mesogenic units
n
Transition temperature/8C (corresponding enthalpy changes/
J gK1), heating/cooling
DT1
DT2
2a
2b
2c
1
2
3
Cr138.9(69.5)I/I129.6(8.2)N114.1(52.9)Cr
–
15.5
20.1
20.9
Cr118.8(48.4)N123.9(5.5)/I120.8(13.8)N100.7(57.6)Cr
Cr122.4(57.4)N131.2(2.7)I/I127.3(8.2)N106.4(64.7)Cr
5.1
8.8
Cr, crystal; N, nematic; I, isotropic.
a
Mesophase temperature ranges on heating cycle.
Mesophase temperature ranges on cooling cycle.
b

138
B.-Y. Zhang et al. / Journal of Molecular Structure 741 (2005) 135–140
DSC /(mW/mg)
↓ exo
2.0
1.0
0
–1.0
–2.0
20
40
60
80
100
120
140
160
Temperature /°C
Fig. 1. DSC thermograms of the mesogenic unit 2b.
temperature (Ti) and it had small enthalpy change
corresponding to the transition from the liquid crystalline
phase to isotropic liquid phase. The mesogenic region (DT)
between Tm and Ti (DTZTiKTm) of 2b and 2c was 5 and
9 8C, respectively. In cooling cycle from isotropic phase, the
thermograms also showed two sharp exothermic peaks: one
occurred at transition from isotropic to liquid crystalline
phase at higher temperature (Tai) and the other occurred at
transition from liquid crystalline phase to crystal at lower
temperature (Tc). The mesogenic region was around 20 8C.
However, when 2a was heated from room temperature, the
DSC trace showed only one sharp endothermic peak at
138.9 8C indicating the melting temperature(Tm). In cooling
cycle from the isotropic liquid phase, the thermogram
showed two exothermic peaks, Tai at 129.6 8C and Tc at
114.1 8C, with the region of liquid crystal phase at 15 8C. In
addition, when the mesogenic units 2a–2b were in the liquid
crystal state, threadlike texture, the typical of nematic phase,
was observed using the crossed polarizer as shown in Fig. 2.
These data indicated that 2b and 2c were enantiotrpic
nematic liquid crystals, and 2a was monotropic nematic
liquid crystal with its mesogenic phase observed only on
supercooling.
low enough. In our studies, however, the DSC curves of
these three-armed star-shaped liquid crystals 3a–3c showed
no crystallization, but vitrifying into a glass state, like most
polymers, during cooling from the isotropic melt. Further-
more, the glass state was quite stable and no melting sharp
peak was observed when the glass was reheated. This was
probably due to steric hindrance which hindered the
crystallization of these star-shaped molecules.
3.3. Thermal properties of three-armed
star-shaped liquid crystals
The thermal properties of three-armed star-shaped liquid
crystals 3a–3c evaluated by DSC were summarized in
Table 3 and the representative DSC thermograms were
shown in Fig. 3. The glass transition temperatures were at
13.2–21.9 8C and 3a–3c had no melting point. As we know,
liquid crystalline polymers, such as side-chain liquid-
crystalline polymers, are atactic and their systems disorder
induces vitrification rather than crystallization during cool-
ing. In contrast, low molecular mass compounds with high
molecular order usually crystallize when the temperature is
Fig. 2. Polarized optical micrograph of 2b (200!). (a) Threadlike texture
of 2b at heating to 120 8C. (b) Threadlike texture of 2b at cooling to 111 8C.

B.-Y. Zhang et al. / Journal of Molecular Structure 741 (2005) 135–140
139
Table 3
Phase transition temperatures of star-shaped liquid crystals
a
b
c
Td
star-shaped liquid
crystals
n
Transition temperature/8C (corresponding enthalpy
changes/J gK1), heating/cooling
DT1
DT2
3a
3b
3c
1
2
3
g13.2 (0.2)dN86.3 (1.4)I/I84.1 (1.5)N7.7 (0.3)dg
g21.9 (0.3)dN126.8 (1.7)I/I125.6 (1.9)N17.1 (0.6)dg
g16.4 (0.6)dN123.6 (5.2)I/I121.9 (5.4)N11.3 (0.8)dg
73.1
104.9
107.2
76.4
108.5
110.6
353
349
357
g, glass state; N, nematic; I, isotropic.
a
Mesophase temperature ranges on heating cycle.
Mesophase temperature ranges on cooling cycle.
Temperature at which 5% weight loss occurred.
DCp in J/(gk).
b
c
d
As summarized in Table 3, 3a–3c were different from
exhibited enantiotropic nematic phases on heating and
cooling cycles (Fig. 4). Taking star-shaped liquid crystal
3b for example, it was in liquid crystalline phase at room
temperature due to its low glass transition temperature,
but with the increase of the temperature, the sample
flowed quickly, showed typical nematic threadlike textures
gradually. However, the texture disappeared at 127 8C, but
nematic droplets appeard followed by threadlike texture
when the isotropic state was cooled to 125 8C. The texture
did not change until the temperature reached room
temperature.
the mesogenic units 2a–2c that they showed enantiotropic
behaviour and a wide mesogenic region. On heating trace,
the mesogenic region of 3a–3c was extended to 73.1, 104.9
and 107.2 8C, respectively. Among them, 3a exhibited the
narrowest while 3c displayed the widest mesophase
temperature range. The reason could be that the terminal
polarity group played an important role during the liquid
crystal phase formation and it maintained the molecular
orientation through the acting forces of molecular induc-
tion and polarization. With the extension of terminal
alkoxy chain (from nZ1 to 3), the molecular cooperative
packing was enhanced and the structural anisotropy was
increased.
X-ray diffraction measurements can provide more
detailed information on the liquid crystalline phase
structures. The quenched samples of three-armed star-
shaped liquid crystals 3a–3c were studied by wide angle
X-ray diffraction (WAXD) and small angle X-ray scattering
(SAXS). Data showed amorphous diffuse peaks at about 2q
of 208, but no sharp peak in the lower Bragg angle region.
This result suggested that 3a–3c exhibited only nematic
mesophases [13], which was consistent with their optical
textures.
The phase transition temperatures displayed in the
Table 3 were reversible and did not change on repeated
heating and cooling cycles. Meanwhile, TGA results
showed that the temperatures were higher than 340 8C
when 5% weight loss occurred (Td), indicating a high
thermal stability of synthesized star-shaped liquid crystals.
In general, the nematic state is restricted to rod-like
molecules which spontaneously order with their (for
calamitic molecules) long axes roughly parallel or restricted
to discotic molecules whose structures are similar to the
calamitic nematic, although in this case the short axes of
3.4. Texture analysis of three-armed
star-shaped liquid crystals
Studies on the textures of three-armed star-shaped
liquid crystals 3a–3c using POM showed that 3a–3c
DSC /(mW/mg)
↓
exo
0.400
0.300
0.200
0.100
0
–0.100
–0.200
–0.300
–0.400
–0.500
0
20
40
60
80
100
120
140
Temperature /°C
Fig. 3. DSC thermograms of the star-shaped liquid crystal 3b.

140
B.-Y. Zhang et al. / Journal of Molecular Structure 741 (2005) 135–140
give rise to the appearance of a nematic liquid crystalline
order (Fig. 5).
4. Conclusion
In this work, a novel three-armed star-shaped liquid
crystals 3a–3c has been synthesized. These molecules do
not crystallize but vitrify to form stable supercooled
mesophase when cooled so that they can be applied as
glassy liquid crystals. The synthesized star-shaped liquid
crystals 3a–3c show wide mesomorphic temperature ranges
and high thermal stability. In addition, the mesomorphic
temperature ranges increase as the terminal alkoxy chain
extends and the liquid crystals exhibit threadlike optical
texture, a typical nematic liquid crystal phase, in heating and
cooling cycles.
Acknowledgements
The authors are grateful to National Natural Science
Fundamental Committee of China and HI-Tech Research
and development program (863) of China, National Basic
Research Priorities Programme (973) of China, and Science
and Technology Research Major Project of Ministry of
Education of China for financial support of this work.
Fig. 4. Polarized optical micrograph of 3b (200!). (a) Threadlike texture
of 3b at heating to 98 8C. (b) Droplets texture of 3b at cooling to 124 8C.
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