Synthesis, characterization, and electro-optic properties of novel bent-core mesogens based on cinnamoyloxy derivatives

Article abstract of DOI:10.1080/15421406.2010.526563

The new series of bent-core molecules based on a 3,4'-biphenyldiol central core has been synthesized. The influence of the central core as well as that of a fluorine and chlorine substituent on the diphenyl rings in the bent-core compound has also been ex

Full text of DOI:10.1080/15421406.2010.526563

Mol. Cryst. Liq. Cryst., Vol. 534: pp. 95–113, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2010.526563
Synthesis, Characterization, and Electro-Optic
Properties of Novel Bent-Core Mesogens Based
on Cinnamoyloxy Derivatives
CHIEN-TUNG LIAO,1 JIUNN-YIH LEE,1 AND
CHIU-CHUN LAI2
1Department of Polymer Engineering, National Taiwan University of
Science and Technology, Taipei, Taiwan, ROC
2Department of Textile Engineering, Chinese Culture University, Taipei,
Taiwan, ROC
The new series of bent-core molecules based on a 3,40-biphenyldiol central core has
been synthesized. The influence of the central core as well as that of a fluorine and
chlorine substituent on the diphenyl rings in the bent-core compound has also been
examined. A few novel series of bent-core compounds containing cinnamoyloxy or
a-methylcinnamoyloxy groups in the side arms have been synthesized, and the meso-
phases exhibited by them have been compared. The mesophases under discussion were
investigated by means of polarizing microscopy, differential scanning calorimetry,
X-ray diffraction, as well as electro-optical experiments. The influence of the molecu-
lar structure on the occurrence of the SmCP phase was investigated. Finally, the
electro-optical properties of the SmCP phase, such as voltage-transmittance effect,
tilt angle, dielectric permittivity, and switching behavior, were also measured. As a
consequence, the correlation between their electro-optical properties and chemical
structures of these compounds is investigated.
Keywords Bent-core mesogens; cinnamoyloxy; dielectric permittivity; tilt angle;
voltage-transmittanceeffect
Introduction
The chiral mesophases could be obtained by using achiral molecules with a bent core
since the proposal by Watanabe et al. [1–9]. The type of liquid crystal has attracted a
great deal of attention. The different liquid crystal-phases, generally denoted as B-type
mesophases, have already been described and reviewed in the literature [10–13].
Together with the B1 mesophase, which is characterized by a columnar disposition
of the molecules, the B2 arrangement, also called the SmCP (smectic C polar) meso-
phase, is the most commonly observed for bent-shaped molecules. It has been stated
that this mesophase can exhibit either ferroelectric or antiferroelectric properties. The
design and synthesis of novel achiral bent-shaped mesogens forming ferroelectric
Address correspondence to Prof. Jiunn-Yih Lee, Department of Polymer Engineering,
National Taiwan University of Science and Technology, Taipei, Taiwan, ROC. E-mail:
jlee@mail.ntust.edu.tw
95

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C.-T. Liao et al.
and=or antiferro-electric smectic or columnar mesophases is one of the most exciting
research activities in the area of thermotropic liquid crystals. Bent-shaped mesogens
broadly require two units: (i) a central unit, which provides the bending angle, and (ii)
two rod-like units with a terminal aliphatic chain that could be attached to the central
unit. In general, the relationship between molecular structure and the mesomorphic
behavior involves different aspects; for example, the position and magnitude of the
bending angle, the number of phenyl rings present in the bent-core, the type and
influence of lateral substituent [14–16], the nature and orientation of linking groups,
and length of the terminal chains. In this article, two new series of symmetrical
bent-shaped mesogens have been synthesized and their mesomorphic properties stud-
ied. We examine some novel series of bent-core compounds containing 3, 40-positions
of a diphenol ring as the central unit with the side wings containing a cinnamoyloxy or
an a-methylcinnamoyloxy group. We investigate the relation between the structure
and the mesomorphic properties. In addition, we add the halogen (F and Cl) in the
30-position of the diphenol and research the affect in the mesomorphic properties.
The electro-optical properties of the SmCP phase is also measured. As a consequence,
the correlation between their electro-optical properties and chemical structures of
these compounds is investigated.
Experimental
Characterization Techniques
1H nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance-
500 spectrometer (National Taiwan University of Science and Technology (Taiwan))
(500 MHz), using CDCl3 as solvents. Elemental analyses for C and H were performed
on a Heraeus Vario EL-III elemental analyzer. The optical textures of mesophases
were characterized by polarizing optical microscopy (POM; model Olympus BH5
(National Taiwan University of Science and Technology (Taiwan))) equipped with
a hot stage (Mettler Toledo FP82HT (National Taiwan University of Science and
Technology (Taiwan))) and a programmable temperature controller (Mettler Toledo
FP90 central processor (National Taiwan University of Science and Technology
(Taiwan))). Temperatures and enthalpies of transitions were determined by differ-
ential scanning calorimetry (DSC; model Perkin Elmer Diamond (National Taiwan
University of Science and Technology (Taiwan))). Powder samples of ca. 3.0 mg were
examined at heating and cooling rates of 5ꢀC minꢁ1 under a nitrogen atmosphere.
Synchrotron powder X-ray diffraction (XRD) measurements were performed in
transmission geometry with synchrotron radiation at beamline BL17A of the
National Synchrotron Radiation Research Center (NSRRC), Taiwan, where the
˚
X-ray wavelength was 1.33366 A. The XRD data were collected using a Mar345
image plate detector mounted orthogonal to the beam with sample-to-detector dis-
tance of 280 mm. The diffraction signals were accumulated for 10 s. The powder
samples were packed into a capillary tube and heated by a heat gun, where the tem-
perature controller is programmable by a PC with a proportional-integral-derivative
(PID) feedback system. The scattering angle theta was calibrated by a mixture of
silver behenate and silicon. For the 2D XRD patterns, the surface-aligned samples
of the mesophases were obtained by very slow cooling (1ꢀC minꢁ1) of a small droplet
of the compound from the isotropic melt on a glass plate treated with commercially
available homeotropic agent.

Novel Bent-Core Mesogens
97
Electro-optical investigations were carried out using commercially available
liquid crystal cells with indium tin oxide (ITO) electrodes coated with antiparallel
rubbed polyimide (from Mesostate Corp. (Taiwan), cell gap ¼ 7.5 mm for active
area ¼ 1 cm2). The sample was filled into the cell in the isotropic phase. A digital
oscilloscope (Yokogawa Electric, DL1640, Taiwan) was used in these measurements,
and a high-power amplifier was connected to an arbitrary function generator
(Tektronix AWG2005). For the switching polarization experiment was measured
by using triangular waveform voltage method under slowly cooling from the iso-
tropic phase. The optical transmission experiments were probed by monochromatic
light using an He-Ne laser beam (632.8 nm). The cell was placed between crossed
polarizers and its orientation was adjusted to obtain the minimum transmission of
light (detected by a photodiode) without any electric field. The photodiode detector
was set at about 20 cm away from the cell sample and the incident laser beam was
normal to the cell. A digital oscilloscope was used in these measurements, and a
high-power amplifier was connected to a function generator and a dc power supply
was utilized in the dc field experiments. During electro-optical measurements, the
modulations of textures by applying electric fields were observed using a polarizing
optical microscope. Finally, we also measured the complex dielectric permittivity
(detected by HP4192A) in the frequency range 102–107 Hz for the planar-aligned
(measuring field is normal to the director) sample on cooling at a rate of about
1ꢀC minꢁ1, keeping the temperature of the sample stable during frequency sweeps.
Materials
All reagents (from Taiwan) and chemicals were purchased from commercial sources
(Alfa (USA), TCI (Japan), and Fluka (USA)) and used as received without further
purification. Tetrahydrofuran (THF) and dichloromethane (CH2Cl2) were distilled
to keep anhydrous before use. Reactions were monitored by thin-layer chromato-
graphy (TLC) on silica gel plates (Merck TLC silica gel 60 F254 aluminum sheets),
which were examined under ultraviolet (UV) light and iodine vapor. Column chroma-
tography was performed using Merck 60-mesh silica gel.
Synthesis
The general synthetic routes of intermediates and target bent-core molecules are
shown in Schemes 1 and 2. The purity and chemical structures of the intermediates
1
and target compound can be easily verified by TLC, H NMR spectroscopy, and
elemental analysis. The synthetic procedures and chemical analyses of each product
are described sequentially below.
4-(Tetradecyloxy)benzaldehyde, 1. This compound was prepared by a modified
literature procedure [17], as a white solid in 84% yield. 1H NMR (300 MHz, CDCl3)
d (ppm): 0.95 (t, J ¼ 6.7 Hz, 3H), 1.21–1.62 (m, 22H), 1.87 (m, 2H), 4.01 (t,
J ¼ 6.6 Hz, 2H), 7.44 (d, J ¼ 8.7 Hz, 2H), 7.82 (d, J ¼ 8.4 Hz, 2H), 9.70 (s, 1H).
Anal. Calc. for C21H34O2: C, 79.19; H, 10.76. Found: C, 79.40; H, 10.66.
(E)-4-tetradecyloxycinnamic acid, 2. A mixture of 4-(tetradecyloxy)benzaldehyde, 1
(10.0 g, 31.4 mmol), malonic acid (4.9 g, 47.1 mmol), and piperidine (1 mL) in dry
pyridine (20 mL) was stirred and heated for 2 h at 60ꢀC and 3 h at 120ꢀC and then

98
C.-T. Liao et al.
Scheme 1. Synthetic routes of intermediates.
cooled down to room temperature. The solution was poured into 3 N HCl (250 mL).
The precipitate was collected by filtration and washed with water. After drying, the
residue was purified by recrystallization from ethanol to give a product as white
1
solid in 87% yield. H NMR (300 MHz, CDCl3) d (ppm): 0.88 (t, J ¼ 6.9 Hz, 3H),
1.23–1.47 (m, 25H), 1.76–1.85 (m, 2H), 3.96 (t, J ¼ 6.6 Hz, 2H), 4.25 (m, 2H), 6.95
(m, 2H), 7.29 (m, 2H), 7.71 (s, 1H). Anal. Calc. for C25H40O3: C, 77.27; H, 10.38.
Found: C, 77.19; H, 10.44.
Ethyl 2-methyl-3-(4-(tetradecyloxy) phenyl) acrylate, 3. To a suspension of sodium
hydride (NaH) (0.97 g, 40.3 mmol, 60% dispersion in mineral oil) in anhydrous
THF (50 mL) under a nitrogen atmosphere was added drop-wise triethyl
2-phosphonopropionate (5.8 g, 24.2 mmol) in THF (20 mL) over 0.5 h, on cooling in

Novel Bent-Core Mesogens
99
Scheme 2. Synthetic routes of target bent-core molecules.
an ice bath. The mixture was stirred for an additional 0.5 h at room temperature to form
a clear solution. After being cooled in an ice bath, compound 1 (7.0 g, 22.0 mmol) in
anhydrous THF (20 mL) was added over 0.5 h. The reaction mixture was allowed to
warm to room temperature for 2h and then quenched with water and extracted with
CH2Cl2. The organic layer was separated and dried over anhydrous magnesium
sulfate. After removal of the solvent by evaporation under reduced pressure, the
residue was purified by column chromatography on silica gel (hexane=ethyl acetate,
1
20:1) to give a product as white solid in 92% yield. H NMR (300 MHz, CDCl3) d
(ppm): 0.86 (t, J ¼ 6.9Hz, 3H), 1.26–1.49 (m, 25H), 1.74–1.83 (m, 2H), 2.13 (s, 3H),
3.98 (t, J ¼ 6.6 Hz, 2H), 4.26 (m, 2H), 6.90 (m, 2H), 7.36 (m, 2H), 7.63 (s, 1H). Anal.
Calc. for C26H42O3: C, 77.56; H, 10.51. Found: C, 77.34; H, 10.34.
2-Methyl-3-(4-(tetradecyloxy) phenyl) acrylic acid, 4. A solution of 3 (6.5 g, mmol)
in ethanol (150 mL) was treated with 5 N solution of potassium hydroxide (KOH).
The reaction mixture was stirred under reflux overnight. The resulting mixture
was acidified with 6 N HCl (200 mL). The white precipitate was collected by
filtration and washed with water. After drying, the residue was purified by

100
C.-T. Liao et al.
recrystallization from ethanol to give a product as white solid in 93% yield. 1H NMR
(300 MHz, CDCl3) d (ppm): 0.88 (t, J ¼ 6.9 Hz, 3H), 1.26–1.46 (m, 22H), 1.74–1.82
(m, 2H), 2.15 (s, 3H), 3.99 (t, J ¼ 6.6 Hz, 2H), 6.93 (d, J ¼ 8.7 Hz, 2H), 7.42 (d,
J ¼ 9.0 Hz, 2H), 7.70 (s, 1H). Anal. Calc. for C24H38O3: C, 76.96; H, 10.23.
Found: C, 76.88; H, 10.33.
4-Benzyloxybenzoic acid, 6. This compound was prepared according to published
procedures [18], as a white solid in 85% yield. 1H NMR (300 MHz, CDCl3) d
(ppm): 5.50 (s, 2H), 7.03 (d, J ¼ 8.4 Hz, 2H), 7.25 (m, 5H), 8.15 (d, J ¼ 8.7 Hz,
2H). Anal. Calc. for C14H12O3: C, 73.67; H, 5.30. Found: C, 73.58; H, 5.45.
3,40-Biphenyl bis(4-benzyloxybenzoate), 8a. To a suspension of compounds 6
(6.82 g, 29.9 mmol) and 7a (2.5 g, 13.6 mmol), dissolved in dry dichloromethane
(100 mL), N,N-dicyclohexylcarbodiimide (DCC; 6.2 g, 29.9 mmol) and 4-(N,N-
dimethylamino) pyridine (DMAP; 0.33 g, 2.72 mmol) were added to react under
nitrogen. The reaction mixture was stirred for 12 h at room temperature. The
precipitated dicyclohexylurea (DCU) was filtered off and washed with an excess of
dichloromethane (50 mL). The filtrate was washed with water and dried over
anhydrous magnesium sulfate. After removal of the solvent by evaporation under
reduced pressure, the residue was purified by column chromatography on silica gel
using chloroform as an eluent. The collected product was crystallized from a
mixture of dichloromethane and 2-propanol to give 8a as a white solid in 84%
1
yield. H NMR (300 MHz, CDCl3) d (ppm): 5.17 (s, 4H), 7.07 (d, J ¼ 8.7 Hz, 4H),
7.18–7.50 (m, 16H), 7.64 (d, J ¼ 9.0 Hz, 2H), 8.16–8.20 (m, 4H). Anal. Calc. for
C40H30O6: C, 79.19; H, 4.98. Found: C, 79.13; H, 5.01.
30-Fluoro-3,40-biphenyl bis(4-benzyloxybenzoate), 8b. This compound was obtained
from the reaction of compounds 6 and 7b by using a similar procedure as described
1
for compound 8a. The product was isolated as a white solid in 80% yield. H NMR
(300 MHz, CDCl3) d (ppm): 5.16 (s, 4H), 7.08 (d, J ¼ 8.7 Hz, 4H), 7.32–7.54
(m, 17H), 8.19 (d, J ¼ 8.7 Hz, 4H). Anal. Calc. for C40H29FO6: C, 76.91; H, 4.68.
Found: C, 76.83; H, 4.87.
30-Chloro-3,40-biphenyl bis(4-benzyloxybenzoate), 8c. This compound was obtained
from the reaction of compounds 6 and 7c, following a similar procedure as
described for compound 8a. The product was isolated as a white solid in 89%
yield. 1H NMR (300 MHz, CDCl3) d (ppm): 5.16 (s, 4H), 7.04–7.08 (m, 4H),
7.20–7.61 (m, 17H), 8.11–8.22 (m, 4H). Anal. Calc. for C40H29ClO6: C, 74.93; H,
4.56. Found: C, 74.88; H, 4.67.
3,40-Biphenyl bis(4-hydroxybenzoate), 9a. A mixture of compound 8a (5.7 g,
9.4 mmol) was dissolved in THF (150 mL) containing a suspension of 10% Pd-C
catalyst (1 g). The mixture was stirred under a hydrogen atmosphere at room
temperature until no further hydrogen was taken up (ca. 10 h). The catalyst was
removed by filtration through Celite and washed with an excess of THF (150 mL).
The solvent was removed by evaporation under reduced pressure and the crude
product was purified by column chromatography on silica gel (chloroform=ethyl
1
acetate, 50:1) to give a white solid in 90% yield. H NMR (300 MHz, CDCl3) d
(ppm): 6.55 (d, J ¼ 8.4 Hz, 4H), 7.26 (m, 1H), 7.36–7.69 (m, 7H), 8.06 (m, 4H),
10.49 (s, 2H). Anal. Calc. for C26H18O6: C, 73.23; H, 4.25. Found: C, 73.13; H, 4.34.

Novel Bent-Core Mesogens
101
30-Fluoro-3,40-biphenyl bis(4-hydroxybenzoate), 9b. This compound was obtained
from compound 8b, following a similar procedure as described for compound 9a.
The product was isolated as a white solid in 75% yield. 1H NMR (300 MHz,
CDCl3) d (ppm): 6.93 (m, 4H), 7.28 (m, 1H), 7.46–7.83 (m, 6H), 8.00 (m, 4H),
10.65 (s, 2H). Anal. Calc. for C26H17FO6: C, 70.27; H, 3.86. Found: C, 70.23; H, 3.97.
30-Chloro-3,40-biphenyl bis(4-hydroxybenzoate), 9c. This compound was obtained
from compound 8c, following a similar procedure as described for compound 9a.
The product was isolated as a white solid in 80% yield. 1H NMR (300 MHz,
CDCl3) d (ppm): 6.95 (m, 4H), 7.24 (m, 1H), 7.49–7.85 (m, 6H), 8.03 (m, 4H),
10.53 (s, 2H). Anal. Calc. for C26H17ClO6: C, 67.76; H, 3.72. Found: C, 67.90; H, 3.50.
Compounds of the series DB and MD were prepared by a similar method as
described for compound 8a. After workup, the crude products were purified by
column chromatography on silica gel using chloroform as an eluent and twice recrys-
tallization from a mixture of chloroform and ethanol as a white solid. The yields of
these products after purification were ca. 57–71%.
Biphenyl-3,40-diyl bis(4-(3-(4-(tetradecyloxy) phenyl) acryloyloxy) benzoate,
DB-H. The compound was prepared from compounds 2 and 9a. The product was
1
isolated as a white solid in 76% yield. H NMR (300 MHz, CDCl3) d (ppm): 0.88
(t, J ¼ 6.9 Hz, 6H), 1.20–1.47 (m, 44H), 1.81 (m, 4H), 4.01 (t, J ¼ 6.6 Hz, 4H), 6.48
(s, 1H), 6.53 (s, 1H), 6.94 (d, J ¼ 8.7 Hz, 4H), 7.22–7.36 (m, 7H), 7.45–7.68 (m,
9H), 7.84 (s, 1H), 7.89 (s, 1H), 8.29 (m, 4H, Ar-H). Anal. Calc. for C72H86O10: C,
77.81; H, 7.80. Found: C, 77.73; H, 7.75.
30-Fluorobiphenyl-3,40-diyl bis(4-(3-(4-(tetradecyloxy) phenyl) acryloyloxy)
benzoate, DB-F. The compound was prepared from 2 and 9b. The product was
1
isolated as a white solid in 72% yield. H NMR (300 MHz, CDCl3) d (ppm): 0.89
(t, J ¼ 6.3 Hz, 6H), 1.20–1.55 (m, 44H), 1.81 (m, 4H), 4.01 (t, J ¼ 6.6 Hz, 4H), 6.48
(s, 1H), 6.53 (s, 1H), 6.94 (d, J ¼ 9.0 Hz, 4H), 7.23–7.56 (m, 15H), 7.84 (s, 1H),
7.90 (s, 1H), 8.27–8.31 (m, 4H). Anal. Calc. for C72H85FO10: C, 76.57; H, 7.59.
Found: C, 76.51; H, 7.52.
30-Chlorobiphenyl-3,40-diyl bis(4-(3-(4-(tetradecyloxy) phenyl) acryloyloxy)
benzoate, DB-Cl. The compound was prepared from 2 and 9c. The product was
1
isolated as a white solid in 78% yield. H NMR (300 MHz, CDCl3) d (ppm): 0.89
(t, J ¼ 6.3 Hz, 6H), 1.21–1.49 (m, 44H), 1.81 (m, 4H), 4.01 (t, J ¼ 6.6 Hz, 4H), 6.48
(s, 1H), 6.53 (s, 1H), 6.94 (d, J ¼ 8.7 Hz, 4H), 7.24–7.58 (m, 15H), 7.84 (s, 1H),
7.90 (s, 1H), 8.29 (m, 4H). Anal. Calc. for C72H85ClO10: C, 75.47; H, 7.48.
Found: C, 75.43; H, 7.56.
Biphenyl-3,40-diyl bis(4-(2-methyl-3-(4-(tetradecyloxy) acryloyloxy) benzoate,
MD-H. The compound was prepared from 4 and 9a. The product was isolated as
a white solid in 82% yield. 1H NMR (300 MHz, CDCl3) d (ppm): 0.88 (t,
J ¼ 6.9 Hz, 6H), 1.27–1.49 (m, 44H), 1.81 (m, 4H), 2.26 (s, 6H), 4.01 (t, J ¼ 6.6 Hz,
4H), 6.95 (d, J ¼ 8.7 Hz, 4H), 7.21–7.53 (m, 14H), 7.61–7.69 (m, 2H), 7.92 (s, 2H),
8.29 (m, 4H). Anal. Calc. for C74H89O10: C, 77.79; H, 7.75. Found: C, 77.63; H, 7.85.
30-Fluorobiphenyl-3,40-diyl
benzoate, MD-F. The compound was prepared from 4 and 9b. The product was
bis(4-(2-methyl-3-(4-(tetradecyloxy)
acryloyloxy)
1
isolated as a white solid in 85% yield. H NMR (300 MHz, CDCl3) d (ppm): 0.88 (t,

102
C.-T. Liao et al.
J ¼ 6.9 Hz, 6H), 1.20–1.56 (m, 44H), 1.81 (m, 4H), 2.28 (s, 6H), 4.01 (t, J ¼ 6.6 Hz,
4H), 6.96 (d, J ¼ 8.7 Hz, 4H), 7.24–7.56 (m, 15H), 7.92 (s, 2H), 8.28–8.31 (m, 4H).
Anal. Calc. for C74H89FO10: C, 76.79; H, 7.75. Found: C, 76.87; H, 7.63.
30-Chlorobiphenyl-3,40-diyl
benzoate, MD-Cl. The compound was prepared from 4 and 9c. The product was
bis(4-(2-methyl-3-(4-(tetradecyloxy) acryloyloxy)
1
isolated as a white solid in 80% yield. H NMR (300 MHz, CDCl3) d (ppm): 0.89
(t, J ¼ 6.3 Hz, 6H), 1.27–1.48 (m, 44H), 1.80 (m, 4H), 2.29 (s, 6H), 4.02 (t,
J ¼ 6.6 Hz, 4H), 6.97 (d, J ¼ 8.7 Hz, 4H), 7.27–7.60 (m, 15H), 7.75 (s, 2H),
8.29–8.35 (m, 4H). Anal. Calc. for C74H89ClO10: C, 75.71; H, 7.64. Found: C,
75.56; H, 7.70.
Results and Discussion
Thermal Properties and Optical Textures
The phase behaviors of all compounds were investigated by combining different
techniques, including POM, DSC, and XRD. Phase transition temperatures
observed by POM match well with the corresponding DSC thermograms. The meso-
morphic properties of the studied compounds, the phase sequence, phase transition
temperatures, and enthalpy data are summarized in Table 1. Upon cooling form the
isotropic state, the compound DB-H displays a characteristic Schlieren texture and
broken fan textures of the mesophase at 110ꢀC (see Fig. 1a), indicating the formation
of the SmCP phase. The other compounds show similar optical texture, thus imply-
ing that they have the same mesophase. All of the compounds exhibit enantiotropic
mesomorphic behavior and only a mesophase. A typical DSC curve of compound
DB-H is shown in Fig. 2a, which shows that one mesophase can be obtained and that
there are no significant differences for different heating or cooling rates. In the DSC
Table 1. Phase transition temperatures (ꢀC) and enthalpies (kJ molꢁ1; in parentheses)
for the bent-core compounds as determined by DSC (scan rate¼ 5ꢀC minꢁ1 a
)
Second heating
first cooling
Second heating
first cooling
Compound
R
H
Cr
SmCP
I
DB-H
.
119.2 (29.5)
98.0 (27.8)
130.6 (26.7)
109.8 (28.0)
111.2 (30.9)
56.2 (21.0)
99.6 (19.3)
73.1 (18.8)
89.4 (23.6)
45.8 (18.1)
58.9 (22.3)
39.6 (21.2)
.
153.9 (15.2)
151.4 (14.2)
149.9 (13.1)
142.9 (13.1)
127.9 (14.8)
125.1 (14.2)
131.5 (15.5)
125.5 (15.8)
119.6 (12.3)
112.3 (11.9)
97.8 (12.5)
91.8 (12.3)
.
DB-F
F
Cl
H
F
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
DB-Cl
MD-H
MD-F
MD-Cl
Cl
aAbbreviations: Cr ¼ crystalline phase; SmCP ¼ polar SmC phase (i.e., B2 phase);
I ¼ isotropic phase (. ¼ phase exists).

Novel Bent-Core Mesogens
103
Figure 1. Polarized optical microscopic textures obtained Schlieren with broken fan-shaped
textures in the SmCP phase on cooling (1ꢀC minꢁ1) from the isotropic state for (a) compound
DB-H at 110ꢀC, (b) compound DB-F at 136ꢀC, (c) compound DB-Cl at 123ꢀC, (d) compound
MD-H at 119ꢀC, (e) compound MD-F at 72ꢀC, and (f) compound MD-Cl at 70ꢀC. (Arrows
are the directions of polarizers and analyzers.).
thermograms, the enthalpy values obtained for the SmCP mesophase-to-isotropic
phase transition were in the range of about 11.9 to 15.8 kJ molꢁ1 (see Table 1).
Comparison of compounds DB-H and DB-F, shows that the compound DB-Cl
is due to the lowest clearing transition temperature upon cooling from the isotropic
liquid. In this case, the chlorine atom has a larger size atom than the fluorine and
hydrogen atoms. As a result, when introduction of the lateral substituent was at
the biphenyl core into the 30-position, the clearing temperature of the compound
was decreased. Thus, size of the lateral substituent has a profound influence on their
mesomorphic behavior. However, series MD, which contain an a-methylcinnamoy-
loxy group in the side wings, showed smaller clearing transition temperature than
series DB. The presence of a methyl group in the cinnamoyl moiety of these five-ring
esters completely destroys the mesophase, probably due to steric reasons.

104
C.-T. Liao et al.
Figure 2. DSC curves of compound DB-H obtained at a rate of 5ꢀC minꢁ1: (a) second heating
and (b) first cooling cycle with respective transition temperatures.
X-ray Diffraction Studies
To further identify the type of mesophases, the determination of structural para-
meter has been further performed by XRD. The theoretical molecular length of
DB-F determined from molecular model estimated using CS Chem3D Ultra 7.0
software (National Taiwan University of Science and Technology, Taiwan) gave
˚
molecular length of ca. 55.0 A, where we suppose that the molecular structure is
coplanar and the alkyl chains are in the fully extended all-trans conformation. As
shown in Fig. 3a, the 2D diffraction pattern including two symmetric intense arcs
in the small-angle region on the meridian XRD pattern of compound DB-F and
the data were integrated in Fig. 3b, as an illustrative example. The resulting XRD
pattern of compound DB-F at 110ꢀC exhibits three Bragg reflections in the
small-angle region at 2h ¼ 1.78, 3.54, and 5.39ꢀ with the d-spacing of 42.5, 21.6,
˚
and 14.2 A, respectively, corresponding to the reciprocal spacing in the ratios 1, 2,
and 3 and to the indexation (hk) ¼ (10), (20), and (30), which can imply the forma-
tion of a layered structure. On the other hand, a diffuse scattering halo in the
ꢀ
˚
wide-angle region centered at 2h around 16.91 with d-spacing of ca. 4.5 A indicates
a liquid-like arrangement of the molecules within the layers. As shown in Fig. 3c, the
temperature dependence of the layer d-spacing of compound DB-F in the SmCP
phase was recorded upon cooling from the isotropic phase. The values of the layer
d-spacing only change slightly within the temperature range of the SmCP phase.
˚
Therefore, the layer d-spacing value of 42.5 A for compound DB-F by XRD
˚
measurement is smaller than the calculated theoretical molecular length of 55.0 A,
which suggests a tilted arrangement of the smectic structure; that is, smectic C phase
with tilt angle of 39.4ꢀ along the layer normal direction. Series DB and MD also
revealed similar diffraction pattern at temperatures within the respective mesophase
temperature range. The d-spacing and tilt angle of series DB were smaller than
the series MD (see Table 2) due to the steric effect of the methyl group in the
series MD.

Novel Bent-Core Mesogens
105
Figure 3. (a) 2D XRD pattern of a partially surface-aligned sample of compound DB-F
obtained in the SmCP phase at 110ꢀC (upon cooling from the isotropic phase). (b) X-ray
diffraction intensity against angle profile obtained in the SmCP phase of (DB-F) at 110ꢀC
and (c) layer d-spacing as a function of the temperature dependence (upon cooling from the
isotropic phase).
Table 2. Summary data of series MD and DB
Compound
Mesophase
Parameters (A)
Measured Spacings (A)
Tilt angle (ꢀ)
Optical tilt angle (ꢀ)
Switching time (ms)a
Maximum Ps=nC (cmꢁ2
DB-H
DB-F
DB-Cl MD-H MD-F MD-Cl
SmCP SmCP SmCP
SmCP SmCP
SmCP
57.0
43.8
39.7
43
˚
55.0
42.3
39.7
40
15.4
890
55.0
42.5
39.4
40
18.0
833
55.0
42.6
39.2
42
57.0
43.7
39.9
42
57.0
43.9
39.6
42
˚
32.2
800
15.9
903
32.0
850
61.7
804
)
aThe data were measured in the range TC – T ¼ 15ꢀC.

106
C.-T. Liao et al.
Electro-Optical Behavior
The switching behavior has been observed in the SmCP phase of the studied com-
pound MD-F using the triangular wave method. Two distinct sharp peaks per half
period of an applied triangular wave voltage are clearly seen under a triangular wave
field of about 180 Vpp at 60 Hz (see Fig. 4a). This is a strong indication of an
antiferroelectric switching process. The temperature dependence of the magnitude
of the Ps value for the compound MD-F obtained by integrating the area under
the two peaks is shown in Fig. 4b and spontaneous polarization (Ps) is about 850
nC cmꢁ2 at 75ꢀC. It can be seen that the Ps is dependent on temperature for com-
pound MD-F. A similar tendency was also observed in the other compounds. The
abrupt increase of Ps around the isotropization temperature indicates that the phase
transformation from the isotropic phase to the SmCP phase is a first-order phase
Figure 4. (a) Switching current response observed in the SmCP phase of compound MD-F
at 92ꢀC under the applied triangular wave voltage (7.5 mm antiparallel rubbed polyimide-coated
ITO cell, 180 Vpp, 60 Hz) and (b) spontaneous polarization (Ps) as a function of temperature.

Novel Bent-Core Mesogens
107
transformation. The Ps value exhibits a maximum polarization (see Table 2). Com-
parably, the spontaneous polarization value of series DB (800–890 nC cmꢁ2) without
the methyl group is similar to the analogous series MD (804–903 nC cmꢁ2) which
have methyl group. It seems that an additional methyl group at linking group in
the core is not able to abate rotation of the molecular. Therefore, the order of Ps
values regarding molecular design is as follows: Ps (without lateral substituent at
the biphenyl core) >Ps (fluoro-substituent at the biphenyl core) >Ps (chloro-
substituent at the biphenyl core). It seems that the dipole moment originating in
the core containing the halogen substituent depresses spontaneous polarization.
To study the tilt sense in the SmCP phase, we carried out dc field experiments. A
sample of compound MD-Cl was cooled slowly from the isotropic phase under a dc
voltage of 100 Vpp to induce the chiral domain. Circular domains with dark brushes
making an angle with respect to the analyzer were obtained, which indicates a syncli-
nic tilt of the molecules in a field-induced state. When the polarity of the applied field
Figure 5. Polarizing optical micrographs of the circular domains in the SmCP phase for com-
pound MD-Cl (on antiparallel rubbing direction LC cells with 7.5 mm thickness) is shown.
Chiral domains obtained by various dc fields: 0 V (SmCAPA), 6 V, 12 V, 18 V, 30 V, 35 V,
140 V, ꢁ140 V (SmCSPF) at 85ꢀC in compound MD-Cl. (Arrows are the directions of polar-
izers and analyzers.)

108
C.-T. Liao et al.
was reversed, the orientation of the brushes rotated in an anticlockwise direction and
the mesophase shifted to another ferroelectric state. After removing electric fields, the
extinction crosses were reoriented along the crossed polarizer position in the off state,
indicating an anticlinic tilt in the antiferroelectric ground state (SmCAPA). As shown
in Fig. 5, by applying electric fields with reverse polarities, the extinction crosses
rotated counterclockwise (positive field) and clockwise (negative field). The
extinction crosses in the switched-on state did not coincide with the crossed polarizer
positions in the off state. The angles between the extinction directions and the crossed
polarizer positions are about 43ꢀ for ꢂ35 V at 85ꢀC in compound MD-Cl, which cor-
responds to the optical tilt angle. The inclined position of the majority of dark extinc-
tion brushes obtained under a dc field indicates a field induced synpolar smectic phase
with a predominately uniform tilt direction of the molecules in adjacent layers
(SmCsPF phase). In the highly birefringent domain the extinction crosses are inclined
with the directions of the polarizers by about 40–45ꢀ. Hence, the molecules in these
domains seem to have 40–45ꢀ tilted synclinic organization. The switching behavior
of the extinction crosses of the domains rotated in opposite directions depending
on the sign of the field, which are characteristic field-responsive textures for chiral
domains of the SmCP phase [19, 20]. Furthermore, the switching process can be stud-
ied by measuring the tilt angle as a function of the applied dc electric fields in the
SmCP phase of series MD (at TC ꢁ T ¼ 10ꢀC, respectively), as shown in Fig. 6. The
tilt angles are estimated from the rotating angles between the extinction brushes
and crossed polarizers by applying electric fields, which indicates the rotated phase
structure from the anticlinic-antiferroelectric (SmCAPA) state to the synclinic-
ferroelectric (SmCSPF) state had occurred. The tilt angles of series MD rise with
increasing the applied voltage and saturate at the fields of ꢃ35 V. In addition, both
series DB and MD have similar tilt angles at lower applied voltages and similar satu-
rated tilt angles around 40ꢀ, as shown in Table 2. However, comparing materials with
series DB and MD, the compound containing the halogen-substituent has a similar
tilt angle. The results also show that, in general, the halogen-substituted materials
Figure 6. Tilt angle of the SmCP phase as a function of the dc electric fields in compounds (on
antiparallel rubbing direction LC cells with 7.5 mm thickness) respectively.

Novel Bent-Core Mesogens
109
have similar tilt angles as nonsubstituted materials. Thus, it can be demonstrated that
the halogen-substituent materials decrease the spontaneous polarization but the tilt
angle is similar.
In order to investigate the dynamics of the polarization switching behavior, the
switching currents of compound DB-Cl were measured with an applied square wave
field (as Vpp ¼ 150 V, f ¼ 10 Hz) in the SmCP phase (at 85ꢀC upon cooling from the
isotropic phase). The switching current curves of compound DB-Cl at 85ꢀC are
Figure 7. (a) Time dependences of the switching current under a square wave field for
compound DB-Cl at 85ꢀC and (b) switching time as a function of temperature for compound
DB-Cl (on antiparallel rubbing direction LC cells with 7.5 mm thickness as Vpp ¼ 150 V,
f ¼ 10 Hz).

110
C.-T. Liao et al.
shown in Fig. 7a as a function of time. The switching time (s) values are determined
from the time elapsed between the appearance of the maximum of the current signal
and the field reversal [21,22]. The switching time as a function of the temperature is
shown in Fig. 7b with the same field and frequency (i.e., Vpp ¼ 150 V, f ¼ 10 Hz).
The decrease of the switching time with increasing temperature for compound DB-Cl
(in the range of 600–17 ms) was observed due to the viscosity decrease. The switching
time values were measured at the temperature of TC ꢁ T ¼ 15ꢀC (see Table 2).
Comparably, the switching time value of series DB (15–32 ms) is smaller than that
of the analogous series MD (16–62 ms). Therefore, the order of switching time values
regarding molecular design is as follows: without lateral substituent at the biphenyl
core < fluoro-substituent at the biphenyl core < chloro-substituent at the biphenyl
core. The results show that switching time may be influenced by viscosity as a
decrease of the switching time with a decrease in viscosity. Therefore, the viscosity
decreases due to the substituent destruction of the molecular interactions.
Finally, we also measured the complex dielectric permittivity in the frequency
range 102ꢁ106 Hz for the planar-aligned (measuring field is normal to the director)
sample on cooling at a rate of about 1ꢀC minꢁ1, keeping the temperature of the sam-
ple stable during frequency sweeps. The temperature dependence of the relative
dielectric permittivity (E0) of compound DB-H at various frequencies [23] is shown
in Fig. 8. The large variation in E0 with frequency in the SmCP phases is due to
the dielectric relaxation modes present in the phase. The large change in the value
of E0 at Iso to SmCP phase transitions is due to the transformation of the paraelectric
structure into an antiferroelectric structure. As the result, highest value of E0 is
observed at ꢃ19.4 in the SmCP phase at 100 Hz. Figures 9a and 9b show the fre-
quency dispersions of real and imaginary parts of dielectric constants, E0 and E00 in
the SmCP phase of compounds DB-H and MD-H. A relaxation mode was detected
at about 500 k Hz. A similar result has also been observed in another bent-core struc-
ture [24–26]. The results were tentatively explained such that relaxation is due to the
rotation around the molecular short axis [27]. The explanation was based on the
Figure 8. Temperature dependence of the dielectric constant (E0) at 102, 103, 104, 105, and
106 Hz for MD-H.

Novel Bent-Core Mesogens
111
Figure 9. Frequency dispersions of real and imaginary parts of dielectric constants, E0 (.) and
E00 ( ), (a) (b) without a dc bias field and (c) (d) under a 35 V dc bias field in the SmCP phase of
ꢀ
compound DB-H and MD-H at TC – T ¼ 25ꢀC.
Figure 10. Cole-Cole plots without a dc bias field and under a 35 V dc bias field in the SmCP
phase of compound MD-H at TC ꢁ T ¼ 25ꢀC.

112
C.-T. Liao et al.
untilted ferroelectric structure instead of the tilted antiferroelectric one. The relax-
ation was explained as the reorientation around the long axis. As mentioned above,
the SmCP phase was confirmed to be antiferroelectric and the dielectric response due
to the Goldstone mode was essentially small in the absence of a dc bias field, because
net polarization in neighboring layers was canceled out [28]. In order to understand
the relaxation mode in detail, a dc bias field was applied [29] (Figs. 9c and 9d). When
a 35 V dc bias field was applied, the antiferroelectric ordering was distorted due to
the molecular rotation on the cone, so that a net polarization emerged. The dielectric
strength increased in the higher frequency and an additional relaxation was detected
in the lower frequency range. The Cole-Cole plot (E00 versus E0) provides valuable
information with regard to the dielectric relaxation process. The relaxation process
of compound MD-H was observed from the Cole–Cole plots (see Fig. 10) at various
biases (0 and 35 V). The relaxation strength (diameter of the circle) increased with
added bias. The center of the circles was shifted along the E0 axis to larger E0 values
with added bias. The result that the dielectric strength of a small relaxation
mode increased with adding dc bias field was explained by the distortion of the
antiferroelectric ordering.
Conclusions
We have achieved the molecular design, synthesis, and characterization of bent-core
liquid crystals that have different linking groups (a-methylcinnamoyloxy group and
cinnamoyloxy group) and different lateral substituents (fluorine and chlorine).
Specifically, we have explored the possibility of stabilizing a wide thermal range
SmCP phase. It exhibits the SmCP phase over a 60ꢀC thermal range. Electro-optic
studies were carried out for the majority of the compounds exhibiting the SmCP
phase. Electrical switching and molecular tilt angle as functions of temperature were
investigated in the SmCP phase. The tilt angle of compound MD-Cl can be up to 43ꢀ
at low fields (40 V), and the switching time of all target compounds were very fast at
150 Vpp in the SmCP phase. Finally, we observed the relaxation process from the
Cole–Cole plots.
Acknowledgment
We thank Dr. Jey-Jau Lee for help with the XRD experiments at theNational
Synchrotron Radiation Research Center (NSRRC, for the beamline BL17A) in
Taiwan. Financial support from the National Science Council of Taiwan (ROC)
through NSC 98-2221-E-011-004 is gratefully acknowledged.
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