Synthesis, characterization, and electro-optic properties of novel ferroelectric liquid crystal with a large tilt angle and wide temperature range of chiral smectic C phase

Article abstract of DOI:10.1080/15421406.2010.526470

The present article is a study of the ferroelectric behavior in low-molar-mass organosiloxane liquid-crystal materials classed. A few novel compounds with large tilt angle and wide temperature range of the SmC* phase have been synthesized, and the mesophases 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 molecular structure on the occurrence of the SmC* phase was investigated. Finally, the electro-optical properties of the SmC* phase, such as 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. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15421406.2010.526470

This article was downloaded by: [Purdue University]
On: 21 March 2013, At: 07:01
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Molecular Crystals and Liquid Crystals
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/gmcl20
Synthesis, Characterization, and Electro-
Optic Properties of Novel Ferroelectric
Liquid Crystal with a Large Tilt Angle
and Wide Temperature Range of Chiral
Smectic C Phase
Chien-Tung Liao a , Jiunn-Yih Lee a & Chiu-Chun Lai b
a Department of Polymer Engineering, National Taiwan University of
Science and Technology, Taipei, Taiwan, ROC
b Department of Textile Engineering, Chinese Culture University,
Taipei, Taiwan, ROC
Version of record first published: 18 Jan 2011.
To cite this article: Chien-Tung Liao , Jiunn-Yih Lee & Chiu-Chun Lai (2011): Synthesis,
Characterization, and Electro-Optic Properties of Novel Ferroelectric Liquid Crystal with a Large Tilt
Angle and Wide Temperature Range of Chiral Smectic C Phase, Molecular Crystals and Liquid Crystals,
534:1, 3-18
To link to this article: http://dx.doi.org/10.1080/15421406.2010.526470
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation
that the contents will be complete or accurate or up to date. The accuracy of any
instructions, formulae, and drug doses should be independently verified with primary
sources. The publisher shall not be liable for any loss, actions, claims, proceedings,
demand, or costs or damages whatsoever or howsoever caused arising directly or
indirectly in connection with or arising out of the use of this material.

Mol. Cryst. Liq. Cryst., Vol. 534: pp. 3–18, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2010.526470
Synthesis, Characterization, and Electro-Optic
Properties of Novel Ferroelectric Liquid Crystal
with a Large Tilt Angle and Wide Temperature
Range of Chiral Smectic C Phase
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 present article is a study of the ferroelectric behavior in low-molar-mass
organosiloxane liquid-crystal materials classed. A few novel compounds with large
tilt angle and wide temperature range of the SmCꢀ phase have been synthesized,
and the mesophases 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 molecular structure on the occurrence of the SmCꢀ phase
was investigated. Finally, the electro-optical properties of the SmCꢀ phase, such
as tilt angle, dielectric permittivity, and switching behavior, were also measured.
As a consequence, the correlation between their electro-optical properties and chemi-
cal structures of these compounds is investigated.
Keywords Chemical reactions; optical microscopy; thermal analysis
Introduction
Ferroelectric liquid-crystal compounds (FLC) of large tilt angle that are useful for
high-speed modulation or switching of optical radiation were developed. Such
FLC are useful in total internal reflection (TIR) switching devices. When an applied
DC voltage rotates the molecules through about a 90ꢁ angle, which changes the per-
ceived refractive index at the FLC layer and permits rapid optical switching in such
TIR switches. For an FLC material to be useful in tilt angle h of approximately 45ꢁ,
since the dipole molecule rotates through an angle of about 2h upon application of a
dc field to the cell. When designing new liquid-crystal molecules with large tilt angle
and wide temperature range, it is necessary to have in mind that their mesogenic
behavior is strongly influenced by the structure of the rigid molecular core [1].
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
3

4
C.-T. Liao et al.
During the last few years, several series of ferroelectric liquid-crystalline materials
containing a 4-hydroxyphenyl 40-hydroxybiphenyl-4-carboxylate core and chiral
octan-2-ol unit have been synthesized [2] and studied with the aim of obtaining
new materials responding to the application demands and contributing to a better
understanding of the chemical structure–to–physical property relationship. The
addition of an oligomeric siloxane end-group to a calamitic mesogen has been shown
to promote lamellar organization and the formation of smectic phases due to the
tendency of siloxane and paraffinic groups to nanosegregate into distinct sublayers
[3–13], forming a so-called virtual siloxane backbone that enables dopant compati-
bilization in a way similar to what is achieved with side-chain liquid-crystalline
copolymers [14]. In this article, we report the synthesis and characterization of
new ferroelectric liquid-crystal material with a 4-hydroxyphenyl 40-hydroxybiphenyl-
4-carboxylate core, chiral octan-2-ol unit, and oligomeric siloxane end-group. We
examined the influence of the addition of different lengths of siloxane and achiral
alkyl chains in the end-group on the phase transition. Finally, the properties of
the smectic Cꢀ phase, such as the spontaneous polarization as a function of tempera-
ture, tilt angle, and relative permittivity, have been measured for target compounds.
Experimental
1H Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker
Avance-500 spectrometer (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) equipped with a hot stage (Mettler Toledo FP82HT)
and a programmable temperature controller (Mettler Toledo FP90 central pro-
cessor). Temperatures and enthalpies of transitions were determined by differential
scanning calorimetry (DSC; model Perkin Elmer Diamond). Powder samples of
ca. 3.0 mg were examined at heating and cooling rates of 5ꢁC minꢂ1 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 PID feedback system. The scat-
tering 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 (1ꢁC minꢂ1) of a small droplet of the compound from the isotropic
melt on a glass plate treated with commercially available homeotropic agent.
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., 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 Elect., DL1640) 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 using a triangular waveform

Novel Ferroelectric Liquid Crystal
5
voltage method under slow cooling from the isotropic phase. 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 1ꢁC minꢂ1, keeping the temperature of the
sample stable during frequency sweeps.
The pitch experimental setup and the preparation of the sample with planar and
homogeneous alignment have been described in detail in a previous paper [15]. The
cell thickness was 25 mm. The monodomain chosen for measurement has a dimension
of 2 ꢃ 1 mmꢂ2. The pitch is measured from the whole length of 30 stripes with a
micrometer eye-piece attached to the microscope.
Materials
All reagents and chemicals were purchased from commercial sources (Alfa, TCI, and
Fluka) 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 chromatography (TLC) on silica gel plates
(Merck TLC silica gel 60 F254 aluminum sheets), which were examined under ultra-
violet (UV) light and iodine vapor. Column chromatography was performed using
Merck 60-mesh silica gel.
Synthesis
The general synthetic routes of intermediates and target bent-core molecules are
shown in Scheme 1. The purity and chemical structures of the intermediates and
target compounds can be easily verified by TLC, 1H NMR spectroscopy, and
elemental analysis. The synthetic procedures and chemical analyses of each product
are described sequentially below.
4-(Benzyloxy) Phenol, 1. This compound was prepared according to published
procedures [16], as a white solid in 81% yield. 1H NMR (ppm, CDCl3): 4.55
(s, 1H), 5.00 (s, 2H), 6.76 (d, J ¼ 9.3 Hz, 2H), 8.87 (d, J ¼ 9.0 Hz, 2H), 7.25–7.44
(m, 5H); HRMS (ESI), m=z, 200.0722; Anal. Calc. for C13H12O2: C, 77.98; H,
6.04; Found: C, 77.94; H, 6.36.
1-(Benzyloxy)-4-(Octan-2-yloxy) Benzene, 2. This compound was prepared
1
according to published procedures [17], as a white solid in 61% yield. H NMR
(ppm, CDCl3): 0.84–0.98 (m, 3H), 1.24–1.77 (m, 13H), 4.16–4.26 (m, 1H), 5.01 (d,
J ¼ 6.9 Hz, 2H), 6.77–6.92 (m, 4H), 7.23–7.44 (m, 5H); HRMS (ESI), m=z,
345.2122; Anal. Calc. for C21H28O2: C, 80.73; H, 9.03; Found: C, 80.67; H, 9.15.
4-(Octan-2-yloxy) Phenol, 3. A mixture of compound 2 (3.2 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

6
C.-T. Liao et al.
Scheme 1. Synthetic routes of intermediates and target molecules.
was removed by evaporation under reduced pressure and the crude product was
purified by column chromatography on silica gel (chloroform=ethyl acetate, 50:1)
1
to give a white solid in 93% yield. H NMR (ppm, CDCl3): 0.88 (t, J ¼ 6.6 Hz,
3H), 1.24–1.87 (m, 13H), 4.09–4.23 (m, 1H), 4.53 (s, 1H), 6.72–6.85 (m, 4H);
HRMS (ESI), m=z, 222.1523; Anal. Calc. for C14H22O2: C, 75.63; H, 9.97; Found:
C, 75.52; H, 10.06.
Methyl 40-Hydroxybiphenyl-4-carboxylate, 4. This compound was prepared
1
according to published procedures [18], as a white solid in 95% yield. H NMR
(ppm, CDCl3): 3.94 (s, 3H), 6.94 (d, J ¼ 8.7 Hz, 2H), 7.54 (d, J ¼ 8.7 Hz, 2H), 7.63
(d, J ¼ 8.4 Hz, 2H), 8.09 (d, J ¼ 8.4 Hz, 4H); HRMS (ESI), m=z, 228.0798; Anal.
Calc. for C14H12O3: C, 73.67; H, 5.03; Found: C, 73.78; H, 5.00.
Methyl 40-(Hex-5-enyloxy)Biphenyl-4-carboxylate, 5a. This compound was
1
prepared according to published procedures [18], as a white solid in 62% yield. H
NMR (ppm, CDCl3): 1.60 (m, 2H), 1.83 (m, 2H), 2.13 (m, 2H), 3.86 (s, 3H),
3.93–4.17 (m, 2H), 4.96–5.08 (m, 2H), 5.85 (m, 1H), 6.97 (m, 2H), 7.57 (m, 4H),
8.07 (d, J ¼ 6.0 Hz, 2H); HRMS (ESI), m=z, 310.1582; Anal. Calc. for C20H22O3:
C, 77.39; H, 7.14; Found: C, 77.42; H, 7.12.

Novel Ferroelectric Liquid Crystal
7
Methyl 40-(Undec-10-enyloxy)Biphenyl-4-carboxylate, 5b. This compound was
obtained from the reaction by using the similar procedure as described for
1
compound 5a. The product was isolated as a white solid in 60% yield. H NMR
(ppm, CDCl3): 1.25–1.58 (m, 12H), 1.76–1.83 (m, 2H), 2.01–2.08 (m, 2H), 3.87 (s,
3H), 4.00 (t, J ¼ 6.0 Hz, 2H), 4.91–5.03 (m, 2H), 5.75–5.88 (m, 1H), 6.97 (d,
J ¼ 9.0 Hz, 2H), 7.57 (d, J ¼ 9.0 Hz, 2H), 7.62 (d, J ¼ 9.0 Hz, 2H), 8.06 (d,
J ¼ 9.0 Hz, 2H); HRMS (ESI), m=z, 380.2415; Anal. Calc. for C25H32O3: C, 78.91;
H, 8.48; Found: C, 78.87; H, 8.47.
40-(Hex-5-enyloxy) Biphenyl-4-carboxylic Acid, 6a. This compound was prepared
1
according to published procedures [18], as a white solid in 87% yield. H NMR
(ppm, CDCl3): 1.56 (m, 2H), 1.76 (m, 2H), 2.10 (m, 2H), 4.02 (t, J ¼ 6.3 Hz, 2H),
4.94–5.07 (m, 2H), 5.77–5.89 (m, 1H), 7.05 (d, J ¼ 9.0 Hz, 2H), 6.68 (d, J ¼ 9.0 Hz,
2H), 7.50 (d, J ¼ 8.7 Hz, 2H), 8.98 (d, J ¼ 8.4 Hz, 2H), 12.88 (s, 1H); HRMS
(ESI), m=z, 296.1398; Anal. Calc. for C19H20O3: C, 77.00; H, 6.80; Found: C,
76.98; H, 6.77.
40-(Undec-10-enyloxy) Biphenyl-4-carboxylic Acid, 6b. This compound was
obtained from the reaction by using the similar procedure as described for
1
compound 6a. The product was isolated as a white solid in 90% yield. H NMR
(ppm, CDCl3): 1.27–1.41 (m, 12H), 1.67–1.76 (m, 2H), 1.96–2.03 (m, 2H), 4.00 (t,
J ¼ 6.3 Hz, 2H), 4.90–5.02 (m, 2H), 5.71–5.85 (m, 1H), 7.04 (d, J ¼ 8.7 Hz, 2H),
7.68 (d, J ¼ 9.0 Hz, 2H), 7.75 (d, J ¼ 8.4 Hz, 2H), 7.98 (d, J ¼ 8.4 Hz, 2H), 12.89 (s,
1H); HRMS (ESI), m=z, 366.2168; Anal. Calc. for C24H30O3: C, 78.65; H, 8.25;
Found: C, 78.64; H, 8.11.
4-(Octan-2-yloxy) Phenyl 40-(Hex-5-enyloxy)biphenyl-4-carboxylate, 7a. To
a
suspension of compounds 6a (8.85 g, 29.9 mmol) and 3 (3.0 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 7a as
a white solid in 81% yield. 1H NMR (ppm, CDCl3): 0.89 (t, J ¼ 7.2 Hz, 3H),
1.30–1.80 (m, 14H), 2.12–2.19 (m, 2H), 4.03 (t, J ¼ 6.6 Hz, 2H), 4.30–4.36 (m, 1H),
4.97–5.08 (m, 2H), 5.78–5.91 (m, 1H), 6.99 (d, J ¼ 8.7 Hz, 2H), 7.02 (d, J ¼ 8.7 Hz,
2H), 7.13 (d, J ¼ 9.0 Hz, 2H), 7.61 (d, J ¼ 8.7 Hz, 2H), 7.70 (d, J ¼ 8.4 Hz, 2H),
8.24 (d, J ¼ 8.7 Hz, 2H); HRMS (ESI), m=z, 500.3022; Anal. Calc. for C33H40O4:
C, 79.16; H, 8.05; Found: C, 79.20; H, 8.03.
4-(Octan-2-yloxy)Phenyl 40-(Undec-10-enyloxy)biphenyl-4-carboxylate, 7b. This
compound was obtained from the reaction of compounds 3 and 6b by using the
similar procedure as descried for compound 7b. The product was isolated as a
white solid in 77% yield. 1H NMR (ppm, CDCl3): 0.89 (t, J ¼ 6.3 Hz, 3H),
1.30–1.61 (m, 24H), 1.70–1.86 (m, 3H), 2.01–2.17 (m, 2H), 4.02 (t, J ¼ 6.3 Hz, 2H),
4.30–4.36 (m, 1H), 4.92–5.03 (m, 2H), 5.77–5.89 (m, 1H), 6.94 (d, J ¼ 9.0 Hz, 2H),

8
C.-T. Liao et al.
7.02 (d, J ¼ 8.7 Hz, 2H), 7.13 (d, J ¼ 9.0 Hz, 2H), 7.61 (d, J ¼ 8.7 Hz, 2H), 7.70 (d,
J ¼ 8.4 Hz, 2H), 8.23 (d, J ¼ 8.4 Hz, 2H); HRMS (ESI), m=z, 570.3665; Anal. Calc.
for C38H50O4: C, 79.96; H, 8.83; Found: C, 79.88; H, 8.93.
Target compounds were prepared by a similar method for that described
for compound C6Si2. After workup, the crude products were purified by column
chromatography on silica gel using toluene=ethyl acetate as an eluent and twice
recrystallization from a mixture of 2-propanol as a white liquid crystal. The yields
of these products after purification were ca. 82–87%.
4-(Octan-2-yloxy)
Biphenyl-4-carboxylate, C6Si2. Compound 7a (0.11 mg, 0.225 mmol), 55 mg
1-hydropentamethyldisiloxane (0.25 mmol), and 0.6 mmol platinum
Phenyl
40-[6-(1,1,2,2,2-pentamethyldisiloxanyl)hexyloxy)
divinyltetramethyldisiloxane as catalyst were dissolved in 5 mL of dry toluene in a
flask fitted with a CaCl2 drying tube. The reaction mixture was stirred at room
temperature overnight. Column chromatography on silica gel with toluene=ethyl
acetate gradient as eluant gave pure product. The collected product was
crystallized from a mixture of 2-propanol to give C6Si2 as a white liquid crystal
1
in 82% yield. H NMR (ppm, CDCl3): 0.00–0.05 (s, 15H), 0.06–0.07 (m, 2H), 0.89
(t, J ¼ 7.0 Hz, 3H), 1.30–1.55 (m, 19H), 1.81 (m, 2H), 4.02 (t, J ¼ 6.5 Hz, 2H),
4.31–4.34 (m, 1H), 6.93 (d, J ¼ 9.0 Hz, 2H), 7.01 (d, J ¼ 8.5 Hz, 2H), 7.13
(d, J ¼ 9.0 Hz, 2H), 7.60 (d, J ¼ 8.5 Hz, 2H), 7.69 (d, J ¼ 8.5 Hz, 2H), 8.22
(d, J ¼ 8.5 Hz, 2H); HRMS (ESI), m=z, 648.3632; Anal. Calc. for C38H56O5Si2: C,
70.32; H, 8.70; Found: C, 70.31; H, 8.71.
4-(Octan-2-yloxy) Phenyl 40-[6-(1,1,2,2,3,3,3-Heptamethyltrisiloxanyl)hexyloxy)
Biphenyl-4-carboxylate, C6Si3. The compound was prepared from 7a and
1-hydroheptamethyltrisiloxane. The product was isolated as a white white liquid
1
crystal in 84% yield. H NMR (ppm, CDCl3): 0.00–0.06 (s, 21H), 0.08–0.10 (m,
2H), 0.89 (t, J ¼ 7.0 Hz, 3H), 1.30–1.55 (m, 19H), 1.82 (m, 2H), 4.02 (t, J ¼ 6.5 Hz,
2H), 4.32–4.33 (m, 1H), 6.93 (d, J ¼ 9.0 Hz, 2H), 7.01 (d, J ¼ 8.5 Hz, 2H), 7.13
(d, J ¼ 9.0 Hz, 2H), 7.60 (d, J ¼ 9.0 Hz, 2H), 7.69 (d, J ¼ 8.5 Hz, 2H), 8.23
(d, J ¼ 8.5 Hz, 2H); HRMS (ESI), m=z, 722.3988; Anal. Calc. for C40H62O6Si3: C,
66.43; H, 8.64; Found: C, 66.20; H, 8.55.
40-[11-(1,1,2,2,3,3,3-Heptamethyltrisiloxanyl) Undecyloxy) Biphenyl-4-carboxylate,
C11Si3. The compound was prepared from 7b and 1-hydroheptamethyltrisiloxane.
1
The product was isolated as a white white liquid crystal in 87% yield. H NMR
(ppm, CDCl3): 0.00–0.01 (s, 21H), 0.06–0.07 (m, 2H), 0.89 (t, J ¼ 7.0 Hz, 3H),
1.29–1.55 (m, 29H), 1.83 (m, 2H), 4.02 (t, J ¼ 6.5 Hz, 2H), 4.32–4.34 (m, 1H), 6.93
(d, J ¼ 9.0 Hz, 2H), 7.01 (d, J ¼ 9.0 Hz, 2H), 7.13 (d, J ¼ 9.0 Hz, 2H), 7.60 (d,
J ¼ 9.0 Hz, 2H), 7.69 (d, J ¼ 8.5 Hz, 2H), 8.23 (d, J ¼ 8.5 Hz, 2H); HRMS (ESI),
m=z, 792.4458; Anal. Calc. for C45H72O6Si3: C, 78.13; H, 9.15; Found: C, 78.09; H, 9.09.
Results and Discussion
Thermal Properties and Optical Textures
The phase behaviors of all compounds were investigated by a combination of POM
and DSC Phase transition temperatures observed by POM match well with the
corresponding DSC thermograms. The mesomorphic properties of the studied

Novel Ferroelectric Liquid Crystal
9
compounds C6Si2, C6Si3; and C11Si3; the phase sequence; phase transition
temperatures; and enthalpy data are summarized in Table 1. Upon cooling from the
isotropic state, the compound C6Si2 displayed a characteristic Schlieren texture and
broken fan textures of the mesophase at 86ꢁC (see Fig. 1), indicating the formation
of the SmCꢀ phase. The other compounds C6Si3 and C11Si3 showed similar optical
textures, thus implying that they have the same mesophase. All of the compounds
exhibit enantiotropic mesomorphic behavior and only a mesophase. A typical DSC
curve of compound C6Si2 is shown in Fig. 2, which shows that one mesophase can
be obtained and that there are no significant differences for different heating or cool-
ing rates. In the DSC thermograms, the enthalpy values obtained for the SmCꢀ meso-
phase to isotropic phase transition were in the range of about 4.16 to 5.58 kJ molꢂ1
.
A series of materials reported by Petrenko show that the inclusion of terminal
siloxane at the end of the molecule increased steric hindrance between layers. Gen-
erally, the siloxane unit is also considered to be a microphase unit, but its size and
flexibility disrupt the interlayer interface. As a result, when the structure adds more
siloxane unit at the end, the clearing temperature of the compound is decreased.
Comparison of compounds C6Si2 and C6Si3 upon cooling from the isotropic liquid
shows that the compound C6Si3 is due to the lowest clearing transition temperature.
Thus, the size and flexibility effect of the siloxane unit have a profound influence on
their mesomorphic behavior. Comparison of compounds C6Si3 and C11Si3, shows
that the clearing transition temperature is increased due to the increased achiral alkyl
chain. The result also conforms to the previous reports.
XRD Studies
To further identify the type of mesophases, determination of structural parameters
has been further performed by XRD. The theoretical molecular length of C11Si3
determined from molecular model estimated using CS Chem3D Ultra 7.0 software
˚
gave molecular length of ca. 48.2 A, where we suppose that the molecular structure
Table 1. Phase transition temperatures (ꢁC) and enthalpies (kJ molꢂ1; iꢂn1paarentheses)
for the target compounds as determined by DSC (scan rate ¼ 5ꢁC min
)
Second heating
first cooling
Second heating
first cooling
Compound
(n, m)
(6, 2)
Cr
SmCꢀ
I
C6Si2
.
1.34 (þ6.15)
ꢂ7.20 (ꢂ5.92)
>ꢂ30
.
88.9 (þ5.58)
87.5 (ꢂ5.56)
65.8 (þ4.16)
64.4 (ꢂ4.25)
106.9 (þ4.40)
105.4 (ꢂ4.19)
.
C6Si3
(6, 3)
.
.
.
.
.
.
>ꢂ30
C11Si3
(11, 3)
46.9 (þ12.4)
32.8 (ꢂ10.5)
aAbbreviations: Cr ¼ crystalline phase; SmCꢀ ¼ chiral SmC phase; I ¼ isotropic phase
(. ¼ phase exists).

10
C.-T. Liao et al.
Figure 1. Polarized optical microscopic textures show Schlieren with broken fan-shaped
textures in SmCꢀ phase on cooling (1ꢁC=min) from the isotropic state for compound C6Si2
at 86ꢁC.
is coplanar and the alkyl chains are fully extended in the 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 C11Si3 and the
data were integrated in Fig. 3b, as an illustrative example. The resulting XRD pat-
tern of compound C11Si3 at 100ꢁC exhibits three Bragg reflections in the small-angle
˚
region at 2h ¼ 1.96, 3.92, and 5.88 with d-spacings of 39.9, 19.5, and 13.0 A, respect-
ively, corresponding to the reciprocal spacing in the ratios 1, 2, and 3, and to the
indexation (hk) ¼ (10), (20), and (30), which implies the formation of a layered struc-
ture. On the other hand, a diffuse scattering halo in the wide-angle region centered at
ꢁ
˚
2h around 16.41 with d-spacing of ca. 4.6 A indicates liquid-like arrangement of the
molecules within the layers. As shown in Fig. 3c, the temperature dependence on the
Figure 2. DSC curves of compound C6Si2 obtained at a rate of 5ꢁC minꢂ1: (a) second heating
scan and (b) first cooling scan.

Novel Ferroelectric Liquid Crystal
11
Figure 3. (a) 2D XRD pattern of a partially surface-aligned sample of compound C11Si3
obtained in the SmCꢀ phase at 100ꢁC (upon cooling from the isotropic phase), (b) X-ray
diffraction intensity against angle profile obtained in the SmCꢀ phase of (C11Si3) at 100ꢁC,
and (c) layer d-spacing as a function of the temperature dependence (upon cooling from the
isotropic phase).
layer d-spacing of compound C11Si3 in the SmCꢀ phase was recorded upon cooling
from the isotropic phase. The values of the layer d-spacing only slightly change
within the temperature range of the SmCꢀ phase. Therefore, the layer d-spacing
˚
value of 39.9 A for compound C11Si3 by XRD measurement is smaller than the
˚
calculated theoretical molecular length of 48.2 A, which suggests a tilted arrange-
ment of smectic structure; that is, a smectic C phase with tilt angle of 34.1ꢁ along
the layer normal direction. Compound C6Si2 and C6Si3 also revealed similar
diffraction patterns at temperatures within the respective mesophase temperature
range (see Table 2).
Electro-Optical Behavior
The switching behavior has been observed in the SmCꢀ phase of the studied com-
pound C6Si2 using the triangular wave method. One distinct sharp peak per half
period of an applied triangular wave voltage is clearly seen under a triangular wave
field of about 80 Vpp at 60 Hz (see in Fig. 4a). This is a strong indication of a ferro-
electric switching process. To clarify this, we also have measured the polarization by
the Diamant-Bridge method (see Fig. 4b) and observed only single hysteresis loop
down to the lowest frequency (1 Hz) available in the measurement, again indicating
ferroelectric-type switching. The temperature dependence of the magnitude of the Ps

12
C.-T. Liao et al.
Table 2. Summary data of target compounds
Compound
Mesophase
Parameters (A)
Measured spacings (A)
Tilt angle (ꢁ)
Optical tilt angle (ꢁ)
Switching time (ms)a
Maximum Ps=nC (cm2)a
C6Si2
C6Si3
C11Si3
SmCꢀ
38.9
29.7
40.2
45
SmCꢀ
40.5
30.8
40.5
41.5
141
SmCꢀ
48.2
39.9
34.1
44
˚
˚
23
91
35
74
62
aThe data were measured at Tc ꢂ T ¼ 20ꢁC.
value for the target compounds obtained by integrating the area under the one peak
is shown in Fig. 4c. The abrupt increase of Ps around the isotropization temperature
indicates the phase transformation from the isotropic phase to the SmCꢀ phase. It is
also observed that the value of Ps increases with decrease in temperature of the
Figure 4. (a) Switching current response observed in the SmCꢀ phase of compound C6Si2 at
75ꢁC under the applied triangular wave voltage (7.5 mm antiparallel rubbed polyimide-coated
ITO cell, 80 Vpp, 60 Hz) and (b) bridge method. Summary of spontaneous polarization (Ps) as
a function of (c) the temperature and (d) voltage observed in the SmCꢀ phase of target
&
compounds (.: C6Si2; : C6Si3; : C11Si3).
ꢁ

Novel Ferroelectric Liquid Crystal
13
Figure 5. Helical pitch as a function of ac electric field in compound C6Si2 at 80ꢁC
(f ¼ 30 Hz).
Figure 6. (a) The time dependences of the switching current under a square wave field for
compound C6Si2 at 30ꢁC and (b) switching time as a function of temperature and (c) voltage
for target compounds (on antiparallel rubbing direction LC cells with 7.5 mm thickness as
&
Vpp ¼ 150 V, f ¼ 10 Hz;. .: C6Si2; : C6Si3; : C11Si3).
ꢁ

14
C.-T. Liao et al.
Figure 7. Behavior of rotational viscosity for target compounds with different (a) temperature
&
and (b) voltage (.: C6Si2; : C6Si3; : C11Si3).
ꢁ
compounds. The increase in the Ps value with decrease in the temperature may be
due to the strong polarization-electric (P-E) field coupling and polarization-tilt angle
(P-A) theta coupling [19]. We also see the behavior of spontaneous polarization with
applied voltage, which is shown in Fig. 4d. Because the molecular helical order in the
sample may not be suppressed completely by the confining substrates, it was
observed that with an increase in the applied voltage, the spontaneous polarization
increased sharply initially. The helix could be unwinding by applied field (see Fig. 5);
result in such a dependence has been observed in FLC materials. However, the
applied field may also induce a transition to a higher order phase that does not exist
in the phase diagram without an applied field. The Ps value exhibits a maximum
polarization of 91 nC cmꢂ2 for compound C6Si2, 62 nC cmꢂ2 for compound
C6Si3, and 74 nC cmꢂ2 for compound C11Si3 at Tc ꢂ T ¼ 20ꢁC (see Table 2).

Novel Ferroelectric Liquid Crystal
15
Figure 8. Tilt angle of the SmCꢀ phase as a function of the dc electric fields in compounds (on
&
antiparallel rubbing direction LC cells with 7.5 mm thickness) (C6Si2: .; C6Si3: ; C11Si3: ).
ꢁ
Comparing, the spontaneous polarization value of target compounds (compounds,
C6Si3 and C11Si3), it appears that compounds with longer alkyl chain length have
higher Ps values at any temperature below the Curie point. It also seems that enable
comparison of Ps values for the length of the siloxane group (compounds C6Si2 and
C6Si3). It appears higher Ps values with lower siloxane group length. The results
also conform to previous reports [9].
In order to investigate the dynamics of the polarization switching behavior, the
switching currents of compound C6Si2 were measured with an applied square wave
field (as Vpp ¼ 80 V, f ¼ 10 Hz) in the SmCꢀ phase. The switching current curves of
compound C6Si2 at 30ꢁC are shown in Fig. 6a 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. The switching time as a func-
tion of the temperature is shown in Fig. 6b (Vpp ¼ 150 V, f ¼ 10 Hz). The decrease of
the switching time with increasing temperature for target compounds was observed
due to the viscosity decrease. The switching time value of 23 ms for compound C6Si2,
141 ms for compound C6Si3, and 35 ms for compound C11Si3 at Tc – T ¼ 20ꢁC was
observed (see Table 2). The switching time with applied voltage has been shown in
Fig. 6c. It was observed that the switching time decreases with increasing voltage.
The combined effect of the switching time and spontaneous polarization can be
seen in the behavior of the rotational viscosity, as shown in Fig. 7a and Fig. 7b.
These three are related with each other by the following relation
cw ¼ sPSE
ð1Þ
where s is switching time, cw is the rotational viscosity, Ps is the spontaneous polar-
ization, and E is the applied field. The rotational viscosity decreases, the spon-
taneous polarization increases, and the switching time decreases in accordance
with Eq. (1). The rotational viscosity value of 336 mPass for compound C6Si2,

16
C.-T. Liao et al.
1, 766 mPass for compound C6Si3, and 640 mPass for compound C11Si3 at Tc ꢂ
T ¼ 20ꢁC were exhibited (Vpp ¼ 150 V, f ¼ 10 Hz).
The optical tilt angles for all compounds in the SmCꢀ phase were measured as a
function of temperature on cooling. The measured optical tilt angle as a function of
temperature for the SmCꢀ phase for target compounds shown in Fig. 8 enables com-
parison of optical tilt angles with different lengths of the alkyl chain and different
lengths of the siloxane group in the tail of the compounds. In general, the optical tilt
angles increase with decreasing temperature and exhibit a maxima. The maximum
optical angles for the three compounds are similar at any temperature below the
Curie point. The tilt angle value exhibits a maximum of 45ꢁ for compound C6Si2,
44ꢁ for compound C6Si3, and 41.5ꢁ for compound C11Si3 at Tc – T ¼ 20ꢁC.
The temperature and frequency dependence of the dielectric constant for C11Si3
is shown in Fig. 9a. For C11Si3, the dielectric constant increased sharply at the
Iso-SmCꢀ phase transition region. The maximum value of the dielectric constant
was observed near the Iso-SmCꢀ phase. As the temperature decreased, the dielectric
constant decreased slightly, corresponding to the increase of the tilt angle in the
2
3
4
0
&
Figure 9. (a) Temperature dependence of the dielectric constant (e ) at 10 (.), 10 ( ), 10 ( ),
ꢁ
5
6
&
10 ( ), and 10 (4) Hz for C11Si3. (b) Cole-Cole plot for C11Si3 at different temperatures:
ꢁ
ꢁ
ꢁ
ꢀ
&
85 C (.), 70 C ( ), and 50 C ( ). (c) Cole-Cole plot of the dielectric permittivity in the SmC
ꢁ
phase of target compounds at Tc ꢂ T ¼ 15ꢁC (C6Si2: .; C6Si3: ; C11Si3: ).
&
ꢁ

Novel Ferroelectric Liquid Crystal
17
Figure 10. Dielectric permittivity (e0) as a function of log of frequency at 0 V (.) and 35 V ( )
ꢁ
&
and again at 0 V ( ) for compound C11Si3.
SmCꢀ phase. The molecules have such a large tilt angle that their rotational freedom
is decreased somewhat by steric hindrance [20]. This result would cause the molecu-
lar motion (Goldstone mode) to decrease. The Cole-Cole plot (E00 versus E00) provides
valuable information with regard to the dielectric relaxation process. Almost
semicircular Cole-Cole plots at various temperatures are shown in Fig. 9b for the
c relaxation of C11Si3. The relaxation strength (diameter of the circle) is similar with
increasing temperature. The Cole-Cole plot of the dielectric permittivity in the SmCꢀ
phase of target compounds at Tc ꢂ T ¼ 15ꢁC was observed (see Fig. 9c). As a result,
the relaxation strength increased with decreasing length of the achiral terminal chain.
Figure 10 shows the dielectric permittivity as function of frequency in C11Si3. As
can be seen from the figure, with the change in bias voltage of the measuring field
from 0 to 35 V, the permittivity value decreases to a minimum, which is due to
suppression of the phason (Goldstone) mode, which occurs due to phase fluctuation
of the molecules [21]. But again taking measurements at zero bias, the sample cell
shows small memory effects.
Conclusions
We have achieved the molecular design, synthesis, and characterization of liquid
crystals with a 4-hydroxyphenyl 40-hydroxybiphenyl-4-carboxylate core, chiral
octan-2-ol unit, and oligomeric siloxane end-group. Specifically, we have explored
the possibility of stabilizing a wide thermal range chiral smectic C (SmCꢀ) phase.
It exhibits the SmCꢀ phase over 100ꢁC thermal range. Electro-optic studies were car-
ried out for the majority of the compounds exhibiting the SmCꢀ phase. Electrical
switching and molecular tilt angle as functions of temperature and voltage were
investigated in the SmCꢀ phase. The tilt angle of C6Si2 compound can reach up
to 45ꢁ at high fields (160 V) in the SmCꢀ phase, and the response time range was
between 23 ms at 160 V in SmCꢀ. The tilt angle, dielectric behavior, and switching
property appearing in the SmCꢀ phase of the target compounds were observed.

18
C.-T. Liao et al.
Acknowledgment
We thank Dr. Jey-Jau Lee for help with the XRD experiments at National Synchro-
tron Radiation Research Center (NSRRC, for the beamline BL17A) in Taiwan.
Financial support from the National Science Council of Taiwan (ROC) through
98-2221-E-011-004 is gratefully acknowledged.
References
[1] Wu, S. L., & Hsu, H. N. (2007). Liq. Cryst., 34, 1159.
[2] Yoshizawa, A., Nishiyama, I., Fukumasa, M., Hirai, T., & Yamane, M. (1989). Jpn. J.
Appl. Phys., 28, 1269.
[3] Coles, H. J., Owen, H., Newton, J., & Hodge, P. (1993). Liq. Cryst., 15, 737.
[4] Sunohara, K., Takatoh, K., & Sakamoto, M. (1993). Liq. Cryst., 13, 283.
[5] Newton, J., Coles, H., Hodge, P., & Hannington, J. (1994). J. Mater Chem., 4, 869.
[6] Poths, H., Wischerhoff, E., Zentel, R., Schonfeld, A., Henn, G., & Kremer, F. (1995).
Liq. Cryst., 18, 811.
[7] Sun, Z. M., Zhang, X., & Feng, D. (1990). Europhys. Lett., 11, 415.
[8] Ibn-Elhaj, M., Skoulios, A., Guillon, D., Newton, J., Hodge, P., & Coles, H. J. (1996). J.
Phys. II, 6, 271.
[9] Corsellis, E., Guillon, D., Kloess, P., & Coles, H. (1997). Liq. Cryst., 23, 235.
[10] Robinson, W. K., Carboni, C., Kloess, P., Perkins, S. P., & Coles, H. J. (1998). Liq.
Cryst., 25, 301.
[11] Naciri, J., Ruth, J., Crawford, G., Shashidhar, R., & Ratna, B. R. (1995). Chem. Mater.,
7, 1397.
[12] Naciri, J., Shenoy, D. K., Keller, P., Gray, S., Crandall, K., & Shashidhar, R. (2002).
Chem. Mater., 14, 5134.
[13] Vlahakis, J. Z., Maly, K. E., & Lemieux, R. P. (2001). J. Mater. Chem., 11, 2459.
[14] Marzec, M., Mikulko, A., Wrobel, S., Dabrowski, R., Darius, M., & Haase, W. (2004).
Liq. Cryst., 31, 153.
[15] Diamant, H., Drenck, K., & Pepinsky, R. (1957). Rev. Sci. Instrum., 28, 30.
[16] Raina, K. K., & Kumar, P. (2009). Journal of the Indian Institute of Science, 89(2), 243.
[17] Brunsveld, L., Zhang, H., Glasbeek, M., Vekemans, J. M., & Meijer, E. W. (2000). J. Am.
Chem. Soc., 122, 6175.
[18] Olsson, N., Helgee, B., Andersson, G., & Komitov, L. (2005). Liq. Cryst., 32, 1139.
[19] Raina, K. K., & Kumar, P. (2009). J. Indian Inst. Sci., 89(2), 243.
[20] Krone, V., & Ringsdorf, H. (1991). Liq. Cryst., 9, 207.
[21] Hiller, S., Biradar, A. M., & Haase, W. (1996). Phys. Rev. E, 53, 641.