Synthesis and characterization of ferroelectric liquid crystalline siloxanes containing 4-hydroxyphenyl(2S,3S)-2-chloro-3-methylvalerate

Article abstract of DOI:10.1080/15421406.2011.599204

New series of organosiloxane ferroelectric liquid crystalline materials have been synthesized, and their mesomorphic and physical properties have been characterized. These new series contain bis-siloxane or tris-siloxane unit attached to the flexible alkyl chain end of (2S,3S)-2-chloro-3-methylvalerate. The siloxane molecule induction is helpful to the chiral smectic C (S _C) formation and chiral S_C* stabilization, and it simultaneously causes the liquid crystal temperature range of chiral S _C* to be broader. The siloxane member is helpful in reducing the smectic C (S_C) transation shift temperature, and the molecule containing tris-siloxane units shows better effect than the bis-siloxane one. The synthesis and characterization of the new FLCs materials which exhibit S_C* phase at room temperature and higher spontaneous polarization are presented.

Full text of DOI:10.1080/15421406.2011.599204

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Synthesis and Characterization of
Ferroelectric Liquid Crystalline Siloxanes
Containing 4-Hydroxyphenyl(2S,3S)-2-
Chloro-3-Methylvalerate
Chih-Hung Lin a
a Center for General Education , Chang Gung University of Science
and Technology, Kwei-Shan , Tao-Yuan , Taiwan , Republic of China
Published online: 27 Dec 2011.
To cite this article: Chih-Hung Lin (2012) Synthesis and Characterization of Ferroelectric Liquid
Crystalline Siloxanes Containing 4-Hydroxyphenyl(2S,3S)-2-Chloro-3-Methylvalerate, Molecular Crystals
and Liquid Crystals, 552:1, 33-42, DOI: 10.1080/15421406.2011.599204
To link to this article: http://dx.doi.org/10.1080/15421406.2011.599204
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Mol. Cryst. Liq. Cryst., Vol. 552: pp. 33–42, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.599204
Synthesis and Characterization of Ferroelectric
Liquid Crystalline Siloxanes Containing
4-Hydroxyphenyl(2S,3S)-2-Chloro-3-Methylvalerate
CHIH-HUNG LIN∗
Center for General Education, Chang Gung University of Science and
Technology, Kwei-Shan, Tao-Yuan, Taiwan, Republic of China
New series of organosiloxane ferroelectric liquid crystalline materials have been syn-
thesized, and their mesomorphic and physical properties have been characterized. These
new series contain bis-siloxane or tris-siloxane unit attached to the flexible alkyl chain
end of (2S,3S)-2-chloro-3-methylvalerate. The siloxane molecule induction is helpful to
the chiral smectic C (SC∗) formation and chiral SC∗ stabilization, and it simultaneously
∗
causes the liquid crystal temperature range of chiral SC to be broader. The siloxane
member is helpful in reducing the smectic C (SC) transation shift temperature, and the
molecule containing tris-siloxane units shows better effect than the bis-siloxa∗ne one.
The synthesis and characterization of the new FLCs materials which exhibit SC phase
at room temperature and higher spontaneous polarization are presented.
Keywords Ferroelectric liquid crystal; siloxane; spontaneous polarization
Introduction
With the development of optoelectronics, the transposition of ferroelectricity to anisotropic
liquids and, in particular, to thermotropic liquid crystals, found in Rochelle salts (potassium
and sodium tartrate) [1] at the beginning of the 20th century, has been a matter of great
interest for the past two decades [1,3]. The molecular design of these compounds was
directly deduced from the original work of Mayer and Coles who predicted, using a
symmetry argument, that tilted smectic phases obtained with chiral molecules have a
transverse dipole moment and should exhibit an electric spontaneous polarization [4,5].
Ferroelectric liquid crystal (FLC) materials exhibiting SmC∗ phase at room temperature
and low melting temperatures are of great interest to researchers in the electro-optic field
[6–11]. These are the basis of a variety of potential devices ranging from large-area flat∗-
panel displays, ultrafast electro-optic modulators, and spatial light modulators [12–16]. SC
materials are suited for gray-scale applications.
FLC molecules consist of a rigid core and end-tail groups with a chiral center located
on one or both groups. For most FLC molecules, the end-tail groups are hydrocarbon
chains. When varying the hydrocarbon chain lengths of the molecules, the macroscopic
properties are modified. For example, the rotational viscosity as well as the tilt angle of
∗Address correspondence to Chih-Hung Lin, Center for General Education, Chang Gung Univer-
sity of Science and Technology, 261, Wen-Hwa 1st Road, Kwei-Shan, Tao-Yuan, Taiwan, Republic
of China. E-mail: chlin@gw.cgit.edu.tw
33

34
C.-H. Lin
∗
the SC molecules increase with increasing chain length. The case of the variation of the
spontaneous polarization, Ps, as a function of chain length is less obvious. To crystallize
FLC molecule easily, increasing the length of the nonpolar moiety helps to dilute the
transverse dipole moment in the molecule which thus decrease the value of Ps, whereas the
increase of the tilt angle should have a tendency to increase Ps.
In this study, we designed and prepared a new series of molecule, which contains
a two-chiral center and the partial substitution of hydrocarbon chains by more flexible
dimethylsiloxyl and trimethylsiloxyl groups. Thus it was expected to enhance the thermal
stability and Ps value of the FLC material [17–19].
Experimental
1H-NMR spectra were recorded on a Varian VXR-300 or Bruker 300 MHz spectrome-
ter. Thermal transitions and thermodynamic parameters were determined using a Seiko
SSC/5200 differential scanning calorimeter (DSC) equipped with a liquid nitrogen cool-
ing accessory. Heating and cooling rates were 10◦C/min. Thermal transition reports were
collected during the second heating and cooling scans. A Nikon Microphot-FX polarized
optical microscope (POM) equipped with a Mettler FP 82 hot stage and a FP 80 central pro-
cessor was used to observe the thermal transitions and to analyze the anisotropic textures.
Polymerization reactions were traced using a Nicolet 520 FT-IR spectrometer.
Synthesis
The general synthetic routes of the intermediates and target molecule are shown in Scheme 1.
The purity and chemical structures of the intermediates and target compounds can be
easily verified by TLC and 1H-NMR spectroscopy. The synthetic procedures and chemical
analyses of each product are described sequentially below.
Compounds (1)–(5)
4-(2-Propenyloxy)biphenyl-4ꢀ-ol (1)
4-(5-Hexenyloxy)biphenyl-4ꢀ-ol (2)
4-(8-Nonenyloxy)biphenyl-4ꢀ-ol (3)
4-(9-Decenyloxy)biphenyl-4ꢀ-ol (4)
4-(10-Undecenyloxy)biphenyl-4ꢀ-ol (5)
Compounds (1)–(5) were prepared by the same method. The synthesis of compound
(1) is described below.
4,4ꢀ-Dihydroxydiphenyl (7.9 g, 0.0425 mol) was added to a solution of potassium
hydroxide (4.76 g, 0.085 mol) in ethanol (300 mL, 95%). Potassium iodide (0.2 g) was also
added and the solution was heated to reflux for 0.5 h. 3-Bromopropene (3.0 g, 0.025 mol)
was then slowly added and the solution was refluxed overnight. The solution was cooled and
poured into 50 mL of 6N HCl, and the solid salt was removed by filtration. The remaining
white solid was recrystallized from ethanol to yield 2.43 g (43%) of white crystals [mp
= 140.5◦C. 1H-NMR (CDCl3, δ, ppm): 4.50–4.70 (t, 2H, O CH2 CH ), 5.00 (s, 1H,
HO ), 5.31 (m, 2H, = CH2), 6.08 (m, 1H, CH2 CH ), 6.80–7.60 (m, 8H, aromatic
protons)].
Compound (6)
(2S,3S)-2-Chloro-3-methylvaleric acid (6)

Ferroelectric Liquid Crystalline Solixanes
35
Scheme 1. Synthesis of compounds (1M)–(5M), (1A)–(5A), and (1B)–(5B).

36
C.-H. Lin
l-Isoleucine (9.7 g, 0.074 mol) was dissolved in 250 mL of 6N HCl and slowly added to
sodium nitrite powder (10.2 g, 0.148 mol). This solution was allowed to stand at 0◦C–5◦C
for 20 h by ice bath. The solution was then extracted by 200 mL of dichloromethane.
The extraction solution was washed with saturated sodium chloride (aq.) and dried over
anhydrous magnesium sulfate. After removal of the solvent by evaporation under reduced
pressure, the residue was purified by distillation under reduced pressure to yield 5.79 g
(52%) of colorless liquids [1H-NMR (CDCl3, δ, ppm): 0.78–1.04 (m, 6H, CH3), 1.20–2.10
(m, 3H, CH CH2 CH3), 4.16 (d, 1H, CHCl COOH), 9.95 (s, 1H, COOH)].
Compounds (1M)–(5M)
4-(2-Propenyloxy)biphenyl-4ꢀ-yl(2S,3S)-2-chloro-3-methylvalerate (1M)
4-(5-Hexenyloxy)biphenyl-4ꢀ-yl(2S,3S)-2-chloro-3-methylvalerate (2M)
4-(8-Nonenyloxy)biphenyl-4ꢀ-yl(2S,3S)-2-chloro-3-methylvalerate (3M)
4-(9-Decenyloxy)biphenyl-4ꢀ-yl(2S,3S)-2-chloro-3-methylvalerate (4M)
4-(10-Undecenyloxy)biphenyl-4ꢀ-yl(2S,3S)-2-chloro-3-methylvalerate (5M)
Compounds (1M)–(5M) were prepared by the same method. The synthesis of com-
pound (1M) is described below.
4-(2-Propenyloxy)biphenyl-4ꢀ-ol (1) (2.5 g, 0.0112 mol) and (2S,3S)-2-chloro-3-
methylvaleric acid(1.8 g, 0.0121 mol) dissolved in dry dichloromethane(150 mL), N,N-
dicyclohexylcarbodiimide (DCC, 2.72 g, 0.0132 mol) and 4-(N, N-dimethyl-amino)-
pyridine (DMAP, 0.1 g) were added to react under nitrogen. The reaction mixture was
stirred for 24 h at room temperature. The solution was filtered and washed with an excess
of dichloromethane. The filtrate was washed with water and dried over anhydrous mag-
nesium sulfate. After removal of the solvent by evaporation under reduced pressure, the
residue was purified by column chromatography on silica gel using hexane/ethyl acetate
1
as eluent to yield 1.37 g (34%) of white crystals [mp = 97.3◦C. H-NMR (CDCl3, δ,
ppm): 0.90–1.06 (m, 3H, CH2 CH3), 1.10–1.18 (m, 3H, CH CH3), 1.30–1.55 (m,
2H, CH2 CH3), 2.15–2.30 (m, 1H, CH CH3), 4.38 (d, 1H, CHCl), 4.58 (m, 2H,
O CH2 CH ), 5.31 (m, 2H, CH2), 6.08 (m, 1H, CH2 CH ), 6.90–7.58 (m, 8H,
aromatic protons)].
Liquid Crystal Siloxane Compounds (1A)–(5A) and (1B)–(5B). Siloxane dimer synthesized
from (1M) to (5M) are marked as (1A)–(5A) and siloxane trimer from (1M) to (5M) are
marked as (1B)–(5B). Compounds (1A)–(5A) and (1B)–(5B) were prepared using the same
method. The synthesis of compound (1B) is described below.
To a degassed solution of compound (1M) (0.29 g, 0.0008 mol) and heptamethyl-
trimethylsiloxyl (0.21 g, 0.0009 mol) in 15 mL of dry, freshly distilled toluene was added
divinyltetramethyldimethylsiloxyl (2.5 mg) as catalyst. The reaction mixture was stirred at
70◦C under nitrogen for 48 h. After the solvent being evaporated, the residue was purified by
column chromatography (silica gel, using n-hexane/ethyl acetate as an eluent) to yield 0.30
g (64%) of white crystals [1H-NMR (CDCl3, δ, ppm): 0.0 (m, 21H, Si CH3), 0.53 (m,
2H, Si CH2 CH2 ), 1.10–2.40 (m, 11H, CH2
,
CH2 CH3, and CH CH3),
4.01 (m, 2H, O CH2 CH2 ), 4.38 (d, 1H, CHCl ), 6.90–7.58 (m, 8H, aromatic
protons)].

Ferroelectric Liquid Crystalline Solixanes
Results and Discussion
37
In this study, we synthesized three series of compounds. Each series includes five different
length spacers, such as 3, 6, 9, 10, and 11 methlene units. The first series of compound
(1M)–(5M) represents a double bond in the end-tail position of the compounds; in the
second series (1A)–(5A), a dimethylsiloxyl group is utilized to replace the double-bond tail
end. For the third series (1B)–(5B), a trimethylsiloxyl group is substituted as the end-tail
group. The molecular structure and the general synthetic procedures of these compounds
are shown in Scheme 1, and all the final products are examined by the nuclear magnetic
resonance spectrometer in order to verify the correction of the molecular structure.
The thermal and mesomorphic properties of compounds (1M)–(5M), (1A)–(5A), and
(1B)–(5B) are measured using DSC and POM. The phase transition temperatures and
enthalpy changes of compounds (1M)–(5M) with five different length spacers are listed in
Table 1. Among them, the phase transition of compound (2M) displays an enantiotropic
SA∗phase, while compounds (3M) and (4M) show an enantiotropic ∗SA and a monotropic
SC phase. Compound (5M) reveals both enantiotropic SA and SC phases. The results
∗
demonstrate that the thermal stability of an SC phase increases as the spacer length
increased.
The phase transition temperatures and enthalpy change of the compounds (1A)–(5A)
and (1B)–(5B) are listed in Tables 2 and 3. The crystallization of compound (1A) is rather
slow, and, therefore, the crystallization temperature cannot be observed during the DSC
cooling scan down to −60◦C. Compound (1B) is a transparent liquid under the room
temperature, and no phase transition is detected by POM and DSC scanning between
−60◦C and 100◦C. All the compounds (2A)–(5A) and (2B)–(5B) reveal both enantiotropic
∗
SC characteristic and SA phases. Figure 1 depicts a DSC analysis diagram of compound
(5A), and its liquid crystalline mesophases are observed by POM; Figure 2(a) shows the
forcal-conic fan texture of SA, while Figure 2(b) displays the broken focal-conic fan texture
∗
of SC
.
In comparison of the thermal transitions of three series of compounds (1M)–(5M),
∗
(1A)–(5A), and (1B)–(5B), some conclusions are summarized as follows: (a) SC phase is
formed when the carbon number of flexible spacer is larger than six for the vinyl precursors
(1M)–(5M). (b) For both siloxane derivatives, (2A) and (2B) containing six methylene
∗
units in the spacer show enantiotropic SC phase. This means that incorporation of the
Table 1. Thermal transition of olefinic compounds (1M)–(5M)
Phase transitions, ◦C(corresponding enthalpy
heating
cooling
Compound
n
changes, Kcal/mole)
K97.2(3.20)I
(1M)
(2M)
(3M)
(4M)
3
I92.3(−3.16)K
K56.3(4.31)aSA63.2(−)aI
6
9
I56.1(−)aSA50.9(4.02)K
K38.5(5.94)SA43.7(1.0)I
I50.5(−0.98)SA33.1(−0.08)S∗C17.6(−1.89)K
K46.0(4.98)SA52.9(0.85)I
10
I48.8(−1.03)SA32.7(−0.06)S∗C22.4(−5.53)K
K49.5(5.03)S∗C55.0(−)bSA56.4(0.83)I
(5M)
11
I53.2(−0.98)SA42.0(−)bSC∗ 26.2(−5.52)K
∗
Note: K = crystal, SA = smectic A, SC = chiral smectic C, I = isotropic.
aOverlapped transition.
bDetermined by polarizing optical microscopic observation.

38
C.-H. Lin
Table 2. Thermal transitions of dimethylsiloxyl-containing compounds (1A)–(5A)
Phase transitions, ◦C(corresponding enthalpy
heating
cooling
Compound
n
changes, Kcal/mole)
K57.8(3.42)I
(1A)
(2A)
(3A)
(4A)
(5A)
3
a
K14.7(0.71)S∗C24.0(−)bSA41.9(0.21)I
6
9
I32.8(−0.16)SA20.1(−)bSC∗ 8.5(−0.69)K
K36.7(0.86)S∗C48.2(0.06)SA57.1(0.52)I
I49.8(−0.23)SA44.4(−0.02)S∗C30.1(−0.92)K
K31.2(1.44)S∗C47.0(0.04)SA61.3(1.18)I
10
11
I57.7(−1.02)SA44.3(−0.04)S∗C19.4(−0.79)K
K27.9(0.91)S∗C52.8(0.05)SA63.3(1.40)I
I60.6(−1.18)SA50.9(−0.03)S∗C25.4(−0.89)K
∗
Note: K = crystal, SA = smectic A, SC = chiral smectic C, I = isotropic.
aEnthalpy is very small.
bDetermined by polarizing optical microscopic observation.
∗
siloxane unit in an LC molecule has the tendency to stabilized the SC phase. (c) The LC
compounds with tris-siloxane unit (2A)–(5A) show much lower melting points and wider
∗
SC temperature ranges than those LC compounds with bis-siloxane unit (2B)–(5B). Both
compounds (4A) and (5B), which reveal an SC∗ phase under the room temperature, can be
used to formulate LC mixtures for display application.
According to the literature [20], side chain liquid crystal polysiloxane containing 4-
hydroxyphenyl(2S,3S)-2-chloro-3-methylvalerate as mesogen owns higher transition tem-
perature of nearly 40◦C than small liquid crystal siloxane compound, although their tem-
perature ranges of liquid crystal are close. Correspondingly, both the polymer with six
carbon spacer length and the compound (2M) with six carbon spacer length have only SA
∗
liquid crystalline mesophase; however, compounds (2A) and (2B) exhibit enantiotropic SC
phases. This result shows that the small siloxane molecule is more helpful than the side
chain polysiloxane for stabilizing the SC∗ liquid crystalline mesophase. If it is assumed that
longer spacer length is good for the formation and the stabilization of an SC∗ phase in terms
of the spacer concept for general liquid crystal mesophase, the molecules of dimethylsiloxyl
and trimethylsiloxyl should be predicted to have the capability to increase the function of
Table 3. Thermal transitions of trimethylsiloxyl-containing compounds (1B)–(5B)
Phase transitions, ◦C(corresponding enthalpy
heating
cooling
Compound
n
changes, Kcal/mole)
(−)a
(1B)
(2B)
(3B)
(4B)
(5B)
3
(−)a
K−2.2(0.34)S∗C18.6(−)bSA36.0(0.41)I
6
9
I25.6(−0.15)SA10.8(−)S∗C−10.0(−0.39)K
K35.2(0.48)S∗C52.1(0.01)SA61.4(0.05)I
I52.4(−0.10)SA44.9(−0.01)S∗C28.8(−0.54)K
K29.5(0.86)S∗C52.2(0.11)SA59.8(0.91)I
10
11
I55.8(−0.71)SA49.5(−0.05)S∗C26.6(−0.84)K
K20.4(0.75)S∗C46.6(0.08)SA56.4(0.71)I
I53.5(−0.51)SA45.0(−0.02)S∗C16.7(−0.51)K
∗
Note: K = crystal, SA = smectic A, SC = chiral smectic C, I = isotropic.
aDid not show any temperature transition when it was measured by DSC from −60◦C to 100◦C.
bDetermined by polarizing optical microscopic observation.

Ferroelectric Liquid Crystalline Solixanes
39
Figure 1. DSC thermograms of compound (5A) (10◦C/min) for (a) second heating scanning and (b)
cooling scanning.
∗
effective spacer length. Besides, in contrast of the SC transition property between com-
pounds (2A) and (5M), dimethylsiloxyl group shows the equal effect as five-carbon length
spacer in the thermoproperty. The rest may be inferred by the analogy, compound (5A) with
the effective spacer length counted as 16 units of carbon chain, and this long carbon chain
certainly produced no side chain crystallization phenomenon. This situation demonstrates
two advantages: the first is that the small siloxane molecule has the increasing function
of effective spacer length; the second is that siloxane molecule will not result in the side
chain crystallization. Therefore, it is suitable for applying to the synthesis of smectic liquid
crystal compound.
Compounds (3M)–(5M), (3A)–(5A), and (3B)–(5B) were filled into a 4-mg thick
cell, made of indium tin oxide glass, whose surface was coated by polyimide which was
rubbed in parallel direction. Voltage (about 5 V/µm) of 50 Hz frequency and 20 Vp (peak

40
C.-H. Lin
Figure 2. Polarizing optical micrographs (magnification ×320) of compound (5A) in (a) SA texture
∗
obtained from isotropic phase after cooling to 55◦C and (b) SC texture obtained after cooling to
40◦C.
voltage) was used, and the measurement area was 0.26 cm2. The sample was heated to an
isotropic temperature, adding voltage to sample at cooling process. Measurements of Ps
value and rotational viscosity were taken every 0.5–1◦C during the SC mesophase range.
∗
Due to the lack of cooling accessory, the lowest temperature of measurement was the room
temperature.
The Ps value and rotational viscosity of various compounds have the tendency to
fall down when the temperatures increase. In Figure 3, the Ps value quickly becomes
larger to 255 nC/cm2 and rotational viscosity also becomes higher when the temperature of
compound (5A) falls down from 26◦C to 25.7◦C. Similarly, the Ps value becomes higher
when the temperature of compound (4B) falls down from 28◦C to 27.6◦C. This result
exhibits that there is a higher Ps value when the temperatures of compounds (5A) and (4B)
∗
fall down from an SC phase to a neighboring crystallization temperature.
In summ∗ary, this study indicates that the synthesized FLCs with siloxane have not only
the wider SC phase, but also a larger spontaneous polarization. These compounds have
great potential to be applied to the ferroelectric liquid crystalline display monitor.

Ferroelectric Liquid Crystalline Solixanes
41
Figure 3. Spontaneous polarization (Ps) and viscosity vs. temperature for compound (5A).
Conclusions
∗
∗
The siloxane molecule induction is helpful to the SC formation and S∗C stabilization,
and simultaneously it causes the liquid crystal temperature range of SC to be broader.
Carbon chain length higher or equal to 6 units of compounds (2A)–(5A) and (2B)–(5B)
∗
will possess an enantiotropic SC phase. The siloxane molecule has the ability to increase
“effective spacer length,” and it is also assistance in the formation of SC phase and
the stabilization of SC liquid crystalline mesophase. Furthermore, the siloxane molecule
∗
∗
will not induce side chain crystallization phenomenon and dilute Ps value. Therefore, the
siloxane molecule can lower the mesophase transition temperature of the liquid crystal
compound, and simultaneously it can widen the temperature range of SC∗ liquid crystalline
mesophase, especially the trimethylsiloxyl group.
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