Synthesis and characterization of ferroelectric liquid crystal dimers containing thioester and carboxylate linking groups in the inner side of the molecule

Article abstract of DOI:10.1016/j.molstruc.2007.11.024

A homologous series of chiral unsymmetrical liquid crystal dimers possessing carboxyl and carbothiol linkages nearer to the chiral center were synthesized using (S)-2-(6-methoxynapthyl-2) propionic acid as chiral starting material. All the dimers were characterized by usual spectral techniques, thermal methods and electro-optical studies. Structural effects on the mesomorphic and physicochemical properties were investigated in terms of variation of achiral chain length at both terminals and compared with our previous investigations that contain both carboxylate and both carbothiloate linkages. The microscopic investigation reveals that these dimeric compounds exhibit only SmC^* and SmA^* mesophases. The liquid crystalline behaviour of the dimers was further confirmed by DSC analysis. It was observed that the SmC^* phase range is increased significantly with increase in the achiral chain length. Whereas SmA^* phase range decreases with increase in achiral chain length. When comparing the mesomorphic behaviour of LC dimers containing -COO- and -COS- linkages, the present compounds do not exhibit metastable states but increased SmA^* and SmC^* mesophase stability. The spontaneous polarization (P_s) and tilt angle values were also measured and a largest of 40.8 nCcm^-2 and 44.5 deg were obtained, respectively.

Full text of DOI:10.1016/j.molstruc.2007.11.024

Available online at www.sciencedirect.com
Journal of Molecular Structure 886 (2008) 175–182
www.elsevier.com/locate/molstruc
Synthesis and characterization of ferroelectric liquid crystal
dimers containing thioester and carboxylate linking groups
in the inner side of the molecule
a,
a
b
*
S. Senthil , V.R. Kamalraj , S.L. Wu
a Department of Chemistry, Anna University, Sarder Patel Road, Chennai, Tamil Nadu 600 025, India
b Department of Chemical Engineering, Tatung University, 40 Chungshan N. Road, 3rd Sec., Taipei 104, Taiwan, ROC
Received 3 July 2007; received in revised form 9 October 2007; accepted 8 November 2007
Available online 23 November 2007
Abstract
A homologous series of chiral unsymmetrical liquid crystal dimers possessing carboxyl and carbothiol linkages nearer to the chiral
center were synthesized using (S)-2-(6-methoxynapthyl-2) propionic acid as chiral starting material. All the dimers were characterized
by usual spectral techniques, thermal methods and electro-optical studies. Structural effects on the mesomorphic and physicochemical
properties were investigated in terms of variation of achiral chain length at both terminals and compared with our previous investigations
that contain both carboxylate and both carbothiloate linkages.
The microscopic investigation reveals that these dimeric compounds exhibit only SmC* and SmA* mesophases. The liquid crystalline
behaviour of the dimers was further confirmed by DSC analysis. It was observed that the SmC* phase range is increased significantly with
increase in the achiral chain length. Whereas SmA* phase range decreases with increase in achiral chain length. When comparing the
mesomorphic behaviour of LC dimers containing –COO– and –COS– linkages, the present compounds do not exhibit metastable states
but increased SmA* and SmC* mesophase stability. The spontaneous polarization (Ps) and tilt angle values were also measured and a
largest of 40.8 nCcmꢀ2 and 44.5 deg were obtained, respectively.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Ferroelectric liquid crystals; Chiral dimers; Spontaneous polarization; Ferroelectric switching; Tilt angle
1. Introduction
the tails and spacer can segregate each other giving rise
to a biaxial structure, where the bent molecules are
packed in the smectic layer with uniform bend direc-
tion [6]. The SmC phase is not an intercalated one,
which generally appears in homologous with relatively
long terminal chains [7,8]. Liquid crystal with a very
wide SmC* phase are very useful material as they
may give long operational temperature ranges with high
spontaneous polarization values [9]. Compound with
the thiobenzoate group at the interface of core and
spacer are reported to enhance the SmC* phase dura-
tion [10,11]. This group shows an interesting suppres-
sion of many smectic phases and meta stable
polymorphism, and enhances the SmC* mesophase
range [12–14]. It was observed that compounds with
thiolo ester linkage shows higher SmC* phase duration
Liquid crystal dimeric compounds consist of mole-
cules containing two mesogenic units separated by a
flexible spacer, most commonly polymethylene chain
[1–3]. The attracting behaviour of liquid crystal dimeric
molecules gaining a significant role in understanding
polymorphism in liquid crystals, because of the remark-
able transition properties usually associated with prop-
erties on the parity [4] and the length of the terminal
as well as the mesogenic linking spacers [5]. If the ter-
minal chains are significantly longer than the spacer,
*
Corresponding author. Tel.: +91 4422203155; fax: +91 4422200660.
E-mail address: shendils@gmail.com (S. Senthil).
0022-2860/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.11.024

176
S. Senthil et al. / Journal of Molecular Structure 886 (2008) 175–182
than the compound with carboxylic ester linking
groups. Interestingly the Ps (spontaneous polarization)
observed for thiolo ester linking compounds were sig-
nificantly larger then the corresponding carboxylate
linking compounds [15]. The variation in mesomorphic
and physiochemical properties are largely ascribed to
the difference in dipole moment and difference in the
bend angle of the –COO– and –COS– which increases
the overall dipole moment considerably [16,17].
Our earlier investigation of LC dimeric molecules con-
taining carboxylic linking group or carbothiolo ester link-
ing group on both sides of dimeric molecule [15] reveal,
the incorporation sulphur group suppress the variety of
polymorphism and increases the SmC* phase stability
and tilt angle of the molecule. To understand the sulphur
effect further, in the present investigation, we have designed
a series of liquid crystal dimers with both carboxylate and
carbothiolate linkages nearest to the each chiral center at
inner side of the molecules and planned to study the struc-
tural property relationship by varying the achiral chain
length. The formula of the target molecules is shown
below.
2. Experimental
2.1. Reagents and preparations
The chiral starting material, (S)-2-(6-methoxynaphthyl-
2)propionic acid,6-mercapto-1-hexanol were purchased
from Tokyo chemical Industry (TCI), Japan, with purity
greater than 99%. Thin layer chromatography was per-
formed with TLC sheets coated with silica; spots were
detected by UV irradiation. Silica gel (MN Kieselgel
60, 70–230 mesh) was used for column chromatography.
The organic solvent dichloromethane was purified by
treatment with CaH2 and distilled before use. The spec-
tral data are in accordance with expected molecular
structure.
2.2. General synthetic procedure
The starting chiral material was esterified with 6-mer-
capto-1-hexanol in the presence of N,N-dicyclohexylcarbo-
diimide (DCC) and 4-(dimethylamino)pyridine (DMAP)
to produce 6-{[(2S)-2-(6-methoxynaphthyl-2)propanoyl]-
thio}hexyl(2S)-2-(6-methoxynaphthyl-2)propanoate. The
methoxy group was removed using BBr3 and the resulting
bisphenol was further esterified with 4-(4-alkoxyphenyl)
benzoic acid using DCC and DMAP to obtain the target
dimers. All the intermediates and final products were
O
O
H2n+1CnO
C
C
OCnH2n+1
O
O
H
O
O
*
*
C C O (CH2)6
CH3
S C CH
CH3
n=8,9,10,11,12
H3CO
H
C
+
*
HO (CH2)n SH
COOH
CH3
DCC/DMAP
CH2Cl2, 3Hr, rt
O
H3CO
H
O
OCH3
*
*
(C
C O H2)6
C
S C CH
CH3
CH3
BBr3/CH2Cl2
-20ºC
O
H
HO
O
*
OH
*
C O (CH )
C
2 6
S C CH
CH3
CH3
DCC/DMAP
CH2Cl2, 24Hr, rt
O
H2n+1CnO
C
OH
O
O
C
H2n+1CnO
C
OCnH2n+1
O
O
H
O
O
*
*
C C O (CH2)6
CH3
S C CH
CH3
n=8,9,10,11,12
Scheme 1. Scheme of synthesis of symmetrical dimers.

S. Senthil et al. / Journal of Molecular Structure 886 (2008) 175–182
177
purified by column chromatography on silica gel, using
dichloromethane as eluent. The synthetic procedures
for the target compounds were carried out as outlined in
Scheme 1.
1735 cmꢀ1 (Aromatic C@C Stretching): 1475 cmꢀ1 (Ar–
O–CH2 Stretching): 1228 cmꢀ1
.
1
*
H NMR (CDCl3, d ppm): 1.07 (d, 3H, CHCH3), 1.1–
1.19 (m, 2H, CH2), 3.82 (1H,*CH), 3.85 (s, 3H, OCH3),
4.05 (t, 2H, OCH2), 2.68 (t, 4H, SCH2), 7.09–7.69 (m,
6H, ArH).
2.2.1. 6-{[(2S)-2-(6-Methoxy-2-naphthyl)propanoyl]thio}-
hexyl(2S)-2-(6-methoxynaphthyl-2)propanoate
(S)-2-(6-Methoxynapthyl-2) propionic acid (2.53 g,
0.011 mol), 6-mercapto-1-hexanol (0.59 g, 0.005 mol) and
DMAP (0.12 g, 0.001 mol) were mixed in dry dichloro-
methane (25 ml). DCC (2.68 g, 0.013 mol) was then added
to the mixture and stirred at room temperature for 3 h. Pre-
cipitated materials were removed by filtration. The solution
was washed with 5% acetic acid (3 ꢁ 50 ml) and saturated
NaCl (3 ꢁ 50 ml), followed by water. The organic layer
was dried over anhydrous MgSO4 and filtered. The solvent
was removed under reduced pressure and purified by col-
umn chromatography using dichloromethane as eluent,
and recrystallized from ethanol giving white solid yield
3.5 g (90%).
2.2.2. 6-{[(2S)-2-(6-Hydroxynaphthy-2l)propanoyl]thio}-
hexyl(2S)-2-(6-hydroxy-2-naphthyl)propanoate
6-{[(2S)-2-(6-Methoxynaphthyl-2)propanoyl]thio}hexyl(2S)-
2-(6-methoxynaphthyl-2)propanoate (2.7 g, 0.005 mol) was
dissolved in dry dichloromethane (25 ml) and cooled to
ꢀ20 °C. BBr3 (2.5 ml) was added slowly under an inert
atmosphere with stirring. The reaction was continuously
followed by TLC. Dichloromethane (200 ml) was added
for quenching soon after the completion of reaction. The
solution was poured into ice-cooled saturated NH4Cl solu-
tion (250 ml). The mixture was stirred and the organic layer
was washed with saturated NaCl solution (3 ꢁ 200 ml),
water (3 ꢁ 200 ml); it was then dried over anhydrous
MgSO4, filtered and the solvent was removed under
reduced pressure. The product was purified by column
IR (KBr): 2933 cmꢀ1 (OCH3 Stretching): 1029 cmꢀ1
(O@C–S Stretching): 1663 cmꢀ1 (O@C–O Stretching):
Fig. 1. 1H NMR spectrum of dimer CTNNB.

178
S. Senthil et al. / Journal of Molecular Structure 886 (2008) 175–182
chromatography using dichloromethane/ethyl acetate (5/1)
mixture, to give a white solid; yield 1.9 g (75%).
78.78, H% 7.63%. A similar procedure was adopted for
the preparation of other compounds in the series using
the corresponding acids (Figs. 1 and 2).
IR (KBr): 2933 cmꢀ1 (OH Stretching): 3400 cmꢀ1
(O@C–S Stretching): 1660 cmꢀ1 (O@C–O Stretching):
1700 cmꢀ1 (Aromatic C@C Stretching): 1474 cmꢀ1 (Ar–
O–CH2 Stretching): 1220 cmꢀ1
.
2.3. Physical measurements
1H NMR (CDCl3, d ppm): 1.06 (d, 3H, *CHCH3), 1.17–
1.19 (m, 2H, CH2), 1.5–1.58 (m, 2H, CH2), 3.81 (q, 1H,
*CH), 4.05 (t, 2H, OCH2), 2.72 (t, 4H, SCH2), 7.09–7.69
(m, 6H, ArH) 9.72 (s, 1H, OH).
The Chemical structure for intermediates and target
materials were analyzed by nuclear magnetic resonance
spectroscopy using Joel EX-400 FT-NMR spectrometer.
Purity was checked by thin layer chromatography and
further confirmed by elemental analysis using Perkin-
Elmer 2400 elemental analyzer. Transition temperatures
and phase transition enthalpies were determined by differ-
ential scanning calorimetry using Perkin-Elmer DSC and
calorimeter at heating rate 5 °C minꢀ1. Mesophases were
principally identified by microscopic texture of the materi-
als sandwiched between two-glass plates by polarizing
optical microscopy using Nikon Microphol-FXA in con-
junction with Instec hot stage. The physical properties
of ferroelectric phase for the materials were measured in
antiparalleled-aligned cells purchased from E.H.C. Co.,
Japan. The spontaneous polarization (Ps) was measured
by triangular wave method [18]. The measurement of
optical transmittance versus applied electric field was con-
ducted using He–Ne laser (5 mW, 632.8 nm) as a probe
beam [19,20], passing through the cell between crossed
polarizers, whose axes were parallel and perpendicular
to the smectic layer normal, was detected by a photodi-
ode. The signals were detected with HP54502A digital
oscilloscope. The voltage applied to the cell was produced
by an arbitrary waveform generator (AG 1200) and
amplified by a homemade power preamplifier. The tilt
angles were measured in 5 lm homogeneously aligned cell
placed between crossed polarizer. A threshold voltage
2.2.3. 6-(1-Methyl-2-([6-((2-6-((40-(octyloxy)biphenyl-4-
yl]carbonyl)oxy)napthyl-2]propyl)oxy)hexyl]thio}-2-
oxoethyl)napthyl-2,40-(octyloxy)biphenyl-4-carboxylate
(CTONB)
6-{[(2S)-2-(6-Hydroxy-2-naphthyl)propanoyl]thio}hexyl-
(2S)-2-(6-hydroxy-2-naphthyl)propanoate (0.5 g, 0.001 mol),
4(4-octyloxyphenyl) benzoic acid (0.85 g, 0.0025 mol) and
DMAP (1.2 g) were mixed in dry dichloromethane
(25 ml). DCC (0.52 g, 0.0025 mol) was then added and then
resulting mixture stirred at room temperature for one day.
Precipitated materials were removed by filtration, and the
solvent removed under reduced pressure. The product
was purified by column chromatography using dichloro-
methane as eluent, and recrystallized from ethanol giving
white solid; yield 1 g (70%).
IR (KBr): 2932 cmꢀ1 (O@C–S Stretching): 1685 cmꢀ1
(O@C–O Stretching): 1733 cmꢀ1 (Aromatic C@C Stretch-
ing): 1474 cmꢀ1 (Ar–O–CH2 Stretching): 1265 cmꢀ1
.
1
*
H NMR (CDCl3, d ppm): 1.54 (d, 3H, CHCH3), 0.8–
1.27 (m, 19H, CH2), 2.8 (t, 4H, SCH2), 1.8–1.82 (m, 4H,
*
CH2 CH2), 3.70 (q, 1H, CH) 4.05 (t, 2H, OCH2), 2.72
(t, 4H, SCH2), 6.09–8.26 (m, 10H, ArH). Elemental analy-
sis calculated for C74H82O9S C 78.89; H 7.64; Found C
Fig. 2. DSC thermogram of dimer CTDNB.

S. Senthil et al. / Journal of Molecular Structure 886 (2008) 175–182
179
(+3 V) was applied across the cell in SmC* phase and then
the stage was rotated to such an angle where the darkest
phase was obtained. Then the potential was switched to
ꢀ3 V, the stage was rotated on the same direction to get
the darkest phase again. The difference between the two
angle measurements was noted as tilt angle [21].
Fig. 3. The representative HOPM photograph of the dimer CTONB.
Table 1
Transition temperature of dimers observed in polarizing microscope
Dimers
Transition temperature (°C)
Crystallizationa
Cr II
Cr I
SmC*
SmA*
Isob
b
b
CTONB (n = 8)
CTNNB (n = 9)
CTDNB (n = 10)
CTUNB (n = 11)
77.8
93.7
84.4
56.7
87.0
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
140
146
145
127
138
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
143
152
160
145
160
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
190
181
178
170
167
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
102.7a
CTDDNB (n = 12)
a
Crystallization point was taken from the DSC thermogram during heating cycle.
All the transition temperatures were observed in polarizing microscope during cooling cycle.
b

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S. Senthil et al. / Journal of Molecular Structure 886 (2008) 175–182
3. Results and discussion
3.1. Mesomorphic properties
200
Iso
180
The representative photograph taken in HOPM during
the first cooling cycle at 20ꢁ is shown in Fig. 3. The meso-
phase and their corresponding phase transition observed
are given in Table 1. All the dimeric molecules exhibit bire-
fringent melt. The melting point at the first heating cycle of
the compounds was in between 138 and 146 °C and isotro-
pic transition temperatures were in between 167 and
190 °C. This series of dimeric compounds exhibits SmA*
and SmC* textures during cooling from the isotropic state.
The SmA* was identified by its focal-conic optical texture
and SmC* phase was identified by its broken focal-conic
optical texture. At the transition from SmA* to SmC*,
the focal-conic fans become crossed with bars which are
transitory in nature.
SmA*
160
140
120
100
80
SmC*
CrI
CrII
Crystallization
CrI
60
The DSC thermogram confirms the formation of
liquid crystalline phase for all the compounds in the ser-
ies. On first heating, the dimeric compounds show gener-
ally two endothermic peaks, this corresponds to the
solid–liquid crystalline (Tm) and liquid crystalline-isotro-
pic (Ti) transition temperatures, respectively. But
CTDNB showed small exothermic peak at 102.7 °C dur-
ing heating with low enthalpy value, this may be due to
the crystal–crystal transitions due to all trans alignment
of methylene units. On cooling all the compounds in
the series shows two exothermic peaks between 190 and
70 °C. A broad peak is observed for crystallization point,
this indicates a large amount of energy is involved, where
as the isotropic transition consumes relatively less
amount of energy. This confirms liquid crystalline phase
formation. After the crystallization no further transition
was observed.
8
9
10
11
12
No of carbons in achiral chain
Fig. 4. Transition temperature on cooling as a function of spacer length.
gular wave voltage with field frequency and amplitude
5VP-P. All the compounds in this series exhibit single
switching current response attributable to the formation
of a ferroelectric mesophase phase in its SmC* phase.
The switching profile is presented in Fig. 5. In order to
investigate the tilt nature of the unidentified SmX* phase,
the same experiment was attempted in SmX* phase temper-
ature, but it does not show any response. It indicates that,
these phases are highly crystalline mesophase and not the
tilted in nature.
3.3. Spontaneous polarization
From the data obtained from the HOPM and DSC ther-
mogram, a phase diagram is constructed and shown in
Fig. 4. This clearly indicates that (Ti) decreases linearly
by about 5–10 °C for every addition of a carbon atom in
achiral chain. The SmC* phase range is increased with
increase in the spacer length of the achiral chain. The
SmA* phase range is decreased with increase in the achiral
chain length except for CTUNB it shows considerable
increase in the SmA* phase duration. Incorporation of sul-
phur and oxygen atom in the internal linking group with
increase in the achiral chain length has created a difference
in the bond angles of –COS– and –COO–, which produces
considerable dipole change and could be responsible for the
significant change in the mesomorphic character, meso-
phase stability and transition temperature when compare
to previously reported dimers containing only –COO– or
–COS– linkages.
The magnitudes of spontaneous polarizations (Ps val-
ues) of the dimeric compounds are plotted. It is interesting
to note that the largest Ps value among all the compounds
3
2
1
0
-1
-2
-3
3.2. Switching behaviour
0
0.05
0.1
0.15
0.2
Time (sec)
The switching current behaviour of all the compounds
was measured in a 5 lm homogeneous cell, under a trian-
Fig. 5. Switching current behaviour of CTDNB in SmC* phase at 155 °C.

S. Senthil et al. / Journal of Molecular Structure 886 (2008) 175–182
181
3.4. Tilt angle measurements
50
40
30
20
10
0
CTONB
CTNNB
CTDNB
CTUNB
CTDDNB
The tilt angles (h) were measured in a 2 lm homoge-
neously aligned cell. The temperature dependence of the
tilt angles is shown in Fig. 7 for three compounds and
the measurement was difficult in other two compounds
viz., CTONB and CTNNB which exhibits very short
SmC* phase. The tilt angle data is in accordance with
Ps measurements. CTDNB shows the maximum tilt angle
(h) of 44.5 deg and it decreases with increase in terminal
spacer.
4. Conclusion
A series of ferroelectric liquid crystal dimeric mole-
cules bearing carboxylate ester and carbothiolate linkages
nearest to the chiral center at the inner side of the mol-
ecule with variety of spacer at the terminal end (n = 8–
12) were synthesized. All the compounds were showing
anisotropic behaviour as observed by hot stage optical
polarizing microscope. They were exhibiting SmC* and
SmA* textures. The liquid crystalline behaviour of the
dimer was confirmed by DSC analysis. SmC* phase
duration is increased significantly with increase in the
achiral chain length. The SmA* phase range decreases
with increase in the achiral chain length except for
CTUNB it shows considerable increase in the phase
range. CTDNB produces the maximum Ps and tilt
angles. This study reveals that, incorporation of sulphur
considerably increase the tilt angle in SmC* phase and
suppress the polymorphism in liquid crystal dimeric
compounds.
0
5
10
15
20
25
Ti-T/ºC
Fig. 6. Spontaneous polarization as a function of temperature.
was observed in CTDNB (40.8 nC/cm2). A plot of Ps
against achiral chain length in SmC* phase temperatures
are shown in Fig. 6. The Ps value increases considerably
with increase in terminal chain up to the decyloxy deriva-
tive, and then decreases by small amount. Maximum Ps
value was observed for decyloxy chain. When comparing
with previously reported dimers [15] belongs to the same
family with different linking groups (–COO–/–COO–,
–COS–/–COS–) these class of compounds containing
mixed linking groups (–COO–/–COS–) produces large
dipole differences in neighbourhood of those two chiral
centers. This could be the reason for getting large Ps for
the present series of compounds.
Acknowledgement
This research was supported by grants from Board of
Research in Nuclear Sciences, Govt. of India (2006/37/
13/BRNS/205).
CTDNB
CTUNB
50
CTDDNB
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