Synthesis and mesomorphic properties of chiral liquid crystal dimers containing lactate units

Article abstract of DOI:10.1016/j.molstruc.2007.07.022

A series of symmetrical chiral ferroelectric liquid crystal dimers viz., alkyl bis{(R)-2-[4-(4′-decyloxyphenyl)benzoyloxyphenoxy]propionates}, ABDBP-n (n = 6-10), containing lactate units were synthesized and characterized. The length of the methylene spacer was increased from 6 to 10 in between the two chiral carbons located at the inner side of dimer. All the synthesized dimers exhibited polymorphism by showing N^*, SmA^*, SmC^* and SmX^* (unidentified) mesophase sequence. The mesophases and their corresponding transition temperatures were identified by polarized microscopy and differential scanning calorimetric measurements. Electro-optic switching studies of these dimers in SmC^* phase has exhibited ferroelectric bistable switching characteristics with the maximum spontaneous polarization of about 60.74 nC cm^-2. The tilt angles of the SmC^* phase was also measured to confirm the tilted phase characteristics.

Full text of DOI:10.1016/j.molstruc.2007.07.022

Available online at www.sciencedirect.com
Journal of Molecular Structure 877 (2008) 50–55
www.elsevier.com/locate/molstruc
Synthesis and mesomorphic properties of chiral liquid crystal
dimers containing lactate units
a,
c
c
d
b
S. Senthil *, D. Srividhya , S. Manjunathan , S. Thirumaran , S.-L. Wu
a
Department of Chemistry, Anna University, Chennai 600 025, India
Department of Chemical Engineering, Tatung University, 40 Chungshan N. Road, 3rd Section, Taipei 104, Taiwan, ROC
b
c
Department of Chemistry, Sona College of Technology, Salem, Tamil Nadu, India
d
Department of Chemistry, Annamalai University, Tamil Nadu, India
Received 1 April 2007; received in revised form 3 July 2007; accepted 11 July 2007
Available online 24 July 2007
Abstract
A series of symmetrical chiral ferroelectric liquid crystal dimers viz., alkyl bis{(R)-2-[4-(40-decyloxyphenyl)benzoyloxyphenoxy]
propionates}, ABDBP-n (n = 6–10), containing lactate units were synthesized and characterized. The length of the methylene spacer
was increased from 6 to 10 in between the two chiral carbons located at the inner side of dimer. All the synthesized dimers exhibited
polymorphism by showing N*, SmA*, SmC* and SmX* (unidentified) mesophase sequence. The mesophases and their corresponding
transition temperatures were identified by polarized microscopy and differential scanning calorimetric measurements. Electro-optic
switching studies of these dimers in SmC* phase has exhibited ferroelectric bistable switching characteristics with the maximum spon-
taneous polarization of about 60.74 nC cmꢀ2. The tilt angles of the SmC* phase was also measured to confirm the tilted phase
characteristics.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Ferroelectric liquid crystals; Mesomorphism; Electro optical properties; Liquid crystal dimmers; Chiral lactates
1. Introduction
reported in twins with the chiral centers located either at
the free end of the molecule [6] or at the inner side of the
molecule. Liquid crystals derived from lactic acid shows,
interesting ferroelectric, antiferroelectric orders and
V-shaped electro-optic switching properties [8–13]. In lactic
acid derivatives, the chiral carbon placed near to the rigid
core produces anticlinic phases, and further addition of
the same type of chiral carbon produce only SmC* phase
with pronounced mesophase stability. This behaviour of
chiral lactates tempts us to investigate the mesomorphic
properties of liquid crystal dimers having chiral lactate
units placed in between rigid rod units. Separation of chiral
centers by methylene chain will help in understanding the
behaviour of lactate units in producing polymorphic meso-
phases. This investigation deals with the synthesis of liquid
crystal dimers containing chiral centers at the inner side of
the molecule separated by methylene spacers. The effect of
Liquid crystalline dimers claim their significant contri-
bution towards the understanding of the mesomorphic
behaviour of liquid crystalline polymers as well as low
molecular weight liquid crystals [1–3]. The length and the
type of spacer between two mesogenic units in twins pro-
vide more information about the appearance of polymor-
phism in liquid crystals [4,5]. Generally, in the case of
liquid crystal dimers, groups attached to chiral center pro-
duce large variations on both mesomorphic and physical
properties [7]. The effect of length and parity of the flexible
spacer in form chirality of the chiral phase has also been
*
Corresponding author. Tel.: +91 44 22203155; fax: +91 44 22200660.
E-mail address: shendils@gmail.com (S. Senthil).
0022-2860/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.07.022

S. Senthil et al. / Journal of Molecular Structure 877 (2008) 50–55
51
the length of the spacer attached to the chiral center on the
2.3. Preparation of materials
mesomorphic and physical properties of such dimers have
been studied. The formula of the target molecule is
depicted as mentioned below.
The starting chiral material, (R)-2-(4-hydroxyphen-
oxy)propionic acid, was purchased from Aldrich Chemical
O
O
O
∗
C10H21
O
OC10H21
O
O
O
(CH2)n-2
O
O
O
∗
O
ABDBP-n(n=6,7,8,9,10)
2. Experimental
2.1. General synthetic procedure
Company with purity greater than 99%. 4-(4-decyloxyphe-
nyl)benzoic acid were prepared by reported method else-
where [17]. Thin layer chromatography was performed
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
reagents and solvents dichloromethane, cyclohexene, etha-
nol and ethyl acetate were purified before use. The sche-
matic synthetic procedures are depicted in the scheme
and the representative synthetic procedures for HBDBP-n
(n = 6) are described as follows. The value of ‘n’ indicates
the number of methylene carbons placed between the two
mesogens.
The hydroxyl group of (R)-2-(4-hydroxyphenoxy)propi-
onic acid, was protected by the benzyl group and was
esterified with various aliphatic diols in presence of N,N-
dicyclohexylcarbodiimide(DCC) and 4-(dimethylamino)
pyridine (DMAP). The benzyl group was deprotected by
using 10% palladium on carbon and cyclohexene. The
resulting bisphenol was esterified with 4-(40-alkoxyphe-
nyl)benzoic acid using DCC and DMAP to get the target
dimers. All the intermediates and final products were puri-
fied by column chromatography passing over silica gel and
using dichloromethane as eluent. The detailed synthetic
scheme is furnished in Scheme 1.
2.3.1. (R)-2-(4-Benzyloxyphenyloxy)propionic acid
(R)-2-(4-Hydroxyphenoxy)propionic acid (3.64 g,
0.02 mole) and potassium iodide (0.5 g) were dissolved in
the solution of KOH (0.85 g, 0.021 mol), water (25 ml)
and ethanol (150 ml). The mixture was heated to reflux.
Benzyl chloride (2 g, 0.02 mol) was added drop wise in eth-
anol (50 ml). After complete addition, the reaction mixture
was further refluxed for 15 h. The mixture was then cooled
and acidified with dilute HCl. The white crystals formed were
filtered and recrystallized using ethyl alcohol (Yield 80%).
1H NMR spectrum (CDCl3, d ppm): 1.63 (d, 3H,
–*CHCH3), 4.7 (q, 1H, –*CH), 5.00 (s, 2H, –CH2–), 6.8–
7.4 (m, 4H, ArH).
2.2. Characterization of materials
The chemical structures of the intermediates and the tar-
get materials were analyzed by nuclear magnetic resonance
spectroscopy using Jeol EX-400 FTNMR spectrometer.
Purity was checked by thin layer chromatography and fur-
ther confirmed by elemental analysis using a Perkin-Elmer
2400 elemental analyzer. Transition temperatures and
phase transition enthalpies were determined by differential
scanning calorimetry using Perkin-Elmer DSC7 calorime-
ter at the heating rate of 5 ꢁC minꢀ1. Mesophases were
identified by microscopic texture of the materials sand-
wiched between two glass plates by polarizing optical
microscopy using a Nikon Microphot-FXA in conjunction
with Instec HS1 hot stage.
The physical properties of ferroelectric SmC* phase for
the materials were measured in antiparallel aligned cells
purchased from E.H.C Co. Japan. The spontaneous polar-
ization (Ps) was measured by the triangular wave method
[14]. The measurement of optical transmittance versus
applied electric field was conducted using He–Ne laser
(5 mW, 632.8 nm) as a probe beam [15,16] passing through
the cell between crossed polarizers, whose axes were paral-
lel and perpendicular to the smectic layer normal, was
detected by a photodiode. The signals were detected by
using HP54502A digital oscilloscope. The voltage applied
to the cell was produced by an arbitrary wave form gener-
ator (AG1200) and was amplified by a homemade power
preamplifier.
2.3.2. Hexyl bis[(R)-2-(4-benzyloxyphenyloxy)propionate]
(R)-2-(4-Benzyloxyphenyloxy)propionic acid (1.5 g,
0.006 mol) 1,6-hexanediol (0.56 g, 0.006 mol) and DMAP
(0.12 g, 0.001 mol) were added to dry dichloromethane
(25 ml). DCC (1.32 g, 0.0065 mol) was then added and the
resulting mixture was stirred at room temperature for 3 h.
Precipitated urea was removed by filtration. The solution
was washed with 5% acetic acid (3 · 50 ml) and saturated
NaCl (3 · 50 ml) followed by water. Organic layer was
dried over anhydrous MgSO4 and filtered. The solvent
was stripped out under reduced pressure. The product
was purified by column chromatography using dichloro-
methane as eluent and recrystallized using ethanol yielding
a white solid. Yield = 1.7 g.
1H NMR spectrum (CDCl3, d ppm): 1.5(d, 3H,
–*CHCH3), 1.45–1.46 (m, 8H, –CH2–), 4.62 (q, 1H, –*CH),
5.02 (s, 2H, –OCH2), 6.8–7.42 (m, 4H, ArH).

52
S. Senthil et al. / Journal of Molecular Structure 877 (2008) 50–55
O
∗
HO
O
OH
KOH/EtOH,KI
Cl
O
∗
O
O
OH
DCC / DMAP
HO (CH2)n OH
O
∗
O
O
O
(CH2)n-2
O
O
O
∗
O
Pd/C, EtOAc,EtOH
CYCLOHEXENE
O
∗
HO
O
O
(CH2)n-2
O
O
OH
∗
O
O
C10H21
O
DCC / DMAP
OH
O
O
O
O
∗
C10H21
O
OC10H21
O
O
(CH2)n-2
O
O
O
∗
O
ABDBP-n(n=6,7,8,9,10)
Scheme 1. Scheme of the synthesis of chiral liquid crystals.
2.3.3. Hexyl bis[(R)-2-(4-hydroxyphenyloxy)propionate]
Bis[hexyl (R)-2-(4-benzyloxyphenyloxy)propionate] (1.56 g,
0.005 mol) was dissolved in a 1:1 mixture (25) of ethyl ace-
tate:ethanol (95%). Cyclohexene (1 ml) was then added to
the mixture and refluxed under slight nitrogen pressure.
The course of the reaction was followed by TLC examina-
tion. After the completion of reaction, the solvents were
removed under vacuum. The resulting product was purified
by column chromatography over silica gel using dichloro-
methane and ethyl acetate mixture (V:V = 5:1) with an
yield of 0.9 g (85%) of pure white solid. 1H NMR spectrum
(DMSO-d6, d ppm): 1.55 (d, 3H, –*CHCH3), 1.44–1.46 (m,
4H, –CH2), 4.14 (q, 1H, –*CH), 4.71 (t, 2H, –OCH2), 6.9–
7.6 (m, 4H, ArH), 9.68 (s, 1H, –OH).
2.3.4. Hexyl bis{(R)-2-[4-((40-decyloxyphenyl)benzoyloxy)
phenoxy]propionate} (ABDBP-6)
Hexyl bis[(R)-2-(4-hydroxyphenyloxy)propionate] (0.25 g,
0.001 mol), 4-(40-decyloxyphenyl)benzoic acid (0.35 g,
0.0011 mol) and DMAP (1.2 g) were added in dry dichloro-
methane (25 ml). DCC (0.31 g, 0.0015 mol) was then added
and the resulting mixture was stirred at room temperature
for one day. Precipitated materials were removed by filtra-
tion. The solvent was stripped out under reduced pressure.
The product was purified by column chromatography using
dichloromethane as eluent and recrystallized using ethanol
givinga white solid. Yield = 0.55 g. 1H NMRspectrum(CDCl3,
d ppm): 1.63 (d, 3H, –*CHCH3), 0.8–2.17 (m, –CH2), 4.16 (q,
1H, –*CH), 4.74 (t, 2H, –OCH2), 6.90–8.21 (m, ArH).

S. Senthil et al. / Journal of Molecular Structure 877 (2008) 50–55
53
3. Results and discussion
160
Iso
3.1. Mesomorphic properties
140
N*
The mesophases and their corresponding phase transi-
tion temperatures observed were summarized in Table 1.
It shows N*, SmA*, SmC* and unidentified SmX* phases
on cooling (Fig. 1). SmA* was obtained by the formation
of focal-conic fan. SmC* phase was characterized by the
existence of broken focal conic textures. SmX* phase was
appeared as paramorphotic short grainy phase which is
shown in Fig. 3. Liquid crystalline property of these phase
confirmed by observing a very low enthalpy transition in
DSC and by observing viscous fluidity while applying slight
mechanical stress on glass plates. The polarized micro-
scopic investigation of the compounds showed that the
shorter methylene spacer chain compounds (ABDBP-6
and ABDBP-7) exhibit N*, SmA*, SmC*, SmX* mesophase
sequence and compounds ABDBP-8 and ABDBP-9 exhibit
SmA*, SmC*, SmX* mesophase sequence, where as
ABDBP-10 show SmA* and SmX* mesophases. A phase
diagram as a function of transition temperatures was
drawn (Fig. 2) which reveals that, the stability of the
SmC* phase was drastically reduced when ‘n’ increases as
expected. In addition, increase of spacer between two core
units produced suppression of multiple mesophases and
enhance the stability of SmA* mesophase. ABDBP-10
exhibited considerably wide SmA* mesophase at about
103 ꢁC.
120
100
80
60
40
20
0
SmA*
m.p.
SmC*
SmX*
Cr
6
7
8
9
10
No. of carbon in spacer
Fig. 1. Plot of transition temperature as a function of spacer length on
cooling.
3
2
1
ABDBP-6
0
-1
-2
-3
3.2. Switching behaviour
The switching current behaviour of all the compounds
was measured in a 5 lm homogeneous cell, under a triangle
wave voltage with field frequency and amplitude 5 VP–P. It
was observed that, compounds ABDBP-n (n = 6–9) show a
single switching current response attributed to the forma-
tion of ferroelectric mesophase as represented by an exam-
ple of ABDBP-8 in Fig. 2. Unidentified SmX* mesophase
formed in the samples were also investigated for its switch-
ing behaviour. But it fails to show the switching response
indicated that, these phases are not tilted by nature.
0
0.05
0.1
Time (sec)
0.15
0.2
Fig. 2. Switching behaviour of ABDBP-6 in SmC* phase in 5 lm
homogeneously aligned cell at the frequency 1 Hz.
3.3. Spontaneous polarization and tilt angle measurements
The magnitudes of the spontaneous polarizations (Ps)
for ABDBP-n (n = 6–9) in the SmC* phase are plotted in
Table 1
Mesophases, transition temperatures (ꢁC) and enthalpies of transition (J/g, in square brackets) observed in the chiral liquid crystals
Compound name Transition temperature (ꢁC)a
M.Pc
Cr
SmX*
SmC*
SmA*
N*
Iso
ABDBP-6
ABDBP-7
ABDBP-8
ABDBP-9
ABDBP-10
61.12 [2.314]
62.46 [3.189]
53.75 [11.52]
59.97 [8.40]
54.69 [8.36]
•
•
•
•
–
24.8 [2.23]
23.56 [3.08]
22.89 [3.78]
18.25 [2.36]
1.525 [2.90]
•
•
•
•
•
58.59 [1.95]
60.09 [1.25]
48.45 [0.93]
42.05 [0.76]
–
•
•
•
•
–
78.45b
•
•
•
•
•
143.48 [6.86]
145.3 [1.68]
–
–
–
•
•
–
–
–
145.56 [3.99]
147.23 [9.75]
137.49 [3.09]
134.47 [7.91]
135.42 [8.81]
•
•
•
•
•
82.23b
55.56b
53.5b
33.37 [2.07]
DSC thermograms obtained on cooling at 5 ꢁC minꢀ1
.
a
Values taken from DSC thermograms at cooling rate of 5 ꢁC minꢀ1. The values in brackets are correspond to enthalpy of the transition.
Enthalpies were too small to calculate by the instrument.
b
c
Melting points were measured on first heating cycle.

54
S. Senthil et al. / Journal of Molecular Structure 877 (2008) 50–55
40
35
30
25
20
15
10
5
ABDBP -6
ABDBP -7
0
5
10
15
20
25
°
Tc-T( C)
Fig. 5. A plot of tilt angle measurement against the temperature of
ABDBP-6 and ABDBP-7 in SmC* phase in 5 lm cell.
between two mesogens, which indicates that the polariza-
tion of the molecule could largely depend on the distance
between the two chiral centers. The tilt angles were mea-
sured in a 5 lm homogeneously aligned cells placed
between crossed polarizers. A positive pulse exceeding the
threshold voltage (+2 to +3 V) applied across the cell.
The stage was rotated to some angle to attain darkest state
of the cell attained and the angle was noted. Then the pulse
was switched to negative and again rotated an angle to
attain the darkest position. The difference between these
two angles was taken as title angle. The experiment was
repeated at different temperature at the SmC* phase. The
temperature dependence of the tilt angles for ABDBP-6
and ABDBP-7 in the SmC* phase was shown in Fig. 5. It
shows that the highest tilt angle of the compounds was
about 34ꢁ and 24.5ꢁ as observed in ABDBP-6 and
ABDBP-7, respectively. The magnitudes of the tilt angle
for compounds ABDBP-8 and ABDBP-9 in SmC* phase
were not measured due to the SmC* phase was formed
monotropically with short temperature range and has crys-
tallized in the homogeneously aligned cells.
Fig. 3. Optical polarizing microscopic picture of ABDBP-7 at (a) 60 ꢁC
(SmC*) and (b) 55 ꢁC (SmX*).
70
ABDBP-6
ABDBP-7
60
50
40
30
20
10
0
ABDBP-8
ABDBP-9
0
5
10
15
20
25
°
Tc-T( C)
Fig. 4. Spontaneous polarization as a function of temperature of ABDBP-
6–9 in SmC* phase in 5 lm cell.
4. Conclusion
A series of liquid crystalline twins having lactate groups
as chiral part in the inner side of the molecule have been
synthesized for the investigation of their mesophases and
the physical properties. ABDBP-n (n = 6–9) displayed fer-
roelectric SmC* phase and ABDBP-10 exhibited only
SmA* and SmX* phases up to room temperature. The
highest Ps value was observed to be 60.5 nC cmꢀ2 and
the highest tilt angle reached 34ꢁ for ABDBP-6 in SmC*
phase.
Fig. 4. The Ps values steeply increases at the interface of
transition from SmA*–SmC* phase and it gradually
decreases before SmC*–SmX* transition. Highest Ps value
was observed for ABDBP-6 (60.74 nC cmꢀ2) and the low-
est value was observed for ABDBP-9 (4.43 nC cmꢀ2). It
is interesting to observe that the Ps value of the twins were
decreasing drastically while increasing the spacer length

S. Senthil et al. / Journal of Molecular Structure 877 (2008) 50–55
55
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The authors are grateful for the financial support from
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