Photo and electrically switchable B7 mesophase exhibiting asymmetric bent-core liquid crystals

Article abstract of DOI:10.1039/c3nj41030g

The first bent-core liquid crystal materials with a photo-active azo linkage exhibiting the B₇ mesophase are reported. The lower homologues of the bent-core compounds display the B₁ phase and higher homologues show the B₇ mesophase. The mesomorphic properties were characterized using polarized optical microscopy, differential scanning calorimetry and X-ray diffraction. The B₇ phase of these materials establishes ferroelectric switching characteristics with anticlinic organization (SmC_AP_F) of the molecules in the layers. The bent-core molecules with an azo linkage are important for photoisomerization studies, which indicate trans to cis isomerization at 30 s, whereas the reverse processes take place over 40 s in chloroform.

Full text of DOI:10.1039/c3nj41030g

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Photo and electrically switchable B7 mesophase
exhibiting asymmetric bent-core liquid crystals†
Murugesan Vijay Srinivasan,a Palaninathan Kannan*a and Arun Royb
Cite this: NewJ.Chem., 2013,
37, 1584
The first bent-core liquid crystal materials with a photo-active azo linkage exhibiting the B7 mesophase are
reported. The lower homologues of the bent-core compounds display the B1 phase and higher homologues
show the B7 mesophase. The mesomorphic properties were characterized using polarized optical microscopy,
differential scanning calorimetry and X-ray diffraction. The B7 phase of these materials establishes ferroelectric
switching characteristics with anticlinic organization (SmCAPF) of the molecules in the layers. The bent-core
molecules with an azo linkage are important for photoisomerization studies, which indicate trans to cis
isomerization at 30 s, whereas the reverse processes take place over 40 s in chloroform.
Received (in Montpellier, France)
14th November 2012,
Accepted 5th March 2013
DOI: 10.1039/c3nj41030g
www.rsc.org/njc
character and supramolecular chirality. The switchable meso-
phase is attributed to the efficient packing of bent molecules into
Introduction
Ferroelectricity is an important property for new functional and
smart materials. Ferroelectric materials exhibit electric properties
that are useful in modern optoelectronics, especially in liquid crystal
display, and they have seen rapid growth in recent years. Ferro-
electric and antiferroelectric properties arising from various forms of
liquid crystals, including calamitic, bent-shaped and polymeric
molecules, is an intense research area in liquid crystals.1–12 Niori
et al.1 first reported ferroelectric switching behavior from achiral
bent-core molecules in the smectic phase. Later, the long axes of the
BC molecules were found to be tilted with respect to the layer
normal in the lamellar B2 phase with antiferroelectric properties.13
In general, most of the bent-shaped mesogens, derived from
a 1,3-phenylene core with five aromatic rings, were found to
form numerous mesophases. These bent-shaped mesogens
exhibit a variety of phases (B1–B8) and these B phases are called
banana phases, which depend on the nature of the mesogen
and terminal alkyl chain. Among them, electro-switching has
been observed for the B2, B5 and B7 phases. The bent-core
molecules with shorter terminal alkyl chains exhibit the B1
phase, which has a columnar structure (Colr) and is generally
non-switchable. Polar switching, however, can only be observed
for long terminal alkyl chain compounds exhibiting the B2 and
B7 phases with a lamellar structure. The switching properties in
the lamellar phase of bent-core compounds is ascribed to polar
smectic layers. In addition to the polar order, if the long axes of
molecules are tilted with respect to the layer normal, the phase is
designated as a SmCP phase (where P stands for polar order). If
the polarizations in adjacent layers are the same, the SmCP phase
behaves as a ferroelectric phase. Whereas if the polarizations in
adjacent layers alternate from layer to layer, the SmCP phase
behaves as an antiferroelectric phase. The B2 phase has four types
of arrangements based on orientation of tilt and polarization in
successive layers, which results in SmCSPA, SmCAPA, SmCSPF and
SmCAPF configurations where the subscripts S and A of C, and F
and A of P, denote synclinic, anticlinic, ferroelectric and antiferro-
electric configuration, respectively.14–16
Pelzl et al.17 first reported B7 phase being of particular
interest because of its unusual nucleation as well as its beautiful
optical textures. The B7 phase forms screw-like, telephone-wire
nuclei, beaded filaments, striped focal conics, banana leaf-like
and circular domains and checkerboard textural elements.17–19
Recently, two different models of the structure of the switchable
B7 phase have been proposed using either modulated/undulated
layered SmCP or SmCPG (G denotes general model, which
explained a double tilted SmCG structure) phases.7,19
Bent-shaped liquid crystals composed of azo and imino
linkages have barely been studied.21–24 Furthermore, bent-core
compounds possessing these linkages have so far not been
reported to exhibit the B7 phase. In the present work, we
report the first example of azo compounds exhibiting the B7
mesophase. The homologous series bent-core compounds are
constructed using different linking groups, such as azo, imino,
ester and biphenyl moieties. As azobenzene is considered to be
a prototype molecular switch, the incorporation of an azo
a Department of Chemistry, Anna University, Chennai 600 025, India.
E-mail: pakannan@annauniv.edu; Fax: +91 44 2220 0660; Tel: +91 44 2235 8666
b Soft Condensed Matter, Raman Research Institute, Bangalore 560 080, India.
E-mail: aroy@rri.res.in; Fax: +91 9480836211; Tel: +91 9480836211
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3nj41030g
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Results and discussions
Mesomorphic properties of the compounds
The mesophase behaviors of the compounds (8a–8d) were investi-
gated using DSC and POM analysis. The homologous series of
asymmetric bent-core compounds (8a–8d) consist of a five-ring
aromatic core and terminal alkyl chains on both sides with even
numbers of carbon atoms (6, 8, 10 and 12). The transition
temperatures and the associated enthalpy values are summarized
in Table 1. A DSC thermogram of the compounds (8a–8d) is shown
in Fig. 1. DSC thermograms of the compounds were measured
at the rate of 5 1C minꢀ1. The compounds 8a, 8b and 8c indicate
three endo and two exo peaks during heating and cooling.
The lowest temperature, intermediate and highest temperature
peaks correspond to crystal–crystal, crystal–liquid crystal and liquid
crystal–isotropic transition, respectively. Compound 8d shows two
crystalline phases both on heating and cooling. DSC analysis
revealed that the clearing temperature of the compounds decreases
on increasing the terminal chain length and the mesophases of all
the compounds are enantiotropic in nature.
Scheme 1 Synthetic route followed for the preparation of bent-core
compounds (8a–8d).
POM textural observations of the four compounds (8a–8d)
were carried out in order to identify the different mesophases.
Among these homologous series of compounds, the com-
pounds with shorter alkyl chains (8a and 8b) show the B1
phase, as expected. The POM texture of compound 8a in the
B1 phase is shown in Fig. 2a. Compound 8b displays a typical
fan-type texture (Fig. 2b–d) of the B1 phase upon cooling from
the isotropic state, which on further cooling transforms to a
crystalline phase (Fig. 2e).32 Usually, the B1 phase is substituted
by the lamellar B2 phase on increasing the length of terminal
alkyl chains and this transition depends on the ratio of core
size to the chain length.33,34 In our case, the higher homologues
(compounds 8c and 8d) with relatively longer alkyl chains
exhibit B7 mesophases, which are depicted in Fig. 2f and 3,
linkage into the bent-core molecule is one of the important
developments in switchable organic molecules.15,25–29 An imino
linkage was introduced in these molecules, as it had been found
to be more conducive to mesomorphism. The compounds with
shorter alkyl chains exhibited the B1 phase, while the com-
pounds with relatively longer alkyl chains showed the B7 meso-
phase. The observed mesophase is further confirmed by DSC
analysis and X-ray diffraction studies. The photo and electrical
switching properties of compounds were studied by UV/Vis
spectrophotometry and an electro-optical method, respectively.
The bent-core compounds (8a–8d) were prepared by follow-
ing the synthetic pathway shown in Scheme 1. (4-(40-Hydroxy-
phenylazo)phenyl)acetamide (1)21,30 was obtained by diazo
coupling reaction of 4-aminophenylacetamide with phenol
using NaNO2/HCl, NaOH at 0 1C, alkylated by Williamson ether
synthesis with an even number alkyl bromide (n = 6, 8, 10
and 12) to obtain (4-(40-n-alkyloxyphenylazo) phenyl)acetamide
(2a–2d)21 and further 4-(40-n-alkyloxyphenylazo)aniline (3a–3d)31
was obtained by hydrolysis of 2a–2d. Simultaneously, ethyl
40-hydroxy biphenyl-4-carboxylate (4)21 was prepared by esterifica-
tion of 40-hydroxybiphenyl-4-carboxylic acid and ethanol with
conc. sulfuric acid. Then compound (4) was alkylated with an
even number alkyl bromide (n = 6, 8, 10 and 12) to obtain
ethyl 40-n-alkyloxybiphenyl-4-carboxylate (5a–5d)22 followed by
hydrolysis of (5a–5d) to obtain ethyl 40-n-alkyloxybiphenyl-4-
carboxylic acid (6a–6d).22 Further, the 3-formylphenyl
40-n-alkyloxy biphenyl-4-carboxylate (7a–7d) was obtained by
esterification reaction of 3-hydroxy benzaldehyde with 6a–6d
using DCC and DMAP as a catalyst. Finally, the target com-
pounds of 3-((4-((40-n-alkyloxyphenyl)diazenyl)phenylimino)-
methyl) phenyl 40-n-alkyloxybiphenyl-4-carboxylate (8a–8d)22
were synthesized by Schiff’s-base reaction using amine
(3a–3d) and aldehyde (7a–7d).22 Detailed syntheses of 7a–7d
and 8a–8d are given in the Experimental.
Table 1 Phase transition temperatures (1C) and enthalpies [J gꢀ1] of compounds
(8a–8d)
DSC (1C)
Sample code/
Heating
Heating
Heating
Heating
alkyl chain
length
Cr1 Cooling Cr2 Cooling B1 Cooling B7 Cooling
I
8a/6
*
*
*
*
142.2
[08.4]
*
*
*
*
178.4
[14.3]
165.4
[16.4]
172.1
[24.4]
154.9
[26.5]
167.0
[40.0]
153.3
[38.7]
121.1
[40.8]
142.4
[26.6]
*
180.2
[15.2]
175.2
[21.3]
175.9
[22.3]
171.4
[31.1]
—
—
*
*
8b/8
137.9
[09.4]
*
*
*
*
8c/10
8d/12
121.1
[09.8]
—
—
171.3
[36.3]
168.1
[42.0]
167.7
[33.0]
165.3
[17.4]
98.4
[05.1]
61.7
*
[06.4]
Note: Cr and Cr2 = crystalline phases; B7 = polar tilted lamellar phase;
B1 = rectangular columnar (Colr) phase; I = isotropic phase. (* = phase
exists; — = phase does not exist).
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Fig. 3 The POM microphotographs of compound 8d showing the growth of
spiral filaments at 164 1C, myelinic, circular, oval and fan-shaped domains in the
B7 phase at 160 1C (magnification: 50ꢁ).
Fig. 1 DSC thermograms of compounds (8a–8d).
X-ray diffraction studies
Compounds 8a and 8b, exhibiting B1 mesophase as indicated, were
further examined using XRD analysis. For example, the powder XRD
profile of compound 8b exhibits four reflections in the small angle
region with spacing d1 = 39.58 Å, d2 = 36.97 Å, d3 = 23.03 Å, d4 =
20.38 Å and a diffuse peak in the wide angle region with spacing
d5 = 4.58 Å. These small angle peaks at d1, d2, d3, d4 could be
indexed as (11), (20), (02) and (22) planes of a center rectangular
columnar structure, respectively. Thus, this phase is a rectangular
columnar B1 mesophase with columnar lattice constants a = 73.94 Å
and b = 46.86 Å. The X-ray diffraction profile of compound 8b at
160 1C is shown in Fig. 4. Compounds 8c and 8d, exhibiting B7
mesophase as indicated in the POM observations, were further
investigated using XRD analysis. As a representative, compound
8d is described in details. The 2D XRD pattern of compound 8d at
160 1C is shown Fig. 5. In the small angle region, two reflections at
d1 = 42.22 Å and d2 = 22.62 Å with the ratio of spacing as 1 : 1/2
indicate a lamellar or smectic type ordering with layer spacing as
42.22 Å.20 Furthermore, a diffuse peak in the wide-angle region with
d spacing of 4.67 Å is ascribed to liquid like in-plane order of
molecules in the layer. The tilt angle calculated from the observed
layer, spacing from XRD pattern and molecular length estimated
Fig. 2 A microphotograph of the B1 phase of compounds observed from POM,
(a) B1 phase of compound 8a at 175 1C, (b) isotropic transforming into B1 phase
of compound 8b at 169 1C, (c) developing of fan like texture of B1 phase of
compound 8b at 167 1C, (d) typical fan-type texture of B1 phase of compound 8b
at 165 1C and (e) crystallization at 154 1C and (f) circular and oval fan-shaped
domains in the B7 phase at 165 1C of compound 8c (magnification: 20ꢁ).
respectively. This new type of mesophase is exhibited during
the slow cooling from an isotropic liquid in which helical
filaments and single- and double-winded helical patterns were
observed (Fig. 3), this is the characteristic observation for the B7
phase. The molecular arrangement in the B7 phase is inter-
mediate between that of the B1 and B2 phases.6,7 Interestingly,
many fascinating and spectacular textures, like helical
filaments, striped focal conics or fan-shaped, oval domains,
myelinic and checkerboard textures have been observed in the
B7 phase.18,19 Fig. 3 exhibits the growth of spiral filaments,
circular, oval and fan-shaped domains in the B7 phase
on cooling the compound 8d from the isotropic phase. The
equidistant stripes in the circular domain or in the fan-shaped
texture or equidistant concentric circles are clearly the result of
a helicoidal periodicity. This periodicity is probably due to a
twisted distribution of the molecular tilt, which corresponds to
the pitch of helical structure, as in the chiral SmC* phase.17
Generally, the biphenyl moiety is rigid in nature, the presence
of azo and imino linkages and a biphenyl ring in the bent-
shaped molecule form stronger asymmetry, which might be
responsible for the B7 phase. Thus stronger asymmetry, rigidity
and longer methylene chain of the bent-core compounds favor
stabilization of the B7 phase.
Fig. 4 The X-ray diffraction intensity profile for the B1 phase obtained for the
compound 8b at 160 1C.
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Fig. 5 The XRD profile in the B7 phase of compound 8d at 160 1C. The inset
shows the profile in the logarithmic scale and a 2D image of the XRD pattern.
Fig. 6 The switching current response in the B7 phase of the compound
8d showing a single polarization current peak for each half period of applied
electric field.
from the molecular formula is found to be about 41.31. How-
ever the longer wavelength modulations of the layers in the B7
phase are not detected in our XRD measurements.20
arrangement (SmCAPF) of the molecules. These observations were
quite similar to those obtained for the B7 mesophase exhibited by
compounds, reported earlier.7,20 Similar electro-switching responses
were also found for compound 8c (ESI,† S5). The spontaneous
polarization (Ps) for the B7 phases of compound 8c and 8d
has been found as 10 nC cmꢀ2 and 15 nC cmꢀ2, respectively.
Electro-optical investigations
To ascertain the effect of electric field on the mesophases of
these bent-core molecules (8a–8d), detailed electro-optical
investigations have been carried out. ITO coated homemade
cells treated with polyimide were used for these experiments.
The sample thickness of 8 mm was adjusted with suitable Mylar
spacers and sample was filled in the cell by capillary action in
Photo-switching studies of bent-core compounds
isotropic phase. The sample was cooled slowly from the iso- The photo-switching properties of compound 8a were
tropic phase (1 1C minꢀ1) under a low frequency electric field to measured in chloroform using UV/Visible spectroscopy in the
obtain a planar alignment of the sample. The current responses absence and presence of UV light exposure at different time
were measured across a 1 kO resistance connected in series intervals. The absorption spectra of compound 8a is carried out
with the sample cell under the application of a triangular wave in chloroform; C = 1.3 ꢁ 10ꢀ5 M. The absorption spectra of
voltage (53 V, 75 Hz). The current responses observed in the compound 8a shows absorption maxima at 307 nm, 378 nm
mesophase of compounds 8a and 8b indicated the absence of and a shoulder band around 477 nm, as shown in Fig. 7. The
spontaneous polarization and B1 phase of these compounds absorbance in the UV region at 378 nm corresponds to the p–p*
was further confirmed by the electro-optical measurements. In transition of the E isomer, which is usually stronger and a weak
contrast, however, interesting electro-optical effects were absorbance in the visible region around 477 nm represents the
observed for the mesophase of compounds 8c and 8d. The n–p* transition of the Z isomer. The absorption spectra of
compound 8d exhibited different textures on applying different compounds 8a–8d are shown in the ESI† (S6). The absorption
electric field, as represented in S4 (ESI†), which form a needle, spectra are similar for all the compounds because of their
fan and distorted fan texture at 43 V, 53 V and 63 V, respectively. similar molecular structure, the only difference is in the
Among these the voltage at 53 is the optimum voltage to form alkyl chain, which does not alter the electronic transitions.
the fan texture. The switching current response (Fig. 6) in the Consequently, compound 8a alone was considered for photo-
mesophase of the compound 8d shows a single polarization isomerization study.15,27
current peak for each half period of applied electric field. This
Sample 8a was illuminated with UV light, resulting in a
single peak could possibly indicate ferroelectric switching decrease in the absorbance at 378 nm with an increase at
behavior of the mesophase; the single peak disappears in the 477 nm, suggesting the transformation of E to Z isomer. After
isotropic state confirming ferroelectric switching. The same is 30 s exposure of UV light, there is no change in absorption
also verified by observing that the field induced polarized state spectra, which confirms photo saturation of the E - Z
remains stable even under very low frequency at 0.5 Hz. The isomerization processes. Fig. 7 displays the E/Z photoisomeri-
single polarization current peak observed in this experiment zation of compound 8a upon exposure to UV light. Two
further supported the B7 character of the mesophase as B7 isobestic points at 338 and 447 nm are noted from Fig. 7,
phase generally found to exhibit only ferroelectric switching confirming the photoisomerization processes.15,27 A structural
characteristics.7 This type of switching behavior is indicative of conformational change of compound 8a during UV/Visible irradia-
regions with supramolecular chirality with an anticlinic-ferroelectric tion is represented in S7 (ESI†). The Z isomer of compound 8a can
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Table 2 Influence of linkage groups on mesophase behavior of five-ring bent-
core compounds
Substituents
No. of aromatic
ring/X
Compound/
sample code
Mesophase/
reference
n
P
A/11d
B/8d
C/8d
12
12
12
6/–NQCH–Ar–COO–
5/–NQCH–
5/–NQCH–
Y = COO
Y = COO
Biphenyl
B2/21
B2/24
B7
We intended to study the effect of the number of aromatic rings
of compound A, thus compound B was synthesized with five-
ring. We observed that the B2 mesophase was once again
retained, similarly the higher homologues exhibit the B2 phase
and the lower ones exhibit the B1 phase and the transition
temperature also reduced. From compounds A and B, the
number of aromatic ring was decreased from six to five,
reducing the rigidity, hence the transition temperature
decreased. Furthermore, compound C was synthesized by the
introduction of a biphenyl ring, which exhibited the unusual B7
mesophase. Here, the presence of azo, imine and biphenyl
moieties are highly asymmetric and the biphenyl moiety is
relatively rigid in nature. This suggests that the presence of a
sequence of azo, imine linkage groups and a biphenyl
ring in bent-core molecules might be responsible for the B7
mesophase. Thus stronger asymmetry, rigidity and higher
homologues of bent-core compounds favor stabilization of
the B7 phase.
The photo-isomerization behavior of compound 8a can be
compared with similar five-aromatic ring containing bent-core
compounds, which have been reported earlier.27 The photo-
isomerization studies reveal that the compound 8a testifies to E
to Z and Z to E isomerization taking place at 30 s and 40 s,
whereas reported compounds were recorded at 50 s and 60 s,
respectively.27 Though all are five-aromatic ring containing BC
compounds, compound 8a has faster photo-isomerization. This
may be attributed to the nature of the linking group (–CHQN–)
in the azobenzene as well as the rigidity of the biphenyl moiety.
The time taken for photoisomerization depends on the nature
of the linking group (electron withdrawing group/donating
group) and size of the substituent in the azobenzene moiety.35
The faster isomerization process is a key factor for optical data
storage applications.
Fig. 7 (a) Absorption spectra of compound 8a with different exposure times to
UV light (b) the absorption spectra of the shoulder band of compound 8a and
(c) the time dependence photoisomerization curve of E isomer of compound 8a
measured at 378 nm.
be converted to the E isomer by shining visible light or keeping
the solution in the dark. The Z isomer of sample 8a is illuminated
by 520 nm light; with subsequent time intervals resulting in an
increase in the absorbance at 378 nm with a decrease at 477 nm.
There is no change in the absorption after 40 s, which confirms
the reversible E/Z-forms of compound 8a upon UV/Visible light
illumination of the photoisomerization reaction represented in S8
(ESI†). Similarly, the Z isomer is slowly converted to the E isomer
when it is placed in the dark and this process is known as
the ‘‘thermal back reaction’’.15,21,24 The bent-core compounds
8a–8d exhibit very strong photoisomerization phenomena
with UV as well as visible light illumination and these are
also thermally reversible. The photoisomerization behavior of
compound 8d during UV as well as visible light illumination is
given in the ESI† S9 and S10.
Comparison of the mesophase and photoisomerization
behavior of compounds with similar structures
The mesomorphic properties of the compound (8d) can be com-
pared with our previously reported21,24 unsymmetrical bent-core
compounds with similar structures, as shown in Table 2.
Conclusion
We have presented the synthesis of asymmetric bent-core
mesogens with variable terminal chain lengths and their meso-
morphic, photo induced isomerization and electro-optical
switching properties were analyzed. Mesomorphic behavior
and physical properties strongly depend on the symmetry of
It can be seen from Table 2 that compound C (8d) exhibits the molecules. In this series, shorter chain homologues exhibit
the B7 phase, whereas compounds A and B display the B2 phase. a rectangular columnar B1 phase and the higher homologues
In this asymmetric BC compound A was synthesized with six- exhibit the B7 phase. The stronger asymmetry and rigidity of the
rings using different linking groups, azo, imine and ester, bent-core molecules appear to favor the stabilization of the B7
which show the B2 phase with a higher transition temperature. phase. The B7 mesophase shows the ferroelectric-anticlinic
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organization (SmCAPF) of molecules in the layers. The photo- (Sankyo, Japan) for UV (365 nm), (Hitachi, Japan) for visible
isomerization properties of compound 8a revealed that E light illumination (520 nm).
to Z isomerization takes place after 30 s, whereas the reverse
Synthetic procedures
process takes place after 40 s in chloroform as solvent. All
the compounds establish photo-induced as well as thermally
Synthesis of 3-formylphenyl 40-hexyloxybiphenyl-4-carboxy-
reversible switching behavior. The photo-isomerization process late (7a). The 3-formylphenyl 40-hexyloxybiphenyl-4-carboxylate
and ferroelectric switching behavior of bent-core compounds was synthesized by esterification reaction of hexyloxybiphenyl-
might be exploited in the fields of optical data storage devices, 4-carboxylic acid (0.04 mol) with 3-hydroxybenzaldehyde
molecular switches and pyroelectric materials.
(0.4 mol) using dicyclohexylcarbodiimide (DCC) (0.4 mol) and
5% wt of 4-dimethylaminopyridine (DMAP) used as a catalyst in
methylene chloride (DCM) (200 mL). The resultant mixture
was stirred for 12 h at room temperature under a nitrogen
atmosphere. After completion of reaction, the precipitated
byproduct urea was filtered out, then the filtrate was evaporated
and product was purified by silica gel column using chloroform
as eluent. After recrystallization in ethanol, 3-formylphenyl
40-6-hexyloxybiphenyl-4-carboxylate (yield: 95%) was obtained
as a white powder. A similar procedure was adopted for
preparation of the other members of the even number alkyl
series, such as octyl, decyl and dodecyl (7b–7d).
7a: C26H26O4; calcd C, 77.59; H, 6.49; O, 15.90; found C,
77.18; H, 6.48; O, 16.34. FT-IR; 2992, 2921, 1772, 1727, 1603,
1467, 1399, 1274, 1234. 1H NMR (CDCl3, 400 MHz), d (ppm):
9.86 (m, 1H, –Ar–CHO), 8.24 (m, 2H, –Ar–H), 7.77–7.63 (m, 4H,
–Ar–H), 7.43–7.33 (m, 4H, –Ar–H), 6.86 (m, 2H, –Ar–H), 3.96 (m,
Experimental
Materials
4-Aminophenylacetamide (Sigma-Aldrich, India), phenol,
sodium nitrate, potassium hydroxide, 40-hydroxybiphenyl-4-
carboxylic acid, sodium hydroxide, potassium carbonate,
hydrochloric acid (Merck, India), 3-hydroxybenzaldehyde,
potassium iodide, N,N-dicyclohexylcarbodiimide (DCC) and
4-(dimethylamino)pyridine (DMAP) (Aldrich), were used as
received. Solvents such as ethanol, dichloromethane, ethyl
acetate, chloroform and acetone were purified by the reported
procedure.30
Instrumentation
The infrared spectra of compounds were obtained from Perkin 2H, –OCH2–), 1.71 (d, 2H, –CH2–),1.31–1.27 (dd, 4H, –CH2–),
1
Elmer FT-IR spectrometer using KBr pellets. H and 13C NMR 0.98 (m, 3H, –CH3). 13C NMR (CDCl3, 400 MHz), d (ppm): 190.9,
spectra were recorded on a Bruker 400 MHz spectrometer in 165.1, 156.4, 151.8, 141.7, 137.1, 130.4, 129.8, 128.4, 127.4,
CDCl3 with TMS as an internal standard. DSC measurements 114.1, 68.2, 31.9, 29.7, 25.7 and 14.1.
were performed at a heating rate of 5 1C minꢀ1, and samples
Synthesis of 3-((4-(40-hexyloxyphenylazo)phenylimino) methyl)
taken in an aluminum pan with a pierced lid, in a dry nitrogen phenyl 40-hexyloxybiphenyl-4-carboxylate (8a). The synthetic
atmosphere with an empty pan as reference on a TA instrument procedure for 3-((4-(40-hexyloxyphenylazo)phenylimino)methyl)
(model Q 10). The LC texture of compounds were observed phenyl 40-hexyloxybiphenyl-4-carboxylate (8a) was as follows:
using
a
Euromex polarizing optical microscope (POM) 4-(40-hexyloxyphenylazo)aniline (3a) (0.0064 mol) and 3-formyl-
equipped with a Linkem HFS91 heating stage and a TP-93 phenyl 40-hexloxybiphenyl-4-carboxylate (0.0064 mol) were
temperature programmer. Samples were placed in between dissolved in ethanol (50 mL) and the reaction mixture was refluxed
two thin glass cover slips and melted with heating and cooling until a yellow color solid precipitated out. After completion of
at the rate of 2 1C minꢀ1. The photographs were taken with a the reaction, the desired product was filtered out and washed
Canon 1000 D camera. The field induced LC textures of with ethanol, then the product was recrystallized from ethanol
compounds were observed by a Nikon microscope and sponta- to obtain a yellowish brown powdery compound 8a (yield: 71%).
neous polarization (Ps) of the B7 phase was measured by a A similar procedure was adopted for the preparation of the
triangular wave method in a planner-aligned cell using a digital other members of the even number alkyl series, such as octyl,
oscilloscope (Agilent, model MSO6012A) and an arbitrary wave- decyl and dodecyl (8b–8c).
form generator (Agilent, model 33220A) coupled with a power
8a: C44H47N3O4; calcd C, 77.50; N, 6.16; H, 6.95; O, 9.39;
amplifier (Trek, model 601). X-ray diffraction (XRD) studies of found C, 77.38; H, 6.88; N, 6.36 O, 9.38. FT-IR; 3042, 2933, 2860,
non-oriented samples filled in Lindemann capillary (diameter 1735, 1405, 1244, 2001, 842. 1H NMR (CDCl3, 400 MHz), d
0.7 mm) were performed using Cu-Ka radiation from a rotating (ppm): 8.54 (s, H, –CHQN–), 8.27 (m, 2H, –Ar–H), 7. 95 (m, 2H,
anode generator (Rigaku Ultrax-18) with a graphite crystal as –Ar–H), 7. 90 (m, 2H, –Ar–H), 7.66 (m, 2H, –Ar–H), 7.50 (m, 2H,
monochromator. The diffraction pattern of each sample was –Ar–H), 7.42 (m, 2H, –Ar–H), 7.37–31 (m, 2H, –Ar–H), 7.30 (m,
recorded on a 2D image plate (Marresearch). Some of the XRD 2H, –Ar–H), 7.02 (m, 2H, –Ar–H), 6.91 (m, 2H, –Ar–H), 4.06 (m,
measurements were performed using 1D-XRD using Pan 4H, –OCH2–), 1.86 (d, 4H, –CH2–), 1.51–1.45 (dd, 12H, –CH2–),
Analytical-DY1042-Empyreau XRD with PDS and Pix cell. 0.88 (m, 6H, –CH3). 13C NMR (CDCl3, 400 MHz): 165.2, 159.7,
The absorbance spectra of compounds were measured on a 155.4, 151.4, 144.3, 141.7, 132.1, 130.6, 129.4, 127.6, 126.4,
Shimadzu UV-1650 spectrophotometer using chloroform as 123.6, 122.9, 120.4, 114.2, 68.3, 32.1, 29.5, 25.7 and 14.4.
solvent. The photoisomerization of azobenzene between the E
Syntheses of compounds 1 to 6a–6d were followed based on
and Z-form was carried out, using a 6 W high-pressure Hg lamp previous reports.21,22
c
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NJC
Paper
15 M. Vijay Srinivasan and P. Kannan, J. Mate. Sci., 2011,
46, 5029.
16 (a) H. Takezoe and Y. Takanishi, Jpn. J. Appl. Phys., 2006,
45, 597; (b) X. Chen, K. T. Kishore, C. Y. Li, Y. Bai, X. Wan,
X. Fan, Q. F. Zhou, L. Rong and B. S. Hsiao, Macromolecules,
2007, 40, 840.
17 G. Pelzl, S. Diele, A. Jakli, C. Lischka, I. Wirth and
W. Weissflog, Liq. Cryst., 1999, 26, 135.
18 C. K. Lee and L. C. Chien, Liq. Cryst., 1999, 26, 609.
19 G. Pelzl, S. Diele, A. Jakli and W. Weissflog, Liq. Cryst., 2006,
33, 1513.
Acknowledgements
M.V and P.K sincerely acknowledge to Council of Scientific and
Industrial Research (CSIR), New Delhi, India [CSIR scheme no.
01(2095)/07/EMR-II] for the financial support. M.V. is honestly
thankful to University Grants Commission, New Delhi, Government
of India [UGC Meritorious Research Fellowship-Fl4-112006(BSR0/
7-7I2007) (BSR)]. Instrumentation facilities provided under
FIST-DST, DRS-UGC to Department of Chemistry, Anna
University, Chennai and Soft Condensed Matter, Raman
Research Institute, Bangalore are gratefully acknowledged.
20 S. Umadevi, A. Jakli and B. K. Sadashiva, Soft Matter, 2006,
2, 215.
21 M. Vijay Srinivasan, P. Kannan and A. Roy, Liq. Cryst., 2012,
39(12), 1465–1475.
22 M. Vijay Srinivasan, P. Kannan and A. Roy, J. Polym. Sci.,
Part A: Polym. Chem., 2013, 51, 936–946.
23 N. G. Nagaveni, A. Roy and V. Prasad, J. Mater. Chem., 2012,
22, 8948.
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