Design, synthesis and mesomorphic behaviour of a four-ring achiral bent-core liquid crystal in the nematic phase

Article abstract of DOI:10.1039/c6ra05125a

Bent-core nematics have attracted growing interest because of the unconventional properties and extraordinary effects exhibited by these liquid crystalline phases. In this report we design, synthesize and characterize four different four-ring achiral bent

Full text of DOI:10.1039/c6ra05125a

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Design, synthesis and mesomorphic behaviour of four-ringDOI: 10.1039/C6RA05125A
achiral bent-core liquid crystal in the nematic phase
a
a
b
b
b
Amina Nafees, Aloka Sinha,* Nandiraju V. S. Rao, Gayatri Kalita, Golam Mohiuddin
b
and Manoj Kumar Paul
Abstract
Bent-core nematics have attracted growing interest because of unconventional properties and
extraordinary effects exhibited by these liquid crystalline phases. In this report we design,
synthesize and characterize four different four-ring achiral bent-core liquid crystals
containing the methyl group in the central phenyl ring. The mesomorphic behaviour of
compounds has been investigated by optical polarizing microscopy, differential scanning
calorimetry, electro-optical investigation, and dielectric spectroscopy. The calculations based
on density functional theory of the molecules are also carried out. The addition of methyl
group in the central phenyl ring not only induces the lowering of the clearing point but also
enhances the nematic phase range. The dielectric spectroscopy reveals a low frequency
relaxation peak and a very large dielectric permittivity; which is the characteristic of
cybotactic clusters. In electro-optic studies, spontaneous polarization peaks are not observed.
However, a periodic pattern is observed under the application of an ac electric field. Finally,
the compounds show a weak tendency to crystallize; which makes it possible to supercool the
cybotactic nematic phase down to room temperature.
1
. Introduction
After the discovery of the bent-core liquid crystals (LCs) that exhibit nematic phases, the
bent-core nematics (BCNs) have been the subject of intense experimental and theoretical
investigation due to their unique properties, extraordinary effects and potential application in
1
-5
the area of fast switching displays. The nematic phases of bent-core compounds possesses
2
unusual physical properties, like giant flexoelectric response, low frequency dielectric
6
7
8
relaxation, unusual electro-convection scenarios, magnetic field induced birefringence;
which may be related to the presence of polar smectic clusters in the nematic phase. The
majority of the bent-core molecules are based on a five- six- or seven-ring rigid core in which
the central ring represents the bend unit, usually a 1,3 substituted benzene with an angle
o
between the two attached arms is 120 . Recently new bent-core compounds, such as a five-
membered heterocycle, either a 2,5-disubstituted 1,3,4-oxadiazole or a 3,5-disubstituted
o
o
6
1
,2,4-oxadiazole possessing a larger bent angle of 134 ~140 , are also reported.
The
9
, 10
nematic phases are rare in bent core systems
because of the constraints imposed by the
shape of the molecules contributing to steric hindrance for free rotation, core-core
interactions promoting the layer formation due to the segregation between extended aromatic
moieties and end alkyl chains rather than interdigitation of molecules along the orientational
direction of the long axes. The incompatibility between the bent aromatic cores and aliphatic
moieties is another limitation for the formation of nematic phases. Additionally, BCNs have
1
1, 12
been regarded as promising candidates for indefinable biaxial nematic
ferroelectric nematic
and polar
1
3, 14
phases. The fluid ferroelectric nematic phase is expected to exhibit
1

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a much faster and easier response to an external electric field in comparison to the
DOI: 10.1039/C6RA05125A
conventional ferroelectric smectic LCs and explore the possibility for the new applications in
the area of electro-optic device technology. On the other hand, biaxiality was first reported in
1
1, 12
a 1, 3, 4-oxadiazole-based bent-core mesogen,
induced; local vs. macroscopic) is still controversial.
but its very nature (spontaneous vs.
1
5
Although, there is an ongoing scientific debate on these topics, there is now a general
consensus that the unique properties of BCNs (Scheme 1a) result from the presence of
nanometre size clusters of molecules (cybotactic groups) characterized by short range smectic
9
,13, 16-19
(
Sm) positional order and biaxial orientational order.
These clusters persist all over
the nematic temperature range and they are now widely recognized as an inherent feature of
the nematic phase of bent-core mesogens, and accordingly, the phase is generally called the
9
cybotactic nematic (Ncyb, Scheme 1b) phase. The four-spot small angle X-ray diffraction
(
SAXRD) pattern typical of BCNs provides the clear evidence of tilted Smectic−C-like order
9
,10,13,15-17
within clusters.
The same pattern was initially interpreted as a signature of long-
range biaxiality. This was recently confirmed by direct observation of cybotactic groups in
1
8
cryo-transmission electron microscope (cryo-TEM) images. Another important property of
the bent-core LC which has become a major focal point for the liquid crystal research is the
1
9
twist-bend nematic phase. Recently, the existence of a new condensed phase of matter with
unique properties designated as twist-bend nematic (NTB or twist-bend, Scheme 1c) phase, a
fundamentally new type of nematic ground state of achiral molecules below the nematic
2
0-23
phase, is confirmed in n-meric (n = dimer, n = 3, trimer, n =4, tetramer) compounds
2
0-23
19,24,25
containing flexible methylene spacers
as well as rigid bent-core
molecular
2
6
materials. This was theoretically conjectured earlier. These compounds exhibit layer-free,
helical liquid crystal ordering of nano-scale pitch made up of three or four molecules (10~15
nm) and have been structurally identified and characterized. The NTB phase was also
observed in the ether and mixed ether/ester-linked bimesogens and it is concluded that the
gross topologies dictate the incidence of this fascinating phase and not molecular properties
2
7, 28
such as dipole moment and bend angle.
Scheme 1: Schematic sketches of bent-shaped molecular arrangement in the (a) nematic phase (N) and (b) cybotactic
nematic phase schematic picture showing the geometry of layers, molecules (projection of bent-shaped molecule) and
X-ray signals in cybotactic nematic phase (NCyb)(c) NTB phase showing smectic type layers (dashed lines) and tilted
molecules (projection of bent-shaped molecule) forming a helical structure with a very short pitch p (10~15 nm).
2

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BCNs are relatively uncommon and it is generally more difficult and challenging to design
DOI: 10.1039/C6RA05125A
and synthesise four-ring bent-core compounds exhibiting only nematic phase. At present,
little is known about the structural parameters that control the induction of nematic phase for
these compounds. Detailed studies and characterizations of different type of bent-core LCs
are needed to further understand the induction of the nematic phase and the physical
properties such as biaxiality and ferro-electric switching. Currently, there are dozens of bent-
core LC molecules synthesized and explored as mesomorphic material, but a majority of
6
them are five-ring bent-shaped molecules based on oxadiazole as central core or 4-
1
0
substituted phenyl ring with the presence of the lower temperature smectic phase in addition
2
9
to nematic phase. Recently few three-ring bent-core compounds exhibiting monotropic
o
nematic phases (~40±10 C) with a low temperature smectic phase are reported. However, for
practical applications, enantiotropic nematic phase with a wider temperature range, which
must be around room temperature and without the presence of the low temperature smectic or
columnar phases, would be required. A few bent-core compounds exhibiting only nematic
6
, 14,9,30
phases have been also reported.
Hence any attempt to synthesize bent-core compounds consisting of few rings say
three or four with a combination of different linking groups and a combination of substituents
to exhibit enantiotropic nematic phase at ambient temperatures is still a challenging goal for
chemists that can lead to detailed understanding of structure–property relationship as well as
its functional characteristics. It cannot be taken for granted a priori that all the new bent-core
molecular structures would still be able to form nematic phase, and it is also conceivable that
there would be a complete loss of mesophase behaviour in some cases. In this paper, the
design, synthesis, dielectric properties, electro-optical and energy optimised molecular
structure with density functional theory calculation of unsymmetrical achiral four-ring bent-
core LCs with a broad nematic phase range are presented. The salient features of these bent-
core liquid crystalline compounds are (i) wide temperature nematic phases which can also be
cooled down to room temperature without the presence of the lower temperature smectic
phase and (ii) the observation of unique 2D periodic pattern with the application of the
electric field which can be obtained and controlled by the applied electric field.
2
. Design of the molecule
The recent experimental support in favour of a twist-bend nematic phase (NTB), biaxial
nematic phase (N ) that is exhibited by rigid as well as dimer bent-core mesogens composed
b
of two rod-like mesogenic wings coupled to a central linking moiety has been debated very
well. The central linking moiety possesses a transverse dipole with an obtuse bent angle
between the two arms. This represents a banana or V-shaped molecule composed of two
uniaxial arms with a central transverse dipole. The study of the influence of dipole–dipole
correlations on the stability of the biaxial nematic phase, in the two-particle-cluster
3
1
approximation,
neighboring molecules substantially favour the stabilization of biaxial nematic phases, and
b) the electrostatic interactions between permanent transverse dipoles of bent-core molecules
revealed that (a) the polar-molecular-shape correlations between
(
also significantly stabilize the biaxial nematic phases. The introduction of a 2-methyl group
in the 1, 3-disubstituted phenyl ring of a bent-core molecular architecture can generate an
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DOI: 10.1039/C6RA05125A
obtuse bond angle of ~145°, which gives rise to an increase in bend angle as well as a dipole
in the bending direction. The reduced bend from 120 to ~145° of the 1,3-phenyl moiety by
the introduction of a methyl group in the 2-position and with the decrease in the number of
rings from five or more to four in the molecular unit, places these compounds at the
borderline between classical rod-like LCs and bent-core mesogens. The four-ring molecules
can also be recognized as true hockey-stick model molecules. Further the use of linking
groups viz., azobenzene or phenyl benzoate or salicylideneimine units in one of the arms of
the bent molecule promote mesomorphism. The introduction of methyl moiety in the central
phenyl ring with an additional methyl moiety in adjacent ring not only changes the
electrostatic interaction between adjacent molecules but also enhances the intermolecular
distance between molecules. In addition, the influence of the bent shape to negate the nematic
phase formation can be decreased by these two methyl moieties in the lateral direction in
adjacent rings. The realization of such a molecular architecture leads to a reduction in
rotational disorder as well as a dipole in the bent direction. If molecular interactions are
strong enough then the molecular structure can promote polar biaxial nematic phases. In this
study, we aimed to combine the lateral dipole in the form of a methoxy moiety at the terminal
position in one of the arms of the bent-core molecule, while the central bent core possesses
methyl substituents projected at a location inside the molecular core to represent the
transverse dipole. The other end of the molecule is linked to 4-n-alkoxysalicylaldehyde
through an imine moiety, which actually seems superior to the benzylidene aniline core and is
more stable towards hydrolysis due to intermolecular hydrogen bonding.
3
3
. Experimental
.1 Synthesis
The starting material in the present study 4-n-alkyloxy-2-hydroxybenzaldehyde was prepared
by Williamson etherification of 2, 4-dihydroxybenzaldehyde with appropriate n-alkyl
bromide. 2-methyl-3-nitrobenzoic acid was converted into corresponding amine by reduction
using 5% Pd-C, which was condensed with 4-n-alkoxy salicylaldehyde in presence of few
drops of glacial acetic acid to get 2-Methyl-3-N-(4-n-alkyloxysalicylidene) amino benzoic
acid in good yields. 2-Methyl-3-N-(4-n-alkyloxysalicylidene) amino benzoic acid was further
esterified with 3-Methyl-4-hydroxy-4ʹ-methoxyazobenzene/2-Methyl-4-hydroxy-4ʹ-methoxy-
azobenzene using DCC-DMAP reaction to yield the designed products GK3, GK4, GK5 and
GK6.(Scheme1) The compounds were further recrystallized repeatedly to get the pure
1
samples. All the compounds were characterized by elemental analysis, FTIR, UV-Vis and H
NMR studies. The synthetic procedures for all the homologous series of compounds are
3
2
followed as detailed for unsubstituted compounds in an earlier publication.
The assigned νO-H—N stretching band confirmed the presence of intermolecular H-
bonding in all the compounds. The introduction of ortho hydroxyl group in benzylidene
moiety not only enhances the stability of the imines through intramolecular H-bonding to
overcome the hydrolytic instability of the molecules towards moisture but also enhances the
transverse dipole-moment. The infrared spectra of all the compounds exhibited characteristic
-
1
-1
-1
bands in region 1460±15 cm (νN=N, azo), 1602~1620 cm (νCH=N, imine), 1740±15 cm
-1
13
(
ν
C=O, ester), 3200±50 cm (νO-H, H-bonding). The C NMR (Figure S2) data of a
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1
3
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DOI: 10.1039/C6RA05125A
representative example GK5 is furnished below. C NMR: δC (100 MHz, CDCl3) 165.9,
1
1
3
63.9, 163.7, 162.7, 161.9, 152.2, 149.6, 148.5, 147.4, 139.2, 134.2, 133.6, 130.5, 128.4,
26.6, 124.8, 124.1, 123.9, 122.6, 119.7, 116.7, 114.2, 113.8, 113.0, 107.8, 101.5, 68.3, 55.6,
1.7, 30.9, 29.0, 25.9, 22.6, 17.6, 15.4, 14.1.
Scheme2: Synthetic details of the compounds GK3, GK4, GK5 and GK6. Reagents and conditions: i. dry
acetone, KHCO3, CnH2n+1Br, KI; ii. 5% Pd/C, H2, EtOAc stirring 48h; iii. Abs EtOH, AcOH, ∆, 6h; iv. HCl,
o
NaNO2, 0~5 C, o-cresol/m-cresol, NaOH; v. DCC, DMAP, DCM, stirring 48h.Where in GK3 and GK4,
3 3
X=CH and Y=H and for GK5 and GK6, X=H and Y=CH
3
.2 Investigation
Indigenous cells with a thickness of 3.2μm were used for the investigation of the electro-
optical and dielectric behaviour. The LC cells were made with indium tin oxide (ITO) -
coated glass substrates. The inner surfaces of the cells were spin coated with Nylon6/6 and
subsequently rubbed in an anti-parallel direction for planar alignment. The cells were
assembled with the glass plates having their coated sides facing one over another and
separated by high precision spacers. The cells were filled with LC via capillary action in the
isotropic phase. The temperature of the cell was controlled by an Instec (HCS302) hot stage
attached to the STC200 temperature controller. The spontaneous polarization was measured
using polarization reversal triangular wave method. The electric field induced patterns were
observed simultaneously under the polarizing microscope. A Tektronix AFG 3021 function
generator, TPS 2024 oscilloscope, and homemade amplifier were used for the electro-optical
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measurement. Dielectric measurements were carried out using an Agilent (E4980A) precision
DOI: 10.1039/C6RA05125A
LCR meter with a probing voltage of 100 mV in a wide frequency range of 20 Hz to 2 MHz
in the nematic phase. The polarizing optical microscope (Olympus BX51P) was used to
record the texture. The textures were captured at 50X magnification in transmission mode
under crossed polarizer. The melting and clearing transition temperature of the samples were
obtained using differential scanning calorimetry (DSC) and further confirmed by polarizing
optical microscopy (POM) studies. The bent-core materials under investigation are denoted
by GK3, GK4, GK5, and GK6. The molecular structures of the compounds are given in
scheme1 and phase transition temperatures in °C (ΔH, ΔS) are summarised in Table 1, where
-1
-1 -1
the enthalpies ΔH (kJmol ) and the entropies ΔS (Jmol K ) are given in parentheses
respectively. Representative DSC thermogram for GK4 is displayed in Figure1.The inset
shows the nematic-isotropic phase transitions.
3
0
2
2
2
2
2
2.4
2.2
2.0
1.8
1.6
GK4
25
138
139
140
C
141
o
Temperature
100
110
120
130
140
o
Temperature C
Figure 1: DSC trace of GK4 obtained during second heating and cooling cycles scanned at a rate 5K/min.
Table1. Phase transition temperatures (°C) and the nematic phase thermal range (ΔN °C)
of compounds GK3 to GK6 were recorded for the heating (first row) and the cooling
-
1
(
second row) cycles at 5°C min using DSC and confirmed by polarized optical
-
1
microscopy. The enthalpies (ΔH in kJ mol ) and entropies (ΔS), are presented in
parentheses.
Compound
GK3
Phase transition temperatures °C (ΔH, ΔS)
Cr 114.6 (58.4, 18.1) N 144.4 (0.619, 0.18) Iso
Cr 65.6 (44.7, 15.8) N 143.8 (0.655, 0.19) Iso
Cr 107.7 (49.5, 15.6) N 139.8 (0.454, 0.13) Iso
No crystallization N 139.2 (0.517, 0.15) Iso
Cr 110.6 (41.7, 13.0) N 135.6 (0.495, 0.14) Iso
No crystallization N 134.9 (0.512, 0.15) Iso
Cr 104.3 (48.2, 15.4) N 132.2 (0.438, 0.13) Iso
No crystallization N 131.4 (0.548, 0.16) Iso
ΔN (°C)
29.8
78.2
32.1
>120
25
>120
27.9
>120
GK4
GK5
GK6
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DOI: 10.1039/C6RA05125A
4
. Results and discussion
4.1 Density functional theory (DFT) calculations
DFT computational studies based on quantum mechanical calculations were performed to
obtain the information related to molecular conformation, bend angle, dipole moment,
molecular polarizability and electrostatic potential distribution of the selected molecules
GK3-GK6. Full geometry optimizations have been carried out without imposing any
3
3
constraints using the Gaussian 09 program package. Spin restricted DFT calculations were
carried out in the framework of the generalized gradient approximation (GGA) using
Becke3– Lee–Yang–Parr hybrid functional (B3LYP) exchange-correlation functional and the
3
4,35
6
-311G (d, p) basis set.
B3LYP functional with the standard basis set 6-311G have been
used due to its successful application for larger organic molecules, as well as hydrogen
3
6-38
39-43
bonded systems in the past,
and bent-core molecules
recently. The molecules with
planar conformation possessing methyl substituent either downward (hindered position) or
upwards (open position) were submitted for optimization. The difference in optimized energy
between the pairs of GK3-GK3up and GK4-GK4up is ~ 0.1kJ/mol. GK3up and GK4up have
the lower energy in comparison to GK3 and GK4. Therefore, the most preferred geometry
will be GK3up and GK4up. Similarly for GK5-GK5up and GK6-GK6up the difference in
energy is ~ 9.6 kJ/mol. Hence, the GK5 and GK6 with lower energy will be the preferred
optimized geometry .The position of the methyl group in GK3 and GK3up contribute to a
small change in dihedral angle between the two neighbouring rings. The methyl group in
GK3 is more hindered whereas in GK3up the hindered environment is relaxed with a
noticeable increase in the dihedral angle thus, facilitating it to be the most preferred
optimized geometry. [Similarly for the GK4 and GK4up molecules]. In case of GK5 and
GK5up, the methyl moiety in GK5up is hindered by the interaction between the lone pair of
N atom of azo group. However, the interaction of methyl moiety in GK5 with the azo linkage
is comparatively smaller. Thus, GK5 will be the preferred optimized geometry. [Same for
o
o
GK6 and GK6up]. The difference in bend angles 142 for GK3up and GK4up and 148 for
GK5 and GK6 is noticeable (Table 1 in SI). The variation in dipole moment is not significant
but the major contribution of the dipole moment component comes from the Y and Z
components for GK5 and GK6, whereas in the case of GK3up/GK4up it comes from X and Z
components. Polarizabilities are crucial for an understanding of the molecular properties in
molecular optics and spectroscopy. Electrostatic intermolecular interaction energy is related
4
4
to this quantity, in particular for systems without a permanent dipole moment.
DFT calculated principal polarizability components (αXX, YY and αZZ), isotropic
α
iso
polarizability component α , polarizability anisotropy Δα = [αXX - (αYY + αZZ)/2] and
asymmetry parameter, η = [(αYY – αZZ)/ (αXX - α )], parameters relative to the molecular
iso
polarizability tensor, α, in the Cartesian reference frame are displayed in Table 2 in SI. The
molecular polarizability component αXX is comparatively larger along the molecular
longitudinal X-axis (Table 2 in SI) for all the molecules than the other two directions. The
iso
asymmetry parameter η = [(αYY – αZZ)/ (αXX - α )] (0.04~0.05) is rather very small for
o
GK3up/GK4up and is dependent on the bending angle, which is found to be 142 . However,
η value is 0.070~0.078 for meta methyl substituted compounds (GK5 and GK6) with a bend
o
angle of 148 . The principle components of the polarizability tensor (αXX,
αYY and αZZ) and
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iso
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α
= (αXX + αYY + αZZ)/3 is rather small. Although the molecular formula and number of
DOI: 10.1039/C6RA05125A
electrons of the two sets of studied molecules are the same, the main component of static
polarizability along the molecular long X-axis differs significantly due to the different
position of the methyl moieties in the phenyl ring adjacent to the central ring which has
manifested the bend in the molecule.
The dihedral angle between the central phenyl ring and phenyl ring with
o
benzylidene linkage is found to be 136~138 in both sets of compounds (Fig.2). The dihedral
o
angle between the central phenyl ring and phenyl ring of azo moiety is 104 in
o
GK3up/GK4up as well as 135 in GK5/GK6. Hence, these three phenyl rings are out of plane
and can be visualized to promote the twist in the molecule. Further a significant deviation in
the dihedral angles between the central phenyl ring and azo phenyl ring between two sets of
o
o
molecules ~135 for GK5/GK6, ~104 for GK3up/GK4up reflects the influence of methyl
moiety substitution in meta-position and ortho position to ester linkage.
Figure 2: DFT optimised molecular structure of GK3up and GK5 in Cartesian coordinate frame
4
.2. Liquid crystalline phase
(
a) Effects of Structural variation on transition temperature and nematic range
Table 1 depicts phase transition temperatures, enthalpy and entropies associated with the
phase transitions and nematic thermal ranges exhibited by the four newly synthesized
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compounds (GK-series). All the compounds exhibit enantiotropic nematic phase over broad
DOI: 10.1039/C6RA05125A
temperature ranges. All the four compounds exhibit only nematic phase without low
temperature smectic phases. Except for GK3, the remaining three compounds can be cooled
down to room temperature without crystallization or any other additional phase transition.
The nematic-isotropic transition temperatures of all the four compounds are in the range
between 144.4°C to 131.4°C which are not very high and decomposition does not occur at the
phase transition even after repeated heating and cooling cycles. The clearing temperatures
decrease with an increase in alkoxy chain length. The melting point decreases with increasing
difference in chain length at both ends of the molecule. The clearing temperature and melting
point depends on the position of the methyl group in the second benzene ring which reflects
the importance of the methyl substituent to decrease the intermolecular interactions. The
4
4-46
parent compound of the four-ring systems
the central phenyl ring which promoted the bent structure, exhibited polarization modulated
1rev tilted phases when identical alkyl chains (n ≥ 10) are present at both ends of the
molecule. The introduction of a methyl group in the angular phenyl moiety at the 2-position
without the methyl moiety in bay position of
B
o
(
bay position of the central core) not only increased the bent angle (>10 ) but also disturbed
the planarity in the molecule. The thermal stability and nature of the mesophase is
determined by the contributions of the polarizability of different segments of the molecule
and geometric anisotropy of the molecule. Hence the competing influences between core-
core interactions, polarizability anisotropy and geometric anisotropy of molecular
conformation contribute to the nature of the mesophase. Accordingly we observed an
o
enhanced mesomorphic range more than 100 C of nematic phase with a changed phase
4
7
variant from the banana family. This is in sharp contrast to the unsubstituted compounds
o
which exhibited banana mesomorphism with a maximum phase range of ~30 C. Further the
identical compounds with only a methyl substituent in the central ring exhibited nematic
48
o
mesomorphism at a high temperature above 100 C.
(
b)Texture observation
All the four compounds exhibited the schlieren and the marble textures, characteristic of the
nematic phase (Fig.3). On cooling from the isotropic phase, the nematic phase appeared as
droplets which coalesce to form schlieren textures as shown in Figure 3(d, e, f) for the texture
of the compound GK6 which is used as a representative example. Upon further decreasing
the temperature in the nematic phase of GK4, GK5, and GK6 the texture changes the
birefringence colour (Fig.4), indicating a strong increase of the birefringence on cooling,
most probably due to an increase of the order parameter as a result of the growth of the
clusters. Similar observations of such textural changes are reported with respect to nematic
phases exhibited by bent-core compounds as a signature of the growth of cybotactic SmC-
9
type clusters in the cybotactic nematic phase (NcybC). Another four-ring bent-core LCs
with a similar molecular structure possessing terminal alkoxy chains are studied by the
Chakraborty et al. The small angle X-ray investigations on their compounds confirmed the
cybotactic nematic phase and the existence of SmC-type cybotactic clusters with a more
4
9
conspicuous four-spot diffraction pattern in the entire nematic phase.
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A
A
A
A
P
P
A
P
P
A
P
P
Figure 3: Microphotographs showing (a, b) schlieren texture of GK3 at 90°C and of GK4 at 130°C; (c, f) the
highly birefringent marble textures of GK5 and GK6 at 127°C and 125°C respectively in the cooling cycle; (d,
e)Isotropic to nematic transition of GK-6 at T=131.5°C. The images were taken under a 50X objective of the
2
microscope using planar cell geometry with a conducting area of 25mm and thickness of 3.2m placed between
two crossed polarizers.
A
A
P
P
A
A
P
P
Figure 4: Microphotographs showing the change in birefringence with decrease in temperature of GK6; (a) at
25°C, (b) 114°C, (c) 113°C, (d) 81°C. The images were taken under a 50X objective of the microscope using
1
2
planar cell geometry with a conducting area of 25mm and thickness of 3.2m placed between crossed
polarizers.
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4
.3. Dielectric Spectroscopy
The dielectric permittivity and the electrical conductivity are the key electric properties
determining the behaviour of a compound in an electric field. The dielectric spectroscopy
yields information on the frequency dependence of the complex dielectric permittivity,
ε*(f) = ε'(f) - iε''(f) where the real part ε'(f) corresponds to the dielectric permittivity, and
ε"(f) is its imaginary parts (the dielectric loss). The dielectric studies on all the four
compounds were carried out in the planar aligned cell of thickness 3.2μm in the frequency
range 20 Hz to 2 MHz. The real part of the dielectric permittivity was calculated as ε' = C/C
o
,
where C is the capacitance of the filled cell and C is the capacitance of the empty cell. The
o
frequency dependence of loss factor (tan δ) and dielectric permittivity at different
temperature for sample GK4 and GK6 is shown in Fig.5 and Fig.6, respectively.
Figure 5: Loss factor (tan δ) verses frequency at different temperature of, (a) GK4 and (b) GK6
Figure 6: Dielectric permittivity verses frequency at different temperature of GK4 and GK6 respectively.
The loss factor (tan δ) curve shows one relaxation peak between 100 to 1500 Hz in the
measured frequency range. The loss peak appears in the low frequency region and shifted
towards the higher frequency side as the temperature is increased. The reduction in relaxation
frequency with decreasing temperature suggests an increase in cluster size as well as decrease
50
in viscosity with decreasing temperature. The low frequency process is attributed to the
collective motion of the molecules characteristic of nematic phase possessing cybotactic
molecular arrangement due to the following reasons. First, the temperature dependence of the
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dielectric permittivity and relaxation frequency of the process shows very large value of
DOI: 10.1039/C6RA05125A
dielectric permittivity (Fig.6). The large dielectric permittivity is a characteristic of a
6
, 14
cybotactic cluster as seen previously for other bent-core liquid materials.
The second
reason is that the dielectric loss peak becomes completely suppressed under the applied bias
voltage (Fig.7) reflecting that it is due to the collective process of the molecules. The same
phenomenon is observed in all the other samples.
Figure 7: Loss factor (tan δ) verses frequency at different bias voltage of, (a) GK4 and (b) GK6
4
.4. Electro-optical studies
In order to know the electro-optical property of the compounds an ac electric field is applied
across the sample. All the four compounds exhibited parallel stripe pattern along the rubbing
direction (Fig. 8 (b, f, h)) initially. However, these stripes transformed to chocolate-grid like
pattern (Fig.8 (c, g)) followed by the appearance of a turbulent state when the amplitude of
the applied voltage is increased (Fig.8d). When the frequency is increased, the formation of
parallel stripe pattern appeared as shown in the Fig.8e (GK3) and Fig.8i (GK6). The parallel
stripe pattern at low frequency was previously observed by the Wiant et al. in a bent-core
7
LC. The possible origin of parallel stripe pattern was thought to be a flexo-electric effect.
The flexo-electric effect has been reported to produce two kinds of patterns composed of
parallel stripes in calamitic liquid crystal-one at low frequencies and another at high
5
1
19
frequencies. A similar stripe pattern is also observed in achiral rigid bent-core mesogen, in
achiral bimesogen, which has a molecular structure composed of differing mesogenic core
2
8
units linked together by a methylene dioxy chain and in a mixture of monomer and
5
2
methylene units linked mesogenic dimer. The periodic stripe pattern parallel to rubbing
direction is observed in the presence as well as in the absence of the electric field. Another
pattern observed is the chocolate-grid like pattern which is observed when the frequency and
the voltage of the applied field are increased (Fig.8 (a-i)). This type of wavy pattern is
observed in the strongly twisted planar aligned cholesteric LC compounds under the
5
3
influence of ac electric field. Both parallel stripe pattern and chocolate-grid like pattern are
observed in the chiral compounds depending on the pitch of the cholesteric liquid crystal.
However, for the first time we have observed a 2D periodic pattern in bent-core LC. The
induction of periodic pattern may be due to the replacement of H-atom by the bulky methyl
group in the second benzene ring which enhances the intermolecular separation. This result is
also corroborated by DFT calculation. The DFT calculations shows a significant deviation
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from the planar shape to a twisted molecular shape (in particular GK3 and GK4,) originating
DOI: 10.1039/C6RA05125A
from the dihedral angles imposed by the methyl substituent replacing H-atom. This may have
resulted in a periodic structure and is characterized by the presence of periodic pattern in the
presence of electric field. The periodic pattern may have resulted because of the induced
twisted in molecular shape. This is also confirmed by our previous study of similar
compounds, in which the only difference is the absence of methyl group in the second
3
2
benzene ring. No change in texture in the presence of the ac electric field was observed in
our previous studied compounds without a methyl substituent, even though the dihedral
angles and polarizabilities do not deviate significantly from GK5 and GK6 (the molecular
structures, (S1) and related data (table1 and table2) are provided in SI. However, to know the
origin and properties of the periodic pattern, detailed study of the material using other
characterization techniques are under investigation.
o
Figure 8: POM texture following the application of a square wave voltage at 120 C of GK3 at (a) 0V, 0Hz,
(b)30V,10Hz (c) 40 V, 15Hz (d) 60 V, 10Hz and (e) 40 V, 5KHz , of GK4 at (f) 40V, 13Hz (g) 50V, 100Hz
and of GK6 at 30V (h) 100 mHz and (i) 100Hz. The image is taken under crossed polarizers and arrow mark
denotes the rubbing direction.
The measurement of the spontaneous polarization using the repolarization current technique
5
4
was also undertaken. A triangular wave voltage of frequency from 1 Hz to 50 Hz and
voltages up to 80 Vpp is applied but no peak corresponding to spontaneous polarization is
observed in all the four samples. When the electric current is measured through the samples
under the triangular electric fields, the current peaks are found to be absent generally in bent-
core materials exhibiting nematic phase polarization. This is due to the symmetry- allowed
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twist and splay instability which often destroy the polar structure and cancel the macroscopic
DOI: 10.1039/C6RA05125A
9
polarization. However, if an applied electric field is strong enough to cooperatively orient the
dipoles and increase the correlation length of polar order in the cybotactic domains,
ferroelectric-like switching may occur. The current response of GK4 is shown in Fig.9 as a
representative example.
Figure 9: Investigation of sample Gk4; (a) the current response at 115°C on applying a triangular wave voltage
at 50Vpp and frequency 10Hz, (b) ) the current response at 100°C on applying a triangular wave voltage at
5
0Vpp and a frequency 10Hz.
5
. Conclusions
Bent-core compounds containing methyl group in the central phenyl ring have been
successfully designed and synthesized. The addition of methyl group enhances the nematic
phase range and also lowers the clearing temperature. All the studied compounds exhibit a
wide range of the nematic phase which can be cooled down to room temperature. The
dielectric spectrum shows the presence of only a lower frequency mode in the entire nematic
range during cooling conditions. The low frequency mode indicates the formation of
temporary polarized clusters even in fluid phases. The large permittivity at low frequency is
unusual for these bent-core LCs and could be seen as another proof of the presence of
cybotactic clusters. No current peak corresponding to spontaneous polarization is observed.
This may be due to large threshold field required for these materials. Electro-optical studies
revealed the existence of 1D and 2D periodic pattern under the influence of applied ac
electric field. The two types of periodic pattern that are observed is the periodic stripe pattern
and another is the chocolate grid like pattern. For the first time, we report here the unusual
chocolate pattern, a 2D periodic pattern in the presence of the electric field in the bent-core
LC. Such a periodic pattern has potential application in variable grating mode device, in
structured nano-composite material and in nano-patterning required for advanced micro-chip
fabrication.
Acknowledgements
Department of Science and Technology (SR/S2/CMP-007/2010) and University Grant
Commission, New Delhi, India is gratefully acknowledged by A. Sinha and A. Nafees,
respectively. The authors acknowledge Dr P. Tandon, Lucknow University for useful
discussions related to DFT studies.
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a
Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India;
E-mail: aloka@physics.iitd.ac.in; Tel: +91-11-26596003 (O).
b
Chemistry Department, Assam University, Silchar-788011, Assam, India;
E-mail: drnvsrao@gmail.com; Tel: +91-9493222541
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Table of contents
Design, synthesis and characterization of four-ring achiral bent-core liquid crystal with a
broad range nematic phase which can be cooled down without crystallization.