Effect of methoxy group instead of polar group in the nematic phase of four-ring bent-core liquid crystals

Article abstract of DOI:10.1039/c4ra12162g

Two achiral four-ring bent-core liquid crystal possessing an alkoxy chain at one end of the molecules and methoxy group at the other end are investigated by using the optical polarizing microscope, electro-optical studies, and dielectric spectroscopy. Bot

Full text of DOI:10.1039/c4ra12162g

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Effect of methoxy group instead of polar group in the nematic
phase of four-ring bent-core liquid crystals
Amina Nafeesa, Gayatri Kalitab, Manoj Kumar Paulb, Aloka Sinhaa and Nandiraju V. S. Raob
Two achiral fourꢀ ring bentꢀcore liquid crystal possessing an alkoxy chain at one end of the molecules and methoxy group
at the other end are investigated by using the optical polarizing microscope, electroꢀoptical studies, and dielectric
spectroscopy. Both the compounds exhibit enantiotropic nematic phase over a wide range of temperature. In the dielectric
spectroscopic studies of the compounds, only one sharp peak in the loss curve is observed in the entire nematic range. In
addition, the dielectric permittivity of the sample is very large. The molecular structures of these two compounds are
similar to the compounds studied by Ghosh et al., which are reported to exhibit the ferroelectricꢀlike switching and electroꢀ
convection pattern. However, the differences are (a) replacement of the polar halogen substituent at one end of the
molecule by a methoxy group and (b) an increased length of the alkyl chain at the other end. These studies confirm the
requirement of a polar moiety for the ferroꢀnematic phase to exist in these fourꢀring bentꢀcore compounds.
1 Introduction
The investigation of liquid crystals composed of bentꢀcore molecules provides us with new and unexpected electroꢀ optical
properties. The most attractive aspects of this new class of liquid crystals are in their polarity 1 and chirality2, 3 despite being
formed from achiral molecules. Some of the smectic phases exhibited by these bentꢀcore liquid crystals are the first
ferroelectric and antiꢀferroelectric liquid crystalline phases realized without introducing chirality. In this class of materials,
the nematic phase is uncommon due to a strong tendency for smectic layering generated by close packing of the kinked
molecules. However, very recently a number of new bentꢀcore compounds with nematic phases (BCNs) have been
7ꢀ10
synthesized.4ꢀ6 Simultaneously, there has been a surge in the theoretical studies
of these compounds predicting
intriguing new thermotropic nematic and isotropic structures. These include biaxial phases, orientationally ordered but
optically isotropic phases, and even spontaneously chiral and polar liquid phases; these are all made theoretically possible
by the bent shape of the molecules. A nematic phase exhibited by these bentꢀcore compounds is rare and less abundant than
the nematic phase exhibited by calamitic counterparts, because of the kink shape and aromatic interactions, which prohibit
translational freedom. As a result, bentꢀcore nematic (BCN) phases exhibit some unique physical properties such as
biaxiality,11 giant flexoꢀelectricity,12 unprecedented scenarios in electroꢀconvection,13 magnetic field induced
birefringence,14 nonꢀNewtonian fluid rheology 15 and ferroꢀnematic phases or polar response in electroꢀoptical switching.16ꢀ
18
Of the aboveꢀdefined properties of the bent –core liquid crystal, one of the LC phase is considered to be very important
in LC research because it exhibits the illusive biaxial nematic phase and ferroꢀnematic phases. Thermotropic biaxial
nematic phases would not only be of fundamental scientific interest for soft matter physics, but also these phases are
considered as possible candidates for next generation LC displays with enhanced switching performance.19 Moreover, the
fluid ferroelectric nematic phase is expected to exhibit a much faster and easier response to an external electric field
compared to conventional ferroelectric smectic liquid crystals; therefore, the discovery of such a phase could open new
avenues in electroꢀoptic device technology. First, Francescangeli et al. reported the ferroelectric response to a switching
electric field in the nematic phase exhibited by 1, 2, 4ꢀoxadiazole derivatives.16 This is the first demonstration of ferroꢀ
nematic phase in low molar mass thermotropic nematic liquid crystal and this is connected with field induced biaxiality.
Later Shanker et al. reported the ferroꢀelectric like switching in the four series of new 1, 2, 4ꢀoxadiazole derived bentꢀcore
liquid crystals.17 They showed that this is a general feature of the nematic phases of structurally different 3, 5ꢀdiphenylꢀ1,
2, 4ꢀoxadiazole derivatives. However, recently Ghosh et al. reported ferroelectricꢀlike switching in the nematic phase of
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unsymmetrical achiral fourꢀring bentꢀcore liquid crystals.18 These bentꢀcore molecules, exhibit a large nematic phase range
(>70° C). It consists of two unequal lengths in two wings and possesses a polar moiety at one end and alkyloxy chain at the
other end. They observed a distinct single polarization current peak under a triangular wave voltage and a low frequency
relaxation mode in the dielectric spectra, indicating the presence of clusters with higher ordering, not only in the entire
nematic phase, but also extended in the isotropic phase.
In this report, a fourꢀ ring bentꢀcore liquid crystal with similar chemical structure as studied by Ghosh et al. but with
minor modification in end substituent is studied. The only difference is the presence of a polar fluoro or chloro substituent
at one end and an alkyloxy group (C6H13O) at the other end in their compounds. However, in our compounds there is a
methoxy group at one end and an alkyloxy group (C7H15O, C8H17O) at the other end of the molecule. The advantages of
these fourꢀring compounds viz., negative bendꢀsplay anisotropy to formulate broad range blue phases, positive
birefringence, positive dielectric anisotropy and relatively low viscosities comparable to rodꢀlike liquid crystals, are
beneficial for electroꢀoptic devices.20 In the present work, the relaxation modes using dielectric measurements, spontaneous
polarization studies by triangular wave method, texture properties using polarizing microscope and texture of the liquid
crystal under the ac electric field were studied.
2 Experimental
2.1 Synthesis
GK1: R = OC7H15; GK2: R = OC8H17
Scheme1 Synthetic details of compounds GK1 and GK2: (i) HCl, NaNO2, 0ꢀ5°C, Phenol, NaOH; (ii) Abs EtOH,
AcOH, ꢁ, 6h; (iii) DCC, DMAP, DCM, stirring 48h.
4ꢀhydroxy 4/ꢀmethoxy azobenzene (4) and 2ꢀmethyl 3ꢀNꢀ(4ꢀnꢀalkyloxysalicylidene)ꢀamino benzoicacid (5) were
synthesized using the appropriate starting materials following the procedures reported earlier as described in Scheme 1.
The synthesis and characterization of the compounds GK1 and GK2 are presented in the ESI. Elemental analyses of all the
compounds were consistent with the proposed molecular formulae. All the compounds were characterized by FTIR, UVꢀ
Vis and 1H NMR studies.
2

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2.2 Investigation
The two GKꢀseries samples with an alkyloxy group at one end and methoxy group at the other end of the molecule are
studied. The phase sequence of these two samples is obtained from differential scanning calorimetry (DSC) (PerkinꢀElmer
Pyrisꢀ1 system) and further confirmed by polarizing optical microscopy (POM) studies. The phase transition temperature
and molecular structures of the GK1 and GK2 compounds are summarised in Table1. The DSC curve of GK2 is shown in
Fig.1 as a representative example. Both the samples are found to exhibit nematic phase. The longitudinal polarizability of
these lengthy molecules aided by the kinkꢀshape of the molecules promotes the formation of layered phase. Since these
compounds do not exhibit smectic phases it is implied that the lateral attraction between molecules is not large enough to
form a layer. However, the optical observations revealed nematic phase with smectic C type cybotactic clusters (NcybC
)
which are described below. The introduction of lateral dipole in the form of oxygen atom in methoxy moiety at one end of
the molecule increases the liquid crystalline range of nematic phase as well as clearing temperature.
Table1 Molecular structure and phase sequence of compounds GK1and GK2. Phase transition temperatures (°C) and the nematic phase
thermal range (ꢁN °C) of compounds GK1 and GK2 were recorded for the heating (first row) and the cooling (second row) cycles at 5°C
minꢀ1 using DSC and confirmed by polarized optical microscopy. The enthalpies (ꢁH in kJ molꢀ1) and entropies (ꢁS in J molꢀ1Kꢀ1), are
presented in parentheses.
GK1
GK2
Compound
Phase transition temperatures °C (ꢁH, ꢁS)
Cr 148.4 (41.9, 99.4) N 182.4 (0.45, 1.00) Iso
Cr 105.8 (31.3, 82.8) N 179.1 (0.32, 0.72) Iso
Cr 149.5 (41.4, 97.9) N 177.4 (0.44, 0.98) Iso
Cr 94.4 (22.8, 62.2) N 172.4 (0.58, 1.30) Iso
ꢁN (°C)
34
GK1
73.3
27.9
78
GK2
An indium tin oxide (ITO) coated planar aligned homemade cell with the thickness of 3.2 ꢂm was used for the study of
given bentꢀcore liquid crystal. The sample was introduced into the cell using capillary action in its isotropic state 5°C above
the clearing temperature. The temperature of the cell was controlled by an Instec HCS302 hot stage attached to the STC200
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temperature controller. The dielectric measurements were carried out using an Agilent (E4980A) precision LCR meter in
the frequency range of 20Hz to 2MHz. The texture of the liquid crystal was studied using the Olympus (BX51P) Polarizing
microscope. The polarizer and analyzer of the microscope were at 90° with respect to each other. For electroꢀoptical
measurement, a Tektronix AFG3021 function generator, TPS2024 oscilloscopes were used. The spontaneous polarization
was measured using polarization reversal triangular wave method. The electroꢀconvection patterns of the liquid crystal
were observed simultaneously under the polarizing microscope with the application of ac electric field.
50
o
G K 2
1 49 .5
C
45
40
35
30
25
20
15
27
26
25
24
23
o
1 77 .4
C
o
1 77 .4
C
o
1 72 .4
C
180
170
o
Temperature
C
o
17 2 .4
180
C
o
9 4.4
C
80
100
120
1 40
160
20 0
o
Te m p e ra tu re
C
Fig. 1 Representative example of DSC heating and cooling curves of compound GK2
.
3 Results and Discussion
3.1 Texture observations
The mesomorphic behaviour of compounds GK1 and GK2 was characterized by polarized optical microscopy (POM) with
polarizer and analyser assembled at 90° with each other. Upon cooling from the isotropic phase, both the samples exhibited
wide range nematic phase, which cooled to crystalline phase without low temperature smectic phase. Thin films of both the
samples sandwiched between a nylonꢀ6/6 treated glass plate and a coverslip exhibited characteristic schlieren and marble
textures with fluidity under crossed polarizers confirming the nematic phase (see Fig.S1 in the ESI). This indicates a
predominantly homogeneous alignment of the sample with the nematic director being on average parallel to the substrate
surface. The characteristic feature of this fourꢀ ring bentꢀcore liquid crystal is a strong increase of the birefringence
immediately below the isotropicꢀtoꢀnematic transition. Upon decreasing the temperature, the texture changes the
birefringence colour (Fig. 2), indicating a strong increase of birefringence on cooling; most probably due to an increase of
the order parameter as a result of the growth of the cybotactic clusters. Similar observations of such textural changes are
reported with respect to nematic phases exhibited by bentꢀcore compounds as a signature of the growth of cybotactic SmC
clusters in the cybotactic nematic phase.21 Chakraborty et al. studied another similar compound, with an identical bentꢀcore
molecular structure possessing terminal alkyloxy chains. A small angle Xꢀray investigations on this sample confirmed the
cybotactic nematic phase and the existence of SmCꢀtype cybotactic clusters with a more obvious fourꢀspot diffraction
pattern in the entire nematic phase range.22 The colours of the texture continuously change with decreasing temperature,
but not as rapidly as near the transition temperature.
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a
b
c
d
Fig. 2 Microphotographs showing the change in texture colour with decrease in temperature of GK1; (a) at 180.3°C, (b) 165°C, (c)
161°C, and (d) 132.7°C
3.2 Dielectric spectroscopy
Dielectric spectroscopy of both the compounds was carried out using the sample sandwiched in a planar aligned cell of
thickness 3.2ꢂm in the frequency range 20Hz to 2MHz. The measuring voltage was 0.1Vpp. The complex dielectric
permittivity in terms of real (ε’) and imaginary (ε") parts is given by the relation:
ε*(f) = ε'(f) ꢀ iε"(f)
(1)
Where, f is the frequency of the applied electric field. The spectrum of the real part of the dielectric permittivity is called
the dispersion curve and that of imaginary part is called the absorption or dielectric loss curve. The frequency dependence
of loss factor (tan δ) at different temperature for GK1 samples is shown in Fig. 3. Fig. 3(a) shows that there is only one
relaxation peak and it appeared in the low frequency region. To further confirm the single relaxation peak we also carried
out frequency dependence of the loss factor (tan δ) at different temperatures in a thicker cell of dimension 7.7 ꢂm and the
results are presented in ESI (see Fig.S2 in ESI). The appearance of single peak in the low frequency domain suggests a
collective mode. The peak of the curve shifted towards the higher frequency side as the temperature is increased. This
implies that the relaxation frequency increased with an increase in the temperature. The increase in relaxation frequency
seen with an increase in temperature is due to a reduction of the viscosity of the material. The frequency dependence of the
real part of dielectric permittivity of the samples GK1 at different temperatures is shown in Fig. 3(b). Fig. 3(b) shows that
the dielectric permittivity is very large for the samples. The large dielectric permittivity is a characteristic of the ferroꢀ
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electric cluster, as reported previously for other bent–core materials.17 Similar kinds of loss factor (tan δ) peaks and large
dielectric permittivity are also observed in the dielectric spectra of the sample GK2 (see Fig.S3 in ESI). The influence of
the bias voltage on the relaxation mode is also studied. The peak is suppressed under the applied voltage (Fig. 3(c) and
3(d)) which shows that this is the collective process.
.
Fig. 3 Investigation of sample GK1: (a) the loss factor (tan δ) as a function of frequency at different temperature, (b) Dielectric
permittivity (real part) as a function of frequency at different temperatures and the loss factor (tan δ) curve of GK1 at different bias
voltage at temperatures (c) 165°C and, (d) 125°C
3.3 Electro-optical study
To verify the presence of polar ordering, the electrical response and switching behaviour of GK1 and GK2 are measured in
the entire nematic range by means of the repolarization current technique. A triangular wave with peakꢀtoꢀpeak voltage of
up to 80Vpp was applied across the sample. No peak corresponding to spontaneous polarization was observed in both the
samples from the frequency range 1Hz to 50Hz. The response curve obtained in both the samples at frequency 10Hz is
shown in ESI (Fig.S4). This observation could be explained by considering the symmetryꢀallowed twist and splay
instability which often destroy the polar structure and cancel the macroscopic polarization. Thus, in general, in bentꢀcore
materials exhibiting polarization in nematic phase, current peaks are found to be absent when the electric current is
measured through the sample under triangular electric fields. 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, ferroelectricꢀ
like switching may occur.20 To study the electroꢀconvection pattern an ac electric field is applied across the sample, but no
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electroꢀconvection pattern was observed for the sample at applied voltage from 10Vpp to 80Vpp or in the frquency range
from 1Hz to 100Hz. There is no change in the texture of liquid crystal under the applied ac electric field (Fig.S5 in ESI)
3.4 Density Functional Theory Calculations
The quantum mechanical calculations of molecular properties were performed using density functional theory (DFT),23 by
employing the combination of Becke3ꢀLeeꢀYangꢀParr (B3LYP) hybrid functional and 6ꢀ311G(d,p) basis set using the
Gaussian 09 package, to obtain the information related to molecular conformation, bend angle and dipole moment of the
compounds with fluoro (7ꢀF), chloro (7ꢀCl), methyl (7ꢀMe) and methoxy (GK1) substituents at one end of the molecule.
Full geometry optimizations have been carried out without imposing any constraints.23 Spinꢀrestricted DFT calculations
were carried out in the framework of the generalized gradient approximation (GGA) using B3LYP hybrid functional,
24,25
exchangeꢀcorrelation functional and the 6ꢀ311G (d,p) standard basis set
due to its successful application for larger
organic molecules, as well as hydrogen bonded systems in past,26ꢀ28 and bent core molecules29ꢀ32 recently. The results are
summarized in table 2.
Table 2 DFT calculated bend angle Θ as shown in Figure , dipole moment components (ꢂX, ꢂY, ꢂZ), and the
resultant dipole moment (ꢂ).
Dipole moment (Debye)
Compounds
Bend angle (o) Θ
µresultant
=
2
µx
µy
µz
(µx2+µy +µz2)1/2
6.59
5.17
5.95
3.23
3.44
2.46
2.26
147
146
142
146
7-F
7.24
7-Cl
7-CH3
7-OCH3
2.76
1.59
1.60
3.13
3.33
1.24
4.61
3.72
The values relative to angles and dipole moment are expressed in degree (o) and Debye (D) respectively.
The three dihedral angles as shown in Fig. 4 between the first phenyl and central phenyl moieties with salicylidene imine
linkage, central phenyl ring and first phenyl ring of azo moiety with an ester linkage are 136o, 135o respectively reflecting
the absence of coplanarity of three phenyl rings with the exception being the two phenyl rings with an azo linkage (~180 o)
in the molecule. The analysis shows that the bendꢀangle is approximately 146o for almost all mesogens and is influenced
by the transverse methyl group in 2ꢀposition of the central phenyl ring. The calculation of the bending angle using DFT
theory gives only a possible estimation. Recently, such relationships were discussed for 2ꢀmethyl substituted threeꢀring
bentꢀcore mesogens also.33 For all the mesogens, the dipole points almost in the lateral direction with respect to molecular
long axis. All molecules possess a considerably larger dipole along the molecule’s long axis (Xꢀaxis) except in methoxy
homologue (GK1). In GK1 or GK2 the outꢀboard dipole moment of CArꢀOꢀCH3 atom in methoxy moiety reduces the
longitudinal dipole moment and hence it may not be sufficient to promote the ferroelectric like polar switching in these
compounds. Further the polarizability component αXX is found to be largest along the longitudinal Xꢀaxis, which indicates
more dispersed electron cloud. Such dispersed electron cloud leads to larger amount of surface contact.
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Fig. 4 DFT optimized molecular structure using B3LYP hybrid functional and the 6ꢀ311G (d, p) basis set. GK1. The bent angle is 146 o
and the two dihedral angles (135o~136o) indicate that the central phenyl ring is outꢀofꢀplane to adjacent phenyl rings. The phenyl rings
with azo linkage are in the same plane
.
4 Conclusions
In conclusion, the dielectric spectroscopy as well as the electroꢀoptical study of two of the fourꢀ ring bentꢀcore liquid
crystals has been undertaken. The studied compounds are similar to the compounds studied by Ghosh et al. The only
difference is in the polar fluoro substituent at one end of the compound. Ghosh et al. have observed the ferroelectric like
polar switching and electroꢀconvection pattern in the entire nematic range. Ferroelectric like polar switching or the electroꢀ
convection pattern have not been observed in the present studied compounds. They observed three peaks in the dielectric
loss curve and in the present case only one sharp peak in the dielectric loss curve has been observed. The fluoro substituent
in organic compounds in particular in an aromatic ring is regarded as so interesting because of the combination of polar and
steric effects leading to large terminal attraction. Despite the high polarity, the fluoro substituent has a low polarizability
which confers low intermolecular dispersion interactions. The fluoro substituent is the smallest, after hydrogen, of all
possible substituents, and like hydrogen it is monoatomic. Even though the methoxy (the outboard dipole) moiety also
contributes to the polarizability and lateral terminal attraction, the outꢀboard dipole reduces the longitudinal dipole moment
and hence it may not be sufficient to promote the ferroelectric like polar switching in these compounds. This suggests that
for the ferroꢀnematic phase, the polar substituent at one end of the molecule with a large dipole moment is required.
Acknowledgements
Department of Science and Technology (SR/S2/CMPꢀ007/2010) and University Grant Commission India are gratefully
acknowledged by A. Sinha and A. Nafees respectively.
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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: +91-9435522541.
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