Evidence of Possible Biaxiality in a Bent-Core Liquid Crystal Compound for Faster Switching Display Applications

Abstract

We report possible biaxiality in the nematic phase of a bent-core liquid crystal compound exhibiting cybotactic clustering in entire nematic liquid crystal phase using dielectric spectroscopy with promising application in faster switching displays.

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Th3A. 6.pdf The International Conference on Fiber Optics and
Photonics 2016 © OSA 2016
Evidence of Possible Biaxiality in a Bent-Core Liquid
Crystal Compound for Faster Switching Display
Applications
Sourav Patranabisha, Aloka Sinhaa, Golam Mohiuddinb, Nazma Begumc, Nandiraju V. S. Raoc
a
Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110 016, INDIA
b
Department of Chemical Science, IISER Mohali, Punjab-140306, INDIA
c
Department of Chemistry, Assam University, Silchar, Assam-788011, INDIA
Author email address: (souravpatranabish@gmail.com, aloka@physics.iitd.ac.in, ranju.mohiuddin@gmail.com)
Abstract: We report possible biaxiality in the nematic phase of a bent-core liquid crystal
compound exhibiting cybotactic clustering in entire nematic liquid crystal phase using dielectric
spectroscopy with promising application in faster switching displays.
OCIS codes: (160.3710) Liquid crystals, (120.2040) Displays,(250.6715) Switching
1. Introduction
Liquid Crystals (LCs) are the fundamental materials in modern displays. The existing display technology uses
calamitic (rod shaped) LCs while the search for new display materials with an advantage over the present
technology still continues. In recent years, banana shaped or bent-core liquid crystals came into existence and are
widely studied. In particular, the bent-core nematic (BCN) LCs are promising candidates to exhibit the biaxial
nematic phase which has potential application in display technologies with faster switching [1]. The biaxial BCN LC
molecules possess a long axis (n) and two short axes (m or l). The fact that the switching of the short axis (m or l)
can be much faster than the long axis (n) makes the biaxial BCN LC more advantageous over the calamitic LCs that
utilize the switching of long axis (n) only in displays [1,2]. Apart from being biaxial, the BCN LCs must also
possess a collective order (grouping of molecules) called the cybotactic clusters to achieve switching of the short
axis that can be employed in displays for enhanced performance [3]. Earlier, Cavero et al. reported a possible biaxial
switching in the bent-core liquid crystal molecule BHC14 in the nematic phase via dielectric studies [4]. However,
the dielectric studies did not indicate any presence of cybotactic clusters in the nematic phase. Gleeson et al. also
reported the existence of biaxial ordering in a bent-core nematic liquid crystal via distinct electro-convection studies
[5]. In this paper, we report the study of a four-ring bent-core liquid crystal compound with possible biaxial
switching in the nematic phase supported by the existence of cybotactic clusters using dielectric spectroscopy and
optical texture observations. Till date the oxadiazole LC compounds have mainly been reported to exhibit the biaxial
BCN phase while we report biaxiality in the BCN phase of a four-ring bent-core LC compound and at relatively
lower temperatures.
2. Experimental
A four-ring achiral bent-core LC compound 14-2M-Cl (C41H48ClN3O4) was studied. The chemical structure of
the LC compound along with the phase sequence and transition temperatures are given in Fig.1. The phase transition
temperatures were determined by differential scanning calorimetry (DSC) measurements during the heating and
cooling cycles at the rate of 5oC/min and different LC phases of the compound were identified and confirmed
through texture observations using polarizing optical microscope (POM).
Fig.1 The chemical structure of the LC compound 14-2M-Cl, its phase sequence and the transition temperatures

Th3A. 6.pdf The International Conference on Fiber Optics and
Photonics 2016 © OSA 2016
The LC sample was filled in a standard ITO coated 5 μm commercial cell (Instec) with planar orientation and
antiparallel rubbing using capillary action in the isotropic phase (155oC-160oC) of the sample LC material. An Instec
HCS302 hot stage attached to an Instec MK1000 temperature controller was used to maintain the temperature of the
LC cell. The textures of the LC sample at different temperatures were recorded using an Olympus BX51P polarizing
optical microscope. An Agilent E4980A precision LCR meter was used to perform the dielectric measurements in
the frequency domain 20 Hz to 2 MHz and in the voltage range 0 V to 20 V. Test signals of amplitude 100 mV were
applied to the LC system to perform the frequency sweep; while, a test signal of frequency 1 kHz was employed
during voltage sweep.
3. Results and Discussion
The textures of the LC sample were recorded using a hot stage containing the LC sample cell placed under a
polarizing optical microscope with crossed polarizers. All the measurements were carried out during slowly cooling
down the sample after heating it to its isotropic phase. The bent-core LC sample 14-2M-Cl, on cooling, shows
appreciable change in the birefringent colour (Fig.2) in the nematic phase extending up to the smectic phase. This
indicates a strong variation in the birefringence reflecting the formation of cybotactic clusters in the entire LC
phase[6,7].
Fig.2 Micrographs of the textures of the LC sample 14-2M-Cl showing sharp colour changes at (a) 147.5
o
C (b) 146.5
o
C (c) 140
o
C (d) 120
o
C ;
rubbing direction is indicated by 'r' with polarizer (P) and Analyzer (A) crossed
Dielectric studies of the sample in a frequency region from 20 Hz to 2 MHz were carried out using planar aligned
cells of thickness 5 μm. The complex dielectric permittivity that is obtained from the dielectric studies of the sample
using an LCR meter is given by,
(1)
where, f is the frequency of the applied electric field [8]. The spectrum associated with the real part () of the
complex permittivity is called the dispersion curve (Fig.3 (a)) while the one associated with the imaginary part ()
is called the loss (or absorption) curve (Fig.3 (b)). The loss curve for the sample 14-2M-Cl shows a peak in the low
frequency region (50 Hz-200 Hz) in the nematic and smectic phase extending up to the isotropic phase. This low
frequency relaxation indicates the presence of collective ordering of molecules (cybotactic clusters) in the entire LC
phase. Suppression of the low frequency peak was observed on application of a DC bias voltage (Fig.3 (c) and (d))
and the peak became completely suppressed when a sufficiently large DC bias voltage was applied to the system.
This further confirmed the presence of collective ordering (cybotactic clusters) in the sample material [6,9].
However, the high frequency peak corresponds to the ITO contribution.
Fig.3 Dielectric investigation of the bent-core LC sample 14-2M-Cl: (a) dielectric permittivity (real) (b) dielectric permittivity (imaginary) as
a function of frequency at different temperatures; dielectric permittivity (imaginary) as a function of frequency under the application of DC bias
voltage at temperatures (c) 147.5
o
C and (d) 140
o
C
The dielectric investigation of the variation of (real) with applied voltage was carried out using a test signal of
frequency 1 kHz. No Fréedericksz transition was observed in the entire LC phase indicating that the LC compound

Th3A. 6.pdf The International Conference on Fiber Optics and
Photonics 2016 © OSA 2016
under examination has a negative dielectric anisotropy (< 0). However, a small increase in the permittivity ()
followed by a rapid decrease in the permittivity value in the nematic phase was observed (Fig.4 (a)). The rise in
permittivity is typically observed in the nematic phase when the applied voltage is between 2.8 V to 4 V. At higher
voltages the permittivity value falls rapidly probably due to induced electrohydrodynamic instabilities exhibited by
the LC system [4]. The increase in the value of permittivity in the nematic phase indicates the possibility of field
induced biaxial switching, i.e. the alignment of the short axis along the applied electric field resulting in an increase
in the dielectric amplitude [4]. This rise in permittivity value as a function of applied voltage appeared very near to
the isotropic-nematic transition temperature and gradually disappeared as the LC approaches the smectic phase from
the BCN phase. A closer analysis (Fig.4 (b)) reveals that the variation in permittivity value () is approximately 0.2
at 148.5oC, 0.15 at 148oC and decreases thereafter with decreasing temperature.
Fig.4 Dielectric permittivity vs. applied voltage (a) in the nematic phase (b) enlarged view at 148.5
o
C and 148
o
C
4. Conclusion
Here we have studied a bent-core LC compound and observed a possible biaxial switching in the BCN phase via
dielectric studies. The textures of the LC compound showed distinct colour change with the variation of temperature
during cooling indicating a strong change in birefringence. The dielectric studies of the real and imaginary part of
the complex dielectric permittivity revealed the formation of cybotactic clusters in the smectic and nematic phase
extending up to the isotropic phase confirmed by the application of DC bias voltage. The variation of the real part of
the complex dielectric permittivity with the applied voltage indicated a possible field induced biaxial switching, i.e.
the alignment of the short axis along the direction of the applied electric field. The presence of cybotactic clusters in
the BCN phase lowers the required field to align the short axis of the possible biaxial LC sample material along the
direction of the applied field. This results in the rise of the value of permittivity on application of an external electric
field. These studies confirmed the existence of a possible biaxial switching in the BCN phase of the LC material 14-
2M-Cl in the context of cybotactic clusters. The existence of biaxiality in the BCN phase of the bent-core LC
material 14-2M-Cl is particularly important because of its promising potential for display device applications with
the advantage of much faster switching and enhanced performance over the conventional uniaxial nematic LC.
Acknowledgements
SERB, Department of Science and Technology (EMR/2015/001897), Government of India is gratefully
acknowledged by A. Sinha and S. Patranabish.
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