Sharmistha Ghosh

Ferroelectric-like switching in the nematic phase of four-ring bent-core liquid crystals

The recent discovery of a ferroelectric response to a switching electric field in nematic phases exhibited by bent-core polar molecules, particularly 1,2,4-oxdiazole derivatives, could lead to new avenues for the development of electro-optic devices. For the first time, we report ferroelectric-like switching under the influence of a triangular wave electric field in the nematic phase exhibited by unsymmetrical achiral four-ring bent-core compounds. These bent-core molecules, exhibiting a large nematic phase range (>70 °C), consist of two unequal lengths in two wings and possess a polar moiety at one end and an alkyloxy chain at the other end. An anomalous variation in spontaneous polarization as a function of temperature in the nematic phase is distinctly observed, which is similar to the results reported only in the low temperature region of the nematic phase. Electro-optical, current-response and dielectric studies of aligned samples corroborate earlier reports of the proposed polar structure of the cybotactic clusters and the ferroelectric-like polar switching of these nematic phases.

Electro-optical and dielectric properties of a high tilt antiferroelectric liquid crystal mixture (W-193B)

The electro-optical properties and dielectric relaxation have been investigated for an antiferroelectric liquid crystal mixture W-193B. The material exhibits smectic A ∗ , smectic C ∗ and a wide range of anticlinic smectic C ∗ phases. The high tilt and broad room temperature smectic C ∗ phase make it a good candidate for antiferroelectric display materials. Dielectric studies have been made in a planarly aligned cell in the frequency range 10 Hz–13 MHz. Dielectric spectroscopy reveals the existence of soft mode in the smectic A ∗ phase and Goldstone mode in the smectic C ∗ phase. In the smectic C ∗ phase the dielectric spectrum of the material exhibits two absorption peaks related to the rotational fluctuation around the short axis of the molecules and antiphase azimuthal angle fluctuation, respectively, and are separated by about two orders of frequency. Electro-optical response using a low frequency triangular wave showed a very high quasi-static contrast ratio of 132 : 1, threshold voltage of around 7 V and saturation of 17 V. Surface-stabilized, low thickness cells of this mixture showed a perfect double hysteresis loop with a 1 Hz triangular signal, reaching different transmission levels for different voltage amplitudes. These levels can be stabilized with a single holding voltage, making it possible for the material to be passively multiplexed at video rate. (Some figures in this article are in colour only in the electronic version)

Effects of conducting polymer poly(3, 4-ethylenedioxythiophene) nanotubes on the electro-optical and dielectric properties of a nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl host

We report the results of the optical transmission and the capacitance behavior as a function dc electric field of a pristine liquid crystal and conducting polymer nanotube-liquid crystal composite measured in twisted nematic cells. The threshold and driving voltages have been determined from transmission-voltage curve. There is remarkable reduction in the threshold and driving voltage in the polymer nanotube doped liquid crystal cell which is good from application point of view. The residual dc is also reduced significantly in the doped cell and the reduction is even more than that observed in the carbon nanotube doped same liquid crystal system.

Fluorescent unsymmetrical four-ring bent-core mesogens: 2D modulated phases

Based on the chemical incompatibility between the aromatic mesogenic cores and the flexible end aliphatic (methylene units) chains, which can lead to the formation of polarization modulated layered undulated phases, four-ring bent-core compounds have been designed and synthesized. The designed compounds are thermally and hydrolytically stable due to intramolecular hydrogen bonding, highly fluorescent and exhibit 2D-polarization modulated layer undulated (PMLU) smectic phases. The phase transition temperatures have been confirmed by differential scanning calorimetry and the phases are confirmed by polarizing optical microscopy and X-ray diffraction studies. For compounds 1-10-11 and 1-15-11 the peaks are indexed to tilted columnar lattice (Colob), while for 2-12-11 the peaks are indexed to the rectangular lattice belonging to the PMLU family The influence of the direction of ester linking group played an important role in the mesophase as well as the evaluated parameters of the dipole moment, the bending angle and the asymmetry parameter by spin-restricted DFT calculations using the B3LYP exchange-correlation functional and the 6-311G(d,p) basis set.

A Comparative Study of Poly (3,4-ethylenedioxythiophene) (PEDOT) Nanotubes Doped Nematic Liquid Crystal (NLC) System and Carbon Nanotubes (CNT) Doped NLC System for Greater Modification of Electro-Optical Properties of the Host NLC 1770-2

The aim of this present paper is to compare the effects of conducting polymer, poly (3,4-ethylenedioxythiophene) nanotubes and carbon nanotubes, on the electro-optical properties of a nematic liquid crystal mixture (1770-2). From the optical transmission and capacitance behavior it has been proved that the conducting polymer – nematic liquid crystal system possesses much lower threshold voltage and driving voltage than the carbon nanotube – nematic liquid crystal system which might be very useful in display applications. The effective elastic constant has been also significantly reduced in the conducting polymer – nematic liquid crystal system.

Amide linkage in novel three-ring bent-core molecular assemblies: polar mesophases and importance of H-bonding

ABSTRACT Here, we report the first examples of achiral unsymmetrical three-ring bent-shaped liquid crystals comprising amide and imine linkages with transverse substituents of methyl and chloro moieties on the central phenyl ring in the core, exhibiting polar banana phases. The extensive intra and inter molecular H-bonding induced novel banana mesomorphic phases. One-dimensional stacking in the mesomorphic phase as well as ferroelectric polar order promoted by intermolecular H-bonding of amide linkage is demonstrated. The compounds exhibit multifunctional properties viz., the enantiotropic liquid crystalline (LC) phase at ambient temperatures, electro-optical response, spontaneous polarisation, emission characteristics with large Stokes shift, and even charge distribution with large voltage holding ratio (VHR) values. The smectic type phase was confirmed by XRD studies and polar order was established by switching current and dielectric investigations. DFT studies revealed the importance of their suitability for display applications. GRAPHICAL ABSTRACT