Golam Mohiuddin

Influence of the chloro substituent on the mesomorphism of unsymmetrical achiral four-ring bent-core compounds: 2D polarization modulated banana phases

The synthesis and characterization of stable achiral unsymmetrical four-ring banana-shaped molecules consisting of a laterally chloro substituted 3,4′-disubstituted biphenyl unit with an ester linkage between the phenyl rings as the central unit in 4-(N-4′-n-tetradecyloxysalicylidene)aminophenyl [2-chloro-5-(N-4′-n-tetradecyloxysalicylidene) aminobenzoate] and 4-(N-4′-n-alkyloxysalicylidene)aminophenyl [4-chloro-5-(N-4′-n-alkyloxysalicylidene) aminobenzoate] are presented. These compounds are thermally and hydrolytically stable due to intramolecular hydrogen bonding and exhibit polarization splay modulated and layer undulated (PMLU) B7/B1Rev/Tilted (2D-polarization modulated layer undulated smectic phase, B1RevTilt phase) phase variants. The phase transitions have been confirmed by differential scanning calorimetry and the phases are characterized by polarized optical microscopy. Two representative examples have been characterized by X-ray diffraction studies. DFT calculations of the bending angle, dipole moments, molecular polarizabilities and voltage holding ratio are also presented.

Cholesterol-based dimeric liquid crystals: synthesis, mesomorphic behaviour of frustrated phases and DFT study

Chiral unsymmetrical dimeric liquid crystals consisting of a cholesterol moiety as chiral entity and a substituted salicylidene imine core (with the substituent being butyl or fluoro or chloro group) interconnected through an even methylene spacer have been synthesised and their mesomorphic properties are characterised. All the dimers exhibit enantiotropic mesophases. The butyl homologue exhibited N* phase only, the fluoro- and chloro-substituted compound exhibited frustrated blue phases (BPs), N* phase and SmC* or twisted grain boundary (TGB) phases. The occurrence of a fluid frustrated phase, the BP, in particular, observed in compounds with a polar moiety and bent optimised conformation by density functional theory (DFT) study, indicates the importance of polar structures and bent shape of the compounds. Theoretical calculation was performed in order to study the optimised conformation, polarity and electron density distribution of the synthesised cholesterol derivatives using DFT. Time-dependent density functional theory (TD-DFT) calculation also had been carried out to investigate the absorption spectra and HOMO–LUMO energies. The experimental and theoretical absorption spectra are also presented.

A new family of four-ring bent-core nematic liquid crystals with a highly polar end-group

A new family of four-ring achiral bent-core compounds derived from 2-methyl 3-amino benzoic acid with the methyl group in the bent direction incorporated into the central core have been designed and synthesised. These compounds possess an alkoxy chain attached at only one end of the bent core molecule, while the other arm consists of a biphenyl moiety possessing a highly polar cyano-group. The molecular structure has been confirmed by elemental analysis and spectroscopic data, and the thermal behaviour and phase characterisation has been investigated by differential scanning calorimetry and polarising microscopy. All the compounds exhibit a wide-range enantiotropic nematic phase. A comparison with non-mesomorphic unsubstituted and 4-methyl-substituted homologues is also presented.

A new family of four-ring bent-core nematic liquid crystals with highly polar transverse and end groups

Summary Non-symmetrically substituted four-ring achiral bent-core compounds with polar substituents, i.e.., chloro in the bent or transverse direction in the central core and cyano in the lateral direction at one terminal end of the molecule, are designed and synthesized. These molecules possess an alkoxy chain attached at only one end of the bent-core molecule. The molecular structure characterization is consistent with data from elemental and spectroscopic analysis. The materials thermal behaviour and phase characterization have been investigated by differential scanning calorimetry and polarizing microscopy. All the compounds exhibit a wide-ranging monotropic nematic phase.

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

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-core liquid crystals containing a 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. Calculations based on density functional theory of the molecules were also carried out. The addition of a methyl group in the central phenyl ring not only induces the lowering of the clearing point but also enhances the nematic phase range. 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.

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.

Impact of terminal polar substitution on elastic, electro-optic and dielectric properties of four-ring bent-core nematic liquid crystals

Here we report the influence of terminal –F, –Cl and –NO2 substitution on the elastic, dielectric and polar switching behavior of four-ring bent-core liquid crystals (LCs). Elastic constants of nematic liquid crystals are the key parameters in determining the threshold voltage and sensitivity to electro-optical response in a device. The elastic properties of bent-core liquid crystal systems show atypical temperature dependence and there is no hard-core theory to explain the behavior. However based on molecular simulation and atomistic calculations it is found in earlier studies that the bend angle dominates the behavior of elastic constants and the terminal or lateral substitutions have very little effect. Here we have studied three bent-core compounds which are differentiated only by their terminal polar substitution. The bend angle is identical (∼146°) for all the three compounds yet they show dramatically different elastic properties. In the fluoro-substituted compound K11 > K33, while for the other two compounds K33 > K11. Thus it is evident that the terminal polar substitution plays vital role in determining the elastic properties of bent-core systems. Correlating the mesophase ranges with the respective dipole moments of the samples it is observed that the fluoro-substituted compound (11-2M-F) with lowest dipole moment favours only nematic phase with smallest mesophase range (46.1 °C), compound 11-2M-Cl with moderate dipole moment favours short range nematic, broad range smectic with moderate mesophase range (53.1 °C), whereas the compound 11-2M-NO2 possesses the widest mesophase range (99.8 °C) with a very narrow nematic and a broad smectic phase amongst the three studied compounds.

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