Subbarao Krishna Prasad

Cholesterol-based nonsymmetric liquid crystal dimers: an overview

Nonsymmetric dimers derived from naturally occurring cholesterol represent an exemplary and emerging class of chiral liquid crystals. They are formed by linking the pro-mesogenic cholesterol moiety to one of the termini of aromatic/aromatic-alicyclic/supramolecular mesogenic core through a flexible spacer of varying length and parity. Hitherto, over 300 dimers comprising fourteen different mesogenic cores have been reported. The stabilization of a variety of highly frustrated fluid structures, reentrant phases and unprecedented phase sequences clearly illustrates their unique molecular structural characteristics. Besides, the employed fragments facilitate modulating the properties of technologically important fluid phases. Thus, they are immensely significant in both fundamental science and practical applications. In this Feature Article an overview of these chiral dimers is provided.

Liquid crystal dimers possessing chiral rod‐like anisometric segments: synthesis, characterization and electro‐optic behaviour

Several new optically active liquid crystal dimers comprising pro‐mesogenic cholesterol and a chiral diphenylacetylene (tolane) segment, covalently linked in an end‐to‐end fashion through a flexible spacer, have been synthesized and investigated for their mesomorphic behaviour with the aid of optical, calorimetric and X‐ray diffraction studies. Five unsymmetrical dimers, designed on the basis of recent work, involve molecular structural variations of the tolane mesogenic entity with a view to stabilizing a wide thermal range smectic A (SmA) phase featuring the electroclinic effect. Three different chiral chains, namely, (S)‐1‐methylheptyloxy, (S)‐2‐methylbutyloxy, (3S)‐3,7‐dimethyloctyloxy, with or without polar (nitro or fluoro) lateral substituents, were incorporated, while keeping the length (C6) of the spacer constant. As expected, all the dimers exhibited a SmA phase. A few also showed chiral nematic (N*) and/or twist grain boundary and/or chiral smectic C (SmC*) phases. Remarkably, some of these oligomesogens, upon melting, had a stable SmA phase over a wide thermal interval (100–150°C); this state seems to be stable for a long period of time. Electro‐optic studies, including optical tilt angle as well as temporal response as a function of temperature, were carried out in the SmA phase. The SmC* phase was also investigated for its electrical switching and optical tilt angle, as well as spontaneous polarization as a function of temperature. These studies showed that the mesophase response to an applied field is weak and is independent of variations in the dimer investigated.

A novel family of salicylaldimine-based five-ring symmetric and non-symmetric banana-shaped mesogens derived from laterally substituted resorcinol: synthesis and characterization

We present the results of our extensive investigations into the phase behavior of several “robust” five-ring banana-shaped compounds. Six homologous series of either symmetric or non-symmetric bent-core molecules, comprising a laterally substituted 1,3-phenylene ring as the central unit covalently linked to thermally and hydrolytically stable rod-like salicylaldimine segments have been synthesized and evaluated for their mesomorphism. The mesophases have been characterized by optical, calorimetric, X-ray diffraction and electro-optical studies. With a methyl group, polarizable chlorine atom or the more polarizable nitro group as the lateral substituent(s) and terminal alkoxy tails of varying length, the structure–property correlation has been determined. Our study clearly demonstrates that these materials, as predicted, display a different behavior compared to the corresponding compounds without the lateral substituents. The phase behavior shows a critical dependence on the nature and position of the lateral substituent(s). The study includes a comparison with the phase behavior of some of the other types of banana-shaped compounds.

The first examples of optically active tris(N-salicylideneaniline)s: manifestation of chirality from molecules to fluid columnar phases

Tris(N-salicylideneaniline)s comprising chiral tails in varying numbers self-organize into a room temperature helical columnar mesophase resulting from the chiral stacking of the constituent molecules differing in their rotational symmetry and ratio as evidenced by optical, chiroptical, calorimetric and X-ray studies.

Supramolecular Helical Fluid Columns from Self‐Assembly of Homomeric Dipeptides

Herein, we demonstrate that with the widespread theme of residue patterning and stereochemical restraints of self-complimenting proteinogenic amino acids, a new and rich class of homomeric dipeptides exhibiting two-dimensional fluid aggregates with hierarchical ordering can be obtained. In particular, a simple way of achieving a class of functional dipeptides, wherein the first and the second residues chosen are L-/D-alanines and L-/D-leucines, has been accomplished. The supramolecules synthesized can be regarded as intermediates between polycatenars and taper-shaped amphiphiles because they possess two lipophilic segments interlinked by a peptide unit (spacer). Two pairs of enantiomers and their respective diastereomers derived from these amino acids are evidenced to self-organize into a helical columnar phase through hydrogen bonding by means of FTIR, UV/Vis, and chiroptical circular dichroism (CD) spectral analyses as well as by optical, calorimetric, electrical switching, and X-ray studies. The CD and X-ray studies have revealed that the form chirality (handedness) and the magnitude of out-of-plane fluctuations of the lattice planes of the fluid supramolecular columnar structures are solely directed by the stereochemistry encoded in the spacer. Of special significance, the less frequently found oblique helical columnar phase formed by a pair of enantiomers derived from L-/D-alanines, unlike those derived from other amino acids, exhibit ferroelectric behavior; the measured spontaneous polarization is as high as 440 nC cm(-2). Besides, all these supramolecules form stable organogels in ethanol and the CD and SEM studies on a representative gel suggest the presence of helical structures.

The biaxial smectic (SmAb) phase in nonsymmetric liquid crystal dimers comprising two rodlike anisometric segments: an unusual behavior

The first nonsymmetric liquid crystal dimers, possessing salicylaldimine and cyanobiphenyl rodlike anisometric segments, exhibiting the biaxial smectic A (McMillan) phase unequivocally evidenced by microscopic, conoscopic, calorimetric and X-ray diffraction studies is reported.

A new thermotropic reentrant behaviour in a chiral liquid crystal dimer: the occurrence of SmA–SmAb–SmA phase sequence

A nonsymmetric liquid crystal dimer formed by covalently linking cyanobiphenyl and chiral salicyladimine mesogens through a heptamethylene spacer exhibits a new reentrant phase sequence, uniaxial smectic A–biaxial smectic A–uniaxial smectic A phase, below the blue phase–chiral nematic–twist grain boundary phases.

Optically biaxial interdigitated smectic A phase: liquid crystalline dimeric bidentate ligands and their metal complexes

A detailed study into the synthesis and characterization of achiral nonsymmetric dimeric bidentate ligands formed by combining salicylaldimine and cyanobiphenyl rodlike anisometric segments through a flexible spacer (varying in its length and parity) and their Cu(II) and Pd(II) complexes is reported. The spacer parity greatly influences the phase behavior of ligands; notably, the dimers with odd-parity spacer stabilize nematic, uniaxial smectic A and biaxial smetic A phases, while the even-members display nematic and / or smectic phase/s. Unlike ligands, both the metal complexes solely show nematic behavior. An odd–even effect, as noted for ligands, was observed for the nematic–isotropic transition temperatures and the associated entropy values of copper complexes, while such behavior was not found in the case of palladium complexes.

Non-linear dielectric response of a ferroelectric liquid crystal

Dielectric spectroscopy is a very useful method for investigation of the structure and dynamics of liquid crystals. However, with few exceptions, most investigations have been only in the linear regime. In this note we present a simple method for extraction of the non-linear contributions to the dielectric constant and as an example give the results obtained for a ferroelectric liquid crystal.

Alterations in Hippocampal Oxidative Stress, Expression of AMPA Receptor GluR2 Subunit and Associated Spatial Memory Loss by Bacopa monnieri Extract (CDRI-08) in Streptozotocin-Induced Diabetes Mellitus Type 2 Mice

Bacopa monnieri extract has been implicated in the recovery of memory impairments due to various neurological disorders in animal models and humans. However, the precise molecular mechanism of the role of CDRI-08, a well characterized fraction of Bacopa monnieri extract, in recovery of the diabetes mellitus-induced memory impairments is not known. Here, we demonstrate that DM2 mice treated orally with lower dose of CDRI-08 (50- or 100 mg/kg BW) is able to significantly enhance spatial memory in STZ-DM2 mice and this is correlated with a significant decline in oxidative stress and up regulation of the AMPA receptor GluR2 subunit gene expression in the hippocampus. Treatment of DM2 mice with its higher dose (150 mg/kg BW or above) shows anti-diabetic effect in addition to its ability to recover the spatial memory impairment by reversing the DM2-induced elevated oxidative stress and decreased GluR2 subunit expression near to their values in normal and CDRI-08 treated control mice. Our results provide evidences towards molecular basis of the memory enhancing and anti diabetic role of the Bacopa monnieri extract in STZ-induced DM2 mice, which may have therapeutic implications.