Showing papers by "D. S. Shankar Rao published in 2001"

Observation of a Reentrant Twist Grain Boundary Phase

Abstract: We report the occurrence of a reentrant twist grain boundary phase, which we designate as Re- ${\mathrm{TGB}}_{A}$ Microscopic observations on a nonsymmetric dimer showed the phase sequence Iso-- ${N}^{*}$-- ${\mathrm{TGB}}_{A}$--Sm- $A$--Re-- ${\mathrm{TGB}}_{A}$-- ${\mathrm{TGB}}_{{C}^{*}}$ Here ${N}^{*}$ and Sm- $A$ stand for the chiral nematic and smectic- $A$ phases, ${\mathrm{TGB}}_{A}$ is the twist grain boundary phase with smectic- $A$ blocks, and ${\mathrm{TGB}}_{{C}^{*}}$ that with smectic- ${C}^{*}$ blocks and exhibiting features of both the smectic- ${C}^{*}$ and TGB phases The reentrance of the ${\mathrm{TGB}}_{A}$ phase is unambiguously demonstrated using x-ray diffraction, selective reflection, and optical rotation data

The first hexagonal columnar discotic liquid crystalline carbazole derivatives induced by noncovalent π–π interactions

Abstract: The synthesis of 13 discotic mesogens is described in which the well-known hexakis(pentyloxy)triphenylene liquid crystalline material has been chemically modified to incorporate one, two, three and six carbazole moieties. These modifications have been achieved by the alkylation or esterification of mono-, di-, tri- and hexa-hydroxytriphenylene derivatives with alkyl bromides and carboxylic acids incorporating the carbazole moiety. The pure compounds are not liquid crystalline in nature but when doped with TNF, hexagonal columnar mesophases are induced, as shown by DSC, OPM and X-ray diffraction. These mesophases exist below room temperature. The mesophase clearing temperatures are dependent on several factors including the chain length separating the carbazole moiety from the triphenylene core, and the nature of the ether or ester linkage, and the degree of TNF doping. The data suggest that the most stable mesophases (highest clearing temperatures) are formed when a 2 ∶ 1 complex is formed between the carbazole derivatives and the TNF, respectively. The correlation length obtained from X-ray diffraction reveals that the columnar order for one of the ether derivatives decreases, whereas the correlation length increases for one of the ester derivatives. This result suggests that the TNF is not only partaking in π–π noncovalent bonding interactions, but also in polar interactions with the CO bond. Such mesogenic carbazole derivatives may have advantageous photorefractive properties over the amorphous polymeric materials.

A switchable salicylaldimine-based achiral bent-shaped mesogen: synthesis and characterization

Abstract: The first achiral bent core banana-shaped mesogen consisting of a thermally and hydrolytically stable salicylaldimine (2-hydroxybenzylideneamine) mesogenic segment has been synthesized and evaluated for its liquid crystalline behavior. This novel compound exhibits a single switchable mesophase over 60 °C temperature range. The textural and electro-optic switching characteristics suggest that the phase could be of the antiferroelectric type.

Experimental studies on the B7 phase of a banana-shaped achiral mesogen

Abstract: We report optical, X-ray, polarization and response time measurements on a new 'bananas-haped' mesogen. The material has a salicylaldimine segment in the linear part of the core, but is otherwise structurally similar to the mesogen first reported to show a banana mesophase. Textural and X-ray studies show that this mesophase has all the characteristics of the recently discovered B7 phase. It is observed that the voltage required to obtain electrical switching is quite low compared with values for the few other materials exhibiting the B7 phase.

Cholesterol-based dimesogenic bidentate ligands and their Cu(II) and Pd(II) metallomesogens

Abstract: The synthesis and evaluation of the liquid crystalline properties of non-conventional liquid crystals, consisting of two non-identical mesogenic segments interconnected via a paraffinic chain spacer, are of considerable current interest In particular, chiral dimesogens possessing a cholesteryl ester unit as the chiral entity joined to other aromatic mesogens through a polymethylene spacer have shown unique and interesting thermal behaviour In continuation of our investigations on this topic, here we present the synthesis and characterization of the first examples of cholesterol-based unsymmetrical dimesogenic bidentate ligands and their Cu(II) and Pd(II) metal-organic systems (metallomesogens) Our studies reveal that the dimesogenic bidentate ligands exhibit multiple mesophases, whereas their metal complexes stabilize only the mesophase

A novel class of banana-shaped azo compounds exhibiting antiferroelectric switching behaviour

Abstract: The first banana-shaped liquid crystal materials with photo-active azo linkages are reported. The higher temperature mesophase X1 (believed to be B6) above the B1 phase exhibited by these azo compounds shows antiferroelectric switching characteristics. The observation of banana phases in azo compounds assumes significance from the point of view of photochromic studies.

Electroclinic Effect In Unsymmetrical Dimeric Liquid Crystals Composed of Two Non-Identical Chiral Mesogenic Entities

Abstract: We report the synthesis and physical characterization of two unsymmetrical dimeric liquid crystals containing two different chiral mesogenic segments, namely, a tolan unit with (S)-2-octyloxy tail and a cholesteryl ester unit which are separated by a n-butyl (even) and n-pentyl (odd) spacers respectively. Both the dimers exhibit a smectic A mesophase, with the dimer having the n-pentyl spacer stabilizing this phase over a temperature range of ˜150°. Electro-optic studies performed on this compound show that it exhibits an observable electroclinic effect over this entire temperature range. At room temperature, the electroclinic tilt angle and the response time observed are comparable to those exhibited by some monomeric materials.

Phase behaviour of the discotic mesogen 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT) under hydrostatic pressure

Abstract: The phase behaviour of the discotic mesogen 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT) was investigated under hydrostatic pressures up to 500 MPa using high pressure optical and DTA measurements. The known enantiotropic phase transitions of HHTT, i.e. crystal (Cr)-helical phase (H), H-hexagonal columnar phase (Colh) and Colh-isotropic liquid (I) were observed up to 32 MPa. Application of hydrostatic pressures above 32 MPa results in the H and Colh phases becoming monotropic, depending upon the applied pressure. The H phase was observed as a monotropic phase in the pressure region between 32 and about 180 MPa. Thus, the I →Colh →H →Cr transition sequence appeared only on cooling under these pressures, while the Cr →Colh →I transition occurred on heating. Further increases in pressure above a second limiting value leads to the Colh phase becoming monotropic. Thus the I →Colh →Cr transition sequence appeared on cooling, while the Cr →I transition was observed on heating. The T vs. P phase diagram based o...

Novel Liquid-Crystalline Mercury Acetylide Complexes

Abstract: Several new mercury acetylide complexes were prepared from substituted phenyl acetylenes. These are of three varieties: i) where normal or branched alkyl chains are attached to the phenyl ring directly, ii) where these chains are attached via an oxygen atom (alkoxy chains), and iii) where the phenyl ring is substituted by two or three alkoxy chains. All the alkoxy derivatives are liquid crystalline. Their mesomorphism has been examined by polarizing microscopy and differential scanning calorimetry.

Liquid crystalline dimeric compounds with an alkylene spacer

Abstract: Two series of compounds with dimeric molecules have been synthesized and characterized. These molecules consist of two non-mesogenic units linked by an alkylene spacer through an ester linkage. The effects of varying the terminal as well as the spacer chain length on the mesomorphic properties have been studied. For compounds with an even number of carbon atoms in the spacer chains, the observed textures in the mesophases are reminiscent of those seen for the smectic phases of banana-shaped molecules.

Cholesterol-based Linear Trimesogens: Synthesis and Evaluation of Mesomorphic Behaviour

Abstract: Several trimesogens consisting of benzalazine as a central mesogenic segment, connected linearly on both the terminals with the cholesteryl ester moieties through either even–even or odd–odd alkylene spacers, have been synthesised and investigated for their liquid crystalline properties

Effect of pressure on the photoinduced nematic-isotropic phase transition.

Abstract: We report the results of high-pressure studies on the photoinduced nematic-isotropic phase transition. It is observed that pressure has a significant influence on the shift in the transition temperature caused by uv radiation. We propose a possible mechanism to explain the observed results.

Crystal Structure of an Unsymmetrical Dimeric Liquid Crystal with a Wide Temperature Range Chiral Smectic A Phase

Abstract: Cholesteryl 6-{4-[4-(1S-methylheptyloxy)phenylethynyl]phenoxy} hexanoate, C110H160O8, crystallises in the monoclinic space group P21 with a = 18.710(12)A, b = 9.740(14)A, c = 27.865(16)A, α = 90.00˚, β = 103.85(5)˚, γ = 90.00˚, V = 4931(8)A3 Z = 2, D(cal) = 1.085Mg/m3, μ = 0.066 mm−1, F 000 = 1768, GooF = 0.83, R1 = 0.07 and wR2 = 0.20.

Liquid Crystals under High Pressure

Abstract: The effect of elevated pressure on liquid crystals, a sub class of anisotropic soft matter, has attracted attention for almost 100 years now, although significant work has appeared only in recent times Despite a plethora of problems faced in such measurements, almost every aspect of liquid crystals has been investigated under pressure Here an overview of these studies is provided with focus on more recent work