Showing papers by "D. S. Shankar Rao published in 2003"

X-ray studies on the columnar structures of discotic liquid crystals

Abstract: Since the discovery of discotic liquid crystals in 1977 [1] the field has grown rapidly with an estimated 3000 compounds belonging to this category. Generally these compounds have flat or nearly flat cores and surrounded by four to eight long chain substituents. A vast majority of these compounds exhibit the columnar mesophase. This phase is characterized by a structure in which thedisc shaped molecules are stacked one on top of the other to form columns, the different columns constituting a two-dimensional lattice. Several variants of the columnar structure have been identified-upright columns, tilted columns, hexagonal lattice, rectangular lattice, etc. The stacking of the discs within the column could be on the one extreme, liquid-like or on the other have a true long range order. Although X-ray measurements on quite a few compounds exhibiting these different structures have been reported, the temperature dependence of the various structural parameters does not seem to have received much attention. We ...

Electrooptic and viewing angle characteristics of a display device employing a discotic nematic liquid crystal

Abstract: Recently we developed [1] a novel liquid crystal display device employing discotic nematic material. This device exhibits the following improvements over a conventional twisted nematic (TN) display device using a calamitic nematic material. The device (i) is simple to fabricate and (ii) has excellent viewing angle characteristics showing a wide and symmetric viewing angle profile, (iii) has much less difference in the pixel capacitance between the ON and OFF states resulting in reduced cross talk problem. However the response time of the device is slower than conventional TN devices. An obvious solution to this problem is to reduce the viscosity of the material. To achieve this we have doped, in small concentrations, a long chain alkane compound to the parent room-temperature discotic nematic material. Systematic studies show that both the switch ON and switch OFF response times show a significant decrease, i.e., the device switches faster, in the case of the mixtures. It should be mentioned that wide and symmetric viewing angle characteristics remain unaffected by the addition of the dopant material. However both the switch ON and switch OFF response times are still an order of magnitude slower compared to that of conventional TN devices. Considering the fact that these response times are not very different from those for the STN displays, the achievement of symmetric and wide viewing angle characteristics with a simple fabrication process makes this device quite interesting. To make them attractive enough to be considered for commercial applications new materials with faster response have to be developed. Efforts in this direction are underway in our laboratory.

Novel chiral dimesogenic bidentate ligands and their Cu(II) and Pd(II) metal complexes

Abstract: The synthesis and characterization of cholesterol-based dimesogenic bidentate ligands and their Cu(II) and Pd(II) metallomesogens are reported in detail. To understand structure-property relationships in these materials the terminal alkoxy chains and the central metal atom have been varied. Our studies reveal that chiral dimesogenic bidentate ligands with n-butyloxy chains exhibit smectic A (SmA), twist grain boundary and chiral nematic (N*) mesophases while substitution with either n -decyloxy or 3,7-dimethyloctyloxy chains also show a ferroelectrically switchable chiral smectic C (SmC*) mesophase. The metal complexes with n-butyloxy chains show only the SmA phase whereas higher chain length derivatives exhibit N* phase irrespective of the metal atom present. The ligands are thermally stable whereas their metal complexes, especially Pd(II) systems, seem to be heat sensitive. Spontaneous polarization, response time and tilt angle measurements have been carried out in the smectic C* phase of the two ligands.

Monodispersive Unsymmetrical Tetramers Exhibiting a Columnar Phase

Abstract: Oligomeric liquid crystals that are formed by joining two or more mesogenic segments axially through a flexible spacer (or multiple spacers) seem to be interesting materials from the points of view of both basic research and practical applications owing to their multifunctional characteristics and properties similar to those of polymers while retaining fluidity, viscosity etc. associated with those of low molar mass liquid crystals. The synthesis and evaluation of mesomorphic behavior of two novel monodispersive unsymmetrical tetramesogens have been presented here in detail. The molecular design of these tetramesogens incorporates the cholesteryl ester (as a chiral entity), biphenyl (as a supportive entity for mesomorphism), azobenzene (as a photoactive mesogen), and tolan (as a half-disc) segments interconnected through even-even-odd and odd-even-odd alkylene spacers. The target liquid crystalline tetramers have been realized by employing convenient synthetic strategies, which involves the preparation of...

Synthesis and thermal behaviour of salicylaldimine-based liquid crystalline symmetrical dimers

Abstract: A series of symmetrical dimers consisting of salicylaldimine moieties connected by flexible alkylene central spacer via ether linkages has been synthesized. In order to validate the empirical rule suggested by Date et al. to account for the smectic behaviour of such dimers, the chain length of the terminal alkoxy chain has been kept constant (C8) while the number of methylene units in the central spacer was varied from C3 to C11. Another aim of the present investigation was to understand structure-property relationship in these dimers in which the salicylaldimine mesogenic segment has been used for the first time in dimers. The mesomorphic behaviour of these dimers was evaluated using optical microscopy and differential scanning calorimetry and the structure of some of the mesophases has been further investigated with the help of X-ray diffraction. Our studies reveal that the dimers consisting of 3 to 8 methylene units in the flexible spacer show only smectic (smectic C and smectic A) phases. For the dime...

Time-resolved measurements of the dynamics of the photoinduced smectic- C α * –smectic- A transition

Abstract: We present the study of the dynamics of the recently reported photoinduced smectic-${C}_{\ensuremath{\alpha}}^{*}$\char21{}smectic-$A$ transition. High resolution time-resolved dielectric dispersion measurements carried out during the photoisomerization process demonstrate that the magnitude of the uv intensity mimics the role played by temperature in determining the behavior of the soft mode relaxation. We also show that the uv intensity dependence of the soft mode relaxation frequency ${f}_{R}$ in the photoinduced smectic-$A$ phase can be described with a functional form similar to that derived for the temperature dependence of ${f}_{R}$ and compare the experimentally determined critical exponent with the theoretically predicted for the Ising and three-dimensional $\mathrm{XY}$ universality classes. Our study illustrates an interesting feature, namely, the magnitude of light intensity can be treated like a thermodynamic variable such as temperature and pressure to study phase transitions in general.

Phase behaviour of thermotropic banana-shaped compounds under pressure

Abstract: The phase behaviour of two achiral bent core banana-shaped compounds, the hexyloxy (compound I) and decyloxy (compound II) members of the 1,3-phenylene bis[N-(2-hydroxy-4-n-alkoxybenzylidene)-4′-aminobenzoate] series was investigated under hydrostatic pressures up to 300 MPa using high pressure differential thermal analysis and light transmission methods. The reversible transition sequence crystal (Cr1)–B1 phase–isotropic liquid (I), observed at room pressure for compound I, remains in the pressure region up to c 70 MPa. At higher pressures a pressure-induced crystalline phase (Cri) appears between the Cr1 and B1 phases, its temperature region becoming wider with increasing pressure. The temperature vs. pressure phase diagram shows a triple point of 72.9 MPa and 160.3°C for the Cr1, Cri and B1 phases, indicating the lower limit of pressure for the Cri phase. In compound II the reversible transition sequence crystal (Cr1)–B2 phase–I is seen over the whole pressure region, and the temperature range of the B...

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

Abstract: The phase behaviour of the discotic mesogen 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT) was investigated under hydrostatic pressures up to 300MPa using wide-angle X-ray diffraction equipped with a high-pressure vessel. The reversible transitions of HHTT, i.e., crystal (Cr)-helical phase (H) -hexagonal columnar phase (Col h ) - isotropic liquid (I), were observed under pressures up to about 30MPa. The transition sequence changed significantly depending on the applied pressure. The H phase appeared monotropically in the pressure region between 50 and 180 MPa, indicating the Cr→Col h →I on heating and I→Col h →H→Cr transition sequences on cooling. Applying higher pressures altered further the phase behaviour: the enantiotropic Col h phase changed to monotropic one under high pressures above 180 MPa, indicating the Cr→I on heating and I→Col h →Cr transition sequences on cooling. The X-ray diffraction pattern of the H phase exhibited a sharp low-angle and a wide-angle reflections corresponding to the intercolumnar and the core-core distances, respectively. This feature is indicative of the three-dimensional ordering present in the H phase. On the other hand, the Col h phase showed only a low-angle reflection. It was found that the intercolumnar distance in the Col h phase decreases with increasing temperature at high pressures. This indicates that the negative coefficient of thermal expansion for the column packing in the Col h phase is held under pressure.

Columnar Mesophase in a Novel Series of Banana-Shaped Compounds Consisting of Functional Nitro Groups

Abstract: A novel series of banana-shaped compounds with the following molecular structure has been synthesized and characterized. All the compounds synthesized in this series are found to be liquid crystalline forming a B 1 phase, which has recently been designated as a columnar phase. the liquid crystalline behaviour of these compounds has been investigated by using the polarizing optical microscopy, differential scanning calorimetry and x-ray studies. the lower homologues of this series are monotropic whereas the higher ones are enantiotropic. we found that introduction of the nitro functional groups affects the mesomorphic properties in comparison to their respective unsubstituted compounds. the lower homologues of the unsubstituted ones exhibit a B 1 phase whereas the higher ones a B 2 phase.

High Pressure Investigations on the Phase Behaviour of Discotic Liquid Crystals

Abstract: Liquid crystals made of disc shaped molecules, now referred to as discotic liquid crystals, were discovered [1] in 1977 and represent a new class of thermotropic liquid crystals. Since the discovery, there are only a few studies [2-9] concerning the effect of pressure on these materials. Here we report high-pressure investigations on the columnar phase of several triphenylene compounds. We selected the compounds such that for one set of materials the number of peripheral substituents, x, was fixed and the length of the alkoxy chains, n, was varied, while for the second set n was fixed and x varied. We observe that with increase in either n or x, the slope of the pressure-temperature phase boundary, i.e., (dT/dP) 1bar shows a strong decrease. The magnitude of the transition enthalpy and the calculated transition volume across the columnar-isotropic transition show a strong decrease with increasing n or x. To explain these features we propose an argument based on the ratio of the volume of the triphenylene core to the total volume of the molecule.

Effect of pressure on liquid crystal dimers

Abstract: We report high pressure investigations on a homologous series of liquid crystalline dimeric molecules in which the terminal chain length is kept constant but the length of the methylene spacer connecting the two mesogenic units is varied. We find that unlike the nematic–isotropic transition temperature and associated entropy change, there is no alternation in the slope of the nematic–isotropic phase boundary in the pressure–temperature plane as a function of the length of the spacer group in the molecule. By applying the Clausius–Clapeyron equation we conclude that the volume change at the transition should exhibit a strong odd–even effect. Measurements on the shortest homologue of the series, which is non-mesomorphic, show the expected result that the application of pressure induces mesomorphism in non-mesomorphic compounds.