S. Chandrasekhar’s research while affiliated with Raman Research Institute and other places

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Publications (81)


Density, Dielectric and X-ray Studies of Smectic A-Smectic A Transitions
  • Article
  • Full-text available

October 2011

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37 Reads

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1 Citation

Molecular Crystals and Liquid Crystals

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C. Nagabhushana

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V. N. Raja

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[...]

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G. Heppke

We report here the results of detailed density, dielectric and x-ray studies on three systems exhibiting different types of Smectic A-Smectic A transitions. It is found that although the layer spacing shows marked changes at the transitions, the corresponding density changes are extremely small. In every case studied, the dielectric anisotropy shows a pronounced decrease on going over to the lower temperature smectic A phase. This decrease can be correlated with the structural changes.

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A Low-Molar-Mass, Monodispersive, Bent-Rod Dimer Exhibiting Biaxial Nematic and Smectic A Phases

June 2004

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21 Reads

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123 Citations

A "peelable banana" is formed when a bent-core molecule is linked to a rodlike mesogen through a flexible aliphatic spacer. This is an appropriate description of this novel low-molar-mass organic system, which displays a transition from a biaxial nematic (N b) phase to a biaxial smectic A phase. The illustration gives a schematic representation of the dimeric molecules in the N b phase as well as the corresponding textural and conoscopic patterns obtained.


Wide viewing angle and fast responding TN LCD

January 2004

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25 Reads

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3 Citations

Molecular Crystals and Liquid Crystals

A new type of TN display device has been fabricated by sandwiching a dual frequency addressable nematic liquid crystal material between two unrubbed polyimide coated glass plates. Between crossed polarizers, the bright state is observed by applying a high frequency electric field, while the dark state by applying a low frequency electric field of similar strength. This dual frequency addressed rubbing free device shows very wide and highly symmetric viewing angle characteristics, with contrast ratio comparable to that of a rubbed TN device, and with very fast response time. Addressing the device in a “dual frequency dual amplitude pulsed mode” leads to only a small increase (∼1.5 times) in power consumption in comparison with the conventional single frequency addressed device.


Phase Behaviour of the Discotic Mesogen 2,3,6,7,10,11-Hexahexyl Thiotriphenylene (HHTT) Under Pressure

June 2003

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30 Reads

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8 Citations

Molecular Crystals and Liquid Crystals Science and Technology Section A Molecular Crystals and Liquid Crystals

The phase behaviour of the discotic mesogen 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT) was investigated under hydrostatic pressures up to 300 MPa 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 30 MPa. 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 M Col h M I on heating and I M Col h M H M 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 M I on heating and I M Col h M 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.


X-RAY Studies on the Columnar Structures of Discotic Liquid Crystals

June 2003

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248 Reads

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90 Citations

Molecular Crystals and Liquid Crystals Science and Technology Section A Molecular Crystals and Liquid Crystals

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 have used an image plate detector to carry out detailed X-ray studies of several compounds exhibiting the columnar phase. Analysis of the X-ray patterns yields, apart from information on the structural symmetry in terms of the planar space groups, quantitative data on (i) the ordering of the chains and the cores of the mesogens, (ii) the core-core correlation length along the column axis and (iii) the intercolumnar spacing. In particular, it is seen that the extent of ordering of the discs within the columns has significant temperature dependence. Data collected as a function of temperature will be presented and questions regarding ordering within the columns addressed, particularly in two compounds, one of which exhibits a columnar to a plastic phase transition and the other, a columnar to a helical phase transition.



Electrooptic and Viewing Angle Characteristics of a Display Device Employing a Discotic Nematic Liquid Crystal

January 2003

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19 Reads

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28 Citations

Molecular Crystals and Liquid Crystals Science and Technology Section A Molecular Crystals and Liquid Crystals

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.


Figure 1. Columnar phases of discotic liquid crystals. (a) Upright columns forming a two-dimensional hexagonal lattice (Col h ). (b) Tilted columns forming a two-dimensional rectangular lattice.
Figure 5. The differential scanning calorimetric traces for HPA in the heating mode for (bottom) the pure compound and (top) the material doped with 0.60 mol % of anthracene.  
Figure 6. The measured values of the conductivity along the column axis (σ ) for three different frequencies (1.0, 3.162 and 10.0 kHz) for the doped and undoped HHTT in the cooling mode. The arrows indicate the transition temperatures.  
Figure 7. σ measured in the doped HPA in cooling mode at three different frequencies (1.0, 10.0 and 100.0 kHz).  
Figure 8. Measured DC electrical conductivity, σ and σ ⊥ , of doped HHTT. The circle represents the value in the isotropic phase determined using the parallel-plate geometry.  
Discotic liquid crystals as quasi-one-dimensional electrical conductors

August 2002

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478 Reads

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32 Citations

We have studied the electrical conductivity of well-aligned samples of hexahexylthiotriphenylene (HHTT) and hexapentyloxyanthraquinone (HPA) in the pure, as well as doped, states. The former compound was doped with 0.62 mol % by weight of the electron acceptor, trinitrofluorenone (TNF), the latter with 0.60 mol % by weight of the electron donor, anthracene. In the columnar phases, doping causes the AC (1 kHz) conductivity along the columnar axis (sigma||) to increase by a factor of 107 or more relative to that in undoped samples; sigma|| attains a value of 10-2 S m-1, which was the maximum measurable limit of our experimental set-up. On the other hand, in the isotropic phase, doping makes hardly any difference to the conductivity. The DC conductivity of doped HHTT exhibits an enormous anisotropy, sigma||/sigmaperp" ALIGN=absbottom BORDER=0> ges 1010, which is seven orders of magnitude higher than that reported for any liquid crystalline system, and, to our knowledge, the largest observed in an organic conductor. Further, the perpendicular conductivity, sigma


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

November 2001

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35 Reads

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24 Citations

Liquid Crystals

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 on the data obtained in the heating mode contains two triple points; one is estimated as 40 MPa, 77.2°C for the Cr-H-Colh triple point and the other is extrapolated as 285 MPa, 118.3°C for the Cr-Colh-I triple point. These triple points define the upper limits for the appearance of the stable H and Colh phases, respectively.


Quasi-one dimensional electrical conductivity and thermoelectric power studies on a discotic liquid crystal

July 1999

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1,094 Reads

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80 Citations

Pramana

We have studied the electrical conductivity of well aligned samples of hexahexylthiotriphenylene (HHTT) in the pure as well as doped states. The dopant used was a small concentration (0.62 mole %) of the electron acceptor trinitrofluorenone (TNF). In the columnar phases, doping causes the AC(1 kHz) conductivity along the columnar axis (σ ‖) to increase by a factor of 107 or more relative to that in undoped samples; σ ‖ attains a value of 10−2S/m, which was the maximum measurable limit of our experimental set up. On the other hand, in the isotropic phase doping makes hardly any difference to the conductivity. The frequency dependence of the conductivity has been investigated. The DC conductivity of doped samples exhibits an enormous anisotropy, σ ‖/σ ⊥ ≥ 1010, which is 7 orders higher than that reported for any liquid crystalline system, and, to our knowledge, the largest observed in an organic conductor. We also report the first thermoelectric power studies on these ‘molecular wires’. The sign of the thermoelectric power is in conformity with the expected nature of the charge carriers, namely, holes.


Citations (47)


... [1][2][3][4][5] The realisations of nematic phases in five-ring bent-core compounds are rare and limited. [9][10] In recent years, the nematic phases exhibited by BCMs made special attention because of molecular biaxiality, [11][12][13] nematic phases composed of tilted smectic (SmC-type) cybotactic clusters (NcybC) with strongly tilted aromatic cores, [14][15][16] ferro-nematic phase, [17] negative bendsplay elastic anisotropy [18] for the generation of long thermal-range blue phases. The conclusive experimental proof of a macroscopic biaxiality in nematic phases is still eluding, and the X-ray diffraction (XRD) measurements unequivocally confirmed the biaxial molecular aggregates of cybotactic nematic phases composed of SmC-like clusters in BCMs. ...

Reference:

Synthesis, mesomorphic, photophysical and computational studies of new achiral four-ring unsymmetrical bent-core mesogens and their Copper(II) complexes
X-ray study of a thermotropic biaxial nematic liquid crystal
  • Citing Article
  • January 1990

... this case, discotic lamella and columnar mesopahses were constructed by the hierarchical supramolecular self-organization of the linear rod-like benzoic acids as the building blocks; i. e., several numbers of mPhDA dimers aggregated to make a disc-like mesogen unit to exhibit discotic lamellar (D L ) and rectangular columnar (Col rd ) mesophases, which are classified according to the assignment and nomenclature reported in our previous study[5]although the term of D L phase has been used in limited cases in the literature. The D L phase is equal to the nematic phase of disc-shaped molecules (N D ).[6][7][8][9]The mesophase system of mPhDAs is an example of the exhibition of discotic liquid crystals based on the linear building block molecules, while a high number of supramolecular liquid crystalline systems have been reported to exhibit columnar mesophases by various kinds of mesogens and intermolecular interactions.[10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]Recently, we also revealed that a mixture of mPhDA and mOBA provided their mixed liquid crystalline system to exhibit both discotic (or columnar) and calamitic (or smectic) mesophases according to their composition and temperature conditions.[25]It ...

Extension of McMillan’s Model to Liquid Crystals of Disc-Like Molecules
  • Citing Chapter
  • January 1984

... During Sm A-N R transition, the smectic layer spacing ðd 0 ¼ 2π = q 0 Þ has been measured experimentally using an X-ray diffraction method [31][32][33] which remains constant with increase in pressure. Chandrasekhar and Rao [34] observed that the smectic layer spacing of 8OCB increases just before the reentrant transition under pressure. From the expression of inverse layer spacing q 0 (Equation (6)), in addition to above experimental observations, one can infer that the coupling parameters b 1 , b 2 and e will have same pressure dependence. ...

The re-entrant phenomenon in 4′-n-octyloxy-4-cyanobiphenyl: a high pressure X-ray study of the smectic A layer spacing
  • Citing Chapter
  • December 1981

... Obviously, this would drastically lower the ε || value, but does not affect the contribution in the lateral direction. A statistical model of antiferroelectric short range order in the nematic [28], to be discussed below, also argues that in polar systems, the anisotropic average permittivity, ε avg [=(ε || + 2ε ⊥ )/3], is lower than ε ext , the extrapolated value from the thermal trend in the isotropic phase. Fig. 4 (c) presents the thermal variation of ε avg for HLC and two CNT composites: Just as in HLC, ε avg is less than ε ext for the composites as well. ...

The role of permanent dipoles in nematic order
  • Citing Article
  • January 1975

... The combination of unknown chemical structure to start with, and experimental ambiguous measurements provided a clean slate for chemists to express their best ingenuity in synthesising candidate molecules. As a results a number of false positive have been reported in the literature since the '70s [28][29][30][31][32][33], and working on the subject was compared with walking through a minefield [34]. ...

A thermotropic biaxial nematic liquid crystal
  • Citing Article
  • November 1998

Current Science

... This liquid-like nature gives them unique optical, electrical, and mechanical properties. There are several types of LCs, including nematic, smectic, and cholesteric phases, each with distinct arrangements of molecules [93,94]. Nematic LCs (e.g., rod-like or disc-like) are characterized by lower order than smectic phases. ...

Liquid Crystals
  • Citing Article
  • August 1980

Annual Review of Materials Science

... This has been identified as a signature of coexistence phase of (SmC*+SmF*). It may be mentioned that similar splitting of inner maxima was reported by S. Krishna Prasad et al. [28] in a coexistence phase of (SmC*+SmI*). It had been argued that the coupling of molecular tilt may induce a weak bond orientation ...

Observation of the Smectic-C—Smectic-I Critical Point

Physical Review Letters

... Like me, BKS was a synthetic organic chemist focusing upon structure-property relationships to find new properties and phases in liquid crystalline (LC) materials. Besides being well known for his coauthorship in the fundamental paper on discotic LCs together with S. Chandrasekhar and K. A. Suresh [1], he is also well recognized for his work on re-entrant nematic phases [2] and on metallomesogens [3,4], with a focus on the search for phase biaxiality. Since the beginning of this century his work was focused on bent-core liquid crystals (BCLCs), especially fluorine substituted [5,6,7] and those forming non-tilted biaxial and polar smectic phases, where he made significant contributions [8,[9][10][11]. ...

Paramagnetic Nematic Liquid Crystals
  • Citing Article
  • November 1986

Pramana

... The most probable basis for such abundant polymorphism of LC materials lies behind their coupled order parameters, multicritical points, different critical domains, layered structure, great range of order parameters of symmetries, defect mediated transitions, multiply reentrant topologies and defect mediated transitions [4]. If more specifically said, the identity of any phase is coined with respect to their diverse molecular arrangements carried by intermolecular interactions [91]. It has also been observed for a long time that the nature of these interactions between the microscopic constituents is very effectively adjusted as one of the various interacting fields is changed in a particular manner. ...

Molecular interactions and dynamics in liquid crystals

Proceedings / Indian Academy of Sciences