S. Chandrasekhar

Bio: S. Chandrasekhar is an academic researcher from Raman Research Institute. The author has contributed to research in topics: Liquid crystal & Biaxial nematic. The author has an hindex of 31, co-authored 124 publications receiving 3368 citations. Previous affiliations of S. Chandrasekhar include Royal Institution & University of Mysore.

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...

Biaxial Nematic Liquid Crystals in Low Molecular Weight Thermotropic Systems

Abstract: The evidence for the occurrence of the biaxial nematic phase in low molecular weight thermotropic systems is examined. The optical evidence for biaxility appears to be quite unambiguous for all the eleven cases discussed here. X-ray diffraction studies on magnetically aligned samples lend support to the optical evidence, but do not provide independent proof of long range biaxial orientational order. No X-ray studies on monodomain specimens have yet been reported. A noteworthy result, intriguing from the theoretical point of view, is that the isotropic phase transforms directly to the biaxial nematic phase in a first order transition for all the eleven compounds.

The role of permanent dipoles in nematic order

Abstract: We propose a new model for the statistical description of short range order in nematic liquid crystals of positive dielectric anisotropy. We consider the molecules to be cylindrically symmet- ric and assume the pair potential to be of the form A*P 1 ,(cos θ ij ) - B*P 2 ( cos θ ij ) which favours an antiparallel arrangement of the perma- 11 nent dipoles. From physical considerations, antiferroelectric long range ordering is not expected to be present in nematics. Applying the Krieger-James approximation, solutions are obtained with an apolar or P 2 (cos θ) type of long range order, but allowing for antiferro- electric short range order. Two important consequences of the theory arc ( i ) the mean dielectric constant should increase slightly on passing from the nematic to the isotropic phase, (ii) the magnetic and electr~c birefringence in the isotropic phase should both vary approximately as ( T-T* ) -1 , where T* is the hypothetical second order transition point, except when the dipolar interactions are very strong, in which case the electric birefringence should exhibit a slower variation at tempe- ratures well above T*. Choosing values of A*/B*, illustrative calculations are given of the long range and short range order parameters, dielectric properties, electric and magnetic birefringence. The results are in general agreement with the experimental data for such compounds.

Handbook of Optical Materials

Abstract: CRYSTALLINE MATERIALS Introduction Physical Properties Optical Properties Mechanical Properties Thermal Properties Magnetooptic Properties Electrooptic Properties Elastooptic Properties Nonlinear Optical Properties GLASSES Introduction Commercial Optical Glasses Specialty Optical Glasses Fused Silica Fluoride Glasses Chalcogenide Glasses Magnetooptic Properties Electrooptic Properties Elastooptic Properties Nonlinear Optical Properties Special Glasses POLYMERIC MATERIALS Optical Plastics Index of Refraction Nonlinear Optical Properties Thermal Properties Engineering Data METALS Physical Properties of Selected Metals Optical Properties Mechanical Properties Thermal Properties Mirror Substrate Materials LIQUIDS Introduction Water Physical Properties of Selected Liquids Index of Refraction Nonlinear Optical Properties Magnetooptic Properties Commercial Optical Liquids GASES Introduction Physical Properties of Selected Gases Index of Refraction Nonlinear Optical Properties Magnetooptic Properties Atomic Resonance Filters APPENDICES Safe Handling of Optical Materials Abbreviations, Acronyms, and Mineralogical or Common Names for Optical Materials Abbreviations for Methods of Preparing Optical Materials and Thin Films Fundamental Physical Constants Units and Conversion Factors

Dendron-Mediated Self-Assembly, Disassembly, and Self-Organization of Complex Systems

Abstract: Dendron-mediated self-assembly, disassembly, and self-organization of complex systems have been investigated. The most ideal building blocks would need to display shape persistence in solution and in the solid state, since only this feature provides access to the use of complementary methods of structural analysis. Most supramolecular dendrimers are chiral even when they are constructed from nonchiral building blocks and are equipped with mechanisms that amplify chirality. This poses additional challenges associated with the understanding of the structural origin of chirality in different supramolecular structures through combinations of structural analysis methods. While many supramolecular structures assembled from dendrimers and dendrons resemble some of the related morphologies generated from block-copolymers, they are much more complex and are not determined by the volume ratio between the dissimilar parts of the molecule.

Discotic Liquid Crystals: From Tailor-Made Synthesis to Plastic Electronics

Abstract: Most associate liquid crystals with their everyday use in laptop computers, mobile phones, digital cameras, and other electronic devices. However, in contrast to their rodlike (calamitic) counterparts, first described in 1907 by Vorlander, disklike (discotic, columnar) liquid crystals, which were discovered in 1977 by Chandrasekhar et al., offer further applications as a result of their orientation in the columnar mesophase, making them ideal candidates for molecular wires in various optical and electronic devices such as photocopiers, laser printers, photovoltaic cells, light-emitting diodes, field-effect transistors, and holographic data storage. Beginning with an overview of the various mesophases and characterization methods, this Review will focus on the major classes of columnar mesogens rather than presenting a library of columnar liquid crystals. Emphasis will be given to efficient synthetic procedures, and relevant mesomorphic and physical properties. Finally, some applications and perspectives in materials science and molecular electronics will be discussed.