Showing papers by "S. Chandrasekhar published in 1993"

Structural characteristics of some metallo-organic discogens

Abstract: We have recently analysed the crystal and molecular structures of six metalloorganic13; discogens with substituted {J-diketone ligands. The molecules consist of a rigid 11 atom core and a fringe made up of four phenyl rings substituted with13; alkyl/alkoxy chains. In complex (i),with four octyloxy chains, there are four oxygen atoms around the core. Complex (ii) is asymmetrically substituted with two13; heptyloxy and two heptyl chains and therefore has two oxygen atoms and complexes (iii) to (vi)have only alkyl chains and hence no oxygen atoms around the core. The metal atom used for coordination has been chosen as Cu/Pd/Ni. Determination of the crystal and molecular structures of these discogens has led to the identification of the following similarities: (1) All the six discogens crystallize in the triclinic space group PI. The recurrence of the space group may be correlated with the structural requirements for efficient packing of the molecules in the crystal lattice. (2) The coordination around the metal atom is square planar. (3) The 11 atom core is only nearly planar. (4)The phenyl rings and the chains are tilted with respect to the core. (5) The molecular conformation in the crystal confers a nearly13; rectangular shape to these discogens. (6)The chains are fully extended in an ail trans conformation. (7) The molecular arrangement is tilted columnar except for the13; crystal structure of complex (ii).

Dielectric studies in the vicinity of the A-C* transition

Abstract: We have studied the soft mode and Goldstone mode relaxations in a ferroelectric liquid crystal compound (referred to as C6 for short) exhibiting a high value of spontaneous polarisation. The results show that the relaxation frequency fs as well as the dielectric strength Δϵ S can be described by a simple power law, but gives non-mean field exponents. However, fitting done to the Δϵ s data using an expression given by the generalised mean field theory, not only yields the mean field susceptibility exponent γ but also helps in determining the Landau coefficients a and C (where a is the electroclinic coefficient and C the polarisation-tilt coupling constant) as well as the soft mode viscosity. We also present the Goldstone mode viscosity and the relevant elastic torque data. Measurements on C7 and C8, the higher homologs of C6, bring out the effect of chain length on the relaxation parameters.

Ferroelectric liquid crystalline polymers with large pyroelectric coefficients for infrared detectors

Abstract: We have synthesized many ferroelectric side-chain liquid crystalline polymers exhibiting the Smectic C* phase over a wide temperature range extending to sub-ambient temperatures. The tuning of the mesophase temperature has been achieved by incorporating two different mesogens as side groups attached to the siloxane backbone. These copolymers exhibit large values of spontaneous polarization which varies linearly through out the Smectic C* phase. The pyroelectric coefficient and dielectric constants of these materials have been measured. The potential of these ferroelectric liquid crystals as pyroelectric detector materials will be examined.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.