-
[show abstract]
[hide abstract]
ABSTRACT: Polarized Raman spectroscopy was used to investigate the development of orientational order and the degree of phase biaxiality in a bent-core mesogenic system. The values of the uniaxial order parameters and , and biaxial order parameters , , and , and their evolution with temperature were determined. The temperature dependence of almost all order parameters reveals a second order transition from the uniaxial to biaxial nematic phase with increasing to ∼0.22 before a first order transition to the smectic-C phase, upon cooling.
Physical Review Letters 07/2010; 105(2):027801. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: X-ray diffraction is one of the most definitive methods to determine the structure of condensed matter phases, and it has been applied to unequivocally infer the structures of conventional calamitic and lyotropic liquid crystals. With the advent of bent-core and tetrapodic mesogens and the discovery of the biaxial nematic phase in them, the experimental results require more careful interpretation and analysis. Here, we present ab-initio calculations of X-ray diffraction patterns in the isotropic, uniaxial nematic, and biaxial nematic phases of bent-core mesogens. A simple Meier-Saupe-like molecular distribution function is employed to describe both aligned and unaligned mesophases. The distribution function is decomposed into two, polar and azimuthal, distribution functions to calculate the effect of the evolution of uniaxial and biaxial nematic orientational order. The calculations provide satisfactory semiquantitative interpretations of experimental results. The calculations presented here should provide a pathway to more refined and quantitative analysis of X-ray diffraction data from the biaxial nematic phase.
The Journal of Physical Chemistry B 04/2009; 113(12):3845-52. · 3.70 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Two azo substituted achiral bent-core mesogens have been synthesized. Optical polarizing microscopy and synchrotron X-ray scattering studies of both compounds reveal the existence of the thermotropic uniaxial and biaxial nematic and three smectic phases at different temperatures in these single component small molecule systems. The transition from the uniaxial to biaxial nematic phase is confirmed to be second order. The transitions from the biaxial nematic to the underlying smectic phase and between the smectic phases have barely discernible heat capacity signatures and thus are also second order.
Journal of the American Chemical Society 01/2006; 127(49):17224-7. · 9.91 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The first examples of electrically switchable and photo-responsive mesophases in a novel class of bent-core azo compounds, consisting of only five aromatic rings and no imino functionality (–CHN–), are reported. The mesophases have been identified as B1, B2 and a new mesophase, Bx, on the basis of the results of polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction, and electro-optical response. Preliminary but profound photo-induced effects on the transition temperature from isotropic to the B2 mesophase of one of the azo compounds are also presented.
Journal of Materials Chemistry 01/2004; 14:1495. · 5.97 Impact Factor
