Thermotropic Biaxial Nematic Liquid Crystals

Abstract

We have synthesized liquid crystal (LC) mesogens based on a nonlinear oxadiazole unit that exhibit nematic phases near 200 degrees C. Polarized microscopy and conoscopy indicate that these LCs are biaxial nematics. Unambiguous and quantitative evidence for biaxiality is achieved using 2H NMR spectroscopy. "2D powder" spectra, obtained by rotating 2H-labeled samples about an axis perpendicular to the magnetic field at approximately 200 Hz, yield phase biaxiality parameters of approximately 0.1 when coupled with rigorous and proven simulations.

Biaxial Nematic Droplets and their Optical Textures: A Lattice Model Computer Simulation Study

“…Experiments by Chandrasekhar et. al [7] and, more recently, investigations on lyotropic [8] and thermotropic [4,5,9] systems seem to support this hypothesis which we have also verified by computer simulations of a schlieren texture, at least in some biaxial nematic films [10]. The problem of the molecular organization and of the optical texture of biaxial nematics in spherical droplets is also of interest in view of the expected differences in topological defect structure [11].…”

“…Biaxial nematics are currently a subject of great interest [4,5] and the question of how to identify a biaxial nematic is still a major problem. One of classical approaches is to examine the optical texture between crossed polarizers and a suggestion originally made by deGennes [6] was that, due to the different symmetry, biaxial nematics should present only two brushes stable defects in schlieren films.…”

Biaxial Nematic Phase in Model Bent-Core Systems

“…The atomistic simulations [21] of the molecular systems studied experimentally [2,3] have confirmed the existence of a weak biaxiality in that system. Similarly, for a system of GB bent-core molecules with long arms the biaxial nematic phase was observed on cooling [22].…”

“…In 2004 a long-awaited discovery was announced [2][3][4] and the thermotropic biaxial nematics were claimed to be found at last, 34 years after its first theoretical prediction by Freiser in 1970 [5]. The discovery was made in systems of the bent-core mesogens, also often called banana-or boomerang-shaped.…”

Atomistic Simulations of a Thermotropic Biaxial Liquid Crystal

“…Although each strategy showed some merit, none of these succeeded in producing a verifiable biaxial phase. Recently this situation changed with both Madsen et al [8] and Acharya et al [9] reporting strong experimental evidence to have discovered the first biaxial phase in low molecular weight thermotropic liquid crystals based on molecules with an oxadiazole core ( Fig. 1).…”