Deuterium NMR Investigation of the Biaxial Nematic Phase in an Organosiloxane Tetrapode

Abstract

Deuterium NMR is used to examine the molecular order exhibited by an organosiloxane tetrapode giving the first experimental evidence, using a bulk sample, for the existence of a biaxial nematic phase in this type of compounds. The temperature dependence of the averaged quadrupolar coupling constant and asymmetry parameter was determined in the compound's nematic phase. Two distinct regimes could be identified, one with a vanishing asymmetry parameter corresponding to a uniaxial nematic phase and another with a significant temperature dependent asymmetry parameter, corresponding to a biaxial nematic phase. The high values obtained for the asymmetry parameter at the lower end of the nematic range are well above experimental error and constitute a definite proof of the biaxial nature of the nematic phase exhibited by the studied compound for those temperatures.

Biaxial order and a rotation of the minor director in the nematic phase of an organo-siloxane tetrapode by the electric field

“…The NMR spectroscopy of one of the two tetrapode systems showed two distinct nematic regions, one with a vanishing asymmetry parameter corresponding to the uniaxial nematic phase and second with a significant temperature dependent asymmetry parameter corresponding to the biaxial nematic phase. 2,3 Biaxiality in its nematic phase was also confirmed by dynamic light scattering 4 where the central Si atom of the tetrapode molecule was replaced by Ge; however the chains remained siloxanes as before. Recently, molecular theory for the relative stability of the uniaxial and biaxial nematic phases in tetrapode-like systems was given by Gorkunov et al 5 Most of the applications of liquid crystals are based on using their anisotropic properties and their large dependencies on pressure, temperature, and external fields.…”

Computer Simulations of Biaxial Nematics

“…These thermotropic systems have been thoroughly characterised by measuring the anisotropy of infrared absorbance, with the addition of conoscopic and textural imaging under polarised light [9], and the temperature dependence of the averaged quadrupolar coupling constant of a deuterated 8CB solute probe by deuterium NMR [10]. Such tetrapodes are quite complex and for the time being represent a challenge for a computer simulation aimed at understanding the molecular origin of their N b phase, especially in relation to their high flexibility which makes difficult to define their shape and interaction anisotropies in terms of simple models.…”

“…At the time of writing a few instances of stable thermotropic biaxial nematics and their characterisation [6,7,8,9,10] have been reported. The papers of Luckhurst [11] and Praefcke [12] and the book of Chandrasekhar [13] also give a critical account of the early synthetic attempts, while assessment of the more recent achievements can be found in [14].…”

Statistical Theory of Biaxial Nematic and Cholesteric Phases

“…The first convincing thermotropic biaxial nematic phase was jointly reported in 2004 by Kumar, Samulski and coworkers [35][36][37] from bent-core mesogens based on a mesogenic oxadiazole core with lateral substituents. In the same year 2004, a novel class of biaxial phases bases on organosiloxane tetrapodes was found by Mehl and coworkers [38][39][40].…”