Predictions of helical twisting powers and molecular chirality

Abstract

When chiral molecules are added to an achiral nematic liquid crystalline solvent, they can transmit their molecular chirality to the whole system over distances many times their molecular length. The helical twisting power, βm, is a measure of the degree of twist a chiral molecule can induce in a nematic liquid crystal. The work in this thesis is primarily concerned with calculating helical twisting powers for a variety of chiral molecules using computational and theoretical methods. The first technique used to calculate βm employed Monte Carlo simulations of an atomistic chiral dopant molecule in a chiral liquid crystal phase composed of generic liquid crystalline molecules. The method was found to be computationally expensive, but provided reasonable predictions of when compared with experimental results. Prior to these calculations, a liquid crystal solvent for use in this method was studied. Computer simulations of L/D = 4 soft repulsive spherocylinder (SRS) molecules were performed and two liquid crystalline phases were found in the SRS phase diagram. The scaled chiral index and the chirality order parameter are quantitative measures of molecular chirality. Both of these methods have been found to show a good correlation with experimentally determined helical twisting powers of relatively rigidchiral molecules. The chiral measures have also been incorporated in Monte Carlo simulations of flexible chiral molecules. This method has been successful in predicting βm for flexible chiral dopants, in predicting the temperature dependence of βm, has demonstrated a temperature induced helical twist inversion, and has been used in a predictive study to aid in future synthetic strategies. The final part of the thesis uses Monte Carlo simulations of a chiral molecule in an achiral liquid crystal phase. These simulations have shown that it is possible to relate the torque the chiral molecule induces in the solvent to βm.