A phenomenological approach to the inversion of the helical twist sense in the chiral nematic phase

Optically active materials that show a temperature dependent inversion of the helical twist sense in the chiral smectic C* and chiral nematic phases have been known for many years. However, it has only recently been found that inversions can occur in compounds which have single chiral centres. II was found previously that the temperature range and the magnitude and sign of the helical twist in the chiral nematic phase are related to the concentration of the optically active material(s) dissolved in a nematic host. In a similar way, we propose to describe the inversion of the helical twist sense in the chiral nematic phase of pure materials containing a single chiral centre. Additionally, we seek to verify the possible validity of the latter model by means of molecular modelling on appropriate compounds, and by deriving a suitable mathematical expression to allow the direct use of experimental data.