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 nemati c phases have been known for many years. However, it has only recently been found that inversions can occur in compounds which have single c hiral centres. II was found previously that the temperature range and the magnitude and sign of the helical twist in the chiral nematic phas e 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 m olecular modelling on appropriate compounds, and by deriving a suitabl e mathematical expression to allow the direct use of experimental data .