Biaxial dielectric properties in the chiral smectic C phase of single compounds and their relation to molecular structure

Abstract

In an attempt towards relating the dielectric biaxiality to the molecular structure, the dielectric anisotropy and biaxiality and their temperature dependence in the chiral smectic C phase have been determined for seven single compounds. It is found that, on a molecular level, compounds with a strong polar group connected with a cyclohexane ring exhibit a relatively large dielectric biaxiality. Among the substances with large dielectric biaxiality, it is noticed that attaching the polar group to the linking point of the cyclohexane ring and the alkyl chain containing the stereocenter results in the largest biaxiality. The studied compounds possess a smectic C phase preceded either by a smectic A or a nematic phase. As a general rule (with exceptions) we found that a large biaxiality is also coupled to a high value of the spontaneous polarisation. While the dielectric biaxiality for all studied materials is positive, the dielectric anisotropy (at 100 kHz) was found to be negative, especially at lower temperatures in the C* phase. The reported numerical data of the dielectric biaxiality were taken at 100 kHz, a frequency which is relevant for electro-optic device applications. Some recently synthesized structures have very large biaxialities, with δε = ε₂ - ε₁ ≈ 5. Control of both δε and δε are important for the switching characteristics of ferroelectric liquid crystals and adjustments in both parameters should be made when varying the addressing wave forms.

Keywords:

• Dielectric biaxiality
• dielectric anisotropy
• chiral smectic C