Comparative dielectric measurements on isomeric bent-core mesogens

Abstract

The physical behaviour, especially the dielectric properties, of two series of isomeric compounds formed by banana-shaped molecules is reported. The feature of all liquid crystalline substances investigated is the formation of a ferroelectric short range order. In one layer of the SmCP_A phase the spontaneous polarisation shows in one direction and in the following in the opposite resulting in an antiferroelectric behaviour. It is demonstrated that the molecular origin of the polarization is connected with the perpendicular dipole moments. From the two relaxation modes seen the first results from ferroelectric units and the second from the reorientation of the molecules about the long axes. The collective character of both motions is demonstrated. The same mechanisms appear in the columnar phase, but they behave in a different way due to the different structure. Strong changes of the dielectric properties are observed at the transition from the isotropic in the liquid crystalline phase, but astonishingly not between both mesophases.

Keywords:

• Liquid crystals
• Banana-shaped molecules
• Antiferroelectric phases
• Dielectric measurements

Acknowledgement

H. Kresse personally is indebted to Prof. J. A. Janik and his coworkers for the scientific cooperation over many years and the friendship.