Molecular origin of ferroelectricity in induced smectic-C* liquid crystalline phases.

Abstract

Based on recent experimental results, a molecular model of the ferroelectric ordering in the induced smectic-${C}^{*}$ phase is proposed in terms of a coupling between dipole and quadrupole ordering of chiral molecules. The analysis of the correlations between the value and sign of the spontaneous polarization and the molecular structure of a broad variety of chiral dopants indicates that several new experimental results cannot be explained in the framework of the existing theory of ferroelectric ordering. In particular, the realistic theory is expected to account for the qualitative difference between the properties of the two different types of dopants that possess a chiral center either in the flexible chains or in the rigid core, respectively. The general statistical theory of ferroelectric ordering induced by a chiral dopant in the nonchiral smectic-$C$ host phase is developed and used to obtain the expression for the spontaneous polarization in terms of the dopant molar fraction, quadrupole order parameter, molecular chirality, and the geometrical parameters that characterize the relative orientation of molecular electric and steric dipoles with respect to the molecular rigid core. The different kinds of polar interactions between chiral molecules, which can be responsible for the appearance of the spontaneous polarization, are discussed in detail. In the context of this model it is possible to explain qualitatively the difference in properties between the two types of chiral dopants used in the experiment. The results of the theory enable one to understand why the spontaneous polarization is sensitive to the molecular structure of the host phase only if the dopant molecule possesses a chiral center in the rigid core. In the context of the same model one explains also the opposite signs of the polarization induced by the same chiral dopant in the smectic-$C$ host phases with the two different orientations of the molecular dipole. The latter result enables one also to understand the origin of the recently observed polarization sign inversion induced by a change of concentration of the chiral dopant. The theoretical conclusions are supported by some recent experimental data presented in this paper.