Ferroelectricity of induced smectic C* phases

Abstract

If non-chiral smectic C host phases are doped with chiral guest molecules, helically structured S_C* phases are formed by symmetry breaking. If the chiral dopants bear transverse dipoles a spontaneous polarization, P_S, will occur in these so-called induced S_C* phases. In ‘classical’ dopants the chiral centre(s) as well as the transverse dipole(s) are situated in alkyl side groups (type I dopants). The tilt-angle, reduced polarization, P_O=P_S/sin θ(θ= tilt angle), in these mixtures are independent of the host with a linear dependence on the guest mole fraction, x_G. As a material parameter the polarization power, δ_P=(∂P_O/∂x_G)_ΔT, has been introduced and is discussed in terms of the molecular structure of the dopant. Deviations of P_O(x_G) from linearity can be considered by a local field correction. Novel chiral dopants bearing the chiral centre(s) and the transverse dipole(s) in a rigid core (type II dopants) exhibit a significant dependence of the magnitude, and sometimes even of the sign, of P_O on the nature of the host. The different behaviours of the two dopant types are discussed by means of a microscopic model and can be understood in terms of a hard core guest-host interaction which influences the rotational distribution of the dopant molecules with respect to their long molecular axes.