Ferroelectric Liquid Crystals Induced by Atropisomeric Dopants: Dependence of the Polarization Power on the Core Structure of the Smectic C Host

Abstract

A series of 11 chiral dopants with an atropisomeric core derived from 4,4‘-dihydroxy-2,2‘,6,6‘-tetramethyl-3,3‘-dinitrobiphenyl were synthesized in optically pure form. These compounds were doped into five different smectic C (SC) liquid crystal hosts to induce a ferroelectric SC* liquid crystal phase, and the reduced polarization Po was measured as a function of the dopant mole fraction xd over the range 0.005 < xd ≤ 0.05. The polarization power δp was found to strongly depend on the core structure of the SC host. For example, the dopant (+)-2,2‘,6,6‘-tetramethyl-3,3‘-dinitro-4,4‘-bis[(4-octyloxybenzoyl)oxy]biphenyl gave δp values of <30 nC/cm2 in a phenyl benzoate SC host and 1738 nC/cm2 in a phenylpyrimidine SC host; the latter is one of the highest polarization power values reported thus far in the literature. In the phenylpyrimidine SC host, the polarization power was found to depend on the length of the dopant side chains and on the position of the atropisomeric core with respect to those of the surrounding SC host molecules, on the time average. The polarization power followed a trend opposite to that followed by the SC* helical pitch. Analysis of these results suggests that chirality transfer from the atropisomeric core of the dopant to those of the SC host molecules plays a key role in amplifying the polarization induced in the phenylpyrimidine host. It is likely that such intercore chirality transfer results in an asymmetric distortion of the SC* lattice, which in turn, further increases the conformational asymmetry of the chiral dopant by virtue of increased diastereomeric bias between the SC* lattice and the chiral conformations of the dopant.