Effect of Molecular Structure on the Electrooptic Performance of Electroclinic Liquid Crystals

The correlation between molecular structure and electrooptic performance is studied for novel room temperature chiral smectic A liquid crystals. It is observed that slight modifications of the hydrocarbon chain connected to the chiral center result in dramatic differences in their electrooptic performance. If the hydrocarbon chain is shortened or if a double bond is localized at the end of the hydrocarbon chain, the tilt angle, electroclinic coefficient, and switching time are significantly suppressed. INTRODUCTION In 1977, Garoff and Meyer observed the electroclinic effect above the smectic-A-smectic C* phase in liquid crystalline materials composed of chiral molecules. In this effect, the electric field applied parallel to the plane of the smectic layers couples to the transverse molecular dipole moment. Since the molecules lack inversion symmetry, a molecular tilt is realized in a plane perpendicular to the electric field. The electroclinic effect has been studied in the bulk chiral smectic A,' higher-order orthogonal smectic phases, nematic phases, and has even been established to exist at a solid interface. Apart from their interesting basic properties, electroclinic materials are promising for applications, such as spatial light modulators, that require analog phase modulation capability (gray scale) and fast switching times. Evolution of this technology will greatly depend on the synthesis of electroclinic materials with large induced tilt angles and electroclinic coefficients, field-independent and fast switching times, and a broad temperature range stable down to sub-ambient temperatures. ^ IC QUALITY INSPECTED 4 'Sswrn^m^'i Development of materials with large electroclinic tilt angles needed for applications requires a fundamental understanding of the relationship between the molecular structure and the magnitude of the induced tilt angle.' Bahr, Heppke, and Klemke studied a homologous of materials to probe the relationship between molecular structure and the electrooptic performance. They observed that a longer alkyl chain on the non-chiral end resulted in a larger electroclinic tilt angle. More recently, we have also observed a similar phenomena in three successive even members of the KN homologous series. These materials are unusual because of their large induced tilt angles in a broad temperature range with no underlying smectic C* phase which is known to enhance field-induced tilt angles. Walba and coworkers have prepared mixtures of electroclinic materials to enhance the tilt angle. In this contribution we present results of our electrooptic studies. The results show a slight decrease in the chain length connected to the chiral center leads to a significant reduction in the molecular tilt angle and electroclinic coefficient. A similar behavior is found to occur if a double bond is localized at the end of the hydrocarbon chain connected to the chiral center. EXPERIMENTAL Four members of the homologous series which we refer to as KN compounds were studied. The structural formula and phase transition temperatures are presented in Figure 1 and Table I. The desirable feature of these materials is that they all exhibit chiral smectic A phases (identified by optical microscopy) to sub-ambient temperatures. No crystallization was observed in any of the samples when they were maintained at room temperatures. Their broad chiral smectic A temperature range is likely due to the proximity of the chiral center to the aromatic core and the presence of the laterally substituted nitro group which prohibits chiral smectic C* formation. Also in these materials the chiral methyl group is very close to the lateral nitro substitute so that the relative rotations of these two moieties about the long axis of the molecule are highly hindered. This steric hindrance most likely results in large dipolar coupling and therefore a large induced tilt angle as we shall see later.