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Three structurally similar esters containing carborane (1A), bicyclo[2.2.2]octane (1B), and benzene (1C) were prepared and their mesogenic properties investigated. All esters exhibited chiral nematic phases, and only 1B showed rich smectic behavior. The esters were used as additives to three structurally different nematic hosts. The resulting cholesteric pitch p was measured as a function of concentration and temperature. The calculated helical twisting power b M followed the order 1A , 1B ¡ 1C, and the temperature dependence of b M was ¢0 for all mixtures except for 1C in host I. These results are discussed in terms of size and conformational properties of each ring system A-C and the role of their ''biaxiality'' in chirality transfer. The observed trends in b M are consistent with non-specific solute-solvent interactions in which the chiral additive breaks the uniaxial symmetry imposed by the phase and differentiates energy of the chiral conformers of the host.

Antiferroelectric order has been known to exist in liquid crystals since more than a decade and is now an intensely studied field of research. The great application potential of antiferroelectric liquid crystals has especially been demonstrated in sophisticated flat‐panel display prototypes, which nevertheless have not yet reached manufacturing, due to the severe intrinsic problem of folds in the smectic layers, which drastically limit the achievable contrast, and which seem impossible to circumvent. By proper molecular design, we have developed and tested a new generic class of antiferroelectric materials that present an elegant solution to this problem. Their optical properties make them unique not only among liquid crystals but among electro‐optical materials in general. The design of this generic class, which we call orthoconic, also gives an illustrative example of the physical meaning of the addition of tensorial properties. Normal surface‐stabilized antiferroelectrics are optically positive biaxial crystals, with an effective optic axis along the smectic layer normal. The surprising optical property of the corresponding orthoconic antiferroelectric can be formulated as a theorem: When the tilt directions in adjacent smectic layers are made perpendicular to each other, the material becomes negatively uniaxial with the optic axis lying perpendicular to the smectic layer normal. The electro‐optic effect in such a material is based on the fact that the optic axis can be switched between three mutually orthogonal directions, corresponding to zero, negative, or positive values of the applied electric field.

Antiferroelectric liquid crystals with a tilt angle of 45 degrees have very interesting optical properties, which seem to have been overlooked so far-perhaps because such materials have hardly been available. We have prepared a four-component mixture of partially fluorinated compounds with a SmC a * phase in the interval between 27.4°C and 121.6°C, in which the tilt angle θ saturates at 45 degrees for T ≤ 80°C, and we investigate the optical properties, theoretically and experimentally. One of the surprising features of 45 degree materials is that they permit a remarkably high contrast by virtue of an excellent dark-state, in spite of the fact that AFLC materials are notoriously difficult to align. This is because a 45° AFLC turns out to be (negatively) uniaxial instead of biaxial. We describe these properties and propose a number of potentially interesting new applications, including a polarizer-free display mode and a three-level "phase-only" modulator. Keywords uniaxial antiferroelectric liquid crystal; 45° tilt; Sm C a *; AFLC dielectric tensor; new electro-optic modes.

So far, it has proven impossible to achieve an acceptable dark state between crossed polarizers for antiferroelectric liquid crystals (AFLCs), which otherwise would have an enormous potential for electro-optic applications, in particular for high-resolution full color displays. The reason lies in static and dynamic spatial fluctuations of the optic axis. As both have intrinsic causes it is not likely that the problem is ever going to be solved by improvement in alignment and addressing methods. We show that if the directors in alternating layers are orthogonal to each other, the AFLC acquires new optical properties such that the problem is eliminated, and a dark state extinction is achieved which is only limited by the quality of the polarizers. After having synthesized such a material, we have been able to demonstrate the predicted unique electro-optical properties of this new class of AFLC materials.

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