De Vries liquid crystals based on a chiral 5-phenylpyrimidine benzoate core with a tri- and tetra-carbosilane backbone

New chiral de Vries smectic liquid-crystalline compounds are designed, synthesized, and investigated for perspective applications in defect-free bistable surface-stabilized ferroelectric liquid-crystal displays. In these compounds, a 5-phenyl-pyrimidine benzoate core is terminated on one side by a tri- or tetra-carbosilane group linked through an alkoxy group and an alkyl spacer and on the opposite side terminated by a chiral 2-octanol group. The stereogenic center contains either a methyl or perfluoromethyl functional group. These compounds exhibit Iso-SmA∗-SmC∗-SmX-Cr phases under cooling from the isotropic state. Measurements of the temperature-dependent smectic layer spacing by x-ray diffraction experiments combined with the measured apparent optical tilt angle and the birefringence reveal that SmA∗ phase in these compounds is of the de Vries type. In addition, the chiral compound with a tetra-carbosilane backbone, DR277, exhibits good de Vries properties with the SmC∗ phase exhibited over a wide temperature range. By varying the carbosilane end group, the de Vries properties are enhanced, that is, the layer shrinkage of ∼1.9% for the tri-carbosilane DR276 is reduced to ∼0.9% for tetra-carbosilane DR277 at 10 C below SmA∗ to SmC∗ transition temperature, TAC. For DR277, the reduction factor R≈0.22 for T=(TAC-10) C is reasonably low and the apparent optical tilt angle θapp=35.1, hence this compound is a "good de Vries smectic" LC. Therefore, synthesis of the chiral mesogen with an even higher number of carbosilane groups may lead to a further reduction or even zero-layer shrinkage exhibited at TAC with SmC∗ phase extending over a wide temperature range close to the room temperature for perspective suitability in device applications. Our results for 5-phenyl-pyrimidine benzoate core-based compounds support a recently drawn conclusion by Schubert et al. [J. Mater. Chem. C 4, 8483 (2016)10.1039/C6TC03120J] from a different compound, namely that a carbosilane backbone in chiral mesogens strongly influences the de Vries properties.