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-$\mathrm{Sm}{A}^{*}-\mathrm{Sm}{C}^{*}-\mathrm{Sm}X$-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 $\mathrm{Sm}{A}^{*}$ 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 $\mathrm{Sm}{C}^{*}$ 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 $\sim 1.9\%$ for the tri-carbosilane DR276 is reduced to $\sim 0.9\%$ for tetra-carbosilane DR277 at $10{}^{\circ }\mathrm{C}$ below $\mathrm{Sm}{A}^{*}$ to $\mathrm{Sm}{C}^{*}$ transition temperature, $T_{\mathrm{AC}}$. For DR277, the reduction factor $R\approx 0.22$ for $T=\left(T_{\mathrm{AC}}-10\right){}^{\circ }\mathrm{C}$ is reasonably low and the apparent optical tilt angle ${\theta }_{\mathrm{app}}=35.1^{\circ }$, 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 $T_{\mathrm{AC}}$ with $\mathrm{Sm}{C}^{*}$ 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)] from a different compound, namely that a carbosilane backbone in chiral mesogens strongly influences the de Vries properties.

• Received 20 December 2017

DOI:https://doi.org/10.1103/PhysRevMaterials.2.025603