Lateral Cyano Substitution as a Tool for Tuning Negative Dielectric Anisotropy in High-Birefringence Liquid Crystals.

A series of 10 novel liquid crystalline compounds based on oligophenyl and tolane cores bearing laterally positioned cyano groups has been designed, synthesized, and systematically investigated. The molecular architecture was tailored by varying the number and position of lateral nitrile substituents, as well as the degree of core extension and terminal functionalization, in order to elucidate structure-property relationships governing mesomorphic, optical, and dielectric behavior. The majority of the compounds exhibit exclusively nematic phases, while no smectic ordering was observed, indicating that lateral cyano substitution effectively disrupts layered packing. The introduction of cyano groups significantly enables precise control over dielectric permittivity and induces negative dielectric anisotropy, with its magnitude and phase stability being directly governed by both the number and location of nitrile substituents. At the same time, extended π-conjugated cores provide relatively high birefringence (Δn ≈ 0.24), despite the presence of strong transverse dipoles, which typically reduce optical anisotropy. Comparative analysis reveals that excessive lateral polarity destabilizes the nematic phase and narrows its temperature range, highlighting the need for a balance between dipolar interactions and molecular anisotropy. The obtained results demonstrate that lateral cyano substitution constitutes an effective strategy for tuning negative dielectric anisotropy while retaining favorable optical properties. Overall, these results identify lateral cyano substitution as a useful molecular-design strategy for developing high-birefringence nematic materials with negative dielectric anisotropy, which may serve as building blocks for future microwave/GHz-oriented liquid-crystalline mixtures.