Influence of side-methyl substitution position on the phase state and microwave dielectric properties of triphenylacetylene-based liquid crystals

Abstract

Liquid crystal materials are well known in display applications, and their unique birefringence and electrical tunability can be utilised in microwave devices. This innovative technology modulates and filters microwave signals, replacing conventional semiconductors for a broad operational frequency band and tunable phase shift. Although isothiocyanatobiphenylacetylene-based liquid crystals exhibit low viscosity and large dielectric anisotropy, their applications in microwave communication are hampered by their broad near-crystalline phase temperature ranges. To address this limitation, this study designed and synthesized six fluorinated biphenylacetylene liquid crystal compounds with various benzene ring side-methyl substitutions (n = 3–5). The molecular structures, liquid crystal phases, and microwave dielectric properties were evaluated. Our findings indicate that compounds with methyl substitution at the Y₂ position exhibited reduced melting points, an expanded nematic phase temperature range (ΔT_n ≈ 92.3 °C), and an absence of near-crystalline phases. These compounds still maintain high microwave dielectric constants within the 9–30 GHz frequency band (Δε_r = 0.9–1.3) and reduced maximum permittivity losses compared to their non-methyl-substituted counterparts, thereby improving the efficiency in the microwave frequency band. In contrast, the Y₁ position substitution results in a significantly narrower nematic phase temperature range (approximately 2.6 °C on average) and a substantial decrease in the dielectric constant, with a Δε_r reduction of about 0.3 compared to the Y₂ substitution. This work shows that the side-methyl substitution can improve the performance of triphenylacetylene-based liquid crystals in microwave communication, providing valuable insight to aid the discovery of novel microwave liquid crystals.