Abstract:

In recent years， with the research and development of liquid crystal microwave phase shifter technology， how to reduce the high frequency dielectric loss of liquid crystal materials has become a key issue in the development of microwave communication devices. In this paper， seven liquid crystal compounds （Ⅰ1-Ⅰ7） containing triphenylacetylene isothiocyanate with a wide nematic temperature range （89.5~127.7 ℃） were designed and synthesized， and Ⅰ1-Ⅰ3 had a relatively low melting point （45.8~67.2 ℃）. These compounds are mixed with a solvent liquid crystal M composed of a variety of side ethyl triphenyldiacetylene liquid crystals in certain proportions， and their dielectric properties in the microwave frequency band （11~33 GHz） are tested by the rectangular resonant cavity perturbation method. The effects of the molecular structure and substituents on dielectric properties of liquid crystal at high frequency are investigated by molecular simulation. The experimental results show that the higher the dipole moment and polarizability， the higher the dielectric constant and higher frequency dielectric loss of the synthesized liquid crystal compounds；the introduction of branched chains in the terminal alkyl group not only reduces the melting point， but also decreases the dipole moment of the molecule， resulting in the decrease of the dielectric constant； the introduction of acetylene bond between benzene rings can increase the π-electron conjugation system and dielectric constant of the molecule， which can reduce the dielectric loss at high frequency； The dielectric anisotropy （\mathrm{\Delta }ε_r＞1.025） of the isothiocyano-linked （—NCS） phenyl ring （X₁ and X₂） compounds is greater than that of the central phenyl ring （X₃ and X₄） compounds， but the dielectric loss of the latter is lower.

Key words: Fluorinated triphenyl acetylene, Isothiocyanic liquid crystal, High dielectric anisotropy, Low dielectric loss