Effect of substituents on the properties of fluorine-containing polyphenyl liquid crystal compounds

In order to explore the influence of molecular structure on the properties of liquid crystal (LC) materials, the dielectric anisotropy, rotational viscosity coefficient and birefringence of different fluorine-containing polyphenyl LC compounds are measured experimentally. The effects of lateral substituents and terminal substituents are analyzed. The experimental results show that the substitution of fluorine substituents in lateral substituents or terminal substituents will lead to the decrease of rotational viscosity coefficient, whether it is fluorine-containing triphenyl LC compounds or fluorine-containing tetraphenyl LC compounds. The dielectric anisotropies of LC compounds (fluorine-containing tetraphenyl LC compounds ND-3 and ND-8, ND-4 and ND-9, and ND-1 and ND-10) in which the terminal isothiocyano substituents are substituted by fluorine substituents all show a downward trend, with the decreasing degrees of 41.3%, 20.7% and 19.2% respectively. In addition to the influence of the replacement of fluorine and isothiocyanato on the terminal substituents, the dielectric anisotropy is affected by different lateral substituents from large to small: fluorine, methyl, and ethyl. The lateral fluorine substitution impact of ND-3, with an increase of 9.4%, is the best among fluorine-containing tetraphenyl LC compounds when compared to the lateral ethyl substitution of ND-2. Besides, compared with -F, -NCS can effectively extend the pi-electron conjugation length of the molecule and improve the birefringence of the LC. This research is of great help to improve the comprehensive properties of LC materials and provides data accumulation for the development and application of LC compounds.