A comparative study of the mesomorphic properties of fluoro-isothiocyanated and fluorinated terphenyl liquid crystals from birefringence, static dielectric permittivity, splay elastic constant and rotational viscosity measurements

A few recently synthesized fluoro-substituted alkyl terphenyl isothiocyanates as well as three analogous fluoro-substituted terphenyl fluorides were investigated by birefringence, static dielectric permittivity, splay elastic constant and rotational viscosity measurements. The influence of the position of fluoro substituent on the physical properties was investigated. In 3,3′ and 3,5-difluoro substitution, although the fluorine atoms point in the same direction, 3,5 substitution leads to higher values of Δε, ε∥, ε⊥, Δn, no and ne than 3,3′-difluoro substitution. The figure of merit values, however, indicate that the 3,5-difluoro substituted isothiocyanated compound is a better component for mixture formulation in comparison with both the 3,3'-difluoro substituted or the 3',3,5-trifluoro substituted compound. Terphenyl isothiocyanated compounds are found to exhibit high birefringence and low rotational viscosity, which are suitable candidates for infrared applications while the fluorinated analogues are for visible applications.