Fluorinated Biphenylpyrimidine as a Possible Matrix for Ferroelectric Liquid Crystal Mixtures

Abstract
The results of the synthesis and study of a new fluorinated achiral smectic C liquid crystal, namely, 2-(3-fluoro-4'-pentylbiphenyl-4-yl)-5-hexylpyrimidine, are presented. The possibility to use this compound as a matrix for ferroelectric liquid crystal (FLC) mixtures is evaluated. Two mixtures have been prepared: one (FLC-691-F) is based on new fluorinated compound and another (FLC-691) on its non-fluorinated analogue. The diester of optically active 2-octanol and terphenyldicarboxylic acid was used as a chiral dopant for both mixtures. Investigation of these FLC mixtures parameters: phase transition temperatures, helix pitch, spontaneous polarization and tilt angle show significant influence of fluorine atom. When comparing the phase sequence of FLC-691 and FLC-691-F mixtures, one can note that in the fluorine containing mixture an additional smectic A phase arises. At the same temperatures, the helix pitch of FLC-691-F is 2 – 2.5 times less than the helix pitch of FLC-691. The tilt angle of FLC-691-F is much smaller than that of FLC-691 but its value approaches the optimal value of 22.5 degrees, which ensures maximum light transmittance of electro-optical cells. At the same time the presence of fluorine atom in the matrix is a reason for the decrease of the mixture spontaneous polarization and a significant decrease (almost 4 times) in the driving voltage of electro-optical light shutters. As a result, it was shown that the fluorinated matrix provides new possibilities for controlling the mixture parameters.

Keywords: luorinated achiral smectic C, ferroelectric liquid crystal mixture, chiral dopant, spontaneous polarization, helix pitch