Fluorinated ferroelectric liquid crystals : synthesis, properties and fluorine-19 nmr

Abstract

A series of new laterally fluorinated liquid crystals containing a single chiral centre on the terminal chain, was synthesised. All the compounds, with a single exception, exhibit a ferroelectric smectic C* phase. Their liquid crystalline properties were investigated using optical polarising microscopy and differential scanning calorimetry, while ferroelectric properties were studied electro-optically. The number/ position of the lateral fluoro-substituents greatly influence the incidence and range of the liquid crystalline phases. The 2,3 - difluorophenyl unit stabilises the cholesteric and smectic C* phases, but suppresses the formation of the smectic A phase. The 2' ,3 - difluorobiphenyl unit, in contrast, discourages the formation of helical phases. A helix inversion occurs in the cholesteric phase of (S)-4-n-octyloxy-2, 3-difluorobiphenyl-4' -yl 3-fluoro-4-(2-fluorooctanoyloxy) benzoate. Due to certain structural features, the phenomenon could not be explained in terms of the conventional model. Instead, competing conformer species created through interaction between fluoro-substituents on the core and at the chiral centre, are suggested. The influence of the lateral fluoro-substituents on the ferroelectric properties was investigated. Two of the compounds exhibit unusual tilt angle behaviour - the tilt angle decreases with decreasing temperature. A helix inversion takes place in the smectic C* phase of (S)-4-n-octyloxy-2' ,3- difluorobiphenyl-4' -yl 2-fluoro-4-(2-fluorooctanoyloxy)benzoate and is thought to be due to the presence of the 2',3-difluorobiphenyl unit. The orientational ordering of several of the liquid crystals was investigated using 19F nmr. A new technique was developed for this purpose and used to determine the order parameter as a function of temperature of (S)-4-n-octyloxy-2,3-difluorobiphenyl-4' -yl 4-(2-chloro-4-methylpentanoyloxy) benzoate from the dipolar coupling constant, DFF. The question of possible coupling between dipoles on the core and the chiral centre was addressed. For the first time it is experimentally shown that such an interaction does exist for two fluorosubstituents {1 9F - 19F dipolar coupling). Conformational analysis revealed a transverse electrostatic gradient. The conformation of the 2' ,3-difluorobiphenyl unit was investigated by using 19F nmr in conjunction with crystal structure data. It appears that the net dipole moment as well as the effective size of this unit play an important role.