Unusual orientational behaviour of liquid crystals: investigation of the effect of bipolar interactions on the ferroelectric properties

Abstract

Two compounds with a single fluorinated chiral centre, (S)-4-n-octyloxydifluorobiphenyl-4′-yl 3-fluoro-4-(2-fluorooctyloxy)benzoates, have been synthesized and their liquid-crystalline properties determined. The orientational behaviour of the two compounds was investigated by comparing the results of a ¹⁹F NMR analysis with the tilt angle measurement. Selected fluorine–fluorine dipolar coupling constants were measured for different molecular segments in both the smectic A and smectic C* phases. The orientational properties of the two liquid crystals are contrasted in terms of their electronic differences due to a small, but highly significant, difference in their core structures (2,3-difluorobiphenyl vs. 2′, 3-difluorobiphenyl). The unusual tilt angle behaviour exhibited by one of the compounds is suggested to be the result of an unusually large transverse dipole moment which would lead to an increased lateral intermolecular association. The dipolar interaction between fluoro-substituents on the core and at the chiral centre (here experimentally shown to exist for the first time) is discussed and a comparison is drawn between the experimental measurements and the calculated molecular structure.