A COMPARISON OF THE STRUCTURE, FLEXIBILITY AND MESOGENIC PROPERTIES OF 4-METHOXY-4'-CYANOBIPHENYL AND THE ALPHA,ALPHA,ALPHA-TRIFLUORINATED DERIVATIVE

The compound no-4'-(alpha,alpha,alpha-trifluoromethoxy)biphenyl (1OCBF 3) has been synthesized. Unlike the fully protonated analogue, 4-cyano -4'-methoxybiphenyl (1OCB), it does not show a liquid crystalline phas e on cooling from the melting point (51-degrees-C) to room temperature . The transition temperature to a monotropic nematic phase was obtaine d as approximately 0-degrees-C by determining the transition temperatu res of mixtures with 1OCB. The structures, conformational properties a nd orientational ordering of both 1OCB and 1OCBF3 as solutes in a nema tic solvent ZLI 1132 have been investigated via the 17 dipolar couplin gs obtained by analysing the proton and fluorine NMR spectra of these solutions. It is concluded that the major difference between the two m olecules lies in the potential, V(phi2), governing rotation about the ring-oxygen bonds. In 1OCB the potential has the same form as in aniso le, with a minimum when the C-O bond is in the plane of the attached r ing (phi2 = 0-degrees), and a maximum of about 15 kJ mol-1 when phi2 i s 90-degrees. In 1OCBF3 the barrier to rotation about the ring-O bond decreases substantially to being near zero.