A comparison of the structure, flexibility and mesogenic properties of 4-methoxy-4?-cyanobiphenyl and the ?,?,?-trifluorinated derivative

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Abstract
The compound 4-cyano-4′-(α,α,α-trifluoromethoxy)biphenyl (1OCBF3) has been synthesized. Unlike the fully protonated analogue, 4-cyano-4′-methoxybiphenyl (1OCB), it does not show a liquid crystalline phase on cooling from the melting point (51°C) to room temperature. The transition temperature to a monotropic nematic phase was obtained as approximately 0°C by determining the transition temperatures of mixtures with 1OCB. The structures, conformational properties and orientational ordering of both 1OCB and 1OCBF3 as solutes in a nematic solvent ZLI 1132 have been investigated via the 17 dipolar couplings obtained by analysing the proton and fluorine NMR spectra of these solutions. It is concluded that the major difference between the two molecules lies in the potential, V(φ2), governing rotation about the ring–oxygen bonds. In 1OCB the potential has the same form as in anisole, with a minimum when the C–O bond is in the plane of the attached ring (φ2 = 0°), and a maximum of about 15 kJ mol−1 when φ2 is 90°. In 1OCBF3 the barrier to rotation about the ring–O bond decreases substantially to being near zero.

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