Anti-Parallel Dimer Formation of 4-Cyano-4’-Alkyl Biphenyls in Isotropic Benzene Solution - Seeds of Liquid Crystalline Phases -

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  • Shikata Toshiyuki
    Division of Natural Resources and Eco-materials, Graduate School of Agriculture, Tokyo University of Agriculture and Technology
  • Minamoto Megumi
    Department of Macromolecular Science, Graduate School of Science, Osaka University

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The formation of anti-parallel (dipole) dimers of liquid crystalline phase forming 4-cyano-4’-alkyl biphenyls (n = 5 (pentyl) and n = 8 (octyl)) was examined in isotropic benzene solution. High frequency dielectric relaxation (DR) measurements up to 50 GHz were performed to probe molecular dynamic processes of nCB molecules and to evaluate a Kirkwood factor (gK), a measure of orientational correlation between dipole moments of cyano (-C≡N) groups sensitive to the formation of the anti-parallel dimers, (nCB)2, which shows gK < 1. DR spectra for solutions at intermediate to high concentrations were decomposed into two relaxation modes. A fast dielectric mode with the relaxation time of ca. 100~120 ps was assigned to free rotation of monomeric nCB molecules, while the other slow relaxation mode with the relaxation time of ca. 400 ps tentatively to the dissociation process of (nCB)2 dimers due to their lifetime. Infrared (IR) absorption spectra for the isotropic solution were also measured in a wavenumber range from 2200 to 2250 cm-1 corresponding to C≡N stretching vibration mode to evaluate molar fractions of nCB molecules forming the (nCB)2 dimers, since the stretching band slightly altered the peak wavenumber due to the formation of anti-parallel (nCB)2 dimers. The results of both the DR and IR measurements revealed that the formation of (nCB)2 dimers even in isotropic solution and the equilibrium constant of a chemical reaction, 2nCB ↔ (nCB)2, in benzene solution remarkably increased with increasing concentrations of nCB. Because most of nCB molecules form the anti-parallel dimers at moderate to high concentrations, the formation of the (nCB)2 dimers is necessary for nCB to undergo a phase transition from isotropic to a nematic liquid crystalline phase in benzene solutions at higher concentrations and also in the bulk state.

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