Molecular Dynamics of Cyanobiphenyl Compounds with Different Terminal Chains

Quasi-elastic neutron scattering (QENS) measurements were performed for five cyanobiphenyl compounds with different terminal chains (5CB, 5*CB, 8OCB, 8*OCB, and 6O2OCB) in their liquid and liquid-crystalline phases to investigate the molecular dynamics. The spectra were successfully decomposed into two components, indicating that two types of molecular motions are involved. The reorientational motion of the whole molecule around the long molecular axis is responsible for the broad component, whereas a diffusive motion of the terminal chain is responsible for the narrow one. For the former component, the temperature dependence did not differ very much among those compounds. For the latter component, the compounds with a branched alkyl-chain (5*CB and 8*OCB) gave significantly smaller diffusion constants than the others.

(by H. Suzuki & A. Inaba)

	Fig. 1. The dynamic structure factor S(Q, ω) of 5*CB at Q=2.18 Å-1 at 301.6 K. The circles are the experimental data. The red curve is the fitted one. The blue curve shows the narrow component and the green one the broad component.
	Fig. 2. The diffusion constants of 5*CB and 5CB for the narrow and broad components of QENS spectra. The lines show the functions fitted. For the narrow component, 5*CB gives significantly smaller diffusion constants than 5CB.
	Fig. 3. The diffusion constants of 8*OCB, 6O2OCB and 8OCB for the narrow and broad components of QENS spectra. The lines show the functions fitted. For the narrow component, 8*OCB gives significantly smaller diffusion constants than 8OCB and 6O2OCB.