Flexible dimers as dopants for liquid crystal display mixtures with faster relaxation times

A series of fluoro-substituted alpha,omega-bis-diphenyloxyalkanes have been investigated as possible dopants for nematic liquid crystal display mixtures to reduce the rotational viscosity and hence response times of the mixtures. The compounds have been prepared using the Williamson synthesis, and the effect of the dopants on the mesophase transition temperature of a suitable Merck display mixture host ( ZLI-4792) has been determined. Measurements of magnetic field-induced relaxation times for director reorientation have been made on the doped and undoped mixtures using a novel electron spin resonance ( ESR) method developed by some of the authors. The relationship between field-induced relaxation times and the rotational viscosity is explained and results are reported for the rotational viscosity of the host material. A brief discussion of the molecular theories of rotational viscosity of nematic liquid crystals identifies the important molecular factors as shape, size, flexibility and orientational order parameter. The influence of the dopant molecules on the orientational order parameter of the doped mixtures has been measured using a static ESR method, and these results enable the separate effects of the dopants on the order parameter of the host and on the rotational dynamics to be identified. Results for field-induced relaxation times and order parameters of doped mixtures are reported for seventeen alpha,omega-bis-fluoro-diphenyloxyalkane dopants. Additionally comparative results are reported for a structurally related compound 1,2-trans-di(4'-propylphenyl) 1',2'- difluoroethane. It is found that particular substitution patterns of fluorine atoms in the phenyl rings of the dopant compounds can result in shorter relaxation times for the nematic display mixture investigated. The use of flexible dimers as dopants provides an additional design aid in the development of nematic mixtures for applications.