New Syntheses.the Liquid Crystal Properties of Some New Mesogens

The 4'-n-alkyl-4-cyanobiphenyls and 4'-n-alkoxy-4-cyanobiphenyls provide colourless liquid crystal phases which are chemically and photochemically stable. In many cases, the mesophases are produced at quite low temperatures, and indeed two members of the n-alkyl series are nematic at room temperature. Eutectic mixtures prepared from members of the two series give wide nematic ranges, e. g., from about 2 OC to above 60 O C . All the compounds are of high positive dielectric anisotropy and either individually or in admixture their nematic phases perform well in twisted nematic displays. The nematic ranges have been further extended, without adversely affecting the stability characteristics or the electrical properties by incorporating certain members of the series of 4"-n-alkyl-4-cyano-p-terphenyls. For example, one eutectic mixture is nematic from 5 OC to 91 O C . Two optically active analogues of the cyanobiphenyls and cyano-p-terphenyls have been prepared by using a branched chain as the alkyl or alkoxy group. Incorporation of these compounds in mixtures with the 4'-n-akyland 4'-rr-alkoxy-4-cyanobiphenyls gives long pitch cholesteric phases which exist in the room temperature range, and which are colourless and both chemically and photochemically stable. These phases perform well in cholesteric-nematic phase change devices. These chiral solutes used in low concentration (1 %) also prevent areas of reversed twist in twisted nematic displays. 4'-n-octyl-4-cyanobiphenyl gives a room temperature smectic phase. This phase and those shown by other higher homologues of the two series have been shown by miscibility to be smectic A. However, the smectic phases have unusual X-ray characteristics. The smectic polymorphism of the 4"-n-alkanoyl-4-bromo-p-terphenyls has been studied ; the heptanoyl compound provides only the third known example of a direct Sn-N transition. The four series of 4"-chloro-, 4"-methyl-, 3"-chloroand 3"-methyl-w-phenylalkyl 4-(4'-cyanobenzy1ideneamino)cinnamates have been studied. It is concluded that the extremely large alternations in the N-I transition temperatures as the series are ascended are explained in terms of changes in the spatial distribution of the terminal ring of the ester function which are determined by the stereochcmistry of the alkylene chain. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975157