Liquid crystal dimers and the twist-bend nematic phase. The preparation and characterisation of the alpha,omega-bis(4-cyanobiphenyl-4 '-yl) alkanedioates

Eleven members of the homologous series of liquid crystal dimers, the alpha,omega-bis(4-cyanobiphenyl-4 '-yl) alkanedioates, have been synthesised and their transitional properties characterised. These dimers consist of two cyanobiphenyl units connected by an alkyl spacer attached via ester linkages. All eleven members exhibit exclusively nematic behaviour. The nematic-isotropic transition temperatures, T-NI, and associated entropy changes, increment S-NI/R, exhibit pronounced alternations as the length and parity of the spacer is varied; this is characteristic behaviour of liquid crystal dimers. The transitional properties of the ester-linked dimers are compared with the corresponding materials having either ether, methylene or carbonate linkages between the spacer and mesogenic units. For short spacer lengths and both odd- and even-membered dimers, the ester-linked materials show the highest values of T-NI and the methylene-linked the lowest. For longer spacer lengths, T-NI of the carbonate-linked dimers fall between those of the corresponding ester- and ether-linked dimers. The ether-linked materials show the largest alternation in increment S-NI/R on varying spacer length and the carbonate-linked dimers the lowest. This behaviour is interpreted in terms of the molecular geometry and it is suggested that the ether- and ester-linked odd-membered dimers have rather similar shapes. A phase diagram constructed using binary mixtures of the pentyl member of this ester-linked series and the known twist-bend nematogen, 1,7-bis(4-cyanobiphenyl-4 '-yl)heptane (CB7CB), is presented. The twist-bend nematic-nematic transition temperature of the mixtures shows a striking convex curvature as the concentration of CB7CB is decreased, and so it is not possible to estimate a virtual twist-bend nematic-nematic transition temperature for the ester-linked material.