Methylene-linked liquid crystal dimers

A range of symmetric liquid crystal dimers which differ in the nature of the link, either ether or methylene, between the spacer and mesogenic units has been prepared and their transitional properties characterized. The nematic-isotropic transition temperature, T-NI, and the associated entropy change, DeltaS(NI)/R, are sensitive to the chemical nature of this link. Specifically, T-NI falls on replacing ether links with methylene links for both odd and even members although this reduction is more pronounced for odd members. In comparison, DSNI/R increases on changing ether links for methylene links for even dimers, but decreases for odd-membered dimers. These observations are completely in accord with the predictions of a model developed by Luckhurst and co-workers in which the difference between the ether-linked and methylene-linked dimers rests exclusively in their shapes. Furthermore, the highly non-linear pentamethylene-linked dimers show a greater tendency to exhibit smectic behaviour; this is interpreted in terms of molecular packing giving rise to an alternating smectic phase.