The Influence of the Imine Bond Direction on the Phase Behaviour of Symmetric and Non-symmetric Liquid Crystal Dimers

The synthesis and characterisation of two series of liquid crystal dimers are reported: the symmetric 1,5-bis(4′-oxyanilinebenzylidene-4-alkane)pentanes (m-O5O-m), and the non-symmetric 1-(4-cyanobiphenyl-4′-yloxy)-5-(4′-oxyanilinebenzylidene-4-alkane)pentanes (CBO5O-m). In the acronyms for each series m refers to the terminal chain length. The m-O5O-m series showed monotropic nematic phases for m = 1-7. Smectic behaviour emerged at m = 7 and nematic behaviour extinguished at m = 8. For the CBO5O-m series, an enantiotropic nematic phase is seen for m = 1-10. In addition, m = 1 shows a twist-bend nematic phase, and m = 9 and 10 an interdigitated smectic A phase. These series are compared to the corresponding dimers in which the imine link in the benzylideneaniline fragments is reversed, the m.O5O.m and CBO5O.m series. For the symmetric dimers, reversing the imine link has a small effect on the nematic-isotropic transition temperatures, TNI, but a much larger effect on the smectic-isotropic transition temperatures, TSmI. In both cases, the m.OnO.m series shows the higher values. The values of TNI are also slightly higher for the CBO5O.m series than the CBO5O-m series. Surprisingly, the values of TSmI are higher for the CBO5O-m than for the CBO5O.m series. These differences are discussed in terms of the changes in shape and electronic distributions arising from reversing the imine link.