Highly non-linear liquid crystal tetramers

Three new homologous series of liquid crystal tetramers in which four mesogenic groups are linked via three alkyl spacers have been synthesised and characterised. In each series the length of the outer two spacers, n, is varied from 3 to 12 methylene units while the central spacer is held at 5 methylene units. The three series differ only in the substitution pattern around the inner two mesogenic units. The series in which the inner spacer is attached to the 4-position on both mesogenic groups is referred to as n-p5p-n while n-p5m-n refers to attachment at the 4-position on one group and the 3-position on the other. n-m5m-n refers to attachment at the 3-position on both groups. All the members of the n-p5p-n and n-p5m-n series exhibit nematic behaviour. The nematic-isotropic transition temperatures exhibit a pronounced alternation as the length and parity of the outer spacers are varied but which attenuates as n is increased. By contrast, the odd-even effects shown by the associated entropy changes are not attenuated as n is increased. 4-p5m-4 and 5-p5m-5 also exhibit smectic behaviour. 4-m5m-4 shows a nematic phase and an alternating smectic phase analogous in structure to that observed for certain semi-flexible main chain liquid crystal polymers. The n-m5m-n compounds with n = 6, 8, 9 and 10 also exhibit a nematic phase and a disordered crystalline phase. The trends in the transitional properties of the series are interpreted in terms of average molecular shapes and a model developed to understand the behaviour of liquid crystal dimers.