Chiral liquid crystal tetramers

Abstract

The synthesis and characterisation of two new series of liquid crystal tetramers in which four mesogenic groups are linked via three alkyl spacers is reported. In each series the length of the outer two spacers, n, is varied between 3, 4, 5, 7, 10 and 11 methylene units while the central spacer length, m, is either 5 or 6 methylene units. The outer two mesogenic groups are cholesteryl-based, Chol, while the inner two are Schiff's bases, SB. The tetramers are referred to by the acronym Chol-n-SB-m-SB-n-Chol. All twelve tetramers exhibit an enantiotropic chiral nematic phase. The chiral nematic-isotropic transition temperatures and associated entropy changes depend critically on the combination of spacers in the tetramer and this is interpreted in terms of average molecular shapes. The tetramers with m = 5 and n = 5, 7, and m = 6 and n = 3, 4, 5, 7, show a monolayer chiral smectic C* phase, the driving force for which may be the mismatch in the cross-sectional areas of the differing mesogenic groups. The tetramers with m=6 and n = 10, 11 show a quadruply intercalated smectic A phase characterised by a smectic periodicity to estimated molecular length ratio of just 0.23. The driving force for the formation of this phase is thought to be specific electrostatic interactions between the unlike mesogenic units while the formation of the phase is strongly dependent on the length and parity of the spacers due to packing constraints. On cooling, a new phase transition is seen, possibly driven by conformational changes in the outer flexible spacers, for both these tetramers from the intercalated smectic A phase to an intercalated alternating chiral smectic C* phase in which the layers consist of either the mixed mesogenic groups or solely the cholesteryl-based moieties.