Non-symmetric chiral liquid crystal trimers

Two sets of non-symmetric chiral liquid crystal trimers have been synthesised and characterised. These consist of molecules containing a cholesteryl-based group, a 4-methoxyazobenzene unit and a central benzylideneaniline group, interconnected in a linear fashion via two flexible spacers. The length of the spacer connecting the 4-methoxyazobenzene moiety to the benzylideneaniline group is held constant at either five or six methylene units while the length of the second spacer is varied. All 12 trimers exhibited an enantiotropic chiral nematic phase. In addition, a smectic A phase was observed for both sets of trimers when the second spacer contained three, five or seven methylene units. The chiral nematic-isotropic transition temperatures and associated entropy changes depend critically on the parity of the flexible spacers. This behaviour is interpreted in terms of the average molecular shape and how this is controlled by the flexible spacers. The ratio of the smectic periodicity to the estimated all-trans molecular length for each of the trimers was approximately 0.3, implying a triply intercalated arrangement in which each layer contains all three mesogenic units. The driving force for the formation of this phase is considered to be electrostatic interactions between electron rich and electron deficient segments of the molecules.