Liquid crystalline paracyclophanes and ansa compounds—series of polyether macrocycles incorporating diacetylene, phenyl, biphenyl, p-terphenyl and 2,5-diphenyl-1,3,4-thiadiazole rigid cores

Abstract

Series of different macrocyclic liquid crystals consisting of two rod-like aromatic cores connected at both terminal ends by polyether chains have been synthesised by various macrocyclisation reactions (template assisted etherification, Yamaguchi macrolactonisation, Glaser-coupling reaction). The nature and length of the rigid core have a dominant impact on the liquid crystalline phases formed. A stepwise increase of the length of one of the rigid cores leads to significant mesophase stabilisation, to the induction of lamellar phases and, in general, to wider mesophase ranges. The same types of mesophases but at much reduced transition temperatures are observed when flexible methylene linkages are incorporated into the macrocycles. Parallel pre-organisation of the calamitic cores and micro-segregation of chemically distinct molecular parts are discussed as the main driving forces for the mesophase behaviour of these macrocyclic compounds. Additionally, chiral macrocyclic liquid crystals exhibiting SmC* phases, cholesteric phases and blue phases have been synthesised. In contrast to the cyclophanes, ansa-compounds incorporating only one aromatic core within the macrocycle show either a monotropic nematic phase only or no liquid crystalline behaviour at all. An architecture combining the cyclophane structure with an ansa-unit and a molecule with a non-parallel pre-organisation of rod-like mesogenic cores as well does not show LC phases. The lack of LC behaviour for these compounds might be due to the molecular architecture resulting in a steric disturbance of packing that, in this way, does not allow for segregation of rigid and flexible units into distinct sub-domains.