On the balance between syn- and anticlinicity in smectic phases formed by achiral hockey-stick mesogens with and without chiral dopants

Abstract

A series of achiral hockey-stick-shaped mesogens forming tilted smectic liquid crystal phases of synclinic SmC- as well as anticlinic SmC_a-type was prepared and characterized. While all homologues exhibit both phases, the balance shifts from anticlinic to synclinic order upon elongation of the terminal chain at the meta- position, defining the hockey-stick shape. The elongation also leads to an increased kinetic hindrance of the transition between syn- and anticlinic phases and a decreased transition enthalpy. These observations indicate that a well-defined kink (short meta-substituted chain) promotes the anticlinic structure while a higher flexibility between kinked and rod-shape (long meta-substituted chain) promotes synclinic order. An intermediate chain-length homologue was selected as host material for doping with syn- and anticlinic rod-shaped chiral dopants, respectively, at varying concentrations. Opposite of what might be expected the balance between syn- and anticlinic order was not simply dictated by the choice of dopant. Instead, both types of tilting order prevailed with roughly the same strength as in the achiral host regardless of which chiral material was added, up to concentrations well beyond normal doping conditions. Thus, at least with hockey-stick-shaped achiral hosts, syn- as well as anticlinic chiral compounds can be used effectively as chiral dopants without necessarily having an important impact on the clinicity of the resulting mixture. The hockey-stick design concept should be useful in producing achiral anticlinic-forming mesogens for low-polarization, long-pitch antiferroelectric liquid crystal mixtures. Finally, we point out that a mixture study like the one carried out here yields a conclusive means of establishing the clinicity of achiral tilted smectics, an endeavour that can sometimes be far from trivial.