H-Shape mesogenic dimers – the spacer parity effect

Abstract

It was found that for H-shape dimers, the parity of the number of atoms in the spacer linking the mesogenic cores has important influence on the mesogenic properties only for the molecules with weak mesogenic core anisotropy. In the case of stronger core anisotropy, the parallel arrangement of mesogenic units is induced regardless of the spacer parity. As a result, phase transition temperatures and layer spacing are only weakly affected by spacer parity. By studying the properties of six new homologue series it was found that for H-shape dimers the orthogonal smectic (SmA) is strongly disfavoured: most compounds form a tilted smectic (SmC) phase, which is related to hindrances in molecular rotation for molecules with large breath to length ratio. For dimers with very long spacer, and thus decoupled mesogenic units, the smectic phase is replaced by a nematic phase, and for stiff polycatenar molecules with very short spacers the cubic phase can appear.