Smectic mesophase behavior of dimeric compounds showing antiferroelectricity, frustration and chirality

We have prepared a series of dimeric compounds, mOAN15AMOm, which comprise two Schiff's base mesogens. These compounds formed three types of fluid smectic liquid crystals. When the alkoxy tail length is short (m = 4 and 6). the alkoxy tail groups mix randomly with the alkyl spacer groups, resulting in the SmCA(s) phase. The compounds with in longer than 14 form the SmCA(b) phase with a bilayer character, in which the spacer and tail groups segregate from each other and consequently two mesogenic layers (so-called bilayer) are included within a repeat unit. In such a bilayer association of twin dimers, spontaneous polarization appears in each bilayer. In this SmCAb phase, furthermore, the molecules are tilted to the layer so that chirality is spontaneously formed in addition to ferroelectricity. The structure was identified as SmCAPA type. The compounds with intermediate in of 8-12 formed the frustrated SmCA(f) phase with a density modulation (frustration) along the layer. On cooling from the smectic phases, crystallization takes place for mOAM5AMOm with m = 4-8. while the chiral Cr-blue phase with the TGB-like helical structure was clearly identified with m = 14-18. The structure and properties of these exotic phases in twin dimers are described in detail.