The mesomorphic behavior and phase structure were examined in the mixture of two kinds of dimeric compounds, alpha,omega-bis(4-alkoxyanilinebenzylidene-4'-carbonyloxy)pentane (mOAM5AMOm), by optical microscopy, X-ray diffraction, polarization switching, and second-harmonic generation measurements. One compound is 4OAM5AMO4 with a short terminal alkyl chain that forms a single-layer smectic phase (SmCAs) with a random mixing of spacer and tail groups. Another compound is 16OAM5AMO16 with a long terminal alkyl chain that forms a chiral, anticlinic, and antiferroelectric bilayer phase (SmCAb) with the bent molecules tilted to the bilayer. By mixing these two compounds, the SmCAs phase of 4OAM5AMO4 is easily destabilized, leading to the wide content region of the bilayer phases. In the bilayer regime, three other smectic phases are newly induced. Two of them are antiferroelectric and ferroelectric phases in which the molecules lie perpendicularly with respect to the layer. The other shows no polar response to an external electric field and behaves like a smectic A. The new appearance of these bilayer phases is discussed as a mixing effect of long and short tail groups.

Twin dimers, mOAM5AMOm, form characteristic smectic phases, single-layer SmCA s and antiferroelectric bilayer SmCA b phases. Structural transformation from SmCA s to SmCA b was examined in two series of binary mixtures of 8OAM5AMO8/ 4OAM5AMO4 and 8OAM5AMO8/16OAM5AMO16. In this transformation, the frustrated SmCA f with density modulation along the layer is formed and the size of modulation increases from 50 to 250 A ˚with variations in mixture contents from an edge near SmCA s to another edge near SmCA b . From polymorphisms shown by some mixtures, it was found that SmCA s and SmCA b are located at high-temperature and low-temperature edges, respectively, and that the SmCA f phase is set between them.