Two-dimensionally periodic phases in mixtures of compounds made of rodlike and bent-core molecules

We report two distinct types of two-dimensionally periodic phases in different concentration regions of a binary system composed of the bent-core (BC) compound 1,3-phenylene bis[4-(3-methylbenzoyloxy)]4^'-n-dodecylbiphenyl 4^'-carboxylate (BC12) and the rodlike (R) compound 4-n-octyloxy0.3em0ex4^'-cyanobiphenyl (8OCB). BC12 exhibits only the lamellar B₂ phase. 8OCB which has a highly polar cyano end group exhibits the smectic-Ad phase with a lamellar structure of partial bilayers made of antiparallel pairs of the polar molecules. The molecular arrangements in the two-dimensionally periodic phases have been deduced based on optical microscopy, polarized infrared and x-ray diffraction studies. The arrangement in the two-dimensionally periodic phase in lower concentration ranges of BC12 (24 to 350.3em0exmol%) is dictated by the partial bilayer structure formed by the rods. The BC molecules arrange themselves with their arrow axes parallel to the long axes of the R molecules and the structure conforms to the c2mm space group symmetry. In higher concentration ranges of BC12 (35 to 600.3em0exmol%) the structure is dominated by the BC molecules, to form the familiar B₁ phase with the p2mg space group symmetry. The R molecules lie at the interfaces of these domains stabilizing the structure.