Molecular Biaxiality in Nematic Liquid Crystals as Studied by Infrared Dichroism

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-octyloxy 4(') -cyanobiphenyl (8OCB). BC12 exhibits only the lamellar B2 phase. 8OCB which has a highly polar cyano end group exhibits the smectic-A(d) 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 35 mol% ) 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 60 mol% ) 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.