Liquid crystalline self-assembly of 2,5-diphenyl-1,3,4-oxadiazole based bent-core molecules and the influence of carbosilane end-groups

Bent-core liquid crystals (BCLCs) exhibit unique properties like spontaneous breaking of the mirror symmetry and polar switching by electric fields, forming the basis of a range of applications currently being developed. Here we report first 2,5-diphenyl-1,3,4-oxadiazole based BCLCs with carbosilane end groups. Their self-assembly in liquid crystalline (LC) phases was investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction, electro-optical and dielectric investigations and was compared with a related non-silylated compound. A transition from non-modulated via modulated synclinic SmC phases to optically almost isotropic sponge phases was observed with growing packing density upon lowering temperature, which was investigated in the bulk state as well as in freely suspended films. In the SmC phases of both series the polar order is only local, leading to polarization randomized paraelectric and superparaelectric phases. Development of macroscopic polar order leads to the formation of polarization modulated smectic phases with oblique lattice, followed by optically isotropic LC phases with grainy and sponge-like 3d modulated layers, and finally isotropic crystalline phases. The silylated and non-silylated 1,3,4-oxadiazole based BCLCs have almost identical phase sequences differing mainly in details of the switching characteristics and in the structure of the optically isotropic low temperature LC phases, being achiral with a local SmCsPA structure for the silylated compounds and chiral with a local SmCaPA structure if non-silylated. Elongation of the bent core unit of the silylated oxadiazoles favours synpolar order, leading to a ferroelectric LC phase, while 3d layer distortion is suppressed. Overall, this work provides a general understanding of the LC phase sequences of the important class of 1,3,4-oxadiazole based BCLCs and of the nature of the distinct types of achiral and mirror-symmetry broken chiral (dark conglomerate type) isotropic LC phases in general.