Study on Helical Structure of the B4 Phase Formed from Achiral Banana-Shaped Molecule

The helical structure of the B4 phase in the banana-shaped molecules, the 1,3-benzene bis[4-(4-n-alkoxyphenyliminomethyl) benzoate] (P-n-O-PIMB) series, has been studied by means of optical microscopy, X-ray diffraction and circular dichroism for homogeneously and homeotropically aligned cells. The homogeneously aligned cells showed a transparent blue color and a circular dichroism at about 400 nm, indicating the existence of a helical structure. Detailed texture observation also reveals the existence of two helical domains with opposite helical senses. The two helical domains are formed with equal probability, and with the helical axes parallel to the layers similar to the twist grain boundary (TGB) phase. This helical structure is also supported by X-ray diffraction. The balance between the two domain areas is broken by doping P-n-O-PIMB with its chiral analogue. The texture of the homeotropically aligned cells suggests two different helical axes, both of which are parallel to the layer but with one parallel and the other perpendicular to the bent-core plane.