Origin of helix in achiral banana-shaped molecular systems

Helical structures were confirmed for both the SmA(b) and SmBlue phases of banana-shaped molecular systems from observations of the microscopic fringe pattern and the selective reflection of blue color, respectively. X-ray and optical microscopy indicate that the helical axes in the SmA(b) and SmBlue phases are normal and parallel to the smectic layer, respectively. In these two helical phases, C-13 NMR spectra show two C=O peaks, suggesting two different configurations of ester group, whereas only one C=O peak appears in the isotropic and crystal phases. This indicates that the two C=O groups in the mesogenic core are not in the same plane but are twisted. The addition of chiral dopant makes the dichroic ratio of the right-and left-circularly polarized scattered light positive or negative in the SmBlue phase, although the wavelength (similar to 430nm) of the scattering peak does not change significantly. The origin of the helix will be discussed in view of the twisted molecular conformation (conformational chirality) and the escape from macroscopic polarization.