ORIGIN OF HELIX IN ACHIRAL BANANA-SHAPED MOLECULAR-SYSTEMS

Helical structures were confirmed for both the SmA(b) and SmBlue phase s of banana-shaped molecular systems from observations of the microsco pic fringe pattern and the selective reflection of blue color, respect ively. X-ray and optical microscopy indicate that the helical axes in the SmA(b) and SmBlue phases are normal and parallel to the smectic la yer, 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 make s the dichroic ratio of the right-and left-circularly polarized scatte red light positive or negative in the SmBlue phase, although the wavel ength (similar to 430nm) of the scattering peak does not change signif icantly. The origin of the helix will be discussed in view of the twis ted molecular conformation (conformational chirality) and the escape f rom macroscopic polarization.