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Banana-shaped Liquid Crystals
Bent-core liquid crystals were synthesized by Vorlander in 1930’s. But, they were not recognized as interesting materials for a long time, since they are bad molecules for liquid crystals. Matsunaga et al. synthesized the bent-core mesogens in in 1993, but reported only the mesomorphic properties. Moreover, at the same time, Cladis and Brand displayed the model structure of SmCP, which has C2v symmetry and is similar to the B2 phase realized in banana-shaped mesogens, in their paper of electrooptic properties of antiferroelectric liquid crystals. Despite of such studies, there were nobody who started to study the physical properties. Prof. Watanabe in our department brought us this compound. Soon after this, we confirmed the polar structure of this compound by switching current measurements. This is how we realized the importance of banana-shaped liquid crystals. We reported the ferroelectric switching in the international liquid crystal conference in Kent (1996), triggering the research of this interesting liquid crystal system. Our first paper has been referred 550 times, which is equivalent to once a week on average.
Banana-shaped liquid crystals are interesting from many viewpoints. Banana-shaped liquid crystals are the first ferroelectric and antiferroelectric liquid crystals, which contain no chiral carbon. This is the most interesting point from the viewpoint of polarity. We are pursuing our research particularly to understand the relationship between their molecular structures and the emergence of ferroelectric and antiferroelectric phases (JACS). Banana-shaped liquid crystals introduce chirality to the systems, although they have no chiral carbons. Particularly in the B4-like phase, we can observe spontaneous chiral segregation under a polarizing optical microscope. Recently, we succeeded in controlling chirality by various methods (Angew. Chem.). Because of the bent-shape and molecular polarity, banana-shaped mesogens form complicated layer structures (Science). Banana-shaped mesogens show not only the highest second-order nonlinear optic susceptibility in liquid crystals but also large chiral nonlinear optic effect such as chiral Pockels (electrogyration) effect and SHG-CD (PRL). Recently we proposed a fast liquid crystal display and demonstrated its performance using SmA-like phase of a banana-shaped liquid crystal (Jpn. J. Appl. Phys. and Phys. Rev. Lett.).
