We have prepared a novel series of banana-shaped molecules with a central bent core based on a 2,7-dihydroxynaphthalene group, the side wings containing a Schiff's based moiety, and alkoxy flexible tails with carbon numbers n ) 6, 8, 10, 12, 14, and 16. Among these, the molecules with n ) 8-16 formed a novel smectic phase. Its smectic layer possessed a liquidlike association of the molecules similar to the SmA or SmC phase, although the texture that developed from the isotropic melt was unconventional. Small fractal domains initially grew and then coalesced into several large domains. Very weak birefringence and fine structure without any anisotropy are characteristic of this phase. It is interesting to note that two different domains exist showing the opposite sign of optical rotation. Strong circular dichroism (CD) with a peak at 430 nm was also observed for each domain with the opposite sign. The results suggest a natural occurrence of the helix in these materials. The right-handed and left-handed helices were formed with equal probabilities, but the left-handed helix was selectively formed by the introduction of a chiral dopant. The molecule with n ) 6 formed a frustrated smectic phase, which exhibited a simple fan-shaped texture.

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.

Achiral banana-shaped molecules with dodecyloxy tail groups, P-12-O-PIMB, N-12-O-PIMB, and S-12-O-PIMB, have exhibited unusual smectic phases which possess chiral and helical structures. In this work, we mixed these banana-shaped molecules with the chiral molecule forming a chiral nematic liquid crystal and found an exclusive effect of the achiral dopant that the twisting power of the chiral nematic phase in the mixtures is significantly increased with the increase of the content of achiral banana-shaped molecules. This characteristic effect in the chiral nematic field seems to offer the rational evidence for the twist conformation of such banana-shaped molecules, since the chirality should be included intrinsically within each chain. The asymmetric twist conformation in the ester linkage group connecting the central core with the side wings is likely to be the origin of enhanced twisting power.

The correlation between chirality of the B2 and B4 phases was examined by measuring the circular dichroism (CD) in the classic bent-core molecule, P-12-O-PIMB, which shows a direct B2 to B4 phase transition on cooling. Among the various structures formed in the tilted B2 phase, the homochiral SmC S P F and SmC A P A layer structures show a distinct CD peak at around 400 nm, while no CD effect is observed in the racemic SmC S P A layer structure. In contrast, the lower temperature smectic B4 phase is essentially chiral and it appears to be independent of the type of layer structures of the initial B2 phase. It always exhibits a CD peak at a similar wavelength to that of the homochiral B2 phase, but with opposite sign. By incorporating the optical microscopic observation with the CD measurements, we found that once the chiral domains are formed in the B4 phase, they are conserved thereafter on successive transformation between the B2 and B4 phases. The resulting chiral structure in the B2 phase is homochiral SmC A P A . For this conservation of chirality, there must be an inherent chiral source for both the B2 and B4 phases. We propose that conformational chirality fills this role.

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