Chiral correlation between low-birefringent phases with twist grain boundary-like helix and highly birefringent phases with layer chirality as elucidated from circular dichroism observations.

The bent-shaped molecule with a central naphthalene core, 2,7-naphthalene-bis[4-(4-dodecylphenyliminomethyl)]benzoate, forms the low-birefringent B2 (LB-B2) phase with the twist grain boundary (TGB)-like helical structure and the low-birefringent B4 (LB-B4) phase in order of decreasing temperature. By applying the electric field, the LB-B2 phase is altered to the highly birefringent B2 (HB-B2) phase because of unwinding of the TGB-like helix. The HB-B2 phase is transformed to the HB-B4 phase without a loss of birefringence on cooling. These four phases show characteristic circular dichroism spectra, showing the consistent correlation through the transformation between these phases. The source of the chirality in the achiral system and the correlation in the chirality between these phases are discussed.

Two-fold helical inversion in the chiral SmC phase of optically active materials derived from (R)-(+)-1-(1-phenyl)ethylamine.

The transfer of chirality from molecules to an anisotropic liquid crystal phase has been a subject of significant interest. A typical example is the spontaneous formation of helical structures in chiral nematic and chiral SmC* phases. One of the most interesting chiral effects in these helical liquid crystal systems is the helical inversion arising from the sensitivity of the chirality to temperature variation. In this study we found a distinct 2-fold helical inversion in the chiral SmC* phase of materials with single chiral centers, in which the chiral phenylethylamine is introduced to the end of the mesogens through the imine linkage and the alkyl tail is attached to the other end. As far as we know, this is the first example of 2-fold helical inversion in a single-component system.

Chirality transfer between weakly birefringent and electric-field-induced highly birefringent B2 phases in a bent-core mesogen.

Electric-field-induced transition was observed for the weakly birefringent chiral B2 phase, which is formed from the banana molecule based on the naphthalene bent core. This phase is considered to possess the twisted grain boundary (TGB)-like helical structure. When an electric field is applied, the TGB-like helix unwinds. The resulting large domain of the SmC(A)P(A) phase shows the high birefringence and simultaneously the antiferroelectric switching between SmC(A)P(A) and SmC(S)P(F) states. Through this field-induced transformation, two interesting features are obtained. First, the initially formed chiral domains are preserved even after the field-induced transformation to the unwound SmC(A)P(A) phase. This indicates the close correlation between the TGB-like helix and the layer chirality in such a way that the helical sense of the TGB-like helix is memorized as the layer chirality of the homochiral SmC(A)P(A) phase. Second, there is a critical temperature, above which the helicoidal structure is stable against the electric field. There is a competition between winding into a TGB-like structure and unwinding due to the electric field, and at higher temperatures, the helicoidal power is too strong to surpass the effect of the electric field.

Polar structures in binary mixtures of bent-core liquid crystals showing ferroelectric and antiferroelectric B2 phases.

We have studied the polar structures in the binary mixtures of bent-core liquid crystals P-n-O-PIMB(n-2)*-(n-4)O showing the ferroelectric smectic- CAPF (Sm-CAPF) (n=8 and 10) and antiferroelectric Sm-CSPA (n=9) B2 phase. Although the polar structure of the one-to-one mixture is governed by the compound with longer end chains, it is much more complicated in the mixtures with slightly less fraction of compounds with longer end chains. Even if the mixtures show the antiferroelectric phase before field application, the ferroelectric domains remain once the field is applied and coexist with the antiferroelectric domains. The coexistence structure was modeled by comparing the microscope real image with that of second-harmonic generation. The fraction of the ferroelectric domains is larger at higher temperature region of the B2 phase and decreases with decreasing temperature. The stable phase structures were discussed based on the phenomenological theory. We suggest that the coupling energy of the layer chirality is higher than the energies of the orientation correlations of tilts and polarizations between adjacent layers.

Interpretation of the odd-even behavior for the emergence of ferroelectricity and antiferroelectricity in bent-core mesogens.

Simple theoretical interpretation has been made on the previously reported odd-even behavior of the emergence of ferroelectricity and antiferroelectricity in homologous series of bent-core mesogens with the same chiral end chains (S,S) ; compounds with even and odd carbon numbers (including oxygen) exhibit ferroelectric and antiferroelectric B2 phases, respectively. The odd-even behavior was confirmed in newly synthesized racemic compounds with chiral end chains of R and S forms (R,S) together with (R,R) and (S,S) forms. According to our theoretical interpretation, ferroelectricity should be more stable in (R,S) compounds than in (S,S) compounds. Actually, the transition temperature from the isotropic phase to the Sm-CP phase was higher and the temperature range of the B2 phase was broader in the (R,S) compound than in the (S,S) compound.

Structural characteristics of the B6 phase for a bent-core molecular system observed through the B1-B6 transition.

A bent-shaped molecule exhibiting the mesophase sequence of B6-B1 was studied to understand the structural characteristics of the B6 phase compared with those of the B1 phase. A well-oriented sample was prepared in a magnetic field and examined by wide-angle x-ray diffraction measurements through the B6-B1 phase transition. The B6 phase has been considered to show only the (002) reflection, but this B6 phase showed broad scattering just inside the (002) reflection. The broad scattering has an intensity maximum at a very similar position to that of the (101) reflection in B1 and evolves into the well-defined (101) reflection on cooling into B1. Thus, B6 has a similar frustrated structure to B1, but the size of the antidomain in B6 may be dynamically distributed while B1 possesses an antidomain of definite size.

Odd-even behavior of ferroelectricity and antiferroelectricity in two homologous series of bent-core mesogens.

Two chiral bent-core mesogens Pn-O-PIMB(n - 2)* (n = 9 and 10) and their oxygen analogues Pn-O-PIMB(n - 2)*-(n - 4)O (n = 8, 9, and 10) with omega-[(S)-amyloxy]alkoxy terminal groups were prepared, and their phase structures were investigated by means of electro-optic, polarization reversal current and second harmonic generation measurements in order to clarify the effect of the interlayer steric interaction on the emergence of polar orderings. The odd-even behavior for the alternative appearance of ferroelectricity and antiferroelectricity was observed in two homologous series; the bent-core mesogens P10-O-PIMB8*, P8-O-PIMB6*-4O, and P10-O-PIMB8*-6O in addition to the previously reported P6-O-PIMB4* and P8-O-PIMB6*, where the length of chains n is even, exhibited ferroelectric phases. On the contrary, the mesogens P7-O-PIMB5*, P9-O-PIMB7*, and P9-O-PIMB7*-5O, where n is odd, showed antiferroelectric phases. It is obvious that the interlayer steric interaction plays a major role for the emergence of a variety of phase structures.