Seng Kue Lee

Mesomorphic behaviour in bent‐shaped molecules with side wings at different positions of a central naphthalene core

Six bent‐shaped molecules were prepared with central bent cores based on 2,7‐, 1,7‐, 1,6‐, 1,3‐, 1,2‐ and 2,3‐dihydroxynaphthalene groups with side wings containing a Schiffs base moiety and dodecyloxy tail, N(2,7), N(1,7), N(1,6), N(1,3), N(1,2) and N(2,3). All of the compounds form fluid smectic mesophases and their mesomorphic behaviour and properties are discussed in terms of their molecular structure. The compounds are categorized into two groups; in one of them, N(2,7), N(1,6) and N(1,3), the bent cores have a bent angle of 120° between the side wings, whereas in the other, N(1,7), N(1,2) and N(2,3), the bent angle is 60°. The typical banana molecule, N(2,7), in the first group, forms a chiral blue phase that shows two distinguishable domains with opposite optical rotations and circular dichroism. Under an electric field, the chiral blue phase is transformed to strongly birefringent B2 phase which possesses a basically homochiral SmCAPA structure. N(1,3) also forms a chiral blue phase, but, in contrast, N(1,6), where the side wings are asymmetrically substituted, forms a typical antiferroelectric B2 phase with the racemic SmCSPA structure. N(1,2) and N(2,3) of the other group assume a U‐like shape to form a conventional SmA phase. Of interest is the fact that N(1,7) forms a chiral blue phase irrespective of having a small bent angle of 60°. This indicates that the distance between side wings as well as the bent angle plays an important role for the formation of banana‐shaped smectic phases.

Formation of banana phases in bent-shaped molecules with unusual bent angles as low as 60°

We prepared a homologous series of bent-shaped molecules with a low bent angle of about 60° using a 1,7-naphthalene central core, a long side wing in which one phenyl ring is attached to the classical wing via an ester linkage and alkoxy tail groups with numbers of carbons of 4 to 14. All the compounds form well-defined mesophases. Two calamitic nematic (N) and SmA phases, and two banana SmAPA and B4 phases were identified with a sequence of N–SmA–SmAPA–B4 upon decreasing temperature. In all these phases, molecules behave as bent molecules irrespective of the low bent angle of approximately 60°, and are packed with the bent direction perpendicular to the n-director. SmAPA is different from SmA only by having biaxiality, and exhibits antiferroelectric switching characteristic of banana smectic phases.

Unusual Formation of Switchable Hexagonal Columnar Phase by Bent-Shaped Molecules with Low Bent-Angle Naphthalene Central Core and Alkylthio Tail

The low-angle bent-shaped molecules with 1,7-naphthalene central core and alkylthio tails can form a novel hexagonal columnar phase and a dark B4 phase. The columnar phase has a large two-dimensional hexagonal lattice with edges of 65–75 A and exhibits polar switching with spontaneous polarization along the column axis. Calculated from the density (∼1 g·cm-3) and unit volume, the number of molecules that are necessary to fill a 4.6-A stratum of each column were found to be ∼11. Such a large number of molecules can be accommodated only in the tube-like assembly, which may be the first example as formed by the usual bent-shaped molecule with a single alkylthio tail.

Effect of Molecular Structure on Smectic Phase Structures in Two Homologues Series of Bent-Shaped Molecules with Asymmetric Central Naphthalene Core

Two homologous series of achiral banana-shaped liquid crystals with an asymmetric central core based on 1,6- and 1,7-dihydroxynaphthalene and with side wings containing Schiffs based moiety, N(1,7)-n-O-PIMB and N(1,6)-n-O-PIMB (n = 4, 8, 12, and 16) were synthesized. The mesophases and phase structures of all the compounds were studied by means of electrooptic, polarization reversal current, circular dichroism, and X-ray diffraction measurements in order to confirm the relationship between the molecular structures and mesomorphic behaviors. Most of N(1,7)-n-O-PIMB compounds (n = 8, 12, and 16) exhibitedjhe chiral blue phase although the bent angle of central bent core is 60° which is unconventional conformation of banana-shaped molecules. In contrast, N(1,6)-n-O-PIMB showed the typical B1 phase with frustrated layer structure or the B2 phase with the racemic SmC S P A structure in accordance with the length of alkyl terminal chain. These results indicate that the size of the bent core, the position as well as the magnitude of the bend, and the length of alkyl terminal chain play an important role for the emergence of particular banana smectic phases.

Synthesis and mesomorphic properties of new chiral banana‐shaped liquid crystals with chiral 3‐(alkoxy)propoxy terminal groups

New chiral banana‐shaped liquid crystals with chiral 3‐(alkoxy)propoxy terminal groups (Pn‐O‐PIMB5*‐4O, n = 7, 8, 9 and 10) were synthesized and their mesomorphic properties and phase structures investigated by means of electro‐optic measurements, polarizing optical microscopy, differential scanning calorimetry, and second harmonic generation measurements. Most of these chiral bent‐core mesogens (n = 7–9) showed the antiferroelectric B2 phase, whereas P10‐O‐PIMB5*‐4O exhibited the B7 phase. Comparing with the previously reported homologue Pn‐O‐PIMB(n‐2)*, we conclude that the terminal chain structure, particularly the position of chiral centres, plays an important role in the emergence of particular phase structures.

Longer-terminal-chain-sensitive phase structures in mixtures and nonsymmetric molecules of bent-core mesogens

Odd–even behavior for the emergence of ferroelectricity and antiferroelectricity has been examined in bent-core compounds containing two different chain lengths; i.e., 1:1 mixtures of neighboring homologues of symmetric bent-core mesogens and nonsymmetric pure compounds. The odd–even behavior was found to be very strictly held for longer terminal chains: longer terminal chains play an important role in determining the polar structure. Namely, the ferroelectric state emerges when the carbon number n of the longer chain is even, whereas the antiferroelectric state emerges for odd n.

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.

Formation of a homochiral antiferroelectric ground state in asymmetric bent-shaped molecules

New asymmetric bent-shaped molecules were synthesised and their mesomorphic properties and phase structures investigated by means of electro-optical measurements. All of the compounds formed fluid smectic phases and showed the homochiral antiferroelectric (SmCAPA) ground structure of the B2 phase. Comparing the new molecules with the previously reported classical symmetric bent-shaped molecules, we concluded that the modification of the chemical structure, especially the asymmetrical design of the bent-shaped molecules, is very significant for the interlayer interaction, influencing the formation of a specific polar order in banana phases.

Electric-field-induced transition between ferro- and antiferroelectric ground states observed in the B7 phase of a bent-shaped molecule with alkylthio tails

We prepared a new bent-shaped molecule, in which dodecylthio tails and typical Schiff-based side wings are substituted on a conventional phenyl central core. This molecule forms the B7 phase in a temperature region from 117 to 142 °C, which shows characteristic banana leaf and circular domain textures. On application of an electric field, the B7 phase is easily transformed to the B2 phase with two different ground states, depending on the temperature. In the higher temperature region, the B2 phase possesses a homochiral ferroelectric (SmCSPF) ground state, whereas the lower temperature B2 phase has a homochiral antiferroelectric (SmCAPA) ground state. These two states transformed into each other reversibly at 124 °C under the applied field, while they are stable over the mesophase temperature on the field-off state.

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.