Fumito Araoka

Liquid Crystalline Corannulene Responsive to Electric Field

An amide-appended corannulene derivative (3) with tribranched paraffinic side chains self-assembles into a hexagonal columnar liquid crystalline (LC) mesophase over a wide temperature range from 154 to -10 degrees C. In contrast with columnar LC assemblies of planar polycyclic aromatic hydrocarbons, the LC assembly of nonplanar 3 responds to an applied electric field and can align homeotropically to the electrode surface. Even after the electric field is switched off, this alignment is memorized for a long period of time unless the material is heated above the clearing temperature.

How doping a cholesteric liquid crystal with polymeric dye improves an order parameter and makes possible low threshold lasing

Lasing conditions in a dye-doped cholesteric liquid crystal (ChLC) have been studied in view of optical modes for the light propagating in ChLCs using a polymeric dye with the transition dipole moment parallel to the local director of the ChLC host. We found that lasing always occurs at the lower-energy edge of the photonic gap. This is because that the optical eigen mode at the lower-energy gap is linearly polarized parallel to the director, while it is perpendicular at the higher-energy gap. Because of this well-defined lasing condition, low-threshold lasing was successfully achieved.

Ferroelectric Columnar Liquid Crystal Featuring Confined Polar Groups Within Core–Shell Architecture

Finessing Ferroelectric Liquid Crystals For a material to show a ferroelectric response, it needs to have segments that can be polarized, with a net polarization that remains when the applied field is removed. However, the fluidity that allows liquid crystal molecules to easily move under an applied force also makes it hard to create a ferroelectric response. Miyajima et al. (p. 209) show that a set of columnar liquid crystal molecules, with polar cyano groups tethered to amide-capped nonpolar chains, can assemble into an umbrella-shaped core–shell architecture, in which hydrogen bonding among the amides keeps the cyano groups confined. With only subtle variations in the tether chemistry, the assemblies can be tuned from having a para-electric to a ferroelectric response, which requires only a small coercive field. Precise positioning of polar components in a liquid crystal architecture modulates the collective electronic properties. Ferroelectric liquid crystals are materials that have a remnant and electrically invertible polar order. Columnar liquid crystals with a ferroelectric nature have potential use in ultrahigh-density memory devices, if electrical polarization occurs along the columnar axis. However, columnar liquid crystals having an axial nonzero polarization at zero electric field and its electrical invertibility have not been demonstrated. Here, we report a ferroelectric response for a columnar liquid crystal adopting a core–shell architecture that accommodates an array of polar cyano groups confined by a hydrogen-bonded amide network with an optimal strength. Under an applied electric field, both columns and core cyano groups align unidirectionally, thereby developing an extremely large macroscopic remnant polarization.

Oriented Salts: Dimension‐Controlled Charge‐by‐Charge Assemblies from Planar Receptor–Anion Complexes

Salts, ionic compounds comprising cations (positive ions) and anions (negative ions), are essential materials for biotic activities. They are also utilized as inorganic minerals for industry. The appropriate arrangement of charged species through electrostatic interactions is a significant issue for constructing ordered nanoscale architectures in various states. For example, most inorganic, organic, and inorganic–organic hybrid salts use electrostatic interactions between ions to form organized three-dimensional (3D) crystal structures. The 3D structures defined herein include not only crystals of isomeric space groups in a cubic system but also non-isomeric crystals. Appropriate pairs of cations and anions yield ionic liquids, which are partially ordered but essentially nondimensional (0D) states. In ionic liquids, bulky geometries of both the cationic and the anionic species effectively prevent crystallization owing to significantly weaker ionic interactions. In contrast to crystals and liquids from ions, soft materials formed by electrostatic interactions between charged components have been reported as liquid crystals on the basis of ionic mesogens. 4] For example, Kato et al. reported various ionic liquid crystals comprising alkyl-substituted imidazolium salts, which afford columnar structures and have ionic conduction. Compared to such ionic mesophases, in which the locations of either cations or anions cannot be confirmed, more rigidly organized structures with a certain level of mobility in their building subunits are also useful for various applications such as ferroelectric materials. In contrast to bulky components, planar cationic and anionic molecules effectively interact with each other and form charge-by-charge assemblies composed of alternately stacking charged components. Aided by supplementary van der Waals interactions along with electrostatic and p–p interactions, dimension-controlled charge-by-charge assemblies will form not only crystals but also soft materials such as supramolecular gels, liquid crystals, and other organized structures. In comparison to p-conjugated cations, which are often based on sp-hybridized planar geometries, p-conjugated planar anionic species are required to delocalize their excess electrons, for example by depositing them in aromatic systems, to prevent them from suffering an electrophilic attack. Focusing on these perspectives, one of the strategies for forming planar anions is the complexation of electronically neutral p-conjugated anion receptors and spherical halide anions. As p-conjugated planes for associating with halide anions, BF2 complexes of 1,3-dipyrrolyl-1,3-propanediones efficiently bind spherical anions with inversion of pyrrole rings (1 and 2 ; Scheme 1 a). 8] Receptor 1 affords single crystals, which are composed of 1D columnar structures of alternately stacking chloride and bromide complexes and tetrapropylammonium (TPA) countercations, and are prepared from a hydrocarbon solvent. In contrast to these crystal states, an alkyl-substituted receptor 2 exhibits the formation of anion-responsive supramolecular octane gel, which is temporally transformed into a solution state by the addition of tetrabutylammonium (TBA) salts owing to the formation of soluble ion pairs comprising fairly aliphatic TBA cations and receptor–anion complexes. 9] Therefore, the introduction of planar cations in place of bulky TBA cations may form fine-tuned supramolecular organized structures as soft materials using p–p stacking and electrostatic interactions along with van der Waals forces. Herein, we present the [*] Y. Haketa, Prof. Dr. H. Maeda College of Pharmaceutical Sciences, Institute of Science and Engineering, Ritsumeikan University Kusatsu 525–8577 (Japan) Fax: (+ 81)77-561-2659 E-mail: maedahir@ph.ritsumei.ac.jp

A racemic layer structure in a chiral bent-core ferroelectric liquid crystal

A fluid smectic phase of a chiral bent-core liquid crystal was found to have a ground state structure that is anticlinic in tilt and ferroelectric in polar order, SmCAPF*. The layer chirality of this structure alternates from layer to layer despite their being composed of chiral mesogens. Observations of the optical second harmonic generation signal from well-aligned domains confirm that the ground state of this phase is bistable ferroelectric. In addition to the ground state two types of metastable domains are also observed.

Enhanced Optical Activity by Achiral Rod-Like Molecules Nanosegregated in the B4 Structure of Achiral Bent-Core Molecules

Chirality in a mixture system consisting of bent-core 1,3-phenylene bis[4-(4-8-alkoxyphenyliminomethyl)benzoates] (P8-O-PIMB) and rod-like n-pentyl-cyanobiphenyl (5CB) molecules has been studied. Precise circular dichroism (CD) spectra using thin sample cells indicate mainly two characteristics: (1) the origin of CD signals is due to chiral-segregated bent-core molecules in the B(4) phase, where 5CB is in the isotropic phase; (2) the enhanced CD signal is detected in the B(X) phase, where 5CB is in the nematic phase. These results suggest that 5CB molecules are embedded in the network of helical nanofilaments formed by P8-O-PIMB and form helical superstructure with the same handedness as the helical nanofilaments in the B(X) phase, resulting in the giant CD signals.

Development of Laser Dyes to Realize Low Threshold in Dye-Doped Cholesteric Liquid Crystal Lasers

Cholesteric liquid crystals (CLCs) have attracted much attention for use in a distributed feedback (DFB) laser cavity and their application to wavelength-tunable dye lasers. [ 1–6 ] For practical applications, however, the lasing threshold must be lowered essentially to zero. Considerable attempts to achieve this have been made concerning 1) the cavity structure (e.g., use of the defect mode), [ 7–9 ] 2) the excitation condition (e.g., excitation by circularly polarized light), [ 10 , 11 ] and 3) CLC materials (e.g., use of highly birefringent CLCs). [ 12 ] Syntheses of highly ordered dye molecules along the CLC local director have also been carried out successfully. [ 13 , 14 ] However, not much effort has been directed toward fi nding ideal dye systems. [ 15 , 16 ] Almost all the LC lasers so far investigated have utilized commercial laser dyes. It is obvious that dyes with higher luminous effi ciency (high molar absorption coeffi cient and high quantum yield) have to be used to increase energy effi ciency and decrease threshold. Very recently, we reported an extremely low threshold using a highly fl uorescent pyrene dye. [ 17 ] In the study reported here, we have systematically designed and synthesized series of pyrene and anthracene derivatives and found that one of the pyrene derivatives shows an even lower lasing threshold, 23 nJ/pulse. Here we suggest a strategy to obtain ideal dyes in terms of luminous effi ciency and radiative decay rate. It is very important to design a high-performance dye in order to achieve continuous wave (cw) operation, the fi nal goal of LC lasers. Pyrene and anthracene derivatives have phenyl, biphenyl, terphenyl, and naphthyl moieties, and the π -extended pyrene with 1,3,6,8-positions and anthracene with 9,10-positions have remarkable material potentials, such as high absorption coeffi cient, high quantum yield, and high stability with respect to heat and oxidation. [ 18–22 ] Therefore we focus on the use of new pyrene and anthracene derivatives as new laser dyes. Currently, because of these important optical properties, these dyes are used in organic light-emitting diodes (OLEDs) [ 21 , 23 ]

An autonomous actuator driven by fluctuations in ambient humidity

Devices that respond to negligibly small fluctuations in environmental conditions will be of great value for the realization of more sustainable, low-power-consumption actuators and electronic systems. Herein we report an unprecedented film actuator that seemingly operates autonomously, because it responds to the adsorption and desorption of a minute amount of water (several hundred nanograms per 10 mm(2)) possibly induced by fluctuations in the ambient humidity. The actuation is extremely rapid (50 ms for one curl) and can be repeated >10,000 times without deterioration. On heating or light irradiation, the film loses adsorbed water and bends quickly, so that it can jump vertically up to 10 mm from a surface or hit a glass bead. The film consists of a π-stacked carbon nitride polymer, formed by one-pot vapour-deposition polymerization of guanidinium carbonate, and is characterized by a tough, ultralightweight and highly anisotropic layered structure. An actuator partially protected against water adsorption is also shown to walk unidirectionally.

Electric‐Field‐Responsive Handle for Large‐Area Orientation of Discotic Liquid‐Crystalline Molecules in Millimeter‐Thick Films

As well-known for liquid-crystalline (LC) displays a variety of rodlike LC molecules that assemble into nematic and smectic phases align unidirectionally under the influence of an electric field (E field). In these cases, the direction of such oriented LC molecules can be controlled as desired. Hence, if semiconducting discotic LC molecules that assemble columnarly can be aligned likewise by an E field, superiorly conducting electronic devices with suitably oriented pathways for carrier transport can be developed. However, discotic LC molecules 6] have not been reported to align under an E field. This is mainly because such LC materials with large discotic cores are poorly fluidic because of a 2D structural order of strongly p-stacked 1D columnar components. Therefore, to develop a strategy for designing discotic LC molecules that can be aligned by an E field is a grand challenge. Here, we report that aromatic amides with branched paraffinic tails, when properly spaced from mesogenic discotic cores, serve as an E-field-responsive handle that enables large-area unidirectional orientation of columnarly assembled LC molecules with extended p-conjugated cores. Also important in the view of device fabrication is that the resulting macroscopic orientation can be maintained even after the E field is switched off. Recently, we reported that a corannulene derivative (1COR ; Figure 1), appended at its bowl-shaped core with 10 aromatic amide groups, forms a columnar LC assembly. Although the core unit of 1COR is very large, the LC columns can be oriented in such a way that their columnar axes are parallel to the direction of an applied E field (Figure 2a). As corannulene has a dipole because of its nonplanarity, we assumed that the large dipole of its columnar assembly accounts for the observed E-field-induced orientation. To support this hypothesis, we newly synthesized 2TP (Figure 1), a planar analogue of 1COR having a triphenylene unit as core part instead of a nonplanar corannulene. Just as other triphenylene derivatives so far reported, this LC molecule columnarly assembled with hexagonal geometry (Colh) in a rather wide temperature range including room temperature (Figure 1). Furthermore, as in the case of 1COR, [5c] the columnar structure is likely stabilized by a hydrogen-bonding interaction of the amide groups along the column (see Figure S22 in the Supporting Information). To investigate if 2TP at its LC mesophase can be aligned by an E field, an isotropic melt of 2TP was introduced into a sandwich-type glass cell composed of patterned ITO electrodes with a gap of 5 mm. A rectangular-shaped 1.0 Hz E field of alternating current (25 Vppmm ) was applied to this sample which was heated at its LC mesophase temperature. Polarizing optical microscopy (POM) of 2TP under crossed polarizers showed a dark field of a part, operating at an E field, of the sample, which was sandwiched by two ITO electrodes, whereas the other non-operating part, located between an ITO electrode and a glass, remained birefringent (Figure 2b). When this sample was heated to allow a Colh-to-Iso phase transition and then cooled without E field to the LC mesophase temperature range, a birefringent texture developed over the entire view of the POM image. However, when the E field was again applied to this sample, the observed birefringence disappeared. Because the dark field, after rotation of the sample, did not display any contrast in the POM images, this observation indicates that the LC columns of 2TP, located under the applied E field, uniformly align homeotropically relative to the substrate. This homeotropic columnar orientation, once developed entirely over the LC sample by an E field, was maintained even after the E field was switched off. The resultant POM image remained dark as long as the oriented sample was kept in the LC mesophase temperature range. These observations exclude the possibility of our initial hypothesis that the nonplanar corannulene core plays a crucial role in the observed E-field responsiveness of the samples. However, these observations beneficially suggest that the amide-appended side-chain motif, commonly incorporated into 1COR and 2TP, might serve as an E-field[*] D. Miyajima, Prof. Dr. T. Aida Department of Chemistry and Biotechnology School of Engineering, The University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan) E-mail: aida@macro.t.u-tokyo.ac.jp

Liquid crystalline amorphous blue phase and its large electrooptical Kerr effect

An amorphous blue phase III with low and wide thermal range (∼20 °C) including room temperature is induced by doping a bent-core nematic with a strong chiral material. We confirm that the electrooptical response is due to the Kerr effect, with the Kerr constant being up to two orders of magnitude larger than conventional Kerr materials such as nitrobenzene.