Positions of Chiral Alkoxy Groups Responsible for Ferroelectricity in a Columnar Liquid Crystal Phase of Diphenylureas with Six Alkoxy Groups.

The columnar polarization direction of ferroelectric columnar liquid crystals can be switched by applying an external electric field, and the polarization direction can be maintained, even after the electric field is removed. If the polarization direction of each column in ferroelectric columnar liquid crystals can be switched and maintained, then ultrahigh-density memory devices can be generated. Recently, we found that the columnar phase of N,N'-bis(3,4,5-tri(S)-citronellyloxyphenyl)urea (Urea-(S)-cit) shows ferroelectricity, whereas that of N,N'-bis(3,4,5-tridecyloxyphenyl)urea (Urea-10) does not. However, the mechanisms by which the six chiral alkoxy groups in Urea-(S)-cit generate ferroelectricity have not been determined. In this study, we regioselectively synthesized four diphenylurea compounds containing (S)-citronellyloxy and decyloxy groups, i.e., N,N'-bis(3,5-di((S)-citronellyloxy)-4-decyloxyphenyl)urea (1), N,N'-bis(4-((S)-citronellyloxy)-3,5-didecyloxyphenyl)urea (2), N,N'-bis(3-((S)-citronellyloxy)-4,5-didecyloxyphenyl)urea (3), and N,N'-bis(3,4-di((S)-citronellyloxy)-5-decyloxyphenyl)urea (4), and investigated which chiral alkoxy group at which position is strongly responsible for the ferroelectricity. The chiral alkoxy groups at 3- and 5-positions of the phenyl groups were clarified to play a significant role in the generation of ferroelectricity. Furthermore, a comparison of these four compounds based on circular dichroism spectroscopy and second harmonic generation experiments revealed the relationship between the helical structure order and the stability of the polarized structure.

Magnetic Supramolecular Spherical Arrays: Direct Formation of Micellar Cubic Mesophase by Lanthanide Metallomesogens with 7-Coordination Geometry.

Here, an unprecedented phenomenon in which 7-coordinate lanthanide metallomesogens, which align via hydrogen bonds mediated by coordinated H2 O molecules, form micellar cubic mesophases at room temperature, creating body-centered cubic (BCC)-type supramolecular spherical arrays, is reported. The results of experiments and molecular dynamics simulations reveal that spherical assemblies of three complexes surrounded by an amorphous alkyl domain spontaneously align in an energetically stable orientation to form the BCC structure. This phenomenon differs greatly from the conventional self-assembling behavior of 6-coordinated metallomesogens, which form columnar assemblies due to strong intermolecular interactions. Since the magnetic and luminescent properties of different lanthanides vary, adding arbitrary functions to spherical arrays is possible by selecting suitable lanthanides to be used. The method developed in this study using 7-coordinate lanthanide metallomesogens as building blocks is expected to lead to the rational development of micellar cubic mesophases.

Janus metallic film with gold and silver luster by electroless deposition of silver using poly(dopamine acrylamide) thin film.

Films that exhibit different metallic luster on the front and back, called Janus metallic films, have broad applications ranging from design materials to optical devices. However, the fabrication of these films is often a complicated process involving multiple metal deposition steps, thermal annealing, and calcination. Herein, we report the simple preparation of a Janus metallic film by electroless deposition of silver on a poly(dopamine acrylamide) (pDOPAm) thin film. pDOPAm was successfully synthesized via the controlled reversible addition-fragmentation chain transfer polymerization of dopamine acrylamide without a protective group using dimethylformamide as the solvent. The synthesized pDOPAm was spin-coated onto a solid substrate, which was then immersed in an aqueous AgNO3 solution to achieve the electroless deposition of silver. Our preparation method will considerably simplify the fabrication of Janus metallic films, enabling their widespread application as decorative or authentication materials.

Molecular Dynamics Studies of the Ho(III) Aqua-tris(dibenzoylmethane) Complex: Role of Water Dynamics.

The seven-coordinate Ho(III) aqua-tris(dibenzoylmethane)(DBM) complex, referred to as Ho-(DBM)3·H2O, was first reported in the late 1960s. It has a threefold symmetric structure, with Ho at the center of three dibenzoylmethane ligands and hydrogen-bonded water to ligands. It is considered that the hydrogen bonds between the water molecule and the ligands surrounding Ho play an important role in the formation of its symmetrical structure. In this work, we developed new force-field parameters for classical molecular dynamics (CMD) simulations to theoretically elucidate the structure and dynamics of Ho-(DBM)3·H2O. To develop the force field, structural optimization and molecular dynamics were performed on the basis of ab initio calculations using the plane-wave pseudopotential method. The force-field parameters for CMD were then optimized to reproduce the data obtained from ab initio calculations. Validation of the developed force field showed good agreement with the experimental crystalline structure and ab initio data. The vibrational properties of water in Ho-(DBM)3·H2O were investigated by comparison with bulk liquid water. The vibrational motion of water was found to have a characteristic mode originating from stationary rotational motion along the c-axis of Ho(III) aqua-tris(dibenzoylmethane). Contrary to expectations, the hydrogen-bond dynamics of water in Ho-(DBM)3·H2O were found to be almost equivalent to those of bulk liquid water except for librational motion. This development route for force-field parameters for CMD and the establishment of water dynamics can advance the understanding of water-coordinated metal complexes with high coordination numbers such as Ho-(DBM)3·H2O.

Crack-Free Structural Color Materials Prepared without Disrupting the Particle Arrangement by Controlling the Internal Stress Relaxation and Interactions of the Melanin Particles.

In fabricating structural color materials with assembled colloidal particles, there is a trade-off between the internal stresses acting on the particles and the interactions between the particles during solvent volatilization. It is crucial to fabricate crack-free materials that maintain the periodic arrangements of the particles by understanding the mechanism for crack initiation. Here, we focused on the composition and additives of melanin particle dispersions to obtain crack-free structural color materials without disturbing the particle arrangements. The use of a water/ethanol mixture as a dispersant effectively reduced the internal stresses of the particles during solvent evaporation. Furthermore, the addition of low-molecular-weight, low-volatility ionic liquids ensured that the arrangement and interactions of the particles were maintained after solvent volatilization. Optimization of the composition and additives of the dispersion made it possible to achieve crack-free melanin-based structural color materials while maintaining vivid, angular-dependent color tones.

A Flexible and Robust Structural Color Film Obtained by Assembly of Surface-Modified Melanin Particles.

In this study, core-shell-hairy-type melanin particles surface modified with a polydopamine shell layer and a polymer brush hairy layer were fabricated and assembled to readily obtain bright structural color films. The hot pressing of freeze-dried samples of melanin particles decorated with a hydrophilic, low glass transition temperature polymer brush results in films that exhibit an angle-dependent structural color due to a highly periodic microstructure, with increased regularity in the arrangement of the particle array due to the fluidity of the particles. Flexible, self-supporting, and easy-to-cut and process structural color films are obtained, and their flexibility and robustness are demonstrated using compression tests. This method of obtaining highly visible structural color films using melanin particles as a single component will have a significant impact on practical materials and applications.

Chiral Self-Sorting and the Realization of Ferroelectricity in the Columnar Liquid Crystal Phase of an Optically Inactive N,N'-Diphenylurea Derivative Possessing Six (±)-Citronellyl Groups.

An axially polar-ferroelectric columnar liquid crystal (AP-FCLC) phase that exhibits both switching and maintenance of the macro-polarity in the column axis direction has been achieved in an N,N'-bis(3,4,5-trialkoxyphenyl)urea compound (rac-1) prepared from (±)-citronellyl bromide. Although it had been thought that chirality is necessary to achieve the AP-FCLC phase from our previous study, the optically inactive compound which is a mixture of 21 stereoisomers, generated an AP-FCLC phase. We confirmed its ferroelectricity and investigated the mechanism for realizing the AP-FCLC phase using optoelectronic experiments, X-ray diffraction, and circular dichroism spectroscopy. As a result, it was suggested that chiral self-sorting occurs in the columnar liquid crystal phase, in which molecules with a similar stereochemistry form a one-handed helical column, and columns with the same helicity gather together to form a chiral domain. Accordingly, we conclude that the optically inactive compound rac-1 also indicates ferroelectricity similar to that of an optically pure urea compound because of chiral self-sorting.

Control of Structural Coloration by Natural Sunlight Irradiation on a Melanin Precursor Polymer Inspired by Skin Tanning.

Natural melanin affects the reflection and absorption of light, and it is known as an important element in producing bright structural colors in nature. In this study, we prepared core-shell particles using a melanin precursor polymer, that is, polytyrosine (PTy), as a shell layer by the oxidative polymerization of tyrosine ethyl ester (Ty) in the presence of cerium oxide (CeO2) core particles. Inspired by skin tanning, irradiating the CeO2@PTy core-shell particles with UV or natural sunlight caused melanization by extending the π-conjugated length of PTy, producing colloidal particles with the ability to absorb light. The pellet samples consisting of CeO2@PTy particles appeared whitish because of multiple scattered light. In contrast, the light absorption capacity of CeO2@PTy UV or CeO2@PTy Sun particles after light irradiation suppressed scattered light, dramatically improving the visibility of the structural color of the pellet samples made from these particles. Thus, a new method has been developed to control the visualization of structural colors to the human eye by irradiating the melanin precursor polymer with light.

Progress in polydopamine-based melanin mimetic materials for structural color generation.

Structural color is a color derived from optical interaction between light and a microstructure and is often seen in nature. Natural melanin plays an important role in bright structural coloration. For example, the vivid colors of peacock feathers are due to structural colors. The periodic arrangement of melanin granules inside the feathers leads to light interference, and the black granules absorb scattered light well, resulting in bright structural color. In recent years, polydopamine (PDA) has attracted attention as a melanin mimetic material. This review article summarizes recent research on structural coloration using PDA-based artificial melanin materials. It also outlines possible applications using bright structural colors realized by artificial melanin materials and future perspectives.

Effect of the Polydopamine Composite Method on Structural Coloration: Comparison of Binary and Unary Assembly of Colloidal Particles.

Melanin influences light reflection and absorption and is known to be one of the elements producing structural color, such as that in the feathers of birds. In this study, we used polydopamine (PDA), an artificial melanin, as a light-absorbing material and examined in detail the effect of its composite method on the structural color. The following two composite methods were investigated using cerium(IV) oxide (CeO2) particles as a core particle: binary coassembly of CeO2 and PDA particles and unary assembly of CeO2@PDA core-shell particles. Although both methods dramatically improved the visibility of the structural color by suppressing the scattered light owing to the light absorption capability of the PDA, there was a difference in the particle arrangement, angle dependence of the structural color, and color tone change. By selecting the PDA composite method, the guidelines for providing high visibility and the desired structural color were presented.

Preparation of liquid crystal nanocapsules by polymerization of oil-in-water emulsion monomer droplets.

Liquid crystal nanocapsules (LC-Nanocapsules) were prepared by miniemulsion polymerization of the oil-in-water emulsion monomer droplets dissolving the liquid crystal (LC) compounds. In order to establish the preparation conditions of LC-Nanocapsules exhibiting the liquid crystallinity, the effects of the capsule wall-forming monomers and the crosslinking agent concentration on the capsule structure were investigated in detail. The monodisperse colloidal products covered with the robust polymer shell wall was successfully prepared by the polymerization of the emulsion monomer droplets obtained through the phase inversion temperature emulsification technique using the amphiphilic block copolymer as an emulsifier. The endothermic peak was observed at the nematic-isotropic phase transition temperature (TNI) of the LC in the differential scanning calorimetry diagram of LC-Nanocapsules. The bright- and dark-field images of the dried thin films of LC-Nanocapsules spread on a glass substrate were found to appear repeatedly by the temperature change below and above TNI by polarized optical microscopic analysis. These results revealed that the LC-Nanocapsules with a complete engulfing morphology were successfully formed by the spontaneous coacervation phenomena between the crosslinked polymer and the LC with a progression of the polymerization, as theoretically predicted from the viewpoint of the spreading coefficients.

Preparation of photochromic liquid core nanocapsules based on theoretical design.

Photochromic materials have attracted considerable attention for their practical applications in optoelectronic devices. In this study, we developed the photochromic liquid core nanocapsules by polymerization of oil-in-water (O/W) nanoemulsion monomer droplets on the basis of Hansen solubility parameters. The thermoresponsive amphiphilic block copolymers (POEGMAm-b-PStn and POEGMAm-b-PMMAn) were synthesized by sequential reversible addition fragmentation chain transfer (RAFT) polymerization of hydrophilic oligo(ethylene glycol) methyl ether methacrylate and hydrophobic styrene or MMA. The O/W nanoemulsion methyl methacrylate (MMA) droplets, dissolving dipropylene glycol methyl-n-propyl ether (DPMNP) and (E)-3-(adamantan-2-ylidene)-4-[1-(2,5-dimethyl-3-furyl) ethylidene]dihydro-2,5furandione (Aberchrome 670) as a core liquid and a photochromic dye, respectively, were obtained through the phase inversion temperature emulsification technique using POEGMAm-b-PStn as a surfactant. As theoretically predicted in terms of the spreading coefficients, the DPMNP solution of Aberchrome 670 was successfully encapsulated by coacervation of the crosslinked PMMA condensed phase. Aberchrome 670 dissolved in a liquid core was found to photoisomerize twice as fast as that dispersed in the solid polymer matrices.

Does Introduction of a Bent Tail Stabilize Biaxiality and Lateral Switching Behavior of Smectic A Liquid Crystal Phases of Rodlike Molecules?

By introducing an oleyl group at the end of the straight rodlike molecule, the effect of the tail shape on the liquid crystallinity, biaxiality, and lateral switching behavior of the smectic A phase was investigated. Three types of molecules possessing a fluorinated phenyl (pentafluorophenyl, 2,3,4-trifluorophenyl, or 2,3-difluorophenyl) group and a cis-octadec-9-enyl group were synthesized, and their liquid crystallinities were compared with the corresponding molecules with a straight alkyl ( trans-octadec-9-enyl or n-octadecanyl) group. In switching experiments, the molecules with a bent terminal chain showed higher spontaneous polarization ( Ps) values than those with a straight terminal chain. Further, the directional changes of the long molecular axes were suppressed for the molecules possessing a bent terminal chain. These results show that the introduction of a bent terminal chain is highly effective for stabilizing ferroelectric switching behaviors.

Ellipsoidal Artificial Melanin Particles as Building Blocks for Biomimetic Structural Coloration.

Inspired by the structural coloration of anisotropic materials in nature, we demonstrate the preparation of structural color materials by the assembly of anisotropic particles. Spherical artificial melanin particles consisting of a polystyrene core and polydopamine shell were stretched asymmetrically to form uniform-sized ellipsoidal particles with different aspect ratios. The aspect ratio and assembly method of the ellipsoidal particles influence the structural coloration, indicating that the particle shape is one of the important parameters for controlling the structural coloration. The discovery of a method to control the structural color using ellipsoidal particles is useful in basic research on structural colors in nature and provides flexibility in material design and extends the application range of structural color materials.

Melanin Precursor Influence on Structural Colors from Artificial Melanin Particles: PolyDOPA, Polydopamine, and Polynorepinephrine.

Polydopamine (PDA) is of interest as a mimetic material of melanin to produce structural color materials. Herein, to investigate the influence of the material composition of the artificial melanin particles on structural color, we demonstrated the preparation of core-shell particles by polymerization of norepinephrine or 3,4-dihydroxyphenylalanine, which are melanin precursors similar to dopamine, in the presence of polystyrene particles. It was revealed that the arrays of the prepared particles produce high-visibility structural color because of absorption of scattering light. Although poly(3,4-dihydroxyphenylalanine) showed the same tendency as PDA which was previous studied, polynorepinephrine can easily produce a smooth and thick shell layer compared with that of PDA, and pellets consisting of the particles showed angle-dependent structural color. Therefore, the artificial melanin particles that produce angle-dependent structural color became stable than ever before. These results indicated that material compositions of artificial melanin particles have influence on structural color, and an important finding for application as a coloring material was obtained.

Adhesion Control of Branched Catecholic Polymers by Acid Stimulation.

Biomimetic material design is a useful method for producing new functional materials. In recent years, catecholic polymers inspired from the adhesion mechanism of marine organisms have attracted attention. Here, we demonstrated the preparation of catecholic polymers by reversible addition-fragmentation chain transfer (RAFT) polymerization of an acetonide-protected catecholic monomer, that is, N-(2-(2,2-dimethylbenzo-1,3-dioxol-5-yl)ethyl)-acrylamide (DDEA). By selecting the specific RAFT reagents, well-defined branched PDDEA and linear PDDEA were obtained. These PDDEA samples showed stronger adhesion strength after deprotection by acid stimulation compared with that before deprotection. In addition, we demonstrated the adhesion control of synthetic polymers by photoirradiation in the presence of photoacid generators, which decompose under light and release an acid.

Polydopamine-Based 3D Colloidal Photonic Materials: Structural Color Balls and Fibers from Melanin-Like Particles with Polydopamine Shell Layers.

Nature creates beautiful structural colors, and some of these colors are produced by nanostructural arrays of melanin. Polydopamine (PDA), an artificial black polymer produced by self-oxidative polymerization of dopamine, has attracted extensive attention because of its unique properties. PDA is a melanin-like material, and recent studies have reported that photonic materials based on PDA particles showed structural colors by enhancing color saturation through the absorption of scattered light. Herein, we describe the preparation of three-dimensional (3D) colloidal photonic materials, such as structural color balls and fibers, from biomimetic core-shell particles with melanin-like PDA shell layers. Structural color balls were prepared through the combined use of membrane emulsion and heating. We also demonstrated the use of microfluidic emulsification and solvent diffusion for the fabrication of structural color fibers. The obtained 3D colloidal materials, i.e., balls and fibers, exhibited angle-independent structural colors due to the amorphous assembly of PDA-containing particles. These findings provide new insight for the development of dye-free technology for the coloration of various 3D colloidal architectures.

A metal-lustrous porphyrin foil.

A metal-lustrous self-standing film, named "porphyrin foil", was formed from a glass-forming polymeric porphyrin. The amorphous glass nature of the porphyrin foil played a key role in spontaneously producing a smooth surface. Its sharp contrast in intense absorption and specular reflection of light at each wavelength provided a brilliant metallic lustre.

Structural Color Tuning: Mixing Melanin-Like Particles with Different Diameters to Create Neutral Colors.

We present the ability to tune structural colors by mixing colloidal particles. To produce high-visibility structural colors, melanin-like core-shell particles composed of a polystyrene (PSt) core and a polydopamine (PDA) shell, were used as components. The results indicated that neutral structural colors could be successfully obtained by simply mixing two differently sized melanin-like PSt@PDA core-shell particles. In addition, the arrangements of the particles, which were important factors when forming structural colors, were investigated by mathematical processing using a 2D Fourier transform technique and Voronoi diagrams. These findings provide new insights for the development of structural color-based ink applications.

Shape-Assisted Self-Organization in Highly Disordered Liquid Crystal Phases.

In achiral rod-like molecules, a nematic phase is the most disordered liquid crystal phase, which only has one-directional order in the direction of the molecular long axis. A dumbbell-shaped molecule (compound 3: R-C6 H10 -CH=CH-C6 H4 -CH=CH-C6 H10 -R, (R=nC5 H11 )), and its liquid crystal phase (X phase) are reported, which exhibit high scattering without thermal fluctuation between two nematic phases under a polarized light optical microscope. The X phase was investigated by X-ray diffraction, scanning electron microscopy, atomic force microscopy, and molecular dynamics simulation. A layered structure was ascertained for which a molecular self-organization mechanism was postulated in which the super-structure is based on lateral intermolecular interlocking. A second nematic phase above the X phase consisted of "rice grain"-shaped particles.