Keiki Kishikawa

Design amphiphilic dipolar π-systems for stimuli-responsive luminescent materials using metastable states

π-Conjugated compounds that exhibit tunable luminescence in the solid state under external mechanical stimuli have potential applications in sensors and imaging devices. However, no rational designs have been proposed that impart these mechano-responsive luminescent properties to π-conjugated compounds. Here we demonstrate a strategy for mechano-responsive luminescent materials by imparting amphiphilic and dipolar characteristics to a luminescent π-conjugated system. The oligo(p-phenylenevinylene) luminophore with a didodecylamino group at one end and a tri(ethylene glycol) ester group at the other end yields segregated solid structures by separately aggregating its hydrophobic and hydrophilic moieties. The segregated structures force the molecules to align in the same direction, thereby generating a conflict between the side-chain aggregation and dipolar stabilization of the π-system. Consequently, these metastable solid structures can be transformed through mechanical stimulation to a more stable structure, from a π-π stacked aggregate to a liquid crystal and further to a crystalline phase with variable luminescence.

Switchable columnar phases

Disclike molecules form various columnar phases. Extensive efforts have been made to make these phases polar. In the course of these attempts, two columnar phases, in which dipoles along the column ferroelectrically switch, were discovered. In this Highlight article, we summarize the attempts to realize polar columnar phases and then introduce the two molecular systems possessing switchable columnar phases.

Supramolecular Polymerization and Polymorphs of Oligo(p‐phenylene vinylene)‐Functionalized Bis‐ and Monoureas

Bis- and monoureas hybridized with the oligo(p-phenylene vinylene) (OPV) pi-electronic segment and 3,4,5-tridodecyloxyphenyl wedge were synthesized and their supramolecular polymerization in diluted solution, gel formation in concentrated solution, and liquid crystallinity in bulk state were investigated. Bisurea 1a featuring a hexamethylene linker showed the highest supramolecular polymerization ability and formed tapelike nanofibers that can gelate various organic solvents. On the other hand, bisurea 1b featuring a dodecamethylene linker and monourea 2 showed a lower degree of supramolecular polymerization, resulting in gel formations in a smaller variety of solvents. These results clearly reflect a high level of cooperativity between the two urea sites and the two OPV segments of 1a upon hydrogen-bonding and pi-pi stacking interactions, respectively. When the gels of 1a, 1b, and 2 were dried, all the compounds self-organized into multilamellar superstructures. Thermal treatment of these lamellae at high temperatures induces columnar liquid-crystalline mesophases as a result of microsegregation between the rigid OPV parts and the molten aliphatic wedges. These results demonstrate that the present molecular constituent is very useful for fabricating dye-based functional assemblies providing nanoscale pi-electronic fibers, and solvent-incorporated and bulk soft materials.

Biomimetic non-iridescent structural color materials from polydopamine black particles that mimic melanin granules

A novel approach for creating non-iridescent bright structural color materials from polydopamine (PDA) black particles is presented. Two biomimetic features, melanin granules and the amorphous structures found in nature, were incorporated into these materials, consisting of PDA black particles to develop a bright and non-iridescent structural color without using any additives.

A colorless functional polydopamine thin layer as a basis for polymer capsules

Herein, we describe a facile method to prepare a colorless functional polydopamine (PDA) thin layer by the in situ oxidative copolymerization of dopamine (DA) and ATRP initiator-bearing DA (DA-BiBB) onto polystyrene (PSt) core particles: PSt@PDA/BiBB2. Surface-initiated ATRP of 2-hydroxyethyl methacrylate (HEMA) was performed on PSt@PDA/BiBB2 particles, followed by the removal of the template particles, which generated PHEMA capsules that were based on a colorless PDA thin layer and have tailored hollow core sizes and capsule wall thicknesses. The PDA thin layer is used as a basis for polymer brushes. The present method does not require cross-linking the polymer brushes. Furthermore, subsequent preparation of functional PHEMA capsules by post-functionalization of hydroxy groups of PHEMA chains was successful.

Facile synthesis of free-standing polymer brush films based on a colorless polydopamine thin layer.

A free-standing polymer brush film with tailored thicknesses based on a colorless polydopamine (PDA) thin layer is prepared and characterized. The surface-initiated atom transfer radical polymerization (ATRP) of 2-hydroxyethyl methacrylate (HEMA) is performed on a PDA layer with thickness of ca. 6 nm, which generated an optically transparent and colorless free-standing PHEMA brush film (1.5 cm × 1.5 cm). Because the cross-linked PDA layer is used as the base for the polymer brushes, the reported method does not require cross-linking the polymer brushes. The free-standing film thicknesses of ≈16-75 nm are controlled by simply changing the ATRP reaction time. The results show that the free-standing PHEMA brush film transferred onto a plate exhibits a relatively smooth surface and is stable in any solvent.

Full-Color Biomimetic Photonic Materials with Iridescent and Non-Iridescent Structural Colors

The beautiful structural colors in bird feathers are some of the brightest colors in nature, and some of these colors are created by arrays of melanin granules that act as both structural colors and scattering absorbers. Inspired by the color of bird feathers, high-visibility structural colors have been created by altering four variables: size, blackness, refractive index, and arrangement of the nano-elements. To control these four variables, we developed a facile method for the preparation of biomimetic core-shell particles with melanin-like polydopamine (PDA) shell layers. The size of the core-shell particles was controlled by adjusting the core polystyrene (PSt) particles’ diameter and the PDA shell thicknesses. The blackness and refractive index of the colloidal particles could be adjusted by controlling the thickness of the PDA shell. The arrangement of the particles was controlled by adjusting the surface roughness of the core-shell particles. This method enabled the production of both iridescent and non-iridescent structural colors from only one component. This simple and novel process of using core-shell particles containing PDA shell layers can be used in basic research on structural colors in nature and their practical applications.

Liquid crystalline amides: linear arrangement of rod-like molecules by lateral intermolecular hydrogen bonding and molecular shape effect

Bent- and straight-rod shaped liquid crystalline amides 1–3 were designed and synthesized, and their thermodynamic behaviors were investigated by polarized optical microscopy and differential scanning calorimetry, and their layer distances were measured by X-ray diffraction. Weak or strong hydrogen bonds were observed in variable-temperature FT-IR spectra for bent-rod shaped compounds 1 possessing a long alkoxy chain (OR2), straight-rod shaped compounds 2, and straight-rod shaped 3 in their smectic C, smectic Cx, and smectic A phases, respectively. Thus, these liquid crystalline amides could generate linear molecular aggregates in their smectic layers by the lateral intermolecular hydrogen bonds.

Stabilization of the blue phases of simple rodlike monoester compounds by addition of their achiral homologues

As novel rodlike liquid crystalline ester compounds exhibiting blue phases (BPs), chiral 4-(4′-alkoxyphenyl)phenyl 4-(4′-alkoxyphenyl)benzoate derivatives (R–C6H4–C6H4–OCO–C6H4–C6H4–R′), 1 (R = R′ = (R)-1-methylheptyloxy, BP range: 1.6 K on heating), 2 (R = (R)-1-methylheptyloxy, R′ = n-octyloxy, BP range: 0.8 K on heating), 4 (R = (R)-1-methylheptyloxy, R′ = H, BP range: 1.0 K on heating), were synthesized and their phase behaviors were investigated. Further, the BP of 1 was stabilized by addition of its achiral homologue ester compound 3 (R = R′ = n-octyloxy). In the range of the composite ratio of 1 : 3 from 10 : 0 to 0 : 10, the BP temperature range became wider with increase of the content of 3 in the mixture. At the ratio of 6 : 4, the BP indicated the widest temperature range in the mixing experiment (5.0 K on heating and 6.0 K on cooling).

Liquid crystalline molecules with hydrogen-bonding networks in the direction of molecular short axes

The salts of 4-alkoxy-3,5-bis(hydroxymethyl)benzoic acids and alkylamine give stable smectic liquid crystals in a wide temperature range. These smectic phases were investigated by polarised optical microscopy, differential scanning calorimetry and powder X-ray diffraction. An inter-digitated structure of the plough-shaped network created by the inter-molecular hydrogen bonding between the hydroxymethyl groups in the direction of the molecular short axis is proposed.