Published papers

Mechanoresponsive materials have been studied to visualize and measure stresses in various fields. However, the high-sensitive and spatiotemporal imaging remain a challenging issue. In particular, the time evolutional responsiveness is not easily integrated in mechanoresponsive materials. In the present study, high-sensitive spatiotemporal imaging of weak compression stresses is achieved by time-evolutional controlled diffusion processes using conjugated polymer, capsule, and sponge. Stimuli-responsive polydiacetylene (PDA) is coated inside a sponge. A mechanoresponsive capsule is set on the top face of the sponge. When compression stresses in the range of 6.67–533 kPa are applied to the device, the blue color of PDA is changed to red by the diffusion of the interior liquid containing a guest polymer flowed out of the disrupted capsule. The applied strength (F/N), time (t/s), and impulse (F·t/N s) are visualized and quantified by the red-color intensity. When a guest metal ion is intercalated in the layered structure of PDA to tune the responsivity, the device visualizes the elapsed time (τ/min) after unloading the stresses. PDA, capsule, and sponge play the important roles to achieve the time evolutional responsiveness for the high-sensitive spatiotemporal distribution imaging through the controlled diffusion processes.

The vapor-phase polymerization of pyrrole in the presence of polystyrene (PS) particles adsorbed at the air-water interface successfully leads to the formation of PS/polypyrrole (PPy) Janus particles where only the surface in contact with the air phase is regioselectively covered with the PPy nanolayer. The coverage area of the PPy nanolayer on the Janus particles, and therefore the contact angle of Janus particles at the air-liquid interface, decreases with a decrease in surface tension of liquid by the addition of isopropanol. The contact angle of particles at the interface decreases after the polymerization, which should be because the pyrrole monomer dissolved in the water phase from the gas phase promotes the wetting of the liquid to the PS particles by decreasing the surface tension of the liquid. Controlling the PS particle size realizes the formation of PS/PPy Janus particles with sizes ranging between 5 and 1000 μm. Furthermore, the Janus particles are demonstrated to be oriented at the air-water interface with the hydrophilic PS side toward the water phase and the hydrophobic PPy side toward the air phase, realizing stabilization of an armored bubble in the water medium.

The size of soft colloids (microgels) is essential; however, control over their size has typically been established empirically. Herein, we report a linear-regression model that can predict microgel size using a machine learning method, sparse modeling for small data, which enables the determination of the synthesis conditions for target-sized microgels.

Layered organic polymers have intercalation capabilities and dynamic properties. In classical intercalation chemistry, the interlayer guests are intercalated in the host layers via electrostatic interaction. The present work shows the organic layered materials with the host-guest interlayer interaction via hydrogen bond. Polydiacetylene (PDA) exhibits color changes from blue to red with the application of external stimuli, such as thermal and mechanical stresses. Here we report on a layered PDA containing 4,4′-bipyridyl in the interlayer space as a hydrogen-bonding guest. Whereas the layered PDA without interlayer guest shows the color transition at 65 °C, gradual color changes with two-stage reversibility are observed in the temperature range of −20–240 °C by the introduction of the hydrogen-bonding guest. The weaker interlayer interaction via the hydrogen bond promotes the dynamic motion directing the thermoresponsive color changes in a wide temperature range.

Development of high-performance organic energy storage is one of the important challenges in recent materials science. Molecular design and synthesis have potential for enhancement of the performances. Efficient exploration and design of the molecules are required in a wide search space. In the present work, a capacity prediction model for organic anodes was constructed on small experimental data by sparse modeling, a method of machine learning, combined with our chemical insights. The straightforward linear regression model facilitated discovery of a high-performance active material for organic anodes in a limited number of experiments. A recommended compound, 5-formylsalicylic acid (SA-CHO), showed one of the highest performances in recent works, i.e., a specific capacity of 873 mA h g-1at 100 mA g-1(sample number: n = 3) with rate performance and cycle stability. The model can be applied to explore organic anode active materials with higher specific capacity.

Surfactant-free polypyrrole (PPy) nanoparticles, which were colloidally stable in aqueous medium, were successfully synthesized by coupling polymerization of pyrrole using Fe(NO3)3 solids in the absence of any colloidal stabilizer. The pyrrole monomers were gradually supplied from the vapor phase, and the coupling reaction of the monomers could proceed to generate PPy in a water medium. The resulting PPy nanoparticles were extensively characterized in terms of diameter, bulk chemical composition, surface chemistry, and colloidal stability by dynamic light scattering, electron microscopy, elemental microanalysis, Fourier transform infrared spectroscopy, Raman spectroscopy, electrophoresis, and X-ray photoelectron spectroscopy. The characterization results indicated that the PPy nanoparticles can be colloidally stable based on the electrostatic stabilization mechanism due to cationic charges generated on the PPy molecules by doping during the polymerization. General chemical oxidative polymerization in aqueous medium using the Fe(NO3)3 oxidant without a colloidal stabilizer as a control experiment resulted in generation of atypical PPy aggregates with over a micrometer size, indicating that the polymerization at low ionic strength is essential for colloidal particle formation. Finally, it was demonstrated that the PPy nanoparticles worked as a surfactant-free black-colored particulate emulsifier by adsorption at the oil-water interface to stabilize Pickering-type oil-in-water emulsions.

The organizational states of functional monomer molecules have a significant impact on the properties of polymer materials. This Perspective summarizes amorphous conjugated polymer networks (CPNs) as a new family of polymerized structures. CPNs have been studied since the 1990s. The number of papers about CPNs increased in the 2010s after the earlier work was summarized in a review in 2005. However, the amorphous types had not attracted much attention in previous articles. Amorphous CPNs have potential structural advantages compared with conventional crystalline polymers and framework materials. Diverse combinations of monomers and linkers are used to synthesize amorphous CPNs. Conjugated monomers as functional units are not densely aggregated but are homogeneously dispersed in the network. The amorphous network contributes to structural flexibility for molecular motion related to dynamic properties. The low-crystalline nature affords control over the nanoscale morphology. This Perspective starts with a brief summary of polymerized structures. Our recent work is introduced to show the structures and properties of amorphous CPNs. Simultaneous and random copolymerization of multiple conjugated monomers provides amorphous CPNs. Enhanced electrochemical performance in energy-related applications was extracted from the resultant amorphous CPNs containing redox-active moieties, thanks to their structural characteristics. These results imply that a variety of advanced functional materials can be developed based on the concept of amorphous CPNs.

Responsiveness to stimuli is important in daily life: natural biological activity is governed by continuous stimulus responsiveness. The design of stimuli-responsive materials is required for the development of advanced sensing systems. Although fully controlled stimuli-responsive systems have been constructed in nature, artificial systems remain a challenge. Conventional stimuli-responsive materials show direct responsiveness to an applied stimulus (Stimulus 1), with structural changes in their molecules and organized states. This feature article focuses on cascading responses as a new concept for integrating stimuli-responsive material design. In cascading responses, an original stimulus (Stimulus 1) is converted into other stimuli (Stimulus 2, 3, …, N) through successive conversions. Stimulus N provides the eventual output response. Integration of multiple stimuli-responsive materials is required to achieve cascading responses. Although cascade, domino, and tandem chemical reactions have been reported at the molecular level, they are not used for materials with higher organized structures. In this article, we introduce functional carriers and sensors based on cascading responses as model cases. The concept of cascading responses enables the achievement of transscale responsivity and sensitivity, which are not directly induced by the original stimulus or its responsive material, for the development of advanced dynamic functional materials.

In this study, we show that doping lanthanides into lamellar crystals reorganizes the lamellar structure and dramatically changes the crystal morphology. Azo-DA, a compound with azobenzene derivatives and carboxylic acids at both ends of the diacetylene moiety, formed plate-like lamellar crystals. The doping of holmium (Ho), a lanthanide, into the film obtained by stacking Azo-DA lamellar crystals, promoted a dramatic change in crystal morphology, resulting in the formation of an Azo-DA/Ho film with a radial lamellar crystal structure. A detailed investigation of the crystal growth process revealed that Azo-DA/Ho, which is slightly formed in the solution phase during Ho doping, acts as a pseudonucleating agent and dramatically changes the morphology of the lamellar crystals. Additionally, the morphological changes in the lamellar crystal films significantly changed the surface properties of the films, such as their appearance and water repellency. Similar morphological changes in lamellar crystals were induced when other lanthanide elements were used instead of Ho, and the type of lanthanide dopant can affect the magnetic properties of the films.

Imaging and measuring compression stresses secure a safe and healthy life. Compression stresses in kPa range are not easily detected by conventional mechanoresponsive materials because microscopic molecular motion of the chromophores is not induced by such weak stresses. Moreover, imaging of the stress distribution is not achieved so far. The present study shows a sponge device combining two stimuli-responsive materials, a capsule releasing interior liquid and color-changing polymer in responses to compression stress and chemical stimulus, respectively. The stimuli-responsive capsule is dispersed on a melamine sponge comprised of the fibers with coating the layered polydiacetylene (PDA). The application of weak compression stresses induces collapse of the capsules, outflow of the interior liquid, and subsequent irreversible color change of PDA. The cascading response in the sponge device colorimetrically enables imaging of the distribution and measuring the strength of the compression stresses in kPa range. Furthermore, the device demonstrates imaging and measuring unknown weak compression stresses applied by the irregular-shaped objects. A couple of clinical issues in surgical operation of intestine are studied using the stress-imaging sponge device. The device and its design strategy can be applied to stress imaging in a variety of fields.

Tetragonal 2D lattices are spontaneously formed by the self-assembly of homogeneous nanocubes. However, ordered arrays consisting of differently sized rectangular nanoblocks have not been achieved because the regulation of the assembly is weakened by the combination of binary units. In the present work, ordered arrays comprising binary nanocubes were investigated using the combination of 10 nm Pt nanocubes and 20 nm BaTiO3 nanocubes. Heterogeneous but ordered 2D tetragonal lattices were successfully produced using differently sized rectangular nanoblocks. The highly ordered self-assembly in the heterogeneous system requires the matching of the size ratio of binary nanocubes with the buffer effect of oleic acid covering the building units.

Conductive polymers have been extensively studied as functional organic materials due to their broad range of applications. Conductive polymers, such as polypyrrole, polythiophene, and their derivatives, are typically obtained as coatings and precipitates in the solution phase. Nanoarchitectonics for conductive polymers requires new methods including syntheses and morphology control. For example, nanoarchitectonics is achieved by liquid-phase syntheses with the assistance of templates, such as macromolecules and porous materials. This minireview summarizes the other new synthetic methods using the solid and vapor phases for nanoarchitectonics. In general, the monomers and related species are supplied from the solution phase. Our group has studied polymerization of heteroaromatic monomers using the solid and vapor phases. The surface and inside of solid crystals were used for the polymerization with the diffusion of the heteroaromatic monomer vapor. Our nanoarchitectonics affords to form homogeneous coatings, hierarchical structures, composites, and copolymers for energy-related applications. The concepts using solid and vapor phases can be applied to nanoarchitectonics for not only conductive polymers but also other polymers toward a variety of applications.

Anisotropic ultrathin 2D materials are used as building blocks to obtain higher-order structures. Tailoring the size, surface chemistry, and assembly state of exfoliated nanosheets is required for the design of functional materials. However, these factors are not easily controlled during the down-sizing exfoliation process, which is a general route for obtaining 2D materials. In the present work, polydispersed nanosheets of a transition metal oxide are prepared based on prediction model for the size distribution constructed by a combination of machine learning and chemical perspective on small data. Self-assembly of the exfoliated nanosheets results in the uneven thin films exhibiting an angle-independent structural color. As the nanosheets vary in thickness and lateral size, their assembled thin films exhibit structural color independent of the incident angle. The structural color is changed by casting only 20 µL of an aqueous solution containing biogenic amines such as histamine within a sensing time of 5 min. The biogenic amines are intercalated into the interlayer space of the stacked nanosheets and the resulting increase in the thickness changes the color of the thin-film device. The new design strategies can be applied to constructing other thin-film sensing devices based on nanosheets.

When mechanical stresses, such as tensile, compressive, and frictional stresses, are applied to objects by various motions, they are accumulated in materials. Conventional mechanoresponsive materials and sensors detect one-time applied stress. However, the accumulated stresses are not visualized or measured in previous works. The present study demonstrated imaging and sensing of not only one-time but also accumulated tensile, compressive, and frictional stresses. Polyurethane (PU) film was combined with 2D layered polydiacetylene (PDA), a stimuli-responsive color-changing polymer. PDA generally exhibits no color changes with the application of tensile and compression stresses because the molecular motion leading to the color change is not induced by such mechanical stresses. Here the versatile mechanoresponsiveness was achieved using a block copolymer guest partially intercalated in the layered PDA. As the interlayer and outerlayer segments interact with PDA and PU, respectively, the applied stresses to the film are transferred from PU to PDA via the block copolymer guest. The color changes of the film imaged and quantified the accumulated work depending on the number and strength of the applied multiple stresses such as tensile, compressive, and frictional stresses. The design strategy of materials and methodology of sensing can be applied to the development of new sensors for accumulated mechanical stresses in a wide range of length and strength scales.

Layered materials exhibit unique properties, such as intercalation and exfoliation, originating from the two-dimensional anisotropic nanostructures. A variety of inorganic and organic layered materials have been synthesized for design of functions. However, conventional host layers are static and rigid to induce dynamic functions originating from the structural flexibility. The present work shows a new dynamic and flexible host layer based on organic macrocycles. A macrocycle containing functional, flexible, and intercalating units, namely macrocyclic diacetylene (MCDA), was designed to form a self-assembled layered structure. After the guest molecules were intercalated in the interlayer space, the host MCDA layers were topochemically polymerized to obtain polydiacetylene. The layered macrocycles containing the π-conjugated main chain and interlayer guests showed gradual thermoresponsive color changes in a wide temperature range of −100 to 230 °C with variation of the effective conjugation length. The flexible host layer affords induction of the dynamic motion of the intrinsically rigid π-conjugated main chain in the wide temperature range. Moreover, the thermo-responsivity was tuned by the intercalated guests. The color-change properties enabled real-time temperature imaging for medical applications. Flexible layered macrocycles can be a new design strategy of soft materials with dynamic functions.

Exfoliation of layered materials provides nanosheets, such as monolayers and few-layers. In recent years, nanosheets have attracted much interest as two-dimensional (2D) materials for their diverse properties and applications originating from the anisotropic characteristic structures. Exfoliation methods have been developed depending on the types of interactions between the layers, such as van der Waals and electrostatic interactions. The present review focuses on exfoliation chemistry based on rigid and soft natures of the layered materials. The rigid inorganic layered compounds are converted to the soft layered materials with interaction of organic guests. The surface-functionalized nanosheets are obtained by exfoliation of the soft layered composites with dispersion in organic media. The exfoliation behavior is governed by the flexibility of the precursor layered materials, i.e. interaction between the interlayer guest and dispersion medium. Although exfoliation is generally an uncontrollable top-down process, materials informatics on our own experimental small data assists elucidation of the control factors toward tailored 2D materials. The exfoliation schemes are applied to a variety of layered materials. The present review shows potential new insights for exfoliation chemistry of soft layered materials.

Dynamic properties are derived from the structural flexibility of 2D polymers. Softening layered structures has the potential for tuning and enhancing the dynamic properties. In the present work, the flexibility of layered polydiacetylene (PDA) is tuned by the interlayer polymeric guests with different branching structures. PDA shows thermoresponsive color-change properties through shortening the effective conjugation length with molecular motion. Whereas the blue-to-red color transition is observed at certain threshold temperatures for the layered PDA without the interlayer guest, the intercalation of the bulky polymer guests lowers the starting temperature and widens the temperature range for the thermoresponsive color changes. The resultant layered composite of PDA and bulky polymer affords the homogeneous coating on substrates on the centimeter scale. The thermoresponsive color-change coating is applied to temperature-distribution imaging. The specific heat of liquids is colorimetrically estimated using the coating on the bottle. The coating on a silk cloth visualizes the temperature distribution on a simulated tissue during surgical operation using an ultrasonic coagulation cutting device. The coating can be applied to thermal imaging in a variety of fields. Moreover, the softening strategy contributes to explore dynamic properties of soft 2D materials.