Published papers

Conducive polymers have a wide range of applications originating from their π-conjugated systems. The redox reactions of conductive polymers with doping and dedoping of anions have been applied to cathodes for charge storage. In contrast, the redox reactions with cations have not been fully studied in anodes for charge storage. Here, we found that the nanostructures of conductive polymers, such as polypyrrole (PPy) and polythiophene (PTp) derivatives, have reversible redox reactions with cations. The PPy and PTp nanostructures acted as anodes with specific capacities of 157 and 44.4 mA h g–1, respectively, at a current density of 20 mA g–1. The introduction of a carboxy group to the pyrrole and thiophene rings enhanced the specific capacities up to 730 and 963 mAh g–1, respectively. The enhanced electrochemical properties were not observed in the bulk-size conductive polymers. The results suggest that conductive-polymer nanostructures have potential for developing metal-free, high-performance charge storage devices.

Exfoliation of layered materials, a typical route to obtain 2D materials, is not easily controlled because of the unpredictable downsizing processes. In particular, the thickness control remains as a complex challenge. Here, we found a correlation between the thickness and lateral size distribution of the exfoliated nanosheets, such as transition metal oxides and graphene oxide. The layered composites of the host metal oxides and interlayer organic guests are delaminated into the surface-modified nanosheets in organic dispersion media. The exfoliation behavior varies by combination of the hosts, guests, and dispersion media. Here, we found that the thick and thin nanosheets were obtained on the monodispersed and polydispersed conditions, respectively. The selective syntheses of the thick and thin nanosheets were achieved using a prediction model of the lateral size distribution. The correlation between the thickness and lateral size distribution can be applied to thickness-selective syntheses of 2D materials.

2D nanostructures including monolayers and few layers exhibit unique properties originating from the anisotropic ultrathin nature. Conventional organic 2D nanostructures, such as covalent organic frameworks, are comprised of the planar π-conjugated macromolecules linked and extended parallel to the layers. The present work shows a new type of exfoliated organic 2D material. The macrocycles are stacked perpendicular to the lateral direction of the nanosheets. Self-assembly of a designed macrocyclic diacetylene monomer with a chair conformation forms the laminated structures containing the layers based on the perpendicularly arranged macrocycles. The exfoliable layered structures are obtained by the topochemical polymerization of the diacetylene moieties. The intercalation and subsequent swelling provide the nanosheets based on the laterally stacked macrocycles in aqueous phase. The colloidal nanosheets possess structural flexibility with the molecular motions originating from the ultrathin nature and conformational diversity. The hydrogel cross-linked with the flexible nanosheets exhibits unique stretching and thermoresponsive shrinking properties. The present work implies the potential of the flexible 2D materials based on the macrocycles.

2D polydiacetylene (PDA) layered composites with added guest molecules have demonstrated an excellent tunable mechanochromic properties toward the development of mechanochromic force sensors in energy-related and medical applications. However, its force sensitivity at the nanoscale is left uncharacterized due to a lack of technique to conveniently apply forces to the 2D films in different directions, limiting the quantitative understanding of the sensitivity-tuning mechanism. In this work, the quantitative friction force microscopy combined with fluorescence microscopy is introduced to measure the force sensitivity of the PDA layered composites with guest organic amines at the nanoscale. Intercalating nonylamine and 1-aminodecane increase the sensitivity of 10,12-pentacosadiynoic acid by 127% and 111%, respectively, whereas adding n-tridecaylamine and stearylamine reduce it by 77% and 86%. This force sensitivity at the nanoscale is inversely correlated to the thermochromic temperature and the Young's moduli of the films, suggesting that the softer film has both higher force- and thermo-sensitivity.

Data-scientific approaches have permeated into chemistry and materials science. In general, these approaches are not easily applied to small data, such as experimental data in laboratories. Our group has focused on sparse modeling (SpM) for small data in materials science and chemistry. The controlled synthesis of 2D materials, involving improvement of the yield and control of the size, was achieved by SpM coupled with our chemical perspectives for small data (SpM-S). In the present work, the conceptual and methodological advantages of SpM-S were studied using real experimental datasets to enable comparison with other machine learning (ML) methods, such as neural networks. The training datasets consisted of ca. 40 explanatory variables (xn) and 50 objective variables (y) regarding the yield, size, and size-distribution of exfoliated nanosheets. SpM-S provided more straightforward, generalizable, and interpretable prediction models and better prediction accuracy for new experiments as an unknown test dataset. The results indicate that machine learning coupled with our experience, intuition, and perspective can be applied to small data in a variety of fields.

We demonstrated a mechanochromic chameleon packaging that quantitatively visualizes damage based on the polydiacetylene blue-to-red color transition. The applied pressures on the damage can be conveniently read by taking a photograph via a homemade mobile app called the “Pressure Analysis App”. The key aspects of the development were 1) the adjustment of the sensitivity by adding guest molecules into the PDA matrix, 2) quantitative calibration, and 3) the accurate reading of the pressure from RGB images based on the calibration. The force sensitivity of polydiacetylene on cellulose was characterized in the range of 30-100 nN (2.59-8.86 MPa) at the nanoscale by dual friction force/fluorescence microscopy, and in the range of 0.7-2.2 N (0.11-0.35 MPa) at the macroscopic scale by a friction tester. The scanning speed was found to be one of the key parameters, where the force sensitivity above 50 mm/s was 6.45 times larger than that below 10 mm/s. The developed mechanochromic packaging system can be used for transporting electronics and medical tools, such as vaccine vials, food, etc., more safely and reliably in the future.

Morphology and size control of insoluble and infusible conjugated polymers are significant for their applications. Development of a precipitation polymerization route without using a surface stabilizer is preferred to control the reaction, morphology, and size. In the present work, precipitation polymerization for an amorphous conjugated polymer network, a new type of polymerized structure containing functional units, was studied for the size and morphology control in the solution phase at low temperature. The random copolymerization of benzoquinone (BQ) and pyrrole (Py) monomers formed microspheres of the BQ-Py network polymers as the precipitates in the solution phase. The particle diameter was controlled in the range of 70 nm and 1 μm by changing the pH of the solution and concentration of the monomers. The resultant nanoparticles were applied to a metal-free electrocatalyst for the hydrogen evolution reaction (HER). The catalytic activity of the BQ-Py nanoparticles was higher than that of the bulk micrometer-sized particles. The results imply that the morphology and size of amorphous conjugated polymer networks can be controlled by precipitation polymerization.