研究者詳細 - 山本　崇史

論文 - 山本　崇史

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Selective C-N Bond Cleavage in Unstrained Pyrrolidines Enabled by Lewis Acid and Photoredox Catalysis

Aida, K; Hirao, M; Saitoh, T; Yamamoto, T; Einaga, Y; Ota, E; Yamaguchi, J

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 146 （ 44 ） 30698 - 30707 2024年10月

ISSN 0002-7863

Cleavage of inert C-N bonds in unstrained azacycles such as pyrrolidine remains a formidable challenge in synthetic chemistry. To address this, we introduce an effective strategy for the reductive cleavage of the C-N bond in N-benzoyl pyrrolidine, leveraging a combination of Lewis acid and photoredox catalysis. This method involves single-electron transfer to the amide, followed by site-selective cleavage at the C2-N bond. Cyclic voltammetry and NMR studies demonstrated that the Lewis acid is crucial for promoting the single-electron transfer from the photoredox catalyst to the amide carbonyl group. This protocol is widely applicable to various pyrrolidine-containing molecules and enables inert C-N bond cleavage including C-C bond formation via intermolecular radical addition. Furthermore, the current protocol successfully converts pyrrolidines to aziridines, γ-lactones, and tetrahydrofurans, showcasing its potential of the inert C-N bond cleavage for expanding synthetic strategies.

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Tailoring Unstrained Pyrrolidines via Reductive C–N Bond Cleavage with Lewis Acid and Photoredox Catalysis

Marina Hirao, Kazuhiro Aida, Tsuyoshi Saitoh, Takashi Yamamoto, Yasuaki Einaga, Eisuke Ota, Junichiro Yamaguchi

（American Chemical Society (ACS)） 2024年07月

Skeletal remodeling of unstrained azacycles such as pyrrolidine remains a formidable challenge in synthetic chemistry. To achieve such remodeling, continuous development of the cleavage of inert C–N bonds is essential. In this study, we introduce an effective strategy for the reductive cleavage of C–N bond in N-benzoyl pyrrolidine, leveraging a combination of Lewis acid and photoredox catalysis. This method involves single-electron transfer to the amide, followed by site-selective cleavage at C2–N bond. Cyclic voltammetry and NMR studies demonstrated that the Lewis acid is crucial for promoting the single-electron transfer from the photoredox catalyst to the amide carbonyl group. This protocol is widely applicable to various pyrrolidine-containing molecules and enables inert C–N bond cleavage including C–C bond formation via intermolecular radical addition. Furthermore, the current protocol successfully converts pyrrolidines to aziridines, lactones, and tetrahydrofurans, demonstrating the potential to expand synthetic strategies in skeletal remodeling.

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Cu, Fe, N-doped Carbon Nanotubes Prepared through Silica Coating for Selective Oxygen Reduction to Water in Acidic Media

Kato M., Abe D., Xie S., Sato S., Fujibayashi N., Matsumoto K., Onoda A., Hayashi T., Mitsui T., Fujiwara K., Yamamoto T., Einaga Y., Tadgell C.A., Kato Y., Asakura K., Yagi I.

Chemcatchem （Wiley） 16 （ 15 ） 2024年05月

ISSN 1867-3880

We report Cu, Fe, N-doped carbon nanotubes, (Cu,Fe)−N−CNT, as electrocatalysts for the oxygen reduction reaction (ORR) in acidic media. (Cu,Fe)−N−CNT was prepared using a silica coating method in pyrolysis to minimize the formation of carbon-coated metal oxide or carbide nanoparticles, which are known to be inactive for the H<inf>2</inf>O<inf>2</inf> reduction. (Cu,Fe)−N−CNT shows a turnover frequency of 0.66 e− site−1 s−1 at +0.8 V vs. RHE and H<inf>2</inf>O<inf>2</inf> yields of <1 % for the ORR with a utilization factor of active sites of 82 %. Kinetic analysis reveals that 4e− transfer rates for (Cu,Fe)−N−CNT are higher than those of a monometallic counterpart of Fe−N−CNT. In situ X-ray absorption spectroscopy enables us to determine redox potentials: E°’(FeIII/FeII)=0.65 V vs. RHE and E°’(CuII/CuI)=0.45 V for (Cu,Fe)−N−CNT, and E°’(FeIII/FeII)=0.65 V for Fe−N−CNT. These results indicate that bimetallic doping into carbon nanotubes gives the effect on kinetic parameters but not on thermodynamic ones. In other words, there is no direct electronic interactions between the Cu and Fe active sites for (Cu,Fe)-N-CNT because such interactions should modulate their redox potentials.

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Designing Molecularly Imprinted Polymer-Modified Boron-Doped Diamond Electrodes for Highly Selective Electrochemical Drug Sensors

Ishii, K; Ogata, G; Yamamoto, T; Sun, SY; Shiigi, H; Einaga, Y

ACS SENSORS （ACS Publications） 9 （ 3 ） 1611 - 1619 2024年03月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 2379-3694

Drug detection in biological solutions is essential in studying the pharmacokinetics of the body. Electrochemical detection is an accurate and rapid method, but measuring multiple drugs that react at similar potentials is challenging. Herein, we developed an electrochemical sensor using a boron-doped diamond (BDD) electrode modified with a molecularly imprinted polymer (MIP) to provide specificity in drug sensing. The MIP is a polymer material designed to recognize and capture template molecules, enabling the selective detection of target molecules. In this study, we selected the anticancer drug doxorubicin (DOX) as the template molecule. In the electrochemical measurements using an unmodified BDD, the DOX reduction was observed at approximately −0.5 V (vs Ag/AgCl). Other drugs, i.e., mitomycin C or clonazepam (CZP), also underwent a reduction reaction at a similar potential to that of DOX, when using the unmodified BDD, which rendered the accurate quantification of DOX in a mixture challenging. Similar measurements conducted in PBS using the MIP-BDD only resulted in a DOX reduction current, with no reduction reaction observed in the presence of mitomycin C and CZP. These results suggest that the MIP, whose template molecule is DOX, inhibits the reduction of other drugs on the electrode surface. Selective DOX measurement using the MIP-BDD was also possible in human plasma, and the respective limits of detection of DOX in PBS and human plasma were 32.10 and 16.61 nM. The MIP-BDD was durable for use in six repeated measurements, and MIP-BDD may be applicable as an electrochemical sensor for application in therapeutic drug monitoring.

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Electrochemical Diagnosis of Urinary Tract Infection Using Boron-Doped Diamond Electrodes

Zhang, ZP; Ogata, G; Asai, K; Yamamoto, T; Einaga, Y

ACS SENSORS （ACS Publications） 8 （ 11 ） 4245 - 4252 2023年11月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 2379-3694

Efficient detection of sodium nitrite in human urine could be used to diagnose urinary tract infections rapidly. Here, we demonstrate a fast and novel method for the selective detection of sodium nitrite in different human urine samples using electrolysis with a bare boron-doped diamond electrode. The measurement is performed without adding any other species, such as enzymes, and uses a simple electrochemical approach with an oxidation step followed by reduction. In the present study, we pay attention to the reduction potential range for the measurement, which is substantially different from many previous literature reports that focus on the oxidation reaction. The determination of added sodium nitrite based on cyclic voltammetry or differential pulse voltammetry is employed for two pooled urine samples and three individual urine matrices. From this, the linear response ranges for sodium nitrite detection are 0.5-10 mg/L (7.2-140 μmol/L) and 10-400 mg/L (140-5800 μmol/L). The results from these urine samples convert well to the calibration curve, with a limit of detection established as 0.82 mg/L (R2 = 0.9914), which is clinically relevant.

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Selective Electro-Oxidation of Cumene Utilizing a Flow Reactor Equipped with Diamond Electrodes

Shogo Nihongi, Riyako Matsuoka, Tsuyoshi Saitoh, Yasuaki Einaga, Takashi Yamamoto

Current Research in Green and Sustainable Chemistry （ELSEVIER） 7 100378 2023年10月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り

Although oxidation of benzylic C(sp3)−H bonds is an important process for synthesizing biologically active compounds, the reactions generically require hazardous reagents, catalysts, and harsh conditions. Since electro-organic synthesis utilizes electricity itself as a reagent, reactions can proceed well even under ambient conditions. Herein, we report a selective oxidation of cumene in an electrochemical flow reactor. Cumyl alcohol was selectively obtained under optimum amount of charge and current density. The selective oxidation of cumene to cumyl alcohol was achieved because overoxidation of cumyl alcohol into acetophenone was suppressed by the electrochemical flow reactor.

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Observation of peripheral airways using ultra-thin fiberscope.

Kinoshita T, Terai H, Ikemura S, Takada N, Fukui T, Irie H, Miyazawa R, Funatsu Y, Okamura T, Koh H, Yamamoto T, Shigematsu R, Ohgino K, Oka K

Respirology (Carlton, Vic.) 28 （ 9 ） 881 - 884 2023年09月

ISSN 1323-7799

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Spin Manipulation in a Metal-Organic Layer through Mechanical Exfoliation for Highly Selective CO2 Photoreduction

Wu, DY; Yin, HQ; Wang, ZS; Zhou, MR; Yu, CF; Wu, J; Miao, HX; Yamamoto, T; Zhaxi, W; Huang, ZT; Liu, LY; Huang, W; Zhong, WH; Einaga, Y; Jiang, J; Zhang, ZM

ANGEWANDTE CHEMIE-INTERNATIONAL EDITION 62 （ 18 ） e202301925 2023年04月

ISSN 14337851

Spin manipulation of transition-metal catalysts has great potential in mimicking enzyme electronic structures to improve activity and/or selectivity. However, it remains a great challenge to manipulate room-temperature spin state of catalytic centers. Herein, we report a mechanical exfoliation strategy to in situ induce partial spin crossover from high-spin (s=5/2) to low-spin (s=1/2) of the ferric center. Due to spin transition of catalytic center, mixed-spin catalyst exhibits a high CO yield of 19.7 mmol g−1 with selectivity of 91.6 %, much superior to that of high-spin bulk counterpart (50 % selectivity). Density functional theory calculations reveal that low-spin 3d-orbital electronic configuration performs a key function in promoting CO<inf>2</inf> adsorption and reducing activation barrier. Hence, the spin manipulation highlights a new insight into designing highly efficient biomimetic catalysts via optimizing spin state.

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Spin Manipulation in a Metal-Organic Layer through Mechanical Exfoliation for Highly Selective CO₂ Photoreduction

Dayu Wu, Hua-Qing Yin, Zeshi Wang, Mingren Zhou, Chengfeng Yu, Jing Wu, Huixian Miao, Takashi Yamamoto, Wenjiang Zhaxi, Zetao Huang, Luying Liu, Wei Huang, Wenhui Zhong, Yasuaki Einaga, Jun Jiang, Zhi-Ming Zhang

Angewandte Chemie International Edition （Wiley-VCH GmbH） 62 （ 18 ） e202301925 2023年04月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り

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Amine-Functionalized Diamond Electrode for Boosting CO₂ Reduction to CO

Tatsuhiko Mikami, Takashi Yamamoto, Mai Tomisaki, Yasuaki Einaga

ACS Sustainable Chemistry & Engineering （American Chemical Society） 10 （ 45 ） 14685 - 14692 2022年11月

研究論文（学術雑誌）, 共著, 責任著者, 査読有り

Carbon capture, utilization, and storage (CCUS) have been attracting much attention as an initiative to achieve carbon neutrality. In the capture and storage strategy, amines are widely used for highly efficient separation and capturing of CO₂ since amines react with CO₂ to form carbamates. In the utilization strategy, CO₂ electroreduction is a promising method to convert CO₂ into value-added compounds because reaction conditions are relatively mild and easily controlled. In this work, toward combining CO₂ capture and storage technologies and CO₂ electroreduction, we focused on amine modification of the electrode surface, in which boron-doped diamond (BDD) was chosen as a sustainable electrode material. CO₂ electroreduction using the amine-modified BDD was performed, and an applied potential dependence of the product selectivity was examined. In situ attenuated total reflectance-infrared measurements showed that the peak intensity of the stretching vibration of the carbonyl group at around 1640 cm^-1 decreased as applied potentials became more negative. The effect of amine modification of the BDD surface lies in the formation of a C-N bond during CO₂ electroreduction, resulting in the enhancement of the selectivity of CO production up to 8 times compared to the unmodified BDD.

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Electro-conversion of cumene into acetophenone using boron-doped diamond electrodes

Mana Kitano, Tsuyoshi Saitoh, Shigeru Nishiyama, Yasuaki Einaga, Takashi Yamamoto

Beilstein Journal of Organic Chemistry （Beilstein-Institut） 18 1154 - 1158 2022年09月

研究論文（学術雑誌）, 共著, 最終著者, 責任著者, 査読有り, ISSN 1860-5397

A straightforward electro-conversion of cumene into acetophenone has been reported using boron-doped diamond (BDD) electrodes.This particular conversion is driven by the addition reaction of a cathodically generated hydroperoxide anion to an anodically generated cumyl cation, where the BDD’s wide potential window enables the direct anodic oxidation of cumene into the cumyl cation. Since electricity is directly employed as the oxidizing and reducing reagents, the present protocol is easy to use, suitable for scale-up, and inherently safe.

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Anodic Oxidation of Phenols: A Key Step for the Synthesis of Natural Products

Takashi Yamamoto, Tsuyoshi Saitoh, Yasuaki Einaga, Shigeru Nishiyama

2021年09月

研究論文（学術雑誌）, 共著, 筆頭著者, 責任著者, 査読有り

Fabrication and electrochemical properties of boron-doped SiC

Kazuki Uchiyama, Takashi Yamamoto, Yasuaki Einaga

Carbon （Carbon） 174 240 - 247 2021年04月

責任著者, 査読有り, ISSN 0008-6223

Silicon carbide (SiC) has excellent properties such as chemical and physical stability, biocompatibility, and high thermal conductivity. However, electrical conductivity of SiC is not high enough for electrochemical applications, which has been a major challenge. Here, in order to improve the conductivity, we prepared boron-doped SiC (SiC:B) and evaluated the electrochemical properties. SiC and SiC:B were fabricated by a microwave plasma chemical vapor deposition method. Structural characterization revealed that the main phase of SiC:B is 3C–SiC where the concentration of doped boron could be controlled by tuning the growth conditions. Electrochemical properties were evaluated by cyclic voltammetry measurements, indicating that the reactivity and sensitivity of the SiC:B electrode was comparable of that of the glassy carbon electrode.

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Weak Antilocalization and Spin Hall Effect in Pt Films Doped with Molecular Spin

Wataru Iwamoto, Takashi Yamamoto, Kaname Tsuchii, Yuya Tazaki, Akio Asami, Hiroki Hayashi, Yasuaki Einaga, Kazuya Ando

ACS Applied Electronic Materials （American Chemical Society (ACS)） 2 （ 7 ） 2098 - 2103 2020年07月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 2637-6113

© 2020 American Chemical Society. A high degree of tunability and versatility of organic chemistry has paved the way toward molecular engineering of electronic and spintronic devices. Here, we report molecular spin doping of a Pt film, the key material in spin orbitronics. The molecular spin doping using a monolayer of a metal terpyridine complex allows to control the interaction between the conduction electrons and localized magnetic moments in the Pt film. The interaction was probed by measuring the weak antilocalization, which demonstrates enhanced magnetic impurity scattering induced by the molecular spin doping. By measuring the spin-orbit torque generated by Pt doped with molecular spins, we found that the spin Hall effect is robust against the magnetic impurity scattering, showing that the significant spin-orbit coupling dominates the charge-to-spin conversion in Pt. These results demonstrate that the molecular spin doping paves the way to explore spin-orbitronics phenomena.

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Surface Rashba-Edelstein Spin-Orbit Torque Revealed by Molecular Self-Assembly

S. Haku, A. Ishikawa, A. Musha, H. Nakayama, T. Yamamoto, K. Ando

Physical Review Applied （Physical Review Applied） 13 （ 4 ） 044069 2020年04月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 2331-7019

© 2020 American Physical Society. © 2020 American Physical Society. We report the observation of a spin-orbit torque (SOT) originating from the surface Rashba-Edelstein effect. We find that the SOT in a prototypical spin-orbitronic system, a Pt/Co bilayer, can be manipulated by molecular self-assembly on the Pt surface. This provides evidence that the Rashba spin-orbit coupling at the Pt surface generates a sizable SOT, which is hidden by the strong bulk and interface spin-orbit coupling. We show that the molecular tuning of the surface Rashba-Edelstein SOT is consistent with density-functional-theory calculations. These results illustrate the crucial role of the surface spin-orbit coupling in the SOT generation, which alters the landscape of metallic spin-orbitronic devices.

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Avian Influenza Virus Detection by Optimized Peptide Termination on a Boron-Doped Diamond Electrode

Teruhiko Matsubara, Michiko Ujie, Takashi Yamamoto, Yasuaki Einaga, Tomo Daidoji, Takaaki Nakaya, Toshinori Sato

ACS Sensors （American Chemical Society (ACS)） 5 （ 2 ） 431 - 439 2020年02月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 2379-3694

Copyright © 2020 American Chemical Society. The development of a simple detection method with high sensitivity is essential for the diagnosis and surveillance of infectious diseases. Previously, we constructed a sensitive biosensor for the detection of pathological human influenza viruses using a boron-doped diamond electrode terminated with a sialyloligosaccharide receptor-mimic peptide that could bind to hemagglutinins involved in viral infection. Circulation of influenza induced by the avian virus in humans has become a major public health concern, and methods for the detection of avian viruses are urgently needed. Here, peptide density and dendrimer generation terminated on the electrode altered the efficiency of viral binding to the electrode surface, thus significantly enhancing charge-transfer resistance measured by electrochemical impedance spectroscopy. The peptide-terminated electrodes exhibited an excellent detection limit of less than one plaque-forming unit of seasonal H1N1 and H3N2 viruses. Furthermore, the improved electrode was detectable for avian viruses isolated from H5N3, H7N1, and H9N2, showing the potential for the detection of all subtypes of influenza A virus, including new subtypes. The peptide-based electrochemical architecture provided a promising approach to biosensors for ultrasensitive detection of pathogenic microorganisms.

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Enhancement of coercivity of self-assembled stacking of ferrimagnetic and antiferromagnetic nanocubes

Keisuke Sawano, Keishi Tsukiyama, Makoto Shimizu, Mihiro Takasaki, Yuya Oaki, Takashi Yamamoto, Yasuaki Einaga, Christian Jenewein, Helmut Cölfen, Hideo Kaiju, Tetsuya Sato, Hiroaki Imai

Nanoscale （Royal Society of Chemistry (RSC)） 12 （ 14 ） 7792 - 7796 2020年

研究論文（学術雑誌）, 共著, 査読有り, ISSN 2040-3364

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An electrochemical aptamer-based sensor prepared by utilizing the strong interaction between a DNA aptamer and diamond.

Asai K, Yamamoto T, Nagashima S, Ogata G, Hibino H, Einaga Y

The Analyst （Analyst） 145 （ 2 ） 544 - 549 2019年11月

共著, 査読有り, ISSN 0003-2654

© 2020 The Royal Society of Chemistry. Stable and continuous biosensing of electroactive species in vivo has been achieved by using boron-doped diamond (BDD) electrodes owing to their outstanding electrochemical properties. However, the present problem in biosensing using BDD electrodes is how to specifically measure/detect the target molecules, including electrochemically inactive species. A possible solution is to fabricate an electrochemical aptamer-based (E-AB) sensor using a BDD electrode. In a preliminary investigation, we found that DNA aptamers strongly adsorb on the BDD surface and the aptamer-adsorbed BDD apparently worked as an E-AB sensor. The present study reports the performance of the aptamer-adsorbed BDD electrode as an E-AB sensor. Doxorubicin (DOX), a widely used chemotherapeutic, was chosen as a target molecule. The sensor could be prepared by just dipping BDD in an aptamer solution for only 30 min, and the electrochemical signals were dependent on the DOX concentration. The adsorption of DNA was strong enough for continuous measurements and even a sonication treatment. Such behaviors were not observed when using gold and glassy carbon electrodes. In a kinetic measurement, distortion by a sluggish response was observed for both association and dissociation phases, indicating that the interaction between DOX and the aptamer involves several kinetic processes. By fitting to a Langmuir isotherm, a limit of detection of 49 nM and a maximum detectable concentration of 2.3 μM were obtained. Although the sensitivity was lower than those of the well-established E-AB sensors of gold, the values are within a drug's therapeutic range. Overall, the present work demonstrates that a DNA aptamer and a BDD electrode is an effective combination for an E-AB sensor with stable sensitivity, and a wide variety of DNA aptamers can be applied without any special treatment.

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Observation of Proton Transfer Coupled Spin Transition and Trapping of Photoinduced Metastable Proton Transfer State in an Fe(II) Complex.

Nakanishi T, Hori Y, Sato H, Wu SQ, Okazawa A, Kojima N, Yamamoto T, Einaga Y, Hayami S, Horie Y, Okajima H, Sakamoto A, Shiota Y, Yoshizawa K, Sato O

Journal of the American Chemical Society （Journal of the American Chemical Society） 141 （ 36 ） 14384 - 14393 2019年08月

共著, 査読有り, ISSN 0002-7863

An important technique to realize novel electron- and/or proton-based functionalities is to use a proton-electron coupling mechanism. When either a proton or electron is excited, the other one is modulated, producing synergistic functions. However, although compounds with proton-coupled electron transfer have been synthesized, crystalline molecular compounds that exhibit proton-transfer-coupled spin-transition (PCST) behavior have not been reported. Here, we report the first example of a PCST Fe(II) complex, wherein the proton lies on the N of hydrazone and pyridine moieties in the ligand at high-spin and low-spin Fe(II), respectively. When the Fe(II) complex is irradiated with light, intramolecular proton transfer occurs from pyridine to hydrazone in conjunction with the photoinduced spin transition via the PCST mechanism. Because the light-induced excited high-spin state is trapped at low temperatures in the Fe(II) complex-a phenomenon known as the light-induced excited-spin-state trapping effect-the light-induced proton-transfer state, wherein the proton lies on the N of hydrazone, is also trapped as a metastable state. The proton transfer was accomplished within 50 ps at 190 K. The bistable nature of the proton position, where the position can be switched by light irradiation, is useful for modulating proton-based functionalities in molecular devices.

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Influence of the Nature of Boron-Doped Diamond Anodes on the Dehydrogenative Phenol-Phenol Cross-Coupling

Gleede B., Yamamoto T., Nakahara K., Botz A., Graßl T., Neuber R., Matthée T., Einaga Y., Schuhmann W., Waldvogel S.

ChemElectroChem （ChemElectroChem） 6 （ 10 ） 2771 - 2776 2019年05月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 2196-0216

© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Boron-doped diamond (BDD) represents a powerful and innovative electrode material. In particular, in combination with fluorinated solvents such as 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), the system exhibits the largest known electrochemical window of approximately 5 V in protic media. Furthermore, the anodic treatment allows the direct formation of oxyl radicals, which are known to exhibit specific reactivity. The electrochemical dehydrogenative phenol-phenol cross-coupling is a versatile and useful transformation to non-symmetric biphenols. This electro-organic conversion can be divided into two regimes: initial oxidation at the anode and the electrolyte-controlled follow-up reaction. This work intends to provide an answer about the influence of BDD electrodes on oxidation reactions in electrosynthesis. Depending on the electro-organic transformation, the support material of BDD, its boron content, and its fabrication method have a significant influence on the electrosynthetic efficiency.

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Thermoresponsive, Freezing-Resistant Smart Windows with Adjustable Transition Temperature Made from Hydroxypropyl Cellulose and Glycerol

Nakamura C., Yamamoto T., Manabe K., Nakamura T., Einaga Y., Shiratori S.

Industrial and Engineering Chemistry Research （Industrial and Engineering Chemistry Research） 58 （ 16 ） 6424 - 6428 2019年04月

共著, ISSN 08885885

© 2019 American Chemical Society. A thermoresponsive smart window that can switch its transmittance to control heating from sunlight is attracting great attention. Such windows made from a hydrogel of a thermoresponsive polymer such as poly(N-isopropylacrylamide) (PNIPAm) or hydroxypropyl cellulose (HPC) have been successful and can switch their transmittance at room temperature. However, such hydrogels occasionally freeze in cold places, degrading their transmittance. Thus, a thermoresponsive hydrogel that can be used in various geographical regions is desired. Here, we produced a thermoresponsive smart window with freezing resistance made from HPC and glycerol. We could adjust its switching temperature by simply changing the amount of added glycerol, letting us easily change it to room temperature for practical use. These smart windows show high cyclic performance, freezing resistance, and heat shielding, demonstrating great potential.

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In situ spectroscopic study on the surface hydroxylation of diamond electrodes

Kasahara S., Ogose T., Ikemiya N., Yamamoto T., Natsui K., Yokota Y., Wong R., Iizuka S., Hoshi N., Tateyama Y., Kim Y., Nakamura M., Einaga Y.

Analytical Chemistry （Analytical Chemistry） 91 （ 8 ） 4980 - 4986 2019年

研究論文（学術雑誌）, 共著, ISSN 00032700

© 2019 American Chemical Society. Carbon-based materials are regarded as an environmentally benign alternative to the conventional metal electrode used in electrochemistry from the viewpoint of sustainable chemistry. Among various carbon electrode materials, boron-doped diamond (BDD) exhibits superior electrochemical properties. However, it is still uncertain how surface chemical species of BDD influence the electrochemical performance, because of the difficulty in characterizing the surface species. Here, we have developed in situ spectroscopic measurement systems on BDD electrodes, i.e., in situ attenuated total reflection infrared spectroscopy (ATR-IR) and electrochemical X-ray photoelectron spectroscopy (EC-XPS). ATR-IR studies at a controlled electrode potential confirmed selective surface hydroxylation. EC-XPS studies confirmed deprotonation of C-OH groups at the BDD/electrolyte interface. These findings should be important not only for better understanding of BDD's fundamentals but also for a variety of applications.

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High-temperature cooperative spin crossover transitions and single-crystal reflection spectra of [Fe^III(qsal)₂](CH₃OSO₃) and related compounds

Takahashi K., Yamamoto K., Yamamoto T., Einaga Y., Shiota Y., Yoshizawa K., Mori H.

Crystals （Crystals） 9 （ 2 ） 2019年

共著, ISSN 2073-4352

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. New Fe(III) compounds from qsal ligand, [Fe(qsal) 2 ](CH 3 OSO 3 ) (1) and [Fe(qsal) 2 ](CH 3 SO 3 )·CH 3 OH (3), along with known compound, [Fe(qsal) 2 ](CF 3 SO 3 ) (2), were obtained as large well-shaped crystals (Hqsal = N-(8-quinolyl)salicylaldimine). The compounds 1 and 2 were in the low-spin (LS) state at 300 K and exhibited a cooperative spin crossover (SCO) transition with a thermal hysteresis loop at higher temperatures, whereas 3 was in the high-spin (HS) state below 300 K. The optical conductivity spectra for 1 and 3 were calculated from the single-crystal reflection spectra, which were, to the best of our knowledge, the first optical conductivity spectra of SCO compounds. The absorption bands for the LS and HS [Fe(qsal) 2 ] cations were assigned by time-dependent density functional theory calculations. The crystal structures of 1 and 2 consisted of a common one-dimensional (1D) array of the [Fe(qsal) 2 ] cation, whereas that of 3 had an unusual 1D arrangement by π-stacking interactions which has never been reported. The crystal structures in the high-temperature phases for 1 and 2 indicate that large structural changes were triggered by the motion of counter anions. The comparison of the crystal structures of the known [Fe(qsal) 2 ] compounds suggests the significant role of a large non-spherical counter-anion or solvate molecule for the total lattice energy gain in the crystal of a charged complex.

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Electrochemical Pinacol Coupling of Acetophenone Using Boron-Doped Diamond Electrode

Nakahara K., Naba K., Saitoh T., Sugai T., Obata R., Nishiyama S., Einaga Y., Yamamoto T.

ChemElectroChem （ChemElectroChem） 6 （ 16 ） 4153 - 4157 2019年

共著, ISSN 2196-0216

© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim An electrochemical pinacol coupling reaction of acetophenone using a boron-doped diamond (BDD) electrode has been reported. This transformation is driven by the one-electron reduction of acetophenone on the BDD cathode, followed by an intermolecular radical coupling. Owing to the BDD's outstanding electrochemical properties, the pinacol-type compound has been obtained in good conversion yield. The roles of a supporting electrolyte and solvent were addressed; (1) lithium ions contribute to increase in the reactivity of the radical intermediate. (2) Addition of water promotes the electron transfer process at the BDD surface. The reaction is relatively tolerant to para-substituted acetophenone derivatives. Since electric current is directly employed as reducing reagents to generate a radical intermediate, the reaction is metal-free, sustainable, and inherently safe.

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Oxidative Cleavage of the Acyl-Carbon Bond in Phenylacetone with Electrogenerated Superoxide Anions

Zhang, Y., Sugai, T., Yamamoto, T., Yamamoto, N., Kutsumura, N., Einaga, Y., Nishiyama, S., Saitoh, T., Nagase, H.

ChemElectroChem （ChemElectroChem） 6 （ 16 ） 4194 - 4198 2019年

共著, 査読有り, ISSN 2196-0216

© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim An oxidative cleavage reaction of the acyl−carbon bond in phenylacetone derivatives under cathodic reduction is reported. This particular transformation is driven by the cathodic reduction of dissolved molecular oxygen to superoxide anions. Various cathode materials were examined, and we discovered that boron-doped diamond (BDD) enables highly efficient molecular conversion, owing to BDD's outstanding electrochemical property for utilizing the small amounts of molecular oxygen effectively. The scope of the reaction is broad and can be applied to a variety of substrates, ranging from ketones to esters and amides. As an electric current is directly employed to generate the superoxide anion, the reaction is metal- and catalyst-free. Electro-organic synthesis using the carbon-based BDD electrode thus provides access to sustainable molecular transformations.

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Controlled decoration of boron-doped diamond electrodes by electrochemical click reaction (e-CLICK)

Yamamoto Takashi, Akahori Miku, Natsui Keisuke, Saitoh Tsuyoshi, Einaga Yasuaki

CARBON （Carbon） 130 350 - 354 2018年04月

ISSN 0008-6223

© 2017 Elsevier Ltd Controlled surface functionalization of electrode materials is an important technique in various fields such as analytical chemistry, molecular electronics, and molecular recognition in biological systems. In this work, we report the control decoration of boron-doped diamond (BDD) electrodes by electrochemically driven copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition of alkyne and azide (e−CLICK) in which the catalyst is produced by electroreduction of copper(II) species. As the e−CLICK strategy enabled to tune the produced amount of the copper(I) catalyst electrochemically, the alkyne-terminated BDD electrodes were decorated with azide-terminated ferrocene in a controlled manner.

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Molecular engineering of Rashba spin-charge converter

Nakayama H., Yamamoto T., An H., Tsuda K., Einaga Y., Ando K.

Science Advances （Science Advances） 4 （ 22 ） eaar3899-1 - eaar3899-7 2018年03月

共著, 査読有り, ISSN 2375-2548

© 2018 American Association for the Advancement of Science. All rights reserved. In heterostructures with broken inversion symmetry, the electrons' motion is coupled to their spin through interface-driven spin-orbit coupling: the Rashba effect. The Rashba effect enables the interconversion between spin and charge currents, offering a variety of novel spintronic phenomena and functionalities. However, despite the significant progress in Rashba physics, controlling the spin-charge conversion in metallic heterostructures remains a major challenge. We show that molecular self-assembly provides a way to engineer the Rashba spin-charge converters. We demonstrate that magnetoresistance and voltage generation originating from the spin-charge conversion in metallic heterostructures can be manipulated by decorating the surface with selfassembled organic monolayers through the cooperative molecular field effect. We also demonstrate reversible phototuning of the spin-charge conversion through light-driven molecular transformations using a molecule that can photoisomerize between the trans and cis states. These findings, with the almost-infinite chemical tunability of organic monolayers, pave the way toward molecular engineering of spin-orbit devices.

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Contribution of Coulomb Interactions to a Two-Step Crystal Structure Phase Transformation Coupled with a Significant Change in Spin Crossover Behavior for a Series of Charged Fe^II Complexes from 2,6-Bis(2-methylthiazol-4-yl)pyridine

Takahashi K., Okai M., Mochida T., Sakurai T., Ohta H., Yamamoto T., Einaga Y., Shiota Y., Yoshizawa K., Konaka H., Sasaki A.

Inorganic Chemistry （Inorganic Chemistry） 57 （ 3 ） 1277 - 1287 2018年02月

研究論文（学術雑誌）, 共著, ISSN 00201669

© 2018 American Chemical Society. A series of [FeII(L)2](BF4)2 compounds were structurally and physically characterized (L = 2,6-bis(2-methylthiazol-4-yl)pyridine). A crystal structure phase transformation from dihydrate compound 1 to anhydrous compound 3 through partially hydrated compounds 2 and 2′ upon dehydration was found. Compounds 1 and 3 exhibited a gradual spin crossover (SCO) conversion, whereas compounds 2 and 2′ demonstrated two-step and one-step abrupt SCO transitions, respectively. An X-ray single-crystal structural analysis revealed that one-dimensional and two-dimensional Fe cation networks linked by π stacking and sulfur-sulfur interactions were formed in 1 and 3, respectively. A thermodynamic analysis of the magnetic susceptibility for 1, 2′, and 3 suggests that the enthalpy differences may govern SCO transition behaviors in the polymorphic compounds 2′ and 3. A structural comparison between 1 and 3 indicates that the SCO behavior variations and crystal structure transformation in the present [FeII(L)2](BF4)2 compounds can be interpreted by the relationship between the lattice enthalpies mainly arising from Coulomb interactions between the Fe cations and BF4 anions as in typical ionic crystals.

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A solvent-directed stereoselective and electrocatalytic synthesis of diisoeugenol

Yamamoto T., Riehl B., Naba K., Nakahara K., Wiebe A., Saitoh T., Waldvogel S., Einaga Y.

Chemical Communications （Chemical Communications） 54 （ 22 ） 2771 - 2773 2018年02月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 13597345

© 2018 The Royal Society of Chemistry. A stereoselective and electrocatalytic coupling reaction of isoeugenol has been reported for the first time in a 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP)/boron-doped diamond (BDD) electrode system. This particular C-C bond formation and diastereoselectivity is driven by a solvate interaction between the radical species and another isoeugenol molecule. Due to an electrocatalytic cycle, only understoichiometric amounts of charge are necessary. Since electric current is directly employed as the oxidant, the reaction is metal and reagent-free. In addition, the electrolysis can be conducted in a very simple undivided beaker-type cell under constant current conditions. Therefore, the protocol is easy to use, suitable for scale-up, and inherently safe.

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Spin-Singlet Transition in the Magnetic Hybrid Compound from a Spin-Crossover Fe(III) Cation and π-Radical Anion

Takahashi Kazuyuki, Sakurai Takahiro, Zhang Wei-Min, Okubo Susumu, Ohta Hitoshi, HYamamoto Takashi, HR, Einaga Yasuaki, Mori Hatsumi

Inorganics 5 （ 3 ） 54-1 - 54-14 2017年08月

共著, 査読有り

Gigantic Photomagnetic Effect at Room Temperature in Spiropyran-Protected FePt Nanoparticles

HYamamoto Takashi, HR, Yurieva Elena A, Tsuda Kento, Hosomi Tomoko, Aldoshin Sergey M, Einaga Yasuaki

Physica Status Solidi - Rapid Research Letters 11 （ 9 ） 1700161-1 - 1700161-5 2017年07月

共著, 査読有り

Photo-modulation of superconducting and magnetic properties

Takashi Yamamoto, Keisuke Natsui, Yasuaki Einaga

Photon-Working Switches （Springer Japan） 285 - 299 2017年05月

In this chapter, photo-modulation of superconducting and magnetic properties is described. First, the superconducting property of a boron-doped diamond is investigated, in which the critical current density varies dramatically between the hydrogen- and oxygen-terminated diamond. Second, the superconducting diamond is modified with an azobenzene molecular layer to modulate the superconductivity upon photoirradiation. In this composite superconductor, the critical current density is reversibly amplified by 55% upon photoisomerization of the azobenzene layer. Third, Prussian Blue, a coordination polymer, is integrated with the semiconducting titanium oxide nanosheets to modulate the magnetic property upon photoirradiation. In this magnetic heterostructure, photoinduced magnetic phase transition of Prussian Blue is present by injecting photoexcited electrons from titanium oxide nanosheets.

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Cooperative Spin-crossover Transition from Three-dimensional purely π-Stacking Interactions in a Neutral Heteroleptic Azobisphenolate Fe^III Complex with N₃O₃ Coordination Sphere

Murata Suguru, Takahashi Kazuyuki, Mochida Tomoyuki, Ohta Hitoshi, Yamamoto Takashi, Einaga Yasuaki

Dalton Transactions 46 （ 18 ） 5786 - 5789 2017年03月

共著, 査読有り

Cooperative Spin-crossover Transition from Three-dimensional purely π-Stacking Interactions in a Neutral Heteroleptic Azobisphenolate FeIII Complex with N3O3 Coordination Sphere

Murata Suguru, Takahashi Kazuyuki, Mochida Tomoyuki, Ohta Hitoshi, Yamamoto Takashi, Einaga Yasuaki

Dalton Transactions （Royal Society of Chemistry） 46 （ 18 ） 5786 - 5789 2017年

査読有り, ISSN 1477-9234

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Single-crystal-to-single-crystal transformation in hydrogen-bond-induced high-spin pseudopolymorphs from protonated cation salts with a π-extended spin crossover Fe(III) complex

Suguru Murata, Kazuyuki Takahashi, Tomoyuki Mochida, Takahiro Sakurai, Hitoshi Ohta, Takashi Yamamoto, Yasuaki Einaga

Polyhedron 2017年

査読有り

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Enhancement of the Coercivity in Co–Ni Layered Double Hydroxides by Increasing Basal Spacing

Zhang Cuijuan, Tsuboi Tomoya, Namba Hiroaki, Einaga Yasuaki, YAMAMOTO Takashi

Dalton Transactions （Royal Society of Chemistry） 45 （ 34 ） 13324 - 13331 2016年06月

共著, 査読有り, ISSN 1477-9234

The magnetic properties of layered double hydroxides (LDH) containing transition metal ions can still develop, compared with layered metal hydroxide salts which exhibit structure-dependent magnetism. In this article, we report the preparation of a hybrid magnet composed of Co-Ni LDH and n-alkylsulfonate anions (Co-Ni-CnSO3 LDH). As Co-Ni LDH is anion-exchangeable, we can systematically control the interlayer spacing by intercalating n-alkylsulfonates with different carbon numbers. The magnetic properties were examined with temperature- and field-dependent magnetization measurements. As a result, we have revealed that the coercive field depends on the basal spacing. It is suggested that increasing the basal spacing varies the competition between the in-plane superexchange interactions and long-range out-of-plane dipolar interactions. Moreover, a jump in the coercive field at around 20 Å of the basal spacing is assumed to be the modification of the magnetic ordering in Co-Ni-CnSO3 LDH.

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Highly Sensitive Detection of Influenza Virus by Boron-Doped Diamond Electrode Terminated with Sialic Acid-Mimic Peptide

Matsubara Teruhiko, Ujie Michiko, HYAMAMOTO Takashi, HR, Akahori Miku, Einaga Yasuaki, Sato Toshinori

Proceedings of the National Academy of Sciences of the United States of America 113 （ 32 ） 8981 - 8984 2016年06月

研究論文（学術雑誌）, 共著, 査読有り

The Role of Coulomb Interactions for Spin Crossover Behaviors and Crystal Structural Transformation in Novel Anionic Fe(III) Complexes from a π-Extended ONO Ligand

Murata Suguru, Takahashi Kazuyuki, Sakurai Takahiro, Ohta Hitoshi, YAMAMOTO Takashi, Einaga Yasuaki, Shiota Yoshihito, Yoshizawa Kazunari

Crystals （MDPI AG） 6 （ 5 ） 49-16pp 2016年05月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 2073-4352

To investigate the pi-extension effect on an unusual negative-charged spin crossover (SCO) FeIII complex with a weak N2O4 first coordination sphere, we designed and synthesized a series of anionic FeIII complexes from a pi-extended naphthalene derivative ligand. Acetonitrile-solvate tetramethylammonium (TMA) salt 1 exhibited an SCO conversion, while acetone-solvate TMA salt 2 was in a high-spin state. The crystal structural analysis for 2 revealed that two-leg ladder-like cation-anion arrays derived from pi-stacking interactions between pi-ligands of the FeIII complex anion and Coulomb interactions were found and the solvated acetone molecules were in one-dimensional channels between the cation-anion arrays. A desolvation-induced single-crystal-to-single-crystal transformation to desolvate compound 2' may be driven by Coulomb energy gain. Furthermore, the structural comparison between quasi-polymorphic compounds 1 and 2 revealed that the synergy between Coulomb and pi-stacking interactions induces a significant distortion of coordination structure of 2.

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Charge-Transfer Phase Transition of a Cyanide-Bridged Fe^II/Fe^III Coordination Polymer

Zhang Kuirun, Kang Soonchul, Yao Zi-Shuo, Nakamura Kazusa, YAMAMOTO Takashi, Einaga Yasuaki, Azuma Nobuaki, Miyazaki Yuji, Nakano Motohiro, Kanegawa Shinji, Sato Osamu

Angewandte Chemie International Edition 55 （ 20 ） 6047 - 6050 2016年04月

研究論文（学術雑誌）, 共著, 査読有り

Charge-Transfer Phase Transition of a Cyanide-Bridged FeII/FeIII Coordination Polymer

Zhang Kuirun, Kang Soonchul, Yao Zi-Shuo, Nakamura Kazusa, Yamamoto Takashi, Einaga Yasuaki, Azuma Nobuaki, Miyazaki Yuji, Nakano Motohiro, Kanegawa Shinji, Sato Osamu

Angewandte Chemie International Edition 55 （ 20 ） 6047 - 6050 2016年04月

査読有り

Preparation of Dihydroquinazoline Carbohydrazone Fe(II) Complexes for Spin Crossover

Shen Fu-Xing, Huang Wei, Takashi Yamamoto, Einaga Yasuaki, Wu Dayu

New Journal of Chemistry （ROYAL SOC CHEMISTRY） 40 （ 5 ） 4534 - 4542 2016年02月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1144-0546

Dihydroquinazoline FeII complexes, namely [Fe(pq-R)(2)](X)(2)center dot CH3OH center dot yCH(3)CN center dot zH(2)O (R = 2-py: X = ClO4-, x = 1, y = 0, z = 2 (1), BF4-, x = 0, y = 2, z = 1.75 (2), CF3SO3-, x = 1, y = 1, z = 0 (3); and X = ClO4-: R = 2-OCH3, x = 0, y = 0, z = 1 (4), R = 3-OCH3, x = 0, y = 0, z = 1 (5)), were prepared and the effects of the solvent, counteranion and ligand substituent on spin crossover properties were discussed. Comparison of X-ray diffraction data for these complexes revealed the sole presence of high-spin FeII at 298 K and the bond distances around the FeII center at low temperature fall much closer to those at high temperature, which is consistent with variable-temperature dc magnetic susceptibility data. However, the loss of solvent induced a significant change in the spin state of complexes 1 and 2. Moreover, fits to magnetic data of the desolvated samples provide crossover temperatures of T-1/2 = 182.9(6), 157.0(8) and 138.3(5) K for 1-des (ClO4-), 2-des (BF4-) and 3-des (CF3SO3-), respectively, indicative of the anion-dependent transition temperature. The cooperativity operating in the complexes is thought to be mainly from the intermolecular pi center dot center dot center dot pi interactions between dihydroquinazoline rings on the neighboring molecules. The ligand substituent effect was also investigated on Fe-II SCO properties. This work revealed the spin crossover properties of new types of dihydroquinazoline Fe(II) complexes, including the transition temperature, the degree of completion and the cooperative nature of the transition, which can be optimally designed when developing new spin-crossover materials.

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Co-Fe Prussian Blue Analogue Intercalated into Diamagnetic Mg‒Al Layered Double Hydroxides

Zhang Cuijuan, YAMAMOTO Takashi, Einaga Yasuaki

Nanomaterials and Nanotechnology （InTech Europe） 6 （ 1 ） 26-7pp 2016年01月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1847-9804

A heterostructure of diamagnetic magnesium-aluminium layered double hydroxides (Mg-Al LDHs) and photomagnetic cobalt-iron Prussian Blue analogue (Co-Fe PBA) was designed, synthesized and then designated as LDH-PB. The cyanide-bridged Co-Fe PBA was two-dimensionally intercalated into the Mg-Al LDH template by the stepwise anion exchange method. LDH-PB showed ferrimagnetic properties with in-plane antiferromagnetic exchange interactions, as well as small photo-induced magnetization by visible light illumination due to the low dimensional structures and the characteristic photo-induced electronic states of the mixed valence of FeIII(low spin, S = 1/2)-CN-CoII(high spin, S = 3/2)-NC-FeII (low spin, S = 0).

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New Family of Anionic Fe^III Spin Crossover Complexes Featuring a Weak-Field N₂O₄ Coordination Octahedron

Takahashi Kazuyuki, Kawamukai Kiko, Okai Mitsunobu, Mochida Tomoyuki, Sakurai Takahiro, Ohta Hitoshi, Yamamoto Takashi, Einaga Yasuaki, Shiota Yoshihito, Yoshizawa Kazunari

Chemistry A European Journal 22 （ 4 ） 1253 - 1257 2015年12月

研究論文（学術雑誌）, 共著, 査読有り

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Novel Fe(II) Spin Crossover Complexes Involving Chalcogen-Bond and π-Stacking Interactions with a Paramagnetic and Nonmagnetic M(dmit)2 Anion (M = Ni, Au; dmit = 4,5-dithiolato-1,3-dithiole-2-thione)

Okai Mitsunobu, Takahashi Kazuyuki, Sakurai Takahiro, Ohta Hitoshi, Yamamoto Takashi, Einaga Yasuaki

Journal of Materials Chemistry C （Royal Society of Chemistry） 3 （ 30 ） 7858 - 7864 2015年05月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 2050-7526

To introduce both an inter-sulphur atom and p-stacking interaction between a spin crossover (SCO) cation and paramagnetic anion, we designed and synthesised two novel Fe(II) SCO compounds involving a terpyridine-type thiazole containing ligand with the paramagnetic Ni(dmit)(2) and nonmagnetic Au(dmit)(2) anion (dmit = 4,5-dithiolato-1,3-dithiole-2-thione). The temperature variations of magnetic susceptibility and Mossbauer spectroscopy revealed that both complexes exhibited an almost complete SCO conversion, and moreover, the Ni(dmit)(2) anion was in a paramagnetic state down to 2 K. The crystal structures for the Ni(dmit)(2) complex at 213 and 400 K indicated that the chalcogen-bond and p-stacking interaction would be enhanced by the electrostatic interaction between the cation and anion, preventing the p-radicals from dimerising. The light-induced excited spin state trapping effect was observed at 5 K.

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Antiferromagnetic Transition in a Novel Star-Shaped High-Spin Fe(III) Tetranuclear Cluster from a Mononuclear Coordination Anion Featuring π-Extended Schiff Base Ligands

Takahashi Kazuyuki, Kawamukai Kiko, Mochida Tomoyuki, Sakurai Takahiro, Ohta Hitoshi, Yamamoto Takashi, Einaga Yasuaki, Mori Hatsumi, Shimura Yasuyuki, Sakakibara Toshiro, Fujisawa Takumi, Yamaguchi Akira, Sumiyama Akihiko

Chemistry Letters （The Chemical Society of Japan） 44 （ 6 ） 840 - 842 2015年04月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0366-7022

A novel star-shaped high-spin Fe(III) tetranuclear cluster was unexpectedly obtained by the reaction of an [Fe-III(phsal)(2)](-) anion with an Fe(III) coordination cation (H(2)phsal: N-(2-hydroxyphenyl)salicylaldimine). Despite a star-shaped Fe(III) tetranuclear structure similar to reported Fe(III) single molecule magnets, the present compound exhibited a bulk antiferromagnetic transition at 0.6 K due to a magnetic exchange coupling through intercluster pi-stacking interactions.

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Photochromism-Induced Ampliﬁcation of Critical Current Density in Superconducting Boron-Doped Diamond with an Azobenzene Molecular Layer

NATSUI Keisuke, YAMAMOTO Takashi, AKAHORI Miku, EINAGA, Yasuaki

ACS Applied Materials & Interfaces （American Chemical Society） 7 （ 1 ） 887 - 894 2014年12月

共著, 査読有り, ISSN 1944-8244

A key issue in molecular electronics is the control of electronic states by optical stimuli, which enables fast and high-density data storage and temporal-spatial control over molecular processes. In this article, we report preparation of a photoswitchable superconductor using a heavily boron-doped diamond (BDD) with a photochromic azobenzene (AZ) molecular layer. BDDs electrode properties allow for electrochemical immobilization, followed by copper(I)-catalyzed alkyne-azide cycloaddition (a click reaction). Superconducting properties were examined with magnetic and electrical transport measurements, such as field-dependent isothermal magnetization, temperature-dependent resistance, and the low-temperature voltage-current response. These measurements revealed reversible amplification of the critical current density by 55% upon photoisomerization. This effect is explained as the reversible photoisomerization of AZ inducing an inhomogeneous electron distribution along the BDD surface that renormalizes the surface pinning contribution to the critical current.

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Synergistic Spin Transition between Spin-Crossover and Spin-Peierls-Like Singlet Formation in the Halogen-bonded Molecular Hybrid System: [Fe(Iqsal)2][Ni(dmit)2]•CH3CN•H2O

Kappei Fukuroi, Kazuyuki Takahashi, Tomoyuki Mochida, Takahiro Sakurai, Hitoshi Ohta, Takashi Yamamoto, Yasuaki Einaga, Hatsumi Mori

Angewandte Chemie International Edition （WILEY-V C H VERLAG GMBH） 53 （ 7 ） 1983 - 1986 2014年02月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1433-7851

To introduce halogen-bond interactions between a cation and an anion, a novel Fe-III complex from iodine-substituted ligands involving a paramagnetic nickel dithiolene anion was prepared and characterized. The compound exhibited the synergy between a spin-crossover transition and a spin-Peierls-like singlet formation. The halogen-bond interactions between the iodine and the sulfur atoms stabilized the paramagnetic state of -spins and played a crucial role in the synergistic magnetic transition between d- and -spins. In addition, the compound showed the light-induced excited spin state trapping effect.

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Structure-Distortion-Induced Photomagnetic Effect in Azobenzene/Polyoxometalate Langmuir–Blodgett Films

Takashi Yamamoto, Yasushi Umemura, Yasuaki Einaga

Dalton Transactions （ROYAL SOC CHEMISTRY） 42 （ 45 ） 16014 - 16020 2013年11月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1477-9226

We have prepared photomagnetic Langmuir-Blodgett films composed of an amphiphilic azobenzene and a magnetic polyoxometalate. The obtained film possesses a well-organized layered structure where polyoxometalate anions are sandwiched by azobenzene cations in a single-layered manner. Reversible photoisomerization of azobenzene was achieved even at low temperature, accompanying intensity changes in the d-d transition of polyoxometalate anions. The photomagnetic effect was observed reversibly upon alternate UV and visible light irradiation. Based on polarized spectroscopy, the observed photomagnetic effect is ascribed to the structure-distortion of polyoxometalate layers.

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A Neutral Fe(III) Compound Exhibiting A Two-Step Spin Transition and Dielectric Anomalies

Zhao-Yang Li, Jing-Wei Dai, Kevin J. Gagnon, Hong-Ling Cai, Takashi Yamamoto, Yasuaki Einaga, Han-Hua Zhao, Shinji Kanegawa, Osamu Sato, Kim R. Dunbar, Ren-Gen Xiong

Dalton Transactions 42 （ 41 ） 14685 - 14688 2013年10月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1477-9226

The mononuclear ferric compound Fe(H-5-Cl-thsa-Me)(5-Cl-thsa-Me) ·H2O (1) was synthesized and characterized using powder/single-crystal X-ray diffraction, Mössbauer spectroscopy, differential scanning calorimetry (DSC), and magnetic susceptibility measurements. This photo-responsive compound shows reversible, two-step spin-crossover behaviour. Moreover, dielectric anomalies were observed during the spin transitions, demonstrating the multifunctional properties of compound 1. © 2013 The Royal Society of Chemistry.

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Modulation of Critical Current Density in Polycrystalline Boron-Doped Diamond by Surface Modification

Keisuke Natsui, Takashi Yamamoto, Takeshi Watanabe, Yoichi Kamihara, Yasuaki Einaga

Physica Status Solidi B 250 （ 8 ） 1943 - 1949 2013年09月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0370-1972

The maximum current flowed with zero resistance, referred to the critical current density, is desired to be sufficiently large in order to apply superconducting wire rods and electromagnets to various devices. Although the critical current density can be enhanced by introduction of non-superconducting impurities, the transition temperature would become diminished in such a case. Here, we report modulation of a critical current density in heavily boron-doped diamonds (BDDs) by changing the surface-termination without introducing any impurity. The surface of a BDD was changed between hydrogen- and oxygen-termination. As a result, the critical current density could be modulated in a reversible manner between the hydrogen- and oxygen-terminated diamonds with maintenance of the superconducting transition temperature. This is because the volume fractions of the superconducting phases were modulated by surface modification. © 2013 WILEY-VCH Verlag GmbH &amp
Co. KGaA, Weinheim.

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Synthesis and Size Control of Iron(II) Hexacyanochromate(III) Nanoparticles and the Effect of Particle Size on Linkage Isomerism

Matthieu F. Dumont, Olivia N. Risset, Elisabeth S. Knowles, Takashi Yamamoto, Daniel M. Pajerowski, Mark. W. Meisel, Daniel R. Talham

Inorganic Chemistry （AMER CHEMICAL SOC） 52 （ 8 ） 4494 - 4501 2013年04月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0020-1669

The controlled synthesis of monodisperse nano-particles of the cubic Prussian blue analogue iron(II) hexacyanochromate(III) is reported along with a kinetic study, using cyanide stretching frequencies, showing the variations of the activation energy (E-a) of the linkage isomerism as a function of the particle size. Highly reproducible, cubic-shaped iron(11) hexacyanochromate(110 nanocrystals, with sizes ranging from 2 to 50 nm, are synthesized using a microemulsion technique, whereas a bulk synthesis yields nonuniform less monodisperse particles with sizes greater than 100 nm. Monitoring the cyanide stretching frequency with FTIR spectroscopy shows that the rate of isomerization is faster for smaller particles. Moreover, a kinetic analysis at different temperatures (255 K <= T <= 321 K) gives insight into the evolution of E-a with the particle size. Finally, time-dependent powder X-ray diffraction and net magnetization confirm the FTIR observations. The data are interpreted within the concept of a simple two-component model with different activation energies for structures near the surface of the solid and within the bulk.

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Paramagnetic Phase of a heavy-Fermion Compound, CeFePO, Probed by ⁵⁷Fe Mössbauer Spectroscopy

Tetsuro Nakamura, Takashi Yamamoto, Masanori Matoba, Yasuaki Einaga, Yoichi Kamihara

Journal of the Physical Society of Japan （PHYSICAL SOC JAPAN） 81 （ 6 ） 064714-4pp 2012年05月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0031-9015

Fe-57 Mossbauer spectroscopy was applied to an iron-based layered compound CeFePO. At temperatures from 9.4 to 293 K, no magnetic splitting was observed in the Mossbauer spectra of CeFePO indicating a paramagnetic phase of the Fe magnetic sublattice. All the spectra were fitted with a small quadrupole splitting, and the Debye temperature of CeFePO was found to be similar to 448 K. The isomer shift at room temperature, 0.32 mm/s, was almost equal to those of LnFeAsO (Ln = La, Ce, Sm). Comparing s-electron density using the isomer shifts and unit cell volumes, it was found that the Fe of CeFePO has a similar valence state to other layered iron-based quaternary oxypnictides except LaFePO.

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Anodic Oxidation on a Boron-Doped Diamond Electrode Mediated by Methoxy Radicals

Takenori Sumi, Tsuyoshi Saitoh, Keisuke Natsui, Takashi Yamamoto, Mahito Atobe, Yasuaki Einaga, Shigeru Nishiyama

Angewandte Chemie International Edition （WILEY-V C H VERLAG GMBH） 51 （ 22 ） 5443 - 5446 2012年04月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1433-7851

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Photomagnetic K_0.25Ni_1-xCo_x[Fe(CN)₆]•nH₂O and K_0.25Co[Fe(CN)₆]_0.75y[Cr(CN)₆]_0.75(1-y)•nH₂O Prussian Blue Analogue Solid Solutions

Daniel. M. Pajerowski, Takashi Yamamoto, Yasuaki Einaga

Inorganic Chemistry （AMER CHEMICAL SOC） 51 （ 6 ） 3648 - 3655 2012年03月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0020-1669

Magnetically bistable solid solutions of Prussian blue analogues with chemical formulas of KaNi1-xCox[Fe(CN)(6)](beta)center dot nH(2)O (Ni1-xCoxFe) and K alpha Co gamma[Fe(CN)(6)](y)[Cr(CN)(6)](1-y)center dot nH(2)O (CoFeyCr1-y) have been synthesized and studied using mass spectrometry, Mossbauer spectroscopy, X-ray diffraction, temperature-dependent infrared spectroscopy, and dc magnetometry. These compounds provide insight into interfaces between the photomagnetic Co-Fe Prussian blue analogue and the high-T-C Ni-Cr Prussian blue analogue that exist in high-T-C photomagnetic heterostructures. This investigation shows that the bistability of Co-Fe is strongly modified by metal substitution, with Ni1-xCoxFe stabilizing high-spin cobalt-iron pairs and CoFeyCr1-y stabilizing low-spin cobalt iron pairs, while both types of substitution cause a dramatic decrease in the bistability of the material.

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Electronic Conductivity in Berlin Green and Prussian Blue

Daniel. M. Pajerowski, Takeshi Watanabe, Takashi Yamamoto, Yasuaki Einaga

Physical Review B （AMER PHYSICAL SOC） 83 （ 15 ） 153202-4pp 2011年04月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1098-0121

We report the temperature dependent (100-300 K) conductivity of Berlin green and Prussian blue powders. The method of electronic conduction in vacuum dried Berlin green is identified as hopping of localized electrons and vacuum dried Prussian blue is reaffirmed as an insulator. The assignment of the conduction mechanism is based upon fits to the experimental data, and the different conductivities of the two samples can be understood in terms of both molecular and simple crystalline energy levels of the solids.

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Block Copolymer Permeable Membrane with Visualized High-Density Straight Channels of Poly(ethylene oxide)

Takashi Yamamoto, Taro Kimura, Motonori Komura, Yukimitsu Suzuki, Tomokazu Iyoda, Sadayuki Asaoka, Haruyuki Nakanishi

Advanced Functional Materials （WILEY-BLACKWELL） 21 （ 5 ） 918 - 926 2011年03月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1616-301X

A simple fabrication, scalable to centimeter scale, of a permeable membrane made of block copolymer containing molecular transport channels is demonstrated by coating photo-crosslinkable liquid-crystalline block copolymer, consisting of poly(ethylene oxide) (PEO) and poly(methacrylate) (PMA) bearing stilbene (Stb) mesogens in the side chains (PEO(114)-b-PMA(Stb)(52)), onto a sacrificial cellulose acetate film substrate. After thermal annealing, perpendicularly aligned and hexagonally arranged PEO cylindrical domains with a surface density of 10(11) cm(-2) were formed and then fixed efficiently by photo-crosslinking the stilbene moieties in the PMA(Stb) domains by [2 + 2] dimerization. The fully penetrating straight PEO cylindrical domains across the 480-nm-thick membrane were well-defined and visualized as molecule-transport channels. After exfoliated by removal of the cellulose acetate layer, the membrane could be transferred onto another substrate by either scooping or a horizontal lifting method. Throughout the processes, the fully penetrating PEO channels across the membrane are preserved to open at both ends. A simple permeation experiment demonstrates that rhodamine dyes permeate efficiently through the PEO cylindrical channels of the annealed membrane but not across a non-annealed one.

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Photoreduction of Prussian Blue Intercalated into Titania Nanosheet Ultrathin Films

Takashi Yamamoto, Naoki Saso, Yasushi Umemura, Yasuaki Einaga

Journal of the American Chemical Society （AMER CHEMICAL SOC） 131 （ 37 ） 13196 - 13197 2009年08月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0002-7863

Ultrathin fims composed of titania nanosheets and Prussian Blue were prepared by using the modified Langmuir-Blodgett technique. The ultrathin films could be deposited in a layer-by-layer manner with lamellar structure. Upon band gap excitation of titania nanosheets, the generated electrons were injected into ferromagnetic Prussian Blue, resulting in reduction to paramagnetic Prussian White. It was revealed that the reaction could proceed only in the presence of hole-scavenging interlayer water molecules.

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Langmuir–Blodgett Films of Molecular Organic Materials

Daniel R. Talham, Takashi Yamamoto, Mark W. Meisel

Journal of Physics: Condensed Matter （IOP PUBLISHING LTD） 20 （ 18 ） 184006-13pp 2008年05月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0953-8984

Langmuir-Blodgett methods are perhaps the original approach for achieving controlled deposition of organic thin films. Molecules are first organized into a monolayer array on the surface of water before transfer as a monolayer onto solid supports. Molecular monolayers, multilayers, and multilayered heterostructures can be achieved. The capability of exercising such control over thin film assemblies has attracted materials chemists and physicists to develop Langmuir-Blodgett films for studies on organic conductors, magnets, non-linear optics, rectifiers, and intermolecular electron transfer. This article reviews objectives in each of these areas and selects some specific examples from the literature to highlight the state of the art, mostly from the point of view of the chemical systems that are studied. Mixed organic/inorganic hybrid films represent a new direction for Langmuir-Blodgett films in materials science, combining conventional inorganic solid-state phenomena with the properties of the organic networks, and recent examples, taken principally from the authors' work, are highlighted.

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Normal Photochromism of Spiropyran in Montmorillonite Interlayer

Naoki Saso, Takashi Yamamoto, Yasushi Umemura, Yasuaki Einaga

Colloids and Surfaces A: Physicochemical and Engineering Aspects （ELSEVIER SCIENCE BV） 317 （ 1-3 ） 309 - 315 2008年03月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0927-7757

Spiropyran-montmorillonitehybrid materials have been fabricated by the Langmuir-Blodgett(LB) and an intercalation method. The LB method using clay minerals could be applied to fabricate hybrid ultrathin films consisting of a non-amphiphilic spiropyran. It is confirmed that hybrid LB films possess the layered structure and exhibit "normal photochromism" without any changes in the layered structure. In case of the intercalation method, hybrid compounds also possess the layered structure. Interestingly, intercalation hybrid compounds exhibit "reverse photochromism". It is suggested that a different process in formation of the layered structure influences the observed photochromism. (C) 2007 Elsevier B.V. All rights reserved.

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Photomagnetic Hybrid Ultrathin Films

Takashi Yamamoto, Yasuaki Einaga

Journal of Solid State Electrochemistry （SPRINGER） 11 （ 6 ） 781 - 790 2007年06月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1432-8488

Two novel types of photomagnetic hybrid ultrathin film (film A and B) of metal cyanides have been fabricated by means of the modified Langmuir-Blodgett method using a smectite clay mineral. Film A is composed of an amphiphilic azobenzene cation, a montmorillonite, and Prussian Blue in which photocontrol in the magnetization was realized by the photoisomerization of azobenzene chromophore. The observed photomagnetic efficiency was large (ca. 11%) due to the well-organized structure of the ultrathin film. Film B is composed of a quaternary ammonium salt, a montmorillonite, and Co-Fe Prussian Blue in which the photoinduced magnetization caused by the electron transfer exhibited an anisotropic response with regards to the direction of the applied magnetic field. This phenomenon is ascribed to the unique structure of Co-Fe Prussian Blue formed onto the clay layer.

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Photochromism Induced Magnetization Changes in Prussian Blue Ultrathin Films Fabricated into the Langmuir–Blodgett Films Composed of an Amphiphilic Azobenzene and a Deoxyribonucleic Acid

Takashi Yamamoto, Yasushi Umemura, Masaru Nakagawa, Tomokazu Iyoda, Yasuaki Einaga

Thin Solid Films （ELSEVIER SCIENCE SA） 515 （ 13 ） 5476 - 5483 2007年05月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0040-6090

A photomagnetic ultrathin film consisting of an azobenzene, a deoxyribonucleic acid, and Prussian Blue has been fabricated by the Langmuir-Blodgett method. Prussian Blue layers in the films were a two-dimensional structure with the height of 50 A and the average coherence length of 210 A. Reversible photoisomerization of the azobenzenes was observed even in the films. The films exhibit the long-range ferromagnetic order below 4.2 K and moreover, upon photoillumination at 2 K, reversible changes in the magnetization were realized with the value of ca. 1.0%. This photoswitching in the magnetization is due to changes in the electrostatic field induced by photoisomerization. (C) 2007 Elsevier B.V. All rights reserved.

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Photomagnetic Langmuir–Blodgett Films Consisting Azobenzene and Prussian Blue: Correlation Between the Film Structure and the Photomagnetic Efficiency

Takashi Yamamoto, Yasushi Umemura, Osamu Sato, Yasuaki Einaga

Science and Technology of Advanced Materials （NATL INST MATERIALS SCIENCE） 7 （ 3 ） 134 - 138 2006年03月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1468-6996

A photomagnetic organized thin film consisting of an amphiphilic azobenzene, a smectite clay, and Prussian Blue has been prepared by utilizing the Langmuir-Blodgett (LB) method and an ion-exchange reaction. The magnetic LB films were revealed to possess an organized layered structure, and reversible photoisomerization of the azobenzene was observed even in the LB films. Upon photoillumination at 2 K, reversible changes in the magnetization were realized with the value of ca. 11%. This photoswitching in the magnetization is due to changes in the electrostatic field induced by photoisomerization of the azobenzene. A large photoinduced change in the magnetization would be owing to the highly ordered structure of the LB films. In order to verify the correlation between the film structure and photoswitching efficiency, the LB films have been fabricated in other condition. As results, it is confirmed that the packing state of the azobenzene plays an important and crucial role in accomplishment of the highly efficient photomagnetic switching. (c) 2006 Elsevier Ltd. All rights reserved.

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Observation of the Anisotropic Photoinduced Magnetization Effect in Co–Fe Prussian Blue Thin Films Fabricated by Using the Clay Langmuir–Blodgett Films as a Template

Takashi Yamamoto, Yasushi Umemura, Osamu Sato, Yasuaki Einaga

Journal of the American Chemical Society （AMER CHEMICAL SOC） 127 （ 46 ） 16065 - 16073 2005年11月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0002-7863

Thin films of cobalt-iron cyanide (Co-Fe Prussian blue) have been fabricated by means of the modified Langmuir-Blodgett (LB) method using a smectite clay mineral (montmorillonite). In this combined method, clay LB films play a template role in the formation of the Co-Fe Prussian blue thin layer. The films were revealed to possess a well-organized structure not only in perpendicular directions to the film surface but also in parallel directions to the film surface. The photoinduced electron transfer from the iron ion to the cobalt through the bridging cyanide in the films occurred at low temperature (8 K), similar to that in the bulk Co-Fe Prussian blue. The films clearly exhibited magnetic anisotropy with regards to the direction of the applied magnetic field. Moreover, the photoinduced magnetization effect in the films was also found to be anisotropic.

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Photomagnetic Co–Fe Prussian Blue Thin Films Fabricated by the Modified Langmuir–Blodgett Technique

Takashi Yamamoto, Yasushi Umemura, Osamu Sato, Yasuaki Einaga

Chemistry Letters （CHEMICAL SOC JAPAN） 33 （ 5 ） 500 - 501 2004年05月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0366-7022

Co-Fe Prussian blue thin films have been prepared by means of a modified Langmuir-Blodgett technique, in which the hybrid monolayer composed of amphiphiles and clay platelets plays a template role for the formation Co-Fe Prussian blue thin films. Photoinduced magnetization was observed in the thin films as observed in the bulk Co-Fe Prussian blue. To the best of our knowledge, this is the first example Of Successful formation of Co-Fe Prussian blue thin films, which possess the ordered structure and, in addition, exhibit photoinduced magnetization.

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Photoswitchable Magnetic Films: Prussian Blue Intercalated in Langmuir-Blodgett Films Consisting of an Amphiphilic Azobenzene and a Clay Mineral

Takashi Yamamoto, Yasushi Umemura, Osamu Sato, Yasuaki Einaga

Chemistry of Materials （AMER CHEMICAL SOC） 16 （ 7 ） 1195 - 1201 2004年04月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0897-4756

A novel photocontrollable magnetic thin film consisting of amphiphilic azobenzene cations, a smectite clay, and Prussian Blue has been designed and prepared by using the Langmuir-Blodgett method and an ion-exchange reaction. This magnetic thin film possesses a well-organized nanoscale layered structure. Reversible photoisomerization of the azobenzene chromophore realized photoswitching of the magnetization. The changes in the magnetization values were estimated to be ca. 11%. This figure was much larger than our previous system, which contained azobenzene multi-bilayered vesicles in a poly(vinyl alcohol) matrix (J. Am. Chem. Soc. 1999, 121, 3745). These larger changes in the magnetization values are due to the highly ordered structure of the hybrid multilayered films.

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Reversible Photocontrollable Magnetic Vesicles Consisting of Azobenzene

Yasuaki Einaga, Takashi Yamamoto, Takeshi Sugai, Osamu Sato

Chemistry of Materials （AMER CHEMICAL SOC） 14 （ 11 ） 4846 - 4850 2002年11月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0897-4756

Photochromic magnetic vesicles have been designed by intercalation of Prussian blue into a lipid bilayer membrane which consists of azobenzene. Reversible photoisomerization of the solid system realized reversible photocontrol of the magnetic properties. Here, we have synthesized an amphiphile azo moiety containing a long alkyl chain, and we have prepared photochromic magnetic vesicles consisting of only this azo moiety. As a result, the magnetic property change brought about by visible light illumination was estimated to be ca. 13%, which is much larger than the figure achieved for the similar system that we reported previously (similar to1%), which contained the double-chain ammonium amphiphile.

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