Published papers

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.

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.

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.

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.

Abstract 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 H2O2 reduction. (Cu,Fe)−N−CNT shows a turnover frequency of 0.66 e− site−1 s−1 at +0.8 V vs. RHE and H2O2 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.

Electrochemical CO2 reduction is a promising technology for converting CO2 into value-added compounds, contributing toward a sustainable society. Controlling product selectivity is a critical aspect of CO2 electroreduction, and various approaches have been developed to achieve high efficiency and selectivity. Bulding on our previous findings that surface modification of boron-doped diamond (BDD) electrodes with aniline enhances the selectivity of CO production through CO2 capture by the nitrogen lone pair, we herein report pyrazole-modified BDD electrodes to further improve performance. The potential dependence of product selectivity was investigated, revealing that CO production was prominent at relatively positive potentials. Significantly, the selectivity of CO production was increased by up to a factor of two compared with the previously reported aniline-modified BDD. This enhanced performance is attributed to the synergistic effect of the adjacent nitrogen atoms: the high nucleophilicity of one nitrogen atom facilitates CO2 capture, while the adjacent NH moiety stabilizes the intermediate through intramolecular hydrogen bonding.

We have successfully observed the chirality-induced spin selectivity (CISS)-based magnetoresistance (MR) effect in Au/N-(3S)-3,7-dimethyloctyl[1]benzothieno[3,2-b]benzothiophene-2-carboxyamide (S-BTBT-CONHR)/Ni78Fe22 nanodevices at room temperature.

Boron-doped diamond (BDD) electrodes are promising candidates for long-term and scalable CO2 electroreduction, a key technology to realize a sustainable society. BDD electrodes have been shown to maintain a high Faradaic efficiency in CO2 electroreduction for over 1,000 h of continuous operation. Furthermore, a preliminary CO2 electroreduction step at low current density performed prior to main electrolysis (hereafter referred to as pre-electrolysis treatment) has been proposed as an effective surface treatment unique to BDD electrodes. However, it remains unclear whether this pre-electrolysis treatment is still effective for CO2 electroreduction at high current density. In this work, we optimized a pre-electrolysis treatment protocol for BDD electrodes to achieve an efficient CO2 electroreduction even at high current density. X-ray photoelectron spectroscopy studies revealed that the proportion of C–O/C=O and COOH bonding states at the BDD surface were increased after the pre-electrolysis treatment. Consequently, linear sweep voltammetry studies in a CO2-saturated electrolyte solution showed that the BDD electrode after the pre-electrolysis treatment exhibited a significant increase in current compared to the BDD electrode without the pre-electrolysis treatment. We propose that interactions between CO2 molecules in the electrolyte solution and the oxygen-containing functional groups introduced at the BDD surface by the pre‑electrolysis treatment enhances the local CO2 concentration, thereby promoting the CO2 electroreduction. With the optimized pre-electrolysis treatment protocol, the Faradaic efficiency for formic acid production was enhanced from 19% to 71%, representing a 3.7-fold increase.

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.

© 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.

© 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.

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.

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.

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.

© 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.

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.

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.

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 CO2 adsorption and reducing activation barrier. Hence, the spin manipulation highlights a new insight into designing highly efficient biomimetic catalysts via optimizing spin state.