Shinya Hayami

A pentanuclear iron catalyst designed for water oxidation

Although the oxidation of water is efficiently catalysed by the oxygen-evolving complex in photosystem II (refs 1 and 2), it remains one of the main bottlenecks when aiming for synthetic chemical fuel production powered by sunlight or electricity. Consequently, the development of active and stable water oxidation catalysts is crucial, with heterogeneous systems considered more suitable for practical use and their homogeneous counterparts more suitable for targeted, molecular-level design guided by mechanistic understanding. Research into the mechanism of water oxidation has resulted in a range of synthetic molecular catalysts, yet there remains much interest in systems that use abundant, inexpensive and environmentally benign metals such as iron (the most abundant transition metal in the Earth’s crust and found in natural and synthetic oxidation catalysts). Water oxidation catalysts based on mononuclear iron complexes have been explored, but they often deactivate rapidly and exhibit relatively low activities. Here we report a pentanuclear iron complex that efficiently and robustly catalyses water oxidation with a turnover frequency of 1,900 per second, which is about three orders of magnitude larger than that of other iron-based catalysts. Electrochemical analysis confirms the redox flexibility of the system, characterized by six different oxidation states between FeII5 and FeIII5; the FeIII5 state is active for oxidizing water. Quantum chemistry calculations indicate that the presence of adjacent active sites facilitates O–O bond formation with a reaction barrier of less than ten kilocalories per mole. Although the need for a high overpotential and the inability to operate in water-rich solutions limit the practicality of the present system, our findings clearly indicate that efficient water oxidation catalysts based on iron complexes can be created by ensuring that the system has redox flexibility and contains adjacent water-activation sites.

Photo‐Induced Spin Transition of Iron(III) Compounds with π–π Intermolecular Interactions

Iron(III) spin-crossover compounds [Fe(pap)(2)]ClO(4) (1), [Fe(pap)(2)]BF(4) (2), [Fe(pap)(2)]PF(6) (3), [Fe(qsal)(2)]NCS (4), and [Fe(qsal)(2)]NCSe (5) (Hpap=2-(2-pyridylmethyleneamino)phenol and Hqsal=2-[(8-quinolinylimino)methyl]phenol) were prepared and their spin-transition properties investigated by magnetic susceptibility and Mössbauer spectroscopy measurements. The iron(III) compounds exhibited spin transition with thermal hysteresis. Single crystals of the iron(III) compounds were obtained as suitable solvent adducts for X-ray analysis, and structures in high-spin (HS) and low-spin (LS) states were revealed. Light-induced excited-spin-state trapping (LIESST) effects of the iron(III) compounds were induced by light irradiation at 532 nm for 1-3 and at 800 nm for 4 and 5. The activation energy E(a) and the low-temperature tunneling rate k(HL)(T-->0) of iron(III) LIESST compound 1 were estimated to be 1079 cm(-1) and 2.4x10(-8) s(-1), respectively, by HS-->LS relaxation experiments. The Huang-Rhys factor S of 1 was also estimated to be 50, which was similar to that expected for iron(II) complexes. It is thought that the slow relaxation in iron(III) systems is achieved by the large structural distortion between HS and LS states. Introduction of strong intermolecular interactions, such as pi-pi stacking, can also play an important role in the relaxation behavior, because it can enhance the structural distortion of the LIESST complex.

Recent progress in applications of graphene oxide for gas sensing: A review.

This paper is a review of the recent progress on gas sensors using graphene oxide (GO). GO is not a new material but its unique features have recently been of interest for gas sensing applications, and not just as an intermediate for reduced graphene oxide (RGO). Graphene and RGO have been well known gas-sensing materials, but GO is also an attractive sensing material that has been well studied these last few years. The functional groups on GO nanosheets play important roles in adsorbing gas molecules, and the electric or optical properties of GO materials change with exposure to certain gases. Addition of metal nanoparticles and metal oxide nanocomposites is an effective way to make GO materials selective and sensitive to analyte gases. In this paper, several applications of GO based sensors are summarized for detection of water vapor, NO2, H2, NH3, H2S, and organic vapors. Also binding energies of gas molecules onto graphene and the oxygenous functional groups are summarized, and problems and possible solutions are discussed for the GO-based gas sensors.

Proton Conductivities of Graphene Oxide Nanosheets: Single, Multilayer, and Modified Nanosheets

Proton conductivities of layered solid electrolytes can be improved by minimizing strain along the conduction path. It is shown that the conductivities (σ) of multilayer graphene oxide (GO) films (assembled by the drop-cast method) are larger than those of single-layer GO (prepared by either the drop-cast or the Langmuir-Blodgett (LB) method). At 60% relative humidity (RH), the σ value increases from 1×10(-6) S cm(-1) in single-layer GO to 1×10(-4) and 4×10(-4) S cm(-1) for 60 and 200 nm thick multilayer films, respectively. A sudden decrease in conductivity was observed for with ethylenediamine (EDA) modified GO (enGO), which is due to the blocking of epoxy groups. This experiment confirmed that the epoxide groups are the major contributor to the efficient proton transport. Because of a gradual improvement of the conduction path and an increase in the water content, σ values increase with the thickness of the multilayer films. The reported methods might be applicable to the optimization of the proton conductivity in other layered solid electrolytes.

Programmable spin-state switching in a mixed-valence spin-crossover iron grid

Photo-switchable systems, such as discrete spin-crossover complexes and bulk iron-cobalt Prussian blue analogues, exhibit, at a given temperature, a bistability between low- and high-spin states, allowing the storage of binary data. Grouping different bistable chromophores in a molecular framework was postulated to generate a complex that could be site-selectively excited to access multiple electronic states under identical conditions. Here we report the synthesis and the thermal and light-induced phase transitions of a tetranuclear iron(II) grid-like complex and its two-electron oxidized equivalent. The heterovalent grid is thermally inactive but the spin states of its constituent metal ions are selectively switched using different laser stimuli, allowing the molecule to exist in three discrete phases. Site-selective photo-excitation, herein enabling one molecule to process ternary data, may have major ramifications in the development of future molecular memory storage technologies.

Photo-induced reverse valence tautomerism in a metastable Co compound

Abstract In our preceding paper, we have reported that a Co complex, [CoIII-LS(tmeda)(3,5-DBSQ)(3,5-DBCat)]·0.5C6H5CH3, exhibits a photo-induced valence tautomerism. The photo-induced metastable state, [CoII-HS(tmeda)(3,5-DBSQ)2]·0.5C6H5CH3, has a back electron transfer band from CoII-HS to 3,5-DBSQ at around 800 nm, indicating that the valence tautomerism should be photo-reversible. In fact, we have found that the metastable state could be photo-pumped back to the original state by exciting the back electron transfer band. The process can be expressed as [ Co II-HS ( tmeda )(3,5 -DBSQ ) 2 ]·0.5 C 6 H 5 CH 3 ( metastable state )→[ Co III-LS ( tmeda )(3,5 -DBSQ )(3,5 -DBCat )]·0.5 C 6 H 5 CH 3 (ground state).

Reversible Electron Transfer in a Linear {Fe2Co} Trinuclear Complex Induced by Thermal Treatment and Photoirraditaion

Bistable materials possess two close-lying states, which can be reversibly interchanged by external stimuli such as temperature, light, pressure, and guest molecules. These materials offer attractive opportunities for the realization of energyefficient, switchable, molecule-based data storage in electronic devices. A current topic for research in this field is the preparation of switchable multifunctional molecules in which more than two properties coexist or interact synergistically. An important multifunctional compound shows a significant change in both magnetic properties and charge distribution (polarization) at the molecular level. Tunable magnetic molecules, such as spin-crossover complexes, are important for magnetic recording devices. Furthermore, switchable polarity is an essential feature for regulating nonlinear optical and ferroelectric properties. In particular, the study of electronic ferroelectricity, in which an electronic charge order without inversion symmetry is responsible for the electric polarization, has recently attracted significant attention. Thus, it is important to design new compounds in which the spin state and charge distribution can be reversibly controlled by external stimuli. To this end, the development of compounds that consist of paramagnetic donors and acceptors is attracting considerable interest because lattice distortion and charge-transfer processes in such compounds involve not only concomitant spin-state changes but also changes in their dielectric properties. 9] Significant changes in the magnetic susceptibility and the dielectric constant were observed near the neutral–ionic phase transition temperature of chargetransfer complexes. Furthermore, dimerization of the donor and the acceptor induces the formation of a polar structure from a nonpolar structure because of the breaking of the inversion center as a result of molecular charge transfer. Moreover, it has been reported that a dinuclear cobalt complex with a dioxolene bridging ligand exhibits charge transfer between the bridging ligand (donor/acceptor) and cobalt (acceptor/donor) induced by a temperature change and light irradiation. This transfer is accompanied by magnetization change and the formation of a polar structure in the cluster, which is a molecular-level representation of the interconversion of both magnetization and electric polarization similar to that in the aforementioned charge-transfer complex. With a rational design, various discrete multinuclear complexes have been synthesized by using different building blocks such as cyanometallates. However, until now, only the cobalt-dioxolene system has been reported to show such magnetization changes and polar–nonpolar transformation through charge transfer in response to both thermal and optical stimuli. Therefore, the preparation of new compounds with such properties remains a challenge. In this work, we aimed at synthesizing linear bimetallic trinuclear clusters with centrosymmetrical structures that are capable of charge transfer between the metal in the center and a metal ion on the edge. The charge-transfer process was expected to induce a change in magnetization because of the change in spin multiplicity. Furthermore, charge transfer in a cluster with an inversion center also induces the formation of a polar structure from a nonpolar one. To synthesize such a centrosymmetric bimetallic trinuclear cluster, we choose [FeTp(CN)3] (Tp = hydrotris(pyrazolyl)borate) as the building block to treat with [Co(Meim)4] 2+ (Meim = N-methylimidazole). One cyanide bridge of the [FeTp(CN)3] unit is thought to coordinate with the Co ion, which tunes the redox potential required for charge transfer. Moreover, the terminal cyanide ligands are thought to form potential hydrogen-bonding interactions with noncoordinated solvent molecules, stabilizing the bistable state through intermolecular cooperative interactions. In fact, we recently synthesized an Fe2Co trinuclear cluster {[FeTp(CN)3]2Co(Meim)4}·6 H2O (1), in which the cobalt ion is sandwiched between two iron building blocks (Scheme 1). Compound 1 exhibited thermally induced, reversible electron transfer with a thermal hysteresis and photoinduced electron transfer by excitation of the charge-transfer band. Single-crystal X-ray diffraction (XRD) analysis revealed that 1 crystallizes in a P 1 space group. The crystal structure comprises neutral {[FeTp(CN)3]2Co(Meim)4} trinuclear clusters (Figure 1 a) with noncoordinated water molecules located between the clusters (Figure 1b). Within the neutral trinu[*] Prof. T. Liu, Dr. D.-P. Dong, Prof. S. Wu, Prof. C. He, Prof. C.-Y. Duan State Key Laboratory of Fine Chemicals, Dalian University of Technology, 2 Linggong Rd., 116024 Dalian (China) E-mail: liutao@dlut.edu.cn cyduan@dlut.edu.cn

Photo-induced spin transition for iron(III) compounds with π–π interactions

Abstract The magnetic susceptibilities of the spin-crossover compound [Fe(pap) 2 ]PF 6 ·MeOH ( 1 ) have been measured and compared with that of the previously reported compound [Fe(pap) 2 ]ClO 4 ·H 2 O ( 2 ). Compound 1 is a spin-crossover compound with spin transition temperature T 1/2 =288 K, and does not show a hysteresis loop around the spin transition temperature, in contrast with compound 2 . Magnetic susceptibilities measured after illumination prove that 1 exhibits the LIESST effect, so does compound 2 . The crystallographic data on the molecular packing show the presence of π stacking between the aromatic rings of pap ligands, which appears to be responsible for the LIESST effect.

Effects of external electric field upon the photonic band structure in synthetic opal infiltrated with liquid crystal

Based on the electrically controlled birefringence effects of a nematic liquid crystal, a tunable photonic band gap crystal has been fabricated by infiltrating nematic liquid crystal into the voids of synthetic opal composed of silica spheres. A reversible shift of the photonic band gap position and a change of transmittance through the composite opal can be induced by applying an external electric field.

A ferromagnetically coupled Fe 42 cyanide-bridged nanocage

Self-assembly of artificial nanoscale units into superstructures is a prevalent topic in science. In biomimicry, scientists attempt to develop artificial self-assembled nanoarchitectures. However, despite extensive efforts, the preparation of nanoarchitectures with superior physical properties remains a challenge. For example, one of the major topics in the field of molecular magnetism is the development of high-spin (HS) molecules. Here, we report a cyanide-bridged magnetic nanocage composed of 18 HS iron(III) ions and 24 low-spin iron(II) ions. The magnetic iron(III) centres are ferromagnetically coupled, yielding the highest ground-state spin number (S=45) of any molecule reported to date.