Yoshimasa Hoshino

Logarithmic relaxation of electrochemical insulating-to-conducting conversion at polyaniline films: interpretation by electric percolation

Abstract Electrochemical switching of conducting polymers coated on an electrode is associated with the hysteresis called the slow relaxation, the memory effect or the first scan effect, which are exemplified by linear dependence of the anodic peak potential on the logarithm of the time of cathodic electrolysis. The faradaic charge of the anodic voltammetric current at the polyaniline-coated electrode was used as a measure of the slow relaxation. Reproducible linearity between the charge and the logarithm of the electrolysis time was obtained at different thicknesses of the films when the time was less than 10 3 s. After then, the charge was constant, indicating termination of the slow relaxation. The ratio of the minimum charge to the maximum varied logarithmically with the thickness of the films. The slope of the charge vs. log( t ) plot was related to the thickness of the film to the power 1.3. A model of the slow relaxation was presented on the concept that conducting domains without electric connection to the electrode are rearranged by thermal fluctuation to recover the electric contact with the electrode. The average volume of the conducting domains in contact with the electrode was expressed as a function of the molar fraction of the conducting species. It was combined with a kinetic equation under the assumption of rapid mass transport or rapid electron exchange. Then, the relative amount of the redox charge exhibited a linear relation to the logarithm of the electrolysis time. The theory explained quantitatively the experimental behavior of the logarithmic relaxation, the termination of the relaxation and the dependence on the film thickness.

Photographic measurements of propagation speeds of the conducting zone in polyaniline films during electrochemical switching

Abstract In order to observe the propagation of a conducting zone of a polyaniline film in response to anodic switching, propagation perpendicular to the film surface was simulated by propagation along the film by mounting the film on an insulated plate. A polyaniline film electrochemically deposited on an indium tin oxide (ITO) electrode was reinforced by casting polymethylmethacrylate solution on the film. It was peeled off the ITO, chemically reduced and then potentiostatically oxidized at one end in 1 mol dm −3 H 2 SO 4 solution. Dynamic growth of the conducting zone was observed through an optical microscope. A well-defined boundary between the conducting zone and the insulating zone was seen. The contrast of the boundary did not vary with the electrolysis time, the growth length or the applied potential. The growth rate decreased slightly with the electrolysis time as a result of IR drop across the film. A differential equation for growth rate involving the effect of the IR drop was presented and solved to give more accurate values of the rate. The logarithmic growth ratio exhibited a linear relation with the applied potential, indicating a charge-transfer rate of the Tafel type. The anodic transfer coefficient was 0.2 and the rate constant was 5 × 10 −5 m s −1 . The propagation on a molecular scale was discussed on the basis of the experimental results.

Coulombic irreversibility at polyaniline-coated electrodes by electrochemical switching

The electrochemical conversion of the conducting polymers between the conducting (C) and the insulating (I) state is associated with growth of the C zone in the film. In order to visually observe the conversion, the growth perpendicular to the film was simulated to that along the film. The I → C conversion exhibited a well-defined growing boundary between the C and the I zone. In contrast, the C → I conversion occurred rapidly and uniformly over the film. The latter conversion was not completed and hence the C species was left behind in the film. The subsequent I → C conversions showed a vague boundary. These variations were examined semi-quantitatively by the time-dependence of the light absorption of the film responding to the switching. As a more quantitative measurement, amounts of the faradaic charge consumed by the C → I conversion were evaluated from cyclic voltammograms. The amount of the anodic charge was larger than that at the cathodic one. The ratio of the charge was independent of the potential sweep rate v for v > 0.6 V s−1. A quarter of the C species had no experience of the I → C conversion owing to the self-cutoff of the electric path from the C zones toward the electrode. This may be equivalent to the threshold of the random percolation, and hence is regarded as a thermodynamically significant quantity.

Synthesis, structure, and preliminary magnetic studies of unprecedented hexacopper(II) barrel clusters with spin ground state S= 3

The structure of a discrete hexacopper(II) barrel cluster and that of a coordination polymer formed by a related hexanuclear repeating unit have been determined. The CuII metals of the hexanuclear units are held together by eight chelating L-prolinato type ligands, which in turn provide the eight oxygen donors trapping a sodium ion in the center. The structure of the [Cu6Na] unit in the discrete system and in the infinite cluster-chain are essentially the same and both display intra-unit ferromagnetic superexchange. A preliminary magnetic study shows that, for both compounds, the copper centers within the [Cu6] unit are ferromagnetically coupled, yielding an S n= 3 spin ground state.

Preparation, structures and properties of oxalate-bridged binuclear iron(III) complex: [(acac)2Fe(μ-ox)Fe(acac)2] and [(acac)2Fe(μ-ox)Fe(acac)2].CH2ClCH2Cl

Abstract An oxalate-bridged binuclear iron(III) complex, [(acac)2Fe(μ-ox)Fe(acac)2], (acac−=acetylacetonate anion and ox2−=oxalate anion) was prepared. The complex crystallized as two types of crystals under different conditions: one had 1,2-dichloroethane as a solvent molecule of crystallization 2, the other did not 1. Both compounds have been characterized by X-ray crystallography, infrared spectroscopy, and thermogravimetric analysis. Compound 1 has also been characterized by UV–Vis and 1H NMR spectroscopies, mass spectrometry, and electrochemistry. In both crystals, each iron(III) is coordinated in an octahedral arrangement by the oxygen atoms of an oxalate-bridging ligand and four oxygen atoms belonging to peripheral acac ligands in an octahedral arrangement. The intermetallic distance of Fe⋯Fe is 5.4368(9) A in 1 and 5.438(2) A in 2. Two iron(III) ions in each crystal are bridged by the oxalate and both lie in the oxalate-plane. The results of thermal analyses imply that the thermal stability of 2 is lower than that of 1. Cyclic voltammograms of 1 in acetonitrile and dichloromethane at low temperature showed two consecutive, quasi-Nernstian, one-electron reduction steps corresponding to the reduction of FeIII–FeIII to FeIII–FeII followed by the reduction of FeIII–FeII to FeII–FeII. The electrochemical comproportionation constants (Kc) of the equilibrium (FeIII–FeIII)xa0+xa0(FeII–FeII)xa0⇌xa02(FeIII–FeII) are 108.9 in acetonitrile medium and 108.5 in dichloromethane, respectively. The considerably large Kc values indicate that the main factor contributing to the stabilization of the FeIII–FeII mixed-valence state is electronic delocalization through the oxalate-bridge.

Amplification of metal-to-metal communication in the ruthenium(IV)-ruthenium(III) mixed-valence state of binuclear β-diketonatoruthenium complexes by inserting thiophene and anthracene units into acetylene linkers

Two new binuclear β-diketonatoruthenium(III) complexes linked by bis(ethynyl)thiophene (1) and bis(ethynyl)anthracene (2) were prepared. Both the binuclear complexes exhibited two well-defined one-electron oxidation steps and two overlapping one-electron reduction steps in triangular-wave voltammograms in dichloromethane medium at 25°C. These reduction and oxidation steps can be assigned to the reduction and the oxidation of the central metals. The difference in reversible half-wave potentials between the first and the second oxidation steps was 175 and 183 mV for 1 and 2, respectively. The comproportionation constants for the RuIV–RuIII mixed-valence state of 1 and 2 were larger by a factor of about 10 than that of an analogous binuclear complex linked by three acetylene linkers.

A mixed-valence diruthenium complex, triply bridged by mixed-moieties of chloro and methoxo ligands

A mixed-valence diruthenium complex, whose metal centres were triply bridged by one chloro and two methoxo ligands, was synthesized and characterized by X-ray structural analysis, electrochemistry and spectroscopy, and its mixed-valence state was classified into Class III.

Synthesis and characterization of a metal coordination polymer consisting of ruthenium(III) β-diketone units linked by butadiyne bridges

Abstract A new metal coordination polymer consisting of ruthenium(III) β-diketone units linked by butadiyne bridges was synthesized, and two mononuclear complexes, [Ru(mESima)3] and [Ru(mEma)3] (mESima−=3-(trimethylsilyl)ethynyl-2,4-pentanedionate ion and mEma−=3-ethynyl-2,4-pentanedionate ion), were also prepared as starting materials for the polymerization. The polymer was soluble in dichloromethane, chloroform, and benzene. The number-average molecular weight, 5400, corresponding to about 11 monomer units, was obtained from the FT-NMR integrated intensities for the protons on the ethynyl group of the mEma ligand end-group relative to the methyl protons of the entire chains. The NMR spectra also showed that the polymer structure was not a star-burst type but a chain-like type. The polymer exhibited one oxidation peak and one reduction peak on differential pulse voltammograms at 25°C; however, the one anodic peak separated into two peaks at −36°C. The difference in peak potentials between the first and the second oxidation waves was 0.23 V, which was very similar to the difference (0.22 V) in peak potentials for the corresponding binuclear complex. This indicated that metal–metal interactions in the polymer operate between only the closest neighboring units. Results of the controlled potential coulometry of the polymer in dichloromethane medium indicated that each monomer unit underwent a one-electron reduction.

Partial depolarization effect of high-numerical-aperture objectives on polarized microfocus Raman spectra of orthorhombic poly(oxymethylene) single crystal

In microfocus Raman spectroscopic measurement of micron-sized, moth-shaped single crystal of orthorhombic pohombic poly(oxymethylene) (metastable phase), the partial depolarization caused by a sharp focusing with objectives of a high magnification was investigated in order to obtain the information about the vibrational assignments. Polarized microfocus Raman spectra in the frequency range covering molecular vibrations were compared before and after the introduction of an aperture in the collimating system. The spectral difference in the (YX) polarization was observed for the bands of A–B1 and B2–B3 split-pairs, which indicated clearly that the scattering intensities of the B2 modes as well as the A modes were decreased selectively by the introduction of the aperture. In the (XX), (YY), and (XY) spectra, the B2 modes were observed as partial depolarization components of light for electric vector component along the depth direction. Utilizing these partial depolarization effects, it is possible to assign Raman-active fundamentals and to decide the unit cell orientation of the micron-sized, moth-shaped single crystal of orthorhombic poly(oxymethylene). Copyright

Resonance Raman spectra of tris(acetylacetonato)iron(III) and ruthenium(III) complexes and their solvent effect

The resonance Raman spectra of tris(acetylacetonatoiron(III)) and ruthenium(III) complexes in various solvents and in water-acetonitrile (W-AN) mixtures were measured. The resonance Raman spectra of both complexes indicated peaks near 460 and around 1580 cm−1. Thev(C-O) peak (around 1580 cm−1) is shifted to low frequency with an increase in the dielectric constant ɛT of the solvents, whereas thev(M-O) (M=Fe and Ru, near 460 cm−1) are constant, independent of ɛT. It implies that the C-O bond in the acac− ligand is lengthened by the polarizability effect of the solvents, while both the Fe-O and Ru-O bonds, which are located in the inside of the complexes, are not influenced by the solvents indicating that the interaction does not depend on the properties of individual solvent molecules but on those of the aggregate.