Masaya Enomoto

Synthesis of Triaxial LiFePO4 Nanowire with a VGCF Core Column and a Carbon Shell through the Electrospinning Method

A triaxial LiFePO4 nanowire with a multi wall carbon nanotube (VGCF:Vapor-grown carbon fiber) core column and an outer shell of amorphous carbon was successfully synthesized through the electrospinning method. The carbon nanotube core oriented in the direction of the wire played an important role in the conduction of electrons during the charge-discharge process, whereas the outer amorphous carbon shell suppressed the oxidation of Fe2+. An electrode with uniformly dispersed carbon and active materials was easily fabricated via a single process by heating after the electrospinning method is applied. Mossbauer spectroscopy for the nanowire showed a broadening of the line width, indicating a disordered coordination environment of the Fe ion near the surface. The electrospinning method was proven to be suitable for the fabrication of a triaxial nanostructure.

Control of charge transfer phase transition and ferromagnetism by photoisomerization of spiropyran for an organic-inorganic hybrid system, (SP)[Fe(II)Fe(III)(dto)3] (SP = spiropyran, dto = C2O2S2).

Iron mixed-valence complex, (n-C(3)H(7))(4)N[Fe(II)Fe(III)(dto)(3)](dto = C(2)O(2)S(2)), shows a spin entropy-driven phase transition called charge transfer phase transition in [Fe(II)Fe(III)(dto)(3)](-)(infinity) around 120 K and a ferromagnetic transition at 7 K. These phase transitions remarkably depend on the hexagonal ring size in the two-dimensional honeycomb network structure of [Fe(II)Fe(III)(dto)(3)](-)(infinity). In order to control the magnetic properties and the electronic state in the dto-bridged iron mixed-valence system by means of photoirradiation, we have synthesized a photosensitive organic-inorganic hybrid system, (SP)[Fe(II)Fe(III)(dto)(3)](SP = spiropyran), and investigated the photoinduced effect on the magnetic properties. Upon UV irradiation at 350 nm, a broad absorption band between 500 and 600 nm appears and continuously increases with the photoirradiation time, which implies that the UV irradiation changes the structure of spiropyran from the closed form to the open one in solid state. The photochromism in spiropyran changes the ferromagnetic transition temperature from 5 to 22 K and the coercive force from 1400 to 6000 Oe at 2 K. In this process, the concerted phenomenon coupled with the charge transfer phase transition in [Fe(II)Fe(III)(dto)(3)](-)(infinity) and the photoisomerization of spiropyran is realized.

Modulation of a Molecular π‐Electron System in a Purely Organic Conductor that Shows Hydrogen‐Bond‐Dynamics‐Based Switching of Conductivity and Magnetism

New important aspects of the hydrogen-bond (H-bond)-dynamics-based switching of electrical conductivity and magnetism in an H-bonded, purely organic conductor crystal have been discovered by modulating its tetrathiafulvalene (TTF)-based molecular π-electron system by means of partial sulfur/selenium substitution. The prepared selenium analogue also showed a similar type of phase transition, induced by H-bonded deuterium transfer followed by electron transfer between the H-bonded TTF skeletons, and the resulting switching of the physical properties; however, subtle but critical differences due to sulfur/selenium substitution were detected in the electronic structure, phase transition nature, and switching function. A molecular-level discussion based on the crystal structures shows that this chemical modification of the TTF skeleton influences not only its own π-electronic structure and π-π interactions within the conducting layer, but also the H-bond dynamics between the TTF π skeletons in the neighboring layers, which enables modulation of the interplay between the H-bond and π electrons to cause such differences.

Progress of Multi Functional Properties of Organic-Inorganic Hybrid System, A[FeIIFeIIIX3] (A = (n-CnH2n+1)4N, Spiropyran; X = C2O2S2, C2OS3, C2O3S)

In the case of mixed-valence systems whose spin states are situated in the spin crossover region, new types of conjugated phenomena coupled with spin and charge are expected. From this viewpoint, we have investigated the multifunctional properties coupled with spin, charge and photon for the organic-inorganic hybrid system, A[FeIIFeIIIX3](A = (n-CnH2n+1)4N, spiropyran; X = dto(C2O2S2), tto(C2OS3), mto(C2O3S)). A[FeIIFeIII(dto)3] and A[FeIIFeIII(tto)3] undergo the ferromagnetic phase transitions, while A[FeIIFeIII(mto)3] undergoes a ferrimagnetic transition. In (n-CnH2n+1)4N [FeIIFeIII(dto)3](n = 3,4), a new type of phase transition called charge transfer phase transition (CTPT) takes place around 120 K, where the thermally induced charge transfer between FeII and FeIII occurs reversibly. At the CTPT, the iron valence state dynamically fluctuated with a frequency of about 0.1 MHz, which was confirmed by means of muon spin relaxation. The charge transfer phase transition and the ferromagnetic transition for (n-CnH2n+1)4N[FeIIFeIII(dto)3] remarkably depend on the size of intercalated cation. In the case of (SP)[FeIIFeIII(dto)3](SP = spiropyran), the photoinduced isomerization of SP under UV irradiation induces the charge transfer phase transition in the [FeIIFeIII(dto)3] layer and the remarkable change of the ferromagnetic transition temperature. In the case of (n-CnH2n+1)4N[FeIIFeIII(mto)3](mto = C2O3S), a rapid spin equilibrium between the high-spin state (S = 5/2) and the low-spin state (S = 1/2) at the FeIIIO3S3 site takes place in a wide temperature range, which induces the valence fluctuation of the FeS3O3 and FeO6 sites through the ferromagnetic coupling between the low spin state (S = 1/2) of the FeIIIS3O3 site and the high spin state (S = 2) of the FeIIO6 site.

Origin of charge transfer phase transition and ferromagnetism in (CnH2n+1)4N[FeIIFeIII (dto)3] (dto=C2O2S2)

Mixed-valence complex (n-C 3 H 7 ) 4 N[Fe II Fe III (dto) 3 ] (dto=C 2 O 2 S 2 ) shows a new-type of phase transition coupled with spin and charge around 120 K, where the charge transfer between the Fe and Fe III sites occurs reversibly, and shows the ferromagnetic transition at 6.5 K. We have investigated the transport and magnetic properties for the single crystal of (n-C 3 H 7 ) 4 N[Fe II Fe III (dto) 3 ]. Both of the intra- and interlayer resistivity showed an anomalous drop due to the charge transfer phase transition. The anisotropy of the magnetization implied that the spins in the ferromagnetic ordered state were aligned parallel to the layer of [Fe II Fe III (dto) 3 ]∞.

Effect of Nonmagnetic Substitution on the Magnetic Properties and Charge-Transfer Phase Transition of an Iron Mixed-Valence Complex, (n-C3H7)4N[FeIIFeIII(dto)3] (dto = C2O2S2)

The iron mixed-valence complex (n-C(3)H(7))(4)N[Fe(II)Fe(III)(dto)(3)] exhibits a novel type of phase transition called charge-transfer phase transition (CTPT), where the thermally induced electron transfer between Fe(II) and Fe(III) occurs reversibly at ~120 K, in addition to the ferromagnetic phase transition at T(C) = 7 K. To investigate the mechanism of the CTPT, we have synthesized a series of magnetically diluted complexes (n-C(3)H(7))(4)N[Fe(II)(1-x)Zn(II)(x)Fe(III)(dto)(3)] (dto = C(2)O(2)S(2); x = 0-1), and carried out magnetic susceptibility and dielectric constant measurements and (57)Fe Mössbauer spectroscopy. With increasing Zn(II) concentration (x), the CTPT is gradually suppressed and disappears at x ≈ 0.13. On the other hand, the ferromagnetic transition temperature (T(C)) is initially enhanced from 7 K to 12 K between x = 0.00 and 0.05, despite the nonmagnetic nature of Zn(II) ions, and then it decreases monotonically from 12 K to 3 K with increasing Zn(II) concentration. This anomalous dependence of T(C) on Zn(II) concentration is related to a change in the spin configuration of the ferromagnetic state caused by the partial suppression of the CTPT.

Isomerization effect of counter anion on the spin crossover transition in [Fe(4-NH2trz)3](CH3C6H4SO3)2?nH2O

We have investigated the spin crossover transition of the triazole bridged one dimensional FeII complexes, [Fe(4-NH2trz)3](o-, m-, p-tos)2nH2O (tosH = toluenesulfonic acid), in order to study the isomerization effect of counter anion on the spin crossover phenomenon by means of 57Fe Mossbauer spectroscopy and magnetic susceptibility measurement. In the heating process, the spin transition of the all salts occurs around room temperatures, i.e. those of o-, m-, p-tos salts are 330 K, 319 K and 320 & 350 K, respectively. In the cooling process, the structural isomerization effect of counter anion on the spin crossover phenomenon is more remarkable than that in the heating process. In the case of o-tos salt, the spin transition occurs at 250 K with thermal hysteresis of 80 K. On the other hand, in the case of the m-tos salt, the spin transition occurs abruptly at 319 K with negligible small hysteresis. In the case of the p-tos salt, the spin transition occurs abruptly at 295 K in the cooling process in spite of stepwise spin transitions in the heating process.

Linkage-isomerization and charge-transfer in the formation of iron mixed valence complexes, (n-C3H7)4N[FeIIFeIII(dto)3] (dto = C2O2S2) and (n-C4H9)4N[FeIIFeIII(mto)3] (mto = C2O3S)

In iron mixed-valence complex, (n-C3H7)4N[FeIIFeIII(dto)3] (dto = C2O2S2), the linkage isomerisation and charge-transfer between FeII FeIII occur in the precipitation process. We succeeded the suppression of linkage-isomerization and charge-transfer in the formation process of (n-C3H7)4N[FeIIFeIII(dto)3] by the synthesis on low temperature condition and the selection of solvent. Moreover, we have found that the charge transfer occurs in the formation of (n-C4H9)4N[FeIIFeIII(mto)3] (mto = C2O3S) without linkage-isomerization.