Masaki Matsuda

Giant Negative Magnetoresistance Reflecting Molecular Symmetry in Dicyano(phthalocyaninato)iron Compounds

Materials containing Fe(Pc)(CN) 2 dicyano(phthalocyaninato)iron molecules show a giant negative magnetoresistance from the interaction between the conduction and the local moment. Under a magnetic field, the resistance becomes two orders of magnitude smaller than the zero-field resistance. The magnetic-field-angle dependence of the magnetoresistance reflects the symmetry of the Fe(Pc)(CN) 2 molecule. We discuss, according to the scaling relation, the correlation between the magnetoresistance and the molecular spin fluctuation.

Contribution of Degenerate Molecular Orbitals to Molecular Orbital Angular Momentum in Molecular Magnet Fe(Pc)(CN)2

We measured the static magnetic susceptibility and the electron spin resonance of the Fe(Pc)(CN) 2 complexes, and investigated the molecular magnetism of the unit Fe(Pc)(CN) 2 . The magnetic susceptibility shows a highly anisotropic Curie behavior. Based on the electron spin resonance, we found a highly anisotropic g -value ( g 1 = 3.62, g 2 = 1.11, and g 3 = 0.52) in the molecular unit Fe(Pc)(CN) 2 . This anisotropy is caused by the molecular orbital angular momentum in the degenerate next highest occupied molecular orbitals of the molecular unit Fe(Pc)(CN) 2 . Since the molecular unit Fe(Pc)(CN) 2 has a unique structure with fourfold symmetry, the molecular orbital angular momentum has a finite value of l z ∼+1 and -1. The anisotropic molecular magnetism of the unit Fe(Pc)(CN) 2 contributes the highly anisotropic Curie behavior. The molecular unit Fe(Pc)(CN) 2 is a good candidate for a molecular magnet having high magnetic anisotropy.

Charge disproportionation in highly one-dimensional molecular conductor TPP[Co(Pc)(CN)2]2

We investigated the ground state of a highly one-dimensional conductor, TPP[Co(Pc)(CN) 2 ] 2 (TPP = tetraphenylphosphonium and Pc = phthalocyanine), by the measurement of the X-ray diffraction, electron spin resonance, nuclear quadrupole resonance, and magnetoresistance. An increase of the magnetic fluctuations was observed below 20 K, where no structural deformation was detected. In the 59 Co nuclear quadrupole resonance, we found an asymmetric broadening of the spectra owing to the intrinsic inhomogeneity of the molecular charge. We propose that the ground state is characterized by a weak charge disproportionation with antiferromagnetic fluctuations due to the high one dimensionality. A large magnetoresistance was observed under a high magnetic field. Spin effects are dominant at low fields. The anisotropic magnetoresistance suggests a change in the ground-state nature above 10 T.

Variable magnetotransport properties in the TPP[Fe(Pc)L2]2 system (TPP = tetraphenylphosphonium, Pc = phthalocyaninato, L = CN, Cl, and Br)

A series of one-dimensional conductors, TPP[FeIII(Pc)L2]2 with L = CN, Cl, and Br, have been prepared. These compounds show giant negative magnetoresistance due to the interaction between conduction π-electrons and localized Fe-centered d-spins, but the magnitude is found to vary based on the axial ligand, suggesting that the strength of the π–d interactions is sensitively influenced by the axial substituent. The magnetic susceptibility also shows the difference in the magnetic interactions by the axial substituent. The ab initio calculation of the isolated molecules has revealed that the Jahn–Teller effect in this system induces splitting of dxz and dyz orbitals, and that the energy shift of the d-orbital accommodating an unpaired electron is varied by the axial substituent. The axial-ligand dependence of the intramolecular π–d interaction is found to correlate with the magnitude of the magnetoresistance.

A light-emitting diode fabricated from horse-heart cytochrome c

We have fabricated a light-emitting diode from horse-heart cytochrome c and measured the electro-luminescence (EL) spectra. The spectra exhibit broad peaks around 530 and 690 nm, and a weak shoulder around 410 nm. The EL spectra are completely different from the photo-luminescence spectra previously reported. The appearance of the 690 nm emission band suggests the charge-transfer between the iron and the axial methionine ligand plays a crucial role in the electrical conduction in the cytochrome c film.

Stable π–π dependent electron conduction band of TPP[M(Pc)L2]2 molecular conductors (TPP = tetraphenylphosphonium; M = Co, Fe; Pc = phthalocyaninato; L = CN, Cl, Br)

The partially-oxidized TPP[M(Pc)L(2)](2) molecular conductors exhibit variable electronic and magnetic transport bulk materials properties due to central metal and axial ligand molecular modifications. The controllable electrical conductivity and giant negative magnetoresistance can be mainly attributable to the varying ligand field energy and physical bulkiness of the axial ligands which cause modulation in the intra-molecular π-d (Pc-M) and inter-molecular π-π (Pc-Pc) interactions in the TPP[M(Pc)L(2)](2) system, respectively. Characterization of the electronic conduction band utilizing one-dimensional (1-D) tight-binding approximation from infrared reflectance and thermoelectric power profile reveal consistent band widths of 0.43 eV-0.62 eV for the Co series (L = Br < Cl < CN) and 0.44-0.56 eV for the Fe series (L = Br < Cl < CN). The fixed band width suggests that stable electron conduction bands (transport pathway) can be constructed which can withstand the molecular π-d interaction modifications that severely alter the bulk electronic and magnetic materials properties of the TPP[M(Pc)L(2)](2) molecular conductors.

Giant magnetoresistance response by the π–d interaction in an axially ligated phthalocyanine conductor with two-dimensional π–π stacking structure

Electrochemical oxidation of [MIII(Pc)(CN)2]− (M = Co and Fe, Pc = phthalocyaninato) and peri-xanthenoxanthene (PXX) in acetonitrile yields partially oxidized salts of [PXX]2[M(Pc)(CN)2]·CH3CN. In the crystal, the Pc ring is formally oxidized by 0.5 e and forms a two-dimensional double-sheet layer. These crystals show semiconducting behavior with small activation energy under ambient pressure, but show metallic behavior under high pressures. When M = Co, the metallic behavior is maintained down to 5 K. On the other hand, in the π–d system with M = Fe, the metallic behavior is disrupted by a steep increase in the resistivity at low temperatures. In this high resistivity state, negative differential resistance appears in the measurements of current–voltage characteristics, suggesting the development of charge disproportionation due to the π–d interaction. The magnetic susceptibility for M = Fe reflects anisotropic molecular magnetism of the Fe(Pc)(CN)2 unit, and shows an anomaly due to antiferromagnetic interaction between the Fe d-spins. The increase in the resistivity at low temperature under high pressures is suppressed by the application of a magnetic field. The magnitude of the negative magnetoresistance effect is extremely large; −99.8% was recorded at a field of 15 T at 3 K under 12 kbar.

Nonlinear Transport Phenomena in Highly One-Dimensional MIII(Pc)(CN)2 Chains with π–d Interaction (M = Co and Fe and Pc = Phthalocyaninato)

The charge transport properties of a partially oxidized salt composed of Co(Pc)(CN) 2 units with a typical one-dimensional electronic system have been suggested to be determined by charge disproportionation. The current–voltage ( I – V ) characteristics show nonlinear behavior at low temperature, which is suppressed by applying pressure. The observed nonlinearity is considered to result from the electric-field-induced delocalization of carriers in the charge disproportionation state. In the isomorphous magnetic Fe(Pc)(CN) 2 system, the nonlinear behavior is observed at higher temperatures, suggesting that charge disproportionation is more developed in a system with local moments. The threshold voltage of negative differential resistance decreases when an external magnetic field is applied, confirming that the localization of charge carriers is released by magnetic field application.

Phthalocyanine-based multi-dimensional conductors

Abstract Electrochemical oxidation of [CoIII(Pc)X2]− yields highly conducting partially oxidized salt crystals. Their crystal structures and dimensionality of the π–π interaction (electronic system) vary by the cationic species in the salts; one-dimensional for TPP (tetraphenylphosphonium) or PTMA (phenyltrimethylammonium) cations, and ladder and two-dimensional for the PXX (peri-xanthenoxanthene) radical cation. Substitution of the axial group (X) or extension of the π-conjugated macrocycle (Pc) has been found to influence the magnitude of the π–π stacking interactions. Their electrical properties characterized by the conductivity and thermoelectric power measurements are found to be dependent on the dimensionality and the magnitude of π–π stacking interactions.

Metamagnetic Transition and Its Related Magnetocapacitance Effect in Phthalocyanine-Molecular Conductor Exhibiting Giant Magnetoresistance

The magnetization and transport properties of iron-phthalocyanine molecular conductors are investigated under pulsed high magnetic fields up to 55 T. The metamagnetic transition is observed at approximately 14 T, where conductivity is enhanced. Below this transition, the tan δ (=e′′/e′) in the dielectric constants shows a monotonic decrease as the magnetic field strength increases, indicating that the magnetic field stabilizes the charge order. On the verge of this transition, the dielectric constants show a hysteresis below the weak-ferromagnetic transition temperature (6 K), suggesting that the π-electron charge order contributes to the weak ferromagnetism.