Studies on Electronic Structures of Quasi One-Dimensional Phthalocyanine Conductors, NiPc(AsF6)0.5 and CoPc(AsF6)0.5

The charge-transfer salts of metallo-phthalocyanines (MPcs) make up a family of quasi-one-dimensional organic conductors such as MPc(X)y, (M = H2, Ni, Co, Cu, Pt; X = I3, BF4, CIO4, AsF6, SbF6; y = 0.33-1.0). Among these compounds, charge-transfer salts of NiPc, CoPc, and PtPc with y = 0.33 and 0.5 form double-chain and two-band systems: the central metal and maclocycle produce one-dimensional 3d and π-bands, respectively. In NiPc(AsF6)0.5 , positive holes are doped into this π-band at ambient pressure, being metallic above the metal-insulator transition temperature of 40K.i The narrow 3d-band is located near the Fermi level of the π-band. Due to this closeness, high pressure induces a metal-ligand charge-transfer between the 3d and π-bands, which was proved by the vibrational spectrum in the infrared region, plasmon absorption in the near-infrared region, and interband transition in the visible region.ii,iii The high-pressure optical experiment suggests that a metal-insulator transition accompanies the pressure-induced metal-ligand charge transfer. To characterize this pressure-induced insulating phase, they have conducted high pressure experiments of electrical resistivity and thermopower. The metal-insulator transition temperature goes up to high temperature side on increasing pressure and reached about 230 K at 1.0 GPa. The thermopower under high pressure decreased linearly against temperature and did not show any anomaly around the metal-insulator transition temperature in the same way as those characteristics taken at ambient pressure. Since thermopower is sensitive to the density of states at the Fermi level, both the metal-insulator transition under high and ambient pressure is not accompanied by the opening of a gap at the Fermi level. Therefore Peierls transition is not compatible with the behavior of the thermopower. These properties resemble Li1+x Ti2-xO4, the thermopower of which exhibits linear decrease in the nonmetallic region of x = 0.15, 0.20, 0.22 and 0.25 in the same way as the metallic region of x = 0. A finite density of states was suggested at the Fermi level of these nonmetallic compounds. In these materials, the nonmetallic state is attributed to the localized states.iv Thus, there is a possibility that the insulating phase of NiPc(AsF6)0.5 is some kind of localized state probably due to the small amount of Ni 3+, in other words, a small amount of holes in the metal chain or narrow 3d band. The localized Ni3+ will be distributed randomly in the crystal and this excess charge produces random potentials, which make an influence on the coherent motions of the charge carriers in the π-band. Since the number of Ni 3+ sites increases due to the metal-ligand charge transfer induced by high pressure, the insulating phase expands to the high-temperature region in a P-T phase diagram. However, the latest data of heat capacity and low-temperature X-ray diffraction study under ambient pressure indicate the possibility of a structural change around the metal-insulator transition temperature such as a torsion of the phthalocyanine molecular "gear" in the a, b-plane accompanying the change of crystal system from orthorhombic to monoclinic. Optical experiment which is very sensitive to the lattice dimerization also did not show any evidence of lattice dimerization. Consequently, these structural change will not directly influence on the electronic state of the one-dimensional chain. Pressure induced d-m charge transfer is regarded as a "band-filling control" by pressure. It is very interesting to investigate the physical properties in the various band fittings which result in the different density of state or effective on-site Coulomb repulsion. However, under high-pressure, the practicable measurement is limited. Therefore, they tried to control the band filling at ambient pressure. In the conventional electrochemical method through solution state, the crystal composition settled down one stable phase as a necessary result through an equilibrium condition. Therefore, as the first stage, they tried to develop a new doping method in the "solid state". They used the MPc-PBC composite films (M = Pt, Ni) as the starting material for doping. TBAPF6 and TBAAsF6 were adopted as supporting electrolytes. The doped film was characterized by X-ray diffraction, EPMA, ICP, ESR, and conductivity measurement. It showed almost the same properties as the crystals produced by the conventional electrochemical crystallization method. As the second stage, they tried to control the band filling by potential control in the range of the oxidation wave. Judging from the result of the X-ray diffraction study, MPc-PBC could not keep the same crystal structure over the potential range of the anodic peak giving the mixturepattern of undoped and doped one. As a result, they succeeded in the development of a new doping method based in solid state, though the continuous filling control by potential control could not be realized by this method. CoPc(AsF6)0.5 shows semiconducting behavior even at ambient temperature in spite of the same band filling and nearly isomorphous crystal structure to NiPc(AsF6)0.5 . CoPc(AsF6)0.5 is a typical π-d system in which a localized d-electron coexist with an itinerant π-electron and it is described by the one-dimensional Kondo lattice model. Recently, it is pointed out that the next-nearest-neighbor Coulomb repulsion among π-electrons, the magnetic coupling between π and d-electrons, and the antiferromagnetic coupling between d-d electrons are essential to open a charge gap using the one-dimensional quarter-filled Heisenberg-Kondo lattice model.v From the experimental viewpoint, CoPc(AsF6)0.5 is regarded as the narrow-gap one-dimensional semiconductor by its conducting behavior in the temperature range 20-500 K. To elucidate the electronic structure of π-d system of CoPc(AsF6)0.5 , they carried out the optical study for CoPc(AsF6)0.5 (d7; magnetic), NiPc(AsF6)0.5 (d8; non-magnetic), and the mixed crystals, CoxNi1-x Pc(AsF6)0.5 . The mixed crystals were characterized by X-ray measurement, EPMA, ESR, and Raman spectrum. The plasma frequency of CoPc(As6)0.5 is nearly 30 % larger than that of NiPc(AsF6)0.5 . This result indicates the formation of one-dimensional d-band in CoPc(AsF6)0.5 . The plasma edge of Cox Ni1-x Pc(AsF6)0.5 is close to that of NiPc(AsF6)0.5 in spite of the small contents of Ni ions. This means that the overlap of Co dz2-orbital was interrupted by the introduction of non-magnetic Ni ions. It is expected that the magnetic studies of these mixed crystals promote their understanding for this π-d system.i K. Yakushi, H. Yamakado, M. Yoshitake, N. Kosugi, H. Kuroda, T. Sugano, M. Kinoshita,A. Kawamoto, J. Tanaka, Bull Chem. Soc. Jpn. 62, 687 (1989).ii T. Hiejima, K. Yakushi, Solid State Commun. 95, 661 (1995).iii T. Hiejima, K. Yakushi, J. Chem. Phys. 103, 3950 (1995).iv I. Maekawa, F. Takagi, Y. Sakai, N. Tsuda, J. Phys. Soc. Jpn. 56, 2119 (1987).v T. Ogawa, K. Yonemitsu, proceedings of ICSM\u2798, in press