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

Abstract

Electrochemical oxidation of [M^III(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.