The magnetoresistance effect in a conducting molecular crystal consisting of dicyano(phthalocyaninato)manganese(iii)

Abstract

A conducting molecular crystal TPP[Mn^III(Pc)(CN)₂]₂ (Mn^III: d⁴, S = 1, TPP = tetraphenylphosphonium and Pc = phthalocyanine) was fabricated. In its crystal structure, the [Mn^III(Pc)(CN)₂] units formed a one-dimensional regular chain along the c-axis with an overlap integral value of 8.6 × 10⁻³. TPP[Mn^III(Pc)(CN)₂]₂ showed a semiconducting behaviour that also has been observed for isostructural TPP[Co^III(Pc)(CN)₂]₂ (Co^III: d⁶, S = 0) and TPP[Fe^III(Pc)(CN)₂]₂ (Fe^III: d⁵, S = 1/2) whose ground states are charge-ordered states. In spite of the local magnetic moment of the Mn^III ion (S = 1) at the centre of the Pc ligand, TPP[Mn^III(Pc)(CN)₂]₂ exhibited an almost isotropic and small negative magnetoresistance (MR) effect (the MR ratio was −8.7% under 8 T at 10.7 K), contrarily to the anisotropic giant negative MR effect of TPP[Fe^III(Pc)(CN)₂]₂. The isotropy was found to be due to a (d_xz)¹(d_yz)¹ electronic configuration, and the smaller MR effect was explained by a weaker antiferromagnetic interaction between Mn^III ions than that between Fe^III ions, as suggested by a Weiss temperature Θ of −3.1 K (|J_dd|/k_B = 1.2 K).