Magnetic Properties and Relaxation Processes in Manganese (III)-Porphyrin-TCNE-Systems

Abstract

One-dimensional magnetic systems aroused always a great interest. It has been known for a long time that magnetic chains cannot magnetically order at temperatures above 0 K. Finite critical temperatures of transition into the ordered state were usually interpreted as a result of weak interchain magnetic interactions. Porphyrin based molecular magnets [MnR₄TPP] [TCNE] ·solvent have a chain structure composed of manganese(III) porphyrin discs MnTPP (TPP = tetraphenylporphyrin) alternating with TCNE⁻ radical (tetracyanoethylene), being the bridge between two Mn^III-ions, see Fig. 8.1. Substituents R are attached at the disc peripheries, while solvent molecules are located in free space between chains. Up to date only a few substances of this family have been structurally characterized (with R = H or Cl) [1, 2]. A number of members of the porphyrin family were for the first time synthesized in our group with the following R: OCH₃, OC₁₀H₂₁, OC₁₂H₂₅, OC₁₄H₂₉, ortho-F, para-F, CN, CF₃ [3] (ortho and para differentiate place of attachment). Hereafter the abbreviation Mn-R-TCNE for their chemical formula will be used. It is known from X-ray investigations at room and higher temperatures that the substances Mn-OC₁₀H₂₁-TCNE, Mn-OC₁₂H₂₅-TCNE and Mn-OC₁₄H₂₉-TCNE form a hexagonal columnar liquid crystalline phase [4], in which the MnR₄TPP-group and the TCNE-radical are only partially ordered. With decreasing symmetry there is a phase transition into the crystalline phase, and the lattice symmetry is expected to underlie a distortion.