Magnetic Study of N,N,N′,N′-Tetramethyl-p-phenylenediamine (Wurster’s Blue) Iodide Cation Radical

One of the Wurster’s blue cation radicals, which is composed of the N,N,N′,N′-tetramethyl-p-phenylenediamine cation and iodine anion moieties, was prepared and its magnetic susceptibility measured from 2.6 to 300 K. The cation radical underwent no phase transition within this temperature range in striking contrast with the first-order phase transition observed in perchlorate and tetrafluoroborate salts, regardless of similar crystallographic structures. In addition, the temperature dependence of the magnetic susceptibility could be well-reproduced by a dimer model: a thermally accessible triplet state lying above a singlet ground state with an energy separation of ΔE⁄k=229 K. This behavior is controversial regarding the crystallographic determination. The unusual magnetic properties of the iodide salt were discussed, together with the crystal structure, ESR and visible absorption data, in comparison with those of the other salts.