Disjoint and coextensive diradical diions

Abstract

N,N,N′,N′- Tetraanisyl-3,3′-diaminobiphenyl and N,N,N′,N′-tetraanisyl- 3,4′-diaminobiphenyl have been synthesised and their oxidations studied by cyclic voltammetry and EPR spectroscopy. A two-electron oxidation of the former species gives a π system which is disjoint and oxidation of the latter gives a π system which is coextensive. Cyclic voltammetry is shown to be a useful tool for revealing the difference between the disjoint and coextensive π systems. The coulombic contribution to the difference between the half-wave potentials for the first and second oxidation steps (ωE), which measures the ‘hole–hole’ repulsion energy, was extracted by measuring ωE as a function of the relative permittivity of the medium. This was substantially greater in the coextensive case. Both diradical diions gave triplet EPR spectra. A comparison of the cyclic voltammetry of N,N,N′,N′-tetraanisyl- 3,4′-diaminobiphenyl and that of a high-spin polymer precursor with the same local topology showed that, in the oxidised polymer, the additional hole–hole repulsion energy associated with the coextensive spin–spin distribution is of the order of 0.1–0.2 V—a noticeable effect but not one large enough to explain the difficulties that have been encountered in achieving high doping levels in this and related high-spin polymers.

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