Electronic Absorption Spectra of Dimers of p-Benzosemiquinone Anion and Würster's Cations in Solution

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Abstract
The dimerization of p‐benzosemiquinone anion and Würster's cations in ethanol has been studied by the electronic absorption spectra of these radicals formed by ultraviolet irradiation and chemical oxidation of the corresponding parent molecules. The ion radicals studied are p‐benzosemiquinone anion (SQ−), p‐phenylenediamine cation (PD+), N,N‐dimethyl‐p‐phenylenediamine cation (DMPD+, and N,N,N',N'‐tetramethyl‐p‐phenylenediamine cation (TMPD+). With an ethanolsolution of p‐phenylenediamine bromide (PD+Br−), the concentration dependence of the two absorption maxima at 4650 and 6050 Å (corresponding to the monomer and dimer, respectively) were studied quantitatively. It has been confirmed that the monomer and dimer are in equilibrium in the solution at low temperature. The equilibrium constants were found to be 2200 and 4400 liters mole−1 at − 72° and − 77°C, respectively. The electronic absorption bands of these dimers were interpreted by considering the symmetry properties of the singly excited electronic configurations, on the assumption of a parallel structure possessing D 2h symmetry.