The Electronic Absorption Spectra of Würster’s Cation Radicals and Their Dimerization in Solution

The visible and ultraviolet absorption spectra of the p-phenylenediamine cation (PD+), the N,N-dimethyl-p-phenylenediamine cation (DMPD+), and the N,N,N′,N′-tetramethyl-p-phenylenediamine cation (TMPD+) in the presence and in the absence of an excess counter anion (X−=Br−, Cl−, and ClO4−) are studied at low temperatures. Analyses of the data show that these cation radicals are in an equilibrium represented by 2R++X−++2X− (or, more exactly, 2R++0.7X−++2(X−)0.7). It is found that the absorption maxima of the dimer and the value of the heat of dimerization are independent of the kind of counter anion. It is concluded that PD+ and DMPD+ form strong dimers (heat of dimerization, ΔH=−8.0 and −8.2 kcal/mol respectively) and that TMPD+ forms a relatively weak one (ΔH=−5.6 kcal/mol). The features of the dimer spectra reflect the strength of the interaction of the radical in the dimer. In a strong dimer, the spectrum in the low-frequency region consists of a strong intermolecular charge-transfer band (C band) and a weak local excitation band (R′ band), and the frequency shifts of the local excitation bands in the dimer (R′ and Y′ bands) from corresponding bands in the monomer (R and Y) are large. On the other hand, the spectrum in the weak dimer consists of relatively weak C and strong R′ bands, and the shifts of the R′ and Y′ bands are small. A model of the dimer is proposed in which the two cations are arranged in a parallel manner and are surrounded by the ionic atmosphere of the counter anions.