Localization versus delocalization in diamine radical cations

The optical absorption spectrum of the radical cation of 1,4-diphenylpiperazine 2a shows a strong transition in the near-IR, and only a weak band at 445 nm, in the region where aniline radical cations normally absorb strongly. This indicates that the charge and spin are delocalized over the two equivalent aniline moieties. Introduction of a 4-methoxy group on the aromatic ring allows increased stabilization of the radical ion character on one moiety, and charge delocalization across the piperazine ring is suppressed, as shown by optical absorption and resonance Raman spectroscopy. The possibility of coexistence of localized and delocalized radical cations of diphenylpiperazine and its derivatives cannot be ruled out at present. If that were the case, the effect of the methoxy substituents is to shift the equilibrium towards the localized species.