Mixed-Valence Cations of Di(carbazol-9-yl) Biphenyl, Tetrahydropyrene, and Pyrene Derivatives

Although bis(diarylamino) mixed-valence radical cations have been quite extensively studied, their bis(carbazolyl) analogues have not, even though the hole-transporting properties of species such as of 4,4′-bis(9H-carbazol-9-yl)-1,1′-biphenyl, CBP, are widely exploited in organic light-emitting diodes. This work reports the generation by chemical oxidation of the radical cations of 4,4′-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-1,1′-biphenyl (a model for the unstable radical cation of CBP), 2,7-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-4,5,9,10-tetrahydropyrene, and 2,7-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)pyrene. The visible and near-IR spectra of these cations have been compared to those of the corresponding dication spectra, to the spectrum of the 3,6-di-tert-butyl-9-(4-(tert-butyl)phenyl)-9H-carbazole radical cation, and to the results of time-dependent density-functional calculations. The biphenyl-and pyrene-bridged species are found to be localized (class-II) mixed-valence compounds, whereas stronger coupling between the redox centers in the tetrahydropyrene-bridged radical cation results in a delocalized (class-III) species. For all three radical cations, the electronic couplings are lower than those obtained for delocalized 4,4′-bis(diarylamino)-1,1′-biphenyl radical cations.