Triplet triplet absorption of biphenyl and related compounds

Triplet triplet absorption spectra of biphenyl, carbazole and phenanthrene have been studied in rigid glass at 77 K. Oscillator strengths and polarizations are reported. A theoretical study has been performed using the Pariser-Parr-Pople approximation in order to support the band assignments.Phenanthrene is shown to be definitely not related to the other members of the series. Triplet-triplet oscillator strengths are decreased in carbazole, as compared to biphenyl. This effect is discussed in terms of a possible dilution of the ground triplet, in addition to the spectral dilution of the upper triplet states, as previously observed in derivatives of alternant molecules.