Nature of the External Heavy-Atom Effect on Radiative and Nonradiative Singlet–Triplet Transitions

The nature of the external heavy-atom effect on radiative and nonradiative singlet–triplet transitions in aromatic hydrocarbons has been investigated experimentally and theoretically. From studies of the perturbed phosphorescence of aromatic hydrocarbons in rigid glass and mixed-crystal systems we conclude that: (i) Radiative T1 ↔ S0 transitions are much more susceptible to a heavy-atom effect than are the corresponding radiationless transitions. (ii) The enhancement of the radiative T1 ↔ S0 transitions is due to mixing of excited singlet states associated with the perturber molecule into the triplet state of the perturbed molecule. (iii) The important pertuber states appear to be highly localized on the heavy atom rather than states which are essentially delocalized [e.g., (π, π*) states]. (iv) It does not seem necessary to invoke charge-transfer interactions to account for the enhancement.