In fluid solutions at room temperature, the high-purity dibenzofuran (DBF) and dibenzothiophene (DBT)
exhibit phosphorescence and delayed fluorescence from monomers as well as intermolecular triplet excimers.
The delayed emissions assigned to the excimer are red shifted with respect to the corresponding emissions
from the monomers. All four delayed emissions exhibit excitation spectra that are identical with the excitation
spectrum of the prompt fluorescence of the corresponding monomer. Consistent with an emission originating
from the bimolecular annihilation of the triplet excimers, the delayed excimer fluorescence is spectrally very
similar to the normal excimer fluorescence of the compounds, and it decays with a lifetime which is one-half
the decay time of the excimer phosphorescence. At longer delay times, where the triplet excimer−monomer
equilibration is expected to occur, the ratio of the intensity of the excimer phosphorescence to that of the
monomer phosphorescence becomes independent of time. Concomitantly, the lifetimes of the two emissions
become identical to each other, as indicated by the very similar lifetimes of the delayed excimer fluorescence
and the delayed monomer fluorescence. These results confirm the formation of intermolecular triplet excimers
in fluid solutions of these compounds.