Time-Resolved Emission Studies of Intermolecular Triplet Excimers in Fluid Solutions: Dibenzofuran and Dibenzothiophene Revisited

Abstract

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.