Time-resolved emission studies of intermolecular triplet excimers in fluidsolutions: Dibenzofuran and dibenzothiophene revisited

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 emis sions assigned to the excimer are red shifted with respect to the correspon ding emissions from the monomers. all four delayed emissions exhibit excita tion spectra that are identical with the excitation spectrum of the prompt fluorescence of the corresponding monomer. Consistent with an emission orig inating from the bimolecular annihilation of the tripler excimers. the dela yed excimer fluorescence is spectrally very similar to the normal excimer f luorescence of the compounds, and it decays with a lifetime which is one-ha lf the decay time of the excimer phosphorescence. At longer delay times, wh ere the triplet excimer-monomer equilibration is expected to occur, the rat io of the intensity of the excimer phosphorescence to that of the monomer p hosphorescence 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 mono mer fluorescence. These results confirm the formation of intermolecular tri plet excimers in fluid solutions of these compounds.