PHOTOASSOCIATION AND PHOTOINDUCED CHARGE-TRANSFER IN BRIDGED DIARYL COMPOUNDS .6. INTRAMOLECULAR TRIPLET EXCIMERS OF DICARBAZOLYLALKANES AND THEIR COMPARISON TO AN INTERMOLECULAR TRIPLET EXCIMER OF CARBAZOLE

A time-resolved emission study of intramolecular triplet excimer forma tion has been carried out for dicarbazolylmethane (DCM) and dicarbazol ylpropane (DCP) in fluid solution at room temperature. The triplet exc imer formation was deduced from the comparison of the phosphorescence with the corresponding emission from the intermolecular triplet excime r of carbazole. It has been found that whereas the triplet excimer for mation in DCP is evident in both polar and nonpolar solvents, the exci mer formation in DCM is observed only in polar solvents at longer dela y times. The result indicates that the conformation favored by the tri plet excimer is more readily attainable in DCP than in DCM. The enhanc ement of the triplet excimer formation by polar solvent, which is also observed for carbazole, suggests that the triplet excimers are stabil ized (at least in part) by charge resonance interactions. Comparison o f the temporal characteristics of the normal delayed fluorescence of D CP with those of the corresponding excimer phosphorescence suggests th at the delayed fluorescence at long delay times is produced by bimolec ular annihilation of the intramolecular triplet excimers. This in turn implies that the excited singlet-state species produced by bimolecula r annihilation of the triplet excimers is unstable and rearranges into monomeric (i.e., non-interacting) conformation prior to its decay by emission of radiation.