Orientational effects in intramolecular electronic energy transfer in bichromophoric molecules II: Triplet-triplet transfer

Intramolecular triplet energy transfer has been investigated in a series of bichromophoric molecules incorporating a cyclic α-diketone and substituted aromatic ring. The distance as well as the relative orientation between the two chromophores varies as a function of the number of methylene groups in the chains joining them. Previous work with these compounds has shown that intramolecular singlet energy transfer between the aromatic and diketone moieties shows a distance dependence which fits with the exchange mechanism. Phosphorescence of the diketone moiety (including temperature dependence) was used to evaluate the efficiency of triplet energy transfer. Direct excitation of the diketone resulted in weak phosphorescence, indicating that intersystem crossing is inefficient. However, excitation of the aromatic moiety resulted in much more intense phosphorescence as a result of the formation of triplet diketone via triplet energy transfer. Complementariness between singlet and triplet energy transfer was evidenced by the fact that some compounds in which singlet energy transfer was more efficient exhibited weaker phosphorescence and vice versa. Quantitative correlations between the triplet energy transfer efficiency and interchromophore distance were not possible.