REVERSIBLE TRIPLET TRIPLET ENERGY-TRANSFER WITHIN A COVALENTLY LINKED BICHROMOPHORIC MOLECULE

The localized photoexcitation of a Ru(II) polypyridine complex covalently attached by a flexible link to pyrene results in a rapid intramolecular transfer of triplet energy with a forward rate constant of (1.3 +/- 0.3) x 10(8) s-1 in deaerated methanol solution. On account of the energetic proximity of the states, reverse transfer occurs with a rate constant lower by a factor of 18 +/- 3. This equilibration is rapid compared with the relaxation processes that are deactivating the system, which have a common lifetime of 11.2 +/- 0.4-mu-s. These results may be the first direct observation of an equilibrium being reached in an intramolecular triplet energy transfer reaction.