Intramolecular energy transfer in rigid model compounds. Singlet and triplet transfer between cyclopentenone and phenanthrene

Abstract

Through synthesis, phenanthrene, and cyclopentenone have been attached to a rigid spiro-cyclopropane–norbornane molecular frame. The energy levels and spectroscopic properties of the two chromophores are appropriate for studying both singlet–singlet and triplet–triplet energy transfer. When separately attached to the same frame, cyclopentenone exhibits easily detectable phosphorescence. Spiro-linked phenanthrene emits efficiently from both the S₁ and T₁ states. In the model compound the phenanthrene fluorescence is substantially diminished relative to its phosphorescence and the cyclopentenone T₁→S₀ emission has vanished. These results are consistent with efficient (but not total) singlet–singlet energy transfer from phenanthrene to cyclopentenone followed by back transfer from the triplet state of cyclopentenone to the lowest triplet of phenanthrene. The rate constants for singlet and triplet transfer were evaluated to be about 5 × 10⁹ and 115 s^–1, respectively. Approximate calculations of expected transfer rates agree with presumed coulomb and exchange interaction at the singlet level and exchange transfer at the triplet level.