Intramolecular singlet-singlet and triplet-triplet energy transfer in adamantyl-linked trichromophores

Intramolecular singlet-singlet energy transfer (SSET) has been observed in 4-(3-(x-phenanthryl)-1-adamantyl)-4'-(3-(2-naphthyl)-1-adamantyl)biphenyl (3), a trichromophoric molecule consisting of phenanthrene, biphenyl, and naphthalene groups linked sequentially by adamantane bridges in which chromophore attachment is at the tertiary 1- and 3-adamantyl positions. W-visible absorption, steady-state and time-resolved fluorescence, low-temperature phosphorescence and room-temperature laser flash photolysis measurements indicate that efficient SSET takes place with equal probability from the central biphenyl group to each of the terminal chromophores with a rate constant, k > 6 x 10(10) s(-1). Slower SSET from the naphthyl chromophore to the phenanthryl group occurs with a rate constant k similar to 9 x 10(6) s(-1). The experimentally determined SSET efficiency and a calculation of the critical Forster distance, when combined with molecular modeling, indicate that a Forster mechanism is sufficient to account for the observed SSET process. Intramolecular triplet-triplet energy transfer (TTET) from the phenanthryl group to the naphthyl chromophore appears to occur by a slow, thermally activated transfer step from the phenanthrene ring to the central biphenyl group followed by rapid exergonic transfer to the naphthyl group. TTET in the reverse direction involving thermal activation of the naphthyl triplet also apparently may take place.