MEDIATED ELECTRONIC-ENERGY TRANSFER - EFFECT OF A 2ND ACCEPTOR STATE

Intramolecular electronic energy transfer was observed in bichromophoric molecules with the general structure donor-spacer-acceptor where indole is the donor, naphthalene is the acceptor, and cyclohexane is the inert spacer. Measurements were performed in a supersonic jet where both the absorption and emission spectra of the two chromophores were well-resolved and indole could be selectively excited with essentially no excitation on naphthalene. The emission produced on excitation of indole showed strong naphthalene emission and relatively weak indole emission. Moreover, the fluorescence lifetime was similar to the fluorescence lifetime of naphthalene, indicating that the energy transfer occurred on a time scale faster than the fluorescence lifetime of the donor. Energy transfer was observed in several conformers of the bichromophore, but the rates were similar for each. The relative energy-transfer rates of various vibronic levels was determined by measuring the ratio of the naphthalene and indole emission intensities. The energy-transfer rate was found to be fairly constant as a function of vibrational state until the vibrational levels of indole became isoenergetic with the naphthalene S2 state. From this point, the rate increased and then decreased with greater excitation energy. A modification of the theory of mediated intersystem crossing was used to account for this effect.