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

Intramolecular electronic energy transfer was observed in bichromophor ic molecules with the general structure donor-spacer-acceptor where in dole is the donor, naphthalene is the acceptor, and cyclohexane is the inert spacer. Measurements were performed in a supersonic jet where b oth the absorption and emission spectra of the two chromophores were w ell-resolved and indole could be selectively excited with essentially no excitation on naphthalene. The emission produced on excitation of i ndole showed strong naphthalene emission and relatively weak indole em ission. Moreover, the fluorescence lifetime was similar to the fluores cence lifetime of naphthalene, indicating that the energy transfer occ urred on a time scale faster than the fluorescence lifetime of the don or. Energy transfer was observed in several conformers of the bichromo phore, but the rates were similar for each. The relative energy-transf er rates of various vibronic levels was determined by measuring the ra tio of the naphthalene and indole emission intensities. The energy-tra nsfer rate was found to be fairly constant as a function of vibrationa l state until the vibrational levels of indole became isoenergetic wit h the naphthalene S2 state. From this point, the rate increased and th en decreased with greater excitation energy. A modification of the the ory of mediated intersystem crossing was used to account for this effe ct.