Gas-phase intramolecular electron transfer was studied in molecules of the type D-SgA where D was a positive charge donor, A was a positive charge acceptor, and Sp was a rigid, inert spacer. In our experiments, the donor was always naphthalene, the acceptors were benzene, indole, and biphenyl, and the spacers were cyclohexane and androstane. The naphthalene chromophore was selectively ionized by means of resonantly enhanced two-photon ionization via the naphthalene SI state.The location of the charge was monitored by observing the resonantly enhanced multiphoton dissociation spectrum of the ion. When the charge was localized on the naphthalene, the naphthalene ion D2 -Do spectrum was observed as an action spectrum (ion yield at a given mass versus wavenumber). When the charge had transferred to the acceptor, the naphthalene ion spectrum was not observed. Using this technique we attempted to measure the charge-transfer lifetime by measuring the intensity of the naphthalene ion action s p t r u m as a function of the delay between the ionizing and dissociating lasers. We found that in all cases where charge transfer was thermodynamically possible, the charge-transfer lifetime was less than 2 ns, the time resolution of the experiment.
IntroductionIn the last decade charge transfer in bichromophores consisting of a donor and an acceptor separated by a rigid spacer has been actively studied both theoretically3 and e~perimentally.~~~ Charge-transfer rates are not diffusion limited in these systems as they are in intermolecular systems. Intramolecular charge transfer also serves as a good model for biological charge transfer. So far all intramolecular chargetransfer studies have been camed
The free energy and entropy changes associated with intermolecular triplet energy transfer (TT) processes were determined by direct measurement of the equilibrium constant using a laser flash photolysis technique. 10,10-Dimethylanthrone and 9,9-dimethylfluorene were synthesized and used as a "rigid" donor and acceptor, respectively, in comparison with 4-methylbenzophenone, a "nonrigid" donor, and 4-methylbiphenyl, a "nonrigid" acceptor, in the TT reactions. Both the nonrigid donor and the nonrigid acceptor lose entropy while going from the twisted ground state to the planar triplet state. The AS values for 4-methylbenzophenone and 4-methylbiphenyl are 0.5-1.0 and 2.5-3.0 gibbs/mol, respectively.
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.
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