ULTRAFAST PHOTOINDUCED ELECTRON-TRANSFER IN RIGID PORPHYRIN-QUINONE DYADS

Three dyad molecules, each consisting of a porphyrin (P) linked to a q uinone (Q) through a rigid bicyclic bridge, have been prepared, and th eir photochemistry has been investigated using time-resolved fluoresce nce and absorption techniques. In all three molecules, photoinduced el ectron transfer from the porphyrin first excited singlet state to the quinone occurs with rate constants of similar to 10(12) s(-1) in solve nts ranging in dielectric constant from similar to 2.0 to 25.6 and at temperatures from 77 to 295 K. The transfer rate is also relatively in sensitive to thermodynamic driving force changes up to 0.4 eV. This be havior is phenomenologically similar to photosynthetic electron transf er. The rapid rate of photoinduced electron transfer and its lack of d ependence on environmental factors suggests that transfer is governed by intramolecular vibrations. Charge recombination of P-.+-Q(.-), on t he other hand, is substantially slower than charge separation and sens itive to both driving force and environmental conditions. Thus, by cha nging conditions, charge recombination rates can be varied over a wide range while photoinduced electron transfer rates are relatively unaff ected. This suggests that rigid dyads of this general type may be usef ul building blocks for more complex molecular devices.