We have investigated intramolecular photoinduced charge separation and recombination in a series of cyclophanebridged porphyrinquinone systems by means of time−resolved fluorescence decay measurements. Rates of charge separation have been determined as a function of the free energy change of the reaction, of the polarity of the solvent, and of the temperature. In some systems a long−lived fluorescence is observed which is attributed to a thermal repopulation of the initially excited state from the charge transfer state. This delayed fluorescence allows the calculation of the rate of recombination in these cases. The observation of delayed fluorescence for a particular donor−acceptor compound in some solvent serves as a reference for the reaction free energy of the respective charge separation (AG, N 0 eV). The free energy change in other systems is estimated by correcting for differences in the redox potentials of the respective porphyrins and quinones. Electronic couplings and reorganization energies are determined by globally fitting standard rate expressions as a function of the free energy change to the experimental rate data. Three different kinds of fits are performed by (a) using both charge separation and recombination within the nonadiabatic approximation, (b) allowing for Landau−Zener adiabaticity corrections, and (c) fitting rates of charge separation (in the normal region) only. A particular focus lies in the specific effects imposed by the compact structure of the porphyrin−uinone cyclophanes. It is shown that electron transfer in these systems is nonadiabatic and dominated by intramolecular reorganization whereas the influence of the surrounding solvent is minimized by the close packing of electron donor and accepto

The singlet-triplet splitting J of the radical pair P+H-in the photosynthetic reaction center critically depends on the energy separation between P+H-and the triplet state 3P*. Due to its large dipole moment, the energy of P+H-can be considerably shifted by the application of an external electric field. Following a simple perturbation theoretical approach, such a shift should enlarge J to values which do not allow singlet-triplet-mixing in PH-, thereby inhibiting the triplet recombination channel P+H--c 3P*. In contrast to this prediction, the recombination dynamics of P+H-and the yield of 3P* in reaction centers of Rb. sphaeroides were found to change only slightly in an external electric field. This insensitivity of the recombination dynamics to the energy of P+H-is predicted in an extended theoretical treatment of J taking into account nuclear oscillations. Following the extended treatment, J is much less sensitive to the energy of P+H-. This has important consequences for the discussion of J in the context of primary charge separation and also for the values of J to be expected in reaction centers in which the energetics have been altered by chemical or genetic modifications.

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