ENERGY-GAP AND TEMPERATURE-DEPENDENCE OF PHOTOINDUCED ELECTRON-TRANSFER IN PORPHYRIN-QUINONE CYCLOPHANES

We have investigated intramolecular photoinduced charge separation and recombination in a series of cyclophane-bridged porphyrin-quinone sys tems' by means of time-resolved fluorescence decay measurements. Rates of charge separation have been determined as a function of the free e nergy change of the reaction, of the polarity of the solvent, and of t he 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 allow s the calculation of the rate of recombination in these cases. The obs ervation of delayed fluorescence for a particular donor-acceptor compo und in some solvent serves as a reference for the reaction free energy of the respective charge separation (Delta G(cs) similar or equal to 0 eV). The free energy change in other systems is estimated by correct ing for differences in the redox potentials of the respective porphyri ns 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 differ ent kinds of fits are performed by (a) using both charge separation an d recombination within the nonadiabatic approximation, (b) allowing fo r Landau-Zener adiabaticity corrections, and (c) fitting rates of char ge separation (in the normal region) only. A particular focus lies in the specific effects imposed by the compact structure of the porphyrin -quinone cyclophanes. It is shown that electron transfer in these syst ems is nonadiabatic and dominated by intramolecular reorganization whe reas the influence of the surrounding solvent is minimized by the clos e packing of electron donor and acceptor.