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 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 (Delta G(cs) similar or equal to 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-quinone 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 acceptor.