Electron-transfer processes in solution are among the most important reactions in chemistry and biology. The huge number of redox reactions of transition metal ions and complexes, many preparatively important oxidations and reductions of organic compounds, photosynthesis. and metabolism are only a few examples where electron-transfer reactions play a pivotal role. This ubiquity, as well as their relative simplicity, makes them excellent models for the study on a molecular level of chemical reactions in solution. A particularly important question in chemical reaction dynamics in solution is the influence of the solvent on the reaction rate. In this context one distinguishes between static and dynamic solvent effects. Static effects refer to the stabilization of reactants, transition state, and products, that is, how the solvent affects the free energies of these species and the energy of activation. This interpretation of solvent effects on all kinds of chemical reactions is well established. A more recent development is the investigation of the influence of solvent dynamics on the rate of a reaction. The transfer of an electron is usually thought to be triggered by a fluctuation of the dielectric polarization in the surrounding solvent. The dynamics of such fluctuations is determined by the finite response time of the orientational polarization of the solvent. Under certain conditions this dielectric response time can become the rate-determining factor of the reaction. In this article I intend to give a review of these modern developments in the theory and experimental study of electron-transfer processes. We shall see that solvent dynamics may lead to a whole plethora of phenomena in reaction dynamics. The concepts needed for their description are not limited to electron transfer but bear relevance to many other chemical reactions in solution.

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The rate of charge separation in a series of cyclophane-bridged Zn-porphyrin-quinone systems has been investigated in nonpolar and polar solvents by means of fluorescence upconversion. In all systems with driving forces in the range 0.3-1.3 eV, ultrafast charge separation occurs with a rate constant of (2-5) × 10 12 s -1 . In combination with previous investigations on free-base porphyrin-quinone systems the driving force dependence can be probed from the (slightly endoergic) normal to the moderately inverted region for the rate of charge separation alone. The (more limited) data for charge recombination in these systems are reasonably well reproduced by the same reorganization energies and electronic couplings that result from the analysis of the charge separation. The data allow, for the first time, a satisfactory quantum mechanical analysis of the driving force dependence in porphyrin-quinone systems of the charge separation alone and, consequently, the testing of the assumption on the comparability of charge separation and recombination by experimental means.

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