The photophysical properties of an oblique bisporphyrin, comprising zinc(I1) and gold(II1) porphyrins separated by a 2,7-diphenyl-1 ,lo-phenanthroline spacer moiety, have been measured in a low-temperature ethanol glass. Comparison with the properties recorded earlier in fluid, polar solution indicates that the energy of the intermediate charge-transfer state (CTS), formed by electron transfer from zinc porphyrin to appended gold porphyrin, is increased by 0.75 eV upon freezing. This is due to a substantial decrease in the static dielectric constant of the solvent which occurs upon moving from a fluid solution to a frozen glass. There is a corresponding decrease in the solvent reorganization energy upon freezing, and, at 77 K, both the reorganization energy and the reaction exergonicity are essentially zero. The rate of formation of the CTS at 77 K is much slower than that predicted for a superexchange mechanism unless there is a change in the molecular architecture upon freezing. Formation of an intermediate C T S has been confirmed by low-temperature EPR studies. The gold porphyrin excited triplet state is unable to abstract an electron from the appended zinc porphyrin, and, instead, quantitative intramolecular triplet energy transfer takes place.One of many interesting features of the photosynthetic bacterial reaction center complex concerns the rapid electron-transfer process which occurs between porphyrinic species within a protein matrix.'" The rate of this reaction shows little dependence on temperature: and, although the mechanism has been subjected to close s c r~t i n y ,~-~ the exact reaction pathway has still to be resolved. Numerous model systems have been constructed which mimic the rapid photoinduced electron-transfer reaction in fluid solution,8-10 but much less attention has been given to studying such artificial systems at low temperature or within solid matrices. Where studies have been made,li-lS it has often been found that electron transfer is inhibited in the solid state. This has been explained in terms of a thermodynamic effect in which the solid matrix destabilizes the ion pair and, thereby, reduces the reaction exergonicity for photoinduced charge separation. Indeed, Wasielewski and co-workersi1 have conducted a comprehensive investigation of this effect and have found that, for a large series of covalently linked porphyrin-quinone compounds, the reaction exergonicity is lowered by about 0.8 eV in a frozen glass relative to a fluid, polar solution. This is a far reaching conclusion since a practical photochemical appliance will most likely be a solidstate device and, in order to produce efficacious model systems, particular attention must be given to quantifying the medium effects on reorganization energies, reaction exergonicities, and electronic coupling between reactants.In previous publication^,^"^^ we have examined in detail the mechanism and kinetics for intramolecular electron-and energytransfer processes that follow upon laser excitation of a bisporphyrin comprised of zinc(I1) a...
