INTRAMOLECULAR PHOTOCHEMICAL ELECTRON-TRANSFER .7. TEMPERATURE-DEPENDENCE OF ELECTRON-TRANSFER RATES IN COVALENTLY LINKED PORPHYRIN AMIDE QUINONE MOLECULES

Intramolecular rate constants k(ET) for electron transfer (ET) from a porphyrin excited singlet S1 state to a linked quinone acceptor have been measured as a function of temperature in several solvents for the molecule PAQ [5,10,15-tritolyl-15-(4-carboxylphenyl)porphine linked to p-benzoquinone via an amide group]. Most of the data could be analyzed successfully using the high-temperature form of the semiclassical Marcus equation. The analysis of temperature dependence data allows the separation of the electronic and nuclear factors in the Marcus equation. The results indicate that both factors are solvent dependent. The electronic factor, involving the electronic coupling energy H(rp), is found to have larger values in polar solvents and smaller values in most nonpolar solvents. On the other hand, the nuclear factor is not found to correlate with solvent polarity. The analysis of the temperature dependence of k(ET) provides a plausible explanation for the scatter found in a former analysis of solvent effects for photoinduced electron transfer in PAQ.