Effect of coordinated ligands on interporphyrin photoinduced-electron-transfer rates

The effect of coordination of various pyridines on the rate of photoinduced interporphyrin electron transfer in a molecular dyad consisting of a zinc porphyrin covalently linked to a free base porphyrin moiety has been investigated using time-resolved fluorescence techniques. Coordination of pyridine itself to the zinc ion results in a nearly 30-fold increase in the rate constant for one of the photoinduced-electron-transfer reactions. In similar studies with a series of pyridine ligands bearing substituents with electron-donating or accepting properties, the data correlate well with the substituent constant under the Hammett linear free energy relationship. The reaction constant ρ has a value of -0.35, which indicates that the rate of electron transfer is increased by electron donation to the zinc porphyrin. Electrochemical experiments with a poorly-coordinating electrolyte suggest that this donation stabilizes the interporphyrin charge-separated state, in which the zinc porphyrin moiety is positively charged, and that this stabilization in turn leads to a larger rate constant for electron transfer. Effects of this type undoubtedly play a role in metalloproteins in which a porphyrin metal ion is ligated to the protein. They may also affect the interpretation of solvent effects noted in electron-transfer reactions involving metallated porphyrins.