EFFECT OF COORDINATED LIGANDS ON INTERPORPHYRIN PHOTOINDUCED-ELECTRON-TRANSFER RATES

The effect of coordination of various pyridines on the rate of photoin duced 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. Coo rdination df pyridine itself to the zinc ion results in a nearly 30-fo ld increase in the rate constant for one of the photoinduced-electron- transfer reactions. In similar studies with a series of pyridine ligan ds bearing substituents with electron-donating or -accepting propertie s, the data correlate well with the substituent constant under the Ham mett linear free energy relationship. The reaction constant rho 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 d onation stabilizes the interporphyrin charge-separated state, in which the zinc porphyrin moiety is positively charged, and that this stabil ization in turn leads to a larger rate constant for electron transfer. Effects of this type undoubtedly play a role in metalloproteins in wh ich a porphyrin metal ion is ligated to the protein. They may also aff ect the interpretation of solvent effects noted in electron-transfer r eactions involving metallated porphyrins.