D. Gust et al., EFFECT OF COORDINATED LIGANDS ON INTERPORPHYRIN PHOTOINDUCED-ELECTRON-TRANSFER RATES, Journal of physical chemistry, 97(51), 1993, pp. 13637-13642
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.