Ligand-Assisted Electron Transfer from the Triplet State of Zinc Tetraphenylporphyrin to 1,4-Benzoquinone

Article abstract of DOI:10.1021/j100346a049

Effects of axial ligands on the electron transfer from excited triplet state of zinc tetraphenylporphyrin (ZnTPP) to benzoquinone (BQ) have been studied by usung a laser flash technique.For the reaction of pyridinate complex of ZnTPP (Py-ZnTPP) as well as ZnTPP, quantum yield of the porphyrrin cation radicals resulting from the electron transfer, Φ_ion, were determined in various solvents of different dielectric constant (ε).Remarkable enhancement in Φ_ion due to axial ligation of pyridine has been found in the solvents having relatively moderate dielectric constant (9<ε<18).A comparison of the ε dependence of Φ_ion for the reaction of the triplet ZnTPP to that for the triplet Py-ZnTPP has demonstrated that the axial pyridine causes partial neutralization of the effective charge of Py-ZnTPP cation radical.When the axial pyridine is replaced by several kinds of the substituted pyridines having different pK_a, chloride ion, or ethanol, a significant change in Φ_ion is revealed in dichloroethane.The change in Φ_ion was found to depend on the ability of the ligand to transfer negative charge toward the porphyrin ring via the zinc ion.The ligand ability should cause reduction of effective charge on the porphyrin ring moiety of the ligand-bound ZnTPP cation radical which interacts with the BQ anion radical.Cosequently it has been concluded that the enhancement in Φ_ion due to axial ligation is ascribed to the reduction of the effective charge of the cation radical which results in easier separation of the cation radical from its complex paired with BQ anion radical.