A Systematic Investigation of the Effects of Axial Ligands and Peripheral Substituents on the Electronic Structures of Zinc(II) Tetraphenylporphyrin ?-Cation Radicals: Electrochemical EPR and Spectroscopic Observation

Article abstract of DOI:10.1246/bcsj.61.2753

A detailed study of the effect of axial ligand and peripheral substituent on the ?-system of zinc(II) tetraphenylporphyrin ?-cation radical has been made using an electrochemical EPR technique.It is confirmed from the EPR spectra that most of the anions and bases examined coordinate the radicals axially.The spin density on the nitrogen nuclei in the radicals increases with the polarizability of the axial ligand estimated from the optical spectra of the parent neutral complexes.For the fourteen complexes with different substituents on the phenyl groups examined here, there is a linear correlation between the Hammett substituent constant and the spin density.It is also shown that the para position of the phenyl group carries the spin density comparable to that of the ortho position, which indicates considerable conjugation between the porphyrin ring and the phenyl groups.In most cases, there exist two kinds of radical species which give rise to broad and sharp EPR spectra at low temperature.It is suggested that the difference in the motional states of the phenyl groups in the radicals is responsible for the different EPR spectra.