Influence of Meso Substituents on Electronic States of (Oxoferryl)porphyrin π-Cation Radicals

(Oxoferryl)porphyrin .pi.-cation radicals generated from porphyrins substituted at the meso positions with highly electron-withdrawing aryl groups were characterized: tetrakis-5,10,15,20-(2,6-dichlorophenyl)-, 5-(2-chloro-6-nitrophenyl)-10,15,20-tris(2,6-dichlorophenyl)-, and 5-(2,6-dinitrophenyl)-10,15,20-tris(2,6-dichlorophenyl)porphyrin (porphyrins 1-3, resp.). The phys.-chem. properties of the oxidized complexes of 1-3 are compared to those of two (oxoferryl)porphyrin .pi.-cation radical complexes substituted with electron-releasing aryl groups: tetramesitylporphyrin (TMP) and 2-iodotetramesitylporphyrin (2-iodoTMP). While all of the complexes examd. show close correspondence in a no. of spectroscopic parameters, some significant differences were obsd. In contrast to observations for the oxidized complexes of TMP and 2-iodoTMP, the resonance Raman marker bands .nu.2 and .nu.11, which are indicators of symmetry state of porphyrin .pi.-cation radicals of 1-3, do not show the expected downfrequency shifts for oxidn. to compd. I analogs in a2u symmetry states (compds. I are protoheme-contg. peroxidase intermediates that are two oxidn. equiv above the resting ferric state). The upfield hyperfine NMR shifts of the pyrrole .beta.-proton signals of the compd. I analogs of 1-3 are much larger than those for TMP and 2-iodoTMP. These data may be explained by admixt. of some a1u character into the ground state of radical cations of 1-3, consistent with the hypothesis that electron-withdrawing meso substituents lower the energy of the a2u MO, favoring an a1u admixt.