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

A series of (oxoferryl)porphyrin pi-cation radicals generated from porphyrins substituted at the meso positions with highly electron-withdrawing aryl groups has been 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)porphyrins (porphyrins 1-3, respectively). The physical-chemical 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 examined show close correspondance in a number of spectroscopic parameters, some significant differences were observed. 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 oxidation to compound I analogs in a(2u) symmetry states. The upfield hyperfine NMR shifts of the pyrrole beta-proton signals of the compound I analogs of 1-3 are much larger than those for TMP and 2-iodoTMP. These data may be explained by admixture of some a(1u) 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 a(2u) molecular orbital, favoring an a(1u) admixture.