Vibrational, Electronic, and Structural Properties of Cobalt, Copper, and Zinc Octaethylporphyrin ? Cation Radicals

Article abstract of DOI:10.1021/j100307a016

Optical and resonance Raman (RR) spectroscopic characterization of the oxidation products of several metallooctaethylporphyrins has been carried out.One-electron oxidation of the macrocycle yields a series of divalent metal substituted octaethylporphyrin ? cation radicals, M^IIOEP^.+ClO4^- (M = Ni, Co, Cu, and Zn).The porphyrin core vibrational frequencies above 1450 cm^-1 of these complexes are described by a linear function of center to pyrrole nitrogen distances.A comparison of these structural correlations and of Raman depolarization ratios with those of the parent MOEP compounds is used to established vibrational mode assignments for the cations.The agreement between the correlation parameters of the MOEP and MOEP^.+ClO4^- suggests similar potential energy distributions in the normal modes of both species.We find that the frequencies of the stretching modes with dominantly C_bC_b character increase, whereas those with C_aC_m and C_aN character decrease in the cation radical relative to the neutral metalloporphyrin.Similar trends in Soret band maxima for the ? cation radicals and their parent compounds reflect changes in the relative energy of the a_2u(?) orbital.These structural correlations seem to be essentially insensitive to ²A_2u vs ²A_1u radical designation.With the vibrational mode correlations as a guide to evaluation of porphyrin core geometry, we have carried out a detailed analysis of the oxidation products of Co^IIOEP and we suggest structures for the two-electron-oxidized species Co^IIIOEP^.+2ClO4^- and Co^IIIOEP^.+2Br^-.Differences in the high-frequency vibrations of these two compounds are interpreted in terms of expansion or possible ruffling of the porphyrrin core in the latter relative to the former compound.RR excitation in the 600-680-nm region of the Co^IIIOEP^.+2Br^- absorptions shows a lack of anomalously polarized scattering and produces spectra similar to those obtained with near-UV excitation.This suggests the absence of strong Herzberg-Teller coupling between the excited electronic states of this ? cation radical.