ELUCIDATION OF FACTORS AFFECTING THE ELECTRONIC-STRUCTURES OF MAGNESIUM(II) AND ZINC(II) TETRAARYLPORPHYRIN RADICAL CATIONS

A series of magnesium acid zinc tetraarylporphyrins and their porphyri n-oxidized derivatives were studied by UV/Vis, ESR, and resonance Rama n spectroscopy at various temperatures. The series included tetra(meta -dichlorophenyl)porphyrinatozinc (5), terra(ortho-dichlorophenyl)porph yrinato-zinc (6), tetra(ortho-difluorophenyl)porphyrinato-zinc and -ma gnesium (9 and 10), and tetra(pentafluorophenyl)porphyrinatozinc and - magnesium (7 and 8). The radical cations (3a-10a) were isolated by che mical one-electron oxidation of their neutral precursors (3-10). Despi te the structural similarity of all these radicals, their electronic g round state varied within the series. The position of the chloro group s was found to play a key role. While the radical cation of the meta-d ichloro-substituted derivative 5a exhibited A(2u) spectroscopic featur es, the ortho-dichlorophenyl derivative (6a) showed A(1u) spectral fea tures. Radicals of the fluoro-substituted porphyrins, especially that of 10, were found to have state-admixed (A(1u)/A(2u)) electronic struc tures, and the relative contributions of the two states was found to v ary with temperature and to depend on the axial ligand. The results in dicate that the fluoro-substituted porphyrins are primarily A(2u) at l ow temperature, even though their room temperature spectroscopic featu res resemble those of A(1u) cations, The elucidation of factors that a ffect the electronic structures of the radicals in the present series is helpful in providing a greater understanding of the spin-spin inter actions in the intermediates of heme-dependant enzymatic reactions and their synthetic analogues.