PHOTOCHEMICAL, ELECTROCHEMICAL, AND CHEMICAL FORMATION OF THE PI-CATION-RADICAL SPECIES OF MAGNESIUM PHTHALOCYANINE - ANALYSIS OF THE ABSORPTION AND MCD SPECTRA OF [MGPC(-1)].+

Photochemical, electrochemical, and chemical methods have been used to form the ring-oxidized, pi-cation-radical species of magnesium phthalocyanine, [MgPc(-1)].+. Photooxidation of MgPc(-2) was carried out by excitation with visible region light into the phthalocyanine Q band at 670 nm, using carbon tetrabromide as an irreversible electron acceptor. The radical cation was also formed by controlled potential coulometry at the first oxidation potential (+0.70 V vs SCE) of MgPc(-2) and by chemical oxidation with nitric acid. Absorption and magnetic circular dichroism (MCD) spectra are reported between 250 and 850 nm for [(L)(n)MgPc(-1)].+ (n = 1,2) pi-cation-radical species, where L = imidazole, 4-methylimidazole, pyridine, methylpyridine, or cyanide. The first low-temperature absorption and MCD spectra (at (200 K) of [MgPc(-1)].+ are reported, under which conditions it is shown that the cation-radical species dimerizes completely. Electron paramagnetic resonance (EPR) spectra confirm that at low temperatures the unpaired spins couple. At elevated temperatures (> 300 K) in dichloroethane, the monomer predominates. Band deconvolution calculations, which couple fits for absorption and MCD spectra, are reported for each of the radical-cation species. These results are compared to previous band assignments and the deconvolution results for [ZnPc(-1)].+ reported by Nyokong, Gasyna, and Stillman (Inorg. Chem. 1987, 26, 1087-1095). At room temperature, the spectra arise from a mixture of monomers and dimers. For [(im)(n)MgPc(-1)].+, nine well-resolved transitions are centered near 825 nm (12 100 cm-1), 717 nm (13 900 cm-1), 507 nm (19 700 cm-1), 422 nm (23 700 cm-1), 372 nm (26 900 cm-1), 319 nm (31 300 cm-1), 276 nm (36 300 cm-1), 266 nm (37 700 cm-1), and 255 nm (39 200 cm-1). Major band maxima in the experimental spectral data of the monomer are located at 825, 505, 411, 387, and 328 nm. Major band maxima in the spectra of the dimer (measured at 200 K) are located at 712, 510, 370, 317, and 277 nm. Deconvolution of the absorption and MCD spectra indicate that all but the 507-nm absorption band arise from degenerate transitions. Calculations provide band centers at 826 (Q), 507, 413 (B1), 387 (B2), 330, 315, and 277 nm for the monomer and at 717 nm (Q), 507, 422 (B1), 372 (B2), 319, 276, and 255 nm for the dimer. The 507-nm band is assigned to a nondegenerate transition from a low-lying molecular orbitals into the half-filled a1u(pi) molecular orbital. Analysis of these data identify transitions specifically related to the 2A1u ground state of the ring, allowing comparisons to be made with the spectral data from [ZnTPP(-1)].+, a species with the 2A2u ground state.