Photochemical, electrochemical, and chemical formation of the .pi.-cation-radical species of magnesium phthalocyanine. Analysis of the absorption and MCD spectra of [MgPc(-1)].bul.+

Abstract

Photochemical, electrochemical, and chemical methods have been used to form the ring-oxidized, {pi}-cation-radical species of magnesium phtalocyanine (MgPc({minus}1)){sup {center dot}+}. Photooxidation of MgPc({minus}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({minus}2) and by chemical oxidation with nitric acid. Absorption and magnetic circular dichroism (MCD) spectra are reported between 250 and 850 nm for ((L){sub n}MgPc({minus}1)){sup {center dot}+} (n = 1,2) {pi}-cation-radical species, where L = imidazole, 4-methylimidazole, pyridine, methypyridine, or cyanide. The first low-temperature absorption and MCD spectra (at 200 K) of (MgPc({minus}1)){sup {center dot}+} 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 both absorption and MCD spectra, are reported for each of the radical-cation species. 53 refs., 18 figs., 4 tabs.