Phthalocyanine .pi.-cation-radical species: photochemical and electrochemical preparation of [ZnPc(-1).+ in solution

Abstract

The ..pi..-cation-radical species of ZnPc, (ZnPc(-1))/sup .+/ (Pc = phthalocyanine), has been formed quantitatively as a stable product in solution following photochemical reactions with visible-region light in the presence of electron acceptors. The photolyses were carried out by excitation into the phthalocyanine's Q band (lambda > 580 nm), with carbon tetrabromide as an irreversible electron acceptor. The neutral parent species could be regenerated following photooxidation by the addition of sodium dithionite. Cyclic voltammetry of neutral ZnPcL species identified the ring oxidation potentials; for ZnPc(py) and ZnPc(im) (py = pyridine; im = imidazole). In dimethylacetamide, there is one oxidation couple at 0.70 and 0.71 V vs. SCE, respectively, and there are two reduction couples, at -0.96 and -1.28 V vs SCE, for the pyridine complex, and at -0.98 and -1.53 V vs SCE, for the imidazole complex. Each of these reactions was reversible on the cyclic voltammetry time scale. The electrochemical and photochemical oxidation products were characterized by absorption, magnetic circular dichroism (MCD), and electron paramagnetic resonance (EPR) spectroscopies. Four clearly resolvable, optical transitions, centered at 440, 500, 720, and 825 nm, are observed in the absorption spectrum of the cation radical species. EPR spectra obtained from frozen solutions of themore » ..pi.. cation radicals gave isotropic g values that are characteristic of the oxidation at the phthalocyanine ligand (the g values were between 2.0055 and 2.0068). 52 references, 4 figures, 2 tables.« less