Photochemistry of manganese porphyrins. Part 2.—Photoreduction

Abstract

Both manganese(II) and (III) porphyrins exhibit low-intensity, short-lived luminescence in ethanol solution at 77 K. Manganese(II) porphyrins are stable towards photodecomposition when irradiated in fluid solution but undergo efficient photoreduction in the presence of a good hydrogen donor such as ascorbic acid. Manganese(III) porphyrins are reduced to the manganese(II) compounds when irradiated with visible light but the quantum yields are very low (≈ 10^–4). In water, the quantum yield depends upon pH and excitation wavelength but is independent of the nature of the water-solubilising group. The predominant reaction pathway involves intramolecular reduction via an axially coordinated water molecule. Manganese(IV) porphyrins, which are better described as the manganese(III)π-radical cations, undergo relatively efficient photoreduction in water. Here, reduction leads to formation of hydrogen peroxide and involves attack at the ring periphery rather than at an axial position.