Photochemistry of manganese porphyrins. Part 4.—Photosensitised reduction of quinones

There is no observable ground-state reaction between MnIII porphyrins and benzo-1,4-quinone, naphtho-1,4-quinone or anthra-9,10-quinone. However, upon irradiation with visible light, MnIII porphyrins sensitise reduction of the quinone to hydroquinone. Quantum yields (ca. 0.05) are dependent upon the quinone used, the pH of the solution and, to some extent, the nature of the porphyrin ligand. With benzo-1,4-quinone, the photoreaction involves an outer-sphere mechanism and results in a one-electron transfer, producing MnIV porphyrin and semiquinone. Both species are unstable under the conditions used and undergo further reactions, the final products being MnIII porphyrin and hydroquinone. Similar reactions take place with other quinones although the quantum yields are lower.Benzo-1,4-quinone has limited stability in alkaline solution but its workable pH range is increased by incorporating the quinone into a polymer matrix. Such polymers also undergo photosensitised reduction but the organic substrate reacts with the photogenerated MnIV porphyrin.