Radiolytic reduction of tetrapyridylporphyrins. Formation of a stable radical from tetrakis(N-methyl-2-pyridyl)porphyrin

Abstract

One-electron and multi-electron reduction of the three isomeric free-base porphyrins derived from TMPyP [tetrakis(N-methylpyridyl)porphyrin] in aqueous solutions under various conditions have been studied by spectrophotometric pulse radiolysis and γ-radiolysis. Similar experiments were carried out with TPyP (tetrapyridylporphyrins) in acidic solutions. Significant differences were found in the behaviour of the three isomers, in the stability of their one-electron and two-electron reduction products and in the effect of pH. An exceptionally stable radical was formed upon one-electron reduction of the ortho isomer, TM2PyP [tetrakis(N-methyl-2-pyridyl)porphyrin], at high pH. The stability of this radical is due to inhibition of protonation, a necessary step in the decay of the radical to final reduction products. Protonation is probably inhibited by the positive charges on the 2-pyridyl groups, which are very close to the meso positions of the porphyrin, and by steric effects of the ortho N-methyl groups, which inhibit deformation of the porphyrin ring and thus retard protonation. Somewhat stable radicals were also found in strongly acidic solutions of TM2PyP and T2PyP.