EFFECTS OF ELECTRON-WITHDRAWING SUBSTITUENTS ON THE ELECTROCHEMICAL OXIDATION OF PORPHYRINS

Effects of 18 substitutions on the oxidation potentials of mesotetraphenylporphyrins are reported and compared with those observed in reduction of the same compounds. Remarkably, the magnitude of the redox potential shift induced by strong electron-withdrawing substituents (CN, NO2) is different in oxidation from that in reduction. Typically, for the free-base tetraphenylporphyrin, the anodic shift resulting from four cyano substituents is +0.35 V in oxidation and +0.98 V in reduction. Also, at variance with results in reduction, the effects of 0 polysubstitution are nonadditive in oxidation, and for me-talloporphyrins, these effects depend on the nonelectroactive central metal. These results may be interpreted by the existence of two sites for the electron transfer—the pyrrolic nitrogens in oxidation and the π-electron system in reduction. © 1979, American Chemical Society. All rights reserved.