The effect of anions on the electrochemistry of zinc tetraphenylporphyrin

Accurate measurements of porphyrin redox potentials are essential for the prediction and rationalization of the rates of electron transfer reactions involving these biologically important electron-donating and accepting chromophores. It is well established that ligation of bases, including anions, to metalloporphyrins lowers the potential for one-electron oxidation of the porphyrin ring. Evidence has been accumulating that the anion of perchlorate electrolytes, commonly used in cyclic voltammetry, may also affect the potentials for oxidation of zinc tetraphenylporphyrin by ligation to the oxidized radical cation species. The present work describes a survey of redox potentials of zinc tetraphenylporphyrin obtained by cyclic voltammetry in dichloromethane, with tetrabutylammonium salts containing a variety of anions as electrolytes. Of the anions tested, hexafluorophosphate appears to have the least ability to ligate the metal, so that potentials measured in its presence as electrolyte should most closely approach those of the unligated porphyrin. With perchlorate electrolyte, the potential for one-electron oxidation is approximately 80 mV lower, enough to affect the interpretation of photochemical electron transfer rates. In general, anions bind much more strongly to the cation radical than to zinc tetraphenylporphyrin itself. The use of reference redox systems based on thymoquinone and ferrocene carboxylate enabled comparison of potentials measured with different electrolytes.