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 r eactions 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-e lectron oxidation of the porphyrin ring. Evidence has been accumulatin g that the anion of perchlorate electrolytes, commonly used in cyclic voltammetry, may also affect the potentials for oxidation of zinc tetr aphenylporphyrin by ligation to the oxidized radical cation species. T he present work describes a survey of redox potentials of zinc tetraph enylporphyrin obtained by cyclic voltammetry in dichloromethane, with tetrabutylammonium salts containing a variety of anions as electrolyte s. Of the anions tested. hexafluorophosphate appears to have the least ability to ligate the metal, so that potentials measured in its prese nce as electrolyte should most closely approach those of the unligated porphyrin. With perchlorate electrolyte, the potential for one-electr on oxidation is approximately 80 mV lower, enough to affect the interp retation of photochemical electron transfer rates. In general, anions bind much more strongly to the cation radical than to zinc tetraphenyl porphyrin itself. The use of reference redox systems based on thymoqui none and ferrocene carboxylate enabled comparison of potentials measur ed with different electrolytes.