SOLVENT EFFECTS ON RATE CONSTANTS FOR ONE-ELECTRON OXIDATION OF PORPHYRINS

Absolute rate constants for one-electron oxidation of free base porphy rins (hematoporphyrin-IX, mesoprophyrin-IX dimethyl ester) by the tric hloromethylperoxyl radical and by the radical cation of N-methylindole have been determined in a variety of solvents by the pulse radiolysis technique. All solvents were air-saturated and contained CCl4 as a so urce for the CCl3O2 radicals. The rate constants for reactions of the se radicals were measured by following the formation of the porphyrin pi-radical cation optical absorption at 700 nm as a function of porphy rin concentration. The rate constants for reaction of CCl3O2 with the porphyrin were found to vary by an order of magnitude in the differen t solvents and were correlated with the solvent cohesive energy densit y. In the second set of experiments, N-methylindole was used in additi on to CCl4 so that CCl3O2 radicals oxidize this solute to its radical cation, which in turn oxidizes the porphyrin. The variations of the r ate constants for oxidation of the porphyrin by the N-methylindole rad ical cation in the different solvents exhibited different trends than those for oxidation by CCl3O2 radicals. For the reaction of N-methyli ndole radical cation with mesoporphyrin-IX dimethyl ester, it was poss ible to measure the oxidation potentials of the two compounds by cycli c voltammetry in seven solvents of widely varying polarities. The rate constants were then correlated with the free energy of the reaction a nd the solvent reorganization energy according to the Marcus equations .