Production of singlet oxygen in electrogenerated radical ion electron transfer reactions

The electrochemical reduction of oxygen generated superoxide ion and the electrochemical oxidation of ferrocene yielded the ferricenium ion. Both of these ion radicals were produced alternately in the same solution by pulsing the electrode potential between the reduction potential of oxygen and the oxidation potential of ferrocene. The homogeneous electron transfer reaction between superoxide ion and the ferricenium ion then produced ferrocene and singlet oxygen. The intermediacy of singlet oxygen was demonstrated by chemical quenching with 1,3-diphenylisobenzofuran (DPBF) which produced o-dibenzoylbenzene (DBB). The electron transfer reaction between superoxide and DPBF cation radical also produced singlet oxygen. he annihilation reaction between ion radicals has T recently been an area of intense study. It has been well documented that the mechanism of these reactions is an electron transfer reaction between a cation radical and an anion radical producing an electronically excited species and a neutral ground state molecule. 1--3 The effect of these reactions is the ultimate emission of radiation characteristic of the fluorescence of either RI or RS. When the ion radicals are generated electro(1) D. M. Hercules, Accounts Chem. Res., 2, 301 (1969); A. Wessberger and B. Rossiter, Ed., “Physical Methods of Organic Chemistry,” 4th ed, Part IIA, Academic Press, New York, N. Y., 1971. (2) A. J. Bard, I<. S . V. Santhanam, S. A. Cruser, and L. R. Faulkner, “Fluorescence,” G. G. Guilbault, Ed., Marcel Dekker, New York, N. Y., 1967, Chapter 14. (3) T. Kuwana, “Electroanalytical Chemistry,” Vol. 1, A. J. Bard, Ed., Marcel Dekker, New York, N. Y., 1966, Chapter 3. e (1) R1 + R1.C R1 + R2* + Rz + hv or IR? + RI* ----f Ri + hv (2) +e Rz R2*chemically, this phenomenon is called electrogenerated chemiluminescence (ecl). In ecl, both ion radicals are generated in the same solution by alternately pulsing the electrode potential from that where the reduction of Rl occurs to that for the oxidation of Rz. In this manner a very high concentration of ion radicals can be produced in a small volume near the electrode. Luminescence was not, however, the desired product in this investigation. Since electronically excited species are generated in ecl when the enthalpy of the Mayeda, Bard / Singlet Oxygen in Electrogenerated Transfer Reactions