Photosensitized Generation of Superoxide Radical in Aprotic Solvents: an EPR and Spin Trapping Study

Abstract

The UV or visible irradiation of pigments such as curcumin, anthralin, benzanthrone, 1,8-dihydroxyanthraquinone, and rose bengal- or eosine-complexes with cationic surfactants in aerated aprotic solvents, such as benzene, toluene, acetone, n-heptane, cyclohexane, in the presence of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) generates EPR spectra with hyperfine splitting constants (hfsc's) a_N = 12.75 G, a^β_H = 10.50 G, a gamma H = 1.26 G in toluene, 12.83 G, 10.64 G, 1.24 G in benzene, 12.75 G, 10.19 G, 1.35 G in acetone and 12.54 G, 10.46 G, 1.38 G in n-heptane and cyclohexane. These spectra are similar to those observed when DMPO reacts with 18-crown-6 ether-solubilized KO₂ in the respective solvents and suggests that the photoinduced EPR spectra can be safely assigned to the DMPO/superoxide radical adduct (1). A correlation between the hfsc's of 1 and solvent parameters, the solvent acceptor number AN and the Kosower Z value, has been evaluated in terms of its usefulness for the identification of the DMPO/superoxide adduct in organic media.

Key Words:

• superoxide radical
• photosensitization
• aprotic solvent
• benzene
• toluene
• acetone
• cyclohexane
• n-heptane
• spin trapping
• 5,5-dimethyl-1-pyrroline N-oxide