ON THE MECHANISM OF QUENCHING OF SINGLET OXYGEN BY AMINES‐III. EVIDENCE FOR A CHARGE‐TRANSFER‐LIKE COMPLEX

Abstract— A series of amines were found to quench singlet oxygen in the order tertiary > secondary > primary, with a reasonable correlation between the log of their rate constant of quenching and their ionization potential. In addition, a Hammett rho plot gave a rho value of ‐ 1.39 for the quenching of singlet oxygen by a series of substituted N, N‐dimethylanilines, in good agreement with the results obtained by a different method. It was found that some of the amines (anilines) quenched the triplet state of the dye‐sensitizer (Rose Bengal) used for the production of singlet oxygen. Corrections in the results were made in the calculations of rates of quenching of singlet oxygen to allow for the triplet‐state quenching. No extensive quenching of the singlet state of the dye was observed at the concentrations of the amines necessary for singlet‐oxygen quenching. In one case (N, N, N′, N′‐tetramethylphenylenediamine) there was no observable chemical reaction between singlet oxygen and the amine. It was concluded that singlet oxygen undergoes physical quenching by the amines via partial charge‐transfer intermediates.