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. 233 ROBERT

Abstract— Flash photolysis was used to study the reduction of the triplet state of methylene blue by both alkyl‐ and aryl‐amines. The extent of the formation of the semireduced form of the dye yielded rate constants of interaction between the triplet state and the amine (k5). A correlation between log k5 and ionization potentials for alkylamines (slope = ‐1.75 eV‐1) was interpreted as evidence for the formation of a partial charge‐transfer intermediate. The rate constants (k5) calculated for aryl‐amines approached the rate of diffusion in many cases. A Hammett plot for a series of N, N‐dimethyl‐anilines yielded a moderately large p value (– 3.28) consistent with the formation of a charge‐transfer intermediate. It was concluded that reaction of amines with triplet methylene blue leads to the formation of a partial charge‐transfer intermediate which may undergo complete electron transfer to yield radicals, or undergo spin inversion and return to the ground state.

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