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

Can. J . Chem. 52.2889Chem. 52. (1974. Rate constants for the quenching of singlet oxygen by a series of s~~b s t i t~~t e d N,A1-dirnethylanilines were obtained by a direct method employing a dye-laser. The Hammett p value obtained from the data (-1.71) suggests that a (partial) charge-transfer complex may be responsible for the quenching action. This rate data was combined with that obtained for the total quenching action o n the sensitized photooxidation of 1,3-diphenylfuran. The quenching action on the photooxidation reaction is due to both the quenching of singlet oxygen and the quenching of the triplet state of the sensitizer (rose bengal or methylene blue). The combination of the data from each series of experiments resulted in rate constants of quenching of the triplet states of the sensitizers. A number of the N,Ar-dimethylanilines quenched the triplet states at the diffusion limit. Hamniett p v a l~~e s ( -1.86 for rose bengal and -4.19 for niethylene blue) indicate that charge-transfer intermediates are probably responsible for the quenching action. This was confirmed by the observation of a transient intermediate assigned to the charge-transfer radical of methylene blue. The t e c h n i q~~e ~~s e d here represents a novel approach to the investigation of triplet states.

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