Publication costs assisted by the U.S. Army Research OfficeQuenching of the direct photochemical trans -cis isomerization of thioindigo dyes by oxygen is attributable to the intermediacy of the triplet state. From a study of the dependence of the quantum yields and photostationary state concentrations on the oxygen concentration for nine such dyes the approximate triplet lifetimes could be estimated. The quantum yields for the cis --+ trans isomerization were found to be unaffected by oxygen, unless the compounds contained chlorine substituents; this indicates the quenching of a common triplet intermediate in the isomerization of the latter. All of the compounds studied also undergo photosensitized isomerization to a common photostationary state the position of which is a function of the oxygen concentration. In addition, the fluorescence quantum yields were determined for the trans isomers. The mechanism of the photoisornerization process and the effect of substituents on the quantum yields and rates of the various excited state processes are discussed. The similarities found between these results and the data from recent reports on the effect of substituenh on the photoisomerization of some thionaphthioindigos and on the photoisomerization and fluorescence of stilbenes suggest that the resonance stabilization of the trans Si state plays an important part in determining the excited state behavior of molecules of this type. IntroductionAlthough the photochemical cis-trans isomerization of olefins is a well-known phenomenon, the mechanism of such reactions has not been fully elucidated to date. Thus the controversy between proponents of the singlet route vs. the triplet mechanism for the direct photoisomerization of stilbene has not been resolved as yet.2 Interestingly enough, evidence has been presented recently to the effect that the photoisomerization of several 4-nitrostilbenes takes place via the triplet state in the trans -+ cis direction, but by way of a singlet mechanism in the opposite dir e~t i o n ,~ attesting to the importance of substituent effects in such reactions.In a recent communication4 we reported preliminary evidence for the intermediacy of the triplet state in the direct photoisomerization of two thioindigo dyes by an oxygen-quenching technique. We now wish to report the results of our investigation of the direct and sensitized photoisomerization of thioindigo and eight of its ringsubstituted derivatives and our conclusions regarding the mechanism of the isomerization of such compounds. In addition, the effect of substituents on the rates and/or quantum yields of the various excited state processes will be discussed and compared to what has been reported in the literature for some related molecules.

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