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Isomerization of Carotenoids by Singlet Oxygen and a Probable Quenching Mechanism1Sir:Numerous reports of cis-trans isomerization of carotenoids by direct or sensitized irradiation have appeared.2 The isomerization of carotenoids sensitized by chlorophyll a appears to occur only in the direction cis -» trans.2•3 We now report that isomerization of 15,15'-m-j3-carotene ( 1) is sensitized by singlet oxygen ( 102), as well as by triplet methylene blue (3MB) with high efficiency. The product is, at least largely, all-?rarts-/3-carotene (2). Under the same conditions, 2 does not appear to undergo appreciable isomerization.

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Isomerization of Carotenoids by Singlet Oxygen and a Probable Quenching Mechanism1Sir:Numerous reports of cis-trans isomerization of carotenoids by direct or sensitized irradiation have appeared.2 The isomerization of carotenoids sensitized by chlorophyll a appears to occur only in the direction cis -» trans.2•3 We now report that isomerization of 15,15'-m-j3-carotene ( 1) is sensitized by singlet oxygen ( 102), as well as by triplet methylene blue (3MB) with high efficiency. The product is, at least largely, all-?rarts-/3-carotene (2). Under the same conditions, 2 does not appear to undergo appreciable isomerization.

It is a reasonable assumption that many of the known cherniluminescent organic compounds react to give a carbonyl group in an excited state. By extension of this hypothesis we were led to test several substituted acridinium salts. Oxidation of these with hydrogen peroxide in the presence of base resulted in the formation of N-methylacridone in high yield, and the emission of light from an excited state of this ketone. Mechanisms for this process are discussed. Interest in indole derivatives aroused by the finding of a tryptamine residue in the luminescent cypridina luciferin, in conjunction with the hypothesis mentioned above, has led to an investigation of some indoles, and in particular, the important isolable peroxide of tetrahydrocarbazole. A preliminary account of this work is given.