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Abstract— In experiments on the interception of reactive intermediates of strongly oxidizing character in dye (S) sensitized photooxidations using p-nitrosodimethylaniline (RNO) as a selective scavenger, it has been observed that some substrates (A) or 1O2 acceptors (like imidazole derivatives) induce the bleaching of RNO as followed spectrophotometrically at 440 nm. Since singlet oxygen (1O2) does not react chemically with RNO, this bleaching is a consequence of 1O2 capture by the imidazole ring which results in the formation of a trans-annular peroxide intermediate [1O2] capable of inducing the bleaching of RNO (-RNO). In the absence of RNO, [1O2] decomposes or rearranges into the final oxygenation product 1O2: 1Δg Thus, the system imidazole plus RNO can be used as a sensitive and selective test for the presence of 1O2 in aqueous solutions. The method can also be applied in the presence of sensitizing dyes which, under visible irradiation, can partially bleach RNO even in the absence of imidazole derivatives. In such a case, the bleaching of RNO is strongly increased by the presence of imidazoles with a characteristic dependence on their concentration. The separation of the product of RNO bleaching by thin layer chromatography can serve as additional proof of the presence of 1O2 in the system. The imidazole plus RNO method has been applied to a number of sensitizing and non-sensitizing dyes.