Formation and Reactions of Long Lived Xanthene Dye Radicals. I. Photochemical Studies on Reactions of Semireduced Fluoresceïn

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Abstract

Abstract In the present paper photochemical reactions of fluoresceïn with leuco‐fluoresceïn are reported. Photoflash excitation of fluoresceïn leads to the formation of semireduced fluoresceïn radicals as a result of a reaction between the dye triplet and the leuco dye. In the dark the radicals disappear via a disproportionation reaction whereas under illumination they are transformed completely into the dye The disproportionation rate is strongly dependent on the p H of the solution and radicals with extremely long lifetimes were detected at p H 12. Irreversible protonation leading to a change in the hybridization of the central C‐atom of the molecule is postulated to explain the p H‐dependence of the disproportionation. The mechanism of this process is quantitatively described. At very high p H‐values, i.e. p H > 12.5, however, the dye triplets react directly with hydroxyl ions

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