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

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