TRANSIENT SPECIES IN THE PHOTOCHEMISTRY OF EOSIN*

Abstract

Abstract— Photochemical reactions of eosin in aqueous solution were studied using the flash photolysis technique. In deaerated solution the dye was converted quantitatively to the triplet state during flashing. The triplet dye decayed by first and second order reactions which partly regenerated the dye in the ground state and partly produced semioxidized and semireduced eosin. These radical species were formed in an electron dismutation reaction between two triplet molecules and also in a reaction between one triplet and one unexcited molecule. The radicals recombine rapidly to give the dye in the ground state. An efficient reversible photooxidation reaction was observed in eosin solutions containing potassium ferricyanide. Semioxidized eosin was formed in high yield by reaction between the triplet dye and the oxidant. The dye was regenerated rapidly in a reverse reaction between the products of the oxidation reaction. An analogous type of reaction was found to occur in eosin solutions containing p ‐pheny‐lene diamine. This reagent reduced the triplet dye to semireduced eosin; the dye was regenerated in the ground state in a very efficient reverse reaction. The protolytic behaviour of semireduced eosin was studied by varying the pH. Absorption spectra of the transient products were determined and rate constants for the observed reactions were measured. The results are compared with results from previous studies of fluorescein.