TRANSIENT PHOTOCHEMISTRY OF SAFRANIN-O

— We have characterized the spectra, acidity constants and decay kinetics of the triplet and semireduced radical species of Safranin-O. Between pH 3.0 and 10.6, there are three triplet species denoted 3DH2 +2, 3DH+ and 3D, the pKas being 7.5 and 9.2. All three triplet species exhibit first order decay, the rate constant for 3DH+ being ca. 5-fold lower than the rate constants of 3DH+ and 3D. Ascorbic acid and ethylenediaminetetraacetic acid (EDTA) quench the triplet state under appropriate pH conditions and the pH dependencies of the yield of semireduced dye indicate that 3DH+ is more reactive than 3DH+ or 3D. With EDTA as the reducing agent, there is the additional requirement that at least one of the amino nitrogens be deprotonated to obtain a significant yield of semireduced dye. In these reactions, ascorbic acid is oxidized reversibly, but EDTA is oxidized irreversibly, so that with the latter reducing agent photolysis causes buildup of the leucodye, which on subsequent photolysis can reduce triplet state dye. With ascorbic acid as the reducing agent, the regeneration of the ground state dye is reversible, the decay of the semireduced radical being second order. In general, the transient photochemistry of Safranin-O resembles that of Thionine, the major difference being that the lifetimes of 3DH2 +2 and 3DH+ are much longer for Safranin-O than for Thionine.