The effect of surfactants on the absorption and emission properties of thionine (TH⁺) have been studied in detail. Among the various surfactants investigated sodium lauryl sulphate (SLS) has marked effect on these properties. Changes in the absorption spectrum and the decrease in fluorescence intensity at [SLS] below the critical micelle concentration (CMC) are attributed to the formation of a dye-surfactant complex. At [SLS] above CMC, the restoration of dye spectrum with increased extinction coefficient at the λ_max and a small but definite red shift of the λ_max are interpreted as due to the incorporation of the dye into the SLS micelle. The absorbance and spectral shift data suggest the thionine cation to be localized near the micelle Stern layer in the case of SLS micelles but completely outside the micelle in the aqueous environment in the case of CTABr. From the absorbance and fluorescence data, the association constant for the formation of the TH⁺-SLS complex in the premicellar region, and the binding constant for the incorporation of the dye into the micelle in the micellar region have been computed. The values of both these constants were found to increase markedly in the presence of electrolytes.

One-electron reduction of thionine has been studied by using the technique of nanosecond pulse radiolysis and kinetic spectrophotometry. H,e_aq⁻ as well as radicals derived from methanol, ethanol, isopropanol, THF, dioxane andt-butanol by H atom abstraction were used as reductants. The rate constants for the transfer of electrons from these radicalts to thionine were directly determined from the pseudo first-order formation rates of the product, semithionine and the one-electron reduction potential of thionine estimated. The absorption spectrum of semithionine in its different conjugate acid-base forms was found to be in agreement with previously reported spectra and the decay of the species was second order. By monitoring transient absorbance changes as a function of pH, twopK_a values were observed and, based on the effect of ionic strength on the second-order decay constants of the species were assigned to the equilibria described.

One-electron oxidation of thionine has been studied using specific oxidizing radicals such as Cl⁻Tl(II) and N₃ generated by pulse radiolysis of aqueous solutions. The semioxidized thionine exhibited threepK’s indicating four conjugate acid-base forms. N₃ radicals were found to be less efficient in oxidizing thionine as compared to Cl₂⁻, Tl²⁺ and Tl(OH)⁺. The rate constants for electron abstraction from thionine by Cl₂⁻, Tl²⁺, Tl(OH)⁺, Tl(OH)₂ and N₃ were evaluated. The spectra of different protonated forms of semioxidized thionine and the extinction coefficients at λ_max are presented. Reaction of OH radicals with thionine gave transient products whose spectra and acid-base properties were different from those of semioxidized thionine. The rate constant for formation of the product transient agrees well with competition kinetic value for reaction of OH with thionine reported earlier.

The technique of nanosecond pulse radiolysis has been used to study the one-electron reduction of toluidine blue. Apart from H ande_aq⁻ various organic reducing radicals derived from isopropanol, tetrahydrofuran, cytosine etc. have been employed. The rate constants for the one-electron reduction was evaluated from build up traces. By monitoring the absorbance changes as a function of pH, twopK_a values were obtained suggesting the existence of the species in three conjugate acid-base forms. The spectra of the different protonated forms were monitored and their characteristic features have been presented here. In the case of H-atoms it has been observed that in addition to the electron-transfer reaction, other processes like adduct formation also appear to be taking place. Using the redox titration method, with radicals of known redox potential the one-electron reduction potential of toluidine blue was also evaluated (vs. NHE).

The one-electron oxidation of toluidine blue by specific oxidising radicals such as Cl₂⁻, Tl(II), N₃, Br₂⁻ etc. has been studied by employing the pulse radiolysis technique. The Br₂⁻ radical was found to be less efficient in oxidising toluidine blue as compared to Cl₂⁻., Tl⁺² and N₃. The semioxidised species exhibited only onepK_a indicating the presence of two conjugate acid-base forms whose spectral and kinetic features were evaluated. Reaction of OH radicals with the dye gave rise to a transient species which exhibited spectral and kinetic features different from that of the semioxidised species indicating that the mode of reaction of OH is different.