The excited states quenching of phenothiazine dyes by p-benzoquinones in polar solvents

Abstract

The quenching of the excited singlet and triplet states of Methylene Blue and the Azures A, B and C by p-benzoquinones was investigated in methanol and acetonitrile. Fluorescence lifetimes were determined for the dyes, and from fluorescence quenching experiments rate constants were obtained for the singlet state process. Singlet quenching rate constants are nearly diffusion controlled in both solvents. Triplet state processes and transient absorption spectra were investigated by laser flash photolysis. Triplet quenching rate constants were obtained from the decay of the triplet at 830 nm as a function of the quinone concentration. The rate constants for the triplet state reaction are lower than those of the excited singlet, but the triplet process is efficient at lower quinone concentrations. From the transient absorption determinations it was concluded that the quenching proceeds by an electron transfer reaction from the dye excited states to the quinones. It is concluded that these dyes may function as very efficient photochemical reductants. ©