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

The quenching of the excited singlet and triplet states of Methylene Blue a nd the Azures A, B and C by p-benzoquinones was investigated in methanol an d acetonitrile. Fluorescence lifetimes were determined for the dyes, and fr om fluorescence quenching experiments rate constants were obtained for the singlet state process. Singlet quenching rate constants are nearly diffusio n controlled in both solvents. Triplet state processes and transient absorp tion spectra were investigated by laser flash photolysis. Triplet quenching rate constants were obtained from the decay of the triplet at 830 nm asa f unction of the quinone concentration. The rate constants for the tripler st ate reaction are lower than those of the excited singlet, but the triplet p rocess is efficient at lower quinone concentrations. From the transient abs orption determinations it was concluded that the quenching proceeds by an e lectron transfer reaction from the dye excited states to the quinones. It i s concluded that these dyes may function as very efficient photochemical re ductants. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.