The excited-states quenching of resazurin and resorufin by p-benzoquinones in polar solvents

The quenching of the excited singlet and triplet states of resazurin and resorufin by p-benzoquinones was investigated in methanol and in aqueous solution at pH 10. Fluorescence lifetimes were determined for the dyes and from the fluorescence quenching experiments rate constants were obtained for the singlet state quenching in MeOH. The rate constants were nearly diffusion controlled. 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 825 nm for resazurin and at 700 nm for resorufin as a function of the quinone concentration. In the case of resazurin, the triplet quenching rate constants were lower than those of the singlet state. For resorufin, the values of the singlet and triplet were in the same order. From the transient absorption spectrum determinations it was concluded that the quenching proceeded by an electron-transfer reaction from the dye excited-states to the quinones. The results suggest that these dyes may function as very efficient photochemical-reductants.