Photophysical properties of thiazine dyes in aqueous solution and in micelles

Abstract

Depending on the pH of the solution, the excitation of azure A, azure B and azure C leads to the formation of unprotonated ( λ max=420 nm) and protonated ( λ max=370 nm) triplet states and radical cations ( λ max=520 nm). The triplet states are quenched by oxygen with a diffusion-controlled rate constant of ∼2×10 9 1 mol −1 s −1 and with the formation of singlet oxygen. The acid-base equilibrium in the triplet state (p K a=7.2±0.1 for azure B) is different from that in the ground state (p K a=0.0±0.2). The lifetimes of the triplet states are prolonged in the micellar medium of sodium dodecyl sulfate. The critical micellar concentration of the tenside and its dependence on the ionic strength of the solution can be found from measurement of the concentration of the triplet states of the dye in dependence on the surfactant concentration. The results obtained for sodium dodecyl sulfate are comparable with the previous methods.