Solvent-Dependent Photophysical Properties of a Phenothiazone Dye as an Optical Probe

Abstract

Abstract A phenothiazone dye, 7-dimethylamino-3H-phenothiazin-3-one, was investigated as an optical probe in homogeneous media. Two absorption bands appeared in aprotic solvents and dioxane–water and exhibited different dependencies on the solvent polarity/polarizability scale, π*. Both of the bands were quite different from that corresponding to the aggregated molecules or the protonated species of the dye. These results suggest the existence of two ground-state species with non-twisted and twisted conformations of the 7-amino group. The π* dependencies of the fluorescence maxima and the rate constants of the fluorescence decays showed three types of behavior in dioxane–water. The Lippert–Mataga plot of the spectral shift was interpreted by assuming two types of behavior in aprotic solvents. These facts suggest that the fluorescence comes from three emitting states which exist in fast equilibria. The strong polarity dependencies and the long wavelengths of the absorption and fluorescence, the large Stokes shift, and the appreciable fluorescence quantum yield make the dye useful as an optical probe of the polarity of a variety of microenvironments.