Fluorescence characteristics of two substituted coumarin dyes have been investigated. The effect of solvent polarity and viscosity on the fluorescence properties of these dyes was examined. It was found that the fluorescence quantum yield (φf) is a steep function of the viscosity. The dependence of φf on viscosity is successfully explained on the basis of the free volume theory of viscosity. Moreover, fluorescence depolarization measurements in media of different viscosities assist in the establishment of an analytical method for microviscosity determination in the interior of some micellar assemblies.
