A comparative study of the fluorescence of selected flexible and rigidized 7-aminocoumarin derivatives (coumarin 6, 138 and its rigidized coumarin 106) has been made in a series of solvents of varied properties. The studies were further augmented by the quantum chemical SCF-CI MO calculations which provide a theoretical basis for interpreting specific interactions and complex formation with the polar solvents in the intramolecular charge transfer singlet state of these dyes. Moreover, rotational relaxation behaviour, as reflected in the fluorescence polarization of these dyes, is correlated with solvent viscosity/free volume. Through measurements of the temperature/viscosity dependence of the fluorescence depolarization (1/P) in glycerol, we are able to establish an Arrhenius-like relationship linking (1/P) to the free-volume fraction of the medium. Furthermore, depolarization data fitted the Perrin equation with a slope proportional to the sensitivity of the molecular structure towards medium fluidity. The results imply the promising utility of these highly fluorescent dyes as fluorescent probes for local fluidity and polarity of the surrounding medium of interest. We demonstrate possible analytical applications of commercially available coumarin 6 as a promising fluorescent probe for medium properties (e.g. fluidity, polarity and for following micellization in the solutions of some ionic surfactants).

The fluorescence of coumarin 314 in water and ethanol is quenched statically via ground state complexation with cerium (III) ions. The temperature dependence of the quenching rate constant, (K_sv), determined from linear Stern-Volmer plots, indicate that the complexation is an enthalpy-dominating process. TheK_sv determined in ethanol is larger than that in water indicating the important competitive role played by the H-bonding interaction. The activation energy is also evaluated. The experimental findings verify the ability of coumarin 314 to form ground and excited state complexes.

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 (φ_f) is a steep function of the viscosity. The dependence of φ_f 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.