Photophysics and dynamics of coumarin laser dyes and their analytical implications

Abstract

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).