Analysis of fluorescence spectra of PRODAN in protic solvents as the superposition of two Gaussian functions

Abstract

The steady-state fluorescence emission spectra of 6-propionyl-2-dimethylaminonaphthalene (PRODAN) in aqueous solutions of methanol, ethanol and 1-propanol were studied in a wide range of concentrations. The flourescence of PRODAN was also studied in aqueous solutions (from 3 Â 10 À3 to 21 Â 10 À3 mole L À1 ) of dodecyltrimethylammonium bromide (DTAB), a micelle-forming ionic surfactant. In all cases, experimental data, as a function of wavelength, are very well fitted by the sum of two Gaussian functions. In alcohol solutions, the maximum of lower wavelength depends linearly both on concentration and polarizability. In DTAB solutions, the maximum of higher wavelength varies smoothly with concentration, but the lower wavelength maximum changes abruptly at the Critical Micellar Concentration. Results are explained by assuming that the fluorescence emission of PRODAN is originated in transitions from two excited states which are close in energy but have dissimilar interactions with the probe environment.