Fluorescence quenching studies of the self-association in ater of fluorescent surfactants. Alkaryl-2-pyrazolines and alkyl-7-hydroxycoumarins

Abstract

This paper reports on the spectral properties of a range of alkaryl-2-pyrazolines and 3-alkyl-7-hydroxycoumarins containing n-alkyl substituents of varying chain length and examines the self-association behaviour of these compounds in water using a variety of spectroscopic techniques. For the higher members of each series, plots of the fluorescence quantum yield vs the solute concentration showed a sharp decrease in fluorescence yield at a well-defined concentration, in contrast to the smoothly decreasing curve that was characteristic of the lower members of the series. The kinetics of the fluorescence quenching were consistent with the formation of micelles from the surfactant monomers. Solubilization of two lipid probes, Nile Red and NBD-dioctylamine, by micelles of long-chain 3-alkyl-7-hydroxycoumarin sulphonates was accompanied by a strong enhancement of their fluorescence; the same probes showed little affinity for binding to micelles of alkaryl-2-pyrazoline sulphonates or quaternary ammonium pyrazoline derivatives. The micellar aggregation number of the highest member of the coumarin series was determined from fluorescence quenching experiments employing tris (2,2'-bipyridine)ruthenium (II) dichloride and 9-methylanthracene.