Prototropic equilibrium of 2-aminofluorene in non-ionic micelles: Tweens

Spectral characteristics of 2-aminofluorene (2-AF) have been studied as a function of the concentration of four Tweens at a given pH, and as a function of pH at a given concentration of the Tweens. The effective dielectric constant (epsilon(eff)) at a site of neutral 2-AF and the site where a prototropic reaction is occurring has been evaluated using the Stokes shifts of neutral 2-AF and the pK(a) values of the monocation-neutral equilibrium, respectively. These results, along with the single exponential decay of the fluorescence intensity of 2-AF, have clearly established that 2-AF is present at the interface of the micelles and water, and is bound to the oxygen atom of the polyoxy ethylene group through hydrogen bonding. The amino group of 2-AF is oriented towards the polar region and the fluorene ring towards the non-polar region of the micelles. The slight disagreement between the values of epsilon(eff) determined by both the methods is due to a small percentage (5%) of the prototropic reaction occurring in the aqueous phase. In both cases, the solvent systems used have been obtained by employing different compositions of dioxane-water mixtures. The values of the binding constants, non-radiative decay constants and epsilon(eff) determined by the above methods confirm that the hydrophobic character of the Tweens increases with increase of Tween number. (C) 1999 Elsevier Science B.V. All rights reserved.