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

Spectral characteristics of 2-aminofluorene (2-AF) have been studied as a f unction of the concentration of four Tweens at a given pH, and as a functio n of pH at a given concentration of the Tweens. The effective dielectric co nstant (epsilon(eff)) at a site of neutral 2-AF and the site where a protot ropic reaction is occurring has been evaluated using the Stokes shifts of n eutral 2-AF and the pK(a) values of the monocation-neutral equilibrium, res pectively. These results, along with the single exponential decay of the fl uorescence 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 ato m of the polyoxy ethylene group through hydrogen bonding. The amino group o f 2-AF is oriented towards the polar region and the fluorene ring towards t he non-polar region of the micelles. The slight disagreement between the va lues of epsilon(eff) determined by both the methods is due to a small perce ntage (5%) of the prototropic reaction occurring in the aqueous phase. In b oth cases, the solvent systems used have been obtained by employing differe nt compositions of dioxane-water mixtures. The values of the binding consta nts, non-radiative decay constants and epsilon(eff) determined by the above methods confirm that the hydrophobic character of the Tweens increases wit h increase of Tween number. (C) 1999 Elsevier Science B.V. All rights reser ved.