Cationic micellar effects on the proton transfer reactions of N-substituted 2-(7-hydroxycoumarin-4-yl)acetamides and related compound in the ground and excited singlet states

A spectroscopic investigation was undertaken to determine cationic micellar effects on the ground-state and excited singlet-state proton transfer reactions of the title compounds with and without triethylamine (TEA). It was found that the presence of hexadecyltrimethylammonium chloride (HTAC) micelles containing TEA apparently lowers the equilibrium constant for the formation of tautomer anions having hydrophobic alkyl or aryl side chains (owing to a decrease of the effective concentration of TEA in the micellar phase) but exerted only a minor effect on the tautomerization of 7-hydroxy-4-methylcoumarin. The latter finding was explained in terms of the adsorption of this guest on the cationic head group with the hydroxy group directed toward the bulk aqueous phase. Analysis of HTAC micellar effects on the fluorescence spectra and lifetimes of the guests and their tautomer anions in the absence of TEA revealed that the orientation of our bichromophoric guest molecules in the micellar phase is subject to dramatic electronic effects of aryl substituents attached at the 4-position of the hydroxycoumarin ring.