Modulation of Excited-State Proton-Transfer Reactions of 7-Hydroxy-4-methylcoumarin in Ionic and Nonionic Reverse Micelles

The prototropic behavior of the dye, 7-hydroxy-4-methylcoumarin (7H4MC), has been studied in cationic benzyldimethylhexadecylammonium chloride (BDHC) and nonionic poly(oxyethylene)(tetramethylbutyl)phenyl ether (TritonX-100, TX-100) reverse micelles using ground-state absorption and steady-state and time-resolved fluorescence measurements. The results have been compared with the previous results in the anionic sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles. Although the probe dye, 7H4MC, is indicated to reside in the interfacial region in all of the reverse micelles studied, significant differences have been observed in the evolution of the different prototropic species. In BDHC reverse micelles, the anionic form is favored over the tautomeric form at the higher w(0) values, which is contrary to the observation in AOT reverse micelles where both of these forms are almost equally produced. The higher propensity for the formation of the anionic form in BDHC reverse micelles has been explained on the basis of the additional electrostatic stabilization of the anionic species in the cationic BDHC reverse micelles compared to that in the anionic AOT reverse micelles. On the other hand, in TX-100 reverse micelles, the anionic form is not very evident, but interestingly, the tautomer form begins to appear beyond w(0) = 2. The appearance of the tautomeric species apparently coincides with the formation of the water pool in the TX-100 reverse micelles. However, due to the more restricted nature of the water molecules within this reverse micelle (mostly dispersed around the oxyethylene chains), deprotonation of the 7H4MC dye and the consequential stabilization of the anionic form are considerably reduced. The results clearly reveal that the aqueous environment in the vicinity of the probe is quite different for the reverse micelles considered, and these differences largely modulate the prototropic processes of the excited dye.