The Fluorescence Response of a Structurally Modified 4-Aminophthalimide Derivative Covalently Attached to a Fatty Acid in Homogeneous and Micellar Environments

The photophysical properties of 11-(4-N,N-dimethylaminophthalimido)undecanoic acid (DAPL), a surfactant covalently labeled with a fluorophore, in homogeneous and micellar media are reported. The remarkable sensitivity of the fluorescence properties of DAPL to the polarity of the media is attributed to the existence of a low-lying nonfluorescent twisted intramolecular charge transfer (TICT) state. DAPL is found to be an excellent fluorescence sensor for following the micellar aggregation process. Although DAPL shows enhanced binding with the micelles, the quenching experiments and the fluorescence spectral and intensity data clearly indicate that the fluorescing moiety does not penetrate into the nonpolar core region of the micelles even though the fluorophore is covalently attached to the nonpolar end group of the fatty acid. It is unambiguously established from the fluorescence data that both of the terminal groups of DAPL are located in the interfacial region of the micelles by folding of the long polymethylene chain.