Spectrophotometric study of the incorporation of NBD probes in micelles: is a long alkyl chain on the fluorophore an advantage?

Fluorescence spectroscopy is widely used as a tool for elucidating the structure and dynamics of the micellar medium. A prerequisite commonly encountered for quantitative approaches is that the fluorophore resides exclusively in the micellar phase. Providing the fluorophore with a long alkyl chain may appear advantageous with regard to fixing the probe into the micelle. The present work was aimed at determining which are the consequences of this process from a spectroscopic viewpoint. The nitrobenzoxadiazolyl (NBD) moiety, which leads to well known fluorescent probes, was directly grafted on three fatty amines, the chain length of which varied from 8 to 18 carbon atoms. The spectroscopic properties of these NBD derivatives were investigated in three different micellar media: SDS, CTAB and TX100. The dyes were incorporated into micelles, where they were located in the interfacial region, whatever the chain length. When the dyes were previously dissolved in ethanol, and subsequently placed in the presence of the surfactant solution, complete solubilization was obtained. However, when the surfactant solution was used to dissolve directly a thin film of dye, a certain amount of dye remained non-incorporated and formed microcrystals, whose quantity and size increased with chain length. These microcrystals were mainly detected by UV/Vis-absorption and fluorescence microscopy. They induced drastic errors in the determination of fluorescence quantum yields, although they hardly interfered with other steady-state measurements and with dynamical fluorescence measurements. In conclusion, it appeared that for a small, non-ionic fluorophore such as NBD, the presence of a long alkyl chain is not an advantage. It slows down the incorporation process, unless some alcohol is introduced in the medium. Short-chain probes are therefore best suited for the study of the micellar medium.