Defining the different phases of premicellar aggregation using the photophysical changes of a surface-probing compound.

Photophysical changes of a cylindrical compound undergoing twisted intramolecular charge transfer may be used as a surface probe to study the different phases of premicellar aggregate formation. The probe molecule, trans-2-[4-(dimethylamino)styryl] benzothiazole (DMASBT), attaches itself to the premicellar and the micellar aggregates of cationic, anionic, and neutral surfactants in different orientations because of its dipolar nature in the excited state. The micelle formation is preceded by a few typical rearrangements of the surfactant molecules. These events need proper inspection that can only be done by compounds that sense environmental changes by residing in the vicinity of the surface of those aggregates. Steady-state and time-resolved fluorescence spectroscopy coupled with steady-state fluorescence anisotropy measurements serve as a very useful tool to monitor premicellar aggregate formation. The dipolar interaction of DMASBT with the surface of the aggregate and its extraordinary capability to sense the polarity of the environment make it a very efficient molecule to use for the purpose.