Aggregation and disaggregation of anionic aluminum phthalocyanines in cationic pre-micelle and micelle media: A fluorescence study

The interactions of two anionic aluminum phthalocyanines (Pcs), the tetrasulfonated (AlPcS4) and the monosulfonated (AlPcS1) species, with the cationic detergent n-dodecyltrimethylammonium chloride (DTAC) were investigated. These studies were performed by UV–visible absorption, steady-state and transient-state fluorescence, fluorescence lifetime imaging microscopy (FLIM), and fluorescence correlation spectroscopy (FCS). For both AlPcS4 and AlPcS1, large complexes of Pc–DTAC were formed at low surfactant concentrations, in the pre-micelle range. These complexes were visualized by the FLIM technique, directly in a drop of the sample. As the DTAC concentration increased, the size of the complexes decreased significantly. Above the detergent critical micelle concentration (CMC ≈ 22 mM), highly fluorescent monomers of both AlPcS4 and AlPcS1 were found to be strongly bound to the DTAC micelles surface: large equilibrium binding constants, Kb = 3.5 × 105 and 6.2 × 104, were obtained for AlPcS4 and AlPcS1, respectively. The aggregation/disaggregation behavior of phthalocyanines likely results from a delicate balance in the system, between hydrophobic attractions (Pc–Pc) and electrostatic attractions (anionic Pc macrocycles – cationic DTAC headgroups) and repulsions (between anionic sulfonated groups of Pc macrocycles).