OLIGOMERIZATION OF CATIONIC PYRENE DERIVATIVES IN THE PRESENCE OF SODIUM DODECYL-SULFATE - A ROUTE TO NANOAGGREGATE FORMATION

The fluorescence of the probe pyrenylmethyl tri-n-butylphosphonium bro mide (PMTP) has been studied in aqueous solution in the presence of th e anionic surfactant sodium dodecyl sulfate (SDS). At the PMTP concent rations used, monomer fluorescence was observed both in pure water and in SDS solutions above the c.m.c. At intermediate surfactant concentr ations, below the c.m.c., there was a decrease in the intensity of the monomer fluorescence and observation of a new band around 480 nm, wit h a similar spectrum to the excimer. The appearance of this new band i s accompanied by changes in the absorption spectrum, while the excitat ion spectrum for emission at this wavelength differs from the monomer emission. Also, differences in the fluorescence decays were observed f or the emission at 480 nm and for the monomer fluorescence. The result s are interpreted in terms of formation of SDS-PMTP aggregates. Studie s of changes in the fluorescence and absorption spectra, and turbidity as a function of time show that these aggregates grow in size over a period of 2-3 h. At longer times, decreases in turbidity are observed, probably due to precipitation. With an excess concentration of SDS, t he initial growth shows pseudo-first-order kinetics, whose rate depend s on both the temperature and the SDS concentration. The aggregate gro wth has also been studied by light scattering, and reveals formation o f particles of nm dimension. These are suggested to involve pi-stackin g of the PMTP probe held together by surfactant molecules.