Oligomerization of cationic pyrene derivatives in the presence of sodium dodecyl sulfate: a route to nanoaggregate formation

Abstract

The fluorescence of the probe pyrenylmethyl tri-n-butylphosphonium bromide (PMTP) has been studied in aqueous solution in the presence of the anionic surfactant sodium dodecyl sulfate (SDS). At the PMTP concentrations used, monomer fluorescence was observed both in pure water and in SDS solutions above the c.m.c. At intermediate surfactant concentrations, 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, with a similar spectrum to the excimer. The appearance of this new band is accompanied by changes in the absorption spectrum, while the excitation spectrum for emission at this wavelength differs from the monomer emission. Also, differences in the fluorescence decays were observed for the emission at 480 nm and for the monomer fluorescence. The results are interpreted in terms of formation of SDS–PMTP aggregates. Studies 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, the initial growth shows pseudo-first-order kinetics, whose rate depends on both the temperature and the SDS concentration. The aggregate growth has also been studied by light scattering, and reveals formation of particles of nm dimension. These are suggested to involve π-stacking of the PMTP probe held together by surfactant molecules.