EVIDENCE FOR INTRAMOLECULAR HYDROPHOBIC ASSOCIATION IN AQUEOUS-SOLUTION FOR PYRENE-END-CAPPED POLY(ETHYLENE OXIDE)

Evidence is presented that in dilute aqueous solutions of a Py-PEO-Py polymer (Py = pyrene, PEO = poly(ethylene oxide)) (M(n) = 8000, M(w)/M(n) = 1.08), 7 % of the polymeric molecules exist in an end-associated conformation. When excited by UV light, dilute (less than 10(-6) M) solutions of the polymer exhibit typical pyrene monomer and excimer fluorescence. The excimer fluorescence is shown to arise from intramolecular association only. Part of the excimer emission arises from diffusional approach and end-to-end cyclization during the lifetime of the excited pyrene monomer. The observation is that a significant portion of the excimer emission arises from direct UV excitation of polymers that exist in an intramolecularly end-associated conformation. We can make a clear distinction between excimers formed via diffusional end-to-end cyclization and direct excitation of associated end pairs by observing that selective quenching of the excited monomer leads to a decrease of the former type and not the latter. Spectral information, as well as dynamic and static quenching data, is presented to support this hypothesis. A kinetic model is developed, and rate parameters are extracted from data.