PHOSPHORESCENCE INVESTIGATION OF THE CONFORMATION OF A BROMONAPHTHALENE-LABELED POLY(ACRYLIC ACID)

Abstract

A copolymer of 4-bromo-1-naphthyl vinyl ketone and acrylic acid (BNPAA) has been prepared by free radical polymerization. The pendant bromonaphthalene (BN) groups endow the polymer with the unusual property of readily observable phosphorescence in solution at room temperature. The phosphorescence of BNPAA was investigated systematically in solution at room temperature as a function of polymer concentration, salt concentration, solvent structure, and other variables and shows that phosphorescence can serve as a probe of polymer conformation. By assuming that self-quenching of a triplet BN moiety by a pendant ground-state BN moiety on the same polymer chain determines the triplet lifetime, the phosphorescence lifetime of the labeled polymer at room temperature can be correlated to the chain conformation of the polymer BNPAA in dilute solution in the following manner: collapsed, globular conformations of the polymer correlate with significant self-quenching and relatively short phosphorescence lifetimes, and extended conformations of the polymer correlate with inhibition of self-quenching and relatively long phosphorescence lifetimes. Employing the phosphorescence lifetime criterion as a probe, the conformation of BNPAA was studied under a variety of conditions. The effect of different solvents on the polymer conformation was investigated and the strength of solvent-polymer interactions was found to decrease in the order water > methanol > dioxane. These results are consistent with those involving a pyrene-labeled poly(acrylic acid), PyPAA, for which the value of the monomer to excimer fluorescence emission ratio is correlated with the same solvent-dependence of polymer conformation. The influence of salt concentration was investigated, and the study of phosphorescence lifetimes as a function of added electrolytes allowed for the determination of the radius of gyration of the polymer in salt solution. The influence of solvent viscosity on polymer mobility and conformation was also studied by the phosphorescence probe technique. Quenching of phosphorescence with an aqueous phase quencher, NaNO2, elucidated the role of polymer conformation on the quenching process.