Diffusion-limited polymer-small molecule interactions: a study by phosphorescence quenching

Diffusion-limited interactions between benzil and anthracene-labeled polystyrene were studied by phosphorescence quenching in polystyrene-toluene solutions. Values of the bimolecular diffusion-limited quenching rate constant, k sub(q), were obtained by measuring the benzil phosphorescence lifetime as a function of anthracene moiety concentration and applying a Stern-Volmer analysis. In the case of interactions of benzil with anthracene which was labeled randomly to phenyl groups along the polystyrene chain (RAPS), k sub(q) is approximately one-third the value of k sub(q) for the benzil anthracene interaction over a broad range of unlabeled polystyrene concentration, from 0 to at least 400 g/L. This indicates that the physics controlling the polystyrene concentration dependence of the benzil-anthracene interaction also controls the polystyrene concentration dependence of the benzil-RAPS interaction in toluene solution. For both interactions, the Vrentas-Duda free volume theory for D sub(c), the solvent self-diffusion coefficient, predicts quantitatively the polymer concentration dependence of k sub(q), with k sub(T)/k sub(q0) = D sub(c)D sub(q0) where the subscript 0 denotes the value at zero polymer concentration. In contrast to the significant effect of polymer concentration, k sub(q) was found to have little dependence on the polymer molecular weight.