DIFFUSION-LIMITED POLYMER SMALL MOLECULE INTERACTIONS - A STUDY OF 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(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(q) is approximately one-third the value of k(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(s), the solvent self-diffusion coefficient, predicts quantitatively the polymer concentration dependence of k(q), with k(q)/k(q0) = D(s)/D(s0) where the subscript 0 denotes the value at zero polymer concentration. In contrast to the significant effect of polymer concentration, k(q) was found to have little dependence on the polymer molecular weight. Benzil-RAPS interactions are compared to interactions of benzil with anthracene which is labeled at the terminus of the polystrene chain (TAPS), showing that the differences in the photophysical properties of RAPS and TAPS should be considered, along with other factors, in making conclusions about the effect of anthracene moiety placement on these interactions.