Diffusion-limited interactions between benziland anthracene-labeled polystyrene were studied by phosphorescence quenching in polystyrene-toluene solutions. Values of the bimolecular diffusion-limited quenching rate constant, kq, 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), kq is approximately one-third the value of kq 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" the solvent self-diffusion coefficient, predicts quantitatively the polymer concentration dependence of feq, with feq/Aqo = DJD& where the subscript 0 denotes the value at zero polymer concentration. In contrast to the significant effect of polymer concentration, kq 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.
Diffusion-limited interactions between benzil and anthracene were studied by phosphorescence quenching in polystyrene-cyclohexane, polystyrene-toluene, poly(methyl methacrylate)-toluene, and polybutadiene-cyclohexane solutions. Values of the bimolecular diffusion-limited quenching rate constant, kq, were obtained by measuring benzil phosphorescence lifetime as a function of anthracene concentration and applying a Stern-Volmer analysis. Besides polymer species and solvent, k" was measured as a function of polymer molecular weight and concentration, up to 560 g/L. kq was found to be independent of polymer molecular weight in polystyrene-cyclohexane solutions and exhibited a slight molecular weight dependence in polystyrene-toluene solutions. The polymer concentration dependence of kq in polystyrene-cyclohexane and polystyrene-toluene solutions was found to mimic the polymer concentration dependence of the solvent self-diffusion coefficient; this result is consistent with the notion that kq/kq0 ~DJD& where Ds is the solvent self-diffusion coefficient and the subscript 0 indicates the value at zero polymer concentration. A very similar polymer concentration dependence of kq was obtained in poly(methyl methacrylate)-toluene solutions. The Vrentas-Duda free volume theory for Ds was found to predict the polymer concentration dependence of kq quantitatively for polystyrene-toluene and approximately for poly(methyl methacrylate)-toluene solutions; over the range of polymer concentrations studied in polystyrene-cyclohexane solutions, the agreement between the Vrentas-Duda theory and experimental measures of kq appears to be less satisfactory. The Fujita-Doolittle theory can also be used to fit experimental measures of kq in selected cases; however, it is possible to obtain unphysical results if the Fujita-Doolittle theory is applied over too wide a range in polymer concentration.
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