Kinetics of quenching of ketone phosphorescence by oxygen in a glassy matrix

Measurements of oxygen diffusion in polystyrene, poly(methyl methacrylate), and poly(vinyl chloride) films varying in thickness from 7 to 57 mu m were obtained at 22 degrees C by following the kinetics of quenching of phosphorescence of camphorquinone dissolved in each polymer. Values of diffusion coefficients D were (2.2 +/- 0.5) x 10(-7), (1.4 +/- 0.3) x 10(-8), and (1.3 +/- 0.1) x 10(-8) cm(2) s(-1), respectively, in reasonable agreement with literature data. The decreasing trend in D in this series is consistent with increasing polymer properties including cohesive energy density and diffusion activation energy. Comparison of the diffusion-limited rate constant k(d), calculated from D, and the bimolecular rate coefficient k(q), determined from lifetime and steady state intensity measurements, indicates that triplet CQ immobilized in glassy polymers is quenched with an efficiency k(q)/k(d) greater than or similar to 4/9. This result is subject to some uncertainty due to the errors in k(q) and k(d); however, a value of k(q)/k(d) > 4/9 must be considered possible in view of recent reports implying that intersystem crossing between excited complexes of all multiplicity is important in systems where the phosphor is a ketone immobilized in a high-viscosity matrix.