SINGLET OXYGEN REACTIONS IN A GLASSY POLYSTYRENE MATRIX

Time-resolved spectroscopic measurements, together with other techniqu es, have been used to investigate chemical reactions between singlet o xygen (a(1) Delta(g)) and organic molecules (M) in a polystyrene glass at 20 degrees C. The kinetics of these reactions differ from those in liquids, owing to small solute diffusion coefficients in the solid wh ich influence the rates of O-2(a(1) Delta(g)) and M association and di ssociation. For molecules that react efficiently with O-2(a(1) Delta(g )), rate constants in the glass are significantly smaller than in liqu id toluene, reflecting a decrease in the rate constant of O-2(a(1) Del ta(g)) and M association to form an encounter pair. For a poor reactan t with O-2(a(1) Delta(g)), however, the overall quenching rate constan t in the glass can be larger than in the liquid, which reflects a rate constant for M-O-2(a(1) Delta(g)) dissociation that is smaller, and h ence an encounter pair lifetime that is longer, in the polymer. Change s in the rate constant for M-O-2(a(1) Delta(g)) dissociation are also reflected in product quantum yields. Thus, in 'self-sensitized' experi ments, in which the molecule that produces O-2(a(1) Delta(g)) also rea cts with O2(a(1) Delta(g)), that fraction of the reaction product aris ing directly from the initial M-O-2(a(1) Delta(g)) encounter pair is m uch higher in the glass than in the liquid. Furthermore, this intracag e reaction is not inhibited by the addition of an O-2(a(1) Delta(g)) p hysical quencher. These results indicate that, in materials containing small concentrations of additives or impurities capable of undergoing self-sensitized O-2(a(1) Delta(g)) reactions, the quantum yield for o xidation chemistry may be higher in a polymer glass than in a liquid a nd that added stabilizers may have no effect on the reaction. (C) 1998 Elsevier Science Limited. All rights reserved.