Photooxidative degradation mechanism of model compounds of poly(p-phenylenevinylenes) [PPVs]

Seven model compounds of poly(p-phenylenevinylenes) [M-PPVs] were designed to perform the photooxidative degradation experiments. The degradation rates of M-PPVs depend on the nature, number, and position of substituents. The transient species including singlet oxygen, superoxide radical anion, M-PPV triplet state, M-PPV radical cation and anion in M-PPV photodegradation were observed and characterized systematically with ESR spin trapping, pulse radiolysis, and laser flash photolysis techniques. By using rose bengal, methylene blue, fullerene, tetraphenyl porphyrin, 9,10-dicyanoanthracene, or biacetyl as sensitizer in dye-sensitized M-PPV photodegradation experiments, we found that singlet oxygen does not play the principal role as previous reported. Results of triplet-triplet energy transfer from biacetyl to M-PPVs and M-PPV triplet state quenching by 1,4-diazabicyclo[2.2.2] octane (DABCO) suggested that the reaction between M-PPV triplet state and oxygen is really the key step in the processes of photodegradation of the conjugated structure. (C) 2002 Elsevier Science B.V. All rights reserved.