Enhanced photostability of poly(1,3-phenylene diphenylvinylene)-derivatives by diphenyl-substitution

We report measurements of the UV-photodegradation of the poly(para-phenylene vinylene) derivatives poly-(4,4′-diphenylene diphenylvinylene) (PDPV) and poly-(1,3-phenylene diphenylvinylene) m-PPV-DP in the presence of oxygen. We find that degradation of both PDPV and m-PPV-DP is slower than that of PPV, and attribute this to the replacement of the vinylene hydrogens by phenyl rings. The photoluminescence decay and the photoluminescence excitation spectra were measured as a function of UV-irradiation time. Initially a reversible luminescence quenching process was observed for films of either PDPV or m-PPV-DP in moist air in the presence of oxygen and/or water. We propose that this arises from conformational changes of the polymer chain produced following optical excitation. At longer irradiation times, FTIR studies reveal that the degradation process is oxidative. This oxidation is rate limited by oxygen diffusion into the film. Models for the photoluminescence properties are presented.