Photoinduced absorption of structurally improved poly(p-phenylene vinylene) - no evidence for bipolarons

We have recently developed a synthetic route to poly(p-phenylene vinylene), PPV, which gives improved intrachain order. This has a large effect on the optical properties. We present data on photoinduced absorption for this form of PPV. The changes as compared to PPV prepared from the standard sulphonium precursor polymer are significant. Firstly, on the time scale of our experiments there is no evidence for photogenerated metastable bipolarons, usually observed in standard PPV. Secondly, the strong induced absorption due to triplet-triplet transitions is red-shifted to about 1.36 eV and is slightly narrower. This triplet absorption has a much stronger temperature dependence than observed in ordinary PPV, and is not seen above ca. 70 K. We consider that in this new form of PPV there are fewer defects which are responsible for the stabilisation of long-lived charged excitations