Spectroscopic and cyclic voltammetric studies of poly( p-phenylene vinylene) prepared from two different sulphonium salt precursor polymers

Abstract

Films of poly( p-phenylene vinylene) (PPV) were prepared by thermal elimination reactions of two different sulphonium salt precursor polymers. The ultra-violet/visible (u.v./vis.), fluorescence and Raman spectra and small-angle X-ray scattering diagrams of each PPV sample were compared, and the oxidation and reduction potentials of each PPV were determined by cyclic voltammetry. The u.v./vis. spectra of PPV films made from a sulphonium salt polymer based on a cyclic sulphide, which was converted to PPV at a temperature about 100°C lower than that required to obtain PPV from an acyclic sulphide sulphonium salt polymer, showed phonon sidebands at room temperature as well as at −200°C. The phonon sidebands were apparently associated with a narrower distribution of crystallite sizes (with an average long period spacing of 300 Å), which was obtained when PPV was made from the cyclic sulphonium chloride precursor polymer. The experimental values for the oxidation and reduction potentials and for the band gap were found to be in good agreement with the values predicted by ab initio calculations and from previous literature reports. The oxidation potential was found to be 0.80 V vs. SCE (saturated calomel electrode), the reduction potential was found to be −1.8 V vs. SCE, and the band-gap energy was found to be around 2.4 eV. The high value for the oxidation potential for PPV, in comparison to the oxidation potential of triiodide (0.52 V vs. SCE), from which an approximate value of 2 × 10 −10 for the oxidation constant was obtained, explains why only very low conductivities were obtained when PPV was ‘doped’ with iodine.