Bertrand Dulieu

Characterization of photochemically converted poly(para-phenylene vinylene)

Abstract The assisted photochemical conversion of precursor polymer synthesized via the procedure using tetrahydrothiophenium group is studied, particularly the influence of the substrate temperature and duration of the illumination period followed by optical absorption measurements The optimal experimental conditions are obtained for a substrate temperature T = 110°C a`nd an illumination period of at least 30 min. The characterization of the optimized photochemically converted poly( p -phenylene vinylene) [PPV] has been investigated by means of optical absorption, infrared absorption, X-ray diffraction and resonant Raman scattering spectroscopy. We find that optical and structural properties of the optimized photoconverted are rather close to those of standard PPV

Synthesis, Fabrication, and Photoluminescence of CaF2 Doped with Rare Earth Ions

Ambient condition wet synthetic techniques were used to synthesize CaF2 doped with europium and erbium. Photoluminescence spectroscopy and energy-dispersive x-ray analyses were used to study the electronic states of rare earth (RE) ions and the degree of heterogeneity/homogeneity in the matrix. Raman spectroscopy was used to study the degree of crystallinity of CaF2. Effects on the photoluminescence spectra were studied as a function of the thermal treatment of CaF2 samples.

Optical properties of polyparaphenyl thin films from oligomers to polymers

Parasexiphenyl and paraoctiphenyl were recently synthesized by an electrochemical method using monobrominated compounds and purified by sublimation. Moreover these oligomers can be processed by vacuum sublimation and high purity films were obtained whatever the substrate. Various characterization methods are allowed which facilited the understanding of the electronic and optical properties of the polyparaphenyls.

Spectroelectrochemical and structural studies of p-doped poly(p-phenylene vinylene)

Abstract The purpose of this study is to characterize the different modifications in the electronic distribution on the poly(p-phenylene vinylene) (PPV) backbone, induced by doping. The effects of in-situ electrochemical oxidative doping by ClO 4 − . is followed by optical absorption spectroscopy and resonant Raman scattering. On the basis on the Raman spectra, we have identified the vibrational modes associated with the positive polarons, bipolarons and neutral segments. The perturbation associated with the coexistence of these polaronic and bipolaronic segments has been described as a quinoid structure growing at the expense of the benzoid one.

Thermal conversion of PPV precursor : characterization at different stages of the process

Abstract The conversion process of PPV precursor containing a tetrahydrothiophene group was studied by different techniques. Conversion temperatures were chosen according to DSC measurements. Conversion included elimination of water and tetrahydrothiophenium groups which leads to the increase of T g , followed by modulated DSC technique, for the precursor because of elimination of plasticizing components. X-ray diffraction showed densification of the material going along with an increase in conjugation length.

Photochemical conversion of poly-p-phenylene vinylene

Abstract The light-assisted conversion of the soluble polyelectrolyte precursor polymer containing a tetrahydrothiophenium group to poly( p -phenylenevinylene) (PPV) is studied. Optical absorption measurements and spectrally resolved photoluminescence show that irradiation under vacuum at 293 K with ultraviolet light ( hv ⩽ 6.2 eV) leads to partially converted PPV. Nevertheless under these conditions extension of the π-conjugated system is hindered by a so far unidentified photochemical reaction. In contrast we found that fully converted PPV is obtained if the cast film is heated at 120 °C in the dark and subsequently UV-irradiated at the same temperature. The onset of optical absorption of the obtained material is E 0 ≈ 2.44 eV and the energy of the 0-0 radiative transition, measured at 77 K, is E 0-0 ≈ 2.35 eV as compared with E 0 = 2.34–2.44 eV and E 0-0 ≈ 2.33 eV for the thermally converted PPV.

Optical Properties of PPV and PPP Polymers

We present experimental optical absorption and Raman data of PPV and PPP. Theoretical calculations of the band shapes of absorption and Raman spectra of both polymers are reported. In particular the dependence of the Raman band intensities as function of the excitation wavelengths is theoretically accounted for, in very good agreement with experimental data. These features together with the absorption band shapes can be simulated if a distribution of conjugated segments of different lengths is introduced in the calculation.

In-situ resonant raman spectroelectrochemical study of p-doped poly(p-phenylene vinylene): influence of the conversion

We purpose to study the electronic structure modifications induced by p type doping of poly(p-phenylene vinylene) (PPV). PPV is obtained by thermal and photochemical conversion of tetrahydrothiophenium precursor. These both polymers have some differences in their conjugation length distribution. The resonant Raman spectra analysis coupled to a dynamic model permits us to describe vibrational modes corresponding to the different localized charged excitations produced along the chain of these polymers after doping. The characteristics of a quinoid structure are put in evidence. This one is independent of the conversion conditions of PPV and of the counter-ion nature.

Raman scattering of phenylene oligomers: influence of sample morphology

Abstract Polyparaphenylene oligomers and the corresponding polymer PPP were studied by Raman scattering from various sample morphologies: powder, thin film, single crystal and single molecule. The Raman intensities of the 1220, 1280 and 1600 cm −1 modes are compared with the molecule length and with the excitation wavelength.

Temperature dependence of photocarrier generation in poly-p-phenylene vinylene

We report on the variations of the photocurrent with temperature in poly-p-phenylene vinylene (PPV) under helium. Large differences in the build-up and decay kinetics are observed between room temperature and 77 K. At high temperature the photocurrent variations are very slow. When the temperature decreases the variations are more rapid. The temperature dependence of the photocurrent shows three distinct regions. The behavior at high temperature can be described with the Continuous Time Random Walk model. In this case the suggested species are some interchain Bound Polaron Pairs accumulated in time under illumination. At very low temperature the behavior can be explained by a trapping process (short lifetime) near the conduction band.