G. Froyer

Fully undoped and soluble oligo(3,4-ethylenedioxythiophene)s:spectroscopic study and electrochemical characterization

Fully undoped oligo(3,4-ethylenedioxythiophene)s have been synthesized from polycondensation of the corresponding dibromomonomer in the presence of a catalytic Ni(0)-based complex in N,N-dimethylacetamide (DMA). HPLC analysis indicated that the material is constituted of three main oligomers which have also been clearly detected by UV-Visible spectroscopy. Due to its partial solubility in DMA, processability is improved compared to the corresponding insoluble polymer and thin films of oligomers can be deposited by evaporation. Electrochemical and electrochromic properties of undoped film have been carried out. Deep purple in its undoped state, the material becomes sky blue in the oxidized form. The mixture of oligomers was characterized by means of Raman scattering, IR absorption and X-Ray dffraction (XRD), and compared to the poly(3,4-ethylenedioxythiophene) obtained by the oxidative route.

Blue electroluminescence with ladder-type poly(para-phenylene) and para-hexaphenyl

Abstract Conjugated polymers and oligomers are very interesting materials, with a number of possible electronic, optoelectronic and photonic applications. New synthesis techniques allow the preparation of extremely pure soluble ladder-type poly(para-phenylenes) (LPPPs), showing a steepness at the band edge comparable to conventional semiconductors. One consequence of this high purity is an improved photoluminescence quantum yield, around 30% for thin films. Oligomers of poly(poly-phenylene), like para-hexaphenyl (PHP) can be produced with extremely high purity, either as homogeneous disordered layers or well-ordered thin films, which show a high photoluminescence quantum yield in excess of 30%. All these materials are applied as the active layer in light-emitting devices (LEDs), giving bright blue light electroluminescence (EL) emission, and we report on their performance and efficiencies. For the LEd device with LPPP as the active layer, we observe the maximum of the EL emission around 460 nm, whereas for the PHP-based EL devices the emission maximum is located between 400 and 450 nm. The light emission of these EL devices is very high and exceeds a luminance of 2000 cd/m2 for a typical PHP multi-heterolayer EL device. For the overall EL quantum efficiency we obtain values up to 4% in homolayer EL devices. Ordering effects in thin films of conjugated materials, polymers and oligomers, strongly influence their optical properties. This ordering occurs in the active layers of EL devices and determines the threshold field for EL mission. Finally, we demonstrate that the emission colour of our EL devices can be controlled by the driving electric field.

Spectroelectrochemical studies of the C14-alkyl derivative of poly(3,4-ethylenedioxythiophene) (PEDT)

Abstract Spectroelectrochemical properties and electrical resistivity of an alkyl derivative of the poly(3,4-ethylenedioxythiophene), PEDT-C 14 were investigated. The oxidative doping process of this polymer was studied by means of cyclic voltammetry, UV-vis-NIR optical absorption, Raman scattering and Electron Paramagnetic Resonance (EPR). The in situ conductivity on a Pt microelectrode was also monitored. Optical spectroscopies permit to follow the evolution of the different species that are present. Combination of these different techniques leads to the characterisation of the charged species that are induced by doping, and a mechanism of the modification of the electronic structure of PEDT-C l4 is proposed.

A polymorph crystal structure of hexaphenyl observed in thin films

Highly oriented thin films of hexaphenyl - which are used in organic opto-electronic applications - are characterised in terms of their crystal structures. Two different crystal structures of hexaphenyl (C 36 H 26 ) are observed when the films are prepared by physical vapour deposition at various substrate temperatures. If the substrate is kept at room temperature, hexaphenyl crystallises within a structure which is already known from single crystal investigations. However, when the thin films are grown at a substrate temperature of 160°C a new crystalline phase appears. This structure was characterised by X-ray and transmission electron diffraction. Due to the strong preferred orientation of the crystallites within the thin films, the lattice constants as well as main features of the new crystal structure could be determined. The lattice is indexed as monoclinic with: a = 7.98 A, b = 5.54 A, c = 27.64 A and β = 99.8°. The new crystal structure has high similarity to the already known crystal structure. Both structures are built by layers of hexaphenyl molecules, within one layer the aromatic planes of the hexaphenyl molecules are packed in a herringbone pattern The characteristic feature of the new structure is that the long axes of the hexaphenyl molecules are arranged absolutely perpendicular to the layers, whereas within the already known structure the long axes show an tilt angle of 17° to the layer normal direction.

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.

Vibrational characterisation of a crystallised oligoaniline: a model compound of polyaniline

Abstract We present a detailed study on the vibrational properties of N , N ′-diphenyl-1,4-phenylenediamine in different crystalline forms. A new triclinic form of the molecule has been obtained through appropriate recrystallization procedure. This polymorphism of the crystalline state was associated to different vibrational features. These results are discussed with regards to the possible conformations of the molecule. In order to complete the study, thin solid films of these materials were also elaborated by vacuum sublimation of the molecule, upon selected conditions of rate, deposition and thickness. Spectroscopic measurements of these layers are showed and compared to those obtained on the crystalline solid forms. We performed convenient oxidation processes of this neutral N , N ′-diphenyl-1,4-phenylenediamine (powder and thin solid film) leading to the formation of the correspondent radical cation species. A comparison with radical cation generated in solution by electrochemical oxidative method is done. Vibrational characterisations of this doped oligomer were achieved in each case and finally, the observed differences are discussed in terms of conformation.

Orientation of molecules in phenylene oligomer thin films: influence of the substrate temperature

Abstract Para -terphenyl, para -quaterphenyl and para -hexaphenyl thin films were obtained by vacuum sublimation on substrates at various temperatures (413K, 300K and 77K). Depending on the substrate temperature two types of oligomer molecule orientation were observed by X-ray diffraction and confirmed by spectroscopic characterizations. This two “types” of preferred orientation represent preferred grown crystallites; within one type obtained at “low temperature” the molecules are oriented parallel to the substrate, the other type is characterized by molecules approximately perpendicular to the substrate surface.

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.

para-Sexiphenylene as a model compound of poly(para-phenylene) during the electrochemical intercalation of lithium and sodium ions in ethylene carbonate-based electrolyte

Abstract The electrochemical intercalation of alkaline ions (Na + , Li + ) into para -sexiphenylene (PSP) has been carried out in ethylene carbonate (EC) using MClO 4 as the alkaline salt (M-Na or Li). Cyclic voltammetry and galvanostatic experiments reveal different behaviour for lithium and sodium. Intercalation of Na + into PSP occurs with formation of successive stages of Na 0.33 (C 6 H 4 ) and Na 0.5 (C 6 H 4 ). After the first cycle, where the irreversible side reactions (such as the decomposition of the electrolyte, the reduction of the binder, etc.) occurred giving important capacity loss, the system exhibits a good reversibility. On the other hand, intercalation of Li + into PSP is partially reversible whatever the cycle. A reversible capacity of 0.2 is reported in this case. The galvanostatic curves do not show any well-defined plateaux as in the case of the intercalation of Na + .

Raman characterization of parasexiphenyle n-doped with sodium and potassium: Solvent influence and modelization

Oligomers play a crucial role in the elucidation of the spectroscopic properties of parent infinite polymers. Thus, parasexiphenyl appears as a model compound of poly-paraphenylene (PPP). N-doping of parasexiphenyl with various organometallic compounds effectively leads to Raman and infrared properties analogous with those observed in n-doped PPP. Although the Raman spectra of the oligomer compound have been well documented in the literature for THF-wet samples, little work has been devoted to the Raman study of the dry corresponding samples. THF provides a huge dipolar effect and appears to greatly influence the vibrational properties of doped parasexiphenyl and PPP.