Synthetic Metals (v.157, #24)

Electrosynthesized polymers of N-methylpyrrole (NMPy) and N-methylpyrrole-2,6-dimethyl-β-cyclodextrin NMPy-β-DMCD were characterized with cyclic voltammetry and in situ conductivity measurements in aqueous and nonaqueous solutions. In situ UV–vis-spectra of PNMPy and poly(NMPy-β-DMCD) films show differences both in band absorbances and in energies of polaronic transitions. For the electrosynthesis of poly(NMPy-β-DMCD), a (1:1) (mole–mole) NMPy-β-DMCD supramolecular cyclodextrin complex of N-methylpyrrole was used as starting material, which was previously characterized with proton NMR spectroscopy. The PNMPy and poly(NMPy-β-DMCD) films were prepared from 0.05 M NMPy and 0.05 M NMPy-β-DMCD complex, respectively, in 0.1 M LiClO4 aqueous solution and 0.1 M LiClO4 nonaqueous solution (acetonitrile) by electropolymerization. A slight positive shift of the oxidation peak and further differences are observed for poly(NMPy-β-DMCD) electrosynthesized in comparison with PNMPy prepared in both aqueous and nonaqueous solutions. Different CVs and ΔE p of films were observed at various scan rates. In situ conductivity values of PNMPy and poly(NMPy-β-DMCD) films prepared in nonaqueous solution (acetonitrile) show higher values than with films prepared in aqueous solutions.
Keywords: β-Dimethyl cyclodextrin; Poly(N-methylpyrrole); Conducting polymer; Electropolymerization;

Measurement of carrier transport and injection in metal-free tetraphenylporphyrin by E. Pinotti; M. Cartotti; A. Sassella; A. Borghesi (1029-1033).
Carrier transport and charge injection are studied in metal-semiconductor structures employing metal-free tetraphenylporphyrin (H2-TPP) as the organic semiconductor. H2-TPP is deposited on an indium tin oxide (ITO) substrate by means of an apparatus for molecular beam depositions, and aluminum is employed as the top electrode. The ITO/H2-TPP/Al structures thus obtained are investigated by a large-signal capacitance-voltage method, and transport and charge injection are simultaneously measured. At low electrical fields a space-charge limited transport is found, and the device behavior is symmetrical, notwithstanding the different energy barriers at the ITO and Al electrodes. At higher electrical fields the transport regime becomes Schottky-barrier limited, with the asymmetry expected from the energy difference between the two contact metals. The charge injected into the device behaves symmetrically at low fields, and shows a peak at the transition voltages between the space-charge and the Schottky regime, both in the positive and negative bias direction.
Keywords: Transport; Charge injection; Mobility; Porphyrin; Organic semiconductors;

Synthesis and photophysical properties of multi-branched ethynyl fluorene-labeled molecules by Hu Xiaodan; Hong Suk Kim; Min Ju Cho; Jung-Il Jin; Young Min Kim; Young Wook Park; Byeong-Kwon Ju; Dong Hoon Choi (1034-1039).
Two kinds of emissive multi-branched molecule based on an ethynyl fluorene were synthesized by cyclotrimerization method. An ethynyl fluorene-labeled three-armed molecule, DBFP was prepared and the other one, CzDBFP has carbazole terminal groups tethered to DBFP. Their absorption and photoluminescent properties were investigated. The spectral analyses indicate that the molecules bearing ethynyl fluorene moieties as a peripheral group can exhibit highly isolated photophysical properties. The multi-branched molecule, CzDBFP was mixed with 2-{2-[2-(4-diethylamino-phenyl)-vinyl]-6-methyl-pyran-4-ylidene}-malononitrile (DCM1) with an optimum concentration. At the low concentration (0.3 wt%) of DCM1, white emission was observed by incomplete energy transfer process from an excited CzDBFP. At the high concentration (3.0 wt%), red emission was predominantly observed, which was attributed to an efficient Förster energy transfer process.
Keywords: Multi-branched molecule; Absorption; Photoluminescence; Förster energy transfer;

A series of the statistical copolymers of tetradecyloxy-3,5-bis-{2-(4-bromo-phenyl)-[1,3,4]oxadiazole}-benzene (4), 1,4-didecyloxy-2,5-diiodobenzene (5) and 1,4-dodecyloxy-2,5-divinylbenzene (6), for efficient light emitting diode were synthesized. As the feed ratio of compound 4 decreased, the absorption band at 320–360 nm decreased in the UV–vis spectra. In addition, as the composition of compound 4 decreased, the absorption intensity ratio at 1541 cm−1 decreased in the FT-IR spectra. The photoluminescence and electroluminescence emission color of P showed green-yellow color (λ max  = 552 nm). However, the maximum wavelength of photoluminescence and electroluminescence of P1 and P2 were 581 and 583 nm, respectively, which were dramatically red-shifted than that of P. Moreover, these values were almost the same as the values of P3 (580 nm). It is concluded that the energy transfer from high energy oxadiazole segments to low energy alkoxy-PPV segments occurred completely. The efficiency of ITO/P2/Al was 0.0181%, which is the best efficiency among the device based on the copolymers. This is presumably due to that P2 has the best balanced hole- and electron-injection/transporting properties among the copolymers.
Keywords: Poly(p-phenylenevinylene); Conjugated 1,3,4-oxadiazole; Electroluminescence; Energy transfer;

A series of luminescent complexes (C∧N)PtQ (where C∧N is bidentate cyclometalating ligand and Q is 8-hydroxyquinoline or quinoline-8-thiol ligand) are investigated using density functional theory (DFT). The optimized ground state geometries show that all these complexes show a nearly square-planar structure. The positions of the lowest absorption bands are blue-shifted for complexes with electron-deficient pyridine-type C∧N ligand while is red-shifted with quinoline-8-thiol ligand. In addition, with the increasing of electron-releasing ability of substituents, the HOMO–LUMO gap decreases while the ΔE values (the energy difference between d–d transition and HOMO–LUMO gap) increase which will benefit the (ligand-to-ligand charge transition) LLCT or (metal-to-ligand charge transition) MLCT. Also, solvent effects are discussed in toluene, chloroform, dichloromethane and acetone solution. In general, all the complexes show a blue-shifted with the increasing of solvent polarity. Furthermore, three methods are used to investigate the electron-rich or -deficient effect of pyridine-type C∧N ligand. These results reveal that this pyridine-type C∧N ligand is an electron-deficient type ligand.
Keywords: Spectra properties; Electron-deficient effect; TDDFT; Solvent effects;

The dielectric behaviour of in-situ polymerized thin polypyrrole (PPy) films on synthetic textile substrates were obtained in the 1–18 GHz region using free space transmission and reflection methods. The PPy/para-toluene-2-sulphonic acid (pTSA) coated fabrics exhibited an absorption dominated total shielding effectiveness (SE) of up to −7.34 dB, which corresponds to more than 80% of incident radiation. The permittivity response is significantly influenced by the changes in ambient conditions, sample size and diffraction around the sample. Mathematical diffraction removal, time-gating tools and high gain horns were utilized to improve the permittivity response. A narrow time-gate of 0.15 ns produced accurate response for frequencies above 6.7 GHz and the high gain horns further improved the response in the 7.5–18 GHz range. Errors between calculated and measured values of reflection were most commonly within 2%, indicating good accuracy of the method.
Keywords: Polypyrrole; Thin films; Electromagnetic interference shielding; Permittivity; Free space method;

Photorefractive polymer composites using a trinitrofluorenone–C60 dyad with a conformationally flexible linker as photosensitizer by Luis Martín-Gomis; Fernando Fernández-Lázaro; Ángela Sastre-Santos; José A. Quintana; José M. Villalvilla; Pedro Boj; María A. Díaz-García (1064-1070).
The photorefractive (PR) properties, via photoconductivity, birefringence and two-beam-coupling experiments, of a composite containing a fullerene–trinitrofluorenone (C60–TNF) dyad in which the two moieties are linked by a conformationally flexible polyether chain are reported. The PR performance of this compound as a sensitizer in polymer composites based on poly(N-vinylcarbazole) (PVK) as hole charge transporter, piperidindicyanostyrene (PDCST) as non-linear optical chromophore and butyl benzyl phthalate (BBP) as plasticizer, has been compared to those of the corresponding C60 and TNF derivatives. Results show that the linkage of C60 and TNF leads to an improvement in the photoconductivity of the composite leading to an increase of the PR speed.
Keywords: Photorefractive properties; Fullerene-[C60]; Trinitrofluorenone; Photoconductivity; Two-beam-coupling; Birefringence;

From conductive polymer composites with controlled morphology to smart materials by G. Boiteux; Ye.P. Mamunya; E.V. Lebedev; A. Adamczewski; C. Boullanger; P. Cassagnau; G. Seytre (1071-1073).
Conductive polymer composites (CPC) based on insulating matrix with conductive dispersed fillers are extensively studied since many years. A specific feature of such materials is a value of the filler content corresponding to the percolation threshold where a sharp transition insulating/conductive state occurs. The objective of this work is first the study of percolation phenomena in composites based on polypropylene (PP), on copolymers of polyamides (CPA) and polymer blends PP/CPA filled with dispersed iron (Fe) related to their structure but also to pay attention to their specific behaviour in temperature so-called PTC effect which can be controlled according the composition and the structure in the heterogeneous polymer matrix of the composites to elaborate intelligent/smart materials.
Keywords: Conductivity; PTC effect; Metallic filler; Polymer blend;

A novel material based on polyaniline doped with [Cs][In(dmit)2], (cesium) [bis(1,3-dithiole-2-thione-4,5-dithiolato)indium (III)] by Paulo H.S. Picciani; Fernando G. Souza; Nadia M. Comerlato; Bluma G. Soares (1074-1079).
A novel conducting hybrid material based on polyaniline doped with a monoanion derived from [cesium][bis(1,3-dithiole-2-thione-4,5-dithiolato)indium(III)], ([Cs][In(dmit)2]), is reported. Fourier transform infrared (FTIR), UV–vis and X-ray photoelectron spectroscopies showed that the [Cs][In(dmit)2] acts as the doping agent for the emeraldine base (insulating form of the polyaniline). This new material presents lower resistivity than the starting materials, and forms a flexible, self-supporting film with good homogeneity, which is an important feature for the development of new devices to be useful in the electroelectronic industries.
Keywords: Polyaniline; Complex counteranion; [Bis(1,3-dithiole-2-thione-4,5-dithiolato)indium(III)][cesium]+; Dmit; Conductivity;

β-Carotene–FSS organic semiconductor/n-type Si structure has been characterized by current–voltage and capacitance–voltage methods. A deviation in IV characteristic of the diode is observed due to effect of series resistance and interfacial layer. Cheung's functions were used to calculate diode parameters. The ideality factor, series resistance and barrier height values of the diode are n  = 1.77, R s  = 10.32 (10.39) kΩ and 0.78 eV. The obtained ideality factor suggests that Au/β-carotene–FSS/n-Si Schottky diode has a metal–SiO2 oxide layer plus organic layer–semiconductor (MIOS) configuration. The capacitance–voltage characterizations of Au/β-carotene–FSS/n-Si diode at different temperatures were performed. The capacitance of the diode changes with temperature. The barrier height and ideality factor obtained from CV curves are 0.67 eV and 1.68. The interface density properties of the diode are analyzed and the shape of the density distribution of the interface states is in the range of E c −0.49 to −0.62 eV. It is evaluated that the FSS organic layer controls electrical charge transport properties of Au/β-carotene/n-Si diode by excluding effects of the β-carotene and SiO2 residual oxides on the hybrid diode.
Keywords: Inorganic/organic device; Organic semiconductor; Ideality factor;

On the porosity of polypyrrole films by Allan Hallik; Ants Alumaa; Heisi Kurig; Alar Jänes; Enn Lust; Jüri Tamm (1085-1090).
Polypyrrole (PPy) films doped with anions of various size, charge and chemical nature (inorganic, surfactant, with aromatic ring) were electrochemically synthesized and investigated by low-temperature N2 sorption experiments at −196 °C. The specific surface area, total pore volume, average pore radius, pore size distribution and other parameters for oxidized PPy films using dodecylsulfate, 2-naphthalene sulfonate, 1,5-naphthalene disulfonate, poly(4-styrenesulfonate), tosylate, perchlorate, nitrate and chloride as dopant ions, were calculated. The obtained data show that although the average pore radius of investigated mesoporous PPy films (17–19 Å) is practically independent of the dopant anion used, however the latter determines the total pore volume and specific surface area values in different PPy materials investigated. As the total pore volumes for PPy films doped with large amphiphilic anions show the smallest values, the corresponding values for PPy/small inorganic anions, are up to 2 times higher.
Keywords: Polypyrrole; Counter-ion; Porosity; Surface area; BET method; Nitrogen adsorption;