Synthetic Metals (v.156, #7-8)

Microwave absorption by polyaniline–carbon nanotube composites by Darren A. Makeiff; Trisha Huber (497-505).
Conductive thermoplastic composites containing both multi-walled carbon nanotubes and polyaniline doped with para-toluene sulfonic acid were formulated using two different methods. In the first method, PAni-pTsA-coated MWNTs were synthesized and processed into an insulating matrix. The second method involved mechanical mixing of separate synthetic PAni-pTsA and non-coated MWNT solids into an insulating matrix. Microwave absorption measurements at X-band frequencies (8–12 GHz) indicated that the former composites are poor absorbers, while the latter are good absorbers and showed stronger absorption than composites containing only PAni-pTsA or MWNTs.
Keywords: Polyaniline; Carbon nanotubes; Nanocomposites; Poly(methylmethacrylate); Conductivity; Permittivity; Microwave absorption;

DBSA-doped polyaniline (DBSA–PANI) powder and HCl-doped polyaniline with Fe3O4 nanoparticles (HCl–PANI–Fe3O4) powder were mechanically mixed to obtain the Fe3O4–polyaniline nanocomposites. Powders of the nanocomposites were pressed to the pellets. Micromorphology, electrical and magnetic properties of the nanocomposite pellets were studied by using scanning electron microscopy and by measuring the conductivity in 100–300 K and the magnetization curve at room temperature. The DBSA–PANI pellets consist of long fibrils while the HCl–PANI–Fe3O4 pellets consist of granular particles. Thus the Fe3O4–polyaniline nanocomposites pellets consist of long fibrils and granular particles. The conductivity of the nanocomposite pellets linearly decreases from 0.19 ± 0.06 to 0.05 ± 0.01 S/cm when the HCl–PANI–Fe3O4 content increases from 0 to 100 wt.%. The variation of conductivity with temperature reveals that the charge transport mechanism can be considered to be one-dimensional variable-range-hopping (1D-VRH). All the Fe3O4–polyaniline nanocomposites show the magnetization curves. The saturation magnetization monotonously increases with increasing HCl–PANI–Fe3O4 content while the coercivity is estimated to be about zero independent of the HCl–PANI–Fe3O4 content. The saturation magnetization of the HCl–PANI–Fe3O4 is 11 emu/g.
Keywords: Polyaniline; Fe3O4 nanoparticle; Nanocomposite; Conductivity; Magnetization;

Accelerated lifetime measurements of P3HT:PCBM solar cells by Rémi De Bettignies; Jocelyne Leroy; Muriel Firon; Carole Sentein (510-513).
We describe in this article the quick optimization of bulk heterojunction organic solar cells based on poly-3-hexylthiophene and [6,6]-phenyl C61 butyric acid methylester blend. Best cells are found to be based on 1:1 in weight ratio and yield 3.6% power conversion efficiency under air-mass 1.5, 100 mW/cm2 illumination. On the optimized cells, ageing measurements have been realized to collect the absolute values of photovoltaic parameters along the time. On this study, accelerated lifetime has been calculated using the Arrhenius model for a first-order kinetic of degradation. According to the model, we can predict a long lifetime keeping more than 80% of the initial short-circuit current density after 1000 h of working.
Keywords: Bulk heterojunction solar cell; Accelerated lifetime measurements; Stability;

Polypyrrole nanostructures formed by electrochemical method on graphite impregnated with paraffin by Jixiao Wang; Xiaoping Mo; Dongtao Ge; Yun Tian; Zhi Wang; Shichang Wang (514-518).
Polypyrrole nanostructures were prepared electrochemically by template-free method on graphite impregnated with paraffin. The experimental parameters, such as testing temperature, polymerization potential, doped ions and the status of the electrode surface have significant effect on the morphology of the formed polypyrrole. Under various experimental conditions, fibrillar, taper and cauliflower polypyrrole can be obtained. By controlling the active sites, the polypyrrole nuclei will grow in one-dimensional pattern, and thus polypyrrole nanowires were obtained. The method should be useful for preparation other materials nanowires.
Keywords: Conductive polymers; Polypyrrole; Nanowires; Mechanism;

Infrared intramolecular vibrations and lattice modes for both the crystalline α-quaterthiophene (4T) polymorph phases are investigated by using the modern theory of polarization combined with density functional theory. For the first time, Born effective charge tensors and far- and mid-infrared responses for both the polymorph phases of 4T have been calculated. We have also proposed an assignment of the main spectral infrared signatures of the 4T polymorph phases. These assignments could be of particular interest in the far-infrared domain for the understanding of the electronic transport of these materials.
Keywords: Polymorph phase; Infrared spectra; Charge tensor; DFT; Oligothiophene;

Three new substituted arylpyridine coordinated iridium complexes Ir(DPP)2(acac), Ir(BPP)2(acac) and Ir(FPP)2(acac) have been synthesized and used as emitter in OLED. The EL devices based on these complexes emitted in the green to yellow spectral range. The results showed that these complexes have strong phosphorescent characters. For the device based on Ir(FPP)2(acac), the maximum brightness reached 14180 cd m−2 and the maximum external efficiency reached 17.0 cd A−1.
Keywords: Iridium complex; Synthesis; Organic light emitting diode;

Synthesis and characterization of chloro and bromo substituted p-phenylene vinylene homopolymers and alternating copolymers by Laura O. Péres; Mauro R. Fernandes; Jarem R. Garcia; Shu Hui Wang; Francisco C. Nart (529-536).
Halogen-substituted poly(p-phenylene vinylene) (PPV) and copolymers containing halogen-substituted PPV were chemically synthesized via Wessling and Wittig routes, respectively. FTIR, 1H NMR, UV–vis and fluorescence spectroscopies and electrochemistry were used to characterize the polymers. The absorption and photoemission spectra show that addition of halogen causes a red shift in both absorption and emission properties for the halogen-substituted PPV and for the segmented copolymers in relation to the non-substituted polymers. The electron affinity (EA) and the ionization potential (IP) show that the halogens bonded to the ring do not change the HOMO and LUMO edges, but the copolymers present higher values for the IP and lower values for the EA with respect to the homopolymers.
Keywords: PPV; Segmented copolymer; Halogen substituent;

A novel crosslinkable hole transport material, a mixed compound of poly(3,4-ethylenedioxythiophene:poly(styrenesulfonate) (PEDOT)/glycidoxypropyl trimethoxysilane (GPS), has been developed to protect indium contamination from indium tin oxide (ITO) substrates. The curing behavior of the PEDOT/GPS was monitored with Raman spectroscopy and indium contamination was observed with X-ray photoelectron spectroscopy. The hole transport material immediately formed a crosslinked siloxane network structure after spin casting and reduced the indium incorporation in the hole transport layer. The addition of GPS was effective in protecting the indium contamination from ITO substrates through a crosslinked network formation.
Keywords: Silane; Network; Hole transport layer; Indium;

Electropolymerization mechanism of N-methylaniline by Di Wei; Carita Kvarnström; Tom Lindfors; Leif Kronberg; Rainer Sjöholm; Ari Ivaska (541-548).
A poly(N-methylaniline) (PNMA) film was obtained by chronoamperometry on tin oxide (TO) in dimethylformamide DMF containing 0.5 M sulphuric acid. The PNMA film formation was monitored by in situ UV–vis spectroscopy. A detailed electropolymerization mechanism and reaction pathway of N-methylaniline (NMA) is proposed based on mass spectroscopy (MS) and NMR analysis. The NMA repeating units (both in the film and in the soluble part) were found to be partially dealkylated during electropolymerization. NMA was also electropolymerized in different solvents such as propylene carbonate (PC) and acetonitrile (ACN). Our results show that protonation is not a necessary condition to initiate the electropolymerization of NMA, but it may play a key role for the propagation of longer conjugated chains. The influence of solvents on the film formation and the function of protons are discussed in this paper.
Keywords: Poly(N-methylaniline); Electropolymerization mechanism; In situ UV–vis spectroscopy; NMR; MS;

Electropolymerization and characterization of poly(N-methylaniline) and poly(N-butylaniline) in mixtures of aqueous and organic solvents by Maija Blomquist; Tom Lindfors; Lari Vähäsalo; Almantas Pivrikas; Ari Ivaska (549-557).
Electropolymerization of N-methylaniline (NMA) and N-butylaniline (NBA) was studied on glassy carbon and optically transparent tin oxide electrodes in mixtures of aqueous and organic solvents. Five different organic solvents, dimethyl formamide, dimethyl sulfoxide, tetrahydrofuran (THF), propylene carbonate (PC) and acetonitrile (ACN) were mixed with aqueous 1.0 M HClO4 solutions. Our results show that NMA and NBA can be electropolymerized in aqueous–organic solvent mixtures with maximum 30 and 50% (v/v) organic solvents, respectively. The choice of organic solvent strongly influences the film formation. With addition of 10–20% THF, ACN and PC to the aqueous polymerization solution, the film formation was highly improved. The poly(N-methylaniline) and poly(N-butylaniline) films were characterized with cyclic voltammetry, in situ UV–vis spectroscopy, size exclusion chromatography and with electrical conductivity measurements.
Keywords: Electrochemical polymerization; Mixed solvents; Conducting polymers; Poly(N-methylaniline); Poly(N-butylaniline); UV–vis absorption; Size exclusion; Conductivity;

Polyaniline (PANI)-C60 membranes were chemically synthesized with fullerene C60 content of 0.2, 0.5, 1, 2 and 3 mol% (relative to aniline fragment) respectively, and then systematically characterized with FTIR, field emission scanning electron microscopy (FESEM), XPS and electrochemical impedance spectroscopy (EIS). It is demonstrated that electron/ion coupled transport across PANI-C60 membrane is possible in the presence of oxidizing agent at one side of the membrane and reducing agent at the other side. If 0.05 M acidic solution of FeCl3 was used as the oxidizing agent and 0.3 M ascorbic acid as the reducing agent, a typical value of transmembrane transport rate of redox equivalents was 3.1 × 10−8  mol s−1  cm−2 with the membrane containing 0.5% C60. This value was one order higher than that for HCl doped PANI membrane at identical conditions, which can be explained by superimposed C60 doping and acid doping. The 0.5% content of C60 is optimal and at higher content the rates of transmembrane redox transport decrease.
Keywords: Fullerene C60; Polyaniline; Membrane; Redox transport;

In situ spectroelectrochemical studies of the copolymerization of o-aminophenol (OAP) with aniline (ANI) were carried out. Electropolymerization at a constant potential was performed on indium tin oxide (ITO)-coated glass electrodes in aqueous sulfuric acid (0.5 M). Spectroelectrochemical results revealed the formation of an intermediate in the initial stage of copolymerization through the cross-reaction of OAP cation radicals and ANI cation radicals resulting in a head-to-tail dimer or oligomer. An absorption peak at 520 nm in the UV–vis spectra was assigned to this intermediate. The FTIR spectral analysis of the copolymer clearly demonstrates the incorporation of OAP units into the polymer backbone during polymerization.
Keywords: Spectroelectrochemistry; Copolymerization; Intermediates; Polyaniline; Poly(o-aminophenol);

CO2-soluble semiconducting polymers synthesized in supercritical carbon dioxide by Hullathy Subban Ganapathy; Haldorai Yuvaraj; Ha Soo Hwang; Jong Su Kim; Byung-Chun Choi; Yeong-Soon Gal; Kwon Taek Lim (576-581).
By modifying the side chain organization of substituted polythiophenes with fluoroalkyl esters, highly CO2-soluble conjugated polymers were obtained. The polymers were prepared by oxidative polymerization of 2-(3-thienyl)acetyl 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1-octanate (SFTE) and 2-(3-thienyl)ethyl perfluorobutyrate (FTE) with FeCl3 in supercritical carbon dioxide (scCO2) at 207 bar and 40 °C. The properties of polymers, such as yield, molecular weight, electrical conductivity, and UV–vis absorption, were investigated and compared with those prepared in chloroform. The polymers showed a good solubility in CO2 at moderate pressure and temperature with higher solubility of PSFTE than PFTE.
Keywords: Polythiophenes; Supercritical carbon dioxide; Conjugated polymers; Fluoroalkyl; Ester functionalized; CO2-soluble;

New fluorene–bithiophene-based trimers as stable materials for OFETs by Heiko Thiem; Peter Strohriegl; Sepas Setayesh; Dago de Leeuw (582-589).
We report the synthesis of five new 5,5′-bis(9,9′-dialkylfluorene-2-yl)-2,2′-bithiophenes 4ae as active materials for the use in OFETs. Depending on the type of alkyl substituents crystalline or amorphous materials are obtained. Cyclovoltammetry shows that the materials are electrochemically stable and have a HOMO level at −5.3 eV. The different morphologies of thin films of 4ac have great influence on the performance of the materials in OFETs. The field effect mobilities are in the range of 10−5  cm2/Vs in an amorphous film of 4c to 3 × 10−3  cm2/Vs in a polycrystalline film of 4a. This high mobility remains constant after 3 months at ambient conditions, which proofs the high environmental stability of this class of materials.
Keywords: OFET; Fluorene; Bithiophene; Suzuki cross coupling; Morphology;

Polyfluorenes with appended polyhedral oligomeric silsesquioxanes (POSSs) were successfully synthesized through Ni(0)-mediated polymerization. The resulting copolymers exhibit good solubility in common organic solvents such as chloroform, chlorobenzene, etc. and good thermal stability. The PL quantum yields of the copolymers were drastically enhanced compared to PDHF because the POSS-functionalized PFs strongly suppressed intermolecular aggregation and/or thermal oxidation and crosslinking. This effective dilution effect and the higher stability of the POSS showed a deep blue emission in electroluminescent devices. LED devices with these polymers showed low turn-on voltage of near 4.0 V and brightness of 260–460 cd/m2.
Keywords: Polyhedral oligomeric silsesquioxane (POSS); Yamamoto coupling; Electroluminescence;

Polypyrrole (PPy) films with well-ordered corrugations were produced using the following three-step procedure. First, an elastic silicone sheet was stretched by 20% and fixed on a frame. Second, it was coated with gold using an ion sputtering technique. In this step, the gold coating spontaneously corrugated. Finally, PPy was synthesized by an electrochemical reaction on the gold coating that was used as the electrode. The obtained PPy film was corrugated because the corrugated gold coating worked as a template. The wavelength of the controlled corrugation ranged from 7.3 to 41.0 μm by controlling the thickness of the gold coating from 10 to 100 nm.
Keywords: Electrochemical deposition; Polypyrrole; Microstructuring;

Ab initio quantum-chemical methods have been used to investigate how the inclusion of ester groups on polythiophene chains can affect some properties of the polymer. For this purpose, MP2/6-31G(d,p) calculations have been performed on three isomers: dimethyl 2,2′-bithiophene-4,4′-dicarboxylate, dimethyl 2,2′-bithiophene-3,3′-dicarboxylate and dimethyl 2,2′-bithiophene-3,4′-dicarboxylate, which mimic the tail-to-tail, head-to-head and head-to-tail polymer linkages. After considering different arrangements of the ester substituents for each isomer, results allow conclude that introduction of ester groups at the 4,4′-positions does not alter the properties of 2,2′-bithiophene. In contrast, 3,3′- or 3,4′-disubstitutions produce significant changes in both the structural and electronic properties. These results may assist in designing soluble polythiophene-based polymers.
Keywords: 2,2′-Bithiophene; Structural properties; Electronic properties;

Electrocatalytic oxidation of ascorbic acid at polypyrrole nanowire modified electrode by Junsheng Wang; Jixiao Wang; Zhi Wang; Shichang Wang (610-613).
Polypyrrole (PPy) nanowire modified electrodes were prepared electrochemically by template-free method based on graphite electrodes. The freshly prepared electrodes were dipped in 10% HClO4 solution at least 24 h for removal of carbonate ions. The modified electrodes toward ascorbic acid were characterized by potentiostatic method. The experiment's results show that the PPy modified electrodes have obvious electrocatalytic effect toward ascorbic acid oxidation. The oxidation current density has a good linearity in the concentration range of 5.0 × 10−4 and 2.0 × 10−2  mol L−1 of ascorbic acid. The determination sensitivity may be significantly affected by the thickness of PPy film and pH of the test solution. The method has promising application in determination of ascorbic acid in the real samples.
Keywords: Polypyrrole; Nanowire; Electrocatalytic oxidation; Ascorbic acid;

Electrochemical and optical studies of the band gaps of alternating polyfluorene copolymers by Shimelis Admassie; Olle Inganäs; Wendimagegn Mammo; Erik Perzon; Mats R. Andersson (614-623).
The electrochemical and optical properties of a series of alternating polyfluorene copolymers with low band gaps were determined. These polymers incorporated fluorene units alternating with groups including electron-withdrawing (A) and electron-donating (D) groups in donor–acceptor–donor (DAD) sequence to achieve the lowering of band gaps. The polymers were solvent-casted on platinum disk electrode and the band gaps were estimated from cyclic voltammetry (CV). These values were compared with values obtained from optical absorption measurements. Although the electrochemically determined band gaps were found to be slightly higher than the optical band gap in most cases, values are well correlated. The values of the band gaps determined range from 2.1 to 1.3 eV.
Keywords: Optical band gap; Electrochemical band gap; Polyfluorene copolymers;

Two-diode organic light amplifiers/converters and peculiarities of photocurrent multiplication by Leonid Lepnev; Andrei Vaschenko; Alexei Vitukhnovsky; Svetlana Eliseeva; Oksana Kotova; Sofia Torgova; Natalia Kuzmina (624-632).
The novel tandem two-diode organic amplifier/converter of light based on successively connected photosensitive and light emission cells with spatial disjunction of processes of photocurrent multiplication (PM) and electroluminescence (EL) has been proposed and realized. The terbium complex Tb(Sal)3(TPPO)2 (HSal – salicylic acid, TPPO – triphenylphosphine oxide) or aluminum tris-(8-hydroxyquinoline (Alq3)) have been used as active layers in light emission cell of device as well as the perylene pigment Me-PTC (N,N″-dimethylperylene-3,4,9,10-bis(decarboximide)) in the photosensitive one. The suggested method of amplification/conversion has some advantages: (1) it avoids the necessity to adjust HOMO- and LUMO levels of photosensitive and electroluminescent layers and thus, extends the range of suitable light emissive materials including ones with high emission characteristics, (2) it also eliminates reabsorption of light by photosensitive part of the system and so, extends spectral band of amplification/conversion. A modernized model of field-activated structural traps has been suggested. Kinetics of PM during light- and voltage switching with time pauses is explained using this model. The peculiarities of PM and its kinetics at different light intensities, temperatures and applied voltages are analyzed too. Optimal values of temperatures, voltages and light excitation intensities providing high PM have been found. The PM gain up to 105-fold has been achieved. The conversion of long-wave (λ  = 600 nm) light into narrow emission bands of Tb(Sal)3(TPPO)2 (λ max 545 nm) and wide-band (from 490 nm) emission of Alq3 has been obtained. Up- and down-conversion of light along with enhancement of emission have been observed in both devices. A computer simulation of the tandem-diode amplifier/converter has been performed and operation conditions providing transition from the up-conversion mode to the enhancement of light mode have been studied. It has been demonstrated that the processes of PM and the bias redistribution between the units of device exert strong influence to one another and only their unification leads to the tandem-diode operation.
Keywords: Organic light-emitting devices; Up-conversion of light; Photocurrent multiplication;

The effect of channel length on turn-on voltage in pentacene-based thin film transistor by Jae Bon Koo; Jung Hun Lee; Chan Hoe Ku; Sang Chul Lim; Seong Hyun Kim; Jung Wook Lim; Sun Jin Yun; Taehyoung Zyung (633-636).
We report on the influence of channel length on transfer characteristics of pentacene-based thin film transistor (TFT). As the channel length is reduced from 50 to 5 μm, turn-on voltage (V turn-on) is shifted to more positive values, regardless of surface treatments and gate insulators. Especially in case of relatively short channel TFTs having the channel length of below 10 μm, multi-channel operation behavior has been observed and resulted in hump-shaped transfer characteristics. The positive shift behavior of V turn-on is similar to the threshold voltage reduction behavior by drain induced barrier lowering in Si devices and multi-channel operation can be explained by the channel length variation inevitably obtained after lift-off patterning of Au/Ti source–drain metal in relatively short channel TFTs.
Keywords: Organic thin film transistor (OTFT); Pentacene; Turn-on voltage; Short channel; Drain induced barrier lowering (DIBL);

Application of soluble poly(3-alkylpyrrole) polymers on textiles by Richard. C. Foitzik; Akif Kaynak; Frederick M. Pfeffer (637-642).
Soluble conducting poly(3-decanylpyrrole) was directly applied to textiles as a nanoparticle emulsion, using a variety of techniques including hand-brushing, dipping and spray painting. These coatings were compared to those formed by chemical polymerization of 3-decanylpyrrole on the surface of the textile by solution, using vapor and spray polymerization methods. The coating formed using chemical polymerization methods had lower surface resistivity than that formed by direct application of a soluble polymer.It was observed that applied coatings of poly(3-decanylpyrrole) showed a smoother surface morphology with a more even dispersion compared to those formed by chemical methods.
Keywords: Conducting polymers; Poly(3-alkylpyrrole); Conductive textiles; Soluble polymers;