Synthetic Metals (v.156, #9-10)

Electrochemical synthesis of polypyrrole macro-tubes on aluminum substrate by Ali Eftekhari; Mohammad Harati; Mohammad Pazouki (643-647).
A novel structure of conductive polymer films was synthesized by electropolymerization on aluminum substrate under potentiostatic condition at a relatively high applied potential. A thick film of polypyrrole was electrodeposited onto aluminum substrate from an aqueous electrolyte solution of NaNO3 with pH 12 by applying a constant potential of 2.0 V versus SCE. This polypyrrole film has a good stability with strong adhesion to the substrate surface. However, the morphological structure of the film is different from those previously observed for conductive polymers. Large tubes (ca. 100–200 μm in diameter) are formed, which are spiraled around the cylindrical substrate electrode. Although, the internal channels of these polypyrrole macro-tubes are very wide, the polypyrrole synthesized is sufficiently dense, guaranteeing excellent mechanical stability for this novel morphological structure. On the other hand, such large walls of the macro-tubes have nano-structures.
Keywords: Large tubes; Morphological structure; Aluminum electrode; Potentiostatic; Spiral tubes;

Convergence of non-linear optical calculations for infinite polymers by P. Leiva; A. Martinez; P. Otto (648-653).
We have selected several polymers with small elementary cells, poly-HF, poly-H2O, poly-C2H2 and poly-CNH in order to study the convergence of static and dynamic (hyper) polarizabilities at the Coupled Perturbed Hartree–Fock crystal orbital (CO) level. The parameters determining the convergence are the number of k-points over the half Brillouin zone, the number of neighbouring cells treated exactly with the complete Hamiltonian, and the effect of long-range Coulomb interactions. We also present for the first time calculations of the static and dynamic second hyperpolarizabilities γ on polymers with middle-sized cells, the five-membered heterocyclic π-conjugated systems, at this level of theory taking into account the results of the convergence study.
Keywords: Polymers; Non-linear optical properties; Hyperpolarizability;

Spectroscopic characterization of polyaniline doped with transition metal salts by Celly M.S. Izumi; Vera R.L. Constantino; Ana Maria C. Ferreira; Marcia L.A. Temperini (654-663).
For the first time resonance Raman, X-ray absorption near edge structure (XANES) of Fe, Cu and N and electronic paramagnetic resonance (EPR) techniques were employed to characterize the complexes formed by the interaction of Fe(III), Cu(II) and Zn(II) with polyaniline (PANI), in emeraldine base form (EB-PANI). The formation of semiquinone segments (radical cation) upon coordination of Cu(II) and Fe(III) to EB-PANI were undoubtedly demonstrated by resonance Raman spectroscopy. The characteristic doublet band at ca. 1330 cm−1 attributed to ν C ― N + • is observed. Through Cu and Fe XANES spectra, no change in the metal ions oxidation state after PANI doping is observed. The EPR spectra of Cu-PANI and Fe-PANI are dominated by metal cation signal showing a broad signal for Cu(II) at g iso  = 2.138 and a typical low spin signal of Fe(III) at g  = 4.421 and 1.903. The presence of radical cations segments is confirmed in Zn-PANI complex both through the very sharp EPR signal at g  = 2.004, and the characteristic radical cation resonance Raman spectrum. In addition, thermogravimetric data of Cu-PANI and Fe-PANI show a remarkable decrease in the polymer stability both in air and in nitrogen atmosphere.
Keywords: Raman spectroscopy; Polyaniline; EPR; Thermogravimetric analysis;

The electrosynthesis of poly(N-methylaniline) (PNMA) and poly(N-ethylaniline) (PNEA) coatings on mild steel in aqueous oxalic acid solutions was carried out by potentiodynamic synthesis technique. The effects of monomer and electrolyte concentrations on electrochemical growth of PNMA and PNEA coatings on mild steel substrates were investigated. Repassivation peak did not appear during electrosynthesis of PNMA and PNEA coatings from solutions containing 0.1 M monomer and 0.1 M electrolyte. The tests for corrosion protection of the polymer coated and uncoated mild steel substrates were done in 3% NaCl solutions by dc polarization and electrochemical impedance spectroscopy (EIS) techniques. Corrosion tests revealed that PNMA and PNEA coatings exhibited effective anti-corrosive properties. The acidity of the polymerization solution was found to influence the anticorrosive behavior of the polymer coating.
Keywords: Poly(N-methylaniline); Poly(N-ethylaniline); Electropolymerization; Mild steel; Corrosion; Impedance spectroscopy;

Bis(arylquinoxalinyl)carbazole derivatives as saturated blue emitters for electroluminescent devices by Chia-Hua Lin; Ya-Ting Chang; Chen-Shun Li; Charng-Hsing Liu; Jiun-Pey Duan (671-676).
Bis(arylquinoxalinyl)carbazole derivatives 9-phenyl-3,6-bis-(3-phenyl-quinoxalin-2-yl)-9H-carbazole (PPQC), 9-phenyl-3,6-bis-[3-(4-methoxy-phenyl)-quinoxalin-2-yl]-9H-carbazole (PMQC) and 9-phenyl-3,6-bis-(3-p-tolyl-quinoxalin-2-yl)-9H-carbazole (PTQC) were conveniently synthesized from the corresponding tetraone and o-phenylenediamine. Electroluminescent devices using PTQC as the dopant emitters were fabricated. These devices all emit saturated blue light from the PTQC doped layer. Device C that consists of CuPc (10 nm)/TCTA (30 nm)/PTQC:TPBI (7%, 30 nm)/TPBI (40 nm) shows the highest performance. The device emits blue light at 446 nm with CIE values of (0.15, 0.08) and shows a maximum external quantum efficiency of 1.31%, current efficiency of 1.00 cd/A, and brightness of 5067 cd/m2.
Keywords: Bis(arylquinoxalinyl)carbazole derivatives; Blue emitter; Electroluminescence; Saturated blue light; Organic light emitting diode;

Vinyloxyethyl-substituted triphenylamine-based hydrazone and its adducts with 9-ethyl-3,6-dihydroxymethylcarbazole and 4,4′-thiobisbenzenethiol were synthesized and studied as glass-forming hole transport materials. The thermal, optical and photoelectrical properties of the synthesized materials were studied. All the synthesized compounds form glasses with the glass transition temperatures ranging from 21 to 114 °C. Their ionisation potentials established by electron photoemission technique range from 5.34 to 5.49 eV. The time of flight hole mobilities in the layers of the solid solutions of one synthesized hydrazone in bisphenol Z polycarbonate reach 10−4  cm2/V s at high electric fields.
Keywords: Hydrazone; Vinyl ether; Molecular glass; Ionisation potential; Charge transport;

Luminescence properties of MEH-PPV and its crosslinked polymer: Effect of crosslink on photoluminescence and electroluminescence by Dong Hoon Choi; Min Ju Cho; Kyu Il Han; In-Hee Chang; Jong Seok Song; Jae Hong Kim; Sang-Hyon Paek; Suk-Ho Choi (685-689).
Photoluminescence (PL) and electroluminescence (EL) of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH-PPV) and its crosslinked polymer have been investigated in this work. EL and PL of MEH-PPV were affected in both their spectral shape and intensity after introducing a small degree of crosslink between the polymer chains. It is suggested that the EL enhancement can be attributed to the chemical structural variation by the crosslink in the network structure.
Keywords: Phenylene vinylene polymer; Crosslink; Electroluminescence; Photoluminescence; Network structure;

Polymer light-emitting diodes with a phenyleneethynylene derivative as a novel hole blocking layer for efficiency enhancements by G. Wantz; O. Dautel; L. Vignau; F. Serein-Spirau; J.P. Lère-Porte; L. Hirsch; J.J.E. Moreau; J.P. Parneix (690-694).
This paper reports on the use of an electron transport layer (ETL) in polymer light-emitting diodes based on poly(2,5-bis(3′,7′-dimethyl-octyloxy)1,4-phenylene-vinylene) (BDMO-PPV). This ETL is inserted between BDMO-PPV and a calcium cathode as a hole blocking layer (HBL). A novel phenyleneethynylene derivative (ImPE) is proposed and compared to well-known materials such as tris(8-hydroxyquinoline) aluminum (Alq3) and bathocuproïne (BCP). Efficient hole blocking is achieved leading to yield improvements at low luminances. With a 8 nm-thick ImPE layer, at 1 cd/m2, the power efficiency reaches 1.2 lm/W whereas a BDMO-PPV-only PLED exhibits a 0.13 lm/W power efficiency. ImPE enables to reach higher performances than Alq3 for low luminances (<20 cd/m2). However, for luminances higher than 350 cd/m2, it is demonstrated that the hole blocking in no more efficient because of a too strong electric field.

In relation to electrochromic (EC) device performance, it is important to avoid unwanted power consumption and to minimize electrochemical degradation by using an EC material, which exhibits the desired EC state under no bias voltage in most of the time. Regarding this point, we prepared single- and dual-type EC devices based on poly(N-methyl-N′-(6-carbazole-1-ylhexyl)-4,4′-bipyridinium dihexafluorophosphate) [P(Cz-V)]. Since polycarbazole is oxidative-coloring and viologen is reductive-coloring, the supplementary EC action provides variety in color, which cannot be achieved by individual EC materials. In the single-type device, the polymer became green P(Czα+-V2+) at 3 V (versus ITO), bleached P(Cz0-V2+) at 0 V, and violet P(Cz0-V• +) at −3 V. In the dual-type device, the complementary action of (Czα+-V2+) and P(Cz0-V• +) upon a bias voltage of 3 V showed higher EC contrast compared to the single-type device.
Keywords: Electrochromism; Electrochromic device; Polycarbazole derivative; Viologen;

A blue fluorescence of N-ethylcarbazole (EtCz) and of its dimer N,N′-diethyl-3,3′-bicarbazyl ((EtCz)2) as dopants of poly(methyl methacrylate) (PMMA) matrices, under laser pulsed UV irradiation, was observed. The fluorescence lifetimes were found to be 16.1 and 11.6 ns for EtCz and (EtCz)2, respectively. Concomitantly, a green phosphorescence occurred at room temperature and peaking at 444 nm for EtCz and 486 nm for (EtCz)2. From both luminescence spectra, singlet–triplet energy transition was estimated to be 0.56 eV for EtCz and 0.45 eV for (EtCz)2. Moreover, under UV irradiation, photoluminescence spectra exhibited a strong degradation with exposure time, accompanied by the appearance of new bands in the 400–500 nm range. Species associated to these new signals exhibited shorter lifetimes than those of the main peaks. Photodegradation of the doped PMMA matrices were not observed under nitrogen atmosphere, whereas in the presence of oxygen gas, they readily appeared. In the same way, thin film of pure (EtCz)2 did not show any modification under vacuum, whereas in air the photochemical evolution clearly appears. Possible causes of the oxygen-sensitive photodegradation of both the EtCz and (EtCz)2-doped PMMA matrices, were discussed.
Keywords: Fluorescence; Phosphorescence; Photodegradation; Carbazoles; Triplet; Doped-PMMA;

The electrochemical and optical absorption properties as well as the thermal stabilities of a series of 1,3,4-oxadiazole dimers 1,3-bis[2-(4-methylphenyl)-1,3,4-oxadiazol-5-yl]benzene (OXD-X) and its derivatives with the different alkoxy substituents on the central benzene ring such as ―O(CH2) n−1CH3 (OXD-An, n  = 3, 7, 10, 16), ―O(CH2) n OC6H5CH3 (OXD-Bn, n  = 6), and ―O(CH2) n OC6H4N=NC6H4OCH3 (OXD-Cn, n  = 3, 6, 10) are studied. The DSC measurements exhibit dramatically elevated glass transition temperatures for OXD-X and OXD-An (120–245 °C) in contrast to the well-known PBD (∼60 °C), indicating the better thermal stabilities. From OXD-X to OXD-An, OXD-Bn or OXD-Cn, the cyclic voltammograms and UV–vis absorption spectroscopy display significant variation, in which the later three species show additional lower energy absorptions at λ  > 330 nm compared with OXD-X and particularly, both of OXD-Bn and OXD-Cn display an oxidation peak at ∼+1.0 V and two successive redox reactions occur for OXD-Cn. Theoretically, the B3LYP/6-31g calculations explore that these extraordinary properties are due to the influence of the substituents on the benzene ring to the frontier molecular orbital distribution, especially the ―O(CH2) n OC6H4N=NC6H4OCH3 groups in OXD-Cn deduce the new pictures of the frontier molecular orbitals, causing the electron-transporting behavior originally happening on the molecular skeleton transferred to the side chain.
Keywords: 1,3,4-Oxadiazole; Electrochemical property; Optical absorption spectrum; Density function calculation; Electronic structures;

A hollow micro-tubule structure comprised of the conducting polymer, PEDOT, was manufactured by the electrochemical polymerization of EDOT into a template membrane containing 1.2 μm pores. The tubules formed were then capped with a conducting PEDOT/PSS aqueous dispersion. In order to assist in effective capping, a piece of a PC membrane containing 0.05 μm pores was attached to the capping film of the aqueous dispersion. The cap was then reinforced with PEDOT via the electrochemical polymerization of EDOT in order to make it insoluble in aqueous solutions. The cavity of this micro-cylinder structure worked as a container retaining water-soluble electroactive materials and played the role as a current collector for the electrochemical reaction of the species retained inside. In this study, K3Fe(CN)6 was used as the probe material stored in the cylinders. The current collector exhibited a fine current response due to the redox reaction of the interior species. These results show that guest materials can be stored in this capped cylinder structure without the problems associated with the use of organic solvents during the manufacturing processes. Moreover, the most important part of the whole process can be monitored using simple electrochemical methods.
Keywords: Conducting composite; Micro-tubule; Capping film; Electropolymerization;

Polyaniline/TiO2 solar cells by Ziran Liu; Jingran Zhou; Hailin Xue; Liang Shen; Huidong Zang; Weiyou Chen (721-723).
We have used sol–gel spin-coating method to prepare nano-crystalline TiO2 films for two kinds of polyaniline-based solar cell. The junctions thus obtained show rectifying behaviour. Their IV characteristics indicate that a built-in electric field has been created at the nano-crystalline TiO2/polyaniline interface. Their IV characteristics and energy conversion efficiency under simulated solar irradiation are measured and discussed.

The electronic, structural, and chemical properties of polypyrrole doped with the hexafluorophosphate ion are investigated as a function of oxidation state. These properties are found to be highly correlated; specifically, they all experience a rapid transition at approximately −0.1 V versus SCE. The electronic conductivity of the film changes by two orders of magnitude through the transition potential, in the well-known doping-induced metal–insulator transition. Also at −0.1 V versus SCE, X-ray diffraction and macroscopic actuation measurements reveal a structural change in the polymer. Results suggest a loss of pi-stacking in the polymer crystals, and a reordering of the dopant ions in the matrix. The dopants appear to exist in an isotropic liquid-like state in the highly oxidized film, changing to a channel structure at the transition point. This change in structure is also consistent with the transition observed in the electrochemical impedance spectrum. The equilibrium charge on the polymer with oxidation state is consistent with a constant DC capacitance, however the AC impedance spectrum shows increased constant phase element behavior in the reduced state. This could be due to a greater range of diffusion constants in the reduced state, which we associate with the heterogeneous ion configuration.
Keywords: Polypyrrole; X-ray diffraction; Impedance spectroscopy; Metal–insulator transition;

The single layer devices utilizing poly[2-(carbazol-9-yl)-5-(2-ethylhcxyloxy)-1,4-phenylene vinylene] (CzEh-PPV) and poly[2-{4-[5-(4-tert-butylphenyl)-l,3,4-oxadiazolyl]-phenyl}-5-(2-ethythexyloxy)-l,4-phenylene vinylene] (OxdEh-PPV) doped with varying weight percent of 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran (DCM-1) were fabricated and their photo luminescence (PL) and electroluminescence (EL) properties were discussed in this investigation. The PL spectra of DCM-1 doped polymers show that the emission is mostly from DCM-1 and Förster energy transfer may occur between DCM-1 and the two polymers. On the other hand, field-dependence of the emission spectra of EL devices was observed in detail. For the CzEh-PPV/DCM-1, the emission at the wavelength of 534 nm remains unchanged as the level of DCM-1 increases, whereas the peak at 572 nm is intensified with increasing both the additive level and applied electric field. For the OxdEh-PPV/DCM-1, the main peak is red-shift as the level of DCM-1 increases and blue-shift as the applied electric field does.
Keywords: Photoluminescence; Electroluminescence; Poly(phenylene vinylene) derivatives; Förster energy transfer; Field-dependence; DCM-1 doping;

In this study, the oxidative polycondensation reaction conditions of 4-[(2-hydroxybenzylidene)amino]benzoic acid (4-HBAB) by using oxidants such as air O2, H2O2 and NaOCl were studied in an aqueous alkaline medium between 40 and 90 °C. The structures of the synthesized monomer and oligomer were confirmed by FT-IR, UV–vis, NMR and elemental analysis. The characterization was made by TG-DTA, size exclusion chromatography (SEC) and solubility tests. At the optimum reaction conditions, the yield of oligo-4-[(2-hydroxybenzylidene)amino]benzoic acid (O-4-HBAB) was found to be 68% (for air O2 oxidant), 70% (for air H2O2 oxidant) 53% (for NaOCl oxidant). According to the SEC analysis, the number-average molecular weight (M n), weight-average molecular weight (M w) and polydispersity index (PDI) values of O-4-HBAB were found to be 932, 1469 g mol−1 and 1.576, using H2O2, and 1895, 2560 g mol−1 and 1.354, using air O2 and 2320, 3015 g mol−1 and 1.300, using NaOCl, respectively. According to TG analysis, the weight losses of 4-HBAB and O-4-HBAB were found to be 96.86% and 73.10% at 1000 °C, respectively. O-4-HBAB was shown higher stability against thermal decomposition. Also, electrical conductivity of the O-4-HBAB was measured, showing that the polymer is a typical semiconductor. Electrochemical HOMO, LUMO and energy gaps (E g) of 4-HBAB and O-4-HBAB were found to be −6.28, −6.36; −2.39, −2.64; 3.89 and 3.72 eV, respectively. According to UV–vis measurements, optical band gap (E g) of 4-HBAB and O-4-HBAB were found to be 3.23 and 3.09 eV, respectively.
Keywords: Oxidative polycondensation; Oligo-4-[(2-hydroxy benzylidene)amino] benzoic acid; Air O2; NaOCl; H2O2; Thermal analysis; Conductivity and band gap;

Lithium surface protection by polyacetylene in situ polymerization by D.G. Belov; O.V. Yarmolenko; A. Peng; O.N. Efimov (745-751).
Direct acetylene polymerization at ambient temperature on a Li metal surface and polyolefin separator with Ziegler–Natta catalyst was developed. Thin conducting polyacetylene films with strong adhesion to Li or polyolefin surface was formed and studied by electrochemical methods. AC impedance measurements and charge/discharge tests were applied for prepared coin cells with LiCoO2 as a cathode material in the presence of 1 M LiPF6 in dimethyl carbonate, ethylene carbonate, propylene carbonate (5:3:2 vol) electrolyte. Three kinds of polyolefins were used to prepare composite separators with polyacetylene: polypropylene (PP), polyethylene (PE) and three-layer type: PP–PE–PP.The polyacetylene film thickness on Li metal or polyolefin separator was easily controlled by catalyst concentration and polymerization time. It was shown that high ionic conductivity (7.5 × 10−5  S cm−1) might be achieved due to higher porosity of a polyethylene separator. Li–polyacetylene electrodes showed highest ionic conductivity and stability in charge/discharge tests.
Keywords: Polyacetylene polymerization; Polyolefin/polyacetylene separator; Li surface protection; Impedance spectra;

A number of laboratories including our own have reported hysteresis, i.e. a memory effect, in the current–voltage characteristics of semiconducting polymers. Here we report further results on the hysteresis observed in electropolymerised films of a carbon-bridged dithiophene. In particular, we present spectroscopic evidence that shows the effect requires a high degree of conjugation and moderate doping of the polymer. The presence of mobile ions per se in the film is insufficient to cause the effect. Successive film samples produced from the same monomer solution show progressive changes in both the UV and FT-IR spectra consistent with reduced conjugation in the polymer coupled with reduced doping by the BF4 counterions. These changes correlate with a reduction in hysteresis and a general lowering of film conductivity. From the spectroscopic evidence we develop a possible band energy scheme which suggests that both aluminium and ITO should make rectifying contacts to the polymer thus confirming our earlier findings.
Keywords: Semiconducting polymers; Electropolymerization; Electronic memory; Polymer electronics; Diodes; Optical spectra;

Anomalous room temperature magnetoresistance in organic semiconductors by V.N. Prigodin; J.D. Bergeson; D.M. Lincoln; A.J. Epstein (757-761).
We report the substantial change in the large room temperature (∼8% at 100 Oe, up to 15% at 1000 Oe) magnetoresistance of thin organic semiconductor films of tris-(8-hydroxyquinoline) aluminum (Alq3) upon doping with PtOEP and Ir(ppy)3 complexes. The origin of magnetic field effects on charge transport properties of organic semiconductors until now has remained obscure. We propose a model for the anomalous magnetoresistance and its change with doping based on the charge transport in these semiconductors being electron–hole (e–h) recombination limited. The process of e–h recombination includes formation of correlated e–h pairs and the subsequent annihilation of e–h pairs with different rates for the singlet and triplet spin states. The e–h pairs may also dissociate back into free charge carriers. We suggest that a magnetic field controls spin interconversion of e–h pairs. In the absence of field the singlet mixes with the entire triplet manifold by hyperfine interaction. The magnetic field lifts the triplet degeneracy, and for strong field, the mixing remains only between the singlet and the T 0 component of the triplet, thus changing the e–h recombination rate and hence the current. The experimental results are consistent with the model.
Keywords: Organic semiconductors; Magnetoresistance; LEDs; TFTs; Transport measurements; Hyperfine interaction;