Synthetic Metals (v.155, #1)
Poly(fluorene-oxadiazole) copolymer-based light-emitting devices on a plastic substrate by Shu-Jen Lee; Joseph R. Gallegos; Julien Klein; M. David Curtis; Jerzy Kanicki (1-10).
We reported on one new series of light-emitting copolymers for polymer light-emitting devices (PLEDs), poly(dioctyl fluorene-co-diphenyl oxadiazole)s (P(DOF-DPO)s). The device structure used in this study was composed of: indium tin oxide (ITO) anode/PEDOT:PSS ((polyethvlenedioxythiophene)–poly(styrene sulfonate)) as hole-transporting layer/P(DOF) or P(DOF-DPO) as emissive layer/calcium + aluminum cathode. Both the photo-luminescence spectra and electro-luminescence spectra were slightly blue-shifted, when we added less than 10% DPO moiety into P(DOF-DPO). The maximum emission efficiency of the PLEDs based on P(DOF-DPO) with less than 10% DPO moiety are comparable to the P(DOF)-based PLEDs. However, when we added more than 20% DPO moiety into P(DOF-DPO), we found that P(DOF-DPO) PLEDs had significantly lower device efficiency than P(DOF) (poly(dioctyl fluorene)) PLEDs.
Keywords: Polymer light-emitting devices (PLEDs); Poly(dioctyl fluorene-co-diphenyl oxadiazole); Device efficiency;
Electrochemical synthesis of polypyrrole within PMMA nanochannels produced by AFM mechanical lithography by Jie-Ming Chen; Shang-Wei Liao; Yu-Chen Tsai (11-17).
A novel approach for the fabrication of polypyrrole nanowires via electropolymerization within poly(methyl methacrylate) (PMMA) nanochannels on an indium tin oxide (ITO) substrate is reported. The nanochannels width and depth obtained by atomic force microscopy (AFM) mechanical lithography on PMMA coated ITO substrate are about 150 and 35 nm. The nanochannels act as templates for electropolymerization of polypyrrole nanowires. The morphology of PMMA nanochannels and polypyrrole nanowires were investigated by AFM. The polypyrrole nanowires are around 350 nm in width and 20 μm in length. The conducting properties of polypyrrole nanowires were identified by AFM with a conducting tip (CT-AFM). The AFM current image shows that the current difference can be distinguished between doped polypyrrole nanowires and PMMA thin film. The present methodology demonstrates the feasibility and effectiveness of electropolymerization of polypyrrole nanowires within PMMA nanochannels produced by AFM mechanical lithography.
Keywords: Conducting polymers; Polypyrrole; Nanowires; Electropolymerization; AFM lithography;
Effects of monomer and electrolyte concentrations on actuation of PPy(DBS) bilayers by S. Maw; E. Smela; K. Yoshida; R.B. Stein (18-26).
Bilayers of gold-coated Kapton and polypyrrole doped with dodecylbenzenesulfonate PPy(DBS), were prepared electrochemically, and their bending movement was studied under electrochemical stimulation. The effects on actuation of monomer and electrolyte concentration during deposition, as well as electrolyte concentration during subsequent electrochemical cycling, are presented. Lowering the pyrrole concentration decreased hysteresis, somewhat increased actuator speed, and unexpectedly eliminated the so-called capacitive current above the oxidation peak. Neither pyrrole nor electrolyte concentration in the deposition solution affected the extent of movement, despite the fact that the former strongly impacted the cyclic voltammograms. The cyclic voltammograms were, therefore, not predictive of the extent of bending, and strain could not be significantly improved by altering the concentration of either monomer or salt in the deposition solution. Raising the deposition electrolyte concentration, and in particular raising it with respect to the pyrrole concentration, did increase the movement to charge ratio, making the actuators more efficient.
Keywords: Polypyrrole; Actuator; Bilayer; Concentration; Electrochemical actuation; Cyclic voltammograms;
Electronic properties of alkoxy derivatives of poly(para-phenylenevinylene), investigated by time-dependent density functional theory calculations by Songwut Suramitr; Teerakiat Kerdcharoen; Toemsak Srikhirin; Supa Hannongbua (27-34).
Geometries and energy gaps of poly(para-phenylenevinylene) oligomers (OPV N ) and their alkoxy derivatives were investigated, based on quantum-chemical calculations. This oligomer series includes poly(para-methoxy-PV) (DMO-OPV N ), poly(para-hexoxy-PV) (DHO-OPV N ) and poly(2-methoxy,5-(2′-ethyl-hexyloxy)-PV) (MEH-OPV N ). Potential energy hypersurfaces of all OPV2 and OPV2-alkoxy derivatives were calculated by the semiempirical AM1 and ab initio method at the HF/3-21G and HF/6-31G levels. The results obtained indicate that OPV2 provide two conformational structures, one coplanar and one twisted. For its alkoxy derivatives, the stable conformation was found to be that in which the two adjacent phenylene rings were coplanar. An intramolecular weak hydrogen bond interaction was also found to occur between the oxygen atom of the alkoxy derivatives and the hydrogen atom of the vinylene linkage. By using these linear relationships, they can be employed to semiquantitatively estimate the first excitation energy. We introduce the relationships with the working function of E expt = 0.604E TDDFT-B3LYP/6-31G + 0.947 and E expt = 0.604E TDDFT-B3LYP/6-31G* + 0.983, based on the geometry obtained from HF/3-21G for corrected the extrapolated energy gaps of DMO-OPV N , DHO-OPV N and MEH-OPV N . It was found that satisfactory linear relationship and TDDFT method can be used to predict the lowest excitation energies for compounds in these systems and applicable to the design of new conducting polymers.
Keywords: Excitation energy; Quantum-chemical calculations; Conductive polymer; Alkoxy derivatives of poly(para-phenylenevinylene);
Writing with conducting polymer by R.A. de Barros; C.R. Martins; W.M. de Azevedo (35-38).
In this paper, we present an original and straightforward route to prepare conducting polymer pattern in substrates such as, plastic, transparence sheet, glossy paper or in any substrate material found in standard working office. This process consist in replacing the conventional ink used in any DeskJet printer, for a solution of transition metal that will be used to print the desired pattern on a substrates previously soaked in an aqueous solution of conducting polymer monomer. Soon after the patterns are written a UV light is used to develop the printed characters. The measured conductivity of the printed conducting polymer patterns in glosss paper is the order of 2 × 10−2 S/cm. This contribution describes the use of a fast and low-cost technology to produce organic microstructures for microelectronic applications.
Keywords: Polyaniline; UV-irradiation; Silver nanoparticles;
Enhanced rectification through polymer-gold nanoparticle interaction by James L. Dewald; Wudyalew T. Wondmagegn; Amanda V. Ellis; Seamus A. Curran (39-44).
In this paper we report the fabrication of a new nanocomposite based on poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenlenevinylene) (PmPV) and gold (PmPV-gold) nanoparticles. Characterization of this new material using UV–visible absorption spectroscopy, Raman spectroscopy and near-field scanning optical microscopy showed encapsulation of the gold nanoparticles (∼20 nm) by the polymer. Wrapping of gold nanoparticles by the polymer provided higher quality electrical characteristics than the polymer alone, indicated by superior rectification and Schottky diode characteristics with ideality factors of 27.69 and 1.32 for pure PmPV and PmPV-gold nanocomposite cells, respectively. Barrier heights, calculated from the reverse saturation current, were 0.865 and 0.559 eV for PmPV and PmPV-gold nanocomposite cells, respectively.
Keywords: Poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenlenevinylene) (PmPV); Schottky diodes; Gold nanoparticles; Raman scattering; Near-field scanning optical microscopy (NSOM);
Electrochemical properties of poly-1-naphthylamine synthesized in the presence of ferrocenesulfonic acid by Xiang Li; Cheng Sun; Zhongbo Wei (45-50).
Ferrocenesulfonic acid (Fc) plays an important role in increasing the electrochemical polymerization rate of 1-naphthylamine (Nap). Poly-1-naphthylamine prepared in the presence of ferrocenesulfonic acid (PNapFc) was charaterized by FTIR, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The above results confirmed that Fc could be doped in poly-1-naphthylamine (PNpa). From FTIR spectra of PNapFc, it was revealed that polymerization reaction occurred by head-to-tail mechanism. Cyclic voltammograms (CVs) show that PNapFc has a rather high electrochemical activity compared with PNap in 0.5 mol dm−3 Na2SO4 solution at pH 5.00. Particularly, both PNpa- and PNapFc-modified electrodes can catalyze the electrochemical reduction of H2O2 in the 0.1 mol dm−3 citrate buffer with pH 5.00, but the catalytic activity of PNapFc is superior to that of PNpa, which is due to that Fc enhance the charge transfer ability in PNapFc film.
Keywords: Poly-1-naphthylamine; Ferrocenesulfonic acid; Electrocatalytic reduction; SEM; FTIR spectrum; H2O2;
Porphyrin dye-sensitization of polythiophene in a conjugated polymer/TiO2 p–n hetero-junction solar cell by Kohshin Takahashi; Youhei Takano; Takahiro Yamaguchi; Jun-ichi Nakamura; Chiho Yokoe; Kazuhiko Murata (51-55).
In the blended solid of poly(3-hexylthiophene-2,5-diyl) (P3HT) and porphyrin (TPP)/TiO2 p–n hetero-junction solar cells, a photo-induced charge transfer between P3HT and TPP accelerated the charge separation in the depletion layer formed at the P3HT + TPP/TiO2 interface, enhancing the photovoltaic properties. For the blended cell containing zinc porphyrin as TPP, the energy conversion yield of 0.26% was obtained under the illumination of solar simulated light AM1.5–100 mW/cm2.
Keywords: Organic solar cell; Porphyrin; Polythiophene; Photo-induced charge transfer;
High-efficiency red electrophosphorescent devices based on new osmium(II) complexes by Jianping Lu; Ye Tao; Yun Chi; Yungliang Tung (56-62).
In this paper, we report on highly efficient red-phosphorescent light-emitting diodes using a series of osmium complexes 1–3 doped into poly(N-vinyl-carbazole) (PVK) matrix as emitters, thermally stable 1,3,5-tris(4′-fluorobiphenyl-4-yl)benzene (F-TBB) as a hole-blocking layer, and Alq3 as an electron injection layer. The CIE 1931 chromaticity coordinates of complexes 1–3 are around (0.650, 0.347), (0.681, 0.317), and (0.696, 0.302), respectively, and remain almost unchanged over a wide range of operation voltages. The maximum luminous efficiencies reached 7.0, 3.5, and 1.2 cd/A for devices based on 10 wt.% of osmium complexes 1, 2 and 3, respectively, even with air stable aluminum as the cathode. We systematically studied the dependence of device performance on the osmium doping concentrations. It was found that the best device performance was observed at 10 wt.% doping concentration for all three osmium complexes. Both maximum luminance and luminous efficiency increased with increasing osmium complex concentrations in the beginning and reached maximum at 10 wt.% doping concentration. However, a further increase in the doping level resulted in a reduction in both device brightness and efficiency due to concentration quenching and triplet–triplet annihilation. This is consistent with the absolute photoluminescence quantum yields of PVK thin films doped with different concentration of osmium complexes.
Keywords: Light-emitting diodes; Electrophosphorescence; Osmium(II) complexes; Pyridyl pyrazolate; Pyridyl triazolate;
Electrical and optical properties of PPV and single-walled carbon nanotubes composite films by H. Aarab; M. Baïtoul; J. Wéry; R. Almairac; S. Lefrant; E. Faulques; J.L. Duvail; M. Hamedoun (63-67).
We present a study of the evolution of the structural and electrical properties of poly(paraphenylene vinylene) (PPV) and single-walled carbon nanotubes (SWNT) composites as a function of the concentration of carbon nanotubes. We used essentially scanning electron microscopy (SEM), resonant Raman scattering and X-ray diffraction measurements to investigate the spectroscopic characteristics of the composites. When the SWNT concentration increases, the optical properties exhibit modifications which are discussed in relation with the shortening of conjugated segments lengths and the increase of disorder in PPV. Electrical conductivity measurements show that conductivity of the resulting films increases by eight orders of magnitude as the carbon nanotubes mass fraction increases from 0% to 64%. This result can be explained by using a simple percolation path theory, resulting in an estimated percolation threshold around 2%.
Keywords: Poly(paraphenylene vinylene); Single-walled carbon nanotubes; Composites; Optical absorption; Raman spectroscopy; X-ray diffraction; Electrical conductivity;
Third-order optical nonlinearities of naphthalocyanine derivatives measured by resonant femtosecond degenerate four-wave mixing technique by Gang Fu; Takefumi Yoda; Kazuo Kasatani; Hiroaki Okamoto; Shunsuke Takenaka (68-72).
The third-order optical nonlinearities of several naphthalocyanine derivatives were measured under resonant conditions by the femtosecond degenerate four-wave mixing (DFWM) technique. Temporal profiles of the DFWM signal were measured with a time resolution of 0.3 ps, and were found to consist of at least two components, the coherent instantaneous nonlinear response and the slow response with a decay time constant longer than several hundred picoseconds. The latter can be attributed to the long-lived excited state and molecular rotational relaxation. The electronic molecular hyperpolarizability, γ e, was evaluated to be in the range (2.1–7.5) × 10−29 esu for these macrocycles, two orders of magnitude greater than the nonresonant values.
Keywords: Third-order optical nonlinearities; Molecular hyperpolarizability; Degenerate four-wave mixing; Naphthalocyanines; Resonant conditions;
Emission color tuning of new fluorene-based alternating copolymers containing low band gap dyes by Jonghee Lee; Nam Sung Cho; Jaemin Lee; Sang Kyu Lee; Hong-Ku Shim (73-79).
New fluorene-based alternating copolymers (PFPhPhCN, PFPhThCN, and PFThThCN) containing different comonomers have been designed and subsequently synthesized via Pd-catalyzed Suzuki polymerization. The synthesized polymers could be well characterized by 1H NMR, FT-IR, and elemental analyses. These polymers were found to be thermally stable and readily soluble in common organic solvents. The UV–vis absorption maxima of PFPhPhCN, PFPhThCN, and PFThThCN were 399, 456 and 499 nm, and the PL maxima were 484, 539 and 620 nm, respectively. The emitting color of the homopolymer, poly(9,9-dioctylfluorene-2,7-diyl) (PDOF), could be tuned by incorporating various low band gap dyes into the polymer main-chain. The absorption and emission maxima of the copolymers were varied according to the type of incorporated aromatic group (thiophene or phenylene). In particular, PFThThCN exhibited almost pure red emission (chromaticity values x = 0.63, y = 0.37).
Keywords: Color tuning; Conjugated polymers; Light-emitting diodes (LED);
Spectroscopic and conductivity studies of doping in chemically synthesized poly(3,4-ethylenedioxythiophene) by William W. Chiu; Jadranka Travaš-Sejdić; Ralph P. Cooney; Graham A. Bowmaker (80-88).
Spectroscopic methods (Raman (514.5 nm excitation), infrared (IR), X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR)) and electrical conductivity measurements were used to characterize the electrically conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) prepared by persulfate oxidation under aqueous conditions. Elemental analysis was carried out on the resulting sulfate-doped PEDOT to determine the sulfate doping level. Due to the difficulty in dedoping highly stable p-doped PEDOT, the sulfate-pre-doped PEDOT was directly treated with iodine solution in order to investigate the secondary doping processes between PEDOT and iodine. Two processes were observed: (1) the further oxidation of the polymer by iodine to produce an increase in the doping level, with triiodide as the dopant and (2) the replacement of sulfate by triiodide in an ion-exchange process involving triiodide in equilibrium with iodine in the iodine solution. Conductivity studies, in conjunction with XPS and EPR experiments, were used to analyse the relationship between the doping level and conductivity that was explained by differences in the nature of SO4 2− and I3 − as dopant ions and a shift in the polaron/bipolaron equilibrium. The Raman spectra (785 nm excitation) of these PEDOT samples were also investigated and a significant change has been observed in the wavenumber of the symmetric C α = C β stretching bands with varying levels of dopants. A correlation was observed between the ratio of the integrated intensities of the symmetric C α = C β stretching bands and the doping level in PEDOT that can be useful for estimating the doping level of a PEDOT sample from its Raman spectrum.
Keywords: Poly(3,4-ethylenedioxythiophene); Raman; Electron paramagnetic resonance; Electrical conductivity; Doping; Ion-exchange;
A flexible strain sensor from polypyrrole-coated fabrics by Y. Li; X.Y. Cheng; M.Y. Leung; J. Tsang; X.M. Tao; M.C.W. Yuen (89-94).
Conductive-polymer coated fabrics have been investigated as intelligent materials in the past years. In this paper, a flexible strain sensor from polypyrrole-coated fabrics which is featured with high sensitivity, good stability is reported. The strategies used for enhancing the sensitivity and stability of the sensor include: (a) the formation of thin coatings of polypyrrole (PPy) on the surface of fabrics using the chemical vapor deposition (CVD) method, (b) low temperature polymerization of pyrrole, (c) introduction of large docecyl benzene sulfonate anion in PPy film and (d) annealing of the PPy-coated conductive fabrics. The conductivity–strain tests reveal that the developed sensor exhibits a high strain sensitivity of ∼80 for a deformation as large as 50%, while its good stability is supported by the small changes in conductivity and sensitivity over a storage time of 9 months. The effect of the temperature and humidity on the conductivity of the strain sensor is investigated.
Keywords: Strain sensor; Polypyrrole; Conductive polymer; Intelligent materials;
Electrocatalytic reduction of nitrate at polypyrrole modified electrode by Xiuling Zhang; Jixiao Wang; Zhi Wang; Shichang Wang (95-99).
Polypyrrole (PPy) nanowire modified electrode was prepared and its electrocatalytic behavior towards nitrate reduction was characterized. The electrode surfaces were investigated by scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX). The electrocatalytic activity of modified graphite electrode was compared with graphite, copper (Cu) and platinum (Pt) electrode. The nitrite concentration produced at PPy modified electrode is lower than that at any other electrodes, and its changing tendency is opposite to any other electrodes. The PPy nanowire modified electrodes were further modified by electrodeposition of Cu. The results show that the produced nitrite concentration gets higher with the increase of Cu content. The primary experimental results show that the modified electrode has promise use in the remove of nitrate from water.
Keywords: Electrocatalytic; Polypyrrole nanowires; Nitrate reduction; Chemical modified electrode;
Synthesis, structure and properties of a novel quinoid compound by Jiecong Tang; Ruifeng Zhang; Jiacong Shen (100-104).
A novel quinoid compound α,α,α′,α′-tetra(4-tert-butylphenyl)-1,3,4-oxadiazole-quinodimethane (TPOQ) was synthesized through oxidative dehydrogenation in basic condition. The structure was characterized by elemental analysis, 1H NMR, and mass spectrum. UV–vis spectra, photoluminescence (PL) spectra, and electron spin resonance (ESR) measurements showed that the compound had a small bandgap and π–π* transition process produced an increase of polarity. Pure red emission at near 670 nm was observed from a single layer electroluminescence (EL) device, the emission color was unchanged with increase of applied voltage, which showed that TPOQ could be used as red emission material.
Keywords: Oxadiazolequinodimethane derivative; Small bandgap; Positive solvatochromism; Electroluminescence; Red emission;
Preparation of organo-soluble polyanilines in ionic liquid by Niyazi Bıçak; B. Filiz Şenkal; Esma Sezer (105-109).
A method for preparation of organo-soluble polyaniline (PANI) is described. Oxidative coupling polymerization of anilium chloride with ammonium persulfate in a new ionic liquid, 2-hydroxyethyl ammonium formate (HAF), gives organo-soluble polyaniline with appreciable molecular weights (M w = 86,400). Interestingly polyaniline (PANI) prepared by this method is highly soluble in many organic solvents such as acetone, tetrahydrofurane, dioxane, dimethyformamide and N-methyl, 2-pyrrolidinone. Thin films of PANI prepared at 0 °C (by solvent casting) show reasonable conductivities (up to 37.0 S cm−1) when doped with p-toluene sulfonic acid.
Keywords: Organo-soluble polyaniline; Ionic liquid; Ammonium persulfate; Oxidative polymerization;
The growth of PANI films at 450 MPa by L.J. Shen; D.W. Gu; J.S. Li; H.Y. Zhou; W.R. Xiao; N.R. Yang (110-115).
A series of polyaniline (PANI) films were prepared on quartz substrates by in situ polymerization with different reaction time under 450 MPa of hydrostatic high pressure. For investigating the effect of high pressure on the growth of the films, two additional series of samples with comparable conditions under ambient pressure were synthesize. The aniline cation radicals nucleated within 2 min and the polyaniline films grew within 10 min at 450 MPa of pressure. This compared with about 10 min for nucleation and 60 min for growth of polyaniline films under comparable ambient conditions. The process of PANI film growth under high pressure was obviously different from that in ambient pressure case. The high pressure used in polymerization affected directly the electrical property, the morphology, and growth of the films. A formation model of polyaniline film growth in high pressure case was proposed.
Keywords: Polyaniline; High pressure; In situ polymerization; Film growth;
Synthesis and electroluminescence of thiophene-based bipolar small molecules with different arylamine moieties by Tik H. Lee; K.L. Tong; S.K. So; Louis M. Leung (116-124).
A series of fluorescent dyes consisted of a thiophene unit, an 1,3,4-oxadiazole unit and four different arylamine moieties were prepared using a facile multi-steps synthetic route with high yield. The four arylamine structures studied were triphenylamine, N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine, diphenyl(1-naphthyl)amine and 9-phenylcarbazole. Experimental results shown that their HOMOs varied from 5.21 to 5.73 eV strongly affected by the arylamine chemistry while their LUMOs remained relatively unchanged. Their corresponding emission colors ranged from UV (393 nm) to bluish green (483 nm). In general, the thiophene unit enhanced the overall thermal stability of the compounds. According to cyclic voltammetry, the compounds are predominantly hole-transporting while OLED results indicated cpd 10 possess both hole and electron transport properties. Single layer OLED fabricated from 10 resulted in ca. 2000 cd/m2 (luminous intensity) and 1.10 cd/A (current efficiency) max, whereas, a multilayer OLED using 10 as the hole transporting layer achieved over 7400 cd/m2 and 2.3 cd/A max.
Keywords: Thiophene; Arylamine; Electroluminescent dyes; OLED;
Luminescent monomer of substituted tetrastyrylpyrene and poly(p-phenylenevinylene) derivative with pyrene segments: Synthesis and photophysics by John A. Mikroyannidis (125-129).
1,3,6,8-Tetrabromopyrene reacted with 1-tert-butyl-4-vinylbenzene to afford the luminescent monomer M. In addition, the reaction of 1,6-dibromopyrene with 1,4-didodecyloxy-2,5-divinylbenzene yielded a poly(p-phenylenevinylene) (PPV) derivative P. M dissolved partially in chloroform and completely in THF. P showed an excellent solubility being readily soluble in common organic solvents. M emitted intense blue–green light in THF solution with photoluminescence (PL) maximum at 507 nm and green–yellow light in thin film with PL maximum at 570 nm. Polymer P displayed in THF a PL maximum at 464 nm and a shoulder at 513 nm. Thin film of this polymer exhibited PL maximum at 572 nm. Both M and P emitted from excimers in solid state. The PL emission quantum yields in THF were 0.35 for M and 0.32 for P. The emission maximum of M thin film was red shifted by 9 nm after annealing at 150 °C for 24 h as a result of conformational changes of the molecules with increase in temperature.
Keywords: Tetrastyrylpyrene; Poly(p-phenylenevinylene); Pyrene; Light-emitting polymer; Photophysics; Synthesis;
PEDOT/PAMPS: An electrically conductive polymer composite with electrochromic and cation exchange properties by Gursel Sonmez; Philippe Schottland; John R. Reynolds (130-137).
Poly(3,4-ethylenedioxythiophene) (PEDOT)/poly(2-acrylamido-2-methyl-l-propane sulfonate) (PAMPS) composite films were electrochemically prepared from a mixture of water and N,N-dimethylformamide (DMF) containing 3,4-ethylenedioxythiophene (EDOT) and the polyelectrolyte, PAMPS. The presence of PAMPS in the PEDOT matrix was confirmed by spectroscopic and electrochemical methods. Depending on the current density, the conductivity of PEDOT/PAMPS free standing composite films reached values of 80 S/cm. Spectroelectrochemistry of neutralized PEDOT/PAMPS composite films showed a maximum absorption at 2.0 eV (615 nm) and a band gap of 1.65 eV, as calculated from the onset of the π–π* transition. Thin PEDOT/PAMPS composite films were found to switch rapidly between oxidized and reduced states in less than 0.4 s with an initial optical contrast of 76% at λ max: 615 nm. The morphology of the polymer composites demonstrates a cauliflower structure. Despite the high molecular weight of the polyelectrolyte, the film density was found to be similar to classical PEDOT (i.e., ca. 1.4 g/cm3), while the surface roughness averaged below 5%. As expected with the use of a sulfonated polyelectrolyte as dopant, cation exchange properties were observed with hexaammineruthenium(III) chloride as an active electrolyte.
Keywords: Conducting polymer; Poly(3,4-ethylenedioxythiophene); PEDOT; Poly(2-acrylamido-2-methyl-l-propane sulfonic acid); Cation exchange; Spectroelectrochemistry;
On the electronic transport properties of pyrrolo[1,2-a][1,10]phenanthroline derivatives in thin films by L. Leontie; I. Druta; R. Danac; G.I. Rusu (138-145).
The temperature dependences of electrical conductivity, σ, and Seebeck coefficient, S, for six new synthesized pyrrolo[1,2-a][1,10]phenanthroline compounds, have been investigated. Thin film samples (d = 0.05–0.66 μm) deposited by an immersion method (with dimethylformamide as solvent) onto glass were used.The crystalline structure of as-prepared organic films was investigated by XRD. The sample surface was examined by means of AFM and optical microscopy techniques.The ln σ = f(103/T) and S = f(103/T) experimental curves are quite typical for polycrystalline semiconducting materials. The examined organic compounds show a p-type electrical conduction. The activation energy of electrical conduction ranged between 1.42 and 2.04 eV, while the ratio of charge carrier mobilities was in the range 0.76–0.90.The study of optical absorption evidenced direct bandgaps ranged between 4.06 and 4.11 eV, as well bandgaps of 2.69–3.58 eV for amorphous phases.Some relationships between materials parameters and molecular structure of the compounds are established. The model based on band gap representation is suitable for the explanation of charge transport in the studied compounds.
Keywords: Heterocycle synthesis; Solution processing; Atomic force microscopy; Transport measurements (conductivity); Polycrystalline thin films; Organic semiconductors based on conjugated molecules;
Third-order nonlinear optical property of poly(2,5-dihexyloxy)-p-phenylene vinylene modified by N+ ion implantation by Baoming Li; Hongcai Wu; Wenhui Yi; Chao Gao; Xiaoqi Li (146-149).
A soluble poly(2,5-dihexyloxy)-p-phenylene vinylene (PDHOPV) was prepared via dehydrochlorination reaction from hydroquinone and n-hexyl bromide as raw materials. The PDHOPV films implanted by 20 keV nitrogen ions (N+) in the dose range of 3.8 × 1015–9.6 × 1016 ions/cm2 were characterized by UV–vis absorption spectra, which showed the optical absorption was raised gradually in the visible region followed by a red shift of optical absorption threshold and the band gap (E g) was reduced from 2.12 eV to 1.71 eV with the increase of ion dose. The third-order nonlinear optical (NLO) susceptibilities (χ (3)) were measured by degenerate four-wave mixing (DFWM) technique at 532 nm. The results demonstrated that the χ (3) value of PDHOPV film could be enhanced effectively by means of N+ ion implantation. When the ion dose was 3.8 × 1016 ions/cm2, the χ (3) value of PDHOPV film was maximized to 1.06 × 10−8 esu which was almost eleven times larger than that of the pristine film. Moreover, the mechanism responsible for the variation of χ (3) value of PDHOPV film with N+ ion dose was also discussed tentatively.
Keywords: Poly(2,5-dihexyloxy)-p-phenylene vinylene; Ion implantation; Degenerate four-wave mixing; Third-order nonlinear optical susceptibility;
Photogeneration and photovoltaic effect in blends of derivatives of hexabenzocoronene and perylene by Jaroslaw Jung; Andrzej Rybak; Agnieszka Slazak; Slawomir Bialecki; Pawel Miskiewicz; Ireneusz Glowacki; Jacek Ulanski; Silvia Rosselli; Akio Yasuda; Gabriele Nelles; Željko Tomović; Mark D. Watson; Klaus Müllen (150-156).
Photogeneration quantum yield in layers of discotic liquid crystals (DLC, hole transport material), perylene derivative (electron transport material) and of their mixtures were determined by using surface potential decay technique. While in pure DLC only very weak photogeneration in the visible range was detected, and in pure perylene derivative only negative mobile charge carriers were photogenerated, in the mixtures a synergetic effect of photogeneration of both positive and negative mobile charge carriers was observed. This is because the excitons photogenerated at perylene derivative dissociate into electron–hole pairs, and then electrons are transported via perylene derivative while the holes are transferred onto the DLC molecules.The obtained results were correlated with performance of the photovoltaic devices with active layer made of the mixtures of the DLC and perylene derivative and of their blends with photoconducting polymer. It was found, that the power conversion efficiency of the device is dependent on the film morphology.
Keywords: Photogeneration quantum yield; Photovoltaic effect; Discotic liquid crystal; Perylene derivative;
Preparation and structures of electrospun PAN nanofibers as a precursor of carbon nanofibers by S.Y. Gu; J. Ren; Q.L. Wu (157-161).
Polyacrylonitrile (PAN) nanofibers as a precursor of carbon nanofibers with diameters in the range of 130–280 nm were obtained by electrospinning of PAN/DMF solution. Morphologies and structures of PAN precursors were investigated by scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD), infrared spectrometer (IR) and differential scanning calorimeter (DSC). Distribution of diameters of PAN precursor showed log normal distribution. Molecular chains were oriented within the electrospun fibers during the electrospinning process. The cyclization exothermic peak shifted to lower temperature for electrospun nanofibers, which suggested that cyclization was more easily initiated at lower temperature for electrospun nanofibers.
Keywords: Nanofibers; Electrospinning; Polyacrylonitrile (PAN); Structures; Thermal characteristics;
Effect of the dilution in polypropylene on photophysical properties of poly(3-alkylthiophenes) by Katsuichi Kanemoto; Makoto Shishido; Tatsuji Sudo; Ichiro Akai; Tsutomu Karasawa; Yasuyuki Agari (162-167).
Optical properties on the basis of absorption and photoluminescence (PL) have been examined for regioregular (RR-) poly(3-octylthiophene) (P3OT) and regiorandom (RRa-) P3OT diluted in polypropylene (PP). RR-P3OT gives blue-shifted spectra in both absorption and PL against the ones in the spin coated (SC) film, contrary to the case when diluted in poly-(methyl methacrylate) (PMMA). Spectroscopic features indicate that isolated species are primarily generated for RR-P3OT by the dilution using PP. The comparison in PL spectra between in the SC film and in diluted sample demonstrates that the species responsible for PL in the SC film are interchain excitons in RR-P3OT and intrachain excitons in RRa-P3OT. We further identified the enhancement in the PL quantum efficiency of RRa-P3OT as well as RR-P3OT by the dilution using PP, indicating that PL of conjugated polymers can be enhanced in the solid state by dilution using moderate inert polymers. We infer that RR-P3OT takes a highly rod-like structure in PP and that RRa-P3OT primarily takes a much less linear conformation in PP.
Keywords: Poly(3-alkylthiophene); Photoluminescence; Optical absorption; Dilution; Polypropylene;
Pure red electrophosphorescent organic light-emitting diodes based on a new iridium complex by Jia Gao; Han You; Junfeng Fang; Dongge Ma; Lixiang Wang; Xiabin Jing; Fosong Wang (168-171).
A new iridium complex was synthesized and demonstrated a saturated red light emission in organic light-emitting diodes (OLEDs). The maximum brightness of 2800 cd/m2 and the external quantum efficiency of 5.5% were achieved in multilayer OLEDs. The peak wavelength of the emission was found to be at 677 nm with the Commission Internationale de l’Eclairage (CIE) coordinates of (0.71, 0.27).
Keywords: Light-emitting diodes; Saturated red; Iridium complex; Phosphorescence;
Organic spintronics: The case of Fe/Alq3/Co spin-valve devices by F.J. Wang; Z.H. Xiong; D. Wu; J. Shi; Z.V. Vardeny (172-175).
We have embarked on studying magneto-transport response of organic spin-valves made of evaporated Alq3 spacer sandwiched between two ferromagnetic (FM) electrodes with spin-injecting capability. Recently, we have fabricated and completed studies on organic spin-valve devices using half-metallic manganites as one of the spin-injecting FM electrode, which have shown giant magnetoresistance (GMR) of up to 40% at 11 K. Also we found that the GMR response decreases at higher temperatures, and actually disappears at temperatures above ∼180 K, partially because the FM manganite loses its magnetic properties at ambient temperature. In order to realize room temperature organic spin-valve devices, we have begun studying spin-valve devices where both spin-injecting FM electrodes have high Curie temperatures, and thus maintain their magnetic properties at ambient temperature. In this paper we report our preliminary investigations of Fe/Alq3/Co spin-valves, where both electrodes are regular, d-band metallic FM's. We found that these devices show GMR with maximum of about 5% at 11 K. However, at elevated temperatures the GMR value steeply decreases, and in fact vanishes at 90 K. We attribute this decrease to the increase of the spin-lattice relaxation rate of the injected spin-1/2 carriers in the Alq3 spacer at elevated temperatures, where there is also change in the spin carrier injection mechanism at the Fe/Alq3 interface.
Keywords: Organic spintronics; Spin-valve devices; Magnetoresistance; Spin-injection; Spin-polarized transport;
Humidity-dependent characteristics of thin film poly(3,4-ethylenedioxythiophene) field-effect transistor by H.S. Kang; H.-S. Kang; J.K. Lee; J.W. Lee; J. Joo; J.M. Ko; M.S. Kim; J.Y. Lee (176-179).
π-Conjugated poly(3,4-ethylenedioxythiophene) (PEDOT) based field-effect transistors (FETs) were fabricated in this study. A thin PEDOT layer (thickness ≈500 nm) with the desired pattern was formed as an active and a gate electrode by a vapor polymerization of 3,4-ethylenedioxythiophene (EDOT) on the photolithographically patterned ferric p-toluenesulfonate (FTS) oxidant layer. Crosslinked poly(vinyl cinnamate) (PVCN) insulating layer was formed by spin-coating and UV crosslinking. The currents (I ds) of the PEDOT active channel decreased with increasing gate bias (V g), implying p-type FET. The dc conductivities (σ dc) and I ds of the PEDOT active channel were measured as a function of V g under various relative humidities (RHs) ranging from 0% to 55%. The σ dc and the I ds of the PEDOT channel rapidly decreased with increasing positive V g under humid conditions, while those of the PEDOT channel showed no change with V g in vacuum, i.e. 0% RH. The threshold gate bias, which is defined as the onset of the curve of I ds versus V g, rapidly decreased with increase of RH. The moisture absorbed in the PEDOT active channel may assist the dedoping of the system by the screening and/or the relative separation of constituent ions from PEDOT chain upon applying a positive V g. When the positive V g was removed (V g = 0), the σ dc of the channel recovered slowly and a relatively long relaxation time was observed, which implies that the PEDOT active layer was slowly redoped. For the PEDOT based FET, we observed the moisture assisted dedoping and the redoping processes of the PEDOT channel upon applying and removal of positive V g under the humid conditions.
Keywords: Poly(3,4-ethylenedioxythiophene); Poly(vinyl cinnamate); Photolithographic patterning; Field-effect transistor; Humidity;
New hole-transporting dihydrazones by E. Jasiukaityte; J. Ostrauskaite; J.V. Grazulevicius; V. Jankauskas; J. Sidaravičius (180-184).
Synthesis and photoelectric properties of new glass-forming diphenylsulfone dihydrazones with the glass transition temperatures ranging from 22 to 123 °C are reported. The ionization potentials of the films of diphenylsulfone dihydrazones measured by the electron photoemission technique range from 5.00 to 5.33 eV. The hole-drift mobilities in the amorphous films of the diphenylsulfone dihydrazones bearing alkyl substituents doped in bisphenol Z polycarbonate (PC-Z) approach 10−5 cm2/(Vs) at high-applied electric fields as characterized by the time-of-flight method.
Keywords: Low-molar-mass glasses; Hydrazones; Ionization potential; Hole-drift mobility;
Soluble poly-3-alkylpyrrole polymers on films and fabrics by Richard C. Foitzik; Akif Kaynak; Jens Beckmann; Frederick M. Pfeffer (185-190).
Conductive textiles with specific properties can be produced by the chemical polymerisation of a range of 3-alkylpyrroles in the presence of textiles. The morphologies of these coatings are altered from the traditional conductive coatings. Comparison using a SEM reveals substantial differences.
Keywords: Conducting polymers; Poly-3-alkylpyrrole; Conductive textiles; Soluble polymers;
Investigation of the effect of substituent on the growth of polymer for 3-substituted polythiophenes via pyrolysis mass spectrometry by Evren Aslan; Levent Toppare; Jale Hacaloglu (191-195).
In this study, direct pyrolysis mass spectrometry analyses of electrochemically polymerized poly(decanedioic acid bis-(2-thiophen-3-yl-ethyl)ester) and poly(terepthalic acid bis-(2-thiophen-3-yl-ethyl)ester) were performed to investigate structural and thermal characteristics. It was found that when the ester linkages contain hydrocarbon chains, the growth of polymer occurred through both 2- and 5-positions. On the other hand, when the ester linkages contain more rigid groups such as phenyl, steric hindrance inhibited the growth of the polymer through 2-position and polymerization proceeded via coupling of thiophene moieties mainly at 5-position yielding a polymer with a lower conductivity.
Keywords: Conduction polymers; Substituted polythiophenes; Pyrolysis mass spectrometry; Thermal degradation;
Synthesis and electrophosphorescent performances of alkyl-substituted bicycloiridium complexes in polymer light-emitting diodes by Hongyu Zhen; Changyun Jiang; Wei Yang; Xingrong Zeng; Chi Zhang; Yong Cao (196-201).
In this paper, two complexes (mppy)2Ir(tmd) and (bppy)2Ir(tmd) were synthesized and doped into PVK. We find that the alkyl substitution of bicycloiridium complexes on acetylacetonate ligand has a great effect on optimizing the PLED performances. The best device performance is observed for the (bppy)2Ir(tmd)-doped PVK–PBD (40 wt%) device with the concentration of 1 wt%. A maximal external quantum efficiency of QEext = 14.2% ph/el and the luminous efficiency of LE = 33 cd/A with a luminance of 2099 mA/cm2 were achieved at a current density of 6.4 mA/cm2.
Keywords: Alkyl substitution; Bicycloiridium complex; PVK; Doping; PLED;
Gap states of charged combined solitons in a pernigraniline-base polymer by Y. Zhang (202-205).
Considering the electron–electron interactions, we have investigated the electronic structures of charged combined solitons in a pernigraniline-base polymer on the basis of Baranowski–Büttner–Voit model. The numerical results show that there exist two bound states in the Peierls gap for both positive and negative combined solitons, one is deep level and the other is shallow level. The energy of the deep level (for S+ is about 1.03 eV and S− 0.98 eV) agree with the photoinduced absorption data (∼1.0 eV) very well. The shallow one is newly discovered and it is about 0.22 eV under the bottom edge of the conduction band (positive soliton) or above the top edge of the valence band (negative soliton). Some other shallow states outside the energy bands are also predicted.
Keywords: Pernigraniline-base; Combined soliton; Electron–electron interaction; Electronic structure;
A novel fluorene derivative containing four triphenylamine groups: Highly thermostable blue emitter with hole-transporting ability for organic light-emitting diode (OLED) by Qiang Fang; Bing Xu; Biao Jiang; Haitao Fu; Wenqing Zhu; Xueyin Jiang; Zhilin Zhang (206-210).
Four triphenylamine groups-substituted fluorene with rather high glass transition temperature, T g, of 165 °C, was prepared and employed together with TPD as a composite hole-transporting layer to give a device with very low turn-on voltage and high efficiency. When a hole block layer was fabricated, the new compound can be used as a light-emitting layer for OLED to give a device emitting blue color.
Keywords: Organic light-emitting diode (OLED); Triphenylamine derivatives; Fluorene; Blue electroluminescent materials; Hole-transporting materials;
Synthesis, characterization and electrical and CO2 sensing properties of triazine containing three dendritic phthalocyanine by Mustafa Sülü; Ahmet Altındal; Özer Bekaroğlu (211-221).
The metal-free phthalocyanine 1a was synthesised by statistically mixed condensation of 4-nitrophthalonitrile and 4-(2-dimethylaminoethylsulfanyl)phthalonitrile in dry 2-(dimethylamino)ethanol. The same route were applied to prepare metal-containing phthalocyanines using metal salts Zn(OAc)3·2H2O and CoCl2, respectively, in dry DMF. The nitro compounds were reduced to amine with sodium sulfide and quarternized by reaction of MeI. These compounds were then reacted with cyanuric chloride in the presence of potassium carbonate as a base in mixture of THF and DMF gave s-triazine containing three amino phthalocyanines 4a–c. The reaction of three phthalocyanines with sodium salt of triethylmethanetricarboxylate in absolute ethanol were synthesised first generation of dendrimer 5a–c. The second generation of dendritic phthalocyanines 6a–c were prepared by the reaction of dentritic phthalocyanines with tris (H2NC(CH2OH)3) using K2CO3 as a base in DMSO. The dendritic phthalocyanines containing tris are slightly soluble in water. The new compounds were characterized by elemental analysis and UV–vis, IR, NMR spectra. The temperature dependence of the dc and ac conductivity of these compounds and the adsorption of CO2 gas on thin film of the compounds were investigated by dc conductivity and impedance spectroscopy technique using an interdigital transducer structure on glass substrate. While exposure to CO2 have no influence on the electrical conductivity (ac and dc) of compound 6a and 6c, the conductivity of the film of 6b strongly depend on the presence of the CO2 gas. Completely reversible sensor signals were obtained for compound 6b. The impedance spectra were obtained at frequencies between 40 and 105 Hz, and in a wide range of CO2 concentrations (from 500 to 8000 ppm) at room temperature. The temperature dependence of dc conductivity show typical Arrehenius behaviour for all compounds. The ac conductivity of the films is represented by the form ω s . The dependency of frequency exponent s on temperature suggests a correlated barrier hopping (CBH).
Keywords: Dendritic phthalocyanine; s-Triazine; CO2 sensing; CBH;
Synthesis, characterization, and electrical, electrochemical and gas sensing properties of a novel cyclic borazine derivative containing three phthalocyaninato zinc(II) macrocycles by Metin Özer; Ahmet Altındal; Ali Rıza Özkaya; Mustafa Bulut; Özer Bekaroğlu (222-231).
A novel borazine derivative of trinuclear phthalocyanine has been prepared by following the multistep reactions of unsymmetrically substituted phthalocyanines. 4-(4-Amino-3-nitrophenoxy)phthalonitrile (3) which is one of the precursor molecules of the phthalocyanine was obtained from 4-nitrophthalonitrile (1) and 4-amino-3-nitrophenol (2) with K2CO3 in DMF at 50 °C. The zinc(II) phthalocyanine (5) containing an unsymmetrical substituted 4-amino-3-nitrophenoxy group was synthesized by statistical condensation of two different phthalonitriles. 4-(4-Amino-3-nitrophenoxy)phthalonitrile (3) and 4,5-bis(hexylthio)phthalonitrile (4) were cyclotetramerized with zinc acetate in DMF at 170–180 °C to yield 2-(4-amino-3-nitrophenoxy)-9,10,16,17,23,24-hexa(hexylthio)phthalocyanine zinc(II) complex (5), which was then separated by column chromatography on silica gel. The unsymmetrically substituted compound was reduced to the diamine form (6) using hydrazine hydrate in the presence of Pd/C catalyst, and the product was purified with chromatographic separation. Compound 6 was then reacted with triisopropoxyborane in refluxing xylene to give 5H,12H,19H-tris[2-(3,4-diaminophenoxy)-9,10,16,17,23,24-hexa(hexylthio)phthalocyaninato zinc(II) diazaborolo] borazine (7). The resulting product was purified by column chromatography on silica gel. All the target unsymmetrical phthalocyanines and borazine derivative were characterized by elemental analysis, IR, UV–vis, and 1H NMR. Boron and zinc(II) percents in 7 were quantified with ICP-MS. Impedance spectroscopy and dc measurements were performed on spin coated 5–7 films as a function of temperature (293–400 K). The dc results showed an activated conductivity dependence on temperature. The ac results gave a temperature dependent frequency exponent s. The results were compared with the prediction of the Quantum Mechanical Tunelling and Correlated Barrier Hopping models. The ac conductivity of the films was well represented by the form Aω s . Gas sensing properties of the films for the volatile organic compounds (VOCs) (chloroform, acetone, carbontetrachloride and ammonia) were also investigated in the temperature range from 293 to 400 K. The operating temperature had a considerable effect on sensing characteristics. Maximum sensitivity to VOCs were observed at room temperature for all films. Cyclic voltammetry of compounds 6 and 7 in solution indicated that these compounds have similar voltammetric behaviour.
Keywords: Zinc phthalocyanine; Borazine; Electrochemistry; Conductivity; Gas sensing;
A new route to synthesize high degree polythiophene in a room temperature melt medium by S. Geetha; D.C. Trivedi (232-239).
Conducting polymers suffer from folds and kinks because of random nucleation and solvation of a free radical cation to yield a cross linked/disordered polymer and therefore a solvent free electrochemical polymerization in a room temperature melt medium is adopted to yield a high degree polymer with high electronic conductivity. Electropolymerization of thiophene was performed on platinum/ITO substrates using cyclic voltametry or galvenostatic mode in chloroaluminate room temperature melt medium to obtain a reddish brown free standing film which can be peeled off from the electrode surface after a minimum of 10 cycles. The conductivity was found to be around 102 S/cm. The degree of polymerization was calculated to be around 44 from IR studies. A layered structure supportive for high degree of polymerization was witnessed from potential step technique. From UV spectra the charge carriers were found to be bipolarons. The morphology of the film was found to be crystalline from SEM and XRD studies. Capacitative impedance properties for doped samples were interpreted from impedance spectroscopy.
Keywords: Polythiophene; Room temperature melt; Conductivity; Electrosynthesis;
Erratum to “SWNTs coated by conducting polyaniline: Synthesis and modified properties” [Synth. Met. 151 (2005) 131–135] by Mohammad Rezaul Karim; Chul Jae Lee; Yong-Tae Park; Mu Sang Lee (240).