Synthetic Metals (v.157, #10-12)

By means of chemical modification, flexible branched alkoxy chains were successfully attached to the para-site of the side phenyl groups of DP-PPV to obtain a new soluble fully conjugated PPV, named poly(2,3-di-[p-(2′-ethylhexoxy)phenyl]-1,4-phenylenevinylene). Spectroscopic characterizations using UV, photoluminescence (PL) and photoluminescent excitation (PLE) showed only the emission of single chromophore of this polymer in the solution state, indicating that the solubility of poly(2,3-diphenyl-1,4-phenylene vinylene) (DP-PPV) was significantly improved by attaching the alkoxy side chains. In the spin-coated film, the emission of the single chromophore with longer conjugating length was observed. In addition, the spectral feature showed the absence of the emission from the aggregate, which was usually observed in the alkoxy-modified PPV, revealing that the polymer chains were well-separated by the side chains. The performance of the single-layer ITO/PEDOT/p-EHDP-PPV/Ca/Al LED device was also reported. The maximum luminescence and efficiency at 11 V was 3735 Cd/m2 and 0.57 Cd/A, respectively.
Keywords: DP-PPV; Alkoxy side chain; Photoluminescence; Electroluminescence;

New fluorophores with rod-, V- or star-shaped structure: Synthesis, photoluminescence and electroluminescence by Zujin Zhao; Xinjun Xu; Lei Xu; Gui Yu; Ping Lu; Yunqi Liu (414-420).
A series of blue light-emitting compounds with different molecular shapes were synthesized by the Pd/Cu-catalyzed Sonogashira coupling reaction. These compounds composed of hole-transporting group (carbazole) and strong fluorescent group (pyrene), which linked through 9,9-dihexyl-2,7-fluorene ethynylene. The photoluminescent (PL) and electroluminescent (EL) behavior of these compounds was investigated.
Keywords: Sonogashira coupling reaction; Photoluminescence; Electroluminescence; Organic light-emitting diodes;

Organic light-emitting device using new distyrylarylene host materials by Nack-Chul Seong; Young-Min Jeon; Tae-Ho Lim; Joon-Woo Kim; Chil-Won Lee; En-Jung Lee; Ji-Geun Jang; Ho-Jeong Jang; Joon-Young Lee; Myoung-Seon Gong (421-426).
Novel distyrylarylene-based blue host materials, 4,4′-bis(1,2-diphenylvinyl)-p-terphenyl (2), 4,4′-bis[1-phenyl-2-(p-tolyl)vinyl]-p-terphenyl (3) and 4,4′-bis[1-phenyl-2-(4,4′-biphenyl)vinyl]-p-terphenyl (4) were successfully synthesized using 4,4′-bisbenzoyl-p-terphenyl (1) through Wittig–Hornor reaction and a blue organic light-emitting diode (OLED) was made from them. The structure of the blue device is ITO/DNTPD/NPB/Host:DSA-Ph/Alq3/Al-LiF. Here, NPB is used as a hole transport layer, DNTPD as the hole injection layer, DSA-Ph as the blue dopant, Alq3 as the electron transport layer and Al as the cathode. The blue device doped with 5%-DSA-Ph shows a blue EL spectrum at 463 nm and a high efficiency of 4.14–5.13 cd/A.
Keywords: OLED; Distyrylarylene; Blue host material;

Undoped yellow-emitting organic light-emitting diodes from a phenothiazine-based derivative by Huayu Cao; Zhijian Chen; Yingliang Liu; Bo Qu; Shengang Xu; Shaokui Cao; Zhihao Lan; Ziyao Wang; Qihuang Gong (427-431).
The single layer and multilayer undoped light-emitting devices were fabricated using a new soluble phenothiazine-based derivative, poly(3,7-N-octyl phenothiozinyl terephthalylidene) (POPTP). Through the optimization of device structures, the multilayer device has a maximum luminance of 1203 cd/m2 at the bias voltage of 9.3 V, using 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as a hole-blocking layer and tris-(8-hydroxyquinoline)aluminium (Alq3) as a electron-injection/transporting layer. The Commision International de L’Eclairage (CIE) coordinates stabilized at (x, y) = (0.46, 0.53) at various bias voltages. Additionally, the dominant wavelength (λ D) of around 575 nm and the color purity of approximately 100% indicated a pure yellow emission property. Therefore, POPTP is a stable candidate material with a pure yellow emission for the undoped organic light-emitting diodes (OLEDs).
Keywords: Phenothiazine; Pure yellow emission; Electroluminescence; OLED;

A method is described to obtain gold–polypyrrole nanostructures by reacting gold chloride with pyrrole at toluene–water interphase. By adjusting the concentration of the stabilizer, namely, pyridine-3-sulphonate, the shape of the nanostructures formed at the interphase can be tailored. This novel method facilitates the fabrication of Au–Ppy composite material that gives wider scope for biosensing applications.
Keywords: Interphase; Nanostructures; Gold; Polypyrrole; Biosensing;

Enhanced thermal conductivity by the magnetic field in heat transfer nanofluids containing carbon nanotube by Haiping Hong; Brian Wright; Jesse Wensel; Sungho Jin; Xiang Rong Ye; Walter Roy (437-440).
In this paper, we report for the first time that the thermal conductivity (TC) of the heat transfer nanofluids could be enhanced by the external magnetic field. The heat transfer nanofluids contain carbon nanotubes (CNTs) and magnetic-field-sensitive nanoparticles of Fe2O3. The reasonable explanation for these interesting results is that the Fe2O3 particles form aligned chains under applied magnetic field and help to connect the nanotubes, which results in improved thermal conductivity. On longer holding in magnetic field, the particles gradually move and form large clumps of particles, causing clumping of CNTs, then decreasing the TC. We also found that the time to reach the maximum peak value of TC is increased as the applied magnetic field is reduced by increasing the gap between two magnets. Scanning electron microscopy (SEM) pictures show that the nanotubes and nanoparticle are aligned well under the influence of magnetic field.
Keywords: Thermal conductivity; Carbon nanotube; Nanofluids; Magnetic field;

A range of commercially available anionic dyes were successfully incorporated as dopants during the electrodeposition of poly(terthiophene) (P(TTh)). Photovoltaic testing revealed that the best photoelectrochemical cell (PEC) was based on P(TTh) doped with Sulforhodamine B, which showed short-circuit-current (I sc) = 178 μA cm−2, open-circuit-voltage (V oc) = 318 mV, fill-factor (FF) = 29.6% and power-conversion-efficiency (PCE) = 0.0334% under white light illumination intensity of 500 W m−2. Photocurrent action spectroscopy of the PECs based on dye-doped P(TTh)s prepared here, and of the PECs prepared in a previous study, showed that only a cationic dye directly contributed to photocurrent, while anionic dyes did not. It is proposed that these observations are due to attractive or repulsive electrostatic interactions between dyes and the electron acceptor I3 at the polymer/electrolyte interface.
Keywords: Poly(terthiophene); Light-harvesting dyes; Photovoltaics;

A novel polycatechol/platinum composite film prepared by electrochemical synthesis by Gongming Qian; Changzhu Yang; Wenhong Pu; Jintao Huang; Jingdong Zhang (448-453).
This work described the electrochemical synthesis of polycatechol/platinum composite film studied by cyclic voltammetry, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The formation of the polycatechol/platinum composite film was initiated by introducing PtCl6 2− anions into a pre-electropolymerized catechol film followed by the electrochemical reduction of PtCl6 2−. The SEM images indicated that uniform spherical platinum particles were dispersed in the polycatechol film. Compared with the electrodeposited platinum film, the polycatechol/platinum composite film significantly improved the electrocatalytic activity toward the oxidation of methanol, attributed to the uniform dispersion of platinum particles on polycatechol film. The electrochemical methods may be suitable for controlling particle dimension and the density of platinum particles.
Keywords: Polycatechol/platinum composite film; Electrochemical synthesis; Electrocatalytic activity;

A study on the preparation of poly(vinyl alcohol) nanofibers containing silver nanoparticles by Wen-Ji Jin; Hyun Jeong Jeon; Jung Hyun Kim; Ji Ho Youk (454-459).
In this study, two practical methods for the facile and controlled preparation of poly(vinyl alcohol) (PVA) nanofibers containing Ag nanoparticles were investigated for use in antimicrobial applications. In the first method, PVA nanofibers containing Ag nanoparticles were successfully electrospun from PVA/silver nitrate (AgNO3) aqueous solutions after first refluxing them. The Ag nanoparticles in the PVA/AgNO3 aqueous solutions were generated by refluxing them. Interestingly, it was found that the Ag nanoparticles were also spontaneously generated during the electrospinning process. In the second method, Ag nanoparticles were generated by annealing the PVA nanofibers electrospun from PVA/AgNO3 aqueous solutions. Residual Ag+ ions and the Ag nanoparticles generated during the electrospinning process in the PVA nanofibers were diffused and aggregated into larger Ag nanoparticles during the annealing process. All of the Ag nanoparticles were sphere shaped and evenly distributed in the PVA nanofibers prepared by the two methods.
Keywords: Electrospinning; Poly(vinyl alcohol) (PVA); Silver nanoparticles; Reflux; Annealing;

An NMR study of PF 6 − ions in polypyrrole by Chien-Hsin Tso; John D. Madden; Carl A. Michal (460-466).
A variety of NMR techniques are employed to examine the abundance, dynamics, and translational diffusion of PF 6 − ions in doped polypyrrole films in various oxidation states. NMR is employed as a non-invasive alternative to elemental analysis for quantifying ion content, which is found to decrease linearly with decreasing electrochemical potential, directly confirming the ion intercalation mechanism of polymer actuation, and demonstrating the capacitive nature of the polypyrrole film/electrolyte system. With known ion content, sample mass, and deposition current, the relative amounts of pyrrole, ions, and solvent in the films can be determined. A T 1 relaxation study along with 1D nuclear Overhauser effect (NOE) difference experiments reveal that the rotational correlation time and solvent accessibility of PF 6 − ions in the oxidized films are similar to those in the solvent, indicating the ions experience a solvated environment, and do not sit at stable sites in the polymer matrix. A drastic decrease of the NOE enhancement, along with changes in relaxation behavior and electrical conductivity in reduced films implies that polypyrrole undergoes a significant structural change when reduced. This change leads to a much less solvated ion environment, and may be responsible for the expansion of films sometimes observed upon reduction. Translational motion of the PF 6 − ions in the oxidized films is probed via diffusion measurements made using pulsed-field gradient NMR. The diffusion coefficients found ( ∼ 5 × 1 0 − 9  cm2/s) are an order of magnitude smaller than those typically extracted from impedance spectroscopy measurements; this is explained in terms of field driven ion migration in the impedance spectroscopy case.
Keywords: Polypyrrole; NMR; Ion diffusion; Hexafluorophosphate;

The copolymers consisting of both unsubstituted and 2,5-dialkoxy-substituted phenylenevinylenes (P(ROPV-co-PV); RO = MeO, EtO and BuO) with ca. 30 mol% of dialkoxy-substituted units were synthesized to evaluate their thermoelectric properties. Iodine-doped P(ROPV-co-PV) exhibited high Seebeck coefficient with relatively high electrical conductivity among electrically conductive polymers ever reported. The effect of the stretching treatment on their thermoelectric properties was examined. Consequently, both P(MeOPV-co-PV) and P(EtOPV-co-PV) constantly showed high Seebeck coefficients with increased electrical conductivities by the stretching alignment because of an increase in the carrier mobility, while the Seebeck coefficient of P(BuOPV-co-PV) varied inversely with a variation of the electrical conductivity. We also evaluated thermal conductivity of the pristine and iodine-doped P(ROPV-co-PV) for calculations of their thermoelectric figure-of-merit ZT. To the best of our knowledge, consequent thermoelectric figure-of-merit ZT of all the iodine-doped copolymer with the stretch treatment is one of the highest thermoelectric performance among conducting polymers reported ever, which are comparable with that of an inorganic thermoelectric materials, such as β-FeSi2.
Keywords: Poly(2,5-dialkoxyphenylenevinylene); Polyphenylenevinylene; Thermoelectric figure-of-merit; Electrical conductivity; Seebeck coefficient; Thermal conductivity;

Synthesis and optical properties of quinoxaline-containing poly(aryl ether)s by Vasilis P. Barberis; John A. Mikroyannidis; Ioakim K. Spiliopoulos (475-480).
Three novel poly(aryl ether)s were synthesized from the reaction of three bisphenols with 2,3-bis(4-fluorophenyl)-quinoxaline via nucleophilic aromatic substitution. The polymers contained electron transporting and emissive moieties separated by ether linkages. Quinoxaline was used as electron transporting segment, while p-distyrylbenzene, 2,6-distyrylpyridine and p-quinquephenyl were used as emissive segments. The low reactivity 2,3-bis(4-fluorophenyl)-quinoxaline towards nucleophilic aromatic substitution results in polymers with limited molecular weights. The polymers were soluble in polar organic solvents and strong organic acids. In THF solutions the polymers showed absorption maxima at 349–354 nm and emission maxima at 417–454 nm, with quantum yields of 22–41%. In solid state the polymers showed absorption maxima at 350–353 nm and emission maxima at 427–461 nm. The optical properties of polymers were influenced by intrachain interactions in solution, while interchain interactions were important in solid state.
Keywords: Poly(aryl ether)s; Quinoxaline; Nucleophilic aromatic substitution; Light emitting polymers; Luminescent polymers;

New growth method of rubrene single crystal for organic field-effect transistor by Kihyun Kim; Min Ki Kim; Han Saem Kang; Mi Yeon Cho; Jinsoo Joo; Ju Hee Kim; Kyung Hwan Kim; Chang Seop Hong; Dong Hoon Choi (481-484).
A rubrene single crystal, for the active material of an organic field-effect transistor (OFET), was directly grown from a rubrene powder in the single growing zone of a furnace, which is different from a conventional organic vapor transport method for the crystal growth. The growth time of the needle-shaped rubrene single crystals was ∼30 min without any purification process. Structural properties and the geometrical shape of the rubrene single crystals were characterized through a single crystal X-ray diffractometer and a scanning electron microscope. The rubrene single crystal OFETs were fabricated through both the bottom and top contact methods. The Si3N4 and polymer were used for a gate insulator in the bottom and top contact OFETs, respectively. From the current–voltage characteristics of the OFETs, a typical p-type transistor nature was observed. The mobility of the top contact OFETs was measured to be ∼1.12 cm2/V s, which was approximately seven times higher than that of the bottom contact OFETs, because of the direct contact of the rubrene single crystals with both Au electrode and polymer gate insulator.
Keywords: Rubrene single crystal; Direct growth; Organic field-effect transistor; Mobility;

On the improvement of stability of polypyrrole films in aqueous solutions by A. Alumaa; A. Hallik; V. Sammelselg; J. Tamm (485-491).
The stability of oxidised polypyrrole (PPy) doped with mobile anion (ClO4 ) was studied in aqueous media. The influence of various components of the solution (dissolved O2, indifferent electrolyte, organic compounds) on the stability of PPy was characterised by cyclic voltammetry and electron-probe X-ray microanalysis (EPMA). It was established that dissolved O2 favours the PPy overoxidation—the rate determining process of the electrochemical corrosion. The high concentration of the indifferent electrolyte on the other hand enhances the stability of the PPy in aqueous solutions to an essential extent. The more effective inhibitors of the PPy corrosion are the surface-active anions (dodecyl sulfate) and polar aromatic compounds (benzyl alcohol, benzonitrile).
Keywords: Polypyrrole; Stability; Degradation; Overoxidation; Adsorption; π-Electronic interaction;

Fabrication of (TMTSF)2PF6 thin crystals in a confined electrode by Yoon-Kyoung Han; Kyung-In Son; Woun Kang; Chang Seop Hong; Dong-Youn Noh (492-496).
In this study of crystal engineering of an organic superconductor, the single crystals of (TMTSF)2PF6 were fabricated in the form of thin crystals by the electrocrystallization method with a confined electrode by applying a low current (10 nA). Almost all of the flattened needle-shape thin crystals grown in the specially designed confined electrode had an enhanced anisotropic morphology with a thickness-to-width ( t / w ) ratio of about 6%, being about 19 μm thick, which is an order of magnitude smaller than that of the representative bulk crystals ( t / w ≈ 57 % ) grown in a free environment. The X-ray crystal structure analysis of the (TMTSF)2PF6 thin crystals revealed that they have the same crystal structure and packing mode as the bulk ones. The ν 4 and ν 3 mode frequencies in the micro-Raman spectra corresponding to the C=C in-phase and out-of-phase stretching vibrations were 1457 cm−1 and 1595 cm−1, respectively. These frequencies are in between those of the neutral TMTSF and TMTSF+ cation. The thin crystals also show the same SDW transition at 12.2 K as do the bulk ones. It is concluded that the fabricated thin crystals retain the properties of bulk crystals.
Keywords: Organic superconductor; Crystal engineering; Electrocrystallization; Crystal fabrication;