Synthetic Metals (v.157, #4-5)

Efficient electroluminescent tertiary europium(III) β-diketonate complex with functional 2,2′-bipyridine ligand by Yan-Yan Wang; Li-Hong Wang; Xu-Hui Zhu; Jie Ru; Wei Huang; Jun-Feng Fang; Dong-Ge Ma (165-169).
A new red electroluminescent complex (2Fphen-bpy)Eu(DBM)3(2Fphen-bpy: 4-(2,4-difluorophenylene)-2,2′-bipyridine; DBM: dibenzoylmethanate) was designed and synthesized. X-ray single crystal structural analysis shows that the dihedral angles involving the difluorophenylene moiety and two pyridine rings of the 2,2′-bipyridine unit in (2Fphen-bpy)Eu(DBM)3 are 42° and 51.6°, indicant of non-conjugation between the two segments. The europium complex was incorporated into the doped electroluminescent device: ITO/TPD(40 nm)/(2Fphen-bpy)Eu(DBM)3:CBP(30 nm, 3%)/BCP(20 nm)/Alq3(30 nm)/LiF(1 nm)/Al(100 nm). At the brightness of 100 cd/m2, the current and power efficiency are about 2.53 cd/A and 0.79 lm/W, respectively. In comparison with the inefficient control device using (2,2′-bipyridine)Eu(DBM)3 as the red emitter in the same configuration, the introduction of difluorophenylene moiety into the Eu(III) complex could possibly account for the improved performance of the title compound.
Keywords: Light-emitting materials; Electroluminescence; Europium complex; Red emission;

Synthesis of polyaniline/NiO nanobelts by a self-assembly process by Genping Song; Jie Han; Rong Guo (170-175).
Polyaniline (PANI) nanotubes (∼180 nm in diameter) were synthesized in the presence of sodium dodecylbenzenesulfonate (SDBS) as a micellar template and dopant, whereas PANI/NiO nanobelts (300–700 nm in diameter) were obtained with the addition of NiO nanoparticles (∼10 nm in diameter). Results showed that the size of PANI/NiO composite nanobelts increased with an increase content of NiO nanoparticles. XRD, Fourier transform infrared (FTIR) and UV–vis spectroscopy were used to characterize the chemical structures of PANI nanotubes and PANI/NiO nanobelts. The thermal stability and conductivity of samples were affected by the content of NiO. The coordination bonds between NiO and aniline were the key factor that resulted in the morphological change of PANI.
Keywords: Polyaniline; NiO nanoparticle; Nanocomposite; Nanotube; Nanobelt;

Conducting polyaniline with electrical conductivity of 2.34 × 10−1  S cm−1 was obtained using ferrocenesulfonic acid as dopant. After the ferrocenesulfonic acid was oxidized with FeCl3, though the electrical conductivity of the doped polyaniline decreased by 1–2 orders of magnitude, the magnetic susceptibility (χ) increased with the increase of the oxidation degree of ferrocenesulfonic acid. EPR spectra showed not only a signal with a g value of around 2, but also a so-called half-field signal with a g value of about 4 even at room temperature. Coexistence of ferromagnetic intrachain interactions and antiferromagnetic interchain interactions in the materials has been suggested.
Keywords: Conducting polyaniline; Ionic ferrocenesulfonic acid; Magnetic susceptibility; Half-field signal;

Magnetic behavior of polyaniline doped with oxidized ferrocenesulfonic acid by Xueming Wu; Xianhong Wang; Ji Li; Fosong Wang (182-185).
Ferrocenesulfonic acid (FSA) was oxidized by iodine (I2) and then used as dopant for polyaniline emeraldine base. The resulted polyaniline showed an electrical conductivity of 4.50 × 10−2  S/cm and a ferromagnetic interaction with a positive Weiss constant of 15 K, the magnetic behavior is attributed to the ferromagnetic coupling between ferrocenium cations in the counter-ions along the polyaniline chain.
Keywords: Oxidized ferrocenesulfonic acid; Doped polyaniline; Ferromagnetic interaction;

All-optical light modulation in anthraquinone dye doped waveguides based on an improved prism coupling method by Dandan Pan; Yuping Chen; Rui Wu; Xianfeng Chen; Qinghua Meng; Zhicheng Sun (186-189).
In this paper, an improved all-optical light modulation scheme has been proposed with respect to photo-induced changes of the refractive index of polymer planar waveguides by the m-line spectroscopy technique. An improved prism coupling method was used to realize it. In this prism coupling configuration, the strong pump (writing) beam excited by an Ar+ laser at 514 nm is illuminated on film sample by passing through the upper base of prism, and induces local refractive index changes by heating. A probe (reading) beam, which is coupled into prism coupling system and forms waveguide modes in film, is modulated for its changes of reflectivity due to changes of refractive index of waveguide film. A shift of sharp guided wave resonance is observed and 20 ms of response time is obtained.
Keywords: Prism coupling; Thermo-optic; m-Line spectroscopy; Polymethyl-methacrylate; 1,4-Diamino-2,3-diphenoxy-anthraquinone;

Solution-processed polycrystalline copper tetrabenzoporphyrin thin-film transistors by Patrick B. Shea; Lisa R. Pattison; Manami Kawano; Charlene Chen; Jihua Chen; Pierre Petroff; David C. Martin; Hiroko Yamada; Noboru Ono; Jerzy Kanicki (190-197).
The demonstration of organic thin-film field-effect transistors (OFETs) using a solution-processable form of the organometallic molecule copper tetrabenzoporphyrin (CuTBP) is reported. A soluble precursor was spun-cast into an amorphous, insulating thin-film, and thermally annealed at 165 °C for 30 min into a polycrystalline organic semiconductor. Absorbance spectroscopy displayed characteristics of porphyrin macrocycles. Microscopy reveals the formation of domains comprising aligned nanorod aggregates with dimensions of 55 nm wide, 300 nm long, and 100 nm tall on the gate insulator surface. OFETs demonstrated field-effect mobilities typically on the order of 0.1 cm2/V s, threshold voltages around 5 V, subthreshold slopes around 4 V/dec, and ON-/OFF-current ratios near 104.
Keywords: Metallotetrabenzoporphyrins; X-ray diffraction; Atomic force microscopy; Optical absorption; Organic field-effect transistors; Solution processing;

Doped and non-doped organic light-emitting diodes based on a yellow carbazole emitter into a blue-emitting matrix by H. Choukri; A. Fischer; S. Forget; S. Chenais; M.-C. Castex; B. Geffroy; D. Adès; A. Siove (198-204).
A new carbazole derivative with a 3,3′-bicarbazyl core 6,6′-substituted by dicyanovinylene groups (6,6′-bis(1-(2,2′-dicyano)vinyl)-N,N′-dioctyl-3,3′-bicarbazyl; named (OcCz2CN)2, was synthesized by carbonyl-methylene Knovenagel condensation, characterized and used as a component of multilayer organic light-emitting diodes (OLEDs). Due to its π-donor–acceptor type structure, (OcCz2CN)2 was found to emit a yellow light at λ max  = 590 nm (with the CIE coordinates x  = 0.51; y  = 0.47) and was used either as a dopant or as an ultrathin layer in a blue-emitting matrix of 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi). DPVBi (OcCz2CN)2-doped structure exhibited, at doping ratio of 1.4 weight %, a yellowish–green light with the CIE coordinates (x  = 0.31; y  = 0.51), an electroluminescence efficiency η EL  = 1.3 cd/A, an external quantum efficiency η ext  = 0.4 % and a luminance L  = 127 cd/m2 (at 10 mA/cm2) whereas for non-doped devices utilizing the carbazolic fluorophore as a thin neat layer, a warm white with CIE coordinates (x  = 0.40; y  = 0.43), η EL  = 2.0 cd/A, η ext  = 0.7%, L  = 197 cd/m2 (at 10 mA/cm2) and a color rendering index (CRI) of 74, were obtained. Electroluminescence performances of both the doped and non-doped devices were compared with those obtained with 5,6,11,12-tetraphenylnaphtacene (rubrene) taken as a reference of highly efficient yellow emitter.
Keywords: Carbazole; OLED; Doped and non-doped devices; White emission;

Due to strict environmental regulations on the usage of chromate in the coating industries, search for effective inhibitive pigment in replacing those chromate pigments has become necessary. In recent years it has been shown that electrically conducting polymers such as polyaniline (PANI) incorporated coatings are able to protect steel due to their passivating ability similar to that of chromates. This work presents the comparative corrosion protection performance of the coatings containing polyaniline and polyaniline–TiO2 composite (PTC) on steel in acrylic binder. The PANI and PTC were prepared by chemical oxidative method of aniline by ammonium persulfate. The polymers were characterized by FTIR, XRD and SEM. The corrosion protection performance of the coatings containing PANI and PTC on steel was evaluated by immersion test in 3% NaCl for 60 days and salt fog test for 35 days. The performance of the coatings in both the tests was investigated by open circuit potential measurements and EIS technique. It has been found that the open circuit potential values of PTC containing coating are more nobler by 50–200 mV in comparison to that of coatings with PANI. Besides, the resistance values of the coating containing PTC were more than 107  Ω cm2 in the 3% NaCl immersion test after 60 days and 109  Ω cm2 in the salt fog test of 35 days which were two orders high in comparison to that of PANI containing coatings. The better performance of PTC containing coatings may be due to uniform distribution of polyaniline which can form uniform passive film on the iron surface.
Keywords: Polyaniline; Polyaniline–TiO2 composite; Acrylic; EIS; Coating; Corrosion protection; Steel;

On the optical properties of fluoranthenopyracylene ladder type molecule series by Katarína Matuszná; Vladimír Lukeš; Peter Rapta; Lothar Dunsch; Adelia Justina Aguiar Aquino; Hans Lischka (214-221).
This work reports on theoretical and spectroscopic study of recently prepared thermally and chemically stable ladder type molecules with extended π-conjugation consisting of fluoranthenopyracylene units. Density functional theory (DFT) and time-dependent DFT calculations were performed for the electronic ground and lowest excited states to obtain optimal geometries. Our calculations indicate both a planar structure of the central ladder part and a negligible influence of lateral phenylene rings. Geometrical changes upon the electronic excitations are indicated in the center of the molecular skeleton. Experimental absorption spectra of the investigated series measured in ortho dichlorobenzene consist of several bands, with well-distinguished vibronic structure. Effective conjugation length for excitation and fluorescence energy was estimated to be 10 and 9 repeating units, respectively.
Keywords: TDDFT; Optical spectra; Fluorescence; Vibronic structure; Polymer limit;

Dichroic photo- and electroluminescence of oligo p-(phenylene vinylene) derivatives by Marcus W. Lauhof; Stephan A. Benning; Heinz-Siegfried Kitzerow; Volkmar Vill; Felix Scheliga; Emma Thorn-Csányi (222-227).
The synthesis and characterization of all trans-configured, monodisperse 2-ethylhexyloxy-substituted PV-oligomers (OPVs) are presented. The synthetic route is based on a step-wise synthesis using the Horner–Wadsworth–Emmons reaction. Bright fluorescence of the OPVs makes them suitable for the use in organic light emitting diodes (OLEDs). Due to the anisotropy of shape, the oligomers can be aligned which leads to linear polarized photoluminescence and electroluminescence. Different alignment layers were tested. The best results were achieved by using poly(tetrafluoroethylene) (PTFE) as alignment layer. The pentamer shows dichroic ratios of 17.1 for fluorescence and 8.5 for electroluminescence, respectively.
Keywords: Oligo p-(phenylene vinylene); Polarized electroluminescence; Liquid crystals; Organic ligth emitting diodes; Dichroism;

Light emission efficiency of red phosphorescent organic light-emitting diodes was improved by using a double doping structure in light emitting layer. Red devices with tris(2-phenylpyridine) iridium (Ir(ppy)3) as a co-dopant for tris(1-phenylisoquinoline) iridium (Ir(piq)3) showed higher efficiency than Ir(piq)3 device. Ir(ppy)3 acted as a sensitizer for energy transfer from (4,4′-N,N′-dicarbazole)biphenyl to Ir(piq)3 and enhanced luminance efficiency of Ir(piq)3 by 30%.
Keywords: Sensitizer; High efficiency; Red emission;

The temperature dependence of a Schottky diode fabricated from an electrospun doped polyaniline nanofiber on an inorganic n-doped silicon substrate has been studied in the temperature range 180 K <  T  < 300 K. The standard thermionic emission model of a Schottky junction with and without a series resistance was utilized to analyze the data. No significant difference in the values of the device parameters were observed via the use of either method. Charge transport in the ON state of the diode was compared to that in an isolated electrospun doped polyaniline nanofiber and the temperature dependence of the diode resistance was seen to deviate from the quasi 1-D variable range hopping that characterizes charge transport in doped polyaniline. The constrained diode architecture permits the simultaneous existence of multiple charge transport mechanisms under normal operating conditions making this an interesting system for further study.
Keywords: Fiber; Diode; Electrospinning;

“In situ” polymerization was carried out in the presence of titanium dioxide (TiO2) to synthesize TiO2/polyaniline (TiO2/PANI) composite in an aqueous medium containing anionic (sodium dodecylbenzenesulfonate), DBSNa, cationic (tetradecyltrimethylammonium bromide), TTAB, and nonionic surfactants (poly(ethylene oxide) (20) sorbitan monolaurate), Tween 20). The composites were characterized by FTIR, UV–vis spectroscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and Gouy balance measurements. The FTIR and UV–vis spectra reveal that the interaction between TiO2/PANI composite and surfactants. The anionic and cationic surfactants incorporated with TiO2/PANI composite structure. The stability of the composites in terms of direct-current electrical conductivity retention was studied in isothermal conditions. Electrical conductivity measurements indicate that the conductivity of TiO2/PANI synthesized in the presence of the cationic surfactant has the highest conductivity value. All composites showed negative mass magnetic susceptibility values.
Keywords: Polyaniline; Titanium dioxide; Composite; Thermal aging; Surfactant;

Carrier concentration dependence of the mobility in organic semiconductors by Ling Li; Gregor Meller; Hans Kosina (243-246).
Charge transport in organic materials as a function of carrier concentration is investigated. An analytical model of the concentration dependent mobility based on the variable hopping range theory is formulated. This model is applied to analyze the discrepancy between the experimental mobilities extracted from FETs and LEDs. The result shows that an exponential density of states (DOS) is a good approximation of the tail states for describing the charge transport in FETs. When applied to the low carrier concentration regime, for example to the LEDs regime, a Gaussian DOS should be assumed.