Synthetic Metals (v.148, #2)

Interpenetrating networks of two conducting polymers by Bjørn Winther-Jensen; Keld West (105-109).
Interpenetrating networks (IPNs) of two conjugated polymers are prepared by a combination of a chemical oxidation step and a vapour phase polymerisation step on non-conducting surfaces. In this work ferric tosylate was used as the oxidant as it gives very smooth and homogeneous coatings, and because its reaction products can be removed efficiently after the formation of the composite. Several combinations of polymers are demonstrated, and the versatility of the proposed method allows extensions to a wide range of conjugated polymers. The IPNs show optical and electrochemical characteristics, which are sums of the characteristics from the participating conducting polymers.
Keywords: Interpenetrating network; Conjugated polymer; Vapour phase polymerisation;

Formation of adherent polypyrrole coatings on Ti and Ti–6Al–4V alloy by Sean T. Earley; Denis P. Dowling; John P. Lowry; Carmel B. Breslin (111-118).
Highly adherent polypyrrole coatings were electrodeposited at etched Ti and Ti–6Al–4V from 0.2 M oxalic acid and 0.2 M pyrrole at a constant applied potential of 0.8 V versus SCE. The substrates were etched in an alkaline peroxide solution at ambient temperature for a 10 min period prior to the electropolymerization reactions. Chemical etching of the substrate gave rise to an increase in the hydrophilicity of the surface, the formation of a micro-etched surface and the formation of a semiconducting titanium oxide film. There was a considerable increase in the density of nucleation sites for polypyrrole growth at the micro-etched surface. Growth of the polypyrrole proceeded slowly from a high number of nucleation sites at the etched surface to give rise to a highly adherent polypyrrole coating. Adhesion of the polypyrrole coating was attributed to micro-etching of the substrate and the conductivity of the oxide film generated during the chemical etching process.
Keywords: Titanium; Polypyrrole; Implant; Adhesion; Electropolymerization;

The specific heat C(H,T) of (DMeFc) (TCNE) in the temperature range of 2–12 K in the presence of applied magnetic fields is reported. Previous results had shown anomaly at 4.82 K corresponding to a transition at T c from a high temperature one-dimensional (1D) magnet (above T c) to a three-dimensional (3D) macroscopic ferromagnet (below T c). The results show a dramatic behavior of the peak associated with the ferromagnetic transition. The peak height decreases with increasing magnetic field, i.e. magnetic fields smear the transition. For small values of applied magnetic fields the peak associated with the ferromagnetic transition is completely smeared. The results are indicative of anisotropic chains with enhanced 1D coherence and frustration in interchain coupling. The spin-wave spectrum is dominated by the 1D chains below T c.
Keywords: Specific heat; Magnetization; Phase transition; Ferromagnet; Magnetic material;

Near infrared electrochromism of lutetium phthalocyanine by Jidong Zhang; Fanli Lu; Haichao Huang; Jun Wang; Hong’an Yu; Jianzhuang Jiang; Donghang Yan; Zhiyuan Wang (123-126).
The near infrared (NIR) electrochromism of lutetium phthalocyanine (LuPc2) is studied both in its solution and vacuum deposited thin film. The LuPc2 in neutral state has near infrared absorption in the region from 1100 nm to 1600 nm. In electrochemical oxidized state (+1.0 V), it has an absorption peak at 880 nm. In electrochemical reduced state (−1.0 V), it shows no absorption in the near IR region. The attenuation at 1300 nm of electrochromic (EC) devices based on solution and thin film of LuPC2 was also studied. Attenuation of 0.31 dB and 0.28 dB was achieved respectively and the response time was 24 s and 1 s, respectively. These results demonstrate that LuPc2 is a kind of NIR EC material and it has potential application for variable optical attenuator.
Keywords: Lutetium phthalocyanine; Near infrared electrochromism;

Acidic phosphate ester (APES) bearing polysilsesquioxane precursor was prepared employing the ring-opening reaction of (3-glycidoxypropyl)trimethoxysilane (GPTMS) with acidic phosphate monoester carrying long hydrophilic tail. Doped with APES, polyaniline emeraldine base (PANI) was successfully dissolved or dispersed in water, and a free-standing film with electrical conductivity as high as 0.05 S/cm was available. The obtained film showed water-resistance due to the confinement from inorganic networks of amorphous polysilsesquioxane characterized by 29Si-MAS NMR spectrum. When the conductive film was soaked in water for 14 days, its electrical conductivity showed a little decline at the first stage of 6 days, and a considerably long platform existed at the following stage. However, for the film without polysilsesquioxane network, the electrical conductivity showed a sharp decline, and the film was broken into pieces for only 1 day soaking. Covalently linked with inorganic network of polysilsesquioxane by P―O―C bond, thermal stability of the conductive polyaniline was also improved compared with that of the conductive hybrid film from physical blending.
Keywords: Water-borne conductive polyaniline; Acidic phosphate ester; Polysilsesquioxane precursor; Water-resistance;

Fluorene-based oxadiazoles: thermally stable electron-transporting materials for light-emitting devices by Fang-Iy Wu; Ching-Fong Shu; Chin-Hsiung Chien; Yu-Tai Tao (133-139).
We have synthesized and characterized a series of fluorene-based oxadiazole derivatives, in each of which two identical oxadiazole moieties are linked to a fluorene unit via an sp3-hybridized carbon atom (C-9) to form a rigid, nonplanar structure. This structural feature leads to a reduction in the tendency to crystallize and an increase in the glass transition temperature. Electrochemical studies revealed that these materials undergo reversible reductions and have high electron affinities. Each fluorene-based oxadiazole exhibited reasonably good performance when it was used as the electron-transporting layer in a multi-layer organic EL device.
Keywords: Oxadiazole; Fluorene; Electron-transporting material; OLED;

A theoretical model is proposed to describe the deformation dependence of the dc-conductivity in composites in which tunneling is the dominant mechanism of charge carrier transport. It is experimentally shown that carbon black filled composites exhibit three different types of deformation behavior of the conductivity depending on the filler concentration. The model proposed explains these types of behavior and shows good agreement with the experimental results obtained for a wide range of deformations and filler concentrations.
Keywords: Conductivity; Composites; Compression deformation; Carbon black; Charge carriers transport; Tunneling;

Application of near-infrared absorbing heptamethine cyanine dyes as sensitizers for zinc oxide solar cell by Masaki Matsui; Yoshimi Hashimoto; Kazumasa Funabiki; Ji-Ye Jin; Tsukasa Yoshida; Hideki Minoura (147-153).
Novel near-infrared absorbing heptamethine cyanine dyes derived from indole, benzoxazole, benzothiazole, and quinoline were synthesized and examined as sensitizers for a zinc oxide solar cell. A 2-carboxyphenylthio-substituted indolium heptamethine cyanine dye showed incident photon-to-current efficiency (IPCE) of 4.17% at 804 nm.
Keywords: Dye-sensitized solar cell; Heptamethine cyanine dyes; Near-infrared absorbing dye; Zinc oxide; Incident photon-to-current efficiency; Sensitizers;

Synthesis of thermally stable and hole-transporting amorphous molecule having four carbazole moieties by Masayoshi Nomuraa ; Yuji Shibasakia ; Mitsuru Ueda; Kouhei Tugitab ; Musubu Ichikawab ; Yoshio Taniguchib (155-160).
A novel thermally stable and hole-transporting amorphous molecule, 9,9-bis(4-[bis-(4′-carbazol-9-yl-biphenyl-4-yl)-amino]-phenyl)fluorene (T-Caz) has been synthesized in two-step reactions from carbazole. T-Caz showed UV–Vis absorption bands at 290 and 340 nm in chloroform solution, and the photoluminescence spectrum showed a maximum peak at 420 nm in a purplish blue region. Differential scanning calorimetry (DSC) showed a high glass transition temperature at 250 °C. The organic light-emitting device prepared by spin-coating T-Caz solution onto the indium-tin oxide (ITO)-coated glass substrate in conjunction with tris(8-quinolinolato)aluminum (Alq) and LiF/Al, as an electron transporting light-emissive layer and a metal cathode, respectively, showed a maximum luminescence of 10,300 cd/m2 at 11 V.
Keywords: Organic light-emitting diodes; Amorphous hole-transporting materials; High glass transition temperature; Tris(8-quinolinolato)aluminum; Spin-coating; Carbazole;

Methyl 2,5-di-(2′-hydroxyethoxy)benzylidenecyanoacetate (3) was prepared by hydrolysis of methyl 2,5-di-(2′-vinyloxyethoxy)benzylidenecyanoacetate (2). Diol 3 was condensed with 2,4-toluenediisocyanate (TDI) and 3,3′-dimethoxy-4,4′-biphenylenediisocyanate (DMBPI) to yield novel T-type polyurethanes (45) with 2,5-dioxybenzylidenecyanoacetate group as a nonlinear optical (NLO) chromophore, which constitutes a part of the polymer backbone. The resulting polyurethanes 45 were soluble in common organic solvents such as acetone and dimethylsulfoxide (DMSO). Polymers 45 showed a thermal stability up to 250 °C in thermogravimetric analysis (TGA) thermograms with glass transition temperature (T g) values from differential scanning calorimetry (DSC) thermograms near 145 °C. The second harmonic generation (SHG) coefficients (d 33) of poled polymer films were around 2.36 × 10−9  esu. These poled polymers exhibited a thermal stability of dipole alignment up to T g, and no SHG decay was observed below 150 °C due to the partial main chain character of the polymer structure, which is acceptable for NLO device applications.
Keywords: Methyl 2,5-di-(2′-hydroxyethoxy)benzylidenecyanoacetate; NLO chromophore; Polyurethanes; DSC; TGA; AFM; Thermal stability; SHG coefficient;

The field-effect mobility of holes in regioregular poly(3-alkylthiophene)s was determined for a series of 5 alkyl chain lengths from C4 (n-butyl) to C12 (n-dodecyl). Contrary to a previous report, a non-monotonic dependence of field-effect mobility on alkyl chain length was found. The average hole mobility varied from 1.2 × 10−3  cm2/Vs in poly(3-butylthiophene) and 1 × 10−2  cm2/Vs in poly(3-hexylthiophene) to 2.4 × 10−5  cm2/Vs for poly(3-dodecylthiophene). We believe that the hexyl side chain is optimum for charge transport because of better self-organization in poly(3-hexylthiophene) compared to other polymers in the series. The present results provide an important structure–carrier mobility relationship for the regioregular poly(3-alkylthiophene)s which are of wide interest for thin film transistors and photovoltaic cells.
Keywords: Poly(3-alkylthiophene)s; Organic transistors; Structure–mobility relationship; Poly(3-hexylthiophene) transistors; Carrier mobility; Alkyl chain length;

Memory applications of a thiophene-based conjugated polymer by photoluminescence measurements by Himadri S. Majumdar; Chiara Botta; Alberto Bolognesi; Amlan J. Pal (175-178).
Data-storage and memory applications of conjugated polymers and organic semiconductors are generally probed by device current or dielectric properties to determine the storage and switching properties. In this article, we use photoluminescence as an alternate method of probing the states of the devices. We have fabricated such devices based on a polythiophene derivative, and recorded photoluminescence (PL) spectra during and after applying bias. The presence of localized charges on the backbone of the polymer has been found to modulate the PL intensity. Since the relaxation of the space charges is slow, we have shown that PL intensity can be used as a probe for memory applications to read the state of the device. PL as a probe for memory applications of polymeric materials offers an intrinsic advantage that the state can be read without affecting the device properties.
Keywords: Conjugated polymers; Data-storage devices; Memory applications; Photoluminescence; Space charges;

Synthesis, theoretical characterisation and spectra of thiophene–fluorene π-conjugated derivatives by Vladimír Lukeš; Daniel Végh; Pavol Hrdlovič; Martin Štefko; Katarína Matuszná; Viliam Laurinc (179-186).
The syntheses, spectral measurements and the theoretical study of 2-((9H-fluoren-9-ylidene)methyl)thiophene (A), ((9H-fluoren-9-ylidene)methyl)-5-(thiophen-2-yl)thiophene (B), 1-(5-((9H-fluoren-9-ylidene)methyl)thiophen-2-yl)propan-1-one (C), 1- (5-((9H-fluoren-9-ylidene)methyl)thiophen-2-yl)pentan-1-one (D), 1,2-bis(1-(5-((9H-fluoren-9-ylidene) methyl)thiophen-2-yl)propylidene)hydrazine (E) and 1,2-bis(1-(5-((9H-fluoren-9-ylidene)methyl)thiophen-2-yl)pentylidene)hydrazine (F) are presented. The electron absorption and fluorescence spectra of prepared molecules are measured in polymer matrix and solution. The influence of the conformation (e.g. cis- and trans-forms in the case of E and F molecules) on electronic spectra are interpreted by ZINDO/S method. Our results indicate that the thiophene rings connected with terminal fluorene chromophores through the double bounded carbon atom seem to be suitable candidates for the subsequent preparation of the electro-optical materials.
Keywords: Thiophene–fluorene derivatives; Syntheses; Electron absorption and luminescence spectra;

Freestanding polyparaphenylene films were obtained on polymerization of benzene at potential of 1.2 V versus Al wire on substrates like platinum/transparent conducting glass as an anode. The electrolyte used was chloroaluminate room-temperature melt, which was prepared by intimate mixing of a 1:2 ratio of cetyl pyridinium chloride and anhydrous aluminum chloride to yield a viscous liquid. This liquid was miscible in all proportions with benzene and other aromatic hydrocarbons in all proportions at room temperature. The polyparaphenylene films deposited on platinum anode exhibited a prominent cyclic voltammetric peak at 0.7 V versus Al wire as reference electrode in chloroaluminate medium. The impedance spectra gave low charge transfer resistance. The diffused reflectance electronic spectra of the film gave the peaks at 386 nm and 886 nm. The PPP films showed electronic conductivity around 3–4 × 104  S/cm by four probe method under nitrogen atmosphere. The polymer was also characterized by IR spectra, thermal studies, and SEM studies.
Keywords: Polyparaphenylene; Conductivity; Freestanding; Room-temperature melt; Cetyl pyridinium chloroaluminate melt;

Microwave-assisted synthesis of polythiophenes via the Stille coupling by Steve Tierney; Martin Heeney; Iain McCulloch (195-198).
Microwave radiation has been employed to synthesise soluble polythiophenes 14 via the Stille coupling, resulting in the preparation of polythiophene 1 with higher molecular weights (M n  > 15,000) and a lower polydispersity (M w/M n  ≈ 2) than when synthesised using conventional heating. Field-effect mobilities of greater than 3 × 10−2  cm2/V s were measured for polymer 1 in an organic field-effect transistors (OFETs) device.
Keywords: Microwave-assisted; Polythiophene; Semiconductor; Solubility;

LESR study on PPV–PPE/PCBM composites for organic photovoltaics by A.L. Konkin; S. Sensfuss; H.-K. Roth; G. Nazmutdinova; M. Schroedner; M. Al-Ibrahim; D.A.M. Egbe (199-204).
Photoinduced charge carrier generation and charge transfer in the new conjugated poly(phenylene vinylene)–poly(phenylene ethynylene) (PPV–PPE) copolymers mixed with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were studied by LESR technique at 77 K. The effective charge separation has been verified by two ESR signals belonging to positive polarons on the polymer chains and to radical anions of PCBM molecules, respectively. The relaxation times T 1 and T 2 of the anion radical of PCBM were estimated by anisotropic linewidth analysis and microwave power saturation studies. The low temperature recombination of long living charge carrier pairs after cessation of photoexcitation was studied and described by a tunnel model in which the recombination time depends on the distances between charge carrier pairs. Results on PPV–PPE/PCBM were compared with those of a poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO–PPV)/PCBM composite.
Keywords: Electron spin resonance; Conjugated polymers; Fullerene derivatives; Charge recombination; Tunnel model; Polymer solar cells;

Highly efficient deep-blue organic light-emitting diodes with doped transport layers by D. Gebeyehu; K. Walzer; G. He; M. Pfeiffer; K. Leo; J. Brandt; A. Gerhard; P. Stößel; H. Vestweber (205-211).
We demonstrate highly efficient, vapor-deposited blue organic light-emitting diodes (OLEDs) operating at low voltage. For reaching deep-blue color, we used two new fluorophores, 9,10-bis(9,9′-spirobi[9H-fluorene]-2-yl)anthracene (Spiro-Anthracene) from Covion, and 4,4′-bis-(N,N-diphenylamino)-tetraphenyl (4P-TPD) from Syntec-Sensient, sandwiched in between p- and n-type doped wide band-gap transport layers and appropriate blocking layers. These p-i-n OLED devices show high luminance and efficiency at low operating voltages. Both dyes emit deep-blue light at color coordinates of x  = 0.15 and y  = 0.09 (4P-TPD) and x  = 0.15 and y  = 0.18 (Spiro-Anthracene). Optimized devices containing Spiro-Anthracene reach a luminance of 100 and 1000 cd/m2 already at a voltage of 2.9 and 3.4 V, respectively. At the same time, a deep-blue color with CIE color coordinates of x  = 0.14 and y = 0.14 as well as good current efficiencies (3.9 cd/A at 100 cd/m2) and quantum efficiencies (3.7% at 100 cd/m2) are reached, which shows that the concept of doped transport layers and appropriate fluorescent emitters can be applied successfully to the preparation of blue OLEDs.
Keywords: Blue organic light-emitting diode; Vacuum co-evaporation; Doped transport layers; Efficiency;

New BETS salt with the square planar [Ni(CN)4]2− anion by A.D. Dubrovskii; N.G. Spitsina; L.I. Buravov; A.N. Chekhlov; O.A. Dyachenko; Y. Okano; A. Kobayashi; S.I. Pesotskii; R.B. Lyubovskii (213-218).
The new radical cation salt based on bis(ethylenedithio)tetraselenafulvalene (BETS) with the square planar [Ni(CN)4]2− anion was synthesized by electrocrystallization: (BETS)4·Ni(CN)4. X-ray analysis, electron transport, magnetotransport and electronic band structure calculations are presented and discussed.
Keywords: Radical cation salt; Bis(ethylenedithio)tetraselenafulvalene (BETS); Tetracyanonickelate(II) [Ni(CN)4]2− dianion; X-ray crystal structure; Electronic band structure calculations; Shubnikov de Haas (SdH) oscillations;