Synthetic Metals (v.156, #21-24)

Bis(2-(2-hydroxyphenyl)benzthiazolate)zinc(II), [Zn(BTZ)2]2, is amongst the best white-light emissive as well as electron transport materials used in organic light-emitting diodes (OLEDs). In order to gain a deeper understanding for its carrier transport properties, we adopt the density-functional theory (DFT) with generalized gradient approximation (GGA) to calculate the electronic band structure and the density of states (DOS) by the Becke exchange plus Lee–Yang–Parr correlation (BLYP) functional. The intermolecular interaction related to transport behavior has been analyzed from the bandwidths and band gaps. Within the effective mass approximation, we find that the mobility of electron is about two times larger than that for the hole. Furthermore, if we consider the bands near Fermi level, we conclude that the interband gaps within the unoccupied bands are generally smaller than those for the occupied bands, which indicate that the electron can hop through scattering from one band to another, much easier than the hole. These facts indicate that, in [Zn(BTZ)2]2, the electron are the dominant carriers in transport, in contrast to most organic materials.

The study of molecular assemblies obtained by ionic interactions between two π-conjugated organic molecules one possessing electron donor properties and the other possessing electron acceptor properties is discussed here.The electron acceptor molecules chosen are two tetra-chlorinated 3,4,9,10-perylene tetracarboxylic diimides bearing pyridine or amine functions ((1) or (2)) and the electron donor is 4-carbazol-9-yl-butane-1-sulfonic acid (3). The electrochemical characterizations and UV–vis/photoluminescence spectroscopy studies of each compound were realized. Moreover, photoluminescence quenching of (1) and (2) solutions is observed when they are protonated with the acid function of (3). Interestingly, with a simple mixture of (1) or (2) with 9-ethyl-carbazole, (1) and (2) solutions fluorescence quenching is very low. Finally, the relative energy levels values of the HOMO and LUMO of (1)–(3) were estimated and the results are compatible with a possible electron transfer from carbazole to perylene unit making these ionic assemblies attractive candidate for an active layer application in solar cell devices.
Keywords: Perylene diimide; Carbazole; Fluorescence quenching; Ionic assembly;

Conducting IPN actuators: From polymer chemistry to actuator with linear actuation by Frédéric Vidal; Cédric Plesse; Guillaume Palaprat; Abderrahmane Kheddar; Johan Citerin; Dominique Teyssié; Claude Chevrot (1299-1304).
Among electroactive actuators, those based on electronic conducting polymers (ECPs) possess several advantages (low actuation voltages, large deformations, etc.) but also some drawbacks (bending movements only, delamination, etc.). In order to overcome the delamination process observed in working three-layered actuators, we have developed a new concept of actuator consisting in an interpenetrating polymer network (IPN) matrix in which 3,4-ethylenedioxythiophene (EDOT) is chemically polymerized. The typical IPN matrix results from the association of the two following cross-linked polymers: the first one, polyethylene oxide (PEO), will ensure the ionic conductivity of the system after salt incorporation while the second one, polybutadiene (PB), will allow to adjust the required mechanical properties. The chemical polymerization of EDOT within the IPN leads to the formation of a PEDOT gradient, its concentration decreasing from the outside faces towards the centre of the IPN. So, such a one-piece actuator is equivalent to a three-layered actuator. Applying a low potential (2–5 V) at a frequency from 0.1 to 15 Hz, up to 106 bending deformations in open air have been observed after incorporation of a room temperature ionic liquid. However, linear deformations are more interesting for a number of applications than simple bending motions. The description of the design of an open air working actuator with linear actuation is presented.
Keywords: Actuator; Electronic conducting polymer; PEDOT; Linear deformation; Interpenetrating polymer network;

A new analytical model is developed for thin-film field-effect transistors (TFTs). The active layer of the devices is considered purely two-dimensional. In the first part, the basic model is developed for intrinsic materials. It is demonstrated that it accurately describes the electrical characteristics and elucidates on the physical meaning of the device and material parameters, such as threshold voltage and sub-threshold current. It also clarifies the nature of so-called contact effects, often used in literature to explain non-linear IV curves. Furthermore, ambipolar devices are treated.
Keywords: Thin-film field effect transistors; Amorphous silicon; Organic semiconductors; Sexithiophene; Two-dimensional electronics; Contact effects;

Based on a new model for thin-film field-effect transistors, in which the active layer is treated as purely two-dimensional, the effects of impurities on the electrical characteristics are discussed. Localized electronic levels are introduced into the model. It is shown that the presence of traps readily accounts for the non-linearities in the current-voltage curves. Trap states can also explain the temperature dependence of the current and mobility, including the so-called Meyer-Neldel Rule. Finally, transients are qualitatively discussed.
Keywords: Thin-film field effect transistors; Amorphous silicon; Organic semiconductors; Traps;

Various conductive composites were prepared by in situ chemical polymerization of aniline in the presence of several water soluble polymers [alginic acid (2a, AA), poly(acrylic acid) (2b, PAA), and poly(vinyl alchol) (2c, PVA)] and/or anionic surfactants [dodecylbenzenesulfonic acid (1a, DBSA) and sodium dodecylsulfate (1b, SDS)] under various polymerization conditions. As a result, the corresponding composites having good film forming property were readily obtained even in the cases with SDS, although PANI prepared in the presence of SDS (PANI/SDS) generally shows extremely poor film forming property due to its low solubility/miscibility and processability in the similar manner as PANI doped with HCl (PANI/HCl). Among the resulting composites, the conductivities of the composites synthesized with SDS tended to be higher than those of the similar composites prepared with DBSA or without anionic surfactants. In particular, the composite prepared by using PVA bearing high molecule weight (PVA-H) and 20 mmol of SDS to aniline monomer was found to show the highest conductivity among the present investigations (32 S/cm), although the conductivity of typical conductive polyaniline doped with HCl, which was synthesized under the similar polymerization conditions, was ca. 3 S/cm at the best. The present PANI composites were characterized by spectroscopic and thermal analysis. Formation of oxidation states of PANIs in these composites was confirmed by the spectroscopic (UV–vis and FT-IR) analysis. The thermal stability of the resulting composite was somewhat lower than those of PANI/SDS itself and PANI/HCl.
Keywords: Conducting polymer; Polyaniline; Composite; Surfactant;

Synthesis, polymerization and wool coating studies of 3-iso-butylpyrrole and 3-iso-pentylpyrrole by R.C. Foitzik; A. Kaynak; F.M. Pfeffer; J. Beckman (1333-1340).
In a four-step method starting from pyrrole, the synthesis of 3-iso-butylpyrrole and 3-iso-pentylpyrrole, was achieved in 45 and 44% yields, respectively. Polymerization studies of these branched alkyl pyrroles are described and the results compared with those obtained for the unbranched structural isomers n-butyl and n-pentylpyrrole. A series of conductive textiles were produced by the chemical polymerization of the iso-alkylpyrroles using both solution and vapour polymerization techniques. Fabrics coated with poly-iso-alkylpyrrole formed using the solution polymerization method had a lower surface resistance than those formed using the vapour polymerization method. These conductivity results were in direct contrast to those previously obtained for 3-n-alkylpyrroles on fabrics. A remarkable crystal-like growth on the surface of the textile fabric was observed when solution polymerization of 3-iso-pentylpyrrole was employed—reinforcing the notion that subtle changes in monomer structure can drastically affect bulk polymer properties.
Keywords: Polymers; Acylation; Substituent effects; Protecting groups; Reductions; Textilecoating; Conducting polymers; Wool;

High thermoelectric performance of poly(2,5-dimethoxyphenylenevinylene) and its derivatives by Yuji Hiroshige; Makoto Ookawa; Naoki Toshima (1341-1347).
Poly(2,5-dimethoxyphenylenevinylene) (PMeOPV) and a series of copolymers consisting of both 2,5-dimethoxy-substituted phenylenevinylene units and unsubstituted units (P(MeOPV-co-PV)) were evaluated from the viewpoint of their thermoelectric properties. Their conjugated polymer films were prepared by pyrolysis of stretched or unstretched films of sulfonium salt precursor polymers, and subsequently doped with iodine vapor to provide electrical conductivity. The power factors P (=S 2 σ), indicating thermoelectric performance, were calculated with the measured electrical conductivity σ and Seebeck coefficient S of the doped films. PMeOPV showed a higher power factor of 7.1 μW m−1  K−2 at 313 K than that of a camphorsulfonic acid-doped polyaniline as reference. P(MeOPV-co-PV) precursor polymers with less than 20 mol% of MeOPV unit content in the monomer feed were stretchable, therefore providing stretched P(MeOPV-co-PV) films with low MeOPV unit content. The stretching treatment for P(MeOPV-co-PV) enhanced its electrical conductivity, but kept the Seebeck coefficient at nearly the same level as that of unstretched polymers. Consequently a 4.4-fold stretched copolymer exhibited an electrical conductivity of 183.5 S/cm and a Seebeck coefficient of 43.5 μV/K at 313 K, and thus, its power factor at 313 K was over 30 μW m−1  K−2. To the best of our knowledge, this is the highest thermoelectric performance ever reported among conducting polymers.
Keywords: Poly(2,5-dimethoxyphenylenevinylene); Polyphenylenevinylene; Thermoelectric property; Electrical conductivity; Seebeck coefficient; Power factor;

Blue electroluminescence in 1H-pyrazoloquinoline derivatives by E. Gondek; I.V. Kityk; A. Danel; A. Wisla; J. Sanetra (1348-1354).
A new series of pyrazoloquinoline copolymers with N-vinylcarbazole were synthesized. The materials possess high quantum efficiency (up to 1.16%) in blue spectral range (440–480 nm). The copolymers possess principal advantageous compared to the traditional polymers mixtures due to substantially higher stability and larger quantum efficiency. The materials seem to be promising for using like material for light emitting diodes emitting in the blue spectral range. It is crucial that appropriately varying the pyrazoloquinoline derivative substitution we received a red spectral shift of the electroluminescent maxima possessing the enhanced quantum efficiency.
Keywords: Blue light emitting diodes; Co-polymers;

Electromagnetic properties of polyaniline/maghemite nanocomposites by Tar-Hwa Hsieh; Ko-Shan Ho; Ching-Hung Huang; Yen-Zen Wang; Zhi-Long Chen (1355-1361).
This article is to carry out researches on the synthesis of the polyaniline/γ-Fe2O3 nanocomposites by a reverse micelle process and the effects of re-doping time on the electromagnetic properties of polyaniline/γ-Fe2O3 nanocomposites, investigated by Fourier transform spectrometer (FT-IR), X-ray photoelectron spectroscopy (XPS), wide angle X-ray diffraction diffractometer (WAXD), impedance analyzer, micro-ohmeter, and superconductor quantum interference device (SQUID). The nanostructure of the polyaniline/γ-Fe2O3 nanocomposites was characterized by micrographs of transmission electron microscopy (TEM). It was found that the dispersed γ-Fe2O3 phase is roughly distributed in the polyaniline matrix. Results showed that, in the presence of γ-Fe2O3, the growth rate of quinoid ring is markedly retarded. The crystallinity and doping level of polyaniline in the nanocomposites increase with re-doping time, the conductivity and dielectric properties (i.e., permittivity and loss factor) of nanocomposites are hence increased and the ionic polarization relaxation time become shorter from 1.71 × 10−9 to 7.48 × 10−10  s. The particle size and γ-Fe2O3 content in the nanocomposites decrease with re-doping time due to the reduction effect resulting from the doped protonic acid of polyaniline. For the room-temperature SQUID analysis, the superparamagnetic behavior is observed, suggesting the presence of the thermal activation energy contribution to the magnetic moment. The saturated magnetization was decreased with decreasing γ-Fe2O3 contents.
Keywords: Polyaniline/γ-Fe2O3 nanocomposites; Reverse micelle; Electromagnetic properties; Dielectric properties; Superparamagnetic behavior;

Anisotropic conductivity–temperature characteristic of solution-cast poly(3-hexylthiophene) films by Chuanjun Liu; Kenji Oshima; Masato Shimomura; Shinnosuke Miyauchi (1362-1367).
For the purpose of developing a positive temperature coefficient (PTC) device, thin films of poly(3-hexylthiophene) (P3HT) were prepared by solution-cast from chloroform. In the present work, the P3HTs of high molecular weight (M w) with various regioregularities were employed, and the structure anisotropy and the temperature dependence of conductivities in directions parallel (σ ) and perpendicular (σ ) to the film surface were investigated. For highly orientated P3HT films, the temperature dependence of σ was found to reflect an amorphous character of charge hopping along this direction. However, the σ decreased greatly above 50 °C, which was attributed to a decrease in the in-plane π-stacking caused both by the melting of crystallized side chains and the enhanced side-chain disturbance with increasing temperature. Different charge transport mechanisms were proposed to explain the anisotropic conductivity–temperature characteristics observed in the two directions. The high conductivity in the parallel direction was deteriorated significantly by thermal recycles, probably due to a reduction of the orientation degree of P3HT crystallites in the bulk film, rather than a change of conjugated length of polymer backbone. A reversible PTC effect was observed for the P3HT film with a high M w and medium regioregularity in the perpendicular direction, which suggested that the high M w was more important than the high regioregularity to design the thin film PTC devices based on the soluble P3HT films.
Keywords: Poly(3-hexylthiophene); Anisotropy; Regioregularity; Thin film devices; Positive temperature coefficient effect;

EPR study of charge transfer in polyaniline highly doped by p-toluenesulfonic acid by V.I. Krinichnyi; S.V. Tokarev; H.-K. Roth; M. Schrödner; B. Wessling (1368-1377).
Magnetic, relaxation and electronic dynamic parameters of paramagnetic centers in crystalline domains of polyaniline highly doped by p-toluenesulfonic acid (PANI-PTSA) as well as PANI-PTSA dispersed in poly(methyl methacrylate) were studied by the 3 cm (9.7 GHz) and 2 mm (140 GHz) wavebands EPR. These centers demonstrate the Lorentzian single line with the Dysonian contribution at both wavebands indicating intrinsic conductivity of metal-like domains near 1500–4000 S/cm at room temperature. Effective conductivity of the polymer is defined by Q3D delocalization of charge carriers within such domains and their Mott variable range hopping between the domains dominating its micro- and macroscopic conductivity. It was shown that the interaction of the charge carriers with the lattice phonons governs intradomain charge transfer at high temperatures. Reversible dipolare interaction of paramagnetic centers with oxygen was revealed. This interaction depends on electron precession frequency and/or on the PANI-PTSA dispersion in an insulating matrix. Charge transfer in PANI-PTSA was analyzed to be non-correlated with spin relaxation and dynamics that evidences the formation of Q3D metal-like domains contrary to the “single conducting chain” model.
Keywords: EPR; Polyaniline; Polaron; Relaxation; Dynamics;

Conjugated palladium complex with poly(3-heptylpyrrole) and its application by Toshikazu Hirao; Yusuke Otomaru; Yohtaro Inoue; Toshiyuki Moriuchi; Tomoyuki Ogata; Yoshiharu Sato (1378-1382).
Poly(3-heptylpyrrole) was demonstrated to serve as an efficient π-conjugated ligand to afford a conjugated complex with Pd(MeCN)2Cl2, which was successfully applied to organic light emitting diode (OLED) devices. An OLED device with the conjugated complex film as a hole injection layer performed the maximum luminance of 11,000 cd/m2 at 10 V, which was 2 V lower than a device with the conventional copper phthalocyanine (CuPc) hole injection layer.
Keywords: Poly(3-heptylpyrrole); Conjugated complex; OLED device; Hole injection layer;

Solution-processable dendric triphenylamine nonamers as hole-transporting and hole-injection materials for organic light-emitting devices by Musubu Ichikawa; Kumiko Hibino; Norimasa Yokoyama; Tetsuzo Miki; Toshiki Koyama; Yoshio Taniguchi (1383-1389).
We demonstrated two solution-processable triphenylamine dendric nonamers: the N-atom-centered nonamer (TPA9-1) and the phenyl-centered nonamer (TPA9-2). The materials were found to have high glass transition temperature (T g) up to almost 200 °C. The fractional difference of the central units of the molecular structures caused different adaptability due to the different ionization potentials (I p). TPA9-1 (N-atom-centered), whose I p was smaller than that of TPA9-2 (phenyl-centered), was suitable as hole-injection layer material, and TPA9-2 was suitable as hole-transporting layer material. Computational chemistry provides a ready explanation for the I p difference between the two materials.
Keywords: Triphenylamine; Dendrimer; Hole transport; Organic light-emitting diode; Solution processable;

New phosphorescent aromatic polyketones with a 9,9-dioctylfluorene copolymerizing unit (▪; Ar =  p-phenylene (Polymer 1) or pyridine-2,5-diyl (Polymer 2); Flu = 9,9-dioctylfluorene-2,7-diyl) were prepared by organometallic polycondensation. Chemical properties of the polymers support a notion that the polymer has an expanded π-conjugated system through the cross π-conjugated –Ar–CO–Ar– unit. Polymer 1 was photoluminescent with an emission peak at about 430 nm and quantum yields of 10 and 21% at room temperature (r.t.) and 77 K, respectively. The light emission from Polymer 1 showed features of phosphorescence with long decay lifetimes of 0.22 and 0.53 s at r.t. and 77 K, respectively. Polymer 2 behaved similarly in light emission.
Keywords: Conjugated polymers; Polyketones; Cross-conjugation; Phosphorescence;

The conductivity of an emeraldine base polyaniline thin film on an interdigitated electrode in hydrochloric acid solutions of various pHs between 2.25 and 6 was investigated by impedance spectroscopy. In general, protonation induces an emeraldine base (EB) to emeraldine salt (ES) transformation and an associated increase in conductivity only when the pH of the protonating bath is less than 4. However, impedance spectroscopy detects changes in the conductivity behavior that occur prior to the onset of this increase in total conductivity change. These changes can also be measured with AC impedance measurements at a single frequency, which expands the pH range over which emeraldine base polyaniline can be used as a sensor.
Keywords: Impedance spectroscopy; Emeraldine base thin film; AC impedance measurements;

A CO2 sensor based on a composite thin film of emeraldine base polyaniline and poly (vinyl alcohol) cast from N-methyl pyrrolidone on an interdigitated electrode was characterized using impedance spectroscopy. The response of the sensor was slow and smaller in magnitude as compared to reports in the literature for similar sensors. Materials characterization indicated that the desired emeraldine base phase was not present after heat treatment. In addition, the mechanism for carbon dioxide detection previously reported in the literature cannot explain the CO2 sensitivity, because the pH established between CO2 and carbonic acid in the water dissolved in the poly (vinyl alcohol) matrix is not sufficient to induce a significant change in the conductivity of the emeraldine base polyaniline.
Keywords: Polyaniline thin film; Materials characterization; Carbon dioxide sensing; Emeraldine base doping characteristic;

Novel blue luminescent twin molecules containing fluorene, carbazole or phenothiazine units by Vasilis P. Barberis; John A. Mikroyannidis (1408-1414).
Three new twin molecules F, C and P that were bridged with a hexamethylene chain and contained fluorene, carbazole or phenothiazine units, respectively, were synthesized by Heck coupling. They were readily soluble in common organic solvents and showed a satisfactory thermal stability and relatively low glass transition temperatures. They absorbed at 302–373 nm with optical band gap of 2.72–3.01 eV. F and C emitted blue light (428–449 nm) both in solution and thin film, while P emitted blue-greenish light (489–503 nm). The highest photoluminescence (PL) quantum yield in THF solution was obtained for F (0.60). The PL emission maximum in thin film was slightly red shifted relative to solution, thus suggesting a low tendency of molecules to aggregate. This was attributed to structural characteristics of the twin molecules.
Keywords: Twin molecules; Fluorenevinylene; Carbazolevinylene; Phenothiazinevinylene; Synthesis; Photophysics;

The magnetic properties of two types of poly[(3-methoxy)4,6-toluene] (P3mt1 and P3mt2) are studied before and after doping with iodine or organic acids (CF3COOH or CH3SO3H) in a range of temperature varying from 110 to 460 K. Under 250 K and independent of the state, the magnetic susceptibility χ obeys Curie law dependence. Up to this temperature, χ is governed by a Pauli law contribution. This fact can be related to localized or delocalized spin. Furthermore, iodine doping induces distortion in the polymers that affects the ESR line distribution. In the case of organic acids doping process, a hyperfine structure with hyperfine coupling constant values between 2.2 and 2.8 G is observed. As the spin concentration decreases, we deduce that in the case of P3mt2, there is a de-doping process.
Keywords: ESR; Radical center; Susceptibility; Hyperfine coupling;