Synthetic Metals (v.157, #2-3)

A comparative study was done between liquid-state photoelectrochemical cells (PECs) consisting of photoactive electrodes of poly(3-hexylthiophene), P3HT, and a composite film of nanocrystalline titanium dioxide (nc-TiO2) and P3HT. The nc-TiO2/P3HT based device shows better performance with an open-circuit voltage of 0.51 V, a short-circuit current of 0.31 mA/cm2, and a fill factor of 0.51 when illuminated with white light intensity of 100 mW/cm2. The IPCE% obtained at 550 nm for P3HT based device was 0.18% while for that of the nc-TiO2/P3HT based device was 4%. In P3HT based devices, P3HT showed its p-type behavior while in nc-TiO2/P3HT based devices, P3HT acted as a sensitizer to nanocrystalline TiO2.
Keywords: Liquid-state photoelectrochemical cell; poly(3-hexylthiophene); Nanocrystalline titanium dioxide; I3 /I redox couple;

Preparation and characterization of a polypyrrole hybrid film with [Ni(dmit)2]2−, bis(1,3-dithiole-2-thione-4,5-dithiolate)nickellate(II) by Antonio Gerson Bernardo da Cruz; James L. Wardell; Marcus V.D. Rangel; Renata A. Simão; Ana M. Rocco (80-90).
The synthesis of a hybrid material obtained by electropolymerization of a solution of pyrrole and [NEt4]2[Ni(dmit)2] (dmit = 1,3-dithiole-2-thione-4,5-dithiolato) in acetonitrile solution is reported. The material was characterized by cyclic voltammetry, UV–vis and infrared spectroscopies, scanning electron microscopy (SEM), atomic force microscopy (AFM) and thermal gravimetric analysis (TGA). The FTIR spectroscopy showed that the [Ni(dmit)2]2− anion has been inserted in the polypyrrole framework and was not destroyed or modified during the polymerization process. The voltammetric analysis indicated that the material has electroactivity and undergoes redox processes associated with the conducting polymer and the counteranion. The cyclic voltammetry results also suggest that the counteranion is not trapped in the PPy matrix undergoing anion exchange during the redox cycle of PPy. The PPy/[Ni(dmit)2]2− exhibits good thermal stability and a intrinsic conductivity value in the range of semiconductors (10−3  S cm−1).
Keywords: Polypyrrole; Hybrid material; Complex counteranion; bis(1,3-dithiole-2-thione-4,5-dithiolate)nickellate(II); dmit;

Study of the influence of the molecular organization on single-layer OLEDs’ performances by Laurent Aubouy; Philippe Gerbier; Christian Guérin; Nolwenn Huby; Lionel Hirsch; Laurence Vignau (91-97).
We have synthesized three new silole derivatives with incrementally flexible structure to tune their packing ability and therefore study the influence of the molecular organization in single-layer OLEDs. This architecture was chosen since the absence of organic layers interfaces allows a better evaluation of the role of the molecular arrangement in the active layer. The examination of the EL properties gives evidences of the prominent role of the molecular organization on the OLED efficiency. A crystalline-like organization of the molecules allows high current density but low luminance efficiency since an excessive electron current flow is involved compared to the hole one, and the recombination rate is poor. On the contrary, disordered assemblies of molecules allows better performances by avoiding unfavourable π-stacking, while keeping good intermolecular orbital overlaps to support charge carrier transport.
Keywords: Silole derivatives; Single-layer OLEDs; π-Stacking;

Electrosynthesis of polyaniline films on titanium by pulse potentiostatic method by Haihui Zhou; Jingbang Wen; Xiaohui Ning; Chaopeng Fu; Jinhua Chen; Yafei Kuang (98-103).
Polyaniline (PANI) was synthesized on titanium electrode from aqueous solution containing 0.3 mol L−1 aniline and 1 mol L−1 HNO3 by pulse potentiostatic method. The chronoamperogram during polymerization process of aniline was recorded. The effects of the synthesis parameters, such as anodic pulse duration (t a), cathodic pulse duration (t c), lower limit potential (E c) and upper limit potential (E a), on the morphology and electroactivity of the PANI films were investigated by scanning electron microscopy (SEM) and cyclic voltammetry (CV). SEM results present that flake, mica-like, quasi-fibrous and nano-fibrous PANI film could be synthesized with various polymerization parameters. Under the following conditions, t a  = 0.8 s, t c  = 0.1 s, E c  = 0 V and E a  = 1.0 V, high quality nano-fibrous PANI film with the best electroactivity was obtained. The CV results show that the PANI films with different morphologies, which were prepared under the same anodic polymerization charge, have obvious different characteristics. This means that the PANI films with different morphologies have different electrochemical activity.
Keywords: Polymers; Electrochemical techniques; Nanostructures;

Effects of annealing of poly(3-hexylthiophene) film on the performance of double-layered EL devices of ITO/polymer/Alq3/Mg–Ag by Joji Ohshita; Yosuke Tada; Atsutaka Kunai; Yutaka Harima; Atsushi Kohno; Yoshihito Kunugi (104-108).
Double layer devices with a structure of ITO/pHT/Alq3/Mg–Ag (ITO = indium tin oxide, pHT = regio-regular or random poly(3-hexylthiophene), Alq3 = tris(8-hydroxyquinoline)aluminium) were fabricated. The device with a random pHT film emitted a green-yellow light in all voltage region, while that having a regio-regular pHT film exhibited a color change from green to red by applying the bias voltage higher than 15 V. Annealing the pHT films prepared on ITO at 200 °C for 1 h in nitrogen, prior to vapor-deposition of the Alq3 layer, improved the device performance with lowering the onset bias voltage by 2–3 V. The EL colors and spectra were also affected by annealing. X-ray reflectivity measurements before and after annealing the pHT film on ITO indicated increased density of the pHT layer and structural changes in the pHT/ITO interface by annealing, which seems to be responsible for the improved EL device performance.
Keywords: Polythiophene; Regio-regularity; Hole-transport; EL device; Annealing; Interface; X-ray reflectivity;

Electrochemical copolymerization of furan and thiophene was performed at constant electrode potential in a binary solvent system consisting of boron trifluoride-ethyl ether and an additional amount of ethyl ether (mole ratio 1:2). The homopolymers and copolymers obtained were studied with in situ UV–vis- and FT-IR-spectroscopy. An electrochemical degradation mechanism for furan–thiophene copolymers in acetonitrile and in aqueous solution is proposed based on FT-IR spectroscopy data.
Keywords: In situ UV–vis-spectroscopy; FT-IR-spectroscopy; Copolymerization; Polyfuran; Polythiophene;

Efficient flexible devices using a statistical copolymer of oxadiazole containing PPV by Hermona K. Christian-Pandya; Subramanian Vaidyanathan; Changhee Ko; Frederick L. Beyer; Mary E. Galvin (120-124).
Flexible light-emitting diodes, with simple device architectures, fabricated using a random copolymer of hole transporting dialkoxy-substituted phenylenevinylene (PV) with an electron transporting oxadiazole containing PV derivative as the emissive layer and higher work function aluminum cathodes have been examined and compared with control devices on glass substrates. In all devices poly(3,4-ethylenedioxythiophene) with poly(styrenesulfonate) (PEDOT:PSS) was used as the hole injection layer and a thin layer of cesium fluoride or lithium fluoride has been used at the polymer/cathode interface to aid electron injection. Devices on plastic substrates with a lithium fluoride interlayer performed the best, exhibiting an average external quantum efficiency (EQE) of 0.8% and luminance of 1600 cd/m2 at 40 mA/cm2 (7.8 V). Stability of this device and morphology of the emissive film have also been investigated.
Keywords: Light-emitting diodes; Electroactive organics; Flexible substrates; Oligomers; Conjugation; Poly(p-phenylenevinylene);

Synthesis and electropolymerization studies of water-soluble pyrrole-ferrocene derivatives towards biochip device application by C. Farre; N. Spinelli; A. Bouchet; C. Marquette; B. Mandrand; F. Garnier; C. Chaix (125-133).
The synthesis of two new pyrrole derivatives is described. These molecules have been designed for the elaboration of performing electrochemical biochip devices by the means of conductive poly(pyrrole) electroaddressing on an electrode network. A ferrocene group has been introduced in monomer structures to serve as efficient transducer of electrochemical response of poly(pyrrole). The two monomers also bear a phosphate diester group in their structure providing an efficient solubility in water (100 mM). These water-soluble monomers appear to be well suited to a concomitant use with biomolecules such as oligonucleotides, peptides or proteins that must be immobilized on biochip plot surface. In this work, electrochemical properties of the two monomers are investigated, on both a conventional gold electrode and a graphite paste electrode biochip device.
Keywords: Polypyrrole; Ferrocene; Electropolymerization; Biochip;

Molecular dynamics simulation of an exciton in polyaniline by Jie Liu; Yong Zhang; Shujuan Sun; Jinlong Tang (134-137).
Adding a weak electron correlation to the extended Ginder–Epstein model for polyaniline, we carried out an adiabatic dynamics simulation for formation of an exciton in a pernigraniline-base polymer, an oxidized form of polyaniline. The results showed: the formation time was ∼50 fs; the correlation speeded up the exciton formation and shortened the exciton radius.
Keywords: Molecular dynamics; Electron correlation; Exciton; Polymer;

White light emission from a blue LED, combined with a sodium salt of fluorescein dye by Ji-Young Jin; Hyung-Gu Kim; Chang-Hee Hong; Eun-Kyung Suh; Youn-Sik Lee (138-141).
The bivalent sodium salt of fluorescein dye was successfully isolated. The maximum absorption and fluorescence emission wavelengths of the isolated fluorescein salt in epoxy were about 470 and 540 nm, respectively. The fluorescence quantum yield of the isolated salt in an epoxy matrix was measured to be 0.94. A hybrid light-emitting diode (LED) was fabricated using a blue light-emitting diode (LED, 460 nm) and the fluorescein salt dispersed in epoxy as the primary light source and luminescence converter, respectively. White light emission was observed when the salt concentration in the epoxy was 0.04 wt%. In the presence of DCM2 (a red dye, 8 wt% based on the salt), the CIE color coordinates of the hybrid LED system were closer to the pure white point. The conversion efficiencies of the system with and without DCM2 were 68 and 84%, respectively, which are higher than those reported so far in the literature.
Keywords: Fluorescein; Bivalent anion; Luminescence converter; Hybrid LED; White light emission;

Third order nonlinear optical properties and two photon absorption in newly synthesized phenyl sydnone doped polymer by Seetharam Shettigar; G. Umesh; K. Chandrasekharan; Balakrishna Kalluraya (142-146).
We have investigated third order nonlinear optical properties of three newly synthesized 3-phenyl sydnones, doped into PMMA and determined both real and imaginary part of χ (3). The measurements were performed using nanosecond laser pulses at 532 nm wavelength by employing Z-Scan technique. The nonlinear refractive index is found to be of the order of 10−14  cm2/W. The magnitude of third order susceptibility is of the order of 10−13  esu. Their two photon absorption coefficient and absorption cross section have also been determined. The optical power limiting behavior of sydnone moieties doped PMMA was also investigated. The results suggest that among the three organic phenyl sydnone moieties, 2-benzylhydrazono-5-(3-p-tolylsydnone-4-yl)1,3,4,-thiadiazine doped PMMA is a promising class of nonlinear optical material due to its simple molecular structure with p-tolyl group. It also shows a very good optical limiting behaviour with a limiting threshold of 0.21 mJ/pulse. The nonlinear parameters obtained are comparable with the values obtained in stilbazolium like dye such as trans-4-[2-(pyrrl)vinyl]-1-methylpyridium iodide. Hence this material may be tailored suitably for applications such as optical power limiters, switches and modulators.
Keywords: Third order nonlinear optical susceptibility; Phenyl sydnones; Z-Scan; Nonlinear refraction; Two photon absorption; Optical power limiting;

Studies of solution properties of polyaniline by membrane osmometry by Huseyin Zengin; Harold G. Spencer; Gulay Zengin; Richard V. Gregory (147-154).
A series of studies have been carried out on polyaniline (emeraldine base; EB) prepared by polymerization of aniline in aqueous solution, in an effort to determine the molecular weight by the primary method membrane osmometry. The osmotic pressure was determined on various concentrations of EB solutions in the solvent N-methylpyrrolidinone (NMP). The reduced osmotic pressure of polyaniline in NMP was measured, molecular weight determined and a theta temperature determined. The second virial coefficients were calculated. The number average molecular weight of the polymer, determined by membrane osmometry, was found to be approximately 64,000 and the theta temperature 57.7 °C at the reported conditions. Various synthesis conditions and different temperatures were applied for preparation of polyaniline and the number average molecular weights of the polymer were determined. The molecular weight results obtained by membrane osmometry were compared with the molecular weight results obtained by gel permeation chromatography. Both methods showed that the molecular weight strongly depended upon the synthesis conditions. The polymers obtained were characterized by various spectroscopic techniques.
Keywords: Solution properties; Conducting polymers; Gel permeation chromatography (GPC); Membranes; UV–vis spectroscopy; Wide angle X-ray diffraction (WAXD);

The mass change of polypyrrole (PPy) due to ion transport was investigated in NaClO4 of various pHs, using an electrochemical quartz crystal nanobalance (EQCN). It was disclosed that the mass of perchlorate-doped PPy (PPy-ClO4) varies with solution pHs. While the mass was increased with lowering pH in general, the greatest mass changes were observed in the pH range of ca. 1.0–2.0 and 10.5–11.5. The magnitude of mass changes with pH was proportional to the film thickness of PPy-ClO4. Repeated acid/base treatments in between pH 1.0 and 11.3 resulted in the retention of ca. 78% of the initial mass response after 100 cycles, which can be compared with ca. 65% retention of the electrochemically induced mass change. In situ atomic force microscopic (AFM) images of PPy in NaClO4 revealed that the volume increase of 12.6% occurs in a solution of pH 1.0, relative to the volume in a solution of pH 11.3.
Keywords: Polypyrrole; pH; Volume change; Surface roughness; Ion transport; EQCN;

Origin of charge transfer complex resulting in Ohmic contact at the C60/Cu interface by S.W. Cho; Y. Yi; J.H. Seo; C.Y. Kim; M. Noh; K.-H. Yoo; K. Jeong; C.-N. Whang (160-164).
The conducting charge transfer complex formed on the C60/Cu interface after heat treatment was investigated by using density functional theory. We optimized the geometries of the C60 and C60–Cu complexes. Then, a comparison of the valence band spectrum with the simulated density of states revealed detailed molecular properties. We confirm that the C60–Cu complex layer formed at the C60/Cu interface enhances charge transfer. The conducting complex creates both an occupied gap state and unoccupied states between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of C60. These states lead to the formation of Ohmic contact between the C60 layer and the Cu layer.
Keywords: Energy-level alignment; Metal–organic interface; Gap state; Charge transfer complex; Dipole formation; Photoemission; Density functional theory calculation;