Synthetic Metals (v.156, #18-20)

Novel red and white PLED devices consisting of PVK blended with blue-emitting fluorene derivatives and carbazole dopants by Hong-Cheu Lin; Chien-Min Tsai; Jiann-T'suen Lin; K.R. Justin Thomas (1155-1160).
Highly fluorescent polymeric LEDs (PLEDs) utilizing blue-emitting FB fluorene derivatives (FBF14 and FBPh4) blended with PVK are demonstrated. Significant red shift of the emission from blue to red can be achieved by increasing the concentration of FB in the blend. The origin of emission was attributed to electromer (or electroplex) formation. Addition of Carb9 in the blend enhanced the electromer (or electroplex) formation, and red- or white-emission device with high brightness (3440 cd/m2 for red; and 17368 cd/m2 for white) was obtained.
Keywords: Poly(N-vinyl carbazole) (PVK); Fluorene; Electroluminescence (EL);

Fluorene and silafluorene conjugated copolymer: A new blue light-emitting polymer by Run-Feng Chen; Qu-Li Fan; Shu-Juan Liu; Rui Zhu; Kan-Yi Pu; Wei Huang (1161-1167).
A novel series of soluble blue light-emitting conjugated random and alternating copolymers derived from 9,9′-dioctylfluorene (FO) and 3,6-dimethoxyl-9,9-dimethyl-9-silafluorene (DMSiF) were successfully synthesized by Suzuki coupling polymerization. The feed ratios of FO to DMSiF were 90:10, 80:20, 75:25, and 50:50. Chemical structures and optoelectronic properties of the copolymers were characterized by NMR, UV absorption, photoluminescence, and cyclic voltammetry. The 1H NMR spactra of the copolymers indicated that DMSiF content in the copolymers was slightly lower than its feed composition. The random copolymers exhibited PFO-segment-dominated UV absorption and PL spectra in THF solution, in comparison with significantly blue-shifted spectra of the alternating copolymer. The blue shift of the spectra became more remarkable in cast film and increased with the increment of DMSiF content. The changes of the UV absorption and PL spectra in solution and film were ascribed to the effect of methoxyl substituents which can hinder the chain rotation and influence the polymer comformation espercially in the solid film. The systematic invesetigations on the solubility, thermostability, electrochemical property, and photophysical property of the copolymers showed that silafluorene was an attractive building unit for optoelectronic materials.
Keywords: Polysilafluorene; Polyfluorene; Diphenylsilole; Light-emitting diode; Blue light-emitting;

We report the synthesis, optical and electrochemical properties of a new series of polyoxadiazoles (P1P3) consisting of 3,4-dialkoxythiophene and 1,4-divinylbenzene units. The polymers are prepared using the precursor polyhydrazide route. The polymers have well defined structure and exhibit good thermal stability with the onset decomposition temperature in nitrogen at around 330 °C. The optical and charge-transporting properties of the polymers are investigated by UV–vis spectroscopy, fluorescence emission spectroscopy and cyclic voltammetry. The UV–vis absorption spectra of polymers in solution showed a maximum at around 380 nm. The polymers depicted bluish-green fluorescence in solutions and green fluorescence in thin films. Cyclic voltammetry studies reveal that these copolymers have low-lying LUMO energy levels ranging from 3.25 to 3.31 eV and HOMO energy levels ranging from 5.48 to 5.56 eV, which indicated that the polymers are expected to provide enhanced charge-transporting (electron transport/hole blocking) properties for the development of efficient polymer light-emitting diodes (PLEDs).
Keywords: Conjugated polymers; Polycondensation; Fluorescence; Light-emitting diodes;

Photophysical properties and electroluminescent applications of donor–acceptor–donor functionalized red electroactive fluorescent materials by Xiaobo Sun; Yunqi Liu; Xinjun Xu; Gui Yu; Shiyan Chen; Wenfeng Qiu; Yongqiang Ma; Zhehui Zhao; Daoben Zhu (1174-1181).
Four electroactive molecular materials with donor-π-acceptor-π-donor structures have been synthesized for investigation of structure–properties relationships, and used as dopant for electroluminescent applications. The optical and electrical properties of these materials are electronically manipulated by attaching different donors. The twisted intramolecular charge transfer (TICT) model is proposed by photophysical behaviors observed with increasing solvent polarity. Red electroluminescence devices were fabricated by spin coating the blend of polymer host and fluorescent guest. Highly efficient energy transfer leads to a saturated red emission with emission peak at 623 nm, narrow bandwidth of 75 nm and an external quantum efficiency of 0.38%.
Keywords: Structure; Electroluminescence; Spin coating;

A phosphorescent hexa-peri-hexabenzocoronene platinum complex and its time-resolved spectroscopy by Bassem El Hamaoui; Frédéric Laquai; Stanislav Baluschev; Jishan Wu; Klaus Müllen (1182-1186).
Photoluminescence properties of photoactive HBCC 8,2 and HBCPt 2 have been studied. The phenylene-ethynyl-Pt moieties were incorporated onto the bromo functions of HBCBr 2 1 via double oxidative addition of Pt(PPh3)4 to 1 followed by CuI-catalyzed dehydrohalogenation reaction of compound 2 and phenylacetylene to give the desired molecule HBC-Pt 2 . Introduction of platinum-phenylacetylene to the hexa-peri-hexabenzocoronenes (HBC) core enhanced the spin–orbit coupling and decreased the phosphorescence lifetime of the triplet state by three orders of magnitude. Photoluminescence measurements revealed unique phosphorescence emission at room temperature and 77 K. On the other hand, symmetric HBC-C 8,2 , bearing no platinum, revealed different delayed emissions depending on the temperature measurement. At room temperature, delayed fluorescence appeared while at 77 K, only phosphorescence could be observed. The triplet energy level has been tuned between 2.3 and 1.7 eV by varying the HBC core substituents. In consequence, combination of both the heavy-atom effect and the increased intersystem crossing in the novel double platinum-substituted HBC lead to the exclusive emission of phosphorescence without any residues of fluorescence in the prompt and delayed emission spectra.
Keywords: HBC; Spectroscopy; Photoluminescence;

Miscible blend of conductive polyaniline/polyurethane (PANI/PU) showed preferable electrical property at low percolation threshold compared to immiscible blend of PANI/polystyrene-isoprene-copolymer (PANI/SIS) and carbon black/PU composite (CB/PU). The time dependence of the electrical conductivity was investigated with these samples aged under different humidity and temperatures. The electrical conductivity of PANI/PU (11.5/88.5, v/v) decreased with aging time and the morphology changed with time in the coexistence of high moisture and high temperature. After the aging treatment, the film of the miscible blend was re-dissolved and re-cast. The morphology and electrical conductivity were found to recover to the same state as the original film. In addition, the recovery mechanism of the morphology and the conductivity was also proposed here.
Keywords: Polyaniline; Dopant; Blend; Moisture; Aging;

The effects of dye dopants on the conductivity and optical absorption properties of polypyrrole by Eva Håkansson; Tong Lin; Hongxia Wang; Akif Kaynak (1194-1202).
Ten anionic compounds, including four acidic dyes, were used to dope polypyrrole powder. The effects of the dopants on density, optical absorption and conductivity of the polypyrroles were studied. The presence of the dopant in the conducting polymer matrix was verified by ATR-FTIR spectroscopy. Density function theory (DFT) simulation was used to understand the effect of the dopants on the solid structure, optical absorption and energy band structures. Anthraquinone-2-sulfonic acid-doped polypyrrole yielded the highest conductivity. The dye-doped polypyrrole showed an enhancement in its UV–vis optical absorption.
Keywords: Polypyrroles; Dyes/pigments; UV–vis spectroscopy; Molecular modelling; Infrared spectroscopy;

Integration of conducting polymer network in non-conductive polymer substrates by Thomas Steen Hansen; Keld West; Ole Hassager; Niels B. Larsen (1203-1207).
A new method for integration of conjugated, inherently conducting polymers into non-conductive polymer substrates has been developed. A layer of the conducting polymer is polymerised by chemical oxidation, e.g. using Fe(III) p-toluene sulfonate (ferri tosylate) followed by washing with a solvent which simultaneously removes residual and spent oxidant and at the same time dissolves the top layer of the polymer substrate. This results in an integration of the conducting polymer into the surface layers of the polymer substrate. Several combinations of conducting polymers and substrates have been tested, with particular focus on poly(3,4-ethylenedioxythiophene) (PEDOT) on PMMA substrates. The structural, electrical and mechanical properties of this system has been characterised by atomic force microscopy, conductance measurements, and tribological tests. Furthermore, measurement of conductivity and optical absorption during sequential reactive ion etching has allowed for analysis of the PEDOT distribution within the surface layer of the PMMA substrate. The surface resistance of the conducting polymer layer remains low while the surface layer at the same time adapts some of the mechanical properties of the substrate, resulting in a highly conducting surface with very good wear resistance.
Keywords: poly(3,4-ethylenedioxythiophene); Polymer integration; Surface resistance; Mechanical stability; Topography;

Synthesis and anticorrosion properties of polydiphenylamine blended vinyl coatings by S. Sathiyanarayanan; S. Muthukrishnan; G. Venkatachari (1208-1212).
Inherently conducting polymers such as polyaniline and polypyrrole containing organic coatings have been found to offer corrosion protection of steel in acid and neutral media. In this study, the use of polydiphenylamine in vinyl coating for corrosion protection of steel in 3% NaCl solution is reported. The polydiphenylamine was prepared by chemical oxidative method of diphenylamine by ammonium persulfate in hydrochloric acid medium. The synthesized polydiphenylamine (PDPA) was blended with vinyl resin at 0–5% concentrations and the corrosion protection performance of the coating on steel was evaluated by EIS for a period of 80 days. It has been found that the coating containing 3% PDPA is able to protect steel more effectively.
Keywords: Polydiphenylamine; Steel; Corrosion; EIS; Vinyl coating;

Photovoltaic properties and inner electric field of ZnO/Zn-phthalocyanine hybrid solar cells by Yuki Yoshida; Makoto Nakamura; Senku Tanaka; Ichiro Hiromitsu; Yasuhisa Fujita; Katsumi Yoshino (1213-1217).
The photovoltaic properties and distribution of inner electric field were studied for ZnO/Zn-phthalocyanine (ZnPc)/Au solar cells, the ZnO films of which were prepared by the metal organic chemical vapor deposition (MOCVD) method. The photocurrent is generated only by the excitation of ZnO, and the excitation of ZnPc does not contribute to the photocurrent generation. An electroabsorption (EA) study indicated that the inner electric field only exists in the ZnO layer. The absence of the inner electric field in the ZnPc layer results in the absence of photocurrent with ZnPc excitation.
Keywords: Solar cells; Zinc oxide; Organic semiconductors; Phthalocyanine; Electroabsorption; Chemical vapor deposition (CVD);

Theoretical studies on one- and two-photon absorption properties of rubrene and its derivatives by Liang Zhao; Guochun Yang; Zhongmin Su; Chunsheng Qin; Shuangyang Yang (1218-1224).
One- and two-photon absorption properties of a very intriguing perpendicular π-electron system 5,6,11,12-tetraphenylnaphthacene (rubrene) and its derivatives have been discussed from the theoretical investigation view. Based on the correct geometries, the INDO/SDCI method was adopted to determine the one-photon absorption (OPA) and two-photon absorption (TPA) properties. The elongation of π-conjugation and the perpendicular phenyl groups on two-photon absorption cross section (δ max) of rubrene are investigated in detail. The results show that the extension of π-conjugation plays an important role on the magnitude of δ max. In addition, the perpendicular phenyl groups favor to enhance the δ max of rubrene, which can be seen from the dominant charge transfer (CT) contributing to TPA obviously. This CT process is a symmetrical intramolecular CT process from the pendent phenyl groups to the main π-backbone. At the same time, the relationship between substituents attached to rubrene and two-photon absorption cross section is also systematically discussed.
Keywords: Rubrene; One-photon absorption; Two-photon cross section; Sum-over-state; Substituent effect;

Impact of hydrogen bonds in polyaniline.AMPSA n /acid solutions by Dali Yang; Phillip N. Adams; Lori Brown; Benjamin R. Mattes (1225-1235).
Water concentration in polyaniline (PANI)/acid solutions containing 2-acrylamido-2-methyl-l-propanesulfonic acid (AMPSA) significantly changes the solubility of PANI and the stability of its solutions. By varying trace amounts of water, we investigated the impact of hydrogen bonds (H-bonds) on PANI chain conformation in solutions of AMPSA/dichloroacetic acid (DCAA) or AMPSA/orthophosphoric acid. UV–vis–NIR spectral changes of these PANI solutions reveal that a competition between H-bond formation and protonation among PANI, AMPSA and water is an on-going process. The H-bonds between the PANI and AMPSA molecules can cause a large red shift in the πb  → πq electron transition (the quinoid peak) from ∼630 nm into the near IR (NIR) region. Absent protonation peak (410–490 nm) and blue color of the solution suggest that the H-bond interaction, instead of protonation, is sufficient to cause PANI chains to adopt an “expanded coil” chain conformation in the PANI.AMPSA n /DCAA or H3PO4 solutions. Protonation leads to the gelation of PANI solutions. The kinetic mechanism of protonation of the PANI.AMPSA n /DCAA solutions is studied. The activation energy of protonation in the PANI.AMPSA0.6/DCAA solutions is ∼60 kJ/mol.
Keywords: Polyaniline (PANI); Hydrogen bond; Solubility; Gelation; AMPSA;

Langmuir–Blodgett film of new phthalocyanine containing oxadiazol groups and its application in field-effect transistor by Shiyan Chen; Yunqi Liu; Yu Xu; Yanming Sun; Wenfeng Qiu; Xiaobo Sun; Daoben Zhu (1236-1240).
A new phthalocyanine, tetra-{4-[5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2-yl]-phenoxy}-zincphthalocyanine (TOPc) was synthesized and its Langmuir–Blodgett (LB) film was fabricated and characterized by ultraviolet–visible (UV–vis) absorption and polarized UV–vis absorption spectra. The results show that the tilt angle of TOPc molecules on substrate is about 32°. Field-effect transistor incorporating with LB film of TOPc as a semiconductor layer was fabricated. A mobility of 1.1 × 10−4  cm2  V−1  s−1 and an on/off ratio up to 103 have been achieved.
Keywords: Langmuir–Blodgett film; Field-effect; Phthalocyanine;

Performance recovery and optimization of poly(3-hexylthiophene) transistors by thermal cycling by Brian A. Mattis; Paul C. Chang; Vivek Subramanian (1241-1248).
We present the results of studies on the electrical and physical modifications to poly(3-hexylthiophene), upon thermal annealing. Thermally induced performance enhancements and thermal stability of polythiophene thin film transistors (TFTs) are explored. We observe substantial mobility improvements in devices annealed at low temperatures (<80 °C), as well as increases in on/off ratios by more than two orders of magnitude at moderate anneal temperatures (∼120 °C). We document changes in conductivity, mobility, on current, and on/off ratio with anneal temperature and total thermal budget. We are able to show the expulsion of environmental contaminants and increases in film density as means to performance recovery and enhancement. This study provides a comprehensive analysis of the effect of thermal cycling of polythiophene TFTs on various device performance metrics, and identifies relevant thermal limits and failure mechanisms.
Keywords: Poly(3-hexylthiophene); Oxygen doping; Organic; Annealing; Performance recovery;

Antistatic coating and electromagnetic shielding properties of a hybrid material based on polyaniline/organoclay nanocomposite and EPDM rubber by Mauro A. Soto-Oviedo; Olacir A. Araújo; Roselena Faez; Mirabel C. Rezende; Marco-A. De Paoli (1249-1255).
Thermal, mechanical, electrical and microwave radiation absorbing properties of conductive composites based on dodecylbenzenesulfonate doped polyaniline/organoclay nanocomposites and propylene–ethylidene–norbornene rubber have been investigated with special interest on the effect of the nanocomposite concentration. Composites were prepared by melt blending using an internal mixer. Morphology studies by scanning electron microscopy of cryofractured surfaces indicated that the conducting nanocomposites produced heterogeneously distributed aggregates in the continuous elastomeric matrix. The composites exhibit high conductivities, up to 10−3  S cm−1 for 40 wt.% of conducting nanocomposite, and good mechanical properties. They also present high microwave attenuation values, in the frequency range of 8–12 GHz. This property depends on the concentration of the conductive nanocomposite and on the film thickness. The composites can be used for antistatic coatings or for electromagnetic shielding.
Keywords: EMI shielding; Polyaniline; Organoclay; EPDM; Nanocomposite;

The process of hopping transport across organic heterojunctions is critical to the function of organic light-emitting diodes (OLED) and many other currently developed organic electronic devices. We consider the case of a hole-only or homopolar heterojunction with Gaussian energetic disorder. We cross-compare the results of our previous multiparticle Monte Carlo simulator to results of the pioneering analytic hopping model of Arkhipov et al. [V.I. Arkhipov, E.V. Emelianova, H. Bässler, J. Appl. Phys. 90 (2001) 2352]. This comparison made it possible to point out and correct some shortcomings of the latter model. This includes the new definition of the injection level at the heterojunction, which brings orders of magnitude change to the current with respect to values obtained for previously used injection levels. Further insight, related to the backward-to-forward hopping ratio at the heterojunction, also brings about the order of magnitude correction for the current. We end up with a rather complete and cross-verified analytical description of the charge transport across energetically disordered homopolar heterojunctions.
Keywords: Organic light-emitting diodes; Dielectric interface; Hopping transport; Energetic disorder; Organic semiconductors;

Organic light-emitting diodes and organic light-emitting electrochemical cells based on silole–fluorene derivatives by F. Habrard; T. Ouisse; O. Stéphan; L. Aubouy; Ph. Gerbier; L. Hirsch; N. Huby; A. Van der Lee (1262-1270).
Silole groups are known to present a high electron affinity. Initially, copolymerization of siloles with fluorene was aimed at improving electron injection into the polymer layer and so improving the electroluminescent properties of organic light-emitting diodes (OLED's) made from fluorene. But it also provides the ability to turn the light emission colour to the green part of the spectrum and to stop the well-known spectral shift degradation occurring in fluorene-based materials. In this paper we report the synthesis and the characterisation of 1,1-dimethyl-2,5-bis(fluoren-2-yl)-3,4-diphenylsilole 4, and of two soluble conjugated random copolymers derived from 9,9-ditetradecylfluorene and 1,1-dialkyl-2,5-diphenylsilole, where the alkyl group is either methyl 11a or n-hexyl 11b. Silole 4 crystallizes in the triclinic P-1 space group with a  = 9.8771(8), b  = 10.6240(10), c  = 16.585(2) Å, α  = 95.775(8), β  = 97.025(7), and γ  = 111.738(8)°. The results obtained with this molecule, operating in a single-layer OLED (luminance ≈450 Cd/m2 at 12 V; η max  = 0.2 Cd/A), give evidences for the complementarity of the silole and the fluorenyl moieties in the improvement of the charge injection processes when compared with 1,1-dimethyl-2,3,4,5-tetraphenylsilole. The results obtained from organic light-emitting electrochemical cells (LEC's) made from silole–fluorene copolymers 11a, 11b and molten salts show an improvement of both the device lifetime and the spectral stability when compared with polyfluorene. To explain devices performances electrical characterisation data and atomic force microscope (AFM) imaging were combined.
Keywords: Light-emitting electrochemical cells; OLED; Fluorene; Silole; Atomic force microscopy;

Synthesis and redox properties of several new oligoTTF containing functional spacer by C. Carcel; L. Kaboub; A.K. Gouasmia; J.M. Fabre (1271-1279).
By using two different routes, series of new bis- and tris-TTF, containing in their linker function(s) able to generate supplementary interactions in the resulting materials, have been prepared. The first route based on deprotection-realkylation strategy allowed to introduce alcohol and ketone functions. The second way, using acid chloride and hydroxy-TTF, allowed to include ester function in the linker. The electron-donating ability of this series of TTFs has been determined by CV and by SQW.
Keywords: Tetrathiafulvalenes; Functional linker; Redox potentials;

Morphological investigations of polypyrrole coatings on stainless steel by A. Ashrafi; M.A. Golozar; S. Mallakpour (1280-1285).
In this paper, electrochemical polymerization of pyrrole on 316L stainless steel substrates was accomplished using a rotating disc electrode (RDE). By applying various current densities and disc rotation speeds, coatings were produced with the aid of galvanostatic technique. Experiments were performed in an aqueous solution containing 0.2 M pyrrole and 0.1 M oxalic acid. Current densities and disc rotation speed ranges were from 0.05 to 1.0 mA/cm2 and from 0 to 1500 rpm, respectively. Using a scanning electron microscope (SEM), the morphology of polypyrrole coatings was studied, and the morphology diagram was determined. The results obtained showed that various morphologies were obtained by changing the current density and/or disc rotation speed. These results also showed that apart from conventional morphologies of polypyrrole coatings reported in the literature, a new semicrystalline morphology was obtained under the conditions of very low current density (0.05 mA/cm2) and disc rotation speed (≤50 rpm). The degree of crystallinity of this morphology was estimated to be 68% by grazing-incidence small-angle X-ray scattering (GISAXS). The elemental analysis (CHN) revealed the ratio of semicrystalline polypyrrole to oxalic acid dopant to be 4:1.
Keywords: Polypyrrole; Electrochemical polymerization; Morphology;

Erratum to “Synthesis, characterization and low frequency AC conduction of polyaniline/niobium pentoxide composites” [Synth. Met. 156 (2006) 1139–1147] by Y.T. Ravikiran; M.T. Lagare; M. Sairam; N.N. Mallikarjuna; B. Sreedhar; S.K. Manohar; A.G. MacDiarmid; T.M. Aminabhavi (1286).