Synthetic Metals (v.157, #6-7)

Raman dispersion in polyaniline base forms by Gustavo M. do Nascimento; Pedro Y.G. Kobata; Ricardo P. Millen; Marcia L.A. Temperini (247-251).
Raman spectra of polyaniline in its emeraldine (EB) and pernigraniline (PB) base forms were obtained using several excitation laser lines and Raman dispersion of the bands was measured. Only the νC=N band value depends on excitation wavelength for both polymeric forms. These changes of vibrational frequencies with excitation wavelength were analyzed using amplitude mode model (AMM). ∏ n ( ω n R / ω n 0 ) 2 versus excitation photon energies plot was linear in the electronic absorption transition region, for EB at ca. 640 nm and for PB at ca. 620 nm. The dispersion values, D (slope of ∏ n ( ω n R / ω n 0 ) 2 versus excitation photon energies plot) were 0.058 and 0.038 eV−1 for EB and PB, respectively, which indicate that EB has the first excited state of even parity presenting lower energy when comparing to that of PB.
Keywords: Polyaniline; Raman dispersion; Resonance Raman; Conjugation length;

Raman study of κ-ET2Cu[N(CN)2]Cl at ambient and ∼300 bars pressures by K.D. Truong; S. Jandl; M. Poirier (252-256).
Anomalies in the temperature dependence of the 246 and 258 cm−1 Raman active phonons below the antiferromagnetic phase transition are reported in κ-ET2Cu[N(CN)2]Cl at ambient pressure and are compared to measurements under ∼300 bars that render the compound superconducting. At ambient pressure, due to the Mott-gap opening and/or to the spin–lattice interaction and exchange-phonon modulation, frequency softenings, enhanced intensities and linewidths narrowings of these modes occur at low temperatures.
Keywords: Organic superconductor; Raman phonons; Spin-lattice interaction;

Non-destructive probing of the anisotropy of field-effect mobility in the rubrene single crystal by Mang-Mang Ling; Colin Reese; Alejandro L. Briseno; Zhenan Bao (257-260).
Organic single crystals are valuable tools for the exploration of charge transport in organic materials. Here, we report two new methods for the non-destructive probing of anisotropic transport in molecular crystals, demonstrating an angular dependence of the field-effect mobility in the ab-plane of the rubrene single crystal. Clear minima and maxima are observed, corresponding to the a and b principle axes of the crystal, as determined by X-ray diffraction and visual inspection. While this phenomenon has been previously reported, the method presented here offers an angular resolution previously undemonstrated, with methods that eliminate the need to move the fragile crystal. The coincidence of this phenomenon between top- and bottom-contact geometries offers strong support for the performance correlation of mobility with specific molecular orientation, and an improved data set for comparison with transport theory.
Keywords: Organic semiconductors; Single crystals; Rubrene; Anisotropic; Mobility; Transistors;

High contrast solid-state electrochromic devices from substituted 3,4-propylenedioxythiophenes using the dual conjugated polymer approach by Javier Padilla; Venkataramanan Seshadri; James Filloramo; Warren K. Mino; Sarada P. Mishra; Bijan Radmard; Anil Kumar; Gregory A. Sotzing; Toribio F. Otero (261-268).
Solid-state electrochromic windows from 3,4-propylenedioxythiophene (ProDOT) derivatives using dual polymer electrochromic architecture were fabricated and their electro-optical characteristics were recorded. DibenzylProDOT (DiBz-ProDOT) and biphenylmethyloxymethyl ProDOT (BPMOM-ProDOT) were used as cathodically coloring polymers, whereas bis(2-(3,4-ethylenedioxy)thienyl)-N-methyl carbazole (BEDOT-NMCz) was used as their complementary anodically coloring polymer. Straightforward assembly of the devices was designed by means of using a UV photo-curable gel. These devices exhibited response speeds of approximately 1 s with photopic contrasts as high as 52% for the complete device, and were found to exhibit good stability under open circuit conditions. Convenience of reporting photopic values to characterize electrochromic devices compared to reporting contrasts at a single wavelength is remarked.
Keywords: Conjugated polymer; Electrochromics; Smart windows; Poly(3,4-ethylenedioxythiophene) (PEDOT); Poly(3,4-propylenedioxythiophene) (PProDOT); Solid-state devices;

Polyaniline nanofibers prepared with hydrogen peroxide as oxidant by Yangyong Wang; Xinli Jing; Junhua Kong (269-275).
Polyaniline (PANI) nanofibers were successfully prepared by a sonochemical way with hydrogen peroxide as the oxidant. In comparison with the polymerization performed with mechanical stirring, the polymerization rate of aniline was greatly enhanced and PANI nanofibers were achieved instead of the particulate PANI, though the yield was decreased relatively, indicating the positive effect of ultrasound in producing PANI nanostructures. The uniformity and lengths of the PANI nanofibers were greatly improved as compared with the PANI nanofibers synthesized with ammonium peroxydisulfate (APS) as oxidant under the same conditions, rendering hydrogen peroxide a better oxidant in producing high-quality PANI nanofibers. The PANI nanofibers exhibited similar FTIR spectra, XRD patterns, and dispersibility, but different UV–vis spectra, to their counterpart synthesized with APS as oxidant. UV–vis spectra showed only the head-to-tail structured PANI molecules were produced in the high-quality PANI nanofibers. The findings will be of some help to elucidation of the formation mechanism of PANI nanofibers.
Keywords: Polyaniline; Nanofiber; Hydrogen peroxide; UV–vis spectra; XRD;

Electrospun polyalkylthiophene/polyethyleneoxide fibers: Optical characterization by Andrea Bianco; Chiara Bertarelli; Simon Frisk; John F. Rabolt; Maria C. Gallazzi; Giuseppe Zerbi (276-281).
The electrospinning technique is used to obtain functionalized polymeric fibers. Blends of polyethyleneoxide (PEO) and poly-3-dodecylthiophene (P3DDT) have been spun from chloroform solution. The fibers diameter is approximately 1 μm and both polymer components occur in the fibers as separated phases. By washing the fibers with acetonitrile the PEO matrix is completely removed. This process is fast and it leaves long and homogeneous fibers of P3DDT. The morphology and structure of the fibers have been studied with microscopy (SEM and fluorescence confocal microscopy), whereas infrared spectroscopy has been used to check the removal of PEO matrix, to understand the conformation of the polythiophene chains and their orientation in the fibers. Measurements with confocal and SEM microscopes show the homogeneity of the washed fibers; from polarized infrared spectroscopy, the polythiophene chains are mostly aligned along the fiber axis. The electrospinning process followed by a suitable washing step makes possible to orient fibers with a high molecular order.
Keywords: Poly-3-dodecylthiophene; Electrospinning; Confocal microscopy; Infrared spectroscopy;

Controlling the dimensions of carbon nanofiber structures through the electropolymerization of pyrrole by Benjamin L. Fletcher; Timothy E. McKnight; Jason D. Fowlkes; David P. Allison; Michael L. Simpson; Mitchel J. Doktycz (282-289).
Electrically conductive polymers, such as polypyrrole (PPy), show promise for modifying the dimensions and properties of micro- and nanoscale structures. Mechanisms for controlling the formation of PPy films of nanoscale thickness were evaluated by electrochemically synthesizing and examining PPy films on planar gold electrodes under a variety of growth conditions. Tunable PPy coatings were then deposited by electropolymerization on the sidewalls of individual, electrically addressable carbon nanofibers (CNFs). The ability to modify the physical size of specific nanofibers in controllable fashion is demonstrated. The biocompatibility, potential for chemical functionalization, and ability to effect volume changes of this nanocomposite can lead to advanced functionality, such as specific, nanoscale valving of materials and morphological control at the nanoscale.
Keywords: Carbon nanofibers; Polypyrrole; Electropolymerization; Surface modification; Nanomaterial;

Electrochemical and optoelectronic characteristics of discotic 1,7,13-heptanoyldecacyclene (C7DC) were studied. Introduction of carbonyl group to decacyclene resulted in a positive shift of the first reduction potential of more than 300 mV. Photoluminescence quenching was observed when combined with poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), and it was confirmed that the tri-substituted decacyclene functioned as a novel acceptor material. Spin-coated photovoltaic devices containing MEH-PPV and C7DC showed a relatively high open circuit voltage of 1.3 V, possibly because of the high lowest unoccupied molecular orbital (LUMO) position of C7DC. The photovoltaic performance of the devices was improved after annealing at 100 °C, liquid crystal transition temperature for C7DC. Morphological study revealed large crystal-like structures of C7DC in the active layer, indicating improvement of electron transport through C7DC after annealing.
Keywords: Electrochemical properties; Discotic liquid crystals; Self-assembly; Organic photovoltaic devices;

The calculation of electronic properties of an Ag/chitosan/n-Si Schottky barrier diode by Kemal Akkılıç; İlhan Uzun; Tahsin Kılıçoğlu (297-302).
In this study, the film of chitosan by adding the solution of chitosan being a polymeric compound on the top of an n-Si substrate and then by evaporating solvent was formed. It was seen that the chitosan/n-Si contact demonstrated clearly rectifying behavior and the reverse curves exhibit a weak bias voltage dependence by the current–voltage (IV) curves studied at room temperature. Average barrier height and ideality factor values for this structure were determined as 0.94 eV and 1.81, respectively. Furthermore, the energy distribution of the interface state density located in the semiconductor band gap at the chitosan/n-Si substrate in the energy range (E c−0.785) to (E c−0.522) eV have been determined from the IV characteristics. The interface state density N ss ranges from 5.39 × 1012  cm−2  eV−1 in (E c−0.785) eV to 1.52 × 1013  cm−2  eV−1 in (E c−0.522) eV. The interface state density has an exponential rise with bias from the midgap towards the bottom of the conduction band.
Keywords: Schottky barriers; Schottky diodes; Polymeric organic–inorganic semiconductor contact; Chitosan;

Polyaniline and polypyrrole oxygen reversible electrodes by Aimei Wu; Everaldo C. Venancio; Alan G. MacDiarmid (303-310).
The effect of oxygen on the potential of reduced forms of polyaniline and polypyrrole were investigated. These forms of polyaniline and polypyrrole were not stable in the presence of oxygen. Zinc–polyaniline, zinc–polypyrrole, polyaniline–polyaniline (PANI–PANI), and polypyrrole–polypyrrole (PPy–PPy) batteries were constructed and studied. The effect of oxygen on the short-circuit current of these batteries showed that the cathode material, i.e., polyaniline and polypyrrole, can be regenerated by oxygen oxidation after discharge. These materials are potential candidates for the use as O2 “fuel cell” type electrocatalytic electrodes.
Keywords: Polyaniline; Polypyrrole; Rechargeable battery; Conducting polymers; Polymer cathode; Fuel cell; Oxygen; Electrocatalytic oxidation;

Synthesis and characterization of dendritic poly(l-lysine) containing porphyrin–fullerene moieties by Keisuke Kobata; Junya Ogawa; Shyam S. Pandey; Hidetoshi Oshima; Toru Arai; Tamaki Kato; Norikazu Nishino (311-317).
Amphiphilic l-lysine dendrons containing porphyrin and fullerene bearing amino acids with the ratio of 2:0, 2:1 and 1:1 were synthesized and characterized using UV–visible absorption spectroscopy, fluorescence emission spectroscopy and transmission electron microscopy. Photoluminescence spectra of these dendrimers showed photoluminescence quenching due to photoinduced electron transfer from porphyrin to fullerene moieties. Transmission electron microscopic observation evidenced the self-association of these dendrimers in water. Controllable and reversible dissociation and re-aggregation of some of the water soluble dendrimers have been investigated using electronic absorption and fluorescence spectra upon the addition of 2-hydroxypropyl-β-cyclodextrin and 1-adamantanecarboxylic acid in their aqueous dispersion. Enhanced ellipticity of dendritic porphyrin–fullerene conjugate compared to that only dendritic porphyrin indicates intramolecular porphyrin–fullerene interactions.
Keywords: Porphyrin; Fullerene; Cyclodextrin; Adamantanecarboxylic acid; Photoluminescence; Circular dichroism;

The head-to-head-type poly(3-alkynylthiophene-2,5-diyl) HH-P3(CCR)Th has been shown to exhibit a strong tendency to self-assemble. The UV–vis spectrum of HH-P3(CCDec)Th (Dec = decyl) in 1,2-dichlorobenzene at 131 °C showed a peak at λ max  = 520 nm, which shifted to λ max  = 553 nm at 25 °C, with a shoulder peak at 608 nm. However, the UV–vis spectrum of an HH-P3(CCDec)Th film showed a more pronounced shift in its UV–vis absorption peak to a longer wavelength. These phenomena are the characteristics of the self-assembly of the polymer molecule. The HH-P3(CCDec)Th film had a large third-order nonlinear optical susceptibility χ (3) of 3.6 × 10−11  esu.
Keywords: Conjugated polymers; Self-assembly; UV–vis spectroscopy; Nonlinear optical susceptibility;

Polypyrrole actuators working at 2–30 Hz by Steen Skaarup; Lasse Bay; Keld West (323-326).
The main challenge is to provide significant forces at the frequencies relevant to tremor in upper limbs: 2–16 Hz. Forces in the range of 0.1–1 kg are required. It has earlier been shown that utilizing the stiffness change instead of the length change may extend the useable maximum frequency by about a factor of 10 [J.D. Madden, R.A. Cush, T.S. Kanigan, I.W. Hunter, Solid State Ionics 113 (2000) 185]. The maximum frequency reached was, however, only about 1 Hz.By optimizing the synthesis method, and the choice of counterion in PPy and in the electrolyte, a polypyrrole actuator able to yield significant force at up to 30 Hz has been made. The stiffness change turns out to be approximately 20 times faster than the change in length. Simple scaling up of the present data leads to a required total thickness of PPy (30 mm wide film) of 0.13 mm at 2 Hz, and 0.32 mm at 15 Hz for the 1 kg limit. The required mass of the actuator itself at 15 Hz is ∼100 mg. The results indicate the feasibility of using PPy actuators for tremor suppression.
Keywords: Polypyrrole actuator; Artificial muscles; Conducting polymer;