Synthetic Metals (v.148, #3)

The synthesis of series of 4,4′-dialkyl-2,2′-bithiazole oligomers, (ABTz)1,2, and their 5-carboxylic acid derivatives is described. The Langmuir–Blodgett film-forming behavior of the several of the bithiazole dimer carboxylic acids was explored. The compounds form LB films with areas per molecule increasing from 32 to 40 Å2 as the length of the alkyl side chain increases from ethyl to hexyl.
Keywords: Bithiazole carboxylates; Dligomers; LB films;

Electrical rectification through cumulenic bridge: a computational study by Sanyasi Sitha; K. Bhanuprakash; B.M. Choudary (227-235).
Frontier molecular orbital analysis for the electrical rectification behavior of some donor–acceptor cumulenes has been carried out at B3LYP/6–31G** level of theory and the potential drop (PD) which is a measure of rectification behavior of the molecular system is estimated from the orbital energies of molecular orbitals responsible for the conduction channel. It is observed that for cumulenes with even numbers of double bonds as bridges, there is complete spatial localization of the frontier molecular orbitals, Highest occupied molecular orbital (HOMO) is localized on donor, Lowest unoccupied molecular orbital (LUMO) is localized on acceptor side and LUMO + 1 is localized again in the donor part, where as for molecules with odd number of double bonds, the frontier molecular orbitals (HOMO as well as LUMO) are delocalized all over the molecule. It is also observed that there is a decrease in PD (from 1.56–0.82 to 0.46 eV for molecules with two, four and six cumulenic double bonds) for increase in the number of double bonds and this is quite the opposite trend to that of the well studied D–σ–A systems where increase in the number of methylenic linkage increases the PD drastically. For the molecules with odd number of double bonds due to their low lying LUMO delocalized all over the molecule, it is suggested that they may find applications as molecular conductors. These donor–acceptor cumelenic compounds which are showing the opposite trend in rectifying behavior to that of the normal D–σ–A systems can be a useful alternate for sigma bonded molecular electronic systems.
Keywords: Cumulenes; Molecular electronics; Rectifier; Conductor; HOMO–LUMO gap (HLG); Potential drop (PD);

Controlled interconversion of semiconducting and metallic forms of polyaniline nanofibers by David M. Sarno; Sanjeev K. Manohar; Alan G. MacDiarmid (237-243).
Self-assembled polyaniline nanofibers doped with 2-acrylamido-2-methyl-1-propanesulfonic acid were prepared by oxidative polymerization of aniline in the presence of a nonionic surfactant. These nanofibers were dedoped to the semiconducting emeraldine base and then redoped to the metallic emeraldine hydrochloride. It was possible to introduce a different dopant anion from that used in the initial synthesis with no significant changes in fiber morphology or diameter, as observed by scanning electron microscopy (SEM). The method of sample preparation for SEM significantly affected the observed morphology. Deposition from aqueous dispersions resulted primarily in nanofibers that ranged in diameter from 28 to 82 nm (average: 56 nm), whereas drying to solid powder resulted in a less fibrous material. UV–vis–NIR absorbance spectroscopy indicated that the electronic structure of the emeraldine base nanofibers was identical to bulk emeraldine base obtained by conventional synthesis. Estimates from X-ray diffraction data suggested that the fractional crystallinity of emeraldine hydrochloride nanofibers did not differ significantly from the bulk powder.
Keywords: Polyaniline; Nanofibers; Doping; Scanning electron microscopy;

IV characteristics of dark and illuminated PPV-PCBM blends solar cells by S.C. Jain; T. Aernout; A.K. Kapoor; V. Kumar; Wim Geens; J. Poortmans; R. Mertens (245-250).
The effect of temperature and composition on the IV characteristics of blends of (poly(2-methoxy-5-(2′-ethyl-hexyloxy)-l,4-phenylene vinylene) (MEH-PPV), and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) sandwiched between a transparent indium tin oxide (ITO) electrode and an Al backside contact (solar cell structure) are investigated. The observed dark IV curves agree well with the trap-controlled space charge limited transport theory. The illuminated IV curves do not obey the theory of space charge limited currents. The short-circuit currents in the illuminated samples depend strongly on the composition. The current is maximum for the 80% PCBM concentration in the blend. The open circuit voltage also depends on the composition. It is maximum for 0% PCBM concentration and decreases monotonically with PCBM concentration. This might lead to the conclusion that the open circuit voltage is due to internal bulk heterojunctions (BHs). However, we have pointed out that this conclusion, arrived at by earlier workers also, is not necessarily valid. The superposition principle is not valid but the discrepancy is small.
Keywords: IV characteristics; PPV-PCBM; Solar cells;

Synthesis and crystal structure of a new organic metal with honeycomb dithiooxalato-bridged anionic network, β″-(ET)2{[NMe3Ph]NaCr(C2S2O2)3(MeCN)} by E.I. Zhilyaeva; G.V. Shilov; O.A. Bogdanova; R.N. Lyubovskaya; R.B. Lyubovskii; N.S. Ovanesyan; S.M. Aldoshin; C. Train; M. Gruselle (251-255).
The first bis(ethylenedithio)tetrathiafulvalene (ET) based radical cation salt with a bimetallic dithiooxalato-bridged anionic network, β″-(ET)2{[NMe3Ph]NaCr(C2S2O2)3(MeCN)} has been prepared. The compound is metallic down to 60 K, at lower temperatures the resistance grows smoothly. The structure of β″-(ET)2{[NMe3Ph]NaCr(C2S2O2)3(MeCN)} has been determined from single-crystal X-ray diffraction and is characterized by alternation a conducting donor layer of the β″-type packing with three insulating layers, two of which are {[NaCr(dto)3]2−} anionic networks and the third one, interleaving them, is formed by [NMe3Ph]+organic cations and acetonitrile solvent molecules.
Keywords: Organic conductors based on radical cation salts; Electrical conductivity; X-ray diffraction;

High near-infrared photoluminescence quantum efficiency from PbS nanocrystals in polymer films by Tung-Wah Frederick Chang; Ahmed Maria; Paul W. Cyr; Vlad Sukhovatkin; Larissa Levina; Edward H. Sargent (257-261).
We investigate the effect of film morphology and phase segregation in polymer/colloidal PbS nanocrystal films on the near-infrared photoluminescence quantum efficiency (PLQE) of the nanocrystals. A direct correlation between the size of closed packed nanocrystal phase separated domains and lower PLQE was established. Films with a near-infrared PLQE of up to 12% have been prepared.
Keywords: Nanocrystal; Polymer; Photoluminescence efficiency; Integrating sphere;

Synthesis of carbon nanotubes with identical dimensions using an anodic aluminum oxide template on a silicon wafer by Ok-Joo Lee; Sun-Kyu Hwang; Soo-Hwan Jeong; Pyung Soo Lee; Kun-Hong Lee (263-266).
Anodic aluminum oxide (AAO) templates with uniform channels of sub-micron length were fabricated on silicone wafers. Carbon was deposited on the wall of the pores via decomposition of acetylene at 800 °C. The synthesized carbon nanotubes have identical dimensions of 900 nm in length and 70 nm in diameter. Raman spectrum showed that the crystallinity of these CNTs is relatively high though no catalyst was used. The proposed technique can be applied to the fabrication of vacuum microelectronic devices.
Keywords: Carbon nanotube; Anodic aluminium oxide (AAO); Thin film aluminium; Thin film alumina; Raman spectrum;

High ON/OFF ratio and stability of amorphous organic field-effect transistors based on spiro-linked compounds by Tobat P.I. Saragi; Thomas Fuhrmann-Lieker; Josef Salbeck (267-270).
We report high ON/OFF ratios for organic field-effect transistors (OFETs) based on amorphous thin films of spiro-linked compounds. Bottom-contact OFET structures are fabricated, using 2,2′,7,7′-tetra-(m-tolyl-phenylamino)-9,9′-spirobifluorene (Spiro-TPD) and 2,2′,7,7′-tetrakis-(diphenylamino)-9,9′-spirobifluorene (Spiro-TAD) as active materials. The field-effect mobility of holes in Spiro-TPD and Spiro-TAD thin films is 7 × 10−5  cm2/Vs. We obtained ON/OFF ratios up to 3.6 × 106 with low OFF currents in the pA range. Time-dependent measurements show that the transistor characteristics do not change significantly over 9 months.
Keywords: Organic field-effect transistors; Spiro-linked compounds; ON/OFF ratio; Stability;

Temperature dependence of the resistance of self-assembled polyaniline nanotubes doped with 2-acrylamido-2-methyl-1-propanesulfonic acid by Nicholas J. Pinto; Pedro L. Carrión; Angel M. Ayala; Margarita Ortiz-Marciales (271-274).
Hollow polyaniline (PANi) nanotubes doped with 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA) have been synthesized and their electrical resistance measured as a function of temperature. The average length of the nanotubes was in the range 2–5 μm and the average diameter was in the range 200–400 nm. Longer reaction times did not affect the morphology of the nanotubes. The resistance of AMPSA doped nanotubes were measured as a function of temperature and compared to emeraldine base PANi doped with AMPSA in the solid state. Dissolving the PANi nanotubes in dichloroacetic acid leads to better polymer chain ordering.
Keywords: Polyaniline; Nanotubes; Synthesis; Template-free; Resistance;

Polyaniline (PANI) films have been deposited on stainless steel (SS) substrate by galvanostatic, potentiostatic and potentiodynamic methods. Electrochemical impedance spectra of the electrodes have been analyzed using a transmission line model consisting of two rails of finite resistances. The impedance spectra of the galvanostatically prepared electrodes are similar in shape to those of potentiostatically grown electrodes. The resistance of aqueous pore (R aq) due to ionic doping–dedoping process is greater than the resistance (R p) of electronic charge-transfer on polymer chain. These parameters exhibit an increase in magnitude with thickness of PANI, as expected from the transmission line theory. The shape of impedance spectra of potentiodynamically grown PANI, however, is different and its R aq  ≤  R p. The impedance parameters of potentiodynamically prepared PANI/SS electrodes have been found to depend on the sweep rate used for the deposition of PANI. Several possible explanations have been discussed to understand the influence of sweep rate during deposition on impedance parameters of the electrodes. In contrast to the behavior of PANI deposited by galvanostatic and potentiostatic methods, R p decreases with an increase in thickness of PANI deposited by the potentiodynamic method. The effect of concentration of aniline used for the deposition of PANI on impedance parameters has also been studied. The results of capacitance of PANI obtained from the ac impedance studies have been found to be comparable with those obtained from the cyclic voltammetric studies.
Keywords: Polyaniline; Impedance; Transmission line model; Capacitance; Electronic resistance; Ionic resistance;

Polycarbonate-based conductive film prepared by coating DBSA-doped PEDOT/sorbitol by Sang-Soo Jeon; Sang-Il Han; Yong Hyun Jin; Seung Soon Im (287-291).
A series of conductive polycarbonate films (CPCFs) were prepared by spin coating with an ethanol-based coating solution comprised of dodecylbenzenesulfonic acid-doped poly(3,4-ethylenedioxythiophene) (PEDOT)/sorbitol. The variation of CPCF surface resistivity was then measured as a function of temperature. CPCF surface resistivity decreased as sorbitol content increased, with the minimum values in the temperature range 175–185 °C. Furthermore, the CPCF-1.6 exhibited significantly improved thermal stability with respect to surface resistivity even at 175 °C. FT-IR analysis indicated that, after thermal treatment, hydrogen bonds arising from the hydroxy groups of sorbitol or hydroxypropyl cellulose and carbonyl groups of polycarbonate were established. The combination of surface resistivity data and FT-IR analysis suggests that the introduction of sorbitol accompanied with thermal treatment allows the PEDOT chains to undergo favorable inter-chain interactions leading to charge transfer and physical cross-linkage via hydrogen bonding.
Keywords: Conducting polymer; Polycarbonate; Spin coating; Poly(3,4-ethylenedioxythiophene); Sorbitol;

Poly(fluorene)s and poly(p-phenylene)s with pyrenyltriazine segments: synthesis and photophysics by John A. Mikroyannidis; Peter G. Persephonis; Vassilis G. Giannetas (293-299).
A Friedel-Crafts reaction between cyanuric chloride and pyrene afforded 2,4-dichloro-6-(pynen-1-yl)-1,3,5-triazine (1). This reacted with 4-bromophenol to yield 2,4-bis(4-bromophenoxy)-6-(pyren-1-yl)-1,3,5-triazine (2). A series of random copolymers PF-Pyr with various compositions were prepared by Suzuki polycondensation from 2,7-dibromo-9,9-di-2-ethylhexylfluorene (3) and 2. In addition, a series of random copolymers PP-Pyr were similarly prepared from 1,4-dibromo-2,5-didodecyloxybenzene (5) and 2. Solutions of copolymers PF-Pyr emitted blue light with photoluminescence (PL) maximum at 414–444 nm. Thin films of these copolymers emitted intense green light with PL maximum near 520 nm. An efficient energy transfer took place in thin films from the fluorene to the pyrenyltriazine segment even the content of the latter in copolymer was 0.5 mol%. Copolymers PP-Pyr behaved as blue light-emitting materials both in solution and solid state. Their PL maximum was red shifted with increasing the pyrenyltriazine content in copolymer. The PL quantum yields in solution were 0.42–0.56 for PF-Pyr and 0.27–0.35 for PP-Pyr.
Keywords: Poly(9,9-dialkylfluorene)s; Poly(2,5-dialkoxy-p-phenylene)s; Suzuki coupling; Photophysics; Energy transfer; Pyrene; 1,3,5-Triazine;

We report on a simple and novel sonochemical method for the preparation of polyaniline (PANI), as well as its composite, with Au nanoparticles. This technique utilizes ultrasound radiation as an energy source to facilitate the reaction by reducing the reaction time, as well as significantly increasing the yield. The tremendous increase in the reaction rates is explained on the basis of the intensive bubble collapse conditions, which rapidly generate the oxidative radicals necessary for polymerization.
Keywords: Sonochemistry; Polyaniline; Au–polyaniline; Cavitation; Conducting polymer;

High-pressure infrared absorption in Cs2TCNQ3 crystals grown under magnetic field by Hasanudin; N. Kuroda; T. Sugimoto; I. Mogi; K. Watanabe (307-312).
Pressure dependence of infrared absorption has been measured in Cs2TCNQ3 single crystals grown under magnetic field of 5 T (5 T crystals). The behavior of the C―CN as well as C―H stretching modes suggest that there occurs a phase transition which resembles to the one related to the insulator to metal transition in the crystals grown without magnetic field (0 T crystals). This transition, however, takes place at 4.1 GPa, slightly higher than the case of 0 T crystals. The charge transfer degree of the neutral and radical molecules are found to be ρ  = 0.30–0.37 and 0.81–0.85, respectively, which are significantly different from that of 0 T crystals. These differences are suggested to arise from the changes in the relative positions of the TCNQ molecules, as indicated from the behavior of the EMV mode.
Keywords: TCNQ complex; Crystal growth; High magnetic field; Optical spectroscopy; High pressure;

A new class of approach for hybrid inorganic/organic materials containing semiconductor ZnO by Tonggang Jiu; Huibiao Liu; Haiyang Gan; Yuliang Li; Shengqiang Xiao; Hongmei Li; Yang Liu; Fushen Lu; Lei Jiang; Daoben Zhu (313-319).
A new route has been developed to prepare ZnO of various shapes using different thiophene acids and ZnO at suitable reaction. Nanosized hybrid materials have a diversity of well-defined morphologies, such as rod-like, star-like sample and particle. The structure and morphology changes of the as-prepared product were investigated by means of transmission electron microscopy (TEM), X-ray powder diffractometry (XRD), infrared spectrum (IR). The required thiophene acids have been prepared by using organometallic synthetic route and characterized by EI, 1H NMR and photoluminescence (PL). The ZnO nanomaterials are expected to present special optical and electrical properties, and facilitate future nanoscale device application.
Keywords: ZnO; Thiophene acid; Hybrid material; Photoluminescence; Morphology; Photocurrent;