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Synthetic Metals (v.158, #3-4)

Editorial Board (pp. iii).

Effect of the light-induced charge generation in fullerene derivative by Jadwiga Szydłowska; Kinga Trzcińska; Renata Bilewicz; Józef Mieczkowski (pp. 75-82).
A light-induced charge generation is reported in a new fullerene malonate derivative. The monovalent fullerene anionic form was detected by ESR method in temperature below freezing point of the system. The origin of the ESR signal was confirmed by ESR studies of negatively charged fullerene derivative obtained in electrolytic process. The anionic form of the malonate fullerene derivative generated electrochemically gives ESR signal stable in changing temperature. Its characteristics ( g value, width and temperature behavior) are similar to that for the narrow signal of the negatively charged C60. Electrochemical and VIS/NIR studies of C60 and the fullerene malonate derivative as well as ESR spectrum of C60 are also reported.

Keywords: Methanofullerene; Radicals C; 60; /C; 60; ; Electron spin resonance (ESR); Electrochemistry; NIR–VIS spectroscopy; The charge separated stated

A stable, low band gap electroactive polymer: Poly(4,7-dithien-2-yl-2,1,3-benzothiadiazole) by Osman Atwani; Canan Baristiran; Ayca Erden; Gursel Sonmez (pp. 83-89).
A highly stable electrochromic polymer, poly(4,7-dithien-2-yl-2,1,3-benzothiadiazole), was electrochemically synthesized and its electrochemical, optical and morphological properties are reported. Poly(4,7-dithien-2-yl-2,1,3-benzothiadiazole) showed a very well-defined electrochemistry with a relatively low oxidation potential of the monomer at 0.9V versus Ag/Ag+, E1/2 at 0.78V and stability to long-term switching up to 1000 cycles. The band gap of the polymer films, 1.1eV, was determined from electrochemistry. Spectroelectrochemistry for p-doped film indicated two absorption peaks at 560 (Vis range) and 1100nm (NIR) corresponding to π–π* transition (onset 800nm) band, and the bi-polarons band, respectively, and a band gap of 1.5eV. The switching ability and kinetic studies of the polymer prove interesting optical properties in p- and n-doping case with a higher % Δ T at the 1100nm peak, thus making the polymer a suitable material for NIR applications. Morphology studies on carbon fibers indicated a three-dimensional growth on carbon fibers with a large area determined by the pores suggesting the material to be used in storage devices and biosensors applications. The homogeneity of the polymer film on ITO-coated glass ensures that poly(4,7-dithien-2-yl-2,1,3-benzothiadiazole) is a potential material for electrochromic device applications.

Keywords: Low band gap; p- and n-dopable polymer; Spectroelectrochemistry

Light emission of a single strand of poly(3,4-ethylenedioxythiophene) (PEDOT) nanowire by Dong Hyuk Park; Hyun Seung Kim; Yong Baek Lee; Jung Min Ko; Jun-Young Lee; Hyun-Jun Kim; Dae-Chul Kim; Jeongyong Kim; Jinsoo Joo (pp. 90-94).
We observed the green and orange-red light emission from the single strand of doped and de-doped poly(3,4-ethylenedioxythiophene) (PEDOT) nanowires, respectively, by means of the laser confocal microscope photoluminescence (PL) experiments. Light-emitting PEDOT nanowires were electrochemically synthesized and the doping level of the systems was electrochemically controlled through cyclic voltammetry in a solution of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) ionic liquid. From the ultraviolet and visible absorbance spectra, we observed the bipolaron band and the π–π* transition peaks for the doped and de-doped PEDOT nanowires, respectively. The PL intensity of the de-doped PEDOT single nanowire was 2–3 times higher than that of the doped one. Comparing the PL intensities of the fresh and aged PEDOT single nanowire, we observed the relatively long-term stable light emission from the PEDOT nanowires.

Keywords: Poly(3,4-ethylenedioxythiophene); Nanowire; Photoluminescence; Confocal microscope

New triscyclometalated iridium complexes for applications in phosphorescent light-emitting diodes by Jianping Lu; Qinde Liu; Jianfu Ding; Ye Tao (pp. 95-103).
A series of new triscyclometalated iridium complexes Ir1–3 containing diphenylamino substituted phenylpyridine derivatives as ligands have been synthesized and characterized. Inter-ligand energy transfer and related emission color tuning were observed in these iridium complexes. The phosphorescence emission peaks red-shifted to longer wavelengths with increasing the conjugation length of the ligands. Heteroleptic Ir2 is highly amorphous and readily dissolved in a wide range of organic solvents. High-efficiency electrophosphorescent light-emitting diodes have been fabricated using iridium complexes Ir1–3 doped into hole-transporting host materials as the emitters. The device structure used in this study is ITO/PEDOT-PSS/Ir complex:host material/F-TBB/Alq3/LiF/Al, where F-TBB is morphologically stable 1,3,5-tris(4′-fluorobiphenyl-4-yl)benzene functioning as a hole- and exciton-blocking layer. It was found that the device performance was strongly influenced by a number of factors, such as the triplet energy and electrochemical properties of host materials, the energy offset between host and guest, and the balance of electron and hole in emitting layers. When doped in PVK host, amorphous Ir2 showed promising EL performance with a turn-on voltage of 5V, a maximum luminance >15,000cd/m2 at a driving voltage of 18V, and a peak current-efficiency of 21cd/A at a luminance of 1200cd/m2.

Keywords: Light-emitting diodes; Electrophosphorescence; Triscyclometalated iridium complexes; Diphenylamino substituted phenylpyridine derivatives

Production of polypyrrole fibres by wet spinning by Javad Foroughi; Geoffrey M. Spinks; Gordon G. Wallace; Philip G. Whitten (pp. 104-107).
Soluble, doped polypyrrole containing no substituent on the pyrrole ring has been synthesized by the incorporation of the di-(2-ethylhexyl) sulfosuccinate dopant anion, which renders the polymer soluble in various organic solvents. The solutions were amenable to a wet-spinning process that produced continuous doped polypyrrole fibres. The ultimate tensile strength, elastic modulus and elongation at break of the obtained fibres were 25MPa, 1.5GPa and 2%, respectively, with an electronic conductivity of ∼3S/cm. Cyclic voltammetry demonstrated the electroactivity of the polypyrrole fibres. These fibres are likely to be important for electronic textile applications.

Keywords: Polypyrrole; Fibre; Wet spinning; Conducting polymer; Electronic textile

Determination of electronic properties of Al/p-Si/composite organic semiconductor (MIOS) junction barrier by current–voltage and capacitance–voltage methods by Fahrettin Yakuphanoglu (pp. 108-112).
The junction properties of Al/p-Si/DB6MEH diode have been investigated by current–voltage and capacitance–voltage methods. The n ideality factor of Al/p-Si/DB6MEH diode was obtained to be 1.92, which is higher than unity, indicating metal–insulator–organic layer-semiconductor configuration rather than an ideal Schottky diode. The effect of series resistance was analyzed by Cheung functions. The series resistance RS values were determined from d V/dln II and H( I)– I plots and were found to be 19.55kΩ and 20.44kΩ, respectively. The shape of the interface state density is in the range of ESS 0.53–0.69eV. The ϕb (0.93eV) value obtained from CV measurement is higher than that of the ϕb (0.75eV) value obtained from IV measurement. The discrepancy between these values is probably due to existence of excess capacitance at the structure or presence of barrier inhomogeneities. The ideality factor and barrier height values for the Al/p-Si/DB6MEH junction at the room temperature are significantly larger than that of the conventional Al/p-Si Schottky diode. It is evaluated that the DB6MEH organic layer controls electrical charge transport properties of Ag/p-Si diode by excluding effects of the SiO2 residual oxides on the hybrid diode.

Keywords: Metal–semiconductor contact; Interfacial state density; Organic semiconductor

Stabilized blue emission from polyfluorene-based light-emitting diodes: The role of triphenylamine by Umberto Giovanella; Mariacecilia Pasini; Silvia Destri; William Porzio; Chiara Botta (pp. 113-119).
A deep study of the optical properties related to the performances in light-emitting diodes (LEDs) of a fluorene-based copolymer containing triphenylamine (TPA), showed a huge reduction of oxidation of the whole material, a macromolecule separation, hence preventing the crystalline aggregation, an improved time-stability of both material chromaticity and morphology compared with commercial poly(9,9-dioctylfluorene) (PFO). LED device prototypes based on this copolymer exploited enhanced performances with respect to commercial PFO prepared under the same conditions.

Keywords: Polyfluorene; Electroluminescence; Fluorenone; Triphenylamine; Antioxidant; Green emission

Red phosphorescent iridium(III) complexes containing 2,3-diphenylquinoline derivatives for OLEDs by Gui Youn Park; Yunkyoung Ha (pp. 120-124).
For the application in organic light-emitting diodes (OLEDs), the efficient new red phosphorescent iridium(III) complexes, bis[2,3-diphenyl-4-methyl-quinolinato-C2,N] iridium(III) acetylacetonate [Ir(4-Me-2,3-dpq)2(acac)] and bis[2,3-di(4-methoxy-phenyl)-4-methyl-quinolinato-C2,N] iridium(III) acetylacetonate [Ir(4-Me-2,3-dpq(OMe)2)2(acac)] were synthesized from the two-step reactions of IrCl3· xH2O with the corresponding ligand. The complexes were characterized by mass spectrometry and1H NMR. In the photoluminescence (PL) spectra, Ir(4-Me-2,3-dpq)2(acac) and Ir(4-Me-2,3-dpq(OMe)2)2(acac) exhibited the luminescence peak at 604 and 592nm, respectively. Their electroluminescent (EL) devices with a configuration of ITO/2-TNATA/NPB/CBP:dopant/BCP/AlQ3/Liq/Al were fabricated. Ir(4-Me-2,3-dpq)2(acac) and Ir(4-Me-2,3-dpq(OMe)2)2(acac) showed the luminous efficiency of 8.10cd/A and 9.81cd/A at J of 20mA/cm2, respectively. The Commission Internationale de L’Eclairage (CIE) coordinates of Ir(4-Me-2,3-dpq)2(acac) and Ir(4-Me-2,3-dpq(OMe)2)2 (acac) were (0.644, 0.352) and (0.615, 0.375), respectively.

Keywords: Iridium complex; 2,3-Diphenylquinoline ligand derivatives; Red phosphorescence; OLED

Narrow band-gap oligomer for solution-processed heterojunction organic solar cells by Mingliang Sun; Li Wang; Bin Du; Yan Xiong; Ransheng Liu; Yong Cao (pp. 125-129).
A novel diphenylaminofluorenyl-capped narrow band-gap oligomer 4,7-[bis-5-(9,9-bis(octyl)-7-diphenylaminofluorene-2-yl)-thiophen-2-yl]-[2,1,3]benzo thiadiazole (DPAFL-DBT-DPAFL which is further abbreviated to DDBTD, presented in first scheme of the article) has been synthesized and characterized. The obtained molecule absorbs light from 300 to 650nm. Photovoltaic cell by using the blend of the obtained oligomer and PCBM as a active layer was fabricated by spin-coating method. The maximum energy conversion efficiency reached 0.22% and photocurrent response was extended over 700nm.

Keywords: Organic solar cells; Bulk heterojunction; Solution-processing; Oligomer

High-efficiency blue multilayer polymer light-emitting diode fabricated by a general liquid buffer method by Shin-Rong Tseng; Hsin-Fei Meng; Chi-Hung Yeh; Huan-Chung Lai; Sheng-Fu Horng; Hua-Hsien Liao; Chain-Shu Hsu; Li-Chi Lin (pp. 130-134).
A bright, highly efficient blue polymer light-emitting diode is fabricated by spin-coating with multilayer structure. By introducing a liquid buffer layer, the dissolution problem is overcome successfully. This general method can be applied to fabricate arbitrary polymer multilayer structure, in particular the addition of the carrier blocking layers to balance the electron and hole currents and improve the device efficiency. By adding an electron-blocking layer, the external quantum efficiency for blue emission can be increased to 5.5% (9.1cd/A) which is even higher than the theoretical limit of 5%. The luminance reaches 26,530cd/m2and the Commission Internationale de L’Eclairage (CIE) coordinates is (0.15, 0.23). Further more the lifetime is increased by three times relative to the single layer device.

Keywords: Polymer light-emitting diodes; Multilayer; Blue; White; High efficiency

Effects of conformational change and segmental aggregation on photoemission of illuminophores in conjugated polymer MEH-PPV: Blue shift versus red shift by Rakchart Traiphol; Nipaphat Charoenthai (pp. 135-142).
In this contribution, we report experimental results to demonstrate the effects of chain collapse and segmental aggregation of conjugated polymer, poly[2-methoxy, 5-(2′-ethylhexyloxy)-1,4-phenylenevinylene](MEH-PPV), on its photophysics. We have found that the photoemission spectrum of isolated chains in toluene constitutes of high- and low-energy peaks at ∼385nm and ∼558nm. This is attributed to simultaneous emissions of short and long illuminophores within the same conjugated chain. When the system is perturbed by addition of a poor solvent, the chains minimize polymer–solvent interactions by undergoing segmental aggregation and/or chain collapse causing drastic change of the optical properties. We demonstrate that the aggregation is responsible for red-shift peaks in absorption and emission spectra detected at ∼550nm and ∼580nm, respectively. The chain collapse, on the other hand, causes a blue shift of the spectra. Furthermore, the measurements of emission spectra by using excitation wavelength of 350nm detect novel high-energy peaks with λmax at ∼414nm. The results obtained from temperature-dependent experiments suggest that the detection of high-energy peaks arises from the collapse of individual chains.

Keywords: Photophysics; Fluorescence spectroscopy; Energy transfer; Solvent effects

3-Mercapto-1,2-propanediol-substituted polyaniline/Ag nanocomposites prepared by concurrent reduction and substitution chemistry by Young-ho Lee; Jun-hwan Park; Young-doo Jun; Dae-wook Kim; Jin-Jae Lee; Young Chai Kim; Seong-Geun Oh (pp. 143-149).
PANI/Ag composite particles were prepared through two-step reaction process comprising the reduction of the PANI backbone by the substitution of 3-mercapto-1,2-propanediol (MPD) and the simultaneous redox reaction between AgNO3 and MPD-substituted PANI. In order to investigate the effects of MPD and AgNO3 on the oxidation state of PANI, MPD and AgNO3 were sequentially added into PANI/NMP solution, and the oxidation state of PANI was investigated by time-dependent UV–vis spectra. On the basis of UV–vis spectroscopic study, MPD-substituted PANI/Ag composite was prepared through following two steps: (1) treating PANI particles with MPD (reduction of PANI backbone); (2) adding AgNO3 solution to MPD-substituted PANI (re-oxidation of PANI backbone). The molar ratio of AgNO3 to aniline unit in the PANI chains was varied from 1:16 to 1:1. The substitution of PANI with MPD and the formation of PANI/Ag composite were characterized by FT-IR, XRD and EDX. The morphologies of PANI/Ag composites were investigated by TEM.

Keywords: Thiol; PANI; Ag; Composite; Redox

Synthesis and characterization of triphenylamine-3-hexylthiophene oligomer hybrids: A triphenylamine core carrying three terthiophene branches and triphenylamine end-capped quaterthiophene by Ji-Cheng Li; Soo-Hyoung Lee; Yoon-Bong Hahn; Ki-Ju Kim; Kyukwan Zong; Youn-Sik Lee (pp. 150-156).
Two different triphenylamine-3-hexylthiophene (TPA-HT) oligomer hybrids were synthesized via multi-steps: a TPA core carrying three terthiophene branches (TPA-3HT3) and a TPA end-capped quaterthiophene (2TPA-4HT). The compounds are amorphous in nature, and thermally stable up to about 400°C under a nitrogen atmosphere. The UV–vis absorption and photoluminescence emission peaks of TPA-3HT3 in chloroform were observed at 408 and 523nm, while those of 2TPA-4HT were observed at 304/398 and 494/512nm, respectively. The lower energy onsets of the absorption band and the onset oxidation potentials for the two compounds were very similar, indicating that the conjugation lengths of TPA-3HT3 and 2TPA-4HT must be similar. The unexpected result relating to the conjugation length was ascribed to the position of the hexyl substituent on the HT which is directly attached to the phenyl group of TPA in 2TPA-4HT, leading to repulsion between the α-methylene hydrogens on the alkyl group and the hydrogen atom at ortho position in the phenyl ring of TPA. The estimated highest occupied molecular orbital (HOMO) energy levels of the two compounds were very close to those of the well-known hole-transporting materials, 4,4′-bis[ N-(1-naphthyl)- N-phenylamino]biphenyl (NPB) and poly(3,4-ethylenedioxy thiophene) (PEDOT).

Keywords: Thiophene oligomer; Triphenylamine; Amorphous; Hole transporting; Conjugation length

Synthesis, crystal structure and thermal decomposition kinetics of the complex [Sm(BA)3bipy]2 by Hai-Yan Zhang; Ke-Zhong Wu; Jian-Jun Zhang; Su-Ling Xu; Ning Ren; Ji-Hai Bai; Liang Tian (pp. 157-164).
The complex of [Sm(BA)3bipy]2 (BA=benzoic acid; bipy=2,2′-bipyridine) was obtained and characterized by elemental analysis, molar conductance, IR spectra, UV spectra, single crystal X-ray diffraction and TG–DTG techniques. The crystal is monoclinic with space group P2(1)/ n. The unit cell parameters are: a=14.157(2)Å, b=15.416(3)Å, c=26.079(4)Å, β=103.445(2)°. Two Sm3+ ions are connected by four carboxylic groups through bridging bidentate mode. Each center Sm3+ is eight-coordinated with a trigondodecahedron. The thermal decomposition behaviors of the title complex in a static air atmosphere were discussed by TG–DTG and IR techniques. The non-isothermal kinetics was investigated by using double equal-double step method and nonlinear integral isoconversional method. The mechanism function of the first decomposition step was determined. Meanwhile, the thermodynamic parameters (Δ H, Δ G and Δ S) and kinetic parameters (activation energy E and pre-exponential factor A) were also calculated. The lifetime equation at mass-loss of 10% was deduced as ln τ=−34.56+19299.15/ T by isothermal thermogravimetric analysis. The title complex can be used in many areas such as extraction, separation, germicide, catalysis and functional materials and so on.

Keywords: Benzoic acid; Samarium complex; Crystal structure; Thermal decomposition; Non-isothermal kinetics

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