Hierarchical structures of self-assembled helical conjugated polymers—Magnetically forced alignment of liquid crystalline poly(meta-phenylene) derivatives

We synthesized poly(meta-phenylene) derivatives with liquid crystalline (LC) groups consisting of phenylcyclohexyl (PCH) or cyanobiphenyl (CB) moiety in the side chains. Thermotropic LC phases and self-assembled structures of the polymers were investigated through polarized optical microscope (POM), differential scanning calorimeter (DSC), and X-ray diffraction (XRD) measurements. The polymers showed discotic liquid crystallinities based on helical structure of the main chains, where the helical chains are self-assembled to form columns along the helical axis. Macroscopic alignments of the polymers were carried out using a magnetic field of 12 T in liquid crystal phases. XRD measurements of the aligned polymers showed that the columns of the main chains are aligned perpendicular to the magnetic field.     

Poly(fluorene)s and poly(p-phenylene)s with pyrenyltriazine segments: synthesis and photophysics

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.     

Optical properties of highly oriented poly(p-phenylene-vinylene)

A review of recent results on the complex refractive index determination of oriented poly(p-phenylene-vinylene) (PPV) by polarized near-normal-incidence reflectance, transmittance and ellipsometry measurements is presented. In particular, the new experimental data obtained for highly stretch-oriented films of PPV and the results of correlated quantum-chemical calculations on the same polymer will be reported and discussed with the purpose of clarify the controversial assignment of the electronic transitions of PPV and its derivatives.     

Optical absorption and luminescence in poly(4,4′-diphenylenediphenylvinylene)

Poly(4,4′-diphenylenediphenylvinylene), PDPV, is a soluble conjugated polymer that shows a degree of conjugation similar to that in poly-(paraphenylene). The optical properties of thin films exposed to AsF₅ show the appearance of features below the $\text{π-π}{}^1$ gap at 3 eV that can be interpreted in a model of dopant-induced polaron and bipolaron defects. When excited above the $\text{π-π}{}^1$ gap, PDPV shows a strong luminescence peaked at 2.4 eV. The Stokes' shift of 1 eV can be accounted for by radiative decay from a photogenerated polaron-exciton defect.     

Syntheses and optical properties of poly(o-phenylene vinylene)s

Poly(p-phenylene vinylene) (PPV) derivatives have been extensively studied as photoluminescence and electroluminescence materials. Recently, meta-phenylene has been incorporated into PPVs for conjugation-length control and luminescence enhancement. Here, we report three poly(o-phenylene vinylenes) (oPVs). Among them, Polymer II exhibits high molecular weight, good processibility and high photoluminescence quantum efficiency in both solution and solid state, making it an attractive candidate for light-emitting diode applications.     

Spectroscopy of conducting and insulating ladder-type poly(para-phenylene) device structures

We have applied the electroabsorption (EA) and charge-induced absorption (CIA) spectroscopic techniques to methyl substituted ladder-type poly(para-phenylene) (m-LPPP) sandwich device structures. The device structures have been either of conducting (ITO/m-LPPP/Al) or insulating (ITO/SiO/m-LPPP/Al) type. For both devices we have observed a CIA band with its 0–0 transition at 1.95 eV. From a comparison to doping induced and photoinduced absorption spectra we assign this band to polarons. In addition, in the insulating device we observed a derivative-like band with zero crossing at 1.15 eV. In the conducting device two derivative bands are observed with zero crossing at approximately 0.8 and 1.2 eV, respectively. Their intensity increases linearly with the modulus of the applied electric field and the features are independent of the modulation frequency. We tentatively assign these derivative-like bands to a Stark-effect related to charge transfer transitions at the ITO and Al electrodes. At high currents in the conducting device we also observed a relatively broad, structureless CIA band peaking at 1.3 eV, that we assign to biplarons.     

Enhanced stability of poly(3-hexylthiophene) transistors with optimally cured poly(methyl methacrylate) dielectric layers

Poly(3-hexylthiophene)-based organic field-effect transistors (OFETs) have been fabricated on poly(methyl methacrylate) (PMMA) gate dielectric layers under different process conditions, resulting in very different device stability in ambient air. The dielectric layers were prepared by spin coating and subsequently curing at various temperatures (120, 150, and 180 °C) or by ultraviolet light (UV) exposure. With respect to the variations of the on/off current ratio and the threshold voltage, dramatically enhanced stability of the OFETs with the PMMA layer cured at 150 °C has been demonstrated when compared to those cured at different temperatures. The devices cured by UV exposure showed even more superior stability, with reliable performance in ambient air for more than 10 days. The differences in the film surface morphology were analyzed and possible mechanisms for the enhanced stability are discussed.     

Optical and electroluminescence properties of poly (methylphenylsilane) containing an anthracene unit

A poly(methylphenylsilane) containing anthracene units in the polymer backbone was synthesized by the Wrutz-type reaction of 9,10-bis(methylpropylchlorosilyl)anthracene and methylphenyldichlorosilane. This polymer exhibited light-blue photoluminescence (emission maximum 480 nm) with an 87% quantum efficiency. A typical single-layer-type electroluminescence (EL) device with a glass substrate/ indium-tin-oxide/polymer layer/aluminum structure was fabricated using this polymer. This device showed electroluminescence with a similar spectrum to the photoluminescence when the forward bias was applied. At present, the external quantum efficiency calculated in terms of emitted photons per transported electronic charge was 0.01%. We also developed a patterning-image-display EL device utilizing photodecomposition of the polymer chain. This device emitted the patterning image without using the complicated electrode patterning.     

Enhanced photovoltaic cells efficiency via incorporation of high electron-deficient oxadiazole moieties on side chains of poly(phenylene vinylene)s and poly(fluorene)s

High charge mobility and fast exciton dissociation rate are essential for photovoltaic cells. In this study, we demonstrate effects of the electron-deficient oxadiazole (OXD) moiety attached on the side chain on the efficiency of photovoltaic cells for poly(phenylene vinylene)s (PPVs) and poly(fluorene)s (PFs) as the active layer by luminescence decay and time of flight measurement (TOF). We find that OXD moiety can increase exciton dissociation rate and promote electron transport. Therefore, the efficiencies of incident photon converting to electron of the OXD-attached polymers, POPD-MEH-PPV(37/63) and OXD50PFO, are higher than those of MEH-PPV and PFO by factors of 2.1 and 29, respectively.     

Synthesis and optical properties of quinoxaline-containing poly(aryl ether)s

Three novel poly(aryl ether)s were synthesized from the reaction of three bisphenols with 2,3-bis(4-fluorophenyl)-quinoxaline via nucleophilic aromatic substitution. The polymers contained electron transporting and emissive moieties separated by ether linkages. Quinoxaline was used as electron transporting segment, while p-distyrylbenzene, 2,6-distyrylpyridine and p-quinquephenyl were used as emissive segments. The low reactivity 2,3-bis(4-fluorophenyl)-quinoxaline towards nucleophilic aromatic substitution results in polymers with limited molecular weights. The polymers were soluble in polar organic solvents and strong organic acids. In THF solutions the polymers showed absorption maxima at 349–354 nm and emission maxima at 417–454 nm, with quantum yields of 22–41%. In solid state the polymers showed absorption maxima at 350–353 nm and emission maxima at 427–461 nm. The optical properties of polymers were influenced by intrachain interactions in solution, while interchain interactions were important in solid state.     

A study of the thermal degradation of poly(vinyl chloride)

A thermal degradation of poly(vinyl chloride) (PVC) was studied. This was an essential step in the investigation of the compatibility of PVC-based systems because mixing in industry and in the preparation of materials is carried out in the viscous flow range close to thermal degradation. Original Russian Text ? K.A. Polozkov, A.E. Chalykh, V.K. Gerasimov, V.V. Matveev, A.D. Aliev, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 5, pp. 537–540.     

Optical and electrical characteristics of poly(3-alkylthiophene) and polydiphenylamine copolymers: Applications in light-emitting devices

In this work, poly (3-methylthiophene) (P3MT) or poly (3-octylthiophene) (P3OT) films were synthesized electrochemically with polydiphenylamine (PDFA), in layers and blended copolymers, in non-aqueous media through the oxidation of the monomers, using a standard three-electrode cell in acetonitrile with 0.100 mol L⁻¹ LiClO₄. The polymeric thin films were deposited on platinum plates for optimal quality control of the process. The electrochemical behaviors of as-prepared and partial dedoped films obtained by these methods was verified by cyclic voltammetry (CV) and have been characterized by UV–vis reflectance, resonance Raman, electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy. Based on data obtained by UV–vis reflectance spectroscopy, it was possible to characterize the chemical species in the polymer matrix using the resonant Raman method. Both Raman and PL spectra results led to the characterization of three structures (aromatic, semi-quinone and quinone forms of thiophene rings) which formed the homopolymers and copolymers chain. The EPR measurements were taken to quantify the semi-quinone species stabilization at 298 and 77 K temperatures. The ionization potential (I_P), electron affinity (E.A.) and gap energy (E_g) parameters were determined based on the technical analyses carried out.     

Performance recovery and optimization of poly(3-hexylthiophene) transistors by thermal cycling

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.     

Synthesis and optical properties of poly(2,11-triphenyleneethynylene-alt-m-phenyleneethynylene)s

Triphenylene units are introduced into the backbone of a poly(m-phenyleneethynylene) (mPE). The new polymer, mPPET, exhibits strong intra-chain folding in good solvents in which the polymer is soluble, such as chloroform, benzene, and THF. In poor solvents, where the polymer is insoluble, such as acetonitrile, hexane, or DMSO, interchain aggregation dominates. In a solvent mixture of a good solvent and a poor solvent with appropriate ratios, mPPET appears to adopt a random coil conformation. Among the three conformations (intra-chain folding, interchain aggregation, and random coil chain), the intra-chain polymeric foldamer exhibits the highest fluorescence quantum yield. These results indicate that introducing a planar polynuclear aromatic compound into mPE not only increases its intra-chain folding propensity, but also may enhance its fluorescent properties. This approach may be applied to synthesize a variety of conjugated foldamers containing planar polynuclear aromatic hydrocarbons with interesting photophysical properties.     

Synthesis and characterization of new poly(cyanoterephthalylidene)s for light-emitting diodes

A serial of new polymers containing both long-chain alkoxy and cyano groups (C_n-CN-PPV) was synthesized using Knoevenagel reaction between the dialdehyde monomer and the appropriate diacetonitrile. The synthesized polymers were highly soluble in common organic solvents. The weight average molecular weights of the resulting polymers were in the range of 3900–4600 with uniform molecular polydispersity indexes range of 1.05–1.07. The decomposition temperatures and the glass transition temperatures were in the range of 395–410 and 174–198°C, respectively. A quasi-reversible and stable electrochemical behavior for these polymers was observed in the reduction process. Their energy band gaps were calculated both by electrochemical measurement and optical measurement. The UV–VIS absorption spectra of the present polymers showed absorption bands around 452 nm, and the photoluminescence spectra showed strong light emission at 560 nm in chloroform and at 611 nm as thin solid film. Electrical characterization of the LEDs with the structure of indium–tin oxide/copper phthalocyanine/C_n-CN-PPV/Al exhibited a typical diode behavior with a moderate high rectification ratios greater than 10². Red light emission was achieved from these devices using the polymers as the emissive layer.     

Structure, hardness and thermal stability of Ti(Al,N) coatings

In the past two decades, coatings of the Ti-Al-N ternary system have attracted considerable research and industrial interest. Nevertheless, the Ti-Al-N system still offers new interesting possibilities for coating developments such as the addition of low N contents to Ti-Al films in order to reach a good compromise between high hardness and a low friction coefficient.Ti-Al-N coatings with low nitrogen content were deposited by closed field unbalanced magnetron sputtering using two facing Ti targets inserted with Al rods. The Al/(Al + Ti) and N/(Al + Ti + N) atomic ratios were varied from 21 to 28 at.% and 0 to 33 at.%, respectively.Hexagonal close-packed (hcp) α-Ti with a preferential <001> orientation was the only phase detected by X-ray diffraction in the as-deposited films. A decrease in the α-Ti c lattice parameter was observed as aluminium was added to the films. Nitrogen addition increased the c lattice parameter and led to a progressive loss of crystallinity until quasi-amorphous films were obtained. A hardness of ≈ 13 GPa was obtained for the as-deposited films without nitrogen. A continuous increase in hardness was observed with increasing nitrogen content. The highest hardness values (up to 27 GPa) were obtained for the quasi-amorphous films. Annealing of the films with low aluminium content (Al/Al + Ti ≈ 21 at.%) did not significantly affect their structure as hcp Ti remains the only phase detected. On the contrary, annealing of the films deposited with higher aluminium contents (Al/(Al + Ti) ≈ 24 and ≈ 28 at.%) resulted in the formation of face centered cubic (fcc) Al or Ti₃Al, showing that the thermal stability of the films decreased with aluminium incorporation.     

Stereoelectronic effects in polythiophenes: poly(3-cyclopropylthiophene) and poly (3-cyclobutylthiophene)

Stereoelectronic effects enable 3-cyclopropyl and 3-cyclobutylthiophene to produce electrochemical polymers. Poly(3-cyclopropylthiophene) is the only good electroconducting poly(3-alkylthiophene) with a branched carbon directly connected to the thiophene ring.     

Triphenylamine- and oxadiazole-substituted poly(1,4-phenylenevinylene)s: synthesis,photo-, and electroluminescent properties

Novel triphenylamine- and oxadiazole-substituted poly(1,4-phenylenevinylene)s (PPV) or CN-PPV (P1–4) were synthesized by the Wittig–Horner, Knoevenagel, and Gilch type polymerization. The polymers exhibited good solubility in common organic solvents, relatively high photoluminescent (PL) efficiency, and high HOMO level for ca. −5.1 eV. The simple double-layer devices of triphenylamine-PPV (P1), ITO/PEDOT: PSS/P1/Cs/Al, exhibited a very high luminance of 510 cd/m² under a low driving voltage of 3 V, demonstrating the effectiveness of triphenylamine moiety as a substituent of PPV derivatives for a light-emitting polymer. Triphenylamine-CN-PPV (P2) exhibited bipolar reversible redox in CV. The bipolar type of PPVs, P2 and triphenylamine-oxadiazole-PPV (P3), showed lower luminance and efficiency than those of the p-type of PPV (P1).     

Synthesis and optical properties of poly[(p-phenyleneethynylene)-alt-(m-phenyleneethynylene)]s

Poly[(p-phenyleneethynylene)-alt-(m-phenyleneethynylene)]s (PpPEmPE) derivatives 3 have been successfully synthesized by using Heck-type coupling. The co-polymers contain two different chromophores 4 and 5, which are randomly distributed along the polymer chain. The chemical structure of the co-polymers is characterized, and their optical properties are studied. The results show that the emission of 3 is essentially from the low-energy chromophore 5, resulting from the efficient intramolecular energy transfer.