Novel route to poly(p‐phenylene vinylene) polymers

Poly(p‐phenylene vinylene) (PPV), poly(2,5‐dioctyl‐p‐phenylene vinylene) (PDOPPV), and poly[2‐methoxy‐5‐(2′‐ethylhexyloxy)‐p‐phenylene vinylene] (MEHPPV) were synthesized by a liquid–solid two‐phase reaction. The liquid phase was tetrahydrofuran containing 1,4‐bis(bromomethyl)benzene, 1,4‐bis(chloromethyl)‐2,5‐dioctylbenzene, or 1,4‐bis(chloromethyl)‐2‐methoxyl‐5‐(2′‐ethylhexyloxy)benzene as the monomer and a certain amount of tetrabutylammonium bromide as a phase‐transfer catalyst. The solid phase consisted of potassium hydroxide particles with diameters smaller than 2 mm. The experimental results demonstrated that the reaction conversions of PPV and PDOPPV were fairly high (～65%), but the conversion of MEHPPV was only 45%. Moreover, gelation was found in the polymerization processes. As a result, PPV was insoluble and PDOPPV and MEHPPV were partially soluble in the usual organic solvents, such as tetrahydrofuran and chloroform. Soluble PDOPPV and MEHPPV were obtained with chloromethylbenzene or bromomethylbenzene as a retardant regent. The molar mass of soluble PDOPPV was measured to be 2 × 10⁴ g mol^?1, and that of MEHPPV was 6 × 10⁴ g mol^?1. A thin, compact film of MEHPPV was formed via spin coating, and it emitted a yellow light. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 449–455, 2003     

Pyrolysis of poly(phenylene vinylene)s with polycaprolactone side chains

The thermal degradation characteristics of a new macromonomer poly(-caprolactone) with central 4,4′-dicarbaldehyde terphenyl moieties and poly(phenylene vinylene)s with well defined (-caprolactone), (PPV/PCL) as lateral substituents were investigated via direct pyrolysis mass spectrometry. The unexpectedly high thermal stability of the macromonomer was attributed to intermolecular acetylation of benzaldehyde yielding a hemiacetal and causing a crosslinked structure during the pyrolysis. Increased thermal stability of the PCL chains was detected for all samples. The increase in stability of PCL chains was much more pronounced than was detected for poly(p-phenylene)-graft-poly(-caprolactone) copolymer (PPP/PCL); the upward temperature shift was about 100 °C for PPV/PCL and only 20 °C for PPP/CL. This pronounced effect may be due to higher thermal stability of PPV compared to PPP and the decrease in steric hindrance for PPV with PCL side chains.     

Syntheses and electroluminescence properties of conjugated poly(p‐phenylene vinylene) derivatives bearing dendritic pendants

A series of new poly(p‐phenylene vinylene) derivatives with different dendritic pendants—poly{2‐[3′,5′‐bis(2″‐ethylhexyloxy)benzyloxy]‐1,4‐phenylenevinylene} (BE–PPV), poly{2‐[3′,5′‐bis(3″,7″‐dimethyl)octyloxy]‐1,4‐phenylenevinylene} (BD–PPV), poly(2‐{3′,5′‐bis[3″,5″‐bis(2?‐ethylhexyloxy)benzyloxy]benzyloxy}‐1,4‐phenylenevinylene) (BBE–PPV), poly(2‐{3′,5′‐bis[3″,5″‐bis(3?,7?‐dimethyloctyloxy)benzyloxy]benzyloxy}‐1,4‐phenylenevinylene) (BBD–PPV), and poly[(2‐{3′,5′‐bis[3″,5″‐bis(2?‐ethylhexyloxy)benzyloxy]benzyloxy}‐1,4‐phenylenevinylene)‐co‐(2‐{3′,5′‐bis[3″,5″‐bis(3?,7?‐dimethyloctyloxy)benzyloxy]benzyloxy}‐1,4‐phenylenevinylene)] (BBE‐co‐BBD–PPV; 1:1)—were successfully synthesized according to the Gilch route. The structures and properties of the monomers and the resulting conjugated polymers were characterized with ¹H and ¹³C NMR, elemental analysis, gel permeation chromatography, thermogravimetric analysis, ultraviolet–visible absorption spectroscopy, photoluminescence, and electroluminescence spectroscopy. The obtained polymers possessed excellent solubility in common solvents and good thermal stability, with a 5% weight loss temperature of more than 328 °C. The weight‐average molecular weights and polydispersity indices of BE–PPV, BD–PPV, BBE–PPV, BBD–PPV, and BBE‐co‐BBD–PPV (1:1) were in the range of 1.33–2.28 × 10⁵ and 1.35–1.53, respectively. Double‐layer light‐emitting diodes (LEDs) with the configuration of indium tin oxide/polymer/tris(8‐hydroxyquinoline) aluminum/Mg:Ag/Ag devices were fabricated, and they emitted green‐yellow light. The turn‐on voltages of BE–PPV, BD–PPV, BBE–PPV, BBD–PPV, and BBE‐co‐BBD–PPV (1:1) were approximately 5.6, 5.9, 5.5, 5.2, and 4.8 V, respectively. The LED devices of BE–PPV and BD–PPV possessed the highest electroluminescent performance; they exhibited maximum luminance with about 860 cd/m² at 12.8 V and 651 cd/m² at 13 V, respectively. The maximum luminescence efficiency of BE–PPV and BD–PPV was in the range of 0.37–0.40 cd/A. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3126–3140, 2005     

Synthesis and electrical conductivity of poly(arylene vinylene). I. Poly(2,5-dimethoxyphenylene vinylene) and poly(2,5-dimethylphenylene vinylene)

The highly conjugated aromatic polymers, poly(2,5-dimethoxyphenylene vinylene) and poly(2,5-dimethylphenylene vinylene), were obtained from their water soluble, sulfonium salt precursor polymers. Films of these polymers were reacted with either AsF₅ or I₂ vapor. Poly(2,5-dimethoxyphenylene vinylene) showed increases in electrical conductivity of up to 14 to 15 orders of magnitude for these two dopants, while an 8 to 9 order of magnitude increase was observed for poly(2,5-dimethylphenylene vinylene) with the same dopants. The synthesis of the precursor polymers, the properties and elimination reactions of films of the precursors, the doping reactions, and the conductivities of the resulting phenylene vinylene films are discussed.     

Structure and electrical properties of undoped oriented poly(phenylene vinylene) films

The influence of anisotropic structure on the electrical properties of undoped oriented poly(phenylene vinylene) (PPV) films was investigated with impedance spectroscopy and structural characterization. The oriented structure of the stretched PPV films was studied with wave-guide coupling and infrared dichroism. It was found that the unstretched PPV film had a highly planar structure, and one-way stretching converted the planar structure into a uniaxial structure. The impedance of undoped PPV thin films was measured along three different directions: through the film thickness direction, parallel to the stretch direction in the film plane, and perpendicular to the stretch direction in the film plane. Two relaxations were observed, one corresponding to the bulk behavior in the high-frequency range and the other to an interfacial contact polarization in the low-frequency range. From equivalent circuit modeling, it was found that the bulk conductivity through the film thickness direction decreased with increasing orientation, whereas the high-frequency dielectric constant through the film direction remained constant. In addition, the conductivities measured in the film plane were at least two orders of magnitude higher than the conductivity in the direction normal to the film surface. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 98–116, 2004     

Enhanced photoluminescence from poly(phenylene vinylene):dendrimer polyelectrolyte assemblies in solution

Poly(2,5-methoxy-propyloxy sulfonate phenylene vinylene)(MPS-PPV) and DAB-Am-16, a generation 3.0 polypropylenimine hexadecamine dendrimer (DAB), are shown to form a tunable photoresponsive polyelectrolyte assembly in aqueous solution with an enhanced emission signal of up to 18-times that of MPS-PPV alone.     

Photocurrent generation in multilayer self-assembly films fabricated from water-soluble poly(phenylene vinylene)

A novel, water-soluble, cationic PPV derivative poly[(2,5-bis(3-bromotrimethylammoniopropoxy)-phenylene-1,4-divinylene)-alt-1,4-(2,5-bis(2-(2-hydroxyethoxy)ethoxy))phenylene vinylene] (BH-PPV) has been synthesized by a Heck coupling reaction. Multilayered assemblies of the BH-PPV and the sodium salt of hexa(sulfobutyl)fullerenes (C(60)-HS) were fabricated successfully by an alternate deposition technique. The multilayer structures were studied by UV/Vis spectroscopy, small angle X-ray diffraction, and atomic force microscopy. The photoinduced charge transfer property of the self-assembled multilayer film was also measured by a three-electrode cell technique. A steady and rapid cathodic 5.5 microA cm(-2) photocurrent response was measured as the irradiation of the multilayer film was switched on and off. Importantly, the response of on/off cycling is prompt and reproducible. A possible mechanism for the electron-transfer process is proposed.     

Oligo(phenylene vinylene)-poly(methylstyrene) hybrids: controlled step-wise molecular wiring of oligo(phenylene vinylene)

Well defined oligo(phenylene vinylene) grafted polymers known as oligo(phenylene vinylene)-poly(methylstyrene) hybrids have been developed using a step-wise synthetic protocol, where the length of the OPV can be controlled systematically to achieve specific optoelectronic properties. The process allows the structural modification of attached OPV at a molecular level either by varying the chain length or by changing functionalities. The step-wise generation of OPV chains on the backbone of a highly soluble polymer ensures solubilization in a variety of solvents and also the exhibition of interesting optical properties.     

Nanotubes of poly(phenylene vinylene) derivative at the air/water interface

Different sizes of nanotubes of poly(2-methoxy-5-(n-hexadecyloxy)-p-phenylene vinylene)(MH-PPV) have been fabricated at the air/water interface by compressing a monolayer of MH-PPV beyond its collapse point, and their structural characteristics were studied by means of TEM, AFM, SAXRD, IRRAS.     

Structural anisotropy in unstretched and uniaxially stretched poly(p‐phenylene vinylene)

Poly(p‐phenylene vinylene) or PPV is gaining increasing importance because of its superior electroluminescent efficiency and electrical conductivity. The most widely followed synthetic route for PPV involves synthesis of a precursor polymer using a sulfonium monomer salt. Previous studies have proven that the monomer salt which contains cyclic sulfonium groups yields better quality PPV than when the monomer containing dialkyl sulfonium groups is used. The structure of PPV synthesized using cyclic sulfonium precursor has not been as widely reported as that synthesized using dialkyl sulfonium monomer. In the current work, the structure of PPV, synthesized using a cyclic viz. tetrahydrothiophenium monomer salt, has been studied in detail using the wide angle X‐ray diffraction (WAXD) technique. The study reveals that even in the cast (unstretched) form, PPV shows considerable biaxial orientation in the plane of the film. This preferred orientation is found to occur during the casting process and is independent of the solvent used and casting substrate. On stretching these films to a final draw ratio of 7 : 1, this biaxial orientation is transformed into uniaxial orientation with nematic ordering of PPV chains along the stretch direction and PPV chains assuming three preferred orientations in the plane of the film. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 605–614, 1999     

Preparation of poly(phenylene vinylene) from cycloalkylene sulfonium salt monomers and polymers

Two new types of p-xylene bis-sulfonium chloride monomers were prepared from cycloalkylene sulfides. The polymerization characteristics of these monomers to form poly(p-xylene sulfonium chlorides), and the thermal elimination reactions of their polymers to poly(p-phenylene vinylene), were compared with those of two monomers prepared from dialkyl sulfides. The cycloalkylene sulfonium chloride monomer polymerized to higher yields and to higher molecular weight polymers, which showed more efficient elimination reactions.     

Highly conductive meta derivatives of poly(phenylene sulfide)

Changes in the backbone structure of several meta derivatives of polyphenylene sulfide upon doping with AsF₅ have been investigated by IR spectroscopy. Poly(m-phenylene sulfide) does not form a conducting complex with AsF₅ unless reacted under conditions where carbon-carbon bonds form intramolecularly between phenyl rings. This assertion has been verified by comparison of the IR spectra of poly(m-phenylene sulfide) and a newly synthesized derivative, poly(thio-3,7-dibenzothiophendiyl), after doping with AsF₅. This new derivative forms a complex with AsF₅ which exhibits a conductivity of 18.5 S/cm. A sulfone-containing derivative has also been synthesized, poly[thio-3,7-(dibenzothiophene-5,5-dioxide)-diyl]. With this polymer, a much lower conductivity, 10^?3 S/cm, was obtained after exposure to AsF₅.     

2H NMR characterization of phenylene ring flip motion in poly(p-phenylene vinylene) films

Solid-state ²H quadrupole echo nuclear magnetic resonance (NMR) spectra and measurements of ²H spin lattice relaxation times have been obtained for films of poly(p-phenylene vinylene) deuterated in phenylene ring positions (PPV-d₄). NMR line shapes show that all the phenylene rings of PPV undergo 180° rotational jumps about the 1,4 ring axis (“ring flips”) at 225°C. The temperature dependence of the ²H line shapes show that the jump motion is thermally activated, with a median activation energy, E_a = 15 kcal/mol, and a distribution of activation energies of less than ±2 kcal/mol. The jump rate was also determined from the magnitude of the anisotropic T₂ relaxation associated with ²H line shapes and from the curvature of inversion recovery intensity data. The experimental activation energy for jumps is comparable to the intramolecular potential barrier for rotation about phenylene vinylene bonds. ²H NMR provides a method for determining the phenylene-vinylene rotational barrier in pristine PPV, and may potentially be used to study conjugation in conducting films.     

About some properties of new poly(phenylene vinylene) derivatives: PPV-ether and C1-4PPV-ether

Although, the presence of the substituted groups induce a difference in physical and chemical properties of the co-polymer, it does not affect its symmetry group. We deduce from IR and RRS analyses of C_1-4PPV-ether, compared to those of PPV-ether, that the geometrical structural modifications with a decrease in the planarity of the chain and the vibrational features are possibly ascribed to conformational effects. The molecular packing effect is also outlined. Moreover, the decrease of the optical gap and the higher energy shifted transition could be assigned to the different lattice relaxations and vibrational behaviors shown by this co-polymer. Added to IR, RRS and optical density results, the PL measurements prove a great delocalization of π-electron system with a red shifted emission.     

Soluble electroluminescent poly(phenylene vinylene)s with balanced electron- and hole injections.

We report a new route for the design of efficient soluble electroluminescent PPV-based copolymers bearing electron-deficient oxadiazole (OXD) moieties on side chains. The introduction of OXD through a long alkylene spacer with PPV backbone provides a molecular dispersion of OXD in the film; both the side chain OXD and the main chain PPV do retain their own electron-transport and emissive properties, respectively. The use of phenylene vinylene derivatives with asymmetric and branched substituents and a long spacer provides solubility for ease of device fabrication as well as amorphous structure to allow a well-mixing of OXD groups with the main chains. By properly adjusting the OXD content through copolymerization, we can tailor the chemical structure of electroluminescent material to give a balance of hole- and electron injections for various metal cathodes, such that the quantum efficiency is significantly improved and the turn-on voltage is reduced for the devices with aluminum and calcium. For the device with calcium fabricated in open air, a maximum brightness of 15000 cd/m(2) at 15 V/100 nm and a maximum luminance efficiency of 2.27 cd/A can be obtained, respectively, about 30 times brighter and 9.4 times more efficient than those with the corresponding homopolymer, poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV). The use of physical blends to simulate the copolymers provides no significant improvement, since phase-separation structures appear, causing an inefficient utilization of OXD and sometimes voltage-dependent emission spectra. The present route permits a fabrication of single layer PLED with high brightness, high efficiency, and low turn-on voltage.     

Supramolecular assembly of poly(phenylene vinylene) with crown ether substituents to form nanoribbons

Poly(para-phenylene vinylene) with crown ether substituents (C-PPV) could form nanoribbons through supramolecular assembly with K(+) in dilute chloroform solution. The length of the nanoribbons increased with an increase in the standing time of the C-PPV/K(+) solution. Experimental evidence to support the interaction between K(+) and crown ether substituents was provided, and the growth mechanism of C-PPV/K(+) nanorods to nanoribbons was proposed. The influence of the length of the nanoribbons on the photophysics of C-PPV was also investigated.     

Syntheses of new poly (arylene vinylene) analogues: Poly (4,7-benzofuran vinylene) and poly (4,7-benzothiophene vinylene)

The reaction of bis(4,7-tetrahydrothiopheniomethyl) benzofuran dibromide with aqueous tetramethylammonium hydroxide leads to a water-soluble polyelectrolyte which can be film cast and thermolytically eliminated to give poly(4,7-benzofuran vinylene) (PBFV). Subjection of bis(4,7-tetrahydrothiopheniomethyl) benzothiophene dibromide to the same reaction sequence gives poly(4,7-benzothiophene vinylene) (PBTV). UV-VIS studies show that PBFV has a band gap of 2.76 eV, while PBTV has a band gap of 2.92 eV. These polymers are members of a new class of conjugated poly (arylene vinylene)s, in which heterocyclic pseudoaromatic rings are fused onto a poly(1,4-phenylene vinylene) backbone. © 1994 John Wiley & Sons, Inc.     

Electroluminescent properties of a partially‐conjugated hyperbranched poly(p‐phenylene vinylene)

In this paper, the electroluminescent properties of a new partially‐conjugated hyperbranched poly (p‐phenylene vinylene) (HPPV) were studied. The single layer light‐emitting device with HPPV as the emitting layer emits blue‐green light at 496 nm, with a luminance of 160 cd/m² at 9 V, a turn‐on voltage of 4.3 V and an electroluminescent efficiency of 0.028 cd/A. By doping an electron‐transport material [2‐(4‐biphenylyl)‐5‐phenyl‐1,3,4‐oxadiazole, PBD] into the emitting layer and inserting a thin layer of tris(8‐hydroxy‐quinoline)aluminum (Alq₃) as electron transporting/hole blocking layer for the devices, the electroluminescent efficiency of 1.42 cd/A and luminance of 1700 cd/m² were achieved. The results demonstrate that the devices with the hyperbranched polymers as emitting material can achieve high efficiency through optimization of device structures. Copyright © 2006 John Wiley & Sons, Ltd.     

Photoluminescent properties of poly(p‐phenylene vinylene thiophene‐co‐siloxane)

A new p‐phenylene–vinylene–thiophene‐based siloxane block copolymer has been synthesized. The copolymer consists of alternating rigid and flexible blocks. The rigid blocks are composed of phenylene–vinylene–thiophene‐based units, and the flexible blocks are derived from 1,3‐dialkyldisiloxane units. The former component acts as the chromophore, and allows fine tuning of band gap for blue‐light emission, while the latter imparts good solubility of the copolymer in organic solvents, and thus, should enhance processibility of the resulting copolymer. The thermal properties of the copolymer have been characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The photoluminescence (PL) of the copolymer in solution and in cast film has been studied. The effects of concentration on the PL intensity of the new copolymer in polymer blends with poly(methyl methacrylate) (PMMA) and poly(vinyl carbazole) (PVK) have also been described. Efficient energy transfer from PVK to the new block copolymer in the blended film was observed. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1450–1456, 2000     

Properties of single‐walled carbon nanotube‐based poly(phenylene vinylene) electroluminescent nanocomposites

Devices with varying concentrations of single‐walled carbon nanotubes (SWNTs) dispersed in three derivatives of poly(p‐phenylene vinylene) are prepared, and their electroluminescent properties evaluated. Increasing the concentration of SWNTs improves the electrical conductivity of the nanocomposites. However, an undesired increase in the electroluminescence (EL) turn‐on voltage is observed for the hybrids, possibly due to photoluminescence quenching of excitons by the SWNTs. At relatively low concentrations of SWNTs, there is an increase in the EL lifetime; in contrast, at relatively high concentrations of SWNTs, due to photoluminescence quenching by the nanotubes, significant reduction in brightness and faster degradation of the EL performance of the devices is observed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011