Synthesis and properties of various PPV derivatives with phenyl substituents

New electroluminescent polymers with various phenyl groups, poly[2-dimethyl(octyl)silyl-5-(4-(dimethyl(octyl)silyl)phenyl)-1,4-phenylenevinylene] (P1), poly[2,5-bis(4-(dimethyl(octyl)silyl)phenyl)-1,4-phenylenevinylene] (P2), poly[2,5-bis(9,9-dihexylfluorenyl)-1,4-phenylenevinylene] (P3), and poly[2,5-bis(4-(4-(2-etylhexyloxy)phenyl)phenyl)-1,4-phenylenevinylene] (P4), have been synthesized by the Gilch polymerization. The maximum absorption peaks of P1-P4 appeared at 388-423 nm in THF solution, and are red-shifted to 404-425 nm in solid thin film. The photoluminescence (PL) emission spectra of P1-P4 show a maximum peak at 482-503 nm in THF solution and at 521-549 nm as the solid film state. The emission spectra in the solid film state are more red-shifted over 40 nm, and the full width at half maximum (fwhm) was 30 nm greater than the solution conditions. The polymer light-emitting diodes (PLEDs) with the configuration of ITO/PEDOT/polymer/Al emitted light with maximum peaks at around 517-546 nm. The various phenyl substituents, with intermolecular interactions in the solid film state, can introduce the color tuning and device performance enhancement of the conjugated polymer as an emissive layer in PLED.     

Synthesis and electroluminescent properties of copolymers based on PPV with fluoro groups in vinylene units

New electroluminescent copolymers with fluoro groups in vinylene unit, poly(2-ethylhexyloxy-5-methoxy-p-phenylenevinylene-co-2-dimethyloctylsilyl-p-phenylenedifluorovinylene) (MEH-PPV-co-DMOS-PPDFV) and poly(2-dimethyloctylsilyl-p-phenylenevinylene-co-2-dimethyloctylsilyl-p-phenylenedifluorovinylene) (DMOS-PPV-co-DMOS-PPDFV), have been synthesized by the Gilch polymerization. The fluoro groups were introduced on vinylene units to increase the electron affinity of the copolymers. In both types of the copolymers, the band gaps were increasing with higher ratio of DMOS-PPDFV. As compared to the PL spectra in solution state, PL spectra in solid state and EL spectra of both types of copolymers showed red shifted and broad peaks caused by the aggregation. The polymer LEDs of MEH-PPV-co-DMOS-PPDFVs emit light with maximum peaks at around 538-577 nm. By adjusting the feed ratios of DMOS-PPDFV in the copolymers, we could tune the emission colors from orange yellow to green. DMOS-PPV-co-DMOS-PPDFVs showed the maximum EL peaks at about 514-543 nm. DMOS-PPV-co-DMOS-PPDFV with 1:9 feed ratio showed the highest luminescence efficiency of 1.31 lm/W.     

Fast response single‐ion transport light‐emitting electrochemical cell based on PPV derivative

We present the electrical and optical characteristics of a single‐ion transport light‐emitting electrochemical cell (SLEC) based on poly(p‐phenylene vinylene) (PPV) derivative containing aryl‐substituted oxadiazole in the backbone (MEH‐OPPV). Ionized polyurethane–poly(ethylene glycol) (PUI) used as polymer electrolyte is introduced into the active layer of the SLEC. The turn‐on voltage of the SLEC is about 3 V according to its current density–voltage (J–V) characteristics. The response time of the SLEC is less than 10 ms, lower than that of normal LECs by two orders of magnitudes roughly. The reasons of the quick response for the SLEC are discussed in the article. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 4253–4255, 2006     

Novel photoluminescent polymers containing fluorene and 2,4,6-triphenyl pyridine moieties: Effects of noncoplanar molecular architecture on the electro-optical properties of parent matrix

In order to investigate the electro-optical property variations among the target topology-varied polymers with diverse molecular architectures, a series of polymers (P0-P2), having sterically hindered side chains, based on fluorene and pyridine derivatives were successfully synthesized via simple synthetic route and fully characterized via various spectroscopic methods and cyclic voltammetry measurements. The experiments demonstrate that all polymers with almost equivalent absorption and photoluminescence emission properties take on a wide band gap with blue emission in THF solution and thin solid film. However, their energy levels derived from the onset of electrochemical potential in thin solid films exhibit significant differences, which are considered to be due to the branched molecular frameworks on the basis of theoretical calculations. It is demonstrated that the electro-optical properties of wide band gap rigid noncoplanar copolymers are tunable by the sterically hindered pyridine moieties, and these experiments can serve as an excellent exploratory example for fine band gap and photophysics control principle in the p-n heterostructure polymers through the partial planarization of the π-system.     

Synthesis and photophysical properties of a charm-bracelet type C60-grafted PPV derivative

Synthesis and photophysical properties of a new soluble C₆₀-grafted PPV polymer have been described. Fluorescence quenching of the PPV moiety in C₆₀-grafted PPV polymer was observed, suggesting the energy- and/or electron transfer within the polymer. The fluorescence decay profiles at around 500 nm of this polymer both in chloroform and benzonitrile display a single exponential decay giving the fluorescence lifetimes of 1.10-1.27 ns, which are slightly shorter than that of PPV (about 1.50 ns). The nanosecond transient absorption band of C₆₀-grafted PPV polymer was observed at 740 nm corresponded to the excited triplet state of C₆₀, which decreased in benzonitrile, indicating the existence of the extra decay path of the singlet excited state of the C₆₀ other than intersystem crossing. Upon heating this polymer exhibits typical semilunar liquid crystalline texture, being closely associated with its polymeric structure with long solubilizing alkoxy side chains.     

High-performance poly(2,3-diphenyl-1,4-phenylene vinylene)-based polymer light-emitting diodes by blade coating method

A new series of super high brightness and luminance efficient poly(2,3-diphenyl-1,4-phenylene vinylene) (DP-PPV)-based electroluminescent (EL) polymers containing methoxy or long branched alkoxy chains were synthesized via Gilch polymerization. The branched alkoxy groups were introduced to enhance solubility for blade and spin-coating processes. Monomers of DMeO-PPV and m-Ph-PPV were used to increase steric hindrance and prevent close packing of the main chain. By controlling the feeding ratio of different monomers during polymerization, DP-PPV derivatives with high molecular weight were obtained. All synthesized polymers possess high glass transition temperatures and thermal stabilities. The maximum photoluminescent emissions of the thin films are located between 544 and 547 nm. Cyclic voltammetry analysis reveals that the band gaps of these light-emitting materials are in the range of 2.75-2.84 eV. Blade coating was used to fabricate multilayer polymer light-emitting diodes. A multilayer electroluminescent device with the configuration of ITO/PEDOT:PSS/TFB/P1/TPBi/LiF/Al exhibited a very high luminescence efficiency (10.96 cd A⁻¹). The maximum brightness of the multilayer EL device ITO/PEDOT:PSS/TFB/P3/CsF/Al reached up to 78,050 cd m⁻² with a low turn-on voltage (4.0 V). For further investigation, polymer P3 was blended with DPPFBNA to achieve white light-emitting device; the multilayer devices generated a maximum brightness of 1085 cd m⁻² and a luminance efficiency of 0.75 cd A⁻¹, with CIE coordinates (0.28, 0.33) at 11 V.     

The thermal conversion reaction of sulphonyl substituted poly(para-xylylene): evidence for the formation of PPV structures

The thermal elimination of the n-octyl-sulphonyl group from poly[1,4 phenylene (1-n-octyl-sulphonyl)-1,2-ethylene] and the phenyl-sulphonyl group from poly[1,4 phenylene (1-phenyl-sulphonyl)-1,2-ethylene] is evaluated by different analytical techniques: thermal gravimetric analysis, direct insert probe mass spectrometry, in situ ultraviolet-visible spectroscopy and in situ Fourier transform infra red spectroscopy. This thermal treatment yields PPV with improved optical properties compared to the conventional PPV concerning UV-absorption and photoluminescence efficiencies. The results obtained are consistent with the formation of a PPV with a more or less restricted conjugation length compared to the PPV material obtained from the conversion of a sulphinyl precursor polymer.     

PPV类聚合物光电材料的研究进展

聚对苯乙炔(PPV)类聚合物是目前研究最热也是最早应用于聚合物显示的一类材料,其大的共轭体系不仅有利于电子的注入、载流子的迁移,也有利于降低驱动电压,提高材料的荧光量子效率。主要对聚对苯乙炔类聚合物结构的修饰改性及其性能进行了综述,对PPV类聚合物的应用领域及其前景进行了展望。     

Synthesis and magnetic properties of polydiphenylamine derivative bearing stable radical groups

Poly[4,4′-diphenylamine (3,5-di-tert-butyl-4-hydroxy)benzylidene] was synthesized with a dehydrative polycondensation by using H₂SO₄ as a catalyst. After treatment with DDQ, the polymer was oxidized with PbO₂ to generate a stable radical group in the polymer. The colour of the polymer changed from green to dark red upon the oxidation. ESR spectra showed not only a signal with a g value of 2.004, but also a so-called ‘half-field’ signal with a g value of 4.288.     

Blue light-emitting diodes from mesogen-jacketed polymers containing oxadiazole units

A series of novel mesogen-jacketed polymers, poly{2,5-bis[(5-tert-butylphenyl)-1,3,4-oxadiazole]styrene} (P-Ct) and poly{2,5-bis[(5-alkoxyl-phenyl)-1,3,4-oxadiazole]styrene} (P-OCn, n is the number of carbons in the alkoxyl groups, n = 6, 8, 10, 12, 14), were incorporated into polymer light-emitting device as the light-emitting layers. All of the polymers had excellent solubility in common organic solvents. The UV and PL spectra for P-Ct were 312 and 400 nm while for P-OCn they were around 328 and 420 nm in the film. There was no red shift in the PL spectrum of P-Ct after heat treatment which maybe related to the jacketed structure. All of the polymers showed high PL quantum yields in THF solution. From the cyclic voltammetry measurement at room temperature, HOMO levels of the polymers were varied from −5.66 to −6.12 eV, LUMO levels were varied from −2.45 to −2.81 eV. Furthermore, the influence of the alkoxyl lengths in four different device configurations was compared. For the device configuration c: ITO/PVK/P-OC8/Ca/Ag, the maximum luminance and maximum external quantum efficiency were obtained for P-OC8 which maybe related to the easier electron injection and transport. Efficiency of P-OC8 was not improved with device d which proved the electron transporting characteristics of P-OC8.     

Benzotrifluoromethyl group‐substituted poly(para‐phenylenevinylene): Effect on solubility,optical, and electronic properties

A new organosoluble benzotrifluoromethyl group containing poly(p‐phenylenevinylene) (BTFM‐PPV) has been synthesized via Gilch polymerization. The polymer is soluble in common organic solvents such as tetrahydrofuran, chloroform, dichloromethane, toluene, and xylene. BTFM‐PPV exhibited fluorescence emission peak with a very high blue shift at 474 nm with an excitation wavelength at 420 nm compared with many other PPV derivatives reported earlier. Incorporation of fluorated bezotrifluoromethyl pendent group in the PPV backbone lowers the HOMO and LUMO energy levels of BTFM‐PPV (2.48 eV) which retarded the hole injection and increase the electron injection in the device. The current–voltage (I–V) characteristic of the polymer was measured by fabricating the polymer as ITO/BTFM‐PPV/Al diode. The device performance was markedly improved by incorporation of 4‐fluoro‐3trifluoromethylphenyl units into the polymer main chain. The turn on voltage of the device observed from the I–V measurements was 7 V. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Fabrication and characterization of WO3/Ag/WO3 multilayer transparent anode with solution-processed WO3 for polymer light-emitting diodes

ABSTRACT: The dielectric/metal/dielectric (DMD) multilayer is suitable for a transparent electrode because of its high optical and high electrical properties; however, it is fabricated by an expensive and inefficient multistep vacuum process. We present a WO3/Ag/WO3 (WAW) multilayer transparent anode with solution-processed WO3 for polymer light-emitting diodes (PLEDs). This WAW multilayer not only has high transmittance and low resistance but also can be easily and rapidly fabricated. We devised a novel method to deposit a thin WO3 layer by a solution process in an air environment. A tungstic acid solution was prepared from an aqueous solution of Na2WO4 and then converted to WO3 nanoparticles (NPs) by a thermal treatment. Thin WO3 NP layers form WAW multilayer with a thermal evaporated Ag layer, and they improve the transmittance of the WAW multilayer because of its high transmittance and refractive index. Moreover, the surface of the WO3 layer is homogeneous and flat with low roughness because of the WO3 NP generation from the tungstic acid solution without aggregation. We performed optical simulation and experiments, and the optimized WAW multilayer had a high transmittance of 85% with a sheet resistance of 4 /sq. Finally, PLEDs based on the WAW multilayer anode achieved a maximum luminance of 35550 cd/m2 at 8 V, and this result implies that the solution-processed WAW multilayer is appropriate for use as a transparent anode in PLEDs.     

The influence of type-I and type-II triplet multiple quantum well structure on white organic light-emitting diodes

We demonstrate high-efficient white organic light-emitting diodes (WOLEDs) based on triplet multiple quantum well (MQW) structure and focus on the influence on WOLEDs through employing different potential barrier materials to form type-I and type-II MQWs, respectively. It is found that type-I MQW structure WOLEDs based on 1,3,5-tris(N-phenyl-benzimidazol-2-yl)benzene as potential barrier layer (PBL) offers high electroluminescent (EL) performance. That is to say, maximum current efficiency and power efficiency are achieved at about 1,000 cd/m² with 16.4 cd/A and 8.3 lm/W, which increase by 53.3% and 50.9% over traditional three-layer structure WOLEDs, respectively, and a maximum luminance of 17,700 cd/m² is earned simultaneously. The achievement of high EL performance would be attributed to uniform distribution and better confinement of carriers within the emitting layer (EML). However, when 4,7-diphenyl-1,10-phenanthroline or 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline is used as PBL to form type-II MQW structure, poor EL performance is obtained. We attribute that to improper energy level alignment between the interface of EML/PBL, which leads to incomplete confinement and low recombination efficiency of carriers, a more detailed mechanism was argued.     

PPV/TiO2 hybrid composites prepared from PPV precursor reaction in aqueous media and their application in solar cells

In this work, poly(phenylene vinylene) (PPV) and TiO₂ nanocomposites containing different amounts of TiO₂ were prepared through PPV precursor reaction in aqueous media. The TiO₂ components were introduced into the systems by two methods, i.e. through in situ sol-gel reaction or by mixing commercially available TiO₂ nanoparticles with the PPV precursor before reaction. The composite prepared by mixing commercially available TiO₂ nanoparticles shows perfect crystal character of the anatase TiO₂, but TiO₂ particles severely agglomerate in the PPV matrix. The composite prepared by introducing TiO₂ nanoparticles through the sol-gel reaction shows uniform nanoscale dispersion of anatase TiO₂ in PPV matrix. The UV-vis and FL spectroscopic analyses confirm the formation of the TiO₂/PPV composites and reveal the enhanced PL quenching effect as the TiO₂ content increases. The PPV/TiO₂ composites can show significant photovoltaic response. Better photovoltaic performance is observed for the solar cells prepared by using the in situ sol-gel reaction method.     

Synthesis,Characterization, and Electroluminescent Properties of Iridium(III) 2-Phenylpyridine-type Complexes Containing Trifluoromethyl Substituents and Various Main-Group Moieties

New heteroleptic cyclometalated iridium(III) 2-phenylpyridine-type complexes with trifluoromethyl substituents and various main-group moieties were synthesized and their photophysical, electrochemical, and electroluminescent (EL) properties studied. The emission color can be tuned by a facile derivatization of the phenyl moiety of 2-phenylpyridine with various main-group moieties, and we have prepared new yellowish green to orange triplet emitters with enhanced charge injection/charge transporting features, which can furnish attractive EL performance in phosphorescent organic light-emitting devices (OLEDs). Attempts were also made to fabricate two-color white-light OLEDs based on a combination of fluorescent blue and phosphorescent orange emitters.     

Synthesis and properties of optically active substituted polyacetylenes having carboxyl and/or amino groups

Polyacetylenes having carboxyl and/or amino groups in the side chain were synthesized by the polymerization of N-(2-propynyloxycarbonyl)-l-alanine (1) and l-alanine N-propargylamide (2) catalyzed with a rhodium cation complex. Poly(1_0.5-co-2_0.5) exhibited a larger CD signal than the homopolymers. The polymer mixtures obtained by the polymerization of 1 in the presence of poly(2), and those obtained by the polymerization of 2 in the presence of poly(1) showed specific rotations larger than calculated. The polymerization of propargylamine in the presence of poly(1) did not exhibit significant effect, while the polymer mixtures obtained by the polymerization of propiolic acid in the presence of poly(2) exhibited [α]_D of positive sign, although poly(2) alone exhibited [α]_D of negative sign.     

An optimal spectral model for phosphor-converted white light-emitting diodes used in the mesopic vision

White light-emitting diodes (WLEDs) for road lighting are required to have both the high scotopic to photopic ratio (S/P) and color rendering index (CRI). However, there is a trade-off between S/P and CRI, and WLEDs commonly having the S/P of 1.68-2.38 usually exhibit a low CRI. In this work, to provide a best solution to the trade-off problem we proposed an optimal spectral model for the phosphor-converted WLEDs (pc-WLEDs) aiming to figure out the optimal phosphor combination. The Monte Carlo Algorithms combined with the Genetic Algorithm were adopted to obtain the optimization of CRI and S/P by varying the spectral power distributions of WLEDs through adjusting the spectral parameters. Considering the spectral requirements of pc-WLEDs based on the mesopic vision, we chose CaAlSiN₃:Eu²⁺ and Y₃(Ga,Al)₅O₁₂:Ce³⁺ as the red- and green-emitting phosphors to prepare WLEDs with both high S/P and CRI, respectively. The simulation based on the optimal spectral model led to an optimal pc-WLED with a high S/P of 2.0-2.14, Ra > 80 and correlated color temperature (CCT) of 4000-5000 K, which matches very well with the experimental results of S/P = 2.064, Ra = 93.9, and CCT = 4981 K for the two-phosphor converted WLEDs. It implies that the optimal spectral model would be used effectively for the spectral design and phosphor selection for WLEDs.     

Electrospray deposition of polymer thin films for organic light-emitting diodes

Electrospray process was developed for organic layer deposition onto polymer organic light-emitting diode [PLED] devices in this work. An electrospray can be used to produce nanometer-scale thin films by electric repulsion of microscale fine droplets. PLED devices made by an electrospray process were compared with spin-coated ones. The PLED device fabricated by the electrospray process showed maximum current efficiency of 24 cd/A, which was comparable with that of the spin-coating process. The electrospray process required a higher concentration of hole and electron transport materials in the inks than spin-coating processes to achieve PLED maximum performance. Photoluminescence [PL] at 407 nm was observed using electrosprayed poly(N-vinyl carbazole) films, whereas a peak at 410 nm was observed with the spin-coated ones. Similar difference in peak position was observed between aromatic and nonaromatic solvents in the spin-coating process. PLED devices made by the electrospray process showed lower current density than that of spin-coated ones. The PL peak shift and reduced current of electrosprayed films can therefore be attributed to the conformation of the polymer.     

Binary solvent mixture-induced crystallization enhancement for a white emissive polyfluorene copolymer toward improving its electroluminescence

The morphology of fluorescent polyfluorene (PF) derivatives has strong effects on their electroluminescent (EL) performance. Herein, the influence of processing solvents on the morphology of PF-based white emissive polymer and its correlation to the EL behavior is studied. It is found that the PF copolymer films prepared from chlorobenzene:toluene (CB:TOL) solvent mixtures show stronger tendency to form crystalline α phase PF upon thermal annealing than those prepared from pure TOL or CB solvents. The evaporation rate difference in solvent mixtures can assist in the formation of crystal nucleus and enhance the crystallinity of PF backbone after thermal annealing. The results also reveal that the solvent mixture-processed fluorescent PF-based white emissive polymer not only shows more efficient blue emission, but also more balanced electron and hole transport. Accordingly, both the white emission purity and the light-emitting efficiency of the light-emitting diodes based on PF-based white emissive polymer are greatly enhanced.     

Theoretical analysis on the electronic structures and properties of PPV fused with electron-withdrawing unit: Monomer, oligomer and polymer

The geometries and electronic properties of five novel co-polymers which are obtained by replacing phenylene ring in poly(p-phenylenevinylene) (PPV) with five different fused heterocycles, as well as their corresponding monomers and co-oligomers, have been studied using DFT method. The dihedral angle, bond length, bond-length alternation, electron density at bond critical points (BCPs), nucleus-independent chemical shift (NICS) and Wiberg bond index (WBI) are analyzed and correlated with conduction properties. The bond length between the CC linkage and the aromatic ring unit in the studied compounds is decreased and the double bond character is increased along with main chain extension. The changes of BCPs, WBIs and NICSs also show that the conjugational degree is increased with main chain extension and the electronic properties of the studied compounds vary systematically with the electron-withdrawing capacity of the fused heterocycles. The changes of NICS also show that the conjugation in central section is stronger than that in outer section. The band structure analysis shows that the energy gaps of the five novel polymers (in the range of 0.17–0.77 eV) are all much lower than that of PPV (1.3 eV). The proposed new coplanar conjugated polymers may be potential conductors.