Microwave assisted novel synthetic route for polyfluorenes containing triphenylamine and solubilizing alkyl moiety for blue emitting diodes

A series of blue electroluminescent polyfluorenes (PFs) containing triphenylamine and various alkyl moieties were synthesized using an Ni(0) mediated C?C Yamamoto coupling reaction assisted by microwaves. The synthesized PFs were characterized by various spectroscopic techniques. Their absorption and photoluminescence properties were investigated in solvent and found to possess characteristic electronic absorption and emission spectra. These PFs were found to emit in the blue region (407?415 nm) with high quantum yield in the range 0.41?0.73. Cyclic voltammetry studies of the PFs revealed that the compounds were stable under redox conditions with highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital in the range 5.24–5.29 eV and 1.98?2.01 eV, respectively. The E_HOMO for the PFs was similar to the most widely used hole transporting materials N,N′‐Di(1‐naphthyl)‐N,N′‐diphenyl‐(1,1′‐biphenyl)‐4,4′‐diamine (NPD), N,N′‐Bis(3‐methylphenyl)‐N,N′‐diphenylbenzidine (TPD) and N²,N²,N²′,N²′,N⁷,N⁷,N⁷′,N⁷′‐octakis(4‐methoxyphenyl)‐9,9′‐spirobi[9H‐ fluorene]‐2,2′,7,7′‐tetramine, Spiro‐OMeTAD (spiro‐OMe‐TAD). The thermal stability observed for the PFs accounts for their use under ambient conditions. The electrochemical studies of the fabricated polymer light emitting diodes suggest that the PFs have potential to be used as hole transporting and blue electroluminescent materials for optoelectronic devices. © 2017 Society of Chemical Industry     

Green electroluminescent polyfluorenes containing 1,8-naphthalimide moieties as color tuner

A series of copolymers (CNPFs) containing low-band-gap 1,8-naphthalimide moieties as color tuner was prepared by a Yamamoto coupling reaction of 2,7-dibromo-9,9-dioctylfluorene (DBF) and different amount of 4-(3,6-dibromocarbazol-9-yl)-N-(4′-tert-butyl-phenyl)-1,8-naphthalimide (Br-CN) (0.05-1 mol% feed ratio). The light emitting properties of the resulting copolymers showed a heavy dependence on the feed ratio. In photoluminescence (PL) studies, an efficient color tuning through the Förster energy transfer mechanism was revealed from blue to green as the increase of Br-CN content, while in electroluminescence (EL) studies, the color tuning was found to go through a charge trapping mechanism. It was found that by introduction of a very small amount of Br-CN (0.1-0.5 mol%) into polyfluorene, the emission color can be tuned from blue to pure green with Commission International de l'Echairage (CIE) coordinates being (0.21, 0.42) and (0.21, 0.48). A green emitting EL single-layer device based on CNPF containing 0.1 mol% of Br-CN showed good performances with a low turn-on voltage of 4.2 V, a brightness of 9104 cd/m², the maximum luminous efficiency of 2.74 cd/A and the maximum power efficiency of 1.51 lm/W. To further improve the EL performances through balancing the charge trapping process, a copolymer (BCNPF05) derived from 0.5 mol% of a triarylamine-containing 4-{3,6-bis-[4″-(4?-bromophenyl-p-tolyl-amino)-phenyl]-carbazol-9-yl}-N-(4′-tert-butyl-phenyl)-1,8-naphthalimide (Br-BCN) and 99.5 mol% of 2,7-dibromo-9,9-dioctylfluorene was also prepared. As expected, a single layer EL device based on BCNPF05 exhibited better performances with a brightness of 14228 cd/m², the maximum luminous efficiency of 4.53 cd/A and the maximum power efficiency of 1.57 lm/W.     

Novel,blue light-emitting polyfluorenes containing a fluorinated quinoxaline unit

A series of fluorinated quinoxaline containing polyfluorenes were synthesized by Suzuki polycondensation. The polymer light emitting diodes based on the resulting copolymers emitted blue light, and no green emission was observed either in the photoluminescence spectra when the film had been annealed at 80–160 °C nor in the electroluminescence spectra when the current densities increased from 5 to 200 mA cm^?2. The fluorescence quantum yields decreased from 79% to 36% with increasing content of BPFQ from 1 to 20 mol%. Peak external quantum efficiency and luminous efficiency were 2.99% and 2.39 cd A^?1, respectively, with CIE coordinates of (0.16, 0.10).     

Green polymer-light-emitting-diodes based on polyfluorenes containing N-aryl-1,8-naphthalimide and 1,8-naphthoilene-arylimidazole derivatives as color tuner

A novel series of green light emitting single polymers were prepared by end-capping of N-aryl-1,8-naphthalimide and 1,8-naphthoilenearylimidazole derivatives into polyfluorene. The electroluminescence (EL) spectra of polymers (P1 ∼ P5) exhibit greenish-blue, bluish-green, pure green, and yellowish-green emission (λ_max = 465 nm, 490 nm, 500 nm, and 545 nm, respectively) from compounds (M1 ∼ M5). It was found that by the introduction of a small amount of compounds (M1 ∼ M5) (5 mol-%) into polyfluorene, the emission color can be tuned from the blue to green region. The color tuning was found to have gone through charge trapping and Förster energy transfer. The device of P4 emits pure green light with Commission Internationale de l'Eclairage (CIE) coordinates of (0.20, 0.41), and exhibits a maximum brightness of 11500 cd/m² at 12 V with a structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) [PEDOT:PSS]/PVK/emission layer/Ca/Ag. The device of P5 emits yellowish green light with Commission Internationale de l'Eclairage (CIE) coordinates of (0.36, 0.56), and exhibits a maximum brightness of 6534 cd/m² at 17 V.     

Photocrosslinkable hyperbranched polyfluorenes containing oxadiazole: synthesis,photophysics and electroluminescence

BACKGROUND: Crosslinkable hyperbranched polymers have great potential for use in organic light‐emitting devices and photovoltaics. With this in mind, two crosslinkable hyperbranched polyfluorenes, P5 and P6, containing oxadiazole as the core, and linear polyfluorene P0 were synthesized via palladium‐catalyzed Suzuki coupling. A device was also made. RESULTS: The resulting polymers were characterized using NMR, gel permeation chromatography, differential scanning calorimetry and thermogravimetric analysis. The spectral behavior of the polymers was also investigated. Compared with uncrosslinked P5 and P6, the photoluminescence spectra of crosslinked P5 and P6 were little influenced by the crosslinking. After annealing for 3 h at 200 °C in air, the photoluminescence of P6 (with a greater content of branched units) showed excellent stability compared with that of P5 and P0. The results for the device revealed that photocrosslinking improves the stability of the electroluminescence. CONCLUSION: The crosslinkable polymers obtained can be used in multilayer devices. Copyright © 2008 Society of Chemical Industry     

Improved electroluminescence efficiency of polyfluorenes by simultaneously incorporating dibenzothiophene-S,S-dioxide unit in main chain and oxadiazole moiety in side chain

A series of efficient and spectrally stable blue light-emitting polyfluorene derivatives containing 3,7-dibenzothiophene-S,S-dioxide (SO) unit in main chain and oxadiazole (OXD) moiety in the side chain were synthesized via Suzuki copolymerization. It was realized that the glass transition temperatures of the resulted copolymers PFSO-OXD increased gradually with the content of OXD, while the UV-vis absorption, photoluminescence spectra, as well as electrochemical properties were not significantly influenced by the molar ratio of OXD unit. Apparent solvatochromism of copolymers PFSO-OXD can be realized by varying polarity of solvents from toluene to dichloromethane. Light-emitting devices based on PFSO-OXD exhibited superior performances to those of PFSO and PF-OXD20 due to the more balanced charge carrier mobility of the devices. The electroluminescence spectra of all copolymers are independent with the current densities and thermal annealing. The best device performance was achieved based on PFSO-OXD20 with a maximal luminous efficiency of 4.9 cd A⁻¹ with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.16, 0.12). The results indicated that the strategy of concurrently incorporating SO and OXD unit into the main chain and side chain of polyfluorenes, respectively has great potential to achieve efficient blue light-emitting polymers.     

Novel green light-emitting polyfluorenes containing dibenzothiophene-S,S-dioxide-arylamine derivatives

Green light-emitting polyfluorenes were synthesized by Suzuki polycondensation via introducing dibenzothiophene-S,S-dioxide-(di-)tri-phenylamine (G₁ and G₂) moieties and dibenzothiophene-S,S-dioxide (SO) unit into the polyfluorene backbone, respectively. PF–SO–G copolymers show a high thermal stability and a moderate photoluminescence quantum yield in the range of 20–40%. The lowest unoccupied molecular orbital (LUMO) levels reduce with increasing the content of SO unit in the polymers. The efficient energy transfer from fluorene segment to G₁ or G₂ unit occurred in the PL process, and the EL emission peaked at about 510 nm was exclusively from G₁ or G₂ unit. Incorporating SO unit into the polymer backbone makes the device performances improved. The maximal luminous efficiency of 9.0 cd A^?1 with the CIE coordinates of (0.27, 0.56) was obtained for PF–SO15-G₂5 based on a single-layered device of ITO/PEDOT:PSS/polymer/CsF/Al. And the polymers (PF–SO15-G5)s exhibited a dramatic LE stability at high current densities, even though at the current density of 200 mA cm^?2, the luminous efficiencies only dropped 10%. SO unit lowers the LUMO level, balances the injection and transportation of both electron and hole in the polymers, and therefore improves the device performances. The hole- and electron-only devices show that the hole and electron flux are well balanced, which demonstrates that (PF-SO15-G5)s are bipolar polymers with a balanced charge carrier transport.     

Optical and electrochemical properties of polyfluorenes with pyridine–triphenylamine bipolar unit

Two new polyfluorenes were synthesized by Suzuki coupling polymerization, and their photophysical and electrochemical properties were studied. Both polymers contained the bipolar unit 4‐(2,6‐diphenylpyridin‐4‐yl)phenyl‐diphenylamine consisting of pyridine (Py) and triphenylamine (TPA) subunits. The bipolar unit was linked to the polymer chain via TPA or Py subunit, which affected the properties of the polymers. The polymers showed a weak intramolecular charge transfer character. They showed emission at 466 nm in solution and at 512‐538 nm in thin film. Their emission could be tuned by protonation and N‐alkylation of Py. Cyclic voltammetry experiments showed a quasi‐reversible oxidation process for both polymers. Their highest occupied molecular orbital levels were estimated at ? 5.13 and ? 5.21 eV. © 2012 Society of Chemical Industry     

Optimization of opto‐electronic property and device efficiency of polyfluorenes by tuning structure and morphology

Polyfluorene‐based oligomers and polymers (PFs) have been studied intensively as active materials for organic optoelectronic devices. In this review, the optimization of the opto‐electronic property and device efficiency of polyfluorenes in the field of light‐emitting diodes (LEDs) and photovoltaic cells (PVs) by tuning structure and morphology are summarized in terms of two typical modification techniques: copolymerization and blending. The relationships between molecular structures, thin film morphologies, opto‐electronic properties and device efficiencies are discussed, and some recent progress in LEDs and PVs is simultaneously reviewed. After the introduction, the basic knowledge of molecular structures and properties of polyfluorene homopolymers is presented as a background for a better understanding of their great potential for opto‐electronic applications. Immediately after this, three different opinions on the origin of low‐energy emission band at 520–540 nm in polyfluorene‐based LEDs are addressed. Rod–coil block copolymers and alternative copolymers are focused on in the next section, which are a vivid embodiment of controlling supramolecular structures and tailoring molecular structures, respectively. In particular, various supramolecular architectures induced by altering coil blocks are carefully discussed. Recent work that shows great improvement in opto‐electronic properties or device performance by blending or doping is also addressed. Additionally, the progress of understanding concerning the mechanisms of exciton dynamics is briefly referred to. Copyright © 2006 Society of Chemical Industry     

Phosphorescent chelating polyelectrolytes and their neutral precursors: Synthesis, characterizations, photoluminescence and electroluminescence

Novel chelating polyfluorene polyelectrolytes and their corresponding neutral precursors with Ir complex incorporated into polymer backbone were synthesized by Suzuki polycondensation reaction. The aminoalkyl or quaternized ammonium group provides good solubility of these phosphorescent polymers in alcohols which has a great advantage in fabrication of multi-layer polymer phosphorescent PLEDs. The photophysical and electroluminescent properties of the phosphorescent polyelectrolytes and their neutral precursors were investigated. The phosphorescent emission bands from both polyelectrolytes and neutral polymers which lie at around 640 nm displayed a saturated red emission. Devices fabricated from these phosphorescent polymers with air-stable high work function metals, such as Al, as cathode showed comparable device performance with low work function metals, such as Ba.     

Synthesis and gas permeability of new polynorbornene dicarboximide with fluorine pendant groups

The new exo-N-3,5-bis(trifluoromethyl)phenyl-7-oxanorbornene-5,6-dicarboximide (TFmPhONDI, 2), was synthesized and polymerized via ring opening metathesis polymerization (ROMP) using tricyclohexylphosphine [1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazolilydene][benzylidene] ruthenium dichloride (I) to produce the corresponding PTFmPhONDI (3). Gas permeability, diffusion and solubility coefficients of PTFmPhONDI (3) were determined by transient permeation for five gases He, CO₂, O₂, N₂ and CH₄. The larger gas permeability and diffusion coefficients of 3 compared to polynorbornene dicarboximides without fluorine pendant groups were attributed to a lower polymer chain packing due to the effect of the CF₃ groups in the lateral phenyl moiety pending at positions 3 and 5.     

Novel white-light-emitting polyfluorenes with benzothiadiazole and Ir complex on the backbone

Novel white-emitting polyfluorenes were synthesized by mixing fluorescence and phosphorescence emission. Benzothiadiazole(BT) and iridium(III)bis(2-(1-naphthalene)pyridine-C^2′,N)-2,2,6,6-tetramethyl-3,5-heptanedione[(1-npy)₂Ir(tmd)] units were incorporated into polyfluorene backbone as green and red chromophores by Suzuki polycondensation. The device from PFG03-IrR07 shows a maximum luminous efficiency (LE) of 5.3 cd/A, a maximum luminance of 9900 cd/m² at a current density of 453 mA/cm² and a CIE coordinate of (0.32, 0.34) with the configuration: ITO/PEDOT:PSS/PVK/emissive layer/CsF/Al. Besides, the EL efficiencies decline slightly with increasing the current density. All emissions located very close to the equi-energy white point (0.33, 0.33) when applied voltage change from 9 to 14 V. Furthermore, the white emission of devices based on these materials shows very good color quality, with high color rendering index range between 84 and 89. Our results indicate that, by incorporation of singlet and triplet species into polymer backbone, the obtained white-emitting materials and devices are promising candidates for display and solid-state-lighting purpose.     

Novel yellow phosphorescent iridium complexes containing a carbazole-oxadiazole unit used in polymeric light-emitting diodes

Yellow iridium complexes Ir(PPOHC)₃ and (PPOHC)₂Ir(acac) (PPOHC: 3-(5-(4-(pyridin-2-yl)phenyl)-1,3,4-oxadiazol-2-yl)-9-hexyl-9H-carbazole) were synthesized and characterized. The Ir(PPOHC)₃ complex has good thermal stability with 5% weight-reduction occurring at 370 °C and a glass-transition temperature of 201 °C. A polymeric light-emitting diode using the Ir(PPOHC)₃ complex as a phosphorescent dopant showed a luminance efficiency of 16.4 cd/A and the maximum external quantum efficiency of 6.6% with CIE coordinates of (0.50, 0.49). A white polymeric light-emitting diode was fabricated using Ir(PPOHC)₃ which showed a luminance efficiency of 15.3 cd/A, with CIE coordinates of (0.39, 0.44). These results indicate that the iridium complexes containing a linked carbazole-oxadiazole unit are promising candidates in high-efficiency electroluminescent devices.     

Ultraviolet/blue light-emitting diodes based on single horizontal ZnO microrod/GaN heterojunction

We report electroluminescence (EL) from single horizontal ZnO microrod (MR) and p-GaN heterojunction light-emitting diodes under forward and reverse bias. EL spectra were composed of two blue emissions centered at 431 and 490 nm under forward biases, but were dominated by a ultraviolet (UV) emission located at 380 nm from n-ZnO MR under high reverse biases. Light-output-current characteristic of the UV emission reveals that the rate of radiative recombination is faster than that of the nonradiative recombination. Highly efficient ZnO excitonic recombination at reverse bias is caused by electrons tunneling from deep-level states near the n-ZnO/p-GaN interface to the conduction band in n-ZnO.     

Novel network polymer electrolytes containing fluorine and sulfonic acid lithium prepared by ultraviolet polymerization

Poly(vinyl alcohol) (PVAL) and vinyl acetate‐vinyl alcohol copolymers (VAVAL) were esterified with 3,5‐dinitrobenzoyl chloride using the cycled urea N,N′‐dimethylpropyleneurea (1,3‐dimethyl‐3,4,5,6‐tetrahydro‐2(1H)‐pyrimidinone) (DMPU) as the solvent. Vinyl alcohol‐vinyl‐3,5‐dinitrobenzoate copolymers (VALVDNB) and vinyl acetate‐vinyl‐3,5‐dinitrobenzoate copolymers (VAVDNB) were obtained. High degrees of esterification were obtained when PVAL was esterified (86%). The degree of transformation was determined by ¹H‐NMR as well as by chemical analysis, and the structure of the resulting polymers by means of IR spectroscopy and ¹H‐ and ¹³C‐NMR. The microstructure of PVA, PVAL, VAVAL copolymers and VALVDNB copolymers were determined from ¹H‐ and ¹³C‐NMR techniques. The sequence distributions for VAVAL copolymers prepared by base‐catalyzed transesterification of PVA were blocky, while the distributions were close to random for VALVDNB copolymers obtained by esterification of PVAL. Thermal properties were studied by DSC. The T_g values of VAVAL, VALVDNB, and VAVDNB copolymers as a function of copolymer compositions were determined. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Hydrophobic methacrylic copolymers containing azobenzene moieties

The functionalization of polymer materials to produce hydrophobic surfaces is an important goal for a number of applications, especially those associated with self-cleaning and anti-adherent surfaces. Azopolymers are known for photoisomerization property that can lead to photoinduced anisotropy, photomechanical effect and surface modification with surface-relief gratings. In this study, we combine the low surface energy property of perfluoroalkyl methacrylates with the photoinduced characteristics of azopolymers, by fabricating cast films of copolymers of 2,2,2 trifluorethyl methacrylate (TFEMA) or 2,2,3,3,4,4,5,5 octafluoropentyl methacrylate (OFPMA) and 4′-[N-ethyl-N-(2methacryloxy-ethyl)]amine-4-nitro-azobenzene (DR13MA). The intended structures of the copolymers synthesized using radicalar polymerization was confirmed with Fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance (NMR). The polymers had much higher thermal stability than conventional azopolymers, and formed hydrophobic surfaces with a water contact angle of ca. 96°. These hydrophobic azopolymers were amenable to the formation of surface-relief gratings at room temperature resulting from an all-photonic mass transport process, which opens the way for a number of new applications to be designed.     

Highly efficient polymer blends from a polyfluorene derivative and PVK for LEDs

The photophysical and electroluminescent properties of blends of a polyfluorene derivative of the PPV type, poly[(9,9-dihexyl-9H-fluorene-2,7-diyl)-1,2-ethenediyl-1,4-phenylene-1,2-ethenediyl] (labeled as LaPPS16) and poly(vinylcarbazole) - PVK are presented and discussed in terms of the operating light emission mechanisms. Static and dynamic fluorescence measurements and morphology data showed a powerful exciton migration from the host (PVK) to the guest (LaPPS16) resulting in emission coming from solely LaPPS16, even when in concentrations small as 1%. Electroluminescence was greatly enhanced with the blending; increases of 18 times in efficiency and 20 times in luminance were achieved in the blend containing 20% LaPPS16, with 3 V applied voltage.     

二苯乙烯基双二甲基苯胺蓝色荧光化合物的合成及电致光谱的研究

以二溴二甲苯和间甲基二苯胺为起始原料,经过乌尔曼,威斯麦尔,Wittig-horner反应,最终生成目标产物二苯乙烯基双二甲基苯胺。并对该化合物进行了器件研究,结果发现该化合物是一类优良的蓝色荧光材料。     

Significant improvement on the electroluminescence characteristics of MEH-PPV by blending with PMMA

A significant improvement on the electroluminescence threshold voltage from 5.5 volts down to 2.9 volts of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) in the Ca/polymer/ITO diode is observed by incorporating a non-conjugated PMMA into MEH-PPV. The threshold voltage of the material decreases with the poly(methyl methacrylate) (PMMA) content. Near-field scanning absorption and photoluminescence micrographs show that the PMMA domains dispersed in the continuous MEH-PPV phase appear to be similar to sesame seeds embedded in the MEH-PPV pancakes. The resistivity of PMMA is two orders lower than that of MEH-PPV. The excellent electrical conduction of PMMA in Ca/PMMA/ITO glass devices is not due to a pin-hole defect mechanism but may result from the electron hopping from Ca through the carbonyl groups in the PMMA. The significant improvement in the threshold voltage may be due to the combination of the electron hopping in the thin PMMA sesame seeds and the thinner thickness effect of MEH-PPV in the light emitting layer.     

Electron-withdrawing groups and strong-field ligands containing iridium(III) complexes and their efficient blue light-emitting

Two new heteroleptic iridium(III) complexes [Ir(4,6-dfppy)₂(PPh₃)L] (4,6-dfppy = 2-(4,6-difluorophenyl)pyridyl, PPh₃ = triphenylphosphine, L = NCS^?, 1; NCO^?, 2) have been synthesized and fully characterized. By introduction of electron-withdrawing groups such as fluorine atoms on the 4- and 6-positions of 2-phenylpyridyl (ppy) and using strong-field ligands for instance PPh₃ and pseudohalogen as ancillary ligands, the HOMO–LUMO electronic energy gaps of 1 and 2 have been increased sufficiently. The photoluminescence (PL) spectra of 1 and 2 in solution show emission maxima at 456 and 458 nm, respectively, corresponding to efficient blue light-emitting. X-ray analyses show that intra- and intermolecular π–π interactions exist in the solid state of 1 and 2. The PL spectra of 1 and 2 in solid state exhibit about 30 nm spectral red shifts compared with those in solution.