Polyfluorene Research Articles

Polarized electroluminescent devices based on ultrathin α-sexithiophene on oriented β-phase polyfluorene films

White polarized light emitting devices are produced by combining oriented β-phase polyfluorene (PFO) films with α-sexithiophene (6T) on top. Highly oriented PFO films are fabricated with the friction transfer method and exposed to toluene to partially change their conformation to the metastable β-phase PFO without loss of orientation. Orange emitting 6T is then deposited by the conventional vacuum deposition method on top of the blue emitting oriented PFO films. We studied the effects of 6T thickness on the emission color and the results indicated that whitening is possible by depositing 6T at a thickness of 1.5 nm on β-phase PFO. High polarization of both PFO and 6T emitting regions in the electroluminescence spectra suggested that 6T molecules are oriented parallel to the PFO molecules.

Implications of the structure for the luminescence properties of NBR–PF blend devices nanostructured as Langmuir–Blodgett films

Properties of polymers in blends can be enhanced if molecular control is achieved. In this paper, we spread nitrile rubber (NBR) and polyfluorene (PF) on the air–water interface to form stable monolayers, as observed with surface pressure–area isotherms and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). Mixed monolayers showed that the isotherms were shifted to lower areas in relation to the areas predicted for an ideal mixture, which indicates attractive interactions between the polymeric constituents. Also, the transfer of the polymeric films to solid supports was possible using the Langmuir–Blodgett (LB) technique and the optical properties of the films were evaluated with infrared, UV–vis and fluorescence spectroscopies. Molecular-level interactions between the components lead to marked changes in the properties of the films, which exhibited optical (photoluminescence) properties that could be controlled at the molecular level.

Ratiometric Luminescent Detection of Bacterial Spores with Terbium Chelated Semiconducting Polymer Dots

We report a ratiometric fluorescent sensor based on semiconducting polymer dots chelated with terbium ions to detect bacterial spores in aqueous solution. Fluorescent polyfluorene (PFO) dots serve as a scaffold to coordinate with lanthanide ions that can be sensitized by calcium dipicolinate (CaDPA), an important biomarker of bacterial spores. The absorption band of PFO dots extends to deep UV region, allowing both the reference and the sensitizer can be excited with a single wavelength (~275 nm). The fluorescence of PFO remains constant as a reference, while the Tb(3+) ions exhibit enhanced luminescence upon binding with DPA. The sharp fluorescence peaks of β-phase PFO dots and the narrow-band emissions of Tb(3+) ions enable ratiometric and sensitive CaDPA detection with a linear response over nanomolar concentration and a detection limit of ~0.2 nM. The Pdots based sensor also show excellent selectivity to CaDPA over other aromatic ligands. Our results indicate that the Tb(3+) chelated Pdots sensor is promising for sensitive and rapid detection of bacterial spores.

Enhancement of external quantum efficiency through steric hindrance of phenazine derivative for white polymer light-emitting diode materials

Copolymers introducing dithienyl-benzophenazine (<0.1mol%) as a dopant have been synthesized on a polyfluorene (PF) backbone based on the Suzuki coupling reaction. The UV–vis spectra of polymers showed similar behaviors in the solution and on the film. However, PL spectra were similar to that of PF in solution, but the peak around 615nm increased as the amounts of dithienyl-benzophenazine were increased in the casting film. Compared with PF-PDTQ05, PF-FDTQ series showed red-shift emission peak by increased conjugation length of dithienyl-benzophenazine. The emission intensity of PF-FDTQ03 and PF-FDTQ05 was more intense compared to that of PF-PDTQ05 due to strong steric-hindrance of FDTQ. In case of PF-FDTQ05, the luminous efficiency and external quantum efficiency were 0.78cd/A and 0.64%, respectively with a maximum brightness of 3112cd/m2.

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.

Synthesis and characterization of white light-emitting polyfluorene-based copolymers containing new red iridium complex in the main chain

We report the synthesis of single chain white light-emitting polyfluorene (PF)-based copolymers containing a novel red iridium complex ((CVz-PhQ)2IrdbmBr), benzothiadiazole (BT), and 2,5-bisphenyl-1,3,4-oxadiazole (OXD) segments. The resulting copolymers were highly soluble in common organic solvents and could be easily spin-coated onto an indium-tin oxide (ITO)-coated glass substrate to obtain high quality optical thin films. The weight-average molecular weight (Mw) and polydispersity of the copolymers were (17.5–36.3)×104 and 1.54–1.65, respectively. We fabricated white polymer light-emitting diodes (WPLEDs) in ITO/PEDOT:PSS/PVK/copolymer/CsF/Ca/Al, both with and without the poly(9-vinylcarbazole) (PVK) interlayer. By carefully controlling the concentrations of the low-energy-emitting species in the resulting copolymers, white light-emission with contributions from each of the three primary colors was achieved. By incorporating 2mol% of an electron transporting material, OXD, as a comonomer into the copolymer backbone and inserting a PVK interlayer, the external quantum efficiency (EQE) and luminance efficiency (LE) of the WPLEDs were enhanced. The turn-on voltages of the copolymers were in the range of 5.5–7.5V, and the maximum EQE and LE were 1.79% and 1.89cd/A with CIE coordinates of (0.32, 0.34), which are close to the standard for white light emission.

Suppression of Keto Defects and Thermal Stabilities of Polyfluorene–Kaolinite Clay Nanocomposites

A solution blending process for preparation of polymer nanocomposites composed of cationic polyfluorene (PF) and dimethyl sulfoxide (DMSO) -intercalated kaolinite (Ka) clay has been taken to evaluate the effect of Ka nanostructure on the nanocomposite structures, morphology, and properties. Composites containing 2, 5, 7.5, and 10 wt % clay have been characterized with the help of X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), UV–visible spectroscopy, photoluminescence studies, etc. Additionally, the keto defect, inhibition of interchain interaction, and photostability of PF in the nanocomposites have been explored. The XRD and HRTEM studies show the exfoliation of Ka layers at lower content in composites. Intercalation of PF chains into the Ka interlayer space occurs at relatively higher clay content. Nanocomposites exhibit higher thermal stability than pristine PF due to lamination of PF into clay nanogallery through the interchange of DMSO by cationic polyfluorene. The presence of an Si–O–Si stretching band in the composites supports the formation of nanocomposites of PF with Ka. The movement of absorption maxima to higher wavelength indicates the increase of overall conjugation length of PF chains in the nanocomposites. Upon formation of nanocomposite with Ka, the keto defect sites of PF are significantly reduced. This can be attributed to the lamination of single PF chains by Ka interlayer gallery that act as a barrier to oxygen and inhibit the exciton diffusion. Current–voltage characteristics of nanocomposite films have also shown good switching behavior with low forward junction potential.

Highly efficient blue and all-phosphorescent white polymer light-emitting devices based on polyfluorene host

We report efficient blue electrophosphorescent polymer light emitting devices with polyfluorene (PFO) as the host and iridium bis[2-(4,6-difluorophenyl)-pyridinato-N,C2] picolinate (FIrpic) as the dopant. Despite the low-lying triplet energy level of the polyfluorene polymer host, phosphorescent quenching can be suppressed by using poly(N-vinylcarbazole) (PVK) as anode buffer layer, resulting in a high luminous efficiency of 26.4cdA−1, which is one of the best results in the literature based on conjugated polymer reported to date. The reduced phosphorescent quenching is found to be associated with the exciton formation and charge carrier recombination within the PVK layer and the PVK/PFO interface due to the accumulation of holes. As compared with the devices based on non-conjugated host polymer PVK, the devices based on PFO showed a lower turn-on voltage (3.6V vs. 4.4V) and higher power efficiency (17lmW−1 vs. 8.3lmW−1) due to the higher mobility of PFO. When doubly doped with a newly synthesized yellow-emitting metallophosphor, white polymer light-emitting devices with superior device performance (a peak device efficiency of 40.9cdA−1, a CIE coordinates of (0.32,0.48), and a power efficiency of 31.4lmW−1) was achieved. These findings can broaden our selection in polymer hosts for highly efficient phosphorescent blue emitting devices and can find potential applications in full color displays and solid-state lighting applications in the future.

Synthesis of novel polyfluorene with defined group in the center using aryl dipalladium complex as an initiator

Dinuclear Pd(II) complex DiPd–DPE with a bridging ligand C6H4OC6H4-4,4′ was successfully synthesized by double oxidative addition reaction of Pd(PCy3)4 with 4,4′-dibromodiphenyl ether in good yield. X-ray crystal structure analysis reveals that the Pd atoms have the nearly square-planar coordination geometry with the similar bond lengths of two Pd–Br as well as four Pd–P, implying that two Pd(II) centers have similar reactivity. Using the dinuclear Pd(II) complex DiPd–DPE as an initiator, novel polyfluorene (PF) with a defined group (C6H4OC6H4-4,4′) in the middle of polymer main chain was prepared, demonstrated by MALDI-TOF analysis.

Effective Light-Harvesting Antennae Based on BODIPY-Tethered Cardo Polyfluorenes via Rapid Energy Transferring and Low Concentration Quenching

Light-harvesting antennae (LHA) were demonstrated using polyfluorenes (PFs) modified with borondipyrromethene (BODIPY) dyes tethered to the cardo structures. PFs work as a light absorber and an energy donor to the BODIPY units. The series of BODIPY-tethering PFs via the cardo carbon including homocardo PFs and alternative polymers with fluorene and the cardo fluorene were synthesized, and their optical properties were investigated. Initially, highly efficient energy transferring was observed from the PF main chains to the BODIPY unit (99%). It was found that PFs can work as an efficient light absorber because of the large molar extinction coefficient and cause the rapid energy transfer through the cardo structure. Next, from the comparison with the emission efficiency of the BODIPY units in the series of the synthetic polymers, the favorable position of the BODIPY units was obtained to avoid the concentration quenching: The alternative polymer with cardo fluorene and dialkyl-substituted fluorene showed th...

A Facile Strategy toward Conjugated Polyelectrolyte with Oligopeptide as Pendants for Biological Applications

We report a facile yet efficient strategy to synthesize biofunctionalized conjugated polyelectrolyte using click reaction between an amphiphilic oligopeptide (R10) and organic soluble polyfluorene (PF) as an example. PF-R10 shows the absorption and emission maxima at ~380 and ~430 nm in water, respectively. In addition, it exhibits enhanced fluorescence in acidic circumstance as compared to that in neutral environment because of reduced aggregation, which is confirmed by laser light scattering and atomic force microscopy studies. In view of the penetration property of the grafted R10 peptide, PF-R10 shows excellent cell uptake and labeling ability in cellular imaging.

Intramolecular CH/π interaction of Poly(9,9-dialkylfluorene)s in solutions: interplay of the fluorene ring and alkyl side chains revealed by 2D 1H–1H NOESY NMR and 1D 1H-NMR experiments

The existence of intramolecular CH/π interaction of four soluble polyfluorenes (PFs) and molecular di-n-octylfluorene in chloroform-d1 at 20 °C was first revealed by cross-peaks of the ring with several protons of alkyl groups in 1H–1H NOESY NMR and marked upfield shift of several alkyl proton signals in 1H-NMR spectra. Only the PF bearing β-branched chiral groups at 9,9-positions among three chiral PFs in chloroform at 20 °C showed intense circular dichroism bands in the π−π* region because of the intramolecular CH/π interaction. Conflex7 and Gaussian 03 MP2 calculation results supported this idea.

Enhanced photoluminescence of polyfluorene by incorporation of Al-doped ZnO nanoparticles

Photoluminescent and structural properties of nanocomposite films consisting of green-emitting polyfluorene (PF) and Al-doped ZnO (AZO) nanoparticles were investigated. Topographical analysis shows that AZO nanoparticles can be homogeneously dispersed in PF films with less than 10wt.% AZO. Photoluminescence of the composite films was investigated under wavelength selective excitation of AZO. Photoexcitation with laser energy below the AZO bandgap produces a slight increase in the PF emission intensity with increasing AZO concentration, accompanied by a spectral blueshift. These effects are attributed to separation of polymer chains and twisting of the PF backbone as revealed by Raman spectroscopy. On the other hand, photoexcitation above the AZO bandgap shows a remarkable enhancement, with the intensity at 5–10 wt.% AZO being three times greater than that of pure PF. The luminescence enhancement is attributed to the efficient transfer of energy from AZO nanoparticles to PF.

Synthesis of Polyfluorene-Polytriarylamine Block Copolymer with Emitting Part at Junction Point for Light Emitting Applications

A block copolymer consisting of polyfluorene (PF) and polytriarylamine (PTAA) functionalized with green emitting phenoxazine moiety at the junction point of two blocks was designed and prepared for electroluminescent application. PF homopolymer was synthesized by Suzuki coupling polymerization, and was reacted with brominated phenoxazine. In the presence of the resulting PF functionalized with phenoxazine, C-N coupling polymerization of 4-(4’-bromophenyl)-4’’-butyldiphenylamine was carried out to afford a triblock copolymer, PTAA-phenoxazine-PF-phenoxazine-PTAA (PF-Ph-PTAA). Two types of random copolymers were also synthesized with fluorene and phenoxazine (PF2) by Suzuki coupling polymerization for comparison. All the polymers were soluble in common organic solvents and readily formed thin films by a solution processing. Prepared polymers exhibited similar UV absorption and PL emission in chloroform solutions. In a film state, the existence of phenoxazine unit drastically changed PL spectra. Although the content of phenoxazine unit in PF-Ph-PTAA was relatively high (13 mol%), it showed similar PL spectrum to that of PF2(phenoxazine content, 0.2 mol%) indicating that phenoxazine unit is isolated in single polymer chain nevertheless the high content. EL device based on PF-Ph-PTAA showed green-emission, suggesting that emission sites predominantly located in the vicinity of phenoxazine moiety because of its shallow HOMO level.

Polyfluorene-Polytriarylamine Block Copolymer as an Additive for Electroluminescent Devices Based on Polymer Blends

Electroluminescent characteristics were investigated for the blue emitting devices fabricated with the blend systems consisting of hole transporting polytriarylamine (PTAA), electron transporting polyfluorene (PF), and a block copolymer with both segments (PF-b-PTAA) as an active layer in order to elucidate the relationship between the chemical nature and morphology of the active layer, and EL performance. The addition of PF-b-PTAA to PF homopolymer afforded the hole injecting and/or electron blocking ability to increase the efficiency. The addition to PF/PTAA blend keeping the chemical composition constant also improved the performance by controlling the morphology and/or the domain size in phase-separated films.

Single-component polyfluorene electrolytes bearing different counterions for white light-emitting electrochemical cells

A series of polyfluorene (PF) electrolytes bearing Br−, BF4-, or PF6- counterions were synthesized and characterized. 2,1,3-Benzoselenadiazole moieties were incorporated into polymer main chains to produce single-component white light-emitting polymers. The thermal stability of Br-containing ionic PF was decreased because of the Hofmann elimination occurred at higher temperature. By replacing Br− with BF4- or PF6- counterions, the thermal stability of polymers was significantly improved. UV–vis absorption and photoluminescence spectra revealed that Br-containing ionic PF generates blue shift in methanol, while BF4 and PF6-containing ionic PFs show blue shift in acetonitrile. Electrochemical analysis revealed that oxidation potentials of BF4 and PF6-containing ionic PFs were decreased, resulting in increase in HOMO and LUMO levels. The optimized spin-coated light-emitting electrochemical cell (LEC) with the configuration of ITO/PEDOT/polymer/Ag showed a maximum luminescence efficiency of 1.56lm/W at a low operation bias of 3V. The single-component LEC device exhibited pure white light emission with CIE’1931 coordinates approaching (0.33, 0.33) and high color rendering index (CRI>85), referring to its potential use in solid-state-lighting application.

White polymer light-emitting diode materials with efficient electron injection backbone containing polyfluorene, oxadiazole and quinoxaline derivatives

Copolymer including dithienylquinoxaline (TQ) (<0.1mol%) as a dopant and oxadiazole derivative as a side chain has been synthesized on a Polyfluorene (PF) backbone based on the Suzuki coupling reaction. The UV–vis spectra of polymers showed similar behaviors in the solution and on the film. However, PL spectra were similar to that of PF in solution, but the peak around 524nm increased as the amounts of TQ were increased in the casting film. With increase in TQ, effective Forster energy transfer (from PF and PFOxd15 to TQ) was observed. According to CV measurement, HOMO and LUMO levels declined as the amount of oxadiazole derivative increased. In case of PFOxd15TQ10, the luminous efficiency and power efficiency were 1.23cd/A and 0.64lm/W, respectively with a maximum brightness of 6940cd/m2. CIE coordinates were close to pure white (0.33, 0.39).

Sequential Printing by Laser-Induced Forward Transfer To Fabricate a Polymer Light-Emitting Diode Pixel

Patterned deposition of polymer light-emitting diode (PLED) pixels is a challenge for electronic display applications. PLEDs have additional problems requiring solvent orthogonality of different materials in adjacent layers. We present the fabrication of a PLED pixel by the sequential deposition of two different layers with laser-induced forward transfer (LIFT), a "dry" deposition technique. This novel use of LIFT has been compared to "normal" LIFT, where all the layers are transferred in a single step, and a conventional PLED fabrication process. For the sequential LIFT, a 50-nm film of an alcohol-soluble polyfluorene (PFN) is transferred onto a receiver with a transparent anode, before an aluminum cathode is transferred on top. Both steps use a triazene polymer dynamic release layer and are performed in a medium vacuum (1 mbar) across a 15 μm gap. The rough morphologies of the single-layer PFN pixels and the PLED device characteristics have been investigated and compared to both bilayer Al/PFN pixels fabricated by normal LIFT and conventionally fabricated devices. The functionality of the sequential LIFT pixels (0.003 cd/A, up to 200 mA/cm(2), at 30-40 V) demonstrates the suitability of LIFT for sequential patterned printing of different thin-film layers.

Controlled Synthesis of Polyfluorenes via Kumada Catalyst Transfer Polycondensation with Ni(acac)2/dppp as the Catalyst

A new catalyst system, i.e., nickel acetylacetonate/1,3-bis(diphenylphosphino)propane (Ni(acac)2/dppp), was explored to catalyze the Kumada catalyst transfer polycondensation (KCTP) of three fluorene monomers with different substituents at 9-position. The “living” nature of the polymerization was confirmed by polymerization kinetic studies, “monomer addition” experiment and block copolymerizations. As a result, poly(9,9-dioctylfluorene)s (PF8s) with the number-average molecular weights (Mns) in the range 2.8–62.2 kDa and polydispersity indices (PDIs) of ∼1.20 were successfully synthesized in a controlled manner. The syntheses of fluorene-fluorene and fluorene-thiophene diblock copolymers with Mns up to 46 kDa were also demonstrated. A complex, i.e. Ni(dppp)(acac)2, with an octahedral coordination geometry was isolated and confirmed by X-ray crystallographic analysis. The polymerization experiments indicated that the in situ formed Ni(dppp)(acac)2 should be the active catalyst. To the best of our knowledge, this is the first report on the controlled synthesis of polyfluorenes (PFs) via KCTP.

Emission color tuning of copolymers containing polyfluorene, benzothiadiazole, porphyrin derivatives

Copolymer containing benzothiadiazole (BT) and porphyrin (POR) derivatives as dopants (<0.3mol%) was synthesized to polyfluorene (PF) backbone using Suzuki coupling reaction. The synthesized polymer was thermally stable and soluble in general organic solvents. UV–vis spectra of the polymer showed the similar behaviors in solution and on film. However, PL spectra was similar to PF in solution, but its peak increased around 520 and 612nm as BT and POR, the dopants, went up in casting film. The more POR increased, the more effective Forster energy transfer was observed by POR than BT in PF. The device was made in ITO/PEDOT:PSS/polymer/BaF2/Ba/Al structure. For PFB02P05 polymer, the luminous efficiency was 0.66cd/A, the power efficiency 0.29lm/W and the maximum brightness 936cd/m2. CIE coordinate (0.36, 0.34) was closer to pure white. For PFB15P20, the luminous efficiency was 1.40cd/A, the power efficiency 0.32lm/W, the maximum brightness 5997cd/m2. PFB15P20 demonstrated the best performance as green emission.