Polyfluorene Research Articles

Micropatterning of emitting layers by microcontact printing and application to organic light-emitting diodes

The microcontact printing (μCP) technique, which is a simple and low damage fabrication technique for thin films, was successfully applied to fabricate patterned emitting layers such as polyfluorene (PF). We fabricated micropatterns by transferring dried and uniform thin films, and observed strong electroluminescence (EL) from the fabricated organic light-emitting diodes (OLEDs) with the patterned emitting layers. The performance of the fabricated device was superior to that of a conventionally fabricated device. This demonstrates the well-controlled interfaces achieved by μCP. Furthermore, we succeeded in fabricating OLEDs with multiple emitting layers. These results show that this technique is promising for application to cost-effective, high luminance and multicolored OLED displays.

Synthesis and Device Performance of a Highly Efficient Fluorene-Based Blue Emission Polymer Containing Bulky 9,9-Dialkylfluorene Substituents

We have prepared a series of highly efficient and pure blue-emitting polyfluorene (PF)-type polymers that contain 9,9-di(9,9-dihexylfluoren-2-yl)fluorene and 9-(9,9-dihexylfluoren-2-yl)-9-(3,4-di(2-methyl)butyloxyphenyl)fluorene units. The photoluminescence (PL) results show that the size of the side groups strongly influences the color purity and color stability because of the superior ability of large groups to prevent interchain interactions between polymer chains. In contrast to n-octyl functional groups, the PF with 9,9-(dihexylfluoren-2-yl) and 3,4-di(2-methyl)butyloxyphenyl side groups exhibits good PL characteristics as a blue-light-emitting polymer (LEP), showing highly enhanced color integrity and color stability against oxidative conditions over PFs based on 9,9-dioctylfluorene. When N4,N4′-bis(4-methoxyphenyl)-N4,N4′-diphenylbiphenyl-4,4′-diamine (13) is incorporated into the polymer backbone, the polymer exhibits a highly efficient electroluminescence performance with a luminous efficiency of...

Origin of bistability in polyfluorene-based organic bistable devices

This study examined the origin of bistability in polyfluorene (PFO)-based organic bistable memory devices (OBDs) by changing the bulk properties of PFO and metal electrodes. Two different PFOs with and without internal trap sites and two Al and Au electrodes were used. Bistability was induced in all devices when Al was deposited on PFO. On the other hand, bistable switching was observed only in the PFO device with internal trap sites when Au was deposited on PFO. Therefore, both the formation of internal trap sites and an organic–metal interface are essential for bistability in PFO-based OBDs.

Nickel as an alternative semitransparent anode to indium tin oxide for polymer LEDapplications

We report on the possibility of using a thin Ni layer, instead of ITO, as a semitransparenthole-injecting electrode for bottom polymer LEDs. Thin metal layers of Ni were depositedby a sputtering technique and their electrical and optical properties with differentdeposition times have been investigated. Both square resistance and transmittance wereseen to decrease with deposition time (thickness). The films showed a transmittance ofaround 30–40%, which is quite low compared to the 86% of ITO, while their squareresistance was higher than that of ITO. Nevertheless, diodes based on a blue emittingpolymer, polyfluorene (PFO), showed the same efficiency for either ITO or thin Nielectrodes, although the Ni transmittance is around 2.5 times lower than the ITOtransmittance. Such preliminary results definitively suggest that indium-free organicdevices can be achieved.

Controlled Radical Synthesis of Fluorene-Based Blue-Light-Emitting Copolymer Nanospheres with Core−Shell Structure via Self-Assembly

We prepared a series of well-defined polyfluorene-based copolymers by using polyfluorene (PF) macroinitiators to initiate atom transfer radical polymerization (ATRP) of γ-methacryloxypropyltrimethoxysilane (MPS). The chemical structures of hydroxy-functionalized PF (PFOH), PF macroinitiators (PFBr), and polyfluorene-b-poly(γ-methacryloxypropyltrimethoxysilane) (PF-b-PMPS) block copolymers were confirmed by 1H NMR. In THF/aqueous solution, amphiphilic PF-b-PMPS block copolymers can self-assemble into core−shell micelles with PMPS as the core and PF as the shell. Base-catalyzed sol−gel process inside the PMPS core results in PF-encapsulated silica core−shell nanospheres. Transmission electron microscopy (TEM) studies reveal the dense grafting of PF on the surface of silica core. The effect of PF-b-PMPS core−shell structures on the optical and electrochemical properties was also investigated. This novel kind of luminescent nanosphere has potential applications in bioanalysis, biomedical, and light-emitting devices.

Hybrid nanocomposites for optical applications

Incorporating inorganic particles into conjugated polymer matrices is an area of current interest in the fields of optoelectronics and solar energy. The hybrid nanocomposites exhibit interesting physical properties thanks to good optical properties of polymers and to high carrier mobility of inorganic semiconductors. A judicious combination of organic/inorganic can therefore provide materials of low cost, ease processing, high stability, with specific electrical and optical properties. In the present study, we briefly review the composite materials that have been successfully utilized in the field of optoelectronics and photovoltaic conversion. We shall describe in particular a family of nanocomposites using polyhedral oligomeric silsesquioxanes (POSS) of general formula (RSiO 3/2) n where R is an organic group as a core. The composites are made by grafting functional polymer groups to the core, which allows the control of their optical properties. Such composites have high mechanical resistance and stability because of the special structure of the core. For illustration, we present a study of polyfluorene (PF)/POSS materials used as an active layer in organic light emitting diodes, with improved performance as compared to those using polymer only, and we discuss the role of the particles in the transport and emission processes in the devices studied.

Application of heteroleptic iridium complexes with fluorenyl-modified 1-phenylisoquinoline ligand for high-efficiency red polymer light-emitting devices

A series of new heteroleptic iridium complexes bearing fluorenyl-modified 1-phenylisoquinoline as the first ligand and different ancillary ligands has been prepared and characterized. These complexes bis(1-(3-(9,9-dimethyl-fluoren-2-yl)phenyl)isoquinoline-C2,N′)iridium(III)acetylacetonate(Ir(DMFPQ) 2acac)), bis(1-(3-(9,9-dimethyl-fluoren-2-yl)phenyl)isoquinoline-C2,N′)iridium(III)(3-(pyridin-2-yl)-1,2,4-triazolate)(Ir(DMFPQ) 2pt) and bis(1-(3-(9,9-dimethyl-fluoren-2-yl)phenyl)isoquinoline-C2,N′)iridium(III)(2-(2-pyridyl)benzimidazolate)(Ir(DMFPQ) 2pbi) showed red phosphorescent emissions of 615–630 nm in dichloromethane solution. The device fabricated with these complexes doped into a host polyfluorene (PFO) blend with 30% of an electron transport material 2-(4-biphenyl)-5-(4- tert-butylphenyl)-1,3,4-oxadiazole (PBD) showed high device efficiencies. Ir(DMFPQ) 2acac exhibited red emission with an external quantum efficiency( η ext) of 14.3% and luminous efficiency( η c) of 7.8 cd/A at 1.2 mA/cm 2 and the maximum brightness reached 10 006 cd/m 2 (Commission Internationale de I’Eclairage(CIE) chromaticity coordinates: (0.67, 0.32)) at 412 mA/cm 2. Ir(DMFPQ) 2pt showed a η ext of 13.0% and η c of 9.2 cd/A at 17 mA/cm 2, 1532 cd/m 2, and the maximum brightness reached 15085 cd/m 2 (CIE: 0.64, 0.34) at 360 mA/cm 2.

Novel conjugated polymers employing the binding of polyfluorene derivatives and C 60

New polyfluorene (PF)-based conjugated polymers, PClBEHF, PClBEHF- co-PDEHF, PN 3BEHF, PN 3BEHF- co-PDEHF, PEHPBF, PC 60BEHF, PC 60BEHF- co-PDEHF, and PEHPBF +PCBA − containing chloro, azido, piperidine, and fullerene (C 60) groups as pendants, were synthesized. In the case of PC 60BEHF and PC 60BEHF- co-PDEHF, the C 60 moiety was introduced by using covalent bond formation between the azido group and C 60. PEHPBF +PCBA − was synthesized by using ionic bond formation between the piperidinium cation of PEHPBF and carboxylate anion of PCBA. The PL and EL spectra of PClBEHF, PClBEHF- co-PDEHF, PN 3BEHF, PN 3BEHF- co-PDEHF, and PEHPBF exhibited red-shifted and broad peaks caused by the aggregation which was induced by the increasing interchain interaction due to the polar pendants. After binding with C 60, covalently and ionically, the photovoltaic properties of PC 60BEHF, PC 60BEHF- co-PDEHF, and PEHPBF +PCBA − were studied. PL was quenched completely, which indicates photo-induced charge transfer from PF to pendant C 60. Even though photovoltaic effects did not occur in short circuit condition, blending with MEH-PPV as the ratio of 1:1 generated the photovoltaic effects.

Multi-Layered Oriented Polyfluorene Films

Multilayered oriented polyfluorene (PF) films were obtained by applying thermal treatment procedure to a multilayered PF film constructed with fluorene derivatives layer formed on top of a highly oriented friction-transferred crystalline poly(9,9-dioctylfluorene) (PF8) film. The orientations in the multilayered PF films were investigated by polarized photoluminescence (PL) spectroscopy and grazing incident X-ray diffraction (GIXD) analysis. The results of the multilayered PF film constructed with spin-coated PF8 on friction-transferred PF8 indicate that the rearrangement of the upper PF8 layer is induced from the orientation of lower PF8 layer by thermal treatment at the nematic phase temperature. Polarized green emission from the multilayered oriented PF film was demonstrated using the blend of PF8 and poly(9,9-dioctylfluorene-co-benzothiadiazol) (F8BT) as green light emitter for upper layer. By this method, the polarized emission color can be tuned using polymer blends for upper layer similar to the liquid-crystalline polymer arrangement without using different materials as an underlying layer such as the rubbed polyimide.

Optical properties of di‐octyl substituted polyfluorene under hydrostatic pressure

AbstractAlmost all polyfluorene (PF) derivatives utilize solubilizing side chain substituents for tailoring their optical and electronic properties. Side chain substitution, in addition to improving the solution processing and film forming characteristics, confers new functionality and dictates the backbone structure of polymers. The backbone conformation in di‐octyl substituted PF (PF8) is strongly dependent upon the conformation of the alkyl side chain. In this paper we present photoluminescence and Raman scattering studies of bulk PF8 as a function of hydrostatic pressure. These results are further contrasted with the optical studies of a branched side group substituted PF under hydrostatic pressure. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

SYNTHESES AND OPTOELECTRONIC PROPERTIES OF A HEXA-ARMED FLUORENE-BENZENE COPOLYMER BASED ON 2,4,6,8,10,12-HEXABENZYL-2,4,6,8,10,12-HEXAAZAISOWURTZITANE CAGED-CORE

A novel kind of hexa-armed fluorene-benzene copolymer based on a hexaazaisowurtzitane core was synthesized through Suzuki coupling polycondensation. The introduction of this bulky caged-core could not only enhance the photoluminescence quantum efficiency, but also improve the electroluminescence properties, especially suppress the common green-color emission of polyfluorenes (PFs) material during device operation. These features can be attributed to the successful suppression of PF's chain aggregation which profits from the introduction of the bulky hexaazaisowurtzitane core and the design of multi-armed architecture.

Synthesis, thermomorphic characteristics, and fluorescent properties of poly[2,7-(9,9-dihexylfluorene)]-block-poly(N-isopropylacrylamide)-block-poly(N-hydroxyethylacrylamide) rod-coil-coil triblock copolymers

New thermoresponsive conjugated rod-coil-coil triblock copolymers were successfully synthesized from terminal azido functionalized poly(N-isopropylacrylamide)-b-poly(N-hydroxyethylacrylamide) (PNIPAAm-b-PHEAA) and alkynyl functionalized polyfluorene (PF) via a click reaction. The azido functionalized PNIPAAm-b-PHEAA copolymers with different block ratio was prepared by atom transfer radical polymerization from an initiator bearing the azide group, whereas alkynyl functionalized PF was synthesized by Suzuki coupling reaction. The lower critical solution temperature (LCST) of the block copolymers increased with an enhanced hydrophilic PHEAA block ratio, since the longer PHEAA segment facilitated the copolymer chains to stretch at an elevated temperature. The micelles of PNIPAAm-b-PHEAA with different block ratio changed into spheres, aggregate spheres, vesicles, and wormlike micelles as the temperature was increased, due to the variation on the hydrophilic/hydrophobic characteristic of PNIPAAm. However, the micellar morphologies became worms, bundles of wormlike micelles, and hollow tubes in the triblock PF-b-PNIPAAm-b-PHEAA, which were probably induced by the π–π interaction among the fluorene segments. The variation of the micelle morphology with temperature was consistent from the results of transmission electron microscopy, atomic force microscopy, and dynamic light scattering. Also, the micelle morphologies of PF-b-PNIPAAm-b-PHEAA showed a thermoreversible property based on its LCST. The photoluminescence characteristics behaved as an on/off fluorescence indicator of temperature, showing an “on-off-on” profile at an elevated temperature in water at a higher block ratio of PNIPAAm and switching to “on-off” as the block ratio of PNIPAAm decreased. The present study suggests that the PF-b-PNIPAAm-b-PHEAA copolymers have tunable morphologies and could be potentially used as thermoresponsive sensory materials.

Synthesis, characterization and self-assembly behavior in water as fluorescent sensors of cationic water-soluble conjugated polyfluorene-b-poly(N-isopropylacrylamide) diblock copolymers

A series of well-defined novel water-soluble diblock copolymers containing conjugated amino-terminal polyfluorene (PF) block and coil-like poly(N-isopropylacrylamide) (PNIPAM) have been successfully synthesized through atom transfer radical polymerization (ATRP) initiated by a 2-bromoisobutyrate end-capped PF macroinitiator using CuCl/HMTETA (1,1,4,7,10,10-hexamethyltriethylenetetramine) as the catalyst. The first-order kinetic plots indicate the presence of a constant number of active species during the polymerization. The molecular weight and molecular weight distribution can be well controlled, implying synthesis of well-defined block structures of the copolymers. The chemical structures of block copolymers have been characterized by 1H NMR, UV–vis, and photoluminescence (PL) spectra. PF aggregates are formed in water with the increase of temperature for the conjugated-ionic diblock copolymer synthesized, as confirmed by dynamic light scattering (DLS). The formation of excimers within the PF aggregates results in improved Förster resonance energy transfer (FRET) efficiencies.

Synthesis of Novel Conjugated Polyelectrolytes for Organic Field‐Effect Transistors Gate Dielectric Materials

AbstractConjugated polyelectrolytes and their neutral precursors: aminoalkyl‐substituted polyfluorenes (PFNs) with different 4,7‐bis(N‐methylpyrrol‐2‐yl)‐2,1,3‐benzothiadiazole (PBT) contents were synthesized by Suzuki coupling reaction. Their quaternized ammonium polyelectrolyte derivatives were obtained through a post‐polycondensation treatment on the terminal amino groups. The resulting copolymers PFNBr–PBT are soluble in polar solvents such as methanol, DMF, and DMSO. Using the conjugated polyelectrolytes PFNBr–PBT5 as gate dielectric material, poly(3‐hexylthiophene) (P3HT) ‐based field‐effect transistors (FETs) show a high drain current of almost 4.5 mA within a gate voltage of only −3 V. The calculated hole mobility of P3HT is about 0.58 cm2 · V−1 · s−1.magnified image

Synthesis, characterization, and application of light‐emitting copolyfluorenes slightly doped with distyrylbenzene derivatives

AbstractCopolyfluorenes PFG1∼PFG4 slightly doped with 0.006–0.5 mol % of 2,5‐dihexyloxy‐1,4‐bis(2‐phenyl‐2‐cyanovinyl)benzene (green chromophore) were synthesized by the Suzuki coupling reaction to be evaluated as hosts for white‐light electroluminescent (EL) devices. Their optical, thermal and electrochemical properties were almost identical to those of polyfluorene (PF) due to minimal chromophore content. However, the electroluminescent (EL) spectra of the PFGn were very different from photoluminescence spectra in film state. Relative intensity of green emission (ca. 521 nm) in EL spectra are much stronger than those in PL spectra, which can be attributed to charges trapping in the chromophores due to its narrow band gap (Eg ≈ 2.56 eV). The performance of EL devices [ITO/PEDOT:PSS/polymer/Ca (50 nm)/Al (100 nm)] were improved with an increase in chromophore content. The PFG4 device revealed the best performance (6790 cd/m2, 1.69 cd/A), and the PFG1 and PFG2 devices exhibited comparable intensity in blue and green emissions. Blend EL devices were fabricated by using the PFGn as the hosts and a red iridium complex [Ir(piq)2(acac)] as dopant. By controlling the amount of the iridium complex, the white‐light emitting device was achieved with PFG2, with maximum brightness and CIE coordinate being 4120 cd/m2 and (0.31, 0.28), respectively. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 149–160, 2009

Fractal Aggregates of Polyfluorene−Polyaniline Triblock Copolymer in Solution State

We report on the aggregate structure of a symmetrical A-B-A 9,9-dialkylfluorene/2-alkylaniline "coil-rod-coil" triblock copolymer (or PF/PANI11112-b-PANI11)--consisting of 2-dodecylanilines as A blocks and 9,9-di(3,7,11-trimethyldodecyl)fluorene)s as B blocks--mixed in deuterated toluene, chloroform, and methylcyclohexane. These mixtures contain micrometer scale aggregates. Small-angle neutron scattering data indicate that the interface between the aggregates and solvents manifests surface fractal-like structure. The upper and lower limit length scales which show fractal character are of the order of > 100 and 15 nm. The surface fractal dimension D(s) varies from 2.2 to 2.8. The data show that stiff polyfluorene (PF) segments (persistence length > or = 9.5 nm, diameter approximately 2 nm) of PF/PANI11112-b-PANI11 are dissolved down to the molecular level. Photoluminescence data indicate that most PF units are isolated from both polyaniline (PANi) units and each others. Together with the scattering data, this implies that the disordered interface consists of stiff isolated PF blocks linked together via domains of associated PANi blocks.

A Deep-Blue Electroluminescence from 2,5-Diphenyl-1,3,4-oxadiazole and Polyfluorene Copolymer End-capped with Triphenylamine

An electron-transporting 2,5-bis(4-bromophenyl)-1,3,5-oxadiazole (OXD) was incorporated by copolymerization into polyfluorene (PF) end-capped with a 4-bromo-4',4"-dimethyltriphenylamine (TPA) main chains on the Yamamoto coupling method to improve the initial electroluminescence (EL) color stability against applied voltage and the device operation lifetime. The copolymer, PF/OXD-TPA, possessed a high molecular weight enough for film-forming ability and excellent solution-processability, that is, good solubility in common organic solvents such as toluene. An EL device fabricated using this copolymer as a polymeric emission layer allowed the generation of a strong deep-blue EL peaking at 424 nm with a comparatively high EL efficiency of 0.47 cd A -1, and its color was extremely stable against applied voltage within 527 V. The incorporation of OXD units into the PF-TPA system also improved the operation lifetime, although it is not a practical level for displays. The results are probably attributed to the great roles of the incorporated OXD units for disturbing the formation of fluorene excimer sites and for tuning the hole/electron mobility balance..

Raman Spectroscopic Studies of Polyfluorenes

Polyfluorenes reveal a complex interplay between emissive properties and intra- and intermolecular structure. Vibrational frequencies and intensities determined by Raman spectroscopy are strongly influenced by variations in the backbone as well as side chain conformations. Changes in the structural and electronic properties of two side group substi- tuted polyfluorenes (PF): ethyl-hexyl substituted PF (PF2/6) and dioctyl substituted PF (PF8) as a function of solvent, thermal cycling, and hydrostatic pressure via Raman scattering are presented. The vibrational frequencies and intensities of fluorene oligomers with various alkyl side chain conformations are calculated using hybrid density-functional theory. A comparison of the computed vibrational spectra of single chain fluorene oligomers with our experimental data shows that the conformational isomers in PF8 are a direct consequence of the side chain conformation.

Polythiophene–polyoxyethylene copolymer in polyfluorene-based polymer blends for light-emitting devices

Polyfluorene (PFO) films doped with different amount of a polythiophene-co-polyoxyethylene (P1) polymer were studied with the aim to obtain white emission. The Förster resonant energy transfer (FRET) is the basic mechanism that takes place in the photoluminescence of these blends. The non-conducting poly(oxyethylene) behaves as a inert spacer leading to a larger donor–acceptor chromophores separation and then a better control of FRET efficiency. As result, the light-emitting device with the architecture Al/Ca/P1:PFO/PEDOT:PSS/ITO was found to be more efficient than either the devices using pure PFO or P1. Near white CIE coordinates of (0.26, 0.33) with a maximum luminance of 2500 cd/m 2 were found for 2 wt.% device. A maximum external quantum efficiency of 1% was obtained, hence, suggesting that the enhancement of the electronic and optoelectronic properties could be achieved by incorporating P1 into the PFO.

Enhanced green electrophosphorescence by using polyfluorene host via interfacial energy transfer from polyvinylcarbazole

For green-emitting Ir complex-doped polymer devices, high efficiencies have been achieved only when non-conjugated poly( N-vinylcarbazole) (PVK) was used as the host polymer. It is commonly believed that conjugated polyfluorenes (PFO) are not suitable for the use as host of green phosphorescent complexes due to the presence of the low-lying triplet state. In this paper we reported that despite very inefficient PL phosphorescent emission of Ir(Bu-PPy) 3 in the PFO films, strong green electroluminescence (EL) and high device efficiencies have been observed for devices from such blend films when multilayer device with PVK was used as anode buffer. We analyze the PL spectra in steady state and transient response and found that the energy transfer via PVK interlayer plays important role in the efficient EL performance. This fact clearly indicates the efficient electrophosphorescence might be achieved even if the triplet energy of phosphorescence dye is higher than that of the host. These findings could significantly broaden our selection in polymer hosts for phosphorescent devices.