Synthesis and light-emitting properties of fluorene-based copolymers

Synthetic Metals 160 (2010) 1672–1678

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Synthesis and light-emitting properties of fluorene-based copolymers Bearing diphenyl imidazole chromophore Qiang Peng ∗ , Guanwen Fu, Dan Su School of Environmental and Chemical Engineering, Nanchang Hangkong University, Nanchang 330063, PR China

a r t i c l e

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Article history: Received 28 January 2010 Received in revised form 4 May 2010 Accepted 28 May 2010 Available online 29 June 2010 Keywords: Conjugated polymers Photoluminescence Light-emitting diodes

a b s t r a c t New type of alternating fluorene-based copolymers containing electron-deficient aryl-substituted imidazole units in the main chains were designed, synthesized and characterized. The resulting copolymers were amorphous and showed excellent solubility in common organic solvents, such as dichloromethane, chloroform, 1,2-dichloroethane, chlorobenzene, toluene, and THF. These polymers also possessed good thermal stability with glass transition temperatures of 96 and 82 ◦ C and decomposition onset temperatures of 365 and 379 ◦ C, respectively. They exhibited good blue photoluminescence properties with high photoluminescence efficiencies. The long-wavelength emission of polyfluorenes had been effectively reduced by introduction of nonlinear structure aryl-substituted imidazole units. Light-emitting devices with an ITO/PEDOT/PVK/copolymer/LiF/Al configuration could emit strong blue light with high external quantum efficiencies of 1.49 and 2.05%. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Polyfluorenes (PFs) have been extensively studied for their potential applications in organic electronic devices, such as lightemitting diodes (LEDs) [1,2], photovoltaic cells (PVCs) [3], and field-effect transistors (FETs) [4]. Recently, there has been considerable interest in developing light-emitting polymers with high efficiency. PFs have been regarded as a promising candidates for polymer LEDs due to their processability, high fluorescence quantum yields, and good charge transport properties [5]. However, a major problem associated with PFs is their tendency to emit red-shifted light during annealing or in the presence of current passages [6]. One possible reason is formation of fluorenone defects upon thermal or photo-oxidation. Various strategies have also been developed to resolve this problem in PFs. They include introduction of bulky side chains or dendronization [7–12] incorporation of spiro-linked [13] or cross-linked [14] structures, copolymerization to induce disorder [13], improving the oxidative stability of pendant groups [10] or chain ends [5], and blending with a high glass transition temperature polymer to limit the chain mobility [15]. PFs as well as most of the conjugated polymers have been found to be hole-transport-dominated materials because the hole mobility is much higher than the electron mobility in these materials. As a result, unbalanced rates of electron and hole injection from the respective negative and positive contacts and a shift of the

∗ Corresponding author. Tel.: +86 0791 3953369; fax: +86 0791 3953369. E-mail address: [email protected] (Q. Peng). 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.05.039

recombination zone toward the region near the polymer/cathode interface are always observed. Electron injection can be improved by introducing a low work function metal, such as Ca and Ba [16,17]. However, low electron mobility in these materials has limited the efficient recombination of holes and electrons, resulting in a decrease in electroluminescence (EL) efficiency. Attempts have also been made to improve the electron-accepting properties of the polymers by introducing electron-deficient moieties into either the side chains or the main chains of the polymers [18,19]. The emission color of PFs will be easily varied over the entire visible region by introducing electron-deficient co-monomers into the polyfluorene backbone [19]. Accordingly, it is still a challenge to develop efficient light-emitting polyfluorenes with high electron affinity to achieve a more balanced injection and transport of carriers. Imidazole derivatives show unique chemical and physical properties because they contain imidazole heterocycles which have better thermal stability and high photoluminescence quantum yields. As a result, luminescent materials of imidazole small molecules have emerged as the attractive light-emitting materials [20]. Simple MOPAC molecular modeling showed that the two benzene rings of the organic fluorescent moleculars were not coplanar with the imidazole ring [21]. The non-coplanarity could prevent intermolecular electrostatic interaction among the fluorescent molecules. This structure is efficient to decrease such chain aggregation and transfer energy without reducing fluorescence. Substituted imidazoles are also an interesting building block with unique linear and nonlinear optical properties [22] and chemical characteristics in acid–base or metal-complexation reactions [23] for promising application in sensor [24], ionic liquids [25] and

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Scheme 1. Synthesis routes for the monomers and copolymers.

in bio-systems [26]. Imidazole containing non-conjugated polymers have been studied and used intensively, however, only very few conjugated polymers with incorporated imidazole heterocycles (4,5 substitutive) are known [27]. In this paper, we first introduce the imidazole units into the backbone of polyfluorene to increase the thermal stability and the electron affinity. On the other hand, the introduced arylsubstituted imidazole can adjust the long linear structure of polyfluorene to avoid excimer formation and improve the device performance. Thus, we report the synthesis of fluorene-based light-emitting copolymers modified with electron-deficient 2,2diphenyl-2H-imidazoles (shown in Scheme 1) and demonstrate the electroluminescent properties of the copolymers. 2. Experimental 2.1. General information Unless stated otherwise, all reagents and solvents were of reagent grade and were used as received. All reactions were performed under a nitrogen atmosphere using the standard Schlenk technique. All solvents were carefully dried and purged with

nitrogen. All manipulations involving air-sensitive reagents were performed in a dry argon atmosphere. 1 H NMR spectra were recorded on a Bruker ACF 300 spectrometer with d-chloroform as the solvent and tetramethylsilane as the internal standard. Number-average (Mn ) and weight-average (Mw ) molecular weights were determined on a HP 1100 HPLC system equipped with a HP 1047A RI detector and Agilent PLgel MIXED-C300 × 7.5 mm (ID) columns (packed with 5 ␮m particles of different pore sizes). The column packing allowed the separation of polymers over a wide molecular weight range of 200–3,000,000. THF was used as the eluent at a flow rate of 10 mL min−1 at 35 ◦ C. Polystyrene standards were used as the molecular weight references. Cyclic voltammetry measurements were carried out on an AUTOLAB potentiostat/galvanostat electrochemical workstation at a scan rate of 50 mV/s, with a platinum wire counter electrode and a Ag/AgCl reference electrode in an anhydrous and nitrogen-saturated 0.1 mol/L acetonitrile (CH3 CN) solution of tetrabutylammonium perchlorate (Bu4 NClO4 ). The polymer was coated on the platinum plate working electrode from a dilute chloroform solution. UV–vis and fluorescence spectra were obtained on a Carry 300 spectrophotometer and a Carry Eclipse photoluminescence spectrophotometer, respectively. The concentration of the CHCl3

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solutions of the polymers was fixed at 1 × 10−6 M. The relative fluorescence quantum yields of the copolymers in CHCl3 and thin solid films were estimated by using 9,10-diphenylanthracene in cyclohexane (˚f = 0.9) and polydihexylfluorene (PDHF) thin film excited at 370 nm (˚f = 0.55) as the standard. Thermogravimetric analyses (TGA) were conducted on a TA Instrument TGA 2050 thermogravimetric analyzer at a heating rate of 10 ◦ C min−1 and under an N2 flow rate of 90 mL min−1 . Differential scanning calorimetry (DSC) measurements were carried out on a Mettler Toledo DSC 822e system under N2 and at a heating rate of 10 ◦ C min−1 . The thickness of films was measured using a Dektak surface profilometer. 2.2. EL device fabrication and characterization For the fabrication of electroluminescence (EL) devices, glass substrates coated with indium–tin oxide (ITO) and having a sheet resistance of 30  square−1 (CSG Co. Ltd., Shenzhen, China) were cleaned consecutively in ultrasonic baths of ionic detergent water, acetone, and anhydrous ethanol, in that order. A thin layer (40 nm) of commercial PEDOT (Batron-P 4083, Bayer A G, Germany) [PEDOT was poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonic acid)] were spin-coated on ITO at 2500 rpm for 30 s, then the PEDOT film was dried at 80 ◦ C for 8 h in vacuum oven. After that, the PVK [poly(vinylcarbazole)] (from 10 mg mL−1 chlorobenzene solution) and the copolymer (from a 10 mg mL−1 toluene solution of the copolymer) were spin-coated onto the top of PEDOT and PVK films in succession. The thickness values of the PVK and copolymer layers were about 40 nm and 80 nm, respectively, as measured by a Dektak surface profilometer. Thin layers of LiF (0.5 nm) and Al (140 nm) were then deposited sequentially on the copolymer film by thermal evaporation under a reduced pressure of 10−4 Pa. The applied dc bias voltages for the EL devices were in a forward direction (ITO, positive; LiF/Al, negative). The current–voltage characteristics were measured on a voltmeter and an amperometer. The EL efficiency and brightness measurements were carried out using a calibrated silicon photodiode. All measurements on the EL devices were carried out in air at room temperature. 2.3. Synthesis of 2,3-bis(4-bromophenyl)-2,2-dioctyl-2H-imidazole(1) A mixture of 4,4 -dibromobenzyl (1.01 g, 2.73 mmol), 9heptadecanone (0.69 g, 2.73 mmol), glacial acetic acid (20 mL) and ammonium acetate (4.89 g, 63.55 mmol) was stirred and refluxed under N2 for 12 h. After the mixture was cooled to room temperature, it was subsequently concentrated under reduced pressure and water was added to the concentrate. The precipitate was filtered, washed with n-hexane. The crude product was then purified by silica gel column chromatography using dichloromethane as the eluent. Compound 1 was obtained as a white solid with a yield of 68%. 1 H NMR (CDCl3 , 300 MHz, ı/ppm): 7.52–7.22 (m, 8H, ArH), 1.95–1.23 (m, 28H, CH2 ), 0.82 (t, 6H, CH3 ). HRMS: (M+H)+ , Calcd. 602.4942, found 602.4946. Anal. Calcd. (%) for C31 H42 Br2 N2 : C, 61.80; H, 7.03; N, 4.65. Found: C, 61.76; H, 7.02; N, 4.69. 2.4. Synthesis of 4,5-bis(4-bromophenyl)-2,2-di(4hexyloxy)phenyl-2H-imidazole(2)

eluent. Compound 2 was obtained as a white solid with a yield of 38%. 1 H NMR (CDCl3 , 300 MHz, ı/ppm): 7.62–7.32 (m, 16H, ArH), 4.04–4.01 (t, 4H, OCH2 ), 1.62–0.89 (m, 22H, CH2 and CH3 ). HRMS: (M+H)+ , Calcd. 730.5810, found 730.5813. Anal. Calcd. (%) for C39 H42 Br2 N2 O2 : C, 64.12; H, 5.79; N, 3.84. Found: C, 64.10; H, 5.76; N, 3.92. 2.5. Synthesis of the polymer P1 9,9-Dihexylfluorene-2,7-bis (trimethylene boronates) (502.31 mg, 1.0 mmol), 1 (602.49 mg, 1.0 mmol) and Pd(PPh3 )4 (22.5 mg, 0.019 mmol) were added to a mixture of toluene (15 mL) and aqueous 2 M potassium carbonate (5 mL). The mixture was vigorously stirred at 85–90 ◦ C for 60 h. The reaction flask was again evacuated and filled with argon once more. The mixture was heated to reflux with stirring and maintained for 48 h under argon. Phenyl boronic acid (18.3 mg, 0.15 mmol) solution in degassed toluene was then added. The solution was refluxed again for 8 h. After that, bromobenzene (23.5 mg, 0.15 mmol) solution in degassed toluene was then added. The solution was further refluxed for another 8 h. After the mixture was cooled to room temperature, it was poured into 200 mL of methanol and deionized water (v/v = 10:1). A fibrous solid was obtained by filtration. The solid was washed with methanol, water and then methanol. After washing with acetone for 24 h in a Soxhlet apparatus, the resulting polymer, P1, was obtained as a pale yellow solid after drying under vacuum. Yield: 83%. 1 H NMR (CDCl3 , 300 MHz, ı/ppm): 7.80–7.62 (m, 6H, Ar-H), 7.46–7.30 (m, 8H, Ar-H), 2.10 (s, 4H, CH2 ), 1.92–0.76 (m, 56H, CH2 and CH3 ). FT-IR (KBr, cm−1 ): 3021, 2922, 2853, 1605, 1535, 1463, 1372, 1246, 4488, 11103, 1014, 866, 814, 752, 693. Anal. Calcd. for C56 H74 N2 (%): C, 86.77; H, 9.62; N, 3.61. Found: C, 86.12; H, 9.52; N, 3.56. 2.6. Synthesis of the polymer P2 9,9-Dihexylfluorene-2,7-bis (trimethylene boronates) (502.31 mg, 1.0 mmol), 2 (730.58 mg, 1.0 mmol) and Pd(PPh3 )4 (22.5 mg, 0.019 mmol) were added to a mixture of toluene (15 mL) and aqueous 2 M potassium carbonate (5 mL). The mixture was vigorously stirred at 85–90 ◦ C for 60 h. Phenyl boronic acid (18.3 mg, 0.15 mmol) solution in degassed toluene was then added. The solution was refluxed again for 8 h. After that, bromobenzene (23.5 mg, 0.15 mmol) solution in degassed toluene was then added. The solution was further refluxed for another 8 h. After the mixture was cooled to room temperature, it was poured into 200 mL of methanol and deionized water (v/v = 10:1). A fibrous solid was obtained by filtration. The solid was washed with methanol, water and then methanol. After washing with acetone for 24 h in a Soxhlet apparatus, the resulting polymer, P2, was obtained as a pale yellow solid after drying under vacuum. Yield: 76%. 1 H NMR (CDCl3 , 300 MHz, ı/ppm): 7.82–7.64 (m, 6H, Ar-H), 7.51–7.32 (m, 16H, Ar-H), 4.04–4.01 (t, 4H, OCH2 ), 2.12 (s, 4H, CH2 ), 1.80–0.78 (m, 44H, CH2 and CH3 ). FT-IR (KBr, cm−1 ): 3024, 2926, 2852, 1607, 1530, 1462, 1396, 1342, 1248, 1220, 1056, 1016, 975, 813, 759, 607. Anal. Calcd. for C64 H74 N2 O2 (%): C, 85.10; H, 8.26; N, 3.10. Found: C, 84.47; H, 8.15; N, 3.04. 3. Results and discussion

A mixture of 4,4 -dibromobenzyl (0.90 g, 2.42 mmol), di(4hexyloxy)benzyl ketone (0.93 g, 2.42 mmol), glacial acetic acid (30 mL) and ammonium acetate (4.28 g, 55.80 mmol) was stirred and refluxed under N2 for 24 h. After the mixture was cooled to room temperature, it was concentrated under reduced pressure and water was added to the concentrate. The precipitate was filtered, washed with n-hexane. The crude product was then purified by silica gel column chromatography using dichloromethane as the

3.1. Synthesis and characterization The synthetic routes of the monomers and the corresponding copolymers are shown in Scheme 1. The monomer, 1 or 2, was synthesized using 4,4 -dibromobenzyl as the starting materials by reaction with 9-heptadecanone or di(4-hexyloxy)benzyl ketone in the mixture of acetic acid and excess ammonium

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acetate. The polymerization reaction was carried out via the wellknown palladium-catalyzed Suzuki coupling reaction between 9,9-dihexylfluorene-2,7-bis(trimethylene boronate) and monomer 1 or 2 in toluene to yield the pale yellow solid of the corresponding copolymer (yield >75%). The chemical structures of the copolymers were confirmed by 1 H NMR spectroscopy and elemental analyses, which supported the formation of the resulting alternating copolymers. Both copolymers have good solubility in common organic solvents, such as THF, CH2 Cl2 , CHCl3 and toluene, resulting from the hexyl side chains attached to the fluorene moiety and n-alkyl or alkoxyl side chains linked to the imidazole or diphenyl imidazole unit. Smooth and optically clear thin solid films on quartz or ITO substrates were obtained by spin-coating the toluene solutions of the copolymers (10 mg mL−1 ) at a spin rate of 1500 rpm. The molecular weights of the copolymers were measured by gel permeation chromatography (GPC) with reference to polystyrene standards. Polymer P1 had an Mw value of 16,700 g/mol and an Mn value of 9600 g/mol with a polydispersity index of 1.74. Under the same GPC conditions, P2 had an Mw value of 16,600 g/mol and an Mn value of 10,200 g/mol with a polydispersity index of 1.63. The thermal properties of the copolymers were determined from TGA and DSC measurements. The thermal stability of the polymers was evaluated by TGA under a nitrogen atmosphere. Polymer P1 and P2 exhibited good thermal stabilities with decomposition onset temperature (Td ) of 365 and 379 ◦ C, respectively. DSC measurements were also performed on these two polymers. The glass transition temperatures (Tg ) of P1 and P2 were determined to be 96 and 82 ◦ C, respectively, which are higher than that of poly(9,9dihexylfluorene) (PHF) (∼55 ◦ C) [28]. Thus, the incorporation of imidazole units into the main chain has successfully increased the Tg of the polyfluorenes. The improved Tg is very important for these copolymers to be used as emissive materials in electroluminescence displays [29]. According to the results, the obtained polymers are amorphous, and this should lead to good mechanical stability for their future utilization in LEDs.

3.2. Electrochemical properties Cyclic voltammetry (CV) was employed to investigate the electrochemical behavior of the copolymers, as well as to estimate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the materials. The copolymer film deposited on a platinum plate electrode was scanned both positively and negatively in 0.10 M anhydrous acetonitrile solution of tetrabutylammonium perchlorate (Bu4 NClO4 ), using a platinum wire as the counter electrode and Ag/AgCl (0.10 M) as the reference electrode. The energy level of the Ag/AgCl reference electrode (calibrated against the FC/FC+ redox system) was 4.49 eV below the vacuum level. As shown in Fig. 1, both copolymers exhibited reversible p-doping and n-doping behavior. Upon the anodic scan, the corresponding onset potentials of 1.45 and 1.48 V occurred for P1 and P2, respectively. When the polymers were cathodically swept, the onset potentials of the n-doping process occurred at about −1.49 and −1.48 V, respectively. From the onset potentials of the oxidation and reduction processes, the band gaps of P1 and P2 were estimated to be about 2.94 and 2.96 eV, respectively. The LUMO and HOMO energy levels were also estimated to be −3.00, −3.01 eV and −5.94, −5.97 eV, respectively, according to the following equation: EHOMO/LUMO (eV) = (−4.8 − Eref − EOX/RED ), where Eref is the potential of the ferrocene reference and EOX/RED is the onset potential for the oxidation or reduction of the polymer film, as described in previous literature [30]. It is obvious that the electron effinity of polyfluorenes is efficiently enhanced by introduction of aryl-substituted imidazole units.

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Fig. 1. Cyclic voltammograms of the copolymer thin films coated onto platinum electrodes in an acetonitrile electrolyte solution of n-Bu4 NClO4 (0.10 M), with a Ag/AgCl (0.10 M) reference electrode at room temperature (Scan rate: 50 mV s−1 ).

3.3. Optical properties The optical properties of the copolymers were investigated both in solution and in the solid state. As shown in Fig. 2, P1 and P2 have similar UV–vis absorption spectra. The respective absorption maxima (max ) at about 283, 364 nm and 292, 373 nm are due to the ␲–␲* transition associated with the conjugated polymer backbone. Uniform and colorless polymer films were prepared on quartz substrates by spin-coating from toluene solution of the respective polymers (10 mg mL−1 ) at a spinning rate of 1500 rpm. The absorption gives the maximum peaks at about 290, 370 nm and 300, 375 nm for P1 and P2, respectively. In comparison, the UV–vis data for P1 and P2 in films and in toluene solutions indicate that the maximum absorptions of P1 and P2 in toluene are red-shifted to about 7 and 8 nm, respectively. The reason could be attributed to the enhanced interaction of the polymer chains in the solid state. From the absorbance onsets in the P1 and P2 films, the optical band gaps can be estimated to be 2.96 and 2.94 eV, respectively. These values are in good agreement with those obtained from CV measurements. These copolymer solutions emit strong blue fluorescence when excited by UV irradiation. The PL emission spectra of the polymers in chloroform solutions (under 370 nm excitation) are shown in Fig. 3. The maxima appear at 460 and 470 nm for P1 and P2, respectively. The emission spectrum seems re-shifted compared

Fig. 2. Absorption spectra of copolymer P1 and P2 in chloroform solutions and solidstate films spin-coated on quartz plates.

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Fig. 3. PL spectra of copolymer P1 and P2 in chloroform solutions and solid-state films spin-coated on quartz plates.

with that of PDHF. The reason is that the incorporation of imidazole unit increases the conjugation length of polymer main chains, and thus causes greater disorder degree for the higher degree of excited state configurational rearrangement. The corresponding PL quantum yields (f) of these polymers in CHCl3 were determined to be 0.59 and 0.63, when measured relative to that of 9,10-diphenylanthrancene (f = 0.9) [31]. Both copolymers also exhibited good blue emission behavior in the solid state (Fig. 3). The PL maxima of the polymers in thin film form appear at about 463 and 477 nm, which are only slightly red-shifted from their values in solution, suggesting that there was little aggregation of the polymer chains in the solid state. The PL quantum yields of the P1 and P2 films were estimated to be 0.41 and 0.48, respectively, by comparing its fluorescence intensity to that of the polydihexylfluorene (PDHF) thin film excited at 370 nm (f = 0.55) [1]. The effect of annealing temperature on the color and luminescence stability is an important parameter in the performance of polymer light-emitting devices (PLED). To examine the thermal stability of the resulting copolymers, the spin-coated polymer films were treated at 200 ◦ C in air and their PL spectra were systematically evaluated. A polydihexylfluorene (PDHF) film was employed as the control. Fig. 4 shows the normalized PL emission spectra of the copolymer and PDHF films before and after thermal annealing. Thermal treatment of the PDHF film at 200 ◦ C for 2 h led to an additional long wavelength emission at about 520 nm in the

Fig. 5. Typical EL of PLED devices based on copolymer P1 (under the bias of 8, 9, 10 V) and P2 (at 9, 10, 11 V).

PL spectrum. The undesirable green emission that often overlaps with the PL and electroluminescence (EL) spectra of PFs has been attributed to ketonic defects degradation processes [1]. However, these copolymers exhibited good thermally stability and their PL spectra did not show any significant red-shifts after annealing at 200 ◦ C for 2 h. Thus, with the introduction of nonlinear structure aryl-substituted imidazole units, the red-shift phenomenon of PFs has been reduced markedly. To exclude the effect of keto-defects, FT-IR measurements of these two copolymers were carried out before and after annealing. The typical fluorenone related signal at about 1721 cm−1 (>C O stretching mode) [1] was not found in both cases. This indicated that the keto defects have been reduced because of the purification of monomers to a high degree prior to their polymerization [6,11]. 3.4. EL properties

Fig. 4. PL spectra (ex = 370 nm) of the copolymer P1, P2 and PFDH films before and after thermal annealing at 200 ◦ C for 2 h in air (PFDH, polydihexylfluorene).

The EL properties of polymer P1 and P2 were examined with a double-layer LED device configuration of ITO/PPEDOT (40 nm)/PVK (40 nm)/copolymer (80 nm)/LiF (0.5 nm)/Al (140 nm)], in which a conducting polymer, PEDOT, was used as the hole-injection layer. Considering a significant barrier for hole injection between the HOMO values of PEDOT (−5.2 eV) [32] and copolymer (−5.94 or −5.97 eV), a PVK layer (with a HOMO value of −5.7 eV) [33] was inserted to improve hole injection. The sheet resistance of the glass substrates coated with ITO in this work was 30  square−1 . The copolymer films were spin-coated on the ITO/PEDOT substrates, and the thicknesses were about 80 nm in these devices. The devices emit strong blue emission under bias. The EL spectra of the copolymers are similar to the PL spectra of the corresponding copolymer films, which indicates both of them originated from the same excited state. As shown in Fig. 5, the maxima in the EL spectra occurred at about 465 nm (CIE: x = 0.15, y = 0.21, measured at 10 V) and 473 nm (CIE: x = 0.15, y = 0.24, measured at 11 V) for the devices based on P1 and P2, respectively.

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4. Conclusions New type of alternating fluorene-based copolymers containing aryl-substituted imidazole units in the main chains have been successfully synthesized through Suzuki coupling method. The resulting polymers possess good solubility in common organic solvents, good thermal stability, improved electron affinity and reasonable molecular weights. The long-wavelength emission of polyfluorenes had also been effectively reduced by introduction of nonlinear structure aryl-substituted imidazole units. Double-layered light-emitting devices with an ITO/PEDOT/PVK/copolymer/LiF/Al configuration could emit strong blue light with external quantum efficiencies of 1.49 and 2.05%, indicating these copolymers are a promising type of efficient blue light-emitting materials for display applications. Acknowledgements The work was financially supported by the Natural Science Foundation of China (NSFC, No: 20802033), Program for New Century Excellent Talents in University (No: NCET-10-0170), Aviation Science Fund of China (No: 2008ZF56014), Open Project Program of Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, China (No.10HJYH03) and Scientific Research Foundation of Jiangxi Provincial Education Department (No: GJJ09178). References

Fig. 6. Current–voltage–luminance characteristics of the devices based on copolymers P1 (a) and P2 (b).

The current–voltage–luminance characteristics of the devices of copolymers P1 and P2 are shown in Fig. 6. The devices showed good diode behavior. Under the forward bias, the current increased exponentially with an increase in the applied voltage after the turn-on voltage was exceeded. In contrast, under reverse bias, no obvious increase in the current density was observed when the applied voltage was increased. As shown in Fig. 6, a bright blue emission from the ITO/PEDOT (50 nm)/P1 (80 nm)/LiF (1 nm)/Al (140 nm) device started at about 6.2 V. An EL external quantum efficiency of 1.49% was obtained at a bias voltage of 8.9 V and a current density of 78.5 mA/cm2 . The turnon voltage was 7.6 V for the ITO/PEDOT (40 nm)/PVK(40 nm)/P2 (80 nm)/LiF (0.5 nm)/Al (140 nm) device. The EL external quantum efficiency was 2.04% at a bias voltage of 9.8 V and a current density of 59.2 mA/cm2 . For comparison, the device of ITO/PEDOT (50 nm)/PDHF (80 nm)/LiF (1 nm)/Al (140 nm) was fabricated and measured. The turn-on voltage was 7.8 V and the EL external quantum efficiency was 1.02%. Actually, the device based on PDHF with Ba cathode was discussed before with relatively low efficiency of just 0.13% [34]. The EL performance of two these copolymers has been improved significantly. The improvement can be attributed to the incorporation of aryl-substituted imidazole units with high electron affinity into the main chains of polyfluorenes. It is worth noting that the introduction of electron-deficient arylsubstituted imidazole can not only improve the electron affinity of polyfluorenes, but also keep the blue-emission property of polyfluorenes. Thus, the results indicate that these two copolymers are promising candidates for efficient blue emitter for display applications.

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