Synthesis and characterization of red-emitting diketopyrrolopyrrole-alt-phenylenevinylene polymers

Synthetic Metals 160 (2010) 1544–1550

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Synthesis and characterization of red-emitting diketopyrrolopyrrole-alt-phenylenevinylene polymers Zhi Qiao a , Yanbin Xu a , Shuimu Lin a , Junbiao Peng b , Derong Cao a,∗ a b

School of Chemistry and Chemical Engineering, South China University of Technology, Wushan Road 381, Guangzhou 510641, China Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510641, China

a r t i c l e

i n f o

Article history: Received 10 February 2010 Received in revised form 14 May 2010 Accepted 14 May 2010 Available online 11 June 2010 Keywords: Light-emitting Polymer light-emitting diodes Diketopyrrolopyrrole Wittig reaction

a b s t r a c t Two novel diketopyrrolopyrrole (DPP) and p-phenylenevinylene alternating copolymers, poly(1,4-(2,5-dicyano)-phenylenevinylene-alt-2,5-dioctyl-3,6-bis(4-vinylenephenyl)pyrrolo[3,4c]pyrrole-1,4-dione) (P1) and poly(1,4-(2,5-diethoxy)-phenylenevinylene-alt-2,5-dioctyl-3,6-bis (4-vinylenephenyl)pyrrolo[3,4-c]pyrrole-1,4-dione) (P2), were synthesized through Wittig reaction in good yields, and were characterized by FTIR, NMR. Two polymers possessed moderate molecular weights and polydispersities, well-defined structures, and were readily soluble in common organic solvents. In particular, P1 and P2 exhibited excellent thermal stability with Td = 393 and 398 ◦ C at 5% weight loss, respectively. Both P1 and P2 in solution and in thin films exhibited strong red photoluminescence. Both electroluminescence devices using ITO/PEDOT/polymer/Ba/Al configuration with P1 and P2 as the emitting layer showed nearly pure red emission with the emission peaks at 646 nm for P1 and 648 nm for P2, and exhibited low turn-on voltages of 4.5 and 3.4 V, respectively. The results show that P1 and P2 are promising candidates for red emissive materials in polymer light-emitting diodes. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Since their electroluminescence (EL) properties were reported firstly by Burroughes in 1990 [1], the light-emitting conjugated polymers have been attracting intensely because of the flexibility available for fine-tuning their luminescence properties through the manipulation of their chemical structures and the feasibility of combining spin-coating or printing processes for preparing large area flat-panel displays [2–6]. Over all the past decade, polymer light-emitting diodes (PLEDs) have been made considerable progress in the possible applications such as flat-panel or flexible display devices [7–9], due to their numerous advantages over cathode ray tubes (CRTs) and liquid crystal displays (LCDs) [10], such as good processability, fast response time, self-emitting property, wide view angle, high contrast, low power consumption, low weight, and facile color tunability over the full visible range, particularly large area color displays and flexibility [11–13]. However, only green PLEDs meet the requirements of commercial use at present [6,13,14]. New red PLEDs still remain among the most wanted performance to be

∗ Corresponding author. Tel.: +86 20 87110245; fax: +86 20 87110245. E-mail address: [email protected] (D. Cao). 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.05.019

improved in full-color displays, and further improvements are necessary [15,16]. In most ␲-conjugated polymers, the injection and transport of holes are more efficient than transport of electrons due to their inherent richness of ␲-electrons [12,17,18]. As a result, an essential issue for PLEDs with conjugated polymers as the emissive layers reported so far lies in the charge imbalance. Therefore, the electron injection is believed to limit the quantum efficiency of PLEDs devices based on p-dope type polymers. In order to improve the device performance of light-emitting diodes (LEDs), realization of n-dope type polymeric emissive materials, while keeping the high emission property, is both interesting and challenging for researchers [19,20]. In order to synthesize new red-emitting polymeric materials with high electron-transporting ability, we take advantage of the electron-accepting property of diketopyrrolopyrrole (DPP) derivatives. Due to their excellent photostability and high quantum yield of fluorescence, DPP derivatives are potential materials for opto-electrical devices [21–33]. In this paper, we designed two novel red-emitting polymers P1 and P2. P1 contained DPP and 1,4-(2,5-dicyano)-phenylenevinylene units, both of which were electron-acceptor. P2 contained DPP as electron-acceptor and 1,4(2,5-diethoxy)-phenylenevinylene as electron-donor, resulting in donor–acceptor charge structure. P1 and P2 were successfully synthesized by Wittig reaction. Preliminary electroluminescent results

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showed that P1 and P2 are promising candidates for red emissive materials in PLEDs.

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C24 H20 N2 O6 (432.43): C, 66.66; H, 4.66; N, 6.48. Found: C, 66.52; H, 4.67; N, 6.47. FTIR (KBr, cm−1 ): 3139, 2983, 2867, 1640, 1601, 1565, 1515 1443, 1401, 1319, 1142, 1093, 1033, 939, 831, 804, 664.

2. Experimental 2.1. Measurement and characterization 1 H NMR spectra were collected on a Bruker DRX 400 spectrometer in CDCl3 with tetramethylsilane as inner reference. FTIR spectra were recorded on a Tensor 27 spectrometer with KBr pellets. Number-average (Mn ) and weight-average (Mw ) molecular weights were determined by a Waters GPC 515-410 in tetrahydrofuran (THF) using a calibration curve of polystyrene standards. Elemental analysis was performed using a Vario EL III instrument. The thermogravimetric analysis (TGA) of polymers was performed under nitrogen atmosphere at a heating rate of 10 ◦ C/min using STA449C thermal analyzer. Cyclic voltammetry (CV) was carried out on an EG&G model 283 computer-controlled potential/galvanostat (Princeton Applied Research) with platinum electrodes at a scanning rate of 50 mV/s against a calomel reference electrode with a nitrogen-saturated solution of Bu4 NPF6 (0.1 M) in spectrumgrade acetonitrile. UV–vis absorption spectra were recorded on a HP 4803 Instrument. PL and EL spectra were recorded on an Instaspec IV CCD spectrophotometer. The absolute PL quantum yields were determined in an Integrating sphere IS080 (Labsphere) with 325 nm excitation of He–Cd laser (Mells Griod), as the percent of the total output photons in all directions vs the total input photons. The luminance–voltage (L–V) was measured using a Keithley 236 source measurement unit and a calibrated silicon photodiode. The luminance was calibrated using a PR-705 SpectraScan spectrophotometer (Photo research). The external quantum efficiency (EQE) was determined as the percent of the total output photons vs the total input electrons from each electrode.

2.2. Materials 4-Cyanobenzaldehyde, p-xylene, hydroquinone, 1bromooctane and potassium tert-butoxide were purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and were used without further purification; other chemicals were commercially analytical-grade quality. The solvents such as chloroform, ethanol, N-methyl-2-pyrrolidone (NMP) and N,Ndimethylformamide (DMF), were dehumidified by 3 Å molecular sieve before use. 4-[1,3]Dioxolan-2-yl-benzonitrile [34], 2,5dicyano-1,4-xylene-bis(triphenylphosphonium bromide) [35] and 2,5-diethoxy-1,4-xylene-bis(triphenylphosphonium bromide) [36] were prepared following the published procedures. 2,5-Dioctyl-3,6-bis[4-(1,3-dioxan-2-yl)phenyl]pyrrolo[3,4c]pyrrole-1,4-dione (2) was synthesized with a slight modification according to the literature [23]. 2.3. Syntheses of monomers and polymers 2.3.1. 3,6-Bis(4-[1,3]dioxolan-2-yl-phenyl)-2,5dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione (1) After sodium (1.472 g, 64 mmol) was dissolved in 40 mL of tamyl alcohol at about 100 ◦ C over 1 h with FeCl3 (74 mg, 0.46 mmol), 4-[1,3]dioxolan-2-yl-benzonitrile (7.000 g, 40 mmol) was added, then diisopropyl succinate (3.232 g, 16 mmol) in 20 mL of t-amyl alcohol was added dropwise over 1 h. The resulting suspension was strongly stirred for 24 h at 100 ◦ C. After the mixture was cooled to room temperature, glacial acetic acid (16 mL) and methanol (200 mL) were added. The mixture was stirred for 4 h, then filtered, washed several times with methanol and water, dried in vacuo to give 1 as red solid (3.871 g, 56%); mp: >300 ◦ C. Anal. calcd. for

2.3.2. 2,5-Dioctyl-3,6-bis[4-[1,3]dioxolan-2yl-phenyl]pyrrolo[3,4-c]pyrrole-1,4-dione (2) A mixture of 1 (3.456 g, 8 mmol), potassium tert-butoxide (1.975 g, 17.2 mmol) and 80 mL of dried NMP was heated to 60 ◦ C, then 1-bromooctane (9.264 g, 48 mmol) in 20 mL of NMP was slowly added, and the mixture was kept for 24 h at 60 ◦ C. After being cooled to room temperature, the mixture was filtered, and the solid was washed with 50 mL of ethyl acetate and water (50 mL × 3). The organic layer was dried over anhydrous MgSO4 , and then concentrated via rotary evaporation. The raw product was purified by column chromatography (petroleum/ethyl acetate, 1/5, v/v) followed by recrystallization in dichloromethane and methanol to give 2 as bright orange crystals (2.414 g, 46%); mp: 130–131 ◦ C. 1 H NMR (CDCl , 400 MHz, ı/ppm): 7.81(d, 4H, Ar–H), 7.62 (d, 4H, 3 Ar–H), 5.87 (s, 1H, Ar–CH), 4.14–4.04 (m, 8H, –OCH2 ), 3.70 (t, 4H, –NCH2 –), 1.54 (br, 4H, –CH2 ), 1.21 (br, 20H, –CH2 ), 0.82 (t, 6H, –CH3 ). 13 C NMR (CDCl3 , 100 MHz, ı/ppm): 162.6, 148.1, 141.0, 128.8, 128.7, 126.9, 119.9, 103.0, 65.4, 41.9, 31.7, 29.4, 29.1, 29.0, 26.7, 22.6, 14.0. Anal. calcd. for C40 H52 N2 O6 (656.85): C, 73.14; H, 7.98; N, 4.26. Found: C, 73.08; H, 7.94; N, 4.23. FTIR (KBr, cm−1 ): 2924, 2854, 1668, 1603, 1511, 1455, 1427, 1381, 1345, 1307, 970, 944, 856, 834, 772, 655.

2.3.3. 2,5-Dioctyl-3,6-bis(4-formylphenyl)pyrrolo[3,4-c]pyrrole1,4-dione (3) A mixture of 2 (1.968 g, 3 mmol), THF (30 mL) and HCl (2 M, 15 mL) was stirred for 2 h at 60 ◦ C. After being cooled to room temperature, 30 mL of ethyl acetate was added, washed with water and dried over MgSO4 . After filtration, the solvent was evaporated and the crude product was purified by recrystallization in dichloromethane and methanol to give 3 (1.67 g, 98%); mp: 128–130 ◦ C. 1 H NMR (CDCl3 , 400 MHz, ı/ppm): 10.08 (s, 2H, –CHO), 8.03–7.94 (m, 8H, Ar–H), 3.74 (t, 4H, –NCH2 ), 1.54 (br, 4H, –CH2 –), 1.18 (br, 20H, –CH2 ), 0.82 (t, 6H, –CH3 ). 13 C NMR (CDCl3 , 100 MHz, ı/ppm): 191.1, 162.2, 147.6, 137.6, 133.3, 129.9, 129.2, 111.0, 42.0, 31.6, 29.4, 29.0, 28.9, 26.6, 22.5, 14.0. Anal. calcd. for C36 H44 N2 O4 (568.75): C, 76.02; H, 7.80; N, 4.93. Found: C, 76.10; H, 7.85; N, 4.97. FTIR (KBr, cm−1 ): 2918, 2852, 2817, 2732, 1700, 1676, 1597, 1574, 1547, 1503, 1467, 1415, 1396, 1359, 1317, 1296, 1217, 965, 811, 763.

2.3.4. Poly(1,4-(2,5-dicyano)-phenylenevinylene-alt-2,5-dioctyl3,6-bis(4-vinylenephenyl)pyrrolo[3,4-c]pyrrole-1,4-dione) (P1) A solution of sodium ethoxide (0.085 g, 1.25 mmol) in 5 mL of anhydrous ethanol was added to a stirred solution of 3 (0.284 g, 0.5 mmol) and 2,5-dicyano-1,4-xylene-bis(triphenylphosphonium bromide) (0.419 g, 0.5 mmol) in 5 mL of ethanol and 5 mL of chloroform at room temperature. The mixture was stirred for 24 h. The product was precipitated from methanol, filtered and then dissolved in chloroform, precipitated in methanol again. The crude product was purified by a Soxhlet extraction with methanol for 24 h to give P1 as red solid (286 mg, 83%). 1 H NMR (CDCl3 , 400 MHz, ı/ppm): 7.92–6.59 (m, 14H, Ar–H, –CH CH–), 3.72 (br, 4H, –N–CH2 ), 1.54 (br, 4H, –CH2 –), 1.17 (br, 20H, –CH2 –), 0.82 (br, 6H, –CH3 ). Anal. calcd. for (C46 H48 N4 O2 )n : C, 80.20; H, 7.02; N, 8.13. Found: C, 80.29; H, 7.09; N, 8.13. FTIR (KBr, cm−1 ): 2922, 2852, 2335, 1672, 1505, 1460, 1390, 1361, 1090, 1053, 960, 722.

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Scheme 1. Synthetic route of the monomers and the copolymers.

2.3.5. Poly(1,4-(2,5-diethoxy)-phenylenevinylene-alt-2, 5-dioctyl-3,6-bis(4-vinylenephenyl)pyrrolo[3,4-c]pyrrole1,4-dione) (P2) P2 was synthesized with a yield of 55% from 3 and 2,5-diethoxy1,4-xylene-bis(triphenylphosphonium bromide) according to the procedure for the synthesis of P1. 1 H NMR (CDCl3 , 400 MHz, ı/ppm): 8.00–6.58 (m, 14H, Ar–H, –CH CH–), 4.10–3.71 (m, 8H, –OCH2 –, –N–CH2 –), 1.59–0.81 (m, 36H, –CH2 –, –CH3 ). Anal. calcd. for (C48 H58 N2 O4 )n : C, 79.30; H, 8.04; N, 3.85. Found: C, 79.24; H, 8.09; N, 3.90. FTIR (KBr, cm−1 ): 2925, 2854, 1674, 1601, 1503, 1391, 1204, 1090, 1047, 959, 852, 733.

2.4. Fabrication and characterization of light-emitting devices P1 or P2 was dissolved in toluene and THF, filtered through a 0.45 ␮m filter. Patterned indium tin oxide (ITO)-coated glass sub-

strates were cleaned with acetone, detergent, distilled water, and 2propanol, subsequently in an ultrasonic bath. After treatment with oxygen plasma, 50–60 nm of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonic acid) (PSS) (Batron-P 4083, Bayer AG) was spin-coated onto the ITO substrate followed by drying in a vacuum oven. A thin film of P1 or P2 was coated onto the anode by spin-casting inside a drybox. The film thickness of the active layer was 70–80 nm, as measured with an Alfa Step 500 surface profiler (Tencor). For EL devices with Ba as cathodes, Table 1 Polymerization results. Polymers

Yield (%)

Color

P1 P2

83 55

Dark red Henna

a

Based on 5% weight loss.

Mw 6947 17,374

Mn 4724 11,145

PDI

Td(◦ C)a

1.47 1.56

393 398

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a thin layer of Ba (4–5 nm) and subsequently 150 nm layers of Al were vacuum-evaporated subsequently on the top of an EL polymer layer under a vacuum of 1 × 10−4 Pa. 3. Results and discussion 3.1. Synthesis and characterizations The synthetic route of monomers and copolymers is shown in Scheme 1. Compound 1 was prepared by reaction of diisopropyl succinate with 4-[1,3]dioxolan-2-yl-benzonitrile in the presence of sodium t-amyloxide. 1 exhibited very low solubility in common solvents due to the two-dimensional networks caused by intermolecular hydrogen bonds (N. . .H. . .O) combined with ␲–␲ and van der Waals interactions of the phenyl substituents between layers of molecules [24], therefore it could not be characterized by 1 H NMR and 13 C NMR. However, 1 was easily purified by washing repeatedly with methanol and water. In order to use the DPP chromophore in light-emitting devices, it is necessary to prepare soluble and steady DPP derivative. Fortunately, 1 was converted to 2 via N-alkylation of lactam units according to a modified method in moderate yield [23], and the n-octyl chains rendered 2 readily soluble in common organic solvents. Monomer 3 was prepared in good yield of 98% by hydrolyzation of 2 with dilute hydrochloric acid, and recrystallized to reach the desirable purity. The polymerization reaction by Wittig reaction between bisphosphonium salt and dialdehyde in a mixed solvent of ethanol and chloroform at room temperature yielded P1 and P2. Obviously, in relation to the methods for preparing DPP-based homopolymers using Ni-promoted and Pd-catalyzed arene coupling reactions, the method using Wittig reaction is more simple. To our knowledge, it was the first synthesis of DPP polymers upon Wittig polycondensation. Most DPP-containing polymers were prepared by palladium-catalyzed Suzuki coupling reaction [21–31]. Wittig polycondensation for the synthesis of DPP-containing polymers proceeded relatively fast under mild conditions at room temperature, which should be an advantage considering that the optoelectronic properties are strongly influenced by chemicals, i.e., electronic defects in the polymers [37]. The chemical structures of the copolymers were confirmed by elemental analysis, 1 H NMR, and FTIR spectroscopy. Their Mw values, determined by gel permeation chromatography (GPC) with polystyrene as the standard and THF as the eluent, were 6947 and 17374 with polydispersity indices (PDIs) of 1.47 and 1.56, respectively. Low molecular weight of the polymers might be mainly attributed to the fact that the polymer was insoluble and precipitated from the reaction medium, which resulted in the termination of the effective propagation step [35]. Prolonging the polymerization time did not increase the final molecular weight [19]. To reduce the amount of ethanol might increase the molecular weight, but it would lower the solubility of sodium ethoxide, which is disadvantage for the reaction. As a result, Mn of P2 was obviously higher than P1, because the diethoxyl chains rendered P2 readily soluble. In addition, the catalyst was important contribution to the molecular weight of polymers [23]. The thermal stabilities of polymers are shown in Table 1. The onset thermal decomposition temperatures (5% weight loss, Td s) of P1 and P2 were 393 and 398 ◦ C, respectively, which indicated that two polymers had good thermal stabilities. 3.2. Optical and photoluminescent properties The thin film for UV–vis and PL measurements was prepared by spin-coating from chloroform solution on a quartz plate followed by drying under vacuum for 4 h at room temperature. As shown in Fig. 1, the absorption spectra of two polymers (1 × 10−5 M,

Fig. 1. UV–vis absorption spectra of the polymers in CHCl3 solution.

Fig. 2. UV–vis absorption spectra of the polymers in thin film.

CHCl3 ) exhibited similar absorption maxima at 505 and 524 nm, respectively, which were attributed to the DPP segments. However, a slight red shift (19 nm) in the absorption maximum of P2 was observed compared with P1, which might be attributed to the intramolecular charge transfer between 2,5-diethoxylphenyl (donor) and DPP (acceptor) that lowers the optical band gap. P1 and P2 showed different absorption peaks in the short wavelength region, i.e., P1 at 282 nm and P2 at 415 nm. However, they showed the same shoulders at 244 and 340 nm. It should be noted that the absorptions of two polymers were rather broad and intense, covering almost the entire range from 200 to 600 nm, which might be useful for polymer photovoltaic cells. As shown in Fig. 2, the absorption spectrum of P1 in thin film was big different from the corresponding absorption spectrum in solution. The absorption maximum at 288 nm corresponded to ␲–␲* transition of the main chain, and the absorption peak at 525 nm was attributed to the DPP unit. However, the absorption spectrum of P2 in thin film was very similar to that in solution. The absorption peaks in long wavelength region of P2 in film (525 nm) and in solution (524 nm) were nearly identical. All data of absorption and photoluminescence are summarized in Table 2. As shown in Fig. 3, the emission maxima of P1 and P2 in solutions exhibited at 590 and 607 nm, respectively. In other words, the emission peak of P1 was blue-shifted in around 17 nm compared with P2, which could be attributed to the effect of different substituted groups (cyano group and ethoxyl) in polymers. Both P1 and P2 had two emission peaks with the maximum 630/668 nm for P1 and 640/670 nm for P2 in thin films, as shown in Fig. 4. The large red shift and broadening of PL spectra in thin films compared

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Table 2 Absorption and PL data for the polymers. Polymers

P1 P2

Solution, max (nm)

Thin film, max (nm)

Thin film

UV

PL

UV

PL

fwhm (nm)

PL (%)

244, 282, 340, 505 244, 340, 415, 524

590 607, 660

288, 525 294, 340, 417, 525

630, 668 640, 670

108 106

2.39 2.56

with those in the solution state suggested that interchain interactions should have an important effect on the excited state emission mechanisms of polymers. Generally, ␲–␲* interactions or aggregation of ␲-conjugated polymer chains in solid state result in a higher conjugation length and more planar conformations of conjugated chains in the solid polymers, leading to a red shift in the emission spectrum relative to the solution state. The intermolecular interactions in the solid state might favor coplanar arrangements of the aromatic rings in the polymer chain and may be responsible for the observed enhanced conjugation [2]. The absolute PL efficiencies of P1 and P2 are listed in Table 2. 3.3. Electrochemical properties of the polymers In the device fabrication and investigation of LED characteristics of polymers, information on the electronic structure of the luminescent polymers was essential. Cyclic voltammetry (CV) was employed to investigate the redox behavior of the polymers. As shown in Fig. 5 in cathodic and anodic scanning, both P1 and P2 possessed a reversible reduction peak without significant changes after several cycles when swept negatively, and an irreversible

Table 3 Electrochemical potentials and energy levels of the polymers. Polymers

Ered (V)

HOMO (eV)

LUMO (eV)

onset (nm)

Eg (eV)

P1 P2

−0.63 −0.79

−5.82 −5.63

−3.77 −3.61

605 615

2.05 2.02

oxidation peak when swept positively. The onset potentials for reduction of P1 and P2 occurred at −0.63 and −0.79 V, respectively. According to the empirical formula LUMO (EA, electron affinity) = −(Ered + 4.4) eV, where Ered are the onset potentials for reduction of the polymer vs the reference electrode [24], the LUMO values were calculated to be −3.77 and −3.61 eV. The band gaps (Eg ) of P1 and P2 could be calculated by the absorbance onset (onset ) of the polymer films (605 and 615 nm, respectively). According to the formula Eg = 1240/onset , the band gaps of P1 and P2 were 2.05 and 2.02 eV. According to the formula Eg = LUMO–HOMO, the HOMO values of P1 and P2 were −5.82 and −5.63 eV, respectively. All the electrochemical data of the polymers are summarized in Table 3. The polymers show high electron affinity due to the incorporation of the electron-deficient DPP units. 3.4. Electroluminescent characterization Double-layer LEDs based on two polymers with the configuration of ITO/PEDOT/polymer/Ba/Al were fabricated. PEDOT doped with PSS was used as the hole injection/transporting layer. A thin layer of Ba was employed as the cathode because its devices with longer lifetime compared with Ca or Mg [12]. The emission maxima and chromaticity coordinates for all polymers are summarized in Table 4. As shown in Fig. 6, P1 and P2 exhibited nearly identical with emitting peaks at 646 and 648 nm, respectively. The values of full width at half-maximum (fwhm) of P1 and P2 were also very close to 131 and 136 nm, and broader than the PL spectra (with fwhm values of 108 and 106 nm for P1 and P2, respectively) in the film state. A similar phenomenon has been reported in the literatures [38,39].

Fig. 3. Photoluminescent spectra of polymers in CHCl3 solutions.

Fig. 4. Photoluminescent spectra of polymers in thin films.

Fig. 5. Cyclic voltammograms of the polymers on a Pt electrode at a scanning rate of 50 mV/s.

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Table 4 EL performances of devices with the configuration of ITO/PEDOT/copolymer/Ba/Al. Polymers

EL emission max (nm)

EL fwhm (nm)

CIE coordinate (x, y)

Turn-on voltage (V)a

Maximum brightness (cd/m2 , voltage)

P1 P2

646 648

131 136

(0.67, 0.32) (0.67, 0.31)

4.5 3.4

(100, 7.1) (60, 5.4)

a

Turn-on voltage corresponds to luminance of about 1.0 cd/m2 .

Fig. 6. Electroluminescent spectra of polymers in devices with the configuration of ITO/PEDOT/copolymer/Ba/Al.

Fig. 9. Voltage–external quantum efficiency and voltage–luminance efficiency of the devices.

Figs. 7 and 8 show the voltage–luminance and voltage–current density plots for the devices based on P1 and P2. The red emission reached a brightness of 100 cd/m2 at a bias of 6.8 V for P1 and 60 cd/m2 at a bias of 5.4 V for P2. Both devices with P1 and P2 had low turn-on voltages, 4.5 and 3.4 V, respectively, which can be attributed to the lower energy barriers of the copolymers between the Ba work function (−2.8 eV) and the LUMO levels of the polymers (−3.77 eV for P1 and −3.61 eV for P2) [40]. The energy barrier between PEDOT work function (−5.0 eV) and HOMO level of P2 (0.63 eV) is lower than that of P1 (0.82 eV), which results in lower turn-on voltage (Fig. 9). 4. Conclusion Fig. 7. Current–voltage and luminance–voltage curves of LEDs with the ITO/PEDOT/P1/Ba/Al configuration (I, current intensity; L, luminance).

In conclusion, two novel DPP-based alternating copolymers P1 and P2 were successfully synthesized in good yields through Wittig polycondensation reaction under mild conditions. The polymers are readily soluble in common organic solvents and exhibit excellent thermal stability. Lower LUMO energy levels of P1 and P2 indicate that they possess high electron affinity. Electroluminescent devices fabricated using P1 and P2 as emissive layer show the maxima brightness of 100 and 60 cd/m2 without much optimization. Particularly, the turn-on voltages of LEDs based on P1 and P2 are as low as 4.5 and 3.4 V, respectively. The preliminary EL results indicate that P1 and P2 show almost pure red emission, which implies that P1 and P2 are promising candidates for red emissive materials in PLEDs. Acknowledgments

Fig. 8. Current–voltage and luminance–voltage curves of LEDs with the ITO/PEDOT/P2/Ba/Al configuration (I, current intensity; L, luminance).

It should be noted that P1 and P2 exhibited pure red emission. Their chromaticity values were (0.67, 0.32) and (0.67, 0.31), which were almost identical to the standard red (0.66, 0.34) demanded by the National Television System Committee (NTSC).

This work was supported by the National Natural Science Foundation of China (20872038), Science and Technology Planning Project of Guangdong Province, China (2007A010500011). References [1] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Nature 347 (1990) 539.

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