Synthesis of New Conjugated Polyfluorene Derivatives Bearing

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Chem. Mater. 2006, 18, 1053-1061

1053

Synthesis of New Conjugated Polyfluorene Derivatives Bearing Triphenylamine Moiety through a Vinylene Bridge and Their Stable Blue Electroluminescence Rupei Tang,†,§ Zhan’ao Tan,‡ Yongfang Li,*,‡ and Fu Xi*,§ Department of Chemical Engineering, Wuhan UniVersity of Technology, Wuhan 430070, China, CAS Key Laboratory of Organic Solids, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China, and State Key Laboratory of Polymer Physics & Chemistry, Joint Laboratory of Polymer Science and Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China ReceiVed October 13, 2005. ReVised Manuscript ReceiVed January 3, 2006

Two new conjugated polyfluorene derivatives bearing triphenylamine moiety through a vinylene bridge, PDPF and PBPF, have been successfully synthesized according to the Suzuki coupling method. The structures and properties of the monomers and the resulting conjugated polymers were characterized by using 1H NMR, 13C NMR, FT-IR, elemental analysis, GPC, TGA, DSC, UV-vis absorption spectroscopy, photoluminescence (PL) spectroscopy, and cyclic voltammetry (CV). The obtained polymers possess excellent solubility in common organic solvents, good thermal stability with 5% weight loss temperature of 415-448 °C, and relatively high glass transition temperature (Tg) at about 91-159 °C. The weightaverage molecular weight (Mw) of PDPF and PBPF were 5.79 × 104 and 1.68 × 104 with the polydispersity index (PDI) of 2.75 and 2.69, respectively. The two-dimensional conjugated system of PFs can effectively suppress π-stacking/aggregation, improve hole injection, and facilitate intramolecular energy transfer. Single-layer PLED devices using PDPF and PBPF with the configuration of ITO/PEDOT: PSS/polymer/Ca/Al were fabricated. The maximum electroluminescence (EL) efficiency of the singlelayer PLEDs based on PDPF and PBPF was about 0.19 and 0.51 cd/A, respectively. Meanwhile, the PLEDs with the optimized configuration of ITO/PEDOT:PSS/polymer/Alq3/Al showed greatly improved performance. The double-layer device based on PDPF emitted very stable greenish blue light with a low turn-on voltage of 3.6 V and showed the maximum luminance of about 2930 cd/m2 at 21.0 V. The maximum EL efficiency of the double-layer PLEDs based on PDPF and PBPF was both about 2.08 cd/A.

Introduction Since the discovery of electroluminescence (EL) in poly(p-phenylene vinylene) (PPV) in 1990,1 polymer lightemitting diodes (PLEDs) have attracted much scientific and technological research interests during the past decades. The main advantages of EL conjugated polymers over inorganic and organic small molecule-based materials for LEDs are completely visible colors, fast response, easy processability, ease of forming large area, and easy design of the molecular structure for EL polymers.2-4 To realize commercial color display devices, EL polymers that emit the three basic colors, * To whom correspondence should be addressed. E-mail: [email protected] (F. Xi); [email protected] (Y. F. Li). Tel: +86-10-6255 7907; Fax: +86-1062559373. † Department of Chemical Engineering, Wuhan University of Technology. ‡ CAS Key Laboratory of Organic Solids, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences. § State Key Laboratory of Polymer Physics & Chemistry, Joint Laboratory of Polymer Science and Materials, Institute of Chemistry, Chinese Academy of Sciences.

(1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L. Nature 1990, 347, 539. (2) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37, 402. (3) Cao, Y.; Parker, I. D.; Yu, G.; Zhang, C.; Heeger, A. J. Nature 1999, 397, 414. (4) Wilson, J. N.; Windscheif, P. M.; Evans, U.; Myrick, M. L.; Bunz, U. H. F. Macromolecules 2002, 35, 8681.

blue, green, and red, are required. Research for new blue light-emitting polymers with higher performance remains one of the major challenges in this field, although a large number of EL polymers have been synthesized and investigated. Among the vast kinds of conjugated polymers, fluorenebased conjugated polymers (PFs) are a very promising class of blue-emitting materials for PLEDs because of their processibility, thermal and chemical stability, and high fluorescent quantum yields in the solid state.5-7 However, some serious problems have occurred with the use of PFs as the active materials in PLEDs, which has hampered the application in optoelectronic devices. PFs tend to lead to a red-shifted emission and reduced efficiency during device fabrication and operation, which was probably attributed to the formation of interchain excimers8-10 and/ (5) Pei, Q.; Yang, Y. J. Am. Chem. Soc. 1996, 118, 7416. (6) Shu, C. F.; Dodda, R.; Wu, F. I.; Liu, M. S.; Jen, A. K. Y. Macromolecules 2003, 36, 6698. (7) Li, Y.; Ding, J.; Day, M.; Tao, Y.; Lu, J.; Diorio, M. Chem. Mater. 2004, 16, 2165. (8) Zeng, G.; Yu, W. L.; Chua, S. J.; Huang, W. Macromolecules 2002, 35, 6907. (9) Kla¨rner, G.; Davey, M. H.; Chen, W. D.; Scott, J. C.; Miller, R. D. AdV. Mater. 1998, 10, 993. (10) Marsitzky, D.; Vestberg, R.; Blainey, P.; Tang, B. T.; Hawker, C. J.; Carter, K. R. J. Am. Chem. Soc. 2001, 123, 6965.

10.1021/cm0522735 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/28/2006

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phenylene-bearing triphenylamine moiety through a vinylene bridge. The light-emitting properties of PDPF and PBPF had been investigated, and enhanced EL properties were obtained. Experimental Section

Figure 1. Chemical structures of new fluorene-based derivatives.

or Keto-defects.11-13 Several methods have been used to prevent the longer wavelength emission, such as copolymerization with other electroactive monomers, introduction of bulky groups at the C-9 position of the fluorene, and optimization of EL device configurations.14-16 Another serious problem associated with PFs is the significant energy barrier for hole injection from the ITO anode because of its HOMO energy level as low as -5.8 eV. Although an additional hole transport layer (HTL) is generally used in polyfluorene-based PLEDs to improve their device performance, the preferred approach to address the deficiency in hole injection is the modification of chemical structures of PFs through the incorporation of electron-donating moieties into the polymer backbone or side chain. Triphenylamine is a preferred electron-donating moiety with excellent hole-transporting, tridimensional steric, and good UV-light harvesting properties. A few triphenylaminesubstituted PFs have been designed and synthesized in which triphenylamine moietties are introduced into the PFs main chain or attached directly to the PFs backbone through carbon-carbon bond.17,18 In this paper, we report the synthesis and luminescence properties of two new conjugated polyfluorene derivatives, PDPF and PBPF (Figure 1). Unlike previous one-dimensional conjugated system of PFs,17,18 this two-dimensional conjugated system of the polymers is expected to suppress aggregation and excimers formation, improve hole injection, and facilitate intramolecular energy transfer, and in turn to better the optoelectronic properties of PFs. For this purpose, we present a facile method for preparation of the two new conjugated polyfluorene derivatives copolymerized with (11) List, E. J. W.; Guentner, R.; Freitas, P. S. D.; Scherf, U. AdV. Mater. 2002, 14, 374. (12) Scherf, U.; List, E. J. W. AdV. Mater. 2002, 14, 477. (13) Zhao, W.; Cao, T.; White, J. M. AdV. Funct. Mater. 2004, 14, 785. (14) Yang, X. Y.; Jaiser, F.; Neher, D.; Llawson, P. V.; Bre´das, J. L.; Zojer, E.; Gu¨ntner, R.; Freitas, P. S.; Forster, M.; Scherf, U. AdV. Funct. Mater. 2004, 14, 1097. (15) Cho, N. S.; Hwang, D. H.; Jung, B. J.; Lim, E.; Lee, J.; Shim, H. k. Macromolecules 2004, 37, 5265. (16) Kulkarni, A. P.; Zhu, Y.; Jenekhe, S. A. Macromolecules 2005, 38, 1553. (17) Ego, C.; Grimsdale, A. C.; Weil, T.; Enkelmann, V.; Muˆllen, K. AdV. Mater. 2002, 14, 809. (18) Fang, Q.; Tamamoto, T. Macromolecules 2004, 37, 5894.

Measurement. 1H and 13C NMR spectra were recorded using a Bruker AM-300 spectrometer, and chemical shifts were recorded in ppm. Elemental analysis was measured on a Flash EA 1112 elemental analyzer. Fourier transform infrared (FT-IR) spectra were recorded on a BIO FTS-135 spectrometer by dispersing samples in KBr disks. Molecular weights and polydispersities of polymers were determined by gel permeation chromatography (GPC) analysis relative to polystyrene calibration (Waters 515 HPLC pump, a Waters 2414 differential refractometer, and three Waters Styragel columns (HT2, HT3, and HT4)) using THF as eluent at a flow rate of 1.0 mL/min at 35 °C. Thermogravimetric analysis (TGA) was conducted on a Perkin-Elmer 7 thermogravimetric analyzer with a heating rate of 20 °C/min under a nitrogen atmosphere. Differential scanning calorimetry (DSC) analysis was performed on 2920 MDSC (TA instruments) under a nitrogen atmosphere with a heating rate of 10 °C/min. The UV-vis spectra were recorded on an Hitachi UV-3010 spectrometer. The photoluminescence (PL) and electroluminescence (EL) spectra were obtained with an Hitachi F-4500 Fluorescence spectrophotometer. The cyclic voltammograms were recorded with a computercontrolled Zahner IM6e electrochemical workstation (Germany) using polymer films on glassy carbon disks as the working electrodes, Pt wire as the counter electrode, and Ag/Ag+ (0.01 M) as the reference. The current-voltage (I-V) and luminance-voltage (L-V) characteristics of the light-emitting devices were recorded with a computer-controlled Keithley (U.S.A.) 236 source measure unit and a Keithley 2000 Multimeter coupled with a Si photomultiplier tube. All the measurements were performed under ambient atmosphere at room temperature. Materials. Toluene and THF was distilled over sodium and benzophenone. Dichloromethane, dichloroethane, and DMF were dried by distillation over CaH2. 2,5-Dibromobenzyltriphenylphosphonium bromide was prepared from 2,5-dibromotoluene according to literature procedures.19 9,9-Dihexylfluorene-2,7-bis(trimethyleneborate) was purchased from Aldrich. 4-Diphenylaminobenzaldehyde (1). Phosphorous oxychloride (3.8 mL, 0.04 mol) was added dropwise to a stirred DMF (6.2 mL, 0.08 mol) at 0 °C. The mixture was stirred for 1 h at 0 °C and additionally stirred at room temperature for 1 h. After the addition of triphenylamine (10 g, 0.04 mol) solution in dichloroethane (20 mL), the mixture was stirred at 90 °C for 2 h. After cooling, the solution was poured into cold water. The resulting mixture was neutralized to pH 7 with 2 M NaOH aqueous solution and extracted with chloroform. The extract was washed with plenty of brine and water, successively. The organic extracts were dried over magnesium sulfate, evaporated, and purified with column chromatography (silica gel, ethyl acetate/petroleum ester (1/3) as eluent) to yield 9.79 g (88%) of 4-diphenylaminobenzaldehyde as a yellowish solid. mp 131-132 °C. 1H NMR (300 MHz, CDCH3): δ (ppm) 6.987.01 (d, 2H, ArH), 7.17-7.19 (d, 4H, ArH), 7.32-7.36 (t, 4H, ArH), 7.67-7.70 (d, 2H, ArH), 9.81 (s, 1H, CHO). Anal. Calcd for C19H15NO: C, 83.49; H, 5.53; N, 5.12. Found: C, 83.37; H, 5.49; N, 5.05. 2-(4′-(Diphenylamino)phenylenevinyl)-1,4-dibromobenzene (2). To a stirred solution of 1 (0.80 g, 2.93 mmol) and 2,5-dibromoben(19) Ahn, T.; Song, S. Y.; Shim, H. K. Macromolecules 2000, 33, 6764.

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Chem. Mater., Vol. 18, No. 4, 2006 1055 Scheme 1. Synthetic Route of PDPF

zyltriphenylphosphonium bromide (1.73 g, 2.93 mmol) in dried THF (40 mL) under argon was rapidly added potassium tert-butoxide (0.40 g, 3.56 mmol). The suspended mixture was refluxed for 24 h. The reaction mixture was cooled to room temperature, and water and chloroform were added successively. The two phases were separated, and the water phase was extracted twice with chloroform. The combined organic extracts were washed three times with water, dried over magnesium sulfate, evaporated, and purified with column chromatography (silica gel, chloroform/petroleum ester (1/3) as eluent) to yield 1.0 g (68%) of 2-(4′-(diphenylamino)phenylenevinyl)-1,4-dibromobenzene as a yellowish oil. FT-IR (KBr, cm-1): 961 (s, trans CHdCH), 853 (s, cis CHdCH). 1H NMR (300 MHz, CDCH3): δ (ppm) 6.41-7.76 (m, 19H, aromatic H and vinylic H). 13C NMR (300 MHz, CDCH3): δ (ppm) 121.53, 122.42, 123.10, 123.47, 124.11, 124.80, 127.95, 129.10, 129.21, 129.33, 129.37, 130.32, 131.15, 132.27, 134.31, 139.30, 147.32, 148.29. Anal. Calcd for C26H19Br2N: C, 61.81; H, 3.79; N, 2.77. Found: C, 61.73; H, 3.76; N, 2.69. Poly(2-(4′-(diphenylamino)phenylenevinyl)-1,4-phenylene-alt9,9-n-dihexyl fluorene-2,7-diyl) (PDPF). To a stirred mixture of 2 (0.20 g, 0.40 mmol), 3 (0.20 g, 0.40 mmol), Pd (PPh3)4 (10.0 mg, 4.0 µmol), and toluene (25 mL) under argon was added tetraethylammonium hydroxide (5.30 mL of 10% aqueous solution, 3.64 mmol). The reaction mixture was kept stirring for 24 h at 100 °C. After cooling, the resulting mixture was filtered and the filtrate added dropwise to stirred methanol. The crude polymer was collected by filtration and washed with methanol, dried under high vacuum, and then dissolved in chloroform with stirring at room temperature overnight. The solution was filtered, and the polymer was precipitated by dropwise addition to methanol. The precipitated polymer was collected, washed with methanol, and dried under high vacuum to yield 238 mg (87%) of poly(2-(4′-diphenylamino)phenylenevinyl-1,4-phenylene-alt-9,9-n-dihexylfluorene-2,7-diyl) as a greenish powder. 1H NMR (300 MHz, CDCH3): δ (ppm) 0.680.79 (m, 10H, CH2, CH3), 1.00-1.12 (m, 12H, CH2), 2.02 (br, 4H, CH2), 6.51-8.06 (m, 25H, aromatic H and vinylic H). Anal. Calcd for C51H51N: C, 90.35; H, 7.58; N, 2.07. Found: C, 90.21; H, 7.65; N, 2.14.

1-(2′-Ethylhexyloxy)-4-iodobenzene (5). To a stirred mixture of 4-iodophenol (24.50 g, 0.11 mol), 2-ethylhexylbromide (23.64 g, 0.12 mol), and DMF (100 mL) under argon was added K2CO3 (27.60 g, 0.20 mol). The reaction mixture was refluxed for 15 h and cooled to room temperature. The resulting mixture was filtered, and then most solvent of the filtrate was evaporated. The obtained oil was dissolved in chloroform, washed with water, dried over magnesium sulfate, evaporated, and purified with column chromatography (silica gel, chloroform/petroleum ester (1/3) as eluent) to yield 33.10 g (89%) of 1-(2′-ethylhexyloxy)-4-iodobenzene as colorless oil. 1H NMR (300 MHz, CDCH3): δ (ppm) 0.88-0.93 (m, 6H, CH3), 1.28-1.48 (m, 8H, CH2), 1.68-1.70 (m, 1H, CHCH2), 3.80-3.82 (d, 2H, OCH2), 6.67-6.68 (d, 2H, ArH), 7.517.53 (d, 2H, ArH). Anal. Calcd for C14H21IO: C, 50.61; H, 6.37. Found: C, 50.56; H, 6.31. Bis(4-(2′-ethylhexyloxy)phenyl)amino-benzene (6). The mixture of 5 (32.56 g, 98.01 mmol), phenylamine (4.15 g, 44.55 mmol), CuCl (0.36 g, 3.59 mmol), phenathroline (0.65 g, 3.59 mmol), KOH (43.91 g, 0.78 mol), and toluene (70 mL) was refluxed for 24 h. After cooling, the resulting mixture was poured into plenty of stirred water and extracted with chloroform. The obtained organic phase was washed several times with water, dried over magnesium sulfate, evaporated, and purified with column chromatography (silica gel, chloroform/petroleum ester (1/5) as eluent) to yield 14.11 g (63%) of bis(4-(2′-ethylhexyloxy)phenyl)amino-benzene as a yellowish oil. 1H NMR (300 MHz, CDCH ): δ (ppm) 0.89-0.95 (m, 12H, CH ), 3 3 1.30-1.51 (m, 16H, CH2), 1.71-1.72 (m, 2H, CHCH2), 3.81 (br, 4H, OCH2), 6.80-7.15 (m, 13H, ArH). Anal. Calcd for C34H47NO2: C, 81.39; H, 9.44; N, 2.79. Found: C, 81.24; H, 9.41; N, 2.88. 4-(Bis(4′-(2′′-ethylhexyloxy)phenyl)amino)benzaldehyde (7). 7 was synthesized according to the procedure described for 1 using 6 (14.08 g, 0.03 mol), phosphorous oxychloride (15.73 mL, 0.17 mol), and DMF (26.11 mL, 0.34 mol). A yellowish oil (11.80 g) of 4-bis(4′-(2′′-ethylhexyloxy)phenyl)amino-benzaldehyde was obtained in 79% yield. 1H NMR (300 MHz, CDCH3): δ (ppm) 0.890.95 (m, 12H, CH3), 1.31-1.50 (m, 16H, CH2), 1.72 (m, 2H, CHCH2), 3.82-3.87 (q, 4H, OCH2), 6.80-6.90 (m, 6H, ArH), 7.07-7.12 (t, 4H, ArH), 7.61-7.63 (d, 2H, ArH), 9.75 (s, 1H,

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CHO). Anal. Calcd for C35H47NO3: C, 79.35; H, 8.87; N, 2.64. Found: C, 79.29; H, 8.82; N, 2.69. 2-(4′-(Bis(4′′-(2′′′-ethylhexyloxy)phenyl)amino)phenylenevinyl)1,4-dibromobenzene (8). 8 was synthesized according to the procedure described for 2 using 7 (1.63 g, 3.08 mmol), 2,5dibromobenzyltriphenylphosphonium bromide (1.82 g, 3.08 mmol), and THF (45 mL). A yellowish oil (1.66 g) of 2-(4′-bis(4′′-((2′′′ethylhexyloxy)phenyl)amino)phenylenevinyl-1,4-dibromobenzene was obtained in 74% yield. FT-IR (KBr, cm-1): 960 (s, trans CHdCH). 1H NMR (300 MHz, CDCH3): δ (ppm) 0.90-0.95 (m, 12H, CH3), 1.32-1.55 (m, 16H, CH2), 1.72 (m, 2H, CHCH2), 3.773.83 (q, 4H, OCH2), 6.82-6.85 (d, 4H, ArH), 6.88-6.90 (s, 2H, ArH), 6.93-6.97 (d, 1H, ArH), 7.05-7.07 (d, 4H, ArH), 7.157.18 (d, 2H, ArH), 7.33-7.35 (d, 2H, ArH), 7.39-7.41 (d, 1H, ArH), 7.74 (s, 1H, ArH). 13C NMR (300 MHz, CDCH3): δ (ppm) 11.52, 14.47, 23.40, 24.15, 29.35, 30.94, 39.81, 71.04, 115.73, 120.09, 121.86, 122.66, 123.31, 127.30, 128.15, 128.51, 129.31,

131.12, 129.37, 132.89, 134.60, 139.94, 140.52,156.44. Anal. Calcd for C42H51Br2NO2: C, 66.23; H, 6.75; N, 1.84. Found: C, 66.11; H, 6.71; N, 1.92. Poly(2-(4′-(bis(4′′-(2′′′-ethylhexyloxy)phenyl)amino)phenylenevinyl)-1,4-phenylene-alt-9,9-n-dihexylfluorene-2,7-diyl) (PBPF). PBPF was synthesized according to the procedure described for PDPF using 8 (0.29 g, 0.38 mmol), 3 (0.19 g, 0.38 mmol), Pd (PPh3)4 (30.0 mg, 12.0 µmol), tetraethylammonium hydroxide (5.00 mL of 10% aqueous solution, 3.43 mmol), and toluene (25 mL). A bright yellow powder (274 mg) of poly(2-(4′-bis(4′′-((2′′′-ethylhexyloxy)phenyl)amino)phenylenevinyl-1,4-phenylene-alt-9,9-n-dihexylfluorene-2,7-diyl) was obtained in 78% yield. 1H NMR (300 MHz, CDCH3): δ (ppm) 0.68-0.93 (m, 22H, CH2, CH3), 1.001.53 (m, 28H, CH2), 1.69 (br, 2H, CHCH2), 2.02 (br, 4H, CH2), 3.79 (br, 4H, OCH2), 6.78-8.04 (m, 23H, aromatic H and vinylic H). Anal. Calcd for C67H83NO2: C, 86.12; H, 8.95; N, 1.50. Found: C, 86.02; H, 8.86; N, 1.59.

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Figure 2. FT-IR spectra of monomers 2 and 8. Table 1. Polymerization Results, Molecular Weights, and Thermal Data of Polymers polymer

yield (%)

Mna (×10-4)

Mwa (×10-4)

PDI

TGA b (°C)

Tgc (°C)

PDPF PBPF

87 78

2.11 0.62

5.79 1.68

2.75 2.69

448 415

159 91

a Determined by GPC in THF based on polystyrene standards. b Temperature at 5% weight loss under nitrogen. c Determined by DSC at a heating rate of 10 °C/min under nitrogen.

Results and Discussion Synthesis and Characterization. The synthesis of the monomers and the corresponding polymers are outlined in Schemes 1 and 2. Triphenylamine was used as a starting material for the preparation of 1, which was then easily synthesized using 2,5-dibromobenzyltriphenylphosphonium bromide to give the key intermediate, substituted 1,4dibromobenzene, 2, totally in 60% yield by the Wittig reaction.20 Another key intermediate, substituted 1,4-dibromobenzene, 8, was prepared in a facile four-step reaction. The alkylation of 4-iodophenol with 2-ethylhexylbromide in the presence of K2CO3 gave 5, and subsequent conversion to 6 according to the modified Ullmann Condensation reaction21 that was in turn converted to give 7 by the Vielsmier reaction.22 The monomer 8 was obtained using 7 and 2,5-dibromobenzyltriphenylphosphonium bromide in 74% yield according to the procedure described for 2. The structures of the monomers (2 and 8) were confirmed by FT-IR, 1H and 13C NMR spectroscopy, and elemental analysis. The Wittig reaction often gives both cis and trans geometric structures,18 and in our case, δ (CH) peaks of the cis and trans vinylene group were observed at 853 and 961 cm-1 in the IR spectra of 2, respectively, as shown in Figure 2. However, only a trans vinylene group peak at about 960 cm-1 was observed in the IR spectra of 8, as also shown in Figure 2. (20) Yang, Z.; Hu, B.; Karasz, F. E. Macromolecules 1995, 28, 6151. (21) Semmelhack, M. F.; Helquist, P.; Jones, L. D.; Keller, L.; Mendelson, L.; Ryono, L. S.; Smith, J. G.; Stauffer, R. D. J. Am. Chem. Soc. 1981, 103, 6460. (22) Li, H. C.; Geng, Y. H.; Tong, S. W.; Tong, H.; Hua, R.; Su, G. P.; Wang, L. X.; Jing, X. B.; Wang, F. S. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 3278.

Figure 3. UV-vis absorption and PL spectra of PDPF and PBPF: (a) in chloroform solution and (b) in film state.

The obtained polymers, PDPF and PBPF, were easily prepared by the Suzuki coupling method23 and identified by 1 H NMR spectroscopy and elemental analysis. The synthesized polymers were easily soluble in common organic solvents, such as chloroform, toluene, and xylene at room temperature. Table 1 summarizes the polymerization results, molecular weights, and thermal data of the present polymers. The weight-average molecular weight (Mw) of PDPF and PBPF were 5.79 × 104 and 1.68 × 104 with the polydispersity index (PDI) of 2.75 and 2.69, respectively. The TGA curves of the polymers reveal a relatively high thermal stability, and the onset weight loss temperatures of PDPF and PBPF were found to be 448 and 415 °C, respectively. DSC curves display glass transition temperatures (Tg) of PDPF and PBPF at 159 and 91 °C, which were higher than that of poly(9,9-di-n-octylfluoene-2,7-diyl) (PDOF) (reported to be about 75 °C).24 The high thermal stability of the EL polymers is closely related to the performance of the LEDs, which prevents morphological change, deformation, and degradation of the emitting layer by current-induced heat during the operation of EL devices.25 Optical and Photoluminescence Properties. The photophysical properties of PDPF and PBPF were measured by UV-vis and fluorescence spectroscopy in chloroform (23) Suzuki, A.; Miyaura, N. Chem. ReV. 1995, 95, 2457. (24) Grell, M.; Bradley, D. D. C.; Inbaekara, M.; Woo, E. P. AdV. Mater. 1997, 11, 2502. (25) Tokito, S.; Tanaka, H.; Noda, K.; Okada, A.; Taga, Y. Appl. Phys. Lett. 1997, 70, 929.

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Table 2. Optical and Photoluminescence Properties of the Polymers soln λmaxa (nm)

film λmaxb (nm)

polymer

Abs

PL

Abs

PL

Egc (eV)

PDPF PBPF

360 357

451 500

363 361

461 485

2.86 2.75

a Measured in chloroform solution. b Polymer film cast from chloroform solution. c Band gap estimated from the onset wavelength of the optical absorption.

Figure 5. EL spectra of the ITO/PEDOT:PSS/polymer/Ca/Al devices.

Figure 4. Cyclic voltammograms of (a) PDPF and (b) PBPF.

solution and thin films on quartz plates and shown in Figure 3. Both polymers in chloroform solution have almost identical absorption spectra with maximum peaks at about 360 and 357 nm, respectively, which is corresponding to a π-π* transition. In addition, the shoulder peak at about 300 nm is mostly attributed to the absorption of a triphenylamine moiety. PDPF and PBPF emit blue and blue-green light in dilute chloroform solution, respectively. As shown in Figure 3a, the photoluminescence (PL) spectra of the polymers are regular and symmetrical, and the maximum emission of PDPF and PBPF in dilute chloroform solution is observed at about 451 and 500 nm, respectively. Figure 3b shows the optical absorption and PL spectra of PDPF and PBPF in the solid state. The absorption spectra of the polymers in the solid state are similar (a little redshifted) to those in chloroform solution, and the maximum absorption of PDPF and PBPF corresponding to π-π* transition is 363 and 361 nm, respectively. Optical band gaps (Eg) determined from the absorption edge of the solid-state spectra of PDPF and PBPF are found to be 2.86 and 2.75

Figure 6. Current density-voltage (I-V) (a) and luminance-voltage (LV) (b) characteristics of the ITO/PEDOT:PSS/polymer/Ca/Al devices.

eV. The PL spectra of PDPF and PBPF in the solid state are almost identical to those in dilute chloroform solution, and the maximum emission peaks of PDPF and PBPF are observed at about 461 and 485 nm, respectively. The film PL of both polymers shows no longer wavelength emission, which demonstrates that the introduction of a large triphenylamine moiety can effectively suppress π-stacking/ aggregation of conjugated polymers in the solid state. Compared to the solution PL spectrum, the film PL spectrum of PDPF is only red-shifted by 10 nm, but the film PL

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Figure 8. Band diagrams of ITO, PEDOT, polymers, Alq3, Ca, and Al electrode. Figure 7. EL current efficiency curves of the ITO/PEDOT:PSS/polymer/ Ca/Al devices. Table 3. Electroluminescence Properties of Type I Devices polymer

EL λma (nm)

Vona (V)

Lmaxb (voltage/V) (cd/m2)

efficiencyc (cd/A)

PDPF PBPF

472 492

5.0 6.1

1760 (11.6) 2970 (14.6)

0.19 0.51

a Turn-on voltage. b Luminance at maximum bias voltage. c Maximum EL efficiency.

spectrum of PBPF is blue-shifted by 15 nm, which is probably due to the less planar conformation resulting from the larger pendant side group of PBPF. Theoretically, the solution and film PL spectra of PBPF should be more blueshifted than those of PDPF because of the decreased conjugated length resulting from the introduction of larger dialkoxy-substituted triphenylamine moiety. Interestingly, the maximum emission peak of PBPF in solution and solid state is significantly red-shifted by 49 and 24 nm compared to that of PDPF, respectively. The results demonstrate that intramolecular energy transfer of PBPF can easily occur from an electro-donating dialkoxy group of side chain to polymer backbone via a vinylene bridge, which is assumed that the EL performance of the two-dimensional conjugated polymers will be improved. The optical and photoluminescence properties of the polymers are summarized in Table 2. Electrochemical Properties. To gain information on the charge injection, a cyclic voltammogram (CV) was employed to estimate the HOMO and LUMO energy levels of the polymers.26 As shown in Figure 4, both PDPF and PBPF showed electrochemically quasi-reversible n- and p-doping processes. The onset potentials for oxidation (Eon ox) were observed to be 0.49 and 0.25 V (vs Ag/Ag+) for PDPF and PBPF, respectively. The HOMOs of PDPF and PBPF were estimated to be -5.20 and -4.96 eV, respectively, according to the following equations: HOMO ) -(Eon ox + 4.71) (eV).27 The LUMOs of PDPF and PBPF were calculated to be 2.34 and 2.21 eV from the values of the band gap and HOMO energy level. The HOMO of the PEDOT layer (26) Li, Y. F.; Cao, Y.; Gao, J.; Wang, D. L.; Yu, G.; Heeger, A. J. Synth. Met. 1999, 99, 243. (27) Sun, Q. J.; Wang, H. Q.; Yang, C. H.; Li, Y. F. J. Mater. Chem. 2003, 13, 800.

Figure 9. EL spectra of PDPF, PBPF, and Alq3, which have ITO/PEDOT: PSS/polymer/Alq3/Al configuration.

(HTL) is known to be -5.0 eV, and the hole injection from PEDOT to the polymers is expected to be easier than other PFs.14-16,23 Electroluminescence Properties of PLED Devices. Initial investigations of the EL properties of PDPF and PBPF were made by fabricating single-layer (type I) PLEDs of ITO/ PEDOT:PSS(25 nm)/polymer(100 nm)/Ca(6 nm)/Al(80 nm). As shown in Figure 5, the EL spectra of PDPF and PBPF exhibited asymmetrical and additional long-wavelength emission, in comparison with the PL spectra, which was

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Tang et al.

Figure 11. EL current efficiency curves of the ITO/PEDOT:PSS/polymer/ Alq3/Al devices. Table 4. Electroluminescence Properties of Type II Devices polymer

EL λma (nm)

Vona (V)

Lmaxb (voltage/V) (cd/m2)

efficiencyc (cd/A)

PDPF PBPF

469 490

3.6 3.8

2930 (21.0) 3460 (20.0)

2.08 2.08

a Turn-on voltage. b Luminance at maximum bias voltage. c Maximum EL efficiency.

Figure 10. Current density-voltage (I-V) (a) and luminance-voltage (LV) (b) characteristics of the ITO/ PEDOT:PSS/polymer/Alq3/Al devices.

probably due to the Keto-defects formed by the diffusion of reactive Ca into the PF layer.28 To investigate the color purity, chromaticity coordinates using the Commision International I’Eclairage (CIE) (1931) color matching function were converted from the EL spectra. The emission colors of PDPF and PBPF at the CIE coordinates of x ) 0.239, y ) 0.354 and x ) 0.262, y ) 0.405 are bluish green and green, respectively. Figure 6 shows current density-voltage (I-V) and luminance-voltage (L-V) characteristics of the single-layer PLED devices. The current density increased exponentially with the increasing forward bias voltage, which was a typical diode characteristic (Figure 6a). The turn-on voltages of the PLEDs based on PDPF and PBPF was approximately 5.0 and 6.1 V, respectively. The maximum luminances of the single-layer PLEDs based on PDPF and PBPF were about 1760 cd/m2 at 11.6 V and 2970 cd/m2 at 14.6 V, respectively. As shown in Figure 7, the maximum EL efficiencies of the devices based on PDPF and PBPF were 0.19 and 0.51 cd/ A, respectively. The characteristics of the type I PLEDs are summarized in Table 3. The energy band diagrams of ITO, PEDOT, the polymers, Ca and Al electrode, and tris(8-hydroxyquinoline)aluminum (28) Gong, X.; Iyer, P. K.; Moses, D.; Bazan, G. C.; Heeger, A. J.; Xiao, S. S. AdV. Funct. Mater. 2003, 13, 325.

(Alq3) are shown in Figure 8. Judging from these energy levels, the poor performance of the type I devices was probably due to the imbalance of the hole and electron injection. Obviously, introducing an electron-transporting/ hole-blocking Alq3 layer may improve the EL performance of the PLEDs. Thus, the double-layer (type II) PLEDs with the device structure of ITO/PEDOT:PSS(25 nm)/polymer(100 nm)/Alq3(30 nm)/Al(80 nm) were fabricated. As shown in Figure 9, the EL spectrum peaks of PDPF and PBPF for the type II devices were almost identical to those of their PL spectra (Figure 3), and the type II PLEDs based on PDPF and PBPF emitted very stable greenish blue and bluish green light under different voltages, respectively. The CIE coordinates of PDPF are almost immovable with the chromaticity values of (x ) 0.138, y ) 0.269) under 10 V, (x ) 0.135, y ) 0.273) under 12 V, and (x ) 0.136, y ) 0.276) under 14 V. The CIE coordinates of PBPF are also almost immovable with the chromaticity values of (x ) 0.157, y ) 0.435) under 10 V, (x ) 0.163, y ) 0.441) under 12 V, and (x ) 0.156, y ) 0.437) under 14 V. Alq3 is wellknown to be a very efficient emitter; to rule out the EL emission of Alq3 in the type II devices, the EL spectrum of pure Alq3 with the device structure of ITO/PEDOT:PSS/Alq3/ Al was measured and also shown in Figure 9. It can be seen that the EL spectrum of Alq3 is entirely different from those of the type II devices. Thus, we could draw a conclusion that Alq3 did not participate in the emission process of the polymers under our experimental conditions. Gong et al.28 reported that the diffusion of reactive Ca into the PF layer and subsequent formation of intermediate products lead to Keto-defects. In the type II devices, incorporating the electron-transporting Alq3 layer between PF emissive layer and the Ca cathode prevented the diffusion of Ca into the

New 2-D Conjugated Polyfluorene DeriVatiVes

PF layer, so that suppressed the red-shifted emission. Our results demonstrated that the optimization of device configurations could suppress the Keto-emission of PFs and produce stable electroluminescence, which is in accordance with the results obtained by Gong et al.28 Figure 10 shows current density-voltage (I-V) and luminance-voltage (L-V) characteristics of the double-layer PLED devices. The turn-on voltages of the PLEDs based on PDPF and PBPF were approximately 3.6 and 3.8 V, respectively. The maximum luminance of PDPF and PBPF was about 2930 cd/m2 at 21.0 V and 3460 cd/m2 at 20.0 V, respectively. As shown in Figure 11, the maximum EL efficiency of the type II PLEDs based on PDPF and PBPF was both about 2.08 cd/A, which was 4-11 times higher than that of the single-layer PLEDs (type I). The characteristics of the type II PLEDs are summarized in Table 4. The results indicate that the optimization of PLEDs could effectively suppress red-shifted emission and greatly improve the performance of the devices.

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Conclusions We have successfully synthesized two new polyfluorene derivatives, PDPF and PBPF, according to the Suzuki coupling method. The resulting polymers possess excellent solubility in common organic solvents, good thermal stability, and high molecular weights. The two-dimensional conjugated system of PFs can effectively suppress π-stacking/aggregation, improve hole injection, and facilitate intramolecular energy transfer. The optimized PLED based on PDPF emitted a very stable greenish blue light with the greatly improved performance, and the maximum EL efficiency reached about 2.08 cd/A. Acknowledgment. The financial support for this work by the Ministry of Science and Technology of China (973 project, No. 2002CB613402) and the National Natural Science Foundation of China (No. 50273042, 50373050, and 20421101) is gratefully acknowledged. CM0522735