Macromolecules 2003, 36, 7453-7460
7453
Synthesis and Optical and Electroluminescent Properties of Novel Conjugated Copolymers Derived from Fluorene and Benzoselenadiazole Renqiang Yang, Renyu Tian, Qiong Hou, Wei Yang, and Yong Cao* Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, China Received January 31, 2003; Revised Manuscript Received April 21, 2003
ABSTRACT: A novel series of light-emitting copolymers derived from 9,9-dioctylfluorene (DOF) and 2,1,3benzoselenadiazole (BSeD) is prepared by means of palladium-catalyzed Suzuki coupling reaction. The feed ratios of DOF to BSeD were 50:50, 85:15, 92:8, and 98:2, respectively. All of the copolymers are soluble in common organic solvents and highly fluorescent in solid state. Devices from such copolymers emit orange-red light with λmax ) 570-600 nm. The maximal EL emissions of the devices slightly redshifted gradually with increasing BSeD’s contents. The maximal external quantum efficiency of the polymer light-emitting devices (PLED) reaches 1.0%, which indicates that this new seleno-containing EL polymer based on fluorene and benzoselenadiazole is a promising candidate for fabricating PLEDs.
Introduction The notion that π-conjugated polymeric materials should possess interesting and useful electronic and optical properties is now well established in the scientific literature.1 Many of these expectations have been realized by the practical demonstration of electronic and optoelectronic devices in which conjugated polymeric materials are responsible for charge transport and/or light generation.2 Examples include polymer lightemitting diodes (LEDs),3 photovoltaic devices,4 and fieldeffect transistors.5,6 The wide-ranging applications of electroactive and photoactive conjugated polymers have attracted great interest in the development of functionalized polyacetylenes, polypyrroles, polythiophenes, polyanilines, polyfluorenes, etc.7 These materials combine the excellent electrical and optical properties of metals or inorganic materials with the advantages of polymeric materials, such as low density, easy processability, and synthetic tunability.8 The exceptional physical properties of conjugated polymers are mainly related to their π-conjugated backbone which leads to a strong absorption in the UV-visible range and allows the generation of stable and mobile charge carriers to have partial oxidation or reduction.7,9 The electronic structure of these conjugated polymers is modified by these reversible redox reactions, leading also to new electronic states within the band gap and associated electrochromic properties.9,10 The electrical and optical activity of these materials relies on the ability of the materials to transport electrical charges: electrons and positive holes through their structures. Such active organic materials, such as oligomers, polymers, or smaller organic compounds, have a commercial advantage over their inorganic counterparts, such as silicon and gallium arsenide. Much of the motivation for studying polymers stems from the potential to tailor desirable optoelectronic properties and processing characteristics by manipulation of the primary chemical structure.11 Thus, it is easy to raise or lower the HOMO and LUMO levels including conjugation length control, as well as the introduction of electron-donating or -withdrawing groups to the * To whom correspondence should be addressed. E-mail:
[email protected].
parent chromophore.12 Regulating the HOMO and LUMO energy levels permits the fine-tuning of charge injection properties. In LED, the HOMO/LUMO energy difference directly controls emission frequency, i.e., emission color. Light-emitting polymers have revolutionized flatpanel display technology. A large number of lightemitting polymers have been introduced during the last 10 years.13 Polymers with aromatic or heterocyclic units generally absorb light with wavelengths 300 to 600 nm due to π-π* transitions.14 The color of light, quantum efficiency (QE) of light emission, turn-on voltage, and stability of the devices must be optimized for LEDs to be applicable to commercial light-emitting devices. There have been many attempts to improve the performance of PLEDs. A high quantum efficient, pure, and stable material is essential for commercial LED performance, so many research groups have been developing suitable materials. A series of electroluminescent materials has even been provided as the promising candidates for high-resolution, full color, flat-panel polymeric light-emitting diode (PLED) displays. Among those polymers, polyfluorenes are an important class of electroactive and photoactive material.15 In the last 4 years, this research field has literally exploded because of polyfluorenes’ exceptional optoelectronic properties for applications in light-emitting diodes. Normally, polyfluorene homopolymers have large band gaps and emit blue light. Emission color of polyfluorenes can be tuned in entire visible region by incorporating narrow band gap comonomer into polyfluorene backbone. Most widely used narrow band gap comonomers are a variety of aromatic heterocycles with S and N atoms.16,17 Recently, we have reported a series of polyfluorene copolymers prepared by Suzuki coupling of fluorene and sulfur-containing heterocycles.18 The efficient energy transfer due to exciton trapping on narrow band gap heterocycle sites has been observed. Significant enhancement in the external quantum efficiencies of electroluminescence by exciton confinement in the random copolymers that have regions of different energy gap within a single chain was reported first by Burn et al.3d In this contribution a series of novel copolymers derived from substituted-fluorene and Se-containing
10.1021/ma034134j CCC: $25.00 © 2003 American Chemical Society Published on Web 07/03/2003
7454 Yang et al.
heterocycles, 2,1,3-benzoselenadiazole synthesized by the palladium-catalyzed Suzuki coupling method, is reported. In contrast to extensively investigated polyfluorene copolymers with S-containing heterocycless thiophene9,15 and benzothiadiazole17,19sto the best of our knowledge, this is a first report on electroluminescent Se-containing conjugated polymer. Since the Se atom has a much larger size and less electronegativity than the S atom, it would have a more important influence for the heteroatoms on the light-emitting properties of the resulting copolymers. Complete synthetic details and device characterization are presented. The emission properties of Se-containing copolymers are discussed in comparison with its sulfur analogue. Experimental Section General Details. All manipulations involving air-sensitive reagents were performed under an atmosphere of dry argon. All reagents, unless otherwise specified, were obtained from Aldrich, Acros, and TCI Chemical Co. and used as received. All the solvents were further purified before use. 1H and 13C NMR spectra were recorded on a Varinan Inova 500 or Bruker DRX 400 spectrometer operating at 500 and 100 MHz, respectively, and were referred to tetramethylsilane. Analytical GPC was obtained using a Waters GPC 2410 in tetrahydrofuran via a calibration curve of polystyrene standards. Elemental analyses were performed on Vario EL elemental analysis instrument (Elementar Co.). The elemental selenium analysis was recorded on a polarized Zeeman atomic absorption spectrophotometer, Hitachi Co.. UV-visible absorption spectra were recorded on a HP 8453. The PL quantum yields were determined on a Integrating Sphere IS080 with 325 nm excitation from a HeCd laser (Mells Griod). EL efficiency and brightness were carried out with calibrated silicon photodiode. PL and EL spectra were recorded on Instaspec 4 CCD spectrophotometer (Oriel Co.). Cyclic voltammetry was carried out on a CHI660A electrochemical workstation with platinum electrodes at a scan rate of 50 mV/s against calomel reference electrode with nitrogen-saturated solution of 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) in acetonitrile (CH3CN). 2,7-Dibromofluorene (1) and 2,7-dibromo-9,9-dioctylfluorene (2) were prepared according to the published procedures.20 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9dioctylfluorene (3) was prepared following the modified procedure from 2,7-dibromo-9,9-dioctylfluorene (2).21 The resulting boronic ester was recrystallized from THF and methanol and then further purification by column chromatography (silica gel, 10% ethyl acetate in hexane) to give the product mentioned in the title as a white solid (3.1 g, 46%), mp 128131 °C. 1H NMR (500 MHz, CDCl3), δ (ppm): 7.80 (d, 2H, fluorene ring), 7.74 (s, 2H, fluorene ring), 7.71 (d, 2H, fluorene ring), 1.99 (m, 4H, H-alkyl), 1.39 (s, 24H, CH3), 1.22-1.00 (m, 20H, H-alkyl), 0.81 (t, 6H, H-alkyl), 0.56 (m, 4H, H-alkyl). 13C NMR (100 MHz, CDCl3), δ (ppm): 150.86, 144.30, 134.04, 129.29, 119.77 (fluorene ring), 84.11 (C-alkyl), 55.57 (C9, fluorene ring), 40.49, 32.18, 30.33, 29.58, 25.33, 23.98, 22.99, 14.48 (C-alkyl). Anal. Calcd for C41H64O4B2: C, 76.74; H, 10.04. Found: C, 76.44; H, 9.90. 4,7-Dibromo-2,1,3-benzoselenadiazole (4).22 Bromine (3.2 g, 0.02 mol) was added to a solution of 2,1,3-benzoselenadiazole (1.83 g, 0.01 mol) and silver sulfate (3.12 g, 0.01 mol) in concentrated sulfuric acid (20 mL). The mixture was shaken at a room temperature for 1.25 h, the precipitate of silver bromide was filtered off, and the filtrate poured into ice-water. The precipitate, from ethyl acetate (500 mL), gave 4,7-dibromo2,1,3-benzoselenadiazole (2.05 g, 60%) as golden yellow needles, mp 285-287 °C. 1H NMR (500 MHz, CDCl3), δ (ppm): 7.63 (s, 2H, phenylene ring).13C NMR (100 MHz, CDCl3), δ (ppm): 157.2, 132.1, 116.5. Anal. Calcd for C6H2Br2N2Se: C, 21.1; H, 0.6; Br, 46.9; N, 8.2. Found: C, 21.4; H, 0.9; Br, 46.9; N, 7.8.
Macromolecules, Vol. 36, No. 20, 2003 General Procedure of Polymerization.23 Carefully purified 2,7-dibromo-9,9-dioctylfluorene (2), 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (3), 4,7-dibromo-2,1,3-benzoselenadiazole (4), (PPh3)4Pd0 (0.5-2.0 mol %), and several drops of Aliquat 336 were dissolved in a mixture of toluene and aqueous 2 M Na2CO3. The solvents were further purified in advance. The solution was refluxed with vigorous stirring for 3 days under argon atmosphere. At the end of polymerization, 2,7-bis(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)-9,9-dioctylfluorene was added to remove bromine end groups and bromobenzene was added as a monofunctional end-capping reagent to remove boronic ester end groups, because boron and bromine units could quench emission and contribute to excimer formation in LED applications.24 The whole mixture was then poured into methanol. The precipitated material was recovered by filtration through a funnel. The solid material was washed for 24 h using acetone to remove oligomers and catalyst residues. The resulting polymers were soluble in THF, CHCl3, and toluene. Yields: ∼65-80% respectively. Poly[2,7-(9,9-dioctylfluorene)-alt-4,7-(2,1,3-benzoselenadiazole)] (5a). 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (3) (1.00 equiv) and 4,7-dibromo2,1,3-benzoselenadiazole (4) (1.00 equiv) were used in this polymerization. Anal. Found: C, 61.38; H, 6.12; N, 3.63; Se, 10.21. 1H NMR (500 MHz, CDCl ), δ (ppm): 8.04 (2H, fluorene 3 ring), 7.92 (2H, fluorene ring), 7.79 (2H, fluorene ring), 7.61 (2H, benzoselenadiazole ring), 7.26 (benzene ring), 2.12 (4H, H-alkyl), 1.25-1.11 (m, 20H, H-alkyl), 0.95 (t, 6H, H-alkyl), 0.81 (m, 4H, H-alkyl). 13C NMR (100 MHz, CDCl ), δ (ppm): 160.44 (C-benzosel3 enadiazole ring), 152.04, 141.22, 137.48, 135.85, 129.04, 128.76, 124.64, 120.23 (C-fluorene ring, C-benzene ring), 55.80 (C9fluorene ring), 40.61, 32.23, 30.54, 30.09, 29.68, 24.47, 23.01, 14.47 (C-alkyl). Poly[2,7-(9,9-dioctylfluorene)-co-4,7-(2,1,3-benzoselenadiazole)] (5b). 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (3) (0.50 equiv), 2,7-dibromo-9,9dioctylfluorene (2) (0.35 equiv), and 4,7-dibromo-2,1,3-benzoselenadiazole (4) (0.15 equiv) were used in this polymerization. Anal. Found: C, 84.96; H, 9.78; N, 1.10; Se, 3.62. 1H NMR (500 MHz, CDCl ), δ (ppm): 8.03 (2H, fluorene 3 ring), 7.89, 7.85, 7.80, 7.77, 7.68 (fluorene ring), 7.60 (2H, benzoselenadiazole), 7.48, 7.37, 7.27 (benzene ring), 2.12 (4H, H-alkyl), 1.25-1.14 (m, 20H, H-alkyl), 0.94 (t, 6H, H-alkyl), 0.82 (m, 4H, H-alkyl). 13C NMR (100 MHz, CDCl ), δ (ppm): 160.47 (C-benzosel3 enadiazole ring), 151.57, 140.35, 126.93, 120.36 (C-fluorene ring, C-benzene ring), 55.35 (C9-fluorene ring), 40.63, 32.27, 30.60, 30.05, 29.88, 24.52, 22.97, 14.08 (C-alkyl). Poly[2,7-(9,9-dioctylfluorene)-co-4,7-(2,1,3-benzoselenadiazole)] (5c). 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (3) (0.50 equiv), 2,7-dibromo-9,9dioctylfluorene (2) (0.42 equiv), and 4,7-dibromo-2,1,3-benzoselenadiazole (4) (0.08 equiv) were used in this polymerization. Anal. Found: C, 83.99; H, 9.84; N, 0.43; Se, 1.35. 1H NMR (500 MHz, CDCl ), δ (ppm): 8.06 (2H, fluorene 3 ring), 7.94, 7.87, 7.80, 7.78, 7.72 (fluorene ring), 7.65 (2H, benzoselenadiazole), 7.51, 7.40, 7.28 (benzene ring), 2.16 (4H, H-alkyl), 1.29-1.17 (m, 20H, H-alkyl), 0.96 (t, 6H, H-alkyl), 0.85 (m, 4H, H-alkyl). 13C NMR (100 MHz, CDCl ), δ (ppm): 160.48 (C-benzosel3 enadiazole ring), 152.23, 140.94, 140.44, 126.58, 121.92, 120.38 (C-fluorene ring, C-benzene ring), 55.76 (C9-fluorene ring), 40.79, 32.19, 30.44, 30.11, 29.61, 24.34, 22.99, 14.44 (C-alkyl). Poly[2,7-(9,9-dioctylfluorene)-co-4,7-(2,1,3-benzoselenadiazole)] (5d). 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (3) (0.50 equiv), 2,7-dibromo-9,9dioctylfluorene (2) (0.48 equiv), and 4,7-dibromo-2,1,3-benzoselenadiazole (4) (0.02 equiv) were used in this polymerization. Anal. Found: C, 87.97; H, 10.14; N,
