Having Pendent Fluorene Groups
CHUN-HAO HUANG, SHENG-HSIUNG YANG, KUEI-BAI CHEN, CHAIN-SHU HSU
Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan, Republic of China
Received 11 August 2005; accepted 29 September 2005 DOI: 10.1002/pola.21163
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Five novel fluorene-containing polymers, poly[(9,9-dimethylfluoren-2-yl)a-cetylene] (PFA1), poly[(1-pentyl-2-(9,9-dimethylfluoren-2-yl)acetylene) (PFA2), poly[1-decyl-2-(9,9-dimethylfluoren-2-yl)acetylene] (PFA3), poly[1-phenyl-2-(9,9-dimethylfluo-ren-2-yl)acetylene] (PFA4), and poly[1-(3,4-difluorophenyl)-2-(9,9-dimethylfluoren-2-yl)acetylene] (PFA5) were synthesized by the polymerization of the corresponding fluorene-substituted acetylenic monomers (M1–M5), using WCl6, MoCl5,and TaCl5as
catalysts and n-Bu4Sn as a cocatalyst. The synthesized polymers were thermally stable
and readily soluble in common organic solvents. The degradation temperatures for a 5% weight loss of the polymers were352–503 8C under nitrogen. PFA1–PFA5 show emission peaks from 402 to 590 nm. Besides, their electroluminescent properties were studied in heterostructure light-emitting diodes (LEDs), using PFA2–PFA5 as an emitting layer. The PFA5 device revealed an orange-red emission peak at 602 nm with a maximum luminescence of 923 cd/m2at 8 V. A device with the ITO/PEDOT/ a mix-ture of PFA2 (98 wt %) and PFA5 (2 wt %)/Ca/Al showed near white emission. Its max-imum luminance and current efficiency are 450 cd/m2 at 15 V and 1.3 cd/A,
respec-tively.VVC 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 519–531, 2006 Keywords: fluorescence; light-emitting diodes (LED); photoluminescence; polyacety-lene
INTRODUCTION
Conjugatedpolymers have been extensively studied for their potential applications in light emitting diodes,1 organic lasers,2 thin film transistors,3 and solar cells.4Polyacetylene is a prototypical con-jugated polymer among many promising conju-gated polymers, which exhibits high conductivity upon doping.5The conductivities of this polymer can reach as high as 105 S/cm.6 Although poly-acetylene shows a very high electrical conductivity by doping, this polymer has rarely been used as a light-emitting polymer for electroluminescent (EL) devices because nonsubstituted polyacetylene is
insoluble, infusible, and unstable in air and ex-hibits almost no photoluminescence (PL) in the visible region.
Substituted polyacetylenes such as poly(phe-nylacetylene) (PPA) were reported by Percec and coworkers.7–10Recently, Masuda and coworkers,11–15 Tang and coworkers,16–19 and our laboratory20 have also synthesized several series of disubsti-tuted polyacetylene derivatives, which were sta-ble in air and solusta-ble in common organic sol-vents. Although the conductivities of these poly-acetylene derivatives are not as high as those of nonsubstituted ones, some show both PL and EL properties in the visible wavelength region. The most commonly used substituents for these poly-acetylene derivatives are the alkyl and phenyl groups. These alkyl or phenyl-substituted poly-acetylenes can emit red to blue light based on dif-ferent substituents. In general, the PL
efficien-Correspondence to: C. S. Hsu (E-mail: [email protected]. edu.tw)
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 519–531 (2006) V
VC2005 Wiley Periodicals, Inc.
cies of disubstituted polyacetylenes are higher than those of monosubstituted polyacetylenes.21 The EL efficiencies of these substituted polyace-tylenes are rather low. The best EL device using poly[1-(p-butylphenyl)-2-phenylacetylene] doped with 2-(4-biphenyl)-5-(4-t-butylphenyl)-1,3,4-oxa-diazole as emitting materials was reported.11Its quantum yield, current efficiency, and lumines-cence were 0.1%, 0.038 cd/A, and 30 cd/m2, re-spectively.
Recently, a naphthalene, a silole, or a carbazole pendant was incorporated into a poly(1-phenyl-1-alkyne) structure by Tang and coworkers.16–18 They found that the materials not only exhibited a higher thermal stability, but also emitted a strong blue light. They showed much higher quantum efficiencies than those of alkyl- and phenyl- sub-stituted polyacetylenes. For example, an EL de-vice17 with the configuration of ITO/poly[1-phe-nyl-5-(a-naphthoxy)pentyne]: PVK/bathocuproine/ Alq3/LiF/Al, emitted blue light of 468 nm with a maximum brightness of 955 cd/m2 and a power efficiency of 0.18 lm/W. Sanda et al.15also reported some polyacetylenes with pendent carbazole groups. The EL devices based on poly(3,6-ditertbutyl-N-(p-ethynylphenyl)carbazole) in conjunction with iridium complexes exhibited brightness from 13 to 18 cd/m2.
The polymers containing fluorene moieties in the main chain or side-chain have attracted con-siderable attention because of their unique prop-erties of EL applications.22–28Recent examples of the polymers containing pendant fluorene moi-eties include poly(vinylfluorene),25 poly(fluoreny-lacetylene),26 and poly(fluorenylphenylene vinyl-ene).27,28The bulky fluorene pendant also affects the polymer packing and reduces chain interac-tion between the polymer backbones. This will improve the performance of polymer light emit-ting diodes (PLED). In this paper, we incorpo-rated a fluorene unit into the polyacetylene back-bones, and investigated their peculiar PL and EL properties.
EXPERIMENTAL
Materials
Bis(triphenylphosphine)palladium(II) chloride (PdCl2(PPh3)2, 99.99%), copper(I) iodide (CuI, 98%), triphenylphosphine (PPh3, 99%), 2-methyl-3-butyn-2-ol (98%), tetra-n-butyltin (n-Bu4Sn), mo-lybdenum(V) chloride (MoCl5, 99.9%), and
tan-talum(V) chloride (TaCl5, 99.99%) were purchased from Aldrich. Tungsten(VI) chloride (WCl6) was purchased from ACROS. All commercial products were used without further purification. Tetrahy-drofuran (THF) was dried over sodium, and diox-ane, triethylamine (Et3N), and N,N-dimethylfor-mamide (DMF) were dried over calcium hydride and then distilled under nitrogen.
Techniques 1
H and 13C NMR spectra (300 MHz) were
recorded on a Varian VXR-300 spectrometer. Fourier transform infrared (FTIR) spectra were recorded as KBr pellets on Perkin Elmer Spectra 1 spectrometer. Mass spectra were obtained on a JEOL JMS-SX 102A mass spectrometer. Thermal transitions and thermodynamic parameters were determined by using a Perkin Elmer Pyris 1 dif-ferential scanning calorimeter equipped with a liquid-nitrogen cooling accessory. Heating and cooling rates were 10 8C/min. Gel permeation chromatography (GPC) was run on a Waters 510 LC instrument equipped with a 410 differential refractometer, a UV detector, and a set of poly-styrene gel columns of 102, 5 102, 103, and 104 A˚ . The oven temperature was set at 40 8C. THF was used as eluent and the flow rate was 1 mL/ min. The UV–visible spectra and photolumines-cence spectra of polymers were measured from an HP 8453 diode-array spectrophotometer and a Hitachi F-4500 luminescence spectrophotometer, respectively. The PL quantum yield (/f) in chloro-form solution was measured by integrated sphere. Meanwhile, cyclic voltammetric (CV) measure-ments were made in acetonitrile (CH3CN) with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte at a scan rate of 50 mV/s. Platinum wires were used as both the counter and working electrodes, and sil-ver/silver ions (Ag in 0.1 M AgNO3solution, from Bioanalytical Systems, Inc.) were used as the reference electrode. The PLED characterization was carried out by a Keithley 2400 source-mea-sure unit and a calibrated silicon photodiode. The brightness was further measured using a Photo Research PR650 spectrophotometer.
Synthesis of Monomers M1–M5
The synthesis of fluorine-substituted acetylenic monomers M1–M5 is outlined in Scheme 1.
2-Iodofluorene (1)
Fluorene (30.0 g, 180 mmol), iodine (23.0 g, 91 mmol), and iodic acid (HIO3) (8.0 g, 45 mmol) were dissolved in 80% acetic acid aqueous solu-tion containing a small amount of sulfuric acid. The reaction mixture was heated at 808C for 4 h under nitrogen atmosphere. After the solution was cooled, the solvent was removed by
decanta-tion. The product was washed with methanol and dried to yield 31.6 g (60%) of brown solid; mp: 130–1328C. 1 H NMR (300 MHz, CDCl3) d (ppm): 3.81 (d, J ¼ 15.9 Hz, 2H), 7.31 (m, 2H), 7.44 (m, 2H), 7.66 (d, J¼ 7.2 Hz, 1H), 7.73 (d, J ¼ 7.2 Hz, 1H), 7.85 (s, 1H).13C NMR (75 MHz, CDCl3) d (ppm): 36.6, 92.5, 120.2, 121.7, 122.3, 127.1, 127.8, 132.2,
135.9, 138.0, 138.6, 153.2, 154.9. MS [Mþ] calcd. for C13H9I 292.12, found 292. ELEM. ANAL. Calcd. for C13H9I: C, 53.45; H, 3.11. Found: C, 53.40; H, 3.18.
9,9-Dimethyl-2-iodofluorene (2)
2-Iodofluorene (25.0 g, 85.6 mmol) was dissolved in THF and treated with potassium tert-butoxide (21.8 g, 0.19 mol) to give a red solution, followed by methylation with methyliodide (28.2 g, 0.19 mol) at room temperature for 3 h. The product was purified by column chromatography (silica gel, n-hexane as eluent, and Rf 0.7) to yield 25.2 g (92%) of yellow solid; mp: 63–658C. 1 H NMR (300 MHz, CDCl3) d (ppm): 1.47 (s, 6H), 7.31 (m, 2H), 7.44 (m, 2H), 7.66 (d, J¼ 7.2 Hz, 1H), 7.73 (d, J ¼ 7.2 Hz, 1H), 7.85 (s, 1H). 13C NMR (75 MHz, CDCl 3) d (ppm): 26.9, 47.1, 92.5, 120.0, 121.7, 122.5, 127.1, 127.8, 132.0, 135.9, 138.0, 138.6, 153.0, 154.9. MS [Mþ] calcd. for C15H13I 320.17, found 320. ELEM. ANAL. Calcd.
for C15H13I: C, 56.27; H, 4.09; I, 39.64. Found: C, 56.21; H, 4.12. 9,9-Dimethyl-2-(3-hydroxy-3-methyl-1-butynyl)-fluorine (3) 9,9-Dimethyl-2-iodofluorene (25.0 g, 78.1 mmol), PdCl2(PPh3)2 (0.56 g, 0.78 mmol), CuI (1.53 g, 5.86 mmol), and PPh3 (0.74 g, 3.9 mmol) were dissolved in Et3N (250 mL), and the mixture was stirred under nitrogen. After all the catalysts were dissolved, 2-methyl-3-butyn-2-ol (7.88 g, 93.7 mmol) was added. The resulting solution was heated at 70 8C for 12 h. After Et3N was removed under reduced pressure, the product was extracted with diethyl ether. The crude product was isolated by evaporating the solvent and purified by column chromatography (silica gel, ethyl acetate: n-hex-ane ¼ 1:5 as eluent, and Rf 0.4) to yield 13.0 g (68%) of brown oil. 1 H NMR (300 MHz, CDCl3) d (ppm): 1.47 (s, 6H), 1.65 (s, 6H), 7.31 (m, 2H), 7.41 (m, 2H), 7.48 (s, 1H), 7.66 (m, 2H).13C NMR (75 MHz, CDCl3) d (ppm): 26.9, 31.4, 46.8, 66.7, 81.2, 90.4, 119.8, 120.3, 122.5, 122.6, 126.4, 127.0, 127.8, 131.2, 138.2, 140.0, 153.9, 154.0. MS [Mþ] calcd. for C20H20O 276.38, found 276. ELEM. ANAL. Calcd. for C20H20O: C, 89.92; H, 7.29; O, 5.79. Found: C, 89.94; H, 7.23.
1-(9,9-Dimethylfluorene-2-yl)acetylene (M1) 9,9-Dimethyl-2-(3-hydroxy-3-methyl-1-butyl)-fluorene (10.0 g, 36 mmol) and KOH (4.44 g, 79.2 mmol) were dissolved in dioxane (300 mL). The mixture was heated at 120 8C for 4 h, cooled to room temperature, and 6 N HCl aqueous solution (12 mL) was added. The resulting solution was extracted with ethyl ether. The crude product was isolated by evaporating the solvent and puri-fied by column chromatography (silica gel, n-hex-ane as eluent, Rf 0.55) to yield 5.8 g (74%) of col-orless oil. 1 H NMR (300 MHz, CDCl3) d (ppm): 1.47 (s, 6H), 3.12 (s, 1H), 7.32 (m, 2H), 7.44 (m, 2H), 7.56 (s, 1H), 7.64 (d, J¼ 7.2 Hz, 1H), 7.67 (d, J ¼ 7.2 Hz, 1H). 13C NMR (75 MHz, CDCl3) d (ppm): 26.9, 46.8, 77.0, 84.4, 119.8, 120.3, 122.5, 122.6, 126.4, 127.0, 127.8, 131.2, 138.2, 140.0, 153.9, 154.0. MS [Mþ] calcd. for C17H14 218.30, found 218. ELEM. ANAL. Calcd. for C17H14: C, 93.54; H, 6.46. Found: C, 93.53; H, 6.47.
1-Pentyl-2-(9,9-dimethylfluoren-2-yl)acetylene (M2), 1-Decyl-2-(9,9-dimethylfluoren-2-yl)
acetylene (M3), 1-Phenyl-2-(9,9-dimethylfluoren-2-yl)acetylene (M4), and 1-(3,4-Difluorophenyl)-2-(9,9-dimethylfluoren-2-yl) acetylene (M5)
Monomers M2–M5 were synthesized by a similar method. The preparation of monomer M2 is described here. 9,9-Dimethyl-2-iodofluorene (10 g, 31.2 mmol), PdCl2(PPh3)2(0.22 g, 0.31 mmol), CuI (0.62 g, 2.37 mmol), and PPh3 (0.24 g, 1.26 mmol) were dissolved in Et3N (100 mL), and the mixture was stirred at room temperature under nitrogen. After all catalysts were dissolved, 1-heptyene (3.6 g, 37.5 mmol) was added. The resulting solution was reacted at 70 8C for 12 h. After Et3N was removed under reduced pressure, the product was extracted with diethyl ether. The crude product was isolated by evaporating the solvent and purified by column chromatogra-phy (silica gel, n-hexane as eluent, and Rf 0.6) to yield 4.76 g (53%) of colorless oil.
1 H NMR (300 MHz, CDCl3) d (ppm): 0.93 (t, 3H), 1.48 (m, 10H), 1.62 (m, 2H), 2.43 (t, 2H), 7.32 (m, 2H), 7.40 (m, 2H), 7.46 (s, 1H), 7.61 (d, J ¼ 7.2 Hz, 1H), 7.63 (d, J ¼ 7.2 Hz, 1H).13 C NMR (75 MHz, CDCl3) d (ppm): 14.0, 19.4, 22.2, 26.9, 28.5, 31.1, 46.7, 81.2, 90.4, 119.7, 120.1, 122.5, 122.6, 125.8, 127.0, 127.3, 130.5, 138.5, 138.6, 153.4, 153.7. MS [Mþ] calcd. for C22H24 288.43,
found 288. ELEM. ANAL. Calcd. for C22H24: C, 91.61; H, 8.39. Found: C, 91.60; H, 8.40. M3 Yield: 51%. 1H NMR (300 MHz, CDCl3) d (ppm): 0.9 (t, 3H), 1.27 (m, 13H), 1.46(m, 7H), 1.62 (m, 2H), 2.43 (t, 2H), 7.32 (m, 2H), 7.39 (m, 2H), 7.46 (s, 1H), 7.61 (d, J¼ 7.2 Hz, 1H), 7.64 (d, J ¼ 7.2 Hz, 1H). 13C NMR (75 MHz, CDCl3) d (ppm): 14.1, 19.4, 22.6, 26.9, 28.8, 29.0, 29.2, 29.3, 29.5, 29.6, 31.9, 46.7, 81.2, 90.4, 119.7, 120.1, 122.5, 122.6, 125.8, 127.0, 127.3, 130.5, 138.5, 138.6, 153.4, 153.7. MS [Mþ] calcd. for C27H34 358.57, found 358. ELEM. ANAL. Calcd. for C27H34: C, 90.44; H, 9.56. Found: C, 90.40; H, 9.60. M4 Yield: 70%. mp: 112–1148C.1H NMR (300 MHz, CDCl3) d (ppm): 1.50 (s, 6H), 7.34 (m, 4H), 7.43 (m, 2H), 7.51 (m, 3H), 7.60 (s, 1H), 7.70 (d, J ¼ 7.8 Hz, 2H).13 C NMR (75 MHz, CDCl3) d (ppm): 26.9, 46.8, 89.5, 90.4, 119.9, 120.3, 121.9, 122.5, 122.6, 123.0, 126.0, 127.0, 127.1, 127.7, 128.1, 128.3, 130.7, 131.6, 138.2, 139.4, 153.9, 154.0. MS [Mþ] calcd. for C23H18 294.40, found 294. ELEM. ANAL. Calcd. for C23H18: C, 93.84; H, 6.16. Found: C, 93.74; H, 6.26. M5 Yield: 67%. mp: 102–1048C.1H NMR (300 MHz, CDCl3) d (ppm): 1.50 (s, 6H), 7.14 (q, 1H), 7.35 (m, 3H), 7.49 (d, 1.5 Hz, 1H), 7.52 (d, 1.5 Hz, 1H), 7.58 (d, 1.5 Hz, 1H), 7.69 (s, 1H), 7.70 (m, 2H). 13 C NMR (75 MHz, CDCl3) d (ppm): 26.9, 46.8, 87.2, 90.3, 117.5 (d), 119.9, 120.3, 120.4 (d), 121.0, 122.5, 122.6, 126.0, 127.1, 127.8, 128.0 (d), 128.1 (d), 130.7, 138.2, 139.8, 148.2 (d), 148.5 (d), 151.9 (d), 152.2 (d), 153.6, 153.8. MS [Mþ] calcd. for 330.38, found 330. ELEM. ANAL. Calcd. for
C23H16F2: C, 83.62; H, 4.88. Found: C, 83.60; H, 4.91.
Polymerization of Monomers M1–M5
Polymerizations were carried out under nitrogen using either an inert atmosphere glove box or a Schlenk tube in a vacuum line system, except for the purification of the polymers, which was done in an open atmosphere. An experimental proce-dure for the polymerization of monomer M1 is given here.
Monomer M1 (0.9 g, 4.12 mmol) was added into a Schlenk tube with a three-way stopcock on the sidearm. The tube was evacuated under vac-uum and then flushed with nitrogen three times through sidearm. Toluene (10 mL) was injected into the tube through a septum to dissolve the monomer. The initiator solution was prepared in another tube by dissolving WCl6 (0.158 g, 0.4 mmol) and n-Bu4Sn (0.28 mg, 0.8 mmol) in 10 mL of dried toluene. Both tubes were aged at room temperature for 30 min. The initiation solu-tion was injected into the monomer solusolu-tion, and the reaction mixture was stirred at 308C under N2for 24 h. The solution was then cooled to room temperature, diluted with 2 mL of THF and added drop-wise through a cotton filter to 300 mL of methanol with stirring. The polymer was separated by filtration, purified by several repre-cipitations from THF into methanol, and dried in vacuum to yield 0.61 g (68%) of PFA1.
1 H NMR (300 MHz, CDCl3) d (ppm): 1.48 (s, 6H), 7.19 (br, 3H), 7.23 (br, 3H), 7.64 (br, 2H).13C NMR (75 MHz, CDCl3) d (ppm): 26.9, 46.8, 119.8, 120.3, 122.5, 122.6, 126.4, 127.0, 127.8, 131.2, 138.2, 140.0, 153.9, 154.0, 155.0. ELEM. ANAL.
Calcd. for C17H14: C, 93.54; H, 6.46. Found: C, 92.64; H, 6.46. PFA2 Yield: 31%. 1H NMR (300 MHz, CDCl3) d (ppm): 0.93 (t, 3H), 1.47 (br, 12H), 2.18 (t, 2H), 7.17 (br, 5H), 7.62 (br, 2H). 13C NMR (75 MHz, CDCl3) d (ppm): 14.0, 19.4, 26.9, 28.5, 31.1, 45.7, 119.7, 120.1, 122.5, 122.6, 125.8, 127.0, 127.3, 130.5, 138.5, 138.6, 140.0, 153.4, 153.7. ELEM. ANAL.
Calcd. for C22H24: C, 91.64; H, 8.39. Found: C, 89.00; H, 8.10. PFA3 Yield: 36%. 1H NMR (300 MHz, CDCl3) d (ppm): 0.93 (t, 3H), 1.27–1.55 (br, 22H), 2.20 (t, 2H), 7.32 (br, 5H), 7.6 (br, 2H). 13C NMR (75 MHz, CDCl3) d (ppm): 14.1, 19.4, 22.6, 26.9, 28.8, 29.0, 29.2, 29.3, 29.5, 29.6, 31.9, 46.7, 119.7, 120.1, 122.5, 122.6, 125.8, 127.0, 127.3, 130.5, 138.5, 138.6, 146.5, 153.4, 153.7. ELEM. ANAL. Calcd. for
C27H34: C, 90.44; H, 9.56. Found: C, 88.54; H, 9.40.
PFA4 Yield: 75%. 1H NMR (300 MHz, CDCl3) d (ppm): 1.58 (s, 6H), 7.40 (br, 6H), 7.55 (br, 4H), 7.70 (br, 2H).13C NMR (75 MHz, CDCl3) d (ppm): 26.9, 46.8, 119.9, 120.3, 121.9, 122.5, 122.6, 123.0, 126.0, 127.0, 127.1, 127.7, 128.1, 128.3, 130.7, 131.6, 138.2, 139.4, 153.9, 153.0. 156.4. ELEM. ANAL. Calcd. for C23H18: C, 93.84; H, 6.16. Found: C, 92.20; H, 5.89. PFA5 Yield: 70%. 1H NMR (300 MHz, CDCl3) d (ppm): 1.58 (s, 6H), 7.15 (br, 1H), 7.35 (br, 3H), 7.50 (br, 3H), 7.68 (br, 3H). 13C NMR (75 MHz, CDCl3) d (ppm): 26.9, 46.8, 117.5 (d), 119.9, 120.3, 120.4 (d), 121.0, 122.5, 122.6, 126.0, 127.1, 127.8, 128.0 (d), 128.1 (d), 130.7, 138.2, 139.8, 148.2 (d), 148.5 (d), 151.9 (d), 152.2 (d), 153.6, 153.8, 156.2. ELEM.
ANAL. Calcd. for C23H16F2: C, 83.62; H, 4.88. Found: C, 82.66; H, 5.22.
Fabrication of EL Devices
The light-emitting devices were prepared on ITO-coated glass substrates, which were pre-cleaned and treated with UV ozone for 3 min before use. The poly(ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) was pur-chased from Bayer Co. The PEDOT layer was spin-coated with a spin rate of 6500 rpm onto the pretreated ITO substrates and cured at 200 8C for 10 min under vacuum. The emissive layer was then spin-coated on the top of the hole-trans-porting layer from a toluene solution of the poly-mer (0.6% in wt/v) with a spin rate of 1200 rpm. The thickness of the organic layer was measured on a Sloan Dektak 3030 surface profiler. The thickness of the PEDOT layer was about 40–50 nm, and the thickness of the polymer layer was about 80–100 nm. A layer of 35-nm thick calcium
(Ca) cathode was vacuum-deposited at a pressure of about 8 107Torr. An additional protecting layer of 100-nm thick aluminum (Al) was then vacuum-evaporated on top of the Ca layer under the same condition.
RESULTS AND DISCUSSION
Polymer Synthesis
Table 1 summarizes the conditions and results of the polymerization of the fluorene-substituted ace-tylenic monomers, using WCl6, MoCl5, and TaCl5 as catalysts, and n-Bu4Sn as a cocatalyst. The poly-merization reactions were carried out in toluene at either 30 or 808C. Monomer M1 belongs to a mono-substituted acetylene. Both mixtures of WCl6 /n-Bu4Sn and MoCl5/n-Bu4Sn are known to be active catalysts for the metathesis polymerization of mono-substituted acetylenes. In our study, the polymer-ization results of both catalyst mixtures are slight-ly different. A higher yield and number–average molecular weight were achieved for PFA1 by using the WCl6/n-Bu4Sn as catalysts. The results demonstrate that M1 can be polymerized even though it contains a bulky fluorene side group. Our results were agreed with many previous reports,29–31 which showed that the mono-sub-stituted acetylenes can be polymerized without steric effect.
Monomers M2–M5 belong to disubstituted ace-tylenes. According to the reports,32,33 the TaCl5/ n-Bu4Sn mixture is known to be an active cata-lyst for the polymerization of sterically hindered diarylacetylene derivatives. Both monomers M2 and M3, which contain an alkyl and a fluorenyl substituents, were polymerized by TaCl5/n-Bu4Sn in toluene at 80 8C for 24 h, yielding PFA2 and
PFA3 with Mn of 8 KDa in 30% yield.
Besides, M4 and M5, which contain a phenyl and
Table 1. Polymerization of Fluorene-Containing Monomers M1–M5a
Monomer Cat. Temp. (8C) Yield (%) Mnb PDI Tg(8C) Td(8C)c
M1 WCl6 30 68.2 40,500 1.88 76.4 372 M1 MoCl5 30 57.6 35,000 1.52 76.0 371 M2 TaCl5 80 30.9 7500 1.35 75.7 377 M3 TaCl5 80 36.0 8900 1.45 76.3 387 M4 TaCl5 80 75.3 315,700 3.12 81.5 462 M5 TaCl5 80 69.2 285,100 2.88 84.4 503
aAll polymerization took place under nitrogen for 24 h in toluene solution and used n-Bu 4Sn as cocatalyst. [M]¼ 0.2M, [WCl6]¼ [MoCl5]¼ [TaCl5]¼ 20 mM, [n-Bu4Sn]¼ 40 mM.
bDetermined by GPC relative to polystyrene. c
a fluorenyl substituents, were polymerized by TaCl5/ n-Bu4Sn in the same conditions, giving PFA4 and PFA5 with Mnof300 KDa in satisfactory yields (70%). Comparing the polymerization results for PFA2–PFA5, we observed that the alkyl side group would hinder the metathesis polymeriza-tion of disubstituted acetylenes to produce PFA2 and PFA3 with lower molecular weights and yields.
Structure Characterization
Figure 1(b,d) shows the IR spectra of polymers PFA1 and PFA2, respectively; for comparison, the spectra of their corresponding monomers M1
and M2 [Fig. 1(a,c)] are also given. The monomer M1 absorbs at 2130, 3293, and 630 cm1, due to
the CBC stretching, BCH stretching, and
bending vibrations, respectively. All these acety-lene absorption bands disappear in the spectrum of PFA1, but the double-bond¼¼CH stretching at 3010 cm1appears, indicating that the acety-lene triple bonds have been transformed to the polyene double bonds by the polymerization reac-tion. The monomer M2 absorbs at 2226 and 2185 cm1, due to the CBC stretching, which disap-pear in the spectrum of PFA2. Instead, the dou-ble bond C¼¼C stretching at 1650 cm1 appears. It has been reported by Simonescus and Percec that the cis- and trans- PPA can be determined by the C¼¼CH out of plane and in plane defor-mation vibrations. The bands at 740, 895, and 1380 cm1are specific for cis-PPA, and the bands at 922, 970, and 1265 cm1are specific for trans-PPA. Comparing the spectra of M1 and PFA1, most of the absorption bands in the figure-print region are very similar. Therefore, it is difficult to identify the cis- and trans- contents of PFA1 by IR spectra.
The NMR analysis offers more detailed infor-mation on the molecular structure of the poly-mers. As shown in Figure 2(b), in the 1H NMR spectrum of PFA1, there are no peaks in the absorption region of the acetylene protons (d 3.1 ppm). Instead, a broad peak appears in the olefin and aromatic protons absorption region (d 6.8–8.0 ppm). It has been reported by Simo-nescu and Percec that trans-olefin proton peak appears at 6.85 ppm and cis-olefin proton peak appears at 5.82 ppm. We can conclude that PFA1
Figure 1. IR spectra for (a) M1, (b) PFA1, (c) M2, and (d) PFA2.
Figure 2. 1H NMR spectra for (a) M1, (b) PFA1, (c) M2, and (d) PFA2.
Figure 3. TGA thermograms of fluorene-containing polyacetylenes PFA1–PFA5.
synthesized by WCl6 catalyst is a 100% trans structure. Since PFA2–PFA5 belong to disubsti-tuted polyacetylenes, they contain no olefinic pro-tons. Therefore, it is impossible to identify their geometric isomers by1H NMR spectra.
The thermal properties of fluorene-containing polyacetylenes (PFAs) were investigated by TGA and DSC and summarized in Table 1. Figure 3 depicts the TGA traces of the polymers. PFAs show good thermal stability and their thermal degradation temperatures (Tds) at 5% weight loss range from 370 to 503 8C. PFA1 shows the lowest Td at 371 8C and PFA5 reveals the high-est Td at 503 8C. The glass-transition tempera-tures (Tg) are in the range from 76 to 84 8C. Again, the diaryl-substituted PFA4 and PFA5 exhibit higher Tgvalues. Furthermore, the poly-mers are soluble in common organic solvents, such as THF, chloroform, toluene, and xylene.
Optical Properties
Figure 4(a,b) depict the UV–vis absorption spec-tra of PFA1–PFA5 in chloroform and in thin film state. For each polymer, both absorption spectra in the solution and in thin film state are very similar except for some peaks, which show a lit-tle redshift in the thin film state. This is reason-able since more chain aggregation occurs in the solid state. Comparing the five spectra of PFA1– PFA5 in the thin film state, all polymers display an absorption peak near 310 nm, which is associ-ated with the absorption peak of the fluorenyl side groups26 and one or two broad absorption bands in the 350–550 nm region, which is attrib-uted to the p–p* interband transition of the main chains.34
Figure 5(a,b) exhibit the PL spectra of PFA1– PFA5 in chloroform and in thin film state. PFA1 with a monofluorenyl substituent shows a
maxi-Figure 4. UV–vis absorption spectra of PFA1– PFA5. (a) in chloroform and (b) in thin film state.
Figure 5. PL spectra of PFA1–PFA5. (a) in chloro-form and (b) in thin film state.
mum emission at 402 nm in chloroform. It’s PL quantum efficiency (wf) in chloroform is only 0.08. Theoretically, low luminescent efficiency is the result of solitons generation of the trans-structure of PFA1. Soliton formation is experi-mentally verified in an iodine-doped PFA1, caus-ing to a new peak in the absorption spectrum. The result agrees with the PPA reported by Sun et al.11PFA2 and PFA3 belong to alkyl- and fluo-renyl-substituted polyacetylenes, and emit a blue-green light at 483 nm in chloroform. Both wf values of PFA2 and PFA3 are 0.79 and 0.85. PFA4 and PFA5 contain the phenyl- and fluo-renyl-substituents in the polyacetyene main chains, and emit a green to yellowish green light at 541 and 552 nm in chloroform. Both wf values are 0.50 and 0.42, respectively. These values are smaller than those of PFA2 and PFA3. This could be due to more interchain quenching in the PFA4 and PFA5 system. Table 2 summarizes the UV–vis absorption and PL properties of PFA1–PFA5.
Electrochemical Properties
Cyclic voltammetry (CV) was employed to evalu-ate the ionization potential (i.e. hole-injection ability) and the redox stability of our PFAs. The oxidation and reduction potentials were used to determine the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbital energy levels, which were calculated using the internal standard value of4.4 eV with respect to the vacuum level. The anodic sweeps of PFAs (Fig. 6) were reversible and the oxidation onsets
occurred at potentials of þ0.64 to þ0.72 V for PFA1–PFA5. These oxidation potentials give
HOMO energy levels ranging from 5.04 to
5.26 eV for PFA1–PFA5. The cathodic wave was not observed in acetonitrile solution for all of the polymers within solvent limit. However, the energy band gaps of PFAs can be estimated by the absorption onsets of their UV spectra. Using this approach, the calculated band gaps of PFA1–PFA5 are in the range from 2.32 to 2.95 eV. The LUMO energy levels of PFA1– PFA5 can be also estimated from optical band gaps and HOMO energy levels. The HOMO and LUMO energies levels of PFA1–PFA5 are listed in Table 2, and the band diagrams of PFAs are shown in Figure 7. In this paper, the effects of substituents on the energy band gaps of the substituted polyacetylenes are investigated. Basically, there are two substitution related effects, electronic effect, and steric hindrance. Comparing the energy band gap of PFA1 with those of the other disubstituted polyacetylenes, PFA1s shows a much smaller energy band gap. This is because PFA1 has less steric effect and has longer conjugation length for the polyene main chains. In PFA2 and PFA4 case, PFA2 contains alkyl and fluoreneyl substituents, while PFA4 contains phenyl and fluoreneyl substitu-ents. PFA2 shows a much larger energy band gap than PFA4. This is because the alkyl sub-stituent has larger steric effect than the phenyl substituent. In the case of PFA4 and PFA5, the difference between two polymers is on the difluoro substituents. PFA5 with a 2,3-difluo-rophenyl substituent exhibits a smaller energy
Table 2. Optical Properties, Quantum Yields, and Redox Properties of PFA Polymers
Polymer kmax(nm) PL Efficiencye (solution, %) Band Gapf (eV) Eox (V) HOMO/LUMOg (eV) Solutiona Filmb absc PLd absc PLc,d PFA1 313 402 311 402 8 2.32 0.64 5.04/2.72 PFA2 312 483 312 486 (482) 79 2.95 0.7 5.10/2.15 PFA3 310 483 310 486 85 2.95 0.69 5.09/2.14 PFA4 320 (382,465) 541 388,459 546 50 2.44 0.86 5.26/2.82 PFA5 330 (391,476) 552 392,470 580 42 2.38 0.72 5.12/2.74 aMeasured in chloroform. b
Prepared by spin-coating from toluene solution. cData in the parentheses are wavelength of shoulders. d
Excited at kmaxof abs.
eMeasured by integral sphere in chloroform.
fThe optical band gap takes as the absorption onset of UV–vis spectrum of the polymer film. g
band gap. This is due to the less electron with-drawing effect for the 2,3-difluorophenyl sub-stituent than that of a phenyl subsub-stituent. The results are consistent with those reported by Sun et al.14
EL Properties and LED Devices
To evaluate the potential application of PFAs in PLED, we fabricated a double-layered device with the configuration of ITO/PEDOT/PFAs/Ca/ Al. The EL spectra (Fig. 8) of the PFAs are
simi-lar to their corresponding PL spectra (Fig. 4). This indicates that both the PL and EL spectra originate from the same radiative decay process of single excitons. Both PFA2 and PFA3 emit a blue light with peaks at 472 and 468 nm, respec-tively, while PFA4 and PFA5 emit an orange-red light with peaks at 576 and 602 nm, respectively. Besides, we don’t observe the aggregate emission at a long wavelength, compared with the PL and EL spectra of polyfluorenes.23 This means that the PFAs are more stable emitting materials than polyfluorenes.
Figure 9 exhibits the current density-lumi-nance-voltage (I-L-V) plots for the devices fabri-cated with an emitting layer of PFA2 and PFA3. The turn-on voltage of a PFA2 device is 6 V, with a maximum brightness of 100 cd/m2 at 14.5 V, whereas the turn-on voltage of a PFA3 device is 6.3 V, with a maximum brightness of 77 cd/m2at 15 V. Both devices have much better performance
Figure 6. Cyclic voltammorgrams of PFA1–PFA5 films.
Figure 7. Energy level diagrams of PFA1–PFA5.
than the blue devices based on poly(1-alkyl-2-phe-nylacetylene) reported in the literature.12 This is because introducing the fluorene groups into the
polyacetylene backbones would improve the
charge injection of polymers, since PFAs show higher HOMO energy levels than poly(1-alkyl-2-phenylacetylene).
Figure 10 depictes the I-L-V characteristics of the PLED devices fabricated with PFA4 and PFA5. The turn-on voltage of a PFA4 device is 4.5 V, with a maximum brightness of 111 cd/m2 at 8.5 V, while the turn-on voltage of a PFA5 device is 2.8 V, with a maximum brightness of 923 cd/m2 at 8 V. The PFA5 device performance is the best for the red light devices based on poly-acetylenes so far reported in the literatures.14,18
Furthermore, it has been proven that a white PLED can be achieved by blending a blue host
polymer and an orange-red guest polymer.35 We used PFA2 as a host material, which was blended with 2 wt % of PFA5 to yield a white light device. Figure 11 shows the EL spectra of the blended device. The EL intensity is increased as the voltage increases. However, no obvious color change is observed when the voltage increases. The CIE device coordinates ranges (0.33, 0.39) from 6 to 10 V. This demonstrates that PFAs are thermally stable when they are used as emit-ting layers in PLED. The I-L-V characteristics of blended polymers are shown in Figure 12. The turn-on voltage of this device is 6.0 V with a maximum current efficiency of 1.4 cd/A and a maximum brightness of 450 cd/m2 at 15 V. This is a first highly efficient white light PLED device
Figure 9. Current density and luminescence inten-sity-voltage characteristics for PFA2 and PFA3 in ITO/ PEDOT/Polymer PFA2 or PFA3/Ca/Al device.
Figure 10. Current density and luminescence inten-sity-voltage characteristics for PFA4 and PFA5 in ITO/ PEDOT/Polymer PFA4 or PFA5/Ca/Al device.
Figure 11. EL spectra measured at different voltage for the device: ITO/PEDOT/PFA2 (98 wt %)þ PFA5 (2 wt %)/Ca/Al device.
Figure 12. Luminescence and yield-voltage charac-teristics for the white light device: ITO/PEDOT/PFA2 (98%)þ PFA5 (2%)/Ca/Al device.
based on polyacetylenes. Table 3 summarizes the performances of the EL devices for the PFAs.
CONCLUSIONS
Five novel polyacetylenes containing fluorene side groups were successfully synthesized and characterized. The fluorene-substituted acety-lene monomers can be polymerized by using WCl6/n-Bu4Sn, MoCl5/n-Bu4Sn, and TaCl5
/n-Bu4Sn as catalysts. Most of the PFAs were
obtained in high molecular weights and reason-able yields. The monofluorenyl-substituted poly-acetylene PFA1 shows a maximum emission at 402 nm. Its PL efficiency in chloroform is only 0.08. Its EL property is too weak to be measured. However, PFA2–PFA5 belong to disubstituted polyacetylenes. The bulky alkyl, phenyl, and flu-orenyl groups were introduced to increase the steric hindrance and prevent close packing of the main chains. Both PFA2 and PFA3 emit blue-green light at 483 nm, while PFA4 and PFA5 emit a green to yellowish-green light at 541 and 552 nm in solution, respectively.
According to the CV results, the HOMO energy levels of PFA2–PFA5 are located from 5.04 to 5.24 eV, which are higher than those of the disubstituted polyacetylenes reported in the literatures.36 It means that the hole injection from ITO anode is much easier. This is the rea-son why PFA2–PFA5 show much higher EL effi-ciency than the other disubstituted polyace-tylenes. A device fabricated by PFA5, with the configuration of ITO/PEDOT:PSS/PFA5/Ca/Al, re-vealed a turn-on voltage at 2.8 V and a maximum brightness of 923 cd/m2at 8 V. This is the highest value for polyacetylenes reported in the litera-ture so far. Furthermore, a white light PLED device based on the blending of 98 wt % of PFA2
and 2 wt % PFA5 was also fabricated. The device shows a maximum current efficiency of 1.3 cd/A at 6 V and a maximum luminance of 450 cd/m2 at 15 V. All these results demonstrate that disub-stituted polyacetylenes are potential candidates for PLED applications.
The authors are grateful to the National Science Coun-cil of the Republic of China (NSC 93-2216-E-009-011) for its financial support of this work.
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