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Macromolecules 2002, 35, 1224-1228
Synthesis and Color Tuning of New Fluorene-Based Copolymers Nam Sung Cho,† Do-Hoon Hwang,‡ Jeong-Ik Lee,§ Byung-Jun Jung,† and Hong-Ku Shim*,† Center for Advanced Functional Polymers, Department of Chemistry and School of Molecular Science (BK21), Korea Advanced Instituted of Science and Technology, Tae-jon 305-701, Korea; Department of Applied Chemistry, Kumoh National University of Technology, Kumi, 730-701, Korea; and Basic Lab., ETRI, Taejon 305-350, Korea Received July 5, 2001; Revised Manuscript Received October 19, 2001
ABSTRACT: A series of copolymers, poly{9,9-bis(2′-ethylhexyl)fluorene-2,7-diyl-co-2,5-bis(2-thienyl-1cyanovinyl)-1-(2′-ethylhexyloxy)-4-methoxybenzene-5′′,5′′′-diyl} (PFTCVB), were synthesized from the monomers 2,7-dibromo-9,9-bis(2′-ethylhexyl)fluorene and 2,5-bis(2-(5′-bromothienyl)-1-cyanovinyl)-1-(2′′ethylhexyloxy)-4-methoxybenzene (BTCVB) through the Ni(0)-mediated polymerization. The copolymers were characterized using FT-IR spectroscopy, UV-vis spectroscopy, TGA, photoluminescence (PL) and electroluminescence (EL) spectroscopy, elemental analysis, and molecular weight studies. The synthesized copolymers showed an absorption maximum at about 380 nm, and between 425 and 600 nm the absorption increased with increasing fraction of thiophene-containing monomer 4 (BTCVB). In PL, the emission maxima of the copolymers were red-shifted as the fraction of BTCVB increases, despite the copolymers showing little variation in UV-vis absorption characteristics. Light-emitting devices were fabricated in an ITO (indium-tin oxide)/PEDOT/polymer/LiF/Al configuration. The EL spectra showed similar emissions to the PL results, and the copolymer containing 15 mol % of BTCVB showed bright-red emission.
Introduction Since poly(p-phenylenevinylene) (PPV) was first reported as an electroluminescene material by Burroughes et al.,1 enormous effort has been devoted to the synthesis of light-emitting polymers because of their potential applications as active materials for organic electroluminescent displays.2-5 PPV and its derivatives have attracted much attention because of their optical and physical properties, and many efforts have been made to improve the performance of electroluminescent devices.6-11 Although all three primary colors (red, green, and blue) have been demonstrated in LEDs, at present only green and orange LEDs meet the requirements of commercial use. A number of polyfluorene (PF) polymers and their derivatives have been studied since poly(9,9-di-n-hexylfluorene) (PDHF) was first reported as a blue-lightemitting polymer. The interest in these polymers arose because they show highly efficient photoluminescence (PL) and electroluminescence (EL), excellent thermal and oxdative stability, and good solubility in common organic solvents.12-18 Major drawbacks of PFs, however, are that they show excimer and/or aggregate formation upon thermal annealing or the passage of current. Moreover, the efficiency of devices based on PFs are not sufficient for applications. Continuing efforts have been made to suppress excimer formation and improve efficiency in PFs, including copolymerization with anthracene,17 end-capping with a sterically hindered groups19,20/hole-trapping moieties,21 introducing sterically hindered substituents at the 9-position of fluorene,22,23 and combining with cross-linked hole-trans†
Korea Advanced Instituted of Science and Technology. Kumoh National University of Technology. § ETRI. * To whom correspondence should be addressed. Tel +82-42869-2827; Fax +82-42-869-2810; e-mail
[email protected]. ac.kr. ‡
porting layer.24,25 However, appropriate blue-emitting materials that meet the requirements for display applications have yet to be obtained, and further improvements are necessary. Meanwhile, several efforts have been made to tune color using fluorene-based polymers. One successful method for obtaining color tuning was the doping of green- or red-emitting materials into PFs. The other method is copolymerization with low band gap comonomers. Inbasekaran et al. at the Dow Chemical Company have reported several fluorene-based copolymers synthesized by alternating copolymerization using 5,5′dibromo-2,2′-bithiophene and 4,7-dibromo-2,1,3-benzothiadiazole, which showed yellow and green emissions, respectively.26 In addition, Lee et al. at IBM have reported fluorene-based copolymers synthesized by random copolymerization using 3,9(10)-dibromoperylene, 4,4′-dibromo-R-cyanostilbene, and 1,4-bis(2-(4′-bromophenyl)-1-cyanovinyl)-2-(2′-ethylhexyl)-5-methoxybenzene.27 A small amount (5 mol %) of the comonomers in the copolymers was sufficient to change the blue emission of PF to yellow and green emission. Basically, the introduction into PF of comonomers with a lower band gap than fluorene monomer leads to successful color tuning based on PF. In contrast to the body of work on blue-emitting copolymers, only a few reports investigate red emission from copolymers based on PF. To address this issue, in the present work we report on the preparation and characterization of copolymers containing a low band gap comonomer, 2,5-bis(2-(5′-bromothienyl)-1-cyanovinyl)-1-(2′′-ethylhexyl)-4-methoxybenzene (BTCVB), which not only has extended π-conjugation but also contains the thiophene moiety. We prepared copolymers covering the full color range from blue to red by changing the feed ratio of the low band gap comonomer, BTCVB. Using this method, we succeeded in synthesizing new fluorene-based copolymers, poly{9,9-bis(2′-ethylhexyl)fluorene-2,7-diyl-co-2,5-bis(2-thienyl-1-cyanovinyl)-1-(2′-
10.1021/ma011155+ CCC: $22.00 © 2002 American Chemical Society Published on Web 01/18/2002
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Scheme 1. Synthetic Scheme of 2-Bromothiophene-(2-cyano)vinylene-2-methoxy-5-(2-ethylhexyl)-1,4-phenylene(1-cyano)vinylene-5-bromothiophene (BTCVB)
Scheme 2. Synthetic Scheme of Poly{2,7-(9,9′-bis(2-ethylhexyl)fluorene)-co-2,5-thiophene-(2-cyano)vinylene2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-(1-cyano)vinylene-2,5-thiophene} (PFTCVB)
ethylhexyloxy)-4-methoxybenzene-5′′,5′′′-diyl} (PFTCVB), which show full color emissions. The synthetic routes and details are given in Schemes 1 and 2. Experimental Section Measurements. NMR spectra were recorded on a Bruker AM 300 MHz spectrometer with tetramethylsilane as internal reference. Elemental analysis was performed by an EA 1110Fisons. FT-IR spectra were recorded using an EQUINOX 55 spectrometer. UV-vis and PL spectra were recorded on Jasco V-530 and Spex Fluorolog-3 spectrofluorometers. Thermogravimetric analysis (TGA) was performed on a DuPont 9900 analyzer with a heating rate of 10 °C/min under a nitrogen atmosphere. The number- and weight-average molecular weights of polymers were determined by gel permeation chromatography (GPC) on a Waters GPC-150C instrument, using tetrahydrofuran (THF) as eluent and polystyrene as standard. Single-layer LED devices were fabricated
on glass substrates coated with indium-tin oxide (ITO). The device configuration was ITO/poly(3,4-ethylenedioxythiophene)(PEDOT)/polymer/LiF/Al. PEDOT was used as the holeinjection layer. The LED structure used in this study consists of an aluminum contact on the copolymer surface that is spin-casted onto an PEDOT-coated ITO glass substrate from a solution of PFTCVBs in p-xylene. The spin-casting yielded uniform films with thickness of approximately 100 nm. Aluninum was deposited onto the polymer films using the vacuum evaporation method at a pressure of 10-6 Torr. EL spectra of the devices were measured using a Minolta CS-1000. Current-voltage-luminance (I-V-L) characteristics were recorded simultaneously with the measurement of the EL intensity by attaching the photospectrometer to a Keithley 238 and Minolta LS-100 as the luminance detector. All measurements were carried out at room temperature under ambient atmosphere. Materials. 4-Methoxyphenol, 2-ethylhexyl bromide, 1,5cyclooctadiene, 2,7-dibromofluorene, 2,2′-dipyridyltoluene (99.8%,
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anhydrous), N,N-dimethylformamide (99.8%, anhydrous), and 5-bromo-2-thiophenecarboxaldehyde were purchased from Aldrich. Bis(1,5-cyclooctadiene)nickel(0) was purchased from Strem. All chemicals were used without further purification. 1,4-Bis(chloromethyl)-2-(2-ethylhexyloxy)-5-methoxybenzene (2) and 2,7-dibromo-9,9-bis(2′-ethylhexyl)fluorene (5) were synthesized according to procedures outlined in the literature.11,28,29 Synthesis of 1,4-Bis(cyanomethyl)-2-methoxy-5-(2′ethylhexyloxy)benzene (3). A mixture of 10.0 g (30 mmol) of compound 2 and 4.4 g (90 mmol) of sodium cyanate in N,Ndimethylformamide was stirred at 45 °C for 72 h. The resulting mixture was extracted with methylene chloride and brine and then dried with MgSO4. The extract was filtered and evaporated in vacuo. The resulting liquid was poured into water, yielding a pale yellow precipitate that was filtered. The precipitate was recrystallized in methylene chloride and hexane two times. The product yield was 60% (5.7 g). 1H NMR (CDCl3, ppm): δ 6.90 (d, 2H), 3.85 (d, 2H), 3.83 (s, 3H), 3.68 (s, 4H), 1.72 (m, 1H), 1.55-1.29 (m, 8H), 0.91 (q, 6H). 13C NMR (CDCl3, ppm): δ 150.43, 150.30, 119.10, 119.00, 117,73, 112.48, 111.82, 71.13, 56.12, 39.48, 30.59, 29.07, 23.98, 22.99, 18.63, 18.58, 14.03, 11.16 Anal. Calcd for C19H26N2O2; C, 72.58; H, 8.33; N, 8.91. Found: C, 72.52; H, 8.11; N, 8.67. Synthesis of 2,5-Bis-{2-(4′-bromothienyl)-1-cyanovinyl}-2-(2′-ethylhexyloxy)-5-methoxybenzene (BTCVB) (4). A mixture of 10.0 g (32 mmol) of compound 3 and 18.5 g (97 mmol) of 5-bromothiophene-2-carbaldehyde was stirred in 100 mL of methanol at room temperature. A catalytic amount of potassium tert-butoxide in methanol was added to this mixture. After 2 days, the bright yellow solid was filtered and dried. The resulting solid was recrystallized in methylene chloride and methanol, and then the solid was dried in vacuo. The resulting product yield was 62% (13.0 g). 1H NMR (CDCl3, ppm): δ 7.95 (s, 1H), 7.79 (s, 1H), 7.31 (d, 1H), 7.28 (d, 1H), 7.10-7.06 (m, 4H) 3.82 (d, 2H), 3.76 (s, 3H), 3.68 (s, 4H), 1.72 (m, 1H), 1.55-1.29 (m, 8H), 0.91 (q, 6H). 13C NMR (CDCl3, ppm): δ 150.94, 239.64, 138.20, 133.22, 132.84, 130.55, 123.84, 118.55, 118.21, 113.85, 113.32, 104.84, 71.79, 56.51, 39.66, 30.81, 29.16, 24.14, 23.01, 14.03, 11.24. Anal. Calcd for C29H28Br2N2O2S2: C, 52.74; H, 4.27; N, 4.84; S, 9.71. Found: C, 52.72; H, 4.10; N, 4.53; S, 9.84. Polymerization. Homopolymer and statistical copolymers were synthesized by nickel(0)-mediated polymerization.30 The feed ratios of BTCVB (4) were 1, 3, 5, and 15 mol % of the total amount of monomer, and the total amount of reactant was 1.8 mmol. Each Schlenk tube containing 5 mL of DMF, bis(1,5-cyclooctadienyl)nickel(0), 2,2′-dipyridyl, and 1,5-cyclooctadiene (the latter three in a molar ratio of 1:1:1) was kept under argon at 80 °C for 30 min, and then 5 mL of anhydrous toluene was added to the mixture. The polymerization was maintained at 80 °C for 72 h. After this time, 0.1 g of 9-bromoanthracene was dissolved in toluene and added to the reaction mixture for end-capping. After the reaction had finished, each polymer was precipitated from an equivolume mixture of concentrated HCl, methanol, and acetone. The isolated polymers were dissolved in chloroform and precipitated in methanol. Finally, the resulting polymers were purified by Soxhlet extaction and dried in vacuo. The resulting polymer yields ranged from 60 to 75% (poly(9,9-bis(2-ethylhexyl)fluorene-2,7-diyl): 60%, copolymer containing 1 mol % of BTCVB: 72%, copolymer containing 3 mol % of BTCVB: 66%, copolymer containing 5 mol % of BTCVB: 75%, copolymer containing 15 mol % of BTCVB: 61%). Elemental analysis for PFTCVBs-PFTCVB1: C, 86.71; H, 10.52; N, 9.91 × 10-2. PFTCVB3: C, 84.97; H, 10.23; N, 0.22. PFTCVB5: C, 86.52; H, 10.33; N, 0.50. PFTCVB15: C, 82.59; H, 9.47; N, 1.20.
Results and Discussion Synthesis and Characterization of Polymers. All of the synthesized copolymers were soluble in common organic solvents. However, this solubility decreased slightly with increasing ratio of BTCVB. The copolymers were spin-coated onto an ITO substrate, giving trans-
Macromolecules, Vol. 35, No. 4, 2002 Table 1. Polymerization Results copolymers
PFTCVB1
PFTCVB3
PFTCVB5
PFTCVB15
Mw Mn PDI (Mw/Mn) polymer yield (%) y ratioa (%)
47000 20000 2.3 72
23000 15000 1.5 66
33000 13000 2.5 75
61000 22000 2.7 61
1.4
3.1
7.0
17.5
a
y ratios are BTCVB ratios in copolymers, and they measured by elemental analysis through calculation of the amount of nitrogen contained in copolymers.
Figure 1. UV-vis absorption and photoluminescence emission spectra of 2,7-dibromo-9,9-bis(2′-ethylhexylfluorene) and BTCVB in CHCl3.
parent and homogeneous thin films. The numberaverage molecular weight (Mn) and the weight-average molecular weight (Mw) of the copolymers, as determined by gel permeation chromatography using polystyrene standard, ranged from 13 000 to 22 000 and 23 000 to 61 000, respectively, with a polydispersity index ranging from 1.5 to 2.7. The resulting polymer yields were 60-75%. All of the PFTCVB and the homopolymer poly(9,9-bis(2′-ethylhexyl)fluorene) (PBEHF) were endcapped with 9-bromoanthracene in an effort to suppress excimer emission.30 The nature of the end groups apparently affects the π-stacking tendency of the fluorene backbone which is determined by the rigidly planarized biphenyl units within the polymer backbone. PBEHF and PFTCVBs terminated with a more sterically hindered end-capper such as 9-bromoanthracene are less prone to excimer formation. The polymerization results of the synthesized copolymers are summarized in Table 1. In addition, the actual fraction of BTCVB in the PFTCVBs, as determined by elemental analysis through calculation of the amount of nitrogen contained in the copolymers, is somewhat higher than the feed ratio. These results indicate that BTCVB is more active than fluorene monomer (5) in polymerization reactions. The infrared absorption of the cyano subsituent at 2210 cm-1 increased with increasing amount of the cyanostilbene monomer in the comonomer feed. All of the PFTCVBs exhibited very good thermal stabilities, losing less than 5% of their weight on heating to approximately 300 °C evaluated by means of TGA under a nitrogen atmosphere. Optical and Photoluminescence Properties. Figure 1 shows the absorption and emission spectra of 2,7dibromo-9,9-bis(2′-ethylhexyl)fluorene and BTCVB comonomer in CHCl3. BTCVB exhibits a large red shift in both absorption and emissions compared with 2,7dibromo-9,9-bis(2′-ethylhexyl)fluorene. The emission maxima of BTCVB are 518 and 628 nm, respectively.
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Table 2. Spectrum Data λmax (nm) copolymers
UV absorptiona
PL emissiona
EL emission
ΦPLb
PBEHF PFTCVB1 PFTCVB3 PFTCVB5 PFTCVB15
380 380 380 380 379
420 536 544 583 620
419 532 535 580 630
0.80 0.69 0.57 0.51 0.34
a Measured in the thin film onto fused quartz plates. b Photoluminescence quantum yield in chloroform determined relative to quinine sulfate in 0.1 M H2SO4 solution; see refs 32-34.
Figure 2. UV-vis absorption spectra of the thin films of PFTCVBs coated onto fused quartz plates.
Figure 4. Electroluminescence spectra of PFTCVBs and PBEHF which have ITO/PEDOT/polymer/LiF/Al configuration.
Figure 3. Photoluminescence spectra of the thin films of PFTCVBs and PBEHF coated onto fused quartz plates.
Figure 2 shows the UV-vis absorption spectra of the thin films of PFTCVBs coated onto fused quartz plates. All of the PFTCVBs exhibit absorption maxima close to 380 nm, regardless of copolymer composition. The absorption maximum of PFTCVB15 is slightly blueshifted because of its high molecular weight, contrary to the copolymer having low molecular weight.30,31 As the fraction of BTCVB increases, however, the absorption between 425 and 600 nm increases. These absorption bands could be due to the BTCVB units incorporated into the polyfluorene main chain, and the increase of this unit affects the absorption intensities because of the low band gap of BTCVB. The film of PBEHF homopolymer showed PL emission maxima at 420 and 440 nm. Interestingly, these two strong and sharp PL peaks reduce in size dramatically in the PL spectra of the PFTCVBs, as shown in Figure 3. In contrast, the emission peaks from the BTCVB part increase greatly, such that even PFTCVB1 (which contains only 1.4 mol % of BTCVB) clearly exhibits this phenomenon. PFTCVB1 greatly shifted its PL maximum to 540 nm with a large and broad shoulder at 575 nm. These large shifts in PL maxima are probably due to intramolecular energy transfer from the BEHF part to BTCVB chromophores. Therefore, the emission from the latter is major in PL spectrum. The emission maxima of the copolymers gradually shift to the long wavelength as BTCVB content increases. This shift in the emission maxima might be explained that the number of BTCVB unit in copolymer increases when the fraction of BTCVB in polymerization increases. As a result, PFTCVB15 showed a PL emission maximum at 620 nm, yielding red emission. Conclusively, we succeeded in embodying
full colors from blue to red with these new fluorenebased copolymers by making small changes in the amount of red-emissive BTCVB. In Table 2 we present a summary of all of the optical absorptions, UV-vis maximum absorptions, PL and EL maximum emissions in film, and PL quantum efficiencies in solution of the synthesized copolymers. As shown in Table 2, the PL quantum efficiencies decreases with increasing fraction of BTCVB.32-34 Electroluminescence Properties and CurrentVoltage-Luminance (I-V-L) Characteristics. The EL spectra of the PFTCVBs are similar to the PL spectra of the copolyemrs with the same components (Figure 4), which indicates that the same energy transfer is involved in EL and PL. It is reasonable to suppose that these results are due to energy transfer from the high-energy-state fluorene moiety to the lowenergy-state BTCVB moiety. As a result, the transferred excitons decay through radiative recombination in BTCVB and then exhibit a red-shifted emission in comparison to the emission of the homopolymer.35 In particular, PFTCVB15 (which contains 17.5 mol % of BTCVB) showed almost pure red emission at 630 nm. Figure 5 shows the current-voltage-luminance characteristics of the LED of ITO/PEDOT/PFTCVB/LiF/Al. The threshold voltages of PFTCVBs ranged from about 5 to 13 V and decreased stepwise as the fraction of BTCVB increased. The EL intensity of the PFTCVBs increased with increasing BTCVB fraction at lower voltages as shown in Figure 5b. A detailed study of these LED devices is currently underway. Summary We have successfully prepared novel fluorene-based copolymers that have luminescent properties from blue to red. The UV-vis absorption spectra of these copolymers showed absorption maxima at almost the same wavelength (380 nm). However, the PL and EL emission
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Figure 5. Current-voltage-luminance (I-V-L) characteristics of PFTCVBs which have ITO/PEDOT/polymer/LiF/Al configuration.
spectra showed quite different trends. Compared to poly(9,9-bis(2-ethylhexyl)fluorene-2,7-diyl) (BEHF), PL and EL maxima for PFTCVBs shifted to longer wavelengths as the ratio of BTCVB was increased. Dramatically, the wavelengths of the PL and EL maxima are red-shifted about 115 nm when only a small fraction (1.4%) of BTCVB is used. In particular, PFTCVB15 showed an EL emission maximum at 630 nm, yielding an almost pure red color. The threshold voltages stepwise decreased as the fraction of BTCVB increased. Acknowledgment. We thank M. S. Lee, Y. S. Suh, and T. Ahn for their lots of help and gratefully acknowledge financial support from the Center for Advanced Functional Polymers through Korea Science and Engineering Foundation (KOSEF) and ILJIN. We also thank Dr. Y. W. Kim of K.R.I.C.T. for carrying out the elemental analysis and K. S. Pyun for assistance with GPC measurements. References and Notes (1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature (London) 1990, 347, 539. (2) Salaneck, W. P.; Lundsto¨rm, I.; Rånby, B. Conjugates Polymers and Related Materials; Oxford University Press: Oxford, 1993; pp 65-169.
Macromolecules, Vol. 35, No. 4, 2002 (3) Margolis, J. B. Conductive Polymers and Plastics; Chapman and Hall: New York, 1989; pp 1-40. (4) Braun, D.; Heeger, A. J. Appl. Phys. Lett. 1991, 58, 1982. (5) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bre`das, J. L.; Logdlund, M.; Salaneck, W. R. Nature (London) 1999, 397, 121. (6) Hwang, D. H.; Shim, H. K.; Lee, J. I.; Lee, K. S. J. Chem. Soc., Chem. Commun. 1994, 2461. (7) Politis, J. K.; Curtis, M. D.; He, Y.; Kanicki, J. Macromolecules 1999, 32, 2484. (8) Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A. B. Nature 1993, 365, 628. (9) Baigent, D. R.; Greenham, N. C.; Gru¨ner, J.; Marks, R. N.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Synth. Met. 1994, 67, 3. (10) Hwang, D. H.; Kim, S. T.; Shim, H. K.; Holmes, A. B.; Moratti, S. C.; Friend, R. H. J. Chem. Soc., Chem. Commun. 1996, 2241. (11) Ahn, T.; Jang, M. S.; Shim, H. K.; Hwang, D. H.; Zyung, T. Macromolecules 1999, 32, 3279. (12) Pei, Q.; Yang, Y. J. Am. Chem. Soc. 1996, 188, 7416. (13) Yang, Y.; Pei, Q. J. Appl. Phys. 1997, 81, 3294. (14) Ohmori, Y.; Uchida, M.; Muro, K.; Yoshino, K. Jpn. J. Appl. Phys. 1991, 30, L1941. (15) Yoshida, K.; Fujii, A.; Ohmori, Y.; Yoshino, K. Appl. Phys. Lett. 1996, 69, 734. (16) Grell, M.; Long, X.; Bradley, D. D. C.; Inbasekaran, M.; Woo, E. P. Adv. Mater. 1997, 9, 798. (17) Kreyenschmidt, M.; Kla¨rner, G.; Fuhrer, T.; Ashenhurst, J.; Karg, S.; Chen, W. D.; Lee, V. Y.; Scott, J. C.; Miller, R. D. Macromolecules 1998, 31, 1099. (18) Leclerc, M. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2867. (19) Kla¨rner, G.; Davey, M. H.; Chen, W. D.; Miller, R. D. Adv. Mater. 1998, 10, 993. (20) Lee, J.-I.; Kla¨rner, G.; Chen, J. P.; Scott, J. C.; Miller, R. D. SPIE. Int. Soc. Opt. Eng. 1999, 3623, 2. (21) Miteva, T.; Meisel, A.; Knoll, W.; Nothofer, H. G.; Scherf, U.; Mu¨ller, D. C.; Meerholz, K.; Yasuda, A.; Neher, D. Adv. Mater. 2001, 13, 565. (22) Lemmer, U.; Hein, S.; Mahrt, R. F.; Scherf, U.; Hopmeir, M.; Wiegner, U.; Go¨bel, R. O.; Mu¨llen, K.; Bassler, H. Chem. Phys. Lett. 1995, 240, 371. (23) Jenekhe, S. A.; Osaheni, J. A. Science 1994, 265, 765. (24) Mu¨ller, C. D.; Braig, T.; Nothofer, H.; Arnoldi, M.; Gross, M.; Scherf, U.; Nuyken, O.; Meerholz, K. Chem. Phys. Chem. 2000, 1, 207. (25) Chen, J. P.; Klaerner, G.; Lee, J.-I.; Markienwicz, D.; Lee, V. Y.; Miller, R. D.; Scott, J. C. Synth. Met. 1999, 107, 129. (26) Inbasekaran, M.; Wu, W.; Woo, E. P. U.S. Patent 5777070, 1997. (27) Kla¨rner, G.; Lee, J.-I.; Davey, M. H.; Miller, R. D. Adv. Mater. 1999, 11, 115. (28) Burn, P. L.; Kraft, A.; Baigent, D. R.; Bradley, D. D. C.; Brown, A. R.; Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Jackson, R. W. J. Am. Chem. Soc. 1993, 115, 10117. (29) Woo, E. P.; Inbasekaran, M.; Shiang, W.; Roof, G. R. Int. Pat. Appl. WO 97/05184, 1997. Grell, M.; Knoll, W.; Lupo, D.; Meisel, A.; Mieteva, T.; Neher, D.; Nothofer, H. G.; Scherf, U.; Yasuda, A. Adv. Mater. 1999, 11, 671. (30) Kla¨rner, G.; Miller, R. D. Macromolecules 1998, 31, 2007. (31) Asawapirom, U.; Scherf, U. Macromol. Rapid Commun. 2001, 22, 746. (32) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991. (33) Hima, S. J.; Ramin, J.; Yitzhak, T. Angew. Chem., Int. Ed. 1999, 38, 2722. (34) Chen, Z. K.; Huang, W.; Wang, L. H.; Kang, E. T.; Chen, B. T.; Lee, C. S.; Lee, S. T. Macromolecules 2000, 33, 9015. (35) Lee, J.-I.; Zyung, T.; Miller, R. D.; Kim, Y. H.; Jeoung, S. C.; Kim, D. J. J. Mater. Chem. 2000, 10, 1547.
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