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Conjugated Polymers Based on Phenothiazine and Fluorene in Light-Emitting Diodes and Field Effect Transistors Do-Hoon Hwang,*,† Suk-Kyung Kim,† Moo-Jin Park,† Ji-Hoon Lee,‡ Bon-Won Koo,§ In-Nam Kang,§ Sung-Hyun Kim,| and Taehyoung Zyung| Department of Applied Chemistry, Kumoh National Institute of Technology, Kumi 730-701, Korea, Department of Polymer Science and Engineering, Chungju National University, Chungju 380-702, Korea, Electronic Materials Lab., Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, Korea, and Basic Research Lab., ETRI, Taejon 305-350, Korea Received December 3, 2003. Revised Manuscript Received January 28, 2004
Poly[10-(2′-ethylhexyl)-phenothiazine-3,7-diyl] (PPTZ), poly[9,9-bis(2′-ethylhexyl)fluorene2,7-diyl] (PBEHF), and their random copolymers, poly(BEHF-co-PTZ), were synthesized through Ni(0)-mediated polymerization. Light-emitting devices were fabricated using these polymers in an ITO (indium tin oxide)/PEDOT:PSS/polymer/Ca/Al configuration. Each EL device constructed with a poly(BEHF-co-PTZ) copolymer exhibited significantly enhanced efficiency and brightness compared to devices constructed from the PPTZ and PBEHF homopolymers. The EL device constructed with poly(88BEHF-co-12PTZ) exhibited the highest power efficiency and brightness (4200 cd/m2 and 2.08 cd/A respectively). This enhanced efficiency of the copolymer devices results from their improved hole injection and more efficient charge carrier balance, which arise due to the HOMO levels (∼5.4 eV) of the poly(BEHF-co-PTZ) copolymers, which are lower than that of the PBEHF homopolymer (∼5.8 eV). An organic field effect transistor was also fabricated using PPTZ as a new p-type channel material and characterized. The measured field effect mobility of the transistor was 0.8 × 10-4 cm2/V‚s and the on/off ratio of the device was ∼103.
1. Introduction Conjugated polymers have attracted much scientific and technological research interest during the past few decades because of their potential use as semiconductors and electro-active materials in diverse applications such as transistors,1 photovoltaic devices,2 nonlinear optical devices,3 and polymer light-emitting diodes (PLEDs).4,5 In particular, interest in PLEDs fabricated from conjugated polymers6,7 has increased because such PLEDs have properties that are well-suited to flat panel displays: good processability, low operating voltages, fast response times, and facile color tunability over the full visible range. * To whom correspondence should be addressed. E-mail: dhhwang@ kumoh.ac.kr. Fax: 82-54-467-4477. † Kumoh National Institute of Technology. ‡ Chungju National University. § Samsung Advanced Institute of Technology. | ETRI. (1) Shirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 1741. (2) Halls, J. J.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498. (3) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Effects in Monomers and Polymers; John Wiley & Sons: New York, 1991. (4) 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 1990, 347, 539. (5) Gustafsson, G.; Cao, Y.; Treacy, G. M.; Klavetter, F.; Colaneri, N.; Heeger, A. J. Nature 1992, 357, 477. (6) Burn, P. L.; Holmes, A. B.; Kraft, A.; Bradley, D. D. C.; Brown, A. R.; Friend, R. H.; Gymer, R. W. Nature 1992, 356, 47. (7) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37, 402.
The development of new materials capable of displaying proper color with high efficiency and stability is essential for the development of a full color PLED. PPV derivatives and polyfluorene derivatives (PFs)8-11 are well-known as promising materials suitable for practical applications. PPV and its derivatives have attracted much attention because of their optical and physical properties, and many attempts have been made to improve the performance of electroluminescent (EL) devices based on these polymers. Although all three primary colors (red, green, and blue) have been produced in PLEDs, so far only the yellow-orange color emitting devices have demonstrated device performances that are suitable for commercial use. Recently, more attention has been paid to polyfluorenes (PFs) for use as the emissive layer of lightemitting diodes than to PPV derivatives because of their high photoluminescence (PL) quantum efficiency, thermal stability, and also their facile color tunability, which can be obtained by introducing low-band-gap comonomers.12-14 The poor efficiency of the polyfluorene ho(8) Grell, M.; Knoll, W.; Lupo, D.; Meisel, A.; Miteva, T.; Neher, D.; Nothofer, H.-G.; Scherf, U.; Yasuda, A. Adv. Mater. 1999, 11, 671. (9) Scherf, U.; List, E. J. W. Adv. Mater. 2002, 14, 477. (10) Mu¨ller, C. D.; Falcou, A.; Reckefuss, N.; Rojahn, M.; Wiederhirn, V.; Rudati, P.; Frohne, H.; Nuyken, O.; Becker, H.; Meerholz, K. Nature 2003, 421, 829. (11) Lee, J. H.; Hwang, D. H. Chem. Commun. 2003, 2836. (12) Neher, D. Macromol. Rapid. Commun. 2001, 22, 1365. (13) Cho, N. S.; Hwang, D. H.; Lee, J. I.; Jung, B. J.; Shim, H. K. Macromolecules 2002, 35, 1224.
10.1021/cm035264+ CCC: $27.50 © 2004 American Chemical Society Published on Web 03/09/2004
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mopolymer has been improved by blending, copolymerization, and end-capping with charge-transporting materials to achieve efficient charge injection.15-17 Another application of organic semiconductors is their use in organic field effect transistors (OFETs) based on molecular and polymeric organic semiconductors, which are of considerable current interest that is motivated by their potential applications in organic integrated circuit sensors,18 low-cost memory, smart cards, and driving circuits for large-area display device applications such as active-matrix flat-panel liquid-crystal displays (AMFPDs), organic light-emitting diodes, electrophoretic materials, and electronic paper displays.19,20 Most of the organic semiconductors used for the fabrication of p-channels in OFETs are derived from thiophene-based π-conjugated systems,21,22 oligo-thiophenes,23,24 acenes,25,26 phthalocyanines,27 regioregular poly-3-hexylthiophene,28 or polyfluorene copolymers.29 The synthesis of a new class of organic semiconductors with high charge mobility is crucial to the realization of high-performance OFETs. Phenothiazine is a well-known heterocyclic compound with electron-rich sulfur and nitrogen heteroatoms. Molecules30,31 and polymers32,33 containing phenoxazine or phenothiazine moieties have recently attracted much research interest because of their unique electro-optical properties and their resulting potential in diverse applications such as light-emitting diodes31,33 and chemiluminescence.32 It has been suggested that inclusion of phenothiazine into a polyfluorene should improve the hole-transporting properties of the polyfluorene and thus improve the EL device efficiency. Jenekhe et al. recently synthesized an alternating copolymer of phenothiazine and fluorene by the Suzuki coupling reaction and characterized its properties.33 We have also inde(14) Ego, C.; Marsitzky, D.; Becker, S.; Zhang, J.; Grimsdale, A. C.; Mu¨llen, K.; MacKenzie, J. D.; Silva, C.; Friend, R. H. J. Am. Chem. Soc. 2003, 125, 437. (15) Herguth, P.; Jiang, X.; Liu, M. S.; Jen, A. K.-Y. Macromolecules 2002, 35, 6094. (16) Ego, C.; Grimsdale, A. C.; Uckert, F.; Yu, G.; Srdanov, G.; Mu¨llen, K. Adv. Mater. 2002, 14, 809. (17) Miteva, T.; Meisel, A.; Knoll, W.; Nothofer, H. G.; Scherf, U.; Mu¨ller, C. D.; Meerholz, K.; Yasuda, A.; Neher, D. Adv. Mater. 2001, 13, 565. (18) Crone, B.; Dodabalapur, A.; Gelperin, A.; Torsi, L.; Katz, H. E.; Lovinger, A. J.; Bao, Z. Appl. Phys. Lett. 2001, 78, 2229. (19) Wisnieff, R. Nature 1998, 394, 225. (20) Dimitrakopoulos, C. D.; Malenfant, P. R. L. Adv. Mater. 2002, 14, 99. (21) Fuchigani, H.; Tsumura, A.; Koezuka, H. Appl. Phys. Lett. 1993, 63, 1372. (22) Katz, H. E.; Bao, Z.; Gilat, S. L. Acc. Chem. Res. 2001, 34, 359. (23) Garnier, F.; Horowitz, G.; Peng, X.; Fichou, D. Adv. Mater. 1990, 2, 592. (24) Dodabalopur, A.; Torsi, L.; Katz, H. E. Science 1995, 268, 270. (25) Klauk, H.; Gundlach, D. J.; Nichols, J. A.; Jackson, T. N. IEEE Trans. Electron Devices 1999, 46, 1258. (26) Dimitrakopoulos, C. D.; Purushothaman, S.; Kymissis, J.; Callegari, A.; Shaw, J. M. Science 1999, 283, 822. (27) Bao, Z.; Lovinger, A. J.; Dodabalapur, A. Appl. Phys. Lett. 1996, 69, 3066. (28) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 1741. (29) Sirringhaus, H.; Wilson, R. J.; Friend, R. H.; Inbasekaran, M.; Wu, E. P.; Grell, M.; Bradley, D. D. C. Appl. Phys. Lett. 2000, 77, 406. (30) Jenekhe, S. A.; Lu, L.; Alam, M. M. Macromolecules 2001, 34, 7315. (31) Higuchi, A.; Inada, H.; Kobata, T.; Shiraota, Y. Adv. Mater. 1991, 3, 549. (32) Lai, R. Y.; Jenekhe, S. A.; Bard, A. J. J. Am. Chem. Soc. 2003, 125, 12631. (33) Kong, X.; Kulkarni, A. P.; Jenekhe, S. A. Macromolecules 2003, 36, 8992.
Chem. Mater., Vol. 16, No. 7, 2004 1299 Scheme 1
Scheme 2
pendently studied the light-emitting properties of phenothiazine-based polyfluorene copolymers. In this article, we report the design and synthesis of conjugated polymers containing phenothiazine and fluorene. Poly[10-(2′-ethylhexyl)-phenothiazine-3,7-diyl] (PPTZ) (Scheme 1), poly[9,9-bis(2′-ethylhexyl)fluorene2,7-diyl] (PBEHF), and their random copolymers, poly(BEHF-co-PTZ) (Scheme 2), were synthesized through Ni(0)-mediated polymerization. The light-emitting properties of PPTZ and the poly(BEHF-co-PTZ) copolymers were compared with those of the PBEHF homopolymer. Further, an OFET that uses PPTZ as a new class of p-type channel material was fabricated and characterized. 2. Results and Discussion Synthesis and Characterization of the Polymers. PBEHF, PPTZ, and the poly(BEHF-co-PTZ) copolymers all dissolve in common organic solvents such as THF, chloroform, and toluene without evidence of gel formation. The weight average molecular weights (Mw) of PBEHF, PPTZ, and the poly(BEHF-co-PTZ) copolymers were determined by gel permeation chromatography using a polystyrene standard and were found to range from 13000 to 79000 with polydispersity indexes ranging from 1.6 to 3.9. The polymer yields were 4353% after purification. These results for the synthesized polymers are summarized in Table 1. In addition, the compositions of the copolymers were determined by elemental analysis of their nitrogen content. The feed ratios of 3,7-dibromo-10-(2′-ethylhexyl)-phenothiazine (PTZ) used in the present work were 3, 10, and 25 mol % of the total amount of monomer, and the resulting ratios of PTZ units in the poly(BEHF-co-PTZ) copolymers were 8, 12, and 26 mol %, respectively. The actual fractions of PTZ in the resulting copolymers were found to be slightly higher than the feed ratios. These results indicate that in polymerization reactions the PTZ
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Table 1. Actual Compositions, Average Molecular Weights, Polydispersity Indices, and Yields of the Synthesized Polymers feed actual PTZ PTZ mole mole ratio ratioa (%) (%) PBEHF poly(92BEHF-co-8PTZ) poly(88BEHF-co-12PTZ) poly(74BEHF-co-26PTZ) PPTZ
3 10 25
8 12 26
Mw 79000 13000 27000 15000 25000
polymer PDI yield (%) 3.2 1.7 2.0 1.6 3.9
50 45 43 43 53
a The actual PTZ fractions in the copolymers were determined by elemental analysis of their nitrogen content.
Figure 1. UV-visible absorption spectra of the polymer films.
comonomer is more active than the fluorene comonomer. All the polymers exhibited very good thermal stabilities, losing less than 3% of their weight on heating to about 350 °C, as determined with TGA under a nitrogen atmosphere. Optical and Photoluminescence Properties. Figure 1 shows the UV-vis absorption spectra of thin films of PBEHF, PPTZ, and their copolymers coated onto fused quartz plates. The PPTZ thin film exhibits peak UV-visible absorption and absorption onset at 285 and 450 nm, respectively, and the PBEHF thin film exhibits maximum UV-visible absorption and absorption onset at 380 and 440 nm, respectively. The UV-visible absorption peaks of the copolymer films become slightly blue-shifted as the fraction of PTZ in the copolymers increases. The optical band gaps of the polymers were determined from the absorption onset. The optical band gaps of PBEHF and PPTZ were found to be 2.91 and 2.77 eV, respectively. The band gaps of the copolymer films were found to decrease as the fraction of PTZ in the copolymers increased. The optical band gaps of poly(92BEHF-co-8PTZ), poly(88BEHF-co-12PTZ), and poly(74BEHF-co-26PTZ) were found to be 2.86, 2.81, and 2.81 eV, respectively. Figure 2 shows the PL emission spectra of thin films of the polymers coated onto fused quartz plates. The maximum PL of the PBEHF film when it is excited at 350 nm with a Xenon lamp appears at 420 nm. The PL emission spectrum of the PPTZ film was slightly redshifted from that of the PBEHF film and exhibited peak emission at about 478 nm. The copolymer films exhibited peak PL emissions at almost the same wavelength as the PPTZ homopolymer film regardless of the copolymer composition. The PL quantum efficiencies of the polymers were obtained in chloroform solution
Figure 2. PL spectra of the polymer films.
Figure 3. EL spectra of the EL devices with ITO/PEDOT/ polymer/Ca/Al configuration. Table 2. Summary of the Results from the Spectra of the Polymers
UVmax (nm)a PBEHF poly(92BEHF-co-8PTZ) poly(88BEHF-co-12PTZ) poly(74BEHF-co-26PTZ) PPTZ
380 373 373 367 285
relative optical PL band quantum gap PLmax ELmax efficiency (eV) (nm)b (nm) (%)b 2.91 2.86 2.81 2.81 2.77
420 474 480 478 478
419 480 484 480 476
100 84 71 37 7.0
a Measured for thin films on fused quartz plates. b Photoluminescence quantum yield in chloroform determined relative to PBEHF; see ref 34.
relative to the PBEHF homopolymer.34 The solution PL quantum efficiencies decreased with increasing fraction of PTZ in the copolymer. Electroluminescence Properties and VoltageLuminance (V-L) Characteristics. EL devices were fabricated in the ITO/PEDOT:PSS(50 nm)/polymer(80 nm)/Ca(50 nm)/Al(200 nm) configuration and characterized as a function of applied voltage. The EL spectra of the devices are shown in Figure 3. The EL spectra of the PBEHF, PPTZ, and copolymer devices are similar to the PL spectra of the corresponding polymer films. All the results from the UV-visible, PL, and EL spectra are summarized in Table 2. Figure 4 shows the luminance-voltage characteristics of the EL devices. In the EL device constructed from the PPTZ homopolymer, the forward current increases with increasing forward bias voltage, and the curve has (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.
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Chem. Mater., Vol. 16, No. 7, 2004 1301
Table 3. Summary of the EL Device Performances and HOMO Levels of the Polymers
PBEHF poly(92BEHF-co-8PTZ) poly(88BEHF-co-12PTZ) poly(74BEHF-co-26PTZ) PPTZ a
Vtrun-on (V)
maximum brightness (cd/m2)
efficiency (cd/A)
CIE 1931 (x,y)a
HOMO (eV)
4.2 4.2 3.8 4.0 6.4
160 1320 4170 4670 9.0
0.04 0.48 2.08 0.78 0.002
(0.17,0.12) (0.16,0.32) (0.17,0.37) (0.17,0.33) (0.16,0.32)
5.79 5.38 5.40 5.40 5.00
The CIE coordinates were measured at 100 cd/m2 brightness, except for the PPTZ device.
Figure 4. L-I curves of the EL devices with ITO/PEDOT/ polymer/Ca/Al configuration.
a shape that is typical of a diode. Light emission from this device was observable at voltages greater than 6.4 V (@1 cd/m2). The maximum brightness of the device was 9.0 cd/m2 with a maximum power efficiency of 0.002 cd/A. Light emission from the PBEHF homopolymer device was observable at voltages greater than 4.2 V (@1 cd/m2). The maximum brightness of this device was 160 cd/m2 with a power efficiency of 0.04 cd/A. Interestingly, all the EL devices constructed from the copolymers exhibited significantly better device performances than the devices constructed from the PBEHF and PPTZ homopolymers. Of the EL devices constructed using the copolymers, the poly(88BEHF-co-12PTZPV) device exhibited the highest efficiency. The EL device constructed from poly(88BEHF-co-12PTZPV) exhibited a maximum brightness of 4200 cd/m2 and a maximum power efficiency of 2.08 cd/A. One explanation for the dramatic improvement in EL device performance achieved by using the copolymers is that the introduction of PTZ units which have lower HOMO levels facilitates hole injection and results in more efficient charge carrier balance. In general, a high barrier to hole injection between ITO and a lightemitting polymer results in poor light-emitting efficiency for polymers with high work functions, such as polyalkylfluorenes (∼5.8 eV). To test this hypothesis, we measured the ionization potentials of the polymer films to determine their HOMO levels. The ionization potentials of the polymers were determined by low-energy photoelectron spectroscopy using a previously reported method.35 The ionization potentials of the PBEHF and PPTZ thin films were measured to be 5.79 and 5.00 eV, respectively. Given that the optical band gaps of the PBEHF and PPTZ films as determined from the absorption onset are 2.91 and 2.77 eV, we conclude that the LUMO levels of the (35) Sano, T.; Hamada, Y.; Shibata, K. IEEE J. Sel. Top. Quantum 1998, 4, 34.
Figure 5. Band diagrams of the PPTZ, PBEHF, poly(BEHFco-PTZ) copolymers, PEDOT, and the Ca electrode.
Figure 6. Device structure of the PPTZ-based field effect transistor.
PBEHF and PPTZ thin films are 2.88 and 2.23 eV, respectively. The ionization potentials of the poly(92BEHF-co-8PTZ), poly(88BEHF-co-12PTZ), and poly(74BEHF-co-26PTZ) thin films were measured to be 5.38, 5.40, and 5.40 eV, respectively. The HOMO level of the PEDOT layer is known to be ∼5.2 eV. Thus, hole injection and transportation from PEDOT to the copolymers are expected to be easier than to the PBEHF homopolymer (5.79 eV), and as a consequence the charge carrier balance is better in the devices constructed from the copolymers. The band diagrams of ITO, PEDOT, the polymers, and the Ca electrode are shown in Figure 5, and the characteristics of the EL devices are summarized in Table 3. Characteristics of the PPTZ Field Effect Transistor. It should be possible to use PPTZ as a p-channel material in a field effect transistor (FET) because phenothiazine is a heterocyclic compound containing electron-abundant sulfur and nitrogen atoms. An organic field effect transistor was fabricated using PPTZ as shown in Figure 6. Figure 7 shows the output characteristics of the PPTZ-based OFET for gate voltages in the range 0 to -30 V. The PPTZ operated as a typical p-channel transistor in accumulation mode. The field-effect mobility was calculated in the saturation regime using the following equation,
IDS ) (W/2L)µCi(Vg - Vth)2 where IDS is the drain-source current in the saturated region, W and L are the channel width and length,
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transistor was fabricated using PPTZ as a new channel material and exhibited a field effect mobility of ∼10-4 cm2/V‚s and an on/off ratio of ∼103. These results demonstrate that conjugated polymers containing phenothiazine are promising new materials for LED and field effect transistor applications. 4. Experimental Section
Figure 7. Output characteristics of the PPTZ-based field effect transistor.
respectively, µ is the field-effect mobility, Ci is the capacitance per unit area of the insulation layer, and Vg and Vth are the gate voltage and the threshold voltage, respectively. The field-effect mobility of the PPTZ device was found to be 0.8 × 10-4 cm2/V‚s at room temperature. The on-off ratio of the device was ∼103. By comparison, the best hole and electron mobilities of solution processible polymers reported to date are 0.1 cm2/V‚s for poly(3-hexylthiophene) (P3HT)29 and 0.1 cm2/V‚s for poly(benzobisimidazobenzophenanthroline (BBL),36 respectively. Although the mobility of the PPTZ-based transistor is not as good as that of the best devices previously fabricated using the solution process,29,36 the carrier mobility of this device could be improved by surface treatment of the gate dielectrics with hexamethyldisilazane (HMDS) or octadecyltrichlorosilane (OTS) monolayers and/or an annealing process after spin coating of the active organic layer. Modification of the structure of PPTZ by introducing different alkyl groups or end-cappers might also improve its mobility, because mobility of polymers is highly dependent on their molecular ordering. Optimization of the PPTZ-based field effect transistor through modification of the structure of PPTZ, and the synthesis of new polymers and oligomers containing phenothiazine, are currently under investigation. 3. Conclusions PPTZ, PBEHF, and poly(BEHF-co-PTZ) copolymers were successfully synthesized through the Yamamoto coupling reaction, and the light emission properties of these polymers were compared. Whereas the EL devices fabricated using the PPTZ and PBEHF homopolymers exhibited poor device efficiency and brightness, the devices fabricated using the copolymers exhibited significantly enhanced device performances in both efficiency and brightness because of their enhanced hole injection and charge carrier balance. Even though the peak EL emissions of the copolymer devices were slightly red-shifted from that of the PBEHF homopolymer device, the copolymer devices still exhibited strong blue emissions at around 480 nm. An organic field effect (36) Babel, A.; Jenekhe, S. A. J. Am. Chem. Soc. 2003, 125, 13656.
Materials. Phenothiazine, 2-ethylhexylbromide, 1,5-cyclooctadiene, 2,2′-dipyridyl, sodium hydroxide, bromine, N,Ndimethylformimide (99.8%, anhydrous), toluene (99.8%, anhydrous), and dimethyl sulfoxide (99.8%, anhydrous) were purchased from Aldrich. Bis(1,5-cyclooctadiene)nickel(0) was purchased from Strem. All chemicals were used without further purification. Poly(ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT/PSS) (Bayer), which was used in the LED fabrication, was filtered through a 0.45-µm nylon filter prior to spin coating. The solvents [tetrahydrofuran (THF) and dichloromethane] were freshly distilled and dried. 2,7-Dibromo-9,9-bis(2′-ethylhexyl)fluorene (BEHF) was prepared according to a previously reported method.14 10-(2-Ethylhexyl)-phenothiazine (1). Phenothiazine (10 g, 0.30 mol) and sodium hydroxide (12.0 g, 0.3 mol) were dissolved in 250 mL of DMSO and stirred for 30 min at room temperature. 2-Ethylhexylbromide (7.7 mL, 0.55 mol) was injected into the reaction mixture, which was then stirred for 24 h at room temperature. The reaction mixture was extracted three times using dichloromethane and water. The organic layer was separated and dried with anhydrous magnesium sulfate, and then the solvent was removed using a rotary evaporator. The crude product was purified by column chromatography using a cosolvent (hexane/petroleum ether ) 2/1) as the eluent. The product yield was 48% (7.5 g). 1H NMR (300 MHz, CDCl3): δ 0.99 (m, 6H), 1.48 (m, 8H), 2.07 (t, 1H), 3.84 (d, 2H), 6.99 (m, 4H), 7.24 (m, 4H). 13C NMR (CDCl3, ppm): δ 10.43, 13.95, 22.98, 23.96, 28.49, 30.64, 35.68, 50.89, 115.76, 122.21, 125.73, 126.96, 127.40, 145.67. Anal. Calcd for C20H25Br2NS: C, 77.12; H, 8.09; N, 4.50; S, 10.29 Found: C, 77.34; H, 8.13; N, 4.45; S, 10.26. 3,7-Dibromo-10-(2′-ethylhexyl)-phenothiazine (2). 10(2-Ethylhexyl)-phenothiazine (7.5 g, 0.024 mol) was dissolved in 50 mL of dichloromethane, and then bromine (8.0 g, 0.05 mol) was injected into the solution using a syringe and stirred for 4 h at room temperature. Dilute aqueous sodium hydroxide was added to the reaction mixture and stood for 30 min. The reaction mixture was extracted three times using dichloromethane and brine, and then the organic layer was separated and concentrated. The crude product was purified using column chromatography using hexane as the eluent. The product yield was 79% (9.0 g). 1H NMR (300 MHz, CDCl3): δ 0.92 (m, 6H), 1.41 (m, 8H), 1.92 (t, 1H), 3.68 (d, 2H), 6.73 (d, 2H), 7.27 (m, 4H). 13C NMR (CDCl3, ppm): 10.49, 14.08, 23.07, 23.93, 28.52, 30.61, 35.71, 51.17, 114.75, 117.07, 127.20, 129.77, 130.04, 144.49. Anal. Calcd for C20H23Br2NS: C, 51.19; H, 4.94; N, 2.98; S, 6.83. Found: C, 52.32; H, 5.01; N, 2.36; S, 6.83. General Procedure for Polymerization. PBEHF, PPTZ, and the statistical copolymers were synthesized by nickel(0)mediated polymerization. The feed ratio of each monomer was 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 last three in a molar ratio of 1:1:1) was kept under argon at 80 °C for 30 min. Five milliliters of anhydrous toluene was then added to the mixture. The polymerization was maintained at 80 °C for 72 h, and then 0.1 g of 9-bromoanthracene was dissolved in toluene and added to the reaction mixture for end-capping. When 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. The resulting polymers were dissolved in toluene and purified by silica gel column chroma-
Phenothiazine and Fluorene in LEDs and FETs tography using toluene as the eluent. Finally, the resulting polymers were purified by Soxhlet extraction using methanol amd then dried in a vacuum. The polymer yields ranged from 43 to 53% after purification. Fabrication of the Light-Emitting Diodes. Each polymer film was prepared by spin-casting a blend solution containing 1% of the polymer by weight in chlorobenzene. Uniform and pinhole-free films with a thickness around 80 nm were easily obtained from the polymer solutions. For the double-layer device, a modified water dispersion of PEDOT [poly(3,4-ethylenedioxy-thiophene)] doped with poly(styrene sulfonate) (PSS) (Bayer AG, Germany) was used as the holeinjection/transport layer. A metal contact (Ca) was deposited on top of the polymer film through a mask by vacuum evaporation at a pressure below 4 × 10-6 Torr, yielding active areas of 4 mm2. In the case of the Ca cathode (∼50 nm), an additional encapsulating layer of Al (∼200 nm) was thermally evaporated onto it. Fabrication of the Organic Field Effect Transistor. An n-doped Si substrate with a gold source/drain contact functions as a gate, and a silicon dioxide layer of 2000-Å thickness is the gate dielectric with a capacitance per unit area of 13.8 nF/ cm2. The experiments were performed on a device with a channel length of 10 µm. PPTZ thin films were formed by spincoating and the thickness of the resulting films was around 1000 Å. Physical Measurements. 1H and 13C NMR spectra were recorded using a Bruker AM-300 spectrometer. The absorption
Chem. Mater., Vol. 16, No. 7, 2004 1303 spectra were measured using a Hitachi spectrophotometer model U-3501 and the steady-state PL spectra were recorded on a Spex FL3-11. The molecular weights of the polymers were determined by gel permeation chromatography (GPC) analysis relative to a polystyrene standard using a Waters highpressure GPC assembly Model M590. Thermal analyses were carried out on a Dupont TGA 9900 thermogravimetric analyzer under a nitrogen atmosphere at a heating rate of 10 °C/min. The ionization potentials of the polymer films were measured with a low-energy photoelectron spectroscope (Riken-Keiki AC2). For the measurement of device characteristics, currentvoltage (I-V) changes were obtained using a current/voltage source (Keithley 238) and an optical power meter (Newport 818-SL). The brightness and 1931 CIE chromaticity of each EL device were recorded with a PR-650 SpectraScan colorimeter. The transistor characteristics were measured with a Keithley 4200 semiconductor characterization system. All the measurements mentioned above were carried out in air at room temperature.
Acknowledgment. This work was supported financially by the Korean Ministry of Science and Technology through the NRL program. The authors thank Dr. S. Y. Song (Samsung SDI) for conducting the ionization potential measurements. CM035264+
