Synthesis of Fluorene-based Hyperbranched Polymers for
Solution-Processable Blue, Green, Red, and White Light-Emitting Devices
Hung-Min Shih, Ren-Chi Wu, Ping-I Shih, Chien-Lung Wang, Chain-Shu Hsu
Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan, Republic of China Correspondence to: C.-S. Hsu (E-mail: [email protected])Received 16 September 2011; accepted 20 October 2011; published online 19 November 2011 DOI: 10.1002/pola.25080
ABSTRACT:For the purpose of making hyperbranched polymer (Hb-Ps)-based red, green, blue, and white polymer light-emit-ting diodes (PLEDs), three Ps terfluorene (TF), Hb-4,7-bis(9,90-dioctylfluoren-2-yl)-2,1,3-benzothiodiazole (Hb-BFBT), and Hb-4,7-bis[(9,90 -dioctylfluoren-2-yl)-thien-2-yl]-2,1,3-benzothiodiazole (Hb-BFTBT) were synthesized via [2þ2þ2] pol-ycyclotrimerization of the corresponding diacetylene-function-alized monomers. All the synthesized polymers showed excellent thermal stability with degradation temperature higher than 355C and glass transition temperatures higher than 50 C. Photoluminance (PL) and electroluminance (EL) spectra of the polymers indicate that Hb-TF, Hb-BFBT, and Hb-BFTBT are blue-green, green, and red emitting materials. Maximum brightness of the double-layer devices of Hb-TF, Hb-BFBT, and Hb-BFTBT with the device configuration of indium tin oxide/ poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate)/light-emitting polymer/CsF/Al are 48, 42, and 29 cd/m2; the maxi-mum luminance efficiency of the devices are 0.01, 0.02, and
0.01 cd/A. By using host–guest doped system, saturated red electrophosphorescent devices with a maximum luminance ef-ficiency of 1.61 cd/A were obtained when Hb-TF was used as a host material doped with Os(fptz)2(PPh2Me2)2as a guest mate-rial. A maximum luminance efficiency of 3.39 cd/A of a red polymer light-emitting device was also reached when Hb-BFTBT was used as the guest in the PFO (Poly(9,9-dioctylfluor-ene)) host layer. In addition, a series of efficient white devices were, which show low turn-on voltage (3.5 V) with highest luminance efficiency of 4.98 cd/A, maximum brightness of 1185 cd/m2, and the Commission Internationale de l’Eclairage (CIE) coordinates close to ideal white emission (0.33, 0.33), were pre-pared by using BFBT as auxiliary dopant.VC 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 696–710, 2012
KEYWORDS:electrophosphorescent devices; host materials; hyperbranched polymers; polymer light-emitting diodes; polyfluorene
INTRODUCTION Polymer light-emitting diodes (PLEDs) have been attracting great interest because of their potential applications in full-color flat panel displays and in solid-state lighting.1–3 Display and lighting applications require the de-velopment of highly efficient red, green, and blue light-emit-ting materials with high stability and purity.4 Polyfluorene
(PF) is the most promising blue-light emitter for PLED appli-cation because of its high photoluminescence (PL) quantum efficiency and good chemical and thermal stability.5–9 To be used in full-color displays and lighting applications, the emis-sion spectra of PF-based polymers have been turned to cover the entire visible region by incorporating electron-deficient monomer into the PF backbone or by physically doping with fluorescent and phosphorescent dyes with green or red emission.10–12 Several electron-deficient monomers have been reported as the comonomers to be incorporated into the polymer backbone of wide band gap polymers. For exam-ple, Jen and coworkers10demonstrated efficient green
emit-ting polymers synthesized from copolymerization of fluorene with an electron-deficient monomer, 2,1,3-benzothiadiazole (BT). Recently, Cao and coworkers11 report a series of fluo-rene and 4,7-dithienyl-BT (DTBT) copolymers.11 Devices based on the copolymers emitted a saturated red light, with kmaxranging from 628 to 674 nm. In these cases, the narrow
band gap unit functioned as an exciton trap, which allowed intramolecular energy transfer from the fluorene segment to the BT or DTBT units; exciton emission was centered on the BT and DTBT units and resulted in green and red emission. Based on the concept, Shu et al. developed a series of single white light-emitting copolymers, which contain fluorene, BT, and red DTBT moieties in their polymer backbones. By con-trolling the feed ratio of BT and DTBT, the resulting copoly-mers emit white light, with contributions from all three pri-mary colors.12 Although the studies demonstrated the success in modifying emission colors of PF-based polymers, the control of color stability among different batches of the
polymers is relatively difficult, because the emission colors of these polymers are also dependent on the molecular weights (Mw) and the actual molar ratio of the
electron-defi-cient moieties in the polymers, which is not readily controllable.
In these regards, monodisperse luminescent oligomers char-acterized with well-defined molecular structures and Mws
are developed.13 For example, Wong et al. demonstrated a series of terfluorene (TF) and oligofluorene oligomers that show bipolar transport properties withkmaxof PL at around
427 nm. These oligomers are used as efficient blue emitters and good host materials in the highly bright-blue organic light-emitting diodes (OLEDs) devices.14–16 Wong et al. also synthesized a highly efficient green emitter (PL, kmax: 544
nm) by incorporating the BT units into the oligoflourenes. The resulting molecules exhibit excellent solid-state PL quan-tum yields.17 Chen and coworkers18 further incorporated various electron-deficient segments into molecular structure of oligofluorenes to obtain a series of green and red emitting oligomers.
Hyperbranched polymers (Hb-Ps) have been widely studied in the past decades because of their unique molecular archi-tectures and specific properties.19–27 Because of their unique architectures, Hb-Ps have been widely reported as chemical sensors, drug delivery carriers, molecular antennae, and immu-nodiagnostic probes.28–34 Recently, Tang and coworkers35–37 developed a facial approach to synthesize conjugated Hb-poly-arylenes (Hb-PAs) through the diyne [2þ2þ2] polycyclotrime-rizations, catalyzed by transition metal complexes. Their Hb-PAs are spin coated through solution processing and show excellent optical properties, such as high light-emitting effi-ciency and superb optical-limiting performance.38–42
To combine the advantageous color stability and purity of oligomeric light-emitting molecules, and the convenience and low cost of the solution processes, in this article, we devel-oped a facile approach to synthesize blue, green, and red emitting Hb-Ps, that is, Hb-TF, Hb-4,7-bis(9,90 -dioctylfluoren-2-yl)-2,1,3-benzothiodiazole (BFBT), and Hb-4,7-bis[(9,90 -dio-ctylfluoren-2-yl)-thien-2-yl]-2,1,3-benzothiodiazole (BFTBT) through the [2þ2þ2] polycyclotrimerization (Scheme 4). As illustrated in Scheme 4, we first synthesized TF, BFBT, and BFTBT as the blue, green, and red chromophores and modi-fied them into diyne-functionalized monomers,M1, M2, and M3. [2þ2þ2] polycyclotrimerization was then introduced to convert the acetylene groups of the monomers into ben-zene rings, which act as the branching points in the result-ing Hb-Ps. As the branch points limit the extension of the conjugation length of the repeating units, the concept retains the advantage of color stability of oligomeric ana-logs and enable solution processability of the resulting materials. The PL and electroluminance (EL) spectra show thatHb-TF, Hb-BFBT, and Hb-BFTBT are solution-process-able blue, green, and red emitters.Hb-TF was further used as host material because of its large Eg. Red
electrophos-phorescent devices were fabricated by using Hb-TF as a
host and a red-emitting osmium complex as a guest in the emitting layer, which demonstrate the potential ofHb-TF as a host material.43 The device efficiency of the red emitter,
Hb-BFTBT, was further improved in a host–guest doped system, where Hb-BFTBT was used as red-emitting guest in the PFO host layer. Furthermore, by adding BFBT, as aux-iliary dopant in the PFO:Hb-BFTBT blend, white PLEDs with Commission Internationale de l’Eclairage (CIE) coordi-nates (0.33, 0.37), which are close to that of ideal white light (0.33,0.33), low turn-on voltage (3.5 V) with highest luminance efficiency of 4.98 cd/A, and maximum brightness of 1185 cd/m2were also achieved.44–46
EXPERIMENTAL
Materials
Tetrahydrofuran (THF) was distilled over sodium under nitrogen. Toluene was dried over calcium hydride and then distilled under nitrogen. Triethylamine (Et3N) was dried over
potassium hydroxide and then distilled under nitrogen. All other solvents and reagents were purchased from commer-cial sources and used without further purification. Com-pounds 1, 2, 6, 10, and 13 were prepared according to reported procedures.47 Os(fptz)2(PPh2Me2)2 (fptz ¼
3-tri-fluoromethyl-5-pyridyl-1,2,4-triazole) was prepared accord-ing to reported procedures.48
9,9,90,90,900,900-Hexaoctylterfluorene (3)
Compounds 1 (2.53 g, 4.90 mmol) and 2 (5.0 g, 9.80 mmol) were mixed with Pd(PPh3)4(0.011 g, 9.8lmol), Aliquat 336
(0.6 g, 1.25 mmol), and aqueous K2CO3 (2 M, 17 mL) in
degassed toluene (60 mL). The reaction mixture was stirred at 110C under nitrogen for 24 h. After cooling to room temper-ature, the solution was poured into 50 mL of water and extracted with ethyl acetate. The combined organic layers were dried over anhydrous MgSO4, and the solvent was removed
under reduced pressure. The crude product was purified by column chromatography (silica gel, hexane was used as eluent) to yield 5.0 g (92%) white crystals, mp: 53C.
1H NMR (CDCl
3): d ¼ 0.72–0.85 (m, 18H, A(CH2)7ACH3),
0.98–1.11 (m, 72H, A(CH2)A(CH2)6ACH3), 2.00–2.01 (m,
12H, (CH2) A(CH2)6ACH3), 7.25–7.35 (m, six aromatic
pro-tons), 7.72–7.90 (m, 14 aromatic protons). 13C NMR (CDCl3):
d ¼ 152.01, 151.70, 151.24, 141.03, 140.72, 127.02, 126.35, 123.18, 121.72, 120.17, 55.40, 40.60, 32.01, 30.26, 26.42, 24.04, 22.82, 14.29. MS (FAB, observedm/z): 1168.
7,70-Dibromo-9,9,90,90,900,900-hexaoctylterfluorene (4) A solution of compound 3 (7.5 g, 6.43 mmol) and FeCl3
(0.036 g, 0.22 mmol) in CHCl3(60 mL) was stirred at room
temperature. Bromine (2.24 g, 14.15 mmol) in CHCl3
(10 mL) was added slowly. The reaction mixture was stirred at room temperature for 12 h and then poured into sodium thiosulfate solution (30 mL) until the red color of bromine disappeared. The solution was extracted with ethyl acetate. The combined organic layers were dried over anhydrous MgSO4, and the solvent was removed under reduced pressure.
(silica gel, hexane was used as eluent) to yield 7.2 g (85%) white solid, mp: 50C.
1
H NMR (CDCl3): d ¼ 0.72–0.85 (m, 18H, A(CH2)7ACH3),
0.98–1.11 (m, 72H, A(CH2)A(CH2)6ACH3), 2.00–2.01 (m,
12H, (CH2)A(CH2)6ACH3), 7.33–7.35 (m, two aromatic
pro-tons), 7.49 (d, two aromatic propro-tons), 7.58–7.66 (m, 10 aro-matic protons), 7.73–7.83 (m, 4 aroaro-matic protons). 13C NMR (CDCl3):d ¼ 153.47, 151.99, 151.70, 151.22, 141.01, 140.69, 140.53, 139.43, 127.20, 127.00, 126.38, 126.25, 123.16, 121.62, 121.30, 120.01, 119.94, 55.40, 40.60, 32.01, 30.26, 26.42, 24.04, 22.82, 14.29. MS (FAB, observedm/z): 1322. 7,70-Bis(trimethylsilylethynyl)-9,9,90,90,900,900 -hexaoctylterfluorene (5)
A mixture of compound 4 (7.0 g, 5.29 mmol), PdCl2(PPh3)2
(0.075 g, 0.11 mmol), CuI (0.08 g, 0.42 mmol), and PPh3
(0.11 g, 0.42 mmol) in Et3N (150 mL) was stirred at 85C
under nitrogen. Trimethylsilyl acetylene (2.1 g, 21.18 mmol) was added slowly. The reaction mixture was allowed to react at 85 C under nitrogen for 12 h. After cooling to room tem-perature, the solution was poured into NH4Cl aqueous solution
and extracted with ethyl acetate. The combined organic layers were dried over anhydrous MgSO4, and the solvent was
removed under reduced pressure. The crude product was puri-fied by column chromatography (silica gel, hexane was used as eluent) to yield 3.0 g (42%) white crystals, mp: 42C.
1
H NMR (CDCl3):d ¼ 0.305 (s, 18H, ACH3), 0.68–0.85 (m, 18H,
A(CH2)7ACH3), 1.08–1.19 (m, 72H, A(CH2)A(CH2)6ACH3),
2.00–2.06 (m, 12H, (CH2)A(CH2)6ACH3), 7.47–7.50 (m, four
aromatic protons), 7.61–7.68 (m, 10 aromatic protons), 7.75–7.83 (m, four aromatic protons).13C NMR (CDCl3):d ¼
152.04, 151.12, 141.22, 140.56, 139.80, 126.42, 121.60, 120.51, 106.52, 55.40, 40.60, 32.01, 30.26, 26.42, 24.04, 22.82, 14.29. MS (FAB, observedm/z): 1359.
7,70-Diethynyl-9,9,90,90,900,900-hexaoctylterfluorene (M1) A mixture of compound 5 (1.5 g, 5.42 mmol) and K2CO3
(0.5 g, 27.08 mmol) in MeOH/THF (6 mL, v/v ¼ 1/3) was stirred at room temperature for 12 h. After cooling to room temperature, the solution was poured into 50 mL of water and extracted with ethyl acetate. The combined organic layers were dried over anhydrous MgSO4, and the solvent
was removed under reduced pressure. The crude product was purified by column chromatography (silica gel, hexane was used as eluent) to yield 1.1 g (82%) yellow oil.
1
H NMR (CDCl3): d ¼ 0.68–0.85 (m, 18H, A(CH2)7ACH3),
1.08–1.19 (m, 72H, A(CH2)A(CH2)6ACH3), 2.00–2.06 (m,
12H, (CH2)A(CH2)6ACH3), 3.17 (s, 2H, BCAH), 7.51–7.54
(m, four aromatic protons), 7.64–7.70 (m, 10 aromatic pro-tons), 7.78–7.85 (m, four aromatic protons). 13C NMR (CDCl3):d ¼ 152.06, 151.12, 141.22, 140.56, 139.80, 126.42,
121.60, 120.51, 106.52, 84.5, 55.40, 40.60, 32.01, 30.26, 26.42, 24.04, 22.82, 14.29. MS (FAB, observedm/z): 1214. 4,7-Bis(9,90-dioctylfluoren-2-yl)-2,1,3-benzothiodiazole (7) Compounds 6 (1.00 g, 3.40 mmol) and 2 (3.86 g, 7.48 mmol) were mixed with Pd(PPh3)4(0.008 g, 0.0034 mmol),
Aliquat 336 (0.22 g, 0.85 mmol), and aqueous K2CO3(2 M,
14 mL) in degassed toluene (40 mL). The reaction mixture was stirred at 110C under nitrogen for 24 h. After cooling to room temperature, the solution was poured into 50 mL of water and extracted with ethyl acetate. The combined or-ganic layers were dried over anhydrous MgSO4, and the
sol-vent was removed under reduced pressure. The crude prod-uct was purified by column chromatography (silica gel, ethyl acetate/hexane ¼ 1/50 was used as eluent) to yield 3.0 g (96%) yellow oil.
1
H NMR (CDCl3): d ¼ 0.72 (m, 12H, A(CH2)7ACH3), 0.98–
1.11 (m, 48H, A(CH2)A(CH2)6ACH3), 2.00–2.05 (m, 8H,
(CH2)A(CH2)6ACH3), 7.31–7.41 (m, six aromatic protons),
7.76–8.05 (m, 10 aromatic protons). 13C NMR (CDCl 3): d ¼ 154.58, 151.54, 151.35, 141.55, 140.88, 136.40, 133.82, 128.36, 128.12, 127.48, 127.06, 124.10, 123.18, 120.17, 119.92, 55.40, 40.60, 32.01, 30.26, 26.42, 24.04, 22.82, 14.29. MS (FAB, observedm/z): 913. 4,7-Bis(7-bromo-9,90 -dioctylfluoren-2-yl)-2,1,3-benzothiodiazole (8)
A solution of compound 7 (3.4 g, 3.72 mmol) and FeCl3
(0.030 g, 0.19 mmol) in CHCl3(40 mL) was stirred at room
temperature. Bromine (1.3 g, 8.22 mmol) in CHCl3(10 mL)
was added slowly. The reaction mixture was stirred at room temperature for 12 h and then poured into sodium thiosulfate solution (20 mL) until the red color of bromine disappeared. The solution was extracted with ethyl acetate. The combined organic layers were dried over anhydrous MgSO4, and the
sol-vent was removed under reduced pressure. The crude prod-uct was purified by recrystallization from THF/MeOH (v/v¼ 1/3) to yield 2.9 g (73%) green crystals, mp: 40C.
1H NMR (CDCl
3): d ¼ 0.72 (m, 12H, A(CH2)7ACH3), 0.98–
1.11 (m, 48H, A(CH2)A(CH2)6ACH3), 2.00–2.05 (m, 8H,
(CH2)A(CH2)6ACH3), 7.47–7.51 (d, four aromatic protons),
7.61–7.65 (d, two aromatic protons), 7.82–8.04 (m, eight aromatic protons). 13C NMR (CDCl 3): d ¼ 154.51, 153.77, 151.36, 140.85, 140.48, 139.89, 136.83, 136.33, 134.02, 133.75, 130.31, 128.57, 128.20, 126.50, 124.15, 123.20, 121.53, 120.05, 55.40, 40.60, 32.01, 30.26, 26.42, 24.04, 22.82, 14.29. MS (FAB, observedm/z): 1071. 4,7-Bis(7-(3-hydroxy-3-methylbut-1-ynyl)-9,90 -dioctylfluoren-2-yl)-2,1,3-benzothiodiazole (9)
A mixture of compound 8 (2.4 g, 2.24 mmol), PdCl2(PPh3)2
(0.22 g, 0.31 mmol), CuI (0.051 g, 0.27 mmol), and PPh3
(0.12 g, 0.46 mmol) in Et3N (50 mL) was stirred at 85 C
under nitrogen. 2-Methylbut-3-yn-2-ol (1.13 g, 13.43 mmol) was added slowly. The resulting solution was allowed to react at 85C under nitrogen for 12 h. After cooling to room temperature, the solution was poured into 50 mL of NH4Cl
aqueous solution and extracted with ethyl acetate. The com-bined organic layers were dried over anhydrous MgSO4, and
the solvent was removed under reduced pressure. The crude product was purified by column chromatography (silica gel, ethyl acetate/hexane ¼ 1/5 was used as eluent) to yield 2.21 g (91%) yellow crystals, mp: 70C.
1
H NMR (CDCl3):d ¼ 0.72 (m, 12H, A(CH2)7ACH3), 0.98–1.11
(m, 48H, A(CH2)A(CH2)6ACH3), 1.67 (s, 12H, ACA(OH)A
(CH3)2), 2.00–2.05 (m, 10H, (CH2) A(CH2)6ACH3AOH), 7.43–
7.46 (d, four aromatic protons), 7.69–7.72 (d, two aromatic protons), 7.83–7.87 (d, two aromatic protons), 7.88 (s, two aromatic protons), 7.94 (s, two aromatic protons), 8.02–8.04 (d, two aromatic protons). 13C NMR (CDCl
3): d ¼ 154.52, 151.56, 141.14, 140.82, 136.84, 133.75, 131.00, 128.52, 128.16, 126.35, 124.12, 121.36, 120.25, 120.04, 94.00, 83.36, 66.02, 55.40, 40.60, 32.01, 30.26, 26.42, 24.04, 22.82, 14.29. MS (FAB, observedm/z): 1077. 4,7-Bis(7-ethynyl-9,90 -dioctylfluoren-2-yl)-2,1,3-benzothiodiazole (M2)
A mixture of KOH (2.08 g, 27.08 mmol) and compound 9 (2.0 g, 5.42 mmol) in toluene (40 mL) was stirred at 110C under nitrogen for 12 h. After cooling to room temperature, the solution was poured into 50 mL of water and extracted with ethyl acetate. The combined organic layers were dried over anhydrous MgSO4, and the solvent was removed under
reduced pressure. The crude product was purified by column chromatography (silica gel, ethyl acetate/hexane ¼ 1/6 was used as eluent) to yield 1.8 g (95%) yellow oil.
1
H NMR (CDCl3): d ¼ 0.72–0.90 (m, 12H, A(CH2)7ACH3),
0.98–1.11 (m, 48H,A(CH2)A(CH2)6ACH3), 2.00–2.05 (m, 8H,
(CH2)A(CH2)6ACH3AOH), 3.17 (s, 2H, : CAH), 7.51–7.54 (d,
four aromatic protons), 7.71–7.74 (d, two aromatic protons), 7.75–7.89 (m, four aromatic protons), 7.95 (s, two aromatic protons), 8.02–8.03 (d, two aromatic protons). 13C NMR (CDCl3): d ¼ 154.28, 151.33, 141.42, 140.46, 139.28, 136.75, 133.52, 131.26, 128.31, 127.95, 126.61, 123.92, 120.43, 120.12, 119.85, 114.06, 84.70, 55.40, 40.60, 32.01, 30.26, 26.42, 24.04, 22.82, 14.29. MS (FAB, observedm/z): 962. 2-Bromo-7-(triisopropylsilylethynyl)-9,90 -dioctylfluorene (11)
A mixture of compound10 (4.0 g, 7.43 mmol), PdCl2(PPh3)2
(0.52 g, 0.74 mmol), and CuI (0.028 g, 0.15 mmol) in piperi-dine (50 mL) was stirred at 40 C under nitrogen. Ethynyl-triisopropylsilane (1.50 g, 8.20 mmol) was added slowly. The resulting solution was allowed to react at 40C under nitro-gen for 12 h. After cooling to room temperature, the solution was poured into 50 mL of NH4Cl aqueous solution and
extracted with ethyl acetate. The combined organic layers were dried over anhydrous MgSO4, and the solvent was removed
under reduced pressure. The crude product was purified by column chromatography (silica gel, hexane was used as eluent) to yield 3.50 g (92%) yellow crystals.
1
H NMR (CDCl3): d ¼ 0.80 (t, J ¼ 6.90 Hz, 6H,
A(CH2)7ACH3), 1.01–1.23 (m, 24H, A(CH2)A(CH2)6ACH3),
1.64 (s, 12H, AC(CH3)2AOH), 1.90 (t, 4H, A(CH2)A
(CH2)6ACH3), 2.11 (s, 2H, AC(CH3)2AOH), 7.37 (d, two
aro-matic protons), 7.51 (s, two aroaro-matic protons), 7.57 (d, two aromatic protons). 13C NMR (CDCl 3): d ¼ 151.34, 141.02, 131.15, 126.43, 121.77, 120.22, 94.28, 83.42, 66.17, 55.58, 40.75, 40.72, 32.18, 31.96, 30.38, 29.63, 24.04, 22.99, 14.48. MS (EI, observedm/z): 592. 2-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-7-(triisopropylsilylethynyl)-9,90-dioctylfluorene (12) n-Butyllithium (5.4 mL, 10.64 mmol, 2.5 M solution in hex-ane) was added into a solution of compound11 (3.5 g, 5.9 mmol) in anhydrous THF (100 mL), which was cooled to 78 C, and the obtained solution was stirred for 2 h, and
then 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3.8 mL, 14.75 mmol) was added slowly. The reaction mix-ture was warmed up slowly to room temperamix-ture and then stirred for 12 h. The mixture was quenched with water and extracted with ethyl acetate. The combined organic layers were dried over anhydrous MgSO4, and the solvent was
removed under reduced pressure. The crude product was purified by column chromatography (silica gel, hexane was used as eluent) to yield 2.7 g (52%) white solid, mp: 25C.
1
H NMR (CDCl3):d ¼ 0.72 (t, 6H, A(CH2)7ACH3), 0.98–1.11
(m, 24H, A(CH2)A(CH2)6ACH3), 1.39 (s, 12H, ACH3), 1.98
(t, 4H, (CH2)A(CH2)6ACH3), 7.25–7.35 (m, three aromatic
protons), 7.70–7.80 (m, four aromatic protons). 13C NMR (CDCl3):d ¼ 151.54, 150.09, 144.36, 141.14, 133.93, 129.05, 127.71, 126.88, 123.15, 120.31, 119.18, 83.92, 55.30, 40.49, 32.02, 30.23, 29.43, 25.17, 23.89, 22.82, 14.31. MS (EI, observedm/z): 640. 4,7-Bis(5-(7-(triisopropylsilylethynyl)-9,90 -dioctylfluoren-2-yl)-thien-2-yl)-2,1,3-benzothiodiazole (14)
Compounds 13 (0.77 g, 1.70 mmol) and 12 (2.70 g, 4.20 mmol) were mixed with Pd2(dba)3(0.05 g, 0.05 mmol),
Ali-quat 336 (0.16 g, 0.63 mmol), and aqueous Cs2CO3(15 mL,
5.8 mmol, 0.4 M) in degassed toluene (40 mL). The reaction mixture was stirred at 110C under nitrogen for 24 h. After cooling to room temperature, the solution was poured into 50 mL of water and extracted with ethyl acetate. The com-bined organic layers were dried over anhydrous MgSO4, and
the solvent was removed under reduced pressure. The crude product was purified by column chromatography (silica gel, hexane was used as eluent) to yield 0.6 g (25%) red oil.
1
H NMR (CDCl3):d ¼ 0.72 (t, 6H, A(CH2)7ACH3), 0.98–1.11
(m, 24H, A(CH2) A(CH2)6ACH3), 1.36 (s, 12H, ACH3), 1.98
(t, 4H, (CH2)A(CH2)6ACH3), 7.25–7.35 (m, three aromatic
protons), 7.60–7.82(m, four aromatic protons). MS (FAB, observedm/z): 1324.
4,7-Bis(5-(7-diethynyl-9,9-dioctylfluoren-2-yl)-thien-2-yl)-2,1,3-benzothiodiazole (M3)
Tetrabutylammonium fluoride (TBAF) (1.3 mL, 1.3 mmol, 1.0 M in THF) was added to a solution of compound 14 (0.60 g, 0.45 mmol) in anhydrous THF (20 mL) at room perature, and the reaction mixture was stirred at room tem-perature for 1 h. The solution was poured into 50 mL of water and extracted with ethyl acetate. The combined organic layers were dried over anhydrous MgSO4, and the solvent was
removed under reduced pressure. The crude product was puri-fied by column chromatography (silica gel, ethyl acetate/hex-ane¼ 1/6 was used as eluent) to give 0.45 g (90%) red oil.
1
H NMR (CDCl3): d ¼ 0.67–0.90 (t, J ¼ 7.05 Hz, 6H,
2.03 (t, J ¼ 3.90 Hz, 4H, A(CH2)A(CH2)6ACH3), 3.17 (s,
2H, :CAH), 7.35 (d, four aromatic protons), 7.50 (d, J ¼ 7.80 Hz, four aromatic protons), 7.66 (d, four aromatic pro-tons), 7.72 (s, two aromatic propro-tons), 7.95 (s, two aromatic protons), 8.16 (d, two aromatic protons). 13C NMR (CDCl3):
d ¼ 150.62, 140.55, 130.81, 126.11, 120.40, 119.53, 84.09, 54.77, 39.79, 39.76, 31.32, 29.51, 29.49, 28.76, 23.21, 22.14, 13.62. MS (FAB, observedm/z): 1012.
General Polymerization Procedure
The monomers were polymerized using TaCl5as the catalyst.
All polymerization reactions and manipulations were carried out under nitrogen using either an inert-atmosphere glove box or Schlenk techniques in a vacuum line system. The pu-rification of the polymers was done in a fume hood. A typical experimental procedure for the polymerization of monomer M1 is given below as an example (Scheme 4). Monomer 1 (0.6 g, 0.49 mmol) was added into a Schlenk tube with a three-way stopcock on the sidearm. The tube was evacuated under vacuum and then flushed with dried nitrogen three times through sidearm. Dried toluene (5 mL) was injected into the tube through a septum to dissolve the monomer. The catalyst solution was prepared in another tube by dissolving TaCl5 (42 mg, 0.118 mmol) in 11 mL of toluene. The
mono-mer solution was then transferred to the catalyst solution using a hypodermic syringe. The resulting mixture was stirred at room temperature under nitrogen for 8 h, and then metha-nol was added to the solution. The mixture was added drop-wise to methanol through a cotton filter with stirring. The polymer precipitate was allowed to stand for 1 h and was then separated by filtration, purified by several reprecipitation steps from toluene solution into methanol, and then dried in a vacuum oven to yield 0.52 g (86%) yellow solidHb-TF. Characterization
1H and 13C NMR spectra were recorded on a Varian-300
MHz spectrometer. Mass spectra were obtained using a JEOL JMS-HX 110 mass spectrometer. Gel permeation chromatog-raphy (GPC) was measured using a Viscotek GPC system equipped with a Viscotek T50A differential viscometer and a Viscotek LR125 laser referactometer. Three 10-lm columns were connected in series in order of decreasing pore size (105, 104, and 103 Å). Polystyrene standards were used for calibration, and THF was used as the eluent. Thermogravi-metric analysis (TGA) was carried out using a Perkin Elmer Pyris 7 instrument. The thermal stabilities of the samples under nitrogen were determined by measuring their weight losses while heating at a rate of 10C/min. Differential scan-ning calorimetry (DSC) was performed on a Perkin Elmer Pyris Diamond DSC unit operated at a heating and cooling rates of 20 and 40 C/min, respectively. The glass transition temperatures (Tgs) were determined from the second
heat-ing scan. UV–Vis spectra were measured usheat-ing an HP 8453 spectrophotometer. PL spectra were obtained using an ARC SpectraPro-150 luminance spectrometer. Cyclic voltammetry (CV) experiments were performed using an Autolab ADC 164 electrochemical analyzer operated at a scanning rate of 50 mV/s; the supporting electrolyte was 0.1 M tetra-n-buty-lammonium tetrafluoroborate (n-Bu4NBF4), which was
dis-solved in acetonitrile. The potentials were measured against an Ag/AgCl reference electrode using ferrocene/ferrocenium (Fc/Fcþ) as the internal standard.
Fabrication and Measurements of PLED
Two polymer light-emitting devices having the configuration of indium tin oxide (ITO)/poly(3,4-ethylene dioxythiophene) (PEDOT):poly(styrene sulfonate) (PEDOT:PSS) (40 nm)/light-emitting polymer (60–80 nm)/CsF (2 nm)/Al (100 nm) and ITO/PEDOT:PSS (40 nm)/light-emitting polymer (60– 80 nm)/TPBI (30 nm)/LiF (1 nm)/Al (100 nm) were fabri-cated. The patterned ITO glass substrates were ultrasonically cleaned with detergent, deionized water, acetone, and isopro-pyl alcohol. The PEDOT:PSS (Baytron P VP AI4083 from H. C. Stack) was spin coated on the cleaned and UV–ozone-treated ITO substrates. The PEDOT:PSS layer was baked at 120 C for 30 min in air to remove residual water and then moved into a glove box under nitrogen. The light-emitting polymers dissolved in chlorobenzene solution were spin coated on top of the PEDOT:PSS layer. Then, the films were baked at 80C for 30 min under vacuum. The TPBI (1,3,5-tris(1-phenyl-1H-benzimidazol-2-ol)benzene) layer, which was grown through thermal deposition, was used as an electron transporting layer that would block holes and confine excitons. The devi-ces were completed by thermal deposition of a CsF (2 nm)/ Al (100 nm) or LiF (1 nm)/Al (100 nm) as cathode. The cur-rent–voltage–luminance characteristics were measured by using an optical power meter PR-650 and a digital source meter Keithley 2400. The EL spectra were measured by using a Photo Research PR-650 spectrophotometer under ambient condition after encapsulation.
RESULTS AND DISCUSSION
Synthesis and Characterization of Monomers and Polymers
Schemes 1–3 illustrate the synthetic routes of the monomers. M1, M2, and M3 were synthesized based on the following procedure. First, reacting 2,7-dibromo-9,9-dioctylfluorene (1) and 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dio-ctylfluorene (2) through Suzuki coupling reaction allows the formation of compound 3. Compound 3 was then bromi-nated with bromide and catalytic amount of Lewis acid, FeCl3, to give compound 4. Reacting the bromo groups of
compound 4 with trimethylsilylacetylene by Sonogashira coupling reaction yielded compound 5.49 Finally, M1 was obtained by deprotecting the terminal acetylene groups of compound5 under basic condition with K2CO3.M2 was
syn-thesized based on the following procedure. Compound7 was also synthesized by reacting compounds 2 and 6 through Suzuki coupling reaction. Compound 7 was then brominated with bromide to give the corresponding compound 8,49
which was further reacted with 2-methyl-3-butyn-2-ol by Sonogashira coupling reaction to yield compound 9. Depor-tecting compound 9 under basic and high temperature con-dition allows the formation of monomerM2. M3 was synthe-sized based on the following procedure. Compound 14 was synthesized by reacting compounds 12 and 13 through Suzuki coupling reaction. Compound 14 was deprotect with
n-Bu4NF under basic condition to allow the formation of
monomerM3.
M1, M2, and M3 were characterized with 1H and 13C NMR spectroscopy and FAB-MS (Supporting Information). In the
1H NMR spectra, the characteristic peak of AC:CAH at d
3.17 ppm was observed for all of the monomers, which indi-cates the success of Sonogashira coupling reactions and the deprotection reactions. Compared to the 1H NMR spectrum of M1, a characteristic singlet peak at d 7.95 ppm that
belongs to the aromatic H on the BT unit can be observed for M2. For monomer M3, the double peaks at d 7.66 ppm represent the protons in the thiophene rings of DTBT. Thus, the1H NMR spectra differentiate the difference in the molec-ular structures of the monomers. In the 13C NMR spectra, the feature signal peak of the carbon on the terminated tri-ple bond of the monomers was also observed atd 84.5 ppm. In the mass spectra, them/z values of 1214, 962, and 1012 were observed for M1, M2, and M3, which closely match
SCHEME 1 Synthetic route of M1. Reagents and conditions: (i) Pd(PPh3)4, K2CO3, Aliquat 336, toluene, H2O, reflux; (ii) FeCl3, Br2, CHCl3, 0C; (iii) trimethylsilylacetylene, PdCl2(PPh3)2, CuI, PPh3, Et3N, 85C; (iv) K2CO3, MeOH/THF (1:3, v/v), r.t.
SCHEME 2 Synthetic route of M2. Reagents and conditions: (i) Pd(PPh3)4, K2CO3, Aliquat 336, toluene, H2O, reflux; (ii) FeCl3, Br2, CHCl3, 0C; (iii) 2-methyl-3-butyn-2-ol, PdCl2(PPh3)2, CuI, PPh3, Et3N, reflux; (iv) KOH, toluene, reflux.
with the calculated monoisotopic masses of the monomers. All the molecular characterization results clearly indicate the success of the reactions and confirm the chemical identify of M1, M2, and M3. The Ps, TF, BFBT, and Hb-BFTBT were synthesized by using M1, M2, and M3 as monomers via [2þ2þ2] polycyclotrimerization (Scheme 4). The Hb-Ps compared with the monomers show much weaker signal for the AC:CAH proton in their 1H NMR spectra through the polycyclotrimerization reaction (Supporting In-formation Fig. S1). Thus, the peak intensities of the AC:CAH proton (d 3.17 ppm) decreased after the polymer-ization reactions, and their remaining intensities depend on the degree of polymerization. The molecular weights and pol-ydispersity indices (PDI) ofHb-TF, Hb-BFBT, and Hb-BFTBT were characterized with GPC and summarized in Table 1. Hb-TF possesses much higher Mw (136.8 103g/mol, PDI:
1.86) than those ofHb-BFBT (6.984 103g/mol, PDI: 1.50)
and Hb-BFTBT (7.853 103 g/mol, PDI: 1.43). The decomposition temperatures (Td) of Hb-TF, Hb-BFBT, and
Hb-BFTBT are 401, 355, and 400 C, which indicate good thermal stability (Supporting Information). The polymers Hb-TF, Hb-BFBT, and Hb-BFTBT reveal Tgs at 110, 51, and
89 C, respectively (Table 1 or Supporting Information Figs. S13–S18). It is evidenced that all the Hb-Ps show good ther-mal behaviors.
Optical Properties
To examine the photophysical properties of these polymers, the absorption and PL spectra of dilute solutions and solid films of Hb-TF, Hb-BFBT, and Hb-BFTBT were measured. Figures 1–3 show the UV–Vis and PL spectra of the dilute solutions and thin films ofHb-TF, Hb-BFBT, and Hb-BFTBT, respectively. The spectral data are summarized in Table 2. In the dilute solutions, Hb-TF shows only one absorption band
SCHEME 3 Synthetic route of M3. Reagents and conditions: (i) PdCl2(PPh3)2, ethynyltriisopropylsilane, CuI, piperidine, 40C; (ii) n-BuLi, 2-isopropyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, THF,78C; (iii) Pd(PPh3)4, K2CO3, Aliquat 336, toluene, H2O, reflux; (iv) n-Bu4NF, THF, r.t.
with kmaxat 365 nm, which is originated from the localized
p–p* transitions of the TF unit, while BFBT and Hb-BFTBT show two distinct absorption bands (Table 2). The shorter wavelength absorption bands of BFBT and Hb-BFTBT can be attributed to the localized p–p* transitions, and the longer wavelength absorption bands come from the intramolecular charge transfer (ICT) between electron-rich donors (fluorene) and electron-deficient acceptors (BT and DTBT).50,51The ICT band ofHb-BFTBT (kmax: 527 nm) is at
longer wavelength than that of Hb-BFBT (kmax: 437 nm).
The bathochromic shift was also observed in the PL kmaxof
the dilute solutions ofHb-TF, Hb-BFBT, and Hb-BFTBT. The PL kmax of Hb-TF, Hb-BFBT, and Hb-BFTBT are 410, 522,
and 612 nm, respectively (Table 2 and Figs. 1–3). In addi-tion, the optical band gaps (Eg) of the Hb-Ps deduced from
the onset of absorption follows the trend that Eg, Hb-TF
(3.04 eV) > Eg, Hb-BFBT (2.45 eV) > Eg, Hb-BFTBT(1.97 eV).
These results clearly indicate that the photophysical proper-ties of the Hb-Ps are strongly affected, when the middle fluo-rene moiety in the TF unit is replaced with an electron-withdrawing group, for example, BT or DTBT. In addition to that, the donating strength of the fluorene moiety is stronger when the fluorene and BT moieties are bridged with a thio-pene ring. The PLkmaxs of the polymers suggest thatHb-TF,
Hb-BFBT, and Hb-BFTBT are blue, green, and red emitters. Thus, the results demonstrate a simple methodology for tun-ing the PL wavelength of hyperbranched conjugated poly-mers by simply introducing the electron accepting units into the electron-rich conjugated systems.
Compared to their dilute solutions, bathochromic shift of the UV, and PL bands, decrease in the PL quantum yields are observed in the thin films of TF, BFBT, and Hb-BFTBT (Table 2 and Figs. 1–3). The phenomenon is attrib-uted to the formation of aggregation of polymers in the solid state. Formation of aggregation in the solid state of these series of Hb-Ps can be expected, as the branching points (benzene rings) possess a planar geometry rather than a three-dimensional structure. Interestingly, the PL spectrum of the Hb-TF thin film is extremely broad and covers exten-sively in the blue and green light region. The result suggests the possibility of coexistence of multiple degrees of aggrega-tions in the thin film. Although the PL spectra of the Hb-BFBT and Hb-BFTBT in thin film red-shifted, they remained green and red emitters in the solid state. Because of its large Eg and wide PL emission, the thin film of Hb-TF therefore
possesses the potential to be used as a host in PLED
TABLE 1 Number-Average Molecular Weights (Mn), Weight-Average Molecular Weights (Mw), PDI, and Thermal Properties of the Hyperbranched Polymers
Polymer Mna (103) Mwa (103) PDIa (Mw/Mn) Tdb (C) Tgc (C) Hb-TF 136.8 255.4 1.86 401 110 Hb-BFBT 6.984 10.528 1.50 355 51 Hb-BFTBT 7.853 11.254 1.43 387 89 a
Weight-average molecular weights (Mw) and PDI of the polymers were
determined by GPC in THF using polystyrene standards.
b
The 5% weight loss of the decomposition temperature measured by TGA under N2.
c
The glass transition temperature measured by DSC under N2.
FIGURE 1 UV–Vis absorption and PL spectra of Hb-TF in solu-tion and in thin film, and the absorpsolu-tion spectrum of Os(fptz)2(PPh2Me2)2in solution.
FIGURE 2 UV–Vis absorption and PL spectra of Hb-BFBT in so-lution and in thin film.
FIGURE 3 UV–Vis absorption and PL spectra of Hb-BFTBT in so-lution and in thin film, and the PL spectrum of PFO in thin film.
applications or as the active layer of a white PLED when blended with suitable amount of red emitters we synthesized in this article.
Electrochemical Properties
CV was used to examine the electrochemical properties and evaluate the HOMO (highest occupied molecular orbital) lev-els of the polymers. Table 3 summarizes theEox, onset,EHOMO,
andELUMO(lowest unoccupied molecular orbital).TF,
Hb-BFBT, and Hb-BFTBT exhibited oxidation onset potentials at about 1.42, 1.44, and 0.87 eV, respectively. Their EHOMO
energy levels are calculated as –5.82, 5.84, and –5.27 eV based on the onset of oxidation potential of the internal standard ferrocene (4.8 eV below vacuum). TheELUMOlevels
of Hb-TF, Hb-BFBT, and Hb-BFTBT are 2.78, 3.39, and 3.30 eV, which were deduced from their EHOMOand optical
band gaps (Eg). The ELUMOof Hb-BFBT, and Hb-BFTBT are
lower than that ofHb-TF, because of the incorporation of BT, an electron-withdrawing group, into the conjugated system.50 As for D-A conjugated polymers, the first oxidation onset potential is normally contributed from the electron-rich seg-ment, the closeEHOMOof Hb-TF and Hb-BFBT can be
inter-preted as the oxidation takes place first at the fluorene moi-eties in the molecules.50 The higher E
HOMO of Hb-BFTBT
could be attributed to the incorporation of thiophene units, an electron-rich unit, into the main chain. The energy level diagrams of the polymers are illustrated in Figure 4.
Device Performance
To evaluate the PLED performances of pristine TF, Hb-BFBT, and Hb-BFTBT, we fabricated the double-layer PLEDs with a device configuration of ITO/PEDOT:PSS (40 nm)/ light-emitting polymer (60–80 nm)/CsF (2 nm)/Al (100 nm). The EL spectra, the current density, and brightness versus
voltage characteristics of the devices are shown in Figure 5. The device performances are summarized in Table 4. As shown in Figure 5(a), the EL kmax of Hb-TF, Hb-BFBT, and
Hb-BFTBT are located at 520, 560, and 664 nm, at maxi-mum luminance efficiency. All the EL kmaxs of the devices
are similar to the PL kmaxs observed in the polymer thin
films. The EL spectrum of Hb-TF shows broad blue-green emission, while the EL spectra of Hb-BFBT and Hb-BFTBT refer to green and red emission. Thus, by introducing the electron-accepting units (BT and DTBT) into an electron-rich conjugated system, the emission spectra of the Hb-Ps-based devices can be easily adjusted, and RGB PLEDs can be obtained using the novel Hb-Ps as the active materials. The broad blue-green emission is due to the aggregation of the polymer chains in the solid state. The Hb-TF-based device had a maximum brightness of 50 cd/m2and maximum lumi-nance efficiency of 0.01 cd/A. The BFBT- and Hb-BFTBT-based devices had a maximum brightness of 42 and 29 cd/m2, and the maximum luminance efficiency of 0.02 and 0.01 cd/A, respectively. The turn-on voltage of Hb-TF, Hb-BFBT, and Hb-BFTBT are 7.1, 6.5, and 4.4 V, respectively (Table 4). Among these polymers, Hb-BFTBT-based device displayed the lowest turn-on voltage than others because of its highest EHOMO, which reduces the energy barrier for the
hole-injection from the PEDOT:PSS layer.
Because of its large Eg,Hb-TF has the potential to be used
as a polymeric host. To evaluate Hb-TF as a host material, we used red electrophosphorescent, Os(fptz)2(PPh2Me2)2 as
a guest material and fabricated a multilayer device with the configuration of ITO/PEDOT:PSS (40 nm)/Hb-TF:X wt % Os(fptz)2(PPh2Me2)2(60–80 nm)/TPBI (30 nm)/LiF (1 nm)/Al TABLE 2 Optical Properties of the Hyperbranched Polymers
Polymer
UV–Vis Absorption
kmax(nm) PLkmax(nm) UPL(%) Egc(eV)
Solutiona Filmb Solutiona Filmb Solutiona Filmb Filmb
Hb-TF 365 366 410 438, 490, 544 37 8 3.04 Hb-BFBT 335, 423 340, 437 522 545 50 30 2.45 Hb-BFTBT 373, 511 376, 527 612 655 18 7 1.97 a In 0.5 wt % toluene. b
Spin-coated from 0.5 wt % toluene.
c
Egwas determined from the onset wavelength of
UV absorption spectra in thin film.
TABLE 3 Electrochemical Properties of the Hyperbranched Polymers
Polymer Eox, onseta(V) EHOMOb(eV) ELUMOc(eV)
Hb-TF 1.42 5.82 2.78
Hb-BFBT 1.44 5.84 3.39
Hb-BFTBT 0.87 5.27 3.30
a
The onset oxidation potential measured by CV.
b
EHOMOis calculated based on the equation, EHOMO¼ 4.8 þ Eox, onset. c
ELUMOis calculated based on the equation, ELUMO¼ Egþ EHOMO.
FIGURE 4 Energy level diagram for the devices having the configuration of ITO/PEDOT:PSS/light-emitting polymer/CsF/Al.
(100 nm). The PL and EL spectra of 7 and 14 wt % Os(fptz)2(PPh2Me2)2-doped blends are presented in Figure 6.
The current density and brightness versus voltage characteris-tics of 7 and 14 wt % Os(fptz)2(PPh2Me2)2-doped devices are
shown in Figure 7. Device performance is summarized in Table 5. The PL spectrum of theHb-TF thin film and the UV-Vis absorption spectrum of Os(fptz)2(PPh2Me2)2in chlorobenzene
solution shown in Figure 1 demonstrate a significant overlap between the PL band ofHb-TF and the metal-to-ligand charge transfer (MLCT) absorption band of Os(fptz)2(PPh2Me2)2.
Thus, an efficient Fo¨rster energy transfer can be expected from the singlet excited state of the host to the MLCT state of the guest, Os(fptz)2(PPh2Me2)2. A fast intersystem crossing
takes place once Os(fptz)2(PPh2Me2)2 is excited and
subse-quent red phosphorescence emission was generated when excited Os(fptz)2(PPh2Me2)2 relaxes from its triplet excited
state.52As indicated in Figure 6(a), the PL spectra of the blends that contained Os(fptz)2(PPh2Me2)2-doped Hb-TF films exhibit
two emission bands withkmaxat 435 and 616 nm. The shorter
wavelength band originates from the residual emission of the host, and the longer wavelength band corresponds to the phos-phorescence of the Os(fptz)2(PPh2Me2)2 guest. The result
implies that partial energy transfer moderated from the host to the guest. Conversely, the EL spectra of Figure 6(b) exhibited only the red emission derived from Os(fptz)2(PPh2Me2)2,
sug-gesting that both energy transfer and direct charge trapping/ recombination at the Os(fptz)2(PPh2Me2)2guest were
responsi-ble for the observed EL.53–55According to the energy level
dia-gram in inset of Figure 7(a), Hb-TF has much deeper HOMO level than Os(fptz)2(PPh2Me2)2. Therefore, holes in the active
layer can be effectively trapped in the Os(fptz)2(PPh2Me2)2
do-main. In this case, efficient charge recombination in the guest domain and dominant phosphorescence emission of the guest was observed.56As shown in Table 5, the EL spectra of 7 and 14 wt % Os(fptz)2(PPh2Me2)2-doped blends show EL kmax at
616 nm, with the CIE coordinates of (0.64, 0.36) close to the standard CIE coordinate of red emission (0.67, 0.33). Maximum brightness of the 7, and 14 wt % Os(fptz)2(PPh2Me2)2-doped
devices are 41 and 84 cd/m2, and the maximum luminance
FIGURE 5 (a) EL spectra and (b) current density and brightness versus voltage characteristics of the devices having the config-uration of ITO/PEDOT:PSS/light-emitting polymer/CsF/Al.
TABLE 4 Performances of the Devices Based on the
Configuration of ITO/PEDOT:PSS/Light-Emitting Polymer/CsF/Al
Polymer Hb-TF Hb-BFBT Hb-BFTBT
Turn-on voltage (V)a 7.1 6.5 4.4
Max. Brightness (cd/m2) 48 @ 12 V 42 @ 13 V 29 @ 9 V
Max. L.E. (cd/A) 0.01 0.02 0.01
ELkmax(nm) 520 560 664
CIE (x, y) (0.30, 0.39) (0.43, 0.54) (0.67, 0.32)
a
Recorded at 1 cd/m2
.
FIGURE 6 (a) PL spectra and (b) EL spectra of the blends pre-pared from Hb-TF as the host doped with the 7 and 14 wt % of Os(fptz)2(PPh2Me2)2as the guest.
efficiency of the two devices are 1.37 and 1.61 cd/A. The turn-on voltage of 7 and 14 wt % Os(fptz)2(PPh2Me2)2-doped
devices are 6.6 and 5.6 V. Thus, increasing the dopant concen-tration not only enhances the device performance of Hb-TF-based PLEDs but also decreases their turn-on voltage. The decrease of turn-on voltage suggests that the charges can be
directly injected into the guest molecules at sufficiently high doping concentration; consequently, the guest domain serves as extra transport channels for the charge carriers and reduces the turn-on voltage.57–62 Hb-TF provides an good packing environment for the dispersed Os(fptz)2(PPh2Me2)2
emitters and improves the device efficiency as a result of the reductions in the degrees of both triplet–triplet annihilation and self-quenching of the Os(fptz)2(PPh2Me2)2guest.
63
Hence, the results demonstrate the potential of Hb-TF as a good polymeric host in the PLED applications.
The PL and EL spectra of pristine Hb-BFTBT illustrate its potential as a red emitter. To further improve the device per-formance ofHb-BFTBT-based PLEDs, we used PFO as a host andHb-BFTBT as the guest and fabricated double layer red PLEDs with the configuration of ITO/PEDOT:PSS (40 nm)/ PFO:X wt % Hb-BFTBT (60–80 nm)/CsF (2 nm)/Al (100 nm). Figure 3 displays the PL spectrum of PFO in thin film. The emission band of PFO overlaps with the absorption band of theHb-BFTBT thin film, suggesting efficient energy transfer from the PFO host to the Hb-BFTBT guest.44,45The PL and EL spectra of the PFO:BFTBT blends with Hb-BFTBT concentrations of 0.25–5 wt % are presented in Figure 8. The current density and brightness versus voltage characteristics of the devices are shown in Figure 9. Device performances are summarized in Table 6. As shown in Fig-ure 8, both of the PL and EL spectra of the blends contain
FIGURE 7 (a) Current density and brightness versus voltage characteristics and (b) luminance efficiency versus current den-sity characteristics of the devices based on the configuration of ITO/PEDOT:PSS/Hb-TF:X wt % Os(fptz)2(PPh2Me2)2/TPBI/LiF/Al. (X¼ 7 and 14). The inset is the energy level diagram for the Os(fptz)2(PPh2Me2)2-doped devices.
TABLE 5 Performances of the Devices Based on the Configuration of ITO/PEDOT:PSS/Hb-TF:X wt% Os(fptz)2(PPh2Me2)2/TPBI/LiF/Al Guest Os(fptz)2(PPh2Me2)2 Dopant concentration (wt %) 7 14 Turn-on voltage (V)a 6.6 5.6 Max. Brightness (cd/m2) 41 @ 10.5 V 84 @ 10.5 V
Max. L.E. (cd/A) 1.37 1.61
ELkmax(nm)b 616 616 CIE (x, y)b (0.64, 0.36) (0.64, 0.36) a Recorded at 1 cd/m2 . b Recorded at 8 V.
FIGURE 8 (a) PL spectra and (b) EL spectra of the blends pre-pared from PFO as the host doped with the 0.25, 0.5, 2, and 5 wt % of Hb-BFTBT as the guest.
two emission bands. The one withkmax at 440 nm, belongs
to the characteristics emission of the PFO host, while the other one with kmax at 609 nm, belongs to the emission of
theHB-BFTBT guest. The degree of energy transfer from the PFO host to theHb-BFTBT guest increases as the doping con-centration increases from 0.25 to 5 wt %.53–55At dopant
con-centration of 5 wt %, the host emission is almost extinct. According to the energy level diagram in inset of Figure 9(a), the EHOMO of Hb-BFTBT is 0.5 eV below the EHOMO of PFO.
Therefore, holes can be effectively trapped in theHb-BFTBT domains and allows efficient emission from the guest mole-cules.56 The EL spectrum of the device using PFO:5 wt % Hb-BFTBT as a emitting layer shows saturated red emission withkmaxat 638 nm and CIE coordinates at (0.63, 0.34). The
maximum brightness and maximum luminance efficiency of the device are 538 cd/m2 and 0.41 cd/A, which exhibited better performance than those of the pristine Hb-BFTBT-based PLEDs.64 The turn-on voltage of the device is 10.1 V, which is higher than that of the pristine Hb-BFTBT. To fur-ther improve the performance of the device, inserting a hole-transporting layer or adding hole-hole-transporting material into the PFO:Hb-BFTBT blend is now in progress in our group. As shown in Figure 9 and Table 6, the device with the lowest Hb-BFTBT dopant concentration (0.25 wt %) show the best performance with turn-on voltage at 5.1 V, a maximum brightness of 1169 cd/m2, and maximum luminance effi-ciency of 3.39 cd/A. The EL spectra of the device contain both blue (kmaxat 440 nm) and red (kmax at 609 nm)
emis-sion bands with similar intensities [Fig. 8(a)]. The CIE coor-dinates of the device is at (0.53, 0.32). The incomplete energy transfer from PFO to Hb-BFTBT provides an oppor-tunity of making white PLEDs via adding an auxiliary green dopant. For this purpose, an efficient green emitter, BFBT was added into the PFO:Hb-BFTBT blend.46 White PLEDs with the double-layer configuration of ITO/PEDOT:PSS (40 nm)/PFO:X wt % BFBT:Y wt % Hb-BFTBT (60–80 nm)/ CsF (2 nm)/Al (100 nm) were fabricated. Several concentra-tions of BFBT:Hb-BFTBT were attempted to achieve the ideal white light emission. The performances of the devices are summarized in Table 7. The current density and bright-ness versus voltage characteristics and the luminance effi-ciency versus current density curves of the devices are shown in Figure 10. The inset of Figure 10(a) shows the EL spectra of the devices at an applied potential of 6 V. At dop-ant concentrations of 0.05, 0.08, and 0.1 wt % for both BFBT and Hb-BFTBT, the devices have a maximum brightness of 1183, 1185, and 1886 cd/m2 and maximum luminance FIGURE 9 (a) Current density and brightness versus voltage
characteristics and (b) luminance efficiency versus current den-sity characteristics of the devices based on the configuration of ITO/PEDOT:PSS/PFO:X wt % Hb-BFTBT/CsF/Al (X¼ 0.25, 0.5, 2, and 5). The inset is the energy level diagram for the Hb-BFTBT-doped devices.
TABLE 6 Performances of the Devices Based on the Configuration of ITO/PEDOT:PSS/PFO:X wt% Hb-BFTBT/CsF/Al Guest Hb-BFTBT Dopant concentration (wt %) 0.25 0.5 2 5 Turn-on voltage (V)a 5.0 5.1 7.7 10.1 Brightness (cd/m2)b 369 329 188 74 L.E. (cd/A)b 1.99 1.97 0.97 0.37 Max. Brightness (cd/m2) 1,187 1,169 643 538
Max. L.E. (cd/A) 2.42 3.39 1.96 0.41
ELc 616 620 620 638 CIE (x, y)c (0.45, 0.29) (0.53, 0.32) (0.60, 0.34) (0.63, 0.34) a Recorded at 1 cd/m2 . b Recorded at 20 mA/cm2 . c Recorded at 11 V.
efficiency of 3.70, 4.98, and 3.71 cd/A. Their CIE coordinates are (0.31, 0.34), (0.33, 0.37), and (0.35, 0.38), which are all very close to the CIE coordinates of the ideal white light, (0.33, 0.33). The turn-on voltages of all the devices are 3.5 V. Further increase of the dopant concentration to 0.25 wt % lowered the device performance, deviated the EL of the device from white emission. These results demonstrated the
potential ofHb-BFTBT as a red emitter, and as a component in the applications of white PLEDs.
CONCLUSIONS
In summary, to achieve good control in the color stability of solution-processable light-emitting materials, we developed a facile approach to synthesize diyne-functionalized TF, BFBT, andBFTBT as the blue, green, and red monomeric chromo-phores and prepared the corresponding blue, green, and red emitting Hb-Ps,Hb-TF, Hb-BFBT, and Hb-BFTBT through the [2þ2þ2] polycyclotrimerization. All the synthesized Hb-Ps show excellent thermal stability, moderate to highTgs, and can
be fabricated into the active layers of PLEDs based on the solu-tion process. PL and EL spectra of the Hb-Ps clearly demon-strate that Hb-TF, Hb-BFBT, and Hb-BFTBT are blue-green, green, and red emitting materials. Because of its large Eg,
Hb-TF was further evaluated as a polymeric host material. Satu-rated red electrophosphorescent devices with CIE coordinates of (0.64, 0.36), turn-on voltage (5.6 V) with a maximum lumi-nance efficiency of 1.61 cd/A, and maximum brightness of 84 cd/m2 were obtained when a Hb-TF was used as a host material doped with Os(fptz)2(PPh2Me2)2 as a guest material.
The device efficiency of the red emitter,Hb-BFTBT, was further improved in a host–guest doped system by usingHb-BFTBT as red-emitting guest in the PFO host layer. Furthermore, a series of efficient white PLEDs were prepared by usingBFBT, as aux-iliary green dopant in the PFO:Hb-BFTBT blend. White PLEDs with CIE coordinates of (0.33, 0.37), which is close to that of ideal white light (0.33, 0.33), low turn-on voltage (3.5 V) with highest luminance efficiency of 4.98 cd/A, and maximum brightness of 1185 cd/m2were also achieved.
The authors thank the National Science Council and Ministry of Education for financial support.
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TABLE 7 Performances of the Devices Based on the Configuration of ITO/PEDOT:PSS/PFO:X wt % BFBT:Y wt % Hb-BFTBT/CsF/Al Guest BFBT:Hb-BFTBT Dopant concentration (wt %) 0.05:0.05 0.08:0.08 0.1:0.1 0.25:0.25 Turn-on voltage (V)a 3.5 3.5 3.5 3.5 Brightness (cd/m2)b 547 648 645 445 L.E. (cd/A)b 2.82 3.26 3.27 2.24 Max. Brightness (cd/m2) 1,183 1,185 1,886 1,037
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