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By Junqiao Ding, Baohua Zhang, Jianhong Lu¨, Zhiyuan Xie, Lixiang Wang,* Xiabin Jing, and Fosong Wang Phosphorescent polymer light-emitting diodes (PPLEDs), in which the triplet emitters are dispersed into the polymeric hosts via physical blending[1–6] or chemical bonding,[7–12] have attracted much attention because of their low-cost preparation from solution either by spin-coating or via inkjet printing as well as the capability to harness both the singlet and triplet excitons to realize theoretical 100% internal quantum efficiency.[13–15] For solid-state lighting applications, the development of power-efficient blue, green and red PPLEDs is essential. However, owing to the scarcity of the appropriate polymeric hosts, the performance of blue PPLEDs is not as good as that of the remaining two primary colors. To date, the most widely reported polymeric host used for blue PPLEDs is poly(9-vinylcarbazole) (PVK), which shows a low-lying highest occupied molecular orbit (HOMO) energy level of 5.9 eV.[16] The existing large holeinjection barrier between poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)[17,18] and PVK leads to a considerable increase of the operating voltage and the power consumption. To overcome the disadvantages of PVK, the design of hosts that are suitable for solution processing is of paramount importance for the development of power-efficient blue PPLEDs. As an effective host of the blue triplet emitter, on one hand, the triplet energy is required to be higher than 2.75 eV[19] in order to prevent the reverse energy transfer from the triplet emitter to the host.[20] On the other hand, because hole injection is found to be more inefficient than electron injection among these PPLEDs,[21] it is desirable that the HOMO level matches the Fermi level of PEDOT:PSS (5.1 eV).[16] Unfortunately, optimizing one physical parameter often adversely affects the other. That is to say, a large triplet energy is automatically associated with a low HOMO level.[22] Therefore it is very difficult to find a polymer that satisfies these two requirements at the same time. This inherent bottleneck has promoted us to turn our interests to conjugated dendrimers, which are believed to exhibit the same

¨, Prof. Z. Xie, Prof. X. Jing, [*] Prof. L. Wang, Dr. J. Ding, B. Zhang, J. Lu Prof. F. Wang State Key Laboratory of Polymer Physics and Chemistry Changchun Institute of Applied Chemistry Chinese Academy of Sciences Changchun 130022 (P. R. China) E-mail: [email protected] B. Zhang Graduate School of the Chinese Academy of Sciences Beijing 100039 (P. R. China)

DOI: 10.1002/adma.200902328

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Solution-Processable Carbazole-Based Conjugated Dendritic Hosts for Power-Efficient Blue-Electrophosphorescent Devices

excellent solution processability as polymers.[23,24] As illustrated in Scheme 1a, several molecular design rules must be considered simultaneously. i) Both the core and the dendrons should possess triplet energies above 2.9 eV so that upon connection the triplet energies of the finally obtained dendrimers do not decrease below a certain limit (2.75 eV). ii) Oligocarbazole units are used as the dendrons owing to their excellent hole-transporting property and tunable HOMO energy level.[25–27] iii) The carbazole dendrons are directly attached to the core via their N-positions, as this unique linkage mode does not have obvious influence on the triplet energies.[22] Moreover, tert-butyl surface groups are employed in the dendrimers to ensure their good solubility in common organic solvents to form high-quality films. Given these considerations, the design and synthesis of solution-processable carbazole-based conjugated dendritic hosts, H1 and H2, are presented in this Communication. With increasing dendron generation the HOMO levels can be tuned well to improve the hole-injection ability, while their triplet energies can be maintained at a level as high as 2.9 eV. As a result, a power-efficient blue-electrophosphorescent device has been realized with an efficiency up to 15.4 lm W1 (27.6 cd A1, 12.7%), which is 86% higher than that of PVK. To the best of our knowledge, this is the first report of dendritic molecules used as the efficient solution-processable hosts for triplet emitters. The dendrimers H1 and H2 were synthesized by a convergent method, as depicted in Scheme 1b. Firstly, the key intermediate 3,6-diiodo-9-(40 -iodophenyl)carbazole (2) was conveniently prepared via a two-step reaction with a moderate total yield of 36%. Then, 2 was coupled with the first and second carbazole dendron via Ullmann reactions to afford H1 and H2 with a yield of 82 and 69%, respectively. For comparison H0 was also synthesized. The structures of the dendrimers were verified with 1H NMR spectroscopy, elemental analysis, and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry. Additionally, thermal gravimetric analysis (TGA) was also performed, giving a decomposition temperatures of 412 8C and 466 8C for H1 and H2, respectively, which suggests that dendrimers H1 and H2 are thermally stable. To examine the effect of the attached dendrons on the singlet and triplet energy, the spectral properties of H1 and H2 compared with the core H0 were studied. As shown in Figure 1a, a red shift of 6 nm is observed from H0 to H1 for the lowest-energy absorption bands in the range of 300–350 nm, caused by the enlargement of the conjugation length resulting from the introduction of the first-generation carbazole dendrons. Correspondingly, the emission maximum of H1 is red-shifted by about

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over polymers as solution-processable hosts for power-efficient blue-electrophosphorescent devices. Blue-electrophosphorescent devices with H1 or H2 as the host and 10 wt% iridium(III) [bis(4,6-difluorophenyl)-pyridinato- N,C2]-picolinate (FIrpic) as the dopant were fabricated. For comparison, the control device with PVK as the host was also prepared under the same conditions. As shown in the inset of Figure 2, the electroluminescence (EL) spectra of H1- and H2-based devices are almost identical with the Commission Internationale de L’Eclairage (CIE) coordinates of (0.15, 0.34), corresponding to the emission of FIrpic. Furthermore, no additional emission coming from H1 or H2 is observed, indicative of efficient energy transfer from the dendritic host to FIrpic. Figure 2 presents the current density– driving voltage–brightness curves of the blueelectrophosphorescent devices. It is worth noting that both the luminance and the current Scheme 1. a) Molecular structures of carbazole-based conjugated dendritic hosts. b) Synthesis of at the same driving voltage of H2 are higher H0-H2. Reagents and conditions are: i) Carbazole, CuCl, KOH, 1,10-phenanthroline, and toluene;120 8C. ii) KI, KIO3, and glacial acetic acid; 120 8C. iii) 3,6-Di-t-butyl-9H-carbazole, CuI, K2CO3, than those of H1. In addition, the onset voltage 2 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU), and 18-crown-6; 190 8C. iv) 3,6,300 , (voltage at the brightness of 1 cd m ) 600 -Tetrakis-t-butyl-90 H-[9,30 ;60 ,900 ] tercarbazole, CuI, K2CO3, DMPU, and 18-crown-6; 190 8C. decreases from 5.6 V of H1 to 5.0 V of H2. These results are consistent with the fact that the injection barrier of holes in H2 is much lower than that in H1, 38 nm. In contrast, from H1 to H2 the bathochromic shift is as discussed before. Consequently, the power efficiency increases almost negligible and both the absorption and photoluminesby 52% from H1 to H2, as shown in Figure 3. Moreover, in cence (PL) spectra of H2 are similar to those of H1. These comparison with PVK, a further improvement of 86% is realized observations suggest that the delocalization of the excited states is for H2 with a power efficiency up to 15.4 lm W1 (27.6 cd A1, mainly limited to the first-generation dendrons and does not obviously extend to the second generation ones. Further evidence 12.7%). This efficiency compares very well to the state-of-the-art is obtained from the magnitude and direction of the singlet performance of small molecular host-based blueenergy shift, as listed in Table 1. When moving from H0 to H1 to electrophosphorescent devices fabricated by vacuum deposiH2, the singlet energy shifts to lower energy in sequence by about tion[28–30] and renders these dendritic molecules promising 0.37 and 0.02 eV, respectively. Different from the singlet energy, solution-processable host materials for triplet emitters, especially interestingly, the triplet energy depends much less on the those with high energy. attached carbazole dendrons. For instance, a decrease of only In summary, novel solution-processable carbazole-based about 0.12–0.15 eV is found for the triplet energies of H1 and H2 conjugated dendrimers have been designed and prepared. after the attachment of the dendrons. As a result, the triplet With these dendrimers as the hosts, power-efficient blueenergies of H1 and H2 still remain at about 2.9 eV, which is electrophosphorescent devices are achieved in terms of the high sufficient to host the blue triplet emitters. triplet energy and matched HOMO energy level. We believe that The HOMO energy levels of H0–H2 were measured by in the future, this conjugated dendritic molecule with good soluelectrochemical cyclic voltammetry (CV). As shown in Figure 1b, tion processability will become a third class of host materials for only p-doping processes of H0–H2 were observed in dichlortriplet emitters apart from small-molecular and polymeric hosts. omethane solutions. The onset values of the p-doping processes are 0.81, 0.52, and 0.31 V and the HOMO energy levels are Experimental estimated to be 5.61, 5.32, and 5.11 eV for H0, H1, and H2, respectively. With increasing dendron generation, apparently, the General Information: All chemicals and reagents were used as received HOMO level shifts to higher energy to facilitate the efficient hole from commercial sources without further purification. Solvents for injection. Especially for H2, the HOMO energy level is well tuned chemical synthesis were purified according to the standard procedures. All chemical reactions were carried out under argon. 1H NMR spectra to approach the Fermi level of PEDOT:PSS, which results in were recorded with a Bruker Avance 300 NMR spectrometer. Elemental a barrier-free hole injection from PEDOT:PSS to H2. In general, analysis was performed using a Bio-Rad elemental analysis system. we note that the aforementioned two criterions, namely, high MALDI-TOF mass spectra were performed on a AXIMA CFR MS apparatus triplet energy and matched HOMO level, have been obtained (COMPACT). The intermediates 3,6-di-t-butyl-9H-carbazole [25], 3,6,300 ,600 simultaneously on the conjugated carbazole-based dendritic tetrakis-t-butyl-90 H-[9,30 ;60 ,900 ] tercarbazole [25], and 1,4-diiodobenzene scaffold. This demonstrates that dendritic molecules may favor [31] were prepared according to the literature procedures.

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COMMUNICATION Figure 2. Luminance and current-density versus driving-voltage characteristics for ITO/PEDOT:PSS/H1 (triangles) or H2 (circles): 10 wt% FIrpic/ TAZ (50 nm)/LiF (1 nm)/Al (100 nm). Inset: EL spectra of H1 and H2-based blue-electrophosphorescent devices at a driving voltage of 8 V.

Figure 1. a) Absorption (dashed lines) and PL (dotted lines) spectra at 298 K, and phosphorescence spectra (solid lines) at 77 K of H0–H2. b) CVs of H0–H2 in dichloromethane solutions.

Synthesis of N-Phenylcarbazole (H0): A mixture of iodobenzene (1.0 g, 5.0 mmol), carbazole (0.8 g, 5.0 mmol), CuCl (50 mg, 0.5 mmol), 1,10-phenanthroline (90 mg, 0.5 mmol), KOH (1.1 g, 20.0 mmol), and toluene (10 mL) was heated at 120 8C for 8 h under argon. After cooling to room temperature, the reaction was quenched with 1 N hydrochloric acid. The mixture was extracted with CH2Cl2, washed with NH3  H2O and water, and dried over Na2SO4. After the solvent had been completely removed, the residue was purified by column chromatography on silica gel using petroleum as eluent to give H0 in 91% (1.1 g) yield. 1H NMR (300 MHz, CDCl3, d): 8.20 (d, J ¼ 7.8 Hz, 2H), 7.60–7.68 (m, 4H), 7.51–7.54 (m, 1H), 7.46 (d, J ¼ 3.7 Hz, 4H), 7.31–7.36 (m, 2H). Anal. Calcd for C18H13N: C, 88.86; H, 5.39; N, 5.76. Found: C, 88.87; H, 5.35; N, 5.63. MALDI-TOF (m/z): 243.8 [Mþ].

Synthesis of N-(4-Iodophenyl)carbazole (1): This compound was prepared according to the procedure for the synthesis of H0, using 1,4diiodobenzene and carbazole, in 54% yield. 1H NMR (300 MHz, CDCl3, d): 8.13 (d, J ¼ 7.7 Hz, 2H), 7.92 (d, J ¼ 8.6 Hz, 2H), 7.37–7.44 (m, 4H), 7.27–7.34 (m, 4H). Synthesis of 3,6-Diiodo-9-(40 -iodophenyl)carbazole (2): A mixture of intermediate 1 (5.5 g, 15 mmol) and KI (3.7 g, 22 mmol) in glacial acetic acid (50 mL) was heated to reflux. Then, KIO3 (2.4 g, 11 mmol) was added and the reaction mixture was further refluxed for 4 h. The resulting precipitate was filtered, washed with 10% aqueous Na2S2O3 solution and water, and dried. Recrystallization from alcohol/chloroform afforded the product in 67% (6.2 g) yield. 1H NMR (300 MHz, CDCl3, d): 8.38 (d, J ¼ 1.7 Hz, 2H), 7.93 (d, J ¼ 8.5 Hz, 2H), 7.67 (dd, J ¼ 8.6, 1.7 Hz, 2H), 7.23 (d, J ¼ 8.5 Hz, 2H), 7.12 (d, J ¼ 8.6 Hz, 2H). Synthesis of H1: A mixture of intermediate 2 (3.1 g, 5.0 mmol), 3,6-di-t-butyl-9H-carbazole (4.3 g, 15.5 mmol), CuI (286 mg, 1.5 mmol), 18-crown-6 (396 mg, 1.5 mmol), K2CO3 (4.2 g, 30.0 mmol), and 1,3dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU) (5 mL) was heated at 190 8C for 36 h under argon. After cooling to room temperature, the reaction was quenched with 1 N hydrochloric acid. The mixture was extracted with CH2Cl2, washed with NH3  H2O and water, and dried over Na2SO4. After complete removal of the solvent, the residue was purified by column chromatography on silica gel using petroleum/ethyl acetate (20:1) as eluent to give H1 in 82% (4.4 g) yield. 1H NMR (300 MHz, CDCl3, d): 8.32 (d, J ¼ 1.8 Hz, 2H), 8.21–8.24 (m, 6H), 7.95–8.03 (m, 4H), 7.82 (d, J ¼ 8.7 Hz, 2H), 7.71 (dd, J ¼ 8.6, 1.9 Hz, 2H), 7.63 (s, 4H), 7.51 (dd, J ¼ 8.7, 1.8 Hz, 4H), 7.41 (d, J ¼ 8.6 Hz, 4H), 1.55 (s, 18H), 1.51 (s, 36H). Anal. Calcd for C78H82N4: C, 87.11; H, 7.68; N, 5.21. Found: C, 86.90; H, 7.70; N, 5.16. MALDI-TOF (m/z): 1074.4 [Mþ]. Synthesis of H2: This compound was prepared according to the procedure for the synthesis of H1 in 69% yield from the intermediate 2 and 3,6,300 ,600 -tetrakis-t-butyl-90 H-[9,30 ,60 ,900 ] tercarbazole. 1H NMR (300 MHz, CDCl3, d): 8.65 (s, 2H), 8.34 (d, J ¼ 11.2 Hz, 6H), 8.21 (d, J ¼ 11.0 Hz, 16H), 8.05 (d, J ¼ 8.7 Hz, 2H), 7.96 (d, J ¼ 8.7 Hz, 2H), 7.92 (d, J ¼ 8.7 Hz, 2H), 7.65–7.78 (m, 10H), 7.48–7.54 (m, 12H), 7.37–7.44 (m, 12H), 1.52

Table 1. The photophysical and electrochemical properties of the dendritic hosts.

H0 H1 H2

Absorbance (log e)[a] [nm]

lem[a] [nm]

S1[b] [eV]

T1[c] [eV]

EHOMO[d] [eV]

ELUMO[d] [eV]

242(4.7), 293(4.3), 341(3.6) 242(5.3), 298(5.0), 347(4.3) 244(5.6), 298(5.4), 349(4.8)

363 401 402

3.59 3.22 3.20

3.04 2.92 2.89

5.61 5.32 5.11

2.05 2.11 1.91

[a] Measured in CH2Cl2 at 298 K and a concentration of 105 M. [b] Estimated from the highest energy peak of the fluorescence spectra at 77 K. [c] Estimated from the highest energy peak of the phosphorescence spectra at 77 K. [d] See Experimental section for details.

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Science Foundation of China (No. 50803062), the Science Fund for Creative Research Groups (No.20621401), and the 973 Project (2009CB623601) for financial support of this research. Received: July 13, 2009 Published online: November 2, 2009

Figure 3. Luminous and power efficiency versus current density for ITO/ PEDOT:PSS/H1 (triangles) or H2 (circles): 10 wt% FIrpic/TAZ (50 nm)/LiF (1 nm)/Al (100 nm). Inset: Comparison of the power efficiency between H2 and PVK. (s, 36H), 1.50 (s, 72H). Anal. Calcd calculated for C174H172N10: C, 86.96; H, 7.21; N, 5.83. Found: C, 86.89; H, 7.19; N, 5.85. MALDI-TOF (m/z): 2402.0 [Mþ þ H]. Measurement of Photophysical and Electrochemical Properties: UV– visible (UV–vis) absorption and photoluminescence spectra were measured with a Perkin-Elmer Lambda 35 UV–vis spectrometer and a Perkin-Elmer LS 50B spectrofluorometer, respectively. Fluorescence and phosphorescence spectra at 77 K were measured in a toluene/ethanol/ methanol (5:4:1) solvent mixture. The highest energy peaks in the fluorescence and phosphorescence spectra at 77 K were referred to as the S0n ¼ 0 S1n ¼ 0 and S0n ¼ 0 T1n ¼ 0 transitions, respectively. CV experiments were performed on an EG&G 283 (Princeton Applied Research) potentiostat/galvanostat system. With regard to the energy level of the ferrocene reference (4.8 eV relative to the vacuum level) [32], the HOMO energy levels were calculated using EHOMO ¼ –e (4.8 V þ Eox), in which Eox was taken from the onset of the oxidation potential. The LUMO energy levels were calculated according to the equation, ELUMO ¼ Eg þ EHOMO, where Eg is the optical band gap estimated from the onset of the absorption spectrum. Device Fabrication and Testing: To fabricate OLEDs, a 50-nm-thick PEDOT:PSS (purchased from H. C. Starck) film was first deposited on the pre-cleaned ITO-glass substrates (20 V per square) and subsequently cured at 120 8C in air for 30 min. 3-(4-biphenyl)-4-phenyl-5-(4-tert-butylphenyl)1,2,4-triazole (TAZ) and FIrpic were prepared in our lab. FIrpic and H1 or H2 were dissolved in chlorobenzene, respectively. For a blueelectrophosphorescent device, 10 wt% FIrpic was doped into H1 or H2 and spin-coated onto PEDOT:PSS as the emissive layer (EML). The samples were annealed at 80 8C for 30 min to remove residual solvent. The thickness of the EML was about 40 nm. Successively, a 50-nm-thick film of TAZ, a 1-nm-thick film of LiF, and a 100-nm-thick film of Al were thermally evaporated on top of the EML at a base pressure less than 106 Torr (1 Torr ¼ 133.32 Pa) through a shadow mask with an array of 14 mm2 openings. The EL spectra and CIE coordinates were measured using a PR650 spectra colorimeter. The current–voltage and brightness–voltage curves of devices were measured using a Keithley 2400/2000 source meter and a calibrated silicon photodiode. All the experiments and measurements were carried out at room temperature under ambient conditions. For comparison, the control device with PVK as the host was fabricated under the same conditions.

Acknowledgements The authors are grateful to the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (CX07QZJC-24), the National Natural

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Adv. Mater. 2009, 21, 4983–4986

Solution-processable carbazole-based conjugated dendritic hosts for power-efficient blue-electrophosphorescent devices.

A novel class of hosts suitable for solution processing has been developed based on a conjugated dendritic scaffold. By increasing the dendron generat...
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