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Solution-processable 1,3,5-tri(9-anthracene)-benzene cored propeller-shaped materials with high Tg for blue organic light-emitting diodes
Organic Electronics 12 (2011) 1716–1723
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Solution-processable 1,3,5-tri(9-anthracene)-benzene cored propellershaped materials with high Tg for blue organic light-emitting diodes Hong Huang a, Qiang Fu b, Shaoqing Zhuang a, Guangyuan Mu a, Lei Wang a,⇑, Jiangshan Chen b,⇑, Dongge Ma b, Chuluo Yang c,⇑ a Wuhan National Laboratory for Optoelectronics, School of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Graduate School of Chinese Academy of Sciences, Changchun 130022, PR China c Department of Chemistry, Wuhan University, Wuhan 430072, PR China
a r t i c l e
i n f o
Article history: Received 10 May 2011 Received in revised form 28 June 2011 Accepted 28 June 2011 Available online 18 July 2011 Keywords: Blue material 1,3,5-Tri(9-anthracene) benzene core Organic light-emitting device
a b s t r a c t This study describes the synthesis and characterization of a series of new blue fluorescent materials, with propeller-like topology, consisting of 1,3,5-tri(9-anthracene)benzene core and various aromatic dendrons, such as naphthalene, 3,5-diphenylbenzene, carbazole, and N,N-diphenylamine. These compounds show excellent thermal and morphological stability with high glass transition temperatures (Tg) (166–231 °C) and high thermal decomposition temperatures (Td) (427–504 °C). Solution-processable double-layered OLEDs fabricated with these materials as the light-emitting layer show stable blue emission and good performance. The nondoped electronic device fabricated using compound 5c exhibits a maximum brightness of 4754 cd/m2 and maximum current efficiency of 2.0 cd/A (power efficiency, 1.71 lm/W) with Commission Internationale d’Eclairage (CIEx,y) color coordinates of (x = 0.16, y = 0.19) and the devices’ threshold voltage are only 3.4 eV. Compound 5d shows an even higher efficiency of up to 4.90 cd/A with CIEx,y color coordinates of (x = 0.17, y = 0.31) when doped with a blue fluorescent dopant, 4,40 -bis[4-(di-p-tolylamino)styryl]biphenyl (DPAVBi). Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction In the past decade, light-emitting dendritic materials have attracted much interest, owing to them combining the advantages of both the small molecules and polymer, such as, precise chemical structures and high purity of small molecular materials and good solution processability of polymeric materials [1–4]. Recently, considerable efforts in this field have been focused on developing stable dendritic materials with efficient blue electroluminescence (EL) [5], which is of particular importance for full-color ⇑ Corresponding authors. Tel.: +86 27 87793032; fax: +86 27 87793032 (L. Wang), tel.: +86 0431 85262901; fax: +86 0431 85262901 (J.S. Chen). E-mail addresses: [email protected] (L. Wang), jschen@ ciac.jl.cn (J. Chen), clyang @whu.edu.cn (C. Yang). 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.06.025
display applications and white solid-state lighting due to their intrinsic wide band-gap [6–8]. Many blue dendritic fluorescent and phosphorescent light emitting materials have been developed. For example, a kinked star-shaped fluorene/triazatruxene co-oligomer has been reported [9] with device efficiency (1.75 cd/A, CIE (0.16, 0.15)) and a pyrene-centered starburst oligofluorene [10] with device efficiency (1.48 cd/A, CIE (0.20, 0.32)). Lo’s research group [11] have reported a host free phosphorescent dendrimer material with an external quantum efficiency of 7.9%, and CIE coordinates of (0.18, 0.35). Despite many exciting reports, highly efficient devices exhibiting both high thermal stability and robust emission still remains rare. Therefore, blue fluorescent and phosphorescent OLEDs based on dendrimers still require further improvements in terms of thermal stability and EL efficiency.
H. Huang et al. / Organic Electronics 12 (2011) 1716–1723
Among the blue-emitting materials, anthracene derivatives are particularly attractive due to their excellent photoluminescence and electroluminescence properties [12–16]. Such as, triarylamines [17,18], or the other moieties [19–21] have been incorporated into anthracene unit to improve hole injection and charge carrier transport properties. Although a number of vacuum-deposited anthrance-based molecules have been reported as efficient blue electroluminescent materials [22,23], solution-processed analogs remain rare and largely unexplored in OLEDs for its poor solubility and aggregating properties. In our previous work [24], we have demonstrated that 1,3,5-tri (9-anthracene) benzene is a good core emitter, and its centered starburst oligofluorenes show robust blue light emission in an OLED device. Although these materials exhibit deep blue emission, and the current efficiency is up to 1.8 cd/A, but the values of Tg are only about 110 °C for the oligofluorenes possessing long alkyl chains. Here, we report the design and synthesis of four symmetric dendrimer molecules based on the following two principles. Firstly, the propeller-like, and highly rigid 1,3,5-tri (9-anthracene) benzene group was used as the core, which can reduce strong intermolecular interactions and suppress fluorescence quenching. Secondly, using stilbenzene as a conjugated bridge, the different electron donor dendrons (naphthalene, diphenylbenzene, carbazole, and diphenylamine) were introduced at the peripheries of the core to improve solubility in common organic solvents, influence the charge transporting properties, enhance the morphological stability and the thermal stability and improve color purity. As we expected, all of these materials possess high glass transition temperatures (Tg) and good morphological properties, and exhibit good EL performance with low driving voltages in solution-processed double-layer OLED devices. These materials are a promising class of solution-processable blue emitters for practical applications.
2. Experimental section 2.1. Material and methods All reagents and solvents were used as purchased from Aldrich and were used without further purification. 1H NMR spectra were recorded using a Bruker-AF301 AT 400 MHz. High resolution mass spectrometric measurements were carried out using a Bruker autoflex MALDITOF mass spectrometer. Fluorescence spectra were obtained on a Perkin Elmer LS55 Luminescence spectrometer and UV–Vis spectra were measured using a Shimadzu3150 UV–Vis–NIR spectrophotometer. The differential scanning calorimetry (DSC) analysis was performed under a nitrogen atmosphere at a heating rate of 10 °C/min using a PE Instruments DSC 2920. Thermogravimetric analysis (TGA) was undertaken using a SEIKO EXSTAR 6000 TG/DTA 6200 unit under nitrogen atmosphere at a heating rate of 10 °C/min. To measure the PL quantum yields (Uf), degassed solutions of the compounds in CH2Cl2 were prepared. The concentration was adjusted so that the absorbance of the solution would be lower than 0.1. The excitation was
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performed at 360 nm and 9,10-diphenylanthracene(DPA) in CH2Cl2 (Uf = 0.9 in CH2Cl2) or solid film(gDPA = 1) was used as a standard [25,26]. Cyclic voltammetry measurements were carried out in a conventional three electrode cell using a Pt button working electrode of 2 mm in diameter, a platinum wire counter electrode, and a Ag/AgCl (0.1 M) reference electrode on a computer-controlled EG&G Potentiostat/Galvanostat model 283 at room temperature. Reductions CV of all compounds were performed in dichloromethane containing 0.1 M tetrabutylammoniumhexafluorophosphate (Bu4NPF6) as the supporting electrolyte. 2.2. Synthesis The structures and synthetic routes of the four welldefined compounds are shown in Scheme 1. 1,3,5tri(anthracen-9-yl)benzene (1) and 1,3,5-tris(10-bromo anthracen-9-yl)benzene (2) were synthesized according to the literature [24]. 2.2.1. Synthesis of compound (3) A 100 ml round-bottomed flask was charged with compound 2 (422 mg, 0.5 mmol), toluene (20 ml), and ethanol (10 ml) under nitrogen atmosphere. Subsequently, 4-formylphenylboronic acid (375 mg, 2.5 mmol), tetrabutylammonium bromide (0.96 g, 2.98 mmol) and aqueous potassium carbonate (15.0 ml, 30 mmol) were added slowly to the solution. 10 min later, tetrakis (triphenylphosphine) palladium (0) (173 mg, 0.15 mmol) was added and the reaction mixture was stirred under reflux for 24 h in the absence of light. After cooling to room temperature, the precipitate was filtrated and washed with distilled water and methanol, a yellow powder was obtained. Yield 0.34 g (74%). 1H NMR: (CDCl3, 400 MHz): d (ppm) 10.21 (s, 3H), 8.30–8.28 (d, J = 8.8 Hz, 6H), 8.15–8.13 (d, J = 8.0 Hz, 6H), 7.90 (s, 3H), 7.68–7.56 (m, 18H), 7.43–7.40 (t, J = 7.6 Hz, 6H). 13C NMR (100 MHz, CDCl3) d 191.98, 145.92, 139.21, 136.94, 135.86, 135.77, 133.98, 132.18, 129.93, 129.87, 129.60, 126.85, 126.64, 125.72, 125.63. MS (FAB): 919.3 (m/z), calcd for C69H42O3: 919.3. Anal. Calcd. For C69H42 O3: C 90.17, H 4.61, O, 5.22; found: C 90.14, H 4.56, O 5.30. 2.2.2. Synthesis of compound (4) A 500 ml three-neck round-bottomed flask was charged with 4-bromobenzylphosphonium bromide (5.21 g, 10.0 mmol), sodium hydride (0.75 g, 30.0 mmol) and THF (200 ml) under nitrogen atmosphere. After stirring for 1 h, at 40 °C, compound 3 (2.3 g, 2.5 mmol) was added. The solution was continuously stirred for 4 h at room temperature and the product was precipitated by the addition of methanol (200 ml). The precipitated solid was filtrated and washed with methanol, a light yellow solid powder was obtained. Yield 3.2 g (93%). 1H NMR: (CDCl3, 400 MHz): d (ppm) 8.30–8.28 (d, J = 8.0 Hz, 6H), 7.89 (s, 3H), 7.78–7.75 (m, 12H), 7.56–7.39 (m, 33H), 7.26–7.21 (m, 3H). 13C NMR (100 MHz, CDCl3) d 136.34, 132.20, 132.10, 131.98, 131.89, 131.82, 131.51, 131.41, 130.81, 130.59, 130.01, 128.58, 128.47, 128.07, 127.85, 127.13, 126.82, 126.62, 125.55, 125.16. MS (MALDI-TOF): 1378.1999 (m/z), calcd for
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H. Huang et al. / Organic Electronics 12 (2011) 1716–1723 CHO
Br
Br
b)
a)
OHC
CHO
Br 1
2
3 R
Br
c)
d)
Br
Br 4
R 5a: R = naphthalene 5c: R = carbazole
R 5b: R = 3,5-diphenylbenzene 5d: R = diphenylamine
Scheme 1. Synthetic route of 5a, 5b, 5c and 5d. Reagents and conditions: (a) NBS, DMF, 30 °C; (b) toluene, K2CO3, ethanol, (4-formylphenyl)boric acid, Pd(PPh3)4, reflux; (c) 4-bromobenzyl-phosphonium bromide, 40 °C, NaH, THF, 1 h; (d) (i) 1-naphthylboronic acid or 3,5-diphenylbenzeneboronic acid, Pd(PPh3)4, toluene, EtOH; (ii) carbazole or diphenylamine, 18-crown-6, CuI, DMPU, K2CO3, 180 °C.
C90H57Br3: 1378.1994. Anal. Calcd. For C90H57Br3: C 78.44, H 4.17, Br, 17.39; found: C 78.50, H 4.15, Br, 17.35. 2.2.3. Synthesis of compound (5a) A 500 ml three-neck round-bottomed flask was added compound 4 (137.8 mg, 0.1 mmol), 1-naphthaleneboronic acid (111 mg, 0.5 mmol), toluene (20 ml), ethanol (10 ml), 2 M K2CO3 (15 ml, 30 mmol) aqueous solution and tetrakis-(triphenylphosphine) palladium (0) (173 mg, 0.15 mmol) in turn, then the reaction mixture was refluxed under nitrogen for 24 h in the absence of light. After cooling to the room temperature, the precipitate was collected by filtration and washed with distilled water and methanol. The crude product was purified by column chromatography and dried under vacuum to yield a yellowish solid. Yield: 92%. 1H NMR: (CDCl3, 400 MHz): d (ppm) compound 5a: 8.30–8.28 (d, J = 8.0 Hz, 6H), 8.00–7.81 (m, 26H), 7.74–7.72 (m, 6H), 7.58–7.26 (m, 40H). 13C NMR (100 MHz, CDCl3) d 140.24, 139.87, 139.32, 138.46, 137.20, 136.67, 136.41, 134.05, 133.88, 131.79, 131.58, 130.50, 130.05, 128.75, 128.60, 128.24, 128.13, 127.87, 127.74, 127.21, 126.88, 126.61, 126.51, 126.09, 125.99, 125.96, 125.82, 125.56, 125.41, 125.17. MS (MALDI-TOF): 1519.6144 (m/z), calcd for C120H78: 1519.6132. Anal. Calcd. For C120H78: C 94.83, H 5.17; found: C 94.58, H 5.42. 2.2.4. Synthesis of compound (5b) The compound was synthesized using a similar procedure as for compound 5a, yield: 85%. 1H NMR: (CDCl3, 400 MHz): d (ppm) 8.32–8.30 (d, J = 8.8 Hz, 6H), 7.91– 7.72 (m, 50H), 7.57–7.35 (m, 40H). 13C NMR (100 MHz,
CDCl3) d 142.45, 141.79, 141.15, 140.36, 139.28, 138.47, 137.17, 136.71, 136.59, 136.41, 131.78, 130.01, 129.17, 128.87, 128.79, 128.60, 128.55, 127.64, 127.59, 127.38, 127.25, 127.18, 127.11, 126.81, 126.60, 126.12, 125.54, 125.32, 125.17, 124.94 MS (MALDI-TOF): 1825.7515 (m/ z), calcd for C144H96: 1825.7546. Anal. Calcd. For C144H96: C 94.70, H 5.30; found: C 94.85, H 5.15. 2.2.5. Synthesis of compound (5c) A mixture of compound 4 (689 mg, 0.5 mmol), carbazole (376 mg, 2.25 mmol), CuI (14.25 mg, 0.075 mmol), 18crown-6 (19 mg, 0.25 mmol), K2CO3 (310.5 mg, 2.25 mmol) and 1,3-di-methyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU, 2.0 ml) was heated to 190 °C for 24 h under a nitrogen atmosphere. After cooling down to room temperature, the mixture was quenched with 1 N hydrochloric acid, then extracted with dichloromethane, washed with NH3H2O and water. The combined organic phases were dried (MgSO4) and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel using hexane as the eluent to give compound 5c as a yellow solid. Yield: 80%. 1H NMR: (CDCl3, 400 MHz): d (ppm) 8.33–8.31 (d, J = 9.2 Hz, 6H), 8.17–8.15 (d, J = 7.6 Hz, 6H), 7.92 (s, 3H), 7.83–7.80 (m, 18H), 7.63–7.38 (m, 42H), 7.33–7.29 (m, 6H). 13C NMR (100 MHz, CDCl3) d 140.80, 140.70, 139.30, 138.69, 137.12, 137.00, 136.51, 136.44, 136.42, 134.04, 131.84, 130.42, 130.03, 129.25, 128.08, 127.90, 127.27, 127.18, 126.84, 126.68, 125.98, 125.57, 125.20, 123.46, 120.35, 120.02. MS (MALDI-TOF): 1636.6458 (m/ z), calcd for C126H81N3: 1636.6461. Anal. Calcd. For C126H81N3: C 92.45, H 4.99, N 2.57; found: C 92.31, H 5.17, N 2.52.
H. Huang et al. / Organic Electronics 12 (2011) 1716–1723
2.2.6. Synthesis of compound (5d) The compound was synthesized using a similar procedure as for compound 5c. Yield: 74%. 8.30–8.28 (d, J = 8.8 Hz, 6H), 8.17–8.15 (d, J = 7.6 Hz, 6H), 7.89–7.88 (s, 3H), 7.78–7.72 (m, 18H), 7.60–7.37 (m, 48H), 7.27–7.21 (m, 6H). 13C NMR (100 MHz, CDCl3) d 139.25, 138.68, 137.19, 136.62, 136.36, 136.29, 134.01, 131.86, 131.79, 131.72, 131.49, 130.57, 129.99, 129.30, 129.10, 128.87, 128.75, 127.81, 127.44, 126.80, 126.54, 126.31, 125.53, 125.17, 124.55, 121.45. MS (MALDI-TOF): 1641.6924 (m/z), calcd for C126H87N3: 1641.6900. Anal. Calcd. For C126H87N3: C 92.11, H 5.34, N 2.56; found: C 92.10, H 5.39, N 2.51. 2.3. Device fabrication To investigate the electroluminescent (EL) properties of the symmetric starburst compounds containing different surface groups, double layer OLEDs were fabricated via solution processing according to the previous literature.[27] The device structures of the OLEDs were as follows: ITO/PEDOT:PSS/5a–5d or (doping DPAVBi) /TPBI/ LiF/Al. Polyethylene dioxythiophene-polystyrene sulfate (PEDOT-PSS) and 2,20 ,200 -(1,3,5-benzenetriyl)tris[1-phenyl-1H-benzimidazole] (TPBI) were used as hole injection and hole-blocking/electron-transporting layer, respectively. The active layer was spin-coated from toluene solution, and TPBI layer was deposited by means of conventional vacuum deposition onto the ITO-coated glass substrates. The J–V–L characteristics of the EL devices were measured using a Keithley 2400 source meter and a Keithley 2000 source multimeter equipped with a calibrated silicon photodiode. The EL spectra were measured using a JY SPEX CCD3000 spectrometer. The thicknesses of the films were measured using the instrument of Profiler DEKTAK150. All of the measurements were carried out at room temperature under ambient conditions. 3. Results and discussion 3.1. Synthesis, Thermal, Film-forming, Optical and Electrochemical properties Four target compounds, 5a, 5b, 5c and 5d, were prepared as shown in Scheme 1. Detailed synthesis of the 1,3,5-tris(10-bromoanthracen-9-yl)benzene (2) has been described in previous work [24]. The intermediate 3 was synthesized by the Pd-catalyzed Suzuki cross-coupling of formylated benzene boric acid with compound 2 with excellent yields, and were easily purified by a simple filtration and washed with distilled water and methanol. Then Wittig reaction of 4-bromobenzyl-phosphonium bromide and compound 3 gave the compound 4 in 93% yield at 40 °C. The target compounds 5a and 5b were obtained in 92% and 85% of isolated yield, by an easy Suzuki coupling reaction, respectively. The target compound 5c and 5d were obtained in 80% and 74% of isolated yield by the CuI-catalyzed Ullman reaction, respectively. All the four target compounds were soluble in common organic solvents (CH2Cl2, toluene), and characterized fully by 1H and 13C NMR, MALDI-TOF, and elemental analysis.
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Thermal properties of these compounds were studied using DSC and TGA. The thermal properties were measured under N2 gas condition at heating rates of 10 °C/min, and the related data are listed in Table 1. As shown in Fig. 1a, compounds 5a–5d exhibit a high glass transition temperature (Tg) at 166, 189, 260, and 231 °C, respectively. The high Tg are attributed to the rigid core and the star shaped dendrons can effectively suppress the crystallization (or arm aggregation). High decomposition temperatures (Td, 5% weight loss) of these compounds were also observed at 484, 504, 497, and 427 °C (Fig. 1b), respectively, which may be due to the highly stability of the 1,3,5-tri (9anthracene) benzene core and the side groups. The stable aromatic side group can supply electron to the styryl group and enhance the thermal stability of material.[28] The relatively high Tg and Td of these star shaped compounds is very desirable for high performance OLED applications. Efficient film-forming properties of light emitting materials used in OLEDs are crucial for the performance of the devices, so the surface morphologies of thin films of the four compounds (5a, 5b, 5c and 5d) were examined. As showed in Fig. 2, the AFM images show smooth and amorphous thin films formed by spin-coating the compounds (5a, 5b, 5c and 5d) from toluene solution. The deposited films were annealed under N2 gas condition at 150 °C for 0.5 h. The annealed film exhibits a fairly smooth surface morphology with a root-mean-square (rms) roughness of 0.5539, 0.6211, 0.5202 and 0.6565 nm for 5a, 5b, 5c and 5d, respectively. This suggests that all of the compounds possess excellent thermal and amorphous stability, which indicates that the rigid propeller core influences the arrangement of the molecules in the thin films, and 1,3,5-tri(9-anthracene)benzene is a good core emitter for star-shaped fluorophore. Fig. 3 shows the normalized UV–Vis and fluorescence spectra of compounds 5a, 5b, 5c and 5d in a CH2Cl2 solution and in the thin film. The related photophysical properties were listed in Table 1. All of these compounds have similarly structured absorption spectra (range: 330– 440 nm) and emission spectra (range: 400–550 nm) in solution. This indicates that the photophysical properties are mainly determined by the core 1, 3, 5-tri(9-anthracene)benzene. The absorption in the range 330–440 nm is assigned to the S1 S0 transition of the anthracene moiety. In the solid state, all of them show strong pure-blue emission with a main peak at 448, 453, 451 and 460 nm, respectively, which shows that the introduction of dipenylamine results in a notable red-shift in PL spectra and peripheral dendrons can affect the molecular interaction. The quantum yields of compound 4a-4d in CH2Cl2 were 0.68, 0.66, 0.67 and 0.64, respectively, which is even higher than the typical blue material m-ADN [25]. The quantum yield in solid state was also measured, the relative high values indicated that no obviously aggregating or quenching phenomena appeared. In order to further investigate the morphological stability of these materials, the Abs and PL spectra before/after annealing state (see Figure S4.) was also researched, the absorbance and PL emission have no clearly change after annealing (150 °C for 1 h), which showed that the morphological stability of material is excellent. The optical energy bandgaps of the compound
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H. Huang et al. / Organic Electronics 12 (2011) 1716–1723
Table 1 The optical, photophysical and thermal properties of 5a–5d. HOMO
5a 5b 5c 5d a b
Ua/gb
LUMO
5.57 5.54 5.52 5.50
Eg (eV)
0.68/0.69 0.66/0.68 0.67/0.64 0.64/0.61
2.65 2.59 2.58 2.57
PL (nm)
2.92 2.95 2.94 2.93
Solution
Film
436 436 436 437
448 453 451 460
Abs (nm) in solution
Tg/Td (°C)
331,358,380,401 340,358,380,401 344,358,380,401 320,358,380,401
166/484 189/504 260/497 231/427
Using DPA as a standard; kex = 360 nm (U = 0.9 in CH2Cl2). Using DPA as a standard; kex = 360 nm (g = 1 in thin film).
10 9
100
a)
90
Weight (%)
Heat flow
8 7 b
6
5a 5b 5c 5d
a
5
d
4 3
5a 5b 5c 5d
b)
c
100
150
200 250 o 300 Temperature ( C)
80
70
60 200
350
300
400
500
600
Temperature (οC)
700
800
Fig. 1. (a) DSC curves of compounds 5a, 5b, 5c and 5d measured under nitrogen at a heating rate of 10 °C/min; (b) TGA curves of star-shaped compounds measured at a heating rate of 10 °C/min under nitrogen.
Fig. 2. AFM topographic images of the four compounds (5a, 5b, 5c and 5d) in thin solid films (ca. 40 nm thick).
1.0
0.6
0.8 0.6
0.4
0.4
0.2
0.2
0.0 300
350
400 450 500 Wavelength (nm)
550
0.0 600
1.0
5a 5b 5c 5d
0.8 0.6
1.0 0.8 0.6
0.4
0.4
0.2
0.2
0.0 300
350
400
450
500
550
600
0.0 650
Normalized PL Intensity (a.u)
0.8
b) Normalized Absorbance (a.u)
5a 5b 5c 5d
Normalized PL intensity (a.u)
Normalized Absorbance (a.u)
a) 1.0
Wavelength (nm)
Fig. 3. (a) Absorption and photoluminescence spectra of compound 5a, 5b, 5c and 5d in CH2Cl2; (b) Absorption and PL spectrum of the compounds 5a, 5b, 5c and 5d in thin film (ca. 40 nm).
5a, 5b, 5c, and 5d are 2.92, 2.95, 2.94, and 2.93 eV, respectively, which is calculated from the threshold of the optical absorption. In addition to the photophysical properties, the electrochemical properties are also very important in evaluating a material usefulness for optoelectronic applications. The
electrochemical behaviors of the compounds 5a, 5b, 5c and 5d were examined by cyclic voltammetry using a standard three-electrode electrochemical cell in an electrolyte solution of 0.1 M tetrabutylammoniumhexafluorophosphate (TBPAPF6) dissolved in dichloromethane [29]. The onset oxidation peaks (Eox) for 5a–5d were 1.26, 1.23,
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H. Huang et al. / Organic Electronics 12 (2011) 1716–1723 Table 2 Electroluminescence Characteristics of the Devices.a
a b c d e
Device
Emitter
Lmaxb (cd/m2) voltages (V)
gc.max c (cd/A)
gp.max d (lm/W)
CIE (x, y)e
Von (1 cd/m2) (V)
EL(peak) (nm)
1 2 3 4 5 6 7 8
5a 5b 5c 5d 5a:DPAVBi 5b:DPAVBi 5c:DPAVBi 5d:DPAVBi
3366, 11.3 2740, 11.1 4754, 8.5 1562, 11.5 3635, 8.6 6807, 12.4 7470, 11.6 11204, 11.4
1.7 1.6 2.0 2.4 3.6 4.2 3.6 4.9
1.10 1.14 1.71 1.54 2.40 2.81 2.25 3.22
(0.15,0.17) (0.15, 0.14) (0.16, 0.19) (0.18, 0.28) (0.16, 0.31) (0.16, 0.30) (0.17, 0.33) (0.17, 0.31)
4.8 4.5 3.4 4.9 3.2 3.6 3.6 3.4
468 456 468 476 472 472 476 472
Devices configuration: ITO/PEDOT:PSS (40 nm)/ 5a–5d (40 nm) or 5a–5d: DPAVBi (5%)(40 nm)/TPBI (40 nm) /LiF (1 nm) /Al (100 nm). Maximum luminance. Maximum current efficiency. Maximum powder efficiency. Commission International de I0 Eclairage coordinates.
4
1000
10
a)
2
0
400
10
-1
10
200
-2
10 0
Current Efficiency (cd/A)
600
Luminance (cd/m )
2
Current Density (mA/cm )
3
10 Device 1 Device 2 102 Device 3 1 Device 4 10
800
b) Device 1 Device 2 Device 3 Device 4
0.1
0.01
-3
3
4
5
6
7 8 9 Voltage (V)
10 10 11 12 13
10
1.0
c) 1
0.1
Normalized intensity (a.u.)
Power Efficiency (lm/W)
1
Device 1 Device 2 Device 3 Device 4
0.01 -2
10
-1
0
1
2
10 10 10 10 2 Current Density (mA/cm )
3
10
-2
10
-1
0
1
10 10 10 2 Current Density (mA/cm )
2
10
3
d) Device 1 Device 2 Device 3 Device 4
0.8 0.6 0.4 0.2 0.0 400
500 600 Wavelength (nm)
700
Fig. 4. (a) J–V–L curve of the four devices; (b) current efficiency-current density characteristics; (c) powder efficiency-current density characteristics; (d) EL spectra of the four devices.
1.21 and 1.19 eV, respectively (see Figure S3). Thus, the HOMO levels were determined to be 5.57, 5.54, 5.52 and 5.50 eV by the method (Eox + 4.31) eV reported by Li et al. [25] (Table 1). The values were very close to that of 9,10-diphenylanthracene (5.54 eV), which indicates the HOMO level of these compounds were mainly decided by 1,3,5-tri(9-anthracene)-benzene center and the side groups were only finely tune the energy gap and the HOMO level. Generally speaking, the electrochemical properties and photophysical properties of compound 5a–5d in dilute solution are very similar, mirroring the anthracene unit’s characteristics. It shows every individual arm’s electrochemical properties are remained unperturbed for the propeller topology serving as a spacer to effectively block conjugation.
3.2. Device performance To study the EL properties of these compounds, doublelayered devices with the configuration ITO/PEDOT:PSS (40 nm)/5a–5d (40 nm)/TPBI (40 nm)/LiF (1 nm) /Al (100 nm) (emitter: 5a, Device 1; 5b, Device 2; 5c, Device 3; 5d, Device 4) were fabricated by spin-coating from a toluene solution (ca. 7 mg/ml). The EL performances of these compounds are listed in Table 2. Double-layer devices utilizing compounds 5a and 5b exhibit a maximum brightness of 3366, 2740 cd/m2, luminous efficiency of 1.7, 1.6 cd/A, with CIE coordinates 5a (0.15, 0.17), 5b (0.15, 0.14). For compounds 5c and 5d, the double-layer devices showed a maximum brightness of 4754, 1562 cd/m2, luminous efficiency of 2.0, 2.4 cd/A, with CIE coordinates 5c
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H. Huang et al. / Organic Electronics 12 (2011) 1716–1723
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Fig. 5. (a) J–V–L curve of the four doped devices; (b) current efficiency-current density characteristics; (c) powder efficiency-current density characteristics; (d) EL spectra of the four doped devices.
the quantum efficiency of the doped device has been improved, which indicated that the better device performance in the doped device is not due to the change of emission colors. More interestingly, the driving voltage of all the devices is low (ca. 3.6 eV), in which device 5 exhibits the lowest driving voltage of 3.2 V. This indicates that the energy levels of the host materials and the dopant DPAVBi are well matched. Additionally, the dopant DPAVBi can also act as the hole trapper in all of doped devices, so the turn on voltage of the most of doped devices are lower than the undoped devices(see Table 2) [30]. Excellent EL stability (see Figure S2) and the efficient color purity render these materials promising candidates for display applications. We believe that optimization could further improve the device
2.5
Quantum efficiency (%)
(0.16, 0.19), 5d (0.18, 0.28). Fig. 4 shows the EL spectra, the current density–voltage-brightness and the current density-efficiency characteristics of the devices (1, 2, 3, 4). EL spectra of the devices based on the emitters 5c and 5d have obviously red-shift compared to the compounds 5a and 5b, it is induced by the carbazole and diphenylamine group’s electronic-donating properties. Especialy, the EL spectrum of the device 4 has a red shift about 20 nm compared to that of device four. Additionally, the electroluminescence (EL) of all devices remained quite stable under different driving voltages (see Figure S1). In order to further improve the device efficiency, we adopted a host-dopant system using these materials as the host and a highly fluorescent blue dye, DPAVBi, as dopant at a concentration of 5%. Table 2 summarizes the key EL performance of doped devices (5, 6, 7, 8), all of them show a strong blue emission with current efficiency of 3.6–4.9 cd/ A. Fig. 5. shows the EL spectra, the current density–voltage-brightness and the current density-efficiency characteristics of devices (5, 6, 7, 8). The device 8 exhibits a maximum brightness of 11204 cd/m2 at 11.4 V, and a maximum current efficiency of 4.9 cd/A with CIE coordinates of (0.17, 0.31). We ascribe the higher efficiency to the more efficient Förster energy transfer because there is more overlap between the emission of compound 5d and the absorption of DPAVBi. As shown in Fig. 5d, the EL spectra of all DPAVBi doped devices (5, 6, 7, 8) exhibit bright blue emissions at 472 nm with a shoulder at around 496 nm, which is the intrinsic characteristic of the emissions from the dopant DPAVBi. This result indicates completed energy transfer from the host to dopant. From the Fig. 6, it is obviously that
Device 1 Device 2 Device 3 Device 4 Device 5 Device 6 Device 7
2.0 1.5 1.0
Device 8
0.5 0.0
0
10
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Current density (mA/cm ) Fig. 6. Quantum efficiency-current density characteristics of devices 1–8.
H. Huang et al. / Organic Electronics 12 (2011) 1716–1723
performance and fully explore the potential of these starburst materials. 4. Conclusion In summary, we present a series of novel blue starshaped molecules, in which the propeller core 1,3,5-tri(9anthracene)benzene and the different dendrons were bridged by the stilbenzene. These materials exhibit high thermal stability (high glass transition temperature in the range 166–231 °C) with good film forming properties. Meanwhile, their optical and electronic properties can be tuned by adjusting the appropriated surface groups. The nondoped OLED devices fabricated using compound 5c exhibits a maximum brightness of 4754 cd/m2 and maximum current efficiency of 2.0 cd/A (corresponding to a power efficiency of 1.71 lm/W) with CIEx,y color coordinates of (x = 0.16, y = 0.19) and the devices’ threshold voltage are only 3.4 eV. Compound 5d shows an even higher efficiency of up to 4.90 cd/A with CIEx,y color coordinates of (x = 0.17, y = 0.31). These results will be beneficial for the molecular design of blue dendritic materials. Acknowledgements We thank the Donghu New & High Technology Development Zone of Wuhan City and Doctoral Fund of Ministry of Education of China (20100142120049) for financial support and thank the Analytical and Testing Centre of Huazhong University of Science and Technology for measurements. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.orgel. 2011.06.025. References [1] O. Usluer, S. Demic, D.A.M. Ebge, E. Birckner, C. Tozlu, A. Pivrikas, A.M. Ramil, N.S. Sariciftci, Adv. Funct. Mater. 20 (2010) 4152.
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[2] J.L. Wang, Y. Zhou, Y.F. Li, J. Pei, J. Org. Chem. 74 (2009) 7449. [3] S.C. Lo, R.E. Harding, C.P. Shipley, S.G. Stevenson, P.L. Burn, I.D.W. Samuel, J. Am. Chem. Soc. 131 (2009) 16681. [4] S.H. Hwang, C.N. Moorefield, G.R. Newkome, Chem. Soc. Rev. 37 (2008) 2543. [5] L. Wang, Y. Jiang, J. Luo, Y. Zhou, J.H. Zhou, J. Wang, J. Pei, Y. Cao, Adv. Mater. 21 (2009) 4854. [6] S.H. Ye, Y.Q. Liu, K. Lu, W.P. Wu, C.Y. Du, Y. Liu, H.T. Liu, T. Wu, G. Yu, Adv. Funct. Mater. 20 (2010) 3125. [7] S. Sax, N.R. Penkalla, A. Neuhold, S. Schuh, E. Zojer, E.J.W. List, K. Müllen, Adv. Mater. 22 (2010) 2087. [8] H. Sasabe, E. Gonmori, T.Y. Chiba, Y.J. Li, D.K. Tanaka, S.J. Su, T. Takeda, Y.J. Pu, K.C. Nakayama, J.J. Kido, Chem. Mater. 20 (2008) 5951. [9] W.Y. Lai, Q.Y. He, R. Zhu, Q.Q. Chen, W. Huang, Adv. Funct. Mater. 18 (2008) 265. [10] F. Liu, W.Y. Lai, C. Tang, H.B. Wu, Q.Q. Chen, B. Peng, W. Wei, W. Huang, Y. Cao, Macromol. Rapid. Commun. 29 (2008) 659. [11] S.C. Lo, R.N. Bera, R.E. Harding, P.L. Burn, I.D.W. Samuel, Adv. Funct. Mater. 18 (2008) 3080. [12] J.H. Huang, J.H. Su, X. Li, M.K. Lam, K.M. Fung, H.H. Fan, K.W. Cheah, C.H. Chen, H. Tian, J. Mater. Chem. 21 (2011) 2957. [13] J.H. Park, D.H. Lee, H. Kong, M.J. Park, I.H. Jung, C.E. Park, H.K. Shim, Org. Electron. 11 (2010) 820. [14] J. Shi, C.W. Tang, Appl. Phys. Lett. 80 (2002) 3201. [15] S.K. Kim, B. Yang, Y. Ma, J.H. Lee, J.W. Park, J. Mater. Chem. 18 (2008) 3376. [16] Y.T. Lyu, J. Kwak, O. Kwon, S.H. Lee, D. Kim, C. Lee, K. Char, Adv. Mater. 20 (2008) 2720. [17] Z.H. Li, M.S. Wong, Y. Tao, M. D’Iorio, J. Org. Chem. 69 (2004) 921. [18] S.B. Jiao, Y. Liao, X.J. Xu, L.P. Wang, G. Yu, L.M. Wang, Z.M. Su, S.H. Ye, Y.Q. Liu, Adv. Funct. Mater. 18 (2008) 2335. [19] K. Albrecht, Y.T. Kasai, K. Yamamoto, J. Inorg. Organomet. Polym. 118 (2009) 118. [20] H.Q. Zhang, S.M. Wang, Y.Q. Li, B. Zhang, C.X. Du, X.J. Wan, Y.S. Chen, Tetrahedron 65 (2009) 4455. [21] C.J. Zheng, W.M. Zhao, Z.Q. Wang, D. Huang, J. Ye, X.M. Ou, X.H. Zhang, C.S. Lee, S.T. Lee, J. Mater. Chem. 20 (2010) 1560. [22] L. Wang, W.Y. Wong, M.F. Lin, W.K. Wong, K.W. Cheah, H.L. Tam, C.H. Chen, J. Mater. Chem. 18 (2008) 4529. [23] Y.H. Yu, C.H. Huang, J.M. Yeh, P.T. Huang, Org. Electron. 12 (2011) 694. [24] H. Huang, Q. Fu, S.Q. Zhuang, Y.K. Liu, L. Wang, J.S. Chen, D.G. Ma, C.L. Yang, J. Phys. Chem. C. 115 (2011) 4872. [25] J.H. Seo, K.H. Lee, B.M. Seo, J.R. Koo, S.J. Moon, J.K. Park, S.S. Yoon, Y.K. Kim, Org. Electron. 11 (2010) 1605. [26] J.A. Osaheni, S.A. Jenekhe, J. Am. Chem. SOC. 117 (1995) 7389. [27] J. Luo, L. Xiao, Z. Chen, Q. Gong, Appl. Phys. Lett. 93 (2008) 133301. [28] J.Y. Park, S.Y. Jung, J.Y. Lee, Y.G. Baek, Thin. Solid. Films. 516 (2008) 2917. [29] Q. Sun, H. Wang, C. Yang, Y.F. Li, J. Mater. Chem. 13 (2003) 800. [30] C.W. Tang, S.A. Van Slyke, C.H. Chen, J. Appl. Phys. 65 (1989) 3610.
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Solution-processable 1,3,5-tri(9-anthracene)-benzene cored propellershaped materials with high Tg for blue organic light-emitting diodes Hong Huang a, Qiang Fu b, Shaoqing Zhuang a, Guangyuan Mu a, Lei Wang a,⇑, Jiangshan Chen b,⇑, Dongge Ma b, Chuluo Yang c,⇑ a Wuhan National Laboratory for Optoelectronics, School of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Graduate School of Chinese Academy of Sciences, Changchun 130022, PR China c Department of Chemistry, Wuhan University, Wuhan 430072, PR China
a r t i c l e
i n f o
Article history: Received 10 May 2011 Received in revised form 28 June 2011 Accepted 28 June 2011 Available online 18 July 2011 Keywords: Blue material 1,3,5-Tri(9-anthracene) benzene core Organic light-emitting device
a b s t r a c t This study describes the synthesis and characterization of a series of new blue fluorescent materials, with propeller-like topology, consisting of 1,3,5-tri(9-anthracene)benzene core and various aromatic dendrons, such as naphthalene, 3,5-diphenylbenzene, carbazole, and N,N-diphenylamine. These compounds show excellent thermal and morphological stability with high glass transition temperatures (Tg) (166–231 °C) and high thermal decomposition temperatures (Td) (427–504 °C). Solution-processable double-layered OLEDs fabricated with these materials as the light-emitting layer show stable blue emission and good performance. The nondoped electronic device fabricated using compound 5c exhibits a maximum brightness of 4754 cd/m2 and maximum current efficiency of 2.0 cd/A (power efficiency, 1.71 lm/W) with Commission Internationale d’Eclairage (CIEx,y) color coordinates of (x = 0.16, y = 0.19) and the devices’ threshold voltage are only 3.4 eV. Compound 5d shows an even higher efficiency of up to 4.90 cd/A with CIEx,y color coordinates of (x = 0.17, y = 0.31) when doped with a blue fluorescent dopant, 4,40 -bis[4-(di-p-tolylamino)styryl]biphenyl (DPAVBi). Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction In the past decade, light-emitting dendritic materials have attracted much interest, owing to them combining the advantages of both the small molecules and polymer, such as, precise chemical structures and high purity of small molecular materials and good solution processability of polymeric materials [1–4]. Recently, considerable efforts in this field have been focused on developing stable dendritic materials with efficient blue electroluminescence (EL) [5], which is of particular importance for full-color ⇑ Corresponding authors. Tel.: +86 27 87793032; fax: +86 27 87793032 (L. Wang), tel.: +86 0431 85262901; fax: +86 0431 85262901 (J.S. Chen). E-mail addresses: [email protected] (L. Wang), jschen@ ciac.jl.cn (J. Chen), clyang @whu.edu.cn (C. Yang). 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.06.025
display applications and white solid-state lighting due to their intrinsic wide band-gap [6–8]. Many blue dendritic fluorescent and phosphorescent light emitting materials have been developed. For example, a kinked star-shaped fluorene/triazatruxene co-oligomer has been reported [9] with device efficiency (1.75 cd/A, CIE (0.16, 0.15)) and a pyrene-centered starburst oligofluorene [10] with device efficiency (1.48 cd/A, CIE (0.20, 0.32)). Lo’s research group [11] have reported a host free phosphorescent dendrimer material with an external quantum efficiency of 7.9%, and CIE coordinates of (0.18, 0.35). Despite many exciting reports, highly efficient devices exhibiting both high thermal stability and robust emission still remains rare. Therefore, blue fluorescent and phosphorescent OLEDs based on dendrimers still require further improvements in terms of thermal stability and EL efficiency.
H. Huang et al. / Organic Electronics 12 (2011) 1716–1723
Among the blue-emitting materials, anthracene derivatives are particularly attractive due to their excellent photoluminescence and electroluminescence properties [12–16]. Such as, triarylamines [17,18], or the other moieties [19–21] have been incorporated into anthracene unit to improve hole injection and charge carrier transport properties. Although a number of vacuum-deposited anthrance-based molecules have been reported as efficient blue electroluminescent materials [22,23], solution-processed analogs remain rare and largely unexplored in OLEDs for its poor solubility and aggregating properties. In our previous work [24], we have demonstrated that 1,3,5-tri (9-anthracene) benzene is a good core emitter, and its centered starburst oligofluorenes show robust blue light emission in an OLED device. Although these materials exhibit deep blue emission, and the current efficiency is up to 1.8 cd/A, but the values of Tg are only about 110 °C for the oligofluorenes possessing long alkyl chains. Here, we report the design and synthesis of four symmetric dendrimer molecules based on the following two principles. Firstly, the propeller-like, and highly rigid 1,3,5-tri (9-anthracene) benzene group was used as the core, which can reduce strong intermolecular interactions and suppress fluorescence quenching. Secondly, using stilbenzene as a conjugated bridge, the different electron donor dendrons (naphthalene, diphenylbenzene, carbazole, and diphenylamine) were introduced at the peripheries of the core to improve solubility in common organic solvents, influence the charge transporting properties, enhance the morphological stability and the thermal stability and improve color purity. As we expected, all of these materials possess high glass transition temperatures (Tg) and good morphological properties, and exhibit good EL performance with low driving voltages in solution-processed double-layer OLED devices. These materials are a promising class of solution-processable blue emitters for practical applications.
2. Experimental section 2.1. Material and methods All reagents and solvents were used as purchased from Aldrich and were used without further purification. 1H NMR spectra were recorded using a Bruker-AF301 AT 400 MHz. High resolution mass spectrometric measurements were carried out using a Bruker autoflex MALDITOF mass spectrometer. Fluorescence spectra were obtained on a Perkin Elmer LS55 Luminescence spectrometer and UV–Vis spectra were measured using a Shimadzu3150 UV–Vis–NIR spectrophotometer. The differential scanning calorimetry (DSC) analysis was performed under a nitrogen atmosphere at a heating rate of 10 °C/min using a PE Instruments DSC 2920. Thermogravimetric analysis (TGA) was undertaken using a SEIKO EXSTAR 6000 TG/DTA 6200 unit under nitrogen atmosphere at a heating rate of 10 °C/min. To measure the PL quantum yields (Uf), degassed solutions of the compounds in CH2Cl2 were prepared. The concentration was adjusted so that the absorbance of the solution would be lower than 0.1. The excitation was
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performed at 360 nm and 9,10-diphenylanthracene(DPA) in CH2Cl2 (Uf = 0.9 in CH2Cl2) or solid film(gDPA = 1) was used as a standard [25,26]. Cyclic voltammetry measurements were carried out in a conventional three electrode cell using a Pt button working electrode of 2 mm in diameter, a platinum wire counter electrode, and a Ag/AgCl (0.1 M) reference electrode on a computer-controlled EG&G Potentiostat/Galvanostat model 283 at room temperature. Reductions CV of all compounds were performed in dichloromethane containing 0.1 M tetrabutylammoniumhexafluorophosphate (Bu4NPF6) as the supporting electrolyte. 2.2. Synthesis The structures and synthetic routes of the four welldefined compounds are shown in Scheme 1. 1,3,5tri(anthracen-9-yl)benzene (1) and 1,3,5-tris(10-bromo anthracen-9-yl)benzene (2) were synthesized according to the literature [24]. 2.2.1. Synthesis of compound (3) A 100 ml round-bottomed flask was charged with compound 2 (422 mg, 0.5 mmol), toluene (20 ml), and ethanol (10 ml) under nitrogen atmosphere. Subsequently, 4-formylphenylboronic acid (375 mg, 2.5 mmol), tetrabutylammonium bromide (0.96 g, 2.98 mmol) and aqueous potassium carbonate (15.0 ml, 30 mmol) were added slowly to the solution. 10 min later, tetrakis (triphenylphosphine) palladium (0) (173 mg, 0.15 mmol) was added and the reaction mixture was stirred under reflux for 24 h in the absence of light. After cooling to room temperature, the precipitate was filtrated and washed with distilled water and methanol, a yellow powder was obtained. Yield 0.34 g (74%). 1H NMR: (CDCl3, 400 MHz): d (ppm) 10.21 (s, 3H), 8.30–8.28 (d, J = 8.8 Hz, 6H), 8.15–8.13 (d, J = 8.0 Hz, 6H), 7.90 (s, 3H), 7.68–7.56 (m, 18H), 7.43–7.40 (t, J = 7.6 Hz, 6H). 13C NMR (100 MHz, CDCl3) d 191.98, 145.92, 139.21, 136.94, 135.86, 135.77, 133.98, 132.18, 129.93, 129.87, 129.60, 126.85, 126.64, 125.72, 125.63. MS (FAB): 919.3 (m/z), calcd for C69H42O3: 919.3. Anal. Calcd. For C69H42 O3: C 90.17, H 4.61, O, 5.22; found: C 90.14, H 4.56, O 5.30. 2.2.2. Synthesis of compound (4) A 500 ml three-neck round-bottomed flask was charged with 4-bromobenzylphosphonium bromide (5.21 g, 10.0 mmol), sodium hydride (0.75 g, 30.0 mmol) and THF (200 ml) under nitrogen atmosphere. After stirring for 1 h, at 40 °C, compound 3 (2.3 g, 2.5 mmol) was added. The solution was continuously stirred for 4 h at room temperature and the product was precipitated by the addition of methanol (200 ml). The precipitated solid was filtrated and washed with methanol, a light yellow solid powder was obtained. Yield 3.2 g (93%). 1H NMR: (CDCl3, 400 MHz): d (ppm) 8.30–8.28 (d, J = 8.0 Hz, 6H), 7.89 (s, 3H), 7.78–7.75 (m, 12H), 7.56–7.39 (m, 33H), 7.26–7.21 (m, 3H). 13C NMR (100 MHz, CDCl3) d 136.34, 132.20, 132.10, 131.98, 131.89, 131.82, 131.51, 131.41, 130.81, 130.59, 130.01, 128.58, 128.47, 128.07, 127.85, 127.13, 126.82, 126.62, 125.55, 125.16. MS (MALDI-TOF): 1378.1999 (m/z), calcd for
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H. Huang et al. / Organic Electronics 12 (2011) 1716–1723 CHO
Br
Br
b)
a)
OHC
CHO
Br 1
2
3 R
Br
c)
d)
Br
Br 4
R 5a: R = naphthalene 5c: R = carbazole
R 5b: R = 3,5-diphenylbenzene 5d: R = diphenylamine
Scheme 1. Synthetic route of 5a, 5b, 5c and 5d. Reagents and conditions: (a) NBS, DMF, 30 °C; (b) toluene, K2CO3, ethanol, (4-formylphenyl)boric acid, Pd(PPh3)4, reflux; (c) 4-bromobenzyl-phosphonium bromide, 40 °C, NaH, THF, 1 h; (d) (i) 1-naphthylboronic acid or 3,5-diphenylbenzeneboronic acid, Pd(PPh3)4, toluene, EtOH; (ii) carbazole or diphenylamine, 18-crown-6, CuI, DMPU, K2CO3, 180 °C.
C90H57Br3: 1378.1994. Anal. Calcd. For C90H57Br3: C 78.44, H 4.17, Br, 17.39; found: C 78.50, H 4.15, Br, 17.35. 2.2.3. Synthesis of compound (5a) A 500 ml three-neck round-bottomed flask was added compound 4 (137.8 mg, 0.1 mmol), 1-naphthaleneboronic acid (111 mg, 0.5 mmol), toluene (20 ml), ethanol (10 ml), 2 M K2CO3 (15 ml, 30 mmol) aqueous solution and tetrakis-(triphenylphosphine) palladium (0) (173 mg, 0.15 mmol) in turn, then the reaction mixture was refluxed under nitrogen for 24 h in the absence of light. After cooling to the room temperature, the precipitate was collected by filtration and washed with distilled water and methanol. The crude product was purified by column chromatography and dried under vacuum to yield a yellowish solid. Yield: 92%. 1H NMR: (CDCl3, 400 MHz): d (ppm) compound 5a: 8.30–8.28 (d, J = 8.0 Hz, 6H), 8.00–7.81 (m, 26H), 7.74–7.72 (m, 6H), 7.58–7.26 (m, 40H). 13C NMR (100 MHz, CDCl3) d 140.24, 139.87, 139.32, 138.46, 137.20, 136.67, 136.41, 134.05, 133.88, 131.79, 131.58, 130.50, 130.05, 128.75, 128.60, 128.24, 128.13, 127.87, 127.74, 127.21, 126.88, 126.61, 126.51, 126.09, 125.99, 125.96, 125.82, 125.56, 125.41, 125.17. MS (MALDI-TOF): 1519.6144 (m/z), calcd for C120H78: 1519.6132. Anal. Calcd. For C120H78: C 94.83, H 5.17; found: C 94.58, H 5.42. 2.2.4. Synthesis of compound (5b) The compound was synthesized using a similar procedure as for compound 5a, yield: 85%. 1H NMR: (CDCl3, 400 MHz): d (ppm) 8.32–8.30 (d, J = 8.8 Hz, 6H), 7.91– 7.72 (m, 50H), 7.57–7.35 (m, 40H). 13C NMR (100 MHz,
CDCl3) d 142.45, 141.79, 141.15, 140.36, 139.28, 138.47, 137.17, 136.71, 136.59, 136.41, 131.78, 130.01, 129.17, 128.87, 128.79, 128.60, 128.55, 127.64, 127.59, 127.38, 127.25, 127.18, 127.11, 126.81, 126.60, 126.12, 125.54, 125.32, 125.17, 124.94 MS (MALDI-TOF): 1825.7515 (m/ z), calcd for C144H96: 1825.7546. Anal. Calcd. For C144H96: C 94.70, H 5.30; found: C 94.85, H 5.15. 2.2.5. Synthesis of compound (5c) A mixture of compound 4 (689 mg, 0.5 mmol), carbazole (376 mg, 2.25 mmol), CuI (14.25 mg, 0.075 mmol), 18crown-6 (19 mg, 0.25 mmol), K2CO3 (310.5 mg, 2.25 mmol) and 1,3-di-methyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU, 2.0 ml) was heated to 190 °C for 24 h under a nitrogen atmosphere. After cooling down to room temperature, the mixture was quenched with 1 N hydrochloric acid, then extracted with dichloromethane, washed with NH3H2O and water. The combined organic phases were dried (MgSO4) and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel using hexane as the eluent to give compound 5c as a yellow solid. Yield: 80%. 1H NMR: (CDCl3, 400 MHz): d (ppm) 8.33–8.31 (d, J = 9.2 Hz, 6H), 8.17–8.15 (d, J = 7.6 Hz, 6H), 7.92 (s, 3H), 7.83–7.80 (m, 18H), 7.63–7.38 (m, 42H), 7.33–7.29 (m, 6H). 13C NMR (100 MHz, CDCl3) d 140.80, 140.70, 139.30, 138.69, 137.12, 137.00, 136.51, 136.44, 136.42, 134.04, 131.84, 130.42, 130.03, 129.25, 128.08, 127.90, 127.27, 127.18, 126.84, 126.68, 125.98, 125.57, 125.20, 123.46, 120.35, 120.02. MS (MALDI-TOF): 1636.6458 (m/ z), calcd for C126H81N3: 1636.6461. Anal. Calcd. For C126H81N3: C 92.45, H 4.99, N 2.57; found: C 92.31, H 5.17, N 2.52.
H. Huang et al. / Organic Electronics 12 (2011) 1716–1723
2.2.6. Synthesis of compound (5d) The compound was synthesized using a similar procedure as for compound 5c. Yield: 74%. 8.30–8.28 (d, J = 8.8 Hz, 6H), 8.17–8.15 (d, J = 7.6 Hz, 6H), 7.89–7.88 (s, 3H), 7.78–7.72 (m, 18H), 7.60–7.37 (m, 48H), 7.27–7.21 (m, 6H). 13C NMR (100 MHz, CDCl3) d 139.25, 138.68, 137.19, 136.62, 136.36, 136.29, 134.01, 131.86, 131.79, 131.72, 131.49, 130.57, 129.99, 129.30, 129.10, 128.87, 128.75, 127.81, 127.44, 126.80, 126.54, 126.31, 125.53, 125.17, 124.55, 121.45. MS (MALDI-TOF): 1641.6924 (m/z), calcd for C126H87N3: 1641.6900. Anal. Calcd. For C126H87N3: C 92.11, H 5.34, N 2.56; found: C 92.10, H 5.39, N 2.51. 2.3. Device fabrication To investigate the electroluminescent (EL) properties of the symmetric starburst compounds containing different surface groups, double layer OLEDs were fabricated via solution processing according to the previous literature.[27] The device structures of the OLEDs were as follows: ITO/PEDOT:PSS/5a–5d or (doping DPAVBi) /TPBI/ LiF/Al. Polyethylene dioxythiophene-polystyrene sulfate (PEDOT-PSS) and 2,20 ,200 -(1,3,5-benzenetriyl)tris[1-phenyl-1H-benzimidazole] (TPBI) were used as hole injection and hole-blocking/electron-transporting layer, respectively. The active layer was spin-coated from toluene solution, and TPBI layer was deposited by means of conventional vacuum deposition onto the ITO-coated glass substrates. The J–V–L characteristics of the EL devices were measured using a Keithley 2400 source meter and a Keithley 2000 source multimeter equipped with a calibrated silicon photodiode. The EL spectra were measured using a JY SPEX CCD3000 spectrometer. The thicknesses of the films were measured using the instrument of Profiler DEKTAK150. All of the measurements were carried out at room temperature under ambient conditions. 3. Results and discussion 3.1. Synthesis, Thermal, Film-forming, Optical and Electrochemical properties Four target compounds, 5a, 5b, 5c and 5d, were prepared as shown in Scheme 1. Detailed synthesis of the 1,3,5-tris(10-bromoanthracen-9-yl)benzene (2) has been described in previous work [24]. The intermediate 3 was synthesized by the Pd-catalyzed Suzuki cross-coupling of formylated benzene boric acid with compound 2 with excellent yields, and were easily purified by a simple filtration and washed with distilled water and methanol. Then Wittig reaction of 4-bromobenzyl-phosphonium bromide and compound 3 gave the compound 4 in 93% yield at 40 °C. The target compounds 5a and 5b were obtained in 92% and 85% of isolated yield, by an easy Suzuki coupling reaction, respectively. The target compound 5c and 5d were obtained in 80% and 74% of isolated yield by the CuI-catalyzed Ullman reaction, respectively. All the four target compounds were soluble in common organic solvents (CH2Cl2, toluene), and characterized fully by 1H and 13C NMR, MALDI-TOF, and elemental analysis.
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Thermal properties of these compounds were studied using DSC and TGA. The thermal properties were measured under N2 gas condition at heating rates of 10 °C/min, and the related data are listed in Table 1. As shown in Fig. 1a, compounds 5a–5d exhibit a high glass transition temperature (Tg) at 166, 189, 260, and 231 °C, respectively. The high Tg are attributed to the rigid core and the star shaped dendrons can effectively suppress the crystallization (or arm aggregation). High decomposition temperatures (Td, 5% weight loss) of these compounds were also observed at 484, 504, 497, and 427 °C (Fig. 1b), respectively, which may be due to the highly stability of the 1,3,5-tri (9anthracene) benzene core and the side groups. The stable aromatic side group can supply electron to the styryl group and enhance the thermal stability of material.[28] The relatively high Tg and Td of these star shaped compounds is very desirable for high performance OLED applications. Efficient film-forming properties of light emitting materials used in OLEDs are crucial for the performance of the devices, so the surface morphologies of thin films of the four compounds (5a, 5b, 5c and 5d) were examined. As showed in Fig. 2, the AFM images show smooth and amorphous thin films formed by spin-coating the compounds (5a, 5b, 5c and 5d) from toluene solution. The deposited films were annealed under N2 gas condition at 150 °C for 0.5 h. The annealed film exhibits a fairly smooth surface morphology with a root-mean-square (rms) roughness of 0.5539, 0.6211, 0.5202 and 0.6565 nm for 5a, 5b, 5c and 5d, respectively. This suggests that all of the compounds possess excellent thermal and amorphous stability, which indicates that the rigid propeller core influences the arrangement of the molecules in the thin films, and 1,3,5-tri(9-anthracene)benzene is a good core emitter for star-shaped fluorophore. Fig. 3 shows the normalized UV–Vis and fluorescence spectra of compounds 5a, 5b, 5c and 5d in a CH2Cl2 solution and in the thin film. The related photophysical properties were listed in Table 1. All of these compounds have similarly structured absorption spectra (range: 330– 440 nm) and emission spectra (range: 400–550 nm) in solution. This indicates that the photophysical properties are mainly determined by the core 1, 3, 5-tri(9-anthracene)benzene. The absorption in the range 330–440 nm is assigned to the S1 S0 transition of the anthracene moiety. In the solid state, all of them show strong pure-blue emission with a main peak at 448, 453, 451 and 460 nm, respectively, which shows that the introduction of dipenylamine results in a notable red-shift in PL spectra and peripheral dendrons can affect the molecular interaction. The quantum yields of compound 4a-4d in CH2Cl2 were 0.68, 0.66, 0.67 and 0.64, respectively, which is even higher than the typical blue material m-ADN [25]. The quantum yield in solid state was also measured, the relative high values indicated that no obviously aggregating or quenching phenomena appeared. In order to further investigate the morphological stability of these materials, the Abs and PL spectra before/after annealing state (see Figure S4.) was also researched, the absorbance and PL emission have no clearly change after annealing (150 °C for 1 h), which showed that the morphological stability of material is excellent. The optical energy bandgaps of the compound
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H. Huang et al. / Organic Electronics 12 (2011) 1716–1723
Table 1 The optical, photophysical and thermal properties of 5a–5d. HOMO
5a 5b 5c 5d a b
Ua/gb
LUMO
5.57 5.54 5.52 5.50
Eg (eV)
0.68/0.69 0.66/0.68 0.67/0.64 0.64/0.61
2.65 2.59 2.58 2.57
PL (nm)
2.92 2.95 2.94 2.93
Solution
Film
436 436 436 437
448 453 451 460
Abs (nm) in solution
Tg/Td (°C)
331,358,380,401 340,358,380,401 344,358,380,401 320,358,380,401
166/484 189/504 260/497 231/427
Using DPA as a standard; kex = 360 nm (U = 0.9 in CH2Cl2). Using DPA as a standard; kex = 360 nm (g = 1 in thin film).
10 9
100
a)
90
Weight (%)
Heat flow
8 7 b
6
5a 5b 5c 5d
a
5
d
4 3
5a 5b 5c 5d
b)
c
100
150
200 250 o 300 Temperature ( C)
80
70
60 200
350
300
400
500
600
Temperature (οC)
700
800
Fig. 1. (a) DSC curves of compounds 5a, 5b, 5c and 5d measured under nitrogen at a heating rate of 10 °C/min; (b) TGA curves of star-shaped compounds measured at a heating rate of 10 °C/min under nitrogen.
Fig. 2. AFM topographic images of the four compounds (5a, 5b, 5c and 5d) in thin solid films (ca. 40 nm thick).
1.0
0.6
0.8 0.6
0.4
0.4
0.2
0.2
0.0 300
350
400 450 500 Wavelength (nm)
550
0.0 600
1.0
5a 5b 5c 5d
0.8 0.6
1.0 0.8 0.6
0.4
0.4
0.2
0.2
0.0 300
350
400
450
500
550
600
0.0 650
Normalized PL Intensity (a.u)
0.8
b) Normalized Absorbance (a.u)
5a 5b 5c 5d
Normalized PL intensity (a.u)
Normalized Absorbance (a.u)
a) 1.0
Wavelength (nm)
Fig. 3. (a) Absorption and photoluminescence spectra of compound 5a, 5b, 5c and 5d in CH2Cl2; (b) Absorption and PL spectrum of the compounds 5a, 5b, 5c and 5d in thin film (ca. 40 nm).
5a, 5b, 5c, and 5d are 2.92, 2.95, 2.94, and 2.93 eV, respectively, which is calculated from the threshold of the optical absorption. In addition to the photophysical properties, the electrochemical properties are also very important in evaluating a material usefulness for optoelectronic applications. The
electrochemical behaviors of the compounds 5a, 5b, 5c and 5d were examined by cyclic voltammetry using a standard three-electrode electrochemical cell in an electrolyte solution of 0.1 M tetrabutylammoniumhexafluorophosphate (TBPAPF6) dissolved in dichloromethane [29]. The onset oxidation peaks (Eox) for 5a–5d were 1.26, 1.23,
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H. Huang et al. / Organic Electronics 12 (2011) 1716–1723 Table 2 Electroluminescence Characteristics of the Devices.a
a b c d e
Device
Emitter
Lmaxb (cd/m2) voltages (V)
gc.max c (cd/A)
gp.max d (lm/W)
CIE (x, y)e
Von (1 cd/m2) (V)
EL(peak) (nm)
1 2 3 4 5 6 7 8
5a 5b 5c 5d 5a:DPAVBi 5b:DPAVBi 5c:DPAVBi 5d:DPAVBi
3366, 11.3 2740, 11.1 4754, 8.5 1562, 11.5 3635, 8.6 6807, 12.4 7470, 11.6 11204, 11.4
1.7 1.6 2.0 2.4 3.6 4.2 3.6 4.9
1.10 1.14 1.71 1.54 2.40 2.81 2.25 3.22
(0.15,0.17) (0.15, 0.14) (0.16, 0.19) (0.18, 0.28) (0.16, 0.31) (0.16, 0.30) (0.17, 0.33) (0.17, 0.31)
4.8 4.5 3.4 4.9 3.2 3.6 3.6 3.4
468 456 468 476 472 472 476 472
Devices configuration: ITO/PEDOT:PSS (40 nm)/ 5a–5d (40 nm) or 5a–5d: DPAVBi (5%)(40 nm)/TPBI (40 nm) /LiF (1 nm) /Al (100 nm). Maximum luminance. Maximum current efficiency. Maximum powder efficiency. Commission International de I0 Eclairage coordinates.
4
1000
10
a)
2
0
400
10
-1
10
200
-2
10 0
Current Efficiency (cd/A)
600
Luminance (cd/m )
2
Current Density (mA/cm )
3
10 Device 1 Device 2 102 Device 3 1 Device 4 10
800
b) Device 1 Device 2 Device 3 Device 4
0.1
0.01
-3
3
4
5
6
7 8 9 Voltage (V)
10 10 11 12 13
10
1.0
c) 1
0.1
Normalized intensity (a.u.)
Power Efficiency (lm/W)
1
Device 1 Device 2 Device 3 Device 4
0.01 -2
10
-1
0
1
2
10 10 10 10 2 Current Density (mA/cm )
3
10
-2
10
-1
0
1
10 10 10 2 Current Density (mA/cm )
2
10
3
d) Device 1 Device 2 Device 3 Device 4
0.8 0.6 0.4 0.2 0.0 400
500 600 Wavelength (nm)
700
Fig. 4. (a) J–V–L curve of the four devices; (b) current efficiency-current density characteristics; (c) powder efficiency-current density characteristics; (d) EL spectra of the four devices.
1.21 and 1.19 eV, respectively (see Figure S3). Thus, the HOMO levels were determined to be 5.57, 5.54, 5.52 and 5.50 eV by the method (Eox + 4.31) eV reported by Li et al. [25] (Table 1). The values were very close to that of 9,10-diphenylanthracene (5.54 eV), which indicates the HOMO level of these compounds were mainly decided by 1,3,5-tri(9-anthracene)-benzene center and the side groups were only finely tune the energy gap and the HOMO level. Generally speaking, the electrochemical properties and photophysical properties of compound 5a–5d in dilute solution are very similar, mirroring the anthracene unit’s characteristics. It shows every individual arm’s electrochemical properties are remained unperturbed for the propeller topology serving as a spacer to effectively block conjugation.
3.2. Device performance To study the EL properties of these compounds, doublelayered devices with the configuration ITO/PEDOT:PSS (40 nm)/5a–5d (40 nm)/TPBI (40 nm)/LiF (1 nm) /Al (100 nm) (emitter: 5a, Device 1; 5b, Device 2; 5c, Device 3; 5d, Device 4) were fabricated by spin-coating from a toluene solution (ca. 7 mg/ml). The EL performances of these compounds are listed in Table 2. Double-layer devices utilizing compounds 5a and 5b exhibit a maximum brightness of 3366, 2740 cd/m2, luminous efficiency of 1.7, 1.6 cd/A, with CIE coordinates 5a (0.15, 0.17), 5b (0.15, 0.14). For compounds 5c and 5d, the double-layer devices showed a maximum brightness of 4754, 1562 cd/m2, luminous efficiency of 2.0, 2.4 cd/A, with CIE coordinates 5c
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H. Huang et al. / Organic Electronics 12 (2011) 1716–1723
10
a)
4
2
2
Current Density (mA/cm )
500
3
400
10
300
10
2
200
1
Device 5 Device 6 Device 7 Device 8
100 0
10
0
10
-1
4
8 Voltage (V)
10
12
10
Device 5 Device 6 Device 7 Device 8
0
10
1
2
10 10 Current density (mA/cm2)
Device 5 Device 6 Device 7 Device 8
0.1
1.0
0.1
0.01
1
0
10
c)
1
b)
0.01
Normalized intensity (a.u)
Power efficiency (lm/W)
10
6
Current Efficiency (cd/A)
10
Luminance (cd/m )
600
1
2
10 2 10 Current Density (mA/cm )
d) Device 5 Device 6 Device 7 Device 8
0.8 0.6 0.4 0.2 0.0
400
450
500 550 600 650 Wavelength (nm)
700
750
Fig. 5. (a) J–V–L curve of the four doped devices; (b) current efficiency-current density characteristics; (c) powder efficiency-current density characteristics; (d) EL spectra of the four doped devices.
the quantum efficiency of the doped device has been improved, which indicated that the better device performance in the doped device is not due to the change of emission colors. More interestingly, the driving voltage of all the devices is low (ca. 3.6 eV), in which device 5 exhibits the lowest driving voltage of 3.2 V. This indicates that the energy levels of the host materials and the dopant DPAVBi are well matched. Additionally, the dopant DPAVBi can also act as the hole trapper in all of doped devices, so the turn on voltage of the most of doped devices are lower than the undoped devices(see Table 2) [30]. Excellent EL stability (see Figure S2) and the efficient color purity render these materials promising candidates for display applications. We believe that optimization could further improve the device
2.5
Quantum efficiency (%)
(0.16, 0.19), 5d (0.18, 0.28). Fig. 4 shows the EL spectra, the current density–voltage-brightness and the current density-efficiency characteristics of the devices (1, 2, 3, 4). EL spectra of the devices based on the emitters 5c and 5d have obviously red-shift compared to the compounds 5a and 5b, it is induced by the carbazole and diphenylamine group’s electronic-donating properties. Especialy, the EL spectrum of the device 4 has a red shift about 20 nm compared to that of device four. Additionally, the electroluminescence (EL) of all devices remained quite stable under different driving voltages (see Figure S1). In order to further improve the device efficiency, we adopted a host-dopant system using these materials as the host and a highly fluorescent blue dye, DPAVBi, as dopant at a concentration of 5%. Table 2 summarizes the key EL performance of doped devices (5, 6, 7, 8), all of them show a strong blue emission with current efficiency of 3.6–4.9 cd/ A. Fig. 5. shows the EL spectra, the current density–voltage-brightness and the current density-efficiency characteristics of devices (5, 6, 7, 8). The device 8 exhibits a maximum brightness of 11204 cd/m2 at 11.4 V, and a maximum current efficiency of 4.9 cd/A with CIE coordinates of (0.17, 0.31). We ascribe the higher efficiency to the more efficient Förster energy transfer because there is more overlap between the emission of compound 5d and the absorption of DPAVBi. As shown in Fig. 5d, the EL spectra of all DPAVBi doped devices (5, 6, 7, 8) exhibit bright blue emissions at 472 nm with a shoulder at around 496 nm, which is the intrinsic characteristic of the emissions from the dopant DPAVBi. This result indicates completed energy transfer from the host to dopant. From the Fig. 6, it is obviously that
Device 1 Device 2 Device 3 Device 4 Device 5 Device 6 Device 7
2.0 1.5 1.0
Device 8
0.5 0.0
0
10
1
10
2
10
3
2
10
Current density (mA/cm ) Fig. 6. Quantum efficiency-current density characteristics of devices 1–8.
H. Huang et al. / Organic Electronics 12 (2011) 1716–1723
performance and fully explore the potential of these starburst materials. 4. Conclusion In summary, we present a series of novel blue starshaped molecules, in which the propeller core 1,3,5-tri(9anthracene)benzene and the different dendrons were bridged by the stilbenzene. These materials exhibit high thermal stability (high glass transition temperature in the range 166–231 °C) with good film forming properties. Meanwhile, their optical and electronic properties can be tuned by adjusting the appropriated surface groups. The nondoped OLED devices fabricated using compound 5c exhibits a maximum brightness of 4754 cd/m2 and maximum current efficiency of 2.0 cd/A (corresponding to a power efficiency of 1.71 lm/W) with CIEx,y color coordinates of (x = 0.16, y = 0.19) and the devices’ threshold voltage are only 3.4 eV. Compound 5d shows an even higher efficiency of up to 4.90 cd/A with CIEx,y color coordinates of (x = 0.17, y = 0.31). These results will be beneficial for the molecular design of blue dendritic materials. Acknowledgements We thank the Donghu New & High Technology Development Zone of Wuhan City and Doctoral Fund of Ministry of Education of China (20100142120049) for financial support and thank the Analytical and Testing Centre of Huazhong University of Science and Technology for measurements. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.orgel. 2011.06.025. References [1] O. Usluer, S. Demic, D.A.M. Ebge, E. Birckner, C. Tozlu, A. Pivrikas, A.M. Ramil, N.S. Sariciftci, Adv. Funct. Mater. 20 (2010) 4152.
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[2] J.L. Wang, Y. Zhou, Y.F. Li, J. Pei, J. Org. Chem. 74 (2009) 7449. [3] S.C. Lo, R.E. Harding, C.P. Shipley, S.G. Stevenson, P.L. Burn, I.D.W. Samuel, J. Am. Chem. Soc. 131 (2009) 16681. [4] S.H. Hwang, C.N. Moorefield, G.R. Newkome, Chem. Soc. Rev. 37 (2008) 2543. [5] L. Wang, Y. Jiang, J. Luo, Y. Zhou, J.H. Zhou, J. Wang, J. Pei, Y. Cao, Adv. Mater. 21 (2009) 4854. [6] S.H. Ye, Y.Q. Liu, K. Lu, W.P. Wu, C.Y. Du, Y. Liu, H.T. Liu, T. Wu, G. Yu, Adv. Funct. Mater. 20 (2010) 3125. [7] S. Sax, N.R. Penkalla, A. Neuhold, S. Schuh, E. Zojer, E.J.W. List, K. Müllen, Adv. Mater. 22 (2010) 2087. [8] H. Sasabe, E. Gonmori, T.Y. Chiba, Y.J. Li, D.K. Tanaka, S.J. Su, T. Takeda, Y.J. Pu, K.C. Nakayama, J.J. Kido, Chem. Mater. 20 (2008) 5951. [9] W.Y. Lai, Q.Y. He, R. Zhu, Q.Q. Chen, W. Huang, Adv. Funct. Mater. 18 (2008) 265. [10] F. Liu, W.Y. Lai, C. Tang, H.B. Wu, Q.Q. Chen, B. Peng, W. Wei, W. Huang, Y. Cao, Macromol. Rapid. Commun. 29 (2008) 659. [11] S.C. Lo, R.N. Bera, R.E. Harding, P.L. Burn, I.D.W. Samuel, Adv. Funct. Mater. 18 (2008) 3080. [12] J.H. Huang, J.H. Su, X. Li, M.K. Lam, K.M. Fung, H.H. Fan, K.W. Cheah, C.H. Chen, H. Tian, J. Mater. Chem. 21 (2011) 2957. [13] J.H. Park, D.H. Lee, H. Kong, M.J. Park, I.H. Jung, C.E. Park, H.K. Shim, Org. Electron. 11 (2010) 820. [14] J. Shi, C.W. Tang, Appl. Phys. Lett. 80 (2002) 3201. [15] S.K. Kim, B. Yang, Y. Ma, J.H. Lee, J.W. Park, J. Mater. Chem. 18 (2008) 3376. [16] Y.T. Lyu, J. Kwak, O. Kwon, S.H. Lee, D. Kim, C. Lee, K. Char, Adv. Mater. 20 (2008) 2720. [17] Z.H. Li, M.S. Wong, Y. Tao, M. D’Iorio, J. Org. Chem. 69 (2004) 921. [18] S.B. Jiao, Y. Liao, X.J. Xu, L.P. Wang, G. Yu, L.M. Wang, Z.M. Su, S.H. Ye, Y.Q. Liu, Adv. Funct. Mater. 18 (2008) 2335. [19] K. Albrecht, Y.T. Kasai, K. Yamamoto, J. Inorg. Organomet. Polym. 118 (2009) 118. [20] H.Q. Zhang, S.M. Wang, Y.Q. Li, B. Zhang, C.X. Du, X.J. Wan, Y.S. Chen, Tetrahedron 65 (2009) 4455. [21] C.J. Zheng, W.M. Zhao, Z.Q. Wang, D. Huang, J. Ye, X.M. Ou, X.H. Zhang, C.S. Lee, S.T. Lee, J. Mater. Chem. 20 (2010) 1560. [22] L. Wang, W.Y. Wong, M.F. Lin, W.K. Wong, K.W. Cheah, H.L. Tam, C.H. Chen, J. Mater. Chem. 18 (2008) 4529. [23] Y.H. Yu, C.H. Huang, J.M. Yeh, P.T. Huang, Org. Electron. 12 (2011) 694. [24] H. Huang, Q. Fu, S.Q. Zhuang, Y.K. Liu, L. Wang, J.S. Chen, D.G. Ma, C.L. Yang, J. Phys. Chem. C. 115 (2011) 4872. [25] J.H. Seo, K.H. Lee, B.M. Seo, J.R. Koo, S.J. Moon, J.K. Park, S.S. Yoon, Y.K. Kim, Org. Electron. 11 (2010) 1605. [26] J.A. Osaheni, S.A. Jenekhe, J. Am. Chem. SOC. 117 (1995) 7389. [27] J. Luo, L. Xiao, Z. Chen, Q. Gong, Appl. Phys. Lett. 93 (2008) 133301. [28] J.Y. Park, S.Y. Jung, J.Y. Lee, Y.G. Baek, Thin. Solid. Films. 516 (2008) 2917. [29] Q. Sun, H. Wang, C. Yang, Y.F. Li, J. Mater. Chem. 13 (2003) 800. [30] C.W. Tang, S.A. Van Slyke, C.H. Chen, J. Appl. Phys. 65 (1989) 3610.