Fluorene derivatives for highly efficient non-doped single-layer blue organic light-emitting diodes

Organic Electronics 15 (2014) 57–64

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Fluorene derivatives for highly efficient non-doped single-layer blue organic light-emitting diodes Xin Jiang Feng a,b,⇑, Shao Fu Chen a,1, Yong Ni a,1, Man Shing Wong b,⇑, Maggie M.K. Lam c,2, Kok Wai Cheah c,⇑, Guo Qiao Lai a,⇑ a

Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University, Hangzhou 310012, PR China Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist University, Kowloon Tong, Hong Kong Special Administrative Region c Department of Physics and Institute of Advanced Materials, Hong Kong Baptist University, Kowloon Tong, Hong Kong Special Administrative Region b

a r t i c l e

i n f o

Article history: Received 13 May 2013 Received in revised form 18 October 2013 Accepted 18 October 2013 Available online 6 November 2013 Keywords: Fluorene derivatives Nondoped Single-layer Blue Organic light-emitting diodes

a b s t r a c t Diphenylamino- and triazole-endcapped fluorene derivatives which show a wide energy band gap, a high fluorescence quantum yield and high stability have been synthesized and characterized. Single-layer electroluminescent devices of these fluorene derivatives exhibited efficient deep blue to greenish blue emission at low driving voltage. The single-layer OLED of PhN-OF(1)-TAZ shows a maximum current efficiency of 1.54 cd/A at 20 mA cm2 with external quantum efficiency (EQE) of 2.0% and CIE coordinates of (0.153, 0.088) in deep blue region, while the single-layer device of oligothienylfluorene PhN-OFOT-TAZ shows a maximum brightness of 7524 cd/m2 and a maximum current efficiency of 2.9 cd/A with CIE coordinates of (0.20, 0.40) in greenish blue. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Organic light-emitting diodes (OLEDs) have been recently realized in consumer electronics for practical applications in small-size active matrix OLED displays and solid-state lightings due to the success of materials development and device engineering [1–6]. Among the three principal colors necessary for full color OLED display applications, blue-emitting materials and devices are still needed to be improved the most in terms of efficiency, lifetime and color purity owing to the far superior stabilities

⇑ Corresponding authors. Address: Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University, Hangzhou 310012, PR China. Tel./fax: +86 571 28862271 (X.J. Feng), fax: +852 34117348 (X.J. Feng, M.S. Wong), fax: +852 34115813 (K.W. Cheah). E-mail addresses: [email protected] (X.J. Feng), mswong@hkbu. edu.hk (M.S. Wong), [email protected] (K.W. Cheah), gqlai@hznu. edu.cn (G.Q. Lai). 1 Fax: +86 571 28862271. 2 Fax: +852 34115813. 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.10.019

and device performances of green and red emitters. Besides, blue-emitting materials can also be the host for dopant emitters because of its intrinsic large energy bandgap. However, only a handful of deep-blue OLEDs based on organic emitters achieved high external quantum efficiecy (EQE) up to date [7–12]. Hence, the development of blue emitters for highly efficient and stable full color OLED applications still remains a challenge because these materials require possessing a high fluorescence yield, a wide energy band gap, a high thermal stability and good thin film morphology [13]. Some materials having both electron-rich and electrondeficient moieties show abilities of electron-transporting and hole-transporting. They can effectively stabilize exciton and balance the charges in the emitting layer, and thus improve device performance in OLEDs [14–18], in spite of their shortcomings [19–27]. Nevertheless, such materials with wide band gap, highly efficient and stable blue emission are still to be developed [28–33]. Oligofluorenes non-conjugated with highly electronrich triphenylamines and electron-deficient triazole (TAZ)

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moieties have been demonstrated to exhibit high thermal stability and good OLED device performance in our previous study [29]. However, significant red-shift was observed for the oligomer with higher conjugation and similar phenomenon was reported by other studies, which was due to the oxidation of 9-position on fluorine [34,35]. So a comparatively small molecule is of advantage over large ones in device fabrication by sublimation method. On the other hand, donor–acceptor oligofluorenes conjugated with diphenylamino and triazole groups have shown high fluorescence quantum yields and excellent multiphoton absorption properties [36]; and we expect good electroluminescence of these conjugated materials with the advantage in intramolecular charge transfer over the corresponding non-conjugated ones. Furthermore, introduction of thienyl ring into a molecule can reduce the energy gap effectively and thus tune the color in device [37,38]. In this contribution, we reported herein the synthesis and electroluminescence performance of three diphenylamino- and triazole-conjugated oligofluorene derivatives, PhN-OF(n)-TAZ (n = 1, 2) and oligothienylfluorene PhNOFOT-TAZ as a non-doped blue emitter in single-layer OLEDs. It has been found that these materials exhibit high quantum yield and thermal stability. The single-layer OLED of PhN-OF(1)-TAZ shows a maximum current efficiency of 1.54 cd/A at 20 mA cm2 (external quantum efficiency (EQE) of 2.0%) with CIE coordinates of (0.153, 0.088) in deep blue region, while the OLED of PhN-OF(2)-TAZ shows a maximum current efficiency of 1.8 cd/A (EQE of 1.1%) and a maximum brightness of 5349 cd/m2 with CIE coordinates of (0.16, 0.23) in blue region. Furthermore, the single-layer device of PhN-OFOT-TAZ shows a maximum brightness of 7524 cd/m2 and a maximum current efficiency of 2.9 cd/A (EQE of 2.0%) with CIE coordinates of (0.20, 0.40) in sky blue to greenish blue. These results are among the best performance reported for blue emission of small molecule OLEDs in non-doped single-layer devices [29,39–41].

2. Experimental 2.1. Synthesis of 4-N-phenyl-5-(4-tert-butylphenyl)-3-{4[90 ,90 -di-n-butyl-70 -diphenylamino-20 -fluorenyl]phenyl}1,2,4-triazole (PhNOF(1)-TAZ) [42] A mixture of 9,9-di-n-butyl-7-diphenylamino-2-fluorenylboronic acid (1, 1.93 g, 3.94 mmol), 2 (1.14 g, 2.63 mmol), Pd(OAc)2 (30 mg, 0.13 mmol), P(o-tolyl)3 (80 mg, 0.26 mmol), 2 M K2CO3 (4 mL), toluene (20 mL) and methanol (8 mL) under nitrogen atmosphere was heated at 75 °C overnight with good magnetic stirring. After cooling to room temperature, the reaction mixture was poured into water and extracted with ethyl acetate (30 mL), followed by dichloromethane (30 mL). The combined organic layer was dried over anhydrous Na2SO4, evaporated to dryness. The residue was purified by silica gel column chromatography using 8:1 dichloromethane:ethyl acetate as eluent affording 1.74 g (83%) of a white solid. 1H NMR (400 MHz, CDCl3,) 7.63–7.47 (m, 10H), 7.37 (d, J = 8.4 Hz, 2 H), 7.31 (d, J = 7.6 Hz, 2 H), 7.27 (m, 6 H), 6.99–7.25 (m, 9 H), 1.85–1.91 (m, 4 H),

1.29 (s, 9 H), 1.04–1.09 (m, 4 H), 0.63–0.71 (m, 10 H). 13C NMR (100 MHz, CDCl3,) 154.8, 154.4, 152.9, 152.4, 151.4, 147.9, 147.3, 142.7, 140.8, 137.9, 135.6, 135.5, 130.0, 129.7, 129.1, 129.0, 128.4, 128.0, 126.9, 125.9, 125.4, 123.9, 123.4, 122.5, 121.0, 120.5, 119.4, 119.2, 55.1, 40.0, 34.7, 31.1, 26.0, 23.0, 13.8. HRMS (MALDI-TOF) m/z calcd for C57H56N4 796.4505, found [M+] 796.4512. 2.2. Measurements 1 H and 13C NMR spectra were recorded on a Bruker AM 400 spectrometer. Mass spectrometric measurements were recorded by a HP5989 mass spectrometer. UV–vis spectra were obtained on a Varian Cary 200 spectrophotometer. Fluorescence spectra were obtained on a Perkin–Elmer LS55 luminescence spectrometer. The differential scanning calorimetry (DSC) analysis was performed under a nitrogen atmosphere on a TA Instruments DSC 2920. Thermogravimetric analysis was undertaken using a TGA instrument (PE-TGA6). To measure the fluorescence quantum yields (UF), degassed solutions of the compounds in different solvents were prepared and quinine sulfate monohydrate (U350 = 0.58) was used as a standard, while the absolute quantum yields UF of the dyes in powder were measured by Edinburgh Photonics FLS920 fluorescence spectrometer. The concentration was adjusted so that the absorbance of the solution would be lower than 0.1. Cyclic voltammetric (CV) measurements were carried out in a conventional three-electrode cell, using a Pt button working electrode 2 mm in diameter, a platinum wire counter electrode, and a SCE reference electrode on a computer-controlled EG&G Potentiostat/Galvanostat model 283 at room temperature. Reduction CV of all compounds was performed in dichloromethane containing Bu4NPF6 (0.1 M) as the supporting electrolyte.

2.3. Device fabrication Prior to the deposition of organic materials, indium–tin oxide (ITO)/glass was cleaned with a routine cleaning procedure and pretreated with oxygen plasma. Devices were fabricated under about 106 Torr base vacuum in a thinfilm evaporation coater following a published protocol [43]. The current–voltage–luminance characteristics were

Fig. 1. Molecular structure of fluorene derivatives.

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Scheme 1. Synthesis of PhN-OF(1)-TAZ.

Table 1 Photophysical, electrochemical, and thermal properties of fluorene derivatives. kabs max (nm)

kem max (nm)

UF

Optical gapd (eV)

e Eoxd 1=2 (V)

3.06

0.34, 0.89 0.35, 0.88 0.34, 0.82

(V)

HOMOf (eV)

LUMOg (eV)

Tgh/Tmh/Tdeci (°C)

1.86

5.48

2.42

112/278/449

1.85

5.48

2.46

135/281/484

1.80

5.86

3.09

124/248/508

e Ered p

Solutiona (film) PhN-OF(1)TAZ PhN-OF(2)TAZj PhN-OFOTTAZj

374 (376) 375 (377) 397 (403)

419b (458) 422b (458) 452b (498)

0.83c (0.47) 0.89c (0.54) 0.61c (0.23)

3.02 2.77

a

Measured in toluene. Excited at the absorption maxima. Using quinine sulfate monohydrate (U350 = 0.58) as a standard. d Determined by absorption cutoff. e E1/2 vs Fc+/Fc estimated by CV method using platinum disc electrode as a working electrode, platinum wire as a counter electrode, and SCE as a reference electrode with an agar salt bridge connecting to the oligomer solution and all the potentials were calibrated with ferrocene, E1/2 (Fc/Fc+) = 0.46 V vs SCE. f Determined by UV photoelectron spectroscopy. g Optical gap = HOMO–LUMO. h Determined by differential scanning calorimeter from re-melt after cooling with a heating rate of 10 °C/min under N2. i Determined by thermal gravimetric analyser with a heating rate of 20 °C/min under N2. j Partially reported in previous paper [36,44]. b

c

measured with a diode array rapid scan system, using a Photo Research PR650 spectrophotometer and a computer-controlled, programmable, direct-current (DC) source. All measurements were carried out in air at room temperature without encapsulation.

3. Results and discussion 3.1. Synthesis The molecular structures of the blue emitting fluorene derivatives are shown in Fig. 1. Syntheses of PhN-OF(2)TAZ and PhN-OFOT-TAZ were carried out according to the literature procedures [36,44]. The synthetic route for PhN-OF(1)-TAZ is outlined in Scheme 1. The target compound was prepared by Palladium catalyzed Suzuki cross-coupling reaction of 4-diphenylaminofluorenylboronic acid, 1 and bromo-triazole derivative 2 in good yield. All the molecules were fully characterized by NMR spectroscopy, elemental analysis, and high-resolution mass

spectrometry and found to be in good agreement with the structure. 3.2. Photophysical, electrochemical, and thermal properties The photophysical, electrochemical, and thermal properties of these oligofluorenes are summarized in Table 1. Fig. 2a and b show the absorption and emission of all compounds in toluene and in thin film, respectively. All the oligomers show similar absorption spectral features, which are composed of two major absorption bands. The stronger absorption band spans from 374 to 397 nm corresponding to the p ? p* transition of the p-conjugated cores and the weaker absorption peak appears around 300 nm due to the n ? p* transition of triarylamine moieties. Solvatochromism effects are observed for the oligomers. For instance, the emission maxima of PhN-OF(2)-TAZ, exhibit solvatochromic shift of 20 nm and 60 nm changing from toluene to chloroform and chloroform to DMF, respectively, which indicates the dipolar or charge-transfer character of these oligomers (see Fig. 2c). There is a significant red-shift in

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Fig. 2. (a) Absorption and emission spectra of oligomers in toluene. (b) Absorption and emission spectra of oligomers in thin film. (c) Solvatochromic effect of PhN-OF(2)-TAZ.

absorption with the introduction of thienyl ring into the fluorenyl skeleton which is attributed to the low resonance energy of thiophene ring [37,38]. Upon excitation at the absorption maximum, these fluorene derivatives also exhibit strong blue fluorescence with peak ranging from 419 nm to 452 nm in toluene and from 458 nm to 498 nm in thin film prepared by evaporation of the dye in dichloromethane onto a quartz plate. Red-shift of 39–

46 nm is observed for the emission maxima in thin film compared to those in solution, indicating the aggregation of molecules in the solid state. Meanwhile, these oligomers show less vibronic structure in thin film than in solution. The fluorescence quantum yields (/) measured in toluene are high in the range of 0.83–0.89 for PhN-OF(1)-TAZ and PhN-OF(2)-TAZ and moderate for PhN-OFOT-TAZ (/ = 0.61) due to heavy atom effect of sulphur of the thienyl

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Scheme 2. Diagram of the device configurations.

Fig. 3. EL spectra of fluorene derivatives based on a single-layer device structure.

ring [45,46], while the corresponding quantum yields in thin film are 0.47, 0.54, and 0.23 for PhN-OF(1)-TAZ, PhN-OF(2)-TAZ, and PhN-OFOT-TAZ, respectively (see Table 1), showing the potential for OLED applications of these materials. To probe the redox properties of these end-capped p-conjugated oligofluorenes, cyclic voltammetry (CV) was performed in a three-electrode cell set-up with 0.1 M of Bu4NPF6 as a supporting electrolyte in CH2Cl2. All the molecules exhibit two reversible one-electron oxidation waves with E1/2 in the range of 0.34–0.35 V and 0.82–0.89 V corresponding to the sequential removal of an electron from the triarylamine and p-conjugated fluorene core, respectively. The ionization potentials (or HOMO levels) of these oligomeric films as determined by UV photoelectron spectroscopy are in the range of 5.48 to 5.86 eV. By adding the optical energy to the HOMO level, their LUMO levels are estimated to be in the range of 2.46 to 3.09 eV (see Table 1).

The thermal properties of these oligofluorenes were determined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) measurements (see Table 1). These oligofluorenes exhibit high thermal stabilities with decomposition temperatures (Td) higher than 449 °C. In addition, they exhibit high glass transition temperature (Tg) spanning from 112 to 135 °C and the Tg increases in the order of PhN-OF(1)-TAZ < PhN-OFOTTAZ < PhN-OF(2)-TAZ, which is consistent with an increasing order of the molecular weight. The high Tg and Td values of these oligomers suggest stable morphological properties, which is desirable for OLEDs with high thermal stability. The strong blue emission, good thermal stability, and relatively large energy gap (Eg) of these oligofluorenes derivatives imply that these compounds can be employed as potential blue emitters and host materials in OLED devices. 3.3. Electroluminescence performance To investigate the EL performance of these materials, four different types of non-doped electroluminescent devices (as shown in Scheme 2) have been fabricated in which their device structures are summarized as follows:

Device A : ITO=CFx=emitter=LiF=Al; Device B : ITO=CFx=emitter=TPBI=LiF=Al; Device C : ITO=CFx=2  TNATA=emitter=LiF=Al; Device D : ITO=CFx=emitter=Alq3 =LiF=Al; 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) and Alq3 were used as an electron-transporting layer (ETL), and 4,40 ,400 -tris[(N-(2-naphthyl)-N-phenyl- amino)] triphenyl-amine (2-TNATA) was used as a hole transporting

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Fig. 4. Current density–voltage–luminance characteristics of single-layer devices.

Table 2 Summary of EL device performance of fluorene derivatives based on non-doped single-layer Device A structure.

a b c d e f g

Emitter/thickness (nm)

Cathode

Vona (V)

V20b (V)

gl,20c (cd/A)

gp,20d (lm/W)

L20e (cd/m2)

EQEf (%)

kEL(fwhm)g (nm)

CIE(x, y)

1 2 3

LiF/Al LiF/Al LiF/Al

3.4 3.5 2.9

5.12 6.7 4.27

1.54 1.8 2.85

0.95 0.9 2.10

308 367 570

2.0 1.1 1.18

440(60) 476(80) 488(90)

0.153, 0.088 0.165, 0.243 0.200, 0.400

PhN-OF(1)-TAZ (70) PhN-OF(2)-TAZ (72) PhN-OFOT-TAZ (70)

Turn on voltage when a brightness of 1 cd/m2 observed. Voltage taken at a current density of 20 mA/cm2. Current efficiency at a current density of 20 mA/cm2. Power efficiency at a current density of 20 mA/cm2. Luminance at a current density of 20 mA/cm2. External quantum efficiency at a current density of 20 mA/cm2. Full-width at half-maximum.

Fig. 5. Energy levels of emitters and the transporting-layer materials. Fig. 6. EL efficiency-current density characteristics of single-layer devices.

layer. Fig. 3 depicts the normalized electroluminescence (EL) spectra of the non-doped single-layer OLEDs (Device A). The EL maxima peaked at 440–476 nm with full width half maximum (FWHM) of 60–80 nm are in the deep-blue to blue range for the devices of PhN-OF(1)-TAZ and PhNOF(2)-TAZ. While for PhN-OFOT-TAZ-based single-layer OLEDs, the EL maxima are in the sky-blue to greenish blue region of 488–512 nm with FWHM of 60–80 nm (see Supporting Information Fig. S1 and Table S1). The EL

spectra of these devices are nearly identical to the PL spectra of these fluorescent dyes in thin film, indicating that all of the EL emissions originated from the singlet-excited state of the blue emitters. There is only very small EL color shift of all the single-layer devices with varying driving current. For example, the CIEx,y color coordinates of Device A with the configuration of ITO/CFx/PhN-OF(1)-TAZ

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(120 nm)/LiF(1 nm)/Al only shifted from (0.156, 0.145) at 0.2 mA/cm2 to (0.161, 0.149) at 400 mA/cm2 with DCIEx,y = (0.005, 0.004). Fig. 4 shows the dependence of current density–voltage–luminance (J–V–L) characteristics of the single-layer devices. A relatively low turn-on voltage (at a brightness of 1 cd/m2) ranging from 2.9 to 3.4 V is observed for these single-layer devices (Table 2). With an increase of layer thickness, device 4 exhibited slight inferior EL performance to device 1, while slight improvement of device performance was observed of the device 5, device 6, and device 7 as compared to device 4 when an electron-transporting layer (TPBi) was added for the single-layer devices, which indicates that a thin TPBi layer is assisting the injection from cathode to PhN-OF(1)-TAZ because of the shallow LUMO of PhN-OF(1)-TAZ compared to that of TPBi. However, adding a thin TPBi layer did not improve the device efficiency for PhN-OFOT-TAZ when EL performance of device 10 was compared with those of device 11 and device 12, which could be attributed to the closely aligned LUMOs of PhN-OFOT-TAZ and TPBi. (see Supporting Information Table S1 and Figs. S1–S3, and Fig. 5). Detailed EL performances of all devices are summarized in Table 2 and Supporting Information Table S1. First of all, single-layer device 1 with emitter PhN-OF(1)-TAZ has a external quantum efficiency of 2.0% and a low driving voltage of 3.4 V in the deep blue region. Among these blue emitters, PhN-OFOT-TAZ gives the best device performance, which could be attributed to its comparatively greenish spectrum and green emission devices usually give better device performance according to photopic luminosity function. Device 3 (see Table 2 and Supporting Information Table S1) shows the highest efficiency among the single-layer devices. The current and power efficiencies of device 3 are 2.85 cd/A (EQE = 1.18%) and 2.10 lm/W with a luminance of 570 cd/m2 at 20 mA/cm2, respectively (see Fig. 6 and Table 2). An addition of 2-TNATA layer could not improve the device performance, which could be attributed to the thick layer of 2-TNATA which do not facilitate the hole injection from anode to PhN-OF(2)-TAZ well. However, for Device 2, PhN-OF(2)-TAZ-based OLED, current efficiency, luminance and EQE of the device were significantly improved by inserting Alq3 as an ETL in Device 9, which can be attributed to the more greenish spectrum compared to that of device 2 deriving from an exciplex formation between Alq3 and PhN-OF(2)-TAZ [47]. Despite a higher driving voltage, Device 9 exhibits a current efficiency up to 2.9 cd/A with EQE of 1.3% and a luminance of 588 cd/m2 at 20 mA/cm2, which indicates the potential of device structure optimization for multi-layer devices. Besides, color-tunable of the oligomers by changing the p-conjugation core facilitates the different needs for OLED utility.

4. Conclusion In summary, a homologous series of diphenylamino and triazole-conjugated oligofluorenes and oligothienylfluorene for efficient blue-light emission of non-doped EL devices have been synthesized and investigated. This class

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of materials show better promising OLED performance than those of the non-conjugated counterparts while keeping the emission within blue region. The single-layer OLED of PhN-OF(1)-TAZ shows a maximum current efficiency of 1.54 cd/A at 20 mA cm2 with external quantum efficiency (EQE) of 2.0% and CIE coordinates of (0.153, 0.088) in deep blue region which are among the best performance reported for fluorene-based materials in a non-doped single layer device. The single-layer PhN-OF(2)-TAZ-based OLED has a maxima brightness of 5349 cd/m2 and a maxima current efficiency of 1.9 cd/A with CIE coordinates of (0.16, 0.23) while the single-layer device of PhN-OFOT-TAZ shows a maxima brightness of 7524 cd/m2 and a maxima current efficiency of 2.9 cd/A with CIE coordinates of (0.20, 0.40) in sky blue to greenish blue region. Furthermore, material with lower conjugation length exhibits better electroluminescence than the longer one, indicating that the material property of this series can be tailored and optimized. Introduction of thienyl ring into the conjugation system could modulate not only the device efficiency but also the fluorescence range. Our findings show that these fluorene derivatives could serve as efficient blue emitters for OLED applications. Acknowledgments We acknowledge General Research Fund (GRF) (HKBU 202408), Hong Kong Research Grant Council, SAR Hong Kong, Zhejiang Provincial Natural Science Foundation of China (ZJNSF LY13E030005) and Hangzhou Normal University (20100836QD) for financial support for this work. Appendix A. Supplementary material Supplementary material associated with this article can be found, in the online version, at http://dx.doi.org/ 10.1016/j.orgel.2013.10.019. References [1] C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913–915. [2] L.S. Huang, C.H. Chen, Mater. Sci. Eng. R 39 (2002) 143–222. [3] F.-C. Chen, T. Yang, M.E. Thompson, J. Kido, Appl. Phys. Lett. 80 (2002) 2308–2310. [4] B.W. D’Andrate, S.R. Forrest, Adv. Mater. 16 (2004) 1585–1595. [5] B. Chen, J. Ding, L. Wang, X. Jing, F. Wang, Chem. Commun. 48 (2012) 8970–8972. [6] M. Zhang, S.F. Xue, W.Y. Dong, Q. Wang, T. Fei, C. Gu, Y.G. Ma, Chem. Commun. 46 (2010) 3923–3925. [7] D. Yokoyama, Y. Park, B. Kim, S. Kim, Y.-J. Pu, J. Kido, J. Park, Appl. Phys. Lett. 99 (2011) 123303. [8] I. Cho, S.H. Kim, J.H. Kim, S. Park, S.Y. Park, J. Mater. Chem. 22 (2012) 123–129. [9] H. Park, J. Lee, I. Kang, H.Y. Chu, J.-I. Lee, S.-K. Kwon, Y.-H. Kim, J. Mater. Chem. 22 (2012) 2695–2700. [10] T.-C. Tsai, W.-Y. Hung, L.-C. Chi, K.-T. Wong, C.-C. Hsieh, P.-T. Chou, Org. Electron. 10 (2009) 158–162. [11] Y. Yang, P. Cohn, L. Dyer, S.-H. Eom, R. Reynolds, K. Castellano, J. Xue, Chem. Mater. 22 (2010) 3580–3582. [12] L. Fisher, E. Linton, T. Kamtekar, C. Pearson, R. Bryce, C. Petty, Chem. Mater. 23 (2011) 1640–1642. [13] Q.-X. Tong, S.-L. Lai, M.-Y. Chan, Y.-C. Zhou, H.-L. Kwong, C.-S. Lee, S.T. Lee, T.-W. Lee, T. Noh, O. Keon, J. Phys. Chem. C 113 (2009) 6227– 6230. [14] Y. Shirota, M. Kinoshita, T. Noda, K. Okumota, T. Ohara, J. Am. Chem. Soc. 122 (2000) 11021–11022.

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