High thermal stability 3, 6-fluorene-carbazole-dendrimers as host materials for efficient solution-processed blue phosphorescent devices

Tetrahedron 69 (2013) 4169e4175

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High thermal stability 3, 6-fluorene-carbazole-dendrimers as host materials for efficient solution-processed blue phosphorescent devices Yinbo Qian a, *, Feng Cao a, Wenping Guo b a b

School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, PR China Wuhan National Laboratory for Optoelectronics, Wuhan 430074, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 November 2012 Received in revised form 23 March 2013 Accepted 2 April 2013 Available online 6 April 2013

A novel series of solution-processable 3,6-disubstituted-fluorene-carbazole based host materials 36FCzG1 and 36FCzG2 are designed and synthesized. Owing to the highly asymmetry tetrahedral configuration, these hosts exhibit high glass transition temperatures (Tg) (161 and 162
C, respectively), high triplet energy levels (2.80 and 2.80 eV, respectively), excellent film forming capabilities, and chemical miscibility. Phosphorescent organic lighting-emitting diodes (OLEDs), which base on these host materials doped with the guests of iridium(III) bis(4,6-difluorophenylpyridinato)-picolinate (FIrpic) by spin coating, possesses a low turn-on voltage of 4.0 V, a maximum efficiency of 18.5 cd/A (8.1 lm/W), and a maximum external quantum efficiency of 10.3%. These results show that the devices are among the excellent solution processable blue phosphorescent OLEDs based on dendrimers. Furthermore, a novel way is developed to construct solution processable small molecules based on 3,6-disubstituted fluorene and carbazole dendrimers combined in a highly rigid configuration. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: PhOLED Dendrimer Solution-processable Carbazole Fluorene

1. Introduction Organic light-emitting diodes (OLEDs) have attracted much attention as it has been recognized as the next-generation highquality full color displays.1e5 The efficiencies of OLEDs have been improved dramatically because of the development of efficient phosphorescent hosts & dopants containing transition metals that can harvest both singlet and triplet excitons for emission, providing the opportunity to realize internal quantum efficiency reach to 100% theoretically.6e10 To date, most of the efficient PhOLEDs have been fabricated through vacuum thermal evaporation in multilayer configurations.11e14 But it requires complex technological processes and a large amount of organic materials are wasted, leading to relatively high fabrication costs, at the same time, pixilation limits large-size scalabilities, and high-resolution applications.15 Solution processes offers an attractive alternative to vacuum deposition techniques, mainly due to their better compatibility with low-cost production techniques and large substrates. Compared with success of developing the red and green PhOLEDs fabricated by solution processing,16,17 high performance solution-processed blue PhOLEDs are still scarce, owing to the lack of appropriate host materials. Generally, for high efficient solution-processed blue

* Corresponding author. E-mail address: [email protected] (Y. Qian). 0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2013.04.004

PhOLEDs, the hosts would possess the following properties: (i) A high triplet energy gap (>2.70 eV), which prevents the reverse energy transfer from the dopant to the host; (ii) Suitable highest occupied/lowest unoccupied molecular orbital (HOMO/LUMO) energy levels matching those of the adjacent layers are required to reduce the operational voltage; (iii) Good solubility and film formation ability. Recently, solution-processed blue PhOLEDs using small organic molecules and conjugated polymers as host materials have been reported.18e22 For polymer host materials, impurities, especially the metal catalyst used in polymer synthesis would dramatically influence the performance of OLEDs. In contrast to the polymer hosts, the small molecule hosts can be easily synthesized and purified, but the configuration isn’t rigid enough so that the material would be recrystallized easily. Dendrimer hosts combine the advantages of both the polymers and the small molecules. As far as we know, almost all of them incorporates a solubilizing group such as an alkyl or alkoxy chain to overcome their poor solubilities and film-forming abilities. It is well known that alkyl or alkoxy groups are electrically insulating, and introducing such groups into a molecule to affect its conductivity. Even worse, by attaching alkyl or alkoxy chains to a molecule, its glass-transition temperature (Tg) drops rapidly, which reduces morphological stability, especially when the molecule is small.23,24 Thus it is very difficult for dendrimers to possess both morphological stability and

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solution processability, especially for blue phosphorescent host materials whose effective conjugation length must be very short to encompass the phosphor emitter. Kakimoto et al. reported that solution-processed green PhOLEDs hosted by triphenylamine/ benzimidazole hybrids, and a maximum current efficiency of 27.3 cd/A was realized by using fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) as a guest, but this host material couldn’t be used as the blue phosphorescent for the low triplet energy level.25 Usluer et al. designed a series of compounds including the 2,7disubstituted fluorene derivatives and carbazole dendrimers, a maximum current efficiency of 7.7 cd/A was harvested by using the Alq3 as emitter layer and these compounds as hole transport layer (HTL).26 Qiu et al. also reported a series of carbazole dendrimer hosts, a maximum current efficiency of 11.5 cd/A was achieved based on the deep-blue phosphorescent emitter FIr6, but the electron transporting material OXD-7 was also added for the double hosts.27 As we all know, 2,7-disubstituted fluorene is often used as common groups in the fluorescence emitters for both high quantum yield and excellent film formation ability, but it isn’t suitable for blue PhOLED materials for the low triplet energy levels. Recently, 3,6-substituted fluorene derivatives were reported as the promising blue hosts for polymer PhOLEDs.28 However, blue PhOLEDs using 3,6-substituted fluorene derivatives as dendrimer host materials have not been explored. In this study, we design and synthesis a series of 3,6-substituted fluorene derivatives including carbazole dendrimers 9,90 -(9-phenyl-9-(9-phenyl-9H-carbazol-3-yl)-9H-fluorene-3,6-diyl)bis(3,6-di-tert-butyl-9H-carbazole) (36FCzG1), 9,90 -(9-phenyl-9-(9-phenyl-9H-carbazol-3-yl)-9Hfluorene-3,6-diyl)bis(3,6-bis(3,6-di-tert-butyl-9H-fluoren-9-yl)-9H -carbazole) (36FCzG2), using a 3,6-substituted fluorene derivatives as the rigid core with two carbazole dendrimer groups linked to the core part through the 3,6-positions. Additionally, tert-butyl groups are introduced into the molecular structure to ensure good solubility and to form high-quality films. As a result, the newly synthesized compounds possess three important characteristics: (I) relative high triplet energy levels (2.80 eV) because of the nonconjugated linkage; (II) appropriate HOMO energy levels (5.21 to 5.36 eV), thereby facilitating the transfer of holes from poly(3,4ethyle-nedioxy-thiophene): poly(styrene-4-sulfonate) (PEDOT: PSS) to the emitting layer; (III) the high thermal stability of forming stable amorphous thin films as a result of the highly twisted configuration of the molecules. In our work, with the newly synthesized hosts and the blue phosphorescent emitter FIrpic, the device performance reaches a maximum efficiency value of 18.5 cd/A. This series of molecules exhibits excellent thermal and morphological stability, good film-forming ability, and solubility making them very promising candidates for optoelectronics.

Scheme 1. Synthetic routes toward the compounds 36FCzG1 and 36FCzG2.

All compounds were purified using the silica gel column method, producing very pure powders. 1H, 13C NMR, mass spectrometry, and elemental analysis were employed to confirm the chemical structures of above mentioned compounds as described in the Experimental section. This result matches the 3D model of the two compounds optimized by the Amsterdam Density Functional 2009.01 (ADF2009.01) program, which is to be discussed in the following section. 3. Thermal analysis For a better insight into the structureeproperty relationship, thermal properties were measured. Fig. 1 shows high thermal stability as determined by thermogravimetric analysis (TGA), and the decomposition temperature (Td), corresponding to 5%-weight-loss, is 464 and 461
C for the compounds 36FCzG1 and 36FCzG2, respectively. The Tg explored by differential scanning calorimetry (DSC) reached 161
C for 36FCzG1, and 162
C for 36FCzG2, compared with N,N0 -di(naphthalen-1-yl)-N,N0 -diphenylbiphenyl-4,40 diamine (NPB) (Tg¼98
C), a commonly used material in OLEDs, our compounds show greatly improved thermal resistance.30 It is obviously related to their increased molecular sizes by the introduction of tert-butyl groups in their structures.

100

36FCzG1 36FCzG2

2. Results and discussion

80

60

40

o

161 C

Exothermnic

The synthetic routes and chemical structures of 36FCzG1 and 36FCzG2, are shown in Scheme 1. The carbazole dendrimers G1 and G2, and 3,6-dibromo-9H-fluoren-9-one were synthesized according to a literature method.28,29 3,6-Dibromo-9-phenyl-9H-fluoren9-ol (1) was synthesized by using Grignard Reagent with a high yield (70%). The following reaction of the compound 1 and Nphenylcarbazole losses a H2O molecule through the strong acid to form 3-(3,6-dibromo-9-phenyl-9H-fluoren-9-yl)-9-phenyl-9H-carbazole (2), and the final products are prepared by the classic Ullmann reaction of the compound 2 and the compounds G1 and G2 in the presence of a catalytic amount of copper(I) iodide and 18crown-6 in 1,3-dimethyltetra-hydropyrimidin-2(1H)-one (DMPU) with high yield 85% and 88%, respectively.

Weight (%)

2.1. Synthesis and characterization

50

100

o

162 C

100

150

200

Temperature (oC)

200

300

400

250

500

600

700

Wavelength (nm) Fig. 1. TGA traces of 36FCzG1 and 36FCzG2 recorded at a heating rate of 10
C/min. Inset: DSC measurement recorded at a heating rate of 10
C/min.

Y. Qian et al. / Tetrahedron 69 (2013) 4169e4175

A high Td makes it possible to endure high temperatures accompanied by thermal vacuum sublimation during device fabrication or further purification by gradient sublimation. All the compounds, which show no glass transition within the experimental temperature range, displayed intrinsic amorphous properties. It is a very desirable feature for photoelectronic applications. Furthermore, atomic force microscopy (AFM) was used to investigate the morphology of films prepared from the compounds doped with FIrpic, through spin-coating a chlorobenzene solution onto the PEDOT:PSS layer. As shown in Fig. 2, the surface of the compounds doped with 20 wt % FIrpic, are free of pinholes and quite smooth, and the root-mean-square (rms) values range from 0.55 to 0.44 nm. No phase separation or any aggregated domains were observed. However, the films prepared from mCP doped with FIrpic onto the PEDOT:PSS layer exhibits a kind of aggregation, or phase segregation. The results demonstrate that these new compounds can form uniform amorphous films upon solutionprocessing, which is highly important in improving the efficiency and lifetime of OLEDs.

Fig. 2. AFM topographic images of 36FCzG1 (left) and 36FCzG2 (right) with 20 wt % FIrpic in thin solid films (40 nm thick).

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be 5.80 and 5.82 eV, for 36FCzG1 and 36FCzG2, respectively, according to the corresponding onset potentials of 1.10 and 1.12 eV, respectively. The HOMO energy levels are clearly raised and approach the work function of PEDOT (5.2 eV). These results reveal that the introduction of two carbazole groups linked through the 3,6 positions of the fluorene unit can lead to the reduction of the hole injection barrier, thereby facilitating the injection of positive charge carriers. All of these results together with absorption spectra were used and then the LUMO energy levels are obtained (Table 1). 3.2. Theoretical calculations To gain insight into the relationship of the compound 36FCzG1 at the molecular level, the geometrical and electronic properties of the compounds were studied using density functional theory (DFT) calculations. The geometries of these new compounds in Fig. 4 showed that the carbazole dendrimers and phenyl units are significantly twisted with the fluorene core, resulting in a non-planar structure in each molecule. The calculated HOMO and LUMO levels are 5.40 and 2.13 eV, respectively, which are in good agreement with the experimental results. These geometrical characteristics can effectively prevent intermolecular interactions between psystems and thus suppress molecular recrystallization and limit the extent of conjugation between the central core and the branches. As a result, it improves the morphological stability of thin film and keeps the triplet energy gap at a very high level in these molecules. Calculated HOMO and LUMO density maps of the compounds are also included in Fig. 4. The LUMO levels of all the compounds are localized predominantly on the fluorene core, while the HOMO levels are distributed over the electron-rich tert-butyl-carbazole fragment. The calculated values of the energy levels are in agreement with the results measured by electrochemical CV.

3.1. Electrochemical analysis 4. Photophysical properties The electrochemical properties of the compounds were studied in solution through cyclic voltammetry (CV) using supporting electrolyte tetrabutylammoniumhexafluorophosphate (TBAPF6) (Fig. 3). During the anodic scan in dichloromethane, 36FCzG1, and 36FCzG2 showed reversible oxidation behavior, which could be attributed to the introduction of two tert-butyl groups at the 3,6positions of carbazole, suggesting that the compounds can be stable upon accepting holes in OLEDs. As revealed in the literature,31 it is important to block the active sites of carbazole derivatives when the compounds transport positive charge carriers in devices.

36FCzG1

2

Current (µA/cm )

4

3

36FCzG2 2 0.0

0.2

0.4

0.6

0.8

1.0

1.2

+

Potential (V vs Ag/Ag ) Fig. 3. Cyclic voltammograms of 36FCzG1 and 36FCzG2 in dilute dichloromethane solution.

Fig. 5 exhibits the room-temperature absorption, photoluminescent (PL) spectra of the compounds in CH2Cl2 and the phosphorescent spectra of the compounds in 2-methyltetrahydrofuran and all the results are summarized in Table 1. The absorption spectra of the two compounds exhibit similar patterns with three absorption bands centered at 250e350 nm, which are nearly identical to those of the unsubstituted carbazole monomer and thus can be attributed to the nep* and pep* transitions33 upon UV excitation, the PL spectra lost vibronic structure and the emission peaks red-shifted to 393e398 nm with respect to the carbazole monomer emission. Due to the steric effect of the two tert-butyl groups at the 3,6-positions of the carbazole, the peaks of absorption and emission of 36FCzG2 are shifted to longer wavelength, relative to the compound 36FCzG1. The emission of 36FCzG2 has the shoulder peak at about 360 nm, which would be attributed to the second generation carbazole dendrimer groups. The highest energy 0e0 phosphorescent emission was used to calculate the triplet energy level, giving a value of 2.80 eV for 36FCzG1, 2.80 eV for 36FCzG1, higher than the values of the commonly used triplet blue-emitter FIrpic (2.62 eV).34 Using host materials that possess high triplet energies is a provision for effective confinement of the triplet excitons on the guest, while it prevents back energy transfer between the host and dopant molecules. Based on the results of the measurements, the newly synthesized compounds with high triplet energy levels are expected to serve as appropriate hosts for FIrpic. 5. Electroluminescent devices

The HOMO energy levels were obtained according to the following equation: HOMO¼(4.7þEoox) eV,32 using Ag/Agþ as a reference anode (4.7 eV in a vacuum). These were calculated to

In view of the high triplet energy (ET¼2.80 eV) of 36FCzG1 and 36FCzG2, their applications as host material for blue

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Table 1 The thermal, electrochemical, and photophysical data of 36FCzG1 and 36FCzG2

36FCzG1 36FCzG2 a b c d

Tg/Td (
C)

lmax,abs (nm)a

lem (nm)a

lem (nm)b

ET (eV)

HOMO/LUMOc (eV)

HOMO/LUMOd (eV)

161/464 162/461

347, 296 348, 296

393 398

443 443

2.80 2.80

5.80/2.32 5.82/2.38

5.40/2.13 d/d

Measured in CH2Cl2. Measured in 2-MeTHF glass at 77 K. Deduced from cyclic voltammetry. Estimated from the optical band gap together with HOMO values.

Fig. 4. Optimized geometries and calculated HOMO and LUMO density pictures for 36FCzG1 according to DFT calculations at the B3LYP/6-31* level.

2

200

0.4

0.4

0.2

0.2

0.0 250

300

1.0

350 400 450 Wavelength (nm)

500

0.0 550

36FCzG1 36FCzG2

0.8 0.6 0.4 0.2 0.0 400

450

500 550 600 Wavelength (nm)

650

Fig. 5. (a) Normalized absorption and emission spectra of 36FCzG1 and 36FCzG2 in dilute dichloromethane solution; (b) the normalized phosphorescence spectra of 36FCzG1 and 36FCzG2 in a frozen 2-methyltetrahydrofuran matrix at 77 K.

10

3

10

2

10

1

10

2

10

1

10

0

10

8 10 Voltage (V)

12

10 14

0

36FCzG1 36FCzG2

-1

-2

10 -1 10

(c)

10

0

1

2

10 10 10 Luminance (Cd/m2)

3

10

2

10

1

10

0

10 4 10

Power Efficiency (lm/W)

0.6

Current Efficiency (Cd/A)

0.6

(b) Normalized PL Intensity (a.u)

0.8

Normalized EL Intensity (a.u)

0.8

1.0

Normalized PL Intensity (a.u)

Normalized Abs Intensity (a.u)

36FCzG1 36FCzG2

4

0 6

(b)

10

2

100

(a) 1.0

36FCzG1 36FCzG2

Luminance (cd/m )

(a) 300

Current Density (mA/cm )

electrophosphorescent devices were investigated by a simple three-layer configuration: indium tin oxide (ITO)/poly(3,4ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) (40 nm)/Host: FIrpic (20 wt %) (40 nm)/TmPyPB (40 nm)/LiF (1 nm)/ Al (100 nm). Where the conducting polymer PEDOT:PSS was used as the hole injection layer; FIrpic with an optimized concentration of 20 wt % was used as the emitter; Both TPBI and TmPyPB were used as the electron transporting layer and the hole- and excitonconfining layer; and LiF was used as the electron injection layer. Fig. 6 presents the current densityevoltageseluminance (JeVeL) characteristics and curves of efficiency versus luminance of the devices. According to the JeV characteristic curves of the devices,

-1

1.0 0.8

36FCzG1 36FCzG2

0.6 0.4 0.2 0.0 400

500 600 Wavelength (nm)

700

Fig. 6. (a) Current efficiencyevoltageeluminance (JeVeL) curves of the two devices; (b) Luminanceecurrent efficiency and luminanceepower efficiency characteristics; (c) EL spectra of the two devices.

Y. Qian et al. / Tetrahedron 69 (2013) 4169e4175

Table 2 Summary of the characteristics of FIrpic doped blue phosphorescent devices Host 36FCzG1 36FCzG2 a b c d e f

Von (V)a

Lmax (cd/m2)b

hc,max

hp,max

hext,max

CIE (x,y)f

5.8 5.5

6302 (12.6 V) 7626 (12.4 V)

18.5 16.1

8.1 7.8

10.3 9.7

(0.15,0.31) (0.15,0.31)

(cd/A)c

(lm/W)d

(%)e

Recorded at 1 cd/m2. Maximum luminance. Maximum current efficiency. Maximum powder efficiency. Maximum external quantum efficiency. Measured at 8 V.

ITO

5.2

6

5.5

5.5

2.6 LUMO 2.9 LiF/Al

TmPyPB

4.7

FIrpic CaG2

4

PEDO T:PSS

3.3

2.0 2.1 3.0

CaG1

2

Potential (eV)

the introduction of two tert-butyl groups in the molecules didn’t lead to a decrease in current density, the turn-on voltages (corresponding to 1 cd/m2) of the devices are similar with the previously reported values based on solution processed blue electrophosphorescent devices. The turn-on voltage indicates charge injection, transport, and combination isn’t efficiently, most likely due to the HOMO energy levels of these hosts aren’t well matched with the PEDOT:PSS and thereby the hole injection of the device is difficult. The devices based on 36FCzG1 and 36FCzG2 show a maximum luminance (Lmax) of 6302 cd/m2 (12.6 V) and 7626 cd/m2 (12.4 V), a maximum efficiency (hc,max, hp,max) of 18.5 cd/A, 8.1 lm/ W and 16.1 cd/A, 7.8 lm/W, corresponding to a maximum external quantum efficiency (hext,max) of 10.3% and 9.7%, respectively. The performances of the devices are far superior to those of the corresponding mCP based devices (Lmax of 8900 cd/m2, hc,max of 6.2 cd/ A and hext,max of 3.1%) in previously reported.19 Table 2 summarizes the performance of the devices based on hosts 36FCzG1 and 36FCzG2. As shown in Table 2, this devices performance is also consistent with the view that the introduction of tert-butyl groups into the host materials can reduce the intermolecular interactions and suppress tripletetriplet annihilation in the devices.35

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5.7 6.7 HOMO

Fig. 7. Energy level diagram of HOMO and LUMO levels (relative to vacuum level) for materials investigated in this work.

molecular recrystallization and limit the extent of conjugation, and exhibited excellent film formation ability, high triplet energy level and high thermal stability. Utilizing these new compounds as hosts, the solution-processed blue phosphorescence OLEDs with FIrpic as a dopant show normal turn-on voltage of 4.0 V, a maximum current efficiency of 18.5 cd/A, a maximum efficiency of 8.1 lm/W and a maximum external quantum efficiency 10.3%. This can be attributed to the high triplet energy level and excellent film forming ability. The performances of the devices are far superior to those of the corresponding mCP-based devices, which is outstanding for a solution-processed blue phosphorescent OLED. This work suggests that highly efficient solution processed blue phosphorescence OLEDs can be achieved in the design of such host materials with high thermal stability, high triplet energy, excellent film-forming ability, and morphological property. 7. Experimental

The electroluminescence (EL) spectra of devices are almost identical with the Commission Internationale de L’Eclairage (CIE) coordinates of (0.15,0.31), corresponding to the emission of FIrpic, which was comparable to the literature value reported by Kido’s group,36 with CIE coordinates of (0.16,0.32). When the driving voltage was increased to 12 V, the CIE coordinates of the emission color remained almost unchanged. Furthermore, no additional emission coming from the host materials was observed, indicative of efficient energy transfer from the host to FIrpic. The obtained results are among the best for solution processed blue PhOLEDs based on carbazole dendrimer molecular host materials. The high performance could be attributed to the excellent film forming ability and the high triplet energy of host materials and balanced charge recombination in the devices. For the compounds 36FCzG1 and 36FCzG2, the turn-on voltages of the devices were determined by the LUMO energy barriers between the TmPyPB and the host 36FCzG1 or 36FCzG2 (Fig. 7). The HOMO levels of the TmPyPB are enough to block the exciton diffusion, the LUMO level barriers between the 36FCzG2 and TmPyPB are larger than that of the 36FCzG2, leading to the device hosted by the 36FCzG2 shows the lower turn-on voltage. 6. Conclusions In summary, we have designed and synthesized a new series of host materials 36FCzG1 and 36FCzG2 for solution processed blue phosphorescent organic light-emitting devices. By substitution at the 3,6- and 9,9-positions of fluorene with the rigid tert-butylcarbazole dendrimer units, the novel compounds showed highly twisted configurations, which effectively suppress

7.1. General information 1

H NMR and 13C NMR spectra were measured on a BrukerAF301 AT 400 MHz spectrometer. Elemental analyses of carbon, hydrogen, and nitrogen were performed on an Elementar (Vario Micro cube) analyzer. Mass spectra were carried out on an Agilent (1100 LC/MSD Trap) using ACPI ionization and the final products were characterized by the MALDI-TOF. UVevis absorption spectra were recorded on a Shimadzu UVevis-NIR Spectrophotometer (UV-3600). PL spectra were recorded on Edinburgh instruments (FLSP920 spectrometers). Differential scanning calorimetry (DSC) was performed on a PE Instruments DSC 2920 unit at a heating rate of 10
C/min from 20 to 250
C under nitrogen. The glass transition temperature (Tg) was determined from the second heating scan. Thermogravimetric analysis (TGA) was undertaken with a PerkineElmer Instruments (Pyris1 TGA). The thermal stability of the samples under a nitrogen atmosphere was determined by measuring their weight loss while heating at a rate of 10
C/min from 30 to 700
C. 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 an Ag/AgNO3 (0.1 M) reference electrode on a computercontrolled 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. The onset potential was determined from the intersection of two tangents drawn at the rising and background current of the cyclic voltammogram. The film surface morphology was measured with AFM (Seiko Instruments, SPA-400).

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7.2. Quantum chemical calculations The geometrical and electronic properties were performed with the Amsterdam Density Functional (ADF) 2009.01 program package. The calculation was optimized by means of the B3LYP (Becke three parameters hybrid functional with LeeeYangePerdew correlation functionals)37 with the 6e31G(d) atomic basis set. Then the electronic structures were calculated at s-HCTHhyb/ 6e311þþG(d, p) level. Molecular orbitals were visualized using ADFview. 7.3. Device fabrication and performance measurements All the devices were fabricated by spin-coating process. The ITO substrate with a sheet resistance of 20 U/, was cleaned with the cleaner and deionized water under the ultrasound for 15 min, respectively. Then the ITO was dried in an oven for 3 h. Finally, the ITO was treated with UV-ozone for 5 min. The device structure of the blue PhoLEDs was ITO/PEDOT:PSS (40 nm)/host: FIrpic (20 wt %) (40 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm). A 40 nm PEDOT:PSS (Baytron P VP CH 8000) aqueous solution was spin coated onto the ITO substrate and baked at 120
C for 10 min to remove the residual water. The substrates were then taken into a nitrogen glove box, where the emitting layers were spin coated onto the PEDOT:PSS layer from 1,2-dichloroethane solution and annealed at 80
C for 30 min. The substrate was then transferred into an evaporation chamber, where 40 nm TmPyPB were evaporated at an evaporation rate of 1e2  A/s under a pressure of 4104 Pa and the LiF/Al bilayer cathode was evaporated at evaporation rates of 0.2 and 10  A/s for LiF and Al, respectively, under a pressure of 1103 Pa. The active area of the device was 9 mm2. The EL spectrums, luminance, CIE coordinates, and the currentevoltageeluminance-efficiency characteristics of the devices were measured with a rapid scan system using a Photo Research PR655 spectrophotometer and a Keithley 2400 digital source. All the date of EL characteristics were measured at room temperature under an ambient atmosphere. 7.4. Materials Compounds N-phenylcarbazole,38 compounds G1, G2,29 and the compound 3,6-dibromo-9H-fluoren-9-one28 were prepared according to published procedures. 9,90 -(9-Phenyl-9-(9-phenyl9H-carbazol-3-yl)-9H-fluorene-3,6-diyl)bis(3,6-di-tert-butyl-9Hcarba-zole) (36FCzG1), 9,90 -(9-phenyl-9-(9-phenyl-9H-carbazol-3yl)-9H-fluorene-3,6-diyl) bis(3,6-bis (3,6-di-tert-butyl-9H-fluoren9-yl)-9H-carbazole) (36FCzG2) were synthesized by the same procedure: A mixture of 3-(3,6-dibromo-9-phenyl-9H-fluoren-9yl)-9-phenyl-9H-carbazole (1.5 mmol), 3,6-di-tert-butyl-carbazole (G1) (or 3,300 ,6,600 -tetra-tert-butyl-90 H-9,30 :60 ,900 -tercarbazole (G2)) (3.2 mmol), CuI (0.10 mmol), K2CO3 (6.0 mmol), 18-Crown-6 (0.10 mmol), and DMPU (5.0 mL) was heated at 170
C for 24 h. After cooling, the mixture was treated with water and extracted with dichloromethane. The organic extract was dried over anhydrous MgSO4 with removal of the volatiles. The residue was purified by column chromatography on silica gel using hexaneedichloromethane as the eluent. 7.4.1. 3,6-Dibromo-9-phenyl-9H-fluoren-9-ol (1). Bromobenzene (16.5 g, 105 mmol) diluted in ether was added to magnesium bar (3.0 g, 125 mmol) slowly to get bromobenzene Grignard. The bromobenzene Grignard reagent was added dropwise to 3,6-dibromofluorenone (5.0 g, 14.9 mmol) dissolved in ether. The yellow troubled solution turns into brown. Then the mixture was heated under refluxed overnight. The pale powder was harvested. Yield: 70%. 1H NMR: (CDCl3, 400 MHz): d (ppm) 7.19 (d, J¼8.1 Hz, 2H), 7.25e7.35

(m, 5H), 7.40 (dd, J¼8.1, 1.8 Hz, 2H), 7.77 (d, J¼1.8 Hz, 2H). APCIþMS (m/z): calcd for C19H12Br2O, 413.9; found, 415.2. 7.4.2. 3-(3,6-Dibromo-9-phenyl-9H-fluoren-9-yl)-9-phenyl-9H-carbazole (2). To 0.8 g 2 and 0.577 g 9-penyl carbazole dissolved in CH2Cl2 was added 0.5 mL CF3COOH, then the mixture was stirred at room temperature for 14 h. Ice cold NaHCO3 aqueous was added, and the product was extracted with CH2Cl2. Yield: 70%. 7.4.3. 36FCzG1. White solid. Yield: 65%. 1H NMR: (CDCl3, 400 MHz): d (ppm) 8.13e8.06 (m, 6H), 7.95e7.94 (d, J¼4.0 Hz, 2H), 7.75e7.73 (d, J¼8.0 Hz, 2H), 7.61e7.53 (m, 6H), 7.49e7.35 (m, 19H), 1.45e1.44 (m, 36H). 13C NMR: (CDCl3, 100 MHz): d (ppm) 150.66, 146.16, 142.91, 141.28, 141.17, 139.97, 139.23, 137.94, 137.65, 137.04, 129.88, 128.53, 128.34, 127.57, 127.48, 127.01, 126.64, 126.50, 126.12, 123.64, 123.45, 123.28, 123.23, 120.42, 119.99, 119.54, 118.49, 116.23, 109.87, 109.35, 65.50, 34.72, 32.00. MS (MALDI-TOF): calcd for C77H71N3: 1037.5642, found, 1037.5654. Anal. Calcd for C77H71N3: C, 89.06; H, 6.89; N, 4.05. Found: C, 89.14; H, 6.78; N 4.08. 7.4.4. 36FCzG2. White solid. Yield: 60%. 1H NMR: (CDCl3, 400 MHz): d (ppm) 8.45e8.38 (m, 1H), 8.35e8.28 (m, 1H), 8.24e8.22 (m, 2H), 8.16e8.06 (m, 11H), 8.05e7.72 (m, 7H), 7.65e7.52 (m, 11H), 7.48e7.31 (m, 26H), 1.46e1.45 (m, 72H). 13C NMR: (CDCl3, 100 MHz): d (ppm) 153.10, 150.62, 147.22, 146.26, 142.91, 142.80, 141.28, 141.17, 139.97, 139.25, 138.10, 137.94, 137.65, 137.04, 129.89, 129.84, 129.27, 129.24, 128.53, 128.35, 128.34, 127.57, 127.48, 127.01, 126.64, 126.50, 126.12, 125.53, 123.64, 123.52, 123.45, 123.28, 123.23, 123.21, 123.09, 120.42, 119.99, 119.54, 118.49, 117.94, 116.23, 116.18, 116.10, 114.42, 109.98, 109.87, 109.34, 65.55, 34.73, 32.04. MS (MALDI-TOF): calcd for C144H131N7: 1923.0500, found, 1923.0547. Anal. Calcd for C77H71N3: C, 88.04; H, 6.86; N, 5.10. Found: C, 88.15; H, 6.74; N 5.11. Acknowledgements This work was supported by National Natural Science Foundation of China (NSFC, grant no. 61008050 and no. 41006019). Also thank the Analytical and Testing Centre Huazhong University of Science and Technology for measurements. References and notes € ssem, B.; Leo, K. 1. Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lu Nature 2009, 459, 234. 2. Chang, C.-H.; Cheng, H.-C.; Lu, Y.-J.; Tien, K.-C.; Lin, H.-W.; Lin, C.-L.; Yang, C.-J.; Wu, C.-C. Org. Electron. 2010, 11, 247. 3. Chang, C.-H.; Tien, K.-C.; Chen, C.-C.; Lin, M.-S.; Cheng, H.-C.; Liu, S.-H.; Wu, C.-C.; Hung, J.-Y.; Chiu, Y.-C.; Chi, Y. Org. Electron. 2010, 11, 412. 4. Helander, M. G.; Wang, Z. B.; Qiu, J.; Greiner, M. T.; Puzzo, D. P.; Liu, Z. W.; Lu, Z. H. Science 2011, 332, 944. 5. Wang, Z.-B.; Liu, Z.-W.; Lu, Z.-H.; Helander, M. G.; Qiu, J.; Puzzo, D. P.; Greiner, M. T.; Hudson, Z. M.; Wang, S. Nat. Photon. 2011, 5, 753. 6. Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151. 7. Baldo, M. A.; O’Brien, D. F. Phys. Rev. B: Condens. Matter 1999, 60, 14422. 8. Cho, Y. J.; Lee, J. Y. Adv. Mater. 2011, 23, 4568. 9. Jeon, S. O.; Jang, S. E.; Son, H. S.; Lee, J. Y. Adv. Mater. 2011, 23, 1436. 10. Chou, H.-H.; Cheng, C.-H. Adv. Mater. 2010, 22, 2468. 11. Han, C.-M.; Xie, G.-H.; Xu, H.; Zhang, Z.-S.; Xie, L.-H.; Zhao, Y.; Liu, S.-Y.; Huang, W. Adv. Mater. 2011, 23, 2491. 12. Lu, K. Y.; Chou, H. H.; Hsieh, C. H.; Yang, Y. H.; Tsai, H. R.; Tsai, H. Y.; Hsu, L. C.; Chen, C. Y.; Chen, I. C.; Cheng, C. H. Adv. Mater. 2011, 23, 4933. 13. Jiang, Z. Q.; Chen, Y. H.; Fan, C.; Yang, C. L.; Wang, Q.; Tao, Y. T.; Zhang, Z. Q.; Qin, J. G.; Ma, D. G. Chem. Commun. 2009, 3398. 14. Ho, C.-L.; Chi, L.-C.; Hung, W.-Y.; Chen, W.-J.; Lin, Y.-C.; Wu, H.; Mondal, E.; Zhou, G.-J.; Wong, K.-T.; Wong, W.-Y. J. Mater. Chem. 2012, 22, 215. 15. Awadalla, S. A.; Chen, H.; Mackenzie, J.; Lu, P.; Iniewski, K.; Marthandam, P.; Redden, R.; Bindley, G.; He, Z.; Zhang, F. J. Appl. Phys. 2009, 105, 114910. 16. Cai, J.-X.; Ye, T.-L.; Fan, X.-F.; Han, C.-M.; Xu, H.; Wang, L.-L.; Ma, D.-G.; Lin, Y.; Yan, P.-F. J. Mater. Chem. 2011, 21, 15405. 17. Chang, Y. J.; Chow, T. J. J. Mater. Chem. 2011, 21, 3091.

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