anthracene hybrids for efficient blue organic light-emitting diodes

Displays 34 (2013) 447–451

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Novel carbazole/anthracene hybrids for efficient blue organic light-emitting diodes Ting Zhang a,b, Hao Dai a, Jiuyan Li a,⇑ a b

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China Department of Chemistry, Anhui Science and Technology University, Fengyang, Anhui 233100, China

a r t i c l e

i n f o

Article history: Available online 23 August 2013 Keywords: OLEDs Blue Fluorescence Carbazole Anthracene

a b s t r a c t Two novel carbazole/anthracene hybrided molecules, namely 2-(anthracen-9-yl)-9-ethyl-9H-carbazole (AnCz) and 2,7-di(anthracen-9-yl)-9-ethyl-9H-carbazole (2AnCz), were designed and synthesized via palladium catalyzed coupling reaction. The anthracene was attached either at the 2-site (AnCz) or at both 2,7-sites (2AnCz) of the central carbazole core to tune the conjugation state and the optoelectronic properties of the resultant molecules. Both of them show good solubility in common organic solvents. They also possess relatively high HOMO levels ( 5.39 eV, 5.40 eV) that would facilitate efficient hole injection and be favorable for high power efficiencies when used in organic light-emitting devices (OLEDs). AnCz and 2AnCz were used as non-doped emitter to fabricate OLEDs by vacuum evaporation. Good performance was achieved with maximum luminance efficiency of 2.61 cd A 1 and CIE coordinates of (0.15, 0.12) for AnCz, and 9.52 cd A 1 and (0.22, 0.37) for 2AnCz. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Organic light-emitting diodes (OLEDs) have attracted a great deal of attention because of their promising applications in fullcolor, large-area flat-panel displays and solid-state lighting [1,2]. Compared to green-light-emitting materials, many efforts are still needed to further improve the performance of blue-light-emitting materials, particularly in terms of luminescent efficiency and color purity [3]. In recent years, much progress has been made to improve deep blue electroluminescence (EL) color with a Commisssion Internationale de L’Eclairage (CIEy) coordinate value of y < 0.15 [4,5]. Non-doped OLEDs fabricated with host emitters which bear proper charge transporting ability are apparently advantageous in terms of ease of device fabrication, low cost especially in mass production, and absence of phase separation or spectra shift with driving voltage. Among the three primary color emitting materials, blue host emitters are especially more significant than other two color ones. Based on their high excited energies, excellent blue-light-emitting materials can not only effectively reduce the power consumption of a full-color OLED panel, but also be served as hosts for exothermic energy transfer to lower-energy fluorophores to facilitate efficient white and other color emission [6]. In fact, the large-band-gap host materials are

⇑ Corresponding author. Tel.: +86 41184986233. E-mail address: [email protected] (J. Li). 0141-9382/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.displa.2013.08.001

relatively rare and always keep as one of the most challenging targets of OLED research. It is thus important to develop high-performance blue-light-emitting materials with good stability and high fluorescent efficiency. Maintaining efficient carrier injection in the light-emitting device is also considered to be a significant issue for achieving high device performance. In this paper, we report the synthesis and luminescent properties of two novel carbazole-based deep blue emitters with anthracene endcaps, namely AnCz and 2AnCz (Scheme 1). Carbazole-based materials have been well known for their excellent thermal stability, hole transporting properties, versatile structural derivatization, and unique ability to form amorphous film [7]. Anthracene is important building block to construct blue luminescent dyes due to its merits such as the excellent fluorescent quantum yield, thermal and electrochemical stability, and ease to be modified [5]. Herein, we select carbazole as the core and anthracene as the end caps at the C2 or C2,7 positions of carbazole. Qiu et al. reported a series of solution-processible carbazole-based host materials, and demonstrated that the 2,7-substituted carbazole compounds show hypsochromic shift in photoluminescence compared to the 3,6-substituted carbazole analogues. We select the carbazole as the core and 9-naphthylanthracene as the end caps at the C2 and C7 positions of carbazole with the expectation to construct true blue emissive materials by this unique linkage mode [8,9]. Furthermore, these molecules are well soluble due to the alkyl substituents at the N9 position of carbazole moieties. The blue OLEDs containing AnCz and 2AnCz as a non-doped emitter show

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T. Zhang et al. / Displays 34 (2013) 447–451

Scheme 1. Synthetic routes for AnCz and 2AnCz.

luminance efficiency of 2.61 cd A 0.12).

1

with CIE coordinates of (0.15,

2. Experimental The synthetic routes for the carbazole-cored derivatives with anthracene endcaps (AnCz, 2AnCz) are shown in Scheme 1. They were prepared by C–C coupling reaction of the boronic acid of the anthracene endcaps (7) and the brominated carbazole core (3 or 6). First, the important intermediates for the synthesis of these target molecules, including anthracene-9-boronic acid (7) [8], 2-bromo-9-ethyl-9H-carbazole (3) and 2,7-dibromo-9-ethyl-9Hcarbazole (6) [10,11], were synthesized according to literature methods, as described in Scheme 1. Finally the target compounds AnCz and 2AnCz were prepared at a high yield of 90% and 60% by Suzuki coupling reactions of the boronic acid 7 with the brominated carbazole 3 and 6, respectively. All the products are highly soluble in common organic solvents such as dichloromethane, chloroform, or toluene and were purified conveniently by column chromatograph and recrystallization to an excellent purity for OLEDs application. The chemical structures of AnCz and 2AnCz were confirmed by 1H NMR spectroscopy, mass spectrometry (TOF-MS-EI), and elemental analysis. 2-(anthracen-9-yl)-9-ethyl-9H-carbazole (AnCz): A mixture of 7 (486 mg, 2.19 mmol), 3 (500 mg, 1.82 mmol) and Pd(PPh3)4 (105 mg, 0.091 mmol) in toluene (15 mL) and 2 M aqueous Na2CO3 solution (4.6 mL, 9.2 mmol) was degassed by pump. The solution was heated at 80 °C for 18 h under argon. After the reaction mixture was cooled to room temperature, dichloromethane and water were added. The organic layer was separated and washed with diluted HCl and brine, then dried over anhydrous MgSO4. The solvent was removed under vacuum and the residue was purified by column chromatograpy over silica gel with petroleum ether/CH2Cl2 (5:1) as the eluent to give AnCz as a light yellow solid (609 mg, 90% yield). 1H NMR (400 MHz, CDCl3, d): 8.52 (s, 1H; ArH), 8.29 (d, J = 8.0 Hz, 1H; ArH), 8.22 (d, J = 8.0 Hz, 1H; ArH), 8.08 (d, J = 12.0 Hz, 2H; ArH), 7.75 (d, J = 8.0 Hz, 2H; ArH), 7.52–7.44 (m,

5H; ArH), 7.35–7.28 (m, 4H; ArH), 4.34 (q, 2H; CH2), 1.40 (t, J = 8.0 Hz, 3H; CH3); MS (TOF-MS-EI, m/z): calcd. for C28H21N 371.47, found 371.17. Anal. calcd. for C28H21N: C, 90.53; H, 5.70; N, 3.77. Found: C, 90.17; H, 5.91; N, 3.91. 2,7-di(anthracen-9-yl)-9-ethyl-9H-carbazole (2AnCz): 2AnCz was prepared according to the method used for AnCz by using 7 (453 mg, 2.04 mmol), 6 (300 mg, 0.85 mmol) and Pd(PPh3)4 (98 mg, 0.085 mmol) in toluene (15 mL) and 2 M aqueous Na2CO3 solution (4.25 mL, 8.5 mmol). The crude product was purified by column chromatograpy over silica gel with petroleum ether/CH2Cl2 (4:1) as the eluent to give 2AnCz as a light yellow solid (265 mg, 60% yield). 1H NMR (400 MHz, CDCl3, d): 8.55 (s, 2H; ArH), 8.41 (d, J = 8.0 Hz, 2H; ArH), 8.10 (d, J = 8.0 Hz, 4H; ArH), 7.80 (d, J = 12.0 Hz, 4H; ArH), 7.55 (s, 2H; ArH), 7.50–7.47 (t, J = 8.0 Hz, 4H; ArH), 7.40–7.34 (q, 6H; ArH), 4.34 (q, 2H; CH2), 1.40 (t, J = 8.0 Hz, 3H; CH3); MS (TOF-MS-EI, m/z): calcd. for C28H21N 547.69, found 547.23. Anal. calcd. for C42H29N: C, 92.11; H, 5.34; N, 2.56. Found: C, 92.47; H, 5.11; N, 2.42. Thermogravimetry analyses (TGA) and differential scanning calorimetry (DSC) measurements were carried out using a Perkin–Elmer thermogravimeter (Model TGA7) and a Netzsch DSC 204 at a heating rate of 10 °C min 1 under a nitrogen atmosphere, respectively. The UV–vis absorption and photoluminescence (PL) spectra of AnCz and 2AnCz were measured in CH2Cl2 solutions and the solid films on quartz plate. The fluorescence quantum yields were determined against quinine sulfate as the standard (U = 0.55, in 0.1 N H2SO4) [12]. Electrochemical measurements were made by using a conventional three-electrode configuration and an electrochemical workstation (BAS 100B, USA) at a scan rate of 100 mV s 1. A glassy carbon working electrode, a Pt-wire counter electrode, and a saturated calomel electrode (SCE) as reference electrode were used. All measurements were made at room temperature on samples dissolved in dichloromethane, with 0.1 M tetra-n-butylammonium tetrafluoroborate (Bu4NPF6) as the electrolyte, ferrocene as the internal standard [13]. The patterned ITO substrates were cleaned by successive ultrasonications in detergent, deionized water, ethanol, and dichloromethane, followed by treatment with UV–Ozone for 20 min.

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T. Zhang et al. / Displays 34 (2013) 447–451

PEDOT/PSS (Bayer AG) was spin-coated on pre-treated ITO substrates from aqueous dispersion and baked at 120 °C for 40 min in air. All the organic layers were deposited by vacuum evaporation in a vacuum chamber with a base pressure less than 10 6 torr. The cathode was completed through thermal deposition of LiF (1 nm) and then capping with Al metal (100 nm). The emitting area of each pixel is determined by overlapping of the two electrodes as 9 mm2. The EL spectra, CIE coordinates, and current–voltage–luminance relationships of devices were measured with computercontrolled Spectrascan PR 705 photometer and a Keithley 236 source-measure-unit. All the measurements were carried out at room temperature under ambient conditions.

(a)

AnCz 2AnCz

3. Results and discussion

0

100

200

300

400

500

600

Temperature ( )

(b)

2nd heating

Endothermic

The thermal properties of AnCz and 2AnCz were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), the data are listed in Table 1. As shown in Fig. 1a, AnCz and 2AnCz exhibit good thermal stability with decomposition temperatures (Td) (corresponding to 5% weight loss) at 308 and 425 °C, respectively. This is an essential merit for organic light-emitting materials especially when they are used under high temperature. Fig. 1b illustrates the DSC thermograms for AnCz. When the AnCz powder was heated for the first run, only the melting point (Tm) was detected at 187 °C. An endothermic transition due to glass transition (Tg) was observed at 79 °C in the 2nd heating circle of the sample. However, even the melting was not observed for 2AnCz under the present measuring conditions. The photophysical properties of AnCz and 2AnCz were investigated by means of electronic absorption and steady state photoluminescence (PL) measurements for both dilute solutions in dichloromethane and the solid films on quartz plates. The pertinent data are summarized in Table 1. As shown in Fig. 2a, The absorption spectrum of AnCz in dilute solution exhibit the characteristic vibronic patterns of the isolated anthracene group with three bands at 350, 368, and 387 nm [14]. A red shift of 6 nm was detected in the absorption spectrum of AnCz thin film in comparison with that of the solution. Upon photoexcitation at the absorption maximum, the AnCz solution exhibits deep-blue fluorescence with emission peak at 431 nm. Similar to the absorption, a red shift of 15 nm was detected in the PL spectra of thin film. The absorption spectra of 2AnCz have similar features with those of AnCz. 2AnCz solution emits blue fluorescence with emission peaks at 433 nm, as shown in Fig. 2b. But it should be noted that the PL spectrum of 2AnCz thin film is quite differ from that of the solution. The broad spectral profile and the large red-shift for 2AnCz film are probably caused by excimer formation [15]. The fluorescence quantum yields (U) of these molecules were determined in dilute dichloromethane solutions as 63% and 58% for AnCz and 2AnCz, respectively, relative to the quinine sulfate as a standard [12]. Fig. 3 shows the cyclic voltammograms of AnCz and 2AnCz. The HOMO energy levels were estimated from the onset potential of the first oxidation wave (Eonset 5.39 and 5.40 eV for AnCz ox ) to be and 2AnCz respectively, according to the equation of HOMO (eV) = (Eonset + 4.8 eV) [13]. Apparently the HOMO levels of these ox molecules are quite close to the widely-used hole-transporting material NPB ( 5.30 eV) [7], indicating that small hole-injecting

1st heating

Tg = 79

60

40

80

100

80

120

160

200

Temperature ( ) Fig. 1. TGA traces of AnCz and 2AnCz (a) and the DSC thermograms for AnCz (b).

barrier can be expected at the NPB/emitting layer interface when they are used together to fabricated OLEDs. The optical band gaps were determined by the absorption edge technique [13]. The LUMO levels were calculated by subtracting the gap from the energy of the HOMO. The detailed electrochemical and electronic data of these molecules are listed in Table 1. Because of their low ionization potentials, good thermal stabilities, and high fluorescence quantum yields, these materials show great potential for use as emitters in OLEDs. Blue-emitting devices with the configuration of ITO/PEDOT:PSS (40 nm)/NPB (40 nm)/ AnCz (device I) or 2AnCz (device II) (15 nm)/TPBI (30 nm)/LiF (1 nm)/Al (200 nm) were fabricated. In these devices, PEDOT:PSS was used as the hole-injection layer, NPB as the hole-transporting layer (HTL) and TPBI as the electron-transporting and hole-blocking layer (ETL), respectively. A 15 nm thick AnCz or 2AnCz film is deposited as the non-doped emitting layer (EML). The devices show the emission peak at 452 nm and CIE coordinates of (0.15, 0.12) for AnCz, and 500 nm and CIE coordinates of (0.22, 0.37) for 2AnCz, respectively, as shown by the EL spectra in Fig. 4a. In addition, the EL spectra are almost identical in spectral features to their PL of films, except with a red-shift of 6–7 nm. This is a commonly observed phenomena that is probably caused by the

Table 1 Summary of physical parameters of AnCz and 2AnCz. Comp.

AnCz 2AnCz

kem max (nm)

kabs max (nm) CH2Cl2

Film

CH2Cl2

Film

350, 368, 387 350, 369, 388

393,373, 357 394,375, 357

431 433

446 493

U (%)

Td (°C)

63 58

308 425

HOMO (eV)

5.39 5.40

Eg (eV)

3.05 3.05

LUMO (eV)

2.34 2.35

T. Zhang et al. / Displays 34 (2013) 447–451

Absorbance (a.u.)

Solution Film

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 300

350

400

450

500

550

600

and a maximum luminance efficiency of 9.52 cd A 1 (at 7.5 V) were obtained. The efficiency roll-off with increasing current density was observed for both devices. This is a frequently observed phenomena for both fluorescent and phosphorescent OLEDs. In comparison with the similar compounds reported in literature [5], the present AnCz device still remained a high efficiency if compared in the same current density range. While the 2AnCz device exhibited a more severe efficiency decrease, which should be because of the excimer formation in the OLED. The detailed performance of both device I and II are summarized in Table 2. Apparently, the performances of AnCz based device I are close to the best data ever reported for non-doped deep-blue fluorescent OLEDs with CIE coordinate of y < 0.15.

0.0 650

1.0

Solution Film

1.0 0.8

0.8 0.6

0.6

0.4

0.4

0.2

0.2

0.0 300

350

400

450

500

550

600

Fig. 2. Absorption and fluorescence spectra of (a) AnCz and (b) 2AnCz in dilute dichloromethane solutions and in solid films.

AnCz 2AnCz

Device I Device lI

0.6 0.4 0.2

400

500

600

700

800

Wavelength (nm) 400

0.0 650

Wavelength (nm)

(a)

0.8

0.0

10000

(b) Current density (mA/cm2)

(b)

PL Intensity (a.u.)

Absorbance (a.u.)

1.0

Normalized Intensity (a.u.)

Wavelength (nm)

Device I Device lI

300

1000

200

100

100

10

0

4

5

6

7

8

9

10

Luminance (cd/m2)

(a)

1.0

PL Intensity (a.u.)

450

1

Voltage (V)

0.0

0.5

1.0

1.5

2.0

Potential (V vs Fc/Fc+) Fig. 3. Cyclic voltammograms of AnCz and 2AnCz measured in CH2Cl2 at a scan rate of 100 mV s 1.

influence of the electrical field on the excited states in most organic light-emitting diodes [16]. The current density–voltage–luminance (J–V–L) characteristics and efficiency versus current density curves of the devices I and II are illustrated in Fig. 4b and c. The AnCz based device I exhibited a maximum brightness of 3111 cd m 2 at 9.0 V, and a maximum luminance efficiency (gL) of 2.61 cd A 1 at 6.5 V, corresponding to a peak power efficiency (gp) of 1.26 lm W 1. While for 2AnCz based device II, a maximum luminance of 9240 cd m 2 (at 10 V)

Luminance efficiency (cd/A)

12

(c)

Device I Device lI

9

6

3

0

0

40

80

120

160

200

240

280

Current density (mA/cm2) Fig. 4. Characteristics of devices I and II. (a) EL spectra, (b) current density–voltage– brightness (J–V–B) curves, and (c) plots of the luminance efficiency (gL) as a function of current density (J) for devices I and II.

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T. Zhang et al. / Displays 34 (2013) 447–451 Table 2 Electroluminescence data for the devices I and II. Device

Vturn-on (V)

Lmax (cd m

I II

4.0 4.8

3111, 9.0 9240, 10

2

, V)

gL(max) (cd A 1, V)

gp(max) (lm W 1, V)

kmax (nm)

CIE (x, y)

2.61, 6.5 9.52, 7.5

1.26, 6.5 4.12, 7.0

452 500

0.15, 0.12 0.22, 0.37

4. Conclusion

References

In summary, novel carbazole-based deep-blue-emitting materials have been developed by attaching anthracene endcaps at the 2or 2,7-positions of carbazole core. Both molecules are thermally stable and have good solubility in organic solvents. A luminance efficiency of 2.61 cd A 1 (1.26 lm W 1) and an almost standard blue coordinates (0.15, 0.12) were obtained in the non-doped OLEDs with AnCz as emitter, which is closed the best performance ever reported for deep-blue OLEDs with a low CIE coordinate y < 0.15. Bluish-green emission with extremely high efficiency of 9.52 cd A 1 was also obtained for 2AnCz based OLED, which contains the component of excimer emission.

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Acknowledgements We thank the National Natural Science Foundation of China (21072026 and 20923006) and the Fundamental Research Funds for the Central Universities (DUT12ZD211) for financial support of this work.