Efficient and stable non-doped deep-blue organic light emitting diode based on an anthracene derivative

Article abstract of DOI:10.1007/s11426-011-4243-9

A new anthracene derivative 9,10-bis[3,5-di(4-tert-butylphenyl)phenyl] anthracene (BPPA) was synthesized via Suzuki coupling reaction and characterized by ¹H NMR spectrum, mass spectrum, and elemental analysis. BPPA exhibits deep-blue emission

Full text of DOI:10.1007/s11426-011-4243-9

SCIENCE CHINA
Chemistry
April 2011 Vol.54 No.4: 666–670
doi: 10.1007/s11426-011-4243-9
• ARTICLES •
Efficient and stable non-doped deep-blue organic light emitting
diode based on an anthracene derivative
WANG ZhiQiang1,2, ZHENG CaiJun1, LIU Heng3,
OU XueMei1 & ZHANG XiaoHong1*
1Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials;
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
2College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471022, China
3Binzhou Polytechnic, Binzhou 256603, China
Received November 26, 2010; accepted January 2, 2011
A new anthracene derivative 9,10-bis[3,5-di(4-tert-butylphenyl)phenyl]anthracene (BPPA) was synthesized via Suzuki cou-
pling reaction and characterized by 1H NMR spectrum, mass spectrum, and elemental analysis. BPPA exhibits deep-blue emis-
sion both in solution and in solid thin film. This compound has a non-planar structure that results in high thermal stability and
the phenomenon of polymorphism. The non-doped device based on this material shows stable deep-blue emission with the
1931 Commission international de I’Eclairage (CIE) coordinate of (0.15, 0.05) under different applied voltages. The device
exhibits the maximum external quantum efficiency of 2.2% at 14.9 mA/cm2 with luminance of 105 cd/m2.
organic light emitting diodes, anthracene derivative, deep-blue emission, theoretical calculations, thermal properties
other important essential for display applications. The 1931
1 Introduction
CIE coordinate of blue emission specified in the National
Television System Committee (NTSC) standard is (0.14,
0.08). In addition, it is well known that the power consump-
tion of a full-color OLED is highly dependent upon the
color of blue emission [7]. The deeper the blue color
(smaller CIE y-value) is, the lower the power consumption
of the device is. Recently, several deep-blue OLEDs with
the 1931 CIE coordinates close to the NTSC standard have
been reported [7–15], but the OLEDs with the coordinate of
y < 0.05 are still rare [16, 17].
Anthracene derivatives usually have a high fluorescence
quantum yield, wide energy gap, and good thermal stability.
Thus, they have been widely used as deep-blue emitting
materials in OLEDs [7, 9, 15, 18–20]. In this paper, we re-
port a novel anthracene derivative 9,10-bis[3,5-di(4-tert-
butylphenyl)phenyl]anthracene (BPPA). The 3,5-di(4-tert-
butylphenyl)phenyl groups end-capped at the 9- and 10-
positions of the central anthracene core are highly twisted to
Since the pioneering work on organic light emitting diodes
(OLEDs) by the Kodak [1] and Cambridge groups [2],
OLEDs have attracted considerable attention due to their
potential application in flat panel displays. Three basic col-
ors, red, green and blue, are essential for the full color
OLEDs. During the past two decades, significant progress
has been made in this field [3–6]. However, the perform-
ances of the blue-light-emitting OLEDs are still relatively
poor in comparison with those of the red- and green-light-
emitting OLEDs. Although the use of dopant emitters in the
host-guest systems is an effective approach to improve the
electroluminescence (EL) efficiency, the non-doped devices
are more preferable for the purpose of the concise fabrica-
tion process. In addition to efficiency, color purity is an-
*Corresponding author (email: xhzhang@mail.ipc.ac.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2011
chem.scichina.com
www.springerlink.com

Wang ZQ, et al. Sci China Chem April (2011) Vol.54 No.4
667
increase steric hindrance and cause molecular non-planarity.
This compound exhibits a high glass transition temperature
(Tg) and a phenomenon of polymorphism. The non-doped
device based on BPPA shows stable deep-blue emission
with the 1931 CIE coordinate of (0.15, 0.05) and high ex-
ternal quantum efficiency of 2.2%.
product was purified by column chromatography (eluent =
dichloromethane/hexane, 1:10 v/v), and then recrystallized
in toluene to give the product as a white solid (yield: 0.53 g,
62%). 1H NMR (400 MHz, CDCl3)  (ppm): 7.87–8.04 (6H,
m), 7.69–7.74 (12H, m), 7.50 (8H, d, J = 6.64), 7.34–7.38
(4H, m), 1.38 (36H, s); MALDI-TOF MS m/z 59.3; Anal.
calcd for C46H34: C, 92.26; H, 7.74; Found: C, 92.14; H,
7.78.
2 Experimental
Indium-tin-oxide (ITO)-coated glass substrates with the
sheet resistance of 50 /□ were cleaned with isopropyl
alcohol and deionized water, dried in an oven over 120 °C,
and treated with UV-ozone. A 30 nm thick film of poly(3,
4-ethylenedioxy thiophene) doped with poly(styrene sul-
fonate) (PEDOT:PSS) was spin-coated on the ITO glass
substrates. After being dried at 120 °C for 30 min under the
nitrogen atmosphere, the substrate was transferred to the
vacuum deposition system with a base pressure of < 5 × 107
torr. The device was fabricated by evaporating organic lay-
ers onto the PEDOT:PSS layer sequentially with an evapo-
ration rate of 1–2 Å/s. After CsF layer was evaporated onto
the organic layers, the Mg:Ag alloy cathode was prepared
by co-evaporation of Mg and Ag at the volume radio of
10:1. EL spectra and CIE color coordinates were measured
with a spectrascan PR650 photometer and the current-voltage-
luminescence characteristics were measured with a com-
puter-controlled Keithley 2400 SourceMeter under ambient
atmosphere.
Commercially available reagents were used without further
purification unless otherwise stated. 1H NMR spectrum was
recorded in CDCl3 solution on a Bruker Avance 400 spec-
trometer with tetramethylsilane (TMS) as the internal stan-
dard. Elemental analysis was performed on a Vario III ele-
mental analyzer. Mass spectrum was obtained on a BIFLEXIII
MALDI-TOF spectrometer. The UV absorption and photo-
luminescence spectra were recorded on a Hitachi U-3010
UV-vis spectrophotometer and a Hitachi F-4500 fluores-
cence spectrophotometer, respectively. With the heating rate
of 10 °C/min under a nitrogen atmosphere, thermal gravim-
etric analysis (TGA) and differential scanning calorimetry
(DSC) measurements were performed on a TA Instrument
TGA2050 and a TA Instruments DSC2910, respectively.
Cyclic voltammetry measurement was performed using a
CHI600A analyzer with the scan rate of 500 mV/s in the
CH2Cl2 solution. The electrolytic cell was a conventional
three-electrode cell in which a Pt disc working electrode, a
Pt wire auxiliary electrode, and a saturated calomel elec-
trode (SCE) as the reference electrode were employed.
Tetra-n-butylammonium hexafluorophosphate (0.1 M) was
used as the supporting electrolyte.
3 Results and discussion
To gain insight into the electronic structure and the stereo-
structure of BPPA, quantum chemical calculations were
carried out using the B3LYP/6-31G(d) method [21]. The
reliability of the method can be found in previous reports
[22–24]. The aryl substituents at the 9- and 10- positions of
the anthracene core are highly twisted toward the anthra-
cene backbone with the torsion angle of 91.9°, which indi-
cates that BPPA has a non-planar structure. The non-planarity
of the structure can limit the intermolecular interactions,
and facilitate the formation of stable amorphous film. The
electron densities of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital
(LUMO) are almost concentrated on the anthracene unit
(Figure 1), suggesting that the absorption and emission of
BPPA are controlled mostly by the central anthracene unit.
Combining the half-wave oxidation potential (E1/2ox) and the
UV-vis absorption edge of BPPA, the HOMO and LUMO
energy levels are estimated to be 5.91 and 2.89 eV, respec-
tively.
BPPA was synthesized via the Suzuki coupling reaction
(Scheme 1). 3,5-Di(4-tert-butylphenyl)benzene boronic acid
(1; 0.85 g, 2.2 mmol), 9,10-dibromoanthracene (2; 0.336 g,
1 mmol), tetrakis(triphenylphosphine)palladium [Pd(PPh3)4]
(20 mg), tetrahydrofuran (THF; 20 mL), and aqueous potas-
sium carbonate (2 M, 5 mL) were sequentially added to a
round bottom flask (50 mL). The resulting mixture was
purged with nitrogen for 30 min, refluxed overnight, and
poured into 100 mL water. The precipitate was filtered,
washed several times with water, and dried. The crude
The UV-vis absorption and photoluminescence (PL)
spectra of BPPA in dilute CH2Cl2 solution are shown in
Figure 2. The absorption spectrum exhibits the characteris-
tic vibration patterns of the isolated anthracene group at 336,
356, 375 and 397 nm [9, 10]. The strong absorption
Scheme 1 Synthetic route of BPPA. Reagents and conditions: THF,
K2CO3, Pd(PPh3)4, reflux.

668
Wang ZQ, et al. Sci China Chem April (2011) Vol.54 No.4
solid film. The PL spectrum of the solid thin film shows a
deep-blue emission with a structureless peak at 438 nm.
The thermal properties of BPPA were investigated
through TGA and DSC under the nitrogen atmosphere.
BPPA exhibits a high thermal stability, the decomposition
temperature (Td), which corresponds to a 3% weight loss
upon heating during TGA, is around 410 °C. DSC meas-
urements were performed from 30 to 405 °C under the ni-
trogen atmosphere. Upon the first heating, an endothermic
solid-solid phase transition was observed at 381 °C before
the melting temperature (Tm) of 391 °C (Figure 3). The
melted sample was rapidly cooled by liquid nitrogen to
form a glassy state. When the amorphous glassy sample was
heated again, a glass transition occurred at 207 °C. The high
glass transition temperature (Tg) indicates that BPPA can
form highly stable amorphous films. Upon further heating
beyond Tg, an exothermic multi-peak due to crystallization
was observed at around 320 °C, and then two endothermic
solid-solid phase transition were observed at 377 and 381 °C,
respectively, which indicates that BPPA has multiple crystal
forms. The phenomenon of polymorphism suggests that
BPPA has different conformers, which is responsible for the
ready formation of the amorphous state.[26]
To investigate the EL properties of BPPA, we fabricated
the non-doped device with a configuration of ITO/PEDOT:
PSS (30 nm)/NPB (15 nm)/TCTA (15 nm)/BPPA (30 nm)/
TPBI (30 nm)/CsF (2 nm)/Mg:Ag. In this device, ITO and
CsF/Mg:Ag are the anode and the cathode, respectively;
PEDOT:PSS is the hole-injection layer (HIL); 4,4′-bis[N-(1-
naphthyl)-N-phenyl amino] biphenyl (NPB) is the hole-
transporting layer (HTL); 4,4′,4″-tri(N-carbazolyl)triphenyl-
amine (TCTA) is used as the hole-transporting and electron-
blocking layer (HT-EBL); 1,3,5-tris(N-phenylbenzimidazol)
benzene (TPBI) is used as the electron-transporting layer and
hole-blocking layer (ET-HBL); BPPA is the emitting layer
(EML).
Figure 1 Calculated HOMO (a) and LUMO (b) orbitals of BPPA.
Figure 2 The absorption and photoluminescence spectra of BPPA in
dilute CH2Cl2 solution (dotted line) and in solid film (solid line).
band at 260 nm can be assigned to the aryl groups at the 9-
and 10-positions of the anthracene core. When BPPA is
excited at 260 or 375 nm, the same PL spectra with max
=
415 and 436 nm are observed, which indicates that the en-
ergy can be transferred from the side groups to the central
anthracene unit. Fluorescence quantum yield (f) of BPPA
in the dilute CH2Cl2 solution has been measured to be 0.95
by using 9,10-diphenylanthracene (f = 0.90) as the calibra-
tion standard [25]. The absorption spectrum of BPPA as a
solid thin film on a quartz disc is almost identical to that of
BPPA in dilute CH2Cl2 solution (Figure 2), which implies
that the intermolecular interactions are very weak in the
Figure 3 The DSC curves of BPPA (dotted line for the first heating of
the original sample, solid line for the second heating of the amorphous
glassy sample).

Wang ZQ, et al. Sci China Chem April (2011) Vol.54 No.4
669
Figure 4 shows the normalized EL spectra of the device
at different applied voltages. All the emission peaks are at
430 nm with very narrow FWHM (the full width at half
maximum) of about 55 nm. The EL spectra show no emis-
sion in the longer wavelength region which is often ob-
served in the case of intermolecular - stacking or charge-
transfer complexes formed at the EML/HTL or EML/ETL
interface. The high quality emission should be attributed to
the non-planar structure of BPPA. While increasing the ap-
plied voltage from 5 to 10 V, the EL spectra remain un-
changed，which is desirable for OLEDs. The 1931 CIE
coordinate (0.15, 0.05) of this device is far beyond the
NTSC blue standard, which may provide an enlarged color
gamut for color displays. Up to now, the devices with CIE
coordinates of y < 0.05 are still very rare to our knowledge
[16, 17].
Schematic energy level diagram of the BPPA based de-
vice is shown in Figure 5. It can be seen that the hole-injection
barrier at the TCTA/BPPA interface and the electron-injection
barrier at the BPPA/TPBI interface is 0.21 and 0.09 eV,
respectively. These small carrier-injection barriers indicate
that both holes and electrons could be readily injected into
the emitting layer. In addition, from the energy level dia-
grams, it is also obvious that there are large hole-injection
Figure 6 The current density-voltage-luminance characteristics of the
BPPA based device.
barrier (0.39 eV) at the BPPA/TPBI interface and large
electro-injection barrier (0.59 eV) at the TCTA/BPPA in-
terface, which should efficiently prevent both holes and
electrons leaving the emitting layer. Figure 6 shows the
current density-voltage-luminance characteristics of the
device. The turn-on voltage is 4.3 V, and the maximum lu-
minance is 1280 cd/m2 at the voltage of 10 V and the cur-
rent density of 420 mA/cm2. The device exhibits the maxi-
mum external quantum efficiency of 2.2% (current effi-
ciency of 0.7 cd/A) at 14.9 mA/cm2 with the luminance of
105 cd/m2.
4 Conclusion
A highly efficient deep-blue anthracene derivative BPPA
was synthesized. This compound has a high thermal stabil-
ity (Td = 410 °C and Tg = 207 °C), and exhibits the phe-
nomenon of polymorphism, which implies that it can form
the stable amorphous thin film readily. The non-doped de-
vice based on this material shows stable deep-blue emission
at 430 nm under different applied voltages. A maximum
external quantum efficiency of 2.2% was achieved at 14.9
mA/cm2 with the luminance of 105 cd/m2. The 1931 CIE
coordinate (0.15, 0.05) of this device is far beyond the
NTSC blue standard, which may provide an enlarged color
gamut for color displays.
Figure 4 The EL spectra of the BPPA based device at different applied
voltages.
This work was supported by the National Natural Science Foundation of
China (50773090, 50825304, 51033007).
1
2
Tang CW, Vanslyke SA. Organic electroluminescence diode. Appl
Phys Lett, 1987, 51: 913–915
Burroughes JH, Bradley DDC, Broun AR, Marks RN, Mackay K,
Friend RH, Burn PL, Holmes AB. Light-emitting diodes based on
conjugated polymers. Nature, 1990, 347: 539–541
3
4
5
Tang CW, Vanslyke SA, Chen CH. Electroluminescence of doped
organic thin films. J Appl Phys, 1989, 65: 3610–3616
Shi J, Tang CW. Doped organic electroluminescent devices with im-
proved stability. Appl Phys Lett, 1997, 70: 1665–1667
Baldo MA, O’Brien DF, You Y, Shoustikov A, Sibley S, Thompson
Figure 5 Relative energy level alignments of the BPPA based device.

670
Wang ZQ, et al. Sci China Chem April (2011) Vol.54 No.4
ME, Forrest SR. Highly efficient phosphorescent emission from
emitting material with high color purity. Adv Mater, 2001, 13:
organic electroluminescent devices. Nature, 1998, 395: 151–154
Sun Y, Giebink NC, Kanno H, Ma B, Thompson ME, Forrest SR.
Management of singlet and triplet excitons for efficient white organic
light-emitting devices. Nature, 2006, 440: 908–912
Zheng CJ, Zhao WM, Wang ZQ, Huang D, Ye J, Zhang XH, Lee CS,
Lee ST. Highly efficient non-doped deep-blue organic light-emitting
diodes based on anthracene derivatives. J Mater Chem, 2010, 20:
1560–1566
Wang L, Jiang Y, Wang J, Pei J, Cao Y. Highly efficient and color-
stable deep-blue organic light-emitting diodes based on a solution-
processible dendrimer. Adv Mater, 2009, 21: 4854–4858
Chien CH, Chen CK, Hsu FM, Shu CF, Chou PT, Lai CH.
Multifunctional deep-Blue emitter comprising an anthracene core and
terminal triphenylphosphine oxide groups. Adv Funct Mater, 2009,
19: 560–566
1690–1693
6
7
16 Wu CC, Lin YT, Wong KT, Chen RT, Chien YY. Efficient organic
blue-light-emitting devices with double confinement on terfluorenes
with ambipolar carrier transport properties. Adv Mater, 2004, 16: 61–65
17 Tang S, Liu MR, Lu P, Xia H, Li M, Xie ZQ, Shen FZ, Gu C, Wang
HP, Yang B, Ma YG. A molecular glass for deep-blue organic light-
emitting diodes comprising a 9,9′-spriobifluorene core and peripheral
carbazole groups. Adv Funct Mater, 2007, 17: 2869–2877
18 Tao SL, Hong ZR, Peng ZK, Ju WG, Zhang XH, Wang PF, Wu SK,
Lee ST. Anthracene derivative for a non-doped blue-emitting organic
electroluminescence device with both excellent color purity and high
efficiency. Chem Phys Lett, 2004, 397: 1–4
8
9
19 Kim YH, Jeong HC, Kim SH, Yang K, Kown SK. High-purity-blue
and high efficiency electroluminescent devices based on anthracene.
Adv Funct Mater, 2005, 15: 1799–1805
10 Shih PI., Chuang CY, Chien CH, Shu CF.. Highly efficient non-
doped blue-light-emitting diodes based on an anthrancene derivative
end-capped with tetraphenylethylene groups. Adv Funct Mater, 2007,
17: 3141–3146
11 Gao Q, Li ZH, Xia PF, Wong MS. Efficient deep-blue organic light-
emitting diodes: Arylamine-substituted oligofluorenes. Adv Funct
Mater, 2007, 17: 3194–3199
12 Moorhy JN, Venkatakrishnan P, Huang DF, Chow TJ. Blue light-
emitting and hole-transporting amorphous molecular materials based
on diarylaminobiphenyl-functionalized bimesitylenes. Chem Commun,
2008, 2146–2148
13 Zhen CG, Chen ZK, Liu QD, Kieffer J. Fluorene-based oligomers for
highly efficient and stable organic blue-light-emitting diodes. Adv
Mater, 2009, 21: 2425–2429
20 Kim SK, Yang B, Ma YG, Lee JH, Park JW. Exceedingly efficient
deep-blue electroluminescence from new anthracenes obtained using
rational molecular design. J Mater Chem, 2008, 18: 3376–3384
21 Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA,
Cheeseman JR. Gaussian 03, Revision A1, Gaussian Inc, Pittsburgh,
PA 2003
22 Lee SJ, Park JS, Yoon KJ, Kim YI, Jin SH, Kang SK. High-efficiency
deep-blue light-emitting diodes based on phenylquinoline/caybazole-
based compounds. Adv Funct Mater, 2008, 18: 3922–3930
23 Wang ZQ, Xu C, Wang WZ, Fu WJ, Niu LB, Ji BM. Highly efficient
undoped deep-blue electroluminescent device based on a novel
pyrene derivative. Solid-State Electron, 2010, 54: 524–526
24 Liu F, Tang C, Chen QQ, Li SZ, Wu HB, Xie LH. Pyrene functioned
diarylfluorenes as efficient solution processable light emitting mo-
lecular glass. Org Electron, 2009, 10: 256–265
14 Liao SH, Shiu JR, Liu SW, Yeh SJ, Chen YH, Chen CT, Chow TJ,
Wu CI. Hydroxynaphthyridine-derived group III metal chelates: Wide
band gap and deep blue analogues of green Alq3 (tri(8-hydroxyquinolate)
aluminum) and their versatile applications for organic light-emitting
diodes. J Am Chem Soc, 2009, 131: 763–777
25 K. Danel, T. H. Huang, J. T. Lin, Y. T. Tao, C. H. Chuen. Blue-
emitting anthracenes with end-capping diarylamines. Chem Mater,
2002, 14: 3860–3865
26 Shirota Y. Organic materials for electronic and optoelectronic devices.
J Mater Chem, 2000, 10: 1–25
15 Kim YH, Shin DC, Kim SH, Ko CH, Yu HS, Kown SK. Novel blue