High-color-purity and high-efficiency non-doped deep-blue electroluminescent devices based on novel anthracene derivatives

Article abstract of DOI:10.1039/c2nj20809a

Two novel anthracene derivatives 2-tert-butyl-9,10-bis(2-biphenyl) anthracene (TBBPA) and 2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene (TBNPA) were synthesized and characterized. The quantum chemical calculations show that TBBPA and TBNPA have hi

Full text of DOI:10.1039/c2nj20809a

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PAPER
High-color-purity and high-efficiency non-doped deep-blue
electroluminescent devices based on novel anthracene derivativesw
Zhi-Qiang Wang,a Chen Xu,*a Wei-Zhou Wang,a Lu-Meng Duan,b Zhen Li,b
Bang-Tun Zhaoa and Bao-Ming Ji*a
Received (in Montpellier, France) 19th September 2011, Accepted 22nd November 2011
DOI: 10.1039/c2nj20809a
Two novel anthracene derivatives 2-tert-butyl-9,10-bis(2-biphenyl) anthracene (TBBPA) and
2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene (TBNPA) were synthesized and
characterized. The quantum chemical calculations show that TBBPA and TBNPA have highly
non-coplanar structures. Both the two anthracene derivatives exhibit deep-blue emission with high
fluorescence quantum yields. The electrochemical properties and energy levels of the two
compounds were investigated by cyclic voltammetry. The non-doped devices based on the two
anthracene derivatives exhibit low turn-on voltage and stable deep-blue emission with high
efficiency (external quantum efficiency of about 3.0%). The Commission Internationale de
l’Eclairage (CIE) coordinates (0.15, 0.07) of the devices are very close to the National Television
System Committee (NTSC) blue standard. The comprehensive performances of the devices are
comparable to the best of the non-doped deep-blue OLEDs with a CIE coordinate of y o 0.10.
Introduction
and stable non-doped deep-blue OLEDs continues to receive
considerable attention.10–16
Since the pioneering work on organic light emitting devices
(OLEDs) by the Kodak group,1 OLEDs have been attracting
considerable attention due to their potential applications in
flat-panel displays and solid-state lighting sources.1–5 For the
full-color OLEDs, highly efficient and stable three basic color
(red, green, and blue) emissions are needed. Although OLEDs have
achieved remarkable development to date, the performance of blue
OLEDs is often inferior to those of red and green ones because of
the intrinsic wide band gap of blue-emitting materials.6–8 The
electroluminescence (EL) efficiency can be improved by using
dopant emitters, but the addition of dopants implies additional
complexity and cost for the mass production of OLEDs. In
addition to efficiency and stability, color purity is another
important criterion for OLEDs. The Commission Internationale
de l’Eclairage (CIE) coordinates of blue emission specified by the
National Television System Committee (NTSC) are (0.14, 0.08).
In addition, the power consumption of a full-color OLED is
highly dependent upon the color of blue emission, and the deeper
the blue color (smaller CIE y-value) is, the lower the power
consumption of the device is.9 Thus, developing highly efficient
It is well known that 9,10-diphenylanthracene (DPA) is an
attractive material for its high fluorescence quantum efficiency
in dilute solution.17 Nevertheless DPA is incapable of forming
stable amorphous thin film because of its tendency to crystallize in
the solid state, and its fluorescence can be easily quenched in the
solid state, which hamper its application in OLEDs.7,8,18,19 More-
over, to obtain excellent color purity, blue emitting materials
should have molecular structures that can prevent excimer or
exciplex formation which brings about broad and red-shifted
electroluminescent emission.20 To mitigate these problems,
several deep-blue anthracene derivatives have been synthesized
by introducing bulky groups at the para-position or meta-position
of phenyl substituents in DPA.7,8,12,14,20–23 In this study, we
designed and synthesized two novel anthracene derivatives
2-tert-butyl-9,10-bis(2-biphenyl) anthracene (TBBPA) and 2-tert-
butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene (TBNPA).24 In
TBBPA and TBNPA, the phenyl group and naphthyl group were
introduced at the ortho-position of phenyl substituents in DPA.
This can further enhance the non-planarity of molecular structures
and reduce the conjugation length of material molecules due to the
higher steric hindrance. The introduction of the tert-butyl group in
the anthracene unit can improve the performances of devices by
reducing the molecular aggregation.25 The quantum chemical
calculations were carried out to investigate their stereostructures
and electronic structures. The photophysical properties and
electrochemical properties of the compounds were studied
in a contrastive manner. The non-doped devices, ITO/bis[N-
(1-naphthyl)-N-phenylamino]biphenyl (NPB, 30 nm)/TBBPA
a College of Chemistry and Chemical Engineering, Luoyang Normal
University, Luoyang 471022, People’s Republic of China.
E-mail: lyhxxjbm@126.com, xubohan@163.com;
Fax: +86 379 65523821; Tel: +86 379 65523821
b Department of Chemistry, Zhengzhou University, Zhengzhou
450052, People’s Republic of China
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2nj20809a
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or TBNPA (30 nm)/1,3,5-tris(N-phenylbenzimidazol)benzene
(TPBI, 30 nm)/LiF (2 nm)/Al, show stable deep-blue emission
with high EL efficiency. The 1931 CIE coordinates (0.15, 0.07)
of the devices are very close to the NTSC blue standard.
Experimental
General
Commercially available reagents were used without purification
unless otherwise stated. 1H NMR spectra were recorded in
CDCl3 solution on a Bruker Avance 400 spectrometer with
tetramethylsilane (TMS) as the internal standard. Elemental
analysis was performed on a Vario III elemental analyzer. Mass
spectrum was obtained on a Bruker Microflex spectrometer.
The absorption and photoluminescence spectra were recorded
on a Hitachi U-3010 UV-Vis spectrophotometer and a Hitachi
F-4500 fluorescence spectrophotometer, respectively. Cyclic
voltammetry was performed on a CHI620C electrochemical
analyzer.
Synthesis
2-tert-Butyl-9,10-dibromoanthracene (compound 2) and 1-(2-
bromophenyl)naphthalene (compound 5) were synthesized
according to the literature.26,27 TBBPA and TBNPA were
synthesized by the Suzuki coupling reaction (Scheme 1).
2-tert-Butyl-9,10-dibromoanthracene (2). Br2 (3.2 g,
20 mmol) in 50 mL CCl4 was added dropwise in 2-tert-
butylanthracene 1 (2.3 g, 10 mmol) in 50 mL CCl4 during
stirring, then reacted for 12 h at room temperature. The
precipitate was collected by filtration, and recrystallized from
ethanol to give the yellow needle-like crystals (yield: 2.4 g, 62%).
1H NMR (400 MHz, CDCl3) d (ppm): 8.57–8.45 (4H, m),
7.74–7.72 (1H, d, J = 8.0), 7.62–7.60 (2H, m), 1.49 (9H, s);
TOF-MS: m/z = 391.9; anal. calcd. for C18H16Br2: C, 55.13; H,
4.11; found: C, 54.95; H, 4.03.
Scheme 1 Synthetic route of TBBPA and TBNPA.
the reaction mixture at ꢀ78 1C, and stirred for 12 h at room
temperature. Then HCl (2 M, 10 mL) was added, and stirred
for 1 h. The resulting solution was extracted with CH2Cl2 three
times, then the combined organic solution was dried over
anhydrous MgSO4 and evaporated with a rotary evaporator.
The crude product was purified by column chromatography
(eluent = dichloromethane/n-hexane, 1 : 3 v/v) to give a white
powder (yield: 0.67 g, 68% for compound 7; 0.63 g, 51% for
compound 8). Compound 7: 1H NMR (400 MHz, CDCl3)
d (ppm): 7.94–7.92 (1H, d, J = 8.0), 7.50–7.40 (7H, m),
7.32–7.30 (1H, d, J = 8.0), 4.32 (2H, s); TOF-MS: m/z = 198.2.
1-(2-Bromophenyl)naphthalene (5). Naphthalene-1-boronic
acid 3 (1.7 g, 10 mmol), 1-iodo-2-bromobenzene 4 (2.82 g,
10 mmol), and tetrakis(triphenylphosphine)palladium [Pd(PPh3)4]
(150 mg) were mixed in toluene (100 mL), then Na2CO3 (2 M,
30 mL) and ethanol (30 mL) were added during stirring, the
mixture reacted for 24 h at 80 1C under the nitrogen atmo-
sphere. After cooling, the resulting solution was extracted with
CH2Cl2 three times, then the combined organic solution was
dried over anhydrous MgSO4 and evaporated with a rotary
evaporator. The crude product was purified by column
chromatography (eluent = n-hexane) to give a white powder
(yield: 2.4 g, 85%). 1H NMR (400 MHz, CDCl3) d (ppm):
7.91–7.89 (2H, d, J = 8.0), 7.74–7.72 (1H, d, J = 8.0),
7.55–7.24 (8H, m); TOF-MS: m/z = 282.0; anal. calcd. for
C16H11Br: C, 67.87; H, 3.92; found: C, 67.78; H, 3.84.
2-tert-Butyl-9,10-bis(2-biphenyl)
anthracene
(TBBPA).
2-tert-Butyl-9,10-dibromoanthracene 2 (0.39 g, 1.0 mmol),
biphenyl-2-boronic acid 7 (0.49 g, 2.5 mmol), and tetrakis-
(triphenylphosphine)palladium [Pd(PPh3)4] (30 mg) were mixed
in toluene (20 mL), then Na2CO3 (2 M, 5 mL) and ethanol
(5 mL) were added during stirring. The mixture was refluxed
under the nitrogen atmosphere for 24 h. After cooling, the
precipitate was collected by filtration and purified by column
chromatography (eluent = dichloromethane/n-hexane, 1 : 10 v/v).
The product was recrystallized in toluene (yield: 0.28 g, 52%).
1H NMR (400 MHz, CDCl3) d (ppm): 7.66–7.52 (6H, m),
Biphenyl-2-boronic acid (7) and 1-(1-naphthyl)phenyl-2-
boronic acid (8). n-Butyl lithium (2.5 M, 6 mmol) was added
dropwise to the solution of 6 (1.2 g, 5 mmol) or 5 (1.4 g,
5 mmol) in anhydrous THF (30 mL) at ꢀ78 1C during half an
hour, and the mixture was stirred for another 1 h at this
temperature. Trimethylborate (1.25 g, 12 mmol) was added to
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7.49–7.45 (3H, m), 7.37–7.30 (3H, m), 7.29 (1H, s), 7.23–7.17
(2H, m), 6.99 (2H, d, J = 6.4), 6.94 (2H, d, J = 7.2), 6.94–6.80
(6H, m), 1.19 (9H, s); TOF-MS: m/z = 538.3; anal. calcd. for
C42H34: C, 93.64; H, 6.36; found: C, 93.46; H, 6.27.
2-tert-Butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene (TBNPA).
The synthesis of TBNPA is similar to that of TBBPA by a
1
reaction of 2 with 8, yield: 0.26 g, 41%. H NMR (400 MHz,
CDCl3) d (ppm): 8.24 (2H, d, J = 8.0), 8.13–7.86 (6H, m),
7.82–7.59 (7H, m), 7.46–7.32 (2H, m), 7.07–7.02 (3H, m),
6.88–6.76 (2H, m), 6.67 (1H, d, J = 8.0), 6.62–6.53 (2H, m),
6.51–6.42 (4H, m), 1.18 (9H, s); TOF-MS: m/z = 638.3; anal.
calcd. for C50H38: C, 94.00; H, 6.00; found: C, 93.83; H, 5.92.
Devices fabrication and measurements. Indium-tin-oxide
(ITO) coated glass substrates were cleaned with isopropyl
alcohol and deionized water, then dried in an oven at 120 1C,
and finally treated with UV-ozone. The devices were prepared in a
vacuum at a pressure of about 1 ꢁ 10ꢀ6 Torr. Organic layers were
deposited onto the substrate at a rate of 0.1–0.2 nm sꢀ1. After the
organic film deposition, LiF and aluminum were thermally evapo-
rated onto the surface of organic layer. The thicknesses of the
organic materials and the cathode layers were controlled using a
quartz crystal thickness monitor. EL spectra and 1931 CIE color
coordinates were measured with a spectrascan PR650 photometer
and the current–voltage–luminance characteristics were measured
with a computer-controlled Keithley 2400 SourceMeter under
ambient atmosphere.
Results and discussion
Quantum chemical calculations
Quantum chemical calculations were carried out to characterize
the stereostructures and electronic structures of each material by
using the B3LYP/6-31G(d) method.28 The stereostructures of
TBBPA and TBNPA are shown in Fig. 1. In TBBPA, the two
benzene rings at the 9- and 10-positions are highly twisted
towards the anthracene backbone to angles of 77.91 and 76.81,
respectively, and the dihedral angle between benzene rings in
biphenyl groups is 53.11 (Fig. 1a). For TBNPA, the dihedral
angles between aromatic rings are slightly larger than those of
TBBPA (Fig. 1b). The calculation results indicate that the
TBBPA and TBNPA have highly non-coplanar structures, which
can limit the intermolecular interactions against fluorescence
quenching and facilitate the formation of stable amorphous thin
films. As shown in Fig. 1, the calculated highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) electron densities of the materials are almost
concentrated on the anthracene moiety. Thus, it is expected that
the absorption and emission properties of TBBPA and TBNPA
are controlled mostly by p–p* transition of the anthracene moiety.
Fig. 1 Calculated stereostructures and frontier orbitals of TBBPA (a)
and TBNPA (b).
to the groups at the 9- and 10-positions of the central
anthracene core. When TBBPA is excited at either 379 nm
or 265 nm, the same PL spectrum (lmax are 421 nm and
443 nm) was observed. In the case of TBNPA, the same PL
spectrum (lmax are 425 nm and 443 nm) can be obtained by
excitation at either 379 nm or 265 nm. It can be found that the
structures of absorption and PL spectra of the compounds are
similar to those of DPA (Fig. 2). The PL quantum yields in
dilute CH2Cl2 solutions were measured to be 0.73 for TBBPA
and 0.69 for TBNPA, respectively, by using DPA as the
Photophysical properties
Fig. 2 shows the absorption and photoluminescence (PL)
spectra of TBBPA and TBNPA in dilute CH2Cl2 solutions.
The absorption spectra of the compounds show the characteristic
vibrational patterns of the isolated anthracene moiety (lmax
are about 341, 361, 379 and 399 nm).18,19 The additional
absorption bands between 250 nm and 340 nm can be assigned
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Fig. 3 Cyclic voltammograms of TBBPA and TBNPA in CH2Cl2
solution.
ferrocene (Fc) with respect to a zero vacuum level.29,30 The band
gaps were determined to be 2.90 eV for TBBPA and 2.89 eV for
TBNPA from the absorption band edges (Fig. 2, 427 nm for
TBBPA, 429 nm for TBNPA). The lowest unoccupied molecular
orbital (LUMO) energy levels were estimated to be ꢀ2.60 eV for
both TBBPA and TBNPA according to the HOMO energy level
values in combination with the band gaps.
Electroluminescence properties of multilayer OLEDs
To investigate the electroluminescence (EL) properties of the
anthracene derivatives, we fabricated the non-doped devices,
device I: ITO/NPB (30 nm)/TBBPA (30 nm)/TPBI (30 nm)/
Fig. 2 Absorption and photoluminescence spectra of TBBPA (a) and
TBNPA (b) in dilute CH2Cl2 solution and solid thin film.
reference standard. The high PL quantum yields and the similar
spectrum structures indicate that the excellent luminescence
properties of DPA are maintained in TBBPA and TBNPA.
The absorption and PL spectra of the TBBPA and TBNPA thin
films are almost identical to those acquired in dilute CH2Cl2
solutions except for a slight red-shift (4 nm for TBBPA, 7 nm
for TBNPA) (Fig. 2), which implies that the significant inter-
molecular interactions do not occur in solid thin films.
In addition, TBNPA exhibits a blue-shifted PL spectrum
and a less difference between the PL spectra of the solution
and thin film compared to its isomer TBDNPA (the naphthyl
group is at the para-position of phenyl substituents in DPA).14
This can be attributed to the less conjugation length and higher
non-planarity of TBNPA compared to TBDNPA (see ESIw).
Electrochemical properties
The electrochemical properties of TBBPA and TBNPA were
investigated by cyclic voltammetry in CH2Cl2 containing
tetrabutylammonium hexafluorophosphate (0.1 M) at a scan
rate of 0.05 V sꢀ1. The electrolytic cell was a conventional
three-electrode cell consisting of a Pt working electrode, a Pt
wire counter electrode and an Ag/AgCl reference electrode. As
shown in Fig. 3, both TBBPA and TBNPA exhibit the reversible
oxidation process with almost same half-wave oxidation potentials
(E1/2ox) (1.19 V for TBBPA, 1.18 V for TBNPA). The highest
occupied molecular orbital (HOMO) energy levels were calcu-
lated to be ꢀ5.50 eV and ꢀ5.49 eV for TBBPA and TBNPA,
respectively, by using the energy level value of ꢀ4.8 eV for
Fig. 4 EL spectra of device I (a) and device II (b) under different
luminance.
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Fig. 5 Current density–voltage–luminance characteristics of the
Fig. 7 External quantum efficiency as a function of the current
devices I and II.
density for devices I and II.
LiF (2 nm)/Al, device II: ITO/NPB (30 nm)/TBNPA (30 nm)/
TPBI (30 nm)/LiF (2 nm)/Al. In these devices, ITO (indium tin
oxide) and LiF/Al are the anode and the cathode, respectively;
4,40-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) is the
hole transporting layer; 1,3,5-tris(N-phenylbenzimidazol)benzene
(TPBI) is used as the electron transporting and hole blocking
layer; TBBPA and TBNPA are the emitting layers (EML).
Fig. 4 shows the normalized EL spectra of the devices I and II
under different luminance. It can be found that the EL spectra of
devices I and II are similar to the PL spectra in dilute CH2Cl2
solutions and thin films, especially for TBBPA. The emission peaks
of the device I based on TBBPA are at 424 nm and 448 nm, the
TBNPA based device II shows an emission peak at 448 nm with a
shoulder peak at 428 nm. The EL spectra of the two devices do not
show an obvious red-shift compared to the PL spectra of TBBPA
and TBNPA thin films, and show no emission at longer wavelength
from the exciplex species. This may be attributed to the highly non-
coplanar structures of TBBPA and TBNPA because of the
presence of the phenyl group or naphthyl group at the ortho-
position of phenyl substituents in DPA. Significantly, the EL
spectra of the devices show a little change under different
luminance from 10 cd mꢀ2 to 1000 cd mꢀ2, which is highly
desirable for OLEDs. The 1931 CIE coordinates (0.15, 0.07) of
the devices are very close to the NTSC blue standard.
The current density–voltage–luminance characteristics of the
devices I and II are shown in Fig. 5. The turn-on voltages
(corresponding to 1.0 cd mꢀ2) are 3.3 V and 3.2 V for the devices
I and II, respectively. The important reason for such low turn-on
voltages is the small injection barriers in these devices. As shown
in the relative energy levels diagram of the devices (Fig. 6), the
injection barriers of both the NPB/EML interface and EML/
TPBI interface are about 0.1 eV. It is noted that, even at a
practical luminance of 100 cd mꢀ2, the applied voltages are only
4.4 V and 4.2 V for devices I and II, respectively, and the external
quantum efficiencies are 2.52% and 2.45% at this luminance. As
shown in Fig. 7, device I achieves a maximum external quantum
efficiency of 3.0% at 3.5 V, which is comparable to the best of
the non-doped deep-blue OLEDs with a CIE coordinate of
y o 0.10.12,14,31,32 Device II achieves a maximum external
quantum efficiency of 2.5% at 4 V with a current density of
about 6.2 mA cmꢀ2, meanwhile, it exhibits a lower efficiency roll-
off. The high performance of devices I and II: low applied voltage,
stable emission, high efficiency and the CIE coordinates matched
to the NTSC blue standard, indicating that TBBPA and TBNPA
are promising candidates for non-doped deep-blue OLEDs.
Conclusions
Two novel anthracene derivatives TBBPA and TBNPA are
synthesized by introducing the phenyl group or naphthyl group
at the ortho-position of phenyl substituents in DPA. Both the two
anthracene derivatives have highly non-coplanar structures, and
show deep-blue emission with high fluorescence quantum yields.
The non-doped devices based on the two anthracene derivatives
exhibit low turn-on voltage and stable deep-blue emission with
high efficiency. The 1931 CIE coordinates (0.15, 0.07) of the
devices are very close to the NTSC blue standard. The high
performances of the devices indicate that TBBPA and TBNPA
are promising candidates for non-doped deep-blue OLEDs.
Acknowledgements
This work was supported financially by the National Natural
Science Foundation of China (No. 21072089, 20902043 and
20872058).
Fig. 6 Relative energy levels diagram of the devices based on TBBPA
and TBNPA.
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