Undoped, red organic light-emitting diodes based on a N,N,N',N'-tetraphenylbenzidine (TPD) derivative as red emitter with a triphenylamine derivative as hole-transporting layer

Article abstract of DOI:10.1016/j.dyepig.2009.08.003

A novel dicyanovinyl-functionalized N,N,N',N'-tetraphenylbenzidine (TPD) derivative, N,N'-bis[4-(1,1-dicyanovinyl)phenyl]-N,N'-bis(4-fluorophenyl)benzidine and a novel triphenylamine-based derivative, 4,4′,4″-tris{[di(p-tolyl)amino]phenyl}triphenylamine w

Full text of DOI:10.1016/j.dyepig.2009.08.003

Dyes and Pigments 84 (2010) 203–207
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Undoped, red organic light-emitting diodes based on a N,N,N’,
N’-tetraphenylbenzidine (TPD) derivative as red emitter with
a triphenylamine derivative as hole-transporting layer
,
,
,
,
*
Xingbo Cao a b, Yugeng Wen a b, Yunlong Guo a b, Gui Yu a, Yunqi Liu a, Lian-Ming Yang a
a Beijing National Laboratory for Molecular Sciences, Laboratory of New Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
b Graduate School of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel dicyanovinyl-functionalized N,N,N’,N’-tetraphenylbenzidine (TPD) derivative, N,N’-bis[4-(1,1-
dicyanovinyl)phenyl]-N,N’-bis(4-fluorophenyl)benzidine and a novel triphenylamine-based derivative,
4,40,400-tris{[di(p-tolyl)amino]phenyl}triphenylamine were synthesized and employed as red molecular
emitter and hole-transporting layer, respectively, in an undoped organic light-emitting diode devices.
The device based on the TPD-FCN emitter and DBTPA hole-transporting material, which was fabricated in
a facile non-doped configuration, achieved maximum luminance of 3400 cd mꢀ2 at a driven voltage of
17.5 V and maximum luminous efficiency of 3.04 cd Aꢀ1. Interestingly, the device reached a brightness of
Received 3 July 2009
Received in revised form
25 August 2009
Accepted 26 August 2009
Available online 4 September 2009
Keywords:
OLEDs
1399 cd mꢀ2 and luminous efficiency of 2.85 cd Aꢀ1 at the low current density of 20 mA cmꢀ2
.
Ó 2009 Elsevier Ltd. All rights reserved.
Red fluorescent material
Hole-transporting layer
Tetraphenylbenzidine derivative
Luminous efficiency
Brightness
1. Introduction
concentration is usually low and the effective doping range is
extremely narrow. The resolution of concentration quenching lies
in the design of novel red fluorescent materials which do not
aggregate and which can themselves be employed as the light-
emitting layer in OLEDs, thus resulting in so-called ‘host-emitting.
undoped’ red OLED devices. Whilst in recent years, undoped red
emitters have attracted increasing attention [17–23], novel red
emitters of high are still sought from the viewpoints of their ease of
synthesis and purification as well as high efficiency, brightness,
color purity, stability, etc.
This paper concerns the use of a novel dicyanovinyl-func-
tionalized TPD derivative, N,N’-bis[4-(1,1-dicyanovinyl)phenyl]-
N,N’-bis(4-fluorophenyl)benzidine (TPD-FCN), as red-emitting
material in an undoped red OLED. The N,N,N’,N’-tetraphenyl-
benzidine (TPD) molecule is an excellent non-planar donor, as it
can be considered as an electron-rich, propeller-shaped, dimeric
triphenylamine (TPA) and the 1,1-dicyanovinyl group is an
excellent acceptor group. It was considered that 1,1-dicyanovinyl-
Organic light-emitting diodes (OLEDs) have been the subject of
intense research since the first reports of molecular [1] and poly-
meric OLEDs [2] as the compounds offer potential application in
flat-panel displays. Primary light-emitting materials for OLEDs not
only display high electroluminescent (EL) efficiency [3–16] but also
must possess good thermal properties, long lifetime and high
chroma. Whilst many organic emitters exhibit high fluorescent
quantum yields in dilute solution, in contrast, they display either
weak or even no fluorescence in both highly concentrated solution
and the solid state [17]. This particular phenomenon, which is
known as concentration quenching, is especially pronounced in the
case of red organic emitters which commonly comprise an intra-
molecular charge transfer (ICT) donor–
extensively -conjugated structure and which, also, are prone to
aggregation in the solid state through either dipole–dipole inter-
actions or intermolecular -stacking. As result, doping is
p–acceptor group or an
p
p
a
universally employed in the fabrication of efficient, red OLEDs [18].
However, doping is difficult to control because the optimum dopant
functionalized TPD derivatives, of A–p–D–p–A structure, should
be non-planar and possess a pair of antiparallel dipoles which will
effectively prevent aggregation occurring through intermolecular
p–p stacking or dipole–dipole interactions, their fluorescence
concentration quenching in the solid state would be substantially
suppressed. Meanwhile, a novel, triphenylamine-based derivative,
* Corresponding author. Tel.: þ86 10 62565609.
E-mail address: yanglm@iccas.ac.cn (L.-M. Yang).
0143-7208/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2009.08.003

204
X. Cao et al. / Dyes and Pigments 84 (2010) 203–207
4,40,400-tris{[di(p-tolyl)amino]phenyl}triphenylamine
was used as hole-transporting material so as to achieve high
luminous efficiency at low driven voltage.
(DBTPA),
158.6, 156.9, 143.5, 140.0, 136.6, 132.0, 128.0, 127.8, 127.4, 125.5,
122.2, 117.7, 116.2, 116.0, 114.0, 113.0, 75.3. MS (EI): m/z 676
(Mþ, 100%). Anal. Calcd for C44H26 F2N6: C, 78.09; H, 3.87; N, 12.42;
F, 5.61. Found: C, 77.69; H, 3.88; N, 12.35; F, 5.56.
2. Experimental
2.2.3. Tri(4-bromophenyl)amine(3)
2.1. General procedures
A solution of triphenylamine(2.45 g,10 mmol) in DMF (25 ml) was
stirred at room temperature to which was added N-bromosuccini-
mide (5.2 g, 32 mmol) in 10 ml DMF in small portions (upon the
addition of N-bromosuccinimide, the color of the reaction mixture
turned to green). After stirring overnight at room temperature, the
reaction mixturewaspoured intowaterand wasextractedwithether.
The organic layer was dried over anhydrous Na2SO4. After the solvent
was evaporated, the crude product was purified by recrystallization
from methanol to afford a write solid in 90% yield (4.35 g). Mp:
1H and 13C NMR spectrawere obtainedona Bruker DMX-400 NMR
Spectrometer using tetramethylsilane as internal standard. Mass
spectra (EI or MALDI-TOF) were recorded on a Micromass GCT-MS
spectrometerand aBruker BIFLEXIII Mass Spectrometer, respectively.
UV–vis absorption and fluorescence spectra were obtained with
Hitachi U-3010 and Hitachi F-4500 fluorescence spectrometer.
Elemental analyses were carried out on a Carlo-Erba 1160 elemental
analyzer. TGA measurements were carried out on a TA SDT 2960
instrument under a dry nitrogen flow, heating from room tempera-
140–141 ꢁC.1H NMR (CDCl3, 400 MHz):
d
7.35 (d, 6H, J ¼ 8.32 Hz), 6.92
(d, 6H, J ¼ 8.28 Hz). MALDI-TOF, MS (m/z): 482.9 (Mþ).
1
ture to 500 ꢁC with a heating rate of 10 ꢁC minꢀ . DSC measurements
were performed on DSC 2010 instrument under a dry nitrogen flow,
2.2.4. 4,40,400-tris{[di(p-tolyl)amino]
phenyl}triphenylamine (DBTPA)
1
heating from ꢀ50 to 280 ꢁC at a rate of 10 ꢁC minꢀ and then cooling
from 280 to ꢀ50 ꢁC at a rate of 40 ꢁC minꢀ1, and again heating from
4-[N,N-di(p-tolyl)amino]phenyl boronic acid (1.59 g, 5 mmol),
3 (0.72 g, 1.5 mmol), K2CO3 (1.03 g) and tetrakis(triphenyl-
phosphine)palladium (caution: light sensitive, air sensitive; avoid
strong oxidizing agents; 100 mg) were slurred in a mixture of THF
(30 ml) and water (5 ml). The mixture was refluxed under a N2
atmosphere for 12 h and, after cooling, the ensuing mixture was
extracted with CH2Cl2, and the organic phase was washed with
water, dried over Na2SO4 and filtered. Solvent removal by rotary
evaporation, followed by column chromatography over silica gel
with a mixture of petroleum ether and CH2Cl2 (4:1, v:v) as eluent,
resulted in the product DBTPA as a white solid in 45% yield (0.71 g).
1
ꢀ50 to 280 ꢁC at a rate of 10 ꢁC minꢀ . Cyclic voltammetric
measurementswerecarriedoutinaconventionalthree-electrodecell
using Pt buttonworking electrode of 2 mm diameter, a platinumwire
counter electrode, and a Ag/AgCl reference electrode on a computer-
controlled CHI660C instruments at room temperature. Reduction CV
of the target compounds was performed in CH2Cl2 containing
Bu4NPF6 (0.1 M) as a supporting electrolyte. The energy levels were
calculated using the ferrocene (EFOC) value of ꢀ4.8 eV as standard,
while EFOC was calibrated to be 0.45 V vs. Ag/AgCl electrode in CH2Cl2
solution. The energy levels of the lowest unoccupied molecular
orbitals (LUMOs) were calculated by the HOMO value and the energy
gap (Eg) from the edge of the absorption spectrum. Reagents and
solvents were available commercially and used as received. Triphe-
nylamine [24], 4-formyl-40-fluorotriphenylamine (1) [25] and 4-
[N,N-di(p-tolyl)amino]phenyl boronic acid [26] were synthesized
according to the literature.
Mp: 153–155 ꢁC. 1H NMR (DMSO, 400 MHz):
d 7.75 (d, 6H,
J ¼ 8.16 Hz), 7.52 (d, 6H J ¼ 8.52 Hz), 7.15–7.05 (m, 18H), 6.97–6.90
(m, 18H), 2.26 (s, 18H). MALDI-TOF MS (m/z): 1058.6 (Mþ). 13C NMR
(CDCl3, 100 MHz):
d 146.24, 144.33, 132.78, 131.51, 128.90, 126.32,
126.18, 123.63, 121.89, 19.83. Anal. Calcd for C78H66N4: C, 88.43;
H, 6.28; N, 5.29. Found: C, 88.49; H, 6.24; N, 5.26.
2.2. Synthesis of materials
2.3. OLED fabrication and measurements
2.2.1. 4-(1,1-Dicyanovinyl)-40-fluorotriphenylamine (2)
CuPc, NPB, Alq3, and BCP (purchased from Aldrich Chemical Co.)
were purified using a train sublimation method. OLED devices were
fabricated on 30 U/Ô indium-tin oxide (ITO) coated glass substrates
A mixture of 4-formyl-40-fluorotriphenylamine (1) (1.46 g,
5 mmol), malononitrile (0.66 g, 10 mmol) and NH4OAc (0.77 g,
10 mmol) in anhydrous ethanol (30 ml) was refluxed for 3 h. After
being cooled to room temperature, the mixture was filtered and the
precipitate washed with anhydrous ethanol, providing the product
2 as an orange solid in 90% yield (1.53 g). Mp: 151–152 ꢁC. 1H NMR
using conventional vacuum vapour deposition employing a vacuum
4
of 2 ꢂ 10ꢀ Pa. The organic layers and electrodes were grown by
means of conventional vacuum deposition. A quartz crystal oscil-
lator placed near the substrate was used to measure the thickness
of the thin films, which were calibrated ex situ using an Ambios
Technology XP-2 surface profilometer. EL spectra were recorded on
the Hitachi F-4500 spectrophotometer. Current-voltage character-
istics for OLEDs were measured with a Keithley 4200-SCS semi-
conductor characterization system. Brightness was measured with
a spectra scan PR 650 photometer. All device tests were carried out
under an ambient atmosphere at room temperature.
(CDCl3, 400 MHz):
d
7.74 (d, 2H, J ¼ 8.8 Hz), 7.52 (s, 1H), 7.39 (t, 2H,
J ¼ 7.7 Hz), 7.24 (t, 1H, J ¼ 7.6 Hz), 7.21–7.15 (m, 4H), 7.09 (t, 2H,
J ¼ 8.5 Hz), 6.91 (t, 2H, J ¼ 8.8 Hz). MS (EI): m/z 339 (Mþ, 100%).
2.2.2. N,N0-bis[4-(1,1-dicyanovinyl)phenyl]-N,N0-bis
(4-fluorophenyl)benzidine (TPD-FCN)
To a solution of 2 (0.85 g, 2.5 mmol) in dichloromethane (30 ml)
was added solid ferric chloride (1.62 g, 10 mmol) at 0–5 ꢁC with
stirring. The ensuing mixture was stirred in an ice bath at 0–5 ꢁC for
1 h and then poured into methanol; the precipitate was filtered and
washed with methanol and water. The residue was dissolved in
dichloromethane and dried over anhydrous Na2SO4 and the solvent
was evaporated; the crude product was purified by column chro-
matography using silica gel employing a gradient of petroleum
ether/dichloromethane (1:1, v/v), providing an orange-red solid in
3. Results and discussion
3.1. Synthesis
Two target compounds, TPD-CN and DBTPA, were prepared as
shown in Figs. 1 and 2. The formylated triarylamine 1 underwent
a condensation reaction with malononitrile to afford the interme-
diates 2 with excellent yields, which were easily purified by
a simple filtration and wash with ethanol. The target compound
TPD-FCN was obtained in 80% of isolated yield by an easy oxidative
80% yield (0.68 g). Mp: 228 ꢁC. 1H NMR (DMSO-d6, 400 MHz):
d 8.23
(s, 2H), 7.84 (d, 4H, J ¼ 8.9 Hz), 7.74 (d, 4H, J ¼ 8.4 Hz), 7.39–7.27
(m,12H), 6.90 (d, 4H, J ¼ 8.9 Hz). 13C NMR (CDCl3, 100 MHz):
d 161.0,

X. Cao et al. / Dyes and Pigments 84 (2010) 203–207
205
146 ꢁC for DBTPA in differential scanning calorimetry (DSC)
measurements. The excellent thermal stability indicates that all the
target compounds form stable films by vacuum sublimation. The
high glass transition shows that DBTPA would be a good hole-
transporting material. In cyclic voltammograms (CV), the target
compounds are electrochemically stable in CH2Cl2 solution when
potential cycling was applied (shown in Fig. 3). Their energy levels of
the highest occupied molecular orbitals (HOMOs) were ꢀ5.39 eV for
TPD-FCN and ꢀ4.93 eV for DBTPA. The data suggested that DBTPA
would be more favorable for hole-injection into emitting layer as
compared to the usually used NPB material, and TPD-FCN with the
relatively low LUMO energy level (ꢀ3.00 eV) would facilitate electron
injection when used as an active layer in OLED devices.
F
F
CH2(CN)2
NH4OAc
EtOH
N
CHO
N
CN
NC
2
1
CN
NC
N
F
FeCl3
CH2Cl2
3.3. Photophysical properties
N
UV–vis absorption and photoluminescence (PL) spectra are
important parameters to testify the designed materials. The
absorption peaks of TPD-FCN in dichloromethane are located
448 nm (shown in Fig. 4). The PL emission peak of TPD-FCN in
dichloromethane appeared at 555 nm, and the one in thin films are
located at 622 nm (shown in Fig. 5). Apparently, the salvation
effects resulted in a blue-shift of 67 nm for the PL emission peak of
TPD-FCN in dichloromethane. Importantly, there is only one PL
emission peak for TPD-FCN whether in solution or in the thin film.
The compound TPD-FCN exhibits a considerably strong photo-
luminescence in the solid state because its molecular structure
could effectively avoid the fluorescence-quenching resulting from
the molecular aggregation. In contrast, TPD-FCN displayed a rather
low fluorescent quantum yield in solutions. For example, its fluo-
rescent quantum yield in CH2Cl2 (measured by using coumarin 307
as standard according to a known procedure [27,28]) was 0.10%.
Such an unusual fluorescence behavior of the non-planar twin
dipolar molecule was observed and explained in the previous
publications [15,21].
F
CN
NC
TPD-FCN
Fig. 1. Synthetic route to the compound TPD-FCN.
coupling reaction, and the target compound DBTPA was obtained in
45% of isolated yield by the Pd (0)-catalyzed Suzuki reaction. The
two target compounds were soluble in most organic solvents, and
characterized fully by 1H and 13C NMR, MS, and elemental analysis.
3.2. Thermal and electrochemical properties
Decomposition temperatures at 5 wt% loss of the materials TPD-
CN and DBTPA were found to be over 400 ꢁC from thermogravimetic
analysis (TGA). Glass transitions appeared at 126 ꢁC for TPD-FCN and
3.4. Electroluminescent properties
Br
The target compound TPD-FCN can easily be deposited as pris-
tine thin films by thermal deposition in a vacuum system. In order to
achieve high performances in organic EL devices, two series of
devices were fabricated with the following configuration: ITO/CuPc
(8 nm)/NPB (20 nm)/TPD-FCN (35 nm)/BCP (15 nm)/Alq3 (5 nm)/LiF
(1 nm)/Al (100 nm) (the device 1), and ITO/CuPc (8 nm)/DBTPA
(20 nm)/TPD-FCN (35 nm)/BCP (15 nm)/Alq3 (5 nm)/LiF (1 nm)/Al
(100 nm) (the device 2). CuPc, Alq3, NPB and BCP are, respectively,
copper (II) phthalocyanine, tris(8-hydroxyquinolinolato)aluminum,
NBS
N
N
Br
CH2Cl2
Br
3
N
0.00006
0.00005
0.00004
0.00003
0.00002
0.00001
0.00000
-0.00001
-0.00002
-0.00003
-0.00004
0.00006
0.00005
0.00004
0.00003
0.00002
0.00001
0.00000
-0.00001
-0.00002
-0.00003
-0.00004
TPD-FCN
DBTPA
N
B(OH)2
N
N
Pd(0)
K2CO3
N
DBTPA
0.0
0.5
1.0
1.5
2.0
Potential (V)
Fig. 2. Synthetic route to the compound DBTPA.
Fig. 3. Cyclic voltammogram of TPD-FCN and DBTPA in CH2Cl2.

206
X. Cao et al. / Dyes and Pigments 84 (2010) 203–207
300
a
1.0
0.8
0.6
0.4
0.2
0.0
DBTPA
1.0
3500
3000
2500
2000
1500
1000
500
device 1
device 1
device 2
device 2
DBTPA
TPD-FCN
TPD-FCN
0.8
200
0.6
0.4
100
0.2
0.0
0
0
0
2
4
6
8
10 12 14 16 18 20
300
400
500
600
700
800
Voltage (V)
Wavelength(nm)
Fig. 4. Absorption and PL spectra of the target compounds.
3.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
b
device 1
device 2
N,N0-diphenyl-N,N0-bis(1-naphenyl)-1,10-biphenyl-4,40-diamine, and
2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline. Herein, TPD-FCN,
CuPc, NPB or DBTPA, BCP, and Alq3 were used as emitting, hole--
injection, hole-transporting, hole-blocking, and electron-
transporting layers, respectively.
Fig. 5 shows the PL spectra of the compound TPD-FCN in the
solid films and EL spectra of the TPD-FCN-based emission layer in
the structurally different non-doped devices 1 and 2. Like the PL of
TPD-FCN’s thin film, the two different devices produced only one
red emission peak located nearly at 632 nm and CIE (0.61, 0.39).
This phenomenon demonstrated that the EL emission spectra of
TPD-FCN-based OLEDs are stable and not interfered by different
device structures.
0
50
100
150
200
250
Current Density (mA/cm2)
Fig. 6 shows the EL properties of TPD-FCN-based two different
devices. The device 1 with NPB as hole-transporting layer only had
Fig. 6. Current density–voltage and luminance–voltage (a) and luminous efficiency–
current density (b) characteristics of two TPD-FCN-based OLED devices.
a
medium performance: the maximum luminance reached
2703 cd/m2 at a driving voltage 19.5 V; the maximum luminous
efficiency reached 1.79 cd/A; and a brightness of 393 cd/m2 and
a luminous efficiency of 1.66 cd/A at a low current density of
20 mA/cm2. The device 2 with DBTPA as hole-transporting layer
achieved higher performances. The device 2 had the maximum
luminance of 3400 cd/m2 at a driving voltage 17.5 V, and the
maximum luminous efficiency of 3.04 cd/A, respectively. It is
noteworthy that the device 2 can reach a high brightness of
1399 cd/m2 and a luminous efficiency of 2.85 cd/A at a low current
density of 20 mA/cm2. Compared to the device 1, the device 2 with
DBTPA achieved higher brightness and higher luminous efficiency
at a low driving voltage, particularly the brightness at a low
current density of 20 mA/cm2. The superior performance should
largely be attributed to new red fluorescent material TPD-FCN
with the low fluorescence-quenching nature and the relatively low
LUMO energy level, as well as new hole-transporting material
DBTPA with the relatively high HOMO energy level.
4. Conclusions
We have designed and synthesized dicyanovinyl-functionalized
TPD derivative TPD-FCN as red fluorescent material and TPA-based
derivative DBTPA as the hole-transporting layer by a simple and
easy procedure. The red OLED devices based on TPD-FCN as red
emitters were fabricated in a facile non-doped configuration with
different hole-transporting layers. The device 2 with DBTPA as
PL of solid film
EL of device (1)
EL of device (2)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
hole-transporting layer achieved
a relatively high maximum
luminous efficiency and high brightness at a low current density of
20 mA/cm2. The red fluorescent material will be anticipated to give
even better performances through further optimization of the
device processing. This work would lead to the development of
more new red fluorescent materials usable in OLEDs for high
performances.
Acknowledgements
450
500
550
600
650
700
750
Wave length (nm)
The authors thank the National Natural Science Foundation of
China (Project Nos. 20872142 and 20825208) for financial support
of this work.
Fig. 5. PL spectra of TPD-FCN in the solid films and EL spectra of two TPD-FCN-based
OLED devices.

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