Multifunctional bipolar triphenylamine/oxadiazole derivatives: Highly efficient blue fluorescence, red phosphorescence host and two-color based white OLEDs

Article abstract of DOI:10.1039/b816264f

Two simple triphenylamine/oxadiazole derivatives were synthesized and fully characterized; their multifunctionality as highly efficient non-doped blue fluorescence, excellent red phosphorescent host and single-doped two-color based white OLEDs has been de

Full text of DOI:10.1039/b816264f

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Multifunctional bipolar triphenylamine/oxadiazole derivatives: highly
efficient blue fluorescence, red phosphorescence host and two-color based
white OLEDsw
a
b
c
a
a
a
Youtian Tao, Qiang Wang, Yuan Shang, Chuluo Yang,* Liang Ao, Jingui Qin,
b
c
Dongge Ma* and Zhigang Shuai*
Received (in Cambridge, UK) 16th September 2008, Accepted 3rd November 2008
First published as an Advance Article on the web 20th November 2008
DOI: 10.1039/b816264f
9
Two simple triphenylamine/oxadiazole derivatives were synthe-
sized and fully characterized; their multifunctionality as highly
efficient non-doped blue fluorescence, excellent red phosphorescent
host and single-doped two-color based white OLEDs has been
demonstrated.
electron-withdrawing moieties by incorporation of steric groups
10
and/or meta-linkage of the two moieties. In our previous
11
work, we firstly reported an ortho-linked carbazole/oxadiazole
hybrid molecule as a phosphorescent host, which achieved a
maximum current efficiency of 77.9 cd/A for green and 13.6 cd/A
for red light.
Phosphorescent organic light-emitting diodes (PHOLEDs) con-
tinue to attract intensive interest because they can approach
In this communication, we report a simple combination of
commonly used hole-transporter triphenylamine and electron-
12
100% internal quantum efficiency theoretically. Highly efficient
transporter oxadiazole. By introducing the triphenylamine
moiety on the o-position relative to the oxadiazole ring of
2,5-diphenyl-1,3,4-oxadiazole, a twisted molecule TPA-o-OXD
was acquired, which shows blue-shifted emission, less intra-
molecular charge-transfer and higher triplet energy levels in
comparison with its para-structured analogue TPA-p-OXD. Both
compounds show promising bipolar transport properties and
highly efficient blue fluorescence. Furthermore, they are proven
to be excellent host materials for red electrophosphorescence.
Finally, a two-color based white OLED (WOLED) was con-
structed in a rather simple way by utilization of their dual roles as
blue-emitter and host for red phosphorescence. The single-doped
and single emissive layer device structure is much simple than the
PHOLEDs with emission covering the whole visible region have
1–4
been reported over the past decade. However great success has
2,5
just been achieved in green-light phosphorescence. Efficient
blue PHOLEDs with Commission Internatinale de Eclairage
1a,b
CIE) y values below 0.2 are very scarce,
(
and thus there is
still a need to develop blue-emitting fluorescent materials with
sufficient luminous efficiency and proper chromaticity. On the
other hand, the red phosphorescent materials investigated were
0
mostly doped into a 4,4 -bis(N-carbazolyl)biphenyl (CBP) host
matrix, which is prone to crystallization when the dopant con-
4a,6
centration is too low. Furthermore, the driving voltages of red
PHOLEDs with the CBP host are usually high because of
7
13
14
15
insufficient and/or unbalanced injection of holes and electrons.
Recently, bipolar hosts have aroused considerable interest in
OLEDs, because they can balance the charge recombination and
widely reported stacked, multi-emissive-layer or triple doped
WOLEDs.
The two compounds TPA-p-OXD (1) and TPA-o-OXD (2)
were readily prepared by a Pd(0)-catalyzed Suzuki cross-coupling
reaction of 4-(diphenylamino)phenylboronic acid with 2,5-bis-
8
simplify device structures. However, there is a dilemma between
the bipolar transporting properties and the band gap of the
material, because the electron-donating and electron-withdrawing
moieties integrated on bipolar molecules unavoidably lower the
band gap of the material due to the intramolecular charge-
transfer; while the low triplet energy of the host can cause reverse
energy transfer from the guest back to the host, consequently
leading to the decreased efficiency of the PHOLEDs. To address
this issue, most recently, molecular design focuses on the
interruption of p-conjugation between electron-donating and
0
0
(4 -bromophenyl)-1,3,4-oxadiazole or 2,5-bis(2 -bromophenyl)-
1,3,4-oxadiazole (Scheme 1). Noticeably, for the steric torsional
2, a high yield of 85% was obtained by using KOH as the base;
2 3
this is in contrast to a low yield (below 20%) when using Na CO
as a base as in the normal Suzuki reaction. The good thermal
stability of 1 and 2 is indicated by the high decomposition
d
temperatures (T , corresponding to 5% weight loss) of 435 1C
(1) and 432 1C (2) in the thermogravimetric analysis. The
a
Department of Chemistry, Hubei Key Lab on Organic and Polymeric
Optoelectronic Materials, Wuhan University, Wuhan 430072,
People’s Republic of China. E-mail: clyang@whu.edu.cn
State Key Laboratory of Polymer Physics and Chemistry,
b
Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences, Changchun 130022, People’s Republic of China.
E-mail: mdg1014@ciac.jl.cn
CAS Key Laboratory of Organic Solids, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100080, People’s Republic of
China. E-mail: zgshuai@iccas.ac.cn
c
w Electronic supplementary information (ESI) available: Synthetic
procedures, DFT calculation, OLEDs fabrication and characteriza-
tion. See DOI: 10.1039/b816264f
Scheme 1 Synthesis of 1 and 2.
This journal is ꢀc The Royal Society of Chemistry 2009
Chem. Commun., 2009, 77–79 | 77

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glass-transition temperature (T
g
) of 116 1C (1) and 94 1C (2)
performances are summarized in Table 2. We note that the
device derived from 1 emits blue light with a maximum emission
of 459 nm and a low energy shoulder at 520 nm; whereas the EL
spectrum based on 2 shows an emission peak at 453 nm and no
shoulder emission. The suppression of shoulder emission may be
contributed to the twisted molecular configuration of 2. This
may provide a new strategy to avoid charge-transfer or excimer
emission in non-doped blue light-emitting devices. A maximum
determined through differential scanning calorimetry (DSC) are
16
much higher than that of CBP (62 1C) and comparable to NPB
17
(1,4-bis[(1-naphythylphenyl)amino]biphenyl) (95 1C), which is
18
essential for the morphological stability of thin films.
Fig. 1 shows the electronic absorption and fluorescence
spectra of 1 and 2. The absorption around 300 nm for both
1
and 2 originates from the triphenylamine-centered n–p*
9b
2
transition. The absorption at 375 nm for 1 can be attributed
to p–p* transition from the electron-donating triphenyamine
moiety to the electron-accepting oxadiazole moiety. Notably,
the p–p* absorption transition is blue-shifted by 40 nm and
remarkably reduced in intensity as only a shoulder for 2, which
implies the inhibition of intramolecular charge-transfer. The
PL spectrum of the ortho-structured 2 also exhibits a hypso-
chromic shift of 13 nm with respect to 1 in the solid state. The
above phenomena can be attributed to the large space torsion
disrupting the p-conjugation between triphenylamine and
oxadiazole moieties for 2. The two compounds are blue-
emitting with an extraordinarily high quantum yield of 93%
brightness of 7364 cd/m at 13.3 V and a maximum current
2
efficiency of 3.9 cd/A at 3.26 mA/cm for 1, and 9441 cd/m at
2
2
12.9 V and 3.1 cd/A at 0.7 mA/cm for 2 were achieved. In
addition, all devices display low turn-on voltages in the range
2.7–3.1 V. These performances are comparable with those of
9a,17,19
non-doped blue-emitting devices reported in literature.
The triplet energies of 1 and 2 were determined to be 2.35 and
2.46 eV, respectively, by the highest-energy vibronic sub-band of
the phosphorescence spectra at 77 K, and therefore they may act
as appropriate host materials for orange/red-emitting phos-
phorescent emitters. The OLEDs using 1 or 2 as hosts for red
phosphorescent iridium complex bis(1-phenylisoquinolinato-
0
for 1 and 74% for 2 in CH
To evaluate the performance of 1 and 2 as blue-emitters,
device A with the structure ITO/MoO (10 nm)/1,4-bis-
(1-naphthylphenyl)amino]biphenyl (NPB, 80 nm)/1 or
2
Cl
2
solution (Table 1).
N,C2 ) iridium(acetylacetonate) [(piq)
2
Ir(acac)] were exploited
(10 nm)/NPB
2
Ir(acac) (6 wt%, 20 nm)/BCP (10 nm)/Alq3
through device B with configurations ITO/MoO
(80 nm)/ 1 or 2:(piq)
3
3
[
2
(30 nm)/LiF (1 nm)/Al (100 nm). Both devices emit red light with
a CIE value of (0.68, 0.32). The EL data are summarized in
2
Table 2. A maximum brightness of 18 333 cd/m and a maximum
(
20 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP,
0 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm) was fabricated.
1
NPB and Alq3 were used as the hole- and the electron-
transporting materials, respectively, and BCP was used as the
current efficiency of 8.0 cd/A was achieved for the 1 based device.
The best EL performance was obtained by using 2 as a host, with
2
a maximum brightness of 24 416 cd/m at 14.9 V, a maximum
hole and exciton blocking layer, while MoO
3
and LiF served as
the hole- and electron-injecting layers, respectively. All devices
are blue-emitting, and representative EL spectra are shown in
Fig. 1. Voltage-luminance-current density and current efficiency-
current density characteristics for representative devices are
shown in Fig. S1 and S2, respectively (ESIw). The device
current efficiency of 11.3 cd/A (corresponding to a maximum
external quantum efficiency of 14.2%) and a power efficiency of
2
8.2 lm/W. When the brightness reaches 1000 cd/m , the current
efficiency is still as high as 9.7 cd/A, which is much better than
2
0
those CBP hosted devices. To the best of our knowledge, these
performance data are amongst the best pure red phosphorescent
4a–c
OLEDs with similar CIE coordinates reported in the literature.
The better device performance achieved from 2 may be
attributed to the enhancement in the localization of charges
and higher triplet energy gap as demonstrated by Kakimoto
9b,c,10a
and coworkers and Su et al.
As shown in Fig. 2, the DFT
calculated results indicated that 2 has nearly complete separation
of the HOMO and LUMO at its hole- and electron-transporting
moieties, respectively; whereas 1 exhibits only partial separation
of the HOMO and LUMO. The optimized dihedral angles for 2
are larger than those for 1, 55.31 to 36.51 and 231 to 0.31. The
more twisted structure of 2 leads to a significantly higher triplet
energy gap for 2 (2.46 eV) than for 1 (2.35 eV).
2 2
Fig. 1 Normalized absorption (in CH Cl solution) and PL (film) spectra
of 1 and 2 (left) and electroluminescence spectra of the devices (right).
Table 1 Physical data of 1 and 2
By utilizing the dual roles of 1 and 2 as both efficient blue-
emitters and excellent hosts for red phosphorescence, white
OLEDs were successfully fabricated in a rather simple way
1
2
a
T
m
/T
g
/T
d
b(1C)
320/116/435
304, 375
467
93
5.28/2.60
270/94/432
309, 335 (sh)
454
74
5.25/2.41
2
with the configuration ITO/NPB (40 nm)/ 1: (piq) Ir(acac)
l
l
abs (nm)
em, max (nm)
c
(0.1 wt%, 20 nm)/BCP (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al
(100 nm) (device C). The EL spectrum at 10 V exhibits the blue
fluorescent emission of 1 and the red phosphorescent emission
of the iridium complex simultaneously (Fig. 1). A maximum
current efficiency of 7.9 cd/A (corresponding to a Zext of 5.2%)
f
d
F (%)
e
HOMO/LUMO (eV)
a
m g d
T : melting point. T : glass-transition temperature. T : thermal
b
decomposition temperature. Absorption maximum, measured in
c
d
Emission maximum, measured in film. Measured in
CH
CH
2
Cl
Cl
2
2
.
2
at 0.02 mA/m and CIE value of (0.39, 0.31) were achieved.
The single-doped device structure was much more simple than
those stacked, multi-emissive-layer or multi-doped WOLEDs.
e
with an integrating sphere. Determined from the onset
2
oxidation/reduction potentials of cyclic voltammetry curve in DMF.
7
8 | Chem. Commun., 2009, 77–79
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Table 2 EL data of the devices
a
on
b
c
c
d
e
p
f
Device
Compound
V
L
max /V
Z
Z
ext
Z
CIE (x,y)
A
1
2
1
2
1
2.8
3.3
2.7
3.1
5.3
7364, 13.3
9441, 12.9
18 333, 12.9
24 416, 14.9
1200, 12.5
3.9
3.1
8.0
11.3
7.9
1.7
2.0
9.8
14.2
5.2
3.6
2.6
7.0
8.2
4.7
0.18, 0.31
0.17, 0.19
0.68, 0.32
0.68, 0.32
0.39, 0.31
B
C
a
b
2
c
d
Turn-on voltage (V). Maximum luminance (cd/m ). Maximum current efficiency (cd/A). Maximum external quantum efficiency (%).
e
f
Maximum power efficiency (lm/W). Commission International de I’Eclairage coordinates.
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In summary, we have developed a simple strategy for designing
bipolar molecules by the ortho-linkage of electron-donating
triphenylamine units and the electron-accepting oxadiazole unit.
The twisted molecule showed blue-shifted emission, less intra-
molecular charge-transfer, and a higher triplet energy level com-
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Chem. Commun., 2009, 77–79 | 79