Bridged triphenylamines as novel host materials for highly efficient blue and green phosphorescent OLEDs

Article abstract of DOI:10.1039/b902950h

Two bridged triphenylamine-triphenylsilane (BTPASi) hybrids have been designed as host materials for phosphorescent OLEDs; devices with the novel host materials achieve maximum external quantum efficiencies as high as 15.4% for blue and 19.7% for green el

Full text of DOI:10.1039/b902950h

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Bridged triphenylamines as novel host materials for highly efficient blue
and green phosphorescent OLEDsw
Zuoquan Jiang,a Yonghua Chen,b Cong Fan,a Chuluo Yang,*a Qi Wang,b Youtian Tao,a
Zhiqiang Zhang,b Jingui Qina and Dongge Ma*b
Received (in Cambridge, UK) 11th February 2009, Accepted 25th March 2009
First published as an Advance Article on the web 27th April 2009
DOI: 10.1039/b902950h
Two bridged triphenylamine–triphenylsilane (BTPASi) hybrids
have been designed as host materials for phosphorescent
OLEDs; devices with the novel host materials achieve maximum
external quantum efficiencies as high as 15.4% for blue and
19.7% for green electrophosphorescence.
the desired criteria for a host material; however, the flexible
TPA molecules lack rigidity, which is disadvantageous to its
thermal and morphological stabilities, and consequently limits
its application as host material in OLEDs. So far, there are
very few triphenylamine-based host materials. Recently, Shu’s
group reported a high triplet energy host based on a
TPA–fluorene hybrid for efficient blue (18.1 lm Wꢀ1, 13.1%)
and green (21 lm Wꢀ1, 12%) electrophosphorescence.9 We
also reported a TPA–oxadiazole derivative as bipolar host for
red electrophosphorescence (8.2 lm Wꢀ1, 14.2%).10
Phosphorescent organic light-emitting diodes (PHOLEDs)
have attracted intense interest because they can in theory
approach 100% internal quantum efficiency.1 The triplet
emitters of heavy-metal complexes are normally doped in a
host matrix to reduce concentration quenching and triplet–
triplet annihilation etc.,2 and thus a suitable host material for
separation of the triplet emitters is vital to achieve efficient
electrophosphorescence. The host for phosphorescent emitters
has to fulfil the requirement that the triplet energy of the host
has to be higher than that of the guest. This should prevent
reverse energy transfer from the guest back to the host and
confine triplet excitons to the guest molecules.3 This require-
ment still remains a challenge in designing a host for blue
electrophosphorescence because one should avoid extended
conjugation to achieve a high triplet energy; on the other
hand, a bulky and sterically hindered molecular configuration
is preferable for forming morphologically stable and uniform
amorphous films.4 Up to now, the hosts for blue electro-
phosphorescence are mainly focused on carbazole-based
derivatives, such as 4,40-bis(9-carbazolyl)-2,20-biphenyl (CBP),1b
1,3-bis(9-carbazolyl)benzene (mCP),3b 3,5-bis(9-carbazolyl)-
tetraphenylsilane (SimCP),5a 9-(4-tert-butylphenyl)-3,6-bis-
(triphenylsilyl)-9H-carbazole (CzSi), etc.5b In addition, a series
of organosilicon compounds with ultrahigh energy gaps as
host materials for deep blue electrophosphorescence have also
been reported.3d–e,6
In our search for new high triplet energy materials capable
of hosting blue-emitting phosphors, the rigidity of the
carbazole structure, as well as the high triplet energy and
good hole-transporting ability of triphenylamine, inspired us
to design a rigid triphenylamine skeleton. Towards this aim,
we synthesized a bridged triphenylamine via a diarylmethene
linkage between two phenyl rings. To have a large energy gap
and molecular size simultaneously, the electrochemically
active sites (para position of the nitrogen) of TPA were blocked
with triphenylsilyl through the non-conjugated sp3-hybridized
silicon atom.5b The bulk and steric spacer triphenylsilyl, as well
as the aryl substituents on the bridgehead carbon atom, are
expected to effectively separate the triplet guest molecules, and
improve their thermal and morphological stabilities. The new
hosts retain the high triplet energy (ca. 2.96 eV) of TPA. Devices
fabricated with the new hosts show maximum external quantum
efficiencies as high as 15.4% (35 lm Wꢀ1) for blue and 19.7%
(64 lm Wꢀ1) for green electrophosphorescence.
The new hosts were synthesized according to the synthetic
route shown in Scheme 1. The initial intermediate 5-tert-butyl-
2-diphenylamino-isophthalic acid dimethyl ester was treated
with aryllithium, and subsequently underwent intramolecular
ring closure through Friedel–Crafts reaction to give the
bridged triphenylamines (BTPA). The BTPAs were
As we know, triphenylamine (TPA) has a high triplet energy
of 3.04 eV7 and good hole-transporting abilities.8 These
characteristics mean that TPA derivatives are close to meeting
a Department of Chemistry, Hubei Key Lab on Organic and Polymeric
Optoelectronic Materials, Wuhan University, Wuhan, 430072,
PR China. E-mail: clyang@whu.edu.cn; Fax: +86 27-68756757;
Tel: +86 27-68756757
b State Key Laboratory of Polymer Physics and Chemistry,
Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences, Changchun, 130022, PR China.
E-mail: mdg1014@ciac.jl.cn; Fax: +86 431-85262357;
Tel: +86 431-85262357
w Electronic supplementary information (ESI) available: Synthesis,
characterization, crystal structure of the compounds. CCDC 714843.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/b902950h
Scheme 1 Synthesis of BTPASi1 and BTPASi2.
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3398 | Chem. Commun., 2009, 3398–3400

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Fig. 1 Molecular diagram of BTPASi2 (for clarity, non-carbon atoms
are shown with 30% thermal ellipsoids probability; carbon atoms are
shown as spheres; hydrogen atoms and solvent molecules are omitted).
Fig. 2 Absorption and emission spectra of BTPASi1 and BTPASi2 in
CH2Cl2 solution at room temperature, and their phosphorescence
spectra in toluene at 77 K (inset).
suppresses intermolecular aggregation.13 The inset of Fig. 2
depicts the phosphorescence spectra of BTPASi1 and BTPASi2
measured on a frozen toluene matrix at 77 K. Their triplet
energies (ET) were determined to be ca. 2.95 eV by the highest-
energy vibronic sub-band of the phosphorescence spectra. This
value is sufficiently high to host red, green, and blue phospho-
rescent emitters, such as the common blue-emitter bis[2-(40,60-
difluorophenyl)pyridinato-N,C20]iridium(III) picolate (FIrpic,
2.62 eV) and green-emitter iridium(III) fac-tris(2-phenyl-
pyridine) (Ir(ppy)3, 2.42 eV).
brominated with NBS under mild conditions, and the obtained
dibromo-BTPAs were treated with n-BuLi at ꢀ78 1C, and then
quenched with chlorotriphenylsilane to afford the desired
products BTPASi1 and BTPASi2. All the compounds were
fully characterized by 1H NMR, 13C NMR, mass spectro-
metry, and elemental analysis (see ESIw). Furthermore, the
molecular structure of BTPASi2 was determined by X-ray
single-crystal crystallographic analysis.z As shown in Fig. 1,
the molecule displays a three-dimensional scissor-like con-
formation, which should be advantageous in effectively iso-
lating the triplet guest molecules, and suppress the intermolecular
aggregation.11
The electrochemical behaviours of BTPASi1 and BTPASi2
were probed by cyclic voltammetry (CV). They all exhibit a
reversible oxidation process. Further more, upon repeated
scanning, both still show reversible oxidation behavior. This
contrasts with the irreversible oxidation process of the pristine
triphenylamine.14 Such electrochemical reversibility can be
attributed to the blocking of the three para-phenyl position
active sites by the tert-butyl and triphenylsilyl moieties, and
should benefit the stability of their electroluminescence devices.
We investigated the utility of BTPASi1 and BTPASi2 as the
host materials for blue and green phosphorescent devices with
the following configurations: device A–B: ITO/MoO3(10 nm)/
NPB(80 nm)/mCP(5 nm)/host: FIrpic(20 nm)/3-(4-biphenylyl)-
4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ, 40 nm)/
LiF/Al; device C–D: ITO/MoO3(10 nm)/NPB(80 nm)/
mCP(5 nm)/host: Ir(ppy)3(20 nm)/TAZ(40 nm)/LiF/Al. NPB
was used as the hole-transporting material; mCP was used to
confine excitons to the emitting layer; TAZ was used as
electron-transporting as well as hole-blocking material;3e
phosphors FIrpic and Ir(ppy)3 doped in the host were used
as the emitting layer, with optimized doping levels of FIrpic
and Ir(ppy)3 at 6% and 9%, respectively; MoO3 and LiF
served as hole- and electron-injecting layers, respectively.15
The luminance–current density–current efficiency (L–J–Zc)
characteristics of the devices are shown in Fig. 3. The device
data are summarized in Table 2.
The good thermal stability of the compounds is indicated by
the high decomposition temperatures (Td, corresponding to 5%
weight loss, see Table 1) of 483 1C for BTPASi1 and 491 1C for
BTPASi2 in thermogravimetric analysis. Their glass transition
temperatures (Tg) were observed at 168 1C for BTPASi1 and
173 1C for BTPASi2, through differential scanning calorimetry
(DSC), which are much higher than those of triphenylamine
analogues, such as 1,4-bis[(1-naphthylphenyl)amino]biphenyl
(NPB, 98 1C), as well as carbazole analogues, such as CBP
(62 1C) and mCP (60 1C).12 The high Tg and Td values of the
two compounds may be mainly attributed to the rigid bridged
TPA core, and can improve the film morphology and reduce the
possibility of phase separation upon heating.
BTPASi1 and BTPASi2 show almost the same electronic
absorption and fluorescence spectra (Fig. 2). The absorption
at 328 nm may be attributed to p–p* transitions of the
bridged-triphenylamine core. Their PL spectra in CH2Cl2
solution exhibit ultraviolet emission at ca. 370 nm. Their
emission maxima in the film are only red-shifted by 2–4 nm
with respect to their CH2Cl2 solutions (Fig. S2w), which
implies that the triphenylsilyl moiety, as well as the peripheral
aryl group on the bridgehead carbon atom, effectively
Table 1 Thermal and optical properties of BTPASi1 and BTPASi2
Abs lmax/nm PL lmax/nm
Device A with BTPASi1 as host and FIrpic as dopant
exhibits a maximum current efficiency of 44 cd Aꢀ1, a
maximum power efficiency of 35 lm Wꢀ1, and a maximum
luminance of 6543 cd mꢀ2. At the practical brightness of
Td/1C Tg/1C solna/film
solna/film
ETb/eV
BTPASi1 483
BTPASi2 491
168
173
328/329
328/331
370/374
369/371
2.95
2.97
100 cd mꢀ2, the current efficiency is still as high as 36.5 cd Aꢀ1
.
To the best of our knowledge, the device efficiencies are among
the highest for the blue phosphorescent OLEDs to date.5b,9
a
b
Measured in CH2Cl2 solution. Triplet energy.
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This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 3398–3400 | 3399

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the 973 program (Grant Nos. 2009CB623602, 2009CB930603,
2009CB623604) for financial support.
Notes and references
z Crystal data of BTPASi2 at 293(2) K: C92H83NO2Si2, Mr = 1290.77,
monoclinic, space group P21/c, Dc = 1.144 g cmꢀ3, Z = 4, a =
14.6954(5) A, b = 19.5226(5) A, c = 26.3298(9) A, a = 901, b =
97.2890(10)1, g = 901, V = 7492.8 (4) A3, m = 0.097 mmꢀ1. Bruker
AXS Smart CCD diffractometer, Mo-Ka radiation, l = 0.71073 A,
number of reflections measured = 59 952, number of independent
reflections = 13 230, Rint = 0.0746, final R(F) = 0.0659 (I 4 2s(I)),
wR(F2) = 0.1657. CCDC 714843.
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Table 2 Summary of device performance
Host
BTPASi1
FIrpic
A
BTPASi2
FIrpic
B
Dopant
Device
Ir(ppy)3
C
Ir(ppy)3
D
Vturnꢀon/V
La/cd mꢀ2
EQEb (%)
LEc/cd Aꢀ1
PEd/lm Wꢀ1
CIE (x, y)
3.9
6543
15.4
44
3.7
24 029
15.0
3.7
1865
8.8
25
17
3.5
16 473
19.7
54
46
71
64
35
(0.26, 0.48) (0.33, 0.60) (0.26, 0.47) (0.33, 0.60)
a
b
Maximum luminance. Maximum external quantum efficiency.
d
Maximum current efficiency. Maximum power efficiency.
c
Device D with BTPASi2 as host and Ir(ppy)3 as dopant shows
a maximum current efficiency of 71 cd Aꢀ1, a maximum
power efficiency of 64 lm Wꢀ1, and a maximum luminance of
16 473 cd mꢀ2. We note that the efficiencies are comparable with
the best green phosphorescent OLEDs we recently reported.2c
In summary, we have designed a type of bridged triphenyl-
amine–triphenylsilane (BTPASi) hybrids as novel host mate-
rials for phosphorescent OLEDs. The rigidification of
triphenylamine significantly improves their thermal and
morphological stabilities without affecting their good hole-
transporting ability and high triplet energy. Devices with the
host materials show maximum external quantum efficiency as
high as 15.4% for blue and 19.7% for green electrophos-
phorescence. This work reveals a very promising application
of triphenylamine-based derivatives as efficient host materials.
The further optimization of device configuration is under way.
We thank the National Natural Science Foundation of
China (Project Nos. 50773057, 20474047 and 20621401),
the Ministry of Science and Technology of China through
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This journal is The Royal Society of Chemistry 2009
3400 | Chem. Commun., 2009, 3398–3400