A facile and versatile approach to efficient luminescent materials for applications in organic light-emitting diodes

Article abstract of DOI:10.1002/asia.201100753

A novel approach for creating efficient solid-state emitters is proposed. In this approach, various polyaromatic hydrocarbon groups are attached to tetraphenylethene, generating a series of luminogens that possess aggregation-induced emission characteristics. These luminogens are highly emissive in the solid state, with emission efficiencies equaling unity. High-performance OLEDs using these luminogens as light-emitting layers are fabricated, exhibiting efficiencies up to 5.6 lm W^-1, 7.3 cd A ^-1, and 3.0 %.

Full text of DOI:10.1002/asia.201100753

COMMUNICATION
DOI: 10.1002/asia.201100753
A Facile and Versatile Approach to Efficient Luminescent Materials for
Applications in Organic Light-Emitting Diodes
Zujin Zhao,*[a] Shuming Chen,[c] Carrie Y. K. Chan,[b] Jacky W. Y. Lam,[b]
Cathy K. W. Jim,[b] Ping Lu,[d] Zhengfeng Chang,[a] Hoi Sing Kwok,[c] Huayu Qiu,[a] and
Ben Zhong Tang*[b]
Design and synthesis of luminophors with high emission
efficiency is a hot research topic due to their practical appli-
cations in optoelectronics such as organic light-emitting
diodes (OLEDs).[1] To respond to the demand, scientists
have prepared a variety of luminogenic materials. Whereas
these materials emit intensely in solution, they become
weakly fluorescent or even non-luminescent in the solid
state. In the solid state, the molecules are located in the im-
mediate vicinity. The aromatic rings of the neighboring fluo-
rophores, especially those with disc-like shapes such as
pyrene and anthracene, experience strong p–p stacking in-
teractions, which promote the formation of aggregates with
ordered or random structures.[2] The excited states of the ag-
gregates often decay via nonradiative pathways, which are
notoriously known as aggregation-caused quenching (ACQ)
of light emission in the condensed phase. This notorious
ACQ effect has prevented many lead compounds carrying
chromophore groups that were identified by solution-based
screening processes in the laboratory from finding real-
world applications and becomes a thorny problem for the
development of efficient OLEDs. Various chemical, physi-
cal, and engineering approaches and processes have been
proposed to alleviate the ACQ effect but have met with
only limited success. The difficulty lies in the fact that aggre-
gate formation is an intrinsic process when luminogenic
molecules are located in close vicinity in the aggregate
phase.[3]
Our group observed a phenomenon of aggregation-in-
duced emission (AIE) that is exactly opposite to the ACQ
effect. Instead of quenching, aggregate formation has turned
a series of propeller-like luminogens to emit intensely in the
solid state.[4,5] This AIE phenomenon is rationalized to be
caused by the restriction of intramolecular rotation (IMR).
In the solution state, the active rotation of aromatic rings ef-
fectively deactivates the excited state through a rotational
energy relaxation channel, thus rendering the luminogens
non-emissive. In the aggregate state, the IMR process is im-
peded, thereby blocking the nonradiative decay pathway
and converting the luminogens into strong emitters.[6]
Among the AIE luminogens, tetraphenylethene (TPE) pos-
sesses a simple chemical structure[7] but shows an outstand-
ing AIE effect. TPE and its derivatives have found an array
of high-technological applications in environmental and bio-
logical sciences.[8] However, TPE itself is not a good light-
emitting material for the construction of efficient OLEDs.
An electroluminescence (EL) device fabricated using TPE
as emitting layer shows inferior performances with maxi-
mum luminance and maximum current efficiency of merely
1800 cdmÀ2 and 0.45 cdAÀ1, respectively, presumably due to
TPEꢀs high tendency to crystallize during the film fabrica-
tion process as well as its low thermal stability.[7a] Linking
two or three TPE units through a silicon atom affords lumi-
nogens with enhanced thermal stability and emission effi-
ciency in the solid state. However, these luminogens exhibit
limited improvement in EL performance.[9] Delightfully,
recent studies show that TPE can be easily attached to some
ACQ chromophores, thus generating new AIE luminogens
with efficient solid-state emissions and outstanding device
performances. Representative examples are given by mono-
substituted TPE derivatives (TPEArs) that are non-emissive
in solution but exhibit high fluorescence quantum yield
(FF), up to unity, in the solid state (Scheme 1).[10] We envi-
sion that such strategy is versatile to solve the ACQ prob-
lem of traditional luminophors and, at the same time, to
create high-performance solid-state emitters for OLEDs. To
further verify the applicability of such synthetic tools, we
prepared new luminogens consisting of TPE cores with dif-
ferent ACQ chromophores at both ends (Scheme 2) and in-
[a] Dr. Z. Zhao, Z. Chang, Prof. H. Qiu
College of Materials, Chemistry and Chemical Engineering
Hangzhou Normal University
Hangzhou 310036 (China)
E-mail: zujinzhao@gmail.com
[b] C. Y. K. Chan, Dr. J. W. Y. Lam, Dr. C. K. W. Jim, Prof. B. Z. Tang
Department of Chemistry
The Hong Kong University of Science & Technology
Clear Water Bay, Kowloon, Hong Kong (China)
E-mail: tangbenz@ust.hk
[c] S. Chen, Prof. H. S. Kwok
Center for Display Research
The Hong Kong University of Science & Technology
Clear Water Bay, Kowloon, Hong Kong (China)
[d] Dr. P. Lu
State Key Laboratory of Supramolecular Structure and Materials
Jilin University
Changchun 130012 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100753.
484
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Chem. Asian J. 2012, 7, 484 – 488

Figure 1. a) Absorption and b) PL spectra of TPEBArs in THF solution
(a) and in the solid state as thin films (b).
Scheme 1. Chemical structures of TPE derivatives with various ACQ
chromophores.
0.37%, which are slightly higher than those of TPEArs
(0.02–0.22%). Such low FF values reveal that they are prac-
tically weak emitters in the solution state. The FF values of
TPEBArs are much lower than those of their ACQ compo-
nents such as anthracene (36%) and pyrene (32%),[11] thus
demonstrating that the TPE unit works as an emission
quencher in the solution state. The multiple phenyl rings of
the TPE unit in an isolated molecule can undergo active
IMR with little restraint in dilute solutions, which effectively
consumes the energy of the excited state, thus rendering the
dye molecules weakly emissive. Although the incorporation
of an additional ACQ chromophore in TPEArs has generat-
ed TPEBArs with slightly enhanced emission efficiency,
their FF values are still low as the emission quenching effect
of the IMR process of the TPE unit is so efficient in the so-
lution state.
vestigated their optical and thermal properties as well as ap-
plications in OLEDs.
Scheme 2. Synthetic route to disubstituted TPE derivatives.
TPEArs were synthesized according to the literature
method.[10b] Whereas their AIE characteristics and electronic
and crystal structures have been presented in our recently
published paper, their thermal and EL properties were in-
vestigated in this study. Their disubstituted counterparts
(TPEBArs) were prepared according to the synthetic route
shown in Scheme 2. A mixture of Z- and E-isomers (approx-
imately 1:1 ratio) of 1,2-bis(4-bromophenyl)-1,2-diphenyle-
thene (2) was prepared by a McMurry coupling reaction of
4-bromobenzophenone (1). Suzuki coupling of 2 with the
On the contrary, all synthesized TPEBArs are highly
emissive in the solid film state. The PL spectra of their thin
films are shown in Figure 1b. The TPEBArs emit light at
474–492 nm (sky blue) and their photoluminescence is bath-
ochromically shifted from that of TPEArs (Table 1), pre-
sumably due to their higher conjugation. The associated FF
values measured by an integrating sphere are much higher
than those in solution, reaching values up to 100%, thus re-
vealing that TPEBArs are completely AIE-active.
corresponding arylboronic acids catalyzed by [PdACHTNUTRGNE(UNG PPh3)4] in
Table 1. Optical and thermal properties of TPEArs and TPEBArs.[a]
basic medium afforded the target products in moderate
yields (52–78%). Similar to 2, these molecules were also
synthesized in a stereorandom fashion. The detailed synthet-
ic procedure and characterization data are given in the Sup-
porting Information. These luminogens are soluble in
common organic solvents, such as tetrahydrofuran (THF),
dichloromethane, chloroform, and toluene, but insoluble in
ethanol and water.
Figure 1a shows the absorption spectra of TPEBArs in
THF. The prepared TPEBArs have an absorption maximum
in the range from 325 to 388 nm and a spectral profile that
varies largely with the type of the ACQ chromophore. All
the TPEBArs display a weak photoluminescence (PL) with
hardly discernible peaks in dilute THF solution (10 mm). The
determined FF values, using 9,10-diphenylanthracene (FF =
90% in cyclohexane) as standard, fall in the range of 0.13–
labs (nm)
Soln
lem (nm)
Film
FF (%)
Tg/Td
Soln[b]
Film[c]
[oC]
TPEBPy
TPEBAn
TPEBPa
TPEBNp
TPEB-2-Np
TPEPy
TPEAn
TPEPa
TPENp
TPE-2-Np
347
388
332
325
340
348
387
323
321
326
488
486
475
474
492
468
450
481
469
474
0.37
0.29
0.34
0.14
0.13
0.22
0.12
0.03
0.02
0.05
100
100
100
100
100
100
100
88
142/494
-/441
129/448
90/387
100/363
-/325
-/353
86/363
63/311
71/284
83
100
[a] Soln=solution (10 mm in THF), labs =absorption maximum, lem =PL
maximum at excitation at 350 nm. [b] Fluorescence quantum yield mea-
sured in THF solution using 9,10-diphenylanthracene (FF =90% in cyclo-
hexane) as standard. [c] Absolute fluorescence quantum yield measured
using an integrating sphere.
Chem. Asian J. 2012, 7, 484 – 488
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485

Z. Zhao, B. Z. Tang et al.
COMMUNICATION
To further confirm the AIE feature of TPEBArs, we
added water, a non-solvent for the luminogens, to their solu-
tion in THF and investigated the PL change. Figure 2a
minogens are readily tuned by incorporation of rigid aro-
matic cyclic rings.
The high PL efficiency and good thermal stability of
TPEArs and TPEBArs in the solid state suggest that they
are good candidates for the fabrication of efficient OLEDs.
Multilayer OLEDs with a configuration of indium tin oxide
(ITO)/N,N-bis(1-naphthyl)-N,N-diphenylbenzidine
(NPB)
(60 nm)/TPEArs or TPEBArs (20 nm)/2,2’,2’’-(1,3,5-benzine-
triyl)tris(1-phenyl-1-H-benzimidazole (TPBi) (10 nm)/tris(8-
hydroxyquinolinolato)aluminum (Alq3) (30 nm)/LiF (1 nm)/
Al (100 nm) (device I) were constructed, in which TPEArs
or TPEBArs functioned as light emitters, NPB worked as
hole transport layer, and TPBi and Alq3 served as hole
blocking and electron transport layers, respectively. The
device performances are summarized in Table 2. OLEDs
Figure 2. a) PL spectra of TPEBPy in THF/water mixtures with different
water fractions (fw). b) Plots of (I/I0À1) values as a function of fw in THF/
water mixtures of TPEBArs, in which I0 is the PL intensity in pure THF
solution.
Table 2. EL performances of TPEArs and TPEABrs.[a]
Von
(V)
Lmax
(cdmÀ2
hP, m ax
(lmWÀ1
hC,max
)
hext,max
EL (nm)
G
)
U
)
N
)
(%)
Device I
shows the PL spectra of TPEBPy in THF/water mixtures
with different water fractions (fw) as an example. Addition
of a large amount of water (fw ꢀ70%) to the THF solution
of TPEBPy causes its molecules to aggregate due to the im-
miscibility of the hydrophobic luminogen with the hydro-
philic medium and, accordingly, increases its emission. The
higher the water content, the stronger is the PL intensity.
Clearly, the emission of TPEBPy is enhanced by aggregate
formation. Other TPEBArs also show similar emission be-
haviors (Figure 2b), thus suggesting that the melding of
TPE and ACQ chromophores at the molecular level has en-
dowed the resultant molecules with a novel feature of AIE.
The thermal properties of TPEArs and TPEBArs were
examined by differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA). Figure 3 shows the TGA
thermograms and DSC curves of TPEBArs recorded under
nitrogen. TPEBArs are thermally stable, showing high onset
decomposition temperatures (Td) at 363–4948C. They are
also morphologically stable, as revealed by their high glass
transition temperatures (Tg) of 90–1428C. Both Td and Tg
values of TPEBArs are much higher than those of TPEArs,
thereby demonstrating that the thermal properties of the lu-
TPEPy
TPEAn
TPEPa
484
486
464
480
492
516
516
488
488
3.6
4.0
5.2
6.0
3.8
4.8
6.0
4.8
5.4
4.4
13400
8410
4600
5.6
3.7
1.6
1.1
4.2
2.0
2.0
3.5
2.3
3.7
7.3
4.6
2.7
2.4
7.3
5.8
5.7
5.5
4.2
6.1
3.0
2.1
1.4
1.3
2.7
2.0
2.0
2.6
1.9
2.5
TPENp
4120
TPE-2-Np
TPEBPy
TPEBAn
TPEBPa
TPEBNp
19800
13370
22600
9250
7130
13500
TPEB-2-Np 494
Device II
TPEBPy
TPEBAn
516
512
4.6
4.4
25500
14750
2.7
3.0
6.0
5.1
2.1
1.9
[a] Device configuration: ITO/NPB (60 nm)/TPEArs or TPEBArs
(20 nm)/TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm) (device I)
and ITO/NPB (60 nm)/TPEBPy or TPEBAn (20 nm)/TPBi (40 nm)/LiF
(1 nm)/Al (100 nm) (device II). lEL =EL maximum, Von =turn-on voltage
at 1 cdmÀ2, Lmax =maximum luminance, hP, m ax =maximum power efficien-
cy, hC,max =maximum current efficiency, and hext,max =maximum external
quantum efficiency.
based on TPEPy, TPEAn, TPENp, and TPE-2-Np show sky-
blue EL at 480–492 nm (Figure 4a), which are slightly red-
shifted from the PL of their films due to the microcavity
effect. The EL of TPEPa is observed at 464 nm, which is be-
tween the PL of its crystals and film.[10b] This suggests that
the TPEPa film fabricated by the vacuum deposition process
is partially crystalline in nature.[7b] All OLEDs with TPEArs
as emitters exhibit good EL performances (Table 2 and Fig-
ure 4b–d), which are much better than those of the TPE-
based device.[7a] Among them, the TPEPy-based device
shows the highest EL efficiency. It is turned on at a low volt-
age of 3.6 V and emits brilliantly with a maximum lumi-
nance (Lmax) of 13400 cdmÀ2. The maximum current effi-
ciency (hC,max), power efficiency (hP, max), and external quan-
tum efficiency (hext,max) attained by the device are 7.3 cdAÀ1,
Figure 3. a) TGA thermograms and b) DSC curves of TPEBArs recorded
under nitrogen at a heating rate of 108CminÀ1
.
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Chem. Asian J. 2012, 7, 484 – 488

A Facile and Versatile Approach to Efficient Luminescent Materials
Figure 5. Performance of multilayer EL devices in which different TPE-
BArs were incorporated, with a configuration of ITO/NPB/TPEBArs/
TPBi/Alq3/LiF/Al. a) EL spectra. b) Current efficiency versus current
density curves. c) Luminance and d) current density as a function of ap-
plied voltage.
Figure 4. Performance of multilayer EL devices in which different
TPEArs were incorporated, with a configuration of ITO/NPB/TPEArs/
TPBi/Alq3/LiF/Al. a) EL spectra. b) Current efficiency versus current
density curves. c) Luminance and d) current density as a function of ap-
plied voltage.
In this work, a series of efficient solid-state emitters were
constructed from TPE and conventional ACQ chromo-
phores and their thermal and optical properties were investi-
gated. These luminogens are almost non-fluorescent in solu-
tion but become strong emitters as nanoparticles in poor sol-
vents or as thin films in the solid state, in which FF values
reach up to 100%. They are morphologically and thermally
stable with high Tg and Td values of up to 142 and 4948C, re-
spectively. The application of these luminogens as light-
emitting layers in OLEDs was explored. Although the
device structures have not yet been optimized, excellent EL
performances have been achieved, thus suggesting that
these luminogens are potential materials for the construc-
tion of efficient non-doped OLEDs. These results reveal
that the attachment of AIE units to ACQ chromophores is a
versatile strategy to solve the ACQ problem and to create,
at the same time, efficient luminescent materials in the solid
state with good thermal stability, efficient solid-state PL,
and promising EL properties.
5.6 lmWÀ1, and 3.0%, respectively. The TPE-2-Np-based
device also shows a good performance, with values of
19800 cdmÀ2 (Lmax), 7.3 cdAÀ1 (hC,max), 4.2 lmWÀ1 (hP, max),
and 2.7% (hext,max).
Similar good EL performances were obtained from devi-
ces fabricated with TPEBArs (Table 2 and Figure 5). These
devices display emissions at 488–516 nm, which are slightly
red-shifted as compared to those of devices based on
TPEArs. The TPEBPy-based device is turned on at 4.8 V
and exhibits an Lmax of 13370 cdmÀ2, a hC,max of 5.8 cdAÀ1, a
hP,max of 2.0 lmWÀ1, and a hext,max of 2.0%. Even better per-
formances were observed in the device based on TPEB-2-
Np, as it possesses a low turn-on voltage of 4.4 V and a
strong radiation, with an Lmax of 13500 cdmÀ2 and hC,max
,
hP,max, and hext,max values of 6.1 cdAÀ1, 3.7 lmWÀ1, and 2.5%,
respectively. As the EL of TPEBPy and TPEBAn was ob-
served at 516 nm, close to that of Alq3, two additional devi-
ces with a configuration of ITO/NPB (60 nm)/TPEBPy or
TPEBAn (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)
(device II) were fabricated to examine whether the emis-
sions in device I originated from the Alq3 layer. The EL
maximum of both devices was still observed at approximate-
ly 516 nm, thus indicating that the EL originates from the
radiative decay of TPEBPy and TPEBAn in devices I and
II. TPEBPy even shows a slightly improved performance in
device II (Figure S1 in the Supporting Information), in
which a lower turn-on voltage of 4.6 V and higher Lmax and
hC,max values of 25500 cdmÀ2 and 6.0 cdAÀ1 were attained.
Acknowledgements
We acknowledge financial support from the Research Project Competi-
tion of HKUST (RPC11SC09), the Research Grants Council of Hong
Kong (604711, 603509, and HKUST2/CRF/10), the University Grants
Committee of Hong Kong (AoE/P-03/08), and the National Natural Sci-
ence Foundation of China (21074028 and 21104012). Z.Z. thanks the Ini-
tial Funding of Hangzhou Normal University (HSQK0085), and the Nat-
ural Science Foundation of Zhejiang Province (Y4110331).
Chem. Asian J. 2012, 7, 484 – 488
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
487

Z. Zhao, B. Z. Tang et al.
COMMUNICATION
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Keywords: aggregation-induced emission · fluorescence ·
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Received: September 6, 2011
Published online: January 9, 2012
488
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