Aggregation-enhanced emission and efficient electroluminescence of tetraphenylethene-cored luminogens

Article abstract of DOI:10.1039/c2cc37928g

A new design of luminescent materials by decorating a tetraphenylethene core with four aromatic chromophores is proposed. The generated luminogens exhibit aggregation-enhanced emission and excellent solid-state fluorescence efficiency (93-99%). Efficient non-doped OLEDs based on them afford remarkable efficiencies up to 11 cd A^-1.

Full text of DOI:10.1039/c2cc37928g

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Aggregation-enhanced emission and efficient
electroluminescence of tetraphenylethene-cored
Cite this: DOI: 10.1039/c2cc37928g
luminogens†
Received 1st November 2012,
Accepted 22nd November 2012
Zhengfeng Chang,a Yibin Jiang,b Bairong He,a Jian Chen,a Zhiyong Yang,c Ping Lu,d
Hoi Sing Kwok,b Zujin Zhao,*a Huayu Qiua and Ben Zhong Tang*c
DOI: 10.1039/c2cc37928g
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A new design of luminescent materials by decorating a tetraphenyl- efficient luminescent materials leads us to conjecture whether
ethene core with four aromatic chromophores is proposed. The they can be designed in an opposite way, that is TPE serves as
generated luminogens exhibit aggregation-enhanced emission a core and conventional chromophores as peripheries? Are they
and excellent solid-state fluorescence efficiency (93–99%). Efficient free of ACQ effect and promising emitters for OLEDs? To answer
non-doped OLEDs based on them afford remarkable efficiencies up these questions, herein, we elaborate three luminogens (Chart 1)
to 11 cd Aꢀ1
.
by decorating a TPE core with four AIE units (TPE) or ACQ
chromophores [TPA and spirobifluorene (SF)]. Their thermal
In 2001, a novel phenomenon of aggregation-induced emission stability and PL and EL properties are investigated.
(AIE) was first found by Tang’s group from propeller-like siloles, To build TPE-cored luminogens, a key TPE derivative bearing
whose emission was very weak in solution but became intense four bromine atoms, 1,1,2,2-tetrakis(4-bromophenyl)ethane
as aggregates formed.1 Such abnormal emission behavior has (TPE-4Br), was readily prepared from 4,40-dibromobenzophenone
drawn great research interest,2 for it is exactly opposite to the by McMurry coupling. A fourfold Suzuki coupling of TPE-4Br
common belief that the emission of chromophores decreases in with the corresponding arylboronic acid generated target pro-
the aggregate state. This intriguing finding paves a new avenue ducts in moderate yields (Scheme 1). The detailed procedures
to tackle the notorious aggregation-caused quenching (ACQ) of and characterization data are given in ESI.† All the luminogens
conventional chromophores. Now, researchers have greatly are soluble in common organic solvents such as dichloro-
expanded their search for AIE luminogens that show specific methane, chloroform, tetrahydrofuran and 1,4-dioxane, but
applications in materials science3 and biological technology.4
insoluble in methanol and water. PTPE, TPE-4SF and TPE-
Recently, we created a series of luminogens by decorating 4TPA are of high thermal stability, with decomposition tem-
conventional chromophores with a typical AIE luminogen, peratures of 445, 495 and 474 1C, respectively, as revealed by
tetraphenylethene (TPE).5 These molecules inherit the AIE thermogravimetric analysis (Fig. S1, ESI†). TPE-4TPA shows a
characteristic from TPE units and their films show highly efficient
photoluminescence (PL) and electroluminescence (EL). For instance,
grafting TPE units as peripheries onto pyrene or triphenyl-
amine (TPA) cores generated remarkable solid-state emitters for
non-doped organic light-emitting diodes (OLEDs), with significantly
high EL efficiency approaching 5%.6 Our continuing pursuit of
a College of Material, Chemistry and Chemical Engineering, Hangzhou Normal
University, Hangzhou 310036, China. E-mail: zujinzhao@gmail.com
b Center for Display Research, The Hong Kong University of Science & Technology,
Kowloon, Hong Kong, China
c Department of Chemistry, Institute of Molecular Functional Materials,
The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon,
Hong Kong, China. E-mail: tangbenz@ust.hk
d State Key Laboratory of Supramolecular Structure and Materials, Jilin University,
Changchun 130012, China
† Electronic supplementary information (ESI) available: Experimental details,
OLED fabrication, aggregate preparation, TGA thermograms, absorption spectra
and curves of fluorescence quantum yields vs. fractions of water in 1,4-dioxane–
water mixtures. See DOI: 10.1039/c2cc37928g
Chart 1 Chemical structures of TPE and TPE-cored luminogens.
c
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Scheme 1 Synthetic routes to TPE-cored luminogens.
Fig. 1 PL spectra of (A) PTPE and TPE-4TPA in 1,4-dioxane–water mixtures with
different water fractions (fw). Inset: photos of PTPE and TPE-4TPA in 1,4-dioxane–
water mixtures (fw = 0 and 99%) taken under the illumination of a UV lamp.
high glass-transition temperature of 192 1C, detected by differ-
ential scanning calorimetry.
PTPE shows an absorption maximum at 342 nm in solution,
associated with the p–p* transition of the molecule. When the
chromophores attached to the TPE core change, the spectral
profiles of the resultant luminogens vary accordingly (Fig. S2,
ESI†). The fluorescence of PTPE in dilute solution is weak. Its
emission maximum is located at B520 nm, with a fluorescence
quantum yield (FF) of 3.4%. Although the value is low, it has
increased obviously in comparison to that of TPE (0.24%),
which should be due to the elongated conjugation and reinforced
stiffness of luminogens.7 Replacing TPE peripheries with con-
ventional chromophores (TPA and SF) generates luminogens
with faint emission in solution like PTPE. The emission maxima
of TPE-4TPA and TPE-4SF are located at B530 nm, and the FF
values are as low as B3.0%. These results suggest that the
intramolecular rotation (IMR) process of the TPE core has con-
sumed the excited state energy of the whole molecule, rendering
weak fluorescence.2 In contrast, these TPE-cored luminogens are
highly emissive in the solid state. The films of PTPE, TPE-4TPA
and TPE-4SF emit intensely, with emission maxima at 510, 521
and 522 nm, and high FF values of 97, 93 and 99%, respectively.
The efficient emissions of films reveal that the TPE-cored lumino-
Table 1 Optical properties of TPE-cored luminogens in the solution (Soln)a and
amorphous (Film)b states
lem (nm)
FF (%)
Solnc
labs (nm)
Soln
Soln
Film
Filmd
PTPE
TPE-4SF
TPE-4TPA
342
372
353
520
528
530
510
521
522
3.4
2.9
3.0
97
93
99
a
b
In 1,4-dioxane solution (10 mM). Film drop-cast on a quartz plate.
c
Determined in 1,4-dioxane solutions using 9,10-diphenylanthracene
d
(FF = 90% in cyclohexane) as standard. Determined in amorphous
films by an integrating sphere.
Table 2 EL properties of TPE-cored luminogensa
Von Lmax
ZP,max
ZC,max
Zext,max
(%)
EL (CIE)
(V) (cd mꢀ2
)
(lm Wꢀ1
)
(cd Aꢀ1
)
PTPE
TPE-4SF
(0.27, 0.50) 4.6 13 700
(0.29, 0.52) 4.4 25 000
6.4
5.8
7.8
11
10
8.8
3.5
3.1
2.7
TPE-4TPA (0.34, 0.55) 3.4 14 100
a
Abbreviations: CIE = Commission International d’Eclairage chroma-
gens virtually possess aggregation-enhanced emission (AEE) ticity coordinates, Von = turn-on voltage at 1 cd mꢀ2, Lmax = maximum
luminance, ZP,max, ZC,max, and Zext,max = maximum power, current, and
characteristic. Although there is only one TPE unit incorporated
as a core in TPE-4TPA and TPE-4SF, the ACQ problem of TPA
external quantum efficiencies, respectively.
and SF peripheries is surmounted well.
To further study the AEE property of TPE-cored luminogens, The device performances are summarized in Table 2. PTPE
their emission spectra were measured in 1,4-dioxane–water mix- shows green EL peaked at 520 nm, which is close to the PL of its
tures. PTPE shows greatly enhanced fluorescence intensity (Fig. 1A) amorphous film (Fig. 2), indicating that both PL and EL
and FF values (Fig. S3, ESI†) when water is added into its originate from the same singlet excitons. The device of PTPE
1,4-dioxane solution. Since PTPE is insoluble in water, aggre- is turned on at 4.6 V and radiates intensely with a maximum
gates of PTPE are formed when the water fraction becomes luminance (Lmax) of 13 700 cd cmꢀ2 at 15 V (Fig. 3A). The
higher in 1,4-dioxane–water mixtures. The IMR process of TPE maximum current (ZC,max), power (ZP,max) and external quantum
units is restricted and the nonradiative decay pathway is (Zext,max) efficiencies attained by the device are 11 cd Aꢀ1
,
blocked, making luminogens fluoresce efficiently. Similar emis- 6.4 lm Wꢀ1 and 3.5%, respectively. These values are improved
sion behavior is also observed for TPE-4SF and TPE-4TPA. The remarkably compared with those obtained from the devices of
emission spectra of TPE-4TPA in 1,4-dioxane–water mixtures TPE dimers (BTPE, 11 180 cd mꢀ2, 7.26 cd Aꢀ1 and 3.17%)8a and
are displayed in Fig. 1B as an example. These results confirm pristine TPE (1800 cd mꢀ2, 0.45 cd Aꢀ1 and 0.4%),8b revealing
that the TPE-cored luminogens are really AEE-active (Table 1). that covalent combination of more TPE units into one molecule
Multilayer non-doped OLEDs with a configuration of ITO/NPB has generated synergistic electronic interaction, which is con-
(60 nm)/emitter (20 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm) ducive to the EL property.
were fabricated using these TPE-cored luminogens as light
TPE-4SF and TPE-4TPA exhibit almost the same EL spectra
emitters. N,N-Bis(1-naphthyl)-N,N-diphenylbenzidine (NPB) and with maxima at 536 nm, which are slightly red-shifted by about
2,20,200-(1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBI) 15 nm from the PL spectra of their amorphous films due to the
were adopted as hole- and electron-transporting layers, respectively. microcavity effect. The devices of TPE-4SF and TPE-4TPA show
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This is very important as the efficiencies of many OLEDs
decline rapidly at high luminance levels. Although the device
structure is not optimized yet, the preliminary results obviously
demonstrate that the TPE-cored luminogens are promising
emitters for OLED applications.
In summary, three thermally stable TPE-cored luminogens
were synthesized and their photophysical properties in solu-
tions and in films were investigated. Although they are weakly
fluorescent in solutions, their solid films are highly emissive,
exhibiting excellent emission efficiencies. The presence of the
TPE core has endowed the luminogens with AEE feature and
surmounted the ACQ effect. High-performance non-doped
OLEDs are accessible based on TPE-cored luminogens. These
results demonstrate that incorporation of TPE as a core into
luminogens provides a feasible and effective approach to
efficient solid-state luminescent materials.
Fig. 2 (A) PL spectra of thin neat films of TPE-cored luminogens. (B) EL spectra of
TPE-cored luminogens.
We acknowledge the financial support from National
Natural Science Foundation of China (51273053, 21104012 and
21074028), Natural Science Foundation of Zhejiang Province
(Y4110331) and the Research Grant Council of Hong Kong
(HKUST2/CRF/10 and N_HKUST620/11).
Notes and references
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Fig. 3 Plots of (A) luminance and current density with the applied voltage and
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lower turn-on voltages (3.4 and 4.4 V, respectively) and higher
current densities at the same driving voltages than the device of
PTPE, presumably due to the better carrier injection and
transport capabilities of TPA and SF than TPE. The device of
TPE-4TPA also gives good results, with a Lmax of 14 100 cd mꢀ2
,
an ZC,max of 8.8 cd Aꢀ1 and an ZP,max of 7.8 lm Wꢀ1. Even better
performances are attained in the device of TPE-4SF, which
shows a Lmax of 25 000 cd mꢀ2 and high EL efficiencies of
10 cd Aꢀ1, 5.8 lm Wꢀ1 and 3.1%. The efficiency roll-off is quite low at
high current density and luminance (Fig. 3B). The current efficiency
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remains as high as 7.5 cd Aꢀ1 at a luminance of 10 000 cd mꢀ2
.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun.