Construction of deep-blue AIE luminogens with TPE and oxadiazole units

Article abstract of DOI:10.1007/s11426-013-4925-6

In this paper, two AIE-active luminogens (Oxa- p TPE and Oxa- m TPE) constructed from tetraphenylethene and oxadiazole units were successfully synthesized and their thermal, optical and electronic properties were investigated. By linking TPE to the oxadiazole core through meta-or para-position, the intramolecular conjugation is effectively controlled. Thanks to the intelligent molecular design and specific AIE feature, when fabricated as emissive layers in non-doped OLEDs, they exhibit blue or deep-blue emission with CIE coordinates of (0.17, 0.23) and (0.15, 0.12), and good efficiencies with η _{C, max} and η _{P, max} up to 1.52 cd A ^-1 and 0.84 Im W^-1, shedding some light on the construction of deep-blue AIE fluorophores.

Full text of DOI:10.1007/s11426-013-4925-6

SCIENCE CHINA
Chemistry
January 2013 Vol.5 No.1: 1–8
doi: 10.1007/s11426-013-4925-6
• ARTICLES •
· SPECIAL TOPIC · The Frontiers of Chemical Biology and Synthesis
doi: 10.1007/s11426-013-4925-6
Construction of deep-blue AIE luminogens with TPE and
oxadiazole units
HUANG Jing1, CHEN PengYu1, YANG Xiao2, TANG RunLi1, WANG Lei2*,
QIN JinQui1 & LI Zhen1*
1Department of Chemistry, Wuhan University, Wuhan 430072, China
2Wuhan National Laboratory for Optoelectronics, School of Optoelectronic Science and Engineering,
Huazhong University of Science and Technology, Wuhan 430074, China
Received May 9, 2013; accepted June 26, 2013
In this paper, two AIE-active luminogens (Oxa-pTPE and Oxa-mTPE) constructed from tetraphenylethene and oxadiazole
units were successfully synthesized and their thermal, optical and electronic properties were investigated. By linking TPE to
the oxadiazole core through meta- or para- position, the intramolecular conjugation is effectively controlled. Thanks to the in-
telligent molecular design and specific AIE feature, when fabricated as emissive layers in non-doped OLEDs, they exhibit blue
or deep-blue emission with CIE coordinates of (0.17, 0.23) and (0.15, 0.12), and good efficiencies with C, max and P, max up to
1.52 cd A1 and 0.84 Im W1, shedding some light on the construction of deep-blue AIE fluorophores.
aggregation-induced emission, tetraphenylethene, deep-blue emitter, organic light-emitting diodes
cated fabrication processes, or jeopardizing the electronic
conjugation in the luminophores.
1 Introduction
In 2001, Tang et al. [11] have discovered a class of non-
planar propeller-shaped luminogens with intriguing charac-
teristic termed as aggregation-induced emission (AIE),
which demonstrated dramatically enhanced emission once
aggregated as nanoparticles or solid films, in contrast to
their weak emission in the solution state. This is totally op-
posite to the ACQ phenomenon. The systematic researches
have shown that the restriction of intramolecular rotation is
the main cause for the AIE effect [12–16]. This unique lu-
minescent property aroused a new research topic of AIE
materials, with tetraphenylethene (TPE) as a prototype AIE
molecule for its facile synthesis and outstanding AIE effect
[17]. Besides the exciting results of TPE-based fluorophors
as bioprobes and chemosensors, OLED devices with
TPE-based fluorophors as the emissive layers, demonstrated
excellent EL performance [18–35]. For example, BTPE,
constructed by two simple TPE units, exhibits outstanding
improvement in EL performance with the current efficiency
Luminescent materials, with wide technological applica-
tions such as biological probes, chemosensors and organic
light-emitting diodes (OLEDs), have attracted worldwide
interests due to their remarkable properties for optoelec-
tronics and biological science [1–5]. When fabricated as
thin solid films in practical applications, most fluorophores
suffer from the notorious aggregation-caused quenching
(ACQ) effect, which is still a big challenge for the design
and utilization of novel luminophors [6, 7]. Although vari-
ous chemical, physical and engineering approaches have
been employed to alleviate the ACQ effect [8–10], for ex-
ample, the attachment of bulky alicyclics, adopting host-
guest systems and blending with transparent polymers.
These methods met with limited success and might also
bring side effects such as more difficult synthesis, compli-
*Corresponding authors (email: lizhen@whu.edu.cn; lei96wang@163.com)
© Science China Press and Springer-Verlag Berlin Heidelberg 2013
chem.scichina.com www.springerlink.com

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Huang J, et al. Sci China Chem January (2013) Vol.56 No.1
up to 7.3 cd A1 [20], compared to TPE (0.45 cd/A) [17],
and the pyrene-cored luminogen TTPEPy is also an excel-
lent emitter with external quantum efficiency and current
efficiency up to 4.95% and 12.3 cd A1, respectively [21].
However, their EL emission has red-shifted at a large extent
to greenish-blue or green for the intrinsic extention of in-
tramolecular conjugation, no longer in the deep-blue region
as TPE, proving that good blue light emitting AIE materials
are still very scarce on another side. Actually, the lack of
highly efficient blue light emitting materials caused by their
intrinsic wide-band-gap nature, is still a critical drawback to
realize full color displays [36, 37]. Therefore, the develop-
ment of blue OLEDs with good performance based on the
AIE materials becomes particular urgent.
TPE itself. Fortunately, the device performance confirmed
our ideas: when fabricated as emissive layers in OLEDs,
Oxa-pTPE and Oxa-mTPE exhibit blue and deep-blue
emissions with CIE coordinates of (0.17, 0.23) and (0.15,
0.12), respectively, and the maximum current efficiency is
up to 1.52 cd A1, much higher than that of TPE (0.45 cd
A1). Herein, we would like to present the synthesis, ther-
mal and photophysical properties and electroluminescence
of the two emitters in detail.
2 Experimental
2.1 Charaterization
1H and 13C NMR spectra were measured on a Mecuryvx300
spectrometer. Elemental analyses of carbon, hydrogen, and
nitrogen were performed on a CARLOERBA-1106 micro-
analyzer. Mass spectra were measured on a ZAB 3F-HF
mass spectrophotometer. UV-Vis absorption spectra were
recorded on a Shimadzu UV-2500 recording spectropho-
tometer. Photoluminescence spectra were recorded on a
Hitachi F-4500 fluorescence spectrophotometer. Differen-
tial scanning calorimetry (DSC) was performed on a
NETZSCH DSC 200 PC unit at a heating rate of 15 °C
min1 from room temperature to 300 °C under argon. The
glass transition temperature (Tg) was determined from the
second heating scan. Thermogravimetric analysis (TGA)
was undertaken with a NETZSCH STA 449C instrument.
The thermal stability of the samples under a nitrogen at-
mosphere was determined by measuring their weight loss
while heating at a rate of 10 °C min1 from 25 to 600 °C.
Cyclic voltammetry (CV) was carried out on a CHI volt-
ammetric analyzer in a three-electrode cell with a Pt counter
electrode, a Ag/AgCl reference electrode, and a glassy car-
bon working electrode at a scan rate of 100 mV s1 with 0.1
M tetrabutylammonium perchlorate (purchased from Alfa
Aesar) as the supporting electrolyte, in anhydrous di-
chloromethane solution purged with nitrogen. The potential
values obtained in reference to the Ag/Ag+ electrode were
converted to values versus the saturated calomel electrode
(SCE) by means of an internal ferrocenium/ferrocene
(Fc+/Fc) standard.
Previously, by properly adjusting the balance of the
elongation of -conjugation and the linkage of additional
rotators, or the utilization of the feature of crystallization
induced blue-shifted emission of AIE molecules, we have suc-
cessfully developed some TPE-based blue emitters [38, 39],
which demonstrated good EL efficiencies (up to 5.0 cd A1),
thanks to their AIE characteristics. In addition, we have also
obtained four deep-blue AIE luminogens simply constructed
from two TPE units with ortha-, meta- or para-linkage
mode, which represent different conjugation degree [40]. As
a result, the conjugation is effectively controlled, and all the
molecules exhibit deep-blue emission ranging from 435 to
459 nm and good electroluminescence efficiencies up to
2.8 cd A1. Thus, to further realize our synthetic ideas and
develop new blue or deep-blue AIE systems, in this work,
we have incorporated TPE moieties to the central elec-
tron-dominated oxadiazole unit through meta- or para-
linkage, and obtained two AIE luminogens Oxa-pTPE and
Oxa-mTPE. It is hoped that the EL performance could be
enhanced with the aid of the oxadiazole group while main-
taining blue or deep-blue emission in comparison with
2.2 Computational details
The geometrical and electronic properties were optimized at
B3LYP/6-31g(d) level using Gaussian 09 program. The
molecular orbitals were obtained at the same level of theory.
2.3 OLED device fabrication and measurement
The hole-transporting material NPB (1,4-bis(1-naphthyl-
phenylamino)-biphenyl), electron-transporting materials 1,3,5-
tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) and tris(8-
hydroxyquinolinato)aluminium (Alq3) were purchased from
Chart 1 Chemical structures of Oxa-mTPE and Oxa-pTPE.

Huang J, et al. Sci China Chem January (2013) Vol.56 No.1
3
Changzhou Mascot Import & Export Co., LTD. The EL
devices were fabricated by vacuum deposition of the mate-
rials at a base pressure of 5×106 Torr onto glass precoated
with a layer of indium tin oxide (ITO) with a sheet re-
sistance of 25  square1. Before the deposition of an or-
ganic layer, the clear ITO substrates were treated with oxy-
gen plasma for 3 min. The deposition rate of organic com-
pounds was 0.9–1.1 Å s1. Finally, a cathode composed of
lithium fluoride (1 nm) and aluminium (100 nm) was se-
quentially deposited onto the substrate in the vacuum of
105 Torr. The L–V–J of the devices was measured with a
Keithley 2400 Source meter and PR655. The EL spectra
were measured by PR655. All measurements were carried
out at room temperature under ambient conditions.
mL) was added. After 2 h, the mixture was slowly warmed
to room temperature. After stirring overnight, the reaction
was terminated by the added brine. The mixture was ex-
tracted with dichloromethane, the organic layer was com-
bined, and dried over anhydrous sodium sulfate. After fil-
tration and solvent evaporation, the crude product was puri-
fied by silica gel column chromatography using petroleum
ether/dichloromethane (v/v = 5/1) as eluent. White powder
1
of 1 was obtained in the yield of 60% (2.30 g). H NMR
(300 MHz, CDCl3, ): 7.53–7.48 (m, 2H), 7.09–7.02 (m,
17H), 1.28 (s, 12H); 13C NMR (75 MHz, CDCl3, ): 143.6,
143.3, 142.9, 141.1, 140.8, 137.5, 134.2, 132.7, 131.3,
131.2, 127.5, 127.0, 126.3, 126.2, 83.6, 24.7; MS (EI), m/z:
458.35 ([M+], calcd for C32H31BO2, 458.40).
Synthesis of Oxa-mTPE
2.4 Preparation of compounds
A mixture of compound 3 (380 mg, 1 mmol), compound 1
(940 mg, 2.05 mmol), Pd(PPh3)4 (0.10 g, 4% mmol) and
potassium hydroxide (560 mg, 10 mmol) in 15 mL of THF
and 3 mL of distilled water in a 50 ml Schlenk tube was
refluxed for 2 days under argon. The mixture was extracted
with dichloromethane. The combined organic extracts were
dried over anhydrous sodium sulfate and concentrated by
rotary evaporation. The crude product was purified by col-
umn chromatography on silica gel using dichloro-
methane/petroleum ether (v/v = 1/1) as eluent to afford the
product as a white powder in the yield of 63.2% (758 mg).
1H NMR (300 MHz, CDCl3, ): 7.55–7.52 (m, 2H), 7.42–
7.33 (m, 4H), 7.09–7.02 (m, 35H), 6.91–6.90 (m, 5H); 13C
NMR (75 MHz, CDCl3, ): 164.5, 143.8, 143.7, 143.6,
143.3, 142.1, 141.4, 140.6, 139.9, 132.0, 131.5, 131.4,
131.3, 131.2, 130.9, 130.4, 129.7, 127.8, 127.7, 127.4,
127.1, 126.5, 122.6; MS (EI), m/z: 882.28 ([M+], calcd for
C66H46N2O, 883.08); Anal. Calcd for C66H46N2O: C, 89.77;
H, 5.25; N, 3.17. Found: C, 89.55; H, 4.94; N, 3.44.
All other chemicals and reagents were obtained from com-
mercial sources and used as received without further purifi-
cation. Solvents for chemical synthesis were purified ac-
cording to the standard procedures. 3-Bromobenzophenone
[41], p-TPE-Br, compound 2 and 3 were synthesized ac-
cording to the literatures [42].
Synthesis of mTPE-Br
A 2.3 M solution of n-butyllithium in hexane (4.09 mmol,
1.78 mL) was added to a solution of diphenylmethane (0.86
g, 5.12 mmol) in anhydrous tetrahydrofuran (40 mL) at 0 °C
under an argon atmosphere. After stirring for 1 h at this
temperature, 3-bromobenzophenone (0.89 g, 3.41 mmol)
was added. After 2 h, the mixture was slowly warmed to
room temperature. Then, the reaction was quenched with an
aqueous solution of ammonium chloride and the mixture
was extracted with dichloromethane. The organic layer was
evaporated after drying with anhydrous sodium sulfate and
the resultant crude product was dissolved in toluene (25
mL). The p-toluenesulfonic acid (0.12 g, 0.68 mmol) was
added, and the mixture was refluxed overnight and cooled
to room temperature. The mixture was evaporated and the
crude product was purified by silica gel column chromatog-
raphy using petroleum ether as eluent to yield a white pow-
Synthesis of Oxa-pTPE
The synthetic procedure was similar to that of Oxa-mTPE,
with compound 2 and 1 as the starting materials. White sol-
1
id. Yield: 68%. H NMR (300 MHz, CDCl3, ): 7.78–7.76
(m, 2H), 7.59–7.40 (m, 6H), 7.086.97 (m, 26H), 6.93–6.88
(m, 12H); 13C NMR (75 MHz, CDCl3, ): 165.3, 144.0,
143.8, 143.7, 143.1, 142.3, 141.5, 140.5, 138.5, 131.6,
131.4, 131.3, 130.5, 128.3, 127.9, 127.8, 127.7, 126.8,
126.6, 123.0; MS (EI), m/z: 882.94 ([M+], calcd for
C66H46N2O, 883.08); Anal. Calcd for C66H46N2O: C, 89.77;
H, 5.25; N, 3.17. Found: C, 89.62; H, 5.23; N, 3.44.
1
der in the yield of 50% (0.70 g). H NMR (300 MHz,
CDCl3, ): 7.20–7.11 (m, 11H), 7.02–7.01 (m, 5H), 6.96–
6.95 (m, 2H); 13C NMR (75 MHz, CDCl3, ): 146.0, 143.4,
143.3, 143.1, 142.2, 139.5, 134.2, 131.3, 130.1, 129.6,
129.3, 128.0, 127.8, 126.9, 126.8, 121.9; MS (EI), m/z:
412.13 ([M+], calcd for C26H19Br, 411.33).
Synthesis of compound 1
3 Results and discussion
A 2.3 M solution of n-butyllithium in hexane (15.0 mmol,
6.5mL) was added to a solution of m-TPE-Br (4.12 g, 10.0
mmol) in anhydrous tetrahydrofuran (60 mL) at 78 °C
under an argon atmosphere. After stirring for 4 h,
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (6.1
3.1 Synthesis
Scheme 1 illustrates the synthetic route to Oxa-pTPE and
Oxa-mTPE. The detailed procedures for the synthesis of

4
Huang J, et al. Sci China Chem January (2013) Vol.56 No.1
the intermediates and target molecules are presented in the
Experimental Section. Briefly, TPE boronic ester 1 or 2 was
obtained from diphenylmethane and 3-bromobenzophenone
or 4-bromobenzophenone in three steps. Compound 3 was
prepared according to the literatures. Suzuki coupling reac-
tions of 3 with 1 or 2 were catalyzed by Pd(PPh3)4 with
good yields. The final products were carefully purified by
column chromatography on silica gel using petroleum
ether-dichloromethane as eluent and fully characterized by
1H and 13C NMR, mass spectrometry, and elemental analy-
sis, which well confirmed their molecular structures. Single
crystal of Oxa-mTPE was isolated from its dichloro-
methane/methanol solution and was crystallographically
characterized. The target molecules were soluble in com-
mon solvents, such as dichloromethane, chloroform and
tetrahydrofuran, but insoluble in water.
investigated by thermal gravimetric analyses (TGA) and
differential scanning calorimetry (DSC). As shown in Fig-
ure 1, their thermal-decomposition temperatures (Td, corre-
sponding to 5% weight loss) were determined to be about
414 and 400 °C (Table 1), respectively. Oxa-pTPE pos-
sessed better thermal stability and exhibited higher glass
transition temperatures (Tg) of 113 °C, than that of
Oxa-mTPE (103 °C). This should be ascribed to the more
planar and rigid conformation of Oxa-pTPE, which was
constructed from TPE moiety and oxadiazole unit through
para-linkage, in another word, it is more conjugated. Thus,
their good thermal stability with high Td and Tg values would
contribute to the preparation of the homogeneous and stable
amorphous emissive layer in OLED device.
3.3 Optical properties
The absorption spectra of Oxa-pTPE and Oxa-mTPE are
shown in Figure 2(a). Their absorption maximums are about
320 and 302 nm, respectively, indicating that the former is
3.2 Thermal properties
The thermal properties of Oxa-pTPE and Oxa-mTPE were
Table 1 The thermal, electrochemical and photophysical data of Oxa-pTPE and Oxa-mTPE
PL max (aggr)f)
PL max (film)
Tda) (°C)
Tg (°C)
Egb) (eV)
EHOMOc) (eV)
ELUMOd) (eV)
abse) (nm)
(nm)
(nm)
Oxa-pTPE
Oxa-mTPE
414
400
113
103
3.30
3.39
5.55
5.57
2.25
2.18
320
302
479
465
480
464
a) 5% Weight loss temperature measured by TGA under N2; b) band gap estimated from optical absorption band edge of the solution; c) calculated from
the onset oxidation potentials of the polymers; d) estimated using empirical equations ELUMO = EHOMO + Eg; e) observed from absorption spectra in dilute THF
solution; f) determined in THF:H2O = 1:99 solution.
Scheme 1 Synthetic route to Oxa-mTPE and Oxa-pTPE.
more conjugated than the latter, as a result of the different
of meta- or para-linkage mode of TPE and oxadiazole unit.
In addition, compared to that of TPE (299 nm), the maxi-
mum absorption wavelength of Oxa-pTPE has red-shifted
about 20 nm, while that of Oxa-mTPE almost remained the
same, further demonstrating our synthetic ideas of utilizing
the different conjugation degree of different linking posi-
tions. Thus, Oxa-pTPE possesses more planar and rigid
conformation than Oxa-mTPE. Actually, the different
linkage is also a method to subtly adjust the electronic
properties of the resultant molecules.
Figure 1 TGA (a) and DSC (second heating cycle) thermograms (b) of
Oxa-mTPE and Oxa-pTPE recorded under N2 at a heating rate of 10 and
15 °C min1.
To investigate the possible AIE characteristic of these

Huang J, et al. Sci China Chem January (2013) Vol.56 No.1
5
Figure 2 (a) UV spectra in THF solution. Concentration (): 12.1 and
10.7 for Oxa-mTPE and Oxa-pTPE, respectively; (b) the PL spectra of
the films of Oxa-mTPE and Oxa-pTPE. The thin films were spin-coated
onto ITO glass from dilute THF solution with concentrations of 1 mg
mL1.
Figure 3 (a) PL spectra of Oxa-pTPE in THF/H2O mixtures with differ-
ent water fractions (fw). Concentration (): 10.7excitation wavelength
(nm): ; (b) plots of fluorescence quantum yields determined in
THF/H2O solutions using 9,10-diphenylanthracene ( = 90% in cyclohex-
ane) as standard versus water fractions. Inset in (b): photos of Oxa-pTPE
in THF/water mixtures (fw = 0 and 99%) taken under the illumination of a
365 nm UV lamp.
two dyes, their fluorescent behaviors were studied. Consid-
ering the good and poor solvents should be well miscible,
tetrahydrofuran and water were chosen as the solvent pair.
Both of Oxa-pTPE and Oxa-mTPE are nonemissive when
molecularly dissolved in organic solvents, but show strong
emission in the aggregate state. Figure 3 demonstrates the
PL change and fluorescent image of Oxa-pTPE in THF and
THF/water mixtures. It is easily seen that in dilute THF
solution, the PL curve is practically flat lines parallel to the
abscissa, confirming that it is nearly nonemissive in the so-
lution state. However, when a large amount of water is
added, intense emission is observed. As shown in Figure 3,
when the water fraction is over 80%, the PL intensity in-
creases swiftly, indicating the formation of the aggregates.
At a fw value of 99%, the PL spectrum peak is at 479 nm for
Oxa-pTPE, and the PL intensity is 120-fold higher than that
in pure THF. Similar behavior was observed for
Oxa-mTPE (Figure 4). However, unlike Oxa-pTPE, when
the water fraction increases from 80 to 99%, the emission
maximum of the PL spectra of Oxa-mTPE in THF/water
mixtures is blue shifted from 490 to 465 nm, possibly due to
the morphological change of the aggregates from amor-
phous to crystalline state, that is, crystallization will
blue-shift the emission. Thus, it seems easier to crystallize
for Oxa-mTPE than Oxa-pTPE, owing to its more twisted
conformation caused by the meta-linkage of TPE and
oxadiazole unit.
The quantitative enhancement of emission was evaluated
by the PL quantum yield (F), using 9,10-diphenylanthracene
as the standard. From pure solution in THF to aggregate
states in 95% or 99% aqueous mixtures, the ФF values of
Oxa-pTPE and Oxa-mTPE increase from 0.0052 and
0.0044 to 0.27 and 0.346. Evidently, these two dyes are
AIE-active.
Closer inspection of the PL spectra of Oxa-pTPE and
Oxa-mTPE in the solid state were carried out, because they
are often fabricated as thin solid films in practical applica-
tions. As shown in Figure 2(b), the film of Oxa-pTPE ex-
hibits sky-blue emission peaked at 479 nm, which is almost
Figure 4 (a) PL spectra of Oxa-mTPE in THF/H2O mixtures with dif-
ferent water fractions (fw). Concentration (): 10.7excitation wave-
length (nm): ; (b) plots of fluorescence quantum yields determined in
THF/H2O solutions using 9,10-diphenylanthracene ( = 90% in cyclohex-
ane) as standard versus water fractions. Inset in (b): photos of Oxa-mTPE
in THF/water mixtures (fw = 0 and 99%) taken under the illumination of a
365 nm UV lamp.
the same as the values obtained from the corresponding PL
spectra of the aggregates with 99% water fraction, suggest-
ing that the aggregates with fw of 99% are in the amorphous
state. However, the film of Oxa-mTPE shows deep-blue
emission peaked at 456 nm, similar to the aggregates with
99% water fraction, but about 34 nm blue-shifted than the
aggregates with 80% water fraction, indicating that the film
is in the crystalline state, and the aggregates with 80% water
fraction are in the amorphous state. This could be attributed
to the twisted conformation of Oxa-mTPE with meta-
linkage between TPE and oxadiazole group, which makes
the molecules more easily crystallized. In addition, the re-
sults have also shown that Oxa-pTPE is more conjugated
than Oxa-mTPE, confirming that the intramolecular conju-
gation could be tuned through different linking positions.
This is in consistent with our design idea.
3.4 Theoretical calculations
To further understand the photophysical properties at the

6
Huang J, et al. Sci China Chem January (2013) Vol.56 No.1
molecular level, Density Functional Theory (DFT) calcula-
tions (B3LYP/6-31g*) were carried out to obtain orbital
distributions of HOMO and LUMO energy levels
ofOxa-pTPE and Oxa-mTPE. The dihedral angle between
the adjacent phenyl blades of TPE and oxadiazole unit is
~54° for Oxa-pTPE, higher than that of Oxa-mTPE (~72°).
Thus, Oxa-pTPE is more planar and conjugated, in good
accordance with the UV data. As shown in Figure 5(a), the
LUMOs of both molecules are located on the electron-
accepting oxadiazole unit, and the HOMO orbitals are
mainly distributed on the TPE moiety, showing the almost
separation of HOMO and LUMO orbitals. Fortunately, we
have obtained the single crystal of Oxa-mTPE from a di-
chloromethane-methanol solution. From the ORTEP draw-
ing (Figure 5(b)), Oxa-mTPE has a twisted conformation
and nearly no intermolecular interactions are found in the
packing arrangement, which would be beneficial to prevent
the formation of species detrimental to emission.
2.25 and 2.18 eV for Oxa-pTPE and Oxa-mTPE, re-
spectively.
3.6 Electroluminescence
The efficient solid-state emission, good thermal and mor-
phological stability of Oxa-pTPE and Oxa-mTPE prompt-
ed us to investigate their electroluminescence properties.
We have fabricated non-doped multilayer organic light-
emitting diodes (OLEDs) with configurations of ITO/NPB
(60 nm)/EML(40nm)/TPBi(10 nm)/Alq3(20 nm)/Al(100 nm)
by vacuum deposition. In the devices, NPB, TPBi and Alq3
act as the hole-transporting layer, hole-blocking layer and
electron-transporting layer, respectively, while Oxa-pTPE
and Oxa-mTPE served as emitters.
As listed in Table 2, device based on Oxa-pTPE possesses
lower turn-on voltage (5.1 eV) than that of Oxa-mTPE
(6.25 eV), showing its smaller injection barrier from trans-
porting layers. However, from the current density-voltage-
luminance curves (Figure 6(a)), we can see that the current
density of Oxa-mTPE increases more quickly than that of
Oxa-pTPE, accompanying by the increasement of the oper-
ating voltage. In addition, the luminance of the device based
on Oxa-mTPE (750 cd m2) is higher than that of Oxa-
pTPE, suggesting the more efficient exciton combination in
the emissive layer for Oxa-mTPE. Altogether, device based
on Oxa-pTPE shows much higher EL efficiencies than that
of Oxa-mTPE. In detail, device of Oxa-mTPE exhibits
maximum current efficiency (C, max) and maximum power
efficiency (P, max) of 0.72 cd A1 and 0.49 Im W1, respec-
tively. Much better EL performance is observed for Oxa-
pTPE with C, max and P, max of 1.52 cd A1 and 0.84
Im W1. It is worthy to note: although the chemical struc-
tures of the two luminogens are nearly the same, that is, the
minor different para- or meta- linkage mode of TPE and
oxadiazole core, their device performance is with big dif-
ference. It is reasonable. The conformation of Oxa-pTPE is
more planar than that of Oxa-mTPE, in another word, it is
more conjugated, which is good for the carrier transporting
when served as emissive layers. For Oxa-pTPE, the corre-
sponding OLED device exhibits sky-blue emission with EL
peak at 470 nm and the CIE coordinate of (0.17, 0.23).
However, much bluer emission is observed for Oxa-mTPE,
with EL peak at 452 nm and CIE coordinates of (0.15, 0.12),
because the meta-linkage of TPE and oxadiazole is less con-
tributable to the elongation of intramolecular conjugation
3.5 Electrochemical properties
Cyclic voltammetry (CV) was carried out to investigate the
electrochemical behaviors of these materials. The highest
occupied molecular orbital (HOMO) energy levels were
estimated from the onset oxidation potentials according to
the equation: EHOMO = (4.8 + Eox) (eV). Therefore, the
HOMO energy levels of Oxa-pTPE and Oxa-mTPE were
calculated to be 5.55 and 5.57 eV, respectively, suggesting
the two molecules possessed similar hole-transporting ability.
And the lowest unoccupied molecular orbital (LUMO) ener-
gy levels could be obtained from optical bandgap energies
(estimated from the onset wavelengths of the UV absorp-
tions) and HOMO values, which gave LUMO values as
Figure 5 (a) Calculated molecular orbital amplitude plots of HOMO and
LUMO levels and optimized molecular structures of Oxa-pTPE and
Oxa-mTPE; (b) ORTEP drawing of Oxa-mTPE.
Table 2 EL performances of Oxa-pTPE and Oxa-mTPEa)
Von (V)
Lmax (cd m2)
615

P, max (Im w1)
0.84

C, max (cd A1)
1.52

ext, max (%)
CIE (x, y)
Oxa-pTPE
Oxa-mTPE
5.10
6.25
0.97
0.17, 0.23
0.15, 0.12
750
0.49
0.72
0.82
a) Abbreviations: Von = turn-on voltage at 1 cd m2, Lmax = maximum luminance, C, max, C, max and ext, max = maximum power, current and external effi-
ciencies, respectively. CIE = Commission International de l’Eclairage coordinates.

Huang J, et al. Sci China Chem January (2013) Vol.56 No.1
7
erating good AIE-active emitters without sacrificing blue or
deep-blue emission.
4 Conclusions
In this work, we have successfully synthesized two TPE-
based luminogens of Oxa-pTPE and Oxa-mTPE. They all
show splendid AIE characteristic and good thermal and
morphological stabilities. By combining the TPE moiety
and oxadiazole group together through meta- or para- link-
age mode, their intramolecular conjugation could be effec-
tively adjusted owing to the different molecular confor-
mations. When served as emissive layers in OLEDs,
Oxa-pTPE and Oxa-mTPE exhibit good EL performance
with Lmax, C, max, and P, max up to 750 cd m2, 1.52 cd A1
and 0.84 Im W1, and CIE coordinates of (0.17, 0.23) and
(0.15, 0.12), respectively. Especially for Oxa-mTPE, the
device exhibits deep-blue emission almost the same as TPE
(445 nm) but with enhanced EL efficiencies. Apparently,
the conjugation could be tuned through different linking
positions, which is also believed to be an efficient approach
to construct deep-blue AIE luminogens for OLEDs.
We are grateful to the National Science Foundation of China (21161160556),
the National Basic Research Program (973 program, 2013CB834700), and
the Open Project of State Key Laboratory of Supramolecular Structure and
Materials (sklssm201302) for financial support.
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