9,10-Bis(N-methylcarbazol-3-yl-vinyl-2)anthracene: High contrast piezofluoro-chromism and remarkably doping-improved electroluminescence performance

Article abstract of DOI:10.1016/j.dyepig.2015.09.032

This paper reported the synthesis, optical and electroluminescent properties of 9,10-bis(N-methylcarbazol-3-yl-vinyl-2)anthracene (MCA). MCA could be facilely prepared by the Wittig-Horner reaction of N-methyl-3-formylcarbazole and 9,10-bis(diethoxyphosphorylmethyl)anthracene and exhibited strong aggregation-induced and crystallization-enhanced emission (AIE/CEE) effect. Simple mechanical force could change MCA solid from initial green to final red emission, affording high contrast piezofluorochromism with large spectral shift of up to 90 nm. When MCA was used as the emissive material in light-emitting diodes, the doped devices showed remarkably improved electroluminescence (EL) performance whose EL efficiency was 8 times higher than that of the non-doped device. Moreover, the dopant concentrations did not affect significantly the fluorescence properties of doped films and EL performances of doped devices, demonstrating the potential advantage of AIE/CEE luminogens applicable for EL dopants.

Full text of DOI:10.1016/j.dyepig.2015.09.032

Dyes and Pigments 125 (2016) 8e14
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
9
,10-Bis(N-methylcarbazol-3-yl-vinyl-2)anthracene: High contrast
piezofluoro-chromism and remarkably doping-improved
electroluminescence performance
a
a
a
a
a
a, b, **
Wei Liu , Shian Ying , Qikun Sun , Xu Qiu , Haichang Zhang , Shanfeng Xue
,
a, *
Wenjun Yang
a
Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Province (QUST), School of Polymer Science & Engineering, Qingdao University of
Science & Technology, 53-Zhengzhou Road, Qingdao 266042, China
State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of
b
Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 July 2015
Received in revised form
This paper reported the synthesis, optical and electroluminescent properties of 9,10-bis(N-methyl-
carbazol-3-yl-vinyl-2)anthracene (MCA). MCA could be facilely prepared by the WittigeHorner reaction
of N-methyl-3-formylcarbazole and 9,10-bis(diethoxyphosphorylmethyl)anthracene and exhibited
strong aggregation-induced and crystallization-enhanced emission (AIE/CEE) effect. Simple mechanical
force could change MCA solid from initial green to final red emission, affording high contrast piezo-
fluorochromism with large spectral shift of up to 90 nm. When MCA was used as the emissive material in
light-emitting diodes, the doped devices showed remarkably improved electroluminescence (EL) per-
formance whose EL efficiency was 8 times higher than that of the non-doped device. Moreover, the
dopant concentrations did not affect significantly the fluorescence properties of doped films and EL
performances of doped devices, demonstrating the potential advantage of AIE/CEE luminogens appli-
cable for EL dopants.
23 September 2015
Accepted 25 September 2015
Available online 8 October 2015
Keywords:
9,10-Bis(N-methyl-carbazol-3-yl-vinyl-2)
anthracene
Aggregation-induced emission
Crystallization-enhanced emission
High contrast piezofluorochromism
Doping-improved electroluminescence
performance
© 2015 Elsevier Ltd. All rights reserved.
WittigeHorner reactions
1. Introduction
into such chromophores [20‒24]. The adjoint problem is that bulky
groups usually dilute fluorophore concentration and hinder carrier
Conjugated organic molecules exhibiting strong solid fluores-
cence and mechanically stable and meta-stable morphologies have
been promising materials applicable in optical recording, active
gain media for optically pumped lasers, and electroluminescence
transport. In 2001, Tang et al. discovered a novel anti-ACQ phe-
nomenon: 1-methyl-1,2,3,4,5-pentaphenylsilole is non-emissive
when dissolved molecularly in good solvents but become highly
emissive upon aggregation in poor solvents and condensation in
solid states, which was named “aggregation-induced emission
(AIE)” [25]. Since then, a large number of AIE luminogens charac-
terized by strongly twisted skeleton bearing rotatable aryl units
have been developed and expected to be promising emitters for
electroluminescence and piezofluorochromism (PFC) due to their
strong solid-state fluorescence and diverse aggregation
morphology [26‒38]. Indeed, AIE luminogens have become the
mainstay of PFC materials reported to date [3,39‒41], and some EL
devices with AIE luminogens as the emissive layer have been
fabricated [42‒46]. However, most present PFC materials based on
AIE dyes show the PFC shifts within tens of nanometers under
simple mechanical force [3,47‒49], and those with very large PFC
shifts (e.g., >80 nm) and well-marked colorimetric fluorescence are
(
EL) fields [1e15]. However, many conventional conjugated organic
molecules suffer from intense intermolecular interactions and
detrimental species such as excimers and exciplexes, which can
induce notorious aggregation-caused quenching (ACQ) effect
[16e19]. To mitigate these problems and obtain highly fluorescent
solid materials, an optional way is to introduce bulky substituents
*
Corresponding author.
* Corresponding author. Key Laboratory of Rubber-Plastics of Ministry of Edu-
cation/Shandong Province (QUST), School of Polymer Science Engineering,
*
&
Qingdao University of Science & Technology, 53-Zhengzhou Road, Qingdao 266042,
China.
E-mail address: ywjph2004@qust.edu.cn (W. Yang).
http://dx.doi.org/10.1016/j.dyepig.2015.09.032
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

W. Liu et al. / Dyes and Pigments 125 (2016) 8e14
9
0 0
over calcium hydride before use. 4,4 -Dicarbazol-9-yl-1,1 -biphenyl
still scarce [48,50]. On the other hand, when AIE luminogens are
used as bulk emissive layers by vapor deposition, their EL perfor-
mances seem to be not as good as expected [42‒46]. This is prob-
ably due to the poor carrier transport caused by strong twisting
skeleton and loose intermolecular stacking. Moreover, most AIE
molecules are characterized by crystallization-enhanced emission
and exhibit usually the decreased fluorescence efficiency in the
amorphous aggregates. It is an innovative attempt to fabricate the
highly fluorescent crystalline films of AIE molecules; however, this
is quite difficult due to that vapor deposition affords amorphous
films in principle. We have ever annealed and solvent-fumed the
vapor-deposition films of AIE-active PFC dyes, but unlike the
amorphized films produced by grinding or pressing, they can not be
converted to high-fluorescent crystalline films. This is probably due
to the lack of residual crystal seeds in vapor-deposition films [51].
To further improve EL performance of AIE dyes, an alternative way
is to optimize device configuration. It is known that the doped
devices could exhibit improved EL performance, but the conven-
tional organic fluorophores usually show dopant concentration-
dependent photoluminescence (PL) and EL properties [52‒54].
We wonder how the EL performance is improved when AIE lumi-
nogens are used as fluorescent dopants. To our knowledge, there
have been no such reports at present.
0
0
(CBP), N,N -di(1-naphthyl)-N,N -diphenylbenzidine (NPB), 1,3,5-
tris(1-phenyl-benzoimidazol-2-yl)benzene (TPBi), and lithium
fluoride (LiF) were purchased from Aldrich Chemical Co. and were
utilized as host, hole-transporting, electron-transporting, hole-
blocking, and interface modify-cation materials, respectively.
Poly(3,4-ethylenedioxythiopene):poly(styrenesulfonate)
(PEDOT:PSS) (Baytron P4083, Bayer AG) was purchased from H. C.
Starck Inc. They were used without further purification.
2.2. Preparation of aqueous dispersion and ground sample
The stock solution of MCA in THF with a concentration of
ꢁ4
1
.0 ꢀ 10 M was prepared and reserved. THF/water mixtures with
different water fractions (aqueous dispersion) were prepared by
slowly adding distilled water into the THF solution of MCA under
ultrasound at room temperature, and the final concentration was
ꢁ5
kept at 1.0 ꢀ 10 M. Ground sample was prepared by grinding
MCA powder on a glass plate with a metal spatula at room
temperature.
2.3. Measurements
Here we have synthesized a simple AIE dye 9,10-bis(N-methyl-
carbazol-3-yl- vinyl-2)anthracene (MCA, Scheme 1), and its PFC
and EL properties are investigated. We now present that MCA
shows high contrast mechanochromic luminescence with very
large PFC shift under simple mechanical force and remarkably
doping-improved and dopant concentration-independent EL
performance.
1H and 13C NMR spectra were recorded on a Bruker AC500
500 MHz) spectrometer at 298 K by utilizing deuterated chloro-
(
form (CDCl ) as the solvent and tetramethylsilane (TMS) as the
3
internal standard. The MALDI-TOF-MS mass spectra were recorded
using an AXIMA-CFRTM plus instrument. Elemental analysis was
performed on a PerkinElmer 2400. UVevis absorption spectra were
recorded on a Hitachi U-4100 spectrophotometer. Fluorescence
measurements were carried out with a Hitachi F-4600 spectro-
2
. Experimental section
photometer. Solution fluorescence quantum yield (F) was deter-
mined at room temperature by the dilution method using
fluorescein in water (pH ¼ 11) as the reference. The quantum effi-
ciencies of solid films were carried out with FLS980 Spectrometer.
Thermal gravimetric analysis (TGA) was undertaken on a Perki-
2
.1. Materials
Starting materials and solvents were purchased from Aldrich
ꢂ
ꢁ1
nElmer thermal analysis system at a heating rate of 10 C min and
Chemical Co., or Energy Chemical Co., China. Tetrahydrofuran (THF)
was distilled over metallic sodium and dimethylformamide (DMF)
ꢁ
1
a nitrogen flow rate of 80 mL min . Differential scanning
Scheme 1. Synthetic route and structure of MCA.

10
W. Liu et al. / Dyes and Pigments 125 (2016) 8e14
calorimetry (DSC) was performed on a NETZSCH (DSC-204) unit at a
heating rate of 10 C min under nitrogen.
liquid (yield: 89.6%). 1H NMR (500 MHz, CDCl
): d 8.12 (d, 2H,
3
J ¼ 10.0 Hz), 7.46 (m, 4H), 7.25 (m, 2H), 0.86 (t, 3H, J ¼ 8.5 Hz) ppm.
ꢂ
ꢁ1
2.4. Device fabrication
2.5.4. N-methyl-3-formylcarbazole
A cooled flask in ice bath was added phosphoryl chloride
(0.6 mL, 5.9 mmol) and anhydrous DMF (1.4 mL, 17.7 mmol) under
nitrogen. This mixture was added 9-methyl-carbazole (0.98 g,
5.4 mmol, in 20 mL sym-dichloroethane). After stirring for 1 h, the
Indium-tin oxide (ITO) coated glass with a sheet resistance of
5e20 U per square was used as the substrate. The substrate was
1
pre-patterned by photolithography to give an effective device size
2
ꢂ
of 6.25 mm . It was then cleaned in an ultrasonic bath with acetone,
reaction temperature was raised to 90 C and kept for 8 h. The
detergent, deionized water, and isopropanol in a sequence, and
dried in an oven. After oxygen plasma cleaning for 4 min, a 40 nm
thick PEDOT:PSS layer was first spin-coated onto the ITO substrate
mixture was cooled and poured into ice water and extracted with
dichloromethane. The solvent was evaporated and the crude
product was purified by column chromatography on silica gel (ethyl
using water solution and then dried by baking in a vacuum oven at
acetate/hexane, 1/10, v/v). A yellow solid was obtained (0.88 g,
ꢂ
1
8
0 C overnight. On the top of that, organic layers were vacuum
78.3%). H NMR (500 MHz, CDCl
3
):
d
10.10 (s, 1H), 8.62 (s, 1H), 8.16
0
0
deposition with N,N -di-1-naphthyl-N,N -diphenylbenzidine (NPB),
emitting layer (EML ¼ CBP: x% MCA), and 1,3,5-tri(phenyl-2-
benzimidazolyl)benzene (TPBi) in a sequence. Finally, a 0.5 nm
thick LiF film and a 100 nm thick Al film were evaporated with a
shadow mask to form the top electrode, at a base pressure of
(d, 1H, J ¼ 11.5 Hz), 8.01 (d, 1H, J ¼ 9.5 Hz), 7.54 (t, 1H, J ¼ 8.0 Hz),
7.47, (t, 2H, J ¼ 9.5 Hz), 7.33, (t, 1H, J ¼ 9.5 Hz), 4.34, (t, 2H,
J ¼ 9.5 Hz), 1.89 (m, 2H), 1.33 (m, 10H), 0.86, (t, 3H, J ¼ 9.0 Hz) ppm.
2.5.5. 9,10-Bis[(N-methylcarbazol-3-yl)vinyl]anthracene (MCA)
ꢁ
4
3
ꢀ 10 Pa. The thickness of the evaporated cathodes was moni-
tored using a quartz crystal thickness/ratio monitor (Model: STM-
00/MF, Sycon). The electroluminescence (EL) spectra and CIE co-
9,10-bis(diethoxyl-phosphorylmethyl)anthracene
(0.50
g,
1.05 mmol) and N-methyl-3-formylcarbazole (0.50 g, 23.0 mmol)
were dissolved in 50 mL of anhydrous THF. Potassium tert-butoxide
(0.78 g, 6.7 mmol) was added and the suspension was stirred for
6 h at room temperature. Methanol was added into the mixture and
a yellow-green solid was collected by filtration. The crude product
was purified by silica gel column chromatography (petroleum
1
ordinates of these devices were measured by a PR650 spectra scan
spectrometer. The luminance-current and density-voltage charac-
teristics were recorded simultaneously with the measurement of
the EL spectra by combining the spectrometer with a Keithley
model 2400 programmable voltageecurrent source. All measure-
ments were carried out at room temperature under ambient
conditions.
ether/methylene chloride ¼ 4/1, v/v). A green solid was obtained
1
(0.51 g, 82%). H NMR (500 MHz, CDCl
3
):
d
8.53 (m, 4H), 8.40 (s, 2H),
8.18 (d, 2H, J ¼ 8.0 Hz), 7.99 (d, 2H, J ¼ 13.0 Hz), 7.88 (d, 2H,
J ¼ 10.0 Hz), 7.49 (m, 10H), 7.28 (t, 2H, J ¼ 9.5 Hz), 7.16 (d, 2H,
J ¼ 9.5 Hz), 4.37, (t, 4H, J ¼ 11.0 Hz), 1.91 (m, 4H), 1.35 (m, 20H), 0.88
2
2
.5. Synthesis
1
3
(
t, 6H, J ¼ 9.5 Hz) ppm. C NMR (125 MHz, CDCl
3
): d 140.92, 140.40,
.5.1. 9,10-Bis(chloromethyl)anthracene
138.21,133.03, 129.76,128.56,126.69, 125.87, 125.04,124.40, 123.28,
122.90, 122.26, 120.53, 119.02, 118.73, 108.98, 53.40, 43.31, 31.79,
29.42, 29.21, 27.29, 22.58, 14.06 ppm m/z 588.26 [Mþ, calcd 589.5].
To a stirred solution of anthracene (1.78 g, 10 mmol), anhydrous
ZnCl (1.64 g, 12 mmol), paraformaldehyde (1.50 g, 50 mmol) in
2
dioxane (20 mL) was slowly added concentrated aqueous hydro-
chloric acid (40 mL) at room temperature. After addition, the
mixture was gentle refluxed for 3 h and allowed to stand for 16 h at
room temperature. The resulting fine granular solid was separated
Anal. Calcd. for C58
H, 7.65; N, 3.59.
60 2
H N : C, 88.73; H, 7.70; N, 3.57. Found: C, 88.62;
3. Results and discussion
by filtration and washed with H
2
O and dioxane to afford a crude
product. The crude product was recrystallized from toluene to give
3.1. Design, synthesis and characterization
9
6
,10-Bis(chloromethyl)anthracene as a yellowish solid (1.8 g,
1
4.1%). H NMR (500 MHz, CDCl
3
):
d
8.40 (m, 4H), 7.62 (m, 4H), 5.61
In previous work, we have synthesized a series of 9,10-bis(N-
alkylcarbazol-3-yl-vinyl-2)anthracenes (ACZn) with different
length of alkyl chains, and found that shorter alkyl-containing ACZn
solids show higher fluorescence efficiencies, and the emission
spectra of pristine crystalline and ground amorphous states are
gradually blue- and red-shifted, respectively [55]. This implies that
the shortest alkyl-containing ACZn might be highly fluorescent
material and exhibits larger PFC shift. Thereby 9,10-bis(N-methyl-
carbazol-3-yl-vinyl-2)anthracene (MCA) is synthesized and inves-
tigated. MCA can be facilely prepared by the WittigeHorner of N-
(
s, 4H) ppm.
2
.5.2. 9,10-Bis(diethoxylphosphorylmethyl)anthracene
solution of 9,10-bis-(chloromethyl)anthracene (7.8 g,
8.3 mmol) and triethyl phosphite (30 mL) was stirred vigorously
A
2
overnight at gentle reflux. The excess triethyl phosphate was
removed by distillation under reduced pressure. The crude product
was separated by silica gel column chromatography using ethyl
acetate/petroleum ether (1/1, v/v) as the eluent. A yellow solid was
obtained (10.4 g, 76.8%). H NMR (500 MHz, CDCl
1
3
):
d
8.32 (d, 4H,
methyl-3-formylcarbazole
and
9,10-bis(diethoxy-phosphor-
J ¼ 9.5 Hz), 7.53 (d, 4H, J ¼ 8.0 Hz), 4.21 (d, 8H, J ¼ 8.5 Hz), 3.78 (d,
ylmethyl)anthracene in high yield of 82% (Scheme 1), and its
composition and structure has been unambiguously characterized
by MALDI-TOF MS, H NMR, C NMR, and elemental analysis.
4
H, J ¼ 9.0 Hz), 1.11 (t, 12H, J ¼ 7.5 Hz) ppm.
1
13
2.5.3. N-methylcarbazole
A mixed solution of 50% aqueous NaOH (12 mL), DMSO (40 mL)
and carbazole (3.0 g, 17.9 mmol) was slowly added iodomethane
3.2. Fluorescence properties in different solvents and aqueous
dispersions
(
0.57 mL, 19.8 mmol) at room temperature. The mixture was stirred
ꢂ
at 90 C overnight and then cooled to room temperature. After that,
the brine was added to the mixture and the organic phase extracted
Since large rigid skeleton and short alkyl chain, MCA is only the
limited soluble in common organic solvents, such as dichloro-
methane, chloroform, tetrahydrofuran (THF), toluene, and dime-
thylformamide (DMF). All the dilute solutions are weakly or non
fluorescent, but the aqueous dispersions with a large amount of
with dichloromethane was dried over MgSO
removed and residue was purified by a column chromatography
silica gel, petroleum ether/DCM ¼ 10/1, v/v) to give 2.90 g of yellow
4
. The solvent was
(

W. Liu et al. / Dyes and Pigments 125 (2016) 8e14
11
non-solvent water are strongly emissive (Fig. 1a). For example, the
fluorescence intensity of MCA in suitable THF/water mixture is 50
times higher than that in THF (Fig. 1b). As-prepared (pristine) MCA
solid emits bright green fluorescence with the peak emission
diffractogram of the ground MCA solid displays broad and
depressed reflections, indicating the formation of crystal defects
and the appearance of some amorphous states. Therefore, there are
residual crystal seeds and amorphized regions existed in the PFC
state, which should crystallize upon annealing or solvent-fuming
and be responsible for reversible PFC behavior.
wavelength of 512 nm and the fluorescence efficiency (
.3. Solid-state photophysical and thermal properties
Exploitation of new PFC materials and investigation on PFC
Ф) of 0.47.
3
3.4. Electroluminescence properties
phenomenon essence and their applications have attracted much
attention. In terms of exploitation on new PFC materials, high
contrast in fluorescence color and large spectral shift under simple
mechanical force are desirable. We investigate PFC behavior of the
mixing sample of MCA and KBr (for the sake of minimizing the
amount of fluorophore) using IR pellet press (30 s at 1500 psi). As
shown in Fig. 2a, the fluorescence color of pressed sample is red.
Since strong fluorescence in crystalline and partly amorphized
states (Fig. 4a), as well as high thermostability (Fig. 4b) and glass
transition temperature (Fig. 3a), MCA is expected to might be
promising material for light-emitting diodes. We first employ MCA
as bulk emissive layer to fabricate the non-doped device with the
configuration of ITO/PEDOT:PSS (40 nm)/NPB (40 nm)/EML
(50 nm)/TPBi (40 nm)/LiF (0.5 nm)/Al (100 nm). This bulk film
device affords the maximal luminance and luminous efficiency of
ꢂ
However, when the sample is annealed for 3 min at 200 C, green
ꢁ
2
ꢁ1
ꢁ2
fluorescence is observed. Repressing the annealed sample affords
again red fluorescence. The red-fluorescence sample could be
changed to green emission by fuming over dichloromethane at
room temperature, and the green fumed sample could also be
changed back to red emission by repressing. This fluorescence color
change is reversible and also very conspicuous. Fig. 2b depicts the
corresponding emission spectra of the above fluorescence color
change. The pressed sample emits red fluorescence with the peak
wavelength of 602 nm, and the fumed and annealed samples emit
green with the peak wavelength of 512 nm. This indicates that
pressing-induced spectral shift is up to 90 nm, which is among the
largest PFC shifts reported to date under simple mechanical force,
to our knowledge. It is noted that the pressed state shows bright red
emission with the commendable fluorescence efficiency of 0.31
measured on a fluorescence integrating sphere, and grinding
experiment affords the same effectiveness as pressing (shown in
Fig. 4a).
7800 cd m at 10 V and 0.57 cd A at 45 mA cm , respectively
(Fig. 5), an unimpressive EL performance. To gain an insight into the
poor EL performance, the emission spectrum and fluorescence ef-
ficiency of MCA bulk film obtained by vapor deposition are
measured. The vapor deposited film could be considered as amor-
phous state, and such bulk film emits yellow fluorescence with the
peak wavelength of 564 nm and the fluorescence efficiency of 0.16
(Fig. 4a and c). Therefore, MCA is not only an aggregation-induced
emission but also a crystallization-enhanced emission dye. The
poor EL performance could be ascribed to the obviously decreased
fluorescence efficiency in amorphous film and the strongly twisted
skeleton impeding carrier transport. It is noted that annealing or
fuming could not change the fluorescence color of vapor deposited
film, which may be because of the absence of the seed of the crystal.
This implies that the residual crystal seeds have played a crucial
role in the reversible PFC behavior based on recrystallization, and
the amorphous state from the vapor deposition is different from
not only the crystalline state but also the partly amorphized ground
state in molecular stacking and fluorescence properties. The
decreased fluorescence efficiency and red-shifted emission
To understand aggregate structure change caused by mechani-
cal force in PFC behavior, wide-angle X-ray diffraction (WXD) and
differential scanning calorimetry (DSC) measurements on MCA
solids are conducted (Fig. 3). The DSC curves upon heating proce-
suggest that MCA molecules have stronger
pep interaction and less
dure indicate that there is no thermal transition for as-prepared
twisted conformation in the amorphous film.
ꢂ
(
pristine) MCA solid but a obvious glass transition at 160 C for
It is known that doped device could alleviate the intermolecular
interaction and enhance the carrier transport. To understand how
the EL performance is improved when AIE luminogens are used as
fluorescent dopants, we have doped MCA into host material CBP
ground solid (Fig. 3a), implying the existence of amorphous state in
ground solid. The diffraction pattern of the pristine MCA solid
shows sharp and intense diffraction peaks, indicative of a well-
ordered microcrystalline structure (Fig. 3b). In contrast, the
0
0
(4,4 -Bis(N-carbazolyl)-1,1 -biphenyl) to fabricate the doped
ꢁ
5
Fig. 1. (a) Fluorescence imaging of MCA solutions (upper) and aqueous dispersions (lower) in different solvents (1.0 ꢀ 10 M); (b) Emission spectra of MCA in THF/H
2
O mixtures at
.0 ꢀ 10ꢁ M.
5
1

12
W. Liu et al. / Dyes and Pigments 125 (2016) 8e14
Fig. 2. Fluorescence images of KBr-MCA mixture under various external stimuli (a) and the corresponding emission spectra (b).
Fig. 3. Differential scanning calorimetric curve of pure MCA solid under pristine and ground states (a) and the corresponding powder wide-angle X-ray diffractogram at room
temperature (b).
ꢁ
2
devices with configuration of ITO/PEDOT:PSS (40 nm)/NPB
40 nm)/CBP:MCA (50 nm)/TPBi (40 nm)/LiF (0.5 nm)/Al (100 nm),
in which the dopant concentrations are 3%, 5%, and 10% by weight,
respectively. As shown in Fig. 5, the doped devices exhibit the
maximal luminances of 15,000e16,800 cd m and the maximal EL
ꢁ
1
(
efficiencies of 4.9e4.2 cd A , which are 2-fold and 8-fold higher
than the corresponding luminance and efficiency of non-doped
device. That is, doping can significantly improve EL performances
Fig. 4. (a) Fluorescence images of pure MCA solid under pristine, ground, annealed, and deposited film states (taken under a 365 nm UV lamp, the numbers are the corresponding
ꢂ
fluorescence efficiencies); (b) TGA thermograms of MCA solid recorded at a heating rate of 10 C/min; (c) Fluorescence spectra of MCA solid under pristine, deposited film and
ground states.

W. Liu et al. / Dyes and Pigments 125 (2016) 8e14
13
Fig. 5. (a) The current densityeluminanceevoltage (JeLeV) and (b) the luminous efficiencyepower efficiencyeluminance (LEePEeL) characteristics under the device configuration
of ITO/PEDOT:PSS (40 nm)/NPB (40 nm)/: EML (z50 nm)/TPBi (40 nm)/LiF (0.5 nm)/Al (100 nm) (bulk film ¼ pure MCA; Doped 3% ¼ CBP: 3% MCA; Doped 5% ¼ CBP: 5% MCA;
Doped 10% ¼ CBP: 10% MCA).
Fig. 6. (a) Fluorescence of doped and no-doped films; (b) Electroluminescence spectra of doped and no-doped devices.
of AIE-active MCA dye, and higher dopant concentrations still
afford commendable EL performances. Meanwhile, we have
measured the fluorescence efficiencies of doped films, and find that
their fluorescence efficiencies are 26%, 28%, and 22% for 3%, 5%, and
aggregation-induced and crystallization-enhanced emission (CEE)
dye, and grinding or pressing can change MCA solid from green to
red emission with the spectral shift of 90 nm. This large spectral
shift is among the best remarkable piezofluorochromism under
simple mechanical force to date. Since the CEE characteristic of
MCA dye, the evaporated bulk film shows the decreased fluores-
cence efficiency and the non-doped device exhibits unimpressive
EL performance. However, when MCA is used as dopant, the doped
films show the increased fluorescence efficiencies and the doped
devices exhibit the significantly improved EL performances. Inter-
estingly, the dopant concentrations do not affect significantly the
fluorescence properties of doped films and EL performances of
doped devices. This work demonstrates that AIE molecules could be
promising dopants for dopant concentration-independent doped
devices with improved EL performances.
10% of MCA concentration, respectively. This implies that the
intermolecular interaction has been alleviated.
pep
Fig. 6 shows the PL and EL spectra under different dopant
concentration. In general, the emission spectra of doped films are
obviously blue-shifted relative to that of the bulk film, and the
dopant concentrations only affect slightly the emission spectra. In
detail, the EL and PL spectra of doped films are more similar
(
difference about 10 nm), but the difference between EL and PL
spectra for bulk film is over 25 nm. This indicates that doping
devices could retain qualitatively the natural fluorescent proper-
ties of the light-emitting layer. The doping-induced blue-shifted
emission could be partly related to the alleviated intermolecular
pep interaction, perhaps MCA could adopt more twisted
Acknowledgment
conformation in doped film, and real reason is not clear. Never-
theless, AIE molecules should be promising dopants due to that
the dopant concentration does not affect significantly the fluo-
rescence properties of doped films and EL performances of doped
devices.
We are grateful for financial support from the National Natural
Science Foundation of China (51173092, 51303091), the Natural
Science Foundation of Shandong Province of China (2014ZRB01296)
and the Natural Science Foundation of Qingdao City of China (13-1-
4-207-jch). We thank the Specialized Doctoral Research Fund of
4
. Conclusions
Ministry of Education (No. 201333719120005, 2014M560537) and
the State Key Laboratories of Supra-molecular Structure and Ma-
terials of Jilin University (SKLSSM2015023) and Luminescent Ma-
terials and Devices of South China University of Technology (2014-
skllmd-08).
We have synthesized a simple molecule 9,10-bis(N-methyl-
carbazol-3-yl-vinyl-2)anthracene (MCA) and demonstrated the
optical and electroluminescence properties. MCA is a strong

14
W. Liu et al. / Dyes and Pigments 125 (2016) 8e14
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