Aggregation-induced emission, mechanochromism and blue electroluminescence of carbazole and triphenylamine-substituted ethenes

Article abstract of DOI:10.1039/c4tc00097h

Carbazole and triphenylamine-substituted ethenes are synthesized [Ph ₂CCPh(R), R = 9-carbazolyl, 9-hexyl-3-carbazolyl and 4-(diphenylamino)phenyl] and their optical properties are investigated. All luminogens are nonemissive when molecularly dissolved in good solvents but become highly emissive in the aggregated state, showing a phenomenon of aggregation-induced emission. High solid-state fluorescence quantum yields up to 97.6% have been achieved in their solid thin films. The luminogens are thermally stable, showing high degradation temperatures of up to 315 °C. They exhibit mechanochromism: their emissions can be repeatedly switched between blue and green colors by simple grinding-fuming and grinding-heating processes due to the morphological change from crystalline to amorphous state and vice versa. Multilayer light-emitting diodes with device configurations of ITO/NPB/dye/TPBi/Alq₃/LiF/Al, ITO/NPB/dye/TPBi/LiF/Al and ITO/dye/TPBi/LiF/Al are fabricated, which emit sky blue light with maximum luminance, current efficiency, power efficiency and external quantum efficiency of 11700 cd m^-2, 7.5 cd A^-1, 7.9 lm W^-1 and 3.3%, respectively.

Full text of DOI:10.1039/c4tc00097h

Journal of
Materials Chemistry C
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PAPER
Aggregation-induced emission, mechanochromism
and blue electroluminescence of carbazole and
triphenylamine-substituted ethenes†
Cite this: J. Mater. Chem. C, 2014, 2,
4320
Carrie Y. K. Chan,a Jacky W. Y. Lam,a Zujin Zhao,b Shuming Chen,c Ping Lu,d
Herman H. Y. Sung,a Hoi Sing Kwok,c Yuguang Ma,d Ian D. Williamsa
abef
*
and Ben Zhong Tang
Carbazole and triphenylamine-substituted ethenes are synthesized [Ph2C]CPh(R), R ¼ 9-carbazolyl, 9-
hexyl-3-carbazolyl and 4-(diphenylamino)phenyl] and their optical properties are investigated. All
luminogens are nonemissive when molecularly dissolved in good solvents but become highly emissive in
the aggregated state, showing a phenomenon of aggregation-induced emission. High solid-state
fluorescence quantum yields up to 97.6% have been achieved in their solid thin films. The luminogens
are thermally stable, showing high degradation temperatures of up to 315 ꢀC. They exhibit
mechanochromism: their emissions can be repeatedly switched between blue and green colors by
simple grinding–fuming and grinding–heating processes due to the morphological change from
crystalline to amorphous state and vice versa. Multilayer light-emitting diodes with device configurations
of ITO/NPB/dye/TPBi/Alq3/LiF/Al, ITO/NPB/dye/TPBi/LiF/Al and ITO/dye/TPBi/LiF/Al are fabricated, which
emit sky blue light with maximum luminance, current efficiency, power efficiency and external quantum
efficiency of 11 700 cd mꢁ2, 7.5 cd Aꢁ1, 7.9 lm Wꢁ1 and 3.3%, respectively.
Received 15th January 2014
Accepted 18th March 2014
DOI: 10.1039/c4tc00097h
www.rsc.org/MaterialsC
their dilute solutions emit strongly under UV irradiation. This is
attributed to the strong intermolecular electronic interactions
Introduction
in the solid state, which favour the formation of detrimental
species such as excimers and exciplexes. Such a phenomenon is
coined as “aggregation-caused quenching” (ACQ) and has been
a textbook-knowledge.2 In contrast to a large number of ACQ
molecules, examples of highly emissive organic solids with high
FF,F values are still rare, especially those with strong solid-state
blue emission. It is therefore challenging to achieve stable and
efficient solid-state blue emitters compared with green and red
emitters as the performance of the current blue OLEDs is still
much beyond than those of the green and red ones. With the
fact that blue is one of the primary colors, it is essential for the
fabrication of full-color displays. In addition, blue-emitting
materials may act as host materials for green and red dopants.
Recently, we and other groups observed a novel phenom-
enon of aggregation-induced emission (AIE) which is exactly
opposite to the ACQ effect: a series of propeller-like molecules
are practically nonluminescent when molecularly dissolved in
good solvents but become highly emissive when aggregated into
nanoparticles in poor solvents or fabricated as thin lms in the
solid state.3,4 The AIE effect enables the molecules to nd high-
tech applications as chemical sensors, biological probes,
stimuli-responsive nanomaterials and active layers for the
fabrication of OLEDs.5 Among the AIE luminogens, tetraphe-
nylethene (TPE) possesses a simple molecular structure with a
splendid AIE effect (Chart 1). However, its EL performance is
Since Tang demonstrated the use of organic light emitting
diodes (OLEDs) with multilayer structures to improve the device
performance, a large number and a wide variety of organic
uorophores have been prepared.1 In OLEDs, the emitters are
fabricated as thin lms and their device performance depends
somewhat on their solid-state uorescence quantum yields
(FF,F). However, many organic uorophores are non-
luminescent or weakly emissive in the solid state, although
aDepartment of Chemistry, Institute for Advanced Study, Institute of Molecular
Functional Materials, Division of Life Science, Division of Biomedical Engineering
and State Key Laboratory of Molecular Neuroscience, The Hong Kong University of
Science & Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong, China.
E-mail: tangbenz@ust.hk
bState Key Laboratory of Luminescent Materials and Devices, South China University of
Technology (SCUT), Guangzhou 510640, China
cCentre for Display Research, The Hong Kong University of Science & Technology, Clear
Water Bay, Kowloon, Hong Kong, China
dState Key Laboratory of Supermolecular Structure and Materials, Jilin University,
Changchun 150012, China
eGuangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory,
South China University of Technology (SCUT), Guangzhou 510640, China
fHKUST-Shenzhen Research Institute, No. 9 Yuexing 1st RD, South Area, Hi-tech Park,
Nanshan, Shenzhen 518057, China
† Electronic supplementary information (ESI) available: Crystal data of 1–3 and
supplementary gures. CCDC 970451–970453. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c4tc00097h
4320 | J. Mater. Chem. C, 2014, 2, 4320–4327
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luminogens and elaborated a multistep reaction route for their
syntheses (Scheme 1). Compound 5 was rst prepared by reac-
tion of carbazole (4) with benzyl bromide in the presence of
tetrabutylammonium bromide under basic conditions. Lith-
iation of 5 followed by reaction with benzophenone and acid-
catalyzed dehydration afforded 1. On the other hand, the
Friedel–Cras acylation of benzoyl chloride (7) with 9-hexyl-
carbazole and TPA furnished 8a and 8b, which were converted
into 2 and 3, respectively, by their reaction with diphenylme-
thyllithium followed by acid-catalyzed dehydration. All the
intermediates and nal products were carefully puried and
fully characterized by NMR and mass spectroscopy, from which
satisfactory analysis data corresponding to their expected
molecular structures were obtained. Single crystals of 1–3 were
Chart 1 Molecular structures of tetraphenylethene and its derivatives
containing carbazole and triphenylamine units.
quite poor with maximum luminance (Lmax), current efficiency grown from their dichloromethane–methanol solutions and
(hC,max) and external quantum efficiency (hext,max) of 1800 cd characterized crystallographically. Their ORTPE drawings are
mꢁ2, 0.45 cd Aꢁ1 and 0.4%, respectively.6 Carbazole and tri- shown in Fig. 1 and the associated crystal data are summarized
phenylamine (TPA) are well-known hole-transporting materials in Table S1 in the ESI.† All the luminogens are soluble in
but suffer from the ACQ effect in the condensed state. Incor- common organic solvents, such as tetrahydrofuran, toluene,
poration of TPE into their structures has solved their ACQ dichloromethane and chloroform, but insoluble in water.
problem and meanwhile generates efficient solid-state emitters
such as TPECa and TPATPE with unity FF,F values. OLEDs
fabricated using TPECa and TPATPE show good device perfor-
mance. For example, the EL device of TPECa with a congura-
tion of ITO/NPB/TPECa/TPBi/Alq3/LiF/Al exhibits a Lmax of 7508
cd mꢁ2 and hext,max of 1.8%.7 The performance of the EL device
based on TPATPE is more impressive, exhibiting Lmax and
hext,max of 26 090 cd mꢁ2 and 3.3%, respectively.8 Both TPECa
and TPATPE can play dual roles as light-emitting and hole-
transporting layers in OLEDs. This simplies the device struc-
ture and helps to reduce the fabrication cost.
Mechanochromic luminescent materials, which show emis-
sion color or intensity change induced by mechanical stress,
have aroused increasing interest in recent years due to their
potential applications in many elds such as sensors, displays
and memory devices.9 Taking advantage of their high FF,F
values, AIE-active molecules are thus considered as promising
candidates for such applications. A variety of AIE luminogens
with various functionalities have been prepared. However, AIE
molecules with mechanochromic uorescence properties are
seldom reported. With such regard, we designed and synthe-
sized new luminogens from triphenylethene, carbazole and TPA
building blocks (1–3; Chart 1). Unlike TPECa and TPATPE, the
carbazole and TPA units serve as skeletons, rather than
substituents, for the construction of 1–3. Luminogens 1–3
exhibit not only AIE characteristics but also intriguing mecha-
nochromism due to the morphological change from the crys-
talline to the amorphous state and vice versa. Multilayer OLEDs
utilizing the new luminogens are fabricated, which show blue
light with high luminance and current efficiency up to 11 700 cd
mꢁ2 and 7.5 cd Aꢁ1, respectively.
Optical properties
The dilute THF solution (10 mM) of 1 exhibits a peak maximum
at 341 nm (Fig. 2). The spectra of 2 and 3, however, are located at
longer wavelength, revealing that they possess a better conju-
gation. Like other AIE luminogens prepared previously, the
photoluminescence (PL) spectra of 1–3 in dilute THF solutions
(10 mM) exhibit only noisy PL signals with no discernible peak
maxima (Fig. 3 and S1 and S2, ESI†). The solution-state uo-
rescence quantum yields (FF,S) of 1 and 2 estimated using 9,10-
diphenylanthracene as standard are merely 0.70% and 0.27%,
respectively, indicating that they are weak emitters when
molecularly dissolved in good solvents. These values are much
lower than those of carbazole (37%)10a and TPA (13.0),10b
Scheme
1 Synthesis routes to carbazole and triphenylamine-
substituted ethenes.
Results and discussion
Synthesis
To enrich the AIE research and widen its practical applications,
we designed the molecular structures of three new AIE Fig. 1 ORTEP drawings of 1–3 (CCDC 970451ꢁ970453).
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Table 1 Absorption and emission of 1–3 in the solution (soln)a,
aggregated (aggr)b, crystalline (cryst) and amorphous (amor)c states
labd (nm)
leme (nm)
Soln (FF,S
Luminogen
Soln
)
Aggr
Cryst
Amor (FF,F)
1
2
3
341
342
350
(0.70)
(0.27)
(1.22)
466
489
499
455
454
429
465 (55.7)
490 (33.1)
500 (97.6)
a
b
In dilute THF solution (10 mM). In the THF–H2O mixture (1 : 9 by
c
d
e
volume). In a solid thin lm. Absorption maximum. Emission
maximum with the quantum yield (%) given in the parentheses. FF,S
¼ uorescence quantum yield in THF solution determined using 9,10-
diphenylanthracene (FF ¼ 90% in cyclohexane) as standard. FF,F
¼
uorescence quantum yield for the thin lm measured by a calibrated
Fig. 2 UV spectra of 1–3 in THF solutions. Solution concentration:
integrating sphere.
10ꢁ5 M.
To gain deep insight into the optical behaviors of the lumi-
nogens, we performed theoretical calculations on their energy
levels. Their highest occupied (HOMO) and lowest unoccupied
molecular orbitals (LUMO) plots are given in Fig. 4. The tri-
phenylvinyl units are twisted from the plane of the carbazole or
TPA unit in all the molecules, which prevents emission
quenching caused by the unfavourable p–p stacking interac-
tion. Whereas the HOMO of 1 and 2 are dominated by the
orbitals from the carbazole ring and part of the triphenylvinyl
unit, the orbitals of their LUMO are located mainly on the latter
one. On the other hand, the electron cloud of the HOMO of 3 is
located mainly on the TPA unit. However, it shows almost no
contribution to the LUMO energy level. The calculated band
gaps for 1–3 are 3.80, 3.85 and 3.61 eV, respectively.
Fig. 3 (A) PL spectra of 1 in THF and THF–H2O mixtures with different
water fractions. Concentration: 10ꢁ5 M; excitation wavelength: 340
nm. (B) Plot of I/I0 values versus compositions of the THF–H2O
mixtures of 1.
Interestingly, the crystals of 1–3 emit at 429–455 nm, which
are 10–71 nm blue-shied from those of the amorphous lms.
suggesting that the triphenylvinyl unit in 1–3 works as a PL The unusual blue shi observed in the crystalline phase may be
quencher in the solution state.
attributable to the conformation twisting of the aromatic rings
Whereas luminogens 1–3 are almost non-uorescent in
solutions, they emit intensely in the aggregated state. As shown
in Fig. 3, the PL intensity of 1 remains low in aqueous mixtures
with less than 80% water content but starts to increase swily
aerwards. The emission intensity reaches its maximum at 95%
water fraction. From the pure THF solution to the THF–H2O
mixture with 90% water content, the PL intensity rises by 350-
fold. Similar phenomena are also observed in 2 and 3 (Fig. S1
and S2†). Clearly, the emissions of 1–3 are induced by aggregate
formation, or in other words, they are AIE-active. In dilute
solution, the rotation of multiple phenyl rings has consumed
the energy of the excitons through the nonradiative relaxation
channel and thus has quenched the light emission of the dye
molecules. In the aggregate state, the intramolecular rotation
(IMR) is restricted, thus allowing the luminogens to emit
intensely. We further investigated the PL behaviors of 1–3 in the
solid state. The emissions of their thin lms are observed at
465–500 nm, which are similar to the PL of their aggregates in
THF–H2O mixtures (Table 1). The FF,F value of 3 (97.6%)
measured using a calibrated integrating sphere is much higher
than those of 1 and 2 (55.7% for 1 and 33.1% for 2), presumably
due to its higher conjugation.
Fig. 4 Molecular orbital amplitude plots of HOMO and LUMO levels of
1–3 calculated using the B3LYP/6-31G* basis set.
4322 | J. Mater. Chem. C, 2014, 2, 4320–4327
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of the luminogens in order to t into the crystalline lattices.
Without such restraint, the molecules in the amorphous state
may assume a more planar conformation and thus show redder
luminescence. To gain further insight into the AIE mechanism,
we checked the geometric structures and packing arrangements
of 1 and 3 in the crystalline state. As shown in Fig. 5, multiple
˚
C–H/p hydrogen bonds with distances of 2.620 and 3.075 A are
formed between the carbazolyl protons of one molecule of 1 and
the p cloud of the carbazole unit of the neighbouring molecule.
Similar interactions are also observed in 3. These multiple
C–H/p hydrogen bonds help rigidify the molecular confor-
mation and have locked the molecular rotation. As a result, the
excited state energy consumed by the IMR process is greatly
reduced, thus enabling 1 and 3 to emit intensely in the solid
state. The dihedral angles between the vinyl core and the
carbazole or triphenylamine unit are 43.7ꢀ, 46.1ꢀ and 53.0ꢀ for
1–3, respectively. This suggests that 3 takes a more twisted
conformation, thus leading to bluer crystal emission and the
largest hypochromic PL shi from aggregated to crystal state.
Upon gently grinding the crystals of 3 using a spatula or
pestle, their emission changes from blue to green (Fig. 6),
demonstrating a property of mechanochromic uorescence.
Since the PL of the mechanically agitated crystals is close to that
of the amorphous lm (Fig. 7A), such a phenomenon should be
associated with the morphological change from crystalline to
amorphous state. This is supported by the result from powder
X-ray diffraction analysis: whereas the diffractogram of the
crystals exhibits many sharp peaks, that of the mashed crystals
shows fewer and broader peaks with much weaker intensities,
indicating that they, to a large extent, are of amorphous nature
(Fig. S3†). Aer solvent fuming to acetone vapour at room
Fig. 6 Reversibly switching between the blue crystalline and green
amorphous states of 3 by grinding–fuming and grinding–heating
cycles. The photographs were taken under 365 nm UV irradiation from
a hand-held UV lamp.
ꢀ
temperature for 10 min or heating at 120 C in air for 10 min,
the green powders recrystallize and are restored nearly to the
blue form. The switching between the blue crystalline and green
Fig. 7 (A) Change in the PL spectrum of crystals of 3 by grinding–
fuming and grinding–heating processes. (B) Repeated switching of the
light emission of 3 by the grinding–fuming cycle.
amorphous states of 3 can be repeated for many cycles without
fatigue because this process is nondestructive in nature
(Fig. 7B). We believe that by applying a larger pressure load
using a hydraulic presser, a more drastic uorescence shi can
be observed. The change in reectivity has been commonly used
as an output in rewritable optical media. A luminescence-based
process is of advantage because it offers higher sensitivity, lower
background noise and potential for two-dimensional imaging.11
Luminogen 3 is thus a promising candidate for innovative
applications in optical information storage systems.
Thermal properties
The thermal stability of 1–3 was evaluated by thermogravimetꢁri1c
analysis (TGA) under nitrogen at a heating rate of 10 ꢀC min
.
As shown in Fig. 8, all the luminogens are thermally stable,
losing 5% of their weights at temperatures (Td) from 294 to
315 ꢀC. Their thermal transitions were investigated by
differential scanning calorimetry (DSC) at a heating rate of
Fig. 5 Perspective view of the packing arrangements in crystals of 1
and 3. The aromatic C–H/p hydrogen bonds are denoted by dotted
lines.
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Fig. 8 (A) TGA curves and (B) DSC thermograms of first (solid lines)
and second (dotted lines) heating scans of 1–3 recorded under
nitrogen at a heating rate of 10 ꢀC min.
10 C minꢁ1. All the compounds exhibited clear melting tran-
ꢀ
sitions (Tm) in the range of 174–207 ꢀC. They also possess good
morphological stability, as suggested by their reasonably high
glass-transition temperatures (Tg) of 72, 50 and 66 C, respec- and (C and D) current efficiency versus current density in the multilayer
tively. Coupled with their efficient solid-state emission, lumi-
nogens 1–3 are thus promising EL materials.
Fig. 9 Plots of (A and B) current density and luminance versus voltage
ꢀ
light-emitting diode of 1–3 with device configurations of (A and C)
ITO/NPB/LEL/TPBi/Alq3/LiF/Al and (B and D) ITO/NPB/LEL/TPBi/LiF/Al.
The inset in (C) and (D): EL spectra of 1–3.
Electroluminescence
Table 2 EL performance of 1–3a
The efficient light emission and high thermal stability of 1–3
lEL
Von
(V)
Lmax
hP,max
(lm Wꢁ1
hC,max
(cd Aꢁ1
hext,max
encourage us to investigate their electroluminescence (EL)
properties. Multilayer OLEDs with congurations of ITO/NPB
(50 nm)/LEL (20 nm)/TPBi (30 nm)/Alq3 (20 nm)/LiF (1 nm)/Al
(100 nm) (Device I) and ITO/NPB (50 nm)/LEL (20 nm)/TPBi
(30 nm)/LiF (1 nm)/Al (100 nm) (Device II) were constructed,
where luminogens 1–3 worked as the light-emitting layer (LEL),
NPB functioned as the hole-transporting material, TPBi served
as the electron-transporting and/or hole-blocking layer and Alq3
worked as the electron-transporting material, respectively. The
performance and EL data are depicted and summarized in Fig. 9
and Table 2. Devices I of 1–3 emit bright sky blue EL at 468–488
nm, which are close to the PL spectra of their amorphous thin
lms. This indicates that both EL and PL originate from the
same radiative decay of the singlet excitons. Among the lumi-
nogens, the OLED based on 3 shows the best performance. The
device is turned on at a low voltage of 3.2 V and exhibits an Lmax
Dye
(nm)
(cd mꢁ2
)
)
)
(%)
Device I: ITO/NPB/LEL/TPBi/Alq3/LiF/Al
1
2
3
468
468
488
5.6
3.4
3.2
2410
2240
7140
0.8
3.2
7.9
1.9
4.3
7.5
1.0
1.8
3.3
Device II: ITO/NPB/LEL/TPBi/LiF/Al
1
2
3
472
492
492
4.4
4.4
4.0
7160
5130
10 900
1.2
2.0
3.7
1.9
3.9
5.2
1.3
1.7
3.1
Device III: ITO/LEL/TPBi/LiF/Al
1
2
3
488
492
488
4.0
4.6
1.0
6300
4390
11 700
0.4
0.5
1.0
1.1
1.0
2.5
0.6
0.4
1.1
a
Abbreviation: LEL ¼ light-emitting layer, lEL ¼ electroluminescence
maximum, Von ¼ turn-on voltage at 1 cd mꢁ2, Lmax ¼ maximum
luminance, hP,max ¼ maximum power efficiency, hC,max ¼ maximum
current efficiency and hext,max ¼ maximum external quantum efficiency.
of 7140 cd mꢁ2. The hC,max, maximum power efficiency (hP,max
)
and hext,max attained by the device are 7.5 cd Aꢁ1, 7.9 lm Wꢁ1
and 3.3%, respectively. Compared to previous results obtained
from other AIE luminogens, these values are pretty high.
Removal of the Alq3 layer in Device I forms Device II, which serve as bifunctional materials. To test such possibility, OLEDs
results in different consequences. Whereas the EL performance with a conguration of ITO/LEL/TPBi (40 nm)/LiF (1 nm)/Al (100
of Device II of 1 is slightly better than that of Device I, those of 2 nm) are constructed, where 1–3 served as both light-emitting
and 3 become poorer though the EL devices exhibit higher Lmax and hole-transporting layers. Again, luminogen 3 shows the
values.
best EL performance. Its EL device emits at 488 nm, showing an
Materials with multi-functional properties, such as light- Lmax, hC,max, hP,max and hext,max of 11 700 cd mꢁ2, 2.4 cd Aꢁ1
,
emitting and hole- and/or electron-transporting capabilities, are 1.0 lm Wꢁ1 and 1.1%, respectively (Fig. 10). These values,
more useful for technological applications and in great demand however, are much inferior to those of Device I and II, leaving
because they can greatly simplify the device structure and much room for further improvement. Nevertheless, the EL data
reduce the fabrication cost. Carbazole and TPA are well-known given in Table 2 demonstrate the great potential of the present
for their light-emitting and hole-transporting properties. Since luminogens as solid light-emitters for the construction of effi-
luminogens 1–3 are constructed from these moieties, they may cient EL devices.
4324 | J. Mater. Chem. C, 2014, 2, 4320–4327
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nitrogen at a heating rate of 10 C minꢁ1. Single crystal X-ray
ꢀ
diffraction intensity data were collected at 100 K on a Bruker–
Nonices Smart Apex CCD diffractometer with graphite mono-
chromated Mo Ka radiation. Processing of the intensity data
was carried out using the SAINT and SADABS routines and the
structure and renement were conducted using the SHELTL
suite of X-ray programs (version 6.10). The ground-state geom-
etries were optimized using the density functional (DFT) with
B3LYP hybrid functional at the basis set level of 6-31G*. All the
calculations were performed using the Gaussian 03 package.
Fig. 10 Plots of (A) current density and luminance versus voltage and
(B) current efficiency versus current density in the multilayer light-
emitting diode of 1–3 with a device configuration of ITO/LEL/TPBi/LiF/
Al. The inset in (B): EL spectra of 1–3.
Device fabrication
The devices were fabricated on 80 nm-ITO coated glasses with a
sheet resistance of 25 U ,ꢁ1. Prior to loading into the
pretreatment chamber, the ITO-coated glasses were soaked in
an ultrasonic detergent for 30 min, followed by spraying with
de-ionized water for 10 min, soaking in ultrasonic de-ionized
water for 30 min and oven-baking for 1 h. The cleaned samples
were treated with peruoromethane plasma with a power of
100 W, gas ow of 50 sccm and pressure of 0.2 Torr for 10 s in
the pretreatment chamber. The samples were transferred to the
organic chamber with a base pressure of 7 ꢂ 10ꢁ7 Torr for the
Conclusions
In this work, we designed and synthesized a group of carbazole
and triphenylamine-substituted ethenes (1–3). All the lumi-
nogens are well characterized by spectroscopic techniques and
their chemical structures are further conrmed by X-ray crys-
tallography. Whereas they are nonemissive in dilute THF solu-
tions, they become strong emitters when aggregated in poor
solvents or fabricated as lms in the solid state, demonstrating
the phenomenon of AIE. These luminogens exhibit high solid-
state uorescence quantum yields of up to 97.6% and enjoy
high thermal stability (Td up to 315 ꢀC). They also show
intriguing mechanochromic uorescence properties: their
emission can be repeatedly switched between blue and green
colors by simple grinding–fuming and grinding–heating
processes due to the morphological change from crystalline to
amorphous state and vice versa. OLEDs using 1–3 as emitters are
fabricated, which show high luminance and current and
external quantum efficiencies of up to 11 700 cd mꢁ2, 7.5 cd Aꢁ1
and 3.3%, respectively.
deposition
of
N,N-bis(1-naphthyl)-N,N-diphenylbenzidine
(NPB), emitter, 2,20,200-(1,3,5-benzinetriyl)tris(1-phenyl-1-H-
benzimidazole) (TPBi) and/or tris(8-hydroxyquinolinato)
aluminium (Alq3) which served as hole-transporting, light-
emitting and hole-blocking and electron-transporting layers,
respectively. The samples were then transferred to the metal
chamber for cathode deposition which composed of lithium
uoride (LiF) capped with aluminium (Al). The light-emitting
area was 4 mm2. The current density–voltage characteristics of
the devices were measured on a HP4145B semiconductor
parameter analyzer. The forward direction photons emitted
from the devices were detected by a calibrated UDT PIN-25D
silicon photodiode. The luminance and external quantum effi-
ciencies of the devices were inferred from the photocurrent of
the photodiode. The EL spectra were obtained using a PR650
spectrophotometer. All the measurements were carried out
under air at room temperature without device encapsulation.
Experimental section
Materials and instrumentation
Dichloromethane (DCM) and tetrahydrofuran (THF) were
distilled from calcium hydride and sodium benzophenone
ketyl, respectively, under nitrogen immediately prior to use. All
the chemicals and reagents were purchased from Aldrich and
used as received without further purication. 1H and 13C NMR
spectra were recorded on a Bruker AV 300 spectrometer in
deuterated chloroform using tetramethylsilane (TMS; d ¼ 0) as
the internal reference. High resolution mass spectra (HRMS)
were recorded on a GCT premier CAB048 mass spectrometer
operating in MALDI-TOF mode. UV spectra were recorded on a
Milton Roy Spectronic 3000 Array spectrophotometer. Photo-
luminescence (PL) was recorded on a Perkin-Elmer LS 55
Preparation of nanoaggregates
Stock THF solutions of the compounds with a concentration of
10ꢁ3 M were prepared. Aliquots of the stock solutions were
transferred to 10 mL volumetric asks. Aer adding appropriate
amounts of THF, water was added dropwise under vigorous
stirring to furnish 10ꢁ5 M solutions with different water
contents (0–95 vol%). The PL measurements of the resulting
solutions were then performed immediately.
Synthesis
Synthesis of 9-benzylcarbazole (5). Into a 100 mL one-necked
spectrouorometer. Thermogravimetric analysis (TGA) was round bottom ask were added carbazole (4, 3 g, 17.9 mmol),
carried out on a TA TGA Q5000 analyzer under nitrogen at a benzyl bromide (2.56 g, 15.0 mmol), tetrabutylammonium
heating rate of 10 ꢀC minꢁ1. The thermal transitions of the bromide (0.48 g, 0.15 mmol), 50 wt% of aqueous KOH solution
luminogens were investigated by differential scanning calo- (30 mL) and benzene (30 mL). The reaction mixture was heated
rimetry (DSC) using a TA DSC Q1000 analyzer under dry to reux overnight. Aer being cooled to room temperature, the
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reaction mixture was extracted with DCM. The organic layer was product was puried by silica-gel column chromatography
washed with water and dried over magnesium sulfate. Aer using hexane–DCM (3 : 2 v/v) as eluent. White solid; yield 84.2%
ltration and solvent evaporation, the crude product was puri- (2.38 g). 1H NMR (300 MHz, CDCl3), d (ppm): 8.60 (s, 1H), 8.10
ed by silica-gel column chromatography using hexane as (d, 1H), 8.04 (d, 1H), 7.84 (d, 2H), 7.60–7.43 (m, 6H), 7.26 (t, 1H),
eluent. A white solid of 5 was obtained in 88.1% yield (3.39 g). 4.33 (t, 2H), 1.94–1.84 (m, 2H), 1.42–1.23 (m, 6H), 0.87 (t, 3H).
1H NMR (300 MHz, CDCl3), d (ppm): 8.13 (d, 2H), 7.45–7.37 (m, 13C NMR (75 MHz, CDCl3), d (ppm): 197.30, 143.70, 141.78,
4H), 7.35–7.23 (m, 5H), 7.14 (d, 2H), 5.51 (s, 2H); 13C NMR (75 139.71, 132.31, 130.57, 129.09, 128.84, 127.08, 124.72, 123.77,
MHz, CDCl3), d (ppm): 141.33, 137.85, 129.44, 128.12, 127.08, 123.08, 121.38, 120.57, 109.89, 108.94, 44.03, 32.20, 29.59,
126.51, 123.69, 121.06, 119.87, 109.56, 47.23; HRMS (MALDI- 27.60, 23.20, 14.68. HRMS (MALDI-TOF): m/z 355.1933 [M+,
TOF): m/z 257.1220 [M+, calcd 257.1204].
calcd 355.1936].
1-(9-Carbazolyl)-1,2,2-triphenylethene (1). To a solution of 5
[4-(Diphenylamino)phenyl](phenyl)methanone
(8b).
(1.2 g, 4.66 mmol) in dry THF (40 mL) was added 4.44 mmol of Compound 8b was prepared from triphenylamine (2 g, 8.15
n-butyllithium solution (2.5 M in hexane) at 0 ꢀC under mmol), benzoyl chloride (1.15 g, 8.15 mmol) and aluminium
nitrogen. The resulting solution containing 6 was stirred at 0 ꢀC chloride (1.14 g, 8.55 mmol) in DCM (40 mL). The procedures
and 4.44 mmol of benzophenone was then added. The reaction were similar to those for the synthesis of 8a. Green solid; yield
mixture was allowed to warm to room temperature and stirred 80.1% (2.28 g). 1H NMR (300 MHz, CDCl3), d (ppm): 7.77 (d, 2H),
for another 6 h. The reaction was quenched by adding an 7.71 (d, 2H), 7.55 (t, 1H), 7.46 (t, 2H), 7.33 (t, 4H), 7.19–7.12 (m,
aqueous solution of ammonium chloride. The organic layer was 6H), 7.03 (t, 2H). 13C NMR (75 MHz, CDCl3), d (ppm): 195.91,
extracted with DCM three times. The resulting organic layers 152.60, 147.17, 139.14, 132.65, 132.38, 130.33, 130.29, 130.20,
were combined, washed with water and dried over anhydrous 128.80, 126.67, 125.32, 120.20. HRMS (MALDI-TOF): m/z
magnesium sulfate. Aer solvent evaporation, the crude alcohol 349.1465 [M+, calcd 349.1467].
(containing excess diphenylmethane) was dissolved in about 50
1-(9-Hexyl-3-carbazolyl)-1,2,2-triphenylethene (2). To a solu-
mL of toluene in a 100 mL two-necked round bottom ask tion of diphenylmethane (0.50 g, 2.95 mmol) in dry THF (30 mL)
equipped with a condenser. A catalytic amount of p-toluene- was added 2.81 mmol of n-butyllithium solution (2.5 M in
sulphonic acid (TsOH) was then added and the mixture was hexane) at 0 ꢀC under nitrogen. The resulting solution con-
heated to reux. Aer being cooled to room temperature, the taining diphenylmethyllithium was stirred at 0 ꢀC and 2.81
organic layer was washed with 25 mL of 10% aqueous sodium mmol of 8a was then added. The procedures were similar to
bicarbonate solution twice and dried over anhydrous magne- those for the synthesis of 1. White solid; yield 42.9% (0.61 g). 1H
sium sulfate. Aer solvent evaporation, the crude product was NMR (300 MHz, CDCl3), d (ppm): 7.87 (d, 1H), 7.75 (s, 1H), 7.40
puried by silica gel column chromatography using hexane– (d, 1H), 7.35 (d, 1H), 7.17–7.02 (m, 18H), 4.21 (t, 2H), 1.85–1.82
1
DCM (4 : 1 v/v) as eluent. White solid; yield 40.0% (0.75 g). H (m, 2H), 1.38–1.29 (m, 6H), 0.88 (t, 3H). 13C NMR (75 MHz,
NMR (300 MHz, CDCl3), d (ppm): 7.98 (d, 2H), 7.27–7.22 (m, CDCl3), d (ppm): 145.20, 144.98, 142.39, 141.28, 140.62, 139.91,
10H), 7.17–7.03 (m, 3H), 7.02–7.01 (m, 3H), 6.91–6.86 (m, 5H). 135.12, 132.27, 132.17, 132.16, 130.28, 129.19, 128.81, 128.29,
13C NMR (75 MHz, CDCl3), d (ppm): 142.27, 142.23, 142.12, 128.23, 126.96, 126.81, 126.74, 126.52, 126.06, 124.01, 123.56,
141.01, 138.30, 132.66, 132.20, 130.54, 129.33, 128.91, 128.76, 122.97, 120.98, 119.31, 109.30, 108.37, 43.79, 32.22, 29.60,
128.56, 128.37, 128.17, 128.12, 126.29, 124.06, 120.63, 120.29, 27.65, 23.24, 14.70. HRMS (MALDI-TOF): m/z 505.3439 [M+,
111.52. HRMS (MALDI-TOF): m/z 421.1860 [M+, calcd 421.1830]. calcd 505.2770].
(9-Hexyl-3-carbazolyl)(phenyl)methanone (8a). 9-Hexyl-
1-[(4-Diphenylamino)phenyl]-1,2,2-triphenylethene (3). To a
carbazole was rst prepared from carbazole (4, 5 g, 29.9 mmol), solution of diphenylmethane (0.45 g, 2.70 mmol) in dry THF
1-bromohexane (4.11 g, 24.9 mmol) and tetrabutylammonium (40 mL) was added 2.58 mmol of n-butyllithium solution (2.5 M
ꢀ
bromide (0.80 g, 2.5 mmol) in 50 wt% of aqueous KOH solution in hexane) at 0 C under nitrogen. The resulting solution con-
(40 mL) and benzene (40 mL). The procedures were similar to taining diphenylmethyllithium was stirred at 0 ꢀC and 2.58
those for the synthesis of 5. Viscous colourless liquid; yield mmol of 8b was then added. The procedures were similar to
90.0% (5.66 g). 1H NMR (300 MHz, CDCl3), d (ppm): 8.10 (d, 2H), those for the synthesis of 1. White solid; yield 38.8% (0.50 g). 1H
7.46–7.38 (m, 4H), 7.22 (t, 2H), 4.28 (t, 2H), 1.90–1.80 (m, 2H), NMR (300 MHz, CDCl3), d (ppm): 7.25–7.21 (m, 4H), 7.19–7.02
1.50–1.24 (m. 6H), 0.86 (t, 3H). 13C NMR (75 MHz, CDCl3), d (m, 21H), 6.85 (d, 2H), 6.79 (d, 2H). 13C NMR (75 MHz, CDCl3), d
(ppm): 141.08, 126.22, 123.46, 121.00, 119.33, 109.31, 43.74, (ppm): 148.28, 146.65, 144.73, 144.47, 144.27, 141.38, 141.16,
32.27, 29.61, 27.66, 23.23, 14.71. HRMS (MALDI-TOF): m/z 138.60, 132.84, 132.06, 129.80, 129.20, 128.82, 128.28, 127.10,
251.1683 [M+, calcd 251.1674].
127.01, 124.90, 123.47, 123.35. HRMS (MALDI-TOF): m/z
Into another 100 mL one-necked round bottom ask were 499.2428 [M+, calcd 499.2300].
added 9-hexyl-carbazole (2 g, 7.96 mmol), benzoyl chloride (7,
1.12 g, 7.96 mmol), aluminium chloride (1.11 g, 8.35 mmol) and
DCM (40 mL). The reaction mixture was stirred overnight. A
Acknowledgements
large amount of cold water was added to quench the reaction This work was partially supported by the National Basic
and the reaction mixture was then extracted with DCM. The Research Program of China (973 Program; 2013CB834701), the
organic layer was washed with water and dried over magnesium Research Grants Council of Hong Kong (HKUST2/CRF/10 and
sulfate. Aer ltration and solvent evaporation, the crude N_HKUST620/11), the Innovation and Technology Commission
4326 | J. Mater. Chem. C, 2014, 2, 4320–4327
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Paper
Journal of Materials Chemistry C
¨
(ITCPD/17-9) and the University Grants Committee of Hong
Kong (AoE/P-03/08 and T23-713/11-1). B.Z.T. thanks the support
of the Guangdong Innovative Research Team Program
(201101C0105067115).
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