Using tetraphenylethene and carbazole to create efficient luminophores with aggregation-induced emission, high thermal stability, and good hole-transporting property

Article abstract of DOI:10.1039/c2jm14914a

Tetraphenylethene (TPE) is an archetypal luminogen that exhibits a phenomenon of aggregation-induced emission (AIE), while carbazole is a conventional chromophore which shows the opposite effect of aggregation-caused quenching (ACQ) of light emission in the condensed phase. Melding the two units at the molecular level generates a group of new luminescent materials that suffer no ACQ effect but depict high solid-state fluorescence quantum yields up to unity, demonstrative of the uniqueness of the approach to solve the ACQ problem of traditional luminophores. All the TPE-carbazole adducts are thermally and morphologically stable, showing high glass-transition temperatures (up to 179 °C) and thermal-degradation temperatures (up to 554 °C). Multilayer electroluminescence devices with configurations of ITO/NPB/emitter/TPBi/Alq ₃/LiF/Al are constructed, which exhibit sky blue light in high luminance (up to 13650 cd m^-2) and high current and external quantum efficiencies (up to 3.8 cd A^-1, and 1.8%, respectively). The devices of the luminogens fabricated in the absence of NPB or hole-transporting layer show even higher efficiencies up to 6.3 cd A^-1 and 2.3%, thanks to the good hole-transporting property of the carbazole unit.

Full text of DOI:10.1039/c2jm14914a

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PAPER
Using tetraphenylethene and carbazole to create efficient luminophores
with aggregation-induced emission, high thermal stability, and good
hole-transporting property†‡
Zujin Zhao,a Carrie Y. K. Chan,b Shuming Chen,c Chunmei Deng,b Jacky W. Y. Lam,b Cathy K. W. Jim,b
Yuning Hong,b Ping Lu,d Zhengfeng Chang,a Xiaopeng Chen,e Ping Lu,e Hoi Sing Kwok,c Huayu Qiua
b
*
and Ben Zhong Tang
Received 30th September 2011, Accepted 22nd December 2011
DOI: 10.1039/c2jm14914a
Tetraphenylethene (TPE) is an archetypal luminogen that exhibits a phenomenon of aggregation-
induced emission (AIE), while carbazole is a conventional chromophore which shows the opposite
effect of aggregation-caused quenching (ACQ) of light emission in the condensed phase. Melding the
two units at the molecular level generates a group of new luminescent materials that suffer no ACQ
effect but depict high solid-state fluorescence quantum yields up to unity, demonstrative of the
uniqueness of the approach to solve the ACQ problem of traditional luminophores. All the TPE–
carbazole adducts are thermally and morphologically stable, showing high glass-transition
ꢀ
ꢀ
temperatures (up to 179 C) and thermal-degradation temperatures (up to 554 C). Multilayer
electroluminescence devices with configurations of ITO/NPB/emitter/TPBi/Alq3/LiF/Al are
constructed, which exhibit sky blue light in high luminance (up to 13 650 cd mꢁ2) and high current and
external quantum efficiencies (up to 3.8 cd Aꢁ1, and 1.8%, respectively). The devices of the luminogens
fabricated in the absence of NPB or hole-transporting layer show even higher efficiencies up to 6.3 cd
Aꢁ1 and 2.3%, thanks to the good hole-transporting property of the carbazole unit.
approaches have been proposed to hamper the formation of
Introduction
luminogenic aggregates and mitigate the ACQ effect. However,
in most cases, these processes have met with only limited success
because they are basically working against a natural process as
aggregate formation is an intrinsic process when luminogenic
molecules are located in close vicinity in the solid state.2
Great efforts have been made to develop efficient solid-state
emitters for their potential applications in the fabrication of
practically useful optoelectronic devices such as organic light-
emitting diodes (OLEDs). A thorny obstacle to the development
of OLEDs is the aggregation-caused quenching (ACQ) effect: the
efficient luminescence processes of many conventional lumino-
phores in dilute solutions are often weakened or even totally
exterminated when their molecules aggregate due to the forma-
tion of detrimental species such as excimers and exciplexes in the
condensed phase.1 Various chemical, physical, and engineering
In 2001, we observed a phenomenon of aggregation-induced
emission (AIE)3,4 that is exactly opposite to the ACQ effect.
Unlike conventional luminophores, the AIE luminogens are
almost non-fluorescent when molecularly dissolved in good
solvents but emit intensely when aggregated as nanoparticles in
poor solvents or fabricated as thin films in the solid state. Among
the AIE luminogens, tetraphenylethene (TPE) is an archetypal
luminophore with a simple molecular structure but possesses
outstanding AIE effect.5 Through a series of designed experi-
ments and theoretical calculations, the AIE effect of TPE is
identified to be caused by the restriction of intramolecular
rotation (IMR). In the solution state, the rotation of the multiple
phenyl rings of TPE has consumed the excited state energy
through the nonradiative process. In the aggregate state, the
IMR process is restricted, which blocks the nonradiative relax-
ation channel and hence makes the molecule highly emissive.6,7
Recent research shows that TPE is a very interesting and useful
molecule, and its derivatives have been extensively investigated
and explored their high-tech applications in biological8,9 and
aCollege of Material, Chemistry and Chemical Engineering, Hangzhou
Normal University, Hangzhou, 310036, China
bDepartment of Chemistry 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
cCenter for Display Research, HKUST, Kowloon, Hong Kong, China
dState Key Laboratory of Supramolecular Structure and Materials, Jilin
University, Changchun, 130012, China
eDepartment of Chemistry, Zhejiang University, China
† Dedicated to the celebration of the 70th birthday of professor Daoben
Zhu.
‡ CCDC reference number 836364. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2jm14914a
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environmental sciences.10 Recently, a new structural design
strategy to solve the ACQ problem of traditional luminophores
without causing any side effects is developed.11 By decorating the
ACQ chromophores with AIE units, new luminogens with AIE
characteristics are generated. An example of such molecules is
represented
by
9-[4-(1,2,2-triphenylvinyl)phenyl]carbazole
(TPECa, Chart 1), which is non-emissive in the solution state but
exhibits the fluorescence quantum yield of unity in the aggregate
state.6d To further verify the applicability of such strategy and
explore more efficient solid-state emitters, in this work, we
prepared counterparts of TPECa by attaching the TPE unit to
carbazole at different positions. We have chosen carbazole as the
ACQ component because of its high thermal stability and good
hole-transporting property. Thus, adducts containing such
chromophores are anticipated to show good thermal stability
and bifunctional property. In this paper, we present their
synthesis as well as their materials properties.
Results and discussion
Synthesis
We designed the molecular structures of a group of TPE–
carbazole adducts and elaborated multi-step reaction routes for
their synthesis (Scheme 1). Intermediates 1–4,12 6,10c and 711a were
prepared according to the literature methods. Suzuki coupling
reactions of 7 with 1, 4, and 2 catalyzed by Pd(PPh3)4 in basic
medium gave the target products TPECaP, BTPECaP, and
TTPECaP, respectively, in ꢂ54 to 71% yields. The procedures for
their synthesis are described in detail in the Experimental section.
The structure and purity of the products were characterized by
standard spectroscopic methods and elemental analysis with
satisfactory results. Single crystals of TPECaP were grown from
its methanol/dichloromethane mixture and analyzed by X-ray
diffraction crystallography. All the products dissolve readily in
common organic solvents, such as THF, toluene, chloroform,
and dichloromethane, but are insoluble in methanol and water.
Scheme 1 Synthetic routes to the TPE–carbazole adducts.
conjugation length. The molar absorptivities become higher with
the increase of the TPE units. The photoluminescence (PL)
spectra of the new luminogens in dilute THF solutions exhibit
only noisy signals without discernible peaks. The fluorescence
quantum yields (FF) of TPECaP, BTPECaP, and TTPECaP
estimated using 9,10-diphenylanthracene are merely 0.068, 0.263,
and 0.095% (Table 1), indicating that they are genuinely weak
emitters when molecularly dissolved in good solvents. Such
values are close to that of TPECa (0.045%) but are much lower
than those of carbazole (37%)13 and carbazole-based chromo-
phores,14 suggesting that the TPE unit in TPECaP, BTPECaP,
and TTPECaP works as a PL quencher in the solution state.11
Whereas the TPE–carbazole adducts are almost non-fluores-
cent in solutions, they become strong emitters in the aggregate
state. Fig. 1B shows their PL spectra in the solid film state.
When their thin films are photoexcited, intense PL emissions at
Optical properties
Fig. 1A shows the absorption spectra of the TPE–carbazole
adducts in THF solutions. All the luminogens exhibit absorption
maxima at ꢂ342 nm, associated with the p–p* transitions of the
molecules, indicating that they possess similar effective
Chart 1 Molecular structures of the TPE–carbazole adducts.
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increase afterwards. The emission intensity reaches its maximum
at a fw value of 99.5%. Similar phenomena are also observed in
TPECaP and BTPECaP (Fig. 2B). Clearly, the emissions of the
TPE–carbazole adducts are induced by aggregate formation,
thus truly verifying their AIE characteristics. In dilute solution,
the rotation of the multiple phenyl peripheries against the olefin
double bond of the TPE unit of the molecules has quenched the
light emission of the carbazole core. In the aggregate state, the
IMR process is restricted, thus allowing the molecules to emit
intensely. Clearly, the attachment of AIE unit to ACQ dye has
created a series of new AIE luminogens with efficient PL in the
solid state.
Fig. 1 (A) Absorption and (B) PL spectra of the TPE–carbazole adducts
(A) in THF solutions (10 mM) and (B) in thin films.
Crystal structure
Table 1 Optical and thermal properties of the TPE–carbazole adductsa
In order to have a better understanding of the photophysical
properties of the present luminogens, we investigated the packing
arrangements of the TPE–carbazole adducts in the crystal state.
However, only crystals of TPECaP can be grown from its
methanol/dichloromethane mixture. Fig. 3 shows the ORTEP
drawing of TPECaP and its representative repeatable units. The
phenyl plane next to the carbazole ring is highly twisted. The
TPECaP molecules are stacked in a head to head fashion, where
the carbazole and TPE units pile up separatively at different
sides. Such kind of stacking pattern is somewhat similar to that
of H-aggregate4a,15 but strictly it is not the case as the two adja-
cent carbazole units lay in an almost mutually perpendicular
labs/nm
Tg/Td
lem/nm
FF/%
Soln
Cryst
Film
Soln
Film
ꢀC
TPECab
TPECaP
BTPECaP
TTPECaP
340
342
341
342
440
455
nd
468
476
490
491
0.045
0.068
0.263
0.095
100
100
100
100
nd/315
112/402
147/479
179/554
nd
a
Abbreviation: labs ¼ absorption maximum, lem ¼ emission maximum,
Soln ¼ solution (10 mM in THF), Cryst ¼ crystals, Film ¼ drop-casted
film, FF ¼ fluorescence quantum yield in THF solution estimated using
9,10-diphenylanthracene (FF ¼ 90% in cyclohexane) as standard, and
that in solid film measured by integrating sphere, Tg ¼ glass-transition
ꢀ
temperature, Td
¼
onset decomposition temperature, nd
¼
not
direction. C–H/p hydrogen bond with a distance of 2.989 A is
formed between the carbazole hydrogen atom in one molecule
determined. b Data taken from ref. 6d for comparison.
and the p-electron cloud of the central core of the carbazole ring
476–491 nm are observed. The associated FF values of solid
films measured by integrating sphere reach 100%, revealing that
these molecules are AIE-active.
To further confirm the AIE characteristics of the new lumi-
nogens, we added water, a non-solvent for the luminogens, into
their THF solutions and studied the PL change. The PL spectra
of TTPECaP in THF/water mixtures with 0 and 99.5% water
fractions (fw) are given in Fig. 2A as an example. The PL
intensity remains almost unchanged at low fw values but starts to
Fig. 2 (A) PL spectra of TTPECaP in THF/water mixtures with 0 and
99.5% water fractions (fw). (B) Plots of (I/I0 ꢁ 1) values versus water
fractions in THF/water mixtures of TPECaP, BTPECaP, and TTPECaP.
I0 was the PL intensity in pure THF solution. The inset in panel A: photos
of TTPECaP in THF/water mixtures (fw ¼ 0 and 99.5%) taken under the
illumination of a UV lamp.
Fig. 3 ORTEP drawing of TPECaP and molecular packing and C–H/
p hydrogen bonds formed between the adjacent molecules of TPECaP in
the crystal state. The solvent molecules are omitted for clarity.
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in the adjacent molecule. Meanwhile, multiple C–H/p
state energy, resulting in non-emissive nature of the luminogens
in the solution state. Other AIE-active TPE derivatives also show
similar electronic structures. In our previous work, we found that
when TPE units were attached as substituents to some electron-
withdrawing chromophores such as benzo-2,1,3-thiadiazole, the
generated luminogens exhibit reasonably high FF values in the
solution state.16 For example, 4,7-bis[4-(1,2,2-triphenylvinyl)
phenyl]benzo-2,1,3-thiadiazole (BTPETD) shows a FF value of
61% in dilute solution. Theoretical calculation shows that the
orbitals of the LUMO of BTPETD are densely populated in
the benzo-2,1,3-thiadiazole core. In this case, the IMR process of
the TPE unit may have thus little influence on the relaxation
process of the excited state. As a result, whether the light emis-
sion of a TPE-substituted ACQ chromophore in solution will be
quenched by the IMR process of its TPE unit should relate
closely to the electron cloud distribution in the LUMO and the
electronic structure of the ACQ component. Systematic investi-
gation on the validity of this hypothesis through theoretical
calculations and photophysical property measurements of a vast
variety of TPE derivatives is in progress in our labs.
ꢀ
hydrogen bonds with distances of 2.786 and 2.677 A are also
formed between the TPE units of adjacent TPECaP molecules.
All these collective effects help rigidify the molecular confor-
mation and have locked the molecular rotation. The excited state
energy consumed by the IMR process is thus greatly reduced,
enabling TPECaP to emit intensely in the solid state. The crystal
of TPECaP emits a strong blue light at 453 nm, which is 21 nm
blue-shifted from that of its thin film. Similar phenomena have
been observed in TPECa and other TPE derivatives5,6d,11a and is
presumably caused by conformational adjustment of their
molecules by twisting their aromatic rings to fit into the crys-
talline lattices. Since there is no such constraint in the amorphous
films, the TPECaP molecules may have adopted a more planar
conformation and arranged randomly, thus leading to red-shift
in the PL spectrum.
Theoretical calculation
To have a deep insight on the optical behaviors of the TPE–
carbazole adducts, the geometrical and electronic properties of
TPECaP, BTPECaP, and TTPECaP are studied by density
functional theory using B3LYP hybrid functional theory with
a 6-31G(d) basis set. Their optimized structures and HOMO and
LUMO orbital distributions are given in Fig. 4. The TPE units
are twisted from the plane of the carbazole core in all the
molecules, which prevents emission quenching caused by p–p
stacking interaction. Whereas the HOMOs of all the luminogens
are dominated by the orbitals from the carbazole core and part of
the TPE unit, the orbitals of the LUMOs are located mainly on
the TPE unit. The carbazole cores in TPECaP and TTPECaP
show almost no contribution to the LUMOs. We infer that the
IMR process of the TPE unit can effectively consume the excited
Thermal stability
To investigate the thermal properties of the TPE–carbazole
adducts, differential scanning calorimetry (DSC) measurement
and thermogravimetric analysis (TGA) were carried out. As
shown in Fig. 5, TPECaP, BTPECaP, and TTPECaP are ther-
mally stable and show high onset degradation temperatures (Td)
ꢀ
ranging from 402 to 554 C (Table 1). They also possess good
morphological stability, as suggested by their high glass-transi-
tion temperatures (Tg) of 112, 147, and 179 ꢀC, respectively. The
Tg and Td values well correlate with the number of TPE units in
the molecular structure, which is understandable as the TPE unit
Fig. 4 Optimized molecular structures and molecular orbital amplitude plots of HOMOs and LUMOs of the TPE–carbazole adducts calculated using
the B3LYP/6-31G(d) basis set.
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however, exhibits bathochromically shifted EL compared with
PL of its film, probably due to the microcavity effect. OLEDs
based on these luminogens show good performances. The device
fabricated from TPECaP is turned on at a low voltage of 3.8 V
and exhibits a maximum luminance (Lmax) of 11 060 cd mꢁ2
(Fig. 6). The maximum current (hC,max), power (hP,max), and
external quantum efficiencies (hext,max) attained by the device
are 3.5 cd Aꢁ1, 2.9 lm Wꢁ1, and 1.7%, respectively. Devices
based on TPECa and TTPECaP exhibit slightly better perfor-
mances with higher Lmax, hC,max, and hext,max values up to
13 650 cd mꢁ2, 3.8 cd Aꢁ1, and 1.8%, respectively. These values
are much better than those of the 3TPATPE-based device11b and
also superior to those of devices based on luminogens con-
structed from carbazole core and conventional planar ACQ
chromophores,12b,18 thanks to the AIE feature of the TPE unit.
The EL results of TPE–carbazole adducts, although prelimi-
nary, suggest that these AIE luminogens are promising candi-
dates for OLEDs.
Fig. 5 (A) TGA thermograms and (B) DSC curves (second heating scan)
of the TPE–carbazole adducts recorded under nitrogen at a heating rate
of 10 ꢀC minꢁ1
.
is mainly constructed from aromatic rings. Generally, lumino-
genic materials constructed from carbazole show higher Tgs and
Tds than those fabricated from triphenylamine.17 For example,
the Tg and Td of 3TPETPA, an AIE luminogen with triphenyl-
amine core decorated with three TPE units, are 143 and 359 ꢀC,
respectively,11bꢀ which are much lower than those of TTPECaP
(179 and 554 C, respectively). The good thermal and morpho-
logical stabilities of these luminogens are advantageous for the
construction of stable and durable optoelectronic devices.
Materials that possess multiple functional properties, such as
light-emitting and hole- and/or electron-transporting capabil-
ities, are more useful for high-tech applications and in great
demand because they can simplify the device structure and
facilitate fabrication procedures. Carbazole is a representative
building block that can be used to construct materials with both
light-emitting19 and hole-transporting properties.20 As a result,
the electron-donating ability of the carbazole core may endow
the present luminogens with good hole-transporting property.
To explore the application of these AIE luminogens as bifunc-
tional materials, OLED of TPECaP with a configuration of ITO/
TPECaP (80 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) (device
II) was constructed, where TPECaP served as both light-emitting
and hole-transporting layers. Device II based on TPECaP emits
at 496 nm (Fig. 6A), which is close to the EL peak in device I.
This indicates that the EL originates from the TPECaP layer.
However, device II exhibits much better performances. It emits
more intensely with luminance up to 12 930 cd mꢁ2, exhibiting
higher hC,max, hP,max, and hext,max values of 5.5 cd Aꢁ1, 3.8 lm
Wꢁ1, and 2.2%, respectively. Device III of TTPECaP fabricated
in the absence of NPB or hole-transporting layer also shows
better performances. It is turned on at a low voltage of 3.6 V,
exhibiting a high hP,max of 4.1 lm Wꢁ1, a hC,max of 6.3 cd Aꢁ1 and
a hext,max of 2.3%. Evidently, the carbazole core retains its good
hole-transporting property after being substituted with TPE,
enabling TPECaP and TTPECaP to perform well as bifunctional
materials in OLEDs.
Electroluminescence
While we have studied the AIE property of TPECa in our
previous paper, its application in OLEDs has yet to be explored.
We thus investigated the electroluminescence (EL) of TPECa as
well as the new luminogens by fabrication of multilayer EL
devices with a configuration of ITO/NPB (60 nm)/emitter (20
nm)/TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm) (device
I) by vapor deposition processes, in which N,N-bis(1-naphthyl)-
N,N-diphenylbenzidine (NPB) functioned as a hole-transporting
layer, TPE–carbazole adducts served as emitting layers, 2,20,200-
(1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimidazole)
(TPBi)
worked as hole-blocking layer, and tris(8-hydroxyquinolinolato)
aluminium (Alq3) functioned as electron-transporting layer. The
EL device of BTPECaP is not available due to its poor film-
forming property. Table 2 summarizes the EL performances of
the devices. TPECaP and TTPECaP show similar EL spectra
with peak maxima at ꢂ488 nm. The EL spectra resemble the PL
spectra of their films, indicating that both EL and PL originate
from the same radiative decay of the singlet excitons. TPECa,
Table 2 EL properties of the TPE–carbazole adductsa
Active layer
lEL/nm
Von/V
Lmax/cd mꢁ2
hP,max/lm Wꢁ1
hC,max/cd Aꢁ1
hext,max/%
TPECaI
484
488
488
496
490
4.0
3.8
4.0
4.4
3.6
7508
11 060
13 650
12 930
9048
2.7
2.9
2.1
3.8
4.1
3.8
3.5
3.8
5.5
6.3
1.8
1.7
1.8
2.2
2.3
TPECaPI
TTPECaPI
TPECaPII
TTPECaPIII
a
Device configuration: IITO/NPB (60 nm)/emitter (20 nm)/TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm), IIITO/TPECaP (80 nm)/TPBi (40 nm)/
LiF (1 nm)/Al (100 nm), and IIIITO/TTPECaP (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) Abbreviations: lEL ¼ EL 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.
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Fig. 6 (A) EL spectra, (B) change in luminance and current density with the applied bias, and (C) plots of current efficiency versus current density in
multilayer EL devices of TPECaP with a configuration of ITO/NPB/TPECaP/TPBi/Alq3/LiF/Al (device I) and ITO/TPECaP/TPBi/LiF/Al (device II).
of TPE unit to the carbazole core has quenched its light emission
in solution due to the active IMR process but endowed the
resultant molecules with salient AIE characteristics. Preliminary
results from theoretical calculation imply that the IMR process
of the TPE units works more effectively to consume excited state
energy when the orbitals of the LUMOs of the molecules are
distributed mainly on the TPE units. Multiple C–H/p hydrogen
bonds formed between the TPE and carbazole units in the solid
state help to restrict the IMR process efficiently and boost the FF
values of the luminogens to unity. The TPE–carbazole adducts
show high thermal and morphological stabilities, whose Tgs and
Tds are comparable or higher than those of adducts constructed
from TPE and triphenylamine. Efficient OLEDs using these AIE
Fig. 7 Cyclic voltammogram of TPECaP measured in dichloromethane
containing 0.1 M tetra-n-butylammonium hexafluorophosphate. Scan
luminogens as active layers are fabricated, which show good
device performances. The carbazole unit shows merits in
improving the thermal stability of the materials and their device
performances, making the present luminogens function as both
light-emitting and hole-transporting materials. The results
obtained here manifest that the ACQ problem of conventional
chromophores can be solved without causing an adverse effect by
introduction of the AIE unit into their molecular structures. The
present work not only gives further insight into the mechanism of
the AIE phenomenon but also demonstrates the applicability of
the new approach for creating efficient materials for optics and
electronics.
rate: 100 mV sꢁ1
.
Fig. 7 shows the cyclic voltammogram of TPECaP. The
HOMO energy level of TPECaP can be estimated from the onset
oxidation potential and is equal to ꢁ5.36 eV. The LUMO energy
level can be obtained by subtraction of the optical band gap
(obtained from the onset absorption wavelength) from the
HOMO energy level and is calculated to be ꢁ2.18 eV.21 The
HOMO energy level of TPECaP is slightly higher than that of
NPB (ꢁ5.4 eV),22 indicating that TPECaP is in favor of hole
injection and transportation. The use of NPB as an additional
hole-transporting layer is thus not necessary in the device fabri-
cation and could even be harmful because it may break the
charge balance in the EL device. These data attest that the
present luminogens are excellent hole-transporting materials, in
addition to being highly emissive in the solid state. This helps to
simplify the device structure, shorten the fabrication process, and
lower production costs.
Experimental
General information
THF was distilled from sodium benzophenone ketyl under dry
nitrogen immediately prior to use. All other chemicals and
reagents were purchased from Aldrich and used as received
without further purification. 1H and 13C NMR spectra were
measured on a Bruker AV 300 spectrometer in deuterated chlo-
roform using tetramethylsilane (TMS; d ¼ 0) as internal refer-
ence. UV spectra were measured on a Milton Roy Spectronic
3000 Array spectrophotometer. PL spectra were recorded on
a Perkin-Elmer LS 55 spectrofluorometer. Elemental analysis
was performed on an Elementary Vario EL analyzer. The high
resolution mass spectra (HRMS) were recorded on a GCT
premier CAB048 mass spectrometer operated in MALDI-TOF
Conclusions
In this work, new AIE luminogens are constructed from TPE and
carbazole and their electronic, thermal, and optical properties
are investigated. All the molecules are almost non-fluorescent in
solutions but become strong emitters when aggregated in poor
solvents or fabricated as thin films in the solid state. Attachment
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mode. Thermogravimetric analysis (TGA) was carried on a TA
TGA Q5000 instrument under dry nitrogen at a heating rate of
independent reflections (Rint ¼ 0.0405), GOF on F2 ¼ 1.045, R1 ¼
ꢁ3
ꢀ
0.0886, wR2 ¼ 0.1743 (all data), De 0.740 and ꢁ0.348 e A
.
10 C minꢁ1. Thermal transitions were investigated by differen-
ꢀ
tial scanning calorimetry (DSC) using a TA DSC Q1000
ꢀ
Synthesis
instrument under dry nitrogen at a heating rate of 10 C minꢁ1
.
9-[40-(1,2,2-Triphenylvinyl)biphenyl-4-yl]-9H-carbazole (TPE-
CaP). A mixture of 1 (0.32 g, 1 mmol), 7 (0.41 g, 1.1 mmol),
Pd(PPh3)4 (0.058 g, 0.05 mmol), and potassium carbonate (0.55 g,
4.0 mmol) in 100 mL of toluene/ethanol/water (8/1/1 v/v/v) was
heated to reflux for 24 h under nitrogen. The mixture was then
poured into water and extracted with dichloromethane. The
organic layer was washed with water and dried over magnesium
sulfate. After filtration and solvent evaporation, the residue was
purified by silica-gel column chromatography using hexane/
dichloromethane mixture as eluent. White solid; yield 71% (0.4 g).
1H NMR (300 MHz, CDCl3), d (TMS, ppm): 8.14 (d, 2H, J ¼ 8.1
Hz), 7.79 (d, 2H, J ¼ 9.0 Hz), 7.60 (d, 2H, J ¼ 8.4 Hz), 7.48–7.38
(m, 6H), 7.30–7.25 (m, 2H), 7.16–7.05 (m, 17H). 13C NMR (75
MHz, CDCl3), d (TMS, ppm): 144.5, 144.0, 142.1, 141.6, 141.2,
140.3, 138.7, 137.5, 132.6, 132.0, 131.9, 128.9, 128.5, 128.4, 128.0,
127.3, 127.2, 126.9, 126.7, 124.0, 121.0, 120.7, 110.5. HRMS: m/z
573.2451 (M+, calcd 573.2457). Anal. Calcd for C44H31N: C,
92.11; H, 5.45; N, 2.44. Found: C, 91.85; H, 5.40; N, 2.36%.
The ground-state geometries were optimized using the density
functional theory with B3LYP hybrid functional at the basis set
level of 6-31G(d). All the calculations were performed using the
Gaussian 03 package. Cyclic voltammetry was performed at
room temperature with use of a standard three-electrode elec-
trochemical cell in dichloromethane containing tetra-n-buty-
lammonium hexafluorophosphate (0.1 M). The working and
reference electrodes were platinum and Ag/AgCl. The reference
electrode was checked versus ferrocene as internal standard as
recommended by IUPAC. All the solutions were deaerated by
bubbling nitrogen gas for a few minutes prior to the electro-
chemical measurements.
Device fabrication
The devices were fabricated on 80 nm ITO coated glass with
a sheet resistance of 25 U ,ꢁ1. Prior to loading into the
pretreatment chamber, the ITO-coated glasses were soaked in
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 by perfluoromethane plasma with a power of 100 W, gas
flow of 50 sccm, and pressure of 0.2 Torr for 10 s in the pretreat-
ment chamber. The samples were transferred to the organic
chamber with a base pressure of 7 ꢃ 10ꢁ7 Torr for the deposition
of NPB, emitter, TPBi, and Alq3. The samples were then trans-
ferred to the metal chamber for cathode deposition which
composed of lithium fluoride (LiF) capped with aluminium (Al).
The light-emitting area was 4 mm2. The current density–voltage
characteristics of the devices were measured by 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 efficiencies of the devices were inferred from the
photocurrent of the photodiode. The EL spectra were obtained by
a PR650 spectrophotometer. All the measurements were carried
out under air at room temperature without device encapsulation.
9-Phenyl-3,6-bis[4-(1,2,2-triphenylvinyl)phenyl]-9H-carbazole
(BTPECaP). The procedure was analogous to that described for
TPECaP. Pale green solid; yield 66%. 1H NMR (300 MHz,
CDCl3), d (TMS, ppm): 8.33 (d, 2H, J ¼ 1.5 Hz), 7.64–7.59 (m,
6H), 7.50–7.47 (m, 5H), 7.43 (d, 2H, J ¼ 8.4 Hz), 7.15–7.03 (m,
34H). 13C NMR (75 MHz, CDCl3), d (TMS, ppm): 144.7, 144.6,
142.9, 141.8, 141.5, 141.5, 140.2, 138.2, 133.5, 132.5, 132.1, 132.0,
130.7, 128.5, 128.4, 128.4, 127.6, 127.2, 127.1, 126.9, 126.0, 124.7,
119.2, 110.9. HRMS: m/z 903.3847 (M+, calcd 903.3865). Anal.
Calcd for C70H49N: C, 92.99; H, 5.46; N, 1.55. Found: C, 92.52;
H, 5.39; N, 1.45%.
9-[40-(1,2,2-Triphenylvinyl)biphenyl-4-yl]-3,6-bis[4-(1,2,2-tri-
phenylvinyl)phenyl]-9H-carbazole (TTPECaP). The procedure
was analogous to that described for TPECaP. Pale green solid;
yield 54%. 1H NMR (300 MHz, CDCl3), d (TMS, ppm): 8.33 (d,
2H, J ¼ 1.5 Hz), 7.79 (d, 2H, J ¼ 8.1 Hz), 7.64–7.60 (m, 4H),
7.49–7.44 (m, 8H), 7.17–7.05 (m, 51H). 13C NMR (75 MHz,
CDCl3), d (TMS, ppm): 144.6, 144.5, 144.4, 144.3, 143.9, 142.7,
142.0, 141.6, 141.3, 141.1, 140.5, 140.2, 139.1, 138.5, 137.3, 133.8,
132.7, 132.6, 132.2, 132.1, 128.9, 128.5, 128.4, 127.7, 127.2, 127.1,
127.0, 126.9, 126.1, 124.7, 119.3, 110.8. HRMS: m/z 1234.3140
(M+, calcd 1234.5307). Anal. Calcd for C96H67N: C, 93.40; H,
5.47; N, 1.13. Found: C, 93.42; H, 5.31; N, 1.05%.
Preparation of nanoaggregates
Stock THF solutions of the compounds with a concentration of
10ꢁ4 M were prepared. Aliquots of the stock solution were
transferred to 10 mL volumetric flasks. After appropriate
amounts of THF were added, water was added dropwise under
vigorous stirring to furnish 10ꢁ5 M solutions with different water
contents (0–99.5 vol%). The PL measurements of the resultant
solutions were then performed immediately.
Acknowledgements
We thank the financial support from the Research Grants
Council of Hong Kong (603509, HKUST13/CRF/08, and
HKUST2/CRF/10), the Innovation and Technology Commis-
sion (ITP/008/09NP), the University Grants Committee of Hong
Kong (AoE/P-03/08), and the National Natural Science Foun-
dation of China (21074028 and 21104012), the Natural Science
Foundation of Zhejiang Province (Y4110331) and the Initial
Funding of Hangzhou Normal University (HSQK0085).
X-Ray crystallography
Crystal data for TPECaP (CCDC 836364): C44H31N$CH2Cl2,
MW ¼ 658.62, monoclinic, P2(1)/c, a ¼ 21.295(2), b ¼
3
ꢀ
ꢀ
16.3989(19), c ¼ 9.7404(11) A, b ¼ 98.263(2), V ¼ 3366.2(7) A ,
Z ¼ 4, Dc ¼ 1.300 g cmꢁ3, m ¼ 0.227 mmꢁ1 (MoKa, l ¼ 0.71073),
F(000) ¼ 1376, T ¼ 173(2) K, 18 495 measured reflections, 5854
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 4527–4534 | 4533

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C. Y. K. Chan, C. K. W. Jim, Z. Wang, C. Wang, P. Lu,
H. S. Kwok, Y. Ma and B. Z. Tang, Pure Appl. Chem., 2010, 82,
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4534 | J. Mater. Chem., 2012, 22, 4527–4534
This journal is ª The Royal Society of Chemistry 2012