Systemic studies of tetraphenylethene-triphenylamine oligomers and a polymer: Achieving both efficient solid-state emissions and hole-transporting capability

Article abstract of DOI:10.1002/chem.201201400

By employing a new synthetic strategy, a series of oligomers and a polymer composed of different number of tetraphenylethene and triphenylamine units was designed and synthesised. The optical physics properties and electroluminescence behaviours were studied comparatively. All the molecules demonstrate an aggregation-induced emission (AIE) phenomenon and bear very high quantum yields in the solid state. The emission wavelengths and quantum efficiencies alternate with the change of the molecular configurations and achieve their maximum at the largest oligomer. The thermal stabilities also become higher along with the increase in the molecular weight. The molecules have suitable HOMO levels that match the work function of the indium tin oxide (ITO) anode. They can act as both light-emitting and hole-transporting materials in OLEDs. Thus the present strategy combines the intrinsic emissive nature of AIE materials and the good hole-transport capability of aromatic amines, thereby achieving a win-win for both optical and electrical properties. Glowing recommendation: A series of oligomers and a polymer composed of different numbers and configuration of tetraphenylethene and triphenylamine units have been synthesised. Their optical and electroluminescence behaviours were studied systematically. All the molecules demonstrate aggregation-induced emission (see figure) and have very high quantum yields in the solid state. Copyright

Full text of DOI:10.1002/chem.201201400

FULL PAPER
DOI: 10.1002/chem.201201400
Systemic Studies of TetraphenylACTHNUTRGNEeNUG thene–Triphenylamine Oligomers and a
Polymer: Achieving Both Efficient Solid-State Emissions and Hole-
Transporting Capability
Yang Liu,[a, b] Xihan Chen,[a] Yun Lv,[b] Shuming Chen,[c] Jacky W. Y. Lam,[a]
Faisal Mahtab,[a] Hoi Sing Kwok,[c] Xutang Tao,[b] and Ben Zhong Tang*[a, d]
Abstract: By employing a new synthet-
ic strategy, a series of oligomers and
wavelengths and quantum efficiencies
alternate with the change of the molec-
ular configurations and achieve their
maximum at the largest oligomer. The
thermal stabilities also become higher
along with the increase in the molecu-
lar weight. The molecules have suitable
HOMO levels that match the work
function of the indium tin oxide (ITO)
anode. They can act as both light-emit-
ting and hole-transporting materials in
OLEDs. Thus the present strategy
combines the intrinsic emissive nature
of AIE materials and the good hole-
transport capability of aromatic
amines, thereby achieving a win–win
for both optical and electrical proper-
ties.
a
polymer composed of different
number of tetraphenylethene and tri-
phenylamine units was designed and
synthesised. The optical physics proper-
ties and electroluminescence behav-
iours were studied comparatively. All
the molecules demonstrate an aggrega-
tion-induced emission (AIE) phenom-
enon and bear very high quantum
yields in the solid state. The emission
Keywords: aggregation · hole trans-
port · luminescence · oligomeriza-
tion · optical physics
Introduction
genic molecules are commonly used in the solid state in real
applications.[3] Since the abnormal phenomenon of “aggrega-
tion-induced emission” (AIE) discovered by our group,
many organic chromophores that have AIE properties have
been explored by us and other groups.[4] This kind of dyes
emits efficiently in the aggregate state and does not suffer
from the common fluorescence “aggregation-caused quench-
ing” effect. High-tech applications based on the AIE lumi-
nogens, such as fluorescence sensors (for explosivity, ions,
pH, temperature, viscosity and pressure), biological probes
and active layers in the fabrication of organic light-emitting
diodes (OLEDs) have been realised.[5] Specifically, OLEDs
that utilise AIE dyes exhibit outstanding performances.[4a,5f,g]
Tetraphenylethene (TPE), a propeller-shaped molecule, is
Organic materials with enhanced solid-state emissions have
been thoroughly established in many optoelectronic applica-
tions for years.[1] For most of the organic conjugate chromo-
phores, however, an intrinsic limitation still exists: they emit
strongly in their dilute solutions but become weak fluoro-
phores when fabricated into solid films or aggregated in
their poor solvents on account of the strong intermolecular
interactions.[2] This problem must be solved because lumino-
[a] Dr. Y. Liu, X. Chen, Dr. J. W. Y. Lam, Dr. F. Mahtab,
Prof. B. Z. Tang
Department of Chemistry
Hong Kong University of Science & Technology
Clear Water Bay, Kowloon, Hong Kong (P.R. China)
Fax : (+852)2358-1594
E-mail: tangbenz@ust.hk
[b] Dr. Y. Liu, Y. Lv, X. Tao
State Key Laboratory of Crystal Materials
Shandong University, Jinan 250100 (P.R. China)
[c] S. Chen, Prof. H. S. Kwok
Center for Display Research
Hong Kong University of Science & Technology
Clear Water Bay, Kowloon, Hong Kong (P.R. China)
one of the most prominent representatives of AIE dyes. Be-
cause of the restriction of intramolecular rotations in aggre-
gates and the severely twisted conformations, TPE deriva-
tives have extreme solid-state fluorescence efficiencies. Fur-
thermore, they have several other advantages such as facile
synthesis, easy functionalization and chemical and thermal
stability.[5b,c,e]
[d] Prof. B. Z. Tang
Department of Polymer Science & Engineering
Key Laboratory of Macromolecular Synthesis and
Functionalization of the Ministry of Education of China
Zhejiang University, Hangzhou 310027 (P.R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201400.
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For optoelectronic applications, in addition to high optical
efficiency, carrier (electron and hole) transportation is an-
other crucial requirement. Normally a single compound
cannot fulfil all the demands. In OLEDs, multilayer archi-
tectures are generally adopted to balance the charge injec-
tion and transportation.[6] In reality, it would be ideal to
have emitters with both efficient solid-state emissions and
good charge-transporting properties because it would simpli-
fy the device fabrication procedure and lower the construc-
tion costs. Triphenylamine (TPA) is well known for its ease
in oxidation of the centre nitrogen and its ability to trans-
port charge carriers through the radical cation species with
high stability.[7] TPA-based organic semiconductors have
been widely studied as hole-transporting materials for vari-
ous optoelectronic applications such as xerography, OLEDs,
photovoltaic cells and organic field-effect transistors
(OFET).[8] In our previous work, by introducing TPA func-
tionalities into TPE derivatives, luminogens with good hole-
transporting ability were demonstrated.[5f,9]
As shown in Scheme 1, four kinds of TPE–TPA deriva-
tives have been studied in our previous work.[5f,9a] The TPE
and TPA moieties were easily combined through classic Pd0-
catalysed Suzuki coupling reactions. As we know, TPA also
suffers from the notorious effect of aggregation-caused
emission quenching in the condensed phase.[9a] Combining
TPA with TPE redeems its emissive ability and at the same
time bestows the hybrid molecules with good hole-transport-
ing properties. Compound 2 showed a hole mobility of 2.6ꢁ
10ꢀ3 cm2 Vꢀ1 sꢀ1 in a thin film OFET.[9b] OLEDs that employ
them as emitting and hole-transporting materials exhibited
quite good performances.
Given that both TPE and TPA are really versatile materi-
als, systemic studies of TPE–TPA oligomers and polymers
are significant in the discovery of promising candidates for
advanced optoelectronics applications and to understand
their structure–property relationships. In this paper, we pres-
ent the synthesis, characterisation and properties of a new
series of TPE–TPA oligomers and polymer. By using a new
synthetic strategy, TPE–TPA oligomers and a polymer with
different numbers and configuration of units were prepared
and their optical physics properties were studied compara-
tively.
bis-ketone intermediate 2BOA produced the TPE–TPA
polymer PTAE. The polymer was purified by precipitation
from methanol/chloroform (30:7) mixtures. PTAE dissolves
well in THF. The molecular weight measured by gas-phase
chromatography (GPC) is 5010 (polydispersity index
(PDI)=1.67). Perhaps because of the rigid structure, poly-
mer molecules with long chains will precipitate from the re-
action solutions and stop growing due to the limited solubili-
ty, and therefore the molecular weight was relatively low.
The reaction of the bis-ketone 2BOA with an excess
amount of diphenylmethyllithium (1:3 molar ratio) afforded
the bis-alcohol intermediate. Then the bis-alcohol intermedi-
ate was dehydrated to obtain the product DTAE with two
TPE moieties under a catalytic amount of p-toluenesulfonic
acid in boiled toluene. In a like manner, the reaction of
Results and Discussion
Synthesis: In our previous studies, TPE and TPA moieties
were combined by Suzuki coupling. In this work, McMurry
coupling[10] and the diphenylmethyllithium-mediated unsym-
metrical synthetic method were adopted.[11] Schemes 2 and 3
depict the synthetic routes. First, TPA was converted to the
key
intermediates
of
[4-(diphenylamino)phenyl]-
ACHTUNGTRENNUNG(phenyl)methanone (BOA) and 4,4’-(phenylazanediyl)-
bis(1,4-phenylene)bis(phenylmethanone) (2BOA) facilely
by AlCl3-catalysed Friedel–Crafts acylation.[12] Then the
McMurry coupling of BOA catalysed by Zn/TiCl4 gave
DATE in a high yield. Similarly, McMurry coupling of the
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Tetraphenylethene–Triphenylamine Oligomers
FULL PAPER
Figure 1. 1H NMR spectra of TPE–TPA oligomers and polymer in
[D]chloroform. The solvent peak is marked with an asterisk.
Figure 2. 13C NMR spectra of TPE–TPA oligomers and polymer in
[D]chloroform.
the molecules have similar chemical structures, their
1H NMR spectra showed similar profiles. Each of the spec-
tra was composed of peaks at dꢁ6.8 ppm (mainly from the
contributions of TPA moieties) and more peaks at low fields
(mainly from the contributions of TPE moieties and the N
meta positions of TPA). The NMR spectroscopic signals of
the oligomers DATE, DTAE and DTDAE are quite sharp;
for the polymer PTAE, the peaks broadened. The presence
of a large number of repeat units in the polymer backbone
and the chemical environment distributions of the units gen-
erally led to broad NMR spectroscopic signals. The rigid
backbone of PTAE might further lower the mobility of the
polymer in solution, which will also result in broad peaks.
The 13C NMR spectra offer clearer profiles. As shown in
Figure 2, the three oligomers showed quite similar carbon
resonance positions. Following the same trend as in the
1H NMR spectra, the signal peaks broadened when it came
to the polymer PTAE. In all of the spectra, no other unex-
pected signals were observed, thus verifying the identity and
high purity of their molecular structures. The identities of
the three oligomers were further proven by the M+ peaks in
their high-resolution mass spectra (Figures S1, S2 and S3 in
the Supporting Information), thereby confirming the occur-
rence of the reactions and the formation of the expected
products. The purity of the molecules was also confirmed by
Scheme 1. Chemical structures of explored triphenylamine-containing tet-
raphenylethenes in our laboratory.
2BOA with diphenylmethyllithium in a molar ratio of 1:1.2
afforded the intermediate with one alcohol and a ketone.
Upon dehydration, the alcohol yielded the intermediate
threo-b-benzyloxyaspartic acid (TBOA). McMurry coupling
of TBOA by using its ketone group produced the oligomer
DTDAE, which has three TPE and two TPA moieties. All
the dye molecules were characterised by NMR spectroscopy.
1
The H and 13C NMR spectra (Figures 1 and 2) all gave sat-
isfactory signal integrations. As shown in Figure 1, because
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B. Z. Tang et al.
According to the 1H NMR
spectra, the E/Z ratio is ambig-
uous. The different isomers
have quite close polarities and
are hard to isolate by chroma-
tography or recrystallization
methods. Later studies are
based on the mixtures of the
isomers. Both the theoretical
and experimental studies re-
vealed that the isomers should
have very similar properties
and the presence of the differ-
ent isomers does not signifi-
cantly affect the overall proper-
ties.
Aggregation-induced emission:
All four kinds of dyes dissolved
readily in common organic sol-
vents such as chloroform and
THF but were insoluble in
water. The three oligomers are
practically non-emissive when
molecularly dissolved in the so-
AHCTUNGTERGlNNUN utions, but emit intensely in
the aggregate state.[9] As shown
in Figure 3, the PL (photolumi-
nescence) spectra of DATE,
DTAE and DTDAE in THF
are basically flat lines parallel
to the abscissa, thereby reveal-
ing that they are genuinely
weak emitters in the solution
state. When a large amount of
water was added to their so-
Scheme 2. Synthetic routes to oligomers DATE and DTAE and the polymer PTAE.
AHCTUNGTERGlNNUN utions in THF, intense PL sig-
nals were recorded under iden-
tical measurement conditions at
521, 501 and 519 nm, respec-
tively. The higher the water
fraction, the stronger the emis-
sion intensity. As shown in
Figure 4, from the molecular
solutions in THF to the aggre-
gate suspension in 90% aque-
ous mixtures, their PL intensi-
ties increase several hundreds
of times. The inset photographs
of Figure 3 and Figure 4 further
demonstrate the non-lumines-
cent and emissive natures of
Scheme 3. Synthetic route to DTDAE.
elemental analysis with satisfactory results. Because of the
McMurry coupling reactions involved in the synthetic routes
of DATE, DTDAE and PTAE, there are E and Z isomers
in the products. (For DTAE, there is no McMurry reaction
in its synthetic route, thus no isomers should be present.)
the molecular isolated species and nano-aggregates. The PL
behaviour of the polymer PTAE is a little different than
that of the three oligomers. In THF, the fluorescence of the
polymer is not totally quenched. A moderate emission at
the short wavelength of approximately 410 nm can be de-
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Tetraphenylethene–Triphenylamine Oligomers
FULL PAPER
Figure 3. PL spectra of A) DATE, B) DTAE and C) DTDAE in THF/water mixtures with different water fractions. Concentration: 10 mm. Bottom: fluo-
rescent images of each dye in corresponding fractions of THF/water mixtures.
Figure 4. Plots of PL intensities versus compositions of the THF/water mixtures of A) DATE, B) DTAE and C) DTDAE. Inset: fluorescent images of
each dye in THF and THF/water mixtures with 90% of water.
tected in the spectrum, and the solution appears violet blue
under UV irradiation. Upon adding water, the emission
band at shorter wavelength decreases its intensity and disap-
pears after the water fractions are larger than 50%. Mean-
while, an emission at longer wavelength of 510 nm appears
and increases along with the increase in the water fractions.
The growing trend of this longer-wavelength emission band
is the same as that of the oligomers. The spectra and photo-
graphs in Figure 5 depict the changes. After aggregation, the
polymer only showed the bluish-green emission with the
wavelength of around 510 nm. We posit that in solutions the
shorter emission band at approximately 410 nm comes from
the TPA moieties of the molecularly dissolved polymer. Be-
cause of the long and rigid backbone of the polymer, the in-
tramolecular rotations of the phenyl blades around the TPE
stator cannot completely deactivate the excited states of the
entire molecule through the rotational energy relaxation
channels. Thus a portion of fluorescence from TPA chromo-
phores is released. While in the aggregate state, the emission
at longer wavelength recovers due to the restriction of intra-
molecular rotations, and the shorter wavelength emission
will quench through Fçrster resonance energy transfer to
the longer-wavelength emission. The decrease in the shorter-
wavelength emission and the increase in the longer one are
both evidence of the process of aggregation formation.
Since water is a non-solvent for all four of the dyes, their
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B. Z. Tang et al.
This should be attributable to
the poor crystallisability of
PTAE because of the long poly-
mer chain.
These dyes are very emissive
in the aggregate state. Figure 7
shows the PL spectra of DTAE,
DATE, DTDAE and PTAE in
the solid state and the corre-
sponding fluorescent images. In
the solid state (cast films),
DTAE shows the most blue
emission wavelength of 470 nm
among the four molecules. For
DATE, although it has similar
molecular weight to that of
DTAE, its two side donor
groups (TPA) make it a D–p–D
structure with intense intramo-
lecular charge transfer, thus its
emission at 500 nm is longer
than that of the former. For the
oligomer DTDAE, which has
three TPE and two TPA units,
the emission comes to 519 nm.
At 525 nm, the polymer PTAE
Figure 5. A) PL spectra of PTAE in THF/water mixtures with different water fractions. Concentration: 10 mm.
B) Plots of PL intensity versus compositions of the THF/water mixtures. Inset: fluorescent images of PTAE in
THF/water mixtures with different water fractions.
molecules must be aggregated in the solvent mixtures with
high water fractions. The aqueous mixtures are, however,
macroscopically homogenous with no precipitates, thereby
suggesting that the aggregates are nano-dimensional.[13] This
is proven by the TEM images. As shown in Figure 6, in the
90% aqueous mixtures, DATE aggregates into nanoparticles
with dimensions of dozens of nanometers, and the electron-
diffraction pattern displays many clear diffraction spots,
which suggests that they are crystalline, whereas the aggre-
gates of the polymer PTAE look like nanoscale vesicles, and
the electron-diffraction pattern only depicts an obscure dif-
fuse halo, thereby suggesting their amorphousness in nature.
Figure 7. PL spectra of DTAE, DATE, DTDAE and PTAE in the solid
state and the corresponding fluorescent images.
has the longest emission wavelength of the four. Their abso-
lute quantum yields were measured by using a calibrated in-
tegrating sphere. Among the three oligomers, the largest
one is DTDAE, which has the most efficient emission with
a quantum yield of 99%; the D–p–D-structured DATE
stands in second place with a yield of 93%; the bluest one,
DTAE, has a relatively low efficiency of 77%.
Figure 6. TEM images of nano-aggregates of A) DATE (scale bar:
50 nm) and B) PTAE (scale bar: 100 nm) formed in THF/water mixtures
with 90% water fractions. Insets: electron diffraction patterns of the
nano-aggregates (scale bar: 21 nm).
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Tetraphenylethene–Triphenylamine Oligomers
FULL PAPER
Table 1. Optical and thermal properties of DATE, DTAE, DTDAE and
PTAE.[a]
We suggest that the reason why these dyes show opposite
luminescence abilities in solution and in aggregates is as fol-
lows: in a dilute solution, the peripheral phenyl rotors in the
TPE moieties can undergo dynamic intramolecular rotations
against their double-bond stator at the excited states, which
will non-radiatively annihilate their energy and render the
molecules non-luminescent, whereas in the aggregates the
intramolecular rotations of the aryl rotors are greatly re-
stricted on account of the spatial constraints. This restriction
of intramolecular rotations blocks the non-radiative pathway
and opens up the radiative channel. As a result, the mole-
cules become emissive in the aggregate state.[4] Furthermore,
the propeller-like TPE and TPA unit would efficiently
hamper the close packing between the chromophores, there-
by preventing the formation of detrimental species such as
excimers and exciplexes that cause emission quenching and
redshift.[14] Therefore all the factors convert these molecules
from weak fluorophores in solution into strong emitters in
aggregates. Additionally, there is another noticeable phe-
nomenon: from aggregates in THF/water mixtures to the
cast films, the emission of DTAE blueshifts by 31 nm;
DATE blueshifts by 21 nm; there is no change in DTDAE;
the polymer PTAE redshifts by 15 nm. That is to say, as the
molecules grow larger, the emission blueshifts from aqueous
suspensions to cast films become smaller and ultimately the
redshift appears in the polymer PTAE. (For details, see
Table 1.) We propose that this evolution is caused by the in-
creasing intermolecular interactions when the molecules
become bigger.[15] In our previous work, we have proven
that the AIE-active dyes normally possess special crystallisa-
tion-induced blueshift characteristics. The emission wave-
length relies heavily on the crystallinity of the materials. For
the smaller molecules, their crystallinities in the cast films
are higher than those in the aqueous suspensions, therefore
they showed blueshifts in the cast films. For the molecules
with longer chains, they have more tendency to form amor-
phous aggregates; and the intermolecular interactions in the
films are stronger than those in the nano-aggregates.
The molecular band gaps of
lem [nm]
Thermal properties
AIE luminogen Solution Aggregate Film (FF) Td [8C]
Tg [8C]
DATE
DTAE
DTDAE
PTAE
513
496
519
404
521
501
519
510
500 (93)
470 (78)
519 (99)
525 (57)
344
374
422
364
80
108
137
172
[a] Abbreviations: lem =emission maximum in THF (solution; 10 mm),
THF/water mixtures (aggregate; 10 mm; 10:90 by volume), and solid thin
films with quantum yields (FF [%]) given in the parenthesis, Td =onset
degradation temperature and Tg =glass transition temperature. The FF
values of the solid films were measured by using a calibrated integrating
sphere.
Figure 8. UV/Vis absorption spectra of the four TPE–TPA derivatives in
THF.
differential scanning calorimetry (DSC) analyses. As shown
in Figure 9, for the three oligomers, as the molecular weight
increases, the degradation temperature (Td) increases from
344 (DATE) to 3748C (DTAE), and then to 4228C
(DTDAE). The Td of the polymer PTAE is 3648C, which is
slightly below that of DTAE. However, the residue percent-
the dyes in solution were inves-
tigated through the UV/Vis ab-
sorption spectra (Figure 8). In
THF, the three oligomers dis-
play two absorption peaks at
around 305 and around 360 nm.
Although the longer absorption
band is quite inconspicuous for
the polymer PTAE, it has a tail
extending to a similar cut-off
wavelength. This demonstrates
that the molecules have similar
band gaps.
Thermal properties: The ther-
mal stabilities of these dyes
were investigated by thermo-
Figure 9. TGA and DSC (second heating cycle) thermograms of DATE, DTAE, DTDAE and PTAE recorded
gravimetric analysis (TGA) and under N2 at a heating rate of (TGA) 20 and (DSC) 108Cminꢀ1
ACHTUNGTRENNUNG .
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B. Z. Tang et al.
age of the polymer is much higher than that of the oligo-
mers. Following a regular trend, the glass transition temper-
ature (Tg) of the molecules increases gradually as the molec-
ular weight increases. From the 808C of DATE, it increases
to 1088C (DTAE), then to 1378C (DTDAE) and finally to
1738C of the polymer PTAE. Except for DATE, all the Tg
values of the molecules are relatively high for optoelectronic
applications. The good morphological stabilities, coupled
with their efficient solid-state emissions, the dyes are thus
promising electroluminescent (EL) materials.[16]
ed. That is, ITO/X/1,3,5-tris(N-phenylbenzimiazole-2-yl)ben-
zene (TPBi; 30 nm)/LiF/Al (100 nm), in which X stands for
NPB (40 nm)/LEL (20 nm) in the “with HTL” devices and
stands for LEL (60 nm) in the “without HTL” devices
(LEL=DATE or DTAE or DTAAE; TPBi serves as the
electron-transporting material). Because we used the evapo-
ration method to fabricate the devices, the polymer PTAE
was not included.
Figure 10 depicts the PL and EL spectra of the solid thin
films of each oligomer. Compared with the PL spectra, the
EL of DATE and DTAE redshift a little (16–32 nm), where-
as that of DTDAE has no change. In the OLEDs a dedicated
HTL does not affect their emission wavelengths, which sug-
gests the recombination region remains unvarying. Figure 11
shows the current density–luminance characteristics of the
devices based on each dye. We can see that the devices with
simple structures (i.e., no dedicated HTL) show comparable
brightness at the same current density. Table 2 provides all
details of the devices. Without an NPB layer, OLEDs based
on DATE exhibit superior performances in terms of both lu-
Electroluminescence: Similar to the TPE–TPA derivatives
in our previous works, these oligomers possess high HOMO
levels of around 5.2 eV derived from the electrochemical
measurements (Figure S5 in the Supporting Information),
which are close to that of the most widely used hole-trans-
porting material, N,N’-di(1-naphthyl)-N,N’-diphenylbenzi-
dine (NPB).[9a] Thus to prove the hole-transporting ability of
the dyes, multilayer OLEDs with and without a dedicated
hole-transporting layer (HTL) were designed and construct-
Figure 10. PL and EL spectra of solid thin films of A) DATE, B) DTAE and C) DTDAE.
Figure 11. Current density–luminance characteristics of EL devices based on A) DATE, B) DTAE and C) DTDAE.
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Tetraphenylethene–Triphenylamine Oligomers
FULL PAPER
Table 2. Electroluminescence performances of DATE, DTAE and
DTDAE.[a]
UV light for 15min through a copper mask, a two-dimen-
sional photoresist pattern was fabricated. Upon excitation,
the unexposed square parts of the pattern emitted bright
green light, whereas the exposed patterned lines suffer from
photo-oxidative bleaching and are thus not emissive. The
sharp line edges of the patterns are clearly observed under
the fluorescence microscope (Figure 13). The generation of
fluorescent patterns is of great importance for the construc-
tions of photonic and electronic devices and biological prob-
ing systems.
Emitter
HTL lEL [nm] Von [V] Lmax [cdmꢀ12
]
CEmax [cdAꢀ1
]
p
516
520
502
506
524
524
4.3
4.4
3.2
3.7
3.6
6.0
14383
15487
13694
13227
40940
18400
6.72
7.04
3.28
3.24
11.2
8.7
DATE
ꢁ
p
DTAE
ꢁ
p
DTDAE
ꢁ
[a] With device configurations of ITO/ACTHNUGTRNEUNG(HTL)/emitter/TPBi/LiF/Al. Ab-
breviations: HTL=hole-transporting layer, lEL =EL maximum, Von
=
turn-on voltage at 1 cdmꢀ12, Lmax =maximum luminance, CEmax =maxi-
mum current efficiency, PEmax =maximum power efficiency and
EQEmax =maximum external quantum efficiency.
minance (15487 versus 14383 cdmꢀ12) and efficiencies (7.04
versus 6.72 cdAꢀ1); for DTAE the difference is negligible
(13227 versus 13694 cdmꢀ12 and 3.24 versus 3.28 cdAꢀ1); for
DTDAE the performance shows some decrease in the devi-
ces without HTL (18400 versus 40940 cdmꢀ12 and 8.7 versus
11.2 cdAꢀ1). Figure 12 depicts the current and power effi-
ciencies versus voltage characteristics of the devices. We can
see that OLEDs without a dedicated HTL exhibited better
stabilities, especially at high voltage. For DATE and DTAE,
the simple devices maintain higher efficiencies at almost all
voltages. Even for DTDAE, the simple device possesses
lower maximum efficiency than that of the device with
HTL; whereas at high voltage, the efficiency becomes great-
er than that of the latter thanks to a more gentle decrease in
efficiency along with the increase in bias. The voltage-de-
pendent stability is very helpful for practical use and should
be attributed to the balanced energy level.[17] Matching the
suitable HOMO level to the work function of the anode en-
ables them to exhibit better electronic characteristics in the
EL devices.
Figure 13. Two-dimensional fluorescent photopattern of PTAE. The pho-
tograph was taken under a fluorescence optical microscope.
Conclusion
In this work, we have designed and synthesised a series of
TPE–TPA derivatives with different numbers of TPE and
TPA units. Their optical physics, thermal stabilities and elec-
troluminescence properties were studied comparatively. All
the molecules demonstrate an AIE phenomenon and have
very high quantum yields in the solid state. Thanks to their
intrinsic hole-transport capability, they can act as both light-
emitting and hole-transporting materials in OLEDs. The
present strategy offers a win–win situation for both optical
and electrical properties. More advanced optoelectronic po-
tentials (e.g., electrical pumping organic laser diodes) were
imagined.[18] By tuning the electrical properties of AIE mol-
ecules, we hope the candidates offer the opportunity for
a breakthrough in these fields.
Photopatterning: To explore the applications of the poly-
mer, its photo-patterns were developed. When a thin film of
PTAE spin-coated on a silicon wafer was irradiated with
Figure 12. Power and current efficiencies of EL devices based on A) DATE, B) DTAE and C) DTDAE.
Chem. Eur. J. 2012, 00, 0 – 0
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B. Z. Tang et al.
Lu, S. Chen, J. W. Y. Lam, Z. Wang, Y. Liu, H. S. Kwok, Y. Ma,
B. Z. Tang, Adv. Mater. 2010, 22, 2159; f) Z. Zhao, S. Chen, J. W. Y.
Lam, P. Lu, Y. Zhong, K. S. Wong, H. S. Kwok, B. Z. Tang, Chem.
Commun. 2010, 46, 2221; g) X. Zhang, Z. Chi, H. Li, B. Xu, X. Li, S.
Liu, Y. Zhang, J. Xu, J. Mater. Chem. 2011, 21, 1788.
Acknowledgements
This project was partially supported by the Research Grants Council of
Hong Kong (603509, 601608, CUHK2/CRF/08 and HKUST2/CRF/10),
the Innovation and Technology Commission (ITP/008/09NP and ITS/168/
09), the University Grants Committee of Hong Kong (AOE/P-03/08),
and the National Science Foundation of China (20974028). B.Z.T. thanks
the Cao Guangbiao Foundation of Zhejiang University for its support.
Y.L. thanks the Independent Innovation Foundation of Shandong Uni-
versity for support (grant no. 2011TB020).
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Received: April 24, 2012
Published online: && &&, 0000
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Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Tetraphenylethene–Triphenylamine Oligomers
FULL PAPER
Solid-State Emitters
Y. Liu, X. Chen, Y. Lv, S. Chen,
J. W. Y. Lam, F. Mahtab, H. S. Kwok,
X. Tao, B. Z. Tang* .......... &&&& — &&&&
Systemic Studies of Tetraphenyl-
AHCTUNGTRENNeGUN thene–Triphenylamine Oligomers and
a Polymer: Achieving Both Efficient
Solid-State Emissions and Hole-Trans-
porting Capability
Glowing recommendation: A series of
oligomers and a polymer composed of
different numbers and configuration of
tetraphenylethene and triphenylamine
units have been synthesised. Their
optical and electroluminescence
behaviours were studied systematically.
All the molecules demonstrate aggre-
gation-induced emission (see figure)
and have very high quantum yields in
the solid state.
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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These are not the final page numbers!