Tuning the electronic nature of aggregation-induced emission luminogens with enhanced hole-transporting property

Article abstract of DOI:10.1021/cm2003269

Triphenylamine (TPA) is a well-known holetransporting material but suffers aggregation-caused emission quenching in the solid state. Tetraphenylethene (TPE), on the other hand, is an archetypal luminogen that shows the phenomenon of aggregation-induced em

Full text of DOI:10.1021/cm2003269

ARTICLE
pubs.acs.org/cm
Tuning the Electronic Nature of Aggregation-Induced Emission
Luminogens with Enhanced Hole-Transporting Property
Yang Liu,† Shuming Chen,‡ Jacky W. Y. Lam,† Ping Lu,† Ryan T. K. Kwok,† Faisal Mahtab,† Hoi Sing Kwok,‡
and Ben Zhong Tang*,†,§
†Department of Chemistry and ‡Center for Display Research, The Hong Kong University of Science & Technology (HKUST),
Clear Water Bay, Kowloon, Hong Kong, China
§Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
S Supporting Information
b
ABSTRACT: Triphenylamine (TPA) is a well-known hole-
transporting material but suffers aggregation-caused emission
quenching in the solid state. Tetraphenylethene (TPE), on the
other hand, is an archetypal luminogen that shows the phenom-
enon of aggregation-induced emission (AIE). In this work, TPA
is attached to the TPE core as peripheral group to generate new
AIE luminogens with enhanced hole-transporting property. The
TPA-TPE adducts, named 1-[40-(diphenylamino)biphenyl-4-yl]-1,
2,2-triphenylethene (TPATPE) and 1,2-bis[40-(diphenylam-
ino)biphenyl-4-yl]-1,2-diphenylethene (2TPATPE) are synthesized
in satisfactory yields by Suzuki coupling of 4-(diphenyla-
mino)phenylboronic acid with 1-(4-bromophenyl)-1,2,2-tripheny-
lethene and 1,2-bis(4-bromophenyl)-1,2-diphenylethene, respectively. Whereas the hybrid molecules are practically nonluminescent in the
solution state, their aggregates in poor solvents and thin films emit intensely with fluorescence quantum yields up to 100%. Both TPATPE and
2TPATPE are thermally and morphologically stable, showing high thermal-degradation (Td up to 430 °C) and glass transition (Tg =119°C)
temperatures. Multilayer electroluminescence (EL) devices are constructed, which emit sky blue and green EL with maximum luminance of
32230 cd/m2 and current efficiency up to 13.0 cd/A. The devices without hole-transporting layers (HTL) show performances comparable to
or better than those with HTL, presumably because of the high hole mobility of TPATPE and 2TPATPE coupled with the matching of their
energy levels with the anode.
KEYWORDS: fluorescence, tetraphenylethene, triphenylamine, aggregation, hole-transport
’ INTRODUCTION
emit stronger, bluer lights than their amorphous counterparts.5
Organic light-emitting diodes (OLEDs) utilizing the AIE mol-
ecules are fabricated, which exhibit outstanding performances.6
To enhance the device performance, scientists have fabricated
OLEDs with multilayers to balance the charge injection and
transportation. In reality, it would be nice to have emitters with
both efficient solid-state emissions and good charge-transporting
properties because it will simplify the device fabrication proce-
dure, shorten the fabrication process, and lower the production
cost.7 Examples of such fluorophores are tris(8-hydroxyquinoli-
nato)aluminum (Alq3) and siloles, which are well-known EL
emitters and electron-transporting materials.8 However, AIE
luminogens with good hole-transporting properties are rare.
Arylamines such as triphenylamine (TPA) are versatile materials
and exhibit good hole-transporting capability.9 TPA derivatives,
however, suffer from the notorious effect of aggregation-caused
emission quenching in the condensed phase.6e For example, N4,N4,
Much effort has been devoted to the creation of efficient
luminescent materials because of their potential applications in
optics and electronics such as electroluminescence (EL), organic
lasers, and fluorescent sensors.1 Whereas many dyes emit strongly in
their dilute solutions, they become weak fluorophors when aggre-
gated as nanoparticle suspensions in poor solvents or fabricated as
thin films in the condensed phase.2 This problem must be solved
because luminogenic molecules are commonly used as solid films in
the real-world applications. Various approaches, such as the attach-
ment of bulky alicyclics, encapsulation by amphiphilic surfactants,
and blending with transparent polymers, have been taken to
interfere with luminogen aggregation but the attempts have, how-
ever, met with only limited success.3
We have recently observed “abnormal” emission behaviors of
aggregation-induced emission (AIE) and crystallization-en-
hanced emission. Instead of quenching, aggregate formation of
some propeller-shaped molecules such as siloles and tetraphe-
nylethenes (TPEs) has enhanced their light emissions.4 While
crystallization commonly weakens and red-shifts light emission,
the crystalline aggregates of some AIE molecules are found to
Received: November 30, 2010
Revised:
April 14, 2011
Published: April 25, 2011
r
2011 American Chemical Society
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Chart 1. Chemical Structures of Triphenylamine-Containing
Tetraphenylethenes TPATPE and 2TPATPE
electron diffraction patterns were obtained using JEOL 2010 transmission
electron microscope (TEM) at an accelerating voltage of 200 KV. Samples
were prepared by casting a drop of suspensions onto copper 400-mesh
carrier grids and dried in open air at room temperature.
Device Fabrication. The devices were fabricated on 80 nm ITO-
coated glass with a sheet resistance of 25Ω/0. Prior to load into the
pretreatment chamber, the ITO-coated glasses were soaked in ultrasonic
detergent for 30 min, followed by spraying with deionized water for 10 min,
soaking in ultrasonic deionized water for 30 min, and then ovenbake for 1 h.
The cleaned samples were treated by CF4 plasma with a power of 100 W, gas
flow of 50 sccm, and pressure of 0.2 Torr for 10 s in the pretreatment
chamber. The glasses were transferred to the organic chamber with a base
pressure of 7 ꢁ 10ꢀ7 Torr without breaking vacuum for depositing a 60 nm
4,4-bis(1-naphthylphenylamino)biphenyl (NPB), a 20 nm emitter, a 10 nm
2,20,200-(1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBi), and a
30 nm Alq3, which serve as hole-transporting, light-emitting, hole-blocking,
and electron-transporting layers, respectively. The samples were then trans-
ferred to the metal chamber for cathode deposition which composed of 1 nm
LiF capped with 100 nm Al. The light-emitting area was 4 mm2 defined by the
overlap of the cathode and with the anode. The current densityꢀvoltage
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 efficiencies of the devices were inferred
from the photocurrent of the photodiode. The EL spectra were obtained on a
PR650 spectrophotometer. All the measurements were carried out under air
at room temperature without device encapsulation.
4
4
N ,N -tetraphenylbiphenyl-4,40-diamine, a TPA dimer, shows a
high fluorescence quantum yield (ΦF) in THF solution (75.6%)
but a 5.5-fold lower efficiency in the film state (13.7%). TPE, on the
other hand, is AIE-active and enjoys facile synthesis and high
thermal stability. Fusing the two components into one system
may lead to the generation of new molecules with combined
advantages of the two components, that is, both AIE-active and
hole-transporting. Intrigued by such possibility, in this paper, we
presented the synthesis of TPA-containing tetraphenylethenes
(TPATPE and 2TPATPE; Chart 1) and their photophysical
properties. Efficient OLEDs using these molecules as both emitters
and hole-transporting layers (HTL) are constructed, whose perfor-
mances are comparable or higher than those with HTL.
0
0
Luminogen Preparation. The TPA-TPE adducts were synthe-
sized according to the synthetic routes shown in Scheme 1. Typical
procedures for their syntheses are shown below.
1-[40-(Diphenylamino)biphenyl-4-yl]-1,2,2-triphenyleth-
ene (TPATPE). Into a stirred mixture of 0.411 g (1 mmol) of BrTPE, 0.289
g (1 mmol) of 1, and 6 mL of 2 M Na2CO3 solution in 15 mL THF was
added 0.01 g of Pd(PPh3)4 under nitrogen. The mixture was heated to 80 °C
for 12 h. After being cooled to room temperature, the solution was extracted
with 50 mL of CH2Cl2 twice, washed with water, and dried over Na2SO4.
After filtration and solvent evaporation under reduced pressure, the product
was purified by silicaꢀgel column chromatography using hexane/dichlor-
omethane as eluent. A pale yellow solid was obtained in 66% yield (0.38 g).
1H NMR (400 MHz, CDCl3), δ (TMS, ppm): 7.43ꢀ7.45 (m, 2H),
7.32ꢀ7.34 (m, 2H), 7.23ꢀ7.27 (m, 4H), 7.00ꢀ7.13 (m, 25H). 13C NMR
(100 MHz, CDCl3), δ (TMS, ppm): 147.67, 147.07, 143.80, 143.77, 143.75,
142.30, 140.98, 140.60, 138.24, 134.60, 131.78, 131.36, 129.24, 127.72,
127.63, 127.47, 126.44, 125.64, 124.32, 123.98, 123.95, 122.86. HRMS
(MALDI-TOF): m/z 575.2480 (Mþ, calcd 575.2613). Anal. Calcd For
C44H33N: C, 91.79; H, 5.78; N, 2.43. Found: C, 91.43; H, 5.82; N, 2.45.
1,2-Bis[40-(diphenylamino)biphenyl-4-yl]-1,2-diphenyle-
thene (2TPATPE). The compound was prepared from 0.4 g
(0.6 mmol) of 2BrTPE, 0.416 g (1.44 mmol) of 1, and 0.02 g of Pd(PPh3)4
using the same procedure described above. Yellow solid; yield 70% (0.34 g).
1H NMR (400 MHz, CDCl3), δ (TMS, ppm): 7.45 (d, 4H, J = 8.4 Hz),
7.36ꢀ7.32 (m, 4H), 7.29ꢀ7.23 (m, 8H), 7.15ꢀ7.00 (m, 30H). 13C NMR
(100 MHz, CDCl3), δ (TMS, ppm): 147.64, 147.05, 143.80, 142.37, 142.35,
140.61, 138.28, 138.20, 134.58, 134.55, 131.79, 131.43, 129.22, 127.75,
127.64, 127.45, 126.47, 126.42, 125.71, 125.61, 124.30, 123.96, 122.84.
HRMS (MALDI-TOF): m/z 818.4846 (Mþ, calcd 818.3661). Anal. Calcd
For C62H46N2: C, 90.92; H, 5.66; N, 3.42. Found: C, 90.53; H, 5.66; N, 3.44.
’ EXPERIMENTAL SECTION
Materials and Instrumentations. Tetrahydrofuran (THF) was
distilled from sodium benzophenone ketyl under nitrogen immediately
prior to use. All the chemicals and other reagents were purchased from
Aldrich and used as received without further purification. 4-(Diphenyla-
mino)phenylboronic acid (1), 1-(4-bromophenyl)-1,2,2-triphenylethene
(BrTPE), and 1,2-bis(4-bromophenyl)-1,2-diphenylethene (2BrTPE) were
prepared according to the literature methods.10,11
1H and 13C NMR spectra were measured on a Bruker AV 400 spectro-
meter in CDCl3 or CD2Cl2 using tetramethylsilane (TMS; δ = 0) as
internal reference. UV spectra were measured on a Milton Roy Spectronic
3000 Array spectrophotometer. Photoluminescence spectra (PL) were
recorded on a Perkin-Elmer LS 55 spectrofluorometer. High resolution
mass spectra (HRMS) were recorded on a GCT premier CAB048 mass
spectrometer operating in MALDI-TOF mode. Thermogravimetric anal-
ysis (TGA) was carried on a TA TGA Q5500 under dry nitrogen at a
heating rate of 20 °C/min. Thermal transitions were investigated by
differential scanning calorimetry (DSC) using a TA DSC Q1000 under
dry nitrogen at a heating rate of 10 °C/min. Cyclic voltammograms were
recorded on a Princeton Applied Research (model 273A) at room
temperature. The working and reference electrodes were glassy carbon
and Ag/AgNO3 (0.1 M in acetonitrile), respectively. The HOMO energy
levels were derived from the onset oxidation potentials (Eonset-ox) according
to the equation: HOMO = ꢀ(Eonset-ox þ 4.8 ꢀ Eferrocene) eV, where the
value of Eferrocene measured in our experiment was 0.11 V. All the solutions
were deactivated by bubbling nitrogen gas for a few minutes prior to
electrochemical measurements. Transmission electron micrographs and
’ RESULTS AND DISCUSSIONS
Synthesis. The synthetic routes to the TPA-containing TPEs
(TPATPE and 2TPATPE) are depicted in Scheme 1. Compound 1
was synthesized by lithiation of 4-(bromophenyl)diphenylamine
followed by boronation with triisopropyl borate.10 On the other
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Scheme 1. Synthetic Routes to TPATPE and 2TPATPE
Figure 1. (A) PL spectra of TPATPE in THF/water mixtures with different water fractions (fw). Concentration, 10 μM; excitation wavelength, 355 nm.
(B) Plots of fluorescence quantum yields versus compositions of the THF/water mixtures. Inset: fluorescent images of TPATPE in THF/water mixtures
with different water fractions.
hand, the key reactants BrTPE and 2BrTPE were prepared by
reaction of diphenylmethyllithium with 4-bromobenzophenone11a
and McMurry coupling of 4-bromobenzophenone,11b respectively.
Suzuki coupling reactions12 of 1 with BrTPE and 2BrTPE are
catalyzed by Pd(PPh3)2 in basic medium, furnishing the desirable
products TPATPE and 2TPATPE, respectively, in satisfactory
yields (see Experimental Section in the Supporting Information
for details). The dye molecules were characterized by NMR
represents that they are mixtures of Z- and E-isomers). This is
proved by the peaks observed at δ 7.6ꢀ7.3 in its 1H NMR spectrum
(see Figure S3 in the Supporting Information), from which a Z/E
ratio of ∼1/1 is deduced. In some occasions, the Z- and E-isomers
can be separated by column chromatography or recrystallization.
Unfortunately, these methods are not applicable to 2BrTPE and
2TPATPE because their isomers possess similar polarities. On the
other hand, DSC and TGA measurements show that 2TPATPE
exhibits only a single glass transition temperature (Tg) and onset
degradation temperature (Td), revealing that the two isomers share
similar thermal properties (see Figure 5). Theoretical calculation
demonstrates that its Z- and E-isomers have almost identical orbital
distributions and hence exhibit similar fluorescence properties (see
Figure 6). All these results make our belief that the existence of both
Z- and E-isomers in 2TPATPE would not alter significantly the
overall properties of the material.
1
spectroscopy and both gave high signal: noise H and 13C NMR
spectra (see Figures S1ꢀS7 in the Supporting Information). The
reaction products, for example, gave Mþ peaks at their high-
resolution mass spectra (see Figures S8 and S9 in the Supporting
Information), confirming the occurrence of the coupling reaction
and the formation of the expected products. The purity of TPATPE
and 2TPATPE was also confirmed by elemental analysis with
satisfactory results. It is noteworthy that McMurry couplings of
monosubstituted benzophenones give TPEs with irregular stereo-
structures. Intermediate 2BrTPE was prepared from 4-bromoben-
zophenone using the same reaction. Consequently, the target
product 2TPATPE possesses also a low stereoregularity (the zigzag
line in the molecular structures of 2BrTPA and 2TPATPE
Aggregation-Induced Emission. Both TPATPE and
2TPATPE dissolve readily in common organic solvents such as
chloroform and THF but are insoluble in water. They are practically
nonemissive when molecularly dissolved in the solutions but emit
intensely in the aggregate state. As shown in Figures 1 and 2, the PL
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Figure 2. (A) PL spectra of 2TPATPE in THF/water mixtures with different water fractions (fw). Concentration, 10 μM; excitation wavelength,
355 nm. (B) Plots of fluorescence quantum yields versus the compositions of the THF/water mixtures. Inset: fluorescent images of 2TPATPE in THF/
water mixtures with different water fractions.
Figure 3. UV spectra of (A) TPATPE and (B) 2TPATPE in THF/water mixtures with different water fractions (fw).
spectra of TPATPE and 2TPATPE in THF are basically flat lines
parallel to the abscissa. The ΦF’s in THF are as low as 0.38 and
0.24%, respectively, revealing that TPATPE and 2TPATPE are
genuinely weak emitters in the solution state. When a large amount
of water is added into their THF solutions, intense PL signals are
recorded at 480 and 505 nm under identical measurement condi-
tions. The higher the water fraction, the stronger is the emission
intensity. Since water is a nonsolvent for TPATPE and 2TPATPE,
their molecules must be aggregated in the solvent mixtures with
high water fractions. From the molecular solution in THF to the
aggregate suspension in 95% aqueous mixture, the ΦF’s ofTPATPE
and 2TPATPE increase ca. 103- and 192-fold, respectively. Appar-
ently, their emissions are induced by aggregate formation; in other
words, they are AIE-active. The aqueous mixtures are, however,
macroscopically homogeneous with no precipitates, suggesting that
the aggregates are of nanodimension, as proved by the level-off tails
at the longer wavelength region of their absorption spectra in the
aqueous mixtures with high water contents due to the scattering
effect of the dye nanoparticles (Figure 3). The photographs in the
lower panels of Figures 1 and 2 further manifest the nonluminescent
and emissive natures of the molecular isolated species and
nanoaggregates.
Closer inspection of the PL spectra of the two compounds in
the aqueous mixtures reveals that the emission maximum is blue-
shifted slightly when the water fraction is increased from 70 to
95%. This is probably due to the change in the packing order of
the aggregates from an amorphous state to a crystalline one.13
This is proved by the TEM images and the electron diffraction
patterns in Figure 4. Whereas the aggregates formed in the
solvent mixtures with 90% water fractions display many clear
diffraction spots, those derived from 70% aqueous mixtures give
only an obscure diffuse halo, suggesting that they are crystalline
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and amorphous in nature, respectively (see Figure S10 in the
Supporting Information).
phenyl blades around the TPE stator may have effectively
deactivated the excited states via the rotational energy relaxation
channels, thus rendering TPATPE and 2TPATPE nonemissive.
In the aggregate state, the intramolecular rotations are restricted,
Like their aggregates suspended in the aqueous media,
TPATPE and 2TPATPE are highly emissive in the solid state.
Upon photoexcitation, their thin films emit at 486 and 505 nm
with ΦF values of 100% (Table 1), which are more than 2-fold
higher than that of unsubstituted TPE (49.2%).14
It is worth mentioning that TPA is emissive in solution.6e The
nonluminescent nature of TPATPE and 2TPATPE in the
solution state suggests that the TPE core quenches the light
emission. In solution, the active intramolecular rotations of the
Figure 6. Optimized geometries and molecular orbital amplitude plots
of HOMO and LUMO energy levels of TPATPE and E-2TPATPE.
Figure 4. TEM images of nanoaggregates of (A) TPATPE and (B)
2TPATPE formed in THF/water mixtures with 95% water fractions.
Insets: electron diffraction patterns of the nanoaggregates.
Table 1. Optical and Thermal Properties of TPATPE and
2TPATPEa
λem (nm)
AIE luminogen soln (ΦF) aggr (ΦF) film (ΦF) Td (°C) Tg (°C)
TPATPE
405 (0.38) 480 (39) 486 (100)
511 (0.24) 505 (46) 505 (100)
360
430
2TPATPE
119
a Abbreviations: λem = emission maximum in THF solutions (soln;
10 μM), THF/water mixtures (aggr; 10 μM; 5:95 by volume), and solid
thin films with quantum yields (ΦF, %) given in the parentheses, Td =
onset degradation temperature, and Tg = glass transition temperature.
The ΦF values in the solution and aggregate states were determined by
using quinine sulfate (ΦF = 54% in 0.1 M sulphuric acid as standard,
whereas that of solid film was measured using a calibrated integrating
sphere.
Figure 7. Changes in current and luminance with the applied voltage in
a single-layer EL device of 2TPATPE with a configuration of ITO/
2TPATPE/LiF/Al. Inset: EL spectrum of the device.
Figure 5. (A) TGA and (B) DSC (second heating cycle) thermograms of TPATPE and 2TPATPE recorded under N2 at a heating rate of (A) 20 and
(B) 10 °C/min.
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which block the nonradiative pathways and populate the radia-
tive decay. On the other hand, the propeller-like TPE unit would
efficiently hamper the close packing between the chromophores,
preventing the formation of detrimental species such as excimers
and exciplexes that cause emission quenching and red-shift in the
PL spectrum. All these factors have converted TPATPE and
2TPATPE from weak fluorophors into strong emitters.
Thermal Properties. The thermal properties of TPATPE and
2TPATPE are investigated by TGA and DSC analyses. As shown
in Figure 5A, both TPATPE and 2TPATPE enjoy high
thermal stability with Td’s of 360 and 430 °C, respectively,
which are much higher than that of TPE (Td = 224 °C).14d
DSC analysis of 2TPATPE during the second heating cycle
detects a Tg at 119 °C, which is higher than that of NPB (Tg =
96 °C). This suggests that 2TPATPE possesses a high mor-
phological stability. Coupled with its efficient solid-state
emission, the dye is thus a promising EL material. The Tg of
TPATPE is not detected, probably because of its low molec-
ular weight.
Electronic Structure. To better understand the photophysi-
cal properties of TPATPE and 2TPATPE, we performed
molecular calculations on their energy levels based on DFT/
B3LYP/6-31G(d) using Gaussian 03. The optimized geome-
tries and HOMO and LUMO plots of TPATPE and E-
2TPATPE are given in Figure 6. Both molecules adopt twisted
conformations, whose geometries are similar to the crystal
structure of TPE.15 The dihedral angles between any two
phenyl rings of the TPE core are in the range from 58 to 80°.
Such propeller-like, nonplanar conformations would hamper
the πꢀπ stacking interactions between the dye molecules,
which enable them to emit efficiently in the condensed phase.
The dihedral angle between the adjacent phenyl blades of the
TPE and TPA units is ∼36°, which makes the two chromo-
phores moderate conjugated. The LUMO of both molecules
are dominated by orbitals from the TPE core. In contrast, the
electron cloud of the HOMO is located mainly on the TPA unit.
Such electron distribution imparts the dye molecules with an
intrinsic intramolecular charge transfer property. The molecu-
lar orbitals are more “delocalized” in E-2TPATPE, suggesting
that it possesses a higher conjugation and thus shows a redder
Figure 8. Energy level diagrams and device configurations of multilayer
EL devices of 2TPATPE with and without NPB or hole-transporting
layers. Abbreviations: NPB = 4,4-bis(1-naphthylphenylamino)biphenyl,
TPBi = 2,20,200-(1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimidazole).
Figure 9. (A) PL and EL spectra of solid thin film of TPATPE. (B) Current densityꢀvoltageꢀluminance characteristics of multilayer EL devices of
TPATPE. Changes in (C) power and current and (D) external quantum efficiencies with the applied voltage in multilayer EL devices of TPATPE.
Device configurations: ITO/(HTL)/TPATPE/TPBi/Alq3/LiF/Al.
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Table 2. Electroluminescence Performances of TPATPE and 2TPATPEa
device
LEL
HTL
λEL (nm)
Von (V)
Lmax (cd/m2)
CEmax (cd/A)
PEmax (lm/W)
EQEmax (%)
√
I
TPATPE
492
492
514
512
3.6
4.2
3.4
3.2
15480 (15 V)
26090 (15 V)
32230 (15 V)
33770 (15 V)
8.6 (5.2 V)
8.3 (6.4 V)
12.3 (4.2 V)
13.0 (4.2 V)
5.3 (4.6 V)
4.9 (5.4 V)
10.1 (3.8 V)
11.0 (3.6 V)
3.4 (5.2 V)
3.3 (6.0 V)
4.0 (4.2 V)
4.4 (4.2 V)
III
II
ꢁ
√
2TPATPE
IV
ꢁ
a With device configurations of ITO/(HTL)/LEL/TPBi/Alq3/LiF/Al. Abbreviations: LEL = light-emitting layer, HTL = hole-transporting layer, λEL
=
EL maximum, Von = turn-on voltage at 1 cd mꢀ2, Lmax = maximum luminance, CEmax = maximum current efficiency, PEmax = maximum power efficiency,
and EQEmax = maximum external quantum efficiency.
Figure 10. (A) PL and EL spectra of solid thin film of 2TPATPE. (B) Current densityꢀvoltageꢀluminance characteristics of multilayer EL devices of
2TPATPE. Changes in (C) power and current and (D) external quantum efficiencies with the applied voltage in multilayer EL devices of 2TPATPE.
Device configurations: ITO/(HTL)/2TPATPE/TPBi/Alq3/LiF/Al.
emission than TPATPE. Similar orbital distribution is also
observed in Z-2TPATPE.
configuration of ITO/2TPATPE(100 nm)/LiF/Al(200 nm) using
2TPATPE as light-emitting layer (LEL). Figure 7 shows the
JꢀVꢀL characteristics of the device. The device emits at 518 nm,
exhibiting maximum luminance (Lmax) and current and power
efficiencies (CEmax and PEmax) of 1630 cd/m2, 0.43 cd/A, and 0.22
lm/W, respectively. Although the result is fair, it already demon-
strates that 2TPATPE possesses good charge-injecting and trans-
porting properties.
To enhance the device performance, multilayer OLEDs with a
configuration of ITO/NPB(40 nm)/LEL(20 nm)/TPBi(10 nm)/
Alq3(30 nm)/LiF/Al(200 nm) (for device I, LEL = TPATPE; for
device II, LEL = 2TPATPE) are fabricated, in which NPB functions
as HTL and TPBi and Alq3 serve as electron-transporting materials.
Since the result from Figure 7 reveals that TPATPE and 2TPATPE
may retain the good hole-transporting properties of their TPA
The electrochemical properties of TPATPE and 2TPATPE
are investigated by cyclic voltammetry. Such method is also
widely used for estimating the energy levels of organic semi-
conductors. Both dye molecules exhibit similar voltammograms
(see Figure S11 in the Supporting Information). Their HOMO
derived from the onset oxidation potentials are calculated to be
5.3 and 5.2 eV, respectively, which are the same to or slightly
higher than that of NPB. Their LUMO are obtained by subtrac-
tion of the optical band gap energies from the HOMO values and
are both equal to 2.4 eV.
Electroluminescence. The efficient light emissions of TPATPE
and 2TPATPE in the solid state prompt us to investigate their EL
properties. We first constructed a single-layer EL device with a
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peripheries, the use of NPB as an additional HTL is thus not
necessary and could be even harmful because it may break the
charge balance in the EL device. With such regard, we fabricated
devices III and IV, in which the NPB or hole-transporting layers in
devices I and II are eliminated. The energy level diagrams and the
device architectures of 2TPATPE are depicted in Figure 8 as
example.
The EL spectra of both devices of TPATPE peak at 492 nm,
which is similar to the PL peak of the thin film (Figure 9A). The
EL performance of device I is impressive, with CEmax, PEmax, and
external quantum efficiency (EQEmax) being 8.6 cd/A, 5.3 lm/W,
and 3.4%, respectively (Table 2). Similar outstanding data (8.3
cd/A, 4.9 lm/W, and 3.3%) are also achieved in device III. At the
same voltage, device III shows better electronic character and
radiates more intensely than device I (Figure 9B). Clearly,
TPATPE is an excellent hole-transporting material, in addition
to being highly emissive in the solid state.
The OLED fabricated from 2TPATPE without a NPD layer
(device IV) shows even better performance. The device turns on
at a low bias of 3.2 V, emitting brilliantly with maximum
luminance of 33770 cd/m2 (Figure 10). The CEmax and PEmax
reach 13.0 cd/A and 11.0 lm/W, which are higher than those
attained by device II (Table 2). The high hole mobility of
2TPATPE and its matched HOMO energy level with the anode
may enable it to exhibit better EL performance in the absence of
NPB. The EQEmax is 4.4% and is comparable or even better than most
of the results reported previously for singlet OLEDs.6g One thing that
needs to be pointed out is that with a similar device structure, the
unsubstituted TPE only shows Lmax and CEmax of 1800 cd/m2 and
0.45 cd/A.15 Apparently, the EL properties of TPE can be readily
tuned by molecular engineering endeavors and incorporation of TPA
as substituent to TPE has resulted in new AIE luminogens with higher
thermal stability, more efficient solid-state emissions, and enhanced
hole-transporting properties. All these attributes make TPATPE and
2TPATPE better EL materials for OLED application.
’ ASSOCIATED CONTENT
Supporting Information. 1H and 13C NMR spectra, UV
S
b
absorption spectra, TEM images of nanoaggregates, and CV
spectra of TPATPE and 2TPATPE. This material is available free
of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: tangbenz@ust.hk. Phone: þ852-2358-7375. Fax:
þ852-2358-1594.
’ ACKNOWLEDGMENT
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 Commis-
sion (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
support from Cao Guangbiao Foundation of Zhejiang
University.
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’ CONCLUSIONS
In this paper, TPA-containing tetraphenylethenes TPATPE
and 2TPATPE are synthesized and their optical properties are
investigated. Whereas they are almost nonluminescent when
molecularly dissolved in the solutions, they are induced to emit
intensely by aggregate formation, demonstrating an AIE phe-
nomenon. The AIE effect has boosted the quantum yields of their
solid thin films to unity. Both dye molecules enjoy high thermal
stability (g360 °C) and are morphological stable. Multilayer EL
devices with configurations of ITO/(NPB)/TPATPE or
2TPATPE/TPBi/Alq3/LiF/Al are constructed, which give sky
blue and green EL with maximum luminance and efficiencies of
33700 cd/m2, 13.0 cd/A, 11.0 lm/W, and 4.4%. The OLEDs
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performances than those with HTL, possibly to the high hole
mobility of the TPA unit in the hybrid molecules.
The present results demonstrate that combining ACQ and
AIE units in one molecule is a good approach to surmount the
notorious ACQ problem. Traditional approaches attempt to
prevent luminophors from forming aggregates but generate
new problems. Our strategy, however, takes advantage of chro-
mophore aggregation without sacrificing other functional prop-
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