Construction of Efficient Deep Blue Aggregation-Induced Emission Luminogen from Triphenylethene for Nondoped Organic Light-Emitting Diodes

Article abstract of DOI:10.1021/acs.chemmater.5b00568

Deep blue emitters are crucial for full color displays and organic white lighting. Thanks to the research efforts by scientists, many efficient light emitters with aggregation-induced emission (AIE) characteristics have been synthesized and found promisin

Full text of DOI:10.1021/acs.chemmater.5b00568

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Article
Construction of Efficient Deep Blue AIE Luminogen from
Triphenylethylene for Non-Doped OLED Applications
Wei Qin, Zhiyong Yang, Yibin Jiang, Jacky W.Y. Lam, Guodong Liang, Hoi Sing Kwok, and Ben Zhong Tang
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00568 • Publication Date (Web): 05 May 2015
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Chemistry of Materials
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Wei Qin,†,‡,┴ Zhiyong Yang,†,‡,┴ Yibin Jiang,§ Jacky W. Y. Lam,†,‡ Guodong Liang,†,‡ Hoi Sing
Kwok,§ and Ben Zhong Tang†,‡,#,
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*
†HKUST-Shenzhen Research Institute, No. 9 Yuexing 1st RD, South Area, Hi-tech Park, Nanshan, Shenzhen, 518057,
China
‡Department of Chemistry, Institute for Advanced Study, Division of Life Science, State Key Laboratory of
Molecular Neuroscience, Institute of Molecular Functional Materials and Division of Biomedical Engineering, The
Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China
§Center for Display Research, HKUST, Clear Water Bay, Kowloon, Hong Kong, China
#Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key Laboratory of
Luminescent Materials and Device, South China University of Technology, Guangzhou, 510640, China
ABSTRACT: Deep blue emitters are crucial for full color displays and organic white lighting. Thanks to the research
efforts by scientists, many efficient light emitters with aggregation-induced emission (AIE) characteristics have been
synthesized and found promising applications in organic light-emitting devices (OLEDs). However, few AIE emitters with
deep blue emissions and excellent electroluminescence (EL) performance have been reported. The contribution here
reports a simple but successful molecular design strategy for synthesizing efficient solid-state emitters for non-doped
OLEDs with both deep blue and white emissions. This strategy utilizes triphenylethene, a weakly conjugated AIEgen as
building block for constructing deep blue emitter, involving no complicated control on emission color through
adjustment of the steric hindrance of chromohphores and enables a wide selection of partnered functional units. The
synthesized AIE luminogen, abbreviated as BTPE-PI, is thermally stable and exhibits high fluorescence quantum
efficiency as well as good chargeinjection capability in the solid state. Non-doped deep blue OLED fabricated from BTPE-
PI shows a very high external quantum efficiency of 4.4% with a small roll-off, whose performance is the best among deep
blue AIE materials reported so far. An efficient white OLED with Commission Internationale de l’Eclairage (CIE)
coordinates of (0.33, 0.33) at theoretical white point was first achieved by using AIE luminogen BTPE-PI as deep blue
emitter. Such molecular design strategy opens a new avenue in the development of efficient solid-state deep blue emitters
for non-dopedOLED applications.
attentions have been placed on fluorophores with this
INTRODUCTION
emission color. Even such emitters are prepared, it is
challenging to utilize them for fabricating OLEDs with
good performance as their wide band-gaps9 tend to create
large charge injection barrier and unbalanced charge
injection and transportation, leading to increased driving
voltage and lowered device efficiency.10 On the other
hand, a problem associated with most blue luminogenic
materials is the aggregation-caused quenching (ACQ) of
light emission in the aggregated state: their intense
emission in the solution state is weakened11 and red-
shifted to undesirable sky blue color when their
molecules are aggregated12 due to the severe
intermolecular interaction.13,14
Development of efficient luminescent materials with
primary RGB (red, green and blue) colors in the solid
state is critical for the applications in organic light-
emitting diodes (OLEDs).1 Among the primary RGB colors,
deep blue emission is important for full color displays and
organic white lighting.2,3 It can also offer a wide-gamut
RGB color space, which possesses high saturated and
"pure" light colors.4 On the other hand, it can be used as
an excitation source to activate other color emissions
through energy transfer.5 However, luminophores with
efficient deep blue emission are rare. Despite of their high
external quantum efficiency in electroluminescence (EL)
devices, organometallic complexes with deep blue
emissions are rarely observed because of the consumption
of the energy of the high level metal-ligand charge
transfer bond by the metal d orbitals through non-
Several strategies have been adopted to solve the
abovementioned problems. To improve the charge
balance in OLEDs and hence enhance the EL efficiency,
introduction of donor (D) and acceptor (A) units into the
luminogenic structure15,16 has been proved to be effective
radiative relaxation channel.68 Therefore, considerable
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Page 2 of 12
method because of the creation of the intramolecular
thermal and carrier injection properties.39,40 Last, covalent
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charge transfer (ICT) effect. However, the extent of ICT
effect should be well controlled as strong D-A pair tends
to induce excessive bathochromic shift and normally
combination of these two parts produce a luminogen,
where one TPE moiety (A) is connected to the phenyl ring
of PI at the para position to impart good conjugation and
hence improved charge transportation.41 The another TPE
unit (B) is linked laterally at the meta position to lower
the conjugation and hence shorten the emission
wavelength (Chart 1). This also endows the molecule a
twisted conformation, which can prevent undesirable
intermolecular interactionsin thesolidstate.
In this paper, we demonstrate a new molecular design
strategy and report the synthesis of an efficient deep blue
AIE luminogen constructed from triphenylethene and PI.
The synthesized luminogen, abbreviated as BTPE-PI, is
thermally stablewith balanced carrier injection properties.
Non-doped multilayer EL devices fabricated by using
BTPE-PI as emitting layer, showed deep blue EL with
external quantum efficiency of as high as 4.4% and a
small roll-off. In addition, efficient white EL device with
Commission Internationale de l’Eclairage (CIE)
coordinates of (0.33, 0.33) at the exact theoretical white
point was first achieved by employing AIE luminogen
(BTPE-PI) as the blue deep emitter. Such good results
indicated that such strategy on molecular design shed
some light on the development of efficient deep blue
luminescent materialsfor OLED applications.
17
impairs the emission color purity. On the other side, to
mitigate the ACQ effect, methods of cross dipole
18
stacking, J-aggregate formation19,20 and introduction of
11,2124
bulky substituents
have been employed. However,
these methods are far from satisfactory. For example, to
achieve a particular stacking mode, careful molecular
design and control on the luminogen aggregation are
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required. Integration of bulky alkyl groups2527 is often
accompanied with severe side effects, such as dilution of
chromophore concentration and obstruction of the
chargeinjection andtransportationin ELdevices.
We observed a phenomenon of aggregation-induced
emission (AIE) in some organic luminophores with
12,2830
twisted conformations.
These molecules are non-
emissive in dilute solutions because of their active
intramolecular motions, which consume the energy of the
excitons through non-radiative relaxation pathways.
Such motions, however, are restricted in the aggregated
state, which enables the excitons to decay radiatively. The
twisted conformations of the molecules also prevent the
formation of detrimental species such as exciplexes and
excimers by strong intermolecular interactions. All these
factors make the AIE luminogens to show strong light
emission in the aggregated state. Integration of AIE units
to conventional ACQ chromophores has been found to be
a good strategy to solve their ACQ problem and generates
new luminogens with high fluorescence quantum
efficiencies and inherited properties from both the parent
molecules.31,32 Such design strategy thus offers
a
formulated platform to generate functional luminescent
materialswith efficient solid-stateemissions.
Limited by the long conjugation length of some
typical AIE luminogens such as tertraphenylethene and
silole,33,34 it is difficult to achieve deep blue emission when
using them as building blocks. Few such emitters have
been prepared but their design involves complicated
control on their steric hindrance.35,36 On the other hand,
as these emitters generally possess highly twisted
structures, their EL performances are only moderate due
to the poor charge transportation. Thus, a simple and
direct method to tackle the above problem is to find an
AIE luminogen with efficient emission but low
conjugation. With such AIE unit, complicated design and
synthesis of luminogens with highly twisted structures or
severe steric hindrance are not needed and meanwhile a
much wider partnered functional units can be selected for
Chart 1. Chemical structuresof PI and BTPE-PI.
EXPERIMENTALSECTION
Materials. THF was distilled from sodium benzophenone
ketyl under dry nitrogen immediately prior to use.
Intermediates 542, 643, 744 and tetraphenylethene45 were
prepared according to the reported experimental
procedures. Other chemicals and reagents were
purchased from Aldrich and used as received without
further purification.
Characterization. 1H and 13C NMR spectra were
measured on a Bruker AV 400 spectrometer in CD2Cl2 or
CDCl3 using tetramethylsilane (TMS; δ = 0) as internal
reference. High resolution mass spectra (HRMS) were
recorded on a GCT premier CAB048 mass spectrometer
operating in MALDI-TOF mode. Elemental analysis was
operated on a ThermoFinnigan Flash EA1112 analyzer. UV
luminogen
construction.
Following
this
idea,
triphenylethene (TPE) is selected as the main molecular
skeleton. Although it possesses a low conjugation, it and
its derivatives show splendid AIE characteristics.37,38
Compared to tetraphenylethene in the solid state, its
emission is much bluer, exhibiting a hypsochromatic shift
in the emission maximum of as large as 25 nm (Figure S1
in the Supporting Information). Such property should
significantly improve the color purity of its derivatives.
On the other side, we utilized phenanthro[9,10-
d]imidazole (PI) as a deep blue building block for its good
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Chemistry of Materials
spectra were measured on a Milton Roy 5 Spectronic 3000
Array spectrophotometer. Photoluminescence spectra
were recorded on Perkin-Elmer LS 55
Preparation of BTPE-PI: Into a stirred mixture of BBr-PI
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(264 mg, 0.5 mmol), 4-(2,2-diphenylvinyl)phenylboronic
acid (600 mg, 2.0 mmol), Pd(PPh3)4 (57 mg) and K2CO3
(1.24 g, 9 mmol), 40 mL of THF and 5 mL of water was
added under nitrogen. The mixture was heated to 70 °C
for 24 h. After cooling to room temperature, the solution
was extracted with dichloromethane (150 mL × 2), washed
with water and dried over Na2SO4. After filtration and
solvent evaporation under reduced pressure, the product
was purified by silica-gel column chromatography using
hexane/dichloromethane as eluent. BTPE-PI was obtained
a
spectrofluorometer. The solution fluorescence quantum
yields were determined by using 9,10-diphenylanthracene
as standard (ΦF = 90% in cyclohexane). The absorbance of
the solutions was kept below 0.1 to avoid the internal
filter effect. The emission efficiencies of thin films of PI
and BTPE-PI were measured on a calibrated integrating
sphere. Their thermal stabilities were measured on TGA
Q5000 and DSC Q1000 instruments under nitrogen at a
heating rate of 20 °C/min. Cyclic voltammetry (CV)
experiments were carried out in dichloromethane
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in 54% yield (237 mg) as a white powder. H NMR (400
MHz, CD2Cl2, δ): 8.82–8.78 (m, 2H), 8.73 (d, J = 8.4 Hz,
1H), 7.86 (dd, J = 8.0, 1.2 Hz, 1H), 7.77–7.74 (m, 2H), 7.69–
7.65 (m, 4H), 7.54–7.49 (m, 4H), 7.40–7.27 (m, 22H), 7.24–
7.21 (m, 2H), 7.20–7.17 (m, 2H), 7.10–7.06 (m, 4H), 7.02 (s,
1H), 7.0 (s, 1H). 13C NMR (100 MHz, CD2Cl2, δ): 150.81,
143.70, 143.56, 143.35, 143.04, 140.92, 140.83, 140.70, 139.71,
138.45, 137.89, 137.81, 137.62, 137.36, 130.99, 130.67, 130.60,
130.51, 130.41, 129.99, 129.87, 129.49, 129.14, 128.69, 128.60,
128.42, 128.02, 127.93, 127.89, 127.70, 127.61, 127.50, 126.88,
126.83, 126.75, 126.69, 125.97, 125.31, 124.49, 123.60, 123.47,
122.90, 121.33. HRMS (MALDI-TOF), m/z: M+, calcd. for
C67H46N2, 878.3661; found, 878.3678. Anal. calcd (%) for
C67H46N2: C 91.54, H 5.27, N 3.19; found: C 91.44, H 5.37, N,
3.20.
solution
with
0.1
M
tetrabutylammonium
hexafluorophosphate as the supporting electrolyte at a
scan rate of 100 mV/s, by using Ag/AgNO as the working
3
electrode and saturated calomel electrode (SCE) as the
referenceelectrode.
Synthesis. Preparation of PI: The compound was
prepared according to the reported methods46 with a
slight modification. Into a stirred mixture of 9,10-
phenanthrenequinone (624 mg, 3 mmol), benzaldehyde
(318 mg, 3 mmol), aniline (1.4 g, 15 mmol) and ammonium
acetate (2.87 g, 37.2 mmol), 20 mL of glacial acetic acid
was added under nitrogen. The mixture was heated to
reflux for 12 h. After cooling to room temperature, a pale
yellow mixture was obtained and poured into a methanol
solution under stirring. The precipitates were filtered,
washed with methanol, and dried to give a pale yellow
solid. After filtration and solvent evaporation under
reduced pressure, the product was purified by silica-gel
column chromatography using hexane/dichloromethane
as eluent. It was further purified by stirring in reflux
ethanol. After subsequent filtration and drying in vacuum,
PI was obtained in 53.2% yield (591 mg) as a white powder.
1H NMR (400 MHz, CD2Cl2, δ): 8.79 (t, J = 8.0 Hz, 2H),
8.74 (d, J = 8.4 Hz, 1H), 7.85–7.83 (m, 1H), 7.76 (td, J = 8.0,
0.8 Hz, 1H), 7.72–7.67 (m, 2H), 7.59–7.46 (m, 7H), 7.35 (td,
J = 8.0, 0.8 Hz, 1H), 7.22 (d, J = 8.4 Hz, 1H). 13C NMR (100
MHz, CD2Cl2, δ): 150.01, 140.19, 137.80, 133.75, 132.45, 131.97,
131.91, 131.21, 129.72, 129.64, 128.69, 128.62, 128.39, 127.83,
127.45, 126.98, 126.22, 125.62, 124.59, 123.89, 123.84, 123.62,
123.13, 122.89, 121.19. HRMS (MALDI-TOF), m/z: M+, calcd.
for C27H18N2,370.1470; found, 370.1479.
Preparation of nanoaggregates. Stock THF solutions of
PI and BTPE-PI were prepared with a concentration of 10-4
M. Aliquots of the stock solutions were transferred to 10
mL volumetric flasks. After appropriate amounts of THF
were added, water was added dropwise under vigorous
stirring to prepare 10-5 M solutions with different water
contents (0–90 vol %). The PL spectra of the resultant
solutionswereimmediatelymeasured.
Device fabrication. The devices were fabricated on 80
nm ITO-coated glass with a sheet resistance of 25 Ω per
square. Before loading into the pretreatment chamber,
the ITO-coated glasses were soaked in ultrasonic
detergent for 0.5 h, followed by spraying with deionized
water for 10 min, soaking in ultrasonic deionizedwater for
0.5 h, and oven-baking for 1 h. The cleaned samples were
treated by fluoroform (CHF3) plasma with a power of 10
W, gas flow of 50 sccm, and pressure of 0.2 Torr for 10 s in
the pretreatment chamber. The solid samples were
transferred to the organic chamber with a base pressure
of 5 × 10–7 Torr for the deposition of NPB, BTPE-PI and
TPBi. The samples were then transferred to the metal
chamber for the deposition of cathode, which was
composed of lithium fluoride (LiF) capped with
aluminium (Al). The light-emitting area was 4 mm2 as
defined by the overlap of the cathode and anode. The
current density–voltage-luminance characteristic curves
Preparation of BBr-PI: The compound was prepared from
9,10-phenanthrenequinone (624 mg,
3
mmol), 4-
bromobenzaldehyde (555 mg, 3 mmol), 3-bromoaniline
(2.58 g, 15 mmol, 1.65 mL), ammonium acetate (2.87 g,
37.2 mmol) and 20 mL of glacial acetic acid by following
the same procedure described above, affording a white
1
solid in 51.0% yield (808 mg). H NMR (400 MHz, CD2Cl2,
δ): 8.79 (t, J = 8.0 Hz, 2H), 8.74 (d, J = 8.4 Hz, 1H), 7.85–
7.83 (m, 1H), 7.76 (td, J = 8.0, 0.8 Hz, 1H), 7.72–7.67 (m,
2H), 7.59–7.46 (m, 7H), 7.35 (td, J = 8.0, 0.8 Hz, 1H), 7.22
(d, J = 8.4 Hz, 1H). 13C NMR (100 MHz, CD2Cl2, δ): 150.01,
140.19, 137.80, 133.75, 132.45, 131.97, 131.91, 131.21, 129.72,
129.64, 128.69, 128.62, 128.39, 127.83, 127.45, 126.98, 126.22,
125.62, 124.59, 123.89, 123.84, 123.62, 123.13, 122.89, 121.19.
HRMS (MALDI-TOF), m/z: M+, calcd. for C27H16Br2N2,
527.9660; found, 527.9673.
of the devices were determined by
a HP4145B
semiconductor parameter analyzer and 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
measured by a PR650 spectrophotometer. All the
measurements were performed at room temperature
under air without deviceencapsulation.
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RESULTS AND DISCUSSION
THF solution (Figures 2A and 2B). Since PI is insoluble in
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water, its molecules should form aggregates in solvent
mixtures with high water contents. Clearly, like most
conventional chromophores, the PL of PI is quenched by
aggregate formation. Similar phenomenon was also
observed upon solution thickening. The emission was
significantly quenched when the solution concentration
was increased from 10 μM to 1 mM (Figure S5). On the
other hand, attachment of TPE units to PI has endowed
the resulting adduct (BTPE-PI) with AIE attribute. As
shown in Figures 2C and 2D, the peak intensity of BTPE-
PI was slightly weakened upon addition of a small
amount of water (fw ≤ 50 vol %). Afterwards, the emission
becomes stronger swiftly. The maximum emission
enhancement was observed at fw = 90%, being ~5-fold
higher than that in pure THF solution. Evidently, BTPE-PI
is AIE-active. Quantitative measurements on their
fluorescence quantum yields (ΦF) in solution and solid
states also draw the same conclusion. The ΦF of PI in pure
THF solution is 58.8%, which drops to 38.2% in the thin
film state accompanied with a ~30 nm red-shift in
emission because of strong intermolecular interactions
(Table 1). In contrast, the thin film of BTPE-PI emits at a
spectral region similar to that in the solution state but
exhibits a much higher ΦF (93.8% vs 14.5%). Thus, this
highly blue emissive luminogen is a promising candidate
for thefabrication ofefficient ELdevice.
Synthesis and Characterization. BTPE-PI and PI were
prepared facilely according to the synthetic routes shown
in Scheme S1. The key intermediate BBr-PI was prepared
from 9,10-phenanthrenequinone (1) according to
previously published synthetic procedures.46 BTPE-PI was
obtained by Suzuki coupling of BBr-PI and 4-(2,2-
diphenylvinyl)phenylboronic acid (7) using Pd(PPh3)4 as a
catalyst under basic conditions. All the intermediates and
product were characterized by standard spectroscopic
techniques, which gave satisfactory analysis data
corresponding to their chemical structures and
demonstrated their high purity. Detailed synthetic
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procedures and examples of H NMR and mass spectra
wereshown in FiguresS2 and S3.
B
A
Toluene
DCM
THF
Toluene
DCM
THF
EA
DMF
EA
DMF
320
420
520
620 350
450
550
650
Wavelength (nm)
Wavelength (nm)
Figure 1. Normalized PL spectra of (A) PI and (B) BTPE-
PI in different solvents. Excitation wavelength (nm): 312
(PI) and 340 (BTPE-PI).
2.0
0-80
fw (vol %)
A
B
90
80
70
60
50
40
30
20
10
0
1.5
1.0
0.5
0.0
Optical properties. BTPE-PI possesses a good solubility
in common organic solvents, such as dichloromethane,
chloroform and tetrahydrofuran (THF), but is insoluble in
water. The UV spectra of PI and BTPE-PI measured in
THF are peaked at 312 and 340 nm, respectively (Figure
S4). The associated molar absorptivity (7.2 × 104 L mol−1
cm−1) for BTPE-PI is 3-fold higher than that of PI due to
its more conjugated structure. In THF solution, PI emits a
strong purple photoluminescence (PL) at 370 and 387 nm,
while the PL spectrum of BTPE-PI was centered in the
blue region at ~449 nm. The PL spectra of PI and BTPE-PI
measured in different solvents are shown in Figure 1.
When the solvent is changed from nonpolar toluene to
polar N,N-dimethylformamide, the emission maximum
red-shifts but in a small extent (~4 nm for PI and ~6 nm
for BTPE-PI). Under similar conditions, the emission of 4-
(N,N-dimethylamino)benzonitrile, a typical strong D-A
molecule, shifts bathochromically by ~100 nm.47 This
suggests that both PI and BTPE-PI are insensitive to
solvent polarity and show a weak D-A interaction, which
is beneficial for the improvement of charge balance in
OLEDs but will not cause excessive emission red-shift.
Their PL properties are also studied in THF/water
mixtures with different water fractions (fw) with a view to
study the effects of solvent polarity and aggregation on
their light emission process. When a small amount of
water (fw <70 vol %) was added to the THF solution of PI,
its PL intensity was enhanced slightly. Further increment
of thewater content, however, weakens the light emission.
At fw = 90%, the PL intensity is merely 50% of that in pure
90
330
380
430
480
530
0
20
40
60
80
100
Wavelength (nm)
Water fraction (vol %)
6
4
2
0
90
C
D
fw (vol %)
90
70
60
50
10
0
70
60
0
10
50
450
350
550
650
0
20
40
60
80
100
Wavelength (nm)
Water fraction (vol %)
Figure 2. PL spectra of (A) PI and (C) BTPE-PI in THF/water
mixtures with different water fractions (fw). Plots of relative
PL intensity (I/I0) versus the composition of THF/water
mixtures of (B) PI and (D) BTPE-PI. I0 = emission intensity in
pure THF solution. Concentration: 10 μM; excitation
wavelength (nm): 312 (PI) and 340 (BTPE-PI).
Electronic properties. To get a further insight into the
photophysical properties of PI and BTPE-PI at the
molecular level, density functional theory calculations are
carried out using a suite of Gaussian 03 program. The
nonlocal density functional of B3LYP with 6-31G (d) basis
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Chemistry of Materials
sets was used for the calculation. The highest occupied gap energy (Eg) estimated from the onset absorption
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molecular orbital (HOMO) of BTPE-PI was dominated by
the orbitals from the PI core and TPE (A) unit on the
same axis. The electron cloud of the lowest unoccupied
molecular orbital (LUMO), however, was located mainly
on the PI core and the lateral TPE (B) unit (Figure S6).
Such electron distribution imparts BTPE-PI an intrinsic
intramolecular charge transfer property. The dipole
moment of BTPE-PI is small and equal to 5.29, which is
indicative of a weak D-A property and is consistent with
the experimental data given in Figure 1. The optimized
structures of PI and BTPE-PI are shown in Figure S7 for
comparison. PI adopts an almost planar conformation,
which favors strong intermolecular interactions that lead
to fluorescence quenching. On the contrary, BTPE-PI is
non-planar. The torsion angles between the imidazole
plane (a) and the adjacent phenyl rings are 18.6° (phenyl
ring b) and 88.9° (phenyl ring f). In such case, the PI core
is well-conjugated with the TPE (A) unit that lies on the
same axis but shows a poor electronic communication
with the lateral one (B). The dihedral angles between the
PI unit and the TPE moieties are both ~35°, while those
between any phenyl ring and the olefin core of the TPE
unit fall in the range from 22.7° to 64.9° (Table S1). The
non-planar conformation of BTPE-PI is mainly attributed
to the propeller-shaped TPE units, which disfavor the
close molecular packing in the solid state. Coupled with
the restriction of intramolecular motions in the
aggregated state, all these made BTPE-PI an efficient
solid-stateemitter.
wavelength from the HOMO value and is calculated to be
−2.45 eV. This value is lower than that ofPI (−2.16 eV) but
close to that of 1,3,5-tri(phenyl-2-benzimidazolyl)-
benzene (TPBi, −2.7 eV), which is widely used as an
electron-injection layer in OLEDs.49 Clearly, the electron
injection ability of BTPE-PI is similar to TPBi but better
than PI. This also suggests a good carrier injection
property in BTPE-PI due to its suitable HOMO and
LUMO energy levels. This property will enable the
fabrication of devices with simple configurations, which is
an advantagefor practical OLED applications.
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Td = 262 o
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Tg = 145 o
C
BTPE-PI
PI
BTPE-PI
PI
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350
500
650
800 50
100
150
200
Temperature (oC)
Temperature (oC)
Figure 3. (A) TGA and (B) DSC (second heating cycle)
thermograms of PI and BTPE-PI recorded under nitrogen
at a heating rateof20o C/min.
105
1000
A
B
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100
10-1
10-2
Device I
Device II
Solid film
800
600
400
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0
Thermal properties. The thermal properties of PI and
BTPE-PI are examined by thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC)
measurements (Figure 3 and Table 1). BTPE-PI is
thermally very stable, exhibiting a 5% weight loss at
460 °C. Such value is much higher than that of PI (Td =
262 °C) (Figure 3A). The glass transition temperature (Tg)
of BTPE-PI was detected at 145 °C, while such thermal
transition was not detected in PI (Figure 3B). No
transition peak associated with crystallization was
observed when BTPE-PI was heated up to 200 °C. The
non-planar TPE units in BTPE-PI hinder close packing,
thus making the luminogen non-crystalline or amorphous
in nature. Luminogens with high thermal and
morphological stabilities are beneficial for the formation
of amorphous thin films by vacuum deposition, which is
important for thestabilityofOLEDs.48
Device I
Device II
350
450
550
650
750
2
5
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17
Voltage (V)
Wavelength (nm)
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Device II
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Device II
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Current density (mA/cm2)
Current density (mA/cm2)
Figure 4. (A) PL and EL spectra of solid thin film of BTPE-PI.
(B) Current density–voltage–luminance characteristics of
multilayer EL devices of BTPE-PI. Changes in (C) current
efficiency and power efficiency and (D) external quantum
efficiency with the applied current density in multilayer EL
devices of BTPE-PI. Device configuration: ITO/NPB/BTPE-
PI/TPBi/LiF/Al.
Electrochemical properties. The electrochemical
properties as well as energy levels of BTPE-PI and PI are
investigated by cyclic voltammetry (FigureS8 and Table 1).
The HOMO of BTPE-PI is estimated from its oxidation
onset potential and is calculated to be −5.50 eV. This
suggests a good hole injection property due to the fine
contribution from the PI unit (EHOMO = −5.48 eV). Its
LUMO can be obtained by addition of the optical band
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Chemistry of Materials
Table 1. Opticaland thermalproperties of PI and BTPE-PIa)
Page 6 of 12
1
2
λem
3
4
5
6
7
8
9
λab
EHOMO
ELUMO
Eg
[nm]
Td/Tg
Compound
PI
[nm]
[eV]
[eV]
[eV]
[°C]
soln (ΦF,s)
aggr
film (ΦF,f)
312, 344, 362
340
–5.48
–5.50
–2.16
–2.45
3.32
3.05
370, 387 (58.8)
449 (14.5)
376, 391
456
397, 415 (38.2)
455 (93.8)
262/nd
460/145
BTPE-PI
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a)
Abbreviation: λab = absorption maximum in THF, EHOMO = highest occupied molecular orbitals calculated from the onset
oxidation potential, Eg = energy band gap calculated from the onset of the absorption spectrum, ELUMO = lowest unoccupied
molecular orbitals estimated by using the equation: ELUMO = EHOMO + Eg, λem = emission maximum in THF solution (soln),
THF/water mixture (1:9 by volume) (aggr) and solid thin film spin-coated from THF solution with fluorescence quantum yield
(ΦF) given in the parentheses, Td = temperature for 5% weight loss, Tg = glass transition temperature, nd = not detected.
Table 2. EL performances of BTPE-PIa)
Von
Lmax
CIE
EL
C,max
[cd/A]
P,max
ext,max
[%]
Device
[V]
[cd/m2]
(x, y)
[nm]
463
[lm/W]
5.3 (2.5)
4.4 (3.1)
8.1 (5.3)
I
3.2
3.2
3.2
20300
16400
19200
5.9 (4.6)
4.9 (3.7)
10.7 (9.8)
4.4 (3.4)
4.0 (3.0)
6.4 (5.9)
0.15, 0.15
0.15, 0.12
0.33, 0.33
II
450
III
450, 511, 618, 674
a)
Abbreviation: EL = EL maximum, Von = turn-on voltage at 1 cd/m2, Lmax = maximum luminance, P,max = maximum power
efficiency, C,max = maximum current efficiency and ext,max = maximum external quantum efficiency. The values in the
parentheses are taken at a luminance of 1000 cd/m2. CIE (x, y) = Commission Internationale de l’Eclairage chromaticity
coordinates collected at 8 V. Device configuration: ITO/NPB(60 nm)/BTPE-PI(20 nm)/TPBi(40 nm)/LiF(1 nm)/Al(100 nm)
(Device I), ITO/NPB(40 nm)/BTPE-PI(20 nm)/TPBi(40 nm)/LiF(1 nm)/Al(100 nm) (Device II) and ITO/NPB(40
nm)/Ir(btp)2(acac)(6 wt %):CBP(10 nm)/Ir(ppy)3(8 wt %):CBP(4 nm)/CBP(3 nm)/BTPE-PI(5 nm)/TPBi(40 nm)/LiF/Al (Device
III).
Electroluminescence. Non-doped deep blue OLED. The
efficient solid-state deep blue emission of BTPE-PI as well
as its high thermal stability and suitable LUMO and
HOMO levels prompts us to investigate its EL properties.
Most deep blue EL devices reported previously involvethe
use of dopants. However, to achieve an optimized device
performance, careful control on the dopant concentration
is required.50,51 On the other hand, performance
degradation due to phase separation upon heating is also
a problem encountered in these devices.5254 Nevertheless,
AIE luminogens are free from the ACQ effect and thus
require no dopant for EL device fabrication. With such
regard, we thus fabricated a non-doped OLED devicewith
a simple configuration of ITO/NPB(60 nm)/BTPE-PI(20
nm)/TPBi(40 nm)/LiF(1 nm)/Al(100 nm) (Device I). In
this device, BTPE-PI works as blue emitting layer (EML),
N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-
diamine (NPB) functions as a hole-transporting layer
(HTL) and TPBi serves as both electron-transporting (ETL)
and hole-blocking layer. The EL spectrum of device I was
peaked at 463 nm, whose patten resembles the PL
spectrum of the solid thin film. This confirms that the EL
is indeed from the emitting layer. The device turns on at a
very low bias voltage of 3.2 V and emits brilliant blue light
determined to be 4.4%, which is rather high and close to
the theoretical limit (5%) for fluorescent materials.
Driven at 1000 cd/m2, the device also performs well with
high ext of 3.4%. In addition, the roll-off is small, which
drops merely 14.4% when the device brightness is
increased from 1000 cd/m2 to 5000 cd/m2. The CIE
coordinates are calculated to be (0.149, 0.147), which fall
in the deep blue region (y <0.15)17, 5557(Figures 5A and 5D).
with maximum luminance (Lmax
)
of 20300 cd/m2,
Figure 5. Photos of (A) Device I, (B) Device II and (C) Device
III. (D) Commission Internationale de l’Eclairage (CIE)
chromaticity coordinates of the three devices.
maximum current efficiency (C,max) and power efficiency
(P,max) of 5.9 cd/A, 5.3 lm/W, respectively (Figure 4 and
Table 2). The external quantum efficiency (ext,max) is
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Chemistry of Materials
deep blue emitters reported previously.17,23,35,36,46,6172 The
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EL devices fabricated from BTPE-PI with standard three-
component active layers (HTL/EML/ETL) exhibit really
high performance and energy conversion as reflected by
their high external quantum and power efficiencies. The
maximum luminance (> 15000 cd/m2) attained by them
are one order of magnitude higher than the previous
results. Some EL devices with excellent ext,max (>5%) have
been reported62,63,71,72 but their device structures are
complicated and involve four-component active layers.
When driven at 1000 cd/m2 for practical applications,
most of these devices show an obvious efficiency roll-off.
The BTPE-PI based EL devices, however, still retain very
high performances with efficiencies of up to 3.1% and 3.0
lm/W. Such results are impressive and are the best among
thedeep blue non-dopedOLEDs.
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10V
12V
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B
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Wavelength (nm)
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D
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Figure 6. Energy level diagrams and device configurations of
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BTPE-PI based (A) deep blue (B) white OLEDs.
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400
500
The excellent EL performance of BTPE-PI is somewhat
related to its good charge injection property as well as its
weak D-A system. To get a further insight, the energy
level diagrams and device configurations of the EL devices
are illustrated in Figure 6A. Thanks to the suitable energy
levels of BTPE-PI, the electron injection barrier at the
TPBi/BTPE-PI junction is only 0.25 eV, while the hole
injection barrier is also small (0.20 eV) at the NPB/BTPE-
PI junction. These small injection barriers enable efficient
injection of both holes and electrons into the emitting
layer at low bias. Moreover, the TPBi layer effectively
blocks the leakage of holes from the emitting layer. These
well matched energy levels are beneficial for charge
balance and efficient exciton recombination at the
emitting layer, thus improving the EL efficiency. By fine
adjustment of the thickness of the hole-transporting layer,
we can shift the EL spectrum to the shorter wavelength
Current density (mA/cm2)
Current density (mA/cm2)
Figure 7. (A) EL spectra of a multilayer WOLED at different
applied voltages. (B) Current efficiency–voltage–luminance
characteristics of a multilayer WOLED. Changes in (C)
current efficiency and power efficiency and (D) external
quantum efficiency with the applied current density in a
multilayer
WOLED.
Device
configuration:
ITO/NPB/Ir(btp)2(acac):CBP/Ir(ppy)3:CBP/CBP/BTPE-
PI/TPBi/LiF/Al.
White light OLED. One of the most important
applications for deep blue emitters is the fabrication of
white OLED (WOLED).73,74 Many WOLEDs containing
primary RGB emitters were fabricated, showing unique
advantages of high stability of blue fluorophores and high
efficiency of phosphors with longer emission wavelength.
Their multilayer structure allows flexible manipulation of
each EML as well as the precise control on the exciton
distribution in different EMLs. We fabricated a white EL
region due to the interference effect.5860 Device II with a
configuration
nm)/TPBi(40 nm)/LiF(1 nm)/Al(100 nm) was also
fabricated, which exhibits a bluer light than Device I with
CIE coordinates of (0.15, 0.12) (Figures 5B, 5D and 6A).
of
ITO/NPB(40
nm)/BTPE-PI(20
device
with
a
configuration
of
ITO/NPB(40
nm)/Ir(btp)2(acac)(6 wt %):CBP(10 nm)/Ir(ppy)3(8
wt %):CBP(4 nm)/mCP(3 nm)/BTPE-PI(5 nm)/TPBi(40
nm)/LiF/Al, where BTPE-PI functions as blue emitting
The device shows high performance with Lmax, C,max
,

P,max and ext,max of 16400 cd/m2, 4.9 cd/A, 4.4 lm/W and
layer,
bis[2-(2'-benzothienyl)pyridinato-
4.0%, respectively (Table 2 and Figure 4). Table S2
compares the EL performances of BTPE-PI with those of
N,C3'](acetylacetonato)iridium(III) Ir(btp)2(acac) and
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Page 8 of 12
tris[2-phenylpyridinato-C2,N]iridium(III) Ir(ppy)3 work as integration of a weakly conjugated AIE luminogen into a
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red and green emitters, respectively, and 4,4'-bis(9H-
carbazol-9-yl)biphenyl CBP was used as host to reduce
the structural heterogeneity and facilitate charge
transport between the adjacent red and green emitting
layers. A 3 nm thickness of 1,3-bis(carbazol-9-yl)benzene
(mCP) buffer layer is inserted between the fluorescent
emitter (BTPE-PI) and the phosphorescent emitter
Ir(ppy)3 to confine the singlet excitons and their
recombination in the BTPE-PI region. This configuration
is expected to harvest both single and triplet excitons.75
The device emits intense white light (Lmax =17900 cd/m2)
through the simultaneous emission from the primary RGB
conventional chromophore. The resulting luminogen
BTPE-PI shows a weak D-A interaction but a typical AIE
characteristic and improved solid-state fluorescence
quantum yield than its parent molecules. It also enjoys
high thermal stability and good carrier injection
capability. Non-doped multilayer EL device fabricated
from BTPE-PI shows a small roll-off and emits a deep blue
emission with high luminance, current efficiency, power
efficiency and external quantum efficiency of 20300 cd/m2,
5.9 cd/A, 5.3 lm/W and 4.4%, respectively. These results
are the best for deep blue AIE emitters reported so far. In
addition, for the first time, an efficient AIE-based
WOLED using BTPE-PI as deep blue emitter was
fabricated and realized pure white light with CIE
coordinates of (0.33, 0.33). It is anticipated that by
following the molecular design strategy present here,
many new efficient deep blue AIE luminophores will be
rationallydesignedandsynthesizedin thefuture.
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emitters and shows excellent performance with C,max
,
P,max and ext,max of, 12.3 cd/A, 9.5 lm/W and 7.3%,
respectively (Figure S9). The CIE coordinates are
calculated to be (0.31, 0.32) at 8 V, which are slightly
deviated from the values of the theoretical white point
(0.33, 0.33)74 due to insufficient green and red
components in the emission spectrum. To further
improve the white color purity, we optimized the device
configuration and chose CBP (ELUMO = −2.8 eV) as the
buffer layer, whose LUMO energy level is 0.4 eV lower
than that of mCP (ELUMO = −2.4 eV). This buffer layer
should facilitate more electrons and thus more
recombination in the green and red emission layers due
to no injection barrier from the buffer layer to the green
and red emitting layers (Figure 6B). Thus, another
WOLED device with a configuration of ITO/NPB(40
nm)/Ir(btp)2(acac)(6 wt %):CBP(10 nm)/Ir(ppy)3(8
wt %):CBP(4 nm)/CBP(3 nm)/BTPE-PI(5 nm)/TPBi(40
nm)/LiF/Al (Device III) was constructed. The device
starts to emit at 3.2 V, showing similar intense white
*Ben Zhong Tang, E-mail: tangbenz@ust.hk.
┴Both authors contributed equally to this work.
This work was partially supported by the National Basic
Research Program of China (973 Program, 2013CB834701), the
University Grants Committee of Hong Kong (AoE/P-03/08)
the Research Grants Council of Hong Kong (604913, 16301614,
16305014 and N_HKUST1604114), and the Innovation and
Technology Commission (ITCPD/17-9). We thank the
support of the Guangdong Innovative Research Team
Program (201101C0105067115). We thank Professor Kam Sing
Wong in Department of Physics in HKUST for the
measurement of fluorescence quantum yields of solid powder
samples.
emission with Lmax, C,max, P,max, andext,max of 19200 cd/m2,
10.7 cd/A, 8.1 lm/W and 6.4%, respectively (Figure 7 and
Table 2). Even at 1000 cd/m2,high C, P andext values of
9.8 cd/A, 5.3 lm/W and 5.9%, respectively, are still
achieved. More importantly, the CIE coordinates of this
device exactly match those of the theoretical white point
(0.33, 0.33) at 8 V (Figures 5C and 5D). To the best of our
knowledge, it is the first WOLED device with intense
emission at theoretical white point fabricated from deep
blue AIE fluorophore. In addition, the device shows
impressive color stability with CIE coordinates of (0.31,
0.32) even at high voltages of 10 V and 12 V (Figure 7A).
This suggests that the hole-electron recombination zone
is well confined in the emitting layers in a wide range of
working voltage. It is noteworthy that all the EL curves in
device I, II and III are smooth, thanks to the high stability
of the devices at different applied voltages. Thus, the
molecular design strategy present here is simple but
successful. It makes full use of the intrinsic advantages of
both TPE and conventional chromophores to create deep
blue AIE luminogens with efficient solid-state emission
and prominent thermal stabilities for high performance
OLED applications.
NMR, MS, UV, PL, CV spectra, and molecular orbital
quantum calculation of PI or BTPE-PI, EL performance of
WOLED device. This material is available free of charge via
the Internet at http://pubs.acs.org.
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Construction of Efficient Deep Blue AIE Luminogen from Triphenylethene for Non-Doped OLED
Applications
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