A combinational molecular design to achieve highly efficient deep-blue electrofluorescence

Article abstract of DOI:10.1039/c7tc04685e

A deep-blue emitter 1-(10-(4-methoxyphenyl)anthracen-9-yl)-4-(10-(4-cyanophenyl)anthracen-9-yl)tetraphenylethene (TPEA) has been successfully prepared by a combinational molecular design, which contains triplet-triplet fusion (TTF) and hybridized local ch

Full text of DOI:10.1039/c7tc04685e

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10.1039/C7TC04685E.
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ISSN 2050-7526
PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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DOI: 10.1039/C7TC04685E
A Combinational Molecular Design to Achieve Highly Efficient
Deep Blue Electrofluorescence
Received 00th January 20xx,
Accepted 00th January 20xx
Mengying Biana, Zifeng Zhaob, Yu Lia, Qing Lic, Zhijian Chena,d,e, Dongdong Zhangf, Shufeng
Wanga,*, Zuqiang Bianb, Zhiwei Liub, Lian Duanf,*, Lixin Xiaoa,d,e,
*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
deep blue emitter 1ꢀ(10ꢀ(4ꢀmethoxyphenyl)anthracenꢀ9ꢀyl)ꢀ4ꢀ(10ꢀ(4ꢀcyanophenyl)anthracenꢀ9ꢀyl)tetraphenylethene
(TPEA), has been successfully prepared by a combinational molecular design, which contains tripletꢀtriplet fusion (TTF)
and hybridized local charge transfer (HLCT) characteristics to increase the ratio of triplet excitons used. Besides, the
tetraphenylethene (TPE) moiety contributes the emitter with aggregation induced emission (AIE) property to enhance solidꢀ
state luminescence efficiency. The crystallographic structure shows that the two anthracene groups are twisted from the
central TPE moiety, which effectively prevents bathochromic shift of emission. In addition, we adopted donorꢀacceptor (Dꢀ
A) structure to improve the charge balance in organic lightꢀemitting diodes (OLEDs). The material possesses high thermal
stability with a glass transition temperature (Tg) of 155 °C. Based on all these advantages, the high performance of the nonꢀ
doped device was achieved with a turnꢀon voltage (Von) of 2.6 V at a luminance of 1 cd m−2, a maximum power efficiency
(
η
PE, max) of 11.1 lm W−1, a maximum current efficiency (
1000 cd m−2. Moreover, a deepꢀblue emission with Commission Internationale de l′Éclairage (CIE) coordinates of (0.15,
0.09), a maximum external quantum efficiency (
ext, max) of 8.0% and the highest ηPE, max of 7.3 lm W−1 among all the TTF
η
CE, max) of 9.9 cd A−1, and a low current efficiency rollꢀoff even at
η
and HLCT deepꢀblue emitters was obtained by doping TPEA into the host of bisꢀ4ꢀ[(Nꢀcarbazolyl)phenyl]ꢀphenylphosphine
oxide (BCPO). These results indicate that combinational molecular design is a promising way for highly efficient deep blue
emitters
.
intersystem crossing (RISC), it remains difficult to develop
numerous efficient blue materials with a suitably wide gap. Besides,
HLCT is a different way to use RISC, which possesses a large
transition moment from a local excited (LE) state and a weakly
bound exciton from a charge transfer (CT) state.10ꢀ12 In addition,
TTF is also a favourable approach to use triplet excitons, in which
the interaction between two triplet states leads to the production of
one singlet excited state and one ground state when 2ET1 (the first
triplet state energy) > ES1 (the first singlet state energy).13 Up to now,
the practical blue emitters still rely on TTF materials.14ꢀ16 In 2014,
Cheng et al. reported PCzSP with a maximum power efficiency (ηPE,
max) of 7.2 lm Wꢀ1, which is the highest TTF deep blue OLED as far
as we know.17 In 2017, You et al. reported 2ꢀ(2′ꢀ
hydroxyphenyl)oxazoles based material which realized the highest
Introduction
Although organic lightꢀemitting diodes (OLEDs) have been
commercialized in displays and solidꢀstate lightings, it is still
difficult to achieve blue emission with high efficiency and colour
purity simultaneously. Deep blue emitters with high efficiency were
still scarce.1ꢀ6 Commercial blue emitters exhibited lower efficiency
compared to red and green counterparts, suggesting that blue
emitters consume the highest proportion of energy in practical
applications, which prompts us to develop blue materials with high
power efficiency (ηPE) to reduce energy consumption, i.e., to
develop high internal quantum efficiency (
with improved charge mobility.
To achieve this, at first, the triplet energy should be used to
break through the limit of 25% ηint for singlet spin states. Although
100% ηint of phosphorescent device can be obtained, noble metal
should be used and highly efficient deep blue is still a challenge.7ꢀ9
Recently, new mechanisms, such as thermally activated delayed
fluorescence (TADF), hybridized local charge transfer (HLCT) and
triplet−triplet fusion (TTF), were used to utilize triplet excitons in
ηint) deep blue emitter
ηPE,
of 4.9 lm Wꢀ1 among HLCT deep blue OLEDs to our
max
knowledge.18
Secondly, luminescence quenching should be avoided.
Aggregationꢀcaused quenching (ACQ) often occurs in solidꢀstate
emitters due to the formation of excimers.4, 19, 20 In 2001, Tang et al.
reported an intriguing phenomenon of aggregationꢀinduced emission
(AIE) because of the restriction of intramolecular rotation (RIR).21, 22
In 2016, Tang et al. utilized 9,10ꢀbis[4ꢀ(1,2,2ꢀtriphenylvinyl)phenyl]
anthracene based (BTPEAn) as the blue emitting layer to fabricate a
order to increase
ηint for fluorescent emitters. Though a maximum
ηint of 100% can be realized via the TADF process by using reverse
nonꢀdoped device with a η
PE, max of 4.3 lm Wꢀ1 and a low rollꢀoff.23
In the present, we adopt a combinational molecular design to
integrate delayed triplet emission of TTF, HLCT (TTFꢀHLCT) and
AIE effect for deep blue emitter. Tetraphenylethene (TPE) is chosen
as the AIE moiety, and anthracene core contributes to TTF
characteristic.24ꢀ26 In addition, anthracene possesses the effect of
energy misalignment that its ET2 (the second triplet state energy) lies
close to lowꢀlying singlet energy ES1, which is in line with the basic
ideas of HLCT materials’ design.27 Moreover, anthracene moiety
also possesses high electron transporting property, which is benefit
28
to getting high ηPE. However, the emissions of anthracene
derivatives would be redꢀshifted dramatically due to aggregation
quenching.29 Thus, to acquire deep blue emission, we employ the
twisted structure of TPE and bring in the vertical structure formed by

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Journal Name
anthracenyl and phenyl group to decrease the πꢀconjugation.30ꢀ32 In donating group and the electronꢀaccepting group are sufficiently
addition, we utilize donorꢀacceptor (DꢀA) structure by using weak weak that they do not produce a strong ICT state. Moreover, we
donor unit methoxy and acceptor unit cyano to endow the emitter could not observe phosphorescence in TPEA solution at a low
with improved charge balance as well as less bathochromic shift temperature of 77 K.
caused by weak intramolecular charge transfer (ICT) as reported by
Kido et al.33, 34 Therefore, we synthesized a blue fluorescent 1ꢀ(10ꢀ
(4ꢀmethoxyphenyl)anthracenꢀ9ꢀyl)ꢀ4ꢀ(10ꢀ(4ꢀcyanophenyl)
Scheme 1. Synthesis routes for compound TPEA.
anthracenꢀ9ꢀyl)tetraphenylethene (TPEA) with a glass transition
temperature (Tg) of 155 ºC which directly manifests its good thermal
stability. The crystallographic structure shows that the anthracene
groups are twisted from the central TPE moiety, which effectively
prevents bathochromic shift of emission. As expected, the
photoluminescence quantum yield
enhanced from 0.04 (in tetrahydrofuran, THF) to 0.60 (as nonꢀdoped
film). When wt.% of TPEA was doped into bisꢀ4ꢀ[(Nꢀ
(ηPL) of TPEA is greatly
4
carbazolyl)phenyl]ꢀphenylphosphine oxide (BCPO), the ηPL of
TPEA could even reach 0.81. Nonꢀdoped blue OLED with TPEA
exhibited extremely low turnꢀon voltage (Von) of 2.6 V at a
luminance of 1 cd m−2, with a ηPE, max of 11.1 lm W−1, a maximum
current efficiency (η
CE, max) of 9.9 cd A−1 and maintained 8.8 cd A−1
at a luminance of 1000 cd mꢀ2. The doped device showed deepꢀblue
emission with a peak value of 445 nm corresponding to Commission
Internationale de l′Éclairage (CIE) coordinates of (0.15, 0.09),
achieving a maximum external quantum efficiency (ηext, max) of
8.0%, and the highest ηPE, max of 7.3 lm W−1 among all the TTF and
18
HLCT deepꢀblue devices.17,
These results indicate that it is a
promising way of integrating TTFꢀHLCT and AIE for highly
efficient deep blue emitters.
Results and discussion
Synthesis and characterization
TPEA was synthesized from 4ꢀbromobenzoic acid via 11 steps as
shown in Scheme 1.
TPE was generated from the nucleophilic substitution of
benzophenone which was synthesized by FriedelꢀCrafts acylation
utilizing AlCl3 as catalyst. Anthracene was linked to TPE through
coupling reaction utilizing bis(pinacolato)diboron and Pd catalysts.
Donor unit methoxy and acceptor unit cyano were brought in via
Suzuki coupling catalyzed by Pd catalysts, realizing DꢀA structure.
These compounds have been fully characterized by nuclear magnetic
resonance (1H NMR and 13C NMR). TPEA has also been
characterized by crystal structure analysis, electrospray ionization
mass spectra (ESIꢀMS) and elemental analyses. TPEA was
sublimated before using.
Figure 1. (a) UVꢀvis absorption of TPEA in hexane, CH2Cl2, THF
and PL spectra of TPEA in hexane, CH2Cl2, THF and neat film; (b)
PL spectra of TPEA in THF/water (H2O) mixtures with different
H2O fractions (fw).
Photophysical properties
The solution and film ultraviolet visible (UV–vis) absorption and PL
emission spectra of TPEA are shown in Figure 1a. The absorption
spectrum in THF exhibits three peaks at 356 nm, 375 nm and 395
nm, respectively, and tailing to 413 nm, corresponding to a optical
band gap (Eop) of 3.0 eV, which are ascribed to the vibrational
levels of the anthracene group.35 It is obvious that no peak of ICT is
presented in the absorption spectrum. The PL spectra in hexane,
dichloromethane (CH2Cl2), THF and film show almost identical blue
emissions, and the bathochromic shift of the emission peak obtained
in THF compared with that obtained in nonpolar hexane is only 5
nm, which demonstrates that the interaction between the electronꢀ
To study the AIE characteristic of TPEA, we chose H2O and
THF as the solvent pair for their miscibility, which enabled fine
tuning of the solvent polarity and the extent of solute aggregation.
Figure 1b shows the PL spectra of the emitter in THF/H2O mixture
with different fw. It is clearly seen that, when molecules were readily
dissolved in pure THF solution, the emission was very faint, which
is attributed to the free rotation of the four phenyl rings that provides
a relaxation channel for the excited state to decay. With the gradual
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addition of H2O, the PL intensities of TPEA remained low in S1, which corresponds to HOMO→LUMO+1 and HOMOꢀ
aqueous mixtures with water content less than 50%, above which 1→LUMO. The LE state transition of TPEA makes it maintain a
they increased swiftly. When the water content reached 90%,
emission intensity rose conspicuously and the bathochromic shift of
the emission peak (compared with pure THF) was 10 nm, which is
comparable to the emission in the solid state.
high Eop and ηPL, and its suitable HOMO and LUMO energy levels
are also conducive to the charge injection.37, 38
Although the ηPL of TEPA in THF is only 0.04, it is greatly
enhanced in the solid state. TPEA showed a
ηPL of 0.81 in BCPO
host with 4 wt.% doping concentration and 0.60 in nonꢀdoped films,
which is beneficial to good performance both in doped and nonꢀ
doped OLEDs.
Electrochemical properties
Cyclic voltammetry (CV) was used to determine the relative energy
levels of TPEA. On the basis of the onset potential for oxidation, we
estimated the highest occupied molecular orbital (HOMO) energy
level with regard to ferrocene (ꢀ4.8 eV below vacuum) (shown in
Figure 2). The HOMO level of TPEA was found to be ꢀ5.7 eV by
using an empirical formula (EHOMO = ꢀ (Eox ꢀ EFc/Fc+) eV + (ꢀ4.8)
eV).31 HOMO energy level of TPEA measured by atmospheric
ultraviolet photoelectron spectroscopy is ꢀ5.8 eV, almost the same
with the result of CV, which is correspond to the anthracence moiety
(EHOMO = ꢀ5.7 eV).36 The HOMO level of TPEA matches with that
of most of the holeꢀtransporting materials, implying an advantage for
carrierꢀinjection from neighbouring materials. We tried to measure
the lowest unoccupied molecular orbital (LUMO) level by using CV
to get the reduction potential. Unfortunately, we cannot observe
reduction peak. We speculate that electrochemical reaction of TPEA
is not reversible, which is the main reason of the absence of
reduction peak.
Figure 3. The relevant MOs density map and the transition sketch
map of TPEA calculated at the B3LYP/6ꢀ31G* level.
Thermal properties
The thermal properties of TPEA were investigated by taking
differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA) measurements. TPEA shows great thermal stability
with a decomposition temperature (Td) of 400 °C, which is high
enough for thermal vacuum deposition method for the fabrication of
efficient OLEDs. The high Tg of 155 °C benefits the morphological
stability of amorphous film at high temperature, contributing to the
enhancement of lifetime of devices.
Crystal structure analysis
The crystallographic data of TPEA are listed in Supporting
Information. As shown in Figure 4, the two anthracene rings are
twisted from the central TPE by dihedral angles of 67.71° in TPEA,
and the torsion angles between anthracenyl and phenyl are 94.72°,
86.61°, respectively. No typical πꢀπ intermolecular interactions can
be ascribed to the nonꢀplanar conformation, which effectively
prevents bathochromic shift of emission and separates HOMO from
LUMO. The nonꢀparallel Jꢀaggregation displayed in Figure S4
explains the bathochromic shift of the absorption wavelength with
increasing water fraction (shown in Figure S1c).
Figure 2. The CV curve for TPEA at a scan rate of 100 mVsꢀ1.
Theoretical calculations
Density functional theory (DFT) calculations were carried out at the
B3LYP/6–31+G (d, p) level by using the Gaussian 09 program, and
the frontier molecular orbital energy levels and the distribution of
molecular orbitals (MOs) are shown in Figure 3. The HOMO of
TPEA is mainly located on the anthracene which lies near the
electronꢀdonating methoxy group moiety, while LUMO is mainly
distributed on the anthracene which is situated near the electronꢀ
accepting cyano moiety. It is obvious that the HOMO and LUMO
orbitals are thoroughly separated from each other because of the
existence of TPE, indicating the forbidden character of the
transitions between the donor and acceptor (from HOMO to
LUMO). According to the calculation results of the singlet excited
states, the maximum absorption corresponds to the excitation of S0ꢀ
Figure 4. Oak Ridge Thermal Ellipsoid Plot view of the crystal
structure of TPEA of single molecule and the solvents were removed
for clarity.
Device performance
To investigate the electroluminescent properties of TPEA, a series of
devices was designed and fabricated. The device structures are listed
as follows:
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Journal Name
Device
A:
ITO/MoO3
(1
nm)/4,4′,4′′ꢀtris(Nꢀcarbazolyl)
(1), the ηexciton value of the TPEA device was calculated to be 46.7%,
triphenylamine (TCTA) (30 nm)/TPEA (20 nm)/diphenylꢀbis[4ꢀ
(pyridinꢀ3ꢀyl)phenyl]silane (DPPS) (40 nm)/LiF (1 nm)/Al (100 nm)
Device B: ITO/MoO3 (1 nm)/TCTA (30 nm)/TPEA (20 nm)/bisꢀ4,6ꢀ
which breaks through the limit of the radioactive exciton ratio of
25% for singlet fluorescent OLEDs, the redundant of 21.7% should
be reproduced from the triplet excitons.
(3,5ꢀdiꢀ3ꢀpyridylphenyl)ꢀ2ꢀmethylpyrimidine
nm)/LiF (1 nm)/Al (100 nm)
(B3PYMPM) (40
Device C: ITO/MoO3 (1 nm)/TCTA: MoO3 (20 wt.%, 10 nm)/TCTA
(20 nm)/TPEA (20 nm)/B3PYMPM (40 nm)/LiF (1 nm)/Al (100 nm)
Device D: ITO/MoO3 (1 nm)/TCTA: MoO3 (20 wt.%, 10 nm)/TCTA
(20 nm)/TPEA (20 nm)/B3PYMPM (50 nm)/LiF (1 nm)/Al (100 nm)
Device E: ITO/MoO3 (1 nm)/N, N′ꢀdicarbazolylꢀ3,5ꢀbenzene (mCP):
MoO3 (20 wt.%, 20 nm)/mCP (20 nm)/BCPO: TPEA (4 wt.%, 20
nm)/1,3,5ꢀtris(Nꢀphenylbenzimidazolꢀ2ꢀyl)ꢀbenzene (TPBi) (70
nm)/LiF (1 nm)/Al (100 nm)
To confine triplet excitons, we adopted high triplet energy
electron transporting materials (ETMs) of DPPS and B3PYMPM. In
device A, a 1ꢀnmꢀthick of MoO3 was used as the hole injecting
layer, a 30 nm layer of TCTA as the hole transporting material, a 20
nm layer of TPEA as the emitting layer, a 40 nm layer of DPPS as
the ETM, LiF as the electron injection layer and Al as the cathode.
Device A yielded CIE coordinates of (0.19, 0.26) and a ηPE, max of
3.3 lm Wꢀ1. The high Von (3.5 V) and low current density may be
ascribed to the low carrier mobility and high LUMO of DPPS
(LUMO = ꢀ2.5 eV) which leads to the carrier unbalance.39 Then, we
used B3PYMPM as the ETM instead of DPPS to fabricate device B,
which showed a η
PE, max of 4.3 lm Wꢀ1 and CIE coordinates of (0.17,
Figure 5. Chemical structures and energy diagram of the materials
used for fabrication of devices.
0.20). As shown in Figure 6, the current density of device B is
apparently much higher and the Von (3.0 V) is decreased in
comparison with device A, which is induced by the relative low
electronꢀinjection barrier and higher electron mobility of B3PYMPM
(LUMO = ꢀ3.4 eV, ꢁe = 10ꢀ5 cm2 Vꢀ1 sꢀ1).40 With a pꢀdoped TCTA
layer (TCTA: MoO3: 20 wt.%), device C demonstrated a Von of 2.6
V and a ηPE, max of 10.1 lm Wꢀ1. The further lower driving voltage,
could mainly be ascribed to better carrier mobility of pꢀdoped
TCTA. With the thickness modification of B3PYMPM, device D
achieved a
of 5.6% while its Von was maintained at 2.6 V. The high power
η η
CE, max of 9.9 cd Aꢀ1, a PE, max of 11.1 lm Wꢀ1 and a ηext,
max
efficiency might be due to the small barrier of the neighbouring
layers, and the high electron mobility of anthracene moiety as well
as the ICT structure might also make some contribution.28 In
addition, it is worth noting that the TPEAꢀbased device also showed
excellent efficiency stability, with small rollꢀoff on the ηCE and ηext
with increasing brightness up to 1000 cd mꢀ2 as shown in Table 1.
The EL spectra of the TPEA device also demonstrated good colour
stability with nearly the same CIE coordinates (0.17, 0.20) over the
whole voltage range. It is apparent that a high concentration of
TPEA does not lead to remarkable concentrationꢀquenching.
The theoretical value of the radioactive exciton ratio was
calculated by the following equation:
Figure 6. (a) Current density–voltage characteristics of device A-D;
(b) Current efficiencyꢀcurrent density of device A-D; (c) Power
efficiencyꢀcurrent density of device A-D; (d) External quantum
efficiencyꢀcurrent density of device A-D.
ηꢀꢁꢂ
ꢃ
η
ꢀꢄꢅ ꢄ
η
ꢆꢇ ꢄ
η
ꢀꢁꢈꢉꢂꢊꢋ ꢄ ηꢊꢌꢂ
(1)
in which ηext is the external quantum efficiency, ηexciton the
radioactive exciton ratio, ηout the light outꢀcoupling efficiency (ca.
20%), ηPL the intrinsic photoluminescence efficiency, and ηe·h the
recombination efficiency of injected holes and electrons, which is
nearly 100% only if holes and electrons are totally balanced and
completely recombined to form excitons. According to Equation
To reduce the bathochromicꢀshift of TPEA caused by
intermolecular aggregation, we fabricated a doped OLED device
(device E) with the structure: ITO/MoO3 (1 nm)/mCP: MoO3 (20
wt.%, 20 nm)/mCP (20 nm)/BCPO: TPEA (4 wt.%, 20 nm)/TPBi
(70 nm)/LiF (1 nm)/Al (100 nm), where TPEA was used as the
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emitting dopant in the BCPO host. From the energy diagram, we can
find that the energy gap of TCTA (3.4 eV) is lower than that of
BCPO (3.6 eV), which can lead to the energy transfer from BCPO to
TCTA. Furthermore, the ET1 of TCTA (2.7 eV) is much lower than
that of BCPO (3.0 eV), which is adverse to the triplet excitons
confinement. And the energy barrier between the LUMO of
B3PYMPM (ꢀ3.4 eV) and the LUMO of BCPO (ꢀ2.2 eV) is too large
to process efficient carrier injection. Thus, we chose mCP and TPBi
to replace TCTA and B3PYMPM to avoid the situations stated as
above in device E. Device E emits deepꢀblue light with the
maximum emission peak at 445 nm, corresponding to CIE
coordinates of (0.15, 0.09), close to the standard blue of National
Television Standards Committee (NTSC) with CIE coordinates of
(0.14, 0.08). The lack of additional peaks from BCPO and
neighbouring materials in the EL spectra of the fabricated device
suggests a full energy transfer from BCPO to the blue emitter.The
the rest of the ηexciton value (> 5%) may be attributed to HLCT. To
confirm this, we employed the holeꢀelectron analysis function of
Multiwfn software to calculate electronic transitions for different
excited states.41 The corresponding theoretical calculation was given
in the supporting information (Figure S3). According to the
calculation results, HLCT could be one of the possible mechanisms
to breakthrough the singlet emission limit of 25%.
device E also achieved a ηext, max of 8.0%, and the highest ηPE, max of
Figure 8. (a) Timeꢀresolved electroluminescence response of the
TPEAꢀdoped BCPOꢀbased device E under various excitation pulse
voltages measured at 450 nm; (b) Detail view of timeꢀresolved
electroluminescence response of the TPEAꢀdoped BCPOꢀbased
device E under various excitation pulse voltages measured at 450
nm.
7.3 lm W−1 among all the deepꢀblue TTF and HLCT emitters
(Figure 7). This may be resulted from the high electron mobility of
anthracene moiety and its ICT structure as mentioned above.
Experimental
General
The 1H NMR spectra were recorded on a Varian 300 MHz and a
Bruker 400 MHz NMR spectrometer. ESIꢀMS were performed in
positive or negative ion mode on a Bruker Apex IV Fourier
transform ion cyclotron resonance mass spectrometer. Elemental
analyses were measured on a VARIO elemental analyzer from
Elementar Analysensysteme GmbH. DSC was performed on a
PerkinꢀElmer Diamond DSC Pyris instrument under nitrogen
atmosphere at a heating rate of 10 °C minꢀ1. TGA was undertaken
using a SEIKO EXSTAR 6000 TG/DTA 6200 unit under nitrogen
atmosphere at a heating rate of 10 °C minꢀ1. UVꢀvisible absorption
spectra were recorded using
a
PerkinꢀElmer Lambda 35
Figure 7. (a) Current density, Luminance–voltage characteristics of
device E; (b) Current efficiencyꢀcurrent density of device E; (c)
Power efficiencyꢀcurrent density of device E; (d) External quantum
efficiencyꢀcurrent density of device E (inset: EL spectra for device E
at 100 cd mꢀ2).
spectrometer. Emission spectra were recorded in a Hitachi F7000
fluorescence spectrophotometer and temperature dependent transient
PL decay was measured on a single photon counting spectrometer
from Edinburgh FLS980 with microꢀsecond pulse lamp as the
excitation source. For measurement of the transient
electroluminescence characteristics, shortꢀpulse excitation with a
pulse width of 15 ꢀs was generated using an Agilent 8114A pulse
generator. The amplitude of the pulse was 9 V, 10 V or 11 V and the
baseline was ꢀ3 V. The period was 50 ꢀs, the delayed time was 25 ꢀs
and the duty cycle was 30%. The decay curves of the devices were
detected using an Edinburg FL920P transient spectrometer. The
timeꢀresolved fluorescence spectra were recorded with a high
To reveal the energy transfer process in device E, its transient
electroluminescence was measured. At the end of the excitation
pulse, the electroluminescence decreased rapidly owing to the short
fluorescence lifetime of TPEA, and a delayed electroluminescence
component was observed. By applying a reverse bias of ꢀ3 V, we
could distinguish that the delayed EL originates from TTF processes
rather than recombination of trapped charges. From Figure 8b, it can resolution streak camera system (Hamamatsu C10910). The pump
beam with a repetition rate of
1
KHz is generated by an
be seen that the delayed electroluminescent signal from triplet fusion
is around 40% and increases with the rising excitation pulse voltage,
evidently due to TTF process. It is clearly seen that the contribution
ratio of direct singlet (25%) and triplet fusion from TTF (< 40%) to
the electroluminescence is under 45%, less than the ηexciton value of
50%, estimated from Equation (1) for device E. Therefore, we come
to the conclusion that the main source of triplet fusion is TTF, and
This journal is © The Royal Society of Chemistry 20xx
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DOI: 10.1039/C7TC04685E
ARTICLE
Journal Name
Table 1. List of device performances
(a)
(b)
(c)
Lmax
[cd m-2]
(c)
(c)
(c)
Von
V100
ηCE, max/100/1000
[cd A-1]
ηPE, max/100/1000
[lm W-1]
ηext, max/100/1000
CIE(b)
[x, y]
[V]
[V]
[%]
3.5
3.0
2.6
2.6
3.3
4.8
3.5
3.2
3.2
5.1
4961
4.5/4.5/3.2
4.7/4.7/4.5
8.9/8.8/8.1
9.9/9.5/8.8
7.2/4.0/3.1
3.3/2.9/1.3
4.3/4.3/2.9
10.0/8.6/6.1
11.1/8.8/6.5
7.3/2.6/1.4
2.6/2.6/1.9
3.0/3.0/2.8
5.4/5.4/5.1
5.6/5.6/5.2
8.0/5.0/3.9
(0.20, 0.26)
(0.17, 0.20)
(0.17, 0.20)
(0.17, 0.20)
(0.15, 0.09)
A
B
C
D
E
8950
12500
13910
6527
(a) at 1 cd mꢀ2; (b) at 100 cd mꢀ2; (c) maximum, at 100 cd mꢀ2, at 1000 cd mꢀ2.
amplified modeꢀlock Ti: Sapphire femtosecond laser system
(Legend, Coherent) and a twoꢀstage optical parametric amplifier
(OperA Solo, Coherent). The sample was excited at 360 nm under
room temperature. CV was carried out in a nitrogenꢀpurged
Synthesis
dioxaborolanꢀ2ꢀyl)phenyl) methanone (2): (4ꢀbromophenyl)(4ꢀ
chlorophenyl)methanone (1) (11.8 g, 40.0 mmol),
of
(4ꢀchlorophenyl)(4ꢀ(4,4,5,5ꢀtetramethylꢀ1,3,2ꢀ
bis(pinacolato)diboron (20.3 g, 80.0 mmol), Pd(PPh3)2Cl2 (281
mg, 0.400 mmol), KOAc (11.8 g, 120 mmol) were mixed in a
flask containing nitrogenꢀsaturated 1,4ꢀdioxane (175 mL), stirring
for 15 minutes at room temperature. The reaction mixture was
dropped in 0.5 mL N,Nꢀdimethylformamide (DMF) and stirred for
30 minutes at room temperature. Then the solution was heated to
85 ºC and reacted for 3.0 hours. After cooling to room
temperature, the solvent was eluted to give the solid mixture. The
solid mixture was extracted with CH2Cl2 (200 mL). The combined
organic extracts were dried with anhydrous MgSO4 and
evaporated. The crude product was successfully purified by
column chromatography using dichloromethane as the eluent to
give the faint yellow solid. Yield: 62% (8.54 g). 1H NMR (CDCl3,
TMS, δ): 7.926ꢀ7.910 (d, J = 8.0 Hz, 2H), 7.752ꢀ7.724 (m, 4H),
CH2Cl2 solution at room temperature using
a CHI600C
voltammetric analyzer with tetrabutylammonium
hexafluorophosphate (TBAPF6) (0.1 M) as the supporting
electrolyte. The conventional three electrodes configuration
consists of a platinum working electrode, a platinum wire
auxiliary electrode, and an Ag/AgCl wire pseudo reference
electrode. HOMO level was determined by atmospheric
ultraviolet photoelectron spectroscopy (Rikken Keiki ACꢀ2). The
single crystal of TPEA was obtained by slow evaporation of its
ethanol (Rigaku Denki) image plate diffractometer using graphiteꢀ
monochromated Mo/Kα (λ = 0.71073 Å) radiation. The crystal
structure was solved by the direct methods. The structure was
refined on F2 by fullꢀmatrix leastꢀsquares methods using the
SHELXTLꢀ97 program package. The current density J, voltage V
and luminance L characteristics of the OLEDs were measured
using a computer controlled Keithley 2400 SourceMeter unit with
a calibrated silicon diode and absolute ηext measurement system
(C9920ꢀ12, Hamamatsu Photonics) with the photonic
multichannel analyzer (PMAꢀ12, Hamamatsu Photonics).
7.462ꢀ7.444 (dt, 2H), 1.369 (s, 12H). 13C NMR (CDCl3): δC
=
156.22, 155.78, 150.37, 150.35, 148.98, 147.97, 142.02, 141.99,
138.72, 136.77, 128.07, 127.96, 127.62, 124.13, 123.68, 122.81,
121.38, 120.73, 120.41, 118.91, 77.25, 77.00, 76.74, 66.27 ppm.
Anal. Calc. for C19H20BClO3: C 66.61, H 5.88, B 3.16, Cl 10.35,
O 14.01; found: C 66.54, H 5.96.
Synthesis of 9ꢀ(4ꢀmethoxyphenyl)anthracene (3): Compound (3)
was prepared by Suzuki reaction catalyzed by Pd catalysts. 9ꢀ
Bromoanthracene (10.2 g, 40.0 mmol), 4ꢀmethoxyphenylboronic
acid (6.39 g, 42.0 mmol), Pd(OAc)2 (90.0 mg, 0.400 mmol),
ligand CXA (288 mg, 0.800 mmol), NaHCO3 (10.1 g, 120 mmol)
were mixed in a flask containing nitrogenꢀsaturated toluene (150
mL), tetrahydrofuran (50 mL) and water (50 mL), stirring for 15
minutes at room temperature. Then the solution was heated to 75
ºC and reacted for 5.0 hours. After cooling to room temperature,
the reaction mixture was extracted with ethyl acetate (200 mL).
The combined organic extracts were dried with anhydrous MgSO4
and evaporated. The crude product was purified by column
Materials
MoO3, TCTA, DPPS, B3PYMPM, mCP, BCPO and TPBi were
purchased from Lumtech Incorporation without further
purification.
Synthesis and characterization
Synthesis of (4ꢀbromophenyl)(4ꢀchlorophenyl)methanone (1): (4ꢀ
bromophenyl)(4ꢀchlorophenyl)methanone (1) was easily prepared
by FriedelꢀCrafts acylation utilizing AlCl3 as catalyst.42 Yield:
50% (7.34 g). 1H NMR (CDCl3, TMS, δ): 7.461ꢀ7.478 (d, J = 8.5
Hz, 2H), 7.642 (s, 4H), 7.716ꢀ7.734 (d, J = 9.0 Hz, 2H).
6 | J. Name., 2012, 00, 1-3
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ARTICLE
chromatography on silica gel (dichloromethane/petroleum ether,
1/3, v/v) to obtain a colorless solid (3). Yield: 76%. (8.60 g).
were dried with anhydrous MgSO4 and evaporated. The crude
product was purified by column chromatography on silica gel
using CH2Cl2 as eluant to obtain a yellow solid (12). Yield: 87%
1
Synthesis
of
9ꢀbromoꢀ10ꢀ(4ꢀmethoxyphenyl)anthracene
(2.67 g). H NMR (CDCl3, TMS, δ): 8.070ꢀ8.053 (d, J = 8.5 Hz,
(OMePAnBr) (4): 9ꢀ(4ꢀmethoxyphenyl) anthracene (3) (8.60 g,
30.3 mmol) and NꢀBromobutanimide (NBS) (8.09 g, 45.4 mmol)
were mixed in a flask containing nitrogenꢀsaturated CH2Cl2 (150
mL), stirring for 15 minutes at room temperature. Then the
solution was heated to 45 ºC and reacted for 5.0 hours. After
cooling to room temperature, the mixture was quenched using
NaHSO3 (100 mL) aqueous solution and extracted with CH2Cl2
(100 mL). The combined organic extracts were dried with
anhydrous MgSO4 and evaporated. The crude product was
recrystallized from ethanol (50 mL) and nꢀhexane (50 mL) to give
2H), 8.002ꢀ7.986 (d, J = 8.0 Hz, 2H), 7.940ꢀ7.924 (d, J = 8.0 Hz,
2H), 7.777ꢀ7.757 (m, 2H), 7.683ꢀ7.664 (m, 2H), 7.623ꢀ7.607 (d, J
= 8.0 Hz, 2H), 7.406ꢀ7.346 (m, 6H), 7.161ꢀ7.143 (d, J = 9.0 Hz,
2H), 3.942 (s, 3H), 1.387 (s, 12H).
Synthesis of (4ꢀ(anthracenꢀ9ꢀyl)phenyl)(4ꢀ(10ꢀ(4ꢀmethoxyphenyl)
anthracenꢀ9ꢀyl)phenyl)methanone (DPAnOMe) (7): DPAnOMeB
(2.67 g, 4.51 mmol) (6), 9ꢀbromoanthracene (1.10 g, 4.29 mmol),
Pd(PPh3)4 (52.0 mg, 0.045 mmol) and NaHCO3 (1.14 g, 13.5
mmol) were mixed in a flask containing nitrogenꢀsaturated
toluene (50 mL), THF (30 mL) and H2O (20 mL), stirring for 20
minutes at room temperature. Then the solution was heated to 85
ºC and reacted for 5.0 hours. After cooling to room temperature,
the solvent was eluted to give the crude product. The crude
product was purified by column chromatography on silica gel
(CH2Cl2/petroleum ether, 1/3, v/v) to obtain a yellow solid (7).
Yield: 86% (2.50 g).
1
yellow powder (4). Yield: 87% (9.56 g). H NMR (CDCl3, TMS,
δ): 8.602ꢀ8.584 (d, J = 8.0 Hz, 2H), 7.702ꢀ7.684 (d, J = 8.0 Hz,
2H), 7.585ꢀ7.555 (dd, J = 6.5 Hz, J = 8.5 Hz, 2H), 7.377ꢀ7.347
(dd, J = 7.5 Hz, 2H), 7.323ꢀ7.296 (d, J = 13.5 Hz, 2H), 7.115ꢀ
7.098 (d, J = 8.5 Hz, 2H), 3.938 (s, 3H). 13C NMR (CDCl3): δC =
177.07, 159.18, 137.61, 132.19, 131.36, 130.35, 130.24, 127.79,
127.45, 126.88, 125.42, 122.51, 113.88, 77.27, 77.01, 76.76,
55.36, 29.52 ppm. MS: expected m/z = 362.0; found m/z = 361.2.
Synthesis of 9ꢀ(4ꢀ(1ꢀ(4ꢀ(anthracenꢀ9ꢀyl)phenyl)ꢀ2,2ꢀdiphenylvinyl)
phenyl)ꢀ10ꢀ(4ꢀmethoxyphenyl)anthracene (TPEPAnOMe) (8): A
stirred solution diphenylmethane (820 mg) and anhydrous THF
(15 mL) was cooled to 0 °C under nitrogen. Addition of nꢀBuLi
(1.7 mL, 3.74 mmol, 2.2 M in hexane) over 5 minutes resulted in
a pale yellow solution. After stirring for 40 minutes at this
temperature, the solution of DPAnOMe (1.20 g, 1.87 mmol) (7) in
THF (25 mL) was added. The solution was stirred for two
additional hours, and was allowed to warm to room temperature
and to react for 2 hours. The mixture was quenched by the slow
addition of saturated NH4Cl solution (50 mL) and stirred
overnight. Extraction with ethyl acetate and evaporation of the
organic layer furnished the crude product. The crude product was
purified by column chromatography on silica gel
(CH2Cl2/petroleum ether, 1/4, v/v) to obtain a yellow solid. The
solid and toluene sulfonic acid (356 mg, 1.87 mmol) were
dissolved in saturated nitrogen toluene (30 mL), and the mixture
was heated to reflux under 110 °C for 1 hour. After cooling to
room temperature, the mixture was quenched by saturated
NaHCO3 solution (50 mL) and extracted with ethyl acetate (100
mL). The combined organic extracts were dried with anhydrous
MgSO4 and evaporated. The crude product was purified by
column chromatography on silica gel (CH2Cl2/petroleum ether,
Synthesis of (4ꢀchlorophenyl)(4ꢀ(10ꢀ(4ꢀmethoxyphenyl)anthracenꢀ
9ꢀyl)phenyl)methanone (DPAnOMeCl) (5): (4ꢀchlorophenyl)(4ꢀ
(4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolanꢀ2ꢀyl)phenyl)methanone
(2) (3.42 g, 10.0 mmol), OMePAnBr (3.44 g, 9.50 mmol) (4),
Pd(PPh3)4 (115 mg, 0.100 mmol), NaHCO3 (2.52 g, 30.0 mmol)
were mixed in a flask containing nitrogenꢀsaturated toluene (50
mL), THF (15 mL) and H2O (15 mL), stirring for 20 minutes at
room temperature. Then the solution was heated to 85 ºC and
reacted for 5.0 hours. After cooling to room temperature, the
solvent was eluted to give the crude product. The crude product
was purified by column chromatography on silica gel
(dichloromethane/petroleum ether, 1/3, v/v) to obtain a colorless
solid (5). Yield: 55% (2.60 g). 1H NMR (CDCl3, TMS, δ): 8.048ꢀ
8.031 (d, J = 8.5 Hz, 2H), 7.940ꢀ7.923 (d, J = 8.5 Hz, 2H), 7.783ꢀ
7.763 (m, 2H), 7.667ꢀ7.623 (m, 4H), 7.554ꢀ7.537 (d, J = 8.5 Hz,
2H), 7.407ꢀ7.351 (m, 6H), 7.163ꢀ7.146 (d, J = 8.5 Hz, 2H), 3.941
(s, 3H). 13C NMR (THF): δC = 132.06, 131.40, 131.36, 129.97,
128.51, 126.98, 126.23, 125.18, 124.81, 113.84, 66.95, 66.77,
66.59, 66.42, 66.24, 66.07, 54.59, 24.83, 24.66, 24.50, 24.34,
24.18, 24.02 ppm. HRMS: expected m/z = 498.14; found m/z =
498.13717.
1
Synthesis of (4ꢀ(10ꢀ(4ꢀmethoxyphenyl)anthracenꢀ9ꢀyl)phenyl) (4ꢀ
(4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolanꢀ2ꢀyl)phenyl)methanone
(DPAnOMeB) (6): DPAnOMeCl (2.60 g, 5.20 mmol) (5),
bis(pinacolato)diboron (6.60 g, 26.0 mmol), KOAc (1.53 g, 15.6
mmol) and Pd(PPh3)4 (60.0 mg, 0.052 mmol) were mixed in a
flask containing nitrogenꢀsaturated 1,4ꢀdioxane, stirring for 15
minutes at room temperature. Then the solution was heated to 125
ºC and reacted for 4.0 hours. After cooling to room temperature,
the solvent was eluted to give the solid mixture. The mixture was
extracted with CH2Cl2 (100 mL). The combined organic extracts
1/2, v/v) to obtain a yellow solid. Yield: 46% (0.677 g). H NMR
(CDCl3, TMS, δ): 8.481 (s, 1H), 8.049ꢀ8.032 (d, J = 8.5 Hz, 2H),
7.751ꢀ7.719 (t, 3H), 7.701ꢀ7.681 (d, J = 10.0 Hz, 3H), 7.482ꢀ
7.431 (m, 7H), 7.407ꢀ7.370 (m, 6H), 7.356ꢀ7.334 (m, 3H), 7.307ꢀ
7.291 (d, J = 8.0 Hz, 2H), 7.268ꢀ7.254 (m, 7H), 7.241ꢀ7.209 (m,
3H), 7.152ꢀ7.134 (d, J = 8.0 Hz, 2H), 3.964 (s, 3H). HRMS:
expected m/z = 790.3; found m/z = 790.32129.
Synthesis
of
9ꢀ(4ꢀbromophenyl)ꢀ10ꢀ(4ꢀ(1ꢀ(4ꢀ(10ꢀ(4ꢀ
methoxyphenyl)anthraceneꢀ9ꢀyl)phenyl)ꢀ2,2ꢀ
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DOI: 10.1039/C7TC04685E
ARTICLE
Journal Name
diphenylvinyl)phenyl)
anthracene
(TPEPAnOMeBr)
(9):
crystallographic structure. In addition, a DꢀA structure was built
by using methoxy and cyano to improve the charge balance in the
devices. In addition, the material possesses high thermal stability
with a Tg of 155 °C. The nonꢀdoped device achieved the high
performance with a Von of 2.6 V at a luminance of 1 cd m−2, a ηPE,
TPEPAnOMe (8) (1.41 g, 1.78 mmol), NBS (476 mg, 2.67 mmol)
were mixed in a flask containing nitrogenꢀsaturated chloroform
(50 mL), stirring for 2 hours under 60 °C. The mixture was
quenched by saturated NaHSO3 solution (100 mL) and extracted
with CH2Cl2 (100 mL), washing with water to neutral. The
combined organic extracts were dried with anhydrous MgSO4 and
max of 11.1 lm W−1, a CE, max of 9.9 cd A−1, and a low
η ηCE rollꢀoff.
The doped device based on TPEA was fabricated to acquire deep
blue emission with CIE coordinates of (0.15, 0.09), showing a
ηext, max up to 8.0% and the highest ηPE, max of 7.3 lm W−1 among
all the TTF and HLCT deepꢀblue emitters. Inspired by these
preliminary results, we believe that the combination of the merits
of TTFꢀHLCT and AIE would be a promising molecular design
principle for exploring highly efficient deep blue emitters.
1
evaporated to obtain a yellow solid (9). Yield: 90% (1.40 g). H
NMR (CDCl3, TMS, δ): 8.617ꢀ8.599 (d, J = 9.0 Hz, 2H), 7.752ꢀ
7.669 (m, 6H), 7.600ꢀ7.585 (t, 3H), 7.451ꢀ7.424 (m, 5H), 7.406ꢀ
7.324 (m, 5H), 7.309ꢀ7.293 (d, J = 8.0 Hz, 2H), 7.267ꢀ7.225 (m,
12H), 7.171ꢀ7.134 (m, 3H), 3.963 (s, 3H).
Synthesis of 1ꢀ(10ꢀ(4ꢀmethoxyphenyl)anthracenꢀ9ꢀyl)ꢀ4ꢀ(10ꢀ(4ꢀ
cyanophenyl)anthracenꢀ9ꢀyl)tetraphenylethene
(TPEA):
Conflicts of interest
TPEPAnOMeBr (9) (1.10 g, 1.27 mmol), 4ꢀCyanophenylꢀboronic
(205 mg, 1.39 mmol), Pd(OAc)2 (3.00 mg, 0.013 mmol), ligand Sꢀ
Phos (11 mg, 0.027 mmol) and NaHCO3 (320 mg, 3.8 mmol)
were mixed in a flask containing nitrogenꢀsaturated toluene (30
mL), THF (10 mL) and H2O (10 mL), stirring for 5.0 hours under
85 °C. The mixture was extracted with CH2Cl2 (150 mL), washing
with water to neutral. The solvent was eluted to give the crude
product. The crude product was purified by column
chromatography on silica gel (CH2Cl2/petroleum ether, 1/3, v/v)
The authors declare no competing financial interest.
Corresponding Author
†Correspondence:
Lixin Xiao, Eꢀmail: xiao66@pku.edu.cn;
Shufeng Wang, Eꢀmail: wangsf@pku.edu.cn;
Lian Duan, Eꢀmail: duanl@mail.tsinghua.edu.cn
1
to obtain a yellow solid (TPEA). Yield: 88% (1.0 g). H NMR
Acknowledgements
(CDCl3, TMS, δ): 7.920ꢀ7.904 (d, J = 8.0 Hz, 2H), 7.755ꢀ7.738
(d, J = 8.5 Hz, 4H), 7.722ꢀ7.706 (d, J = 8.0 Hz, 2H), 7.624ꢀ7.607
(d, J = 8.5 Hz, 2H), 7.544ꢀ7.526 (d, J = 9.0 Hz, 2H), 7.482ꢀ7.450
(t, 4H), 7.416ꢀ7.372 (m, 7H), 7.357ꢀ7.337 (m, 3H), 7.325ꢀ7.289
(m, 4H), 7.274ꢀ7.250 (m, 8H), 7.239ꢀ7.222 (m, 2H), 7.181ꢀ7.134
(d, J = 8.5 Hz, 2H), 3.962 (s, 3H). 13C NMR (CDCl3): δC = 55.42,
111.66, 113.91, 118.93, 124.94, 125.17, 125.75, 126.03, 126.76,
127.00, 127.10, 127.35, 127.73, 129.43, 129.88, 130.70, 130.87,
131.38, 131.47, 131.61, 131.63, 132.33, 132.41, 134.45, 136.91,
137.41, 138.25, 140.95, 142.41, 142.66, 143.03, 143.67, 143.74,
144.59, 159.08 ppm. HRMS: expected m/z = 891.4; found m/z =
891.4. Anal. Calc. for C68H45NO: C 91.55, H 5.08, N 1.57, O
1.79; found: C 91.36, H 4.90, N 1.66.
This study was partly financially supported by the National
Key R&D Program of China (No. 2016YFB0401000) and by
NSFC (grants 61575005, 61775004, 11574009 and
U1605244).
Notes and references
The Supporting Information is available free of charge on the
ACS Publications website at DOI:
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high EL efficiencies and the merits of AIE. The anthracene groups
are twisted from the central TPE moiety, which effectively
prevents bathochromic shift of emission as shown in its
8 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry C
Page 10 of 10
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DOI: 10.1039/C7TC04685E
Journal Name
ARTICLE
A Combinational Molecular Design to Achieve Highly Efficient Deep Blue Electrofluorescence
Keyword：deep blue, aggregation induced emission, triplet-triplet fusion, hybridized local charge transfer
Mengying Bian, Zifeng Zhao, Yu Li, Qing Li, Zhijian Chen, Dongdong Zhang, Shufeng Wang,* Zuqiang Bian,*
Zhiwei Liu, Lian Duan,* Lixin Xiao*
A novel high performance deep blue emitting material TPEA which combines the merits of AIE and TTF plus
HLCT is demonstrated.
This journal is © The Royal Society of Chemistry 20xx
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