Rational design of efficient orange-red to red thermally activated delayed fluorescence emitters for OLEDs with external quantum efficiency of up to 26.0% and reduced efficiency roll-off

Article abstract of DOI:10.1039/c9tc05892c

The development of high efficiency orange-red/red thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLEDs) with reduced efficiency roll-off remains a huge challenge. By fusing two weak electron-withdrawing benzoyl and quinoxaline units into one molecule, a strong π-accepting phenyl(quinoxalin-6-yl)methanone unit is introduced as an acceptor to develop three orange-red to red TADF emitters, (2-(4-(phenoxazin-10-yl)phenyl)quinoxalin-6-yl)phenylmethanone (PXZ-PQM), (2,3-bis(4-(phenoxazin-10-yl)phenyl)quinoxalin-6-yl)(phenyl)methanone (DPXZ-PQM) and (3,6-di(phenoxazin-10-yl)dibenzophenazin-11-yl)phenylmethanone (DPXZ-DPPM). By tuning the numbers of the donor and electron-withdrawing characteristics of the acceptor, the emission wavelength can be regulated from orange-red to red (592-642 nm). The rigid planar constituent moieties and highly twisted molecular structures induce high photoluminescence quantum yields of 61-88% in doped films and extremely small singlet-triplet energy splitting. The orange-red TADF-OLED using DPXZ-PQM as an emitter achieves a maximum external quantum efficiency (EQE) as high as 26.0% at 590 nm, which is among the highest EQE values reported for orange-red TADF-OLEDs with similar wavelengths. More importantly, a low efficiency roll-off is rendered with EQEs of 20.1 and 13.7% at a luminance of 100 and 1000 cd m^-2, respectively.

Full text of DOI:10.1039/c9tc05892c

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ARTICLE
Rational Design of Efficient Orange-Red to Red Thermally
Activated Delayed Fluorescence Emitters for OLEDs with External
Quantum Efficiency up to 26.0% and Reduced Efficiency Roll-off
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Jixiong Liang, Chenglong Li,* Yuanyuan Cui, Zhiqiang Li, Jiaxuan Wang, and Yue Wang*
The development of high efficiency orange-red/red thermally activated delayed fluorescence (TADF) organic light emitting
diodes (OLEDs) with reduced efficiency roll-off retains a huge challenge. By fusing two weak electron-withdrawing benzoyl
and quinoxaline units into one molecule, a strong π-accepting phenyl(quinoxalin-6-yl)methanone unit is introduced as
acceptor to develop three orange-red to red TADF emitters, (2-(4-(phenoxazin-10-yl)phenyl)quinoxalin-6-
yl)phenylmethanone (PXZ-PQM), (2,3-bis(4-(phenoxazin-10-yl)phenyl)quinoxalin-6-yl)(phenyl)methanone (DPXZ-PQM)
and (3,6-di(phenoxazin-10-yl)dibenzophenazin-11-yl)phenylmethanone (DPXZ-DPPM). By tuning the numbers of the donor
and the electron withdrawing character of the acceptor, the emission wavelength can be regulated from orange-red to red
www.rsc.org/
(592-642 nm). The rigid planar constituent moieties and highly twisted molecular structures induce high
photoluminescence quantum yields of 61-88% in doped films and extremely small singlet-triplet energy splitting. The
orange-red TADF-OLED using DPXZ-PQM as an emitter achieves a maximum external quantum efficiency (EQE) as high as
2
6.0% at 590 nm, which is among the highest EQE values reported for orange-red TADF-OLEDs with similar wavelength.
-2
More importantly, a low efficiency roll-off is rendered with EQEs of 20.1 and 13.7% at a luminance of 100 and 1000 cd m ,
respectively.
to the Franck-Condon transition principle, small orbital overlap
means a small probability of electronic transition and thereby
Introduction
Organic light emitting diodes (OLEDs) based on thermally decreases the oscillator strength (f) between the S1 and
activated delayed fluorescence (TADF) materials without any ground (S ) states, which is not conducive to obtain high
0
noble-metal are raising ever-increasing interest due to their PLQY.13 Therefore, this is a contradiction between realizing
potential in achieving 100% internal quantum efficiency (IQE) simultaneously the efficient reverse intersystem crossing
in theory via efficient up-conversion from non-radiative triplet (RISC) process and the high PLQY in the one molecule for
1
, 2
(T
1
) to radiative singlet (S
1
).
Over the past few years, the efficient orange-red/red TADF emitters. Many efforts have
record-high external quantum efficiencies (EQEs) of been devoted towards exploiting highly efficient orange-
3
8.15ꢀ±ꢀ0.42 and 37.8 ± 0.6% have been realized for blue and red/red TADF molecules, such as increasing the twist angle
3
, 4
green TADF-OLEDs, respectively.
In contrast, the EQEs of between the donor and acceptor by introducing multiple
orange-red to red TADF-OLEDs with emission maxima beyond donors or acceptors, adjusting donor and acceptor strength of
5
-7
14-17
5
80 nm rarely exceed 25%. The most decisive reason is that the molecule, and enlarging the molecular rigidity.
A rigid
the overlap of vibration coupling between ground state and planar 1,8-naphthalimide derivative based orange-red TADF-
excited state will increase upon the decrease of energy gap OLED has achieved a state-of-the-art EQE of 29.2% by Yang
and thereby decreases the photoluminescence quantum yields and co-worker, which is the most efficient orange-red/red
8
5
(
PLQYs) of the emitters according to the energy-gap law. On TADF-OLED reported to date. Unfortunately, the device
the other hand, a small singlet-triplet energy splitting (ΔEST) is showed the serious efficiency roll-off at high current density
9
, 10
essential for efficient RISC process.
In general, small ΔEST due to the presence of triplet-triplet or singlet-triplet exciton
can be realized by minimizing the overlap integral between the annihilation. Therefore, the development of high efficiency
highest occupied molecular orbital (HOMO) and the lowest orange-red to red TADF-OLEDs with reduced efficiency roll-off
1
1, 12
unoccupied molecular orbital (LUMO).
However, according is a severe challenge for OLED application.
In this work, we designed and synthesized three orange-red
to red TADF emitters, named (2-(4-(phenoxazin-10-
yl)phenyl)quinoxalin-6-yl)phenylmethanone (PXZ-PQM), (2,3-
State Key Laboratory of Supramolecular Structure and Materials
College of Chemistry, Jilin University, Changchun 130012, P. R. China
E-mail: chenglongli@jlu.edu.cn, yuewang@jlu.edu.cn
bis(4-(phenoxazin-10-yl)phenyl)quinoxalin-6-yl)
(phenyl)-
†
Electronic Supplementary Information (ESI) available: TGA, DSC, CV curves,
methanone (DPXZ-PQM) and (3,6-di(phenoxazin-10-
additional emission spectra, transient decay curves of the doped films, additional
electroluminescence data of the OLED devices and crystal data. CCDC 1943981. yl)dibenzophenazin-11-yl)phenylmethanone (DPXZ-DPPM). As
See DOI: 10.1039/x0xx00000x
we all know, benzoyl and quinoxaline units are often used as
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Scheme 1 Synthetic procedures and chemical structures of PXZ-PQM, DPXZ-PQM and DPXZ-DPPM.
the acceptor moieties in high efficiency blue and green TADF sublimation approach. The chemical structures of these
1
8-20
1
emitters due to their weak electron withdrawing character.
Through integrating benzoyl and quinoxaline units into one
molecule to strengthen the acceptor capacity, two acceptor synthetic details and characteristics data are described in the
cores, phenyl(quinoxalin-6-yl)methanone (PQM) and Experimental Section.
compounds were fully characterized and confirmed by H and
1
3
C NMR, mass spectrometry, and elemental analysis. Their
dibenzo[a,c]phenazin-11-yl(phenyl)methanone (DPPM) with
stronger electron withdrawing character are developed. PXZ is
employed as the electron donor segments because of its
strong electron-donating ability and rigid structure. These
emitters show orange-red to red emissions with high PLQYs,
small ΔEST values, and excellent TADF properties. The DPXZ-
PQM-based electroluminescence (EL) device achieves an EQE
as high as 26.0% with an orange EL peaking 590 nm, which
represents the state-of-the-art device performance for
orange–red TADF-OLEDs. More importantly, this device
indicates a decent efficiency roll-off and their EQEs still
maintained as 20.1 and 13.7% at a luminance of 100 and 1000
Single Crystal Structures Analysis
To further make out structure-properties relationship, we
grew single crystals of PXZ-PQM from the mixture solvent of
methylene chloride and methanol. As shown in Fig. 1a, the
dihedral angle between PQM group and phenylene linker is
3
7
1.1° and that between phenylene linker and PXZ moiety is
4.9°, indicating a highly twisted structure for the conjugated
framework of PXZ-PQM. Two consecutive large twisted angles
along the donor and acceptor groups are likely the key for the
HOMO and LUMO separation. Each molecule shows two types
of π⋯π interactions, one is between the PXZ donor in a small
overlap area with a distance of 3.471 Å (Fig. 1c and 1d), and
the other one between the PQM acceptors shows a weak
interaction with a distance of 3.745 Å (Fig. 1b and 1d). In
addition, the three-dimensional framework is formed based on
the multiple hydrogen bonding interactions of C-H···O and C-
H···N to stabilize the crystal and suppress non-radiative
transition (Fig. 1e). Whole crystal data of PXZ-PQM are shown
in Table S1.
-2
cd m , respectively. This study not only obtains a record-high
EQE in orange-red TADF-OLEDs, but also provides a feasible
pathway for constructing highly efficient orange-red/red TADF
emitters.
1
. Results and discussion
Synthesis and Characterization
Density Functional Theory (DFT) Calculations
The molecular structures and synthetic procedures of the
three emitters are shown in Scheme 1. The key intermediates
Br-PQM, DBr-PQM and DBr-DPPM were synthesized via the
cyclization reaction, followed by the Buchwald-Hartwig cross-
coupling reaction to prepare the three TADF emitters (PXZ-
PQM, DPXZ-PQM and DPXZ-DPPM) in good yields. Finally,
these target compounds were further purified by vacuum
To better understand electronic structures and the frontier
molecular orbital distributions of these emitters, the quantum-
chemical calculations were carried out using the Gaussian 16
program package with the B3LYP/6-31G(d) level in the gas
phase. The optimized geometries and the HOMO and LUMO
distributions of these compounds are depicted in Fig. 2. For
2
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is mainly distributed on the DPPM acceptor unit. The
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DOI: 10.1039/C9TC05892C
separated frontier molecular orbitals lead to extremely small
theoretical ΔEST values of 32.5, 2.2, and 41.0 meV for PXZ-
PQM, DPXZ-PQM and DPXZ-DPPM, respectively, calculated
using time-dependent density functional theory (TDDFT)
calculation. In comparison with the other two emitters, DPXZ-
PQM possesses the lowest ΔEST which accelerates the RISC
process from triplet to singlet, indicating that DPXZ-PQM may
2
4-27
be a better TADF emitter.
Thermal Stability and Electrochemical Properties
Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) were performed under nitrogen atmosphere
to study their thermal stabilities. As shown in Fig. S1, DPXZ-
PQM and DPXZ-DPPM exhibit a thermal decomposition
temperature at 475 and 465 °C at a 5% weight reduction,
which is higher than that of PXZ-PQM at 368 °C due to larger
molecular
mass.
Furthermore,
the
glass-transition
temperature values were observed as 84, 137 and 173 °C for
PXZ-PQM, DPXZ-PQM and DPXZ-DPPM, respectively (Fig. S2).
These results clearly establish DPXZ-PQM and DPXZ-DPPM
more excellent thermal stability compared with PXZ-PQM,
indicated that DPXZ-PQM and DPXZ-DPPM can form more
stable amorphous film for the OLED fabrication by vacuum
2
8
deposition.
Electrochemical properties of these three emitters were
carried out by cyclic voltammetry (CV) measurements. These
three compounds exhibited characteristics of reversible
Fig. 1 Single crystal structure (a), packing mode (b, c) and intermolecular oxidation behavior from the PXZ donor. According to the onset
interaction (d, e) of PXZ-PQM in single crystals.
of the oxidation and reduction curves (vs
a
ferrocene/ferrocenium reference) (Fig. S3), the HOMO/LUMO
levels of PXZ-PQM, DPXZ-PQM and DPXZ-DPPM are
established to be - 5.43/- 3.04, - 5.46/- 3.09, and - 5.49/- 3.24
eV, respectively. The HOMO energy levels of these emitters
are almost identical to each other, due to their identical donor
moieties. Nevertheless, DPXZ-DPPM has a deeper LUMO
energy level relative to PXZ-PQM and DPXZ-PQM owing to
increasing the conjugated degree of the acceptor. Their band
PXZ-PQM, a small dihedral angle of 19.8° between the PQM
acceptor and the adjacent phenylene linker is observed. As the
increase of PXZ donors, the large steric hindrance induces
DPXZ-PQM the larger dihedral angles of 52.7° and 53.2°. In
addition, these three compounds exhibit similar highly twisted
structures with large dihedral angles of approximately 80°
between the PXZ donors and adjacent benzene rings, which
2
1-23
can promote efficient separation of the HOMO and LUMO.
g
gaps (E ) are calculated to be 2.39 eV for PXZ-PQM, 2.37 eV for
As expected, The HOMOs of these emitters are mainly located
on PXZ segments due to its good electron-donating and hole-
transporting properties, while the LUMOs of PXZ-PQM and
DPXZ-PQM are localized on the PQM acceptors unit with a
slight extent on the phenylene linker, and that of DPXZ-DPPM
DPXZ-PQM and 2.25 eV for DPXZ-DPPM, respectively, and all
electrochemical data are summarized in Table 1. These results
reveal that the integration of two kinds of acceptor moieties
29
has a great effect on the LUMO level.
Table 1 Photophysical and electrochemical data of PXZ-PQM, DPXZ-PQM, and DPXZ-DPPM
λ
nm)
abs
λ
PL
S
1
T
(eV)b)
1
ΔEST
(eV)c)
HOMO
(eV)
LUMO
(eV)
E
(eV)
g
PLQY
(%)f)
τ
P
τ
d
(μs)g)
Emitter
a)
a)
(eV)b)
d)
d)
e)
(ns)g)
(
(nm)
PXZ-PQM
DPXZ-PQM
DPXZ-DPPM
326, 440
325, 432
381, 494
592
2.23
2.20
0.03
0.02
0.05
-5.43
-5.46
-5.49
-3.04
-3.09
-3.24
2.39
2.37
2.25
70
21.8
5.37
594
642
2.25
2.13
2.23
2.08
88
61
38.7
25.6
3.77
3.53
a)Measured in toluene (1×10-5 M) at room temperature. b)The lowest excited singlet (S
reduction potential in DCM and DMF solution by cyclic voltammetry. Band gap energy. Photoluminescence quantum yield in doped film. PL lifetimes of prompt
) and triplet (T
) energies. c)ΔEST = S
– T
. d)Measured from the oxidation and
1
1
1
1
e)
f)
g)
p d
fluorescence (τ ) and delayed fluorescence (τ ) for the 5 wt% doped films at 300 K.
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Fig. 2 The optimized geometries, HOMO and LUMO distributions of (a) PXZ-PQM, (b) DPXZ-PQM, and (c) DPXZ-DPPM, respectively.
Fig. 3 (a) Normalized UV-vis absorption and fluorescence spectra of PXZ-PQM, DPXZ-PQM and DPXZ-DPPM in toluene at room temperature. (b) The temperature-
dependent transient PL decays of 5 wt% PXZ-PQM, DPXZ-PQM and DPXZ-DPPM doped in DCzDPy films measured at 100−300 K.
Photophysical Properties
cores, respectively. In comparison with PXZ-PQM and DPXZ-
PQM, DPXZ-DPPM exhibits significantly red-shifted absorption
maxima, which can be ascribed to the increased conjugated
degree. The PL spectra of PXZ-PQM and DPXZ-PQM in dilute
toluene solution are almost identical with the maximum
emission wavelength at 592 and 594 nm, which is a result of
cooperation of conjugation effect and charge transfer effect.
DPXZ-DPPM shows a large red-shift of 50 nm with an emission
peak at 642 nm, suggesting more obvious ICT effect owing to
Ultraviolet-visible (UV) absorption and photoluminescence (PL)
spectra of these three emitters in dilute toluene solvent (10-5
M) are shown in Fig. 3a and their key photophysical data are
summarized in Table 1. PXZ-PQM, DPXZ-PQM and DPXZ-
DPPM show weak and broad absorption bands absorption
maxima at 440, 432 and 494 nm, which can be attributed to
the intramolecular charge-transfer (ICT) transition from the
PXZ donor to the acceptor cores. And the intense high-energy
absorption bands with peaks at 326, 325 and 381 nm can be
assigned to the π-π* and n-π* transitions of PXZ and acceptor
8
, 30
the enhanced acceptor ability.
The fluorescence and
phosphorescence spectra of three emitters were recorded in
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Fig. 4 (a) Device diagram and chemical structures of the materials used for the devices. (b) External quantum efficiency versus current density characteristics. The
inset figure shows EL spectra of the devices. (c) Power efficiency and current efficiency versus current density characteristics.
toluene solution at 77 K, respectively, and the singlet (S
1
) and doped films give broad orange-red to red emission peaking at
triplet (T ) levels were estimated from the onsets of the 588, 586, and 630 nm, respectively, and corresponding
1
fluorescence and phosphorescence spectra. As shown in Fig. photoluminescence quantum yields (PLQYs) of 70%, 88%, and
S4a-S4c, the phosphorescence spectra of these compounds are 61%. To get better insight into the TADF natures of these
broad and structureless, illustrating that the phosphorescence materials, the transient PL decay characters of their doped
derives from the charge transfer (CT) state radiation transition films were performed. As shown in Fig. S7, PXZ-PQM, DPXZ-
of the triplet excitons. The S
calculated to be 2.23 and 2.20 eV for PXZ-PQM, 2.25 and 2.23 profile containing a nanosecond-scale fluorescence prompt
eV for DPXZ-PQM, and 2.01 and 2.06 eV for DPXZ-DPPM. component (τ ) of 21.8, 38.7, and 25.6 ns, and a microsecond-
Therefore, ΔESTs of PXZ-PQM, DPXZ-PQM and DPXZ-DPPM are scale delayed component (τ ) of 5.37, 3.77, and 3.53 μs,
determined to be 0.03, 0.02 and -0.05 eV, respectively, which respectively. These results are distinctly shorter than the most
1 1
and T energy levels are PQM and DPXZ-DPPM exhibit two component emission decay
P
d
1
4-17, 32
are basically consistent with the results of the theoretical of reported orange-red/red TADF emitters.
calculations. The negative ΔEST value for DPXZ-DPPM measured the temperature-dependent transient PL decays of
originates from the higher triplet excited state (T ), which can the blend films from 100 K to 300 K (Fig. 3b-d). The results
be explained by the four-level model suggested by Kobayashi showed that the contributions of delayed components to the
et al.31 Because the values of S
and T determined from the PLQY gradually increased with the increase of the
We also
n
1
1
solution could be influenced by the polarity of the solvent, we temperature, indicating their typical TADF natures of these
measured the fluorescence and phosphorescence spectra of emitters (Table S2).3
3-36
Moreover, the related photophysical
DPXZ-DPPM in n-hexane at 77 K (Fig. S4d). A positive ΔEST of rate constants of these doped films are calculated according to
.05 eV is obtained for DPXZ-DPPM. Such small ΔEST Equation S1−S6, and the detailed data are summarized in Table
demonstrate that efficient RISC process from T to S and high S3. Profiting from highly rigid molecular structures, the rate
constants of fluorescent (k ) values of these TADF emitters in
The solid state photophysical properties of three molecules the range of 10 s , which are much higher than most of the
0
s
1
1
2
4
triplet exciton utilization can be expected.
F
7
-1
3
7-39
were prepared and investigated by doping 5 wt% emitters in reported TADF emitters.
Their rate constants of RISC (kRISC)
5
5
5
-1
DCzDPy (5,5'-bis(carbazol-9-yl)-3,3'-bipyridine)). The are estimated to be 0.91 × 10 , 2.05 × 10 and 1.8 × 10 s ,
absorption, fluorescence (298 K) and phosphorescence (77 K) respectively. The highest PLQY and the maximum kRISC confirm
spectra of DCzDPy were shown in Fig. S5. As described in Fig. that DPXZ-PQM may realize best EL performance among them.
S6, the PL spectra of PXZ-PQM, DPXZ-PQM and DPXZ-DPPM
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Table 2 Summarized device performances of the devices based on PXZ-PQM, DPXZ-PQM, and DPXZ-DPPM.
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V
V)
on
a)
L
max
-2 b)
CE
(cd A-1) c)
PE
(lm W-1) d)
EQE
(%) e)
λ
EL
CIE
(x, y)
Emitter
f)
g)
(
(cd m )
(nm)
PXZ-PQM
DPXZ-PQM
DPXZ-DPPM
3.4
2.8
3.6
21900
14140
6017
45.2
60.7
13.6
37.7
67.4
10.6
20.4, 17.5, 11.3
26.0, 20.1, 13.7
11.5, 10.5, 6.8
592
590
630
(0.53, 0.46)
(0.51, 0.48)
(0.61, 0.38)
a)
Turn-on voltage at 1 cd m-2. b)Maximum luminance. c)Maximum current efficiency. d)Maximum power efficiency. e)Maximum EQE, and values at 100 and 1000 cd m-2.
f)
-2 g)
−2
The wavelength of the EL peak at 100 cd m . CIE (x,y): CIE coordinates measured at 100 cd m .
Electroluminescent Properties
small ΔEST value and the maximum kRISC of DPXZ-PQM. More
importantly, these three devices exhibit a mild efficiency roll-
off feature. At a luminance of 100 and 1000 cd m , the EQE
values still maintain to be a high level of 17.5 and 11.3% (roll
off: 14.2 and 44.6%) for the PXZ-PQM-based device, 20.1 and
To investigate the EL properties of these TADF materials,
multilayer devices were fabricated using PXZ-PQM, DPXZ-PQM
and DPXZ-DPPM doped in DCzDPy as the emitters,
respectively, with the following optimized device structure:
ITO/TAPC (45 nm) /TCTA (10 nm)/DCzDPy: dopant (5 wt%) (20
nm)/B3PymPm (50 nm)/LiF (1 nm)/Al (100nm), where ITO
-2
1
3.7% (roll off: 22.7 and 47.3%) for the DPXZ-PQM-based
device, 10.5 and 6.7% (roll off: 8.7 and 40.9%) for the DPXZ-
DPPM-based device, respectively (Table 2). The EQE value of
(indium tin oxide), LiF and Al are used as the anode, the
2
0.1% is also among the best device performance reported for
electron-injecting layer and the cathode, respectively; TAPC
-2 40-43
orange-red/red TADF-OLEDs at a luminance of 100 cd m .
In order to manifest their prominent EL performances, the
theoretical maximum EQE can be expressed by the following
Equation 1:
EQE = γ × (0.25 × Φ
(1,1-bis[4-[N,N-di(p-tolyl)-amino]phenyl]cyclohexane)
and
B3PymPm
(4,6-bis(3,5-di(pyridin-3-yl)phenyl)-2-
methylpyrimidine)) are used as the hole-transporting layer and
electron-transporting layer, respectively; TCTA (4,4’,4’’-
tris(carbazol-9-yi)triphenylamine) acts as the electron-blocking
layer. The chemical structures and energy level diagram of the
materials used as well as the device architectures of the
devices are shown in Fig. S8 and 4a. The current density–
voltage–luminance (J–V–L) characteristics, EL spectra and
efficiency versus luminance curves are shown in Fig. S9, 4b and
c, and the device performance parameters are summarized in
Table 2.
4
6
F
+ 0.25 × ΦTADF + 0.75 × ΦTADF/ΦISC) × ηout
(1)
Where γ is the charge recombination factor, which is ideally
00% if holes and electrons are completely recombined to
form excitons, Φ is the fluorescence efficiency, ΦTADF is the
1
F
TADF efficiency, ΦISC is the intersystem crossing quantum
efficiency, ηout is the light-out-coupling efficiency. Therefore,
the theoretical maximum EQEs of these three devices can be
calculated to be 20.9% for PXZ-PQM, 26.4% for DPXZ-PQM
and 17.9% for DPXZ-DPPM by assuming ηout = 0.3,
respectively, in good agree with their experimental maximum
EQE values, implying high exciton utilization up to 98.5% in
OLEDs.
As presented in Fig. S9, the turn-on voltages of PXZ-PQM,
DPXZ-PQM, and DPXZ-DPPM based devices are 3.4, 2.8 and
3
.6 V, respectively. The PXZ-PQM-, DPXZ-PQM-, and DPXZ-
DPPM-based devices display orange-red to red emission with
peaks at 592, 590, and 630 nm, respectively, corresponding to
the Commission Internationale de L’Eclairage (CIE) coordinates
of (0.53, 0.46), (0.51, 0.48) and (0.61, 0.38). The DPXZ-DPPM-
based device shows a maximum current efficiency (CEmax) of
-1
1
3.6 cd A , a maximum power efficiency (PEmax) of 10.6 lm W-1
and a maximum external quantum efficiency (EQEmax) of
1.5%. Furthermore, the PXZ-PQM-based device displays
1
-1
-1
higher efficiencies of 45.2 cd A , 37.7 lm W and of 20.4%,
which may be ascribed to the higher PLQY of PXZ-PQM.
Encouragingly, the DPXZ-PQM-based device achieves the best
-1
-1
EL performance with CEmax of 60.7 cd A , PEmax of 67.4 lm W
and EQEmax of 26.0%. Fig. 5 shows the device performances of
reported orange-red to red TADF-OLEDs with emission maxima
at 580-650 nm. Obviously, a state-of-the-art EQE of 26.0% is
among the highest values for TADF-OLEDs emitting in orange-
red to red range without any light out-coupling
enhancement.5
, 7, 14-17, 32, 39-45
Compared with PXZ-PQM and
DPXZ-DPPM, more superior performance of the DPXZ-PQM-
based device can be attributed to the highest PLQY, a relatively
6
| J. Name., 2015, 00, 1-3
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J Po lue ran sael od fo Mn aot te rai ad l jsu Cs ht emm ai rs gt ri yn sC
Journal Name
ARTICLE
Fig. 5 The EQEs summary of orange-red to red TADF OLEDs with emission peaks from
80 to 650 nm.
dried over anhydrous sodium sulfate. The crude product was
View Article Online
DOI: 10.1039/C9TC05892C
5
purified
by
using
column
chromatography
with
dichloromethane as the eluent to obtain an orange solid in
1
7
8
2
2
3% yield. H NMR (500 MHz, Chloroform-d): δ 9.48 (s, 1H),
.54 (t, J = 1.3 Hz, 1H), 8.50 – 8.46 (m, 2H), 8.31 (d, J = 1.2 Hz,
H), 7.97 – 7.87 (m, 2H), 7.69 – 7.64 (m, 1H), 7.63 – 7.59 (m,
H), 7.56 (m, J = 7.7 Hz, 2H), 6.74 (dd, J = 7.8, 1.6 Hz, 2H), 6.69
2
. Conclusions
In summary, by fusing two weak electron-withdrawing units,
the rigid planar PQM and DPPM acceptors were used to
constructed orange-red to red TADF emitters. The rigid and
planar constituent groups and large steric hindrance between
donor and acceptor units endow these emitters high PLQYs
and small ΔESTs. Among them, DPXZ-PQM exhibits the highest
(
t, J = 7.5 Hz, 2H), 6.63 (td, J = 7.6, 1.7 Hz, 2H), 6.05 (dd, J = 8.0,
13
1
1
1
.5 Hz, 2H). C NMR (101 MHz, Chloroform-d): δ 195.62,
52.40, 144.26, 144.13, 144.03, 141.60, 140.85, 138.32,
37.09, 136.40, 133.99, 133.04, 132.36, 131.89, 130.60,
PLQY as high as 88%, a relatively small ΔEST of 0.02 eV and the 130.54, 130.23, 130.18, 128.64, 123.36, 121.77, 115.72,
+
maximum kRISC. As a result, the orange-red TADF doped device 113.36. EIMS m/z: 491.38[M] . Anal. calcd for C H N O : C
3
3 21 3 2
based on DPXZ-PQM shows an extremely high EQEmax of up to 80.64, H 4.31, N 8.55, O 6.50; found: C 80.56, H 4.31, N 8.61, O
6.0% with a peak at 590 nm, which is among the highest 6.52.
values for orange-red to red TADF-OLEDs without any light Synthesis
2
of
(2,3-bis(4-bromophenyl)quinoxalin-6-
yl)phenylmethanone (DBr-PQM)
out-coupling enhancement. More importantly, a low efficiency
roll-off is rendered and the EQEs still maintain 20.1 and 13.7%
A suspension of 1,2-bis(4-bromophenyl)ethane-1,2-dione (1.10
at a luminance of 100 and 1000 cd m . This study provides a g, 3.0 mmol) and (3,4-diaminophenyl)phenylmethanone (0.64
-2
new insight for developing orange-red/red TADF emitters with g, 3.0 mmol) in acetic acid (20 mL) was heated to reflux for 8
high PLQY and short τ
d
.
hours. After cooled to room temperature, poured the resulting
mixture into ice-water (200 mL), and then filtered. The solid
was washed with water several times. The crude product was
purified by column chromatography to obtain a light yellow
3
. Experimental Section
1
solid in 80% yield. H NMR (400 MHz, Chloroform-d): δ 8.50 (t,
Synthesis of (2-(4-bromophenyl)quinoxalin-6-yl)phenylmethanone
Br-PQM)
J = 1.2 Hz, 1H), 8.27 (t, J = 1.2 Hz, 2H), 7.93 – 7.86 (m, 2H), 7.68
(
1
3
–
7.62 (m, 1H), 7.56 – 7.49 (m, 6H), 7.47 – 7.39 (m, 4H).
C
To a mixture of 4-bromophenacyl bromide (1.10 g, 4.0 mmol)
and (3,4-diaminophenyl)phenylmethanone (0.93 g, 4.4 mmol)
in 30 mL of water and hexadecyltrimethylammonium bromide
NMR (101 MHz, Chloroform-d): δ 195.68, 153.63, 153.10,
1
1
1
C
42.96, 140.28, 138.76, 137.27, 137.24, 137.09, 133.00,
32.34, 131.86, 131.52, 131.40, 130.34, 130.19, 129.76,
(CTAB) (0.36 g, 2.5 mmol) were added under nitrogen
+
28.62, 124.34, 124.17. EIMS m/z: 543.76[M] . Anal. calcd for
o
atmosphere and stirred for 12 hours at 120 C. Then, the
reaction mixture was poured into cold water, and the
precipitate was filtered and dried under vacuum. The crude
product was purified by column chromatography with
dichloromethane as the eluent to obtain a white solid in 57%
27 2 2
H16Br N O: C 59.59, H 2.96, Br 29.36, N 5.15, O 2.94; found:
C 59.48, H 2.99, Br 29.43, N 5.19, O 2.91.
Synthesis of (2,3-bis(4-(phenoxazin-10-yl)phenyl)quinoxalin-6-
yl)(phenyl)methanone (DPXZ-PQM)
According to the procedure similar to that of PXZ-PQM,
1
yield. H NMR (400 MHz, Chloroform-d): δ 9.38 (s, 1H), 8.52 –
compound DPXZ-PQM was synthesized using DBr-PQM (1.09 g,
8
7
7
1
1
1
C
.47 (m, 1H), 8.29 – 8.23 (m, 2H), 8.16 – 8.12 (m, 2H), 7.93 –
.88 (m, 2H), 7.75 – 7.71 (m, 2H), 7.69 – 7.62 (m, 1H), 7.57 –
.52 (m, 2H). 13C NMR (101 MHz, Chloroform-d): δ 195.57,
52.12, 144.05, 143.86, 140.75, 138.13, 137.08, 135.09,
32.98, 132.54, 132.32, 130.48, 130.19, 130.07, 129.21,
1
2
.0 mmol) to obtain an orange-red solid in 81% yield. H NMR
(500 MHz, Chloroform-d): δ 8.61 (d, J = 1.6 Hz, 1H), 8.39 – 8.33
(m, 2H), 7.96 – 7.92 (m, 2H), 7.85 – 7.77 (m, 4H), 7.69 – 7.65
(m, 1H), 7.56 (t, J = 7.7 Hz, 2H), 7.44 – 7.37 (m, 4H), 6.70 (ddd, J
=
1
7.9, 4.4, 1.4 Hz, 4H), 6.63 (m, J = 7.8 Hz, 4H), 6.52 (qd, J = 7.7,
.5 Hz, 4H), 5.95 (t, J = 8.8 Hz, 4H). C NMR (101 MHz,
+
28.60, 125.77. EIMS m/z: 388.16[M] . Anal. calcd for
H13BrN O: C 64.80, H 3.37, Br 20.53, N 7.20, O 4.10; found:
21 2
1
3
Chloroform-d): δ 195.68, 154.31, 143.98, 143.14, 140.46,
C 64.60, H 3.48, Br 20.60, N 7.21, O 4.11.
Synthesis of (2-(4-(phenoxazin-10-yl)phenyl)quinoxalin-6-
yl)phenylmethanone (PXZ-PQM)
1
1
1
40.37, 140.25, 139.01, 138.62, 137.08, 133.89, 133.07,
32.71, 132.71, 132.59, 132.45, 131.09, 130.21, 128.67,
23.46, 121.73, 115.68, 113.17. EIMS m/z: 748.42[M] . Anal.
+
Br-PQM (0.97 g, 2.5 mmol), phenoxazine (0.51 g, 2.8 mmol),
sodium tert-butoxide (0.48 g, 5.0 mmol) were combined in a
32 4 3
calcd for C51H N O : C 81.80, H 4.31, N 7.48, O 6.41; found: C
8
1.71, H 4.33, N 7.49, O 6.47.
2
50 mL flask and cycled under nitrogen and vacuum three
times, before tris(dibenzylideneacetone)dipalladium (0.10 g,
.1 mmol), 2.5 mL of a 1.0 M solution of tri-tertbutyl
Synthesis
of
(3,6-dibromodibenzophenazin-11-
yl)phenylmethanone (DBr-DPPM)
0
According to the procedure similar to that of DBr-PQM,
compound DBr-DPPM was synthesized using 3,6-
dibromophenanthrene-9,10-dione (0.64 g, 6.0 mmol) to obtain
phosphine in toluene, and 50.0 mL of o-xylene were added.
The reaction was performed for 12 hours at 140 °C, water (25
mL) was added to quench the reaction, the mixture was then
extracted with dichloromethane. The organic solution was
1
a red solid in 83% yield. H NMR (400 MHz, Chloroform-d): δ
9
.13 (d, J = 8.6 Hz, 1H), 9.05 (d, J = 8.6 Hz, 1H), 8.59 (d, J = 1.2
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2015, 00, 1-3 | 7
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J Po lue ran sael od fo Mn aot te rai ad l jsu Cs ht emm ai rs gt ri yn sC
Page 8 of 10
ARTICLE
Journal Name
8
9
Q. Zhang, J. Li, K. Shizu, S. Huang, S. Hirata, H. VMiewiyAarzticalkeiOanlninde
Hz, 1H), 8.46 (d, J = 2.3 Hz, 2H), 8.35 – 8.32 (m, 2H), 7.98 – 7.95
C. Adachi, J. Am. Chem. Soc., 2012, 134, 14706-14709.
DOI: 10.1039/C9TC05892C
(
m, 2H), 7.79 (ddd, J = 12.9, 8.7, 1.9 Hz, 2H), 7.72 – 7.67 (m,
S. Hirata, Y. Sakai, K. Masui, H. Tanaka, S. Y. Lee, H. Nomura,
N. Nakamura, M. Yasumatsu, H. Nakanotani, Q. Zhang, K.
Shizu, H. Miyazaki and C. Adachi, Nat. Mater., 2015, 14, 330-
336.
1
3
1
1
1
1
5
2
H), 7.62 – 7.57 (m, 2H). C NMR (101 MHz, Chloroform-d): δ
95.77, 143.73, 142.73, 142.36, 141.17, 138.39, 137.23,
32.97, 132.78, 132.54, 132.25, 131.82, 130.23, 129.93,
1
1
0 Q. Zhang, D. Tsang, H. Kuwabara, Y. Hatae, B. Li, T.
Takahashi, S. Y. Lee, T. Yasuda and C. Adachi, Adv. Mater.,
28.93, 128.64, 128.35, 128.06, 126.26, 125.99. EIMS m/z:
+
41.73[M] . Anal. calcd for C27
2 2
H14Br N O: C 59.81, H 2.60, Br
2
015, 27, 2096-2100.
9.47, N 5.17, O 2.95; found: C 59.73, H 2.62, Br 29.51, N 5.22,
1 Y. Geng, A. D'Aleo, K. Inada, L. S. Cui, J. U. Kim, H. Nakanotani
and C. Adachi, Angew. Chem., Int. Ed., 2017, 56, 16536-
16540.
O 2.92.
Synthesis
of
(3,6-di(phenoxazin-10-yl)dibenzophenazin-11-
1
1
2 A. Endo, M. Ogasawara, A. Takahashi, D. Yokoyama, Y. Kato
and C. Adachi, Adv. Mater., 2009, 21, 4802-4806.
3 J.-A. Lin, S.-W. Li, Z.-Y. Liu, D.-G. Chen, C.-Y. Huang, Y.-C. Wei,
Y.-Y. Chen, Z.-H. Tsai, C.-Y. Lo, W.-Y. Hung, K.-T. Wong and P.-
T. Chou, Chem. Mater., 2019, 31, 5981-5992.
yl)phenylmethanone (DPXZ-DPPM)
According to the procedure similar to that of PXZ-PQM,
compound DPXZ-DPPM was synthesized using DBr-DPPM (1.08
g, 2.0 mmol) to obtain a red solid in 84% yield. H NMR (500
1
1
1
4 J. X. Chen, W. W. Tao, Y. F. Xiao, K. Wang, M. Zhang, X. C.
Fan, W. C. Chen, J. Yu, S. Li, F. X. Geng, X. H. Zhang and C. S.
Lee, ACS Appl. Mater. Interfaces, 2019, 11, 29086-29093.
5 F. M. Xie, H. Z. Li, G. L. Dai, Y. Q. Li, T. Cheng, M. Xie, J. X.
Tang and X. Zhao, ACS Appl. Mater. Interfaces, 2019, 11,
MHz, Chloroform-d): δ 9.75 (dd, J = 8.6, 2.1 Hz, 1H), 9.67 (dd, J
=
8
1
6
8.6, 2.1 Hz, 1H), 8.78 (s, 1H), 8.54 (dd, J = 8.9, 2.4 Hz, 3H),
.43 (d, J = 8.8 Hz, 1H), 8.03 (d, J = 7.6 Hz, 2H), 7.81 (dd, J =
9.7, 8.5 Hz, 2H), 7.73 (t, J = 7.5 Hz, 1H), 7.63 (t, J = 7.6 Hz, 2H),
.76 (t, J = 6.2 Hz, 4H), 6.69 (q, J = 6.9 Hz, 4H), 6.61 (q, J = 7.0
2
6144-26151.
1
3
16 T. Yang, B. Liang, Z. Cheng, C. Li, G. Lu and Y. Wang, J. Phys.
Chem. C, 2019, 123, 18585-18592.
1
Hz, 4H), 6.06 (t, J = 9.3 Hz, 4H). C NMR (101 MHz,
Chloroform-d): δ 195.80, 144.02, 143.23, 142.86, 141.99,
7 J. X. Chen, K. Wang, C. J. Zheng, M. Zhang, Y. Z. Shi, S. L. Tao,
H. Lin, W. Liu, W. W. Tao, X. M. Ou and X. H. Zhang, Adv. Sci.,
2018, 5, 1800436.
1
1
1
41.78, 141.47, 138.63, 137.23, 134.49, 134.20, 134.02,
33.07, 132.93, 131.30, 130.30, 130.13, 130.06, 129.81,
1
1
2
8 M. Godumala, S. Choi, M. J. Cho and D. H. Choi, J. Mater.
Chem. C, 2016, 4, 11355-11381.
28.70, 125.96, 123.38, 121.88, 121.86, 115.79, 113.34. EIMS
+
m/z: 746.46[M] . Anal. calcd for C51
30
H N
4
O
3
: C 82.02, H 4.05, N
9 D. N. Kanekar, S. Chacko and R. M. Kamble, Dyes Pigm.,
7
.50, O 6.43; found: C 81.90, H 4.11, N 7.55, O 6.44.
2
019, 167, 36-50.
0 K. Zheng, H. Yang, F. Ni, Z. Chen, S. Gong, Z. Lu and C. Yang,
Adv. Opt. Mater., 2019, DOI: 10.1002/adom.201900727,
1900727.
Conflicts of interest
The authors declare no competing financial interests.
2
2
1 Z. Yang, Z. Mao, Z. Xie, Y. Zhang, S. Liu, J. Zhao, J. Xu, Z. Chi
and M. P. Aldred, Chem. Soc. Rev., 2017, 46, 915-1016.
2 Y. Im, M. Kim, Y. J. Cho, J.-A. Seo, K. S. Yook and J. Y. Lee,
Chem. Mater., 2017, 29, 1946-1963.
Acknowledgements
23 Y. Liu, C. Li, Z. Ren, S. Yan and M. R. Bryce, Nat. Rev. Mater.,
2
018, 3, No. 18020
This work was supported by National Basic Research Program
of China (2015CB655003), the National Natural Science
Foundation of China (51803069 and 21772064) and China
Postdoctoral Science Foundation (2018T110244 and
2
4 K. Suzuki, S. Kubo, K. Shizu, T. Fukushima, A. Wakamiya, Y.
Murata, C. Adachi and H. Kaji, Angew. Chem., Int. Ed., 2015,
54, 15231-15235.
2
2
2
2
2
5 H. S. Kim, H. Park, S.-R. Park, S. H. Lee, Y. Ahn, Y. S. Lee and
M. C. Suh, J. Phys. Chem. C, 2018, 122, 28576-28587.
6 W. Zeng, S. Gong, C. Zhong and C. Yang, J. Phys. Chem. C,
2
018M630319).
2
019, 123, 10081-10086.
7 W. Li, X. Cai, B. Li, L. Gan, Y. He, K. Liu, D. Chen, Y. C. Wu and
S. J. Su, Angew. Chem., Int. Ed., 2019, 58, 582-586.
8 Z. Li, C. Li, Y. Xu, N. Xie, X. Jiao and Y. Wang, J. Phys. Chem.
Lett., 2019, 10, 842-847.
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Journal of Materials Chemistry C
Page 10 of 10
Graphical Abstract
View Article Online
DOI: 10.1039/C9TC05892C
A class of novel donor-acceptor orange-red to red thermally activated delayed fluorescence (TADF) emitters
with the maximum external quantum efficiency (EQE) up to 26.0% and extremely low efficiency roll-off
have been designed and synthesized.