Nondoped deep-blue fluorescent organic electroluminescent device with CIEy = 0.06 and low efficiency roll-off based on carbazole/oxadiazole derivatives

Article abstract of DOI:10.1016/j.orgel.2019.02.028

Deep-blue organic light-emitting diodes (OLEDs) with the Commission Internationale de L'e’clairage (CIE) coordinates matching the European Broadcasting Union (EBU) standard blue CIE (x, y) coordinates of (0.15, 0.06) are very limited and in enormous deman

Full text of DOI:10.1016/j.orgel.2019.02.028

Accepted Manuscript
Nondoped deep-blue fluorescent organic electroluminescent device with CIE = 0.06
y
and low efficiency roll-off based on carbazole/oxadiazole derivatives
Yu Tan, Zhende Wang, Chen Wei, Zhiwei Liu, Zuqiang Bian, Chunhui Huang
PII:
S1566-1199(19)30097-7
DOI:
https://doi.org/10.1016/j.orgel.2019.02.028
ORGELE 5134
Reference:
To appear in: Organic Electronics
Received Date: 25 November 2018
Revised Date: 26 January 2019
Accepted Date: 24 February 2019
Please cite this article as: Y. Tan, Z. Wang, C. Wei, Z. Liu, Z. Bian, C. Huang, Nondoped deep-
blue fluorescent organic electroluminescent device with CIE = 0.06 and low efficiency roll-off
y
based on carbazole/oxadiazole derivatives, Organic Electronics (2019), doi: https://doi.org/10.1016/
j.orgel.2019.02.028.
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ACCEPTED MANUSCRIPT
Nondoped deep-blue fluorescent OLEDs were fabricated.
101
101
100
10-1
10-2
100
2.3
2.4
2.6
2.7
3.4
EQEmax: 4.0%
Low roll-off
10-1
CIE: (0.16, 0.06)
1
10
100
1000
Luminance (cd/m2)
5.5 5.3
5.8
5.9
6.2

ACCEPTED MANUSCRIPT
Nondoped deep-blue fluorescent organic electroluminescent device
with CIEy = 0.06 and low efficiency roll-off based on
carbazole/oxadiazole derivatives
Yu Tan, Zhende Wang, Chen Wei, Zhiwei Liu*, Zuqiang Bian and Chunhui Huang
ABSTRACT
Deep-blue organic light-emitting diodes (OLEDs) with the Commission
Internationale de L’e´clairage (CIE) coordinates matching the European Broadcasting
Union (EBU) standard blue CIE (x, y) coordinates of (0.15, 0.06) are very limited and
in enormous demands. Here, we designed and synthesized four blue-emitting
compounds with D-A structures based on carbazole and oxadiazole units. After the
systematic studies of their photophysical, thermal and electrochemical properties, it is
found that their photoluminescent properties were tuned successfully via changing the
electronic structures of Ar group connecting with the oxadiazole and they all have
high photoluminescent quantum yields (PLQYs). Then we selected the outstanding
one as the emitter to prepare nondoped deep-blue OLEDs. The devices have
maximum brightness up to 4406 cd m-2 and high maximum external quantum
efficiency (EQE) up to 4.0% along with very small efficiency roll-off. Furthermore,
the CIE coordinates of (0.16, 0.06) almost reaches the EBU standard blue. The
comprehensive performance of the device is competitive among the best reported
nondoped deep-blue fluorescent OLEDs with EBU standard blue CIE coordinates.
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Keywords: Deep-blue OLED; Nondoped; EBU; Carbazole/oxadiazole derivatives
Introduction
Organic light-emitting diodes (OLEDs) have achieved tremendous development in
the last decades and even been called as the new generation of display and lighting
technologies[1-9]. But some issues still need to be resolved, among which the
significant one is the lack of deep-blue emitting materials[10] especially with the
Commission Internationale de L’e´clairage (CIE) coordinates matching the National
Television System Committee (NTSC) standard blue with CIE coordinates of (0.14,
0.08) or further the European Broadcasting Union (EBU) one of (0.15, 0.06)[11-19].
As a matter of fact, deep-blue emitters play very important roles in displays and
lightings. They can not only cut down power consumptions and extend color gamut,
but also manage the color temperature and generate other visible emissions[13, 17,
19-22]. Recently, some deep-blue emitters have been developed with special
luminous mechanisms such as thermally activated delayed fluorescence (TADF)[5,
23-25], hybridized local and charge transfer (HLCT)[12, 19, 26, 27] and triplet-triplet
annihilation (TTA)[28-30] etc. Although many of the OLEDs based on them achieved
high efficiency, but the number of ones with CIEy below 0.10 is still very limited,
letting alone that close to 0.08 or 0.06. Moreover, most of these OLEDs adopted
doped device structures which needs hosts with a wider energy gap. Although this
doped method can indeed improve the performance of OLEDs, it not only increases
the difficulties in the rational match of host-dopant systems, but also raises the costs
of device fabrication and easily leads to the phase separation problem[10, 31-34]. In
comparison, nondoped devices can meet the requirements of simplifying device
structures and reducing preparation costs. But as the pure emitting layer, an emitting
material had better be capable of balancing the charge injection and transport in the
nondoped device. To achieve this, a “D-A” type structure is usually designed for an
emitting molecule, in which “D” is electronic donor while “A” is electronic
acceptor[19, 20, 27, 35-37]. But another factor need to be considered, that is the D
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and A groups must be selected deliberately to realize an energy gap as wide as
possible for a satisfying low CIEy[19, 24]. In addition, a certain degree of conjugacy
and twist of the molecular structure also need to be equipped to ensure high
photoluminescent quantum yields (PLQYs) in neat film state[13, 20, 28, 37-39].
Carbazole unit is widely used to construct blue or deep-blue emission compounds
because of its deep highest occupied molecular orbital (HOMO) energy level, high
luminous efficiency, good hole transporting property and stability[40, 41]. Oxadiazole
unit possesses outstanding electron transporting property and is frequently utilized to
build electron transporting layer materials or bipolar fluorescent compounds[23,
42-44].
Hence, we used a simple molecular constructing strategy to design a series of D-A
structure blue-emitting compounds, which contain carbazole as electronic donors
while oxadiazole as acceptors. Their molecular structures and synthetic routes are
showed in Scheme 1. They all have high PLQYs. Based on one emitter, we prepared
nondoped deep-blue OLEDs. The device shows high maximum external quantum
efficiency (EQE) up to 4.0% along with very small efficiency roll-off. Moreover, the
CIE coordinates of (0.16, 0.06) are very close to the EBU standard blue.
Scheme 1. Synthetic routes and molecular structures of Cz-Oxa-Ph, Cz-Oxa-Py, Cz-Oxa-Pz and
Cz-Oxa-BPh.
Results and discussion
Photophysical properties
Firstly, the photophysical properties in solution (CH2Cl2) of the four compounds
were studied, as Figure 1a shows. They demonstrated similar absorption bands
shapes, where the absorption peaks at about 340 nm should be assigned to the charge
transfer transitions between carbazole and central electron-withdrawing oxadiazole[18,
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45, 46]. From the onset of absorption spectra, the energy gaps (Eg) were estimated to
be 3.28 eV, 3.22 eV, 3.11 eV and 3.25eV for Cz-Oxa-Ph, Cz-Oxa-Py, Cz-Oxa-Pz
and Cz-Oxa-BPh respectively. They all showed blue emission in solution with the
maximum photoluminescent (PL) peaks at 420 nm, 434 nm, 498 nm and 422 nm
respectively in the above order (Figure 1a). The key photophysical data is listed in
Table 1. It is found that for the three compounds (Cz-Oxa-Ph, Cz-Oxa-Py and
Cz-Oxa-Pz), the energy gaps were decreasing and the emission spectra were
red-shifted with the “N” heteroatom sequentially introduced into the Ar group.
Moreover, the Eg and emission of Cz-Oxa-BPh are close to that of Cz-Oxa-Ph,
indicating that a more phenyl didn’t have an obvious influence on these properties as
the “N” heteroatom worked. Hence, we used density functional theory (DFT)
calculations to understand these behaviors in depth, as Figure 2 and Table 1 shows.
Their HOMOs distributions are almost the same, concentrating on carbazole and the
adjacent phenyl, so the differences between their HOMOs values are small.
Comparing their lowest unoccupied molecular orbitals (LUMOs) values, we can find
that the LUMOs energy levels gradually become deeper for Cz-Oxa-Ph, Cz-Oxa-Py
and Cz-Oxa-Pz, respectively. Analyzing the reason, it should be responsible for this
tendency that the more introduction of “N” heteroatom reduces the electron cloud
density of Ar group[46]. The eventual presentation is the lower energy gaps and
red-shifted PL spectra for these three compounds, which is consistent with the
experimental results. As for Cz-Oxa-BPh and Cz-Oxa-Ph, their LUMOs values are
close to each other, which could explain their close energy gaps and emission spectra
as mentioned above. In their optimized geometry structures, all of them have a certain
degree of twist between carbazole and the adjacent phenyl with similar dihedral
angles (about 54 ), which suggests that they are potentially suitable to serve as
efficient emitters in neat film states.
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(a)
(b)
Cz-Oxa-Ph
Cz-Oxa-Py
Cz-Oxa-Pz
Cz-Oxa-BPh
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Cz-Oxa-Ph
Cz-Oxa-Py
Cz-Oxa-Pz
Cz-Oxa-BPh
300
400
500
600
700
400
500
600
700
Wavelength (nm)
Wavelength(nm)
Figure 1. (a) UV-vis absorption spectra in CH2Cl2 and (b) PL spectra in neat films of the Cz-Oxa
series compounds.
Figure 2. DFT-calculated results of the Cz-Oxa series compounds, including the optimized
geometry structures and HOMOs-LUMOs distributions.
Table 1. The key photophysical, thermal and electrochemical properties of Cz-Oxa series
compounds.
a
b
c
f
λabs
λPL
λPL
PLQYd Tse/Tg
Eg
HOMO/LUMOg
[eV]
Compd.
[nm]
[nm] [nm]
[%]
[ ]
[eV]
3.28
3.22
3.11
3.25
Cz-Oxa-Ph
Cz-Oxa-Py
Cz-Oxa-Pz
283, 341
286/341
290, 341
420
434
498
422
413
432
461
417
87/98
96/69
89/64
98/84
341/63
336/73
338/81
394/88
-5.73/-2.03
-5.76/-2.19
-5.79/-2.48
Cz-Oxa-BPh 293, 341
-5.72/-2.10
b
a The maximum absorption wavelength, measured in CH2Cl2 (1×10-5 mol L-1); the fluorescent
emission peak in CH2Cl2 (1×10-5 mol L-1) at room temperature; c the fluorescent emission peak in
d
e
neat film;
photoluminescence quantum yields in CH2Cl2 and neat film respectively;
corresponding to 5% weight loss; f optical gaps calculated from the absorption onset; g theoretical
values, based on DFT calculations at the B3LYP/6-311G** levels.
We also measured their PLQYs in dilute solution, as Table 1 shows. They all have
very high PLQYs in solutin. Next, we investigated their photoluminescent properties
in neat films. Figure 1b shows the PL spectra of these films. In comparison, the
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emissions of Cz-Oxa-Ph and Cz-Oxa-BPh films locate in the bluest region with PL
peaks below 420 nm. Moreover, they both have PLQYs above 80%, indicating that
they may act well as nondoped deep-blue emitters in OLEDs.
Furthermore, to explore whether these compounds have TADF characteristics, we
measured the transient PL decay spectra of the Cz-Oxa series films in short and long
time range, as Figure S1 (see Supporting Information) shows. Obviously, four
compounds all decay without long-lifetime components with microsecond magnitude,
but show short lifetime with nanosecond one (Table S1). Further, in order to evaluate
their energy level differences (∆EST) between singlet state and triplet state, we
measured the fluorescence spectra at room temperature and phosphorescence spectra
at 77 K in 2-MeTHF (~10-5 mol L-1), as Figure S2 shows, in which the
phosphorescence spectra were achieved after being delayed by 5 ms. From the highest
energy peaks of phosphorescence spectra, the energy levels of triplet (ET1) can be
estimated to be 2.66 eV, 2.66 eV, 2.61 eV and 2.52 eV for Cz-Oxa-Ph, Cz-Oxa-Py,
Cz-Oxa-Pz and Cz-Oxa-BPh respectively. Then, their ∆EST can be calculated, which
are 0.62 eV, 0.56 eV, 0.50 eV and 0.73 eV for Cz-Oxa-Ph, Cz-Oxa-Py, Cz-Oxa-Pz
and Cz-Oxa-BPh respectively. Their ∆EST are large that the reverse intersystem
crossing (RISC) process could not be realized effectively. These evidences indicate
that the Cz-Oxa series compounds are probably not TADF emitters. These
experimental data are summarized in Table S1.
Thermal properties
Good thermal stability plays a positive role in fabricating device with vacuum
evaporation method and enhancing the long-term operation of device, including high
sublimation/decomposition temperature (Ts/Td) and glass transition temperature (Tg).
The thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC)
were used to investigate the thermal properties of the four compounds, the results
showed in Figure 3 and Table 1. They have good thermal stability (Figure 3a), while
the Tg are less than 100 (Figure 3b), among which Cz-Oxa-BPh has the highest Tg
(88 ) likely due to its heaviest molecular weight. Considering the joule heat
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generated in an operational device, a high Tg is necessary to stabilize film morphology.
Hence, Cz-Oxa-BPh is more suitable to be employed as an emitter in OLEDs in
terms of thermal properties.
(a)
(b)
Cz-Oxa-Ph
Cz-Oxa-Pz
Cz-Oxa-Py
Cz-Oxa-BPh
100
80
60
40
20
0
-2.0
-2.5
-3.0
87.7 °C
Cz-Oxa-Ph
Cz-Oxa-Py
Cz-Oxa-Pz
Cz-Oxa-BPh
81.1 °C
73.3
80
°C
63.2 °C
100
200
300
400
500
50
60
70
90
100
Temperature (°C)
Temperature (°C)
Figure 3. (a) TGA and (b) DSC curves of the Cz-Oxa series compounds.
Electrochemical properties
We used cyclic voltammetry to study their electrochemical properties, the results as
Figure 4 shows. Their oxidation processes present considerable reversibility,
indicating they have good electrochemical properties and are capable of
accomplishing well the functions of charge carriers in the operation of devices. From
the oxidation potentials, the HOMOs values can be evaluated[47] and then the
LUMOs values obtained according to the calculated HOMOs and the energy gaps
deduced from the absorption onsets.
4.5
0.81 V
0.83 V
Cz-Oxa-Pz
Cz-Oxa-Py
Cz-Oxa-Ph
Cz-Oxa-BPh
Ferrocene
3.0
1.5
0.0
0.86 V
0.89 V
0
1
2
3
Potential (V)
Figure 4. Cyclic voltammetry curves of the Cz-Oxa series compounds.
Electroluminescent properties
After the study of aforementioned properties of the four compounds, Cz-Oxa-BPh
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stands out as a deep-blue emitter on the whole. So, we used Cz-Oxa-BPh as pure
emitting material to fabricate nondoped OLEDs with three-layers structures below:
ITO/MoO3 (2 nm)/NPB(30 nm)/mCP(10 nm)/Cz-Oxa-BPh (20 nm)/TPBi(40
nm)/LiF (1 nm)/Al (100 nm) (named as device 1) and ITO/MoO3 (2 nm)/mCP: MoO3
(20 wt%, 20 nm)/mCP(20 nm)/Cz-Oxa-BPh (20 nm)/TPBi(40 nm)/LiF (1 nm)/Al
(100 nm) (named as device 2). The energy-level diagram of these materials used in
nondoped OLEDs is showed in Figure 5a, in which ITO (indium tin oxide) and Al
were used as anode and cathode respectively. 2 nm-thickness MoO3 was used to
modify the work function of ITO to enhance hole injection[20, 48] and LiF was used
as the electron injecting layer. TPBi (1, 3, 5-tris (1-phenyl-1H-benzimidazol-2-yl)
benzene) was used as electron transporting layer as well as hole blocking layer
because of its low-lying HOMO level. Comparing two devices, the hole transporting
layers adopted different configurations. In device 1, NPB (N, N’-di-1-naphthyl-N,
N’-diphenylbenzidine) was used as hole transporting layer and mCP (1,
3-bis(N-carbazolyl) benzene) as exciton-blocking layer. In device 2, the hole
transporting layer was changed into a 20 nm-thickness mCP doped with MoO3 (20
wt%) followed by a pure mCP layer, which is intended further to enhance hole
injection[49]. The device performance is showed in Figure 5 and Table 2.
2.3
2.4
(a)
(b)
1.0
0.8
0.6
0.4
0.2
0.0
2.6
2.7
device 1
device 2
3.4
@ 1000 cd/m2
300
400
500
600
700
800
5.5 5.3
Wavelength (nm)
5.8
5.9
6.2
(c)
(d)
101
100
10-1
101
103
800
100
10-1
10-2
102
101
100
400
0
device 1
device 2
device 1
device 2
4
6
8
10
12
14
1
10
100
1000
Luminance (cd/m2)
Voltage (V)
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Figure 5. (a) Energy-level diagram of the materials used in nondoped OLEDs, (b) EL spectra at
1000 cd m-2, (c) luminance-voltage-current density (L-V-J) curves of the devices and (d) power
efficiency-luminance-external quantum efficiency (PE-L-EQE) curves of the devices.
The two devices showed deep-blue emission with EL peaks at 423 nm (Figure 5b).
The EL spectra are similar to the PL ones, indicating that the emissions were from
only Cz-Oxa-BPh rather than other layers or any exciplex formed. From Figure 5c,
we see that both devices have high maximum brightness with 2683 cd m-2 and 4406
cd m-2 respectively. Moreover, both devices have good EQE with 3.1% and 4.0% for
device 1 and device 2 respectively, as Figure 5d shows. But in comparison, the EQE
roll-off of device 1 is severer than that of device 2. For device 2, the EQE can still
retain 3.8% and 3.1% at 100 cd m-2 and 1000 cd m-2 respectively. Therefore, the
performance of device 2 is more excellent than that of device 1, which can be
attributed to the balanced charge injection and transport in the device after the
optimization of the hole transporting layers. In addition, device 2 has the CIE
coordinates of (0.16, 0.06) at a high brightness (3000 cd m-2), which is actually very
close to the EBU standard blue. Comparing with the reported nodoped deep-blue
fluorescent OLEDs with EBU emission (see Table 3), the comprehensive
performance of our device is competitive among the best ones regardless of the EQE
or the efficiency roll-off. The brilliant performance of the nondoped device is
inseparable from the emitting material itself, which indicates that our molecular
design strategy is successful in obtaining a good deep-blue emitter.
Moreover, we adopted two hosts (DPPOC[50] and CBP, their molecular structures
as Figure S6 shows) to prepare doped devices with the same structure of device 2.
The comparison of device performance for nondoped and doped OLEDs is shown in
Figure S7 and Table S3. We can see that the performance parameters of doped
devices are inferior to that of nondoped one. Analyzing the reason, the device
structure is optimal for nondoped device rather than for doped ones, which led to
imbalanced injection and transporting of carriers and poor exciton confinement in the
doped devices. Moreover, the choice of host materials also has great influence on the
device performance. So there are still substantial room to further improve the
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performance of doped devices from some respects such as the optimization of
functional transporting layer or host materials.
Table 2. Key device performance of nondoped deep-blue OLEDs based on Cz-Oxa-BPh.
a
b
c
d
e
Von
λEL
Lmax
CEmax
PEmax
EQEf
[%]
Device
[V]
4.0
3.3
[nm]
[cd m-2]
[cd A-1]
[lm W-1]
device 1
device 2
423
2683
1.3
1.0
3.1/2.0/1.4
423
4406
1.4
1.3
4.0/3.8/3.1
a
b
d
e
At 1 cd m-2; at 1000 cd m-2; c maximum brightness; maximum current efficiency; maximum
power efficiency; f maximum EQE, the values at 100 cd m-2 and 1000 cd m-2, respectively.
Table 3. Key performance parameters of the recently reported nondoped deep-blue OLEDs with
CIE coordinate close to EBU standard.
Compound
Cz-Oxa-BPh
AFpTPI
Von [V]
3.3
4.8
3.5
3.2
3.2
4.1
3.6
3.0
3.0
3.5
4.0
4.75
3.6
3.6
2.0
-
EQEa [%]
4.0/3.8/3.1
4.56b
λEL [nm]
423
434
-
CIE (x, y)
0.16, 0.06
0.15, 0.06
0.16, 0.06
0.16, 0.06
0.16, 0.06
0.153, 0.067
0.155, 0.058
0.15, 0.06
0.16, 0.06
0.157, 0.055
0.16, 0.06
0.166, 0.064
0.162, 0.057
0.165, 0.068
0.15, 0.06
0.16, 0.056
0.16, 0.06
Ref.
This work
[51]
PTPA
1.97/-/-
3.63/3.61/3.36
4.2/-/2.96
5.80/-/-
6.11/-/1.30
5.4/-/-
[52]
m-BBTPI
An-1
428
424
-
[53]
[54]
pBFCz-26DPPM
MBAn-(4)-F
POA
[18]
440
-
[55]
[56]
3nPI-BP-4PI
CzS1
4.95/-/-
4.21/4.20/3.19
3.69/-/-
-
436
426
436
420
424
428
424
428
428
[15]
[57]
BDPP
[58]
CzB-PIM
DPACFPPI
DPACTPI
PATPA
[59]
3.03/-/-
2.31/-/-
0.72/-/-
3.02/-/-
-
[26]
[26]
[60]
M2
[61]
TPA-(3)-F
5.8
[62]
a EQE corresponding to the maximum value, the value at brightness of 100 cd m-2 and 1000 cd m-2,
respectively; b at 300 cd m-2.
Conclusion
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In this work, we designed and synthesized four blue-emitting compounds with D-A
structures based on carbazole and oxadiazole. Through changing the electronic
structures of Ar group connecting with the oxadiazole, the photophysical properties
including the emission spectra and PLQYs of these materials were successfully tuned.
Nondoped deep-blue OLEDs were fabricated based on one emitter. The device has a
maximum brightness up to 4406 cd m-2 and high maximum EQE up to 4.0% along
with very small efficiency roll-off. Moreover, the CIE coordinates of (0.16, 0.06) are
extremely close to the EBU standard blue. These results are competitive with the best
nondoped deep-blue fluorescent OLEDs with EBU CIE coordinates. We think this
work will have a positive effect on exploring new deep-blue emitters which can be
applied to nondoped OLEDs.
Experimental section
General information
1
All chemicals were received from commercial resources. H-NMR spectra were
recorded on a Bruker-400M NMR spectrometer using CDCl3 or DMSO as the solvent.
13C-NMR spectra were recorded on a Bruker-500M NMR spectrometer using CDCl3
as the solvent. Mass spectra were measured on a Bruker Apex IV FTMS. Elemental
analyses were performed on a VARIO EL analyzer (GmbH, Hanau, Germany).
Synthesis
4-(9H-carbazol-9-yl)benzonitrile (1)
Carbazole (8.37 g, 50 mmol), 4-fluorobenzonitrile (6.06 g, 50 mmol) and
anhydrous potassium carbonate (20.7 g, 150 mmol) were added into in DMSO (50
mL). The mixture was stirred at 140 for 12 h under a nitrogen atmosphere. After
cooling to room temperature, the reaction mixture was poured into 1 L water and a lot
of gray solids were obtained and then followed by vacuum drying. The crude product
was purified by silica gel chromatography (dichloromethane: petroleum ether = 1:3)
and then recrystallization (ethanol: dichloromethane = 1:1). The colorless needle-like
crystals were obtained as the final product (5.36 g, yield: 40%). 1H-NMR (400 MHz,
CDCl3, δ): 8.15-8.13 (d, 2H), 7.91-7.88 (d, 2H), 7.75-7.72 (q, 2H), 7.47-7.41 (m, 4H),
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7.35-7.31 (m, 2H). MS: m/z: [MH+] = 269.1.
9-(4-(1H-tetrazol-5-yl)phenyl)-9H-carbazole (2)
1 (10.7 g, 40 mmol), sodium azide (2.86 g, 44 mmol) and ammonium chloride
(2.35 g, 44 mmol) were dissolved in DMF (100 mL). The mixture was stirred at 100
for 24 h. After cooling to room temperature, the reaction mixture was poured into 1 L
ice water and then acidified with dilute hydrochloric acid to get white precipitate.
After filtration, the filter cake was washed with water for 2-3 times and then dried in
vacuum chamber. After recrystallization (absolute ethanol), the colorless needle-like
crystals were obtained as the final product (10.6 g, yield: 85%). 1H-NMR (400 MHz,
DMSO-d6, δ): 8.35-8.33 (d, 1H), 8.29-8.27 (d, 2H), 7.93-7.91 (d, 2H), 7.53-7.46 (q,
4H)，7.35-7.31 (t, 2H), 4.36 (m, 1H). MS: m/z: [MH+] = 312.1.
2-(4-(9H-carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole (Cz-Oxa-Ph)
2 (2.49 g, 8 mmol) was firstly dissolved in super-dry pyridine (20 mL). Then
benzoyl chloride (1.23 g, 8.8 mmol) was added dropwise into the above solution
under nitrogen protection. The mixture was refluxed for 2 h. After cooling to room
temperature, the reaction mixture was poured into 200 mL water and then extracted
with dichloromethane to get an orange solution. The solution was dried by anhydrous
sodium sulfate and then the solvent was removed in reduced pressure. A pale yellow
solid was obtained (2.61 g) and then further purified by thermal gradient sublimation
(200 °C-150 °C-100 °C) at ultralow pressure to get colorless crystals (1.60 g, yield:
1
51%). H-NMR (400 MHz, CDCl3, δ): 8.38 (dd, 2H), 8.24-8.11 (m, 4H), 7.78 (dd,
2H), 7.61-7.40 (m, 7H), 7.33 (t, 2H). 13C-NMR (126 MHz, CDCl3, δ): 164.82, 164.04,
140.97, 140.35, 131.87, 129.17, 128.60, 127.28, 127.03, 126.25, 123.92, 123.86,
122.56, 120.63, 120.50, 109.74. MS: m/z: [MH+] = 388.1. EA: C:80.67%, H:4.53%,
N:10.75% (theoretical value: C, 80.60%; H, 4.42%; N, 10.85%).
2-(4-(9H-carbazol-9-yl)phenyl)-5-(pyridin-2-yl)-1,3,4-oxadiazole (Cz-Oxa-Py)
Picolinic acid (1.49 g, 12 mmol) and N, N'- dicyclohexyl carbodiimide (2.49 g, 12
mmol) were dissolved in toluene (25 mL) and the mixture was stirred for 0.5 h. Then
2 (2.49 g, 8 mmol) was added rapidly into the solution and the mixture reacted at 90
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for 24 h under nitrogen protection. After cooling to room temperature, the reaction
mixture was further cooled in the freezing chamber until white precipitate was
obtained. Filtered, the filtrate was retained and then the residual toluene was removed
by distillation to afford black oily matter, which was subsequently purified by silica
gel chromatography (ethyl acetate: petroleum ether = 3:2) to afford light brown solids
(0.93 g, yield: 20%). The product further underwent recrystallization (absolute
ethanol) and thermal gradient sublimation (200 °C-150 °C-100 °C) at ultralow
pressure and colorless crystals were obtained finally. 1H-NMR (400 MHz, CDCl3, δ):
8.86-8.85(d, 1H), 8.49-8.46(d, 2H), 8.39-8.36 (d, 1H), 8.17-8.15(d, 2H), 7.97-7.92(t,
13
1H), 7.81-7.79(d, 2H), 7.53-7.50(t, 3H), 7.47-7.43(t, 2H), 7.35-7.31(t, 2H). C-NMR
(126 MHz, CDCl3, δ): 165.02, 164.08, 150.39, 143.65, 141.26, 140.30, 137.24,
128.98, 127.19, 126.26, 125.89, 123.87, 123.35, 122.19, 120.64, 120.49, 109.74. MS:
m/z: [MH+] = 389.1. EA: C: 77.52%, H: 4.27%, N: 14.51% (theoretical value: C,
77.30%; H, 4.15%; N, 14.42%).
2-(4-(9H-carbazol-9-yl)phenyl)-5-(pyrazin-2-yl)-1,3,4-oxadiazole (Cz-Oxa-Pz)
The synthetic process of Cz-Oxa-Pz was similar to that of Cz-Oxa-Py, the
difference was just replacing picolinic acid with pyrazine-2-carboxylic acid. Finally,
yellow solids were obtained (2.34 g, yield: 50%). The product was also further
purified
through
recrystallization
and
thermal
gradient
sublimation
1
(200 °C-150 °C-100 °C). H-NMR (400 MHz, CDCl3, δ): 9.59(m, 1H), 8.80(m, 2H),
8.48-8.46(d, 2H), 8.17-8.15(d, 2H), 7.83-7.81(d, 2H), 7.53-7.51(d, 2H), 7.48-7.43(t,
2H), 7.36-7.32(t, 2H). 13C-NMR (126 MHz, CDCl3, δ): 165.41, 162.24, 146.50,
144.63, 144.35, 141.57, 140.21, 139.69, 129.05, 127.25, 126.27, 123.89, 121.73,
120.71, 120.50, 109.70. MS: m/z: [MH+] = 390.1. EA: C: 73.62%, H: 3.80%, N:
17.93% (theoretical value: C, 74.02%; H, 3.88%; N, 17.98%).
2-(4-(9H-carbazol-9-yl)phenyl)-5-([1,1'-biphenyl]-4-yl)-1,3,4-oxadiazole
(Cz-Oxa-BPh)
2 (1.86 g, 6 mmol) was firstly dissolved in super-dry pyridine (60 mL). Then
4-biphenylcarbonyl chloride (1.32 g, 6.06 mmol) was added dropwise into the above
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solution under nitrogen protection. The mixture was refluxed for 2 h. After cooling to
room temperature, the reaction mixture was poured into 200 mL water and white
precipitates then were separated out. After filtration, the filter cake was washed with
water for 2-3 times and then dried in vacuum chamber. The crude product was
purified by silica gel chromatography (dichloromethane) to afford colorless solids
(2.23 g, yield: 80%), which also finally underwent thermal gradient sublimation
(245 °C-195 °C-100 °C) at ultralow pressure. 1H-NMR (400 MHz, CDCl3 δ): 8.41 (d,
2H), 8.25 (d, 2H), 8.16 (d, 2H), 7.79 (d, 4H), 7.67 (d, 2H), 7.56-7.39 (m, 7H), 7.33 (t,
2H). 13C-NMR (126 MHz, CDCl3, δ): 164.73, 164.04, 144.66, 140.97, 140.35, 139.82,
129.04, 128.61, 128.26, 127.79, 127.47, 127.28, 127.19, 126.26, 123.87, 122.63,
122.57, 120.63, 120.51, 109.75. MS: m/z: [MH+] = 464.2. EA: C: 83.07%, H: 4.50%,
N: 9.12% (theoretical value: C, 82.92%; H, 4.57%; N, 9.07%).
Photophysical measurements
UV-vis absorption spectra were measured on a Shimadzu UV-3100 spectrometer.
The fluorescence spectra were measured on an Edinburgh Analytical Instruments
FLS980 spectrophotometer. PLQYs were measured on C9920-02 absolute quantum
yield measurement system (Hamamatsu Company).
Thermal property measurements
Thermogravimetric analysis was measured with a Q600SDT instrument. The
thermal stability of the samples under a nitrogen atmosphere was determined by
measuring their weight loss while heating at a rate of 10 °C min-1 from 25 to 560 °C.
Differential scanning calorimetry was performed on a Q100DSC instrument unit at a
heating rate of 10 °C min-1 from 20 to 250 °C under nitrogen. The glass transition
temperature was determined from the second heating scan.
Cyclic voltammetry measurements
Cyclic voltammetry was carried out in degassed CH2Cl2 solution (10-3 M) at room
temperature with a CHI600C voltammetric analyzer. Tetrabutylammonium
hexafluorophosphate (TBAPF6, 0.1 M) was as the supporting electrolyte and
ferrocene as an external standard. The conventional three-electrode configuration
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included a platinum working electrode, a platinum wire auxiliary electrode, and an
Ag/AgCl wire pseudo-reference electrode. The scan rate is = 100 mV s-1.
Theoretical calculations
The optimized molecular structures of ground state (S0) and HOMOs-LUMOs
distributions were calculated through DFT at the B3LYP/6-311G** levels using a
Gaussian suite of programs (Gaussian 09, Revision A.01).
OLED fabrication and measurements
The ITO-coated glass, MoO3, electron transporting material TPBi, hole transporting
materials NPB and mCP, electron injection material LiF and cathode Al were
available commercially. ITO anode has a sheet resistance of 17~18 Ω square-1 and
80-nm thickness. The ITO substrate was cleaned with detergent, deionized water,
acetone and ethanol before loading into a deposition chamber. The organic and metal
layers materials were evaporated in different vacuum chambers with a base pressure
lower than 8×10-5 Pa. The devices were capsulated in a glove box under a nitrogen
atmosphere. All electrical testing and optical measurements were performed under
ambient conditions. The EL spectra, current density-voltage-luminance (J-V-L) and
EQE characteristics were measured by computer controlled Keithley 2400 source
meter and absolute EQE measurement system (C9920-12, Hamamatsu Photonics)
with the photonic multichannel analyzer (PMA-12, Hamamatsu Photonics).
ASSOCIATED CONTENT
Supporting Information
Supporting information related to this article can be available.
AUTHOR INFORMATION
Corresponding Author
*E-mail: zwliu@pku.edu.cn (Zhiwei Liu)
ACKNOWLEDGMENTS
We gratefully acknowledge financial support from the National Key R&D Program of
China (No. 2016YFB0401001).
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