Highly efficient green organic light emitting diodes with phenanthroimidazole-based thermally activated delayed fluorescence emitters

Article abstract of DOI:10.1039/c7tc05576e

Here, a phenanthroimidazole moiety was firstly introduced into the construction of thermally activated delayed fluorescence (TADF) emitters, and two novel TADF emitters consisting of phenanthroimidazole as the acceptor group and phenazine as the donor group, 1-(4-(tert-butyl)phenyl)-2-(4-(10-phenylphenazin-5(10H)-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (PPZTPI) and 1-phenyl-2-(4-(10-phenylphenazin-5(10H)-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (PPZPPI) were designed and synthesized. The highly twisted conformation between phenazine and phenanthroimidazole in the molecules results in effective spatial separation of the HOMO and LUMO and small singlet-triplet splittings. Both compounds possess obvious TADF features. Their crystallographic properties, electronic structures, thermal stabilities, photophysical properties, and energy levels are studied systematically. Organic light-emitting diodes (OLEDs) using these two green TADF emitters demonstrate high external quantum efficiencies of 21.06% for PPZPPI and 20.52% for PPZTPI, respectively, which is comparable to most of the previously reported TADF OLEDs.

Full text of DOI:10.1039/c7tc05576e

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Journal of
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huang, S. Xiang, Q. Zhang, X. Lv, S. Ye, S. Zhuang and R. Guo, J. Mater. Chem. C, 2018, DOI:
1
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Volume
4
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January 2016 Pages 1–224
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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
rsc.li/materials-c

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Highly efficient green organic light emitting diodes with
phenanthroimidazole-based thermally activated delayed
fluorescence emitters
Zhi Huang, Songpo Xiang, Qing Zhang, Xialei Lv, Shaofeng Ye, Runda Guo, Lei Wang*
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology,
Wuhan, 430074, P. R. China
*Email: wanglei@mail.hust.edu.cn
Abstract: Here, phenanthroimidazole moiety was firstly introduced to the
construction of thermally activated delayed fluorescence (TADF) emitters, and two
novel TADF emitters consisting of phenanthroimidazole as the acceptor group and
phenazine
as
ꢀ(4ꢀ(tertꢀbutyl)phenyl)ꢀ2ꢀ(4ꢀ(10ꢀphenylphenazinꢀ5(10H)ꢀyl)phenyl)ꢀ1Hꢀphenanthro[
,10ꢀd]imidazole (PPZTPI) and
ꢀphenylꢀ2ꢀ(4ꢀ(10ꢀphenylphenazinꢀ5(10H)ꢀyl)phenyl)ꢀ1Hꢀphenanthro[9,10ꢀd]imidazo
the
donor
group,
1
9
1
le (PPZPPI) were designed and synthesized. The highly twisted conformation
between phenazine and phenanthroimidazole in the molecules results in effective
spatial separation of the HOMO and LUMO and small singlet−triplet splittings. Both
compounds possess obvious TADF features. Their crystallographic properties,
electronic structures, thermal stabilities, photophysical properties, and energy levels
are studied systematically. Organic lightꢀemitting diode (OLEDs) using these two
green TADF emitters demonstrate high external quantum efficiencies of 21.06% for
PPZPPI and 20.52% for PPZTPI, respectively, which is comparable to the most of
previously reported TADF OLEDs.
Introduction
1
Since the first organic lightꢀemitting diode (OLED) was made by Tang , OLEDs
have drawn great attention for their superior applications in displays and lighting.
According to spin statistics, traditional fluorescent emitters could only harvest 25% of
singlet excitons to produce light. In order to utilize the triplet excitons, researchers

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developed preciousꢀmetalꢀbased phosphorescent emitters to harvest both singlet and
2, 3
triplet excitons by heavyꢀatom effect.
Although phosphorescent OLEDs could
achieve high efficiencies, high costs and the short operation lifetime of blue
phosphorescent OLEDs limit their widespread application. Fortunately, in 2012,
Adachi and coꢀworkers reported nearly 100% internal quantum yield for pure organic
4
thermally activated delayed fluorescence (TADF) molecules. TADF emitters could
convert triplet excitons to singlet excitons through efficient reverse intersystem
1 1
crossing (RISC), in which a small energy gap (ꢁEST < 0.2eV) between the S and T
states is required to accelerate the RISC process. Therefore, the TADF molecule could
be designed with donor and acceptor moieties in twisted structures, which can
effectively separate the highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO), and achieve a small ꢁE . Meanwhile,
ST
appropriate orbital overlap should be retained to promote the radiative transition.
To date, different kinds of TADF molecules have been developed. Donor parts
5
6
were mostly limited to aromatic amines, such as triphenylamine , carbazole ,
7
8
9
acridine , phenoxazine and phenothiazine . While there were a variety of acceptor
1
0
11
12, 13
14, 15
moieties, including sulfone , ketone , phosphine oxide
, triazine
and some other electron deficient
. Phenanthroimidazole is an ideal building block for organic
,
1
6, 17
4
18ꢀ20
pyrimidine
, dicyanobenzene , triarylboron
21ꢀ24
aromatic rings
lightꢀemitting materials. It is a weak electron donor when an electronꢀwithdrawing
moiety is attached to the imidazole ring, but it shows electron withdrawing properties
2
5
when linked with an electron donor. Ma et al. reported a deepꢀblue molecule with
phenanthroimidazole as a weak electron donor, it showed a CIE coordinates of (0.152,
2
6
0
.077) and high EQE of 6.8%. Wang et al. created a deepꢀblue emitter PPIꢀPPIPCz
based on phenanthroimidazole with CIE coordinates of (0.15, 0.07) and high EQE of
.1%, which also realized highꢀperformance red, yellow and green phosphorescent
devices using it as host. Our group reported a series of phenanthroimidazoleꢀbased
8
2
7, 28
bipolar host material
, and the corresponding green phosphorescent OLEDs
29
showed a maximum EQE of 22.6% . There are still a lot of phenanthroimidazole
30ꢀ33
34, 35
derivatives for highly efficient blue emitters
and host materials
, whereas

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phenanthroimidazole have not been used in TADF materials. The main reason is that
the energy gap between the singlet and triplet excited state of phenanthroimidazole is
27
very large.
Recently, our group reported three donorꢀacceptor (DꢀA) type materials based on
36
phenanthroimidazole and 9,9ꢀdiphenylꢀ9,10ꢀdihydroacridine. Although there was a
great twist angle between the donor and acceptor, it exhibited a large ꢁEST and not
any TADF phenomenon. According to these, we introduced a strong donor phenazine
instead, which led to a small ꢁEST about 0.11eV and showed distinct TADF
phenomenon. In this work, we designed and synthesized two novel TADF materials
based
on
ꢀ(4ꢀ(tertꢀbutyl)phenyl)ꢀ2ꢀ(4ꢀ(10ꢀphenylphenazinꢀ5(10H)ꢀyl)phenyl)ꢀ1Hꢀphenanthro[
,10ꢀd]imidazole (PPZTPI) and
ꢀphenylꢀ2ꢀ(4ꢀ(10ꢀphenylphenazinꢀ5(10H)ꢀyl)phenyl)ꢀ1Hꢀphenanthro[9,10ꢀd]imidazo
phenanthroimidazole,
1
9
1
le (PPZPPI). Moreover, their crystallographic properties, electronic structures,
thermal stabilities, photophysical properties, energy levels and device performances
were investigated. Finally, the green TADF OLED device based on PPZPPI showed a
maximum EQE of 21.06%. These results prove that phenanthroimidazole could be an
excellent acceptor for the design of TADF materials.
Results and discussion
Synthesis and thermal properties
The synthetic routes and molecular structures are depicted in Scheme 1.
2ꢀ(4ꢀbromophenyl)ꢀ1ꢀ(4ꢀ(tertꢀbutyl)phenyl)ꢀ1Hꢀphenanthro[9,10ꢀd]imidazole
and
2ꢀ(4ꢀbromophenyl)ꢀ1ꢀphenylꢀ1Hꢀphenanthro[9,10ꢀd]imidazole were synthesized
37
according to the literature . Due to the unstability of monosubstituted
38
5
,10ꢀdihydrophenazine under aerated conditions , the final products were synthesized
in a oneꢀpot reaction. The detailed synthetic methods and analysis are described in the
Experimental section. The chemical structures of the compounds were confirmed by
1
H NMR, elemental analysis, mass spectrometry and singleꢀcrystal Xꢀray analysis.
Before fabrication of the OLED devices, the two new compounds were further

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purified by repeated temperatureꢀgradient vacuum sublimation.
Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC)
were carried out to investigate the thermal properties of the two compounds under
nitrogen atmosphere. As presented in Fig. 1 and Table 1, compounds PPZTPI and
d
PPZPPI exhibit high decomposition temperatures (T , corresponding to 5% weight
loss) of 430℃ and 419℃, indicating their excellent thermal stability. In addition,
they did not show any glass transition temperatures up to 250℃. The excellent
thermal properties of the two compounds can be ascribed to their highly rigid and
twisted structures.
2 2 4 2
Scheme 1. Synthetic routes and chemical structures of the materials: (a) Na S O , EtOH, H O; (b)
Pd(OAc) , [(tꢀBu) PH]BF , tꢀBuONa, toluene, 110℃.
2
3
4
1
00
PPZTPI
PPZPPI
8
6
4
2
0
0
0
0
0
0
100
200
300
400
500
600
Temperture (
℃)
Fig. 1. TGA curves of PPZTPI and PPZPPI.
Single-crystal structure
We obtain the single crystal by slow evaporation of a solution of PPZTPI in a
mixture of tetrahydrofuran and methanol. The crystal structure (Fig. 2) showed that

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the phenazine group is orthogonal to its connected phenyl rings with a large dihedral
angle of 89.8º. Such a twisted structure, arising from the steric repulsion by the
hydrogen atoms in the phenyl and phenazine unit, results in effective spatial
separation of the HOMO and LUMO and reduction of ꢁEST. There were also large
dihedral angles between the Nꢀsubstituted benzene and phenanthroimidazole plane.
Due to the twisted conformation, there are no close π−π stacking among PPZTPI
fragments. Multiple intermolecular C−H···π short contacts (see Fig. S2) are observed,
which could lock intramolecular rotation and depress the nonradiative decay.
Fig. 2. Crystal structure (CCDC 1577557) of PPZTPI.
Theoretical calculations
To gain a better insight into the molecular configuration and electronic structures
of these phenanthroimidazole based materials, we performed quantum chemical
calculations using density functional theory (DFT) at the B3LYP/6ꢀ31G(d) level. Fig.
3
shows the optimized structures, and the HOMO / LUMO distribution of the two
compounds. The HOMO is mainly distributed on the donor phenazine moiety, and the
LUMO is well spread over the acceptor phenanthroimidazole plane. The small spatial
overlap between HOMO and LUMO indicates strong charge transfer transition and
small electron exchange energy. On the other hand, the dihedral angles between the
phenylene and the donor are 88.77º and 89.54º, respectively for PPZTPI and
1 1
PPZPPI, which is consistent with the singleꢀcrystal. We next calculated the S and T

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excited energies using timeꢀdependent density functional theory (TDꢀDFT) at the
B3LYP/6ꢀ31G(d) level in the gas phase based on the ground state geometries of
compounds. The calculated ꢁEST values of PPZTPI and PPZPPI are 0.01 eV and
0.008 eV, respectively, which is small enough for efficient TADF.
Fig. 3. HOMO and LUMO distributions, and calculated singlet (S
1
) and triplet (T
1
) energy levels
for (a) PPZTPI and (b) PPZPPI.
Table 1. Physical properties of PPZTPI and PPZPPI.
a
c
d
g
PL
nm)
E
g
S
1
/T
1
HOMO/
T
d
a
b
e
Compound
Abs (nm)
Φ
S
1
/T
1
(eV)
f
(
(eV)
2.52
2.51
(eV)
LUMO (eV)
(℃)
430
419
PPZTPI
PPZPPI
322, 364, 422
328, 364, 423
532
531
0.731
0.990
2.47/2.36
2.48/2.36
2.369/2.359
2.327/2.319
ꢀ4.73/ꢀ2.21
ꢀ4.73/ꢀ2.22
a Measured in dilute toluene solution at room temperature.
b PLQY of these emitters doped into CBP films (5 wt%) under oxygenꢀfree conditions at room temperature.
c Measured by the onset absorption wavelength of solution state.
d Obtained from the peak of fluorescence spectra and phosphorescence spectra of these emitters in 2ꢀMeTHF at
7K.
7
e Calculated by TDꢀDFT at the B3LYP/6ꢀ31G(d) level.
f HOMO was determined from the onset of oxidation potentials with ferrocene as reference; LUMO was
g
determined from E and HOMO level.
g Measured from TGA.
Photophysical properties
The UVꢀvis absorption and photoluminescence (PL) spectra of the two
compounds are measured in dilute toluene, as shown in Fig. 4. Key optical parameters
are summarized in Table 1. The two compounds display similar absorption features in

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toluene solution. The absorption bands in the range from 320 to 380 nm are attributed
to the πꢀπ* transition of the phenanthroimidazole moiety. In addition, the longer
wavelength absorption peaks of the two compounds at around 420 nm are assigned to
g
the intramolecular charge transfer (CT) transition. The energy gaps (E ) of PPZTPI
and PPZPPI obtained from the onset of the absorption spectra are 2.52 eV and 2.51
eV, respectively. The peak wavelengths of the PL spectra of PPZTPI and PPZPPI in
diluted toluene are 532 nm and 531 nm, respectively. In order to unveil the
solvatochromic effect chromatically, the fluorescence of PPZTPI and PPZPPI in
different polar solvents was measured, as shown in Fig. S3. As the solvent polarity
increased, their fluorescence was redꢀshifted from blue to orange, suggesting the
strong CTꢀstate characteristic of the two compounds. Fig. S4 shows the PL spectra of
the materials in doped films. The PL spectra of 5 wt% PPZTPI doped CBP film
showed a peak at 527 nm, it was a slight blueꢀshift compare with the peak of 533 nm
for PPZPPI. The blue shift resulted from the large size of tertiary butyl, which will
avoid the intermolecular interaction between host and dopant. High PLQY values of
0.731 and 0.990 are obtained for PPZTPI and PPZPPI, which suggest that they will
be potentially good emitters in OLEDs. The relative lower PLQY for PPZTPI should
be attributed to the poor energy transfers from host to dopant.
To evaluate ꢁEST values of the compounds, their fluorescence and
phosphorescence spectra in 2ꢀmethyltetrahydrofuran (2ꢀMeTHF) solutions were
measured at 77 K. The fluorescence peaks of PPZTPI and PPZPPI at 77 K were 502
nm and 501 nm, which resulted in S energy levels of 2.47 and 2.48 eV, respectively.
1
And the phosphorescence peaks of PPZTPI and PPZPPI were found to be the same
1
at 525 nm, and their corresponding T energy levels were 2.36 eV. According to Fig. 4,
the ꢁEST values were estimated to be 0.11 eV for PPZTPI and 0.12 eV for PPZPPI
in 2ꢀMeTHF. The ꢁEST values were low enough for the efficient RISC process.
In order to confirm the delayed fluorescence behavior of phenanthroimidazole
based TADF emitters, the transient PL decay curves of both doped films were carried
out at room temperature. As shown in Fig. 5, both the doped films of the compounds
showed clear secondꢀorder exponential decays at 300 K. PPZTPI possess a short

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lifetime of 3.26 ns and a long lifetime of 126.5 ꢂs, and 3.0 ns and 118.0 ꢂs for
PPZPPI. The short lifetime component was attributed to the relaxation from S to S ,
1
0
and the delayed component should be assigned to the thermal upꢀconversion of
excitons from T to S , and then from S to S . PPZTPI shows longer delayed
1
1
1
0
lifetime than PPZPPI in the doped films, suggesting that the tertiary butyl blocks the
intermolecular interaction between host and dopant and makes the excitons lasting
longer time. The TADF property is further confirmed by the temperature dependent
transient PL decay from 77K to 300K (see Fig. S5). With the temperature increasing,
the ratio of the delayed component gradually increases and the delayed lifetime
21, 39, 40
gradually decreases
, which demonstrated the typical TADF characteristic of the
two compounds. The detail kinetic parameters are summarized in Table S2.
(b)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
(
a)1.0
1.0
0.8
0.6
0.4
0.2
0.0
PPZPPI
abs
PPZTPI
abs
PL
PL
77K PL
77K PL delay 25ms
0
0
0
0
0
.8
.6
.4
.2
.0
77K PL
7K PL delay 25ms
7
3
00
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 4. Absorption and PL spectra of PPZTPI and PPZPPI in dilute toluene at room temperature,
and their fluorescence and phosphorescence spectra at 77 K.
2
1
1
0
3
2
1
0
1
1
1
1
0
0
0
0
1
0
0
20
40
60
80
Time (ns)
CBP_5%PPZTPI 300K
CBP_5%PPZPPI 300K
0
200
400
600
800
1000
Time (µs)
Fig. 5. Transient PL characteristics of PPZTPI and PPZPPI doped in CBP films (5 wt%) at 300
K.
Electrochemical properties
The electrochemical properties and energy levels of the two compounds were

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investigated by cyclic voltammetry using the conventional three electrode system. As
shown in Fig. 6, PPZTPI and PPZPPI show well reversible oxidation behavior
arising from phenazine unit. The onset voltages of these oxidation curves are the same,
+
at ꢀ0.09 V versus the Ag/Ag redox couple. The HOMO energy levels of PPZTPI and
PPZPPI are estimated to be ꢀ4.73 eV with regard to ferrocene. The LUMO energy
levels of PPZTPI and PPZPPI are calculated from HOMO level and E
g
, and found to
be ꢀ2.21 eV and ꢀ2.22 eV, respectively. It is worth noting that the stronger
36
electronꢀdonating group of phenazine makes the HOMO level much shallower,
which is not favorable for the hole transporting from the hole transporting layer or
host.
PPZTPI
PPZPPI
ꢀ0.5
0.0
0.5
1.0
+
Potential (V vs Ag/Ag )
Fig. 6. Cyclic voltammograms of PPZTPI and PPZPPI.
Electroluminescent devices
To investigate the electroluminescent (EL) properties of the two
phenanthroimidazole based TADF emitters, multilayer OLEDs were fabricated with
doped films as the emitting layer (EML). The device configuration was ITO/MoO
3
(10 nm)/TAPC (80 nm)/TCTA (10 nm)/EML (30 nm)/TmPyPB (40 nm)/LiF (1
nm)/Al (100 nm), in which ITO (indium tin oxide) was used as the anode, MoO
3
was
TAPC
4,4'ꢀ(cyclohexaneꢀ1,1ꢀdiyl)bis(N,Nꢀdiꢀpꢀtolylaniline)) was used as the hole
transporting layer, TCTA (tris(4ꢀ(9Hꢀcarbazolꢀ9ꢀyl)phenyl)amine) was used as an
exciton blocking layer, PPZTPI or PPZPPI doped in CBP
4,4'ꢀdi(9Hꢀcarbazolꢀ9ꢀyl)ꢀ1,1'ꢀbiphenyl) host was used as the EML, TmPyPB
1,3,5ꢀtri(mꢀpyridꢀ3ꢀylꢀphenyl)benzene) was used as the electron transporting layer,
used
as
hole
injection
layer,
(
(
(
LiF was used as electron injecting layer and Al was used as cathode. Fig. 7a shows

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the energy diagram of the materials. The doping concentration of emitters was fixed
to be 5% in the EML after optimization of the control device efficiency. The devices
show the highest EQE at a doping concentration of 5 wt%. As shown in Fig. S6, with
the doping concentration increasing, both emitters exhibit slightly redꢀshifted EL
spectra, which should be ascribed to the interaction between emitter and host under
high doping concentration.
(
b)1.0
PPZTPI
PPZPPI
0
0
0
0
0
.8
.6
.4
.2
.0
4
00
500
600
700
800
Wavelength (nm)
4
3
2
1
0
PPZTPI
PPZPPI
(
c)
PPZTPI
PPZPPI
10
(d)
3
2
1
00
10
10
10
10
00
00
0
1
0.1
10
0
2
4
6
8
(
10
12
14
1
10
1020
cd/m )
1000
Voltage
V)
Luminance
(
2
Fig. 7. (a) Energyꢀlevel diagram of the materials used in the devices. (b) EL spectra at 100cd/m .
c) current density–voltage–luminance (J–V–L) characteristics. (d) EQE–luminance
characteristics.
(
The current density–voltage–luminance (J–V–L) characteristics, external
quantum efficiency (EQE) curves and EL spectra of these 5 wt% doped devices are
shown in Fig. 7. Both of the devices turned on at a low turnꢀon voltage of 3.0 V. Their
EL spectra are close to their PL spectra and there are no other emission peaks from
host materials, which suggests the complete energy transfer from the host materials to
the dopants. PPZPPI based device exhibited green electroluminescence with
emission peak at 528 nm and CIE coordinates of (0.363, 0.572), with the maximum
external quantum efficiency, current efficiency and power efficiency of 21.06%, 67.18
cd/A and 68.11 lm/W, respectively. PPZTPI based device showed similar green
electroluminescence with emission peak at 528 nm and CIE coordinates of (0.359,

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0.575), with the maximum external quantum efficiency, current efficiency and power
efficiency of 20.52%, 65.90 cd/A and 68.98 lm/W, respectively. These EQE of the
devices exceed the theoretical limit of conventional fluorescent OLEDs, indicating
efficient upꢀconversion from triplet excitons to singlet excitons. Meaningfully, the
performance of both the devices was comparable to the most of previously reported
4
, 5, 7, 11, 16, 18, 21, 41ꢀ46
TADF OLEDs.
PPZPPI based device showed slightly lower efficiency rollꢀoff, which was
attributed to the short delayed lifetime and fast RISC and radiative rate. While the
2
EQE is less than 10% at high brightness beyond 1000 cd/m for both. Holeꢀonly
devices and electronꢀonly devices were fabricated to study exciton transport and
formation in the EML with the structure of ITO/MoO
3
(10 nm)/TAPC (80 nm)/TCTA
(
10 nm)/EML (30 nm)/TCTA (10 nm)/MoO (10 nm)/Al (100 nm) and ITO/LiF (1
3
nm)/TmPyPB (40 nm)/EML (30 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm). As
shown in Fig. S7, the current density of holeꢀonly devices increase with the doping
concentration of emitters in the emission layer and that of electronꢀonly devices are
opposite. The shallow HOMO cause holes to be almost trapped into and transported
by emitter. Electron injection and transport are predominantly transported via the host,
emitters capture more electron under high doping concentration and lead to a smaller
current density. The efficiency rollꢀoff at high brightness should be mainly attributed
to unbalanced charge carriers and the direct combination of electron and hole on the
dopant. Although PPZTPI has a lower PLQY than PPZPPI, PPZTPI based device
also achieve relative high efficiency, which means the poor energy transfers from host
to dopant has little effect to the performance. We anticipate that the device
performance can further be improved by using optimized device structures with better
hosts and carrier transporting materials.
Conclusion
In summary, we designed and synthesized two novel green TADF emitters
(PPZTPI and PPZPPI) based on phenanthroimidazole derivatives in this work. The
two compounds employ phenanthroimidazole as the acceptor unit and phenazine as

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the donor unit to form nearly vertical DꢀA structure. Both of these emitters possess
obvious TADF features, good thermal stability, high PLQYs, a small ꢁEST of around
0.1 eV and moderate delayed lifetimes. The OLEDs using PPZTPI and PPZPPI as
the emitters achieve maximum EQE, PE and CE of 20.52%, 65.90 cd/A and 68.98
lm/W, respectively, and 21.06%, 67.18 cd/A and 68.11 lm/W, respectively. The results
provide a promising opportunity for the design and construction of highly efficient
TADF materials based on phenanthroimidazole derivatives.
Experimental section
Materials and measurements
In our previous studies, most of the experimental conditions and equipment have been
36
described. All the reagents and solvents used for the syntheses and measurements
were purchased from commercial suppliers and were used without further purification
1
unless otherwise noted. H NMR spectra was conducted on a BrukerꢀAF301 AT 400
MHz spectrometer. Mass spectra were recorded on a BrukerꢀDaltonics microflex
LT/SH mass spectrometer. Elemental analyses of carbon, hydrogen, and nitrogen were
performed on an Elementar (Vario Micro cube) analyzer. Thermogravimetric analyses
(
TGA) were undertaken using a PerkinElmer Instruments (Pyris1 TGA) under
nitrogen atmosphere at a heating rate of 10 /min from 30 to 600 . The differential
scanning calorimetry (DSC) was performed on a TA Q2000 DSC instrument under
nitrogen at a heating rate of 10 /min from 30 to 250 . The glass transition
temperature (T ) was determined from the second heating scan. UVꢀVis spectra were
℃
℃
℃
℃
g
measured using
a
Shimadzu UVꢀVISꢀNIR Spectrophotometer (UVꢀ3600).
Photoluminescence (PL) spectra were recorded on a Hitachi Fꢀ7000 fluorescence
spectrophotometer. The transient PL decay curves were measured by a single photon
counting spectrometer from Edinburgh Instruments (FLS920). Absolute PLQYs were
obtained using a QuantaurusꢀQY measurement system (C11347ꢀ11, Hamamatsu
Photonics). Cyclic voltammetry (CV) was performed on a computerꢀcontrolled
EG&G Potentiostat/Galvanostat model 283 at room temperature with a conventional
three electrode cell, which consisted of a platinum wire counter electrode, an

Page 13 of 17
Journal of Materials Chemistry C
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DOI: 10.1039/C7TC05576E
Ag/AgNO
diameter. Oxidations of all compounds were performed in dried dichloromethane
containing 0.1 M tetrabutylammonium hexafluorophosphate (Bu NPF ) as supporting
3
(0.1 M) reference electrode and a Pt carbon working electrode of 2 mm
4
6
electrolyte. The onset potential was determined from the intersection of two tangents
drawn at the rising and background current of the cyclic voltammogram. The
solutions were purged with a nitrogen stream for 10 min before measurements.
Synthesis of 5,10-dihydrophenazine
A solution of sodium dithionite (17.4 g, 100 mmol) in water (200 mL) was added to a
boiling solution of phenazine (1.8 g, 10 mmol) in ethanol (50 mL). The mixture was
stirred and heated to reflux for 2 h. A white precipitate was collected by filtration and
dried under reduced pressure to afford 5,10ꢀdihydrophenazine (1.62 g, 8.9 mmol).
1
This material was used in the next step without further purification. H NMR (400
6
MHz, DMSOꢀd ) δ [ppm]: 7.26 (s, 2H), 6.24ꢀ6.22 (m, 4H), 5.99ꢀ5.97 (m, 4H).
Synthesis
of
1
-(4-(tert-butyl)phenyl)-2-(4-(10-phenylphenazin-5(10
H
)-yl)phenyl)-1
H
-phenanth
ro[9,10-d]imidazole (PPZTPI)
A
mixture
of
2
5
ꢀ(4ꢀbromophenyl)ꢀ1ꢀ(4ꢀ(tertꢀbutyl)phenyl)ꢀ1Hꢀphenanthro[9,10ꢀd]imidazole (2.52 g,
mmol), 5,10ꢀdihydrophenazine (1.62 g, 8.9 mmol), tꢀBuONa (0.96 g, 10 mmol),
triꢀtertꢀbutylphosphonium tetrafluoroborate (87 mg, 0.3 mmol) and palladium acetate
33 mg, 0.15 mmol) was dissolved in dry toluene (50 mL), and purged three times by
(
nitrogen/vacuum cycle. The reaction mixture was refluxed for 2 hours, and then
bromobenzene (5 mL) was added by syringe. The reaction mixture was refluxed for
another 10 hours. After cooling, petroleum ether (100 mL) was added to the cooled
mixture and the solid was separated by filtration. The crude product was recrystallized
1
from hexane and dichloromethane to produce yellow solid (1.77 g) in 52% yield. H
NMR (400 MHz, Benzeneꢀd
6
) δ [ppm]: 9.49 (d, J = 8.0 Hz, 1H), 8.63 (d, J = 8.4 Hz,
1H), 8.59 (d, J = 8.4 Hz, 1H), 7.91 (d, J = 8.4 Hz, 2H), 7.68 (t, J = 7.6 Hz, 1H), 7.51

Journal of Materials Chemistry C
Page 14 of 17
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DOI: 10.1039/C7TC05576E
(t, J = 7.6 Hz, 1H), 7.43 (d, J = 8.0 Hz, 1H), 7.28 (t, J = 7.6 Hz, 1H), 7.16ꢀ6.97 (m,
12H), 6.29ꢀ6.26 (m, 4H), 5.84ꢀ5.80 (m, 4H), 1.14 (s, 9H). MS (MALDIꢀTOF):
+
calculated for C49
H
38
N
4
, 682.31; found 681.58, [M] . Elemental analysis calculated
38 4
(%) C49H N : C 86.19, H 5.61, N 8.20; found: C 86.23, H 5.51, N 8.23.
Synthesis
-phenyl-2-(4-(10-phenylphenazin-5(10
azole (PPZPPI)
PPZPPI was prepared with the similar procedure as that of PPZTPI but with
ꢀ(4ꢀbromophenyl)ꢀ1ꢀphenylꢀ1Hꢀphenanthro[9,10ꢀd]imidazole (2.24 g, 5 mmol)
of
1
H
)-yl)phenyl)-1
H
-phenanthro[9,10-
d
]imid
2
instead
ꢀ(4ꢀbromophenyl)ꢀ1ꢀ(4ꢀ(tertꢀbutyl)phenyl)ꢀ1Hꢀphenanthro[9,10ꢀd]imidazole.
of
2
The
crude product was recrystallized from hexane and dichloromethane to produce yellow
1
solid (1.72 g) in 55% yield. H NMR (400 MHz, Benzeneꢀd
6
) δ [ppm]: 9.48 (d, J =
7
2
7
.6 Hz, 1H), 8.62 (d, J = 8.4 Hz, 1H), 8.58 (d, J = 8.4 Hz, 1H), 7.88 (d, J = 8.4 Hz,
H), 7.68 (t, J = 7.6 Hz, 1H), 7.51 (t, J = 7.2 Hz, 1H), 7.43 (d, J = 8.0 Hz, 1H),
.30ꢀ7.25 (m, 2H), 7.16ꢀ6.88 (m, 12H), 6.29ꢀ6.27 (m, 4H), 5.85ꢀ5.80 (m, 4H). MS
+
(MALDIꢀTOF): calculated for C45
H
30
N
4
, 626.25; found 625.96, [M] . Elemental
30 4
analysis calculated (%) C45H N : C 86.24, H 4.82, N 8.94; found: C 86.33, H 4.73, N
8.87.
Computational details
The geometrical and electronic properties were computed using the Gaussian 09
program package. The groundꢀstate geometry was optimized using density functional
theory (DFT) with B3LYP hybrid functional at the basis set level of 6ꢀ31G(d). The
singlet and triplet states were calculated by timeꢀdependent density functional theory
(TDꢀDFT) with the B3LYP functional at the same basis set level. Molecular orbitals
were visualized using Gauss view 5.0 program.
Device fabrication and measurement

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DOI: 10.1039/C7TC05576E
The carrierꢀinjection material MoO
3
, lithium fluoride (LiF), and carrierꢀtransporting
material
4,4'ꢀ(cyclohexaneꢀ1,1ꢀdiyl)bis(N,Nꢀdiꢀpꢀtolylaniline)
(TAPC),
3,3'ꢀ(5'ꢀ(3ꢀ(pyridinꢀ3ꢀyl)phenyl)ꢀ[1,1':3',1''ꢀterphenyl]ꢀ3,3''ꢀdiyl)dipyridine (TmPyPB),
exciton blocking material tris(4ꢀ(9Hꢀcarbazolꢀ9ꢀyl)phenyl)amine (TCTA) and host
material 4,4'ꢀdi(9Hꢀcarbazolꢀ9ꢀyl)ꢀ1,1'ꢀbiphenyl (CBP) were commercially available.
Commercial ITO coated glass with a sheet resistance of 20 ꢃ per square was used as
the substrate. Before device fabrication, the ITO glass substrates were preꢀcleaned
3
carefully and treated with oxygen plasma for 5 min. MoO was firstly deposited on
the ITO substrate, followed by TAPC, TCTA, the emission layer, and TmPyPB.
Finally, a cathode composed of LiF and aluminum was sequentially deposited onto
ꢀ6
the substrate in the vacuum of 2×10 Torr. For all of the OLEDs, the emitting areas
2
were determined by the overlap of two electrodes as 0.09 cm . The EL spectra, CIE
coordinates and JꢀVꢀL curves of the devices were measured using a PHOTO
RESEARCH SpectraScan PR655 photometer and a KEITHLEY 2400 SourceMeter
constant current source. The efficiency values were calculated according to previously
reported methods. All measurements were carried out at room temperature under
ambient conditions.
Acknowledgements
This research work was supported by the NSFC/China (51573065, 51727809), China
Postdoctoral Science Foundation(2017M620321), the science and technology support
program of Hubei Province (2015BAA075) and the Analytical and Testing Centre at
Huazhong University of Science and Technology.
.
References
1
2
3
C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913ꢀ915.
Y. G. Ma, H. Y. Zhang, J. C. Shen and C. M. Che, Synth. Met., 1998, 94, 245ꢀ248.
M. A. Baldo, D. O'brien, Y. You, A. Shoustikov, S. Sibley, M. Thompson and S. Forrest,
Nature, 1998, 395, 151ꢀ154.
4
H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi, Nature, 2012, 492, 234ꢀ238.

Journal of Materials Chemistry C
Page 16 of 17
View Article Online
DOI: 10.1039/C7TC05576E
5
H. Wang, L. Xie, Q. Peng, L. Meng, Y. Wang, Y. Yi and P. Wang, Adv. Mater., 2014, 26,
198ꢀ5204.
5
6
7
D. Zhang, M. Cai, Y. Zhang, D. Zhang and L. Duan, Mater. Horiz., 2016, 3, 145ꢀ151.
G. Xie, X. Li, D. Chen, Z. Wang, X. Cai, D. Chen, Y. Li, K. Liu, Y. Cao and S. J. Su, Adv.
Mater., 2016, 28, 181ꢀ187.
8
9
1
L. Yu, Z. Wu, G. Xie, C. Zhong, Z. Zhu, H. Cong, D. Ma and C. Yang, Chem. Commun., 2016,
52, 11012ꢀ11015.
J. Huang, H. Nie, J. Zeng, Z. Zhuang, S. Gan, Y. Cai, J. Guo, S. J. Su, Z. Zhao and B. Z. Tang,
Angew. Chem. Int. Ed., 2017, 56, 12971ꢀ12976.
0
Q. Zhang, J. Li, K. Shizu, S. Huang, S. Hirata, H. Miyazaki and C. Adachi, J. Am. Chem. Soc.,
2
012, 134, 14706ꢀ14709.
S. Y. Lee, T. Yasuda, Y. S. Yang, Q. Zhang and C. Adachi, Angew. Chem. Int. Ed., 2014, 53,
402ꢀ6406.
11
6
12
13
14
Y. K. Wang, Q. Sun, S. F. Wu, Y. Yuan, Q. Li, Z. Q. Jiang, M. K. Fung and L. S. Liao, Adv.
Funct. Mater., 2016, 26, 7929ꢀ7936.
C. B. Duan, J. Li, C. M. Han, D. X. Ding, H. Yang, Y. Wei and H. Xu, Chem. Mater., 2016, 28,
5667ꢀ5679.
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,
330ꢀ336.
15
16
17
T. A. Lin, T. Chatterjee, W. L. Tsai, W. K. Lee, M. J. Wu, M. Jiao, K. C. Pan, C. L. Yi, C. L.
Chung, K. T. Wong and C. C. Wu, Adv. Mater., 2016, 28, 6976ꢀ6983.
K. Wu, T. Zhang, L. Zhan, C. Zhong, S. Gong, N. Jiang, Z. H. Lu and C. Yang, Chemistry,
2016, 22, 10860ꢀ10866.
K.ꢀC. Pan, S.ꢀW. Li, Y.ꢀY. Ho, Y.ꢀJ. Shiu, W.ꢀL. Tsai, M. Jiao, W.ꢀK. Lee, C.ꢀC. Wu, C.ꢀL.
Chung, T. Chatterjee, Y.ꢀS. Li, K.ꢀT. Wong, H.ꢀC. Hu, C.ꢀC. Chen and M.ꢀT. Lee, Adv. Funct.
Mater., 2016, 26, 7560ꢀ7571.
18
19
20
21
22
23
24
25
26
Y. K. Wang, S. F. Wu, Y. Yuan, S. H. Li, M. K. Fung, L. S. Liao and Z. Q. Jiang, Org. Lett.,
2017, 19, 3155ꢀ3158.
Y. J. Shiu, Y. C. Cheng, W. L. Tsai, C. C. Wu, C. T. Chao, C. W. Lu, Y. Chi, Y. T. Chen, S. H.
Liu and P. T. Chou, Angew. Chem. Int. Ed., 2016, 55, 3017ꢀ3021.
Y. J. Lien, T. C. Lin, C. C. Yang, Y. C. Chiang, C. H. Chang, S. H. Liu, Y. T. Chen, G. H. Lee,
P. T. Chou, C. W. Lu and Y. Chi, ACS Appl. Mater. Interfaces, 2017, 9, 27090ꢀ27101.
M. Li, Y. Liu, R. Duan, X. Wei, Y. Yi, Y. Wang and C. F. Chen, Angew. Chem. Int. Ed., 2017,
56, 8818ꢀ8822.
F. Ni, Z. Wu, Z. Zhu, T. Chen, K. Wu, C. Zhong, K. An, D. Wei, D. Ma and C. Yang, J. Mater.
Chem. C, 2017, 5, 1363ꢀ1368.
S. Wang, X. Yan, Z. Cheng, H. Zhang, Y. Liu and Y. Wang, Angew. Chem. Int. Ed., 2015, 54,
13068ꢀ13072.
Y. Yuan, Y. Hu, Y. X. Zhang, J. D. Lin, Y. K. Wang, Z. Q. Jiang, L. S. Liao and S. T. Lee, Adv.
Funct. Mater., 2017, 27, 1700986.
X. Tang, Q. Bai, Q. Peng, Y. Gao, J. Li, Y. Liu, L. Yao, P. Lu, B. Yang and Y. Ma, Chem.
Mater., 2015, 27, 7050ꢀ7057.
C. Li, S. Wang, W. Chen, J. Wei, G. Yang, K. Ye, Y. Liu and Y. Wang, Chem. Commun., 2015,

Page 17 of 17
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC05576E
5
1, 10632ꢀ10635.
H. Huang, Y. Wang, S. Zhuang, X. Yang, L. Wang and C. Yang, J. Phys. Chem. C, 2012, 116,
9458ꢀ19466.
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
1
H. Huang, Y. Wang, B. Wang, S. Zhuang, B. Pan, X. Yang, L. Wang and C. Yang, J. Mater.
Chem. C, 2013, 1, 5899ꢀ5908.
B. Wang, X. Lv, J. Tan, Q. Zhang, Z. Huang, W. Yi and L. Wang, J. Mater. Chem. C, 2016, 4,
8
473ꢀ8482.
W.ꢀC. Chen, Y. Yuan, S.ꢀF. Ni, Q.ꢀX. Tong, F.ꢀL. Wong and C.ꢀS. Lee, Chem. Sci., 2017, 8,
599ꢀ3608.
3
W. C. Chen, Y. Yuan, Y. Xiong, A. L. Rogach, Q. X. Tong and C. S. Lee, ACS Appl. Mater.
Interfaces, 2017, 9, 26268ꢀ26278.
A. Obolda, Q. Peng, C. He, T. Zhang, J. Ren, H. Ma, Z. Shuai and F. Li, Adv. Mater., 2016, 28,
4
740ꢀ4746.
X. L. Li, M. Liu, Y. Li, X. Cai, D. Chen, K. Liu, Y. Cao and S. J. Su, Chem. Commun., 2016,
2, 14454ꢀ14457.
5
T. Shan, Y. Liu, X. Tang, Q. Bai, Y. Gao, Z. Gao, J. Li, J. Deng, B. Yang, P. Lu and Y. Ma,
ACS Appl. Mater. Interfaces, 2016, 8, 28771ꢀ28779.
Y. Li, X.ꢀL. Li, X. Cai, D. Chen, X. Liu, G. Xie, Z. Wang, Y.ꢀC. Wu, C.ꢀC. Lo, A. Lien, J.
Peng, Y. Cao and S.ꢀJ. Su, J. Mater. Chem. C, 2015, 3, 6986ꢀ6996.
Z. Huang, B. Wang, Q. Zhang, S. Xiang, X. Lv, L. Ma, B. Yang, Y. Gao and L. Wang, Dyes
Pigments, 2017, 140, 328ꢀ336.
Z. Wang, P. Lu, S. Chen, Z. Gao, F. Shen, W. Zhang, Y. Xu, H. S. Kwok and Y. Ma, J. Mater.
Chem., 2011, 21, 5451ꢀ5456.
E. Terada, T. Okamoto, M. Kozaki, M. E. Masaki, D. Shiomi, K. Sato, A. Takeji Takui and K.
Okada, J. Org. Chem., 2005, 70, 10073ꢀ10081.
A. Niwa, T. Kobayashi, T. Nagase, K. Goushi, C. Adachi and H. Naito, Appl. Phys. Lett., 2014,
104, 213303.
Y. Geng, A. D'Aleo, K. Inada, L. S. Cui, J. U. Kim, H. Nakanotani and C. Adachi, Angew.
Chem. Int. Ed., 2017, 56, 16536ꢀ16540.
J. X. Chen, W. Liu, C. J. Zheng, K. Wang, K. Liang, Y. Z. Shi, X. M. Ou and X. H. Zhang,
ACS Appl. Mater. Interfaces, 2017, 9, 8848ꢀ8854.
Q. S. Zhang, B. Li, S. P. Huang, H. Nomura, H. Tanaka and C. Adachi, Nat. Photonics, 2014,
8
, 326ꢀ332.
P. Rajamalli, N. Senthilkumar, P. Gandeepan, P. Y. Huang, M. J. Huang, C. Z. RenꢀWu, C. Y.
Yang, M. J. Chiu, L. K. Chu, H. W. Lin and C. H. Cheng, J. Am. Chem. Soc., 2016, 138,
628ꢀ634.
44
45
46
C. Tang, T. Yang, X. D. Cao, Y. T. Tao, F. F. Wang, C. Zhong, Y. Qian, X. W. Zhang and W.
Huang, Adv. Optical Mater., 2015, 3, 786ꢀ790.
K. Wang, W. Liu, C.ꢀJ. Zheng, Y.ꢀZ. Shi, K. Liang, M. Zhang, X.ꢀM. Ou and X.ꢀH. Zhang, J.
Mater. Chem. C, 2017, 5, 4797ꢀ4803.
D.ꢀQ. Wang, M. Zhang, K. Wang, C.ꢀJ. Zheng, Y.ꢀZ. Shi, J.ꢀX. Chen, H. Lin, S.ꢀL. Tao and
X.ꢀH. Zhang, Dyes Pigments, 2017, 143, 62ꢀ70.