Novel triazine derivatives with deep LUMO energy levels as the electron-accepting components of exciplexes

Article abstract of DOI:10.1039/d0tc04751a

Although a variety of electron-accepting molecules (acceptors) have been reported to develop exciplexes, their LUMO energy levels are still limited from ?2.9 to ?3.3 eV. This narrow range seriously restricts the development of exciplexes. In this work, three novel triazine-based acceptors, 2,4-diphenyl-6-(3-(phenylsulfonyl)phenyl)-1,3,5-triazine (TRZ-1SO₂), 2-phenyl-4,6-bis(3-(phenylsulfonyl)phenyl)-1,3,5-triazine (TRZ-2SO₂) and 2,4,6-tris(3-(phenylsulfonyl)phenyl)-1,3,5-triazine (TRZ-3SO₂), were developed with deep LUMO energy levels of ?3.38, ?3.58 and ?3.74 eV, respectively. Particularly, TRZ-3SO₂exhibits the deepest LUMO energy level of ?3.74 eV among the reported acceptors so far. By combining with an electron-donating molecule, 1,3-bis(9,9-dimethylacridin-10(9H)-yl)benzene (13AB), three exciplexes, 13AB:TRZ-1SO₂, 13AB:TRZ-2SO₂and 13AB:TRZ-3SO₂, were constructed with the emission from green to orange-red. The OLED using 13AB:TRZ-3SO₂as the emitter exhibits an orange-red emission peaking at 572 nm with a maximum external quantum efficiency of 5.5%. Meanwhile, green and red phosphorescent OLEDs (PhOLEDs) were fabricated using interfacial exciplex hosts by combining these acceptors with 4,4′-bis(carbazole-9-yl)biphenyl (CBP), which successfully exhibits superior performance compared with that of reference devices. These results demonstrate that the three novel acceptors have great potential in the development of exciplexes.

Full text of DOI:10.1039/d0tc04751a

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DOI: 10.1039/D0TC04751A
ARTICLE
Novel triazine derivatives with deep LUMO energy levels as the
electron-accepting components of exciplexes
Jing-Guo, a Cai-Jun Zheng, *a Ke Ke, a,b Ming Zhang, a,b Hao-Yu Yang, a Jue-Wen Zhao, a Ze-Yu He, a
Received 00th October 2020,
Accepted 00th October 2020
Hui Lin, a Si-Lu Tao, *a and Xiao-Hong Zhang b
DOI: 10.1039/x0xx00000x
Although a variety of electron-accepting molecules (Acceptors) have been reported to develop exciplexes, their LUMO
energy levels are still limited within -2.9 to -3.3 eV. Such a narrow range seriously restricts the development of exciplexes.
In this work, three novel triazine-based Acceptors, 2,4-diphenyl-6-(3-(phenylsulfonyl) phenyl)-1,3,5-triazine (TRZ-1SO2), 2-
phenyl-4,6-bis(3-(phenylsulfonyl) phenyl)-1,3,5-triazine (TRZ-2SO2) and 2,4,6-tris(3-(phenylsulfonyl) phenyl)-1,3,5-triazine
(TRZ-3SO2) were developed with deep LUMO energy levels of -3.38, -3.58 and -3.74 eV, respectively. Particularly, the TRZ-
3SO2 exihits the deepest LUMO energy level of -3.74 eV among reported Acceptors so far. Combining with electron-donating
molecule 1,3-bis(9,9-dimethylacridin-10(9H)-yl) benzene (13AB), three exciplexes 13AB:TRZ-1SO2, 13AB:TRZ-2SO2 and
13AB:TRZ-3SO2 were constructed with the emission from green to orange-red. The OLED using 13AB:TRZ-3SO2 as the emitter
exhibits an orange-red emission peaking at 572 nm with the maximum external quantum efficiency of 5.5%. Meanwhile,
green and red phosphorescent OLEDs (PhOLEDs) were fabricated with interfacial exciplex hosts by combining these
Acceptors with 4,4'-bis(carbazole-9-yl) biphenyl (CBP), which successfully exhibit superior performance than reference
devices. These results demonstrate that three novel Acceptors have great potential in the development of exciplexes.
and hosts of OLEDs in recent years.17-21 Till now, exciplex-based
OLEDs have achieved significant progress and show great
application prospects. However, they still have to solve two
main problems. First, the photoluminescence quantum yields
(PLQYs) of exciplex emitters are relatively low, especially for red
exciplex emitters. Meanwhile, the stabilities of exciplex-based
OLEDs need further improvement. Thus, to meet the
requirements of commercialized OLEDs, it is still urgent to
improve the performance of exciplex-based OLEDs by
developing new exciplex systems.
According to our previous researches, the formations and
energy gaps of exciplexes are mainly depended on the HOMO
energy level of Donor and the LUMO energy level of Acceptor.22-
25 Thus, abundant candidates of Donor and Acceptor are crucial
for the development of exciplexes. As common electron-
donating groups, like triarylamine, carbazole, phenoxazine, and
acridine possess the evidently different electron-donating
ability, the HOMO energy levels of current Donors can vary in a
large range from -5.1 to -6.1 eV.26-31 Reversely, although the
Acceptors have also been developed based on different
electron-withdrawing groups, such as pyridine, phenanthroline,
pyrimidine, triazine, phenylphosphine oxide and so on, their
LUMO energy levels are generally limited within -2.9 to -3.3
eV.32-35 Such a narrow range seriously restricts the development
of exciplexes, till now, most reported exciplexes have energy
gaps over 2.3 eV with blue, green or yellow emission.36-39 To
address this issue, it is essential to develop new Acceptors with
deeper LUMO energy levels.
Introduction
Exciplexes have attracted enormous attention in the organic
light-emitting diode (OLED) field in recent years due to their
natural thermally activated delayed fluorescence (TADF)
characteristic.1-5 Formed between electron-donating and
electron-accepting molecules (Donor and Acceptor), exciplexes
are characterized with intermolecular charge-transfer (CT)
transition, and their highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) are located on
Donor and Acceptor, respectively.6-8 Therefore, exciplexes
possess completely separated frontier molecular orbitals and
intrinsically have extremely small energy gaps (ΔESTs) between
their S1 and T1 energy levels,9 which will promote the reverse
intersystem crossing process and theoretically realize the full
utilization of triplet excitons.10-12 Furthermore, the constituting
Donor and Acceptor generally possess hole- and electron-
transporting properties, respectively, thus exciplexes are also
natural bipolar mixed systems.13 The balanced carriers-
transporting can be even achieved by adjusting the mixing ratio
between Donor and Acceptor, which is beneficial to simplify the
device structures and improve device performance.14-16
Therefore, exciplexes are widely reported as the TADF emitters
a. School of Optoelectronic Science and Engineering, University of Electronic Science
and Technology of China, Chengdu 610054, P. R. China.
b.
Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University,
Suzhou 215123, P. R. China.
Among familiar electron-withdrawing groups, triazine
exhibits relatively deep LUMO energy level and stable
†
Electronic
Supplementary
Information
(ESI)
available.
See
DOI:10.1039/x0xx00000x
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Page 2 of 9
ARTICLE
Journal of Materials Chemistry C
electrochemical properties. In this work, we worked on triazine respectively. The 0.1 MBu4NPF6 and ferrocenium/ferrocene
View Article Online
+
DOI: 10.1039/D0TC04751A
derivatives to develop new Acceptors. To further lower their (Fe /Fe) were applied as supporting electrolyte and standard
LUMO energy levels, diphenyl sulfone group, which also has material, respectively. The reduction potentials of ETMs were
strong electronegativity and good electron-transporting conducted in DMF.
property, was introduced into the molecules with different OLEDs Fabrication
numbers. And three novel triazine-based Acceptors, 2,4-
diphenyl-6-(3-(phenylsulfonyl) phenyl)-1,3,5-triazine (TRZ-
The patterned ITO substrates (135 nm,15 Ω square−1) were
cleaned with detergent and deionized water, then dried in an
1SO2),
2-phenyl-4,6-bis(3-(phenylsulfonyl)
phenyl)-1,3,5-
oven at 120 °C for two hours, treated with UV-ozone for 30
minutes. All materials were deposited by vacuum evaporation
in a base pressure of about 5 × 10−4 Pa. The evaporation rates
of all the materials were monitored by quartz crystals. Organic
materials were deposited at a rate of less than 1.5 Å s−1 and the
LiF and Al were deposited at the rates of 0.1 and 10 Å s−1,
respectively. The current density-voltage curves were obtained
by using a Keithley 2400 source meter. The EL spectrum and CIE
color coordinates were measured by a Spectra scan PR655
photometer. The EQE of devices was calculated from the
current density, luminance, and EL spectrum, assuming a
Lambertian distribution. All the measurements were performed
in ambient conditions.
triazine (TRZ-2SO2) and 2,4,6-tris(3-(phenylsulfonyl) phenyl)-
1,3,5-triazine (TRZ-3SO2) were developed. TRZ-1SO2, TRZ-2SO2
and TRZ-3SO2 successfully exhibit deep LUMO energy levels of -
3.38, -3.58 and -3.74 eV, respectively. The LUMO energy level of
-3.74 eV is almost the deepest among all reported Acceptors so
far. By using 1,3-bis(9,9-dimethylacridin-10(9H)-yl) benzene
(13AB) as the Donor, three exciplexes 13AB:TRZ-1SO2,
13AB:TRZ-2SO2 and 13AB:TRZ-3SO2 were constructed with the
emission from green to orange-red. And the OLED using
13AB:TRZ-3SO2 as the emitter exhibits an orange-red emission
peaking at 572 nm with the maximum external quantum
efficiency (EQE) of 5.5%. To make full use of these new
Acceptors, we further constructed interfacial exciplex hosts in
the OLEDs by combining these Acceptors with 4,4'-
bis(carbazole-9-yl) biphenyl (CBP). And the green and red
phosphorescent OLEDs (PhOLEDs) successfully exhibit superior
performance compared with the 3,3'-(5'-(3-(pyridin-3-yl)
phenyl)-[1,1':3',1''-terphenyl]-3,3''-diyl) dipyridine (TmPyPB)-
based contrastive devices. We believe the remarkable
performance of these devices effectively proves the great
prospects of three new Acceptors in the development of
exciplexes.
Materials and Synthesis
All commercially available reagents and solvents were used
without further purification.
Synthesis of 2,4-diphenyl-6-(3-(phenylsulfonyl) phenyl)-1,3,5-
triazine (TRZ-1SO2). A mixture of 2(3-Bromophenyl)-4,6-
diphenyl-1,3,5-triazine (5 mmol, 1.94 g) and sodium
benzenethiolate (6 mmol, 793 mg), potassium t-butoxide (10
mmol, 1.12 g) was added to a two-port reaction flask. Then, add
80 ml of DMF solvent under N2 protection, refluxed at 150 °C for
20 h. After the reaction is completed, it is cooled to room
temperature, the organic phase is extracted with a large
amount of water and dichloromethane, and dried over
anhydrous sodium sulfate. Finally, the product was purified by
column chromatography using petroleum ether and
dichloromethane. After drying, a white gummy solid B1was
Experimental Section/Methods
General Information
All materials were purchased from commercial suppliers and
used without further purification. The 1H- and 13C-NMR spectra
were respectively measured by using Bruker AV-400 and AV-
600 NMR spectrometers to clarify the molecular structures of
synthesized Acceptors. High-resolution mass spectra were
obtained by using a WATERS mass spectrometer. Differential
scanning calorimetry (DSC) measurements were performed on
a NETZSCH DSC204 calorimeter at a heating rate of 5 ℃ min-1
1
obtained (1.52 g, 73% yield); H NMR (400 MHz,CDCl3) δ 8.60-
8.52 (m, 4H), 8.21 (dt, J = 7.5, 1.6 Hz, 1H), 7.62 (t, J = 1.5 Hz, 1H),
7.54 (t, J =7.5 Hz, 1H), 7.48 (dp, J = 5.6, 2.0 Hz, 6H), 7.45 (dt, J =
7.5, 1.5 Hz, 1H), 7.42-7.35 (m, 2H), 7.35-7.29 (m, 2H), 7.29-7.22
(m, 1H). MS (EI) m/z: calcd for C27H19N3S ([M]+), 417.53; found,
417.21.
Add product B1 (1.52g), 10 ml of hydrogen peroxide and 80 ml
of acetic acid solution to a 200 ml round bottom flask, reflux at
108 °C to carry out oxidation reaction. After 6 h of reaction, it
was cooled to room temperature, filtered, and finally
recrystallized from petroleum ether to remove impurities to
obtain 1.38 g of white solid TRZ-1SO2 (84% yield); 1H NMR (400
MHz, CDCl3) δ 9.31 (t, J = 1.8 Hz, 1H), 8.96 (dd, J = 7.8, 1.6 Hz,
1H), 8.78-8.74 (m, 4H), 8.15 (dd, J = 7.8, 1.7 Hz, 1H), 8.07-8.03
(m, 2H), 7.72 (t, J = 7.9 Hz, 1H), 7.66 - 7.62 (m, 2H), 7.61-7.57 (m,
6H), 7.56-7.52 (m, 1H).13C NMR (151 MHz, CDCl3) δ 171.90,
169.96, 142.46, 141.41, 137.79, 135.69, 133.39, 132.85, 131.06,
129.72, 129.41, 129.05, 128.72, 127.97, 127.85. MS (EI) m/z:
calcd for C27H19N3O2S ([M]+), 449.97; found, 450.21. Anal. Calcd.
for C27H19N3O2S (%): C, 72.14; H, 4.26; N, 9.35; O,7.12; S, 7.13.
Found: C, 72.21; H, 4.19; N, 9.42; O, 7.02; S, 7.16.
from 50 to 200
℃
under a nitrogen atmosphere.
Thermogravimetric analysis (TGA) was performed on a TAQ 500
thermogravimeter by measuring the weight loss while heating
from 50 to 550 ℃ under nitrogen atmosphere. Absorption and
PL spectra were measured using
a
Hitachi UV–vis
spectrophotometer U-3010 and a Hitachi F-4600 fluorescence
spectrometer, respectively. The single-molecule and mixed
constituting molecules in dichloromethane or chlorobenzene
were spin-coated on quartz glass and annealed at 100 °C for 10
min to form films. Their phosphorescence spectra were
measured at 77 K using a Hitachi F-4600 fluorescence
spectrometer. Cyclic voltammetry was performed by using a
CHI660E electrochemical analyzer. A Pt disk, Pt wire, and Ag
were used as the working, counter, and reference electrodes,
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ARTICLE
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DOI: 10.1039/D0TC04751A
S
Br
O
S
N
O
H2O2
N
N
AcOH
DMF
S
t-BuOK
N
N
N
N
1080C
N
Na
N
1500C
A1
B1
TRZ-1SO2
O
S
S
Br
O
NH
NH2
H2O2
AcOH
Br
t-BuOK
N
N
DMF
S
KOH
N
N
N
N
O
1080C
S
Br
S
O
N
Na
N
N
1500C
O
HCl
A2
B2
TRZ-2SO2
S
O
S
Br
O
N
N
O
H2O2
AcOH
S
N
N
DMF
S
N
O
t-BuOK
N
N
1080C
N
Na
N
1500C
S
S
O
S
Br
Br
O
TRZ-3SO2
A3
B3
Scheme 1. Synthesis of TRZ-1SO2, TRZ-2SO2, and TRZ-3SO2.
Synthesis of 2-phenyl-4,6-bis(3-(phenylsulfonyl) phenyl)-1,3,5- 137.90, 136.52, 132.46, 130.37, 128.75, 128.61, 128.53, 126.58,
triazine (TRZ-2SO2). Add 3-bromobenzamide hydrochloride (10 125.13, 124.76, 124.40, 124.10, 123.30, 123.12.MS (EI) m/z:
mmol, 2.35 g) and KOH solid (15 mmol, 0.84 g) to a 200 ml calcd for C33H23N3O4S2 ([M]+), 589.68; found, 589.95. Anal.
single-mouth reaction flask, 80 ml of ethanol as solvent, and Calcd. for C33H23N3O4S2 (%): C, 67.22; H, 3.93; N, 7.13; O, 10.85;
finally, add 0.5 ml of benzaldehyde with a syringe. The mixture S, 10.87. Found: C, 67.37; H, 4.04; N, 7.11; O, 10.69; S, 10.79.
was stirred at room temperature for 8 hours. When the milky Synthesis of 2,4,6-tris(3-(phenylsulfonyl) phenyl)-1,3,5-
white suspension appeared in the reaction liquid, the stirring triazine (TRZ-3SO2). TRZ-3SO2 was synthesized in the same way
was stopped. Filtered and dried to give 1.82 g of a white solid
A2 (yield 78%); H NMR (400 MHz, CDCl3) δ 8.60-8.51 (m, 2H), 2,4,6-tris(3-bromophenyl) -1,3,5-triazine. The final product was
8.30 (dt, J = 7.5, 1.5 Hz, 2H), 7.64-7.57 (m, 4H), 7.53-7.43 (m, purified and dried to give a white solid (0.85g, 53% yield); H
as TRZ-1SO2, except that the reaction material was changed to
1
1
3H), 7.38 (t, J = 7.5 Hz, 2H). MS (EI) m/z: calcd for C21H13Br2N3 NMR (400 MHz, Chloroform-d) δ 9.26 (d, J =2.0 Hz, 3H), 8.92 (d,
([M]+), 467.16; found, 466.98.
J =7.8 Hz, 3H), 8.20 (d, J = 7.8 Hz, 3H), 8.08 (dd, J = 7.6, 2.0 Hz,
A mixture of solid A2 (3.9 mmol, 1.82 g) and sodium benzene- 6H), 7.76 (t, J = 7.8 Hz, 3H), 7.59 (d, J = 7.2 Hz, 9H). 13C NMR (151
thiolate (11.7 mmol, 1.54 g), potassium t-butoxide (7.8 mmol, MHz, CDCl3) δ 170.69, 142.88, 141.13, 136.70, 133.58, 133.42,
875 mg) was added to a 200 ml two-port flask and place in the 131.62, 120.02, 129.59, 128.22, 127.90. MS (EI) m/z: calcd for
reaction station. The reaction solution was stirred while adding C39H27N3O6S3([M]+), 729.84; found, 729.95. Anal. Calcd. for
50 ml of DMF solvent under N2 protection, refluxed at 150 °C for C39H27N3O6S3 (%): C, 64.18; H, 3.73; N, 5.76; O, 13.15; S, 13.18.
20 h after the reaction is completed, cooled to room Found: C, 63.95; H, 3.84; N, 5.87; O, 13.09; S, 13.25.
temperature. Then extracted the organic phase with a large
amount of water and dichloromethane and dried with
anhydrous sodium sulfate, evaporated, and purified by column
Results and discussion
chromatography using petroleum ether and dichloromethane
to give a white gummy solid B2 (1.25g, yield 61%); 1H NMR (400
MHz, CDCl3) δ 8.59-8.53 (m, 2H), 8.20 (dt, J = 7.5, 1.6 Hz, 2H),
7.60 (t, J = 1.5 Hz, 2H), 7.56-7.43 (m, 7H), 7.41-7.30 (m, 8H),
7.25-7.17 (m, 2H). MS (EI) m/z: calcd for C33H23N3S2 ([M]+),
525.69; found, 525.26.
Design and Synthesis of Acceptors
The synthetic routes of TRZ-1SO2, TRZ-2SO2, and TRZ-3SO2 are
shown in Scheme 1. These materials were prepared in similar
synthesis steps. B1, B2, and B3 were prepared by the
nucleophilic addition reaction of thiophenol sodium with
corresponding bromo compounds A1, A2, and A3 respectively,
where A2 was prepared via the heterocyclic addition reaction of
3-bromobenzamide hydrochloride with benzaldehyde. Then all
of the intermediates were oxidized to obtain final compounds
of TRZ-1SO2, TRZ-2SO2, and TRZ-3SO2. The final compounds
The product B2 (1.25 g) was added in a 200 ml round bottom
flask, 10 ml of hydrogen peroxide and 80 ml of acetic acid
solution were also added, and the mixture was heated to reflux
at 108 °C to carry out an oxidation reaction. After 6 hours, it was
cooled to room temperature, filtered, and recrystallized with
petroleum ether to remove impurities, and finally 1.04 g of a
1
were further purified by sublimation and confirmed by H
NMR,13C NMR, mass spectrometry, and elemental analysis
(detail shown in Supporting Information). 40
Theoretical Calculation
1
white solid TRZ-2SO2 was obtained (yield 74%); H NMR (400
MHz, CDCl3) δ 9.28 (t, J=1.7 Hz, 2H), 8.94 (dq, J=7.8, 1.4 Hz, 2H),
8.75 (dt, J=8.6, 1.5 Hz, 2H), 8.18 (ddt, J=7.8, 2.0, 1.1 Hz, 2H),
8.09-8.05 (m, 4H), 7.74 (t, J = 7.8 Hz, 2H), 7.68-7.65 (m, 1H),
7.63-7.54 (m, 8H). 13C NMR (151 MHz, CDCl3) δ 167.52, 165.52,
To reveal the influence of the sulfone group on the frontier
molecular orbitals and excited states of three new Acceptors,
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density functional theory (DFT) calculations have been LUMO energy levels determined from the onsetVoiefw rAertdicluecOtniloinne
DOI: 10.1039/D0TC04751A
performed in Gaussian program at the B3LYP/6-31G (d) level. waves were -3.38, -3.58 and -3.74 eV for TRZ-1SO2, TRZ-2SO2
The theoretical HOMO and LUMO distributions of three and TRZ-3SO2, respectively, as referred to the redox couple of
Acceptors are depicted in Figure S1a. It is shown that the ferrocenes. It can be observed that as the number of sulfone
LUMOs are mainly dispersed on triphenyltriazine core, while groups increases, the LUMO energy levels of the Acceptors get
HOMOs gradually transfer from triphenyltriazine to deeper. And the value of -3.74 eV is almost the deepest LUMO
diphenylsulfone for TRZ-1SO2, TRZ-2SO2 and TRZ-3SO2, energy level among all reported Acceptors.45 We believe these
respectively. These results should be ascribed to the strong new Acceptors with deep LUMO energy levels will benefit to
conjugations between triazine core and diphenylsulfone unit, develop various exciplexes. Deduced from the LUMO values and
enhancing the electron deficiency of triazine core from TRZ- the absorption edges, the HOMO energy levels of three
1SO2 to TRZ-3SO2. 41 The strong conjugations are also confirmed compounds were -6.73, -6.88 and -7.02 eV for TRZ-1SO2, TRZ-
by the small twisted angles between the triazine core and 2SO2 and TRZ-3SO2, respectively.
diphenylsulfone branch as shown in Figure S1b. As shown in
15
Figure S1c, the theoretical values of HOMO, LUMO, S1 and T1
energy levels were also calculated for three Acceptors. It can be
observed that these Acceptors exhibit almost same S1/ T1
energy levels, while their HOMO and LUMO energy levels get
deeper as the number of diphenylsulfone unit increases.
Thermal Properties
TRZ-1SO2
TRZ-2SO2
TRZ-3SO2
10
5
0
-5
The thermal properties of all Acceptors were measured by
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) under nitrogen atmosphere.42 As shown in
Figure S2, TRZ-1SO2, TRZ-2SO2 and TRZ-3SO2 exhibit excellent
thermal stability with high decomposition temperature (Td) of
352, 436 and 477 °C, respectively. As the Td is positively related
to the molecular weight, Tds of three new Acceptors are
improved greatly with the increase of diphenylsulfone units.
And no obvious glass transition temperature (Tg) is observed in
the range from 25 to 200 °C for all three Acceptors, which
indicates these compounds are not easy to crystallize.43
Consequently, those Acceptors are ideal and excellent for the
vacuum deposition process.
-10
-15
-2.0
-1.5
-1.0
Potential/V
-0.5
Figure 1. The Cyclic voltammograms of TRZ-1SO2, TRZ-2SO2 and TRZ-3SO2.
To study the electron-transporting properties of these
Acceptors, electron-only devices were fabricated with the
structure of ITO/Al (30 nm)/LiF (1 nm)/ETL (100 nm)/LiF (1
nm)/Al (100 nm). In these devices, LiF is used as a buffer layer
between the organic layer and the electrode. The ETL of these
devices were TmPyPB, TRZ-1SO2, TRZ-2SO2, and TRZ-3SO2,
respectively. Electric-field dependent mobilities evaluated by
using the space charge-limited current (SCLC) method are
presented in Figure S3. The electron mobilities of TmPyPB, TRZ-
1SO2, TRZ-2SO2 and TRZ-3SO2 are varied within the range from
Table 1. The physicochemical properties of TRZ-1SO2, TRZ-2SO2, and TRZ-3SO2.
λabs
λPL
λPhos
Egd
HOMO/LUMO
(eV)
T1
Td
(℃)
a
b
c
e
Acceptors
2.7 10-5 to 3.5 10-5 cm2 v-1 s-1, 1.6 10-6 to 6.1 10-6 cm2 v-1 s-1,
(nm) (nm) (nm) (eV)
(eV)
2.95
2.95
2.95




7.3 10-6 to 1.2 10-5 cm2 v-1 s-1 and 1.1 10-5 to 3.1 10-5 cm2




-1 -1
5
-1
v s , respectively with fields varying from 6.410 V cm to
TRZ-1SO2
TRZ-2SO2
370
375
378
412
412
412
420
420
420
3.35
3.30
3.28
-6.73/-3.38
-6.88/-3.58
-7.02/-3.74
352
436
477
6
-1
1.010 V cm . Meanwhile, with more sulfone groups, TRZ-
2SO2 and TRZ-3SO2 exhibit better electron mobilities than TRZ-
1SO2.
Photophysical Properties
TRZ-3SO2
(a)
(b)
a
Determined from the absorption band edges of their toluene solution at
room temperature; b Determined from the first peak of fluorescent spectra
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
TRZ-1SO2
TRZ-2SO2
TRZ-3SO2
TRZ-1SO2
TRZ-2SO2
TRZ-3SO2
1.0
0.8
0.6
0.4
0.2
0.0
c
of their toluene solution at room temperature; Determined from the first
phosphorescent emission peak of their dimethyl tetrahydrofuran solution at
77 K; d Calculated from their absorption band edge; e Estimated from the first
peak of phosphorescence spectrum at 77K.
300 350 400 450 500 550 600 650
350
400
450
500
550
600
Electrochemical and Carrier Transporting Properties
Wavelength (nm)
Wavelength (nm)
To determine the frontier molecular orbitals energy levels of
three Acceptors, their electrochemical properties were
measured by cyclic voltammetry (CV) in dimethyl formamide
(DMF) under nitrogen atmosphere.44 As presented in Figure 1,
all Acceptors exhibit quasi-reversible reduction curves. And the
Figure 2. (a) UV-vis absorption and fluorescence spectra (excited at 360 nm) of TRZ-1SO2,
TRZ-2SO2, TRZ-3SO2 in toluene solutions at room temperature; (b) Phosphorescence
spectra (excited at 340 nm) of TRZ-1SO2, TRZ-2SO2, TRZ-3SO2 in dimethyl tetrahydrofuran
solution at 77 K.
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ARTICLE
To determine the photophysical properties of TRZ-1SO2, TRZ- about 2.1-2.3 V. And the turn-on voltages of E2 and E3 are
View Article Online
2SO2 and TRZ-3SO2, the ultraviolet-visible (UV-vis) absorption slightly lower than that of E1 due to thDeOdI:e1e0p.1e03r-9l/yDin0TgCL0U47M51OA
and photoluminescence (PL) spectra were investigated in energy level of TRZ-2SO2 and TRZ-3SO2. The emission maxima of
toluene solutions at room temperature. As shown in Figure 2a, the EL spectra are 532, 548, and 572 nm for E1, E2, and E3,
all Acceptors exhibit strong absorptions around 310 nm in respectively, which corresponds to the green, yellow, and
toluene solution, which can be ascribed to the local excited orange-red emission, respectively. As the LUMO energy levels
transitions. From the absorption band edges, the energy band of three Acceptors deepen with the increase of diphenylsulfone
gaps (Eg) of TRZ-1SO2, TRZ-2SO2 and TRZ-3SO2 are assumed to units, the EL spectra are gradually red-shifted with the smaller
be 3.35, 3.30 and 3.28 eV, respectively. And the fluorescence energy gaps. And obviously, each of the EL spectra does not
(FL) spectra of TRZ-1SO2, TRZ-2SO2 and TRZ-3SO2 are the same agree to any monomer emission, its exciplex characteristic is
in the whole range, including two peaks at 412 and 435 nm, in confirmed again.50 As shown in Figure S6, the EL spectra of three
accord with their similar electronic states. The phosphorescent devices are almost independent with the increased driving
spectra of three Acceptors measured in dimethyl voltages, indicating stable exciplex emission. In addition, as
tetrahydrofuran solution at 77 K were shown in Figure 2b, and shown in Figure 3c and 3e, the OLED using 13AB:TRZ-3SO2 as
the triplet energy levels (T1s) of TRZ-1SO2, TRZ-2SO2 and TRZ- the emitters exhibits an orange-red emission peaking at 572 nm
3SO2 were all estimated as 2.95 eV, calculated from the first with the maximum EQE of 5.5%. The remarkable performance
peak of their phosphorescence spectra.46 The detailed of devices effectively proves the great prospects of these
physicochemical data are summarized in Table 1.
Acceptors in the development of exciplexes.
Exciplex Emitters Based on New Acceptors
Since an exciplex formation requires an efficient charge transfer
between Donor and Acceptor, it is important to select an
appropriate Donor combined with these Acceptors.47 Here,
13AB is selected as the Donor, three exciplexes 13AB:TRZ-1SO2,
13AB:TRZ-2SO2 and 13AB:TRZ-3SO2 were constructed. And their
photophysical properties were investigated in thin films at room
temperature. As shown in Figure S4, all systems exhibit nearly
identical absorption spectra to those of the constituting
molecules, and no new absorption band appears at longer
wavelengths, suggesting that there exists no formation of new
ground-state transition in the mixed films.48 And the PL spectra
of mixed films exhibit typical exciplex characteristics with a peak
at 507, 538 and 554 nm respectively, which is significantly
redshifted and broadened relative to those of the constituting
molecules. Moreover, the PLQYs of three exciplex emitters
were also measured in nitrogen atmosphere, and their values
are 56.3% for 13AB:TRZ-1SO2, 41.8% for 13AB:TRZ-2SO2 and
25.4% for 13AB:TRZ-3SO2, respectively. The decrease of the
PLQY values is because the non-radiative transition will be
enhanced with the decrease of energy gap for exciplex emitters.
To evaluate the capability of using 13AB:Acceptors (TRZ-
1SO2, TRZ-2SO2 and TRZ-3SO2) as TADF emitters in the OLEDs,
three devices E1-E3 were fabricated with a simple structure:
ITO/TAPC (40 nm)/13AB (10 nm)/EML (30 nm)/Acceptor (45
nm)/LiF (1 nm)/Al (100 nm). In these devices, ITO and LiF/Al are
used as the anode and the cathode, respectively. TAPC is used
as the hole-injection layer. 13AB and Acceptor are used as the
hole-transporting layer and electron-transporting layer,49
respectively. 13AB:Acceptor with a weight ratio of 1:1 is used as
the EML. Figure 3a shows the device structure and schematic
diagram of the energy levels of materials used in devices. And
the chemical structures of the materials used in this paper are
depicted in Figure S5.
(b)
600
(c)
E1 (13AB:TRZ-1SO2
E2 (13AB:TRZ-2SO2
E3 (13AB:TRZ-3SO2
)
)
)
104
103
102
101
100
500
400
300
200
100
0
10
1
E1
E2
E3
0.1
101
102
Luminance(cd/m2)
103
104
2
4
6
8
Voltage (V)
(d)
(e)
103
102
101
100
1.0
E1
E2
E3
0.8
0.6
0.4
0.2
0.0
10
E1
E2
E3
1
101
102
103
104
105
300
400
500
600
700
800
Wavelength (nm)
Luminance (cd/m2)
Figure 3. (a) device structures, energy level diagram of used materials in exciplex-based
OLEDs. the EL characteristics of exciplex-based OLEDs: (b) current density-voltage-
luminance curves; (c) EQE-luminance curves; (d) current efficiency-luminance-power
efficiency and (e) EL spectra.
Exciplex hosts Based on New Acceptors
To make full use of three new Acceptors, we further
constructed interfacial exciplex hosts in the OLEDs by combining
these Acceptors with CBP.51 The absorption and PL spectra of
CBP, Acceptors and CBP:Acceptors in thin films are displayed in
Figure S7. The PL spectra of mixed films show representative
exciplex characteristics, which are observably red-shifted and
broadened in comparison to those of the constituent parts.52
Before utilizting as the hosts, the EL spectra of exciplexes
formed between CBP and three Acceptors were first measured.
And Device E4-E6 were fabricated with a structure of ITO/TAPC
Figure 3b-e display the electroluminescence (EL)
characteristics of these devices, and the performance data are
summarized in Table 2. Device E1-E3 exhibit low turn-on voltage
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Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/D0TC04751A
Table 2. Electroluminescence of Data for Devices E1-E3, G1-G4, and R1-R4.
a
b
c
Von
(V)
CEmax
(cd/A)
PEmax
(lm/W)
EQEmax
(%)
PE, EQE100
PE, EQE500
λmax
(nm)
Device
CIE d
(lm/W, %)
(lm/W, %)
E1
E2
E3
G1
G2
G3
G4
R1
R2
R3
R4
2.3
2.1
35.7
28.5
12.5
88.6
85.9
55
37.4
35.8
14.6
106
108
11.1
8.5
39.0, 10.7
35.9, 8.4
35.1, 10.6
31.6, 8.0
532
548
572
524
524
524
524
612
608
604
612
(0.33, 0.57)
(0.40, 0.60)
(0.49, 0.51)
(0.32, 0.64)
(0.32, 0.64)
(0.32, 0.64)
(0.32, 0.64)
(0.62, 0.38)
(0.61, 0.39)
(0.61, 0.39)
(0.63, 0.37)
2.1
5.5
14.9, 5.4
12.0, 5.0
2.5
23.2
22.7
14.8
20.0
23.1
24.4
18.6
22.4
84. 1, 23. 0
80.9, 21.1
49.5, 14.5
63.8, 18.6
21.2, 22.8
24.4, 20.1
22.6, 18.6
22.0, 22.4
63.7, 21.2
53.8, 17.1
41.9, 14.7
61.2, 19.8
15.8, 21.9
15.9, 17.0
17.0, 17.8
17.8, 21.8
2.4
2.2
44.3
65.2
32.1
47.4
29
2.9
75.8
36.6
40.8
30.6
32.2
2.92
2.66
2.69
3.29
27.1
a Turn-on voltage at 1 cd/m2; b PE and EQE at a brightness of 100 cd/m2; c PE and EQE at a brightness of 500 cd/m2; d CIE coordinate at 1000 cd m-2
CBP:TRZ-3SO2 can’t. Then green and red phosphorescent
devices are fabricated by using interfacial exciplexes
CBP/Acceptor as the hosts. The structure of green devices
(Device G1-G3) is as follows: ITO/TAPC (40 nm)/CBP:
Ir(PPy)2acac (8 wt%, 30 nm)/Acceptor (45 nm)/LiF(1 nm)/Al (100
nm). For comparison, Device G4 was fabricated under the same
structure except for using TmPyPB as an Acceptor.
Figure 4a shows the device structure and schematic
diagram of the energy levels of materials used in devices. As
shown in Figure 4b, G1-G3 exhibit lower turn-on voltage than
G4, which should be attributed to the superiority of interfacial
exciplex hosts.53 In G4, without interfacial exciplex formed
between CBP and TmPyPB, the excitons should be formed on
CBP. While in G1-G3 with interfacial exciplex hosts, the excitons
are directly formed between the LUMO energy level of Acceptor
and the HOMO energy level of CBP, corresponding to lower
energy gaps.54 Therefore, CBP/Acceptor interfacial exciplex
hosts exhibit lower turn-on voltages than CBP single host.
Nevertheless, as TmPyPB has higher electron mobility than
three new Acceptors, the driving voltages of TmPyPB-based
device are lower than other devices. From Figure S9, all the
devices exhibit a single emission peak wavelength at around
524 nm, which indicates that the energy transfers from hosts to
emitters. As presented in Figure 4c, G1-G2 exhibit higher
maximum PEs respectively of 106 and 108 lm W-1 than G4,
which should be ascribed to their low turn-on voltages and high
maximum EQEs of 23.2% and 22.7%, respectively. Meanwhile,
G3 shows the lowest maximum EQE of 14.8%, which is because
CBP/TRZ-3SO2 can’t fully harvest all excitons to Ir(PPy)2acac.55
(b)
500
(c)
105
104
103
102
101
100
103
102
101
100
G1
G2
G3
G4
400
300
200
100
0
101
G1
G2
G3
G4
100
101
102
Luminance(cd/m2)
103
104
105
1
3
5
7
9
11
Voltage (V)
Figure 4. (a) device structures, energy level diagram of used materials in green devices
based on CBP/Acceptors interfacial exciplex hosts. the EL characteristics of green
devices: (b) current density-voltage-luminance curves; (c) EQE-luminance-power
efficiency curves.
(40 nm)/CBP (20 nm)/EML (30 nm)/Acceptor (45 nm)/LiF (1
nm)/Al (100 nm), where CBP:Acceptor with a weight ratio 1:1)
is used as the EML. As shown in Figure S8, all EL spectra of three
CBP-based exciplexes are consistent with their PL spectra with
peaks at 468, 512 and 528 nm for CBP:TRZ-1SO2, CBP:TRZ-2SO2
and CBP:TRZ-3SO2, respectively. These results indicated
CBP:TRZ-1SO2 and CBP:TRZ-2SO2 have high enough energy
levels to sensitize green phosphorescent emitters, while
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Journal of Materials Chemistry C
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significantly better than TmPyPB-based contrastive devices. The
View Article Online
DOI: 10.1039/D0TC04751A
outstanding performance of these devices not only suggests the
great potential of three new Acceptors for the development of
exciplex emitters but also proves the superiority of interfacial
exciplex hosts to develop high-performance PhOLEDs.
Conflicts of interest
There are no conflicts to declare.
(b)
500
(c)
105
104
103
102
101
100
R1
R2
R3
R4
100
10
1
400
300
200
100
0
Acknowledgements
10
The acknowledgements come at the end of an article after the
conclusions and before the notes and references. This work was
supported by the National Natural Science Foundation of China
(Grant Nos. 51773029, 61775029 and 51533005), the
Fundamental Research Funds for the Central Universities (Grant
No. ZYGX2016Z010) and the International Cooperation and
Exchange Project of Science and Technology Department of
Sichuan Province (Grant Nos. 2019YFH0057 and 2019YFH0059).
R1
R2
R3
R4
1
101
102
Luminance(cd/m2)
103
104
105
1
3
5
7
9
11
13
15
Voltage (V)
Figure 5. (a) device structures, energy level diagram of used materials in green devices
based on CBP/Acceptors interfacial exciplex hosts. the EL characteristics of green
devices: (b) current density-voltage-luminance curves; (c) EQE-luminance-power
efficiency curves.
Red phosphorescent OLEDs were fabricated with the
similar structure: ITO/TAPC (40 nm)/CBP: Ir(MDQ)2acac (6 wt%,
30 nm)/Acceptor (70 nm)/LiF (1nm)/Al (100 nm), where
Acceptor was TRZ-1SO2, TRZ-2SO2, TRZ-3SO2, and TmPyPB,
respectively (Device R1-R4). The device structure and schematic
diagram of the energy levels of each layer and EL characteristics
are exhibited in Figure 5. It can be observed that R1-R3 exhibit
higher maximum PEs (32.1, 47.4, and 29 lm W-1, respectively)
than R4, which can be attributed to their low turn-on voltages
and high maximum EQEs. Particularly, R3 exhibits the poor
performance of the lowest maxium EQE of 18.6% comparing to
R1 and R2. It can be explained by EL spectra displayed in Figure
S10. The EL spectrum of R3 not only shows emission maximum
centered at 604 nm but also an obvious bulge around 520 nm
that is attributed to the emission of CBP/TRZ-3SO2 exciplex.
Thus, the energy is not completely transferred from host to
emitter, leading to the lowest EQE.56-58 The detailed
performance data of green and red devices are summarized in
Table 2.
Notes and references
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J. Mater. Chem. C
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Journal of Materials Chemistry C
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DOI: 10.1039/D0TC04751A
Table of contents
Three novel triazine-based Acceptors with deep LUMO energy levels were successfully developed. TRZ-3SO2
exihits the deepest LUMO energy level of -3.74 eV. The OLED using 13AB:TRZ-3SO2 as the emitter exhibits an
orange-red emission peaking at 572 nm with the maximum external quantum efficiency of 5.5%.