Management of thermally activated delayed fluorescence using a secondary electron accepting unit in thermally activated delayed fluorescent emitters

Article abstract of DOI:10.1039/c9tc01354g

A molecular design approach adopting secondary electron accepting units in addition to a main electron acceptor was investigated as a strategy to manage the thermally activated delayed fluorescent (TADF) performance of the emitters. A main skeleton posses

Full text of DOI:10.1039/c9tc01354g

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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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Journal of Materials Chemistry C
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Management of thermally activated delayed fluorescence using a secondary
electron accepting unit in thermally activated delayed fluorescent emitters
1
1
1*
2*
Ha Lim Lee , Kyung Hyung Lee , Jun Yeob Lee , Wan Pyo Hong
1School of Chemical Engineering, Sungkyunkwan University
2
066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, 440-746, Korea
2
LG Chem, Ltd, LG Science Park, 30, Magokjungang 10-ro, Gangseo-gu, Seoul, 07796, Korea
Fax: (+) 82-31-299-4716
*
E-mail: leej17@skku.edu, whongw@lgchem.com
Abstract
A molecular design approach adopting secondary electron accepting units in addition to a main
electron acceptor was investigated as a strategy to manage the thermally activated delayed
fluorescent (TADF) performances of the emitters. A main skeleton possessing a
benzofurocarbazole donor, a phenyl linker and a t-butylated diphenyltriazine acceptor was a
platform of the molecular structures for high efficiency in the TADF devices and CN units was
attached to the phenyl linker as the secondary accepting unit. Either one or two CN units were
introduced to examine the effect of the number of secondary electron accepting unit on the
TADF characteristics. Correlation of the number of secondary electron accepting unit with the
emission properties of the TADF emitters revealed that the introduction of one CN secondary
electron accepting unit is ideal in terms of TADF performances of the emitters. The TADF
emitter without the CN secondary acceptor realized one of the highest efficiency of 19.0% in
the deep blue TADF devices with a y color coordinate below 0.10 by introducing the t-butylated
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DOI: 10.1039/C9TC01354G
diphenyltriazine acceptor. Whereas, the TADF emitter with one CN secondary electron
accepting unit displayed even higher external quantum efficiency of 26.6%.
Key words : electron acceptor, delayed fluorescence, efficiency, emission color
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Journal of Materials Chemistry C
Introduction
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DOI: 10.1039/C9TC01354G
The progress of thermally activated delayed fluorescent (TADF) organic light-emitting diodes
OLEDs) has been remarkable for the last several years with the advent of molecular structures allowing
(
the up-conversion of triplet excitons into singlet excitons.1-9 The core structure for the up-conversion
process is the donor-acceptor (D-A) backbone structure because it can minimize the energy difference
between singlet and triplet excited states by stabilizing the singlet excited state using a charge transfer
(CT) emission process rather than the local emission (LE) process which is common in conventional
fluorescent emitters.2, 5, 7, 10-12 Therefore, the main backbone structure of the TADF dopants was mostly
derived from the D-A structure.
Although the main D-A skeleton was shared in the TADF molecular design, the chemical structure of
the TADF emitters was diversified by engineering the arrangement of the donor and acceptor units to
manage the TADF characteristics. For example, D-A-D type backbone structure with an additional donor
unit and A-D-A structure with an extra acceptor unit could increase the external quantum efficiency
(
EQE) of the TADF devices.2, 4, 5, 7, 10, 13 Multi-donor and multi-acceptor combined structure was a
modified version of the D-A structure.14-16 Other than these, multi-donor and one acceptor merging was
also an effective approach to control the emission color and enhance EQE of the D-A base structure of
the TADF emitters. Generally, the increase of the number of donors had an EQE boosting effect by large
overlap of the molecular orbital for high radiative transition probability.7, 17, 18 In many cases, the donor
part rather than the acceptor part of the TADF emitters was managed. However, the acceptor part of the
TADF emitter is as important as the donor part because the emission from the TADF emitter is CT
emission from the donor to acceptor and the key parameter of singlet-triplet energy gap (EST) is
governed by both the donor and acceptor.1-10, 12 Therefore, it is essential to engineer the acceptor part of
the TADF emitter for enhanced TADF performances.
Herein, we describe the molecular design approach introducing secondary acceptor units in addition
to a main acceptor as a strategy to manage the TADF properties of the TADF emitters. The main acceptor
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of the TADF emitter was t-butylated diphenyltriazine and the secondary acceptor was a CN unit.
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DOI: 10.1039/C9TC01354G
Correlation of the number of secondary electron accepting unit with the emission properties of the TADF
emitters revealed that the introduction of one CN secondary electron accepting unit is ideal in terms of
TADF performances of the emitters. The pristine molecule without the CN unit showed high EQE of
1
9.0% with a color coordinate of (0.15, 0.09) and achieved one of the highest EQEs of the deep blue
TADF devices with a y color coordinate below 0.10. The TADF emitter with one CN secondary electron
accepting unit displayed even higher EQE of 26.6%.
Results and discussion
The design concept of the TADF emitter introducing the secondary acceptor in the D-A backbone
structure is to precisely manage the TADF properties by additionally attaching weaker acceptors than the
main acceptor. As the main acceptor is a strong acceptor, the intrinsic acceptor character of the main
acceptor can be maintained and the secondary acceptor can adjust the electron acceptor character for
improved performances. Main skeleton of the TADF emitters was the D-A structure with a phenyl linker
between the donor and acceptor units, where the donor was benzofurocarbazole and the acceptor was t-
butylated diphenyltriazine. The secondary acceptor was the CN unit which is weaker than
diphenyltriazine as the electron acceptor. The secondary acceptor was attached to the phenyl linker
connecting the donor and acceptor. The number of the CN secondary acceptor was controlled from 0 to
2
to examine the effect of the secondary acceptor on the TADF properties.
The chemical structures and synthetic processes of the TADF emitters with the CN secondary
acceptors, TrzBFCz, TrzCNBFCz, and Trz2CNBFCz are shown in Scheme 1. The TrzBFCz has the
conventional D-A structure, the TrzCNBFCz has one CN secondary acceptor in the D-A structure, and
the Trz2CNBFCz has two CN secondary acceptors in the D-A structure. As the number of the CN
secondary acceptors was systematically controlled, the effect of the secondary acceptor on the TADF
performances can be examined using the three TADF emitters. The synthesis of the TrzBFCz was simply
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carried out by NaH mediated amination reaction between benzofurocarbazole and 2,4-bis(4-(tert-
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DOI: 10.1039/C9TC01354G
butyl)phenyl)-6-(4-fluorophenyl)-1,3,5-triazine prepared by Suzuki coupling reaction between 2,4-bis(4-
(tert-butyl)phenyl)-6-chloro-1,3,5-triazine and (4-fluorophenyl)boronic acid. The synthetic process of
the TrzCNBFCz was quite similar to that of the TrzBFCz except that (3-cyano-4-fluorophenyl)boronic
acid was adopted instead of (4-fluorophenyl)boronic acid in the preparation of the 5-(4,6-bis(4-(tert-
butyl)phenyl)-1,3,5-triazin-2-yl)-2-fluorobenzonitrile intermediate. However, the Trz2CNBFCz could
not be obtained from the same synthetic process because 5-(4,6-bis(4-(tert-butyl)phenyl)-1,3,5-triazin-2-
yl)isophthalonitrile intermediate for amination reaction was not available by direct Suzuki coupling
reaction. In this case, a two-step synthetic method employing a Suzuki coupling reaction between 2,4-
bis(4-(tert-butyl)phenyl)-6-chloro-1,3,5-triazine
and
5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)isophthalonitrile followed by lithiation using Lithium diisopropylamide to generate a Br functional
unit between two CN units was used. Final reaction step was CuI mediated amination reaction between
5
-(4,6-bis(4-(tert-butyl)phenyl)-1,3,5-triazin-2-yl)-2-bromoisophthalonitrile and benzofurocarbazole.
Work-up and purification processes including extraction, column chromatography, recrystallization and
1
vacuum sublimation provided the pure final products. Conventional chemical analysis methods of H and
13C nuclear magnetic resonance (NMR), mass analysis, and high performance liquid chromatography
identified the TADF emitters.
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Scheme 1. Synthesis scheme of TrzBFCz, TrzCNBFCz and Trz2CNBFCz
Electronic molecular orbital distribution of the TADF emitters was examined to study the highest
occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) change by
the CN secondary acceptors. The calculation results by Gaussian software using the B3LYP 6-31G basis
set pictured the HOMO and LUMO of the TADF emitters in Figure 1. Overall pictures of the HOMO
and LUMO look quite similar in the three TADF emitters in that the HOMO and LUMO are extensively
separated, but minor change of the HOMO and LUMO was noticed by the CN secondary acceptor. The
addition of the CN secondary acceptor localized the HOMO of the TADF materials. The HOMO of the
TrzBFCz and TrzCNBFCz was largely extended from the benzofurocarbazole donor to the phenyl linker,
but the HOMO extension became narrow in the Trz2CNBFCz. The LUMO dispersion was also further
localized as the number of CN unit is increased. In the case of the TrzBFCz, the LUMO was distributed
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Journal of Materials Chemistry C
from the diphenyltriazine unit to the phenyl linker, but it was spread over the triazine unit and phenyl
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DOI: 10.1039/C9TC01354G
linker without any dispersion on the phenyl unit of the diphenyltriazine in the Trz2CNBFCz. The HOMO
and LUMO change can be interpreted as the effect of the electron withdrawing character of the CN unit.
The HOMO was not widely extended from the donor to the phenyl linker by the electron deficiency of
the CN attached phenyl linker, whereas the LUMO was shifted to the phenyl linker by the electron
deficiency. The oscillator strength of the TrzBFCz, TrzCNBFCz and Trz2CNBFCz was 0.3341, 0.2195
and 0.1250, respectively. The addition of second CN unit reduced the oscillator strength due to the
isolation of the HOMO and LUMO.
Figure 1. Ground state orbital distribution of TrzBFCz, TrzCNBFCz and Trz2CNBFCz calculated with
B3LYP 6-31G basis set
The orbital distribution of the three materials was also estimated in singlet and triplet excited states.
Figure 2 pictured the highest occupied natural transition orbital (HONTO) and the lowest unoccupied
natural molecular orbital (LUNTO) of TrzBFCz, TrzCNBFCz and Trz2CNBFCz emitters. In all three
emitters, the HONTO and LUNTO in singlet excited state were spatially separated, reflecting the charge
transfer (CT) nature of the singlet emission. However, the HONTO and LUNTO in triplet excited state
were dissimilar in the three emitters. The large overlap of the HONTO and LUNTO was featured in the
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TrzBFCz, but the overlap was significantly reduced by the CN secondary acceptor in the TrzCNBFCz
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DOI: 10.1039/C9TC01354G
and Trz2CNBFCz, suggesting that the origin of the lowest triplet excited state is CT state.
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Figure 2. Excited state orbital distribution of TrzBFCz, TrzCNBFCz and Trz2CNBFCz calculated with B3LYP 6-31G basis set
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Figure 3. Optimized Structure of TrzBFCz, TrzCNBFCz and Trz2CNBFCz in ground state and excited
state.
In addition to the orbital distribution, the geometrical structure of the TADF emitters was affected by
the CN secondary acceptor. The optimized geometrical structure of the three emitters in ground state and
excited state are shown in Figure 3. The dihedral angles between the benzofurocarbazole donor and the
o
phenyl unit were 53.0, 56.5 and 66.9 in the TrzBFCz, TrzCNBFCz and Trz2CNBFCz emitters in ground
state. The CN unit distorted the benzofurocarbazole donor from the phenyl plane by weak steric
hindrance. The benzofurocarbazole donor of TrzCNBFCz and Trz2CNBFCz was largely distorted in
o
excited state, affording close to 90 dihedral angle. The perpendicular orientation of the donor from the
phenyl plane agrees with the strong CT nature of the excited state. However, the dihedral angle of the
o
TrzBFCz was only 48.3 in excited state, suggesting weak CT character in excited state.
The effect of the secondary acceptor can be clearly observed in the oxidation and reduction potential
measurement by cyclic voltammetry (CV) to estimate the HOMO and LUMO. Both the oxidation and
reduction in Figure 4 were affected by the strongly electron withdrawing CN unit. The HOMO/LUMO
levels of the TrzBFCz, TrzCNBFCz and Trz2CNBFCz were -6.04/-3.21, -6.16/-3.41, and -6.37/-4.08 eV,
respectively. The HOMO and LUMO were gradually deepened by adding extra CN units. In particular,
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the LUMO was largely deepened by the CN secondary acceptor, which resulted in the small HOMO-
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DOI: 10.1039/C9TC01354G
LUMO gap in the TADF emitters with the CN secondary acceptor.
Figure 4. CV measurement of TrzBFCz, TrzCNBFCz, Trz2CNBFCz
The photophysical analysis of the TADF emitters was carried out by ultraviolet-visible (UV-vis)
absorption and photoluminescence (PL) measurements. Figure 5(a) shows the UV-vis absorption spectra
of the three emitters in tetrahydrofuran solution. Main absorption peaks of the UV-vis spectra assigned
to electronic transition of the backbone structure were analogous, but the CT absorption peaks were
shifted to long wavelength according to the number of the CN secondary acceptor. The red shift of the
CT absorption peaks suggests strengthened CT character of the TADF emitters by adding the CN
secondary acceptor. PL emission spectra of the TADF emitters at room temperature and low temperature
are shown in Figure 5(b) and 5(c), respectively. The PL spectra at room temperature and at low
temperature (77 k) were also shifted to long wavelength in the order of TrzBFCz < TrzCNBFCz <
Trz2CNBFCz. The addition of the CN unit intensified the CT character of the emitters by the augmented
acceptor strength, shifting the emission spectra to long wavelength. The PL spectra at room temperature
were also broadened by adding the CN secondary acceptor due to strong CT character. The origin of the
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singlet emission was CT emission from the featureless broad emission, but that of the triplet emission
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was a hybrid emission from CT and local triplet excited states. The singlet energy/triplet energy values
of the TrzBFCz, TrzCNBFCz and Trz2CNBFCz emitters were 3.22/2.95, 3.04/2.91, and 2.87/2.77 eV,
respectively, which afforded EST values of 0.27, 0.13, and 0.10 eV. The extra CN secondary acceptor
reduced the EST by the strong acceptor character.
Figure 5. (a) Uv-Visible absorption spectra, (b) PL emission spectra at room temperature, (c) PL
emission spectra at low temperature with 2 microsecond delay
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Figure 6. (a) Prompt decay curves, (b) delayed decay curves of TrzBFCz, TrzCNBFCz and
Trz2CNBFCz
The photophysical behaviour of the emitters was further studied by analysing the transient PL decay
and absolute PL quantum yield measurements to extract kinetic parameters for the TADF transition
processes. The transient PL decay data of the emitters are presented in Figure 6(a) and (b). The decay
times for the fast fluorescence process were 8.3, 13.8, and 18.5 ns in the TrzBFCz, TrzCNBFCz and
Trz2CNBFCz emitters, and those for the delayed fluorescence process were 26.3, 9.4, and 3.1 s. The
CN secondary acceptor accelerated the delayed fluorescence process, but retarded the fast fluorescence
transition. The acceleration of the delayed fluorescence transition by the CN secondary acceptor is due
to small EST. However, the PL quantum yield of the Trz2CNBFCz was only 62.0% because of small
oscillator strength caused by extensive separation of the HOMO and LUMO. As the radiative transition
is activated by large HOMO and LUMO overlap, the HOMO and LUMO separation by the two strong
electron withdrawing CN units limited the radiative transition probability and PL quantum yield. The PL
quantum yield was close to 100% in the TrzBFCz and TrzCNBFCz due to high oscillator strength.
Although it was slightly reduced in the TrzCNBFz, the reduced oscillator strength was compensated by
small E .Therefore, the addition of one CN secondary acceptor was ideal to reach high PL quantum
ST
yield and shortened delayed fluorescence lifetime. As a result, the reverse intersystem crossing (RISC)
rate constant of the TrzCNBFCz was much higher than that of the TrzBFCz. The rather high RISC rate
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constant of the Trz2CNBFCz is due to short delayed fluorescence lifetime in spite of low PL quantum
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yield. All data of photophysical property was summarized on Table 1.
Table 1. Summarized photophysical data of TrzBFCz, TrzCNBFCz and Trz2CNBFCz
Egap EHOMO ELUMO UV gap
ES
ET
EST
(eV)
τ
τ
p
d
Φtotal
Φp
Φd
(
eV) (eV) (eV)
(eV)
(eV)
(eV)
(ns)
(μs)
TrzBFCz
2.83 -6.04 -3.21
3.08
2.95
2.74
3.22
3.04
2.87
2.95
2.91
2.77
0.27
99.8%
82.3%
17.5%
8.3
26.3
9.4
TrzCNBFCz 2.75 -6.16 -3.41
Trz2CNBFCz 2.29 -6.37 -4.08
0.13 100.0% 47.5%
0.10 62.0% 28.1%
52.5% 13.8
33.9% 18.5
3.1
-
5
*
Uv-vis absorption spectra was measured in THF solution with 10 M concentration
** Singlet and triplet energy was calculated from onset of low temperature emission spectra in frozen THF matrix at 77K without and
with 2 ms delay time.
*** PLQY and transient PL was measured with the DPEPO film doped 10 wt% of emitters in N
2
atmosphere.
**** Prompt component and delayed component was calculated by integrating transient PL data.
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Figure 7. (a) Current density-Voltage-Luminance curves, (b) Quantum Efficiency-Luminance
curves and (c) Electroluminescence spectra of TrzBFCz device
The TADF devices of the three emitters were fabricated by doping in the common (oxybis(2,1-
phenylene))bis(diphenylphosphine oxide) (DPEPO) host. The doping concentration range of the TADF
emitters was from 10% to 30%. Firstly, the device performances of the pristine TrzBFCz without the CN
units were investigated. All device performances including current density (J)-voltage (V), luminance
(
L)-V, EQE-L, and electroluminescence (EL) spectra of the TrzBFCz devices are in Figure 7 according
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to doping concentration. A general trend showing the increase of J and L according to doping
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concentration was noticed by carrier hopping through the dopant molecules as reported in other DPEPO
2
0
hosted TADF devices. The carrier trapping effect can be confirmed in the energy level diagram in
Figure 8. The EQE was optimized as 19.0% at 10% doping concentration and then reduced according to
the doping concentration by concentration quenching effect. Considering that the color coordinate of the
TrzBFCz device was (0.15, 0.09), the 19.0% EQE of the TrzBFCz device is one of the highest EQEs of
the deep blue TADF OLEDs with a y color coordinate below 0.10. Compared with the TADF emitter
which had a diphenyltriazine acceptor without the t-butyl unit, the y color coordinate was shifted from
1
9
0
1
.18 to 0.09 by the electron donating character of the t-butyl group. The EQE was also increased from
6.4% to 19.0% possibly due to intermolecular interaction suppressing role of the t-butyl unit in the
diphenyltriazine acceptor. In the EL spectra, peak wavelength of the device was 447 nm at 10% doping
concentration and it was shifted to 453 nm at 30% doping concentration. The bathochromic shift of the
EL spectra at high doping concentration was marginal because of the weak CT character and t-butyl
blocking groups in the acceptor unit. The color coordinates of the TrzBFCz devices were (0.15, 0.09),
(0.15, 0.10), and (0.15, 0.11) at 10, 20, and 30% doping concentrations.
Figure 8. Energy level diagram of TrzBFCz, TrzCNBFCz and Trz2CNBFCz devices
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The device data of the TrzCNBFCz and Trz2CNBFCz devices are shown in Figure 9 and 10. The J
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DOI: 10.1039/C9TC01354G
and L dependence on the doping concentration was similar in all TADF devices by exhibiting high J and
L at high doping concentration. The EQE was optimized either at 10% or 20% doping concentration in
the TADF devices. The maximum EQEs of the TrzCNBFCz and Trz2CNBFCz devices were 26.6 and
1
5.8%, respectively. The highest EQE was observed in the TrzCNBFCz device because of high PL
quantum yield and efficient RISC process as reflected in the high RISC rate constant. The EQE of the
TrzCNBFCz device was much higher than that of the TrzBFCz device. Therefore, the addition of one
CN secondary acceptor was ideal to achieve high EQE in the TADF devices. The EQE of the
Trz2CNBFCz was rather low because of low PL quantum yield of the emitter in spite of high RISC rate
constant. However, the efficiency roll-off of the devices followed the order of the delayed fluorescence
lifetime because it relies on the triplet exciton associated quenching process. The current density up to
5
0% of the maximum EQE was in the order of Trz2CNBFCz > TrzCNBFCz > TrzBFCz.
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Figure 9. (a) Current density-Voltage-Luminance curves, (b) Quantum Efficiency-Luminance
curves and (c) Electroluminescence spectra of TrzCNBFCz device
The EL spectra shift according to doping concentration was observed in the TADF devices by polar
interaction between dopant molecules. The strong donor-acceptor type TrzCNBFCz and Trz2CNBFCz
showed large red shift of the emission spectra at high doping concentration compared with the weak
donor-acceptor type TrzBFCz. A large red shift of the EL spectra by adding the CN secondary acceptor
was noticed by the strengthened CT character. At 10 wt% doping concentration, the peak wavelengths
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of the TrzCNBFCz and Trz2CNBFCz devices were 473 and 511 nm, respectively, which corresponded
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DOI: 10.1039/C9TC01354G
to color coordinates of (0.16, 0.24) and (0.27, 0.52). All device performances were summarized in Table
2
. Current efficiency and power efficiency of the devices are in supporting information.
Figure 10. (a) Current density-Voltage-Luminance curves, (b) Quantum Efficiency-Luminance
curves and (c) Electroluminescence spectra of Trz2CNBFCz device
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Table 2. Summarized device data of TrzBFCz, TrzCNBFCz and Trz2CNBFCz devices
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DOI: 10.1039/C9TC01354G
Vd
QE(%)
EL spectrum
(nm)
CIE
2
(
V)
8.1
6.9
5.3
10.2
6.5
6.0
9.0
7.0
6.9
[1000cd/m ] [Max]
TrzBFCz (10%)
TrzBFCz (20%)
447
451
453
473
481
482
511
524
528
(0.15, 0.09)
(0.15, 0.10)
(0.15, 0.11)
(0.16, 0.24)
(0.17, 0.31)
(0.17, 0.33)
(0.27, 0.52)
(0.33, 0.57)
(0.35, 0.58)
4.3
8.0
19.0
18.0
13.1
20.9
26.6
21.8
15.8
13.7
9.2
TrzBFCz (30%)
8.3
TrzCNBFCz (10%)
TrzCNBFCz (20%)
TrzCNBFCz (30%)
Trz2CNBFCz (10%)
Trz2CNBFCz (20%)
Trz2CNBFCz (30%)
5.9
16.8
14.8
6.9
10.1
7.4
2
*
V
d
: Driving voltage at 1,000 cd/m
*
* All data was measured at 1000 cd/m2
Conclusions
Three TADF emitters based on the molecular platform of t-butylated diphenyltriazine acceptor, a
phenyl linker, and benzofurocarbazole donor combined with a CN secondary electron accepting unit
were synthesized to study the effect of the CN secondary acceptor on the TADF properties. Main role of
the CN secondary accepting unit was to increase the RISC rate constant for efficient up-conversion. The
addition of one CN secondary acceptor facilitated the RISC process while maintaining the high PL
quantum yield of the pristine TADF emitter, enabling high EQE of 26.6% in the sky-blue TADF devices.
However, the addition of two secondary acceptors reduced the PL quantum yield of the TADF emitter,
resulting in low EQE. Moreover, the pristine molecular design having the t-butylated diphenyltriazine
acceptor achieved high EQE of 19.0% with a color coordinate of (0.15, 0.09), which is one of the highest
EQEs of the deep blue TADF OLEDs with a y color coordinate below 0.10. Therefore, the molecular
design approach introducing the secondary acceptor is useful to manage the RISC process while keeping
high PL quantum yield of the emitters.
Experimental
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General information
,4-bis(4-(tert-butyl)phenyl)-6-chloro-1,3,5-triazine, 5H-benzofuro[3,2-c]carbazole wDeOrIe: 10s.y10n3t9h/Ce9sTiCz0e1d354inG
View Article Online
2
our previous work. (3-cyano-4-fluorophenyl)boronic acid, 5-bromoisophthalonitrile was supplied from
LG Chem. Ltd. Bis(pinacolate) diboron was purchased from INCO Co. (4-fluorophenyl)boronic acid,
1
,2-dibromotetrachloroethane were products of Alfa Aesar, Thermo Fisher Scientific Inc. [1,1'-
bis(diphenylphosphino)ferrocene]palladium(II) dichloride, tetrakis(triphenylphosphine) palladium(0)
were purchased from P&H Tech Co. Sodium hydride, Copper iodide, lithium diisopropylamide, (±)-
trans-1,2-diaminocyclohexane were products of Sigma Aldrich Co. Potassium acetate, potassium
carbonate, cesium carbonate, 1,4-dioxane, N,N-dimethylformamide (DMF) were supplied from
Duksan Sci. Co. n-hexane, methylene chloride, tetrahydrofuran, chloroform were purchased from
Samchun Pure Chemical Co., Ltd. Tetrahydrofuran (THF) was distilled over sodium and calcium hydride.
UV-vis spectrophotometer (JASCO, V-730) was used to measure absorption spectra of each emitters. The
L
mass spectrometry was measured using Advion, Expresion CMS spectrometer with APCI mode. The
HOMO and LUMO were measured and calculated by using a cyclic voltammetry (Ivium Tech.,
Iviumstat). The PL spectra was measured with a fluorescence spectrophotometer (PerkinElmer, LS-55).
The singlet and triplet energy of each emitters was determined from frozen THF solution at 77K with
1
13
and without 2 ms delay time. H-nuclear magnetic resonance (NMR) and C-NMR were analyzed by
using AVNACE III HD (Bruker, 500 MHz) spectrometer. The PL quantum yield and transient PL data
were obtained by using Quantaurus-QY system (Hamamatsu, C11347-11) and Quantaurus-Tau system
(
Hamamatsu, C11367-31) under N atmosphere.
2
Synthesis
,4-Bbis(4-(tert-butyl)phenyl)-6-(4-fluorophenyl)-1,3,5-triazine
,4-Bis(4-(tert-butyl)phenyl)-6-chloro-1,3,5-triazine (1.50 g, 3.95 mmol) and (4-fluorophenyl)boronic
2
2
acid (0.83 g, 5.92 mmol) were dissolved in tetrahydrofuran solvent (20 ml) and directly poured into 2-
neck round-bottom flask. Then potassium carbonate was dissolved in deionized water (10 ml) at 2M
21

Journal of Materials Chemistry C
Page 22 of 28
concentration and injected into mixed solution of the starting materials. After slight heating for 10 min,
View Article Online
DOI: 10.1039/C9TC01354G
tetrakis(triphenylphosphine) palladium(0) (0.14 g, 0.12 mmol) was poured into the reaction mixture and
refluxed for 12 h. After cooling down to room temperature, the reaction mixture was extracted using
methylene chloride. Extracted organic layer was then concentrated and adsorbed into silica gel. Adsorbed
mixture was purified with column chromatography using eluent solvent (methylene chloride: n-
hexane=1:4). A white powder was obtained after all purification processes. (1.31 g, Yield 75%)
1H NMR (500 MHz, CDCl
) : δ 8.79-8.76 (m, 2H), 8.66 (d, 4H, J=9.0 Hz), 7.59 (d, 4H, J=7.0 Hz), 7.25-
3
+
7
.21 (m, 2H), 1.40 (s, 18H), MS (APCI) m/z: Found 440.2 [(M+H) ] ; Calculated for C29
H
30FN , 439.57
3
5
-(4-(4,6-Bis(4-(tert-butyl)phenyl)-1,3,5-triazin-2-yl)phenyl)-5H-benzofuro[3,2-c]carbazole
(TrzBFCz)
Sodium hydride (0.16 g, 6.82 mmol) was added into a pressure tube. Then DMF (3 ml) was added into
the pressure tube. 5H-benzofuro[3,2-c]carbazole (0.70 g, 2.73 mmol) was dissolved in DMF (5 ml) and
poured into the pressure tube. After 30 min with stirring, 2,4-bis(4-(tert-butyl)phenyl)-6-(4-
fluorophenyl)-1,3,5-triazine (1.00g, 2.27 mmol) in DMF (10 ml) was injected into the reaction solution
and the mixture was refluxed for 24 h. After the reaction, deionized water was poured into the reaction
mixture for sodium hydride quenching. Then the mixture was extracted using chloroform. Separated
organic layer was concentrated by rotary evaporator and the crude product was purified with column
chromatography using eluent solvent (Chloroform: n-hexane=1:1). Final product was purified using train
sublimation. A greenish powder was obtained after all purification. (0.81 g, Yield 53%)
1H NMR (500 MHz, CDCl
) : δ 9.00 (d, 2H, J=7.0 Hz), 8.71 (d, 4H, J=7.0 Hz), 8.58 (d, 1H, J=7.5 Hz),
3
7
7
.96 (d, 1H, J=7.0 Hz), 7.93 (d, 1H, J=8.5 Hz), 7.82 (d, 2H, J=6.75 Hz), 7.73 (d, 1H, J=8.0 Hz), 7.62-
1
3
.57 (m, 5H), 7.50 (t, 2H, J=8.0 Hz), 7.44 (t, 1H, J=7.75 Hz), 7.37 (t, 1H, J=7.0 Hz), 1.41 (s, 18H), C
) : δ 171.9, 170.8, 156.5, 156.4, 151.4, 141.5, 141.1, 140.4, 135.8, 133.7, 130.8,
29.1, 127.2, 126.2, 125.9, 125.6, 125.2, 123.1, 123.0, 121.6, 121.2, 120.0, 118.3, 117.1, 111.9, 110.2,
NMR (125 MHz, CDCl
3
1
1
+
09.3, 105.8, 35.3, 31.5, MS (HR-FAB+) m/z: Found 677.3277 [(M+H) ]; Calculated for C47
H
40
N O,
4
22

Page 23 of 28
Journal of Materials Chemistry C
6
76.8467.
View Article Online
DOI: 10.1039/C9TC01354G
5
-(4,6-Bis(4-(tert-butyl)phenyl)-1,3,5-triazin-2-yl)-2-fluorobenzonitrile
Synthetic method of 5-(4,6-bis(4-(tert-butyl)phenyl)-1,3,5-triazin-2-yl)-2-fluorobenzonitrile was almost
same that of 2,4-bis(4-(tert-butyl)phenyl)-6-(4-fluorophenyl)-1,3,5-triazine except that (3-cyano-4-
fluorophenyl)boronic acid (0.98 g, 5.92 mmol) was used as the starting material instead of (4-
fluorophenyl)boronic acid. A white powder was obtained after purification (1.70 g, Yield 93%).
1H NMR (500 MHz, CDCl
d, 4H, J=7.5 Hz), 7.39 (t, 1H, J=8.5 Hz), 1.41 (s, 18H), MS (APCI) m/z: Found 465.2 [(M+H)+];
) : δ 9.04 (d, 1H, J=6.5 Hz), 9.00-8.97 (m, 1H), 8.64 (d, 4H, J=6.75 Hz), 7.61
3
(
Calculated for C30
H
29FN , 464.58
4
2
-(5H-benzofuro[3,2-c]carbazol-5-yl)-5-(4,6-bis(4-(tert-butyl)phenyl)-1,3,5-triazin-2-
yl)benzonitrile (TrzCNBFCz)
-(4,6-Bis(4-(tert-butyl)phenyl)-1,3,5-triazin-2-yl)-2-fluorobenzonitrile (1.00 g, 2.15 mmol), 5H-
5
benzofuro[3,2-c]carbazole (0.66 g, 2.58 mmol) and cesium carbonate (2.10 g, 6.46 mmol) were added
into a pressure tube. Then DMF (15 ml) was added into the mixture. After that, the reaction was
proceeded at reflux condition for 12 h. The reaction mixture was cooled down to room temperature and
extracted using chloroform. The extracted organic layer was evaporated using a rotary evaporator and
adsorbed into a silica gel. The adsorbed crude product was purified by using column chromatography.
After column purification, the final product was sublimated using vacuum sublimation. After all
purification, a bright green powder was obtained. (1.21 g, Yield 80%)
1H NMR (500 MHz, CDCl
) : : δ 9.33 (s, 1H), 9.16 (d, 1H, J=8.0 Hz), 8.70 (d, 4H, J=7.0 Hz), 8.59 (d,
3
1
7
H, J=7.5 Hz), 7.98-7.96 (m, 2H), 7.86 (d, 1H, J=8.5 Hz), 7.74 (d, 1H, J=8.5 Hz), 7.63 (d, 4H, J=7.0 Hz),
1
3
.54-7.44 (m, 3H), 7.38 (t, 2H, J=7.0 Hz), 7.28 (d, 1H, J=8.5 Hz), 1.42 (s, 18H), C NMR (125 MHz,
) : δ 172.2, 169.0, 157.0, 156.6, 151.3, 143.5, 140.9, 140.3, 137.3, 135.4, 134.6, 133.2, 130.0,
29.2, 126.4, 126.0, 125.9, 125.0, 123.3, 123.2, 122.0, 120.1, 118.6, 117.9, 116.2, 113.0, 112.0, 110.1,
CDCl
3
1
23

Journal of Materials Chemistry C
Page 24 of 28
+
1
7
09.8, 105.7, 35.4, 31.4, MS (HR-FAB+) m/z: Found 702.3232 [(M+H) ]; Calculated for C48
H
39
N O,
5
View Article Online
DOI: 10.1039/C9TC01354G
01.8562
5
-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)isophthalonitrile
5
-Bromoisophthalonitrile (5.00 g, 24.15 mmol), 4,4,4',4',5,5,5',5'-octamethyl-2,2'-bi(1,3,2-
dioxaborolane) (9.20 g, 36.23 mmol), potassium acetate (7.11g, 72.45 mmol) and [1,1′-
bis(diphenylphosphino)ferrocene]dichloropalladium(II) (0.53 g, 0.72 mmol) were poured into a 2-neck
round-bottom flask. Then 1,4-dioxane (50 ml) was injected into mixture. The reaction was proceeded at
reflux condition for 3 h. After the reaction, the reaction solution was filtered with celite and silica gel.
Then the filtered solution was concentrated and adsorbed into silica gel. The adsorbed crude product was
purified with column chromatography using a mixed eluent (methylene chloride: n-hexane=1:4) A white
powder was obtained after purification (4.70 g, Yield 77%).
1H NMR (500 MHz, CDCl
) : δ 8.28 (s, 2H), 7.99 (s, 1H), 1.26 (s, 12H), MS (APCI) m/z: Found 254.8
, 254.09
3
+
[
(M+H) ]; Calculated for C14
H15BN
2
O
2
5
2
1
-(4,6-Bis(4-(tert-butyl)phenyl)-1,3,5-triazin-2-yl)isophthalonitrile
,4-Bis(4-(tert-butyl)phenyl)-6-chloro-1,3,5-triazine (3.08 g, 8.10 mmol) and 5-(4,4,5,5-tetramethyl-
,3,2-dioxaborolan-2-yl)isophthalonitrile (2.47 g, 9.72 mmol) were dissolved in tetrahydrofuran solvent
(
60 ml) and directly poured into a 2-neck round-bottom flask. 2M aqueous potassium carbonate solution
30 ml) was injected into the mixed solution. Then tetrakis(triphenylphosphine) palladium(0) (0.28 g,
(
0
.24 mmol) was poured into the reaction mixture. The reaction mixture was stirred and heated at reflux
condition for 12 h. After cooling down to room temperature, the reaction mixture was filtered. A white
powder was obtained after all purification. (2.10 g, Yield 56%)
1H NMR (500 MHz, CDCl
) : δ 9.17 (s, 2H), 8.61(d, 4H, J=8.5 Hz), 8.07 (s, 1H), 7.62 (d, 4H, J=8.0 Hz),
3
+
1
.43 (s, 18H), MS (APCI) m/z: Found 472.2 [(M+H) ]; Calculated for C31
H
29
5
N , 471.60
24

Page 25 of 28
Journal of Materials Chemistry C
5
-(4,6-Bis(4-(tert-butyl)phenyl)-1,3,5-triazin-2-yl)-2-bromoisophthalonitrile
View Article Online
DOI: 10.1039/C9TC01354G
5
-(4,6-Bis(4-(tert-butyl)phenyl)-1,3,5-triazin-2-yl)isophthalonitrile (2.30 g, 4.88 mmol) was poured into
a 2-neck round-bottomed flask and connected with a vacuum pump to remove residue solvent and
moisture for 24 h. Then the round-bottomed flask was maintained under nitrogen. After that, the distilled
tetrahydrofuran (100 ml) was injected to dissolve starting materials. Then the reaction solution was
cooled down to -98°C using n-hexane/liquid nitrogen bath. After cooling for 2 h, lithium
diisopropylamide (3.70 ml, 7.32 mmol) was dropped into reaction solution very slowly. Then, the
lithiation was proceeded for 10 min and 1,2-dibromotetrachloroethane (3.18 g, 9.75 mmol) dissolved in
distilled tetrahydrofuran (10 ml) was directly injected into the reaction mixture. Then the reaction was
carried out at the low temperature for 12 h and room temperature for additional 12 h. After that, the
reaction mixture was filtered. An ivory-white product was obtained after all purification. (1.35 g, Yield
4
9%).
1H NMR (500 MHz, CDCl
) : δ 9.17 (s, 2H), 8.64 (d, 4H, J=8.5 Hz), 7.63 (d, 4H, J=8.5 Hz), 1.42 (s,
3
+
1
8H), MS (APCI) m/z: Found 550.1 [(M+H) ]; Calculated for C31
H
28BrN , 550.49
5
2
-(5H-benzofuro[3,2-c]carbazol-5-yl)-5-(4,6-bis(4-(tert-butyl)phenyl)-1,3,5-triazin-2-
yl)isophthalonitrile (Trz2CNBFCz)
-(4,6-Bis(4-(tert-butyl)phenyl)-1,3,5-triazin-2-yl)-2-bromoisophthalonitrile (0.70 g, 1.27 mmol), 5H-
5
benzofuro[3,2-c]carbazole (0.36 g, 1.40 mmol), copper iodide (0.12 g, 0.64 mmol), and potassium
carbonate (0.53 g, 3.81 mmol) were added into a pressure tube. Then DMF (10 ml) was added into the
mixture. After that, (±)-trans-1,2-diaminocyclohexane (0.10 ml, 0.64 mmol) was injected and the
reaction mixed was stirred under heating to 120°C for 2 h. After end of reaction, the reaction mixture
was cooled down to room temperature and extracted using methylene chloride. The extracted organic
layer was filtered with celite/silica gel and concentrated using a rotary evaporator. Concentrated crude
product was adsorbed into a silica gel and the adsorbed crude product was purified using column
chromatography using a mixed eluent (methylene chloride: n-hexane=1:1). After column purification,
25

Journal of Materials Chemistry C
Page 26 of 28
the final product was further purified using a vacuum train sublimation. After all purification, a bright
View Article Online
DOI: 10.1039/C9TC01354G
yellow powder was obtained. (0.21 g, Yield 23%)
1H NMR (500 MHz, CDCl
) : : δ 9.51 (s, 2H), 8.71 (d, 4H, J=8.5 Hz), 8.63 (d, 1H, J=7.0 Hz), 8.05 (d,
H, J=8.5 Hz), 8.00 (d, 1H, J=7.5 Hz), 7.77 (d, 1H, J=8.0 Hz), 7.66 (d, 4H, J=8.5 Hz), 7.58-7.53 (m, 2H),
.49 (t, 1H, J=7.75 Hz), 7.41 (t, 1H, J=7.25 Hz), 7.25-7.24 (m, 1H) , 7.17 (d, 1H, J=8.5 Hz), 1.43 (s,
3
1
7
1
1
1
1
3
8H), C NMR (125 MHz, CDCl ) : δ 172.6, 167.5, 157.5, 156.8, 151.3, 145.5, 140.5, 139.9, 138.8,
3
38.5, 132.7, 129.3, 126.8, 126.2, 124.9, 123.7, 123.3, 122.8, 122.5, 120.3, 119.0, 118.8, 116.1, 114.4,
+
12.1, 110.4, 109.8, 105.3, 35.5, 31.4, MS (HR-FAB+) m/z: Found 727.3185 [(M+H) ]; Calculated for
C
49
H
38
6
N O, 726.8656
Device Fabrication and measurements
All of organic compounds were deposited on pattered indium tin oxide (ITO) substrate. All of TADF
device structures were the same except for the emission layer (EML).
ITO (50 nm)/PEDOT:PSS (60 nm)/TAPC (20 nm)/mCP (10 nm)/EML (25 nm)/TSPO1 (5 nm)/TPBi
(20 nm)/LiF (1.5 nm)/Al (200 nm)
The hole injection layer was poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS).
The hole transport layer was 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC).
1
,3-Di(9H-carbazol-9-yl)benzene (mCP) was used for blocking triplet excitons in the EML. The EML
consisted of (oxybis(2,1-phenylene))bis(diphenylphosphine oxide) (DPEPO) host and TrzBFCz or
TrzCNBFCz or Trz2CNBFCz emitter. The doping concentration of each emitter was from 10 wt% to 30
wt%. Electron type exciton blocking layer was diphenyl(4-(triphenylsilyl)phenyl)phosphine oxide
(
TSPO1). 1,3,5-Tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene (TPBi) was used as an electron
transport layer. All of fabricated devices were analyzed by using 2400 source measurement unit and CS
000 spectroradiometer.
2
26

Page 27 of 28
Journal of Materials Chemistry C
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