Fluorine substituted triazine acceptor based thermally activated delayed fluorescent emitter as an assistant dopant of fluorescent emitter

Article abstract of DOI:10.1016/j.dyepig.2020.108549

A fluorine substituted triazine acceptor was developed for blue thermally activated delayed fluorescent (TADF) emitters. 9,9'-((6-(4-Fluorophenyl)-1,3,5-triazine-2,4-diyl)bis(2,1-phenylene))bis(3,6-di-tert-butyl-9H-carbazole) (FTrzTCz) was synthesized by connecting 4-fluorophenyltriazine with 3,6-di-tert-butylcarbazole at ortho position of a phenyl linker. The FTrzTCz emitter showed high efficiency of 18.5% and improved efficiency roll-off. The FTrzTCz also showed high quantum efficiency of 17.8% as an assistant dopant in the green fluorescent devices. The FTrzTCz worked well as a TADF emitter and an assistant dopant of hyperfluorescent devices by suppressed Dexter energy transfer.

Full text of DOI:10.1016/j.dyepig.2020.108549

Journal Pre-proof
Fluorine substituted triazine acceptor based thermally activated delayed fluorescent
emitter as an assistant dopant of fluorescent emitter
Ju Hui Yun, Kyung Hyung Lee, Jun Yeob Lee
PII:
S0143-7208(20)30736-1
DOI:
https://doi.org/10.1016/j.dyepig.2020.108549
DYPI 108549
Reference:
To appear in: Dyes and Pigments
Received Date: 26 March 2020
Revised Date: 11 May 2020
Accepted Date: 13 May 2020
Please cite this article as: Yun JH, Lee KH, Lee JY, Fluorine substituted triazine acceptor based
thermally activated delayed fluorescent emitter as an assistant dopant of fluorescent emitter, Dyes and
Pigments (2020), doi: https://doi.org/10.1016/j.dyepig.2020.108549.
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Ju Hui Yun: Investigation, Writing - Original Draft, Kyung Hyung Lee: Investigation, Jun Yeob Lee:
Supervision, Writing-Review & Editing:

Fluorine substituted triazine acceptor based thermally activated delayed
fluorescent emitter as an assistant dopant of fluorescent emitter
Ju Hui Yun, Kyung Hyung Lee, and Jun Yeob Lee*
School of Chemical Engineering, Sungkyunkwan University
2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, 440-746, Korea
Fax: (+) 82-31-299-4716
E-mail: leej17@skku.edu
* To whom correspondence should be addressed
Abstract
A fluorine substituted triazine acceptor was developed for blue thermally activated delayed fluorescent
(TADF) emitters. 9,9'-((6-(4-Fluorophenyl)-1,3,5-triazine-2,4-diyl)bis(2,1-phenylene))bis(3,6-di-tert-
butyl-9H-carbazole) (FTrzTCz) was synthesized by connecting 4-fluorophenyltriazine with 3,6-di-tert-
butylcarbazole at ortho position of a phenyl linker. The FTrzTCz emitter showed high efficiency of
18.5% and improved efficiency roll-off. The FTrzTCz also showed high quantum efficiency of 17.8%
as an assistant dopant in the green fluorescent devices. The FTrzTCz worked well as a TADF emitter
and an assistant dopant of hyperfluorescent devices by suppressed Dexter energy transfer.
Key words : high efficiency, Dexter energy transfer, delayed fluorescence, sensitizer
1

Introduction
In recent years, thermally activated delayed fluorescence (TADF) devices have emerged as third
generation organic light-emitting diodes (OLEDs) because TADF is one of the methods to resolve the
low efficiency of fluorescent OLEDs. The TADF emitters can harvest the wasted triplet excitons of
conventional fluorescent emitters through reverse intersystem crossing (RISC) and theoretically reach
100% internal quantum efficiency[1-6]
.
Generally, TADF materials are designed based on a molecular platform with donor and acceptor
structure to induce the RISC transition[1,7-17] and the emission characteristics of the TADF emitters are
determined by the donor or acceptor moieties[18-22]. Therefore, molecular engineering of the donor and
acceptor moieties is of great importance to the TADF emitters. There have been many studies for the
donor and acceptor units. In the case of the acceptors, diphenyltriazine, diphenylsulfone and
benzonitrile have been mostly used as acceptor moieties for TADF emitters[1,7,8,10-17]. In particular, the
diphenyltriazine has been one of the most widely used acceptors of TADF emitters due to moderate
acceptor strength and good stability[23-28]. A lot of diphenyltriazine derived TADF materials have been
documented in the literatures and showed good TADF features. However, the acceptor strength of the
diphenyltriazine acceptor itself is rather limited, and the development of even stronger acceptor moiety
than the diphenyltriazine is necessary while keeping the merit of the diphenyltriazine.
In this work, 9,9'-((6-(4-fluorophenyl)-1,3,5-triazine-2,4-diyl)bis(2,1-phenylene))bis(3,6-di-tert-
butyl-9H-carbazole) (FTrzTCz) was synthesized as a TADF emitter and an assistant dopant for green
fluorescent OLEDs. A fluorine substituted triazine was developed as an acceptor moiety to control the
acceptor strength and 3,6-di-tert-butylcarbazole was introduced as a donor moiety for blue emission
energy and suppressing the Dexter energy transfer. The FTrzTCz showed a high EQE of 18.5% as the
TADF emitter without color shift according to doping concentration. As an assistant dopant, FTrzTCz
worked efficiently by providing high EQE of 17.9% in the hyperfluorescent devices.
2

Results and discussion
Diphenyltriazine is a common acceptor mostly used for green and blue TADF materials. It was
already proven that it can assist achieving high efficiency and long lifetime in the TADF OLEDs as a
building block of the TADF materials[23-28]. However, the diphenyltriazine is a moderate acceptor,
which restricts the application of the diphenyltriazine acceptor in the TADF material design. Therefore,
fluorophenyltriazine was developed as a triazine derived acceptor unit to increase the acceptor strength
while maintaining the advantages of triazine. The F functional group in the fluorophenyltriazine has an
electron withdrawing character and can strengthen the acceptor character of the triazine unit. This
would be advantageous in the development of TADF emitters because the singlet-triplet energy gap
(∆EST) can be reduced and the RISC process can be facilitated. However, the planar structure of
fluorophenyltriazine can induce strong intermolecular interaction and bathochromic shift of emission
energy in film state due to hydrogen bonding. Moreover, it negatively influences emission
performances of the hyperfluorescence OLEDs because of the Dexter energy transfer from the TADF
emitter to fluorescent emitters. Therefore, the FTrzTCz was designed to have 3,6-di-tert-butylcarbazole
donor at ortho position to prevent intermolecular interaction and suppress the Dexter energy transfer
through the protective function of the bulky t-butylcarbazole group.
The synthetic method of FTrzTCz was based on multi-step reaction procedure. Fluorophenyltriazine
acceptor was synthesized by Grignard reaction of 1-bromo-4-fluorobenzene with 1,3,5-trichlorotriazine.
Brominated phenylcarbazole with t-butyl groups at 3 and 6 positions of phenylcarbazole borylated
through lithiation reaction followed by Suzuki coupling with the fluorophenyltriazine produced the
FTrzTCz compound. The FTrzTCz was synthesized in good yield and purified by column
chromatography to obtain high purity above 99%. The detail synthesis was described in Scheme 1.
3

Optical properties and molecular electronic distribution of the FTrzTCz were simulated using
Gaussian software with B3LYP 6-31G basis set. The highest occupied molecular orbital (HOMO) was
localized in t-butylcarbazole because of its electron donating character. The lowest unoccupied
molecular orbital (LUMO) was mainly localized on triazine and phenyl linker. The LUMO was spread
to a fluorine atom due to its strong electronegativity. In the geometrical configuration, the triazine was
out of plane by large steric hindrance between triazine and ortho positioned t-butylcarbazoles. The
dihedral angles between triazine and phenyl linker were nearly 40°. The t-butylcarbazole donor was
o
distorted from the phenyl linker by more than 60 . The large dihedral angle may increase charge
transfer (CT) character of the TADF materials and promote RISC rate through small ∆EST. The
calculated results are depicted in Figure 1.
The HOMO and LUMO of FTrzTCz were determined by oxidation and reduction potentials of
cyclic voltammetry (CV). The measured value was corrected with ferrocene (-4.8 eV) standard and
0.1M tetrabutylammonium perchlorate solution in acetonitrile was used as an electrolyte. The measured
CV curves are presented in Figure 2. The HOMO and LUMO energy levels were -6.13 and -3.42 eV
from the onset potentials of the CV measurements. The measured potentials were calibrated with
ferrocene to obtain the HOMO and LUMO energy levels. In comparison with 9,9'-((6-phenyl-1,3,5-
triazine-2,4-diyl)bis(2,1-phenylene))bis(9H-carbazole) (BisCz-Trz) TADF emitter reported in other
works[29], the HOMO level was similar, but the LUMO level of FTrzTCz was deeper than that of
BisCz-Trz by employing a fluorine atom in the triazine acceptor.
Photophysical properties were measured by ultraviolet-visible (UV-Vis) absorption and
photoluminescence (PL) analysis. The broad absorption from 350 to 400 nm in the UV-vis spectrum
was due to CT absorption of FTrzTCz based on the donor and acceptor structure. Main absorption
below 300 nm was assigned to π-π* transition of the backbone structure. The singlet energy (ES) and
triplet energy (ET) were estimated from onset values of fluorescence at room temperature and
phosphorescence spectra at 77K in toluene solution. Broad fluorescence spectrum indicates that the
4

fluorescence is from CT based singlet excited state. The vibronic peaks in phosphorescence spectrum
suggest that the triplet emission is from local excited (LE) state. The ES and ET of FTrzTCz were 2.97
and 2.75 eV, respectively, and ΔEST was 0.22 eV. The analysed data are presented in Figure 3.
To further figure out the PL emission behaviour, transient PL of FTrzTCz was analysed (Figure 4).
In the transient PL data, the decay curve of FTrzTCz clearly showed delayed emission with long decay
time. The excited state lifetime of the prompt component was 53.7 ns and that of delayed component
was 5.91 μs. The temperature dependent transient PL data in Figure S1 clearly confirms the TADF
character by displaying intensified delayed fluorescence according to temperature. The absolute PL
quantum yield (PLQY) was 85% under a N2 atmosphere at doping concentration of 10%. The PLQY of
FTrzTCz was higher than that of BisCz-Trz (76%). The PLQYs of prompt and delayed fluorescence
were 28% and 57%, respectively. The measured photophysical properties and calculated rate constant
are summed up in Table 1. Additionally, the thermal analysis data in Figure S2 suggest good thermal
stability by showing thermal decomposition temperature over 400 oC.
The FTrzTCz was used as an emitter in the TADF devices to identify the device performances. The
device structure was depicted in Figure 5. The TADF emitter was doped at concentrations of 10~50%.
Current density (J) and luminance (L) according to applied voltage (V) are provided in Figure 6(a).
The J and L were dramatically increased in accordance with the doping concentration because the
FTrzTCz acted as trap sites due to the large energy barrier between DPEPO and FTrzTCz. As the
doping concentration increased, the carriers were dominantly transferred by hopping process through
emitters, increasing the J and L.
The EQEs of the FTrzTCz devices are presented in Figure 6(b). Maximum EQE was 18.5% at
doping concentration of 20%, which was higher than 12.2% of the BisCz-Trz device. The high PLQY
of FTrzTCz enhanced the EQE of the TADF device. Furthermore, the efficiency drop at high
luminance was improved because triplet exciton quenching process was suppressed successfully by
short delayed fluorescence lifetime. The F functional group in the acceptor moiety mainly contributed
5

to the weakened EQE roll-off at high luminance by shortening the delayed fluorescence lifetime
through small ΔEST without sacrificing the PLQY. Although the PLQY was high at 30% doping
concentration (Table S1), the large contribution of triplet excitons in the electric field driven emission
process induces exciton quenching and decreases the EQE at high doping concentration.
The electroluminescence (EL) spectra of the FTrzTCz devices depending on doping concentration
are shown in Figure 6(c). The EL spectra showed no significant change although the doping
concentration was increased from 10% to 50%. The constant EL spectra indicates that the bulky t-butyl
groups and largely distorted structure of FTrzTCz disturbed intermolecular interaction between emitter
molecules. The peak wavelength was in the range of 495~497 nm. All device data are summarized in
Table 2.
Motivated by the intermolecular interaction suppressing effect of the FTrzTCz emitter as reflected in
the EL spectra, the FTrzTCz was applied as an assistant dopant of green fluorescent emitter.
Hyperfluorescent OLEDs were fabricated using 10-2-benzothiazolyl-1,1,7,7-tetramethyl-2,3,6,7-
tetrahydro-1H,5H,11H-benzo [l]pyrano [6 7 8-ij]quinolizin-11-one (C545T) and 2,6-di-tert-butyl-
N9,N9,N10,N10-tetrakis (4-(tert-butyl)phenyl)anthracene-9,10-diamine (6tBPA)[30] as green
fluorescent emitters at 1% doping concentration. The EQEs of the green hyperfluorescent OLEDs were
11.5% and 17.9% for C545T and 6tBPA doped devices, respectively. The EQE of the 6tBPA device
was higher than that of the C545T device because of effective Dexter energy management through the
t-butyl blocking groups in the 6tBPA emitter. The high EQE of the 6tBPA demonstrated that the triplet
excitons of FTrzTCz harvested through RISC transition were effectively used for fluorescent radiative
decay of the 6tBPA. The highly twisted structure and t-butyl blocking groups in the donor structure
were responsible for the effective control of the energy transfer from the FTrzTCz to 6tBPA. The EQE
plots are presented in Figure 7(a). The EQE of the pure C545T and 6tBPA devices was only about 6%
as presented in Figure S3 of supporting information.
6

The EL spectra of hyperfluorescent devices are provided in Figure 7(b). The EL spectra showed
C545T or 6tBPA emission, verifying that the energy transfer from the assistant dopant to fluorescent
emitters occurred efficiently. Color coordinates of the C545T and 6tBPA hyperfluorescent devices
were (0.24, 0.64) and (0.24, 0.58), respectively. The hyperfluorescent device performances were
tabulated in Table 3.
Conclusions
The FTrzTCz was designed as an efficient sky-blue TADF emitter and assistant dopant of green
fluorescent OLEDs. A fluorine atom in triazine acceptor induced strong CT character by intensified
acceptor strength, short delayed fluorescent lifetime and fast RISC. The EQE of the FTrzTCz device
was 18.5% and the efficiency roll-off was improved. The FTrzTCz performed well as an assistant
dopant of green fluorescent OLEDs and enabled high maximum EQEs of 17.9% in the green
hyperfluorescent device. Therefore, the F functionalized triazine acceptor can be a potential candidate
of the acceptor to design TADF materials and assistant dopants of hyperfluorescent OLEDs.
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List of Tables
Table 1. Photophysical properties and rate constant of FTrzTCz
Table 2. Device characteristics of FTrzTCz
Table 3. Device characteristics of hyperfluorescence OLEDs
11

Table 1. Photophysical properties and rate constant of FTrzTCz
T
ES/ET
ΦF/ΦTADF
τp
τd
kISC
kRISC
knr
(eV)[a]
(%)[b]
(ns)[c]
(μs)[d]
(107s-1)[e]
(105s-1)[f]
(104s-1)[g]
3.53
FTrzTCz
2.97/2.75
28/57
53.7
5.91
1.35
4.83
[a] Singlet and triplet energies were onset values of fluorescence and phosphorescence emission
spectrum
[b] ΦF and ΦTADF represent the PLQYs of prompt and delayed fluorescence component
[c] τp is decay time of prompt component
[d] τd is decay time of delayed component
[e] kISC is rate constant of intersystem crossing
[f] kRISC is rate constant of reverse intersystem crossing
[g] knrT is rate constant of non-radiative decay from T1 to S0
12

Table 2. Device characteristics of FTrzTCz
Doping
ηmax (%)[a]
η1000 (%)[b]
Color coordinate
λEL (nm)[c]
concentration (%)
10
20
30
50
16.9
18.5
17.4
15.3
5.1
(0.20, 0.44)
(0.20, 0.45)
(0.20, 0.46)
(0.20, 0.47)
497
496
495
497
14.5
15.9
14.4
FTrzTCz
[a] ηmax is maximum external quantum efficiency
[b] η1000 is external quantum efficiency at 1000cdm-2
[c] λEL is peak wavelength of EL spectrum
13

Table 3. Device characteristics of hyperfluorescence OLEDs
Von (V)[a]
ηmax (%)[b]
η1000 (%)[c]
Color coordinate
λEL (nm)[d]
C545T
6tBPA
7.06
7.11
11.5
17.9
7.1
(0.24, 0.64)
(0.24, 0.58)
515
517
14.0
[a] Von is turn-on voltage at 1cdm-2
[b] ηmax is maximum external quantum efficiency
[c] η1000 is external quantum efficiency at 1000cdm-2
[d] λEL is peak wavelength of EL spectrum
14

List of Figures
Scheme 1. Synthetic procedure of FTrzTCz
Figure 1. Computational calculated result of FTrzTCz
Figure 2. Cyclic voltammetry (CV) curve of FTrzTCz
Figure 3. UV-Vis absorption and photoluminescence (PL) spectra of FTrzTCz using toluene solution.
Fluorescence spectrum was obtained at room temperature and the phosphorescence spectrum was
obtained at 77 K.
Figure 4. Transient PL curve of FTrzTCz
Figure 5. Device structure of TADF and hyperfluorescence OLEDs
Figure 6. (a)Current density-voltage-luminance, (b)external quantum efficiency (EQE) plots and
(c)electroluminescence (EL) spectra of FTrzTCz according to doping concentration
Figure 7. (a)EQE plots and (b)EL spectra of hyperfluorescence OLEDs at 1% doping concentration of
C545T and 6tBPA
15

Scheme 1. Synthetic procedure of FTrzTCz
16

Figure 1. Computational calculated result of FTrzTCz
17

Figure 2. Cyclic voltammetry (CV) curve of FTrzTCz
18

Figure 3. UV-Vis absorption and photoluminescence (PL) spectra of FTrzTCz using toluene solution.
Fluorescence spectrum was obtained at room temperature and the phosphorescence spectrum was
obtained at 77 K.
19

Figure 4. Transient PL curve of (a)prompt and (b)delayed component of FTrzTCz
20

Figure 5. Device structure of TADF and hyperfluorescence OLEDs
21

Figure 6. (a)Current density-voltage-luminance, (b)external quantum efficiency (EQE) plots and
(c)electroluminescence (EL) spectra of FTrzTCz according to doping concentration
22

Figure 7. (a)EQE plots and (b)EL spectra of hyperfluorescence OLEDs at 1% doping concentration of
C545T and 6tBPA
23

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-
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Fluorine substituted triazine acceptor for the design of thermally activated
delayed fluorescent emitter
High efficiency and small efficiency roll-off in the thermally activated delayed
fluorescent devices
High efficiency in fluorescent organic light-emitting diodes by energy transfer

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: