Rational molecular design of deep blue thermally activated delayed fluorescent emitters for high efficiency fluorescent and hyperfluorescent devices

Article abstract of DOI:10.1016/j.orgel.2019.105604

Two deep blue thermally activated delayed fluorescent (TADF) emitters with peripheral alkyl branches surrounding the donor and acceptor moieties were synthesized to explore ideal design of deep blue TADF emitters for fluorescence and hyperfluorescence in terms of quantum efficiency. The TADF emitters having the multiple alkyl branches protecting the donor and acceptor moieties performed better than the emitter without the alkyl branch or with the alkyl branch only in the donor or acceptor unit. The TADF emitters with the peripheral alkyl branches both in the donor and acceptor units showed a high quantum efficiency above 22.0%, a deep blue color coordinate of (0.15, 0.20), and doping concentration independent emission color as TADF emitters, and 13.7% quantum efficiency in the TADF sensitized hyperfluorescent device. Therefore, the multiple alkyl modification method of both donor and acceptor units of TADF emitters can resolve the issues of TADF emitters for ideal device performances.

Full text of DOI:10.1016/j.orgel.2019.105604

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Rational molecular design of deep blue thermally activated delayed fluorescent
emitters for high efficiency fluorescent and hyperfluorescent devices
Yu Jin Kang, Ju Hui Yun, Jun Yeob Lee
PII:
S1566-1199(19)30631-7
DOI:
https://doi.org/10.1016/j.orgel.2019.105604
ORGELE 105604
Reference:
To appear in:
Organic Electronics
Received Date:
Accepted Date:
27 October 2019
21 December 2019
Please cite this article as: Yu Jin Kang, Ju Hui Yun, Jun Yeob Lee, Rational molecular design of
deep blue thermally activated delayed fluorescent emitters for high efficiency fluorescent and
hyperfluorescent devices, Organic Electronics (2019), https://doi.org/10.1016/j.orgel.2019.105604
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Rational molecular design of deep blue thermally activated delayed fluorescent emitters
for high efficiency fluorescent and hyperfluorescent devices
Yu Jin Kang+, Ju Hui Yun+, Jun Yeob Lee*
School of Chemical Engineering, Sungkyunkwan University
2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi, 440-746, Korea
E-mail: leej17@skku.edu
Y. J. Kang and J. H. Yun contributed equally.
Keywords: t-butyldelayed fluorescence fluorescent device energy
transferhyperfluorescence
Abstract
Two deep blue thermally activated delayed fluorescent (TADF) emitters with peripheral alkyl
branches surrounding the donor and acceptor moieties were synthesized to explore ideal
design of deep blue TADF emitters for fluorescence and hyperfluorescence in terms of
quantum efficiency. The TADF emitters having the multiple alkyl branches protecting the
donor and acceptor moieties performed better than the emitter without the alkyl branch or
with the alkyl branch only in the donor or acceptor unit. The TADF emitters with the
peripheral alkyl branches both in the donor and acceptor units showed a high quantum
efficiency above 22.0%, a deep blue color coordinate of (0.15, 0.20), and doping
concentration independent emission color as TADF emitters, and 13.7% quantum efficiency
in the TADF sensitized hyperfluorescent device. Therefore, the multiple alkyl modification
method of both donor and acceptor units of TADF emitters can resolve the issues of TADF
emitters for ideal device performances.
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Introduction
Organic light-emitting diodes (OLEDs) are in the main stream of display technology because
of unique device characteristics such as high contrast ratio, short response time, and design
freedom. However, OLEDs suffer from poor device performances like low efficiency and
short lifetime in blue devices. Therefore, it is urgently needed to upgrade the efficiency and
lifetime of blue OLEDs simultaneously[1-4]
.
There are several solutions to overcome the efficiency and lifetime issues of blue OLEDs,
which are phosphorescent OLEDs (PhOLEDs)[5-10], thermally activated delayed fluorescent
(TADF) OLEDs[11-15], and TADF sensitized hyperfluorescent OLEDs[16-18]. These three
technologies have a potential as the high efficiency blue OLED system although each
technology has merits and demerits. Currently, PhOLEDs are ahead of other blue OLED
technologies in terms of efficiency and lifetime[19-23]. However, the device characteristics of
the blue TADF OLEDs and hyperfluorescence OLEDs have been rapidly advanced for the
last couple of years although the lifetime of the TADF devices is still very short[24-28]
.
Therefore, more materials which can upgrade the device performances of TADF OLEDs and
hyperfluorescence OLEDs are strongly demanded. In particular, an organic compound which
can afford high efficiency and lifetime in the TADF OLEDs and hyperfluorescence OLEDs
would be ideal in the search of high performance blue OLEDs.
In this work, a molecular design protecting the donor and acceptor moieties of TADF emitters
using peripheral alkyl branches were proposed as a strategy to resolve the low external
quantum efficiency (EQE), poor color purity and doping concentration dependent color shift
issues of TADF and hyperfluorescence OLEDs. Two TADF emitters with multiple alkyl
branches surrounding the donor-acceptor core structure, 9,9'-(5-(4,6-bis(4-(tert-butyl)phenyl)-
1,3,5-triazin-2-yl)-1,3-phenylene)bis(3,6-di-tert-butyl-9H-carbazole) (tDCztTrz) and 9,9'-(5-
(4,6-bis(3,5-di-tert-butylphenyl)-1,3,5-triazin-2-yl)-1,3-phenylene)bis(3,6-di-tert-butyl-9H-
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carbazole) (tDCz2tTrz), were developed for universal use in deep blue TADF OLEDs and
hyperfluorescence OLEDs. The tDCztTrz and tDCz2tTrz emitters showed a high EQE over
22%, a deep blue emission color of (0.15,0.20), and doping concentration independent
emission color as TADF emitters, and provided high EQE over 13% in the hyperfluorescence
OLEDs.
Results and discussion
Among the many blue TADF emitters reported until now, triazine type blue TADF emitters
are differentiated from other emitters in that they have a potential as both high efficiency and
long lifetime blue emitters. However, the triazine type emitters have a drawback of strong
intermolecular interaction with other molecules or by themselves, which shifts the emission
color to long wavelength at high doping concentration or by doping in a highly polar host.
Therefore, the color purity of the blue devices with the triazine type emitters is degraded in
the TADF OLEDs. Moreover, it works poorly in the hyperfluorescence OLEDs due to
significant Dexter energy transfer from the TADF emitter to a fluorescent emitter caused by
close contact between planar triazine unit and fluorescent emitter. As the Dexter energy
transfer degrades the EQE of the hyperfluorescence OLEDs due to triplet exciton quenching
of the TADF sensitizer, the challenging issue of the Dexter energy transfer should be handled
for high EQE in the hyperfluorescence OLEDs.
Our idea of avoiding the intermolecular interaction and orbital overlap is to introduce bulky
alkyl branches in the periphery of the donor-acceptor core structure of the TADF emitters to
separate two molecules. The alkyl branch was a t-butyl unit, and the donor-acceptor core
structure was 9,9'-(5-(4,6-diphenyl-1,3,5-triazin-2-yl)-1,3-phenylene)bis(9H-carbazole)
(DCzTrz)[26]. Two DCzTrz derivatives, tDCztTrz and tDCz2tTrz, with multiple t-butyl units
around the DCzTrz core were designed and synthesized. The tDCz2tTrz had an additional t-
butyl unit in the phenyl unit of diphenyltriazine compared with the tDCztTrz. The multiple t-
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butyl units around the core may separate the TADF molecules by large size of the substituent,
eventually weakening intermolecular interaction with other molecules or by themselves. 9,9'-
(5-(4,6-bis(4-(tert-butyl)phenyl)-1,3,5-triazin-2-yl)-1,3-phenylene)bis(9H-carbazole) (CztTrz)
possessing the t-butyl substituents only in the acceptor moiety[29] and 9,9'-(5-(4,6-diphenyl-
1,3,5-triazin-2-yl)-1,3-phenylene)bis(3,6-di-tert-butyl-9H-carbazole) (tCzTrz) having the t-
butyl substituents only in the donor moiety were used as reference materials to study the
effect of peripheral t-butyl branches on the device performances and to establish a design rule
of TADF emitters for high efficiency and long lifetime.
The synthesis of the two emitters was conducted by a multi-step synthetic process including
Grignard reaction of the t-butyl substituted phenyl unit and Suzuki coupling reaction of the t-
butyl substituted carbazole as described in Scheme 1. Two emitters were prepared in good
yields over 60% and general purification processes of column chromatography,
recrystallization, and temperature gradient sublimation afforded highly pure TADF emitters.
Purity level of the compounds identified by high performance column chromatography was
over 99%.
Quantum chemical calculation of the TADF emitters was carried out using Gaussian
molecular simulation software equipped with B3LYP 6-31G basis set. Molecular calculation
results of tDCztTrz and tDCz2tTrz are presented in Figure 1. Overall distribution of the
highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) was not affected by the t-butyl branches as projected from the HOMO localization
on the donor parts and LUMO localization on the acceptor parts. The HOMO and LUMO
were effectively overlapped at the central phenyl unit of the two emitters.
The HOMO and LUMO projected from the quantum chemical calculation results were
determined using experimental measurement by cyclic voltammetry (CV). The
HOMO/LUMOs of tDCztTrz, and tDCz2tTrz were -6.19/-3.48 eV and -6.17/-3.40 eV,
respectively, from the oxidation and reduction data of the emitters in Figure 2. Similar
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HOMO levels were observed in the tDCztTrz and tDCz2tTrz emitters because the same
donors were included in the molecular structure. However, the LUMO became shallow in the
tDCz2tTrz by electron donating effect of two t-butyl units in the phenyl unit of
diphenyltriazine. The measurement data are summarized in Table 1.
Photophysical characterization of the two emitters was performed to study the TADF related
properties. Firstly, photoluminescence (PL) analysis to determine the singlet energy (ES) and
triplet energy (ET) was performed (Figure 3(a) and (b)). Room temperature PL measurement
of fluorescence and low temperature PL measurement of phosphorescence provided the ES
and ET of the TADF emitters. Solution samples with the TADF emitters dissolved in toluene
were used to characterize the fluorescence and phosphorescence. The ES/ET values of
tDCztTrz and tDCz2tTrz were 3.03/2.99 and 3.04/3.02 eV, respectively. There was little
difference of the ES and ET values in the two emitters. A very small singlet-triplet energy gap
(EST) of 0.04 and 0.02 eV was observed in the tDCztTrz and tDCz2tTrz emitters. Strong
donor character of the t-butyl modified carbazole strengthening charge transfer (CT) property
reduced the EST. The strong CT character of the tDCztTrz and tDCz2tTrz emitters was
reflected in the ultraviolet-visible (UV-vis) absorption data exhibiting weak CT absorption
from 350 nm to 420 nm.
PL emission behavior of the emitters was further examined by characterizing transient PL
decay from excited state. Transient PL characterization data of the tDCztTrz and tDCz2tTrz
emitters are shown in Figure 4. Both tDCztTrz and tDCz2tTrz emitters can be categorized
into typical TADF emitters considering the fast prompt fluorescence and slow delayed
fluorescence in the transient PL decay. Excited state lifetimes for delayed fluorescence were
55.7 and 118.7 s in the tDCztTrz and tDCz2tTrz emitters, respectively. PL quantum yield
(PLQY) of the two emitters was characterized by absolute PLQY measurement using an
integrating sphere. The PLQY of tDCztTrz and tDCz2tTrz was 0.10 under oxygen, while it
was increased to 0.54 and 0.59 under nitrogen, respectively. The dramatic increase of the
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PLQY under nitrogen reflects triplet exciton contribution to singlet emission, supporting the
TADF character of the two emitters. Material characterization data of the TADF emitters are
summed up in Table 1.
The photophysical properties of the emitters characterized by PL analysis suggested that the
tDCztTrz and tDCz2tTrz emitters are potentially good TADF emitters, which allowed the
fabrication of TADF devices using the two emitters. The emitters were doped in the same
bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) host for full harvesting of triplet
excitons by up-conversion process. Doping concentration of the emitters was optimized from
10% to 30%. Basic electrical and photometric data represented by current density and
luminance are presented in Figure 5. As is clearly documented in several publications[30-33]
gradual increase of current density and luminance by increased doping concentration was
detected in the two devices by facilitated carrier hopping between TADF emitters at high
doping concentration.
,
EQE of the TADF devices is presented in Figure 6. In addition to the EQE of the tDCztTrz
and tDCz2tTrz devices, the EQE data of tCzTrz, and CztTrz devices were also added for
comparison in Table 2. Maximum EQE of the tDCztTrz and tDCz2tTrz devices was 22.6%,
which was much higher than those of tCzTrz (17.4%) and CztTrz (14.5%) devices. This
indicates that the t-butyl units only around the donor moiety or the acceptor moiety are not
useful. Simultaneous protection of the donor and acceptor moieties using alkyl branches is
critical to the EQE of the TADF devices. The rather high EQE of the TADF devices might be
due to horizontal dipole orientation of the emitters.
Doping concentration dependent electroluminescence (EL) spectra of the tDCztTrz and
tDCz2tTrz devices are presented in Figure 7(a) and (b). Although the doping concentration
was changed from 10% to 30%, there was little change of the EL spectra. This behavior is
quite different from the device data of common TADF emitters showing doping concentration
dependent red-shift of the EL spectrum[26,34,35]. As described in the material design concept,
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the peripheral t-butyl units would hinder intermolecular donor-acceptor interaction,
prohibiting the EL spectrum change according to doping concentration. A blue color
coordinate of (0.15, 0.20) was maintained at all doping concentrations.
Hyperfluorescent OLEDs were also developed using the tDCztTrz and tDCz2tTrz emitters in
comparison with tCzTrz and CztTrz emitters. The EQE of the hyperfluorescent OLEDs of the
four emitters is plotted in Figure 8. Fluorescent blue emitter of the hyperfluorescent OLEDs
was 2,5,8,11-tetra-tert-butylperylene (TBPe). The tDCztTrz and tDCz2tTrz based
hyperfluorescent OLEDs showed high EQE of 13.7%, whereas the tCzTrz and CztTrz
emitters exhibited EQE of 10.4% and 5.9%, respectively. The EQE was comparatively high in
the tDCztTrz and tDCz2tTrz devices, suggesting the superiority of the alkyl branch approach
for high EQE in both TADF and hyperfluorescent OLEDs.
The EQE improvement in the hyperfluorescent OLEDs by the alkyl modifying design method
was analyzed from the viewpoint of Dexter energy transfer from the TADF emitter to TBPe
because main role of the alkyl branch would be to separate TADF emitters from TBPe. The
Dexter energy transfer was indirectly investigated by monitoring delayed fluorescence of the
TADF emitters. A well-known compound, 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP), with a
singlet energy of 3.32 eV and a triplet energy of 2.49 eV[36] was doped in the TADF emitters
to study the Dexter energy transfer. As the singlet energy transfer from the TADF emitters to
CBP is difficult by high singlet energy of CBP, the change of the delayed fluorescence
component can be correlated with triplet energy transfer. Transient PL decay data in Figure 9
apparently indicate that the delayed fluorescent component of CztTrz is largely suppressed by
CBP doping, but that of tDCz2tTrz is weakly affected even after CBP doping. This result
suggests that the Dexter energy transfer is effectively suppressed by peripheral t-butyl units
surrounding the donor-acceptor core structure[37,38]
.
Conclusions
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In conclusion, two deep blue TADF emitters with peripheral alkyl branches surrounding the
donor and acceptor moieties were synthesized to explore ideal design of deep blue TADF
emitters for fluorescence and hyperfluorescence in terms of EQE. The TADF emitters with
the peripheral alkyl branches both in the donor and acceptor units showed a high quantum
efficiency above 22.0%, a deep blue color coordinate of (0.15, 0.20), and doping
concentration independent emission color as TADF emitters, and 13.7% quantum efficiency
in the TADF sensitized hyperfluorescent device. Therefore, the multiple alkyl modification
method of both donor and acceptor units of TADF emitters can resolve the issues of TADF
emitters for ideal device performances.
Experimental
Synthesis
2,4-bis(4-(tert-butyl)phenyl)-6-chloro-1,3,5-triazine and 2-chloro-4,6-bis(3,5-di-tert-
butylphenyl)-1,3,5-triazine were synthesized according to our previous work[29]
.
9,9'-(5-bromo-1,3-phenylene)bis(3,6-di-tert-butyl-9H-carbazole)
Sodium hydride (6.22 g, 259 mmol) washed with n-hexane was added to dimethylformamide
(DMF, 20 ml). 3,6-Di-tert-butyl-9H-carbazole (31.9 g, 114 mmol) dissolved in DMF (60 ml)
was added to the solution dropwisely. After 30 min, 1-bromo-3,5-difluorobenzene (10.0 g,
51.8 mmol) was added and the mixture was refluxed overnight. The reaction was quenched
with distilled water and the mixture was extracted with methylene chloride (MC). The crude
product was purified by column chromatography and a white powder was obtained as a final
product.
Yield : 29.2 g, 79.1%, MS(APCI) 712.70 m/z . 1H NMR (500 MHz, CDCl3): δ 8.133 (s,
4H), 7.797 (s, 2H), 7.768 (s, 1H), 7.508-7.460 (m, 8H), 1.463 (s, 36H). LC/MS (m/z): found,
712.70 ([M + H]+); Calcd. for C46H51BrN2, 711.81.
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9,9'-(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-phenylene)bis(3,6-di-tert-butyl-
9H-carbazole)
9,9'-(5-Bromo-1,3-phenylene)bis(3,6-di-tert-butyl-9H-carbazole) (5.0 g, 7.02 mmol),
bis(pinacolato) diboron (2.14 g, 8.43 mmol), potassium acetate (2.76 g, 28.1 mmol) and [1,1′-
bis(diphenylphosphino)ferrocene]dichloropalladium(II) (0.15 g, 0.21 mmol) were dissolved in
1,4-dioxane (60 ml) and refluxed for 5 hours. The reaction mixture was extracted with MC
and purified by column chromatography. A white powder was obtained as a product.
Yield : 3.1 g, 58.2%, MS(APCI) 759.35 m/z . 1H NMR (500 MHz, CDCl3): δ 8.072 (s,
4H), 7.793 (s, 2H), 7.695 (s, 1H), 7.457 (d, 4H, J=8.5 Hz), 7.304 (d, 4H, J=8.0 Hz), 1.463 (s,
36H), 1.356 (s, 12H). LC/MS (m/z): found, 759.35 ([M + H]+); Calcd. for C52H63BN2O2,
758.88.
9,9'-(5-(4,6-bis(4-(tert-butyl)phenyl)-1,3,5-triazin-2-yl)-1,3-phenylene)bis(3,6-di-tert-butyl-
9H-carbazole) (tDCztTrz)
2,4-Bis(4-(tert-butyl)phenyl)-6-chloro-1,3,5-triazine (0.5 g, 1.32 mmol) and 9,9'-(5-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-phenylene)bis(3,6-di-tert-butyl-9H-carbazole) (1.20
g, 1.58 mmol) were dried in vacuum and dissolved in THF (20 ml).
Tetrakis(triphenylphosphine) palladium(0) (0.076 g, 0.066 mmol) and 2M potassium
carbonate aqueous solution (10 ml) were added to the solution. The mixture was refluxed
overnight and extracted with MC. The crude product was purified by column chromatography
using an eluent of n-hexane/MC. After vacuum train sublimation, the product was obtained as
a white powder.
Yield : 0.94 g, 73.4%, MS(APCI) 977.58 m/z . 1H NMR (500 MHz, CDCl3): δ 9.033 (s,
2H), 8.650 (d, 4H, J=8.5 Hz), 8.188 (s, 4H), 7.998 (s, 1H), 7.559-7.499 (m, 12H), 1.486 (s,
36H), 1.373 (s, 18H). 13C NMR (125 MHz, CDCl3): δ 172.09, 170.60, 156.58, 143.55, 140.46,
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140.35, 139.25, 133.43, 129.16, 128.19, 125.89, 125.60, 124.18, 123.88, 116.63, 109.45,
35.31, 35.00, 32.23, 31.39, 29.92. MS (m/z): found, 976.5765 ([FAB]+); Calcd. for C69H77N5,
976.3832.
9,9'-(5-(4,6-bis(3,5-di-tert-butylphenyl)-1,3,5-triazin-2-yl)-1,3-phenylene)bis(3,6-di-tert-
butyl-9H-carbazole) (tDCz2tTrz)
The synthetic route of tDCz2tTrz was the same as that of tDCztTrz. 2-Chloro-4,6-bis(3,5-di-
tert-butylphenyl)-1,3,5-triazine (0.6 g, 1.22 mmol) was used instead of 2,4-bis(4-(tert-
butyl)phenyl)-6-chloro-1,3,5-triazine. The product was obtained as a yellowish white powder.
Yield : 0.86 g, 64.7%, MS(APCI) 1089.40 m/z . 1H NMR (500 MHz, DMSO): δ 9.022 (s,
2H), 8.660 (s, 4H), 8.181 (s, 4H), 8.124 (s, 1H), 7.713-7.696 (m, 6H), 7.527 (d, 4H, J=9.0 Hz),
1.489 (s, 36H), 1.418 (s, 36H). 13C NMR (125 MHz, CDCl3): δ 172.64, 171.01, 151.48,
143.70, 140.36, 140.33, 138.81, 135.43, 127.32, 126.26, 124.30, 124.18, 124.02, 123.44,
116.60, 109.54, 35.26, 35.00, 32.23, 31.66. MS (m/z): found, 1088.6636 ([FAB]+); Calcd. for
C77H93N5, 1088.5958.
Device fabrication and measurement
Device fabrication and measurement were described in our previous work[29]. The optimized
device structure was ITO/PEDOT:PSS (60 nm)/TAPC (20 nm)/mCP (10
nm)/DPEPO:tDCztTrz or tDCz2tTrz (25 nm)/TSPO1 (5 nm)/TPBi (40 nm)/LiF (1.5 nm)/Al
(200nm). PEDOT:PSS is poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), TAPC is
4,4′-cyclohexylidenebis[N,N -bis(4-methylphenyl) aniline], mCP is 1,3-bis(N-
carbazolyl)benzene, TSPO1 is diphenylphosphine oxide-4-(triphenylsilyl)phenyl and TPBi is
1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene. tDCztTrz and tDCz2tTrz were doped in
DPEPO host as doping concentration of 10% to 30%. All layers was thermally evaporated
under vacuum pressure of 1.0 × 10−6 torr.
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References
[1]. Y. Wang, Y. Hua, X. Wu, L. Zhang, Q. Hou, F. Guan, N. Zhang, S. Yin, X. Cheng, Org.
Electron. 2008, 9, 692.
[2]. J. Shi, C. W. Tang, Appl. Phys. Lett. 2002, 80, 3201.
[3]. J. Y. Hu, Y. J. Pu, F. Satoh, S. Kawata, H. Katagiri, H. Sasabe, J. Kido, Adv. Funct.
Mater. 2014, 24, 2064.
[4]. M. Zhu, C. Yang, Chem. Soc. Rev. 2013, 42, 4963.
[5]. J. Lee, H. F. Chen, T. Batagoda, C. Coburn, P. I. Djurovich, M. E. Thompson, S. R.
Forrest, Nat. Mater. 2016, 15, 92.
[6]. H. Sasabe, J. Takamatsu, T. Motoyama, S. Watanabe, G. Wagenblast, N. Langer, O. Molt,
E. Fuchs, C. Lennartz, J. Kido, Adv. Mater. 2010, 22, 5003.
[7]. J. Zhuang, W. Li, W. Su, Y. Liu, Q. Shen, L. Liao, M. Zhou, Org. Electron. 2013, 14,
2596.
[8]. X. C. Hang, T. Fleetham, E. Turner, J. Brooks, J. Li, Angew. Chem. Int. Ed. 2013, 52,
6753.
[9]. Y. Im, S. Y. Byun, J. H. Kim, D. R. Lee, C. S. Oh, K. S. Yook, J. Y. Lee, Adv. Funct.
Mater. 2017, 27, 1603007.
[10]. K. S. Yook, J. Y. Lee, Adv. Mater. 2012, 24, 3169.
[11]. Q. Zhang, B. Li, S. Huang, H. Nomura, H. Tanaka, C. Adachi, Nat. Photon. 2014, 8, 326.
[12]. S. Hirata, Y. Sakai, K. Masui, H. Tanaka, S. Y. Lee, H. Nomura, N. Nakamura, M.
Yasumatsu, H. Nakanotani, Q. Zhang, K. Shizu, H. Miyazaki, C. Adachi, Nat. Mater. 2015,
14, 330.
[13]. W. Song, T. Kim, J. Y. Lee, Y. Lee, H. Jeong, J. Ind. Eng. Chem. 2018, 68, 350.
11

Journal Pre-proof
[14]. S. K. Jeon, H. L. Lee, K. S. Yook, J. Y. Lee, Adv. Mater. 2019, 31, 1803524.
[15]. T. Hatakeyama, K. Shiren, K. Nakajima, S. Nomura, S. Nakatsuka, K. Konoshita, J. Ni,
Y. Ono, T. Ikuta, Adv. Mater. 2016, 28, 2777.
[16]. H. Nakanotani, T. Higuchi, T. Furukawa, K. Masui, K. Morimoto, M. Numata, H.
Tanaka, Y. Sagara, T. Yasuda, C. Adachi, Nat. Commun. 2014, 5, 4016.
[17]. S. Y. Byeon, D. R. Lee, K. S. Yook, J. Y. Lee, Adv. Mater. 2019, 31, 1803714.
[18]. J. S. Jang, S. H. Han, H. W. Choi, K. S. Yook, J. Y. Lee, Org. Electron. 2018, 59, 236.
[19]. Y. Zhang, J. Lee, S. R. Forrest, Nat. Commun. 2014, 5, 5008.
[20]. J. Lee, C. Jeong, T. Batagoda, C. Coburn, M. E. Thompson, S. R. Forrest, Nat. Commun.
2017, 8, 15566.
[21]. M. Kim, J. Y. Lee, Adv. Funct. Mater. 2014, 24, 4164.
[22]. H. Cho, J. Lee, J. I. Lee, N. S. Cho, J. H. Park, J. Y. Lee, Y. Kang, Org. Electron. 2016,
34, 91.
[23]. H. Shin, J. H. Lee, C. K. Moon, J. S. Huh, B. Sim, J. J. Kim, Adv. Mater. 2016, 28, 4920.
[24]. D. Zhang, M. Cai, Y. Zhang, D. Zhang, L. Duan, Mater. Horiz. 2016, 3, 145.
[25]. L. S. Cui, Y. L. Deng, D. P. K. Tsang, Z. Q. Jiang, Q. Zhang, L. S. Liao, C. Adachi, Adv.
Mater. 2016, 28, 7620.
[26]. M. Kim, S. K. Jeon, S. H. Hwang, J. Y. Lee, Adv. Mater. 2015, 27, 2515.
[27]. D. H. Ahn, S. W. Kim, H. Lee, I. J. Ko, D. Karthik, J. Y. Lee, J. H. Kwon, Nat. Photon.
2019.
[28]. H. Noda, H. Nakanotani, C. Adachi, Sci. Adv. 2018, 4, eaao6910.
[29]. Y. J. Kang, J. Y. Lee, Dyes Pigm. 2017, 138, 176.
[30]. R. J. Holmes, B. W. D'Andrade, S. R. Forrest, X. Ren, J. Li, M. E. Thompson, Appl.
Phys. Lett. 2003, 83, 3818.
[31]. Y. J. Cho, K. S. Yook, J. Y. Lee, Adv. Mater. 2014, 26, 6642.
[32]. C. W. Seo, J. H. Yoon, J. Y. Lee, Org. Electron. 2012, 13, 341.
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[33]. J. Yang, J. Shen, J. Appl. Phys. 1998, 84, 2105.
[34]. Y. H. Park. Y. Kim, H. Sohn, K. An, J. Phys. Org. Chem. 2012, 25, 207.
[35]. M. Kim, S. K. Jeon, S. H. Hwang, S. S. Lee, E. Yu, J. Y. Lee, Chem. Commun. 2016, 52,
339.
[36]. V. Jankus, C. Winscom, A. P. Monkman, J. Chem. Phys. 2009, 130, 074501.
[37]. J. H. Park, S. H. Back, H. M. Jeong, D. J. Ahn, Macromol. Res. 2018, 26, 1099.
[38]. H. J. Jang, J. Y. Lee, J. Kwak, D. Lee, J.-H. Park, B. Lee, Y. Y. Noh, J. Inf. Disp. 2019,
20, 1.
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List of tables
Table 1. Electrochemical and photophysical properties of tDCztTrz and tDCz2tTrz.
Table 2. Quantum efficiency, power efficiency and color coordinate of tDCztTrz, tDCz2tTrz,
tCzTrz and CztTrz
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Table 1. Electrochemical and photophysical properties of tDCztTrz and tDCz2tTrz
Electrochemical properties
Photophysical properties
Delayed
fluorescence
lifetime
HOMO LUMO Band gap
ES
ET
ΔEST
(eV)
PLQY
(%)
(eV)
(eV)
(eV)
(eV)[a] (eV)[b]
(μs)
tDCztTrz
-6.19
-6.17
-3.48
-3.40
2.71
2.77
3.03
3.04
2.99
3.02
0.04
0.02
54
59
55.7
tDCz2tTrz
118.7
[a] Singlet energies were calculated from edge of fluorescent spectra in 1 wt% doped
polystyrene film.
[b] Triplet energies were calculated from edge of phosphorescent spectra in tetrahydrofuran
solution at 77K.
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Table 2. Quantum efficiency, power efficiency and color coordinate of tDCztTrz, tDCz2tTrz,
tCzTrz and CztTrz
Quantum efficiency (%)
Power efficiency (lm/W)
Color
coordinate
Maximum
100 cd/m2
Maximum
100 cd/m2
tDCztTrz
tDCz2tTrz
tCzTrz
22.6
22.6
17.4
14.5
15.5
17.6
14.5
3.3
25.6
27.3
27.6
8.7
11.5
15.2
16.2
1.0
(0.15, 0.20)
(0.15, 0.20)
(0.17, 0.28)
(0.15, 0.12)
CztTrz
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List of figures
Scheme 1. Synthetic procedure of tDCztTrz and tDCz2tTrz
Figure 1. HOMO and LUMO distribution of tDCztTrz and tDCz2tTrz
Figure 2. Cyclic voltammetry (CV) curves of tDCztTrz and tDCz2tTrz
Figure 3. Photoluminescence (PL) spectra of (a) tDCztTrz and (b) tDCz2tTrz
Figure 4. Transient PL curves of tDCztTrz and tDCz2tTrz
Figure 5. Current density and luminance plots of (a) tDCztTrz and (b) tDCz2tTrz
Figure 6. External quantum efficiency (EQE) plots of (a) tDCztTrz and (b) tDCz2tTrz
Figure 7. Electroluminascence (EL) spectra of (a) tDCztTrz and (b) tDCztTrz
Figure 8. EQE of the hyperfluorescent OLEDs of tDCztTrz, tDCz2tTrz, tCzTrz and CztTrz
Figure 9. Transient PL curves of (a) CztTrz and (b) tDCz2tTrz doped with CBP 1wt%
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H
N
O
O
O
O
O
O
B
B
Br
Br
B
NaH, DMF
KOAc, PdCl2(dppf)
1,4-Dioxane
F
F
N
N
N
N
O
O
B
N
N
Cl
N
Br
N
N
N
N
N
N
Mg, I2, THF
K2CO3, Pd(PPh3)4
THF/H2O
N
Cl
Cl
N
Cl
N
N
tDCztTrz
O
O
B
N
N
Cl
N
Br
N
N
N
N
N
N
Mg, I2, THF
K2CO3, Pd(PPh3)4
THF/H2O
N
Cl
Cl
N
Cl
N
N
tDCz2tTrz
Scheme 1
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Figure 1
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Figure 2
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(a)
(b)
Figure 3
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Figure 4
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(a)
(b)
Figure 5
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(a)
(b)
Figure 6
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(a)
(b)
Figure 7
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Figure 8
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(a)
(b)
Figure 9
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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: