A strategy to construct multifunctional TADF materials for deep blue and high efficiency yellow fluorescent devices

Article abstract of DOI:10.1039/d0tc00039f

Three deep blue thermally activated delayed fluorescence (TADF) molecules are designed by connecting three different sterically hindered electron-donating groups with 2,6-diphenylpyrimidine acceptors. The rigid structures based on carbazole derivatives en

Full text of DOI:10.1039/d0tc00039f

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The Strategy to Construct Multifunctional TADF Material
for Deep Blue and High Efficiency Yellow Fluorescent
Devices
Xialei Lv, Shuaiqiang Sun, Qing Zhang, Shaofeng Ye, Wei Liu, Yaxiong Wang, Runda
Guo‡*, Lei Wang†*
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and
Technology, Wuhan, 430074, P. R. China
Email: †wanglei@mail.hust.edu.cn；‡runda_guo@hust.edu.cn
Abstract
Three deep blue thermally activated delayed fluorescence (TADF) molecules are
designed by connecting three different sterically hindered electron-donating groups
with 2,6-diphenylpyrimidine acceptor. The rigid structures based on carbazole
derivatives endow them excellent thermal stability and high photoluminescence
quantum yield (PLQY) about 0.72-0.74. A deep blue doped device based on
SFI34pPM with a CIEy of 0.09 and a high external quantum efficiency (EQE) of 8.2%
is achieved. Also, 3CzPhpPM realizes a maximum EQE of 4.9% with a CIEx,y (0.15,
0.06). The high PLQY and triplet energy (~2.84 eV) allow three deep blue compounds
to be used as hosts for yellow TADF emitter, which realized high-performance yellow
devices (EQEmax = 18.6%) with alleviated efficiency roll-off. What’s more,
3CzPhpPM combining with TADF sensitizer and fluorescence material TBRb
achieves a high EQE of 19.1%, which is among the best yellow fluorescent devices.
The bipolar transporting ability and harvest of triplet excitions together ensure highly
efficient yellow devices. Accordingly, these results shed light on the development of
designing multifunctional deep blue TADF materials, particularly offer a smart way to
construct high efficiency yellow fluorescence devices.
Keywords: thermally activated delayed fluorescence, deep-blue emitters, host, high
efficiency, yellow fluorescence devices

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.
Introduction
Organic light-emitting diode (OLED) is known as dreamlike display device. In recent
years, a variety of OLED products have entered our lives, such as smart phone, TV,
notebook, wearable device and so on. Until now, the cost of OLED applications is still
one of the bottlenecks restricting the rapid development of OLED industry. Organic
luminescent materials are the core of OLED devices. Emitter molecules, which are able
to harvest both singlet excitons and triplet excitons into emitted photons to achieve high
device efficiency, become the focus of researchers. Except for expensive
phosphorescence materials, the most efficient, inexpensive and easily available emitters
are TADF materials.1, 2 The device performance of TADF devices gradually catches up
with that of phosphorescent devices.3 OLED display consists of red, green, and blue
pixels. Since the eyes of human are relatively insensitive to blue light, more blue pixels
are required in all kinds of display devices.4 If the power consumption of the blue pixel
is reduced, the power consumption of the entire display will be reduced largely.
According to statistics, up to 30% of the electric energy is used for direct lighting, such
as large-area TV or portable device display lighting. Thus, stable and efficient deep
blue OLED is a major economic issue.5-7
In recent years, deep blue TADF materials have aroused extensive attention given
the scarcity in OLED industry.8-11 Several great-performance deep-blue emitters has
been reported in recent years.12 Blue TADF materials based on boron-containing
molecules have been extensively investigated due to their narrow full-width at half-
maximum (FWHM) and high efficiency.13-17 However, the synthetic yields of these
compounds are relatively low. Triazine-based deep-blue emitters have also been
reported, which need introduce methyl groups in the proper position to moderate
electronic interaction.18, 19 The synthesis process is relatively complicate adding methyl
groups to the target compounds. The device stability is also important for blue devices,
however the PO-containing and SO-containing materials may not satisfy the industrial
demand.20, 21 Briefly, our goal is to design chemically stable and easily available high-
efficiency deep blue TADF materials. In view of this, this paper designed three deep
blue materials basing on chemical-stable carbazole derivatives, employing readily

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available 2,6-diphenylpyrimidine as the acceptor. Apart of this, rigid donors such as
carbazole derivatives could not only largely increase the radiative rate constant but also
endow the molecules with rigid skeleton, suitable to be host materials.22 Rare deep blue
TADF has been reported with Commission International de L'Eclairage (CIE)
coordinates of about (0.14, 0.08), which is the primary blue according to the National
Television System Committee (NTSC) standard.23, 24 In this work, SFI34pPM-based
device realizes a high external quantum efficiency (EQE) of 8.2% with a CIEy of 0.09.
And the doped device based on 3CzPhpPM shows a maximum EQE of 4.9% with a
CIEx,y of (0.15, 0.06).
The reported deep blue TADF materials mainly falls into two categories, one is
emitter with low PLQY and a relatively small ΔEST (singlet-triplet energy splitting),25,
26 the other is molecule with high PLQY and larger ΔEST.27-30 The latter ones are easy
to obtain, but usually suffer from seriously device efficiency roll-off due to the long
delayed lifetime of triplet excitons.31 However, they can be ideal host materials for
traditional fluorescence, phosphorescence and TADF materials because of the bipolar
charge transport ability of D-A structure of TADF materials, small optical energy gap
for carrier injection and relatively higher triplet energy compared with the common
bipolar hosts and harvest of triplet excitons.32-34 The efficient fluorescence OLEDs
composed of a wide-energy-gap host, a TADF-assistant dopant and a fluorescent
emitter dopant have been studied widely before. Adachi and his coworkers reported
three fluorescence-based organic light-emitting diodes with TADF-assisted hosts and
conventional hosts realizing EQE as high as 13.4-18.0% for blue, green, yellow and red
emission.35 Recently, Duan et al found, if the wide-gap host had TADF properties, the
potential exciton loss pathways, involving Dexter energy transfer from the T1 of both
the host and the TADF sensitizer to that of the fluorescence dopant, could be reduced
greatly.36 However, the wide-energy-gap TADF host materials are hard to find for
fluorescence
dopant
TBRb
(2,8-di[t-butyl]-5,11-di[4-(t-butyl)phenyl]-6,12-
diphenylnaphthacene), which is a normal yellow emitter in white devices.35, 37-39 In the
present work, we allow three deep blue TADF materials to be used as hosts for an
yellow TADF emitter 34AcCz-Trz (5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-

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13,13-dimethyl-8-phenyl-8,13-dihydro-5H-indolo[3, 2-a]acridine) to realize high
efficiency yellow TADF devices with greatly relieved efficiency roll-off compared to
traditional host CBP. Additionally, when using 3CzPhpPM as the wide-gap host and
combing with 34AcCz-Trz sensitizer and TBRb, the yellow device reached a high EQE
of 19.1%. All of these demonstrates that 3CzPhpPM is not only a good deep-blue
emitter but also an excellent host material.
Results and Discussion
Synthesis and Characterization
Scheme 1 depicts the synthetic routes for SFI34pPM as well as SFI23pPM and
3CzPhpPM, which named as PM derivatives in following description. Firstly, the
important intermediate 4-(4-bromophenyl)-2,6-diphenylpyrimidine was prepared
according to the reported method previously.40, 41 Then, the synthesis steps of SFI34
and SFI23 were already reported in our previous work.42 And 3CzPh was facilely
synthesized by Suzuki cross coupling reaction (Scheme S1, ESI†). Finally, the desired
products were obtained from Buchwald-Hartwig coupling reaction with good yields.
They were subjected to recrystallization, followed by temperature-gradient vacuum
sublimation before the thermodynamics tests, photophysical measurements and device
fabrication. The detailed synthesis steps and the confirmation of the chemical structures
of the intermediate compounds and final products were described in experimental part.
H
N
N
N
N
SFI34pPM
H
N
N
Pd(OAc)2, [(t-Bu)3PH]BF4
t-BuONa, toluene, N2
N
N
N
N
SFI23pPM
Br
H
N
N
N
N
3CzPhpPM
Scheme 1. The synthesis process of three target compounds.

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Theoretical calculations
Fig 1. Molecular structures, HOMO/LUMO distributions and their energy level diagram for low-
lying singlet and triplet excited states of three compounds.
To investigate the highest occupied molecular orbitals (HOMOs)/the lowest
unoccupied molecular orbitals (LUMOs) distributions and excited energy states of
these carbazole derivatives, density functional theory (DFT) and time-dependent
density functional theory (TD-DFT) calculations using the B3LYP/6-31G(d) method
were carried out, as illustrated in Fig 1. All the compounds show well-separated HOMO
and LUMO orbitals, and the HOMO distributions are mainly located on carbazole and
the connected five-membered ring and benzene, whereas the LUMO orbital
distributions are similarly dispersed on 2,4,6-triphenylpyrimidine except for the 2-
substituted benzene. Before that, the calculations of three donor groups were conducted
(as seen in Fig S1). It’s obvious that the HOMO energy levels of the donors are
gradually become deeper from SFI34 to SFI23 to 3CzPh, suggesting the electron-
donating ability decreased gradually. The HOMO energy levels of SFI34pPM,
SFI23pPM and 3CzPhpPM follow the same trends, whereas the LUMO energy levels

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do not change apparently. In addition, a small change of the dihedral angles between
donors and acceptor of these compounds is seen, about 52.6o-54.5o, which render them
a larger conjugation degree and a higher oscillator strength (f = 0.2390-0.3009)
compared to that with more twisted-structure materials. As calculated, the theoretical
energy gap between the lowest singlet and triplet states are 0.22-0.32 eV for these PM
derivatives, which demonstrates these emitters are potential TADF materials.
Thermal and electrochemical properties
3CzPhpPM
SFI23pPM
SFI34pPM
-2.4 -1.8 -1.2
0.6
1.2
Potential (V vs. Ag/Ag+)
Fig 2. The electrochemical properties of the four compounds.
Thermal stability of the compounds was examined by thermogravimetric analysis
(TGA) and differential Scanning Calorimeter (DSC) under N2 atmosphere at a heating
rate of 10 °C/min, as shown in Fig S2. The decomposition temperature (Td) is
corresponding to the 5% weight loss, and all dyes show Td in the range of 405 °C-
449 °C. The glass transition temperature (Tg) of 3CzPhpPM is 110 °C. However, no
obvious Tg is seen for SFI34pPM and SFI23pPM from 30 oC-300 oC. Above all, these
compounds have excellent thermal stability. Cyclic voltammetry (CV) measurements
on molecules in Fig 2 were performed in dichloromethane (DCM) and N,N-
dimethylformamide (DMF) via a traditional three-electrode system. The reversible
redox peaks indicate their good electrochemical stabilities. Besides, the oxidation peaks
are deriving from the electron-donating ability of the donors, which show more variety

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than reduction peaks. The HOMO energy levels of SFI34pPM, SFI23pPM and
3CzPhpPM are calculated to be -5.48 eV, -5.51 eV and -5.57 eV respectively, by
calibrating the onset potential with ferrocene, which are well in good agreement with
calculation data. The LUMO energy levels are also estimated from the reduction peaks
(as seen in Table 1).
Photophysical and TADF properties
3CzPhpPM
SFI23pPM
1.0
SFI34pPM
0.5
0.0
300
400
500
600
Wavelength (nm)
Fig 3. The UV-vis absorption and PL spectra of the compounds in toluene solution.
The photophysical properties of these compounds were investigated by UV-vis
absorption and fluorescence spectroscopy in toluene. The absorption band before 314
nm could be ascribed to the π-π∗ and n-π∗ transition of the donor groups, and the
absorption appearing in the wavelength range 352-367 nm is assigned to the
intramolecular charge transfer (ICT) transition. Also, the extinction coefficient () of
three compounds in toluene was measured as seen in Fig S3, and it can be concluded
that 3CzPhpPM possesses the highest ICT transition probabilities among them. From
the PL spectra, the emission peaks of 3CzPhpPM, SFI23pPM and SFI34pPM are 412
nm, 414 nm and 424 nm, respectively. Compared with 3CzPhpPM, the observed
bathochromic shifts of SFI23pPM and SFI34pPM can be attributed to the enhanced
ICT state, which was resulted from the increased electron-donating capacity of the
donors. As shown in Fig S4, the emissions of 3CzPhpPM, SFI23pPM and SFI34pPM
red-shifted with the increase of solvent polarity from n-hexane to dichloromethane,
which further manifests their ICT characteristic. Furthermore, the fluorescence and

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phosphorescence spectra of three compounds at 77 K in dimethyl tetrahydrofuran are
obtained in Fig S5. The first singlet/triplet energies can be calculated from the onset of
the emission, and the S1/T1 are 3.08/2.84 eV, 3.20/2.84 eV and 3.22/2.83 eV for
SFI34pPM, SFI23pPM and 3CzPhpPM, respectively. And the energy gaps between
singlet and triplet (ΔESTs) are 0.24 eV, 0.36 eV and 0.39 eV, respectively, which are
common values for deep blue materials. Further, NTO calculations were carried to
demonstrate the property of the excited states as shown in Fig S6. Three compounds
revealed the typical CT first singlet states, and CT T1 exited states accompanied by
small contributions from locally excited (LE) states, which were consistent with the
measured phosphorescence spectra.
Fig 4. The transient PL decay curve (air and vacuum) at 300 K for 10 wt% of three PM derivatives
dopant in zeonex films (a), (b), (c); and in DPEPO films (d), (e) and (f).
To study the influence of host materials on photophysical properties of these
compounds, we measured transient PL delay characteristics in zeonex43 and DPEPO
host films as seen in Fig 4. Zeonex was used because it has a similar polarity to nonpolar
solvents such as methyl cyclohexane, whereas DPEPO has a high polarity. DPEPO host
possesses high S1 and T1 energies (ES/ET = 3.5/3.1 eV), which are large enough to
suppress a back energy transfer from the guest emitter to the host and to confine the T1
excitons within the guest molecules. It is obvious that the decay curves show difference

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between measuring in air and vacuum conditions no matter in zeonex or in DPEPO
host, which indicates three compounds are typical TADF materials. Besides, the
difference between air and vacuum becomes more apparent for three compounds in
zeonex than that of in DPEPO, which could be ascribed to the smaller singlet-triplet
energy splitting of the charge-transfer singlet state (1CT) and the lowest locally excited
triplet state (3LE) of materials in higher polarity matrix. The lifetimes of the prompt
components of doped films in DPEPO were 5.2-6.6 ns, which demonstrates a fast
radiative decay rate. And the lifetimes of the delay parts are in the range of 118-189 μs.
Moreover, the absolute PL quantum yield (PLQY) of SFI34pPM, SFI23pPM and
3CzPhpPM are 0.74, 0.72, and 0.73, respectively. The delayed component included
the effect of nonradiative decay, internal conversion decay, intersystem crossing decay
and reverse intersystem crossing decay rate, thus the shorter the delayed lifetime the
better the TADF emitter.
Table 1. Basic Photophysical and Electrochemical Parameters of PM Derivatives.
d
e
f
HOMO/
LUMOc(eV)
S1/T1
(eV)
S1/T1
(eV)
Tg/Td
(℃)
b
Compounds λabsa(nm) λPLa(nm) Eg (eV)
SFI34pPM
SFI23pPM
314,367
308,363
293,352
424
414
412
3.09
3.17
3.18
-5.48/-2.64
-5.51/-2.63
-5.57/-2.61
2.90/2.68 3.08/2.84
3.01/2.73 3.20/2.84
-/440
-/449
3CzPhpPM
3.09/2.77 3.22/2.83 110/405
a
Measured in toluene solution at 300 K ； bEstimated from the absorption edges in toluene ； c
Determined by the cyclic voltammetry (CV) measurement.；d Calculated by TDDFT；e Calculated
from the onset of the fluorescence spectra and phosphorescence in 2Me-THF at 77 k；f Measured by
Electroluminescent properties
In order to assess the electroluminescent (EL) properties of these compounds, firstly,
we constructed the doped devices A1-A3 using DPEPO as host. The configuration of
the device A1-A3 was indium-tin oxide (ITO)/ MoO3 (10 nm)/TAPC (50 nm)/mCP (10-
15 nm)/DPEPO: Emitters (20 nm)/DPEPO (5 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al
(Emitters：10 wt% SFI34pPM for device A1; 20 wt% SFI23pPM for device A2; 20 wt%
3CzPhpPM for device A3), where 1,1-bis[4-[N,N-di(ptolyl)-amino]phenyl]
cyclohexane (TAPC) and 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) were used
as hole- and electron-transport materials, respectively. Then, the nondoped devices B1-

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B3 were also fabricated since they all bear steric structures (the detailed devices
structures and EL performance are displayed in Fig S7). Inspired by the high PLQY
and the matched HOMO/LUMO energy levels with yellow materials, we adopted a
yellow TADF material synthesized by our group 34AcCz-Trz (5-(4-(4,6-diphenyl-
1,3,5-triazin-2-yl)phenyl)-13,13-dimethyl-8-phenyl-8,13-dihydro-5H-indolo
a]acridine) using as the dopant and these PM derivatives as hosts.44
[3,2-
Fig 5. The detailed device configuration of A1-A3, B1-B3, C1-C3, and D1-D3 and the structure of
34AcCz-Trz and TBRb.

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Fig 6. (a) Current density–voltage–luminance (C–V–L) curves; (b) CE-Luminance-PE; (c) EQE-
Luminance; (d) Normalized EL spectra for doped blue device A1-A3 of three PM derivatives.
Regarding OLED based on the DPEPO: PM derivatives emitting layer (EML), a
maximum external quantum effciency (EQEmax) of 8.2% was achieved by SFI34pPM,
accompanied with a turn-on voltage of 3.7 V and CIEx,y (0.15, 0.09) as shown in Fig 6.
Although the doped concentrations are different in emitting layers of device A1-A3, the
turn-on voltages of device A1-A3 are the same, because the emitters have a similar
LUMO energy levels with TmPyPB (the electron transporting layer) and the electron
could directly move on the emitter molecules. Addtionally, the other two materials
based deep blue devices reach EQEmax of 5.1% and 4.9% with CIEx,y (0.15, 0.10) and
(0.15, 0.06) for SFI23pPM and 3CzPhpPM respectively, because of the relatively
larger ΔESTs about 0.33 eV for SFI23pPM and 0.37 eV for 3CzPhpPM as seen in Fig
S8. The exact device data of device A1-A3 are summarized in Table 2.
The performances of the nondoped device B1-B3 based on three compounds are
shown in Fig S7 and data are listed in Table 2. Interestingly, the EL spectra of device
B1 and B2 exhibit almost no redshift compared to that of the doped device A1 and A2,
which may stem from the steric resistance effect and rigid structure of their donors.
However, the donor group of 3CzPhpPM has more planar configuration, which leads
to stronger intermolecular interactions and larger bathochromic-shift, and device B3
shows a CIEx,y of (0.16, 0.12) compared to (0.15, 0.06) of device A3. On the whole, the
maximum EQE of device B1-B3 are 4.6%, 4.1% and 4.5%, respectively. The good
nondoped device performances inspired us to utilize these deep blue emitters as host
materials since their bipolar transporting ability and less concentration annihilation
effect.
Herein, a yellow TADF material 34AcCz-Trz reported by our group 36 is chosen as
the emitter using three PM compounds as hosts (Fig S9), and making three yellow
devices of C1-C3. The detailed structures of C1-C3 are displayed in Fig 5, as ITO/MoO3
(10 nm)/TAPC (80 nm)/TCTA (5 nm)/SFI34pPM/ SFI23pPM/ 3CzPhpPM: 3 wt%

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34AcCz-Trz (25 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al. Besides, a contrast device C4
hosted by CBP (4,4'-Bis(9H-carbazol-9-yl)biphenyl) with the same structure was made.
Finally, the reference device C4 realized a maximum EQE of 14.5%, 13.2% at 1000
cd/m2 luminance, and 10.7% at 10000 cd/m2 luminance. Besides, device C1, C2 and C3
showed higher EQEmax of 17.8%, 17.9% and 18.6%, respectively. More importantly,
the efficiency roll-off of device C1-C3 is relieved greatly, as the EQEs are 17.5%, 17.3%
and 17.7% at 1000 cd/m2 luminance, and 14.3%, 14.1% and 14.9% at 10000 cd/m2
brightness, respectively. This result indicates that PM derivatives can not only achieve
highly efficient deep blue TADF OLED, but also efficiently improve the
electroluminescence performances of the hosted yellow TADF devices. Moreover, the
bipolar transporting property is confirmed by hole-only and electron-only devices as
seen in Fig S10. The results proved that 3CzPhpPM represent a more balanced carrier
transportation ability, which explained the highest EQE of device C3. Nevertheless, the
efficiency roll-off depends on both TADF strength and the bipolar transportation
property, since SFI34pPM shows the smallest efficiency roll-off.

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Fig 7. (a) The energy transfer process for device D4; (b) the PL decay curves of yellow and yellow
doped films; (c) Current density–voltage–luminance (C–V–L) curves; (d) EQE-Luminance and
inset: Normalized EL spectra for device D4.
As we all know, fluorescence OLED is the most stable devices compared to
phosphorescence and TADF OLEDs. Finally, the yellow fluorescence device D4 was
designed using 3CzPhpPM as the host, 34AcCz-Trz as sensitizer and TBRb as the
dopant. From the doping concentration dependent device performances of the TADF
sensitized fluorescent devices in Fig S12, the device structure of D4 was more
reasonable. The yellow fluorescence device D4 based on TBRb was constructed with
the detailed structure of ITO/MoO3 (10 nm)/TAPC (60 nm)/TCTA (5 nm)/
3CzPhpPM: 4wt% 34AcCz-Trz: 1wt% TBRb (30 nm)/TmPyPB (40 nm)/LiF (1
nm)/Al. Before this, reference device D1-D3 were made and hosted by CBP and
34AcCz-Trz respectively, and CBP (host)+34AcCzTrz (sensitizer) in Fig S11.
Apparently, TADF host +TADF sensitizer improved the device performance greatly,
which can be ascribed to the reduced triplet exciton loss compared to that of
conventional host (as shown in Fig 7(a)). In order to probe the energy transfer in the
yellow device C1-C3 and D4, the decay curves of doped films were investigated. Since

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the delayed lifetime of PM derivatives functioning as the host of 34AcCz-Trz became
much shorter than that of doped in DPEPO host, the three PM-compounds are talented
host materials (as seen in Fig 7(b)). Apart from this, the delayed lifetime of the film of
3CzPhpPM+4wt% 34AcCzTrz+1wt% TBRb is the shortest one among the yellow
and yellow films, which indicated that utilization of triplet excitons with TADF host
and TADF sensitizer are the most effective among them. Thus, device D4 realized a
high efficiency of 19.1%, with negotiable efficiency roll-off at 1000 cd/m2 luminance,
as shown in Fig 7(c) and (d). From Fig S13, it can be confirmed that the complete
energy transfer happened in the TADF sensitized fluorescence devices. This excellent
yellow device performance based on TBRb enlightens more usage of the blue TADF
materials with long delayed lifetime.
Table 2. The EL performance of three compounds.
a
CEb
PEb
EQEb
CIEd
Roll-offc
Von
(%)
Device
Emitter
(V)
(cd/A)
(lm/W)
(%)
(0.15, 0.09)
(0.15, 0.10)
(0.15, 0.06)
(0.15, 0.10)
(0.16, 0.09)
(0.16, 0.12)
(0.39, 0.55)
(0.38, 0.56)
(0.38, 0.56)
(0.40, 0.56)
(0.49, 0.50)
(0.45, 0.53)
(0.43,0.54)
A1
A2
SFI34pPM
SFI23pPM
3CzPhpPM
SFI34pPM
SFI23pPM
3CzPhpPM
34AcCzTrz
34AcCzTrz
34AcCzTrz
34AcCzTrz
TBRb
3.7
3.7
3.7
3.9
4.1
3.7
3.1
3.4
3.5
3.3
<3.3
3.2
4.0
8.04
5.81
3.08
4.48
3.41
4.90
55.41
56.44
57.72
45.10
56.79
60
7.89
5.36
2.65
3.57
2.80
4.09
44.31
41.39
43.07
38.51
47.39
58
8.2
5.1
-
-
-
-
-
-
A3
4.9
B1
4.6
B2
4.1
B3
4.5
1.7/19.8
3.5/21.4
5.0/20.0
9.1/26.3
0/23.5
C1
17.8
17.9
18.6
14.5
19.1
18.0
19.1
C2
C3
C4
D4
39.4/-
Ref35
Ref37
TBRb
12.6/-
TBRb
62
-
a.
b.
turn-on voltage (at a brightness of 1 cd m-2); the maximum CE (current efficiency), PE (power
efficiency) and EQE (external quantum efficiency); c. the EQE roll-off at 1000 cd m-2and 10000 cd m-2;
d. CIE: Commission Internationale de l’Eclairage chromaticity coordinates at 100 cd m-2.
Conclusions
Finally, we have designed and synthesized a series of bipolar deep blue TADF materials
based on 2,6-diphenylpyrimidine (PM) and carbazole derivative donors for use as blue
emitters and host materials for yellow TADF emitter. Among them, the doped device

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DOI: 10.1039/D0TC00039F
based on SFI34pPM shows a relatively high EQE of 8.2% with CIEx,y of (0.15, 0.09).
3CzPhpPM also realized a good deep-blue device with a EQEmax of 4.9% and CIEx,y
of (0.15, 0.06). Besides, these compounds using as host materials not only improve the
device efficiency, but also relieve the efficiency roll-off apparently in yellow TADF
devices. Especially, the more balanced carrier transporting ability endows the yellow
device hosted by 3CzPhpPM a high EQE of 18.6%, which is much higher than that of
the reference device hosted by CBP (14.5%). On the other hand, SFI34pPM using as
the host exhibits the smallest efficiency roll-off because of the stronger TADF strength.
In the end, 3CzPhpPM is also an excellent host in TADF-sensitizing-fluorescence
system, which realizes a high efficiency of 19.1% based on TBRb. Accordingly, deep
blue materials with rigid structures are ideal candidate for high TADF hosts, and we
also present a new approach for design simple and promising TADF molecules as
versatile materials in fluorescence devices.
Experimental Section
The details of experiment conditions and equipments are cited the earlier reports36
unless otherwise illustrated.
Synthesis of 5'-(4-(2,6-diphenylpyrimidin-4-yl)phenyl)-5'H-spiro[fluorene-
9,12'-indeno[1,2-c]carbazole] (SFI34pPM):
SFI34pPM was synthesized according to the similar procedure as described in reported
paper43, using 5'H-spiro[fluorene-9,12'-indeno[1,2-c]carbazole] and intermediate 1 as
1
the reactants (yield 81%). H NMR (400 MHz, CDCl3) δ [ppm]: 8.79-8.77(d, J = 8.0
Hz, 2H), 8.55-8.53 (d, J = 8.4 Hz, 2H), 8.6-8.33 (d, J = 7.6 Hz, 2H), 8.11 (s, 1H), 8.05-
8.03 (d, J = 7.6 Hz, 2H), 7.99-7.97 (d, J = 8.4 Hz, 1H), 7.84-7.82 (d, J = 7.2 Hz, 1H),
7.76-7.74 (d, J = 8.8 Hz, 2H), 7.61-7.56 (m, 7H), 7.44-7.40 (t, J = 7.6 Hz, 2H), 7.32-
7.27 (m, 2H), 7.18-7.14 (t, J = 8.0 Hz, 1H), 7.09-7.05 (t, J = 7.6 Hz, 2H), 7.03-6.99 (t,
J = 7.6 Hz, 1H), 6.84-6.82 (d, J = 7.6 Hz, 2H), 6.74-6.71 (m, 2H), 6.64-6.62 (d, J = 7.6
Hz, 1H).13C NMR (100 MHz, CDCl3) δ [ppm]:165.08, 164.73, 163.85, 149.41, 147.38,
142.23, 141.57, 141.47, 141.44, 140.98, 139.98, 138.01, 137.43, 136.73, 136.14,
130.97, 130.83, 129.00, 128.93, 128.54, 128.11, 128.00, 127.84, 127.36, 126.56,

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125.65, 124.08, 123.68, 123.16, 122.05, 120.42, 120.25, 120.15, 119.02, 117.72,
110.30, 109.89, 109.15, 66.05. MS (APCI): cal for C53H33N3: 711.2674, found
712.2651[M+H]+.
Synthesis of 5'-(4-(2,6-diphenylpyrimidin-4-yl)phenyl)-5'H-spiro[fluorene-9,7'-
indeno[2,1-b]carbazole] (SFI23pPM)
The synthesis process is referred to SFI34pPM, employing 5'H-spiro[fluorene-9,7'-
indeno[2,1-b]carbazole] and intermediate 1 as the reactants (yield 73%). 1H NMR (400
MHz, CDCl3) δ [ppm]: 8.77-8.75 (d, J = 7.6 Hz, 2H), 8.65 (s, 1H), 8.41-8.39 (d, J = 7.6
Hz, 2H), 8.35-8.30 (m, 3H), 8.13-8.02 (m, 2H), 7.86-7.86 (d, J = 7.6 Hz, 2H), 7.61-
7.58 (m, 8H), 7.49-7.37 (m, 6H), 7.16-7.09 (m, 3H), 6.70 (s, 1H), 6.85-6.83 (d, J = 7.6
Hz, 2H), 6.72-6.70 (d, J = 7.2 Hz, 1H). 13C NMR (100 MHz, CDCl3) δ [ppm]: 164.91,
164.60, 163.80, 149.67, 148.94, 147.82, 141.91, 141.68, 141.12, 141.01, 139.77,
137.91, 137.34, 136.17, 135.34, 130.94, 130.79, 128.96, 128.90, 128.50, 127.86,
127.74, 127.63, 127.32, 127.10, 126.96, 126.10, 124.20, 123.95, 123.85, 123.68,
120.47, 120.30, 120.06, 119.39, 111.37, 110.26, 109.95, 105.53, 66.03. MS(APCI): cal
for C53H33N3: 711.2674, found 712.2679 [M+H]+.
Synthesis of 9-(4-(2,6-diphenylpyrimidin-4-yl)phenyl)-3-phenyl-9H-carbazole
(3CzPhpPM)
The synthesis of 3CzPhpPM is similar to that of SFI34pPM, using intermediate 1 and
1
intermediate 2 as the reactants (yield 80%). H NMR (400 MHz, CDCl3) δ [ppm]：
8.80-8.78 (d, J = 7.2 Hz, 2H), 8.55-8.53 (d, J = 8.0 Hz, 2H), 8.38 (s, 1H), 8.36-8.34 (d,
J = 7.6 Hz, 2H), 8.23-8.21 (d, J =7.6 Hz, 1H), 8.10 (s, 1H), 7.82-7.80 (d, J = 8.0 Hz,
2H), 7.76-7.74 (d, J =7.6 Hz, 2H), 7.72-7.70 (d, J = 8.4 Hz, 1H), 7.61-7.46 (m, 11H),
7.39-7.34 (m, 2H). 13C NMR (100 MHz, CDCl3) δ [ppm]: 165.03, 164.67, 163.82,
141.87, 141.00, 140.02, 137.99, 137.40, 136.36, 133.93, 130.99, 130.86, 129.01,
128.97, 128.84, 128.56, 127.37, 127.09, 126.70, 126.37, 125.69, 124.24, 123.84,
120.52, 118.94, 110.28, 110.11, 110.02. MS (APCI): cal for C40H27N3: 549.2205, found
550.2211[M+H]+.

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Conflicts of interest
There are no conflicts to declare.
Acknowledgments
This research work was supported by the NSFC/China (51573065, 51727809), China
Postdoctoral Science Foundation (2017M620321, 2018M632841), and the Science and
Technology Support Program of Hubei Province (2015BAA075). We are thankful to
SCTS/CGCL HPCC of HUST for providing computing resources and technical support.
The Analytical and Testing Center at Huazhong University of Science and Technology
is acknowledged for characterization of new compounds. The Center of Micro-
Fabrication and Characterization (CMFC) of Wuhan National laboratory for
Optoelectronics (WNLO) of HUST is acknowledged for fabrication of the OLED
devices. In particular, we are thankful for Wuhan Sunshine Optoelectronics Technology
Co., Ltd supply of the intermediate (3-bromocarbazole).
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Table of Contents
A series of multifunctional deep blue TADF materials is designed and
synthesized, when utilizing as the wide-energy-gap host in TADF
sensitization system, a high EQE of 19.12% orange fluorescent device is
obtained.