A ternary phosphine oxide host featuring thermally activated delayed fluorescence for blue PHOLEDs with >20% EQE and extremely low roll-offs

Article abstract of DOI:10.1039/c8tc02010h

Development of high-performance blue organic light-emitting diodes (OLEDs) is one of the main challenges because efficient host-dopant energy transfer is necessary to develop highly efficient OLEDs. In this study, an effective molecular design strategy for blue phosphorescent hosts with high triplet (T₁) energy and the feature of thermally activated delayed fluorescence (TADF) has been demonstrated using a ternary donor-acceptor-acceptor (D-A-A) molecule, DCDPOTZ, in which carbazole, triazine and diphenylphosphine oxide are incorporated as donor and acceptors. DCDPOTZ achieves the typical characteristics of TADF with a high T₁ energy level of 2.93 eV and a small singlet-triplet splitting of 0.27 eV, accompanied by the ambipolar property. Its host characteristics are further improved by its diphenylphosphine oxide group with large steric hindrance for suppressing intermolecular interaction-induced quenching. By adopting bis(4,6-difluorophenylpyridinato)tetrakis((1-pyrazolyl)borate)iridium(iii) (FIr6) as a dopant, DCDPOTZ successfully realizes 100% host-dopant energy transfer efficiency. As a consequence, DCDPOTZ endowed its FIr6-based devices with extremely low driving voltages, the state-of-the-art external quantum efficiency (EQE) of up to 22.7%, and 100% internal quantum efficiency. At 1000 nits, the EQE is still beyond 20%, corresponding to a roll-off as low as 11%, which is one of the lowest values reported to date for blue phosphorescent OLEDs.

Full text of DOI:10.1039/c8tc02010h

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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
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Ternary Phosphine Oxide Host Featuring Thermally Activated
Delayed Fluorescence for Blue PHOLEDs with > 20% EQE and
Extremely Low Roll-offs
Received 00th January 20xx,
Accepted 00th January 20xx
Chenyu Li,†a Xuefeng Fan,†a Chunmiao Hana and Hui Xua*
DOI: 10.1039/x0xx00000x
High-performance blue device is one of main challegens for organic light-emitting diodes (OLED), which
requires the efficient host-dopant energy transfer. In this work, an effective molecular design strategy of
blue phosphorescent hosts with high triplet (T1) energy and the feature of thermally activated delayed
www.rsc.org/
fluorescence (TADF) was demonstrated with
a ternary donor-acceptor-acceptor (D-A-A) molecule
DCDPOTZ, in which carbazole, triazine and diphenylphosphine oxide are incorporated as the donor and
acceptors. DCDPOTZ achieves the typical TADF characteristics with the high T1 energy level of 2.93 eV and
the small singlet-triplet splitting of 0.27 eV, accompanied by the ambipolar property. Its host
characteristics are further improved by its diphenylphosphine oxide group with large steric hindrance for
suppressing intermolecular interaction-induced quenching. By adopting bis(4,6-difluorophenylpyridinato)
tetrakis(1-pyrazolyl)borate) iridium(III) (FIr6) as dopant, DCDPOTZ successfully realizes the 100% host-
dopant energy transfer efficiency. As consequence, DCDPOTZ endowed its FIr6-based devices with the
extremely low driving voltages, the state-of-the-art external quantum efficiency (EQE) up to 22.7% and 100%
internal quantum efficiency. At 1000 nits, the EQE was still beyond 20%, corresponding to the roll-off as
low as 11%, which is one of the lowest values reported so far for blue phosphorescence OLEDs.
host matrixes is also one of the key issues.29-40
Introduction
Taking PHOLED as an example, the T1 energy levels of the
blue phosphors are ~2.8 eV, which requires the T1 energy
Organic light-emitting diode (OLED) has been studied
extensively due to its advantages of high efficiency, ultra-
thinness, lightness and flexibility for the applications of flat
panel display and solid-state lighting.1-3 It is rational that the
highly efficient electroluminescence (EL) should be based on
harvesting both singlet and triplet excitons for 100% internal
quantum efficiency (IQE), which has been demonstrated by
phosphorescence OLEDs (PHOLED)4-6 and thermally activated
delayed fluorescence (TADF) diodes7, 8. These two technologies
can make all the excitons converge to radiative excited states
of their phosphors and TADF emitters, namely the first triplet
(T1) and singlet (S1) excited states, through intersystem
crossing (ISC)4 and reverse ISC (RISC)7, respectively.
Nonetheless, the high-performance blue OLEDs are still a big
challenge, although indispensable for high-quality display and
lighting source.9-12 Besides the difficulty in developing dee-blue
phosphors and TADF dyes,13-28 the insufficiency in efficient
levels of their hosts around 2.9~3.0 eV. In most cases of the
conventional high energy-gap hosts with the singlet-triplet
splitting (ꢀEST) of ~1.0 eV, their energy gaps between the
highest occupied (HOMO) and the lowest unoccupied
molecular orbitals (LUMO) beyond 4.0 eV would reduce and
unbalance the carrier injection and transport (Scheme 1 left).41
More importantly, the charge transfer (CT)-featured excited
states of blue phosphors render their smaller ꢀEST less than 0.3
eV. In this case, the extremely high S1 energy levels of these
hosts simultaneously restrain ISC and Förster resonance
energy transfer (FRET). To solve this issue, an effective strategy
is direct charge capture and recombination by the dopant,
which can indeed realize the state-of-the-art external quantum
efficiency (EQE) of ~20%, but accompanied by serious
concentration quenching.42-45 As the result, only few blue
PHOLEDs revealed EQE roll-off less than 15% at 1000 cd m-2.9,
46, 47 Obviously, TADF molecules are promising as hosts for blue
phosphors, whose small ꢀEST can not only improve the FRET to
the S1 energy level of the dopants, but also dynamically adjust
the singlet and triplet exciton allocation through ISC-RISC
balance (Scheme 1 right). In the view of exciton harvesting,
TADF hosts can lead to 100% exciton convergence on the T1
level of the phosphors via energy transfer. It is beneficial to
suppress concentration quenching and efficiency roll-offs.
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Scheme 1. Comparison on energy transfer and transition processes of conventional
high energy-gap hosts and TADF-featured hosts for blue phosphors. FRET and DET refer
to Förster resonance energy transfer and Dexter energy transfer, respectively. ISC and
RISC are intersystem crossing and reverse ISC, respectively.
Actually, TADF molecules have been used as hosts for blue,
green and yellow fluorescent and phosphorescent emitters.48-
54
In our most recent work, we have demonstrated that blue
Fig. 1. (a) Molecular design of DCDPOTZ as a compact D-A-A system; (b) Single-crystal
packing diagram of DPDPOTZ. Blue and yellow dash lines indicate the P=OꢀH
TADF dyes can be used as hosts to achieve the state-of-the-art
EQE of 22% from its yellow PHOLEDs.55 Nevertheless,
developing TADF hosts for blue OLEDs is more challenging,
since most of TADF molecules as donor-acceptor (D-A) systems
with CT-featured excited states, which are inferior in realizing
the T1 energy level higher than 2.9 eV. In this contribution, we
report an effective molecular design strategy of TADF hosts
with high T1 energy. On the basis of a ternary D-A-A structure,
DCDPOTZ reveals the typical TADF characteristics with the T1
hydrogen bond and edge-to-centroid
π-π interactions; (c) Single-crystal packing
diagram of DPDPOTZ. Besides blue lines for hydrogen bond, edge-to-centroid
π-π
interactions are marked with lines in other colours.
DPDPOTZ and DCDPOTZ can be readily synthesized
through two-step reactions with moderate yields of ~50%
(
Scheme 2). Their chemical structure characteristics were
established with NMR, MS and elemental analysis. Their
molecular configurations and intermolecular interactions in
solid state were further confirmed by single-crystal X-ray
diffraction (SCXRD) analysis (Fig. S1 and Fig. 1bc). It is showed
energy level of 2.93 eV and
ambipolar property.
difluorophenylpyridinato)
ꢀ
EST less than 0.3 eV, as well as the
Through adopting bis(4,6-
tetrakis(1-pyrazolyl)borate)
iridium(III) (FIr6) as dopant, besides the state-of-the-art EQE
up to 22.7%, the EQE roll-off of its blue PHOLEDs was reduced
to 11%, which is one of the lowest values reported so far.
that on the basis of a moderate P=OꢀH hydrogen bond and
an edge-to-centroid interaction between adjacent
π-π
molecules, DPDPOTZ can form one-dimensional chains for
directional carrier transport. On the contrary, DCDPOTZ
reveals four different molecular orientations, owing to the
markedly reduced intermolecular interactions by its large
steric hindrance. Nevertheless, the molecular packing of
DCDPOTZ is denser in three-dimensional space, accompanied
by four types of intermolecular interactions. In this sense, the
contrary effect of intermolecular interaction on quenching and
Results and discussion
Design, Structures and Thermal Properties
With the aim to develop a blue TADF host, one of the most
popular high-energy-gap hosts, 1,3-bis(9- carbazolyl)benzene
(mCP), was firstly chosen as the template. Then, its phenylene
carrier transportation is more balanced in DCDPOTZ
.
was displaced with a strongly electron-withdrawing triazine (TZ)
group. Finally, a diphenylphosphine oxide (DPO) group was
introduced at the meta-position of carbazole (Cz) groups to
The thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) show that the incorporation of rigid
Cz groups enhances the thermal stability of DCDPOTZ (Fig. S2
form the D-A-A structure of DCDPOTZ
. Besides further
and Table S1). The decomposition temperatures (Td) at 5%
weight loss of DCDPOTZ reaches to 412 oC, which is 70 oC
higher than that of DPDPOTZ. Furthermore, it is noted that the
enhancing intramolecular CT (ICT), DPO group can restrain
intermolecular interactions with its steric effect, which is one
of the main host characteristics for supressing collision-
induced quenching (Fig. 1a). Another unipolar host DPDPOTZ
containing two phenyls instead of Cz groups in DCDPOTZ was
also prepared for comparison.
o
melting point (Tm) of DCDPOTZ is as high as 292 C, which is
116 oC higher than that of DPDPOTZ. The inferiority of
DPDPOTZ should be ascribed to the absence of intermolecular
interactions between adjacent chains as shown in its single-
crystal packing diagram. On the contrary, the compact packing
mode of DCDPOTZ provides the weak but abundant
intermolecular interactions, which can establish the sufficient
carrier transport channels.
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DCDPOTZ; while, the intensity of the phenyl-attributed peaks
is in accord with the number of phenyls in these two
compounds. In comparison to DPDPOTZ, besides its Cz-
originated band, DCDPOTZ reveals an absorption tail from 350
to 400 nm, ascribed to its ICT transition. As designed,
according the absorption edge, the S1 energy level of DPDPOTZ
is estimated as high as 3.88 eV. In opposite, the formation of
low-energy CT states dramatically reduces the S1 energy level
of DPDPOTZ to 3.20 eV, establishing the perfect matching with
the S1 energy level of FIr6 (3.0 eV).
N
P
ii
i
N
N
N
O
Br
N
N
Cl
DPDPOTZ
N
N
P
N
iii
ii
N
N
N
N
N
O
N
N
N
H
Cl
DCDPOTZ
Scheme 2. Synthetic procedure of DPDPOTZ and DCDPOTZ: i. Mg, I2, THF, 60 oC, 1 h,
2,4,6-trichloro-1,3,5-triazine, room temperature, 12 h; ii. Diphenylphosphine, Pd(OAc)2,
DMF, 130 oC, 24 h; iii. n-BuLi, THF, 0 oC, 1 h, 2,4,6-trichloro-1,3,5-triazine, room
temperature, 12 h.
Theoretical Simulation
Density functional theory (DFT) simulation shows that the
HOMO and LUMO of mCP are mainly located on it electron-
donating Cz groups, revealing its feature of hole-predominant
injection and transport (Fig. 2 and Fig. S3). Meanwhile, the TZ-
involved HOMO and LUMO verify the electron-predominant
characteristics of DPDPOTZ. On the contrary, the HOMO and
LUMO of DCDPOTZ are mainly contributed by its Cz and TZ
groups, respectively, with limited overlaps. Its HOMO and
LUMO energy levels of -5.86 and -1.74 eV are just between
those of mCP and DPDPOTZ, manifesting its ambipolar
characteristics. The transition characteristics of DCDPOTZ were
further investigated with natural transition orbital (NTO)
analysis on the basis of time-resolved DFT method. For S0→S1
excitation, the highest occupied (HONTO) and the lowest
unoccupied NTO (HONTO) are respectively dispersed on Cz and
TZ groups, indicating the CT-featured S1 excited state of
DCDPOTZ. However, the situation for NTOs of S0→T1 excitation
is opposite that the HONTO and LUNTO are thoroughly
overlapped on Cz group, corresponding to the locally excited
state (LE). It was demonstrated that the D-A systems with CT-
featured S1 state and LE-type T1 state can also realize the TADF
emission through a two-step process as internal conversion
from 3ππ* to 3CT* and then RISC to 1CT*.56 Furthermore, in
contrast to the TADF systems with CT-type T1 states and
Fig. 2. Contours and energy levels of the highest occupied (HOMO) and the lowest
unoccupied molecular orbitals (LUMO) for mCP, DPDPOTZ and DCDPOTZ, and the
natural transition orbital (NTO) analysis results of DCDPOTZ. The HONTO and LUNTO
refer to the highest occupied and the lowest unoccupied NTOs, respectively. E, σ and f
are the excited energy level, associated weight and oscillator strength. The subscripts
“S” and “T” refer to singlet and triplet, respectively.
smaller
ꢀ
distributions could enlarge
EST, although the different singlet and triplet NTO
EST and weaken ISC and RISC
ꢀ
transitions, the higher stability of LE-feature T1 state is
beneficial to reduce triplet-involved quenching. In this sense,
regarding to excited-state characteristics, DCDPOTZ realizes a
good balance between TADF feature and host performance.
Correspondingly, in dilute solution, DPDPOTZ reveals the
ultraviolet emission with three peaks at 348, 365 and 380 nm,
assigned to its 0→0~2 transitions from the 0 vibration energy
level of S0 to 0~2 vibration energy levels of S1. The full widths
at half maximum (FWHM) of DPDPOTZ is only 54 nm, which is
consistent with its LE-featured excited-state transition. In
contrast to the narrow and multipeak fluorescence (FL) of
Photophysical Properties
In dilute solution (10-6 mol L-1 in CH2Cl2), the electronic
absorption spectrum of DCDPOTZ consists of three bands
peaked at 332, 282 and 228 nm, assigned to n→π* transitions
of Cz and TZ groups and π→π* transitions of phenyls (Fig. 3a
and Table S1). The TZ-originated absorption band in the
spectrum of DPDPOTZ is nearly overlapped with that of
DPDPOTZ
, the FL emission of DCDPOTZ is remarkably
broadened and shifted red with the FWHM of 78 nm and the
peak wavelength at 420 nm, giving rise to the structureless
profile, which further indicates its CT-featured radiative
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transition. The triplet transitions of DPDPOTZ and DCDPOTZ lifetime of 8.6
Journal Name
µ
s (Fig. 3b). In contrast to PLQY of 75% from
were investigated with the time-resolved phosphorescence mCP-hosted films, the efficient energy transfer from DCDPOTZ
(PH) spectra, which were record at 77 K after 100 s to get rid to FIr6 is further demonstrated by the blue emission peaked at
µ
of FL interference. It is noted that PH spectra of these two 460 and 488 nm with PLQY of 91% from FIr6-doped DCDPOTZ
molecules are almost overlapped in profile, reflecting their films, corresponding to ~100% apparent host-to-dopant
similar LE-featured T1 transitions in accord with TDDFT results. energy transfer efficiency, which was estimated according to
According to the 0→0 transitions between the 0 vibration the PLQY of FIr6 in dichloroethane as 96 ± 3% (Fig. S4).57
energy levels of S0 and T1, the T1 energy levels of DPDPOTZ and
Electrical Properties
DCDPOTZ are estimated as 2.95 and 2.93 eV, respectively. In
this case, the D-A-A structure of DCDPOTZ remarkably reduces
the S1 energy level without negative influence of the T1 energy
level, which should be attributed to the selective modulation
of its transition characteristics as indicated by TDDFT
simulation. As the result, the ꢀEST of DCDPOTZ is successfully
reduced to 0.27 eV, making RISC and TADF emission feasible.
Fig. 4. (a) Cyclic voltammograms of DPDPOTZ and DCDPOTZ measured at room
temperature with the scanning rate of 100 mV s-1; (b) Current density (J)-voltage
relationship of single-carrier transporting devices with configurations of ITO|LiF (5
nm)|Host (100 nm)|LiF (5 nm)|Al (100 nm) for electron-only and ITO|MoO3 (5
nm)|Host (100 nm)|MoO3 (5 nm)|Al (100 nm) for hole-only, respectively.
The cyclic voltammograms (CV) of DPDPOTZ and DCDPOTZ
were measured to figure out their electrochemical properties
and charge injection abilities (Fig. 4a and Table S1). DPDPOTZ
displays a single irreversible oxidation peak around 2.16 V with
onset voltage of 1.96 V, corresponding to the HOMO energy
level of -6.74 eV, which should be ascribed to the oxidation of
DPO group. Although DCDPOTZ also reveals a single-peak
anodic curve, which is remarkably wider as the mixture peaks
from Cz and DPO groups, the onset voltage of 1.51 V
corresponds to the HOMO energy level of DCDPOTZ as -6.29
eV characteristic of Cz-originated oxidation. On the contrary,
the cathodic curves of DPDPOTZ and DCDPOTZ are similar,
which consist of one reversible and one irreversible peak,
corresponding to the reduction of TZ and DPO groups,
respectively. Their TZ-originated peaks are almost overlapped
Fig. 3. (a) Absorption, fluorescence (FL) and time-resolved phosphorescence (PH)
spectra of DPDPOTZ and DCDPOTZ in dilute CH2Cl2 solution (10-6 mol L-1). PH spectra
were recorded at 77 K after a delay of 100 µs; (b) Steady-state photoluminescence (PL)
spectrum of the vacuum-evaporated DCDPOTZ-doped DPEPO films and time decay
curve of the DPDPOTZ (black) and DCDPOTZ (blue)-hosted films.
The films of DCDPOTZ in bis(2-(diphenylphosphino)phenyl)
ether oxide (DPEPO) matrix (T1 = 3.4 eV) were prepared
through vacuum evaporation, whose broad CT-featured
emission further shifts to 434 nm, accompanied by the
photoluminescence quantum yield (PLQY) of 92% (inset in Fig.
3b). In contrast to the ns-scale lifetime of DPDPOTZ-based film,
the time decay curve manifests the TADF-featured emission of
DCDPOTZ composed of a prompt fluorescence (PF) with a
lifetime of 45 ns and a delayed fluorescence (DF) with a
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with the identical onset voltages of -1.37 V, corresponding to based analogues when the driving voltage higher than 5 V,
the LUMO energy level of -3.41 eV, which is equal to those of which should be attributed to the HOMO and LUMO energy
TZ-based electron transporting materials.58
levels of mCP matched with those of FIr6 for more efficient
The carrier transporting abilities of DPDPOTZ and carrier and exciton confinement on the latter.
DCDPOTZ were evaluated by I-V characteristics of their
nominal single-carrier transporting devices with the
configurations of ITO|LiF (5 nm)|Host (100 nm)|LiF (5 nm)|Al
(100 nm) for electron-only and ITO|MoO3 (5 nm)|Host (100
nm)|MoO3 (5 nm)|Al (100 nm) for hole-only, respectively, in
which MoOx and LiF were hole- and electron-injecting layers,
respectively (Fig. 4b). In accord with the unipolar structure of
DPDPOTZ, it is rational that electron-only current density (J) of
DPDPOTZ-based devices was significantly larger than the hole-
only J by three orders of magnitude at the same voltages. The
one-dimensional chain structure of DPDPOTZ further enhances
its electron transporting ability. The involvement of Cz groups
in DCDPOTZ dramatically improved its hole-transporting ability,
which leads to the increased hole-only J of its devices by more
than two orders of magnitude. Furthermore, despite its TZ
group embedded by Cz groups, the electron J of DCDPOTZ
-
based devices was still comparable to that of DPDPOTZ-based
analogues, indicating the contribution of DPO group as
peripheral acceptor group in recovering electron transport.
Estimated with field-dependent space charge limited current
(SCLC) mode, the electron mobility of DPDPOTZ and DCDPOTZ
was comparable as 3.8 and 2.6
DCDPOTZ revealed the balanced carrier transporting ability
with the hole mobility of 1.8
10-5 cm2 V-1 s-1, which is two
orders of magnitude larger than that of DPDPOTZ (4.8
10-7
×
10-5 cm2 V-1 s-1. However,
×
×
cm2 V-1 s-1). As expected, DCDPOTZ achieves the ambipolar
characteristics on the basis of its D-A structure and effective
intermolecular interactions.
OLED Performance
To further verify the superiority of TADF hosts in blue OLEDs,
mCP, DPDPOTZ and DCDPOTZ were used as matrixes to
fabricated the blue PHOLEDs with a configuration of ITO|MoO3
(6 nm)|NPB (60 nm)|mCP (5 nm)| Host:FIr6 (10wt%, 20
nm)|DPDPOTZ (35 nm)|LiF (1 nm)|Al (100 nm), where 4,4’-
bis[N-(1-naphthyl)-N-phenylamino]-1,1’-biphenyl (NPB) and
DPDPOTZ were adopted as hole and electron transporting
layers, respectively, and mCP was simultaneously used as
exciton-blocking layer (Fig. 5a). All devices displayed the FIr6-
Fig. 5. (a) Energy level diagram of the OLEDs and the chemical structures of the
employed materials; (b) Luminance-Current density (J)-Voltage curves and EL spectra
originated blue EL emissions with the Commission (inset) of the devices; (c) Efficiencies vs. luminance relationship of the devices.
Internationale Ed I'eclairage (CIE) coordinates of (0.15, 0.25)
(inset in Fig. 5b and Table S2).
Furthermore, DCDPOTZ-based devices revealed the state-
of-the-art efficiencies with the maxima of 36.6 cd A-1 for
current efficiency (CE), 41.8 lm W-1 for power efficiency (PE)
and 22.9% for EQE, respectively, corresponding to ~100% IQE,
which were about 1.7 and 5.1 folds of those of mCP and
Owing to the balanced carrier injection and transport,
DCDPOTZ supported its devices with the lowest driving
voltages of 2.8, 4.3 and 6.3 V at 1, 100 and 1000 nits and the
highest luminance beyond 12000 nits, which are among the
best results of FIr6-based devices.47 The turn-on voltage was
already equivalent to the photon energy for FIr6 (2.75 eV). In
comparison to DCDPOTZ, the operation voltages of mCP and
DPDPOTZ-based devices were increased by 0.2~1.7 V,
accompanied by the maximum luminance reduced by 50~70%.
It is noteworthy that with the biggest J, the luminance of
DPDPOTZ-based analogues (Fig. 5c, Fig. S5 and Table S2). It is
noted that mCP and DPDPOTZ rendered the almost equal EQE
roll-offs beyond 30% at 1000 nits for their devices, due to not
only their unipolar characteristics but also their inefficient
energy transfer to FIr6. Significantly, with the superiority in
these two aspects, DCDPOTZ endowed its devices with the
extremely low efficiency roll-offs. At 1000 nits, the EQE of its
DPDPOTZ-based devices was even lower than that of mCP
-
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devices still maintained 20.3%, corresponding to roll-off of 11%, (d, J = 7.6 Hz, 4H), 8.085-8.033 (m, 4H), 7.649-7.599 (m, 4H),
which is among the lowest values to date for blue PHOLEDs.13 7.568-7.530 ppm (m, 8H); GC-MS: m/z(%): 433 (100) [M+];
It is notable that among these three hosts DCDPOTZ and mCP elemental analysis(%) for C27H20N3OP: C 74.82, H 4.65, N 9.69;
are superior in energy transfer to FIr6 and direct charge and found: C 74.84, H 4.66, N 9.73. CCDC No. 1838785.
exciton confinement on FIr6, respectively; while, DPDPOTZ has
no advantages in these two aspects. In this case, the order of Ar, carbazole (3.64 g, 22 mmol) was dissolved in 44 mL of
EL performance as DCDPOTZ mCP
DPDPOTZ solidly anhydrous THF, and then cooled to 0 oC. n-BuLi (8.8 ml, 22
manifests the predominance of host-dopant energy transfer mmol was added dropwise into the solution at 0 oC for 1 h and
9,9'-(6-Chloro-1,3,5-triazine-2,4-diyl)bis(9H-carbazole): In
>
>
for developing high-efficiency blue PHOLEDs.
stirred for an addition hour. The mixture was then added
dropwise into the solution of 2,4,6-trichloro-1,3,5-triazine
(1.84 g, 10 mmol) in 20 mL of anhydrous THF at
0
oC. After
o
Conclusions
addition, the system was heated to 80 C and stirred for 12 h.
After cooling to room temperature, the reaction was quenched
with water. The mixture was exacted with CH2Cl2 (3×10 mL).
The organic phase was combined and dried with anhydrous
Na2SO4. The solvent was removed in vacuum. The residue was
purified by column chromatography over silica gel to afford
white powder (2.45 g) with a yield of 55%. 1H NMR (TMS,
In summary, DCDPOTZ with the feature of TADF was
developed to manifest the superiority of TADF hosts in high-
efficiency blue PHOLEDs. The excellent comprehensive EL
performance of DCDPOTZ is the combined result of its
ambipolar characteristics, 100% host-to-dopant energy
transfer efficiency and effective quenching suppression, the
first two factors of which stem from its TADF feature. This
work demonstrates a feasible strategy to overcome the
bottleneck of OLEDs in blue region, through integrating the
advantages of TADF and PH technologies.
CDCl3, 400 MHz): δ = 8.916 (d, J = 8.0 Hz, 4 H)
，8.053 (d, J =
7.6 Hz, 4H) 7.509 (t, J = 8.0 Hz, 4 H), 7.439 ppm (t, J = 8.0Hz,
，
4 H); GC-MS: m/z(%): 445 (100) [M+]; elemental analysis(%) for
C27H16ClN5: C 72.73, H 3.62, N 15.71; found: C 72.74, H 3.62, N
15.75.
9,9'-(6-(Diphenylphosphoryl)-1,3,5-triazine-2,4-diyl)bis(9H-
carbazole) (DCDPOTZ): DCDPOTZ was synthesized according to
the same procedure of DPDPOTZ, except for using 9,9'-(6-
Experimental
All the reagents for synthesis were purchased from Alfa and
used directly without further purification.
Chloro-1,3,5-triazine-2,4-diyl)bis(9H-carbazole) (0.445 g,
1
mmol) instead of 2-Chloro-4,6-diphenyl-1,3,5-triazine. The title
compound was obtained as yellowish white powder (0.30 g)
1
with a yield of 49%. H NMR (TMS, CDCl3, 400 MHz):δ = 8.667
2-Chloro-4,6-diphenyl-1,3,5-triazine: In Ar, bromobenzene (2.2
mL, 22 mmol) diluted in 20 mL of anhydrous THF was added
dropwise into a mixture of Mg (0.5 g, 22 mmol), I2 (10 mg) and
10 mL of anhydrous THF. After initiation, the system was
(d, J = 7.6 Hz, 4H), 8.142-8.091 (m, 4H), 8.048 (dd, J 1 = 8.0 Hz, J 2=
1.2 Hz, 4H), 7.728 (td, J 1= 7.6 Hz, J 2 = 1.2 Hz, 2H), 7.635 (td, J 1=
8.0 Hz, J 2 = 2.8 Hz, 4H), 7.436-7.347 ppm (m, 8H); LDI-TOF MS:
m/z(%): 611 (100) [M+]; elemental analysis (%) for C39H26N5OP:
C 76.59, H 4.28, N 11.45; found: C 76.59, H 4.27, N 11.47. CCDC
No. 1838786.
stirred at 60 oC for 1 h and then cooled to 0 C. The generated
o
Grignard reagent was further added dropwise into a solution
of 2,4,6-trichloro-1,3,5-triazine (1.84 g, 10 mmol) in 20 mL of
anhydrous THF at 0 oC. After reacted at room temperature for
12 h, the reaction was quenched with water. The mixture was
exacted with CH2Cl2 (3×10 mL). The organic phase was
combined and dried with anhydrous Na2SO4. The solvent was
removed in vacuum. The residue was purified by column
chromatography over silica gel to afford white powder (1.73 g)
with a yield of 65%. 1H NMR (TMS, CDCl3, 400 MHz): δ = 8.635
(d, J = 8.0 Hz, 2 H), 8.517 (d, J = 8.0 Hz, 2H), 7.681-7.617(m, 2
H), 7.576-7.520 ppm (m, 4 H); GC-MS: m/z(%): 267 (100) [M+];
elemental analysis(%) for C15H10ClN3: C 67.30, H 3.77, N 15.70;
found: C 67.32, H 3.78, N 15.74.
Acknowledgements
This project was financially supported by Changjiang Scholar
Program of Chinese Ministry of Education (Q2016208), NSFC
(21672056, 61605042 and 21602048), National Postdoctoral
Program for Innovative Talents (BX201600048), Science and
Technology Bureau of Heilongjiang Province (JC2015002) and
Harbin Science and Technology Bureau (2015RAYXJ008).
2-(Diphenylphosphoryl)-4,6-diphenyl-1,3,5-triazine
(
DPDPOTZ): In Ar, 2-chloro-4,6-diphenyl-1,3,5-triazine (0.27 g,
Notes and references
1 mmol), diphenylphosphine (0.35 mL, 2 mmol), Pd(OAc)2 (3
mg, 0.01 mmol) was dissolved in 5 mL of anhydrous DMF. The
mixture was heated to 130 oC and stirred for 24 h. After
cooling to room temperature, the mixture was poured into
water and exacted with CH2Cl2 (3×10 mL). The organic phase
was combined and dried with anhydrous Na2SO4. The solvent
was removed in vacuum. The residue was purified by column
chromatography over silica gel to afford white powder (0.25 g)
with a yield of 57%. 1H NMR (TMS, CDCl3, 400 MHz): δ = 8.656
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A TADF-featured host supported deep-blue PHOLEDs with EQE of 22.9% for
maximum and 20.3% at 1000 nits.