Exciplex Formation and Electromer Blocking for Highly Efficient Blue Thermally Activated Delayed Fluorescence OLEDs with All-Solution-Processed Organic Layers

Article abstract of DOI:10.1002/chem.201904415

Highly efficient solution-processable emitters are greatly desired to develop low-cost organic light-emitting diodes (OLEDs). The recently developed thermally activated delayed fluorescence (TADF) materials are promising candidates, but blue TADF materials compatible with the all-solution-process have still not been achieved. Here, a series of TADF materials, named X-4CzCN, are developed by introducing the bulky units through an unconjugated linker, which realizes high molecular weight to enhance the solvent resistance ability without disturbing the blue TADF feature. Meanwhile, the peripheral wrapping groups efficiently inhibit the triplet–triplet and triplet–polaron quenching by isolating the energy-transfer and charge-transporting channels. The photophysical measurements indicate that a small variation in peripheral unit will have a noticeable effect on the luminescence efficiency. The enlarged volume of peripheral units will make the electroluminescent spectra blueshift, while enhancing the energy transfer of exciplex and blocking the energy leakage of electromer can facilitate the exciton utilization. As a result, the fully solution-processed blue OLED achieves a CIE of (0.16, 0.27), a low turn on voltage of 2.9 eV, and a high external quantum efficiency of 20.6 %. As far as we known, this is the first report of all-solution-processed TADF OLEDs with blue emission, which exhibits a high efficiency even comparable to the vacuum-deposited devices.

Full text of DOI:10.1002/chem.201904415

A Journal of
Accepted Article
Title: Exciplex Formation and Electromer Blocking for Highly Efficient
Blue Thermally Activated Delayed Fluorescence OLEDs with All-
Solution-Processed Organic Layers
Authors: Xinxin Ban, Feng Chen, Jie Pan, Yan Liu, Aiyun Zhu, Wei
Jiang, and Yueming Sun
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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To be cited as: Chem. Eur. J. 10.1002/chem.201904415
Link to VoR: http://dx.doi.org/10.1002/chem.201904415
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Exciplex Formation and Electromer Blocking for Highly Efficient
Blue Thermally Activated Delayed Fluorescence OLEDs with All-
Solution-Processed Organic Layers
Xinxin Ban,*[a, b] Feng Chen, Jie Pan, Yan Liu, Aiyun Zhu, Wei Jiang* and Yueming Sun
[a]
[a]
[a]
[a]
[b]
[b]
Abstract: Highly efficient solution-processable emitters are greatly
desired to develop low-cost organic light-emitting diodes (OLEDs).
The recently developed thermally activated delayed fluorescence
behind its vacuum-deposited counterparts due to the lack of
functional TADF emitters, especially the blue one.
The molecular design plays a critical role in achieving highly
efficient solution-processable TADF materials. One of the useful
methods is the exploitation of small molecular or polymer TADF
emitters with high solubility in aromatic solvents and good film
forming ability through wet-process.[4] By using these soluble
TADF materials as emitters, solution-processed OLEDs with the
EQE around 20.0% have been successfully achieved.[ However,
due to the severe aggregation induced exciton quenching (ACQ)
effect, these small molecular or polymer TADF emitters must be
doped in appropriate host materials to achieve high
electroluminescence efficiency.[ Unfortunately, the doping
structure with small molecular hosts make the device not able to
be fabricated by fully solution-process, because the solvent used
to deposit upper layer will dissolve the bottom layers beneath it.[
Thus, most of the reported solution-processed TADF OLEDs
usually contain a solution-processed emission layer (EML) and a
vacuum-deposited electron transporting layer (ETL).[8] In fact, the
advantage of solution-processed devices for low-cost and large-
size OLEDs require all the organic materials to be solution-
processed.[9] Normally, to design TADF emitters compatible with
fully solution-process, the multifunctional properties must be
centralized into one molecule, such as (1) good film forming ability
through solution-process, (2) excellent resistivity toward
orthogonal solvent, and (3) restricted exciton quenching effect in
the congested non-doped film state.[10] However, it is difficult to
fulfill the tradeoff between blue TADF and solvent resistance,
which require material not only enlarge π-conjugation and
molecular weight to resist the solvent erosion, but also keep the
small ΔEST and blue emission unchanged. Unfortunately, the fully
solution-processed TADF blue OLEDs have been still unable to
be achieved due to the lack of such multifunctional TADF blue
emitters until now. Therefore, the developing of novel blue TADF
emitters compatible with fully solution-process and deeply
investigating the relationship between molecular structure and
device performance is an urgent problem to be solved.
(TADF) materials are promising candidates, but blue TADF materials
compatible with the all-solution-process still not be achieved. Here, a
serial of TADF materials, named X-4CzCN, are developed by
introducing the bulky units through unconjugated linker, which realizes
high molecular weight to enhance the solvent resistance ability
without disturbing the blue TADF feature. Meanwhile, the peripheral
wrapping groups efficiently inhibit the triplet-triplet and triplet-polaron
quenching by isolating the energy transfer and charge transporting
channels. The photophysical measurements indicate that a small
variation in peripheral unit will have a noticeable effect on the
luminescence efficiency. The enlarged volume of peripheral units will
make the electroluminescent spectra blue-shift, while enhancing the
energy transfer of exciplex and blocking the energy leakage of
electromer can facilitate the exciton utilization. As a result, the fully
solution-processed blue OLED achieves a CIE of (0.16, 0.27), a low
turn on voltage of 2.9 eV and a high external quantum efficiency of
5]
6]
7]
20.6%. As far as we known, this is the first report of all-solution-
processed TADF OLEDs with blue emission, which exhibits a high
efficiency even comparable to the vacuum-deposited devices.
Introduction
Thermally activated delayed fluorescent (TADF) materials have
been extensively investigated due to their potential ability to
harvest 100% internal quantum efficiency through a reverse
intersystem crossing (RISC) process from the triplet state to the
singlet state.[1] Despite being a new technology, the maximum
EQE values of vacuum-deposited TADF organic light emitting
diodes (OLEDs) are over 30%, which is comparable to the state-
of-the-art phosphorescent OLEDs.[2] Recently, the intensive
researches have been devoted to the development of solution-
processed TADF OLEDs, which is cost effective both in
[3]
preparation technics and material systems. However, the device
performance of solution-processed TADF OLEDs are still far
In this work, we present three solution-processable blue TADF
emitters, MCz-4CzCN, PhCz-4CzCN and PhDCz-4CzCN, by
utilizing 4CzCN as emissive core, which is chosen for its clear
TADF feature with small ΔEST and efficient triplet harvesting.
Photophysical investigation indicates that the introduced bulky
units can effectively reduce the intermolecular interactions
between the TADF cores, which ensure them to be used in non-
doped devices. Meanwhile, the increased molecular weight
makes the materials possess good resistivity toward orthogonal
solvent, which enable the fabrication of fully solution-processed
OLEDs. Furthermore, the detail relationship between molecular
structures and EL performances indicates that the increased
steric volume of wrapping units will make the electroluminescent
[
[
a]
b]
Dr. X. X. Ban, Dr. F. Chen, Dr. J. Pan, Dr. Y. Liu, Dr. A. Y. Zhu
Jiangsu Key Laboratory of Function Control Technology for
Advanced Materials, School of Chemical Engineering,
Jiangsu Ocean University, Jiangsu, 222005, China
E-mail: banxx@hhit.edu.cn
Title(s), Initial(s), Surname(s) of Author(s)
School of Chemistry and Chemical Engineering,
Southeast University, Nanjing, 211189, P. R. China
E-mail: jiangw@seu.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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Chemistry - A European Journal
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FULL PAPER
spectra blue-shift, while the blocking of energy leakage from
TADF core to the peripheral electromer state is quite important for
enhancing the exciton utilization. By using these new designed
materials as emitters, fully solution-processed blue TADF OLEDs
are successfully fabricated for the first time with high efficiency
and stable emission. Particularly, PhCz-4CzCN based device
achieves the considerable high performance with the current,
-
1
-
power and external quantum efficiency of 40.8 cd A , 27.5 lm W
1
and 20.6%, respectively, which are comparable to the reported
devices with vacuum-deposited ETL. Importantly, the turn on
voltage was only 2.9 V and the EQE remains as high as 20.0%
-2
and 18.2% at the luminescence of 100 and 1000 cd m ,
respectively. The extremely low turn-on voltage and small
efficiency roll-off proves the superiority of exciplex formation in
enhancing the charge injection and exciton utilization. This
research provides reference for the further design of more
efficient TADF materials for fully solution-processed OLEDs.
Scheme 1. The chemical structure of the encapsulated TADF emitter and its
emissive core.
Results and Discussion
steric shields to reduce the interaction between emission cores,
but also play the role of hole transporting groups in the molecule.
The calculated HOMO and LUMO energy levels are -5.12/-1.87,
Synthesis and characterization: Scheme
1 exhibits the
chemical structures of the designed TADF emitters and the
detailed synthetic routes of the materials are shown in Scheme
S1. The emissive core 4CzCN was synthesized in one-step
reaction from a commercially available starting material 2,3,5,6-
tetrafluorobenzonitrile with 4-methoxy-9H-carbazole. According
to the same strategy, the other three encapsulated compounds
were synthesized from the corresponding branch frameworks.
The peripheral units, MeCz, PhCz and PhDCz were achieved by
connecting 4-methoxy-9H-carbazole to the halogenated
compounds with different molecular-sized through Ullmann
reaction. After demethylating by boron tribromide, the bulky units
were combined with 4-(6-bromohexyloxy)-9H-carbazole by
nucleophilic substitution. Finally, MeCz-4CzCN, PhCz-4CzCN
and PhDCz-4CzCN were easily synthesized through aromatic
nucleophilic substitution reaction with high yields above 80%. The
-
5.14/-1.88 and -5.21/-1.92 eV for MeCz-4CzCN, PhCz-4CzCN
and PhDCz-4CzCN, respectively (Table 1). Additionally, the
HOMO levels of wrapping units are calculated to be -5.13, -5.14
and -5.22 eV for MeCz, PhCz and PhDCz, respectively.
Comparing with HOMO energy level of the emissive core (-5.24
eV for 4CzCN), the HOMO levels of the encapsulated emitters are
similar to those of wrapping units, which further demonstrate that
the hole-injection and transporting capabilities would be
dominated by the peripheral units. The oscillator strength for
-
absorption (f) are calculated to be 6.34, 7.36, 7.51 and 7.56 × 10
2
for 4CzCN, MeCz-4CzCN, PhCz-4CzCN and PhDCz-4CzCN,
respectively. The enhanced
f
also demonstrates that
encapsulated molecules possess the larger steric hindrance by
the introduction of bulky wrapping units, which can suppress the
geometries changes of the excited states and reduce non-
radiative deactivation.[11]
molecular structures of the newly designed emitters were
characterized by 1H-NMR, 13C-NMR, mass spectrometry and
The calculation of singlet and triplet energies were also
performed by using time-dependent DFT (TD-DFT). The
calculated singlet and triplet energies of MeCz-4CzCN, PhCz-
elemental analysis.
Theoretical calculation: To understand the electronic property
of the compounds, theoretical calculations based on density
functional theory (DFT) at the b3lyp/6-31g(d) level were
performed for the emission core and encapsulated molecules.
The calculated electron cloud distribution and geometrical
structure are displayed in Figure 1. As can be seen, the whole
molecule can be divided into three functional fragments: (1) the
parent core ensures the emissive properties of the molecule; (2)
the peripheral bulky units offer the steric shielding effect to reduce
the intermolecular interaction of the emissive cores; (3) the
flexible alkyl chains act as non-conjugated linker to connect the
emission and wrapping units. The lowest unoccupied molecular
orbital (LUMOs) are mainly centered on the benzonitrile moieties
of the emission core, while the highest occupied molecular orbital
4
CzCN and PhDCz-4CzCN are all to be 2.64 and 2.74 eV, which
are identical to that of 4CzCN. As expected, an extremely small
ΔEST of 0.10 eV is achieved for the molecules, which would lead
to an effective RISC process and benefit the utilizations of triplet
excitons. Meanwhile, the singlet and triplet energies of the
peripheral units are also calculated. As can be seen from Table
1 1
S1, the S /T of MeCz, PhCz and PhDCz are 4.09/3.14, 3.99/3.15,
and 3.84/3.14 eV, which are much higher than that of emissive
core. Thus, the designed wrapping units can efficiently suppress
the back energy transfer from the emissive core to the peripheral
groups. To demonstrate exciton confinement effect, the spin-
1
density distribution (SDD) at T state was further calculated. The
SDDs of the encapsulated molecules are only distributed in the
emission core, no triplet spin-densities are dispersed to peripheral
units. Thus, the
(
HOMOs) are mainly delocalized over the peripheral carbazolyl
moieties, which indicates that the peripheral encapsulated units
not only act as
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units. The blue shift emission peak and reduced half peak width
indicate that the intermolecular aggregations of emission units are
inhibited by efficient molecular encapsulation.
Figure S1 depicts the phosphorescence (77 K) spectra of the
emitters and encapsulated units in toluene solution. The triplet
energy (T ) of MeCz-4CzCN, PhCz-4CzCN and PhDCz-4CzCN
1
are calculated to be 2.67 eV from the onset of the
phosphorescence spectra, which is equal to the emissive core
4
CzCN. As estimated from the first peak of phosphorescence
spectra, the peripheral units, MeCz, PhCz and PhDCz, have the
same triplet energy of 3.02 eV, which is high enough to suppress
the reverse energy transfer and locate the triplet excitons in the
emission core. The singlet energy (S
calculated to be 2.74 eV based on the fluorescence spectra at 300
K. Therefore, according to the energy-splitting of S and T , the
1
) of the emitters are
1
1
ΔEST of MeCz-4CzCN, PhCz-4CzCN and PhDCz-4CzCN are
calculated to be 0.22 eV. The PLQY of 4CzCN, MeCz-4CzCN,
PhCz-4CzCN and PhDCz-4CzCN are measured to be 0.36, 0.53,
Figure 1. Optimized HOMO, LUMO and SDD density maps for X-4CzCN.
0
.62 and 0.65, respectively, in neat films at room temperature.
introduced bulky units around the emissive core will act as
envelopes to effectively inhibit the intermolecular interaction in
The enhanced PLQY of the encapsulated emitters can be
attributed to the efficient wrapping effect by the introduced
peripheral units, which eliminates the collision-induced
intramolecular non-radiation transitions and bimolecular
processes. Furthermore, the orderly increased PLQY of MeCz-
excited T
1
states. In contrast, the exposed triplet distribution of
4
CzCN will lead to the severe triplet-triplet interaction induced
exciton quenching in the tight stacking state, which is detrimental
to obtain high efficiency in non-doped device. Moreover, the
calculated molecular weight of MeCz-4CzCN, PhCz-4CzCN and
PhDCz-4CzCN are 1943.9, 2191.9, 2852.2, respectively, which
are much higher than that of 4CzCN (833.3). All the above
theoretical results prove that the introduction of bulky unit by
unconjugated alkyl chain can efficiently increase the molecular
weight without disturbing the excited energy and energy-splitting
property of the emitter.
4
CzCN, PhCz-4CzCN and PhDCz-4CzCN indicates that the
larger the skeleton-volume of the wrapping units, the better the
encapsulation effect.
Photophysical properties: The UV-vis absorption and
photoluminescence (PL) spectra of MeCz-4CzCN, PhCz-4CzCN
PhDCz-4CzCN and their parent core 4CzCN in toluene solution
–5
(
10 M) are measured at room temperature. As shown in Figure
a, the absorption spectra of the encapsulated emitters are similar
to the parent core 4CzCN. The low-energy broad bands between
70 and 430 nm were attributed to the charge transfer (CT)
2
3
excitations of the emissive core, while the strong absorption
peaks at 280 and 330 nm can be assigned to the n–π* and π–π*
transitions of the carbazolyl units. Thus, when the CT absorption
intensity is fixed, the characteristic absorption peaks of carbazole
are enhanced with the increased carbazolyl units. The optical
energy gaps of MeCz-4CzCN, PhCz-4CzCN and PhDCz-4CzCN
are all estimated to be 3.10 eV according to the absorption edges,
which are equal to that of 4CzCN. Moreover, the emission peaks
of the three encapsulated emitters are 452 nm, which is also
identical to 4CzCN. Therefore, it can be concluded that the
introduction of MeCz, PhCz and PhDCz groups through non-
conjugated alkyl chains did not affect the ground or emissive
excited state energies of the emissive core, which is consistent
with the theoretical calculation. However, the photophysical
properties of the molecules in condensed state are changed. The
PL spectra in films are blue shift from 478 nm for 4CzCN to 470
nm for MeCz-4CzCN, PhCz-4CzCN and PhDCz-4CzCN.
Furthermore, the half peak widths of the spectra are gradually
diminished with the increased skeleton-volume of the wrapping
Figure 2. (a) Ultraviolet-visible (UV-vis) absorption and photoluminescence (PL)
spectra in toluene solution; (b) PL spectra in film state. (c) Cyclic voltammogram
2 2
of 4CzCN and X-4CzCNmin CH Cl solution; (d) Transient PL decay
characteristics of the 4CzCN and X-4CzCN films.
Electrochemical properties: The electrochemical properties of
the materials were examined by cyclic voltammetry with ferrocene
as the reference. As shown in Figure 2c, all the materials exhibit
multiple irreversible oxidation curves. According to the onset of
the oxidation peak, the HOMO energy levels of 4CzCN, MeCz-
4
5
CzCN, PhCz-4CzCN and PhDCz-4CzCN are estimated to be -
.51, -5.33, -5.36 and -5.41 eV, respectively. The HOMO levels of
the encapsulated molecules are smaller than their emissive core,
which means the introduced wrapping units really influence the
frontier orbitals. The electrochemical properties of the
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Table 1. Physical properties of 4CzCN and X-4CzCN.
λ
ems
E
[eV]
g
S
[eV]
1
/T
1
∆EST
[eV]
HOMO
[eV]
LUMO
[eV]
f
PLQY
[%]
-
2
[
nm]
[10 ]
a
3.37c
2.84d
3.25c
2.74/2.64c
2.90/2.68e
2.74/2.64c
0.10c
0.22e
0.10c
-5.24c
-5.51f
-1.87c
-2.67g
-1.87c
4
4
4
4
4
4
4
4
52
78
52
73
52
71
52
69
4
CzCN
6.34
0.36
0.53
0.62
0.65
b
a
b
a
b
a
b
c
-5.12
MeCz-4CzCN
PhCz-4CzCN
PhDCz-4CzCN
7.36
7.51
7.56
d
e
e
-5.33f
-5.14c
-5.36f
g
2.81
2.90/2.68
2.74/2.64
2.90/2.68
2.74/2.64
2.90/2.68e
0.22
-2.52
-1.88
-2.55
-1.92
3.26c
2.81d
3.29c
c
0.10
0.22
c
c
e
e
g
c
c
c
c
0.10
0.22e
-5.21
d
-5.41f
g
2.83
-2.58
a
b
c
d
Measured in toluene solution at 300 K. Measured in deposited films at 300 K. Obtained from Gauss simulation. Estimated from the absorption edges in
e
f
g
Toluene. Estimated from fluorescent and phosphorescent spectra. Determined by the CV measurement. Calculated from the energy gap and HOMO level.
wrapping units are also investigated and the HOMO levels of
MeCz, PhCz and PhDCz are measured to be -5.30, -5.39 and -
wrapping units will have a noticeable effect on the encapsulation
effect.
5
.44 eV, respectively (Figure S2). It clearly can be seen that the
Thermal and morphological properties: Figure S5 shows the
thermal analysis data of MeCz-4CzCN, PhCz-4CzCN and
PhDCz-4CzCN. The glass transition temperatures (T ), measured
g
through differential scanning calorimetry (DSC), are 101, 115 and
148 °C for MeCz-4CzCN, PhCz-4CzCN and PhDCz-4CzCN,
g
respectively. The high T is important for the formation of uniform
HOMO levels of the encapsulated compounds are basically equal
to their corresponding wrapping units and change as the
molecular structure changes. Thus, the peripheral donor units will
dominate the hole transporting of the molecules. Based on the
optical energy band gaps, the LUMO energy levels of 4CzCN,
MeCz-4CzCN, PhCz-4CzCN and PhDCz-4CzCN are calculated
to be -2.67, -2.52, -2.55 and -2.58 eV, respectively. These
experimentally calculated results are in agreement with the
previous estimated ones from Gaussian simulation.
amorphous films after annealing in solution-process, which will
facilitate the improvement of device efficiency and long-term
stability. The decomposition temperatures (T
determined through thermogravimetric analysis (TGA). According
to 5% weight loss, the T of MeCz-4CzCN, PhCz-4CzCN and
PhDCz-4CzCN were 399, 406, and 416 °C, respectively. It is
noteworthy that the T and T are sequential increased with the
d
) were also
Transient PL properties: The transient PL decay of these
compounds are measured in film state. As shown in Figure 2d,
d
the prompt lifetimes (τ
CzCN, PhCz-4CzCN and PhDCz-4CzCN are assigned to
conventional fluorescence decay, whereas the delayed lifetimes
) of 1.4, 2.6, 3.3, and 3.5 μs are caused by the reverse
p
) of 10, 13, 13 and 15 ns for 4CzCN, MeCz-
d
g
4
enlarged peripheral bulky units, which imply that the thermal
properties of the encapsulated molecules can strongly enhanced
by adjusting the functional groups introduced at the peripheral
(τ
d
intersystem crossing process of triplet excited state. The clearly
observed nanosecond-scale prompt and microsecond-scale
delayed components at room temperature confirm their TADF
characteristics. Moreover, time-resolved PL spectra of X-4CzCNs
illustrated in Figure S4 shows the identical prompt and delayed
PL spectra, which also proves the TADF emission of these
compounds. Compared with the unprotected emissive core, the
encapsulated emitters show delayed components with longer
lifetimes due to the reduced ICT effect and dipole-dipole
interactions between the emissive fragments. Based on the PLQY
and the lifetime data, the photoluminescence quantum efficiency
d g
position. As a result, the high T and T values resulting from the
bulky substituent of carbazoles and the nonplanar structures
demonstrate the good thermal stability of these newly designed
encapsulated compounds.
of the delayed part (Φ
0
4
d
) are calculated to be 0.23, 0.47, 0.56 and
.58 for 4CzCN, MeCz-4CzCN, PhCz-4CzCN and PhDCz-
CzCN, respectively (Table S2). The increased Φ with the
d
volume of the peripheral units indicates that the larger steric
wrapping units will facilitate to the reducing of triplet-triplet
annihilation. Intriguingly, despite the same ∆EST, the rate of ISC
process (kRISC) of MeCz-4CzCN, PhCz-4CzCN and PhDCz-
5
-1
4
CzCN are calculated to be 0.90-1.23 × 10 s , which are higher
5
-1
than that of 4CzCN (0.42 × 10 s ). Besides, the efficiency of the
RISC (ΦRISC) of the encapsulated molecules are also enhanced
comparing the emissive core, and the bigger steric shields will
result in larger RISC efficiency. Therefore, the introduction of
wrapping units to the emissive core can significantly increase the
PL efficiency of the molecule, and a small variation in the size of
Figure 3. Device structure and fabrication process of the fully-solution-
processed OLEDs.
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Chemistry - A European Journal
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organic layers (HIL, EML and ETL) in the devices are fabricated
through solution-process. The OLEDs containing each of the
three encapsulated emitters, MeCz-4CzCN, PhCz-4CzCN and
PhDCz-4CzCN, are denominated as Device A, Device B and
Device C, respectively. In addition, we also fabricated a reference
device (Device D), which contains the parent core 4CzCN as
emitter to estimate the other device results. Figure 4a and b show
current density–voltage–luminance (J-V-L) curves and EQE-
luminance curves. Table 2 summarize the electroluminescent
(
EL) data of the devices. All devices showed the blue EL spectra,
which indicates the emission from emissive core. The Device A
based on MeCz-4CzCN exhibits a low turn-on voltage of 2.9 V at
-2
the luminescence of 1 cd m . The maximum current efficiency
(
(
CE), power efficiency (PE) and external quantum efficiency
EQE) of Device A are 46.5 cd A , 35.6 lm W and 21.8%,
-1
-1
Figure 4. (a) Current density-voltage-luminance (J-V-L) characteristics; (b)
External quantum efficiency versus current density plots; (c) EL spectra of the
devices and the EL color coordinates on the CIE 1931 chromaticity diagram; (d)
Photographs of the blue EL emission from X-4CzCN based devices.
respectively. The maximum luminescence of Device A is above
14200 cd m . Moreover, the efficiencies still remain as high as
-2
-
1
-1
41.2 cd A , 25.8 lm W and 19.0% at the high luminescence of
-2
1000 cd m , which fully demonstrate the superiority of
encapsulated TADF emitter in solution-processed OLEDs.
Nevertheless, the EL spectrum of Device A is not blue enough
with a peak of 486 nm and a Commission Internationale de
L’Eclairage (CIE) coordinates of (0.17, 0.35).
In order to investigate the film forming ability, the morphologies
of the spin-coating films were characterized by atomic force
microscopy (AFM). As shown in Figure S6, the surface of MeCz-
4
CzCN, PhCz-4CzCN and PhDCz-4CzCN are quite smooth with
the root-mean-square (RMS) values in the range of 0.39-0.52 nm.
Although 4CzCN based film also do not exhibit any pinholes after
annealing, the RMS value of 4CzCN film is 1.02 nm, which is a
little high than that of encapsulated materials. The above results
fully prove that the introduced wrapping units by flexible linkers
can enhance the film forming ability of the materials, which
facilitates the formation of amorphous defect-free films through
solution process. Since the molecules are designed for multilayer
solution-processed OLEDs, the solvent resistant abilities of the
materials are tested. Figure S7 shows the UV-Vis absorption
spectra of the corresponding films before and after rinsing
process by isopropanol. The spectral intensity of the MeCz-
Similar to Device A, the Device B based on PhCz-4CzCN also
has a low turn on voltage of 2.9 V, a high luminescence of 18800
-2
cd m , and high device efficiency with the maximum CE, PE and
-1
-1
EQE of 40.8 cd A , 27.5 lm W and 20.6%, respectively.
Importantly, the EL emission peak of Device B is blue shift to 475
nm with a CIE of (0.16, 0.27) due to the enlarged wrapping units,
which can further isolate the emission core and reduce the
aggregation states. Moreover, the efficiency roll-off of Device B
was further diminished especially at high luminance with high
current density, which can be attributed to the more efficient
wrapping effect of PhCz units. Although Device C exhibits a
further blue shift emission with EL peak at 465 nm due to the
larger bulky unit of PhDCz, the device efficiencies of Device C are
4
CzCN, PhCz-4CzCN and PhDCz-4CzCN remained almost
-1
-1
similar to that of pristine films, which indicates that the
encapsulated molecules with high molecular weight possess high
resistance to alcohol. In contrast, the 4CzCN film was severely
dissolved by alcohol, because only less than half of the absorption
intensity was left. Therefore, it can be predicted that by using
these encapsulated materials as non-doped emitter for fully
solution-processed OLEDs, the orthogonal solvent will not
damage the preceding emission layer during the sequential
deposition alcohol-soluble electron transporting layer (ETL).
Electroluminescence properties: The OLEDs with fully
solution-processed organic layers are fabricated and evaluated,
Figure 3 shows the device structure and fabrication process. The
optimal device architecture consisted of ITO/PEDOT:PSS (20
moderate with CE, PE and EQE of 18.9 cd A , 11.2 lm W and
10.0%, respectively. To the best of our best knowledge, these
devices are the first report of blue-emitting fully solution-
processed TADF-OLEDs, which demonstrates that the molecular
encapsulation is a practical way to enable the molecule to
withstand the sequential orthogonal solvent possess without
sacrificing their parent TADF nature. Besides, the different
efficiency of Device A, B and C indicates that the molecular
engineering of wrapping units is critical for achieving high
performance encapsulated TADF emitters.
Figure S8 shows the EL spectra of the devices at different
applied voltages and their corresponding device pictures. Since
the parent material 4CzCN do not withstand the solvent erosion,
the luminous point of Device D was destroyed. Thus, the detection
of device efficiency of Device D is unavailable. Fortunately, the
spectrum test can still be performed. As shown in Fig 4c, the EL
spectrum of Device D was peaked at 497 nm and the CIE at 100
mA m-2 was (0.22, 0.41). With the increase of applied voltage, a
new emission was appeared at the longer wavelength region,
which indicates 4CzCN tends to form severe aggregated state
under electro-excitation.
2 3
nm)/Blue TADF (60nm)/PHPO (40 nm)/Cs CO (2 nm)/Al (100
nm), where the blue TADF materials were used as non-doped
emitters to investigate their electroluminescence (EL)
performance. The PEDOT:PSS was used as a hole injection layer
(
(
HIL) to decrease the energy barrier between the emission layer
EML) and the ITO anode (indium tin oxide). In order to optimize
the carrier balance, PHPO was introduced as the electron
transport layer (ETL). It should be noted that all the functional
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Chemistry - A European Journal
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Table 2. Device Performances of the fully solution-processed TADF OLEDs.
a
b
PEmaxc
[lm W-1]
EQEmaxd
[%]
EQE100e
[%]
EQE1000f
[%]
g
V
on
CEmax
L
max
TADF emitter
CIE [x, y]h
-
1
-2
[
V]
[cd A ]
[cd m ]
MeCz-4CzCN
PhCz-4CzCN
PhDCz-4CzCN
2.9
2.9
3.1
46.5
40.8
18.9
35.6
27.5
11.2
21.8
20.6
10.0
21.3
20.0
9.8
19.0
18.2
7.9
14200
18800
9600
(0.17, 0.35)
(0.16, 0.27)
(0.19, 0.28)
a)
‒2 b)
c)
d)
V
on = turn-on voltage at 1 cd m , CEmax = maximum current efficiency, PEmax = maximum power efficiency, EQEmax = maximum external quantum
e)
-2 f)
-2 g)
h)
efficiency, EQE at 100 cd m . EQE at 1000 cd m .
Lmax = maximum luminance, CIE = the Commission Internationale de L’Eclairage coordinates at
-
2
the luminance of 1000 cd m .
In contrast, the EL spectra of Device A and B are nearly
unchanged in the applied voltage of 5-15 V. The stable EL
emission of Device A and B indicates that the molecular
encapsulation by introducing bulky units can efficiently suppress
the aggregation state of the emissive core. It is worthy to note that
the EL spectra of the devices based on encapsulated emitters
were all blue shift comparing to emissive-core based Device D,
which exhibits an EL peak at 497 nm. Moreover, with increasing
the steric volume of the wrapping units, the EL peak positions
were gradually shifted to 486, 475 and 465 nm for Device A, B
and C, respectively, which can be assigned to the further
decreased intermolecular interaction between emission cores by
more bulky peripheral wrapping units. As a result, the CIE
coordinates of the MeCz-4CzCN, PhCz-4CzCN and PhDCz-
emission peaks at 460 and 650 nm. To identify the attribution of
the peaks, the PL spectra of MeCz, PhCz and PhDCz films are
investigated.
As shown in Figure S10, MeCz, PhCz and PhDCz exhibit
ultraviolet emission at the range around 350 nm. The fine
structure of the emission peaks indicates that the excited excitons
of MeCz, PhCz and PhDCz are local states, which are easily to
form the exciplex emission with an electron acceptor unit. In order
to investigate the exciplex formation ability, the 1:1 mixed films of
MeCz:PhPO, PhCz:PhPO and PhDCz:PhPO are measured
under photoexcitation. The emission spectra of MeCz:PhPO,
PhCz:PhPO and PhDCz:PhPO are broad and red-shifted to 418,
400, and 404 nm compared with the 343, 340, 355, and 362 nm
for MeCz, PhCz, PhDCz, and PhPO, respectively. The
unstructured new emission of the mixed films can be attributed to
exciplex states, which are formed through the intermolecular
interaction between the excited state donor and acceptor units.
Therefore, the blue range in the EL spectra of MeCz, PhCz and
PhDCz based devices are exciplex emission between EML and
ETL layers. The triplet energies of exciplexes are 2.96, 3.10 and
3.07 eV for MeCz:PhPO, PhCz:PhPO and PhDCz:PhPO,
respectively, which are higher than that of 4CzCN (2.68 eV). Thus,
the exciplex formation between the wrapping units and ETL
molecules will not induce energy leakage of emissive core.
Moreover, thanks to the exciplex formation, the turn-on voltage of
these encapsulated TADF emitters are only around 2.9-3.1 V,
which are lower than most of the reported solution-processed
TADF blue OLEDs even with a vacuum deposited ETL.
4
0
CzCN based devices were (0.17, 0.35), (0.16,0.27) and (0.19,
.28), respectively, which are all in the blue emission range.
Exciplex and electromor properties: In addition to the high
efficiency, the relationship between the molecular structure and
device performance is also important. Since the device
conformation and emissive core of the three emitters are the
same, the relatively low EL performance of PhDCz-4CzCN
comparing to MeCz-4CzCN, PhCz-4CzCN indicates that the
introducing of PhDCz units may cause other potential energy
leakage pathway. In the past reported investigations, much
attention had been focused on the influence of the wrapping units
on the emissive core. However, the strong interaction between
the wrapping units themselves would also produce severe impact
on the EL property, which is usually ignored in the previous
studies. Moreover, the exciplex formation between the introduced
peripheral units and electron transporting material may also
participate the exciton formation and energy transfer process
during the electroexcitation of the devices. Thus, deeply exploring
the exciton formation and energy transfer process are benefit for
fully understanding the relationship between molecular structure
and device performance.
In order to investigate the fundamental exciton harvest process
of the emitters and discover the real reason of different efficiency,
the solution-processed devices only based on the wrapping
groups are fabricated. As shown in Figure 5, since the wrapping
units are electron-rich donors (D), the excited state (D*) could take
part in three decay pathways: (1) straight back to the ground state
to result in the intrinsic light of monomer emission; (2) combination
with the ground state donor unit to exhibit the electromer
emission; or (3) interaction with the ground state ETL molecule to
form the exciplex state. As shown in Figure S9, the EL spectra of
MeCz and PhCz based devices exhibit a deep blue emission with
the peaks around 420 nm, while PhDCz based devices exhibit two
Figure 5. Exciton formation and energy transfer diagram of the
encapsulated molecules.
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Chemistry - A European Journal
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Table 3. Comparison of solution-processed TADF OLEDs with blue emission.
V
on
L
max
CEmax
[cd A ]
EQEmax
[%]
Device structures of organic layers
CIE [x, y]
-
2
-1
[
V]
[cd m ]
18800
2700
N
PEDOT:PSS│PhCz-4CzCN│PhPO
2.9
5.4
5.0
3.9
5.2
6.2
3.8
3.2
3.9
3.2
4.0
40.8
20.6
0.16, 0.27
0.18, 0.30
0.19, 0.37
0.15, 0.28
0.18, 0.27
0.26, 0.31
0.20, 0.29
0.18, 0.27
0.15, 0.26
0.22, 0.37
0.21, 0.42
0.16, 0.25
0.15, 0.19
0.17, 0.30
0.24, 0.40
0.18, 0.40
0.23, 0.40
0.18, 0.23
0.21, 0.33
PEDOT:PSS│POCz-DPS│TPBi
12.6
8.8
7.3
[13]
[8c]
[14]
[15]
[16]
[4c]
[4d]
[17]
[18]
[19]
[5]
PEDOT:PSS│PhCzDSO2│TPBi
4546
4.0
PEDOT:PSS│m-DTPACO│TmPyPB
4.8
2.4
PEDOT:PSS│DCzDMAC-DPS│TPBi
PEDOT:PSS│TPPOCz│TmPyPB│TPBi
PEDOT:PSS│PBD-10│DPEPO│TmPyPB
PEDOT:PSS│P-Ac95-TRZ05│TSPO1│TmPyPB
PEDOT:PSS│TFB│B-oTC│DPEPO│TyPMPB
PEDOT:PSS│poly-TPD│P3-P12│TmPyPB
PEDOT:PSS│PVK│TB14CZ-ACTRZ│TPBi
PEDOT:PSS│PVK│SiCz:4CzFPN│EMPA│TPBi
PEDOT:PSS│PVK│CzSi:MA-TA│TSPO1│TPBi
PEDOT:PSS│PVK│SiCz:5CzCN│TSPO1│TPBi
PEDOT:PSS│mCP:PCzDP-10│TmPyPB
PEDOT:PSS│CzSi:TB-2PXZ│TmPyPB
PEDOT:PSS│polymers:mCP│TmPyPB
PEDOT:PSS│PYD2:emitter│TPBi
3.8
2.2
6119
3.2
2.0
13.5
24.8
37.3
9.0
7.3
6150
4351
12.1
19.1
4.0
2622
13.9
36.1
5.5
D
20.0
22.1
19.7
16.1
8.9
[6c]
[6d]
[6b]
[8d]
[8e]
[3b]
[3c]
38.6
21.0
26.0
9.5
5.8
3.9
4.5
3.9
8.7
10 413
5.6
PEDOT:PSS│mCP:5CzCF3Ph│TmPyPB
11.8
5.2
a)
N = Non-doped device; b) D = Doped device; c) Blue font means the organic materials are fabricated by solution-processed; d) Red
font means the organic materials are fabricated by vacuum deposition.
The exciplex formation will not only reduce the turn-on voltage
of the device, but also eliminate the triplet-polaron quenching
to a sever energy leakage during device operation, which is the
main reason of the low device efficiency of PhDCz-4CzCN. In
contrast, the bimolecular interaction of |MeCz* MeCz ⟩ and |PhCz*
PhCz ⟩ was extremely weak, which result in a negligible
electromer comparing with the dominated exciplex emissions in
MeCz and PhCz based device. Notably, the high device
efficiencies were obtained in MeCz-4CzCN and PhCz-4CzCN
based device, which means most of the excited excitons will
achieve radiative transition in the emission core without energy
leakage from the emissive core to the electromer state. However,
the severe electromer formation in PhCz-4CzCN based device
will not only decrease the electroluminescence efficiency, but also
damage the EL purity. Therefore, it is can be concluded that
blocking the serious exciton quenching channel of low energy
electromer state is important for design efficient wrapping units for
encapsulated TADF materials, which was usually ignored by the
previous reports.
(
TPQ) of the TADF emitter. Since the hole carrier are transported
in the peripheral wrapping units and the triplet state are mainly
located in the core, the exciplex formation at the interface of the
encapsulated emitter and ETL molecule will lead to the
independent channels for charge transport and energy transfer.
Therefore, the triplet state of the emissive core will not directly
contact with the polarized charge transporting units. It has been
reported that triplet–polaron interactions are the main reason for
the unstable EL spectra and reduced device efficiency during the
continuous operation.[12] The steric bulky units introduced to our
designed TADF emitters could efficiently increase the distance
from exciton to polaron and reduce the TPQ effect. As a result,
both Me-4CzCN and PhCz-4CzCN can exhibit the high device
efficiency at high luminescence and stable EL emission with
unchanged spectra in
a
wide range applied voltages.
Nevertheless, the EL spectra of PhDCz-4CzCN exhibits a slight
change at the long emission range, and the color purity is not
satisfactory comparing to Me-4CzCN and PhCz-4CzCN, even
though PhDCz-4CzCN possesses a bigger wrapping unit. Thus,
the 650 nm emission in PhDCz based devices indeed has some
influence on the EL performance of PhDCz-4CzCN.
In addition to the interaction with the ETL to form the exciplex
state, the excited wrapping unit will also interact with the ground
state itself to form the electromer emission. Moreover, the
electromer and exciplex formation will compete with each other
under electric field, and which pathways will dominate the decay
process really depends on the molecular structure of the wrapping
units. As shown in the EL spectra, the intense electromer
emission band at 650 nm was observed in PhDCz based device.
Such low energy electromer state of |PhDCz* PhDCz ⟩ will lead
Based on the above investigations, designing efficient wrapping
units for encapsulated TADF blue emitters, the high triplet energy
to confine the reverse energy transfer is not the only consideration,
the other three factors should also be taken into account. (1) The
larger steric volume of the wrapping units will provide more
effective shield effect to reduce the aggregation of the emissive
cores, which would suppress the concentration quenching and
lead to a blue shift EL spectrum. (2) The exciplex formation
between the wrapping units and the ETL will facilitate the charge
transportation and injection, which can inhibit the triplet-polaron
quenching and lead to a low turn-on voltage. (3) The electromer
state of the wrapping units under electro-excitation is a severe
energy leakage pathway, which will destroy the color purity and
reduce the device efficiency. The newly designed wrapping unit
PhCz, which possess suitable bulky volume, efficient exciplex
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Chemistry - A European Journal
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formation and almost no electromer state, make the PhCz-4CzCN
based device achieve a blue emission with the emission peak of
Quantum chemical calculations
The optimized structures of the designed materials were calculated by
Gaussian 09 at the B3LYP functional with 6-31G (d) basis sets. The
excitation energies in the singlet and triplet states were obtained using the
TD-DFT method based on an optimized molecular structure for the ground
state.
4
1
75 nm, a low turn-on voltage of 2.9 eV, a high luminescence of
8800 cd m-2 and a high device efficiency with EQE above 20 %.
To the best of our knowledge, this is the first report of non-doped
TADF blue OLEDs with fully solution-processed organic layers,
and the device efficiencies are among the highest of the recently
reported solution-processed TADF OLEDs (Table 3).
Device Fabrication and Measurements
The ITO-coated glass with a sheet resistance of 10 Ω sq were used as
-1
the substrate, which were rinsed in deionized water and then
ultrasonicated sequentially in acetone and ethanol. After cleaning, the ITO
substrate was treated in a UV-ozone oven for 20 min. Then 40 nm
Conclusions
o
PEDOT:PSS was spin-coated onto the ITO substrate and dried at 200 C
for 10 min. The substrates were then taken into a nitrogen glove box,
where the 60 nm emission layer was spin coated onto the PEDOT:PSS
layer from chlorobenzene and annealed at at 100 °C for 30 min. After that,
In summary, we have successfully achieved highly efficient fully
solution-processed TADF blue OLEDs by designing and
synthesizing three encapsulated TADF blue emitters.
Photophysical investigation demonstrate that the introduced bulky
wrapping units to the emissive core not only enable the high
molecular weight for sequential orthogonal solvent process, but
also build a large steric hindrance effect for enhancing the RISC
and suppressing the TTA, which makes the encapsulated
emitters can be competent with non-doped devices. The
thoroughly study of the EL properties indicates that even the small
modulation in the molecular structure of wrapping units will
dramatically affect the encapsulation effect. The enlarged volume
of wrapping units will make the electroluminescent spectra blue-
shift, while enhancing the energy transfer of exciplex and blocking
the energy leakage of electromer can facilitate the exciton
utilization of the devices. As a result, the achieved fully solution-
processed TADF blue OLEDs exhibits a low turn-on voltage and
an excellent EL efficiency, which outperform most of the reported
doped device with vacuum deposited EML. This work would
provide critical guidelines for the design of more efficient TADF
blue materials used to future cost-effective solution-processed
organic electroluminescence application.
40 nm PhPO was spin coated onto the EML as the electron transporting
layer. Finally, Cs CO (2 nm) and Al (100 nm) were deposited as the
2
3
cathode for the devices. The EL spectra, device luminance, and current
density-voltage characteristics were recorded using a combination of a
Photo-Research PR-655 SpectraScan and a Keithley 2400 Sourcemeter.
The device was tested in the condition of the atmosphere with no
protective encapsulation.
Synthesis and characterization
All the reagents used for the synthesis of the compounds were purchased
from Aldrich and Acros companies and used without further purification.
The anhydrous tetrahydrofuran (THF) solvent was dewatered according to
standard procedures. The intermediates M1 and M2 are synthesized
according to the references.[20]
4-methoxy-9-phenyl-9H-carbazole (PhCz)
A mixture of M1 (1.97 g, 10.0 mmol), iodobenzene (2.04 g, 10.0 mmol),
CuI (0.06 g, 0.30 mmol), 1,10-phenanthroline (0.06 g, 0.30 mmol), and
K CO (0.78 g, 6.0 mmol) was added to 30 mL DMF solution. The mixture
2 3
was refluxed under nitrogen for 24 h. After cooling, the mixture was poured
into 100 mL water and filtered. The product was then obtained by column
1
chromatography on silica gel to yield a white solid (1.64 g, 61%). H NMR
(600 MHz, DMSO, δ): 8.27 (d, J = 7.4 Hz, 1H), 7.72 – 7.66 (m, 2H), 7.61
(dt, J = 8.4, 1.7 Hz, 2H), 7.56 – 7.53 (m, 1H), 7.41 – 7.32 (m, 3H), 7.27 (dt,
J = 14.1, 3.6 Hz, 1H), 6.96 (d, J = 8.0 Hz, 1H), 6.86 (d, J = 7.9 Hz, 1H),
Experimental Section
4
1
1
.07 (s, 3H). 13C NMR (126 MHz, CDCl
, δ): 156.35, 142.36, 140.22,
3
37.88, 129.83, 127.49, 127.29, 126.68, 125.02, 123.19, 122.76, 120.13,
12.58, 109.19, 102.78, 100.94, 55.58. HRMS (ESI) m/z: calcd for
General methods
1
H and 13C NMR spectra were measured on a Bruker NMR spectrometer
C
19
H
15NO, 273.1154; found, 273.0745. Anal. calcd for C19H15NO: C 83.49,
operating at 600 MHz with tetramethylsilane (TMS) as the internal
standard. Molecular masses were measured by a BRUKER DALTONICS
H 5.53, N 5.12; found: C 83.52, H 5.51, N 5.12.
Matrix-assisted
spectrometry (MALDI-TOF-MS). TGA was performed on a Netzsch
simultaneous thermal analyzer (STA) system (STA 409PC) under a N
laser
desorption-ionization
time-of-flight
mass
4
-methoxy-9-methyl-9H-carbazole (MeCz)
This compound was prepared according to the procedure for the synthesis
of PhCz, giving a yield of 70%. H NMR (600 MHz, DMSO, δ): 8.17 (d, J =
2
1
-
1
flow at a heating rate of 10 °C min . DSC measurements were operated
on a DSC 2910 modulated calorimeter under a N flow at a heating rate of
0 °C min-1 and a cooling rate of 20 °C min . UV–vis absorption spectra
7
.7 Hz, 1H), 7.56 (d, J = 8.2 Hz, 1H), 7.45 – 7.31 (m, 2H), 7.27 – 7.12 (m,
2H), 6.77 (d, J = 7.9 Hz, 1H), 4.03 (s, 3H), 3.86 (s, 3H). 13C NMR (126
MHz, CDCl , δ): 156.34, 142.52, 140.25, 126.49, 124.75, 123.08, 122.20,
19.13, 111.88, 107.89, 101.52, 100.06, 55.49, 29.30. HRMS (ESI) m/z:
calcd for C14 12NO, 211.0997; found, 211.1032. Anal. calcd for C14
C 79.59, H 6.20, N 6.63; found: C 79.58, H 6.22, N 6.68.
2
-1
1
3
were recorded on a SHIMADZU UV-2450 spectrophotometer while
photoluminescence (PL) spectra, photoluminescence quantum yields
1
H
H12NO:
(PLQYs) and low tempertrature phosphorescence spectra were measured
using a HORIBA FLUOROMAX-4 fluorescence spectrophotometer. Cyclic
voltammogram (CV) was measured on a CHI750C voltammetric analyzer
in a typical three-electrode cell with a nitrogen-saturated CH
2
Cl
2
solution
9-(4-(9H-carbazol-9-yl)phenyl)-4-methoxy-9H-carbazole (PhDCz)
of 0.1 mol L-1 tetrabutylammonium hexafluorophosphate as conductive
medium, a platinum plate working electrode, a platinum wire counter
electrode, and a silver wire reference electrode. The transient PL spectra
were measured with Edinburgh FL920 fluorescence spectrophotometer.
This compound was prepared according to the procedure for the synthesis
1
of PhCz, giving a yield of 55%. H NMR (600 MHz, DMSO, δ): 8.31 (dd, J
= 7.6, 3.5 Hz, 3H), 7.97 – 7.89 (m, 4H), 7.62 (d, J = 8.2 Hz, 2H), 7.58 (d, J
= 8.2 Hz, 1H), 7.51 (ddd, J = 8.2, 7.1, 1.1 Hz, 2H), 7.48 – 7.41 (m, 2H),
7
(
.37 – 7.30 (m, 3H), 7.20 (d, J = 8.1 Hz, 1H), 6.92 (d, J = 7.9 Hz, 1H), 4.10
s, 3H). 13C NMR (126 MHz, CDCl
3
, δ):156.43, 142.19, 140.78, 140.05,
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Chemistry - A European Journal
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1
1
36.82, 136.68, 128.50, 128.31, 126.90, 126.15, 125.20, 123.58, 123.34,
22.96, 120.47, 120.28, 112.78, 109.79, 109.16, 102.70, 101.30, 55.62.
for C36
32 2 2 32 2 2
H N O , 524.2464; found, 524.2341. Anal. calcd for C36H N O : C
82.41, H 6.15, N 5.34; found: C 82.39, H 6.17, N 5.33.
22 2
HRMS (ESI) m/z: calcd for C31H N O, 438.1732; found, 483.1738. Anal.
calcd for C31 O: C 84.91, H 5.06, N 6.39; found: C 84.86, H 5.09, N
22 2
H N
4
Cz)
-(6-(9-methyl-9H-carbazol-5-yloxy)hexyloxy)-9H-carbazole (MeCz-
6.40.
This compound was prepared according to the procedure for the synthesis
1
9-phenyl-9H-carbazol-4-ol (PhCz-OH)
of PhCz-Cz, giving a yield of 80%. H NMR (600 MHz, CDCl
3
, δ): 8.34 (dd,
PhCz (2.73 g, 10 mmol) was dissolved in 100 mL dry CH
2
Cl
2
. The solution
J = 15.1, 7.7 Hz, 2H), 8.04 (s, 1H), 7.46 – 7.34 (m, 2H), 7.31 (t, J = 8.1 Hz,
1H), 7.22 (t, J = 7.3 Hz, 2H), 7.05 – 6.99 (m, 2H), 6.67 (t, J = 7.9 Hz, 2H),
4.27 (q, J = 6.4 Hz, 4H), 3.84 (s, 3H), 2.08 (s, 4H), 1.80 (s, 4H). 13C NMR
was stirred at 0 °C and BBr (1.2 mL 1 M solution in CH Cl
3
2
2
, 12 mmol) was
added dropwise. After stirring for 12 h, the reaction was returned to room
temperature and carefully quenched with methanol. After that, a saturated
3
(126 MHz, CDCl , δ): 155.75, 155.73, 142.54, 140.92, 140.24, 138.69,
solution of NaHCO
for three times. The organic layers were combined and dried with MgSO
3
was added, and the mixture was washed with water
126.69, 126.48, 124.85, 124.66, 123.04, 123.02, 122.82, 122.30, 119.64,
119.11, 112.72, 111.94, 109.89, 107.81, 103.26, 101.25, 101.10, 100.77,
4
.
The product was then obtained by column chromatography on silica gel to
yield a brown solid (2.0 g, 79%). H NMR (600 MHz, DMSO, δ): 10.28 (s,
30 2 2
67.85, 67.82, 29.49, 29.30, 26.13. HRMS (ESI) m/z: calcd for C31H N O ,
1
30 2 2
462.2307; found, 462.2401. Anal. calcd for C31H N O : C 80.49, H 6.54,
1H), 8.26 (d, J = 7.7 Hz, 1H), 7.70 – 7.65 (m, 2H), 7.60 (dd, J = 8.4, 1.2
N 6.06; found: C 80.46, H 6.58, N 6.02.
Hz, 2H), 7.53 (t, J = 7.4 Hz, 1H), 7.38 – 7.33 (m, 1H), 7.31 (d, J = 8.1 Hz,
1H), 7.26 – 7.23 (m, 1H), 7.20 (t, J = 8.0 Hz, 1H), 6.80 (d, J = 7.7 Hz, 1H),
6
.69 (d, J = 7.4 Hz, 1H). 13C NMR (126 MHz, DMSO, δ): 154.37, 142.55,
139.91, 137.63, 130.55, 128.02, 127.52, 127.20, 125.36, 123.00, 122.99,
120.40, 111.61, 109.35, 106.14, 100.95. HRMS (ESI) m/z: calcd for
9
9
-(4-(4-(6-(9H-carbazol-5-yloxy)hexyloxy)-9H-carbazol-9-yl)phenyl)-
H-carbazole (PhDCz-Cz)
This compound was prepared according to the procedure for the synthesis
of PhCz-Cz, giving a yield of 75%. H NMR (600 MHz, CDCl
1
3
, δ): 8.46 (d,
C
18
H
13NO, 259.0997; found, 259.1012. Anal. calcd for C18
H13NO: C 83.37,
J = 7.7 Hz, 1H), 8.35 (d, J = 7.7 Hz, 1H), 8.19 (d, J = 7.6 Hz, 2H), 8.04 (s,
H), 7.81 (s, 4H), 7.57 (d, J = 8.1 Hz, 2H), 7.53 (d, J = 7.7 Hz, 1H), 7.48 (t,
H 5.05, N 5.40; found: C 83.34, H 5.06, N 5.41.
1
J = 7.4 Hz, 2H), 7.43 (t, J = 7.6 Hz, 1H), 7.35 (ddd, J = 28.5, 20.6, 12.3 Hz,
8H), 7.23 (d, J = 7.0 Hz, 1H), 7.15 (d, J = 7.8 Hz, 1H), 7.04 (d, J = 8.1 Hz,
1H), 6.77 (d, J = 7.5 Hz, 1H), 6.68 (d, J = 7.5 Hz, 1H), 4.33 (t, J = 5.3 Hz,
9
-methyl-9H-carbazol-4-ol (MeCz-OH)
This compound was prepared according to the procedure for the synthesis
of PhCz-OH, giving a yield of 80%. H NMR (600 MHz, DMSO, δ): 10.10
1
2H), 4.29 (t, J = 5.6 Hz, 2H), 2.11 (d, J = 6.0 Hz, 4H), 1.83 (s, 4H). 13
NMR (126 MHz, CDCl , δ): 155.85, 155.73, 142.22, 140.94, 140.78,
C
(s, 1H), 8.17 (d, J = 7.7 Hz, 1H), 7.52 (d, J = 8.1 Hz, 1H), 7.39 (t, J = 7.6
3
Hz, 1H), 7.24 (t, J = 7.9 Hz, 1H), 7.16 (t, J = 7.4 Hz, 1H), 7.00 (d, J = 8.0
Hz, 1H), 6.61 (d, J = 7.7 Hz, 1H), 3.81 (s, 3H). 13C NMR (126 MHz, DMSO,
δ): 154.08, 143.04, 140.30, 127.01, 124.81, 122.61, 122.20, 119.08,
140.05, 138.70, 136.87, 136.63, 128.49, 128.30, 126.91, 126.71, 126.13,
125.13, 124.87, 123.56, 123.28, 123.08, 123.04, 122.83, 120.49, 120.45,
120.25, 119.65, 112.83, 109.90, 109.78, 109.11, 103.28, 102.45, 102.00,
101.11, 99.99, 68.02, 67.82, 29.50, 26.15. HRMS (ESI) m/z: calcd for
1
10.82, 108.82, 104.99, 100.47, 29.56. HRMS (ESI) m/z: calcd for
11NO, 197.0841; found, 197.0884. Anal. calcd for C13 11NO: C 79.16,
H 5.62, N 7.10; found: C 79.15, H 5.65, N 7.12.
C
13
H
H
C
48
H
39
N
3 2
O
, 689.3042; found, 689.3218. Anal. calcd for C48
39 3 2
H N O : C
83.57, H 5.70, N 6.09; found: C 83.52, H 5.75, N 6.12.
9-(4-(9H-carbazol-9-yl)phenyl)-9H-carbazol-4-ol (PhDCz-OH)
2,3,5,6-tetrakis(4-methoxy-9H-carbazol-9-yl)benzonitrile (4CzCN)
This compound was prepared according to the procedure for the synthesis
of PhCz-OH, giving a yield of 80%. H NMR (600 MHz, DMSO, δ): 10.36
In the nitrogen atmosphere, the prepared M1 (0.88 g, 4.5 mmol) was
added to the anhydrous THF solution (40 mL) contain of NaH (0.60 g, 5
mmol). The mixture was stirred at 0 °C for 0.5 h, then 2,3,5,6-
tetrafluorobenzonitrile (0.17 g, 1 mmol) in anhydrous THF (20 mL) was
added dropwise. The solution was stirred for another 12h at room
temperature. After the reaction is over, the reaction was quenched with the
1
(
s, 1H), 8.31 (dd, J = 7.0, 6.3 Hz, 3H), 7.96 – 7.87 (m, 4H), 7.61 (d, J = 8.2
Hz, 2H), 7.55 (d, J = 8.2 Hz, 1H), 7.51 (ddd, J = 8.2, 7.1, 1.1 Hz, 2H), 7.46
7.40 (m, 1H), 7.37 – 7.33 (m, 2H), 7.32 – 7.25 (m, 2H), 7.03 (d, J = 7.8
Hz, 1H), 6.75 (d, J = 7.5 Hz, 1H). 13C NMR (126 MHz, DMSO, δ): 154.42,
–
1
1
1
42.45, 140.54, 139.83, 136.55, 136.14, 128.76, 128.67, 127.66, 126.84,
25.50, 123.34, 123.14, 123.06, 121.08, 120.77, 120.69, 111.74, 110.31,
addition of 250 mL water. The mixture was extracted with CH
times. The combine organic layer was dried with anhydrous MgSO
2
Cl
2
for three
and
4
09.61, 106.41, 101.16. HRMS (ESI) m/z: calcd for C30
H
20
N
2
O, 424.1576;
the solvent was removed under vacuum. The precipitate was purified by
column chromatography on silica gel, resulted in the bright green product
found, 424.1345. Anal. calcd for C13
C 79.15, H 5.65, N 7.12.
H11NO: C 79.16, H 5.62, N 7.10; found:
1
(0.75 g, 83%). H NMR (600 MHz, DMSO, δ): 8.58 (s, 1H), 7.94 (dd, J =
1
2
4
=
–
4.9, 7.8 Hz, 6H), 7.87 (dd, J = 13.3, 6.4 Hz, 2H), 7.59 (dd, J = 8.2, 4.1 Hz,
H), 7.50 (ddd, J = 8.4, 7.2, 2.5 Hz, 2H), 7.25 (ddd, J = 10.3, 5.3, 2.7 Hz,
H), 7.18 (ddd, J = 7.9, 6.8, 4.2 Hz, 4H), 7.14 (t, J = 7.0 Hz, 2H), 7.09 (t, J
7.5 Hz, 2H), 6.73 (dd, J = 8.1, 1.6 Hz, 2H), 6.68 (d, J = 8.1 Hz, 2H), 3.95
4
-(6-(9-phenyl-9H-carbazol-5-yloxy)hexyloxy)-9H-carbazole (PhCz-
Cz)
A mixture of PhCz-OH (1.26 g, 5.0 mmol), M2 (1.75 g, 5.0 mmol) and
CO (1.7 g, 15.0 mmol) was added to 100 mL acetone solution. The
3.84 (s, 12H). 13C NMR (126 MHz, CDCl
3
, δ): 156.04, 156.02, 155.97,
K
2
3
reaction was heated at 60 oC under nitrogen for 24 h. After cooling, the
mixture was poured into 200 mL water. The crude product was filtered and
140.88, 140.41, 140.38, 138.73, 138.26, 138.11, 137.21, 137.14, 126.76,
1
1
26.73, 124.93, 124.86, 123.64, 123.62, 123.30, 123.25, 123.15, 123.09,
21.40, 121.35, 121.17, 121.11, 118.48, 113.50, 113.16, 112.76, 109.26,
purified by silica gel column chromatography to give the white product
1
108.79, 108.72, 102.86, 102.60, 102.56, 102.31, 102.27, 102.22, 55.41.
MALDI-TOF MS: calcd for C59 , 883.3; found, 883.2 [M+]. Anal.
calcd for C59 : C 80.16, H 4.67, N 7.92; found: C 80.11, H 4.72, N
.85.
(2.25 g, 82%). H NMR (600 MHz, CDCl
3
, δ): 8.42 (d, J = 7.7 Hz, 1H), 8.33
41 5 4
H N O
(t, J = 13.5 Hz, 1H), 8.01 (s, 1H), 7.62 – 7.51 (m, 4H), 7.44 (dd, J = 17.5,
41 5 4
H N O
10.4 Hz, 1H), 7.40 – 7.33 (m, 4H), 7.32 – 7.25 (m, 3H), 7.22 (d, J = 7.0 Hz,
1H), 7.05 – 6.94 (m, 2H), 6.71 (d, J = 7.8 Hz, 1H), 6.66 (d, J = 7.9 Hz, 1H),
4
.32 – 4.22 (m, 4H), 2.09 (d, J = 6.2 Hz, 4H), 1.81 (s, 4H). 13C NMR (126
7
MHz, CDCl
3
, δ): 155.89, 155.80, 142.49, 141.02, 140.34, 138.80, 137.96,
2,3,5,6-tetrakis(4-(6-(9-methyl-9H-carbazol-5-yloxy)hexyloxy)-9H-
carbazol-9-yl)benzonitrile (MeCz-4CzCN)
1
1
1
29.92, 127.55, 127.35, 126.89, 126.79, 125.13, 124.95, 123.28, 123.10,
23.01, 122.85, 120.32, 119.71, 112.73, 110.11, 109.34, 103.47, 102.67,
01.83, 101.17, 68.07, 67.92, 29.58, 26.23, 26.22. HRMS (ESI) m/z: calcd
This compound was prepared according to the procedure for the synthesis
1
of 4CzCN, giving a yield of 85%. H NMR (600 MHz, CDCl
3
, δ): 8.32 (dd,
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201904415
FULL PAPER
J = 20.1, 7.9 Hz, 5H), 8.11 – 8.02 (m, 4H), 7.43 – 7.36 (m, 6H), 7.35 – 7.27
supported by Scientific and Technological Program of
Lianyungang (JC1603, CG1602), Postgraduate Research and
Practice Innovation Program of Jiangsu Province (KYCX19-2257,
KYCX18-2581 and KYCX18-2602).
(
(
m, 10H), 7.21 (dt, J = 7.9, 4.4 Hz, 4H), 7.18 – 7.14 (m, 4H), 7.13 – 6.98
m, 12H), 6.98 – 6.93 (m, 4H), 6.60 (ddd, J = 12.2, 7.9, 3.7 Hz, 4H), 6.57
–
1
2
1
1
1
1
6
6.48 (m, 4H), 4.22 – 4.14 (m, 8H), 4.07 (d, J = 4.8 Hz, 8H), 3.77 (d, J =
2.4 Hz, 12H), 2.04 – 1.95 (m, 8H), 1.95 – 1.86 (m, 8H), 1.67 (ddd, J =
1.2, 15.0, 7.3 Hz, 16H). 13C NMR (151 MHz, CDCl
, δ): 155.71, 155.68,
3
42.50, 142.48, 141.04, 140.20, 140.19, 138.22, 137.33, 126.71, 126.45,
26.43, 124.83, 124.78, 124.63, 124.60, 123.77, 122.99, 122.94, 122.28,
22.24, 119.09, 119.03, 118.51, 113.59, 111.92, 111.89, 107.76, 101.19,
01.17, 100.79, 100.77, 100.73, 77.23, 77.02, 76.81, 67.87, 67.84, 67.78,
7.73, 29.37, 29.30, 29.23, 29.21, 26.03, 25.99, 25.92. MALDI-TOF MS:
Keywords: blue emission • TADF • fully solution-process •
exciplex • organic light emitting diodes
[
[
[
1]
2]
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4
117 9 8
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C
131 117 9 8
H N O : C 80.88, H 6.06, N 6.48; found: C 80.85, H 6.02, N 6.47.
2,3,5,6-tetrakis(4-(6-(9-phenyl-9H-carbazol-5-yloxy)hexyloxy)-9H-
carbazol-9-yl)benzonitrile (PhCz-4CzCN)
This compound was prepared according to the procedure for the synthesis
1
of 4CzCN, giving a yield of 82%. H NMR (600 MHz, CDCl
3
, δ): 8.38 (dd,
J = 15.9, 7.7 Hz, 4H), 8.31 (s, 1H), 8.13 – 8.04 (m, 4H), 7.58 – 7.51 (m,
16H), 7.46 – 7.41 (m, 4H), 7.40 – 7.29 (m, 12H), 7.28 – 7.20 (m, 10H),
7.19 – 6.99 (m, 14H), 6.99 – 6.94 (m, 4H), 6.64 (dd, J = 13.4, 5.4 Hz, 4H),
6.61 – 6.48 (m, 4H), 4.21 (dt, J = 8.9, 6.4 Hz, 8H), 4.14 – 4.05 (m, 8H),
2
.06 – 1.97 (m, 8H), 1.92 (t, J = 22.0 Hz, 8H), 1.77 – 1.60 (m, 16H). 13C
a) T. A. Lin, T. Chatterjee, W. L. Tsai, W. K. Lee, M. J. Wu, M. Jiao, K. C.
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Sci. 2018, 5, 1800436; d) W. Zeng, H. Y. Lai, W. K. Lee, M. Jiao, Y. J.
Shiu, C. Zhong, S. Gong, T. Zhou, G. Xie, M. Sarma, K. T. Wong, C. C.
Wu, C. Yang, Adv. Mater. 2018, 30, 1704961; e) T. Fleetham, G. Li, J.
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S.-J. Su, Adv. Funct. Mater. 2018, 28, 1704927; b) N. Jürgensen, A.
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C. Zheng, Y. Tao, X. Zhang, W. Huang, Chem. Commun. 2015, 51,
13024.
NMR (151 MHz, CDCl
3
, δ): 155.72, 155.69, 142.32, 140.52, 140.48,
1
1
1
1
2
40.16, 138.20, 137.89, 137.85, 137.31, 137.21, 129.76, 127.39, 127.37,
27.23, 127.21, 126.66, 126.65, 124.90, 124.89, 123.09, 123.04, 122.84,
22.81, 120.11, 120.06, 118.51, 112.59, 112.56, 109.10, 109.09, 102.47,
02.44, 101.66, 101.64, 101.60, 101.59, 77.23, 77.01, 76.80, 67.88, 67.84,
9.40, 29.38, 29.33, 29.27, 26.92, 26.05, 26.03, 26.00, 25.96, 25.95, 22.66.
MALDI-TOF MS: calcd for C151
H
125
N
9 8
O , 2191.9; found, 2191.9 [M+]. Anal.
calcd for C151
N 5.71.
H N O : C 82.68, H 5.74, N 5.75; found: C 82.65, H 5.70,
125 9 8
2,3,5,6-tetrakis(4-(6-(9-(4-(9H-carbazol-9-yl)phenyl)-9H-carbazol-5-
yloxy)hexyloxy)-9H-carbazol-9-yl)benzonitrile (PhDCz-4CzCN)
This compound was prepared according to the procedure for the synthesis
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1
of 4CzCN, giving a yield of 80%. H NMR (600 MHz, CDCl
3
, δ): 8.42 (dd,
J = 16.2, 7.8 Hz, 4H), 8.33 (s, 1H), 8.17 (d, J = 7.8 Hz, 8H), 8.14 – 8.05
m, 4H), 7.78 – 7.73 (m, 16H), 7.54 (dd, J = 8.2, 0.9 Hz, 8H), 7.47 (tt, J =
2.6, 6.5 Hz, 12H), 7.42 – 7.36 (m, 6H), 7.36 – 7.26 (m, 18H), 7.22 – 6.95
(
1
(
(
m, 20H), 6.69 (td, J = 8.6, 4.8 Hz, 4H), 6.64 – 6.52 (m, 4H), 4.27 – 4.19
m, 8H), 4.16 – 4.06 (m, 8H), 2.09 – 1.99 (m, 8 H), 1.98 – 1.89 (m, 8H),
1
1
1
1
1
2
.78 – 1.62 (m, 16H). 13C NMR (151 MHz, CDCl
3
, δ): 155.82, 155.79,
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42.16, 140.74, 140.00, 137.33, 136.83, 136.80, 136.60, 136.57, 128.42,
28.25, 126.90, 126.11, 125.10, 123.54, 123.25, 123.21, 123.07, 123.04,
20.50, 120.43, 120.25, 120.23, 112.77, 109.77, 109.75, 109.10, 109.09,
02.43, 102.40, 102.03, 102.02, 101.98, 101.97, 67.94, 67.90, 67.85,
9.39, 29.34, 29.28, 26.06, 26.03, 25.99, 25.97, 22.69, 22.67, 22.62.
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Lee, Adv. Opt. Mater. 2016, 4, 688.
MALDI-TOF MS: calcd for C199
Anal. calcd for C199 : C 83.73, H 5.40, N 6.38; found: C 83.70, H
.45, N 6.29.
153 13 8
H N O , 2852.1; found, 2852.1 [M+].
H
153
N
13
O
8
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5
Acknowledgements
We are grateful for the Grants from the National Natural Science
Foundation of China (21875036, 21805106), the Natural Science
Foundation of Jiangsu Province (BK20181073), the Natural
Science Fund for Colleges and Universities in Jiangsu Province
5, 5756.
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(
(
17KJB150007), the Postdoctoral Science Foundation of China
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This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201904415
FULL PAPER
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Chemistry - A European Journal
10.1002/chem.201904415
FULL PAPER
Entry for the Table of Contents
FULL PAPER
The molecular engineering of TADF
emitter has been successfully used to
enable the fabrication of fully solution-
processed blue OLEDs. The excellent
device efficiency with a high EQE
above 20% can be attributed to the
efficient exciplex formation and
electromer blocking.
Xinxin Ban,* Feng Chen, Jie Pan, Yan
Liu, Aiyun Zhu, Wei Jiang* and Yueming
Sun
Page No. – Page No.
Exciplex Formation and Electromer
Blocking for Highly Efficient Blue
Thermally Activated Delayed
Fluorescence OLEDs with All-
Solution-Processed Organic Layers
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