Enabling high efficiency and long lifetime of pure blue phosphorescent organic light emitting diodes by simple cyano modified carbazole-based host

Article abstract of DOI:10.1016/j.dyepig.2020.109118

Herein, we report two simple and efficient high triplet energy bipolar host materials named 2'-(9H-carbazol-9-yl)-[1,1′-biphenyl]-2-carbonitrile (CzBPCN) and 9-(2′-cyano-[1,1′-biphenyl]-2-yl)-9H-carbazole-3-carbonitrile (CNCzBPCN) by connecting carbazole type donor and cyano acceptor on 2 and 2′ positions of a biphenyl linker, respectively. The effect of cyano substitution on carbazole building block on photophysical and electroluminescence properties was unveiled. Both the compounds revealed high triplet energy (E_T) of above 3.0 eV attributed to the interrupted interchromophoric electronic interactions between the donor and acceptor units. The computational studies and carrier transport analysis proved that the compounds secured good bipolar charge transporting feature. The applicability of these materials as hosts for blue phosphorescent organic light emitting diodes (PhOLEDs) was tested. Interestingly, the CNCzBPCN hosted device exhibited superior performance with three fold improved external quantum efficiency (EQE_max) of 20.2% over the CzBPCN hosted device which showed EQE_max of 7.1%. Importantly, the CNCzBPCN hosted device demonstrated 11 fold extended operational lifetime of 121 h up to 50% of its intimal luminance (L₀ = 200 cd/m²) compared to CzBPCN. We believe that this work demonstrates the cost-effective and efficient high triplet energy bipolar host materials for simultaneous achievement of high EQE over 20% and long operational lifetime of blue PhOLEDs.

Full text of DOI:10.1016/j.dyepig.2020.109118

Dyes and Pigments 187 (2021) 109118
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Enabling high efficiency and long lifetime of pure blue phosphorescent
organic light emitting diodes by simple cyano modified
carbazole-based host
Rajendra Kumar Konidena, Won Jae Chung, Jun Yeob Lee*
School of Chemical Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Gyeonggi, 440-746, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
Host
Herein, we report two simple and efficient high triplet energy bipolar host materials named 2’-(9H-carbazol-9-
yl)-[1,1′-biphenyl]-2-carbonitrile (CzBPCN) and 9-(2′-cyano-[1,1′-biphenyl]-2-yl)-9H-carbazole-3-carbonitrile
(CNCzBPCN) by connecting carbazole type donor and cyano acceptor on 2 and 2′ positions of a biphenyl linker,
respectively. The effect of cyano substitution on carbazole building block on photophysical and electrolumi-
nescence properties was unveiled. Both the compounds revealed high triplet energy (ET) of above 3.0 eV
attributed to the interrupted interchromophoric electronic interactions between the donor and acceptor units.
The computational studies and carrier transport analysis proved that the compounds secured good bipolar charge
transporting feature. The applicability of these materials as hosts for blue phosphorescent organic light emitting
diodes (PhOLEDs) was tested. Interestingly, the CNCzBPCN hosted device exhibited superior performance with
three fold improved external quantum efficiency (EQEmax) of 20.2% over the CzBPCN hosted device which
showed EQEmax of 7.1%. Importantly, the CNCzBPCN hosted device demonstrated 11 fold extended operational
lifetime of 121 h up to 50% of its intimal luminance (L0 = 200 cd/m2) compared to CzBPCN. We believe that this
work demonstrates the cost-effective and efficient high triplet energy bipolar host materials for simultaneous
achievement of high EQE over 20% and long operational lifetime of blue PhOLEDs.
High efficiency
Phosphor
Blue device
1. Introduction
commercial devices [13–19]. However, efficient and stable host mate-
rials for blue PhOLEDs with superior triplet energy (ET) above 3.0 eV,
Since the first demonstration, phosphorescent organic light emitting
diodes (PhOLEDs) received tremendous impetus from the scientific
community as potentially high efficiency devices for next generation
solid state lighting applications and displays because of their unitary
internal quantum efficiency by harnessing 100% excitons [1–15].
However, phosphors typically possess long exciton lifetime and there-
fore they need to be embedded in an appropriate host material to nullify
the efficiency detrimental factors such as exciton diffusion, concentra-
tion quenching and triplet-triplet annulation (TTA) [13,14]. As the
major portion of the emitting layer is occupied by the host matrix, un-
doubtedly the hosts play a crucial role in governing the charge trans-
portation, location of recombination zone and lifetime of the devices
[13,14]. Thus, the host materials are equally important as the dopant
emitters in achieving the best performances. Presently, potential host
materials for green and red PhOLEDs have been developed for achieving
high EQE and long lifetime together and successfully applied in
suitable HOMO/LUMO energy levels and bipolar charge transporting
feature are scarcely reported because it is challenging to achieve all
these credentials simultaneously [20–23]. Importantly, blue PhOLEDs
possess far inferior operational lifetime compared to their green and red
counterparts [24–26]. Thus, it is urgent to promote the operational
lifetime of blue PhOLEDs without scarifying their EQE. Although several
factors affect the lifetime of PhOLEDs, the stability of host material plays
a vital role [24–26]. Therefore, the development of stable and high
triplet energy hosts for blue PhOLEDs is highly desirable. Besides, in the
field of organic materials for electronics, synthetic procedures play a
crucial role. Especially for industrial applications, the synthesis of host
materials for PhOLEDs should be simple, scalable and cost-effective
[15].
In comparison to unipolar hosts, the bipolar host materials
comprised of both the electron donor and acceptor units together in a
single molecule are ideal to improve the charge carrier mobility,
* Corresponding author.
E-mail address: leej17@skku.edu (J.Y. Lee).
https://doi.org/10.1016/j.dyepig.2020.109118
Received 17 May 2020; Received in revised form 8 December 2020; Accepted 22 December 2020
Available online 5 January 2021
0143-7208/© 2020 Elsevier Ltd. All rights reserved.

R.K. Konidena et al.
Dyes and Pigments 187 (2021) 109118
broaden the recombination zone and boost the device performance [13,
14]. However, the development of host materials with bipolar charge
transport and superior ET (>3.0 eV) is challenging due to their intrinsic
charge transfer interactions [20–26]. To solve this, numerous molecular
design approaches have been developed, among which the connectivity
of donor and acceptor units in non-conjugative mode of linkage through
materials. The accomplished synthetic procedure for the materials is
shown in Scheme 1. The cesium carbonate mediated N-arylation of
carbazole/cyanocarbazole with 1-bromo-2-fluorobenzene (1) afforded
intermediate 1a/1b, respectively. Subsequently, the intermediates 1a
and 1b were reacted with (2-cyanophenyl)boronic acid under palladium
catalyzed Suzuki-Miyaura reaction conditions to yield the desired host
materials, CzBPCN and CNCzBPCN, in reasonable yields after column
chromatography and vacuum train sublimation purification methods.
The chemical structures of the intermediates and host materials were
confirmed by the nuclear magnetic resonance spectroscopy (NMR) and
mass spectral methods.
high triplet energy linker is promising to interrupt effective π-conjuga-
tion and strong charge transfer interactions [27–30]. Generally, carba-
zole has been used as stable hole transporting unit for designing high ET
blue hosts [13,14,20–30]. Several acceptor units such as triazine,
phosphineoxide, benzimidazole, oxadiazole and cyano were reported as
electron transporting units, in which the cyano moiety has drawn sig-
nificant attention over unstable phosphine oxide, low ET triazine and
benzimidazole due to its robustness and less extended conjugation [13,
14,20–37]. As a linker, biphenyl unit is attractive due to its high ET, good
chemical stability and wide scope for chemical modification [35]. It is
anticipated that modification of biphenyl unit at 2 and 2′ positions with
donor and acceptor units would effectively hamper the strong inter-
chromophoric interactions and limit the conjugation for high triplet
energy.
2.2. Photophysical properties
The optical properties such as electronic absorption and emission of
the compounds were investigated using ultraviolet–visible (UV–Vis)
absorption and photoluminescence (PL) measurements. The absorption
and emission spectra of the compounds collected in tetrahydrofuran
(THF) solution are displayed in Fig. 1 and the relevant data are pre-
sented in Table 1. The compounds showed more than two indistin-
guishable absorption bands in the range of 274–341 nm. The short
wavelength absorption appearing below ca. 300 nm is assigned to the
Inspired by the above discussions, in this work we designed and
synthesized two simple and easily accessible high triplet energy bipolar
host materials named, CzBPCN and CNCzBPCN comprising of carba-
zole/cyanocarbazole donor and cyano acceptor connected through
biphenyl linker at 2 and 2′ positions, respectively. The effect of cyano
substituent on carbazole building block on photophysical, electro-
chemical, thermal and electroluminescence properties were investigated
in detail. Both the compounds showed high triplet energy over 3.0 eV.
Theoretical studies and single carrier analysis witnessed good bipolar
charge transporting feature for the materials. The potency of these
materials as hosts for blue PhOLEDs was tested by employing Ir(cb)3
blue dopant. The CNCzBPCN hosted device demonstrated excellent
performance with three fold improved EQEmax of 20.2%, current effi-
ciency (CE) of 30.8 cd/A compared to its congener CzBPCN, which
exhibited EQEmax of 7.1% and CE of 10.3 cd/A. Notably, the simple
cyano modification extended the operational lifetime time of the
CNCzBPCN device by 11 fold with LT50 of 121 h compared to the
CzBPCN device with LT50 of 11 h.
localized
and biphenyl units. Whereas, the long wavelength absorption band
stemming above ca. 300 nm is attributed to the n- * transitions of the
π-π* electronic transitions of the carbazole/cyanocarbazole
π
carbazole building block. Both CzBPCN and CNCzBPCN revealed high
optical band gaps of 3.59 eV and 3.53 eV, respectively, indicating their
wide band gap nature. The solutions of the materials showed deep-blue
emission on exposure to UV–Vis light with maximum wavelength fell in
the range of 380–420 nm. Indeed, the CzBPCN showed ca. 40 nm red-
shifted emission compared to CNCzBPCN. This could be attributed to
the strong CT character of CzBPCN due to the presence of strong
carbazole donor compared to cyanocarbazole in CNCzBPCN. This is also
supported by the featureless broad emission profile of CzBPCN. The
CNCzBPCN showed vibrational pattern characteristic of weak CT
emission [34,38]. To assess the credibility of the materials to function as
hosts for PhOLEDs, their lowest triplet excited states were analyzed by
recording the phosphorescence spectra of the materials in THF at low
temperature of 77 K. Similar to fluorescence features, the phosphores-
cence spectra of the compounds witnessed vibronic profile, indicating
that the phosphorescence of the materials stems from the local triplet
excited states [32,33]. The triplet energy (ET) of the materials was
determined from the highest energy vibronic peaks and was found to be
3.04 for CzBPCN and 3.06 eV for CNCzBPCN. The high ET is attributed
to the interrupted conjugation between the chromophores by virtue of
non-conjugation mode of linkage on 2 and 2’ positions of biphenyl unit
[34]. Though, the singlet energy is different for these materials, but
triplet energy is similar. This anomaly can be explained as follows.
Generally, the singlet energy is strongly affected by the CT property of
the organic compounds. The strong CT of CzBPCN lowered the singlet
energy, while the weak CT of CNCzBPCN did not. As the CT excited state
2. Results and discussion
2.1. Molecular design and synthesis
The molecular design of this work involved stable building blocks
such as carbazole/cyanocarbazole as hole transporting units, biphenyl
as a π-linker and cyano unit as an electron transporting unit. We pre-
sumed that substitution of donor and acceptor units on 2 and 2′-posi-
tions of biphenyl linker would hamper the effective conjugation
between donor and acceptor units and interrupt the strong electronic
communication between them, which would inherit high triplet energy
without sacrificing the bipolar charge transporting property for the host
Scheme 1. Synthetic route for the desired bipolar host materials.
2

R.K. Konidena et al.
Dyes and Pigments 187 (2021) 109118
Fig. 1. Electronic absorption and photoluminescence spectra of the materials recorded in THF solution.
energy of the emitter is smaller than the local excited state energy, the
singlet energy is dominated by the CT excited state energy. However, the
triplet energy is governed by the local triplet excited state energy
because it is smaller than the CT triplet excited state energy. Therefore,
the CT property has no effect on the triplet energy in the current two
hosts, resulting in similar triplet energy in the two compounds. The ET of
the CzBPCN and CNCzBPCN was high enough to serve as potential host
materials for the blue phosphor for efficient forward energy transfer
while preventing back energy transfer (see Fig. 2).
2.3. Electrochemical and thermal properties
To ascertain the electrochemical behavior of these materials, cyclic
voltammetry (CV) measurements were performed on their dilute solu-
tions in dichloromethane using 0.1 M tetrabutlyammonium perchlorate
as a supporting electrolyte and ferrocene as an internal standard to
approximate the peak potentials (Fig. S1 and Table 1). The compounds
showed an irreversible oxidation process with an anodically shifted
oxidation potential compared to the ferrocene reference, suggesting the
removal of electron from the appended carbazole/cyanocarbazole units.
Indeed, the CzBPCN revealed facile oxidation as witnessed by its low
oxidation potential (Eox) of +1.06 V compared to CNCzBPCN (+1.10 V)
due to the presence of relatively strong carbazole donor in CzBPCN
compared to cyanocarbazole in CzBPCN. The HOMO energy level of the
compounds was determined from their onset potential of the first
oxidation wave with reference to ferrocene and was ꢀ 5.86 eV for
CzBPCN and ꢀ 5.90 eV for CNCzBPCN. The LUMO energy level of the
compounds was estimated by subtracting the HOMO energy level from
the electrochemical band gap and found to be ꢀ 2.70 eV for CzBPCN and
ꢀ 2.77 for CNCzBPCN.
Fig. 2. Phosphorescence spectra of the bipolar hosts collected at 77 K in
THF matrix.
Table 1
Physicochemical and thermal parameters.
Compound
λabs, nm
λem (nm)a
ET
HOMO/
LUMO
(eV)c
Eg
T5d
(
εmax, Mꢀ 1
(eV)b
(eV)d
(oC)e
cmꢀ 1
103)a
×
CzBPCN
292 (21.4),
323 (sh),
338 (6.3)
274 (73.3),
326 (sh),
341 (6.8)
409
3.04
3.06
ꢀ 5.86/-
3.16
3.13
287
325
2.70
Thermal stability of the compounds was ascertained by thermogra-
vimetric (TGA) analysis under an inert atmosphere (Fig. 3). The com-
pounds showed decent thermal stabilities evident from their high
thermal decomposition temperature (T5d) above 287 ◦C corresponding
to 5% weight loss. Interestingly, the CNCzBPCN displayed marked
CNCzBPCN
345,
ꢀ 5.90/-
363.4,
377.5,
393 (sh)
2.77
a
Collected in THF solution.
thermal stability with T5d of 325 ◦C compared to CzBPCN (T5d
=
b
c
Determined from phosphorescence spectra.
HOMO and LUMO deduced from the CV.
Electrochemical band gap.
287 ◦C). This highlights the importance of cyano substituent in
improving the thermal robustness of the compounds. We also performed
differential scanning colorimetric measurements, but no definite glass
transition temperature (Tg) was observed for the compounds due to the
d
e
Thermal decomposition temperature (T5d).
3

R.K. Konidena et al.
Dyes and Pigments 187 (2021) 109118
materials. In addition, the highly twisted geometry is advantageous to
improve the thermal and morphological stability of the materials. As
shown in Fig. 4, the HOMO orbitals are localized on the carbazole/-
cyanocarbazole building blocks, while the LUMO orbital was delo-
calized on the biphenyl linker and cyano acceptor. The well separated
HOMO and LUMO orbitals on the donor and acceptor, respectively,
further supports the bipolar charge transporting property for the mate-
rials. The calculated HOMO/LUMO/band gap of the compounds
CzBPCN and CNCzBPCN are of ꢀ 5.45 eV/-1.49 eV/3.96 eV and ꢀ 5.91
eV/-1.71 eV/4.20 eV, respectively, and the trend is in good agreement
with the experimentally deduced values.
2.5. Carrier transport properties
Bipolar charge transport of the host materials is one of the most
important parameters for balanced charge injection and transportation,
broad recombination zone and superior device performances of PhO-
LEDs. Thus, to understand the relationship between the molecular
structure and charge transporting properties of these materials, single
carrier devices were constructed by fabricating hole only device (HOD)
and electron only device (EOD). The current density (J)-Voltage (V)
plots of the compounds are shown in Fig. 5. Both the compounds
exhibited decent HOD and EOD current densities compared to conven-
tional mCBP host material, indicating that the materials possess good
hole and electron transporting ability and secured bipolar charge
transporting feature. Indeed, the compound CzBPCN showed high hole
current density compared to CNCzBPCN due to the presence of rela-
tively strong carbazole donor compared to cyanocarbazole, while the
CNCzBPCN witnessed high electron current density compared to
CzBPCN due to the presence of two cyano acceptors. Nonetheless, the
single carrier devices of both the compounds showed sufficiently high
current densities for efficient hole and electron injection and trans-
portation and proved that the materials possess bipolar charge trans-
porting ability. In addition, the hole and electron motilities of the
materials quantified by the space charge limited current (SCLC) method
and were found to be 4.50 × 10ꢀ 6 cm2 Vꢀ 1 sꢀ 1 and 3.45 × 10ꢀ 7 cm2 Vꢀ 1
Fig. 3. TGA traces of the bipolar host materials.
small molecular weight of the two compounds for local motion of the
molecular structure. The marked thermal stabilities suggest these ma-
terials can tolerate heat during vacuum processed OLED device
fabrication.
2.4. Theoretical studies
To understand the electronic structure of these materials, density
functional theoretical computations were executed on the compounds
by employing B3LYP/6-31G* functional [40]. The optimized geometries
and frontier molecular orbital diagrams are shown in Fig. 4. The low
energy conformation of the compounds displayed a non-planar three
dimensional geometry with a large twisting angle between appended
donor and biphenyl linker of >70◦ and between two phenyl units of
biphenyl of >58◦. This can be due to the severe steric repulsions be-
tween the chromophores at the 2-position of biphenyl linker. These re-
sults suggest that the effective conjugation between the donor and
biphenyl would interrupt and hamper the strong charge transfer in-
teractions between donor and acceptor, which is beneficial for securing
high triplet energy with bipolar charge transporting feature for the host
s
ꢀ 1 for CzBPCN and 2.33 × 10ꢀ 6 cm2 Vꢀ 1 sꢀ 1 and 8.98 × 10ꢀ 7 cm2 Vꢀ 1
sꢀ 1 for CNCzBPCN, respectively. This proves that the CNCzBPCN has
better bipolar charge transport properties for balanced carrier density
and efficient exciton formation.
Fig. 4. a) Optimized geometries b) FMO distribution of the bipolar hosts calculated using B3LYP/6-31G* basis set on Gaussian 09 program.
4

R.K. Konidena et al.
Dyes and Pigments 187 (2021) 109118
Fig. 5. I–V plots of a) hole-only and b) electron-only device.
2.6. Electroluminescence properties
operational time of the blue devices are portrayed in Fig. 7 and data
are summarized in Table 2.
The high triplet energy and bipolar charge transporting feature of
the CzBPCN and CNCzBPCN hosts encouraged us to employ them as
bipolar host materials for deep-blue phosphorescent emitter, [[5-(1,1-
dimethylethyl)-3-phenyl-1H-imidazo[4,5-b]pyrazin-1-yl-2(3H)-yli-
dene]-1,2-phenylene]bis[[6-(1,1-dimethylethyl)-3-phenyl-1H-imidazo
As shown in Fig. 7, EL spectra of both the compounds displayed
single peak blue emission with a peak maximum of 469 nm, which
corresponded to the emission of the blue Ir(cb)3 dopant. The EL spectra
were in agreement with the PL spectra of the emitting layer film
(Fig. S2). The absence of any other emissions from the host or any layers
in the device supports efficient exothermic energy transfer from the
hosts to Ir(cb)3 dopant. The Commission Internationale de l’Elcairage
(CIE) coordinates of the devices lie in the range of pure blue coordinates
of x ~ 0.14 and 0.18 < y < 0.19. The CzBPCN hosted device exhibited
maximum EQE (EQEmax) of 7.1%, current efficiency (CE) of 10.3 cd/A
and power efficiency (PE) of 6.5 lm/W. Whereas, the CNCzBPCN hosted
device showed EQEmax of 20.2%, CE of 30.8 cd/A and PE of 26.9 lm/W.
Certainly, the CNCzBPCN hosted device demonstrated superior perfor-
mance over CzBPCN based device with three-fold improved EQE and
CE. This can be attributed to the good charge transporting ability. Be-
sides, the operational lifetime is one of the important issues of blue
PhOLEDs, thus we have measured the lifetime of the devices and pre-
sented the L versus time plot at an initial luminance (L0) of 200 cd/m2 in
Fig. 7. The CNCzBPCN hosted device exhibited 11 times extended
[4,5-b]pyrazin-1-yl-2(3H)-ylidene]-1,2-phenylene]iridium
(Ir(cb)3).
The Ir(cb)3 was chosen as the phosphor because of pure emission color,
high efficiency and moderate lifetime [24,25]. The optimized device
structure of the multi-layered deep-blue PhOLED device was
ITO/BPBPA:HATCN/BPBPA/mCBP/CzBPCN
or
CNCzBPCN:Ir
(cb)3/DBFTrz/ZADN/LiF/Al. Here, BPBPA:HATCN and BPBPA were
used as a hole injection and hole transporting layers, respectively. The
mCBP and DBFTrz served as hole transporting type and electron
transporting type exciton blocking layers and ZADN was employed as
an electron transporting layer. The emitting layer was developed by
doping 30 wt% of Ir(cb)3 either in CzBPCN or CNCzBPCN host. The
energy level alignment plot of the device and chemical structures of
each layer used in the device are shown in Fig. 6. The current density
(J)-voltage (V)-luminance (L) plots, EQE vs L, EL plot and relative L vs
Fig. 6. Energy level diagram and the chemical structures of materials used in the device.
5

R.K. Konidena et al.
Dyes and Pigments 187 (2021) 109118
Fig. 7. a) I–V-L plots, b) EQE vs L, c) EL plot and d) Relative luminance vs time plots of the blue devices.
Table 2
EL parameters of the blue devices.
a
Host
Voltage (Von
)
EQE (%)
Max.
CE (cd/A)
Max.
PE (lm/W)
Max.
CIE (x, y)
LT50 (h)b
@1000 cd/m2
@1000 cd/m2
@1000 cd/m2
CzBPCN
3.2
3.1
7.1
5.8
10.3
30.8
8.1
6.5
3.9
0.14, 0.18
0.14, 0.19
11
CNCzBPCN
20.2
14.8
21.2
26.9
11.4
121
a
turn-on voltage (V) at 1 cd/m2 bL0 = 200 cd/m.2.
operational lifetime up to 50% of its initial luminance (LT50) of 121 h
compared to CzBPCN hosted device with LT50 of 11 h. The long lifetime
of the CNCzBPCN device may be attributed to the formation of broad
recombination zone in emitting layer due to the balanced carrier
mobility by addition of CN unit. The similar hole and electron mobilities
assist enhancing the device lifetime by reducing TTA and triplet-polaron
annihilation through the wide emission zone. Moreover, the large LUMO
gap between the mCBP electron blocking layer and CNCzBPCN host
suppresses electron leakage damaging the mCBP layer, elongating the
device lifetime. The device lifetime of the CNCzBPCN device was much
longer than that of the conventional mCBP device. Furthermore, it is
even longer than that of the CNCzCN1 device reported in our previous
work considering the color coordinate of the blue PhOLEDs [41].
Although the absolute device lifetime was about 200 h in previous work,
it was obtained from the sky-blue PhOLEDs and the hosts could work
only in the sky-blue devices. Therefore, the CNCzBPCN can be regarded
as the deep blue host providing long lifetime. These results would pro-
vide insightful information to design simple and efficient high triplet
energy bipolar host materials for simultaneous achievement of high EQE
over 20% and long operational lifetime of blue PhOLEDs.
efficient high triplet energy bipolar host materials viz., CzBPCN and
CNCzBPCN for blue PhOLEDs by modifying biphenyl with carbazole
type donor and cyano acceptor at 2 and 2′ positions, respectively.
Photophysical properties revealed that the donor and acceptor in-
teractions were effectively minimized due to large dihedral angles,
which resulted in high triplet energy above 3.0 eV for the host materials.
The CNCzBPCN hosted device demonstrated three-fold improved per-
formance over CzBPCN with EQEmax of 20.2% and CE of 30.8 cd/A.
Notably, the CNCzBPCN revealed 11 times extended lifetime with LT50
of 121 to CzBPCN (LT50 = 11 h). These results are either comparable or
superior to the reported deep-blue PhOLEDs (Table S1) [25,41–45]. We
believe that these results would provide valuable guidelines to design
cost-effective and efficient high triplet energy host materials for blue
PhOLEDs to reach high EQE and long lifetime together.
4. Experimental section
4.1. General methods
The commercially purchased chemicals and solvents were used as
received. All the reactions were performed under an inert atmosphere.
The intermediates and desired host materials were purified using col-
umn chromatography methods using silica as a stationary phase. The
nuclear magnetic resonance spectroscopy (NMR) data were recorded on
3. Conclusions
In conclusion, we have designed and synthesized two simple and
6

R.K. Konidena et al.
Dyes and Pigments 187 (2021) 109118
500 MHz JEOL spectrometer and tetramethylsilane was used as an in-
ternal standard to approximate the chemical shift values. The solutions
used for analytical studies were prepared at room temperature. The
absorption and photoluminescence measurements were carried out on
UV–vis absorption and photoluminescence spectrophotometer. Ther-
mogravimetric analysis data were collected under a N2 atmosphere at a
heating rate of 10 ◦C/min. Cyclic voltammetry measurements were
carried out on CHI electrochemical analyzer using three electrode sys-
tem consisting of carbon working electrode, platinum counter electrode
and Ag/AgNO3 reference electrode. Density functional computations
were performed on Gaussian 09 program using B3LYP/6-31G* basis set.
(200 nm). The emitting layer was made by doping 30 wt% Ir(cb)3 dopant
either in CzBPCN or CNCzBPCN hosts. The vacuum evaporation process
was used under 3.0 × 10ꢀ 7 Torr for fabrication of the devices. The de-
vices was equipped with a glass lid filled with nitrogen and stored in
glove box to protect them from oxygen, and all device performances
were measured outside the glove box. The characterization of the de-
vices was performed using a Keithley 2400 source meter, and optical
characterization was carried out using a CS 2000 spectroradiometer.
Author statement
Rajendra Kumar Konidena: Investigation, Writing - Original Draft,
Won Jae Chung: Investigation, Jun Yeob Lee: Supervision, Writing-
Review & Editing.
4.2. Synthesis
The starting materials, 1-bromo-2-fluorobenzene (1) and (2-cyano-
phenyl)boronic acid, were purchased from TCI chemicals and interme-
diate 9-(2-bromophenyl)-9H-carbazole (1a) and 9-(2-Bromophenyl)-
9H-carbazole-3-carbonitrile (1b) was prepared as described in the
literature [39,40].
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
2’-(9H-carbazol-9-yl)-[1,1′-biphenyl]-2-carbonitrile (CzBPCN).
The 1a (1.0 g, 3.1 mmol), (2-cyanophenyl)boronic acid (5.4 g, 3.72
mmol), potassium carbonate (1.2 g, 9.3 mmol), and tetrakis
(triphenylphosphine)-palladium(0) (0.2 g, 0.15 mmol) were dissolved
in 20 mL of THF:H2O (3:1) mixed solvent. The reaction was allowed to
Acknowledgement
◦
reflux at 80 C for 6 h. The reaction was monitored with TLC exami-
This work was supported by the Basic Science Research Program
(Grant No. 2016M3A7B4909243 and 2018R1D1A1B07048498) through
the National Research Foundation (NRF) of Korea funded by the Min-
istry of Science, ICT, and Future Planning.
nation. After completion of the reaction, extraction in dichloromethane
was performed and the organic layer was dried on anhydrous MgSO4.
The solvent was removed using rotary evaporation. The resulted black
crude mixture was purified on column chromatography using 1:2 ratio
of DCM:hexanes, which resulted in CzBPCN as a white solid, Yield (0.85
g and 85%): 1H NMR (500 MHz, CDCl3, δ ppm): 8.0 (d, J = 10.0 Hz, 2H),
7.75–7.73 (m, 1H), 7.68–7.66 (m, 2H), 7.58–7.57 (m, 1H), 7.53–7.51
(m, 1H), 732–7.31 (m, 2H), 7.19–7.13 (m, 4H), 7.06–7.03 (m, 1H),
6.94–6.91 (m, 1H), 6.78 (d, J = 8.0 Hz, 1H). 13C NMR (125 MHz, CDCl3,
δ ppm): 142.22, 137.72, 135.55, 133.35, 132.31, 132.06, 130.78,
130.37, 129.23, 129.15, 128.05, 126.15, 123.14, 120.35, 120.00,
118.74, 112.13, 109.98. MS (APCI) m/z 345.41 [(M + H)+]. Elemental
analysis: Found: C, 88.33; N, 7.98; H, 4.84%; molecular formula
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2020.109118.
References
[1] Baldo MA, O’Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, Forrest SR.
Highly efficient phosphorescent emission from organic electroluminescent devices.
Nature 1998;395:151.
C
25H16N2 requires C, 88.18; N, 8.13; H, 4.68%
[2] Adachi C, Baldo MA, Forrest SR. High-efficiency organic electrophosphorescent
devices with tris(2-phenylpyridine)iridium doped into electron-transporting
materials. Appl Phys Lett 2000;77:904.
9-(2′-Cyano-[1,1′-biphenyl]-2-yl)-9H-carbazole-3-carbonitrile
(CNCzBPCN). The CNCzBPCN was synthesized as per the procedure
used for CzBPCN using 1b (1.0 g, 2.88 mmol), (2-cyanophenyl)boronic
acid (0.5 g, 3.45 mmol), potassium carbonate (1.2 g, 8.64 mmol), and
tetrakis(triphenylphosphine)-palladium(0) (0.16 g, 0.14 mmol). White
solid, Yield 0.87 g and 87%: 1H NMR (500 MHz, CDCl3, δ ppm): 8.29 (s,
1H), 8.02 (d, J = 10.0 Hz, 1H), 7.75–7.72 (m, 3H), 7.59–7.58 (m, 3H),
7.44 (s, 1H), 7.30–7.26 (m, 3H), 7.23–7.11 (m, 1H), 7.09–6.98 (m, 1H),
6.97–6.93 (m, 1H). 13C NMR (125 MHz, CDCl3, δ ppm): 141.58, 137.79,
134.30, 133.37, 132.54, 132.19, 131.04, 129.88, 129.52, 128.69,
128.45, 127.70, 125.57, 123.35, 121.29, 120.71, 120.30, 118.29,
112.80, 110.76, 110.63. MS (APCI) m/z 370.42 [(M + H)+]. Elemental
analysis: Found: C, 85.80; N, 11.34; H, 4.58%; Molecular formula
C26H15N3 requires, C, 85.63; N, 11.37; H, 4.49%
[3] Duan L, Qiao J, Sun Y, Qiu Y. Strategies to design bipolar small molecules for
OLEDs: donor-acceptor structure and non-donor-acceptor structure. Adv Mater
2011;23:1137.
[4] Sasabe H, Kido J. Recent progress in phosphorescent organic light-emitting
devices. Eur J Org Chem 2013:7653.
[5] Yang X, Zhou G, Wong WY. Functionalization of phosphorescent emitters and their
host materials by main-group elements for phosphorescent organic light-emitting
devices. Chem Soc Rev 2015;44:8484.
[6] Wang Y, Yun JH, Wang L, Lee JY. High triplet energy hosts for blue organic light-
emitting diodes. Adv Funct Mater 2020. https://doi.org/10.1002/
adfm.202008332.
[7] Pal AK, Krotkus S, Fontani M, Mackenzie CFR, Cordes DB, Slawin AM,
Samuel DWF, Colman EZ. High-efficiency deep-blue-emitting organic light-
emitting diodes based on iridium(III) carbene complexes. Adv Mater 2018;30:
1804231.
[8] Zhu ZQ, Park CD, Klimes K, Li J. Highly efficient blue OLEDs based on metal-
assisted delayed fluorescence Pd(II) complexes. Adv Optic Mater 2019;7:1801518.
[9] Wu C, Wang B, Wang Y, Hu J, Jiang J, Ma D, Wang QA. A universal host material
with a simple structure for monochrome and white phosphorescent/TADF OLEDs.
J Mater Chem C 2019;7:558.
4.3. Device fabrication details
[10] Li W, Li J, Liu D, Wang F, Zhang S. Bipolar host materials for high-efficiency blue
phosphorescent and delayed-fluorescence OLEDs. J Mater Chem C 2015;3:12529.
[11] Shin H, Lee JH, Moon CK, Huh JS, Sim B, Sky-Blue J J Kim. Phosphorescent OLEDs
with 34.1% external quantum efficiency using a low refractive index electron
transporting layer. Adv Mater 2016;28:4920.
The blue PhOLED devices were fabricated by employing the
following device configuration; indium tin oxide (ITO, 50 nm)/PEDOT:
PSS (60 nm)/N4, N4,N4′,N4’ -tetra([1,1′-biphenyl]-4-yl)-[1,1′-
biphenyl]-
4,4′-
diamine
(BPBPA):1,4,5,8,9,11-Hexaaza-
[12] Swensen JS, Polikarpov E, Ruden AV, Wang L, Sapochak LS, Padmaperuma AB.
Improved efficiency in blue phosphorescent organic light-emitting devices using
host materials of lower triplet energy than the phosphorescent blue emitter. Adv
Funct Mater 2011;21:3250.
triphenylenehexacarbonitrile (HATCN) (20 nm)/N4, N4,N4′,N4’ -tetra
([1,1′-biphenyl]-4-yl)-[1,1′-biphenyl]-4,4′-diamine (BPBPA) (10 nm)/
3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP) (10 nm)/emitting layer
(EML) (25 nm)/2,8- 5 bis(4,6-diphenyl-1,3,5-triazin-2-yl)dibenzo[b,d]
furan (DBFTrz) (5 nm)/2-(4-(9,10-di(naphthalen-2-yl)anthracen-2-yl)
phenyl)-1-phenyl-1H-benzo[d]imidazole(ZADN) (20 nm)/LiF(2 nm)/Al
[13] Tao Y, Yang C, Qin J. Organic host materials for phosphorescent organic light-
emitting diodes. Chem Soc Rev 2011;40:2943.
[14] Chaskar A, Chen HF, Wong KT. Bipolar host materials: a chemical approach for
highly effi cient electrophosphorescent devices. Adv Mater 2011;23:3876.
7

R.K. Konidena et al.
Dyes and Pigments 187 (2021) 109118
[15] Sicard LJ, Li HC, Wang Q, Liu XY, Jeannin O, Berthelot JR, Liao LS, Jiang ZQ,
Poriel C. C1-Linked spirobifluorene dimers: pure hydrocarbon hosts for high-
performance blue phosphorescent OLEDs. Angew Chem Int Ed 2019;58:3848.
[16] Zhu XD, Zhang YL, Yuan Y, Zheng Q, Yu Y, Li Y, Jiang ZQ, Liao LS. Incorporating a
tercarbazole donor in a spiro-type host material for efficient RGB phosphorescent
organic light-emitting diodes. J Mater Chem C 2019;7:6714.
solution-processable green phosphorescent organic light-emitting diodes. Org Lett
2012;14:4786.
[30] Chen WC, Yuan Y, Zhu ZL, Jiang ZQ, Su SJ, Liao LS, Lee CS. De novo design of
D–σ–A molecules as universal hosts for monochrome and white phosphorescent
organic light-emitting diodes. Chem Sci 2018;9:4062.
[31] Liang Q, Han C, Duan C, Xu H. Blue thermally activated delayed fluorescence-
emitting phosphine oxide hosts for ultrasimple and highly efficient white organic
light-emitting diodes. Adv Opt Mater 2018;6:1800020.
[17] Godumala M, Choi S, Park SY, Cho MJ, Kim HJ, Ahn DH, Moon JS, Kwon JH,
Choi DH. Chromenopyrazole-based bipolar blue host materials for highly efficient
thermally activated delayed fluorescence organic light-emitting diodes. Chem
Mater 2018;30:5005.
[32] Konidena RK, Chung W, Lee JY. Molecular engineering of cyano-substituted
carbazole-based host materials for simultaneous achievement of high efficiency
and long lifetime in blue phosphorescent organic light-emitting diodes. Adv
Electron Mater 2020;6:2000132.
[18] Wang F, Liu D, Li J, Ma M. Modulation of n-type units in bipolar host materials
toward high-performance phosphorescent OLEDs. ACS Appl Mater Interfaces 2017;
9:37888.
[33] Konidena RK, Chung WJ. J. Y. Lee. C1-, C2-, and C3-modified carbazole derivatives
as promising host materials for phosphorescent organic light-emitting diodes. Org
Lett 2020;22:2786.
[19] Song W, Shi L, Gao L, Hu P, Mu H, Xia X, Huang J, Su J. [1,2,4]Triazolo[1,5-a]
pyridine as building blocks for universal host materials for high-performance red,
green, blue and white phosphorescent organic light-emitting devices. ACS Appl
Mater Interfaces 2018;10:5714.
[34] Konidena RK, Lee KH, Lee JY. Molecular design and synthetic approach to C2,C3,
C4-modified carbazoles: high triplet energy bipolar host materials for efficient blue
phosphorescent organic light emitting diodes. Chem Commun 2019;55:8178.
[35] Konidena RK, Thomas KRJ, Pathak A, Dubey DK, Sahoo S, Jou JH. Tuning the
photophysical and electroluminescence properties in asymmetrically
tetrasubstituted bipolar carbazoles by functional group disposition. ACS Appl
Mater Interfaces 2018;10:24013.
[20] Song W, Xu Q, Zhu J, Chen Y, Mu H, Huang J, Su J. Imidazo[1,2-b]pyridazine as
building blocks for host materials for high-performance red-phosphorescent
organic light-emitting devices. ACS Appl Mater Interfaces 2020;12:19701.
[21] Im Y, Byun SY, Kim JH, Lee DR, Oh CS, Yook KS, Lee JY. Recent progress in high-
efficiency blue-light-emitting materials for organic light-emitting diodes. Adv
Funct Mater 2017;27:1603007.
[36] Tagare J, Vaidyanathan S. Recent development of phenanthroimidazole-based
fluorophores for blue organic light-emitting diodes (OLEDs): an overview. J Mater
Chem C 2018;6:10138.
[22] Bai Q, Liu H, Yao L, Shan T, Li YJ, Gao Y, Zhang Z, Liu Y, Lu P, Yang B, Ma Y.
Adjusting nitrogen atom orientations of pyridine ring in tetraphenylsilane-based
hosts for highly efficient blue phosphorescent organic light-emitting devices. ACS
Appl Mater Interfaces 2018;8:24793.
[37] Reddy MR, Han SH, Lee JY, Seo SY. Synthesis and characterization of quinoxaline
derivative for high performance phosphorescent organic light-emitting diodes.
Dyes Pigments 2018;153:132.
[23] Chang SY, Li GT, Cheng YC, Hunag JJ, Chang CL, Lin CF, Lee JH, Chiu TL,
Leung MK. Construction of highly efficient carbazol-9-yl-substituted benzimidazole
bipolar hosts for blue phosphorescent light-emitting diodes: isomer and device
performance relationships. ACS Appl Mater Interfaces 2018;10:42723.
[24] Jung M, Lee KH, Lee JY, Kim T. A bipolar host based high triplet energy electroplex
for an over 10000 h lifetime in pure blue phosphorescent organic light-emitting
diodes. Mater Horizon 2020;7:559.
[38] Konidena RK, Chung WJ, Lee JY. Molecular engineering of cyano-substituted
carbazole-based host materials for simultaneous achievement of high efficiency
and long lifetime in blue phosphorescent organic light-emitting diodes. Adv
Electron Mater 2016;2:1500241.
[39] Fan C, Zhao F, Gan P, Yang S, Liu T, Zhong C, Ma D, Qin J, Yang C. Simple bipolar
molecules constructed from biphenyl moieties as host materials for deep-blue
phosphorescent organic light-emitting diodes. Chem Eur J 2012;18:5510.
[40] Frisch MJ, et al. Gaussian 09, revision A.02. Wallingford, CT: Gaussian, Inc.; 2009.
[41] Shin SK, Lee KH, Lee HL, Lee JY. Management of triplet energy and charge-
transport properties of hosts by CN position engineering. J Phys Chem C 2019;123:
8531.
[25] Maheshwaran A, sree VG, Park HY, Kim H, Han SH, Lee JY, Jin SH. High efficiency
deep-blue phosphorescent organic light-emitting diodes with CIE x, y (≤ 0.15) and
low efficiency roll-off by employing a high triplet energy bipolar host material. Adv
Funct Mater 2018;28:1802945.
[26] Choi KH, Lee KH, Lee JY, Kim T. Simultaneous achievement of high efficiency and
long lifetime in deep blue phosphorescent organic light-emitting diodes. Adv Opt
Mater 2019;7:1901374.
[42] Byeon SY, Kim JH, Lee JY. CN-modified host materials for improved efficiency and
lifetime in blue phosphorescent and thermally activated delayed fluorescent
organic light-emitting diodes. ACS Appl Mater Interfaces 2017;9:13339.
[43] Yang CY, Chung WJ, Jeong H, Lee Y, Lee JY. A negative polaron resistant p-type
host for extended lifetime in deep blue phosphorescent organic light-emitting
diodes. J Mater Chem C 2020;8:5131.
[27] Im Y, Han S, Lee JY. CN substituted indolocarbazole as a core structure of exciton
harvesting and lifetime extending host for green thermally activated delayed
fluorescent emitter. Dyes Pigments 2019;164:233.
[28] Jiang W, Tang J, Ban X, Sun Y, Duan L, Qiu Y. Ideal bipolar host materials with bis-
benzimidazole unit for highly efficient solution-processed green
[44] Ko SB, Kang S, Kim T. A silane-based bipolar host with high triplet energy for high
efficiency deep-blue phosphorescent OLEDs with improved device lifetime. Chem
Eur J 2020;26:7767.
electrophosphorescent devices. Org Lett 2014;16:5346.
[29] Huang H, Fu Q, pan B, Zhuang S, Qang L, Chen J, Ma D, Yang C. Butterfly-shaped
tetrasubstituted carbazole derivatives as a new class of hosts for highly efficient
[45] Cho YJ, Lee JY. Tetraphenylsilane-based high triplet energy host materials for blue
phosphorescent organic light-emitting diodes. J Phys Chem C 2011;115:10272.
8