Fine tuning of bipolar side group on dual anthracene core derivatives for highly efficient blue emitters

Article abstract of DOI:10.1016/j.dyepig.2020.108555

Three bipolar blue fluorescent materials were newly synthesized by attaching diphenylamine (DPA), triphenylamine (TPA), and 9-phenylcarbazole amine (PCA) as an electron donating group and 2,4- diphenyl-1,3,5-triazine (DPT) as an electron withdrawing group to the dual anthracene core moiety. Compounds are [10'-(4,6-Diphenyl-[1,3,5]triazin-2-yl)-[9,9']bianthracenyl-10-yl]-diphenyl-amine (DPA-AA-DPT), {4-[10'-(4,6-Diphenyl-[1,3,5]triazin-2-yl)-[9,9']bianthracenyl-10-yl]-phenyl}-diphenyl- amine (TPA-AA-DPT), and 9-(4-(10'-(4,6-diphenyl-1,3,5-triazin-2-yl)-[9,9′-bianthracen]-10-yl)phenyl)- 9H-carbazole (PCA-AA-DPT), respectively. The optical, thermal, and electroluminescence (EL) properties of the synthesized materials were measured. Newly bipolar materials were found to have blue emission in the solution state and to acquire high PLQY values. A device doped with PCA-AA- DPT showed a very high efficiency of 6.86 cd/A and EQE of 5.53% at a high current density without roll-off phenomenon.

Full text of DOI:10.1016/j.dyepig.2020.108555

Dyes and Pigments 181 (2020) 108555
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Fine tuning of bipolar side group on dual anthracene core derivatives for
highly efficient blue emitters
1
1
*
Seokwoo Kang , Hayoon Lee , Yeonhee Shim, Sunwoo Park, Jongwook Park
Department of Chemical Engineering, Kyung Hee University, Gyeonggi, 17104, South Korea
A B S T R A C T
Three bipolar blue fluorescent materials were newly synthesized by attaching diphenylamine (DPA), triphenylamine (TPA), and 9-phenylcarbazole amine (PCA) as
an electron donating group and 2,4- diphenyl-1,3,5-triazine (DPT) as an electron withdrawing group to the dual anthracene core moiety. Compounds are [10’-(4,6-
Diphenyl-[1,3,5]triazin-2-yl)-[9,9’]bianthracenyl-10-yl]-diphenyl-amine (DPA-AA-DPT), {4-[10’-(4,6-Diphenyl-[1,3,5]triazin-2-yl)-[9,9’]bianthracenyl-10-yl]-
0
phenyl}-diphenyl- amine (TPA-AA-DPT), and 9-(4-(10’-(4,6-diphenyl-1,3,5-triazin-2-yl)-[9,9 -bianthracen]-10-yl)phenyl)- 9H-carbazole (PCA-AA-DPT), respec-
tively. The optical, thermal, and electroluminescence (EL) properties of the synthesized materials were measured. Newly bipolar materials were found to have blue
emission in the solution state and to acquire high PLQY values. A device doped with PCA-AA- DPT showed a very high efficiency of 6.86 cd/A and EQE of 5.53% at a
high current density without roll-off phenomenon.
1
. Introduction
Therefore, development of blue materials with high efficiency and
long life-time is still needed [7,8]. A typical OLED electroluminescence
(EL) device consists of an anode, hole transporting layer (HTL), emitting
layer (EML), electron transporting layer (ETL), and cathode. The per-
formance of the EL device is highly dependent on the recombination
efficiency of holes and electrons in the EML [9]. In general, in the
organic semiconductor compound, since mobility of holes is faster than
that of electrons, injection and movement of charges are an important
key for improving EL performance [10,11]. Since light emitting mate-
rials having bipolar characteristics have both hole and electron trans-
porting parts, they can efficiently form the stable exciton by balancing
holes and electrons [12,13]. In addition, bipolar materials are widely
applied to OLEDs because they exhibit low roll-off due to the excellent
charge balance even at high luminance [14,15]. However, it is difficult
to realize blue emission through bipolar characteristics because charge
transfer occurs due to the introduction of the donor-acceptor moiety
[16–18]. In order to implement bipolar blue emitters, it should have the
optimized bipolar characteristics. For this, it is possible to implement
bipolar blue emitters by controlling electron donating and acceptor
ability or by introducing a phenyl- or diphenyl-group between donor
and acceptor [19–25].
Since organic light emission was first discovered in the anthracene
crystal in 1963, and research on OLED was first started by Dr. Tang in
987, it is currently used in various applications such as mobile phones,
1
TVs, and lightings. OLED has occupied the display market with many
advantages such as wide viewing angle, thin panel, light weight, fast
response time, and high saturation [1–3]. During the growing OLED
market, it has brought great attention to the research and development
of OLED materials around the world. In principle, fluorescence has an
internal quantum efficiency of only 25%, but phosphorescence has an
internal quantum efficiency of 100% [4]. Currently, in red and green
materials, various phosphorescent emitters with high efficiency and
long life-time have been developed and commercialized in the market.
However, phosphorescent emitters contain heavy metal atoms such as
iridium (Ir) and platinum (Pt), which generally do not exhibit the blue
phosphorescence emission associated with wide band gaps. In addition,
blue phosphorescent organic light-emitting diodes (PhOLEDs) have
serious problems with roll-off of efficiency at high current density and
short device lifetimes at high brightness [5,6]. Also, unlike the cases of
red and green light emitters, it is difficult for the hole and electron in-
jections to achieve in blue light emitters due to their wide intrinsic band
gaps. A high voltage is required to operate a wide bandgap emitter in
device. The higher the voltage, the more heat is generated, so the blue
light emitter is usually low in efficiency and short device life-time.
In this study, DPA-AA-DPT, TPA-AA-DPT, and PCA-AA-DPT were
successfully synthesized by introducing diphenylamine, triphenylamine,
phenylcarbazole amine as a donor moiety and 2,4-diphenyl-1,3,5-
0
triazine as an acceptor moiety to AA (9,9 -bianthracene) main core
*
Corresponding author.
E-mail address: jongpark@khu.ac.kr (J. Park).
Seokwoo Kang and Hayoon Lee contributed equally to this work as first authors.
1
https://doi.org/10.1016/j.dyepig.2020.108555
Received 28 April 2020; Received in revised form 15 May 2020; Accepted 15 May 2020
Available online 19 May 2020
0
143-7208/© 2020 Elsevier Ltd. All rights reserved.

S. Kang et al.
Dyes and Pigments 181 (2020) 108555
0
�
moiety. Since 9,9 -bianthracene has a twisted structure at 90 , it is
possible to implement blue emitting materials. Furthermore, the mole-
cule was designed to enable blue emission by fixing the electron
acceptor group and by the variation of the electron donating ability. The
related photophysical and electroluminescence (EL) properties were
investigated.
reactions. When the reaction was complete, extract with chloroform and
distilled water. The organic layer was filtered through anhydrous
4
MgSO , and the solution was concentrated under reduced pressure.
Column purification with chloroform: hexane ¼ 1: 1 gives a yellow solid.
(0.82 g, 65%)
1
H NMR (300 MHz, [D8]THF): δ ¼ 8.86 (d, 4H), 8.06 (d, 2H), 7.99
(
d, 2H), 7.65-7.56 (m, 6H), 7.51 (d, 2H), 7.42-7.34 (m, 10H), 7.31 (d,
2
. Experimental
4H), 7.22 (q, 8H), 7.12 (t, 2H). HRMS (FAB-MS, m/z): calcd. for
þ
C61H41N4, 829.3287; found: 829.3327 [MþH] . Anal. calcd for
2
.1. Synthesis
C61H40N4: C 88.12, H 5.25, N 6.63; found: C 88.61, H 4.85, N 6.71.
0
Compound (1), (2), (3) were synthesized according to the literature
26]. The final product was unable to measure 13C NMR due to its low
2.1.5. Synthesis of 9-(4-(10’-(4,6-diphenyl-1,3,5-triazin-2-yl)-[9,9 -
bianthracen]-10-yl)phenyl)-9H-carbazole (PCA-AA-DPT)
[
solubility.
2 3
Pd(OAc) (0.014 g, 0.062 mmol), (cyclohexyl) P (0.034 g, 0.12
mmol) was added to Compound 5 (1.0 g, 1.51 mmol), 9-(4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-carbazole (0.61 g,
1.65 mmol) in nitrogen conditions. The mixture was refluxed with
0
2
.1.1. Synthesis of 2-([9,9 -bianthracen]-10-yl)-4,6-diphenyl-1,3,5-
triazine (Compound 4)
Pd(PPh (0.20 g, 0.18 mmol) was added to compound 3 (2.0 g,
.16 mmol) and 2-Chloro-4,6 diphenyl [1,3,5]triazine (1.34 g, 0.18
mmol) under a nitrogen conditions. The mixture was refluxed with
�
3
)
4
stirring in anhydrous toluene 80.0 mL. At 50 C, added tetraethy-
4
lammonium hydroxide (20 wt%) 10.0 mL to the reaction. When the
reaction is complete, extract with chloroform and distilled water. The
�
stirring in anhydrous toluene 150 mL and ethanol 250 mL. At 50 C, add
M K CO solution (45 mL) to the reaction. After stirring for 5 h under
4
organic layer was filtered through anhydrous MgSO , and the solution
2
2
3
was concentrated under reduced pressure. After column purification
under chloroform: hexane ¼ 1 : 1, reprecipitation with chloroform and
ethanol gives a yellow solid. (0.95 g, 77%)
nitrogen condition, the reaction is terminated and extracted with chlo-
roform and D.I water. The organic layer was filtered through anhydrous
1
MgSO
4
, and the solution was concentrated under reduced pressure. After
H NMR (300 MHz, DMSO, TMS): δ ¼ 8.77 (d, 4H), 8.35 (d, 2H), 8.06
column purification under chloroform: hexane ¼ 1 : 1, reprecipitation
(d, 2H), 8.01-7.95 (m, 6H), 7.79-7.66 (m, 8H), 7.60-7.50 (m, 6H), 7.41
with chloroform and ethanol gives a beige solid. (1.76 g, 72%)
(q, 6H), 7.20 (t, 4H). HRMS (FAB-MS, m/z): calcd. for C61H39N4,
1
þ
H NMR (300 MHz, [D8]THF): δ ¼ 8.86 (d, 4H), 8.80 (s, 1H), 8.22 (d,
827.3130; found, 827.3184 [MþH] . Anal. calcd for C61H38N4: C
2
2
H), 8.04 (d, 2H), 7.68-7.55 (m, 6H), 7.49-7.44 (m, 2H), 7.40-7.35 (m,
H), 7.23 (d, 8H).
88.59, H 4.63, N 6.77; found: C 88.60, H 4.63, N 6.74.
2
.2. Materials, measurements, and OLED fabrication
0
0
2
1
.1.2. Synthesis of 2-(10 -bromo-[9,9 -bianthracen]-10-yl)-4,6-diphenyl-
,3,5-triazine (Compound 5)
Reactant diphenylamine, 4-(diphenylamino)phenylboronic acid, and
Compound 4 (1.5 g, 2.56 mmol) was dissolved in 100 mL of chlo-
9-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-carba-
zole were purchased from Sigma-Aldrich and Alfa-Aesar. Reagents and
solvents were purchased as reagent grade and were used without further
purification. Analytical TLC was carried out on a Merck 60 F254 silica
gel plate, and column chromatography was performed on Merck 60
roform and N-bromosuccinimide was added. Add 10 mL of acetic acid
and reflux. When the reaction was completed, add 100 mL of acetone
and stir for 1 h. After extraction with chloroform and distilled water,
reprecipitation with chloroform and ethanol gives a yellow solid. (1.41
g, 83%)
1
silica gel (230–400 mesh). The H NMR spectra were recorded on Bruker
1
H NMR (300 MHz, [D8]THF): δ ¼ 8.86 (d, 4H), 8.71 (d, 2H), 8.06
Advance 300 spectrometers. The FAB þ -mass and EI þ -spectra were
recorded on a JMS-600 W, JMS-700, 6890 Series, and Flash1112 or
Flash2000. The optical UV–Vis absorption spectra were obtained using a
Lambda 1050 UV/Vis/NIR spectrometer (PerkinElmer). A PerkinElmer
luminescence spectrometer LS55 (Xenon flash tube) was used to perform
PL spectroscopy. The photoluminescence quantum yield (PLQY) was
obtained using Quantaurus-QY Absolute PL quantum yield spectrometer
(
(
d, 2H), 7.67-7.58 (m, 8H), 7.40 (t, 2H), 7.31-7.25 (m, 4H), 7.17-7.14
m, 4H).
2
.1.3. Synthesis of [10’-(4,6-Diphenyl- [1,3,5]triazin-2-yl)-[9,9’]
bianthracenyl-10-yl]-diphenyl-amine (DPA-AA-DPT)
Pd(OAc) (0.015 g, 0.065 mmol) was added to compound 5 (1.0 g,
.51 mmol), diphenyl amine (0.38 g, 2.25 mmol) and sodium tert-
2
1
C11347 (Hamamatsu). The glass transition temperatures (T
g
) and
butoxide (0.73 g, 7.55 mmol) in nitrogen conditions. The mixture
melting temperatures (T ) of the compounds were obtained on a Dif-
m
�
refluxed with stirring in anhydrous toluene 90.0 mL solvent. At 50 C,
ferential Scanning Calorimetry (DSC) Discovery DSC25 (TA in-
struments) under a nitrogen atmosphere. Compounds were heated to
tri-tert-butylphosphine (0.27 mL, 1.12 mmol) was added to the reaction.
After stirring for about 5 h under nitrogen conditions, the reaction was
filtered with toluene. Reprecipitation with chloroform and ethanol gives
a yellow solid. (0.52 g, 45%)
�
�
�
350 C at rate of 5 C/min and cooled at 5 C/min. Degradation tem-
peratures (T ) values of the compounds were measured with Thermal
Gravimetric Analysis (TGA) using a TGA4000 (PerkinElmer). Samples
d
1
�
�
H NMR (300 MHz, DMSO, TMS): δ ¼ 8.76 (d, 4H), 8.27 (d, 2H), 8.08
were heated to 700 C at a rate of 10 C/min. The HOMO energy levels
were determined with ultraviolet photoelectron spectroscopy (Riken
Keiki AC-2). The LUMO energy levels were derived from the HOMO
energy levels and the band gaps. For the EL devices, all organic layers
(
d, 2H), 7.78-7.65 (m, 6H), 7.54 (t, 4H), 7.38-7.28 (m, 8H), 7.21 (m,
8
7
4
H), 7.00 (t, 2H). HRMS (FAB-MS, m/z): calcd. for C55H36N4,
þ
52.2940; found, 752.2942 [M] . Anal. calcd for C55H36N4: C 87.74, H
À 6
.82, N 7.44; found: C 87.87, H 4.74, N 7.37.
were deposited under 10 torr, with a rate of deposition of 1 Å/s to give
2
a deposition area of 4 mm . The LiF and aluminum layers were
2
.1.4. Synthesis of {4-[10’-(4,6-Diphenyl- [1,3,5]triazin-2-yl)-[9,9’]
continuously deposited under the same vacuum condition. The current-
voltage-luminance (I-V-L) characteristics of the fabricated EL devices
were obtained with a Keithley 2400 electrometer. Light intensities were
obtained with a Minolta CS-1000 A. The device was stored in a glovebox
for stability against moisture and air.
bianthracenyl-10-yl]-phenyl}-diphenyl-amine (TPA-AA-DPT)
Pd(OAc) (0.02 g, 0.087 mmol), (cyclohexyl) P (0.04 g, 0.14 mmol)
2
3
was added to Compound 5 (1.0 g, 1.51 mmol), 4-(diphenylamino)phe-
nylboronic acid (0.52 g, 1.80 mmol) in nitrogen conditions. The mixture
was refluxed with stirring in anhydrous toluene (80.0 mL) solvent. At 50
�
C, tetraethylammonium hydroxide (20 wt%) 10.0 mL was added in
2

S. Kang et al.
Dyes and Pigments 181 (2020) 108555
3
. Result and discussion
three molecules obtained through the Gaussian density functional the-
ory (DFT) calculation was confirmed that the β values of the three
�
3
.1. Molecular design, synthesis, and optical properties
compounds were about 76.5–105.9 being a highly twisted structure
(
Fig. 4). The UV–vis absorption spectra of the three bipolar compounds
Scheme 1 shows a synthetic method for making DPA-AA-DPT, TPA-
shows that the spectra shape and position do not significantly change as
the polarity of the solvent increases (Fig. 2). This means that the elec-
tronic structure of the ground state in different solvents is independent
of the solvent polarity [27]. On the contrary, in PL result, it was
confirmed that three synthesized bipolar compounds showed a tendency
to red shift as the polarity of the solvent increases. In general, the
strength of the electron donating group used is the order of diphenyl-
amine, triphenylamine, and phenylcarbazole amine [28].
AA-DPT, and PCA-AA-DPT. The synthesized three bipolar materials have
an asymmetric structure as shown in Fig. 1. 9,9-Bianthracene was
selected as the main core and 9,9-bianthracene chromophore has high
luminous efficiency, is made of highly twisted structure, and has the
various choices for substitution linkage, so the variation of the chemical
structures is broad when designing molecules [26]. To introduce bipolar
properties, diphenylamine, triphenylamine, and phenylcarbazole amine
were introduced as electron donating groups, and 2,4-diphenyl-1,3,
As shown in Table 1, it can be seen that DPA-AA-DPT among the
three materials in hexane, a non-polar solvent, has PLmax of 471 nm, the
longest wavelength region. This is because the diphenylamine group has
the highest electron donating ability. In the case of TPA-AA-DPT, it can
be seen that the tendency for red shift with increasing solvent polarity is
larger than the other two materials (Table 1). It might be explained by
that the transition dipole moment value is larger than that of the other
two materials. In the film state, DPA-AA-DPT, TPA-AA-DPT and PCA-
AA-DPT have UVmax of 361–440 nm. The synthesized materials in the
film state show a tendency to red-shift the PLmax value due to the ag-
gregation effect. In the film state, the PLmax values of DPA-AA-DPT, TPA-
AA-DPT and PCA-AA-DPT are 513, 509 and 487 nm, respectively.
Table 2 shows the highest occupied molecular orbital (HOMO)
values of the three synthesized compounds. The HOMO values of DPA-
AA-DPT, TPA-AA-DPT, and PCA-AA-DPT are À 5.52, À 5.55, and
À 5.87 eV, respectively. These figures tend to have a tendency depending
on the ability of the introduced electron donating group. The absolute
quantum efficiency of the three compounds synthesized in the solution
and film state was measured and summarized in Table 1. In solution
state, PLQY of DPA-AA-DPT, TPA-AA-DPT, and PCA-AA-DPT was 60, 48,
5
-triazine was introduced as an electron withdrawing group.
DPA-AA-DPT, TPA-AA-DPT, and PCA-AA-DPT were synthesized using
Suzuki Miyaura cross-coupling reaction and Buchwald-Hartwig cross--
coupling reaction. The synthesized compound was purified through
recrystallization and column chromatography. The synthesized com-
pounds are characterized by nuclear magnetic resonance (NMR) spec-
troscopy, mass spectroscopy, and elemental analysis.
Fig. 2 shows the UV–visible (UV–vis) absorption and photo-
luminescence (PL) spectra of newly synthesized bipolar compounds in
various solvent solution states, respectively. In the case of TPA-AA-DPT
compound, it was not possible to measure because the solubility was
poor in DMSO solvent. Fig. 3 shows data of UV–visible absorption and PL
spectra using the vacuum deposition film of the synthesized compounds.
These optical properties are summarized in Table 1. In solution state,
DPA-AA-DPT, TPA-AA-DPT, and PCA-AA-DPT show UVmax vibronic
0
absorption peaks at 360–440 nm. (Fig. 2). In the case of 9,9 -bian-
thracene, it effectively prevents conjugation expansion due to the highly
twisted structure, showing the same characteristics of a single anthra-
cene core. The dihedral angle value of the optimized structure of the
Scheme 1. Synthetic routes of newly synthesized compounds.
3

S. Kang et al.
Dyes and Pigments 181 (2020) 108555
Fig. 1. Chemical structure of newly synthesized bipolar materials.
Fig. 2. UV–vis absorption (left) and PL spectra (right) of newly synthesized bipolar materials in different solvents at 1 ꢀ 10À 5 concentration: (a) DPA-AA-DPT, (b)
TPA-AA-DPT, and (c) PCA-AA-DPT.
and 75%, respectively. In film state, PLQY of DPA-AA-DPT, TPA-AA-
DPT, and PCA-AA-DPT was 21, 16, and 47%, respectively. PCA-AA-DPT
shows a high PLQY of 75% and 47% in solution and film state, respec-
tively. The reason why PLQY value of PCA-AA-DPT is higher than the
other two materials in solution and film state is not clear. However, we
believe that there is more twisted angle between two anthracene planes
in PCA-AA-DPT compared to other two compounds and it caused the
higher PLQY.
and Table 2. The T
d
values of DPA-AA-DPT, TPA-AA-DPT and PCA-AA-
�
DPT are 459, 487, and 499 C, respectively, which show very high
thermal stability. The reason why PCA-AA-DPT shows the highest T
d
value is because the rigidity increased through the introduction of the
carbazole moiety, a cyclic compound. In addition, the glass transition
temperature (T
g
) of the synthesized two compounds TPA-AA-DPT and
�
PCA-AA-DPT was confirmed to be 201 and 230 C, respectively, and
DPA-AA-DPT was not confirmed during even several scans. The Tm
�
values of TPA-AA-DPT and PCA-AA-DPT are 407 and 358 C. DPA-AA-
DPT, TPA-AA-DPT and PCA-AA-DPT having such high thermal stabil-
ity have stable advantages since there is little change in material caused
by heat generated when the OLED device is operated [6].
3
.2. Thermal properties
The thermal properties of DPA-AA-DPT, TPA-AA-DPT and PCA-AA-
DPT were confirmed by thermogravimetric analysis (TGA) and differ-
ential scanning calorimetry (DSC). The data are summarized in Fig. 5
4

S. Kang et al.
Dyes and Pigments 181 (2020) 108555
3
.4. Electroluminescence properties
OLED devices were fabricated by using the synthesized three mate-
rials as dopants of the emitting layer. Each device was fabricated with
0
00
the configuration of ITO/4,4 ,4 -Tris[2-naphthyl(phenyl)amino]triphe-
0
0
nylamine (2-TNATA), 55 nm/N,N -bis(naphthalene-1-yl)-N,N -bis
phenyl)benzidine (NPB), 15 nm/
; β–ADN: synthesized emitting ma-
(
α
terials (4%), 35 nm/1,3,5-tri(1-phenyl-1H-benzo[d]imidazole-2-yl)
phenyl (TPBi), 30 nm/LiF, 1 nm/Al, 200 nm. 2-TNATA was used as the
hole-injection and NPB was used as the transporting layer. TPBi was
used as the electron-transporting and hole-blocking layer. The reason for
using
film state of
synthesized compounds in the solution state are well overlapped
Fig. S1). In addition,
α
,β-ADN as the host material is as follows. PL wavelength in the
α
,β-ADN and the UV absorption wavelength of the newly
(
α,β-ADN is well-known blue host compound as the
commercialized material.
EL performance of the OLED devices is shown in Fig. 7, Fig. 8, and
Table 3. Fig. 7a shows plots of current density and luminance versus
voltage; the graphs show typical diode characteristics. Fig. 7b–d shows
current efficiency, power efficiency, and external quantum efficiency
versus current density. The three graphs show stable efficiency without
Fig. 3. UV–vis absorption (left) and PL spectra (right) of the vacuum-deposited
film for the synthesized materials.
3
.3. Theoretical calculations
DFT calculation was performed to check the electron transition when
0
bipolar properties were introduced into 9,9 -Bianthracene (Fig. 6). The
highest occupied molecular orbitals (HOMOs) of DPA-AA-DPT, TPA-AA-
DPT, and PCA-AA-DPT are all widely distributed in electron donating
groups and anthracene. This shows that the electron donating group
plays a decisive role in the conjugation extension and HOMO level ac-
cording to the electron donating effect. On the other hand, in the case of
PCA-AA-DPT, it can be confirmed that the electron density distribution
is weakly formed by the relatively weak electron donating ability.
Regarding the lowest unoccupied molecular orbitals (LUMOs) of DPA-
AA-DPT, TPA-AA-DPT, and PCA-AA-DPT, the concentration of elec-
trons is localized at the anthracene and triazine moiety.
Table 2
Electronic energy and thermal properties of DPA-AA-DPT, TPA-AA-DPT and
PCA-AA-DPT.
a
b
c
Compounds
HOMO
(eV)
LUMO
(eV)
Band gap
(eV)
Tg
Tm
Td
�
�
�
( C)
( C)
( C)
DPA-AA-
DPT
À 5.52
À 5.55
À 5.87
À 3.04
À 2.78
À 3.01
2.48
2.77
2.86
–
–
459
TPA-AA-
DPT
201
230
407
358
487
499
PCA-AA-
DPT
a
Ultraviolet photoelectron spectroscopy (Riken-Keiki, AC-2).
b
c
Band Gap ¼ ELUMO-EHOMO
.
T
d
is thermal-decomposition temperature at a weight percentage of 95%.
Table 1
Optical properties of the synthesized compounds.
a
b
Compounds
Solution
UV (nm)
Tol
Film on glass
PL (nm)
Hex
ф
F
(%)
UV (nm)
PL (nm)
F
ф (%)
Tol
Chloro
THF
DMSO
DPA-AA-DPT
TPA-AA-DPT
PCA-AA-DPT
361, 381, 395, 440
360, 380, 402
471
464
464
494
492
472
511
545
497
515
568
518
544
–
60
48
75
363, 483, 400, 440
361, 389, 409
513
509
487
21
16
47
361, 381, 403
583
364, 385, 408
a
À 5
Hexane, Toluene, Chloroform, THF and DMSO solution (1.00 ⅹ 10 M).
b
Film thickness is 50 nm on the glass.
Fig. 4. Optimization structure and dihedral angle of synthesized compounds (calculated at the B3LYP/6-31G(d)).
5

S. Kang et al.
Dyes and Pigments 181 (2020) 108555
Fig. 5. Thermal properties of newly synthesized bipolar materials: (a) TGA and (b) DSC.
Fig. 6. Electron density distributions of the HOMO and LUMO orbits of DPA-AA-DPT, TPA-AA-DPT, PAC-AA-DPT (calculated at the B3LYP/6-31G(d)).
roll-off phenomenon as the current density increases. DPA-AA-DPT,
TPA-AA-DPT, and PCA-AA-DPT have current efficiency (CE) values of
can be realized.
Fig. 8a–c shows EL spectra of devices, CIE (x, y) value, and band
diagram of materials using the fabricated OLED device. The EL spectra of
the three compounds were correctly implemented according to the size
of the band gap. The CIE (x, y) value of DPA-AA-DPT shows greenish sky
blue as (0.17, 0.44), and PCA-AA-DPT shows typical real blue as (0.15,
0.17).
1
5.3, 8.37, and 6.86 cd/A, respectively. The reason that DPA-AA-DPT
shows the highest CE is that the band gap is relatively small and holes
and electrons smoothly move, so it shows higher efficiency than those of
the other two materials. In addition, due to the bipolar property, the
charge balance is excellent, so there is no accumulation of charge, so
there is no roll-off result and high efficiency can be achieved. Fig. S2
shows the EL spectra according to the voltage increase of the OLED
device. The EL wavelength does not change as the voltage increases. It
can be confirmed that the newly synthesized compounds are stable at
high current density region and the charge injection of the device is
balanced very well. The power efficiency (PE) values of DPA-AA-DPT,
TPA-AA-DPT, and PCA-AA-DPT are 7.10, 3.84, and 3.18 lm/W,
respectively. Since PE was obtained through the efficiency of CE, it was
confirmed that the tendency of the results was the same. The external
quantum efficiency (EQE) of DPA-AA-DPT, TPA-AA-DPT, and PCA-AA-
DPT achieved efficiencies of 6.91, 5.22, and 5.53%, respectively. DPA-
AA-DPT showed the highest EQE value of 6.91%, but the band gap
was small and the CIE y value was 0.44, corresponding to the sky-blue
region. On the other hand, the other two substances showed results
that exceeded the theoretical limit of EQE of 5% in the blue region. This
is a possible result because the regenerated singlet exciton effect was
4. Conclusions
DPA-AA-DPT, TPA-AA-DPT, and PCA-AA-DPT with bipolar proper-
ties were successfully synthesized by introducing diphenylamine, tri-
phenylamine, phenylcarbazole amine as a donor moiety and 2,4-
0
diphenyl-1,3,5-triazine as an acceptor moiety to 9,9 -bianthracene
core moiety. DPA-AA-DPT, TPA-AA-DPT, and PCA-AA-DPT materials
emitted blue light with maximum wavelengths of 471, 464, and 464 nm,
respectively, in the solution state. The three compounds showed high
�
thermal stability, with T
g
greater than 200 C and T
d
greater than 450
�
C. Of the three materials, PCA-AA-DPT exhibited the highest absolute
quantum efficiency of 75%. When the synthesized materials were
applied as dopants, PCA-AA-DPT achieved the high efficiency of 6.86
cd/A and EQE of 5.53%, even at the high current density and shows blue
emission with CIE value of (0.15, 0.17).
0
confirmed in the recently reported 9,9 -bianthracene [29]. In addition,
in the case of PCA-AA-DPT material, the PLQY value is 75%, which is
higher than that of the other two materials, so that a better EQE value
6

S. Kang et al.
Dyes and Pigments 181 (2020) 108555
Fig. 7. EL characteristics of devices DPA-AA-DPT, TPA-AA-DPT, and PCA-AA-DPT as EMLs dopant: (a) l-V-L curve, (b) luminance efficiency versus current density,
(
c) power efficiency versus current density, and (d) external quantum efficiency versus current density.
Fig. 8. (a) EL spectra of devices and (b) CIE x, y values of the synthesized compounds, (c) band diagram, and (d) chemical structures using OLED device.
Declaration of competing interest
relationships which may be considered as potential competing interests:
I declare that the above items are certain.
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.
CRediT authorship contribution statement
The authors declare the following financial interests/personal
Seokwoo Kang: Conceptualization, Methodology, Formal analysis,
7

S. Kang et al.
Dyes and Pigments 181 (2020) 108555
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a
b
c
d
Compounds
V
CE
PE
EQE
(%)
CIE
ELmax
(nm)
FWHM
(nm)
e
(
V)
(cd/
A)
(lm/
(x, y)
c
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7.29
7.47
7.30
15.3
8.37
6.86
7.10
3.84
3.18
6.91
5.22
5.53
0.17,
0.44
0.16,
0.25
0.15,
0.17
490
471
459
57
69
60
TPA-AA-
DPT
PCA-AA-
DPT
[
a
2
Operating voltage at 10 mA/cm .
Current efficiency.
b
c
Power efficiency.
d
e
organic light emitting devices based on imidazole-
Chem Mater 2011;24:61.
π-triphenylamine derivatives.
External quantum efficiency.
Commission International de l’Eclairage.
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[
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Project administration, Funding acquisition.
[
[
[
[
[
[
[
π
diodes. Adv Mater 2008;20:3947.
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Acknowledgements
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and non-doped deep-blue emitters (CIEy¼ 0.04) based on phenylcarbazole and 2-
This research was supported by National R&D Program through the
National Research Foundation of Korea (NRF) funded by the Ministry of
Science & ICT (No. 2020M3A7B4002030). This work was supported by
ITECH R&D program of MOTIE/KEIT (No. 20009872, Development of
high throughput colorimetric chemical materials). This research was
supported by the Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Educa-
tion (No. 2020R1A6A1A03048004).
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2-phenyl-2H-1, 2, 4-triazol-3-yl) pyridine groups. J Mater Chem C 2017;5:4455.
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with an intercrossed excited state for highly efficient, deep-blue
electroluminescence. Adv Funct Mater 2012;22:2797.
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Sulfone hybrid D- -A-structured bipolar fluorophores for high-efficiency blue-
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material for high efficient non-doped fluorescent organic light-emitting diode
approaching standard deep blue. Dyes Pigments 2016;129:34.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2020.108555.
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1
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Rational molecular design of bipolar phenanthroimidazole derivatives to realize
highly efficient non-doped deep blue electroluminescence with CIEy ˂ 0.06 and
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