Oxadiazole-based highly efficient bipolar fluorescent emitters for organic light-emitting diodes

Article abstract of DOI:10.3390/molecules23040843

In this study, a series of bipolar fluorescence emitters named 2DPAc-OXD, DPAc-OXD, 2PTZ-OXD and PTZ-OXD were designed and synthesized with excellent yields. The characterization of materials was investigated by using nuclear magnetic resonance (NMR) (¹H, ¹³C), mass spectrometry and thermogravimetric analysis (TGA). To investigate device efficiencies, two different OLED devices (Device 1, Device 2) were fabricated with two different host materials (Bepp₂, DPEPO). The Device 2 with 2PTZ-OXD as fluorescent emitter exhibited excellent power and current efficiencies of 6.88 Lm/W and 10.10 cd/A, respectively. The external quantum efficiency of 2PTZ-OXD was around 3.99% for Device 2. The overall device properties of phenothiazine donor were better than acridine derivatives.

Full text of DOI:10.3390/molecules23040843

molecules
Article
Oxadiazole-Based Highly Efficient Bipolar
Fluorescent Emitters for Organic
Light-Emitting Diodes
Qiong Wu 1,2, Ramanaskanda Braveenth
and Kyu Yun Chai *
1
ID
, Heng Qiang Zhang , Il-Ji Bae , Miyoung Kim *
2
3
3,
1
,
ID
1
Division of Bio-Nanochemistry, College of Natural Sciences, Wonkwang University Iksan City,
Chonbuk 570-749, Korea; wuqiong9308@126.com (Q.W.); braveenth.czbt@gmail.com (R.B.)
Department of Chemistry, Hebei Normal University for Nationalities, Chengde 067000, China;
zhanghengqiang80@163.com
Nano-Convergence Research Center, Korea Electronics Technology Institute, Jeonju 54853, Korea;
ijbae@keti.re.kr
2
3
*
Correspondence: miy1kim@keti.re.kr (M.K.); geuyoon@wonkwang.ac.kr (K.Y.C.);
Tel.: +82-63-219-0011 (M.K.); +82-63-850-6230 (K.Y.C.);
Fax: +82-63-219-0049 (M.K.); +82-63-841-4893 (K.Y.C.)
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀ
ꢁꢂꢃꢄꢅꢆ
Received: 14 March 2018; Accepted: 5 April 2018; Published: 7 April 2018
Abstract: In this study, a series of bipolar fluorescence emitters named 2DPAc-OXD, DPAc-OXD,
PTZ-OXD and PTZ-OXD were designed and synthesized with excellent yields. The characterization
2
1
13
of materials was investigated by using nuclear magnetic resonance (NMR) ( H, C), mass
spectrometry and thermogravimetric analysis (TGA). To investigate device efficiencies, two different
OLED devices (Device 1, Device 2) were fabricated with two different host materials (Bepp , DPEPO).
2
The Device 2 with 2PTZ-OXD as fluorescent emitter exhibited excellent power and current efficiencies
of 6.88 Lm/W and 10.10 cd/A, respectively. The external quantum efficiency of 2PTZ-OXD was
around 3.99% for Device 2. The overall device properties of phenothiazine donor were better than
acridine derivatives.
Keywords: bipolar; oxadiazole; acridine; phenothiazine; organic light-emitting diodes; fluorescent
1
. Introduction
Organic light-emitting diodes (OLEDs) have developed rapidly since the first efficient double
layer structure device was reported by Tang [
for next generation flat-panel displays and solid-state lighting sources due to various advantages,
such as high-quality color, low energy cost, light weight, and flexibility [ ]. Small OLED displays,
1]. They are expected as the promising candidates
2
which are commercially available now, are applied to mobile displays, television and flat panels.
Nevertheless, there are still some issues related to device performance and reliability need to improve,
especially the efficiency of device with fluorescent emitting materials. To the best of our knowledge,
the maximum external quantum efficiency (EQE) of fluorescent material is 8.5% [
functions as the recombination site of holes and electrons which are injected from anode and cathode,
respectively [ ]. Therefore, the development of high efficient fluorescent-emitting material is a key
3]. The emitting layer
4
issue for high performance OLED devices.
The charge-recombination factor is dominated by the balance of holes and electrons in the emissive
layer and the current balance is directly proportional to the emission [5]. Therefore, emitting layer
materials should meet the requirements of energy level matching for charge carrier injection and
acceptance of both holes and electrons. Meanwhile, they should possess the properties of electron
Molecules 2018, 23, 843; doi:10.3390/molecules23040843
www.mdpi.com/journal/molecules

Molecules 2018, 23, 843
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transporting and hole-transporting character to permit the formation of both stable cation and anion
radicals, which known as bipolar in nature [ ]. Electron-donating units have the potential to transport
4
holes, while electron-accepting units are capable of transporting electrons. Therefore, the most
prevalent strategy of achieving bipolar materials is the incorporation of donor-acceptor (D-A) units into
the same molecule via a linker to facilitate the injection and transport of both holes and electrons [
6].
Recently, A-D and D-A-D types materials are often explored for OLED fluorescent materials [3,7–16]
to improve charge balance and reduce driving voltage; they are also widely designed for thermally
activated delayed fluorescence (TADF) emitters and host martials for phosphorescence and TADF
dopants [17
–
19]. The emission from A-D and D-A-D type molecular design can originate from
21].
intramolecular charge-transfer (ICT) excited states between the donor and acceptor moieties [20
,
Therefore, judicious combination of the appropriate donor and accepter could facilitate simultaneous
manipulation of the highest occupied orbital (HOMO)/the lowest unoccupied orbital (LUMO) levels
and the emission color of the bipolar structure molecule [22].
Triazine, oxadiazole, triazole, cyno-substituted benzene and benzothiadiazole are usually used
as acceptor units, while carbazole, arylamine, phenothiazine and their derivatives are commonly
adopted as the donor moieties for bipolar structures for OLEDs [15
phenothiazine and 9,9-diphenyl-9,10-dihydroacridine as the donor unit, while oxadiazole derivatives
,5-bis(4-bromophenyl)-1,3,4-oxadiazole and 2-(4-bromophenyl)-5-phenyl-1,3,4-oxadiazole are
employed as the acceptors to synthesize four distorted A-D and D-A-D type fluorescent materials,
,5-bis(4-(10H-phenothiazin-10-yl)-1,3,4-oxadiazole (2PTZ-OXD), 2-(4-(10H-phenothiazin-10-yl)
1,3,4-oxadizaole (PTZ-OXD), 2,5-bis(4-(9,9-diphenyl-9,10-dihydroacridine)phenyl)-1,3,4-oxadizaole
2DPAc-OXD) and 2-(4-(9,9-diphenyl-9,10-dihydroacridine)phenyl)-1,3,4-oxadiazole (DPAc-OXD).
–17,20,23–29]. Herein, we utilize
2
2
-
(
We also investigated their photophysical properties, thermal stability and performed quantum
calculations to evaluate the molecular orbitals distribution for all molecules.
2
. Results and Discussion
2
.1. Synthesis
Scheme 1 displays the synthetic process of designed bipolar fluorescent emitters. Compounds
1
,
2
,
3
and
4
were known and synthesized by following the methods from previously reported
32]. 2DPAc-OXD, DPAc-OXD, 2PTZ-OXD and PTZ-OXD were synthesized by using
literatures [30
–
the well-known Buchwald Hartwig cross-coupling reaction between the oxadiazole (2 and 4) and
phenothiazine or 9,9-diphenyl-9,10-dihydroacridine derivatives by using palladium-based catalyst.
2
.2. Thermal Properties
The thermal stabilities of four fluorescence emitters were studied on the basis thermal
decomposition temperature at 5% weight reduction and were measured by using thermo gravimetric
analysis (TGA). All four emitters, which are depicted in Figure 1 exhibited higher decomposition
◦
temperature over 358 C. The acridine donor-based emitters, DPAc-OXD and 2DPAc-OXD showed
◦
thermal stability of 375 and 455 C, respectively. The above values are higher than that of phenothiazine
donor based PTZ-OXD (358 C) and 2PTZ-OXD (419 C) due to its higher rigidity when compare to
acridine donor-based molecules (Table 1). Additionally, we noticed that two site molecules (2PTZ-OXD,
◦
◦
2DPAc-OXD) were excellent with their thermal strength while the thermal strength of single site
molecules (PTZ-OXD, DPAc-OXD) were lower.

Molecules 2018, 23, 843
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Scheme 1.
Synthesis of 2,5-bis(4-(9,9-diphenyl-9,10-dihydroacridine)phenyl)-1,3,4-oxadizaole
(
2
-
2DPAc-OXD), 2-(4-(9,9-diphenyl-9,10-dihydroacridine)phenyl)-1,3,4-oxadiazole (DPAc-OXD),
,5-bis(4-(10H-phenothiazin-10-yl)-1,3,4-oxadiazole (2PTZ-OXD) and 2-(4-(10H-phenothiazin
10-yl)-1,3,4-oxadizaole (PTZ-OXD). Reagents and conditions (a): Pd(OAc) , K CO , t-Bu P, anhydrous
2
2
3
3
◦
toluene, 110 C.
Figure 1. Thermal decomposition temperature (by thermogravimetric analysis, TGA) of fluorescent
emitters.

Molecules 2018, 23, 843
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Table 1. Thermal and physical properties of fluorescent emitters.
a
UV-vis b PL max c
HOMO d
(eV)
LUMO e
(eV)
Eg f
(eV)
ET g
(eV)
T
(
d
Dopant
◦
C)
(nm)
(nm)
PTZ-OXD
358
419
375
455
376
384
396
402
500
512
435
442
−5.48
−5.48
−5.69
−5.68
−2.18
−2.25
−2.56
−2.60
c
3.30
3.23
3.13
3.08
2.48
2.42
2.85
2.81
2
PTZ-OXD
DPAc-OXD
DPAc-OXD
2
a
d
f
Decomposition temperature at 5% weight reduction; b On set UV absorption; Photoluminescence emission (PL);
Highest occupied molecular orbital energy (HOMO); Lowest unoccupied molecular orbital energy (LUMO);
Band gap energy; Triplet energy.
e
g
2
.3. Photophysical and Electrochemical Properties
The photophysical properties were analysed by using UV-visible and photoluminescence spectral
measurements. UV-Vis spectra showed on set values of 376, 384, 396 and 402 nm for PTZ-OXD,
PTZ-OXD, DPAc-OXD and 2DPAc-OXD, respectively which were used to calculate the band gap
2
energies of four emitters (Figure 2). The above values are summarized in Table 1. The triplet
energy of PTZ-OXD, 2PTZ-OXD, DPAc-OXD and 2DPAc-OXD were 2.48, 2.42, 2.85 and 2.81 eV,
respectively. Those above values were evaluated from phosphorescence emission spectrum according
to literature [33]. The triplet energy values were matched with their photoluminescence (PL)
emission of 500 (PTZ-OXD), 512 (2PTZ-OXD), 435 (DPAc-OXD) and 442 nm (2DPAc-OXD). When we
noticed the PL values of phenothiazine donor-based molecules, PTZ-OXD and 2PTZ-OXD were
revealed bathochromic shift related to their lower triplet energy. HOST material is one of the
key factors for enhancing the device efficiencies by controlling holes and electrons balance at the
emission layer. We have fabricated two different OLED devices with different host materials of
Bis[2-(2-pyridinyl)phenolato]beryllium(II) (Bepp ) and Bis[2-(diphenylphosphino)phenyl] ether oxide
2
(
DPEPO), which have higher triplet energy of 2.70 and 2.99 eV for latest [34
,35]. DPEPO host material
has enough potential to prevent the energy transferring from dopant to host due to its higher triplet
energy than our fluorescence emitters. In the case of Bepp host material, it can effectively work with
2
PTZ-OXD and 2PTZ-OXD due to their lower triplet energies.
Figure 2. (a) UV-Vis absorption spectra (b) Photoluminescent spectra of fluorescent emitters.
The electrochemical properties were analysed on the basis of HOMO and LUMO energies.
The HOMO value of acridine donor based DPAC-OXD and 2DPAC-OXD were the same with
while phenothiazine donor based PTZ-OXD and 2PTZ-OXD were almost identical to each other
5.48 eV). Consequently, LUMO energy values were calculated by subtracting the band gap energies
from HOMO energies and the values were 2.18, 2.25, 2.56 and 2.60 eV for PTZ-OXD, 2PTZ-OXD,
−
5.68 eV,
(
−
−
−
−
−
DPAc-OXD and 2DPAc-OXD, respectively. The frontier molecular energies contributed to constructing
the device structure substantiated with proper energy flow with adjacent layers.

Molecules 2018, 23, 843
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Density functional theory (DFT) calculations were investigated by using Gaussian 9 program with
TD-SCF method B3LYP and 6-31G basis set [36]. The frontier molecular orbital (FMO) distributions
are shown in Figure 3. All four materials had clear HOMO and LUMO separations and we did not
notice any overlapping between the donor and acceptor. HOMO of all molecules was distributed over
phenothiazine and acridine moieties due to their strong electron donating nature, which could be
explained by the electron localization on donor molecules. The electron delocalization was observed at
oxadiazole due to its electron withdrawing nature. The clear separation of FMOs are supported by
their lower singlet-triplet energy gap difference of 0.35, 0.29, 0.46 and 0.44 eV for PTZ-OXD, 2PTZ-OXD,
DPAc-OXD and 2DPAc-OXD, respectively (Table 2). The rotation angle between phenothiazine donor
◦
3
◦
and oxadiazole was around 78 , but acridine and oxadiazole showed higher angle of 81 as bulky
phenyl groups attached with sp carbon atom of acridine molecule. The above results indicate that
four A-D and D-A-D type fluorescence emitters were successfully constructed with bipolar charge
transfer properties.
Figure 3. Calculated Frontier molecular orbital distribution.
Table 2. Calculation data of four dopant materials.
Calculation Values
PTZ-OXD
2PTZ-OXD
DPAc-OXD
2DPAc-OXD
S (eV) a
T1 (eV)
3.71
3.36
0.35
77.05
−5.975
−2.185
3.63
3.34
0.29
78.91
−5.485
−2.255
3.83
3.37
0.46
81.26
−5.695
−2.565
3.78
3.34
0.44
81.70
−5.685
−2.605
1
b
c
∆
EST (eV)
◦
D-A rotation( )
HOMO (eV)
LUMO (eV)
a
Singlet energy; b Triplet energy; c Triplet and singlet energy difference.

Molecules 2018, 23, 843
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2
.4. Device Characteristics
The device characteristics were studied after fabricating OLED devices with two different host
materials (Figure 4). The device structures were (Device 1) ITO (150nm)/ NPB (20 nm)/ TCTA (10 nm)/
mCP (10 nm)/ Bepp : dopant 6%(10 nm)/TPBi (55 nm)/ Liq (3 nm)/ Al (100 nm) and (Device 2) ITO
2
(150 nm)/ NPB (30 nm)/ mCP (10 nm)/ DPEPO: dopant 6%(15 nm)/DPEPO (10 nm)/ TPBi (40 nm)/
Liq (3 nm)/ Al (100 nm).
Figure 4. The fabricated device structures of 1 and 2 based on fluorescent emitters.
The turn on voltages of device 1 were between 4.1 to 4.4 V, which was lower than that of device
(5.6 to 6.2 V). PTZ-OXD and 2PTZ-OXD showed lower driving voltage than acridine based DPAc-OXD
2
and 2DPAc-OXD, which proved that phenothiazine donor in cooperating with oxadiazole acceptor can
enhance the device efficiencies (Figure 5). The current efficiency of device 1 was better than device 2,
but 2PTZ-OXD based Device 1 exhibited current efficiency of 9.20 cd/A while Device 2 was 10.10 cd/A.
The above value is higher than reported current efficiency of fluorescence-based emitters in OLEDs.
2
PTZ-OXD based Device 1 showed higher power efficiency of 6.88 Lm/W, which is related with lower
driving voltage of 5.9 V. PTZ-OXD revealed power efficiency of 3.59 Lm/W for its higher driving
voltage (6.8 V). The current and power efficiencies of acridine based DPAc-OXD and 2DPAc-OXD
were lower than phenothiazine-based molecules which is an evidence that acridine and oxadiazole
do not provide balance charge transfer system due to strong electron donating ability of acridine
molecule than that of phenothiazine. Consequently, external quantum efficiency (EQE) of 2PTZ-OXD
based Device 1 was 3.38% and Device 2 was 3.99%. The EQE value of PTZ-OXD, DPAc-OXD and
2DPAc-OXD were 1.94%, 1.84% and 1.81% respectively (Table 3). However, the efficiencies at high
current density were dropped from peak value due to efficiency roll-off. Which attributed to quenching
mechanism such as molecular exciton annihilation occurs at high current densities [37,38].

Molecules 2018, 23, 843
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Figure 5. Current density-voltage-luminance (J-V-L) and luminescence -current and power efficiency
of the fabricated Devices 1 and 2.
Table 3. Device characteristics of fluorescent emitters PTZ-OXD, 2PTZ-OXD, DPAc-OXD and
2
DPAc-OXD.
Device properties
PTZ-OXD
2PTZ-OXD
DPAc-OXD
2DPAc-OXD
4
6
6.8
.4 a
4.2 a
4.1 a
4.1 a
Turn on voltage (V)
Driving voltage (V)
Current (mA)
b
b
b
b
.0
5.6
5.9
0.03
0.03
6.0
7.6
0.04
0.06
1.30
0.72
0.99
0.38
6.2
7.5
0.05
0.10
1.28
0.66
0.98
0.33
a
a
a
a
a
b
a
b
a
b
a
b
a
a
a
0
.05
.05
.03
.97
.59
.60
.94
.26
b
b
b
0
5
4
3
2
1
2
a
a
a
9.20
Current efficiency (cd/A)
Power efficiency (Lm/W)
b
b
b
10.10
6.88
5.67
3.38
a
a
a
b
b
b
a
a
a
1.84
1.08
1.81
0.88
EQE (%)
Luminance (at 1000 cd/m2)
CIE (x,y)
b
b
b
3.99
1008
1020
1029
992
a
a
a
a
(
(
0.38, 0.50)
(0.40, 0.53)
(0.16, 0.12)
(0.17, 0.14)
0.43, 0.14) b
(0.31, 0.49) b
(0.18, 0.12) b
(0.18, 0.15) b
a
Device 1; b Device 2.
The electroluminescence study was carried out to find the emission wavelength and colour purity
Table 3). DPAc-OXD and 2DPAc-OXD showed similar emission peak at 450 nm along with CIE (x, y)
(
colour coordinate of (0.16, 0.12) and (0.17, 0.14), respectively. The emission of PTZ-OXD was observed
at 540 nm, while 2PTZ-OXD showed emission at 550 nm with little bathochromic shift. We did not
notice any other emissions other than our fluorescence dopants (Figure 6).

Molecules 2018, 23, 843
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Figure 6. Normalized electroluminescent (EL) spectra of Device 1.
. Materials and Methods
3
3
.1. General Procedures
All reagents and solvent were purchased from commercial suppliers and were used without
1
13
further purification otherwise stated. H- and C-NMR (Nuclear Magnetic resonance) spectra were
recorded by using a JNM-ECP FT-NMR spectrometer (JEOL, Peabody, MA, USA) and operating
at 500 MHz. Absorbance spectra were recorded from a S-4100 UV-visible spectrophotometer
(
SINCO, Seoul, Korea). The Band gaps (E
spectra while photoluminescence (PL) spectra were measured by using a HR800 Spectro fluorimeter
Horiba Jobin Yvon, Paris, France). HOMO level was calculated by AC-2 using a Photoelectron
g
) were estimated from the onset of the absorbance
(
spectrometer (RIKEN, Saitama, Japan). LUMO was calculated by subtraction of the band gap from
the HOMO energy. Thermal gravimetric analysis was conducted on a DSC Q200 V24.9 Build 121
◦
thermal analysis system (TA instruments, New castle, DE, USA) with the heating rate of 10 C/min.
Mass analysis were carried out by using a Xevo TQ-S spectrometer (Waters, Milford, MA, USA).
Current density-voltage-luminescence (J-V-L) efficiencies were measured by an OLED I-V-L test system
(Polarmix M6100, Suwon, Republic of Korea). The electroluminescence (EL) spectra analysis was
carried out by using a spectroradiometer (Konica Minolta CS-2000, Japan). The molecular distributions
were carried out by using a Gaussian 09 program (Wallingford, CT, USA) with DFT (density functional
theory) and TD-SCF method B3LYP with a 6/31G basic set.
3
.2. Synthetic Procedures
0
3
.2.1. 4-Bromo-N -(4-bromobenzoyl)benzohydrazide (1)
◦
4
-Bromobenzoyl chloride (3.0 g, 13.67 mmol) was dissolved in 30 mL THF and stirred at 0 C for
30 min, then hydrazine monohydrate (0.28 mL, 5.44 mmol) was added dropwise in to the mixture.
After adding, the mixture was stirred for another 3 h at room temperature. The resulting precipitate
was filtered, washed with saturated aqueous NaHCO solution (3
and then dried to get white solid (1.94 g) with the yield of 84%.
×
15 mL) and water, respectively,
3
1
H NMR (500 MHz, DMSO-d ) δ 10.65 (s, 2H), 7.88–7.83 (m, 4H), 7.75 (dd, J = 6.42, 4.53 Hz, 4H).
6
3
.2.2. 2,5-Bis(4-bromophenyl)-1,3,4-oxadiazole (2)
0
4
-Bromo-N -(4-bromobenzoyl)benzohydrazide(5 g, 12.56 mmol) was placed in 250 mL two neck
flask equipped with condenser. Then vacuumed it for 15 min and 30 mL of dry toluene and 30 mL

Molecules 2018, 23, 843
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POCl were injected. Under N atmosphere, the mixture was refluxed until the reaction was finished.
3
2
After completion of the reaction, the solvent and excesses POCl were removed by a rotary evaporator
3
under reduced pressure. Finally, the mixture was recrystallized with ethanol. The white crystals were
filtered and dried (4.75 g, 99.5% yield).
1H NMR (500 MHz, CDCl3)
δ
8.00 (td, J = 8.00, 1.72 Hz, 4H), 7.68 (td, J = 7.99, 1.70 Hz, 4H); 13C NMR
(
500 MHz, CDCl ) δ 164.15,132.59, 128.44, 126.73, 122.72.
3
3
.2.3. 2-(4-Bromophenyl)-5-phenyl-1,3,4-oxadiazole (4)
The synthesis of 2-(4-bromophenyl)-1,3,4-oxadiazole is similar with OXD-2Br, and the
0
only difference lies in the starting material 4-bromo-N -(4-bromobenzoyl)benzohydrazide to
0
N -benzoyl-4-bromobenzohydrazide. After recrystallization, white crystals were filtered and dried in
the oven.
1
Yield 93%. H NMR (500 MHz, CDCl )
(
δ
8.15 (dd, J = 5.30, 3.40 Hz, 2H), 8.04–7.99 (m, 2H), 7.63–7.58
3
13
m, 2H), 7.57–7.52 (m, 1H), 7.52–7.46 (m, 2H); C NMR (500 MHz, CDCl3) δ 144.85, 143.55, 133.66,
1
32.69, 132.08, 131.93, 130.09, 128.75, 128.71, 126.87.
3
.2.4. 2,5-Bis(4-(9,9-diphenyl-9,10-dihydroacridine)phenyl)-1,3,4-oxadiazole (2DPAc-OXD)
,5-Bis(4-bromophenyl)-1,3,4-oxadiazole (5 g, 13.16 mmol), 9,9-diphenyl-9,10-dihydroacridine
9.64 g, 28.94 mmol), potassium carbonate (12.04 g, 87.09 mmol) and palladium acetate (0.18 g,
.79 mmol) were added in 500 mL two neck flask equipped with condenser and vacuumed it for
5 min. Then 300 mL of dry toluene was injected. The mixture was continued with stirring and heating,
2
(
0
1
after 10 min 7.3 mL tri-tert-butylphosphine (10% in toluene) was injected. The above mixture was
stirred and heated under reflux condition until the starting material disappear. After cooling to room
temperature, the mixture was extracted with dichloromethane (DCM) and deionized water. Then
the organic layer was dried over anhydrous MgSO and filtered. The organic layer was concentrated
4
on a rotary evaporator under reduced pressure. Finally, the residue was purified by silica column
chromatography to afford 8.76 g of (white solid) 2DPAc-OXD with the yield of 75.2%.
1H NMR (500 MHz, CDCl3)
.99 (dd, J = 7.67, 1.77 Hz, 8H), 7.08 (td, J = 8.38, 4.34, 4.34 Hz, 4H), 7.29–7.21 (m, 16H); 13C NMR
δ 8.33–8.29 (m, 4H), 6.46 (d, J = 8.24 Hz, 4H), 6.92 (d, J = 4.01 Hz, 8H),
6
(
1
500 MHz, CDCl3)
δ
164.36, 146.28, 144.46, 143.93, 141.83, 132.13, 130.45, 130.27, 129.27, 127.77, 127.05,
+
26.47, 120.78, 114.24, 108.01; MS (APCI): 886.09 for C H N O [M+H ].
64
44
4
3
.2.5. 2-(4-(9,9-Diphenyl-9,10-dihydroacridine)phenyl)-1,3,4-oxadiazole (DPAc-OXD)
-(4-bromophenyl)-5-phenyl-1,3,4-oxadiazole (3 g, 9.96 mmol), 9,9-diphenyl-9,10-dihydroacridine
3.65 g, 1.96 mmol), potassium carbonate (4.54 g, 32.87 mmol) and palladium acetate (0.07 g, 0.299 mmol)
2
(
were added in 250 mL two neck flask equipped with condenser and subjected it for 15 min. Then 80 mL
of dry toluene was injected, followed by 2.8 mL of tri-tert-butylphosphine (10% in toluene) was added.
After the completion of reaction, mixture was extracted with DCM and water. The organic layer was
dried over anhydrous MgSO then filtered and evaporated under reduced pressure. The residue was
4
purified by silica column chromatography to afford 4.7 g of DPAc-OXD as white solid with the yield
of 85.4%.
1H NMR (500 MHz, CDCl3)
.31–7.24 (m, 8H), 7.08 (ddd, J = 8.46, 5.57, 3.22 Hz, 2H), 7.00 (dd, J = 7.89, 1.70 Hz, 4H), 6.97–6.89
m, 4H), 6.46 (d, J = 8.05 Hz, 2H); 13C NMR (500 MHz, CDCl3)
164.94, 164.13, 146.32, 144.29, 141.86,
32.12, 131.99, 130.46, 130.27, 130.20, 129.25, 127.77, 127.09, 127.05, 126.47, 123.89, 123.70, 120.74, 114.22;
δ 8.33–8.29 (m, 2H), 8.16 (dd, J = 7.83, 1.71 Hz, 2H), 7.61–7.55 (m, 3H),
7
(
δ
1
+
MS (APCI): 553.75 for C H N O [M+H ].
39
27
3

Molecules 2018, 23, 843
10 of 13
3
.2.6. 2,5-Bis(4-(10H-phenothiazin-10-yl)-1,3,4-oxadizole (2PTZ-OXD)
,5-Bis(4-bromophenyl)-1,3,4-oxadiazole (5 g, 13.15 mmol), phenothiazine (5.76 g, 28.93 mmol),
2
potassium carbonate (12.03 g, 87.05 mmol) and palladium acetate (0.18 g, 0.79 mmol) were added
in 250 mL two neck flask equipped with condenser and vacuumed it for 15 min. Then 120 mL of
dry toluene was injected while stirring. Then 7.3 mL of tri-tert-butylphosphine (10% in toluene) was
injected in to the mixture. The above mixture was stirred and refluxed. After reaction completed,
mixture was cooled to room temperature and extracted with dichloromethane (DCM) and deionized
water. Then the organic layer was dried over MgSO , followed by concentrated on a rotary evaporator
4
under reduced pressure. Finally, the residue was purified by silica column chromatography to afford
7
.88 g (yellow solid) 2PTZ-OXD with the yield of 97.2%.
1H NMR (500 MHz, CDCl3)
7
δ
8.17–8.13 (m, 2H), 7.38–7.33 (m, 2H), 7.28–7.22 (m, 2H), 7.12 (dt, J = 7.75
,
.66, 1.50 Hz, 2H), 7.05–7.02 (m, 2H), 6.84 (d, J = 8.07 Hz, 2H); 13C NMR (500 MHz, CDCl3)
δ
160.15,
1
46.50, 142.60, 128.88, 128.03, 127.47, 127.23, 124.65, 123.80, 121.54, 120.08; MS (APCI): 618.07 for
+
C H N OS [M+H ].
38
24
4
2
3
.2.7. 2-(4-(10H-phenothiazin-10-yl)-1,3,4-oxadiazole(PTZ-OXD)
-(4-Bromophenyl)-5-phenyl-1,3,4-oxadiazole (3 g, 9.96 mmol), phenothiazine (2.18 g, 10.96 mmol),
2
potassium carbonate (4.56 g, 32.98 mmol) and palladium acetate (0.07 g, 0.299 mmol) were added
in 250 mL two neck flask equipped with condenser and vacuumed it for 15 min. Then 80 mL dry
toluene was injected while stirring and 2.8 mL tri-tert-butylphosphine (10% in toluene) was added.
The mixture was stirred and refluxed until the reaction complete. After cooling to room temperature,
the mixture was extracted with dichloromethane (DCM) and deionized water. The organic layer
was dried over MgSO , followed by concentrated on a rotary evaporator under reduced pressure.
4
Finally, the residue was purified by silica column chromatography afford 4.18 g (light yellow solid) of
PTZ-OXD with the yield of 98.5%.
1H NMR (500 MHz, CDCl3)
.31–7.24 (m, 8H), 7.08 (ddd, J = 8.46, 5.57, 3.22 Hz, 2H), 7.00 (dd, J = 7.89, 1.70 Hz, 4H), 6.97–6.89
m, 4H), 6.46 (d, J = 8.05 Hz, 2H); 13C NMR (500 MHz, CDCl3)
164.94, 164.13, 146.32, 141.86, 132.12,
31.99, 130.47, 130.27, 129.25, 127.78, 127.09, 127.05, 126.47, 123.70, 120.74, 114.22; MS(APCI): 419.80 for
δ 8.33–8.29 (m, 2H), 8.16 (dd, J = 7.83, 1.71 Hz, 2H), 7.61–7.55 (m, 3H),
7
(
δ
1
+
C H N OS [M + H ].
26
17
3
3
.3. OLED Fabrication and Characterization
The device substrate was made by using an ITO (indium tin oxide) with the
thickness of 150 nm.
Then substrate was subjected to ultra-sonication with isopropyl
alcohol and deionized water, followed by ultraviolet and ozone treatment.
Further
0
device fabrication continued as follow: 4,4 -bis(N-phenyl-1-naphthylamino)biphenyl (NPB)
was used as hole transporting material (HTM), 1,3-bis(N-carbazolyl)benzene (mCP) was
used for exciton blocking layer, bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) and
bis[2-(2-pyridinyl)phenolato]beryllium(II) (Bepp2) host materials doped with 6% fluorescence emitters,
0
00
2
,2 ,2 -(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBI) was electron transporting material,
8-quinolinolato lithium (Liq) used for hole injecting material and aluminium was used for cathode
with thickness of 100 nm. All organic layers were deposited by thermal evaporating system under
−
7
5
2
× 10 torr pressure (Sunicel plus, Seoul, Republic of Korea). The active area of the devices were
mm × 2 mm.
4
. Conclusions
In summary, we have designed and synthesized four bipolar fluorescence emitters for OLEDs.
All four materials were synthesized by using Buchwald-Hartwig amination with excellent yield

Molecules 2018, 23, 843
11 of 13
over 75%. Here, we used oxadiazole acceptor with acridine derivatives (DPAc-OXD, 2DPAc-OXD)
and phenothiazine derivatives (PTZ-OXD, 2PTZ-OXD) to construct bipolar fluorescence materials.
All four materials exhibited higher thermal stabilities which were expressed by thermal decomposition
temperature at 5% weight reduction. In order to investigate the device efficiencies, we have fabricated
two different devices with two different host materials Bepp2 (Device 1) and DPEPO (Device 2).
The Device 1 performances were considerably higher than that of Device 2. The phenothiazine
donor based 2PTZ-OXD emitter revealed the best current and power efficiencies of 10.10 cd/A and
6
.88 Lm/W, respectively. The external quantum efficiency of 2PTZ-OXD was 3.99%, while PTZ-OXD
showed 2.26%. When we compared the overall performances, 2PTZ-OXD exhibited excellent properties
while acridine derivatives based 2DPAc-OXD and DPAc-OXD showed lower efficiencies due to its
strong donating power. We found that balance charge donating and withdrawing nature were very
important for bipolar fluorescence emitters in OLEDs to enhance the efficiencies.
Acknowledgments: This research was supported by basic science research program through the National Research
Foundation of Korea (NRF) funded by the education (NRF-2016R1D1A3B01015531).
Author Contributions: K.Y.C. designed the project; Q.W. and H.Q.Z. performed experiment; R.B., I.J.B. and M.K.
fabricated device and analyzed the data; R.B. and Q.W. wrote the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are not available from the authors.
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2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).