3,3-Bicarbazole-Based Host Molecules for Solution-Processed Phosphorescent OLEDs

Article abstract of DOI:10.3390/molecules23040847

Solution-processed organic light-emitting diodes (OLEDs) are attractive due to their low-cost, large area displays, and lighting features. Small molecules as well as polymers can be used as host materials within the solution-processed emitting layer. Herein, we report two 3,3-bicarbazole-based host small molecules, which possess a structural isomer relationship. 9,9-Di-4-n-butylphenyl-9H,9H-3,3-bicarbazole (BCz-nBuPh) and 9,9-di-4-t-butylphenyl-9H,9H-3,3-bicarbazole (BCz-tBuPh) exhibited similar optical properties within solutions but different photoluminescence within films. A solution-processed green phosphorescent OLED with the BCz-tBuPh host exhibited a high maximum current efficiency and power efficiency of 43.1 cd/A and 40.0 lm/W, respectively, compared to the device with the BCz-nBuPh host.

Full text of DOI:10.3390/molecules23040847

molecules
Article
3,30-Bicarbazole-Based Host Molecules for
Solution-Processed Phosphorescent OLEDs
Jungwoon Kim 1, Suhan Lee 2, Jaemin Lee 3 ID , Eunhee Lim 1,* and Byung Jun Jung 2,
*
1
Department of Chemistry, Kyonggi University, 154-42 Gwanggyosanro, Yeongtong-gu, Suwon-si,
Gyeonggi 16227, Korea; 003polychem@gmail.com
2
3
Department of Materials Science and Engineering, The University of Seoul, 163 Seoulsiripdaero,
Dongdaemun-gu, Seoul 02504, Korea; dltnghks88@nate.com
Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-ro,
Yuseong-gu, Daejeon 34114, Korea; jminlee@krict.re.kr
*
Correspondence: ehlim@kyonggi.ac.kr (E.L.); jungbj@uos.ac.kr (B.J.J.);
Tel.: +82-31-249-9663 (E.L.); +82-2-6490-2412 (B.J.J.)
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 20 March 2018; Accepted: 5 April 2018; Published: 8 April 2018
Abstract: Solution-processed organic light-emitting diodes (OLEDs) are attractive due to their
low-cost, large area displays, and lighting features. Small molecules as well as polymers can
be used as host materials within the solution-processed emitting layer. Herein, we report
two 3,30-bicarbazole-based host small molecules, which possess a structural isomer relationship.
9,90-Di-4-n-butylphenyl-9H,90H-3,30-bicarbazole (BCz-nBuPh) and 9,90-di-4-t-butylphenyl-9H,90H-
3,30-bicarbazole (BCz-tBuPh) exhibited similar optical properties within solutions but different
photoluminescence within films. A solution-processed green phosphorescent OLED with the
BCz-tBuPh host exhibited a high maximum current efficiency and power efficiency of 43.1 cd/A and
40.0 lm/W, respectively, compared to the device with the BCz-nBuPh host.
Keywords: solution-processed OLED; phosphorescent OLED; host; bicarbazole
1. Introduction
Ever since organic light-emitting diodes (OLEDs) were first reported by Tang and VanSlyke
with an organometal compound of 8-hydroxyquinoline aluminum and an aromatic diamine
compound [
the development of OLEDs, one great breakthrough was the utilization of a triplet emission called
phosphorescence [ ]. Ir complexes, such as phosphorescent dyes, exhibit a high phosphorescence yield
at room temperature [ ]. The external quantum efficiency of phosphorescent OLEDs (PHOLEDs) has
exceeded that of fluorescent OLEDs [ ]. In PHOLEDs, many carbazole molecules have been employed
as host materials in the emitting layers [
phosphorescent host molecule from early PHOLED reports [
1], they have comprised the core technology of mobile and flat-panel displays. During
2
3,4
5
0
0
6
,7
]. As 4,4 -bis(N-carbazolyl)-1,1 -biphenyl (CBP) is a famous
3,8
], it has been employed as a general
host material in studies pertaining to PHOLED materials and devices [9,10].
Structural isomers possess different properties. 9,90-Diphenyl-9H,90H-3,30-bicarbazole (BCzPh),
which was reported as a phosphorescent host in vacuum-deposited OLEDs by Sasabe et al. [11],
is an isomer of CBP (Chart 1). BCzPh exhibits a higher glass transition temperature than CBP [11
Bicarbazole derivatives have been studied as host materials [11 12], hole-transporting materials [13],
and emitters [14 15] in thermally activated delayed fluorescent (TADF) and exciplex OLEDs.
,12].
,
,
Recently, solution-processed OLEDs have attracted considerable attention due to their low-cost and
non-vacuum fabrication through methods such as ink-jet printing [16]. In solution-processed OLEDs,
poly(9-vinylcarbazole) (PVK) is a typical host material [17
,18]. Cai et al. reported that the small
molecule CBP could be used as a good host material in solution-processed OLEDs [19].
Molecules 2018, 23, 847; doi:10.3390/molecules23040847
www.mdpi.com/journal/molecules

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In this paper, we report two structural isomers based on 3,30-bicarbazole (BCz) that have
different alkyl groups (n-butyl and t-butyl groups) for solubility. 9,90-Di-4-n-butylphenyl-9H,90H-
3,30-bicarbazole (BCz-nBuPh) and 9,90-di-4-t-butylphenyl-9H,90H-3,30-bicarbazole (BCz-tBuPh) have
the same molecular formula and molecular weight. However, they possess slightly different molecular
structures. We investigated the thermal, optical, and electrochemical properties of the molecules as
OLED host materials. Moreover, we evaluated solution-processed OLEDs using new host materials.
Chart 1. The chemical structures of CBP and BCzPh.
2. Results and Discussions
2.1. Synthesis and Thermal Analysis
The synthetic routes and chemical structures for 3,30-bicarbazole (BCz), BCz-nBuPh
,
and BCz-tBuPh can be seen in Scheme 1. Initially, 3,30-bicarbazole was obtained via the chemical
oxidative coupling of carbazoles in the presence of FeCl3 according to a procedure described in the
literature [20]. Additionally, a detailed reduction procedure using zinc powder can be found in the
materials section. The designed new host molecules, BCz-nBuPh and BCz-tBuPh, were synthesized
through an Ullmann coupling reaction between 3,30-bicarbazole and two iodobenzenes with n-butyl
and t-butyl groups, respectively. The two compounds with alkyl groups were much more soluble
in common organic solvents, such as chloroform and toluene, compared to the intermediate
compound, BCz.
Scheme 1. The synthesis of host materials for solution processed organic light-emitting diodes (OLEDs).
◦
Reagents and conditions: (i) FeCl3, CHCl3, room temperature (RT); (ii) Cu, K2CO3, DMF, 145 C.

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Vacuum sublimation of the molecules yielded different aspects. BCz-nBuPh easily melted and
then sublimed. BCz-tBuPh was sublimed as a powder and featured higher sublimation yields than
◦
BCz-nBuPh. The melting point and glass transition temperature◦of BCz-tBuPh (295 and 136 C,
respectively) was much higher than those of BCz-nBuPh (146 and 58 C, respectively) as can be seen in
the differential scanning calorimetry (DSC) curves (Figure 1a). This difference could be attributed to the
stiffness of the t-butyl group compared to the n-butyl group. In contrast to the melting points, thermal
degradation of the two isomers was similar as can be seen in the thermogravimetric analysis (TGA)
curves (Figure 1b). Both molecules were sufficiently stable up to 400 ◦C; their thermal decomposition
temperatures (Td) were 410 ◦C for BCz-nBuPh and 423 ◦C for BCz-tBuPh.
Figure 1. DSC curves on the first heating process (
curves (b) of BCz-nBuPh and BCz-tBuPh.
a) and the second heating process (inset) and TGA
2.2. Photophyscial and Electrochemical Properties
Figure 2 shows the UV-visible absorption and photoluminescence (PL) spectra of BCz-nBuPh and
BCz-tBuPh in dilute chloroform and as films. In the solution spectra (Figure 2a), the photophysical
properties of the two isomers were nearly unchanged by the different alkyl groups. This meant that
the two host materials possessed nearly the same optical properties from the
same conjugated bicarbazole core.
π-π* transition of the
Figure 2. UV-visible absorption and photoluminescence (PL) spectra of BCz-nBuPh and BCz-tBuPh
in solution (a) and in films (b).

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On the other hand, with regard to the film PL spectra (Figure 2b), BCz-nBuPh exhibited a
relatively broad shoulder emission peak over 450 nm compared to BCz-tBuPh. Because the n-butyl
group in BCz-nBuPh exhibited less steric hindrance than the t-butyl group in BCz-tBuPh, molecules
of BCz-nBuPh within the film exhibited a strong degree of molecular aggregation. t-Butyl groups have
been introduced to blue emitting molecules to prevent molecular quenching [21,22]. BCz-tBuPh with
t-butyl groups was expected to yield better host materials than BCz-nBuPh with regard to thermal
and optical properties.
Optical band gaps were calculated from an absorption edge of 374 nm in the films, yielding
values of 3.32 eV for both BCz-nBuPh and BCz-tBuPh. This value was the same as that of BCzPh
(3.32 eV) [11] and enough to cover OLED dopants within visible light as the host materials.
The electrochemical properties of the two host molecules and N,N0-bis(3-methyl-phenyl)-
0
0
[l,l -biphenyl]-4,4 -diamine (TPD) were measured via cyclic voltammetry (CV) measurements (Figure 3).
The ionization potential (IP) energy levels of the molecules were estimated according to an empirical
relationship: E(IP) = E[ox, onset vs. Fc/Fc+] + 5.1 (eV) [23–25], where E[ox, onset vs. Fc/Fc+] (V) is the onset
potential of oxidation versus the redox potential of ferrocene/ferrocenium. The oxidation potentials
of the two host molecules were almost the same. The IP energy levels were estimated to be 5.55 eV
for both BCz-nBuPh and BCz-tBuPh. The thermal, optical, and electrochemical properties of the
molecules are summarized in Table 1.
Figure 3. Cyclic voltammetry (CV) curves of BCz-nBuPh, BCz-tBuPh, and TPD in dichloromethane
solution for oxidation.
Table 1. The thermal, optical, and electrochemical properties of BCz-nBuPh and BCz-tBuPh.
a
d
Host
Materials
Tm/ Tg
(oC)
λmax, solution (nm)
λmax, film (nm)
UV-vis, PL
Eox, onset
Eg c (eV)
IP e (eV)
b
Td (oC)
EA f (eV)
(V versus Fc/Fc+)
UV-vis, PL
BCz-nBuPh
BCz-tBuPh
146/58
295/136
410
423
305, 409
304, 409
308, 413
308, 411
3.32
3.32
0.45
0.45
5.55
5.55
2.13
2.13
a Melting temperature and glass transition temperature, b decomposition temperature corresponding to a 5% weight
e
loss, c optical band gap, d Onset potential of oxidation versus ferrocene/ferrocenium redox system. measured via
CV, f electron affinity as subtracting the optical bandgap from the ionization potential (IP).
2.3. Solution-Processed OLEDs
To evaluate the two host materials in solution-processed OLEDs, we fabricated green
phosphorescent OLED devices using tris[2-(p-tolyl)pyridine]iridium(III) (Ir(mppy)3) as a dopant.
In vacuum-deposited OLEDs, tris[2-phenylpyridine]iridium(III) (Ir(ppy)3) is usually used as
a green dopant without solubility limits. In solution-processed OLEDs, Ir(mppy)3 is often
used [17,19,26]. The emitting layer (EML) composition was host:TPD:PBD:Ir(mppy)3, where PBD
is 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole. TPD and PBD were blended to enhance the
hole and electron transport properties of the EML, respectively. The weight ratios of each component

Molecules 2018, 23, 847
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were 61:9:24:6 according to the previous report for solution-processed OLEDs using PVK or CBP
hosts [19].
The device configuration was indium tin oxide (ITO) (150 nm)/poly(3,4-ethylenedioxy
thiophene):poly(4-styrenesulfonate (PEDOT:PSS) (35 nm)/spin-coated EML (20 nm)/1,3,-bis(3,5-
dipyrid-3-yl-phenyl)benzene (BmPyPB) (50 nm)/LiF (0.7 nm)/Al (70nm). As shown in Figure 4a,
PHOLEDs using BCz-nBuPh and BCz-tBuPh exhibited green emission of Ir(mppy)3 with an
emission maximum at 510 nm. Figure 4b–d present OLED characteristics, such as the current
density-voltage-luminance (J-V-L) curves and efficiency-luminance curves. Table 2 summarizes the
OLED performance. Devices with BCz-tBuPh exhibited maximum current efficiency (CE), power
efficiency (PE), and external quantum efficiency (EQE) values of 43.1 cd/A, 40.0 lm/W, and 12.5%,
respectively; devices with BCz-nBuPh exhibited values of 30.8 cd/A, 28.1 lm/W, and 8.8%, respectively.
Moreover, devices with BCz-tBuPh exhibited a high current efficiency of 42.8 cd/A and 38.8 cd/A at
100 and 1000 cd/m2 of the display-relevant luminance range, respectively. The efficiency of BCz-tBuPh
was lower than the highest reported efficiency in solution-processed green PHOLEDs, but comparable
to the efficiency of devices based on several newly synthesized host materials [18,27].
Figure 4. Electroluminescence (EL) spectra (
efficiency-luminance ( ), and power efficiency-luminance characteristics (
green PHOLEDs using BCz-nBuPh and BCz-tBuPh, respectively.
a
), current density-voltage-luminance (
b), current
c
d) of solution-processed

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Table 2. The performance of solution-processed green PHOLEDs using BCz-nBuPh and BCz-tBuPh.
CE
PE
EQE
max, 1000
(%) f
c
Host
Materials
V1 , V1000
(V) b
Lmax
λ
(nm)
CIE (x, y) a
max, 1000
max, 1000
EL
(cd/m2)
(cd/A) d
(lm/W) e
BCz-nBuPh
BCz-tBuPh
510
510
(0.301, 0.620)
(0.294, 0.621)
3.3, 5.8
3.1, 4.9
6598
8372
30.8, 27.6
43.1, 38.8
28.1, 14.9
40.0, 24.9
8.8. 7.9
12.5. 11.2
a
b
Commission International deI’Eclairage (CIE) coordinates, driving voltage at a luminance of 1 cd/m2 and
1000 cd/m2, d maximum current efficiency (CE) and CE at 1000 cd/m2, e maximum power efficiency (PE) and PE at
1000 cd/m2, e maximum external quantum efficiency (EQE) and EQE at 1000 cd/m2.
As can be seen in Figure 4c,d, BCz-tBuPh exhibited superior efficiency compared to BCz-nBuPh
at all degrees of luminance. This result could be attributed to a difference in effective transfer from the
host to dopant. As BCz-tBuPh with t-butyl groups did not exhibit aggregation emission in the film PL
spectrum, the material served a better host role in OLED devices compared to BCz-nBuPh. Further
device works, such as the reports of other host materials [28,29], will be reported elsewhere.
3. Materials and Methods
3.1. General Procedures
Carbazole, potassium carbonate (K2CO3), copper, iodobenzene, chloroform, and
dimethylformamide (DMF) were purchased from Aldrich (St. Louis, MO, USA). 1-N-butyl-4-
iodobenzene, 1-t-butyl-4-iodobenzene, and FeCl3 were purchased from Aldrich (St. Louis, MO, USA).
Zinc powder, ethyl acetate, and acetic acid were purchased Showa chemical Inc. (Tokyo, Japan),
Duksan (Seoul, Korea), and Samchun (Seoul, Korea), respectively. All commercial reagents were used
as-is without further purification.
NMR spectra were recorded on a Bruker AVANCE II 400 spectrometer (Billerica, MA, USA).
UV-vis spectra were obtained using a UV-Vis spectrophotometer (UV-3150, Shimadzu, Kyoto, Japan).
Photoluminescence (PL) spectra were obtained using a F-4500 fluorescence spectrophotometer (Hitachi,
Tokyo, Japan). Films of small molecules used for the UV-vis and PL measurements were prepared
via solution spin-coating. Optical energy band gaps (Eg) were estimated from the absorption onset
wavelengths (Eg (eV) = 1240/λonset) of the small molecule films. Differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA) were performed with a differential scanning calorimeter
(DSC 1, Mettler Toledo, Columbus, OH, USA) and TGA Q500 (TA instruments, New Castle, DE, USA),
respectively. Cyclic voltammetry (CV) data was obtained with a BAS 100B electrochemical analyzer
(Bioanalytical Systems, West Lafayette, IN, USA).
3.2. Synthesis
3.2.1. 3,30-bicarbazole (BCz)
Carbazole (12.0 g, 72.0 mmol) and anhydrous FeCl3 (47.0 g, 288 mmol) were dissolved in 240 mL
of chloroform and stirred for 30 min under nitrogen. The solution was subsequently poured into a
larger quantity of methyl alcohol. The precipitated green powder and zinc powder (18.7 g, 288 mmol)
were dissolved in a solution of acetic acid and ethyl acetate (180 mL, 1:1) under nitrogen. The mixture
was heated at 50 ◦C overnight. The reaction mixture was filtered and washed with hot ethyl acetate.
The filtered solution was poured into water and extracted with ethyl acetate. The organics were
combined, washed with brine, and dried with sodium sulfate. After solvent removal, 5.94 g of a brown
powder was obtained. This crude product was purified via vacuum sublimation. Obtained as a while
1
solid was 2.53 g (8.0 mmol, 22%) of BCz. H-NMR (400 MHz, DMSO-d6):
δ (ppm) 11.28 (s, 2H), 8.51
(s, 2H), 8.25 (d, J = 7.7 Hz, 2H), 7.81 (dd, J = 8.4 Hz, J = 1.4 Hz, 2H), 7.58 (d, J = 8.4 Hz, 2H), 7.51 (d,
J = 8.1 Hz, 2H), 7.41 (t, J = 7.6 Hz, 2H), 7.19 (t, J = 7.4 Hz, 2H). 13C-NMR (100 MHz, DMSO-d6):
δ (ppm)
140.7, 139.2, 132.8, 126.1, 125.4, 123.6, 123.2, 120.9, 119.0, 118.6, 111.7, 111.5.

Molecules 2018, 23, 847
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3.2.2. 9,90-di-4-n-butylphenyl-9H,90H-3,30-bicarbazole (BCz-nBuPh)
3,30-Bicarbazole (1.5 g, 4.5 mmol), 1-n-butyl-4-iodobenzene (2.5 mL, 18 mmol), copper powder
(1.70 g, 27.1 mmol), and K2CO3 (3.73 g, 27.1 mmol) were dissolved in 10 mL of anhydrous DMF under
nitrogen. After heating the mixture at 145 ◦C overnight, the reaction solution was filtered through a
plug of celite. The filtered solution was poured into water and extracted with chloroform. The organics
were combined, washed with brine, and dried with sodium sulfate. After solvent removal, the product
was purified via column chromatography with 1:1 (v/v) dichloromethane–hexane as the eluent to
yield 1.41 g (2.36 mmol, 52%) of BCz-nBuPh as a white solid. For OLED devices, the product was
further purified twice via vacuum sublimation. The sublimation yield was 33%. Mp 146 ◦C. 1H-NMR
(400 MHz, CDCl3):
7.52–7.49 (m, 6H), 7.44–7.42 (m, 8H), 7.32–7.30 (m, 2H), 2.76 (d, J = 7.6 Hz, 4H), 1.77–1.69 (m, 4H),
1.51–1.42 (m, 4H), 1.01 (t, J = 7.3 Hz, 6H). 13C-NMR (100 MHz, CDCl3):
(ppm) 142.3, 141.5, 140.2,
δ (ppm) 8.45 (s, 2H), 8.23 (d, J = 7.7 Hz, 2H), 7.77 (dd, J = 8.5 Hz, J = 1.6 Hz, 2H),
δ
135.2, 134.3, 129.8, 126.9, 126.0, 125.8, 123.9, 123.5, 120.4, 119.8, 118.9, 110.1 101.0, 35.4, 33.7, 22.5, 14.1.
Anal. calcd. for C44H40N2 (%): C, 88.55; H, 6.76; N, 4.69; found: C, 88.07; H, 7.00; N, 4.80%.
3.2.3. 9,90-di-4-t-butylphenyl-9H,90H-3,30-bicarbazole (BCz-tBuPh)
3,30-Bicarbazole (1.0 g, 3 mmol), 1-t-butyl-4-iodobenzene (2.06 mL, 11.5 mmol), copper powder
(1.1 g, 17 mmol), and K2CO3 (2.4 g, 17 mmol) were dissolved in 10 mL of anhydrous DMF under
nitrogen. After heating the mixture at 145 ◦C overnight, the reaction solution was filtered through a
plug of celite. The filtered solution was poured into water and extracted with chloroform. The organics
were combined, washed with brine, and dried with sodium sulfate. After solvent removal, the product
was purified via column chromatography with 1:2 (v/v) chloroform–hexane as the eluent to yield 1.23 g
(2.01 mmol, 69%) of BCz-tBuPh as a white solid. For OLED devices, the product was further purified
◦
1
twice via vacuum sublimation. The sublimation yield was 74%. Mp 295 C. H-NMR (400 MHz,
CDCl3): (ppm) 8.45 (d, J = 1.4 Hz, 2H), 8.24 (d, J = 7.7 Hz, 2H), 7.77 (dd, J = 8.5 Hz, J = 1.8 Hz, 2H),
7.65–7.62 (m, 4H), 7.56–7.52 (m, 6H), 7.48–7.41 (m, 4H), 7.33–7.29 (m, 2H), 1.45 (s, 18H). 13C-NMR
(100 MHz, CDCl3): (ppm) 150.4, 141.5, 140.1, 135.0, 134.3, 126.8, 126.5, 125.9, 125.8, 123.9, 123.5, 120.4,
δ
δ
119.8, 118.9, 110.2, 110.0, 34.8, 31.5. Anal. calcd. for C44H40N2: C, 88.55; H, 6.76; N, 4.69; found: C,
88.00; H, 6.84; N, 4.80%.
3.3. Device Fabrication and Characterization
Pre-patterned indium tin oxide (ITO) glass substrates were cleaned via ultrasonication with
acetone, isopropyl alcohol, and deionized water. The substrates were treated with ultraviolet-ozone for
20 min using a UV-ozone cleaner (AH1700, AHTECH LTS Co., Ltd., Anyang, Gyeonggi, Korea).
The poly(3,4-ethylenedioxy thiophene):poly(4-styrenesulfonate) (PEDOT:PSS, AI 4083, Heraeus,
Hanau, Germany) solution was filtered through a 0.45 µm PVDF syringe filter and spin-coated
at 4000 rpm for 40 s to form the hole-injection layer (HIL) onto ITO substrates. The PEDOT:PSS-coated
◦
substrates were placed onto a hotplate at 150 C for 30 min under air. Prior to depositing the
emissive layer (EML), a 1 wt. % chlorobenzene solution was prepared in the weight ratio of 61◦:9:24:6
for BCz-nBuPh:TPD:PBD:Ir(mppy)3 and BCz-tBuPh:TPD:PBD:Ir(mppy)3 and stirred at 70 C for
1 h. The solutions were subsequently filtered through a 0.45
µm PTFE syringe filter; the EML
was spin-coated onto the HIL-coated substrate at 2500 rpm for 60 s under nitrogen. Thereafter,
the electron-transporting layer of BmPyPB, the electron injection layer of LiF, and the Al cathode
were deposited via thermal evaporation in a vacuum chamber under 5
×
10−7 torr. The OLED
devices were encapsulated with a glass lid, a moisture getter, and epoxy glue in a glove box.
The current-voltage-luminance characteristics of the OLEDs were obtained with a Keithley 2400 source
meter (Cleveland, OH, USA) and a Konica Minola CS-2000 spectroradiometer (Tokyo, Japan) in air.

Molecules 2018, 23, 847
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4. Conclusions
0
We synthesized 3,3 -bicarbazole-based molecules, BCz-nBuPh and BCz-tBuPh, for phosphorescent
OLED host materials through an Ullmann coupling reaction between 3,30-bicarbazole and iodobenz◦ne
◦
with n-butyl and t-butyl groups. BCz-nBuPh exhibited a low melting point of 146 C compared to 295 C
for BCz-tBuPh. Molecular aggregation emission was observed in the PL spectra of the BCz-nBuPh
films. BCz-tBuPh did not exhibit aggregation emission in the solid state and was more suitable as a
host material in solution-processed EMLs compared to BCz-nBuPh, exhibiting a maximum current
efficiency of 43.1 cd/A with a high current efficiency of 38.8 cd/A at 1000 cd/m2 for green PHOLEDs.
Acknowledgments: This work was supported by the 2014 Research Fund of the University of Seoul for
Byung Jun Jung.
Author Contributions: J.L., E.L. and B.J.J. designed the research; J.K. performed the synthesis and characterization
of molecules under supervision of E.L.; Thermal and electrochemical analysis was conducted by J.K. and J.L.;
OLED fabrication was performed by S.L.; J.K. and B.J.J 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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