New Emitting Materials Based on HTL Moiety with High Hole Mobility for OLEDs

Article abstract of DOI:10.1080/15421406.2015.1075847

New green emitting compounds based on tris(N-methylindolo)benzene (NMT), anthracene and pyrene were synthesized. NMT-An and NMT-Py were used as an emitting layer in OLED device to examine emitting property. OLED device containing NMT-An emitting layer and conventional hole transporting layer (HTL) of NPB was found to exhibit better characteristics compared to NMT-Py. And that device showed maximum EL emission at 502 nm and 550 nm, CIE coordinates (0.38, 0.48), and a luminance efficiency of 2.06 cd/A. Also when NMT and NMT-An were used as a HTL instead of NPB, the device including NMT-An emitter showed 2.67 cd/A and 2.29 cd/A in luminance efficiency.

Full text of DOI:10.1080/15421406.2015.1075847

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
New Emitting Materials Based on HTL Moiety with
High Hole Mobility for OLEDs
Hyeonmi Kang, Hayoon Lee, Hwangyu Shin, Seokwoo Kang, Beomjin Kim &
Jongwook Park
To cite this article: Hyeonmi Kang, Hayoon Lee, Hwangyu Shin, Seokwoo Kang, Beomjin
Kim & Jongwook Park (2015) New Emitting Materials Based on HTL Moiety with High
Hole Mobility for OLEDs, Molecular Crystals and Liquid Crystals, 618:1, 47-54, DOI:
10.1080/15421406.2015.1075847
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1075847
Published online: 07 Oct 2015.
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Date: 11 October 2015, At: 07:20

Mol. Cryst. Liq. Cryst., Vol. 618: pp. 47–54, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2015.1075847
New Emitting Materials Based on HTL Moiety
with High Hole Mobility for OLEDs
HYEONMI KANG, HAYOON LEE, HWANGYU SHIN,
SEOKWOO KANG, BEOMJIN KIM, AND JONGWOOK PARK∗
Department of Chemistry, The Catholic University of Korea, Bucheon, Korea
New green emitting compounds based on tris(N-methylindolo)benzene (NMT), an-
thracene and pyrene were synthesized. NMT-An and NMT-Py were used as an emitting
layer in OLED device to examine emitting property. OLED device containing NMT-An
emitting layer and conventional hole transporting layer (HTL) of NPB was found to
exhibit better characteristics compared to NMT-Py. And that device showed maximum
EL emission at 502 nm and 550 nm, CIE coordinates (0.38, 0.48), and a luminance effi-
ciency of 2.06 cd/A. Also when NMT and NMT-An were used as a HTL instead of NPB,
the device including NMT-An emitter showed 2.67 cd/A and 2.29 cd/A in luminance
efficiency.
Keywords Emitting material layer; Hole transporting later; Anthracene; Pyrene; OLED
Introduction
Many studies on organic light emitting diode (OLED) have been extensively investigated
because of various applications such as mobile display, flexible display, and lightings.[1-3]
Many researchers are engaged in ongoing efforts to systematically elucidate the correlation
between their molecular structures and properties, with the aim of developing materials
with good performance and stability for particular optoelectronic applications. Most con-
ventional studies of emitters use molecules with excellent fluorescence characteristics such
as anthracene, pyrene and fluorene as core or side moieties.[4] In order to fabricate full color
OLED displays, we need high performance red-, green-, and blue-light emitters with high
electroluminescence (EL) efficiencies, good thermal properties, long device lifetimes, and
pure color coordinates.[5] Furthermore, balanced carrier injection and carrier transportation
between device layers are required for high device efficiency. Unbalanced hole and electron
current form a narrow combination zone, which leads to low efficiency with high roll-off
and shorter lifetime.[6] In general, multi-layer structure is fabricated in OLED devices
using vacuum deposition method for efficient carrier injection and carrier transportation.
Reducing the number of layers can result in simplification of the fabrication process and
shortening of preparation time.
In this study, new anthracene derivative and pyrene derivative were synthesized
as materials in which hole transporting layer and emitting material layer can be used
∗Address correspondence to Jongwook Park, Department of Chemistry, The Catholic University
of Korea, 43-1, Yeokgok, Wonmi, Bucheon 420-743, Korea(ROK). E-mail: hahapark@catholic.ac.kr
[257]/47

48/[258]
H. Kang et al.
together. Several studies have reported 5,10,15-trimethyl-10,15-dihydro-5H-5,10,15-
triaza-diindeno[1,2-a;1ꢀ,2ꢀ-c]fluorine (NMT) as a good hole transport material.[6,7]
Anthracene and pyrene derivatives including NMT as a core were prepared as HTL and
EML in OLED devices.
Experimental
Measurements
1H-NMR spectra were recorded on Bruker Avance 300 spectrometers. The optical absorp-
tion spectra were obtained by HP 8453 UV-VIS-NIR spectrometer. Perkin Elmer lumi-
nescence spectrometer LS50 (Xenon flash tube) was used for photo-luminescence (PL)
spectroscopy. Thermal degradation temperature (Td) of the compounds was measured by
thermo-gravimetric analysis (TGA) using SDP-TGA2960 (TA instrument). The HOMO
energy levels were determined with ultraviolet photoelectron yield spectroscopy (Riken
Keiki AC-2). The LUMO energy levels were derived from the HOMO energy levels and
the band gaps. All DFT calculations (B3LYP/6-31G(d)) were performed with the Gaussian
09 program.
Fabrication of OLED
EL devices were fabricated as the following structure : ITO/ 2-TNATA 60 nm/ NPB or
NMT 15 nm/ emitting layer(EML) 35 or 50 nm/ Alq3 20 nm/ LiF 1 nm/ Al 200 nm, where
4,4ꢀ,4ꢀꢀ-tris(N-(2-naphthyl)-N-phenyl-amino)-triphenylamine (2-TNATA) was used as a
hole injection layer (HIL), N,Nꢀ-bis(naphthalene-1-ly)-N,Nꢀ-bis(phenyl)benzidine (NPB)
as a hole transporting layer (HTL), the synthesized materials as emitting layer (EML),
Tris-(8-hydroxyquinoline)aluminum (Alq3) as an electron transporting layer (ETL) and
hole blocking layer (HBL), lithium fluoride (LiF) as an electron injection layer (EIL), ITO
as anode and Al as cathode. The organic layer was vacuum deposited by using thermal
evaporation at a vacuum base pressure of 10−6 torr and the rate of deposition being 1 Å/S to
give an emitting area of 4 mm and the Al layer was continuously deposited under the same
vacuum condition. The current-voltage-luminance (I-V-L) characteristics of the fabricated
EL devices were obtained by Keithley 2400 electrometer and light intensity was obtained
by Minolta CS 1000A.
Synthesis
Synthesis of N-methyloxindole (1). N-methylaniline (20 mL, 186 mmol) was dissolved in
toluene (250 mL), and chloroacetyl chloride (15.8 mL, 198 mmol) was added slowly. Then
the mixture was refluxed for 2h. Water (50 mL) was added after the mixture was cooled
to room temperature and the mixture was stirred for 3h to decompose the chloroacetyl
chloride. Toluene layer was washed first with 10% K2CO3 in water solution. And it was
continuously added with HCl (2M) before drying overnight with sodium sulfate. After
toluene was evaporated, AlCl3 (82 g) was added, and the mixture was stirred for 10 min
at 70◦. Then the above mixture was stirred at 210◦ for 90 min. The reaction mixture was
slowly poured into ice water, and then concentrated HCl (50 mL) was added. The mixture
was extracted with chloroform and chloroform layer was dried over sodium sulfate. After
evaporating chloroform, the solid was recrystallized with hexane. N-methyloxindole (16 g)
was prepared in total yield 66.6% of the two step.

New Emitting Materials Based on HTL
[259]/49
1H-NMR (300MHz, CDCl3) δ(ppm) : 7.29–7.20 (m, 2H), 7.05–7.00 (t, 1H), 6.81–6.78
(d, 1H), 3.47 (s, 2H), 3.18 (s, 3H).
Synthesis of 5-bromo-N-methyloxindole (2). N-methyloxindole (0.5 g, 3.4 mmol) in ace-
tonitrile (5 mL) was stirred at 0◦C, and then NBS (0.62 g) in 10 mL acetonitrile was added
dropwise. The mixture was stirred at that temperature for 1h and then stirred for 2h at am-
bient temperature. Then the solution was evaporated and the solid was dissolved in CHCl3,
washed twice with water. After CHCl3 was evaporated, the brown solid was recrystallized
with hexane. 5-bromo-N-methyloxindole (0.44 g) was prepared in 58% yield.
1H-NMR (300MHz, CDCl3) δ(ppm) : 7.42–7.39 (d, 1H), 7.26 (s, 1H), 6.71–6.68 (d,
1H), 3.52–3.20 (s, 2H), 3.19 (s, 3H).
Synthesis
of
3,8,13-Tribromo-5,10,15-trimethyl-10,15-dihydro-5H-5,10,15-triaza-
diindeno[1,2-a;1ꢀ,2ꢀ-c]fluorine (NMT-Br) (3). A mixture of 5-bromo-N-methyloxindole
(5 g, 22 mmol) and POCl3 (40 mL) was stirred and refluxed for 2h. After cooling, the
mixture was poured into ice water, and then extracted with dichloromethane (MC) for
three times. The combined organic layers were dried over anhydrous magnesium sulfate,
filtered, and concentrated. The solid was washed with MC, white solid NMT (2.6 g) was
prepared. Yield was 61%.
1H-NMR (300MHz, THF) δ(ppm) : 8.58 (s, 3H), 7.63–7.60 (d, 3H), 7.57–7.53 (d, 3H),
4.40 (s, 9H).
Synthesis of 5,10,15-trimethyl-10,15-dihydro-5H-5,10,15-triaza-diindeno[1,2-a;1ꢀ,2ꢀ-
c]fluorine (NMT) (4). Compound NMT was synthesized according to the method
described for compound NMT-Br. The white powder was purified by using column
chromatography with ethyl acetate : hexane (1:10) as the eluent to afford a white solid
(0.3 g, 12%).
1H-NMR (300MHz, CDCl3) δ(ppm) : 8.48–8.46 (d, 3H), 7.61–7.58 (d, 3H), 7.50–7.45
(t, 3H), 7.37–7.32 (t, 3H), 4.47(s, 9H).
Synthesis of 9-boroanthracene (5). 9-bromoanthracence (10.0 g, 38.9 mmol) was dissolved
in anhydrous THF solution (500 mL) and stirred at −78◦C. Then, 1.6 M n-BuLi (29.3 mL,
46.7 mmol) was added. Triethyl borate (9.3 mL, 54.5 mmol) was added to the reaction after
30 min. After the reaction was finished, the solution was acidified with 2 N HCl solution
at room temperature and extracted with ethyl acetate and water. The organic layer was
dried with anhydrous MgSO4 and filtered. The solution was evaporated. The residue was
redissolved in hexane and added to ethyl acetate. The precipitate was filtered and washed
1
with hexane to obtain a beige compound (7.70 g, 89%). H-NMR (300 MHz, CDCl3):
δ(ppm) = 8.47 (s, 1H), 8.14 (d, J = 9.2 Hz, 2H), 8.04 (d, J = 9.8 Hz, 2H), 7.53–7.44 (m,
4H), 5.07 (s, 2H).
Synthesis of 1-boropyrene (7). 1-Bromopyrene (1.0g, 3.56 mmol) was dissolved in anhy-
drous THF solution (20 mL) and stirred at −78◦C. Then, 1.6 M n-BuLi (2.1 mL) was added.
Triisopropyl borate (1.2 mL) was added to the reaction after 30 min. After the reaction
was finished, the solution was acidified with 2 N HCl solution at room temperature and
extracted with ethyl acetate and water. The organic layer was dried with anhydrous MgSO4
and filtered. The solution was evaporated. The residue was redissolved in hexane and added
to ethyl acetate. The precipitate was filtered and washed with hexane to obtain a beige
compound (0.63 g, 72%).

50/[260]
H. Kang et al.
1H-NMR (300 MHz, DMSO): δ(ppm) = 8.72-8.69 (d, 1H) 8.60 (s, 2H) 8.30-8.15 (m,
7H) 8.06-8.04 (t, 1H).
Synthesis of 3,8,13-Tri-anthracen-9-yl-5,10,15-trimethyl-10,15-dihydro-5H-5,10,15-
triaza-diindeno[1,2-a;1ꢀ,2ꢀ-c]fluorine (NMT-An) (6). NMT-Br (0.3 g, 0.48mmol),
9-boroanthracene (0.42g, 1.89mmol), Pd2(dba)3 (0.04 g), sodium-tert-butoxide (0.36 g),
and tri-tert-butylphosphine (0.08 g) were added to anhydrous ortho-xylene (90 mL)
solution. Then 9-boroanthracene (0.42 g) was added to reaction mixture at 80◦C. The
mixture was heated to 80◦C for 5 h under nitrogen. After the reaction was finished, the
reaction mixture was extracted with xylene and water. The organic layer was dried with
anhydrous MgSO4 and filtered. The solution was evaporated. The yellow powder was
purified by using column chromatography with CHCl3 : n-hexane as the eluent to afford a
yellow solid (0.2g, 46%).
1H-NMR (300 MHz, THF): δ(ppm) = 8.67 (s, 3H), 8.59 (s, 3H), 8.12–8.10 (d, 6H),
7.88–7.85 (d, 6H), 7.77–7.44 (d, 3H), 7.49–7.44 (m, 9H), 7.36–7.30 (m, 6H) 4.45 (s, 9H).
Synthesis of 5,10,15-Trimethyl-3,8,13-tri-pyren-1-yl-10,15-dihydro-5H-5,10,15-triaza-
diindeno[1,2-a;1ꢀ,2ꢀ-c]fluorine (NMT-Py) (8). Compound NMT-Py was synthesized ac-
cording to the method described for compound NMT-An. The beige powder was purified
by using column chromatography with ethyl acetate : n-hexane as the eluent to afford a
yellow solid (0.45g, 9.5%).
1H-NMR (300 MHz, THF): δ(ppm) = 8.88 (s, 3H), 8.48–8.45 (d, 3H), 8.35–8.33 (d,
3H), 8.26–8.16 (m, 15H), 8.13–8.09 (d, 3H), 8.06–8.00 (t, 3H), 7.83–7.80 d, 3H), 7.75–7.72
(d, 3H), 4.60 (s, 9H).
Results and Discussion
Triphenylene-fused triazatruxene derivatives have HOMO values of 4.9–5.2 eV and good
hole carrier mobility of 0.03–0.8 cm2/Vs7. NMT which is one of these derivatives has
HOMO value of 5.03 eV and can be used as HTL material in OLED applications. As
shown in Scheme 1, NMT has a propeller structure fused with carbazole. Considering
optical properties of NMT in Fig. 1, PL maximum peak in solution state is 401 nm and
shows sharp deep blue. In film state, unique emission peak appears at 414 nm along with
excimer peak at 454 nm. While rigid structure of NMT increases carrier mobility, it may
form excimer by allowing for packing in film state under optical properties.
Scheme 1. Chemical structures of the synthesized compounds.

New Emitting Materials Based on HTL
[261]/51
Scheme 2. Synthetic routes of NMT, NMT-An and NMT-Py.
Using NMT with good hole mobility as the core, anthracene and pyrene as emitting
materials were substituted to synthesize NMT-An and NMT-Py, EML materials with HTL
characteristics. According to Fig. 1, while NMT-An and NMT-Py show narrow emission
similar to NMT in solution state, excimer peak also appears in film state.
NMT-An and NMT-Py, new emitting materials based on HTL moiety, were applied
to OLED device as EML materials. Energy levels of the materials used in the device are
shown as a diagram in Fig. 2(a). Comparing between device 1 and device 4 in Fig. 2(b) and
Table 1, NMT-An used as EML has luminance efficiency of 2.06 cd/A, which is higher than
0.5 cd/A of NMT-Py. As for the reason, electron injection at Alq3 may be more difficult for
Table 1. Optical, electrical and thermal properties of the synthesized compounds
Solutiona Filmb Solutiona Filmb
UVmax UVmax PL max PL max HOMO LUMO ꢁEexp Td (◦)
c
Compound
(nm)
(nm)
(nm)
(nm)
(-eV)
(-eV)
(eV)
(◦)
NMT
NMT-An
NMT-Py
319
325
401
400
461
414, 454 5.03
477, 545 5.38
489, 613 5.08
1.76
2.63
2.27
3.27
2.75
2.81
380
500
529
326, 369 329, 394
346 349
a Solution in THF (1.00 × 10−5M).
b Film on glass.
c Experimental band gaps.

52/[262]
H. Kang et al.
Figure 1. UV-visible absorption spectra and PL spectra of the synthesized materials: (a) NMT in
chloroform solution (1.00 ×10−5 M) and the thin film state, (b) NMT-An and NMT-Py in chloroform
solution (1.00 ×10−5 M), (c) NMT-An and NMT-Py in the thin film state.

New Emitting Materials Based on HTL
[263]/53
Figure 2. EL characteristics of devices using the synthetic materials as EML: (a) energy diagrams
of the compounds, (b) luminance efficiency against current density.
NMT-Py than NMT-An due to high LUMO as shown in Fig. 2(a). In addition, excimer peak
is found in film PL of both NMT-An and NMT-Py. When the original peak of materials is
compared to excimer peak, original peak is larger for NMT-An and excimer peak is larger
for NMT-Py. This means that NMT-Py can show low efficiency from excimer quenching
in the device.
In the DFT calculations of Fig. 3, twist angle of the NMT core and substituent is 78◦
for NMT-An and 56◦ for NMT-Py. While the two compounds have rigid structure and form
excimer based on good packing, NMT-Py has smaller twisted angle between NMT and
pyrene compared to NMT-An, more strongly inducing excimer formation based on better
packing. A device that uses relatively efficient NMT-An as EML and NMT as HTL in place
of NPB is represented by device 2.
When NMT is applied as HTL instead of NPB, NMT has high LUMO value of 1.76eV
and can increase efficiency by performing the role of an electron blocking. In fact, luminance
efficiency of device 2 is 2.67 cd/A, which is about 30% higher than efficiency of device1.
Device3 includes NMT-An one layer of same thickness as EML and HTL. Compared to
device1 where NPB was used as HTL, luminance efficiency was increased by about 11%.
These results imply that the synthesized NMT-An can be used as EML as well as HTL
material without the need of other HTL materials.
Table 2. EL performances of multi-layered devices
Volt(V) L.E.(cd/A) P.E.(lm/W) E.Q.E.(%)
CIE (x,y)
Elmax(nm)
Device 1
Device 2
Device 3
Device 4
7.64
6.77
7.17
8.69
2.06
2.67
2.29
0.50
0.93
1.38
1.10
0.02
0.90
1.07
0.94
0.49
(0.38, 0.48)
(0.40, 0.50)
(0.41, 0.51)
(0.39, 0.59)
502,550
510,561
510,562
628
Device1: ITO / 2-TNATA (60 nm) / NPB (15 nm) / NMT-An (35 nm) / Alq3 (20 nm) / LiF / Al
Device2: ITO / 2-TNATA (60 nm) / NMT (15 nm) / NMT-An (35 nm) / Alq3 (20 nm) / LiF / Al
Device3: ITO / 2-TNATA (60 nm) / NMT-An (50 nm) / Alq3 (20 nm) / LiF / Al
Device4: ITO / 2-TNATA (60 nm) / NPB (15 nm) / NMT-Py (35 nm) / Alq3 (20 nm) / LiF / Al

54/[264]
H. Kang et al.
Figure 3. Molecular structures of the NMT-An (a) and NMT-Py (b) optimized by B3LYP/6-311G(d).
Conclusion
NMT-An and NMT-Py compounds that can be simultaneously applied to HTL and EML
to simply the fabrication process of OLED devices, were successfully synthesized. Among
the synthesized compounds, NMT-An shows higher EL efficiency and lower operating
voltage compared to NMT-Py. Luminance efficiency of the device that used NMT-An as
EML without HTL was increased by about 11% compared to the device that used NPB as
HTL material.
Funding
This research was supported by a grant from the Technology Development Program for
Strategic Core Materials funded by the Ministry of Trade, Industry & Energy, Republic of
Korea (Project No. 10047758).
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