New pyridopyrazine skelecton-based electron-transporting materials

Article abstract of DOI:10.1166/jnn.2012.5910

2,3-Di(pyridine-2-yl)-7-(4-triphenylsilyl)phenyl)pyrido[2,3-b]pyrazine (DPPP) containing pyridopyrazine was designed and synthesized as a new electron-transporting material for organic lightemitting devices (OLEDs). The obtained material forms homogeneous and stable amorphous film. The new synthesized showed the reversible cathodic reduction for hole blocking material and the low reduction potential for electron transporting material in organic EL devices. The molecule possess excellent thermal stability with glass transition temperature (T_g) of 115 °C in nitrogen. DNTPD (60 nm)/NPD (30 nm)/CBP:Irppy 6% (40 nm)/BAlq (10 nm)/ETL (30 nm)/LiF (0.5 nm)/Al structured device were fabricated using DPPP as electron transport material. The maximum luminance reached at 25000 cd/m². The current efficiency was 10.9 cd/A even high current. Copyright

Full text of DOI:10.1166/jnn.2012.5910

Copyright © 2012 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 12, 4370–4374, 2012
New Pyridopyrazine Skelecton-Based
Electron-Transporting Materials
Ran Kim1, Dae Hwan Oh1, Moon Chan Hwang2, Jang Yeol Baek3,
Sung Chul Shin1, Soon-Ki Kwon2ꢀ∗, and Yun-Hi Kim1ꢀ∗
1Department of Chemistry and RINS, Gyeongsang National University, Jinju 660-701, Republic of Korea
2School of Materials Science and Engineering and ERI, Gyeongsang National University, Jinju 660-701, Republic of Korea
3EL Materials Development Division, Duksan Hi-Metal Co., Ltd., Seongnam, 463-402, Republic of Korea
2,3-Di(pyridine-2-yl)-7-(4-triphenylsilyl)phenyl)pyrido[2,3-b]pyrazine (DPPP) containing pyridopy-
razine was designed and synthesized as a new electron-transporting material for organic light-
emitting devices (OLEDs). The obtained material forms homogeneous and stable amorphous film.
The new synthesized showed the reversible cathodic reduction for hole blocking material and the
low reduction potential for electron transporting material in organic EL devices. The molecule pos-
ꢀ
sess excellent thermal stability with glass transition temperature (Tgꢀ of 115 C in nitrogen. DNTPD
(60 nm)/NPD (30 nm)/CBP:Irppy 6% (40 nm)/BAlq (10 nm)/ETL (30 nm)/LiF (0.5 nm)/Al struc-
tured device were fabricated using DPPP as electron transport material. The maximum luminance
reached at 25000 cd/m2. The current efficiency was 10.9 cd/A even high current.
Keywords: Electron-Transporting Materials, Pyridopyrazine, Amorphous, Tetraphenylsilane.
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Copyright: American Scientific Publishers
1. INTRODUCTION
It is also reported that electron transporting organic
semiconducting materials should be readily sublimable,
forming uniform amorphous films.16 In order to develop
of fulfilling several requirements, many research groups
reported that low molar mass compound or polymeric
structures containing ꢁ electron deficient heterocyclic
moieties such as pyridine, oxadiazole, triazole, triazine,
quinoline, or quinoxaline and electron deficient boron-
containing or aluminum-containing amorphous materials
have been utilized as a hole blocking layer and electron
transporting layer.3ꢀ14–18
The most promising candidate for the next generation
energy-saving lighting and environmentally-friendly flat
panel display systems is a high efficiency organic light-
emitting devices (OLEDs).1–16 Typical OLEDs consist of
multi layers: hole transporting (HT), emissive, and electron
transporting (ET) layers, and so on.16 To reduce driving
voltage, which lead to reduction of energy consumption,
barrier height for the charge injection from the electrodes to
the organic layers must be low and the high charge mobility
of the transporting layers are required.17
Generally, electron mobilities in organic materials are
several orders of magnitude lower than hole mobilities, thus
electron-transporting materials with high electron mobil-
ity are required to further improve OLED performance.18
Efficient ET materials provide some advantages, such as
lowering the operating voltage and power consumption.
Moreover, if the ET materials have high triplet energy and
wide band gaps, in other words, deeper highest occupied
molecular orbital (HOMO), such ET materials can also
work as hole blocking (HB) materials. The complete con-
finement of a hole in an emissive layer by the HB layer
raises the quantum efficiencies of electroluminescence
(EL).16ꢀ18
Recently, we have developed 2,5-bis-(4-triphenylsilanyl-
phenyl)-[1,3,4]oxadiazole (BTSO), that functions as good
hole blocker and electron transporting layer in the phos-
phorescent devices.10
Through continuous material improvements, we design,
synthesis and characterization of amorphous molecular
material, 2,3-di(pyridine-2-yl)-7-(4-triphenylsilyl)phenyl)
pyrido[2,3-b]pyrazine (DPPP) with pyridyl-pyridopyra-
zine, which is new high electron attractive ꢁ conjugated
system. The introduction of pyridines on the 2,3-position
of pyridopyrazine can not only enhance electron affin-
ity but also reduce intermolecular interaction of fused
pyridopyrazine.20 Based on the concepts that the incor-
poration of rigid moieties increases glass transition tem-
peratures (Tgꢂ and that the enlargement of molecular
size enhances the stability of the amorphous glassy state,
∗Authors to whom correspondence should be addressed.
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J. Nanosci. Nanotechnol. 2012, Vol. 12, No. 5
1533-4880/2012/12/4370/005
doi:10.1166/jnn.2012.5910

Kim et al.
New Pyridopyrazine Skelecton-Based Electron-Transporting Materials
Table I. Physical property of DPPP.
ꢃaabs (nm)
ꢃbPL (nm)
Solution
463
red
Tgc, Tm,
Tdd (ꢀC)
E
HOMOcal/LUMO
onset
Solution
352
Film
355
Film
477
(v)
ꢄꢅ (eV)
ꢄE (eV)
DPPP
115, 230, 370
−0ꢆ83
6.75, 3.71
3.04
Notes: aAbsortion maximum, measured in Chloroform solution and film. bPL maximum, measured in Chloroform solution and film. cGlass transition temperature. dThermal
decomposition temperature.
tetraphenylsilyl group was introduced.10ꢀ13–14 It is expected
that silicon derivative is fabricated device, leading to high
efficiency and operational stability of the device as well as
providing high triplet energy level, thermal and chemical
stability.20
2.3. Synthesis of 7-Bromo-2,3-Di(pyridine-2-yl)
Pyrido[2,3-b]Pyrazine (3)
1,2-Di(pyridine-2-yl)ethandione (2 g, 9.4 mmol) and 5-
bromopyridine-2,3-diamime (2.1 g, 11.3 mmol) were dis-
solved in 200 mL of ethanol. The reaction mixture was
refluxed in N2 for 5 h. After the solvent was evapo-
rated, the crude product was purified by column chro-
matography (eluent = EA), the product was recrystallized
2. EXPERIMENTAL DETAILS
2.1. Synthesis of (4-Bromophenyl)Triphenyl-Silane (1)
1
from methanol. Yield: 49% (1.5 g). H-NMR (300 MHz,
1,4-Dibromobenzene (7.5 g, 32 mmol) was dissolved in
160 mL of dehydrated diethyl ether with agitation under
flow of N2. n-Butyl lithium (2.5 M in hexane, 12.7 mL,
32 mmol) was then added into the solution after cooling to
−78 ꢀC using liquid nitrogen. Chlorotriphenylsilane (13 g,
CDCl3ꢂ [ppm] ꢇ 9.2 (d, 1 H), 8.7 (d, 1 H), 8.4–8.2 (m,
3 H), 8.0–7.8 (m, 3 H), 7.3–7.2 (m, 3 H). FT-IR (KBr);
ꢈ = 3054 (aromatic C–H str), 1627 (C N str), 1076 cm−1
(C–Br str).
ꢀ
45 mmol) was added to the solution at −78 C, and after
2.4. Synthesis of 2,3-Di(pyridine-2-yl)-7-(4-
triphenylsilyl)phenyl)Pyrido[2,3-b]
Pyrazine (DPPP)
2 h the reaction was allowed to warm to room temperature
and left for a further 12 h. Finally, water was added to
quench the reaction. The product was extracted by methy-
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lene dichloride (MC), and then dried over MgSO . After
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4
7-Bromo-2,3-di(pyridine-2-yl)pyrido[2,3-b]pyrazine (2 g,
5.5 mmol) and 4-(triphenylsilyl)phenylboronic acid (2.5 g,
the solvent was evaporated, the crude pCroodpuycritgwhat:sApmureifiriecdan Scientific Publishers
by column chromatography (eluent = MC). Yield: 68%
(8.8 g). 1H-NMR (300 MHz, CDCl3ꢂ [ppm] ꢇ 7.7 (d, 2 H),
7.55–7.49 (m, 3 H), 7.46–7.41 (m, 6 H), 7.39–7.35 (m,
6 H), 7.35 (m, 2 H). FT-IR (KBr); ꢈ = 3058 (aromatic
C–H str), 1106 (Si–Ph str), 1006 cm−1 (C–Br str).
6.6 mmol) were mixed in 50 mL dry tolune. K2CO3 (2.0
M, 20 mL) was added, and mixture was stirred. The
mixture was degassed and tetrakis(triphenylphosphine)-
palladium (0.32 g, 5 mol%) was added in one portion
under an atmosphere of N2. The solution was then heated
under reflux for 24 h under N2. After the reaction mixture
cooled and added to an aqueous solution of HCl (2 N). The
resulting mixture was extracted with ethyl acetate (EA),
and then dried over MgSO4. After the solvent was evap-
orated, the crude product was purified by column chro-
matography (eluent = EA), the product was recrystallized
2.2. Synthesis of 4-(Triphenylsilyl)Phenyl-
Boronic Acid (2)
(4-Bromophenyl)triphenylsilane (9.7 g, 23.4 mmol) was
dissolved in 180 mL of dehydrated diethyl ether with agi-
tation under flow of N2. n-Butyl lithium (2.5 M in hexane,
12.1 mL, 30.4 mmol) was then added into the solution
1
from hexane. Yield: 53% (1.8 g). H-NMR (300 MHz,
ꢀ
CDCl3ꢂ [ppm] ꢇ 9.5 (d, 1 H), 8.76 (d, 1 H), 8.44–8.38
(m, 2 H), 8.3 (d, 1 H), 8.0 (d, 1 H), 7.95–7.78 (m, 6 H),
7.66–7.65 (dd, 6 H), 7.52–7.41 (m, 9 H), 7.35–7.28 (m,
2 H). FT-IR (KBr); ꢈ = 3050 (aromatic C–H str), 1585
(C N str), 1110 cm−1 (Si–Ph str). EI-MS (m/z): calcd
619.2, found 619 (M+ꢂ.
after cooling to −78 C using liquid nitrogen. Triethylbo-
rate (11.4 mL, 70 mmol) was added to the reactant solution
at −78 ꢀC, and after 2 h the reaction was allowed to warm
to room temperature and left for a further 12 h. Finally,
water was added to quench the reaction. The product was
extracted by methylene chloride (MC), and then dried over
MgSO4. After the solvent was evaporated, the crude prod-
uct was filtered and washed with the hexane. Yield: 79%
2.5. Device Fabrication
1
(7 g). H-NMR (300 MHz, CDCl3ꢂ [ppm] ꢇ 7.7 (d, 2 H),
EL devices were fabricated by successive vacuum depo-
sition of the organic materials onto an ITO-coated glass
substrate at a deposition rate of 0.1 nm/s at 10−6 torr.
Then Al was deposited onto the organic layer. The evap-
oration rate and the thickness of the film were measured
7.55–7.49 (m, 3 H), 7.46–7.41 (m, 6 H), 7.39–7.35 (m,
6 H), 7.35 (m, 2 H). H-NMR (300 MHz, CDCl3ꢂ [ppm]
ꢇ 9.2 (d, 1 H), 8.7 (d, 1 H), 8.4–8.2 (m, 3 H), 8.0–7.8 (m,
3 H), 7.3–7.2 (m, 3 H). FT-IR (KBr); ꢈ = 3428 (O–H str)
3062 (aromatic C–H str), 1106 cm−1 (Si–Ph str).
1
J. Nanosci. Nanotechnol. 12, 4370–4374, 2012
4371

New Pyridopyrazine Skelecton-Based Electron-Transporting Materials
Kim et al.
with a quartz oscillator. OLED performance was studied
by measuring the current–voltage–luminescence (I–V–L)
characteristics, EL, and PL spectra at room temperature.
I–V–L characteristics, EL spectra and CIE color coordi-
nates were measured with a Keithley SMU238 and Spec-
trascan PR650. EL spectra of the devices were measured
utilizing a diode array rapid analyzer system (Professional
Scientific Instrument Corp.) Fluorescence spectra using a
spectrofluorimeter (Shimadzu Corp.).
form a stable glassy state. This result may be explained by
the introduction of rigid and bulky tetraphenylsilyl group
having tetrahedral molecular skeleton in solid state.
Cyclic voltammetry (CV) was used to study reversible
redox behavior and electrochemical stability and to obtain
information about HOMO and lowest unoccupied molec-
ular orbital (LUMO) energy values of hole and elec-
tron transport materials and to examine the barriers for
charge injection. The electrochemical behavior of DPPP
was investigated by cyclic voltammetry with a standard
three electrodes electrochemical cell in a 0.1 M Bu4NClO4
solution in anhydrous acetonitrile at room temperature
under nitrogen with a scanning rate of 200 mV/s. A plat-
inum working electrode and an Ag/AgNO3 (0.1 M) refer-
ence electrode were used. The reduction onset potentials
were measured to be −0ꢆ83 V. The result indicate that the
electron accepting property of the DPPP, which is stronger
3. RESULTS AND DISCUSSION
Scheme 1 illustrates the chemical structure of DPPP
along with the corresponding synthetic route. 7-Bromo-
2,3-di(pyridine-2-yl)pyrido[2,3-b]pyrazine was obtained
by 1,2-di(pyridine-2-yl)ethandione and 5-bromopyridine-
2,3-diamime through the condensation reaction. The
DPPP was synthesized by Suzuki coupling reaction of
4-(triphenylsilyl)phenylboronic acid and 7-bromo-2,3-di
(pyridine-2-yl)pyrido[2,3-b]pyrazine. The obtained DPPP
was confirmed by various spectroscopic analyses such as
NMR, IR and Mass analysis.
red
1/2
than those of boron derivatives (E = −2ꢆ5 V) and that of
Alq3 (E1re/d2 = −2ꢆ01 V).21–23 Thus, the obtained DPPP ful-
fill not only the requirement of reversible cathodic reduc-
tion for hole blocking material in organic EL devices but
also the requirement of low reduction potential for elec-
tron transporting material in EL devices. According to
the UV edge of band gap energy (3.04 eV) and LUMO
(−3ꢆ71 eV), the energy values of the HOMO was calcu-
lated to be −6ꢆ75 eV.
The thermal properties of DPPP were evaluated by
means of thermogravimetric analysis (TGA) and differen-
tial scanning calorimetry (DSC) in a nitrogen atmosphere
(Fig. 1). The decomposition temperature of DPPP, cor-
responding to 5% weight loss upon heating in TGA, is
around 400 ꢀC and DSC exhibits a distinct glass transition
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To investigate the photophysical properties, the UV-
IP: 46.161.57.127 On: Wed, 01 Jun 2016 20:42:03
visible absorption and photoluminescent spectra (PL) of
DPPP were measured both in chloroform solution and in
Copyright: American Scientific Publishers
ꢀ
around 115 C. The DSC results confirm that DPPP can
OH
triethylborate
chlorotriphenylsilane
Br
Br
Br
Si
1
Si
2
B
n-BuLi/Et2O
n-BuLi/Et2O
OH
O
O
H2N
H2N
Br
N
N
3
Br
N
N
ethanol
reflux
N
N
N
N
N
Pd(PPh3)4/2MK2CO3
tolune
2
+
3
N
Si
N
DPPP
Scheme 1. Synthetic scheme of DPPP.
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Kim et al.
New Pyridopyrazine Skelecton-Based Electron-Transporting Materials
(a)
Fig. 3. Current efficiency–voltage plot and luminance–voltage plot of
DPPP device.
(b)
HOMO level of BTSO is obtained by the optical band gap
and the LUMO level, the HOMO level is comparable with
BCP which is well known for suitable hole blocking layer
due to its large HOMO level (HOMO = 6.4 eV).24–25 From
the results of HOMO and LUMO level, the obtained DPPP
can be expected to have not only function as good electron
transporter but also double function as good hole blocker
and good electron transporter.
The devices consisted of an indium tin oxide anode
(120 nm), a 60 nm DNTPD (N,Nꢁ-[p-di(m-tolyl)amino-
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phenyl]-N,Nꢁ-diphenyl bendizine) hole injection layer,
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ꢁ
ꢁ
Copyright: American Scientific Publishers
a 30 nm ꢉ-NPD (N,N -di(naphtyl)-N,N -diphenyl ben-
dizine) hole transporting layer, a 40 nm 4,4ꢁ-bis(N-carba-
zoyl)biphenyl (CBP) doped with 6% of Ir(ppy)3 emissive
layer, a 10 nm hole blocking layer, 30 nm DPPP as elec-
tron transporting layer, 5 nm LiF as electron injection layer
and Al cathode. Figure 3 shows voltage–luminance and
voltage–current efficiency characteristics of OLED. The
maximum luminance was reached to 25000 cd/m2 with
10.9 current efficiency. Additionally, the maximum exter-
nal quantum efficiency (Fig. 4) of the device was 3.36%.
Fig. 1. TGA and DSC thermograms of DPPP.
thin films (Fig. 2). As shown in the absorption spectra, the
maximum peak is at 352 nm in solution and at 355 nm
in the film, respectively. The optical energy band gap of
DPPP was estimated to be 3.04 eV from the energy thresh-
old of its electronic absorption spectrum. Although the
Fig. 2. Uv-vis absorption and PL spectra of DPPP.
Fig. 4. Quantum efficiency–luminance plot of DPPP device.
J. Nanosci. Nanotechnol. 12, 4370–4374, 2012
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New Pyridopyrazine Skelecton-Based Electron-Transporting Materials
Kim et al.
4. CONCLUSION
7. D. S. Leem, S. O. Jung, S. O. Kim, J. W. Park, J. W. Kim, Y. S. Park,
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(2009).
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Oh, J. Kim, S. K. Kwon, and Y. Kang, Org. Electron. 10, 1066
(2009).
9. J. W. Park, S. J. Park, Y. H. Kim, D. C. Shin, H. You, and S. K.
Kwon, Polymer 50, 102 (2009).
10. S. O. Jung, J. W. Park, D. M. Kang, J. S. Kim, S. J. Park, P. Kang,
H. Y. Oh, J. H. Yang, Y. H. Kim, and S. K. Kwon, J. Nanosci.
Nanotechnol. 8, 4838 (2008).
11. Y. S. Park, J. W. Kang, D. M. Kang, J. W. Park, Y. H. Kim, S. K.
Kwon, and J. J. Kim, Adv. Mater. 12, 1957 (2008).
12. J. U. Kim, H. B. Lee, J. S. Kim, Y. H. Kim, Y. K. Joe, H. Y. Oh,
C. G. Park, and S. K. Kwon, Synth. Met. 150, 27 (2005).
13. Y. H. Kim, H. C. Jeong, S. H. Kim, K. Yang, and S. K. Kwon, Adv.
Funct. Mater. 15, 1799 (2005).
We have demonstrated tetraphenylsilane substituted pyri-
dopyrazine derivative (DPPP) as electron-transporting
material for OLED. The pyridopyrazine with bipyridyl
was efficient electron affinity group. The bulky, sterically
hindered tetrahedron molecular skeleton of tetraphenyl
silyl group hindered close packing and crystallization as
well as increased its molecular rigidity, leading to amor-
phous material having pronounced morphological stability.
It is characterized by high oxidation potentials, low lying
HOMO and LUMO level. The fabricated devices exhib-
ited maximum luminance of 25000 cd/m2 and current effi-
ciency of 10.9 cd/A.
14. S. Kwon, K. R. Wee, A. L. Kim, and S. O. Kang, J. Phys. Chem.
Lett. 1, 295 (2010).
15. S. O. Jung, Y. H. Kim, S. K. Kwon, and J. H. Yang, Org. Electron.
8, 349 (2007).
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Mater. 19, 1260 (2009).
Acknowledgment: This research was financially sup-
ported by Basic Science Research Program through the
National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology
(2011-0003336, 2011-0003336, 2010-0029084 and 2011-
0027499). Ran Kim, Dae Hwan Oh and Moon Chan
Hwang thank the support by MKE and KIAT through the
Workforce Development Program in Strategic Technology.
19. T. H. Hauang, W. T. Whang, J. Y. Shen, Y. S. Wen, J. T. Lin,
T. H. Ke, L. Y. Chen, and C. C. Wu, Adv. Funct. Mater. 16, 1449
(2006).
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