New deep blue emitting materials based on indenopyrazine core with high thermal stability

Article abstract of DOI:10.1166/jnn.2011.3693

New deep blue emitting materials 2,8-bis(3,5-diphenylphenyl)-6,6,12,12- tetraethyl-6,12-dihydrodiindeno[1,2-b:1′,2′-e]pyrazine (DPP-EPY) and 2,8-bis(3′,5′-diphenylbiphenyl-4-yl)-6,6,12,12-tetraethyl-6,12- dihydrodiindeno[1,2-b:1′,2′-e]pyrazine (DPBP-EPY)

Full text of DOI:10.1166/jnn.2011.3693

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Journal of
Nanoscience and Nanotechnology
Vol. 11, 4639–4643, 2011
New Deep Blue Emitting Materials Based on
Indenopyrazine Core with High Thermal Stability
Chang-Hun Seok1, Young-Il Park1, Soo-Kang Kim1, Ji-Hoon Lee2, and Jongwook Park1ꢀ∗
1Department of Chemistry, The Catholic University of Korea, Bucheon, 420-743, Republic of Korea
2Department of Polymer Science and Engineering and Photovoltaic Technology Institute,
Chungju National University, Chungju, 380-702, Korea
New deep blue emitting materials 2,8-bis(3,5-diphenylphenyl)-6,6,12,12-tetraethyl-6,12-dihydro-
diindeno[1,2-b:1ꢀ,2ꢀ-e]pyrazine (DPP-EPY) and 2,8-bis(3ꢀ,5ꢀ-diphenylbiphenyl-4-yl)-6,6,12,12-
tetraethyl-6,12-dihydrodiindeno[1,2-b:1ꢀ,2ꢀ-e]pyrazine (DPBP-EPY) were synthesized through
introduction of m-terphenyl or triphenylbenzene bulky side groups in a new indenopyrazine core.
These materials all showed high thermal stability and highly reduced intermolecular interaction.
DPP-EPY and DPBP-EPY showed PL maxima of 456 nm and 460 nm in deep blue region and
narrow PL spectra with full-width at half-maximum (FWHM) of 46 nm and 52 nm, respectively. As
a result of making non-doped OLED devices using these synthesized materials as emitting layers,
DPP-EPY showed EL spectrum of 452 nm, very narrow FWHM of 46 nm, luminance efficiency of
1.04 cd/A with current density of 10 mA/cm2 and CIE coordinate of (0.161, 0.104), creating a deep
blue OLED close to the National Television System Committee (NTSC) blue standard.
Keywords: Indenopyrazine, Blue Emitting Material, OLED, Fluorescence.
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1. INTRODUCTION
and electroluminescence (EL) becomes broader with inter-
molecular interaction as the material moves from solution
state to film state. This is a unique property of organic
ꢀ-conjuated materials. Therefore, a study on materials
with high thermal stability and pure blue color coordinates
is necessary for full color OLEDs.3ꢁ4
After the report by Tang on green emitting organic light
emitting device (OLED) with improved stability with for-
mation of double-layer small molecule organic thin-films
in 1987, efforts for development of OLED display using
small molecule materials began in earnest.1 Commercial-
ization of full color display for such OLEDs requires
high efficiency, thermal stability, long device lifetime and
pure color coordinates of organic materials emitting red,
green and blue colors.2 According to reports by recent
studies, blue emitting materials show low efficiency due
to incorrect carrier balance between electrons and holes
resulting from different electronic levels between high-
est occupied molecular orbital (HOMO) and lowest unoc-
cupied molecular orbital (LUMO) because of large band
gap.2 Also, blue emitting materials require higher operat-
ing voltage compared to red or green emitting materials
because of difficulty in injection of holes and electrons.
On the other hand, heat resulting from high voltage can
damage the device and shorten the lifetime. Therefore,
thermal stability of material is important. It is also dif-
ficult to emit a pure deep blue color because full-width
at half-maximum (FWHM) of photoluminescence (PL)
In our previous study, indenopyrazine newly reported
as a core material for blue OLED with excellent thermal
property was used to increase thermal stability of mate-
rials.3 In this study, two indenopyrazine derivatives 2,8-bis
(3,5-diphenylphenyl)-6,6,12,12-tetraethyl-6,12-dihydrodii-
ndeno[1,2-b:1ꢀ,2ꢀ-e]pyrazine (DPP-EPY) and 2,8-bis(3ꢀ,
5ꢀ-diphenylbiphenyl-4-yl)-6, 6,12,12-tetraethyl-6, 12-dihy-
drodiindeno[1,2-b:1ꢀ,2ꢀ-e]pyrazine (DPBP-EPY) were
synthesized as new deep blue emitting materials by
breaking the planar structure of indenopyrazine core with
introduction of bulky side groups such as m-terphenyl or
triphenylbenzene connected in 2,8 position. These materi-
als include not only different new side groups of biphenyl
benzene and triphenyl benzene, but also different link
position compared to previous material, TP-EPY includ-
ing meta-terphenyl side group.3 In here, we compare the
optical- and electrical property according to conjugation
length of new side group. Thermal stability, optical and
electrical properties of the two materials were measured
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2011, Vol. 11, No. 5
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doi:10.1166/jnn.2011.3693
4639

New Deep Blue Emitting Materials Based on Indenopyrazine Core with High Thermal Stability
Seok et al.
and compared, and the synthesized materials were used as
emitting layer in non-doped OLED devices.
organic extract was dried with MgSO4 added, and then
filtered the solvent removed in vacuo. The resulting crude
mixture was passed through a short-column of silica with
THF as the eluent and then recrystallized from THF to
obtain DPP-EPY as a white solid.
2. EXPERIMENTAL DETAILS
2.1. General Method
2.3. Synthesis of
1H-NMR and 13C-NMR spectra were recorded on Bruker,
Advance 500 and Fast atom bombardment (FAB) mass
spectra were recorded by JEOL, JMS-AX505WA, HP5890
series II. The optical absorption spectra were obtained by
HP 8453 UV-VIS-NIR spectrometer. Perkin Elmer lumi-
nescence spectrometer LS50 (Xenon flash tube) was used
for photo- and electro-luminescence spectroscopy. The
melting temperatures (Tmꢂ, glass-transition temperatures
(Tg), crystallization temperatures (Tc), and degradation
temperatures (Tdꢂ of the compounds were measured by
carrying out differential scanning calorimetry (DSC) under
a nitrogen atmosphere using a DSC2910 (TA Instruments)
and thermogravimetric analysis (TGA) using a SDP-
TGA2960 (TA Instruments). OELD devices were fabri-
cated as the following structure: ITO/NPB (30 nm)/TCTA
(20 nm)/synthesized materials (30 nm)/Alq3 (30 nm)/LiF
(1 nm)/Al (200 nm), where NꢁNꢀ-bis(naphthalen-
1-yl)-NꢁNꢀ-bis(phenyl)benzidine (NPB), 4,4ꢀ,4ꢀꢀ-tri(N-
carbazolyl)triphenylamine (TCTA) as hole transporting
layer, 8-hydroxyquinoline aluminum (Alq ꢂ as electron
transporting layer, the synthesized materials as emitting
layers, lithium fluoride (LiF) as electron injection layer,
ITO as anode and Al as cathode. The organic layer was
vacuum-deposited 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 mm2, and the Al
layer was continuously deposited under the same vac-
uum condition. The current–voltage (I–V ) characteristics
of the fabricated EL devices were obtained by Keithley
2400 electrometer. Light intensity was obtained by Minolta
CS-1000.
2,8-Bis(3,5-diphenylphenyl)-6,6,12,12-Tetraethyl-
6,12-Dihydrodiindeno[1,2-b:1ꢀ,2ꢀ-e]Pyrazine
(DPP-EPY)
1
The final yield was 90%. H NMR (500 MHz, CDCl3ꢂ:
ꢃ (ppm): 8.21 (d, 2H). 7.88 (s, 4H), 7.81 (m, 4H), 7.75
(m, 10H), 7.42 (t, 4H), 2.41 (m, 4H), 2.17 (m, 4H), 0.43
(t, 12H), 13C NMR (300 MHz, CDCl3ꢂ: 163.2, 152.2,
150.5, 142.8, 142.6, 142.3, 141.4, 138.9, 129.1, 127.8,
127.6, 127.0, 125.5, 122.0, 121.7, 54.3, 31.5, 9.0, FT-IR
(KBr cm−1ꢂ: 3056, 2960, 2929, 2875, 1594, 1496, 1455,
1371, 1288, 1221, 1178, 1124, 1029, 873, 840, 700, Fab+-
MS m/e: 825.
2.4. Synthesis of 2,8-Bis(3ꢀ,5ꢀ-diphenylbiphenyl-
4-yl)-6,6,12,12-Tetraethyl-6,12-
Dihydrodiindeno[1,2-b:1ꢀ,2ꢀ-e]Pyrazine
(DPBP-EPY)
1
The final yield was 42%. H NMR (500 MHz, CDCl3)
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ꢃ (ppm): 8.20 (d, 1H), 7.86 (t, 7H), 7.78 (d, 1H), 7.74
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(d, 5H), 7.51 (t, 4H), 7.41 (t, 2H), 2.42 (m, 2H), 2.17
Copyright: American Scientific Publishers
(m, 2H), 0.46 (t, 6H), 13C NMR (300 MHz, CDCl3ꢂ: 163.2,
150.5, 142.7, 142.0, 141.8, 141.3, 140.7, 140.5, 138.8,
129.1, 128.0, 127.9, 127.8, 127.6, 126.7, 125.2, 121.8,
121.7, 54.3, 31.5, 8.9, FT-IR (KBr cm−1ꢂ: 3062, 2960,
2921, 2875, 1594, 1517, 1498, 1455, 1413, 1367, 1309,
1261, 1226, 1176, 1122, 1078, 1027, 919, 881, 831, 705,
Fab+-MS m/e: 976.
3. RESULTS AND DISCUSSION
Synthetic processes of DPP-EPY and DPBP-EPY are sum-
marized in Scheme 1. Four ethyl groups were intro-
duced in positions 6 and 12 of indenopyrazine core
through tetraalkylation of dibromoindenopyrazine to form
dibromo-tetraethylindenopyazine (EPY). And then, DPP-
EPY and DPBP-EPY were synthesized through Suzuki
Ar–Ar coupling reaction of borated m-terphenyl or triph-
enylbenzene with EPY dibrominated at positions 2 and 8.
Synthesized materials were refined with silica gel column
2.2. General Synthesis of New Emitters
These emitters were synthesized by Suzuki aryl–aryl
coupling reaction using Pd catalyst. A typical syn-
thetic procedure was as follows: To 2,8-dibromo-
6,6,12,12-tetraethyl-6,12-dihydro-diindeno [1, 2-b;1ꢀ, 2ꢀ-e]
pyrazine (1 g, 1.9 mmol) and 4,4,5,5-Tetramethyl-2-
[1,1ꢀ;3ꢀ,1ꢀꢀ]terphenyl-5ꢀ-yl-[1,3,2]dioxaborolane (1.49 g,
4.19 mmol) in
a 500 mL round-bottomed flask
1
under a nitrogen atmosphere were added Pd(OAC)2
(0.042 g, 0.19 mmol), tri-cyclohexyl-phosphine (0.053 g,
0.19 mmol) and toluene. The temperature was increased
chromatography and the structure was verified with H-
NMR, 13C-NMR, FT-IR and Fab+-mass. Optical proper-
ties of these two materials in the solution state and thin-
film state were summarized in Figure 1 and Table I. As
shown in Figure 1(a), synthesized materials in solution
state showed UV maximum wavelengths of 404 nm and
409 nm, respectively for DPP-EPY and DPBP-EPY. DPP-
EPY was blue-shifted slightly because conjugation length
ꢁ
to 50 C, and tetraethylammonium hydroxide (13.5 mL,
19.0 mmol, 20 wt% in water) was added. Stirring was con-
tinued at this temperature and the reaction was monitored
by TLC. When the reaction was complete, extraction of
the product was performed with water and toluene. The
4640
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Seok et al.
New Deep Blue Emitting Materials Based on Indenopyrazine Core with High Thermal Stability
N
N
O
B
O
N
Br
P
Br
Pd(OAc)2,
DPP-EPY
3
N
+
(t-Ethyl)4N+OH–, Toluene
EPY
N
O
B
O
N
DPBP-EPY
Scheme 1. Synthetic routes of new chemical structures.
was shortened by one less phenyl unit in the side group.
In addition, PL maxima of DPP-EPY and DPBP-EPY in
solution state were 428 nm and 438 nm in deep blue
region, respectively. Full-width at half-maximum (FWHM)
of these two materials was narrow at 45 nm and 46 nm.
This suggests an advantage of showing pure color coordi-
nates in OLED devices. PL maximum value in film state
was respectively 456 nm and 460 nm also in deep blue
region, and FWHM represented sharp spectra with 46 and
52 nm. Looking at the broadening of FWHM of tetraethyl
indenopyrazine PL spectrum from 46 nm to 75 nm with
change in the state from solution to film, side groups
m-terphenyl and triphenylbenzene can be interpreted as
(a)
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effective in prevention of packing of molecules.3
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In particular, FWHM of DPP-EPY and DPBP-EPY in
Copyright: American Scientific Publishers
film state is narrow at 46 nm and 52 nm. Narrow FWHM
resulted from reduction in intermolecular interaction with
change in the state of material to film state and can be
interpreted as the effect of bulky side group. Especially,
DPP-EPY shows a spectrum blue shifted more than DPBP-
EPY, allowing a purer deep blue emission.6
In order to verify thermal stability of synthesized mate-
rials, thermal gravimetric analysis (TGA) and differential
scanning calorimetry (DSC) were performed to measure
Tg, Tc, Tm and Td summarized in Figure 2 and Table II. Tg
(b)
ꢁ
was not observed in DPP-EPY but was 154 C in DPBP-
ꢁ
ꢁ
EPY. Tm was 402 C and 379 C respectively and Tc of
ꢁ
DPBP-EPY was found to be 277 C.
Looking at the TGA/DSC result, high Tm suggested
an excellent thermal stability.2 Also, since Tg was only
observed in DPBP-EPY, DPBP-EPY probably has a
more amorphous property compared to DPP-EPY. Organic
Table I. Optical properties of synthesized materials.
Solutiona
Film on glass
UVmax PLmax FW HM UVmax PLmax FW HM
Compounds
(nm)
(nm)
(nm)
(nm)
(nm)
(nm)
DPP-EPY
DPBP-EPY
404
409
428
438
45
46
415
415
456
460
46
52
Fig. 1. UV-Visible absorption and PL spectra (a) in THF solution (b) in
film state: UV-Visible absorption spectra of DPP-EPY (line), DPBP-EPY
(dash) and PL spectra of DPP-EPY (-ꢀ-), DPBP-EPY (-ꢄ-).
aSolution in THF(1.0∗10−6 M).
J. Nanosci. Nanotechnol. 11, 4639–4643, 2011
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New Deep Blue Emitting Materials Based on Indenopyrazine Core with High Thermal Stability
Seok et al.
Fig. 2. Second DSC scan data after N2 Treatment of DPP-EPY (dash)
Fig. 3. EL spectra of OLED device using synthesized EML. The insert
shows OLED device structure (DPP-EPY (-ꢀ-), DPBP-EPY (-ꢄ-)).
and DPBP-EPY (line), 10 ꢁC/min, N2 condition.
materials can be changed or damaged by heat during
operation of devices, and thus thermal stability is impor-
tant. This is closely related to the lifetime of OLED
devices.5ꢅbꢂꢁ7 As a result, the synthesized materials are
expected to show more extended lifetime of OLED devices
due to excellent thermal stability.
wavelength was blue shifted further in DPP-EPY at
452 nm than DPBP-EPY at 464 nm.
This also results from the shorten conjugation length
of side group. Especially as shown in film PL spectrum
of DPP-EPY, the EL spectrum was also extremely narrow
with FWHM of 46 nm. Accordingly, DPP-EPY showed
excellent CIE coordinate of (0.161, 0.104) in the deep blue
Non-doped OLED devices were developed using the
synthesized materials as emitting layer. Structure of device
region close to the National Television System Committee
(NTSC) blue standard.
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is ITO/NPB (30 nm)/TCTA (20 nm)/synthesized materi-
als (30 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (200 nm). As
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summarized in Table III, luminance efficiency and power
efficiency of DPP-EPY and DPBP-EPY were respectively
1.04, 0.61 cd/A and 0.27, 0.17 lm/W. DPP-EPY showed
better efficiencies compared to DPBP-EPY. Figure 3 shows
the EL spectra of the OLED devices. Both synthetic
materials show deep blue spectra with maximum emit-
ting wavelengths of about 450∼460 nm. EL maximum
4. CONCLUSION
In this study, we successfully synthesized two new deep-
blue emitters based on indenopyrazine derivatives by pre-
venting intermolecular interaction from planar molecular
structure with introduction of m-terphenyl and triphenyl-
benzene as bulky side groups in the new indenopyrazine
core. As a result of measuring optical and thermal proper-
ties of two materials in solution and film states, both mate-
rials showed highly reduced intermolecular interaction and
excellent thermal stability. Especially, very narrow FWHM
was shown even in film state because intermolecular inter-
action was reduced by two bulky side groups. Synthesized
materials were used as emitting layer to create non-doped
OLED devices, realizing a deep blue OLED device with
DPP-EPY close to NTSC blue standard with EL maximum
of 452 nm and excellent CIE coordinates of (0.161, 0.104).
Table II. Thermal properties of synthesized materials.
Compounds
Tg/ꢁC
Tc/ꢁC
Tm/ꢁC
Td/ꢁC
DPP-EPY
DPBP-EPY
—
154
—
277
402
379
419
449
Tg: glass transition temperature, Tc: crystallization temperature, Tm: melting-point
temperature, Td: decomposition temperature (5% weight loss).
Table III. EL performance of synthesized materials: ITO/NPB
(30 nm)/TCTA (20 nm)/Synthesized materials (30 nm)/Alq3 (30 nm)/LiF
(1 nm)/Al (200 nm) at 10 mA/cm2.
Luminance
efficiency
(cd/A)
Power
efficiency
(lm/W)
Acknowledgments: This research was supported by a
grant (Catholic University) from the Fundamental R&D
Program for Core Technology of Materials funded by the
Ministry of Knowledge Economy (MKE), Republic of
Korea. This work was supported by the National Research
Foundation of Korea (NRF) grant funded by the Korea
government (No. 20100000422).
ELmax
(nm)
FW HM
(nm)
CIE
(xꢁy)
Emitters
DPP-EPY
452
464
46
50
1.04
0.61
0.27
0.17
(0.161,
0.104)
DPBP-EPY
(0.179,
0.159)
4642
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Seok et al.
New Deep Blue Emitting Materials Based on Indenopyrazine Core with High Thermal Stability
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J. Nanosci. Nanotechnol. 11, 4639–4643, 2011
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