New blue-violet emitters based on an indenopyrazine core for OLEDs: Effects of the position of m-terphenyl side group substitution on optical and electroluminescence properties

Article abstract of DOI:10.1016/j.orgel.2010.01.030

A promising class of blue-violet emitters based on a new functional core, the indenopyrazine group, has successfully been synthesized for the first time by substitution with bulky m-terphenyl side groups in the ortho, meta, or para positions. There are larger blue shifts in the UV-visible absorption and PL spectra of the synthesized ortho- and para-substituted derivatives than in those of the meta-substituted derivative. Molecular calculations verified that these differences are due to the variation in the π-conjugation length of the derivatives with the position at which the side group is attached to the indenopyrazine core. When the synthesized compounds were used as emitting layers in non-doped OLED devices, a related trend was observed in their optical properties. In particular, the OLED containing the para-substituted derivative was found to exhibit excellent characteristics, with maximum EL emission at 423 nm, a full width at half maximum of 42 nm, pure violet emission with CIE coordinates (0.173, 0.063), and an external quantum efficiency of 1.88%.

Full text of DOI:10.1016/j.orgel.2010.01.030

Organic Electronics 11 (2010) 864–871
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
New blue-violet emitters based on an indenopyrazine core for OLEDs:
Effects of the position of m-terphenyl side group substitution on optical
and electroluminescence properties
Youngil Park a, Sookang Kim a, Ji-Hoon Lee b, Dong Hyun Jung c, Chung-Chih Wu d,
Jongwook Park a,
*
a Department of Chemistry/Display Research Center, The Catholic University of Korea, Bucheon 420-743, Republic of Korea
b Department of Polymer Science and Engineering and Photovoltaic Technology Institute Chungju National University, Chungju-si,
Chungbuk 380-702, Republic of Korea
c Insilicotech Co. Ltd., Kolontripolis, 210, Geumgok-Dong, Seongnam, Gyeonggi-Do, Republic of Korea
d Department of Electrical Engineering, Graduate Institute of Electro-Optical Engineering and Graduate Institute of Electronics Engineering,
National Taiwan University, Taipei, Taiwan 10617, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A promising class of blue–violet emitters based on a new functional core, the indenopyr-
azine group, has successfully been synthesized for the first time by substitution with bulky
m-terphenyl side groups in the ortho, meta, or para positions. There are larger blue shifts in
the UV–visible absorption and PL spectra of the synthesized ortho- and para-substituted
derivatives than in those of the meta-substituted derivative. Molecular calculations verified
Received 24 October 2009
Received in revised form 22 January 2010
Accepted 30 January 2010
Available online 4 February 2010
that these differences are due to the variation in the p-conjugation length of the derivatives
Keywords:
OLED
with the position at which the side group is attached to the indenopyrazine core. When the
synthesized compounds were used as emitting layers in non-doped OLED devices, a related
trend was observed in their optical properties. In particular, the OLED containing the para-
substituted derivative was found to exhibit excellent characteristics, with maximum EL
emission at 423 nm, a full width at half maximum of 42 nm, pure violet emission with
CIE coordinates (0.173, 0.063), and an external quantum efficiency of 1.88%.
Ó 2010 Elsevier B.V. All rights reserved.
Luminescence
Indenopyrazine
Density functional calculations
Blue–violet emitter
m-Terphenyl
1. Introduction
the aim of developing materials with good performance
and stability for particular optoelectronic applications.
The interest in organic functional materials based on
p
-
The synthesis of 6,12-dihydro-diindeno[1,2-b;10,20-
e]pyrazine (indenopyrazine) was first reported by Ebel
and Deuschel in 1956, but no follow-up studies examined
the derivatives and potential applications of this molecule
[8]. Given the intense current interest in organic func-
tional materials based on aromatic compounds such as
fluorene, bifluorene, and anthracene for use in OLEDs in
next-generation flat panel displays, new core structures
could potentially yield materials with improved physical
properties for diverse applications [9–28]. We recently re-
ported new derivatives based on an indenopyrazine core
structure that could be used as blue emitters in OLEDs.
conjugated molecules has been rapidly growing in diverse
fields. Many remarkable results have been reported, espe-
cially in the field of optoelectronics (organic light-emitting
diodes [OLEDs], solar cells, and organic thin-film transis-
tors [OTFTs]) [1–7]. Many researchers are engaged in ongo-
ing efforts to systematically elucidate the correlation
between their molecular structures and properties, with
* Corresponding author. Tel.: +82 2164 4821; fax: +82 2 2164 4764.
E-mail address: hahapark@catholic.ac.kr (J. Park).
1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2010.01.030

Y. Park et al. / Organic Electronics 11 (2010) 864–871
865
These compounds were found to exhibit a high external
raethyl-3,9-bis-[1,10;30,100]terphenyl-40-yl-6,12-dihydro-
quantum efficiency of 4.6% and a pure deep blue emission
with CIE coordinates (0.154, 0.078) [29]. In other research
in this area, Yamashita’s group used an indenopyrazine
derivative, difluoro-diindeno[1,2-b;10,20-e]pyrazine-6,12-
dione, as the channel material in an n-type OTFT, with
excellent results (mobility, 0.17 cm2/Vs; on/off ratio, 107)
[30]. Indenopyrazine is therefore gaining recognition in
optoelectronics as a potentially useful core structure for
a new family of organic functional materials. However, if
indenopyrazine derivatives with enhanced performance
suitable for diverse optoelectronics applications are to
be developed, more systematic studies of their structures
diindeno[1,2-b;10,20-e]pyrazine (p-TP-EPY).
2. Experimental
2.1. Synthesis of o-, m-, and p-TP-EPY
The compounds o-, m-, and p-TP-EPY were synthesized
following literature procedures [29]: to 2,8-dibromo-
6,6,12,12-tetraethyl-6,12-dihydro-diindeno[1,2-b;10,20-e]
pyrazine (1 g, 1.9 mmol) and 4,4,5,5-tetramethyl-2-
[1,10;30,100]terphenyl-40-yl-[1,3,2]dioxaborolane
(1.49 g,
4.19 mmol) in a 500 mL round-bottomed flask under a
nitrogen atmosphere were added Pd(OAC)2 (0.042 g,
0.19 mmol), tricyclohexylphosphine (0.053 g, 0.19 mmol)
and toluene. The temperature was then increased 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, the product was
extracted with water and toluene. The organic extract was
dried over anhydrous MgSO4 and filtered, after which 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 m-
TP-EPY as a white solid. The purification method of o-
and p-TP-EPY was same as m-TP-EPY.
and properties must be conducted.
*
In a previous study, we hypothesized that the
p–p
stacking of molecules in the thin-film state due to the
plate-like molecular structure of indenopyrazine would
give rise to a reduction in its fluorescence yield and a bath-
ochromic effect on its emission wavelength. To remove this
effect, we introduced short alkyl groups at the 6- and 12-
positions of indenopyrazine and a bulky m-terphenyl side
group at positions 2 and 8. An OLED device based on the
resulting molecule was found to exhibit excellent charac-
teristics, with high efficiency and a pure deep blue color
[29]. In this study, we investigated the effects on the opti-
cal and electroluminescence properties of varying the sub-
stitution position of the side groups (Scheme 1). In
particular, we examined the bulky m-terphenyl substitu-
tion positions 2 and 8 (meta positions), 1 and 7 (ortho posi-
tions), and 3 and 9 (para positions).
Although varying the substitution positions of side
groups on emitting cores offers important information
about the intrinsic properties of the molecules, it is diffi-
cult to change the side group substitution positions for
most emitting cores (e.g. fluorene, bifluorene and anthra-
cene) [9–28]. In this study, however, varying the substitu-
tion pattern was facilitated by the use of bromo-indanones
with Br substituted at positions 4, 5, and 6 as the starting
compounds. This procedure enabled the facile synthesis
of three compounds in which m-terphenyl is substituted
at the ortho, meta, and para positions: 6,6,12,12-tetra-
ethyl-1,7-bis-[1,10;30,100]terphenyl-40-yl-6,12-dihydro-diin-
deno[1,2-b;10,20-e]pyrazine (o-TP-EPY), 6,6,12,12-tetra-
ethyl-2,8-bis-[1,10;30,100]terphenyl-40-yl-6,12-dihydro-diin-
deno[1,2-b;10,20-e]pyrazine (m-TP-EPY), and 6,6,12,12-tet-
2.1.1. Synthesis of 6,6,12,12-tetraethyl-1,7-bis-[1,10;30,100]
terphenyl-40-yl-6,12-dihydro-diindeno[1,2-b;10,20-e]pyrazine
(o-TP-EPY)
The final yield was 1.06 g, 68%. 1H NMR (500 MHz,
CDCl3): d (ppm) 8.00–7.99 (d, 2H), 7.74–7.72 (m, 6H),
7.59 (d, 2H), 7.48–7.45 (t, 4H), 7.41–7.36 (m, 12H), 7.16–
7.14 (m, 4H), 7.08 (s, 2H), 2.10 (m, 2H), 1.85 (m, 2H),
1.74 (m, 2H), 1.62 (m, 2H), 0.27–0.20 (m, 6H), ꢀ0.51 to
ꢀ0.60 (m, 6H). 13C NMR (500 MHz, CDCl3): 162.7, 152.5,
145.4, 141.5, 141.1, 140.9, 139.0, 138.4, 134.1, 131.7,
130.7, 129.6, 129.1, 128.2, 127.8, 127.4, 126.9, 126.5,
125.0, 120.4, 56.2, 30.6, 30.1, 8.9, 8.8, 7.8. FT-IR (KBr
cmꢀ1): 3056, 2961, 2928, 2873, 1599, 1486, 1469, 1456,
1400, 1367, 1291, 1244, 1204, 1182, 1103, 1076, 1030,
776, 755, 742, 698. HRMS Calcd for C62H53N2 (M+H)+,
825.4209, found: 825.4229.
N
N
N
N
N
N
o-TP-EPY
p-TP-EPY
m-TP-EPY
Scheme 1. Chemical structures of o-, m-, and p-TP-EPY.

866
Y. Park et al. / Organic Electronics 11 (2010) 864–871
2.1.2. Synthesis of 6,6,12,12-tetraethyl-3,9-bis-[1,10;30,100]
terphenyl-40-yl-6,12-dihydro-diindeno[1,2-b;10,20-e]pyrazine
(p-TP-EPY)
bardment (FAB) mass spectrometry. 1H and 13C NMR
spectra were recorded on Bruker Avance 300 and Avance
500 spectrometers. The FT-IR spectra were recorded on a
Thermo Electron Nicolet IR-200 spectrometer. FAB-mass
The final yield was 0.89 g, 57%. 1H NMR (500 MHz, THF-
d8): d (ppm) 8.09–7.76 (s, 2H), 7.76–7.75 (m, 8H), 7.71–
7.69 (d, 2H), 7.46–7.43 (t, 4H), 7.35–7.34 (t, 2H), 7.27–
7.22 (m, 6H), 7.19–7.15 (m, 6H), 7.04–7.03(d, 4H), 2.47–
2.28 (m, 4H), 2.06–1.99 (m, 4H), 0.38–0.36 (t, 12H). 13C
NMR (300 MHz, CDCl3): 162.9, 152.4, 148.0, 141.7, 141.3,
140.9, 140.7, 140.6, 139.8, 139.5, 131.7, 131.3, 130.2,
129.4, 129.0, 128.0, 127.6, 127.3, 126.8, 126.3, 122.4,
122.2, 53.8, 31.3, 8.8. FT-IR (KBr cmꢀ1): 3055, 3025, 2962,
2919, 2874, 2853, 1599, 1486, 1462, 1442, 1317, 1288,
1233, 1178, 1124, 895, 826, 763, 748, 697, 486, HRMS
Calcd for C62H53N2 (M+H)+, 825.4209, found: 825.4214.
spectra were recorded on
a
JEOL, JMS-AX505WA,
HP5890 series II. The melting temperatures (Tm), glass-
transition temperatures (Tg), crystallization temperatures
(Tc), and degradation temperatures (Td) of the com-
pounds were measured by carrying out differential scan-
ning calorimetry (DSC) under
a nitrogen atmosphere
with a DSC2910 (TA Instruments) and thermogravimetric
analysis (TGA) with a SDP-TGA2960 (TA Instruments).
The optical absorption spectra were obtained with a HP
8453 UV–Vis–NIR spectrometer. A Perkin Elmer lumines-
cence spectrometer LS50 (xenon flash tube) was used for
photo- and electroluminescence spectroscopy. The redox
potentials of the compounds were determined with cyc-
lic voltammetry (CV) by using a WBCS 3000 system with
a scanning rate of 100 mV/s. We used a synthetic mate-
rial coated ITO as the working electrode, a saturated Ag/
AgNO3 reference electrode, and acetonitrile (AN) with
0.1 M tetrabutylammonium perchlorate (TBAP) as the
electrolyte. Ferrocene was used for potential calibration
and for reversibility criteria.
2.1.3. Synthesis of 1,7-dibromo-6,6,12,12-tetraethyl-6,12-
dihydro-diindeno[1,2-b;10,20-e]-pyrazine (1c)
2.1.3.1. 4-Bromo-2,3-dihydro-2-(hydroxyimino)indene-1-one
(1a). The yield was 79%. 1H NMR (300 MHz, acetone-d6): d
(ppm) 11.90 (s, 1H), 7.95–7.92 (d, 1H), 7.79–7.76 (d, 1H),
7.50–7.45 (t, 1H), 3.76 (s, 2H).
2.1.3.2. 1,7-Dibromo-6,12-dihydro-diindeno[1,2-b;10,20-
e]pyrazine (1b). The yield was 56%. 1H NMR (300 MHz,
CDCl3): d (ppm) 8.17–8.15 (d, 2H), 7.65–7.63 (d, 2H),
7.45–7.42 (d, 2H), 4.02 (s, 4H).
2.3. Fabrication of the OLEDs
2.1.3.3. 1,7-Dibromo-6,6,12,12-tetraethyl-6,12-dihydro-
diindeno[1,2-b;10,20-e]pyrazine (1c). The yield was 38%. 1H
NMR (500 MHz, CDCl3): d (ppm) 8.08–8.05 (d, 2H), 7.58–
7.56 (d, 2H), 7.32–7.29 (t, 2H), 2.72–2.55 (m, 4H), 2.37–
2.29 (m, 4H), 0.27–0.24 (t, 12H). 13C NMR (500 MHz,
CDCl3): 163.0, 152.2, 145.9, 142.5, 142.3, 134.1, 133.9,
129.4, 129.3, 128.5, 120.4, 120.2, 119.8, 57.5, 57.4, 28.3,
8.8. Fab+-MS m/e: 526.
All organic layers were deposited under 10ꢀ6 Torr,
with a rate of deposition of 1 Å/s to give an emitting area
of 4 mm2. In these devices, 4,40,400-tris(N-(2-naphthyl)-N-
phenyl-amino)-triphenylamine [2-TNATA] was used as
the hole injection layer, N,N0-bis(naphthalen-1-yl)-N,N0-
bis(phenyl)benzidine [NPB] and 4,40,400-tri(N-carbazol-
yl)triphenylamine [TCTA] were used as the hole trans-
porting and exciton blocking layers, respectively, and 8-
hydroxyquinoline aluminum [Alq3] was used as the elec-
tron transporting layer [31]. The LiF and aluminum lay-
ers were continuously deposited under the same
vacuum conditions. The device structures were ITO/2-
TNATA (30 nm)/NPB (10 nm)/TCTA (10 nm)/o- or m- or
p-TP-EPY (30 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (200 nm).
The current–voltage (I–V) characteristics of the fabri-
cated EL devices were obtained with a Keithley 2400
electrometer. Light intensity was obtained with a Minol-
ta CS-1000A.
2.1.4. Synthesis of 3,9-dibromo-6,6,12,12-tetraethyl-6,12-
dihydro-diindeno[1,2-b;10,20-e]pyrazine (3c)
2.1.4.1. 6-Bromo-2,3-dihydro-2-(hydroxyimino)indene-1-one
(3a). The yield was 77%. 1H NMR (300 MHz, acetone-d6): d
(ppm) 11.83 (s, 1H), 7.89–7.85 (m, 2H), 7.64–7.61 (d, 1H),
3.82 (s, 2H).
2.1.4.2. 3,9-Dibromo-6,12-dihydrodiindeno[1,2-b:1,2-
e]pyrazine (3b). The yield was 53%. 1H NMR (300 MHz,
CDCl3): d (ppm) 8.37–8.36 (s, 2H), 7.64–7.60 (d, 2H),
7.54–7.38 (d, 2H), 4.04 (s, 4H).
2.4. Computational details
2.1.4.3. 3,9-Dibromo-6,6,12,12-tetraethyl-6,12-dihydro-
diindeno[1,2-b;10,20-e]-pyrazine (3c). The yield was 63%. 1H
NMR (500 MHz, CDCl3): d (ppm) 8.23–8.21 (s, 2H), 7.58–
7.56 (d, 2H), 7.33–7.26 (d, 2H), 2.33–2.28 (m, 4H), 2.06–
2.02 (m, 4H), 0.38–0.33 (t, 12H), 13C NMR(300 MHz,
CDCl3): 163.1, 151.8, 148.5, 141.1, 132.3, 124.9, 124.6,
121.7, 54.1, 31.2, 8.7. Fab+-MS m/e: 526.
The HOMO and LUMO energy levels of the indeno-
pyrazine derivatives were optimized by using the DMol3
program of Materials Studio 4.3Ò, which carries out
quantum mechanical calculations with density func-
tional theory (DFT). The Perdew, Burke and Ernzerhof
(PBE) functional and double numeric polarization basis
set was used in the calculations. The structures were
optimized by using delocalized internal coordinates; to
confirm that each optimized conformation was the min-
imum structure we performed numerical frequency
analysis.
2.2. Measurements
The structures of the synthesized compounds were
characterized by using NMR, FT-IR, and fast-atomic bom-

Y. Park et al. / Organic Electronics 11 (2010) 864–871
867
to that of m-TP-EPY. In the PL spectra, the maxima for o-
TP-EPY (411 nm) and p-TP-EPY (415 nm) were blue-shifted
26 and 22 nm, respectively with respect to that of m-TP-
EPY at 437 nm, thereby moving these spectra from the blue
region to the violet region. These findings suggest that
changing the substitution position alters the intrinsic opti-
cal properties of the molecule.
These phenomena were also observed for the thin-film
states, as shown in Fig. 2: the Abs peaks of o-TP-EPY
(389 nm) and p-TP-EPY (390 nm) are blue-shifted 19 and
18 nm with respect to that of m-TP-EPY (408 nm), and
the PL spectra maxima of o-TP-EPY (418 nm) and p-TP-
EPY (426 nm) are blue-shifted 25 and 17 nm, respectively
with respect to that of m-TP-EPY at 443 nm. Moreover,
the minute differences of 6–11 nm between the PL spectral
values of the solution and thin-film states for each deriva-
tive indicate that the short alkyl chains (ethyl) at carbon
positions 6 and 12 of indenopyrazine and the m-terphenyl
side groups on the aromatic rings effectively suppress the
3. Result and discussion
3.1. Synthesis and characterization
The indenopyrazine derivatives were synthesized
according to literature procedures [29]. As shown in
Scheme 2, we used bromo-indanones with Br substituted
at positions 4, 5, and 6 as the starting compounds. In the
first step, the oximes (1a, 2a, 3a) were synthesized via
the reaction with isoamylnitrile dropped in under acid
conditions. The oximes were then cyclized to synthesize
the key intermediate dibromo-indenopyrazines (1b, 2b,
3b) with bromine present at the ortho, meta, and para posi-
tions in high yield (overall yield approximately 40%). Since
the 6- and 12-position carbons of the synthesized dibro-
mo-indenopyrazines are acidic, these carbons were capped
with four ethyl groups (1c, 2c, 3c), and then the tetraethy-
lated dibromo-indenopyrazines were coupled with a bo-
rated m-terphenyl group by carrying out Suzuki Ar–Ar
coupling reactions. This procedure enabled the facile syn-
thesis of o-, m-, and p-TP-EPY, in which m-terphenyl is
substituted at the ortho, meta, and para positions respec-
tively. The structures of the synthesized compounds were
characterized by using NMR, FT-IR, and FAB-mass analysis.
p–p* stacking of the molecules regardless of whether the
substitution pattern is ortho, meta, or para. In addition,
the full widths at half maximum (FWHMs) of the PL spec-
tra for the thin-film state are 43 nm for o-TP-EPY and p-TP-
EPY, but 54 nm for m-TP-EPY. The sharper spectra of o-TP-
EPY and p-TP-EPY probably arise because these molecules
have fewer electron transition states than m-TP-EPY.
The energy bands, and the highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) levels of o-, m-, and p-TP-EPY were estimated with
cyclic voltammetry (CV) and through absorption edge
3.2. Optical and electrochemical properties
The UV–visible absorption (Abs) spectra and photolumi-
nescence (PL) spectra for the solution states of o-, m-,
and p-TP-EPY are shown in Fig. 1, and the spectral features
are summarized in Table 1. The PL quantum yields of the
solution states of m-TP-EPY, p-TP-EPY, and o-TP-EPY were
determined to be 73%, 61%, and 42%, respectively. As
shown in Fig. 1, the Abs peaks of o-TP-EPY and p-TP-EPY
were found to be blue-shifted 17 and 15 nm with respect
analysis by using a plot of (h
are the absorbance, Planck’s constant, and the frequency
m) vs. (ahm a, h, and
)2, where
m
of light; the results are summarized in Table 1 [23]. The
HOMO levels of o-TP-EPY and p-TP-EPY are 6.13 and
6.07 eV, respectively, which are slightly lower by 0.10
R1
O
O
R3
R2
1) Na2S2O4, NH4OH
Ethanol
R3
R2
N
R3
R2
HCl, Isoamylnitrite
Benzene
R2
NOH
N
2) H2O, Reflux
R3
R1
R1
R1
1a R1=Br, R2=H, R3=H (79 %)
2a R1=H, R2=Br, R3=H (81 %)
3a R1=H, R2=H, R3=Br (77 %)
1b R1=Br, R2=H, R3=H (56 %)
2b R1=H, R2=Br, R3=H (49 %)
3b R1=H, R2=H, R3=Br (53 %)
N
N
N
N
R1
R3
O
B
Bromoethane
n-Bu4NBr
N
O
R2
R2
N
KOH, DMSO
3P
Pd(OAC)2
R3
R1
o-TP-EPY
p-TP-EPY
(t -ethyl)4N+OH-
Toluene
1c R1=Br, R2=H, R3=H(38 %)
2c R1=H, R2=Br, R3=H(54 %)
3c R1=H, R2=H, R3=Br(63 %)
N
N
m-TP-EPY
Scheme 2. Synthetic routes for o-, m-, and p-TP-EPY.

868
Y. Park et al. / Organic Electronics 11 (2010) 864–871
of the m-terphenyl side group, we performed frontier
molecular orbital calculations to determine the electron
density distributions of ethyl indenopyrazine (EIP-core),
and o-, m-, and p-TP-EPY (Fig. 3). The structures of the
indenopyrazine derivatives were optimized by using the
DMol3 program of Materials Studio 4.3Ò, which carries
out quantum mechanical calculations with density func-
tional theory (DFT). The Perdew, Burke and Ernzerhof
(PBE) functional and double numeric polarization basis
set was used in the calculations [32–34]. The most remark-
able result obtained from the molecular orbital analysis is
that the lobe of the orbital at the junction points of the
ortho and para positions of the EIP-core and side groups
is smaller than the lobe of the orbital at the junction point
of the meta position, in both the HOMO and LUMO as
shown in Fig. 3. In particular, a node was found at the junc-
tion points of the ortho and para positions for the HOMO
and the LUMO. Therefore, whereas there is an extension
of the conjugation in m-TP-EPY from the EIP-core to the
m-terphenyl group, the nodes observed for o-TP-EPY and
p-TP-EPY indicate that these molecules are less conjugated
than m-TP-EPY. This difference in conjugation means that
the energy gaps decrease in the order o-TP-EPY > p-TP-
EPY > m-TP-EPY, as shown in Table 1. In addition, this
trend explains the spectral phenomena shown in Fig. 1,
such as the larger blue shift in the Abs and PL spectra in-
duced by substitution of the m-terphenyl group at the
ortho and para positions, and the wider band gaps. These
findings establish that the p-conjugation of the indenopyr-
azine derivatives can be adjusted by placing side groups at
the node or lobe of the electron distribution of the indeno-
pyrazine core system (Fig. 3(a)). Thus, in efforts to develop
optoelectronics devices based on p-conjugated molecules,
physical characteristics such as their optical and electronic
properties can be systematically controlled according to
the electron density distribution of the orbital at the posi-
tion where the side group attaches to the molecular core.
Fig. 1. Normalized UV and PL spectra of o-TP-EPY (j), m-TP-EPY ( ), and
p-TP-EPY ( ) in CHCl3 (1.00 ꢁ 10ꢀ5 M) solution.
and 0.04 eV, respectively than the HOMO level of m-TP-
EPY at 6.03 eV. The band gaps of o-TP-EPY and p-TP-EPY
were found to be 0.21 and 0.24 eV wider, respectively than
that of m-TP-EPY.
3.4. Thermal properties
The thermal properties of the synthesized substances
were determined by using thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC). The re-
sults are summarized in Table 2. The Tm values of the syn-
thesized o-, m-, and p-TP-EPY derivatives were found to be
365, 338, and 317 °C, respectively, which are more than
110 °C higher than the Tm of the commercial blue luminous
material 4,40-bis(2,2-diphenylvinyl)-1,10-biphenyl (DPVBi),
3.3. Theoretical calculations
In an attempt to theoretically explain the observed vari-
ations in the optical and electronic properties of the
indenopyrazine core system with the substitution position
Table 1
Optical and electrical properties of the synthesized compounds.
UVmax (nm) PLmax (nm) Q.Yb (%) UVmax (nm) PLmax (nm) FWHMc HOMO (eV) LUMO (eV)
D
Eexp (eV)
DEcal (eV)
a
a
c
c
d
e
o-TP-EPY
m-TP-EPY 403
p-TP-EPY 388
386
411
437
415
42
73
61
389
408
390
418
443
426
43
54
43
6.13
6.03
6.07
3.02
3.16
2.99
3.11
2.87
3.08
2.56
2.38
2.53
a
b
c
Solution in CHCl3 (1.00 ꢁ 10ꢀ5 M).
Q.Y. (quantum yield) was measured by using a calibrated integrating sphere system [38,39].
Film on glass.
d
e
Experimental band gaps.
Band gaps obtained with frontier molecular orbital calculations.

Y. Park et al. / Organic Electronics 11 (2010) 864–871
869
Fig. 3. Electronic density distributions of the frontier molecular orbitals
(HOMO and LUMO) of the EIP-core (a), and of o-, m-, and p-TP-EPY (b).
Table 2
Thermal properties of o-, m-, and p-TP-EPY.
Fig. 2. Normalized UV–Vis and PL spectra of thin-films of o-TP-EPY (j),
m-TP-EPY ( ), and p-TP-EPY ( ).
Tg (°C)
Tc (°C)
Tm (°C)
Td (°C)
o-TP-EPY
m-TP-EPY
p-TP-EPY
–
129
127
–
167
165
365
338
317
403
441
442
which is 204 °C. The Tg values of m-TP-EPY and p-TP-EPY
are 129 and 127 °C, respectively, which are approximately
twice as high as that of DPVBi, 64 °C [29]. In the case of a
thin-film of m-TP-EPY, the average surface roughness (Ra)
measured with atomic force microscopy (AFM) was found
to undergo almost no change in comparison to that of a
DPVBi film after storage at 65 °C for 24 h [29]. Improved
lifetimes of the synthesized substances in OLED devices
are anticipated [35].
in good agreement with the shifts from the blue region
to the violet region observed in the PL spectra of the
thin-films. In addition, the EL spectra of o-TP-EPY and p-
TP-EPY are sharper than the m-TP-EPY EL spectrum, as re-
flected in the FWHMs (43 and 42 nm for o-TP-EPY and p-
TP-EPY, respectively, compared to 47 nm for m-TP-EPY).
Such narrow emission spectra can result in pure colors,
which is advantageous for emitting materials. In terms of
the Commission Internationale de l’Eclairage (CIE) coordi-
nates, the y-axis values of o-TP-EPY and p-TP-EPY are
0.068 and 0.063 that is, closer to the violet region than
the value of 0.078 for m-TP-EPY. These CIE coordinates
3.5. Electroluminescence properties
To determine whether the changes with substitution
position in the intrinsic molecular properties are reflected
in OLED (a representative optoelectronics device) perfor-
mance, non-doped OLED devices were fabricated with the
synthesized compounds as the emitting material layer
(EML). The results are summarized in Table 3 and Fig. 4.
The most important observation is that the electrolumi-
nescence (EL) spectra of o-TP-EPY and p-TP-EPY have max-
ima at 419 and 423 nm, respectively, which are blue-
shifted 21 and 17 nm with respect to the peak for m-TP-
EPY at 440 nm (see Fig. 4). This trend in the EL spectra is
show that adjusting the p-conjugation by varying the junc-
tion position of the side group attached to the indenopyr-
azine core system changes the optical and electronic
properties, and that such changes can be exploited in
OLEDs. The EL devices were found to exhibit power effi-
ciencies and external quantum efficiencies (EQE). in the
ranges 0.16–0.9 lm/W and 1.2–4.6%, respectively, at
10 mA/cm2 (see Table 3). The EQE of the m-TP-EPY device

870
Y. Park et al. / Organic Electronics 11 (2010) 864–871
Table 3
EL performances of multi-layered devices with the synthesized emitters. Device: ITO/2-TNATA (30 nm)/NPB (10 nm)/TCTA (10 nm)/o- or m- or p-TP-EPY
(30 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (200 nm) at 10 mA/cm2.
Compounds
Volt (V)
L.E.a (cd/A)
P.E.b (lm/W)
E.Q.E.c (%)
ELmax (nm)
FWHMd (nm)
CIEe (x,y)
o-TP-EPY
m-TP-EPY
p-TP-EPY
8.8
8.2
8.7
0.41
2.13
0.59
0.16
0.90
0.24
1.27
4.61
1.88
419
440
423
43
47
42
(0.175, 0.068)
(0.157, 0.079)
(0.173, 0.063)
a
b
c
Luminescence efficiency.
Power efficiency.
External quantum efficiency.
FWHM (full width half maximum).
d
e
1931 Commission Internationale de l’Eclairage/NTSC (National Television System Committee): CIE (0.14, 0.08).
structure, affects the
p-conjugation of the molecule and
hence alters the optical properties. These changes in opti-
cal properties are reflected in the behavior of the non-
doped OLED devices fabricated using the synthesized com-
pounds. In particular, the device containing p-TP-EPY (high
Tg, 127 °C) was found to exhibit excellent properties: an EL
spectrum with a peak at 423 nm for violet emission at CIE
coordinates (0.173, 0.063) and an EQE of 1.88%.
Acknowledgements
This research was supported by a grant (Catholic Univ.)
from the Fundamental R&D Program for Core Technology
of Materials funded by the Ministry of Knowledge Econ-
omy (MKE), Republic of Korea. This work was supported
by the National Research Foundation of Korea (NRF) grant
funded by the Korea government (No. 20090080199). This
study was supported by the Research Fund, 2009 The Cath-
olic University of Korea (M2009B000200181).
Fig. 4. EL spectra of o-TP-EPY (j), m-TP-EPY ( ), and p-TP-EPY ( ) in
OLED devices. The inset shows the CIE (x, y) coordinates of the o-, m-, and
p-TP-EPY devices.
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