Asymmetric indolylmaleimides as non-dopant type red color emitting dyes

Article abstract of DOI:10.1016/j.orgel.2009.12.025

The photophysical properties of asymmetric indolylmaleimide derivatives (4a-4f) with a variety of aryl substituents at C(4) were analyzed. Their absorption spectra vary in the region of aryl substituents, whereas their emission spectra are nearly identical. The red emission is derived from a charge transfer transition along the indole-maleimide chromophore, which is not significantly disturbed by the introduction of a nearby aryl substituent at C(4). All compounds displayed intensive emissions both in solutions as well as in the form of solid films. Light emitting diodes (LED) were fabricated successfully by using the neat film of these materials as emitters. The performance of devices among the series varied slightly as a result of different film morphology. The best performance was found on the compound with a pyrene substituent, which exhibited optimal external quantum efficiency of 1.1%. This device could be turned on at 4 V and reached to optimal brightness of 9750 cd/m². The red color, according to the Commission Internationale de l'Eclairage (CIE) coordinates at (0.65, 0.34), was very close to pure red (0.67, 0.33).

Full text of DOI:10.1016/j.orgel.2009.12.025

Organic Electronics 11 (2010) 604–612
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Asymmetric indolylmaleimides as non-dopant type red color emitting dyes
Ying-Shin Lee a, Zhenghuan Lin b,d, Yu-Yuan Chen c, Ching-Yang Liu c, Tahsin J. Chow d,
*
a Department of Chemistry, National Central University, Jhongli, Taoyuan 300, Taiwan
b Department of Chemistry and Engineering, Huanghuai University, Zhumadian 463000, PR China
c Department of Applied Chemistry, Chinese Culture University, Taipei 111, Taiwan
d Institute of Chemistry, Academia Sinica, Taipei 105, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
The photophysical properties of asymmetric indolylmaleimide derivatives (4a–4f) with a
variety of aryl substituents at C(4) were analyzed. Their absorption spectra vary in the
region of aryl substituents, whereas their emission spectra are nearly identical. The red
emission is derived from a charge transfer transition along the indole-maleimide chromo-
phore, which is not significantly disturbed by the introduction of a nearby aryl substituent
at C(4). All compounds displayed intensive emissions both in solutions as well as in the
form of solid films. Light emitting diodes (LED) were fabricated successfully by using the
neat film of these materials as emitters. The performance of devices among the series var-
ied slightly as a result of different film morphology. The best performance was found on the
compound with a pyrene substituent, which exhibited optimal external quantum effi-
ciency of 1.1%. This device could be turned on at 4 V and reached to optimal brightness
of 9750 cd/m2. The red color, according to the Commission Internationale de l’Eclairage
(CIE) coordinates at (0.65, 0.34), was very close to pure red (0.67, 0.33).
Received 1 September 2009
Received in revised form 28 December 2009
Accepted 29 December 2009
Available online 4 January 2010
Keywords:
Indolylmaleimide
Charge transfer emission
Non-dopant
Red luminescence
OLED
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
sive as those of the symmetrical ones. The mono-
substituted maleimide derivatives provide us an extra flex-
In the past decades active research investigations in bis-
indolylmaleimides have revealed that these derivatives
may exhibit multidisciplinary functionalities on diversified
applications, such as biological activities for pharmaceuti-
cal applications [1] and luminescent properties for making
organic light emitting diodes (OLEDs), etc. The structure of
maleimides expresses a twofold symmetry when both sub-
stituents on the olefinic carbons are identical. Remarkable
performances utilizing such type of symmetrical com-
pounds as red color emitters in OLEDs have been done by
Chow and Chen et al. [2,3]. It has been recognized, how-
ever, that maleimides containing a single substituent at
C(3) may exhibit distinctively different physical properties
from those of the symmetrical ones [4]. Through our works
we have found that the characteristic red emission deriv-
ing from the asymmetrical derivatives is nearly as inten-
ibility for modifying the structure of the second substitu-
ent at C(4), so that tuning mechanism may be
a
developed on this site. Either the chromaticity or intensity
of the luminescence may become adjustable by the control
of the second substituent. Report on unsymmetrical malei-
mides in this regard is not known to date, except a limited
number of publications focusing on certain special phar-
maceutical effects [5].
In this report a serial of asymmetric indolylmaleimide
derivatives, in which an aromatic group is attached at
C(4), are prepared. Their physical properties are discussed
with a particular attention on their photo and electrolumi-
nescence (EL) characteristics. Maleimides are among the
rare materials, which display intensive red emission even
in the solid state, e.g., either in crystalline form or as neat
film. Their high quantum efficiencies are attributed to their
ready formation of an amorphous phase with a high glass-
transition temperature. The asymmetric indolylmalei-
mides are expected to perform in a similar manner.
* Corresponding author. Tel.: +886 2 27898552; fax: +886 2 27884179.
E-mail address: tjchow@chem.sinica.edu.tw (T.J. Chow).
1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2009.12.025

Y.-S. Lee et al. / Organic Electronics 11 (2010) 604–612
605
Successful fabrication of LEDs with these materials will
light up a way toward future designs in luminescence sen-
sors. A typical device made of asymmetric indolylmalei-
mide with a pyrene substituent, as shown in this report,
can be turned on at 4.0 V with maximal intensity close to
104 cd/m2.
icon photodiode, respectively. All measurements were
completed under ambient conditions.
2.3. Synthesis of 3-bromo-4-(20-methyl-30-indolyl)-N-
benzylmaleimide (2)
To
a toluene (130 mL) solution of 2-methylindole
(3.35 g, 25.50 mmol) was added MeMgCl (25.51 mmol,
9.87 mL) at an ice temperature under a nitrogen atmo-
sphere, and the solution was warmed up gradually and
heated to 60 °C for 1 h. A solution of 2,3-dibromo-N-ben-
zylmaleimde (1, 8.00 g, 23.2 mmol) in toluene (10 mL)
was then added to the above solution, and it was heated
to reflux for 3 h. The solution was cooled to room temper-
ature and diluted with ethyl acetate (100 mL). The solution
was washed successively with 1 N aqueous HCl (150 mL),
water (150 mL) and brine (150 mL). The solution was then
dehydrated over anhydrous MgSO4 and evaporated in va-
cuo to form a yellow residue. The crude product was puri-
fied by silica gel column chromatography with ethyl
acetate/hexane (1:4) as an eluant to afford compound 2
in 68% yield (6.22 g, 15.8 mmol). 1H NMR (CD3COCD3,
400 MHz): d 2.51 (s, 3H), 4.81 (s, 2H), 7.06–7.10 (m, 1H),
7.12–7.16 (m, 1H), 7.28–7.30 (m, 1H), 7.33–7.36 (m, 2H),
7.38–7.43 (m, 3H), 7.47–7.49 (m, 1H), 10.78 (s, 1H). 13C
2. Experimental section
2.1. General
Absorption spectra were taken on a Hewlett–Packard
8453 spectrophotometer and emission spectra on a Hitachi
F-4500 fluorescence spectrophotometer. Infrared spectra
were taken on a Perkin–Elmer L118-F000 FT-IR spectrom-
eter. 1H and 13C NMR spectra were recorded on Bruker
AV400, AV500, and AMX400 superconducting FT spec-
trometers. The signal of tetramethylsilane at d 0.00 was
used as an internal standard for both. Microanalyses were
completed on a Perkin–Elmer 2400 elemental analyzer.
Mass spectra were taken on a Joel JMS 700 double-focusing
spectrometer, facilitated with fast atom bombardment
(FAB) for sample ionization. Melting points of the com-
pounds were uncorrected. Differential scanning calorime-
try was done on a Perkin–Elmer DCS-7 instrument. The
fluorescence quantum yields were determined in CH3CN
NMR (CD3COCD3, 125 MHz):
d 13.34, 41.99, 101.52,
110.99, 119.94, 120.45, 121.67, 126.66, 127.50, 127.84,
128.54, 136.09, 136.87, 138.41, 139.63, 165.86, 168.53,
205.14, 205.40. IR (KBr, cmꢀ1): 3381, 3061, 3033, 2928,
2360, 2340, 1773, 1709, 1624, 1610, 1458, 1430, 1396,
1351, 739, 431.
solutions at 293 K against Nile red as
(Up = 0.78) [6]. Cyclic voltammetry measurements were
carried out on BAS 100B electrochemical analyzer
a reference
a
equipped with a conventional three-electrode system, i.e.,
glassy carbon, platinum wire, and Ag/AgNO3 as the work-
ing, counter, and reference electrodes, respectively. De-
gassed acetonitrile solution with 0.1 M tetra-n-
butylammonium hexafluorophosphate was used as the
electrolyte. Eox and Ered data were measured with reference
to a ferrocene (Fc) internal standard that was calibrated
to +350 mV against a Ag/AgNO3 reference electrode [7].
2.4. General procedure for synthesis of 3a–3f
To a THF (20 mL) solution of 2 (2.00 g, 5.1 mmol) under
a
nitrogen atmosphere was added Pd(PPh3)4 (0.29 g,
0.25 mmol), and the mixture was stirred for 15 min. To
the mixture was then added arylboronic acid (5.6 mmol)
and aqueous Na2CO3 (2 M, 20 mL). Then the mixture was
heated to 110 °C for 24 h. After cooled to room tempera-
ture, the mixture was diluted with ethyl acetate (20 mL)
and followed by washing with water (30 mL) and brine
(30 mL). It was then dehydrated over anhydrous MgSO4
and dried in vacuo, forming a red residue. The crude prod-
uct was purified by silica gel column chromatography with
ethyl acetate/hexane (1:3) as an eluant to afford com-
pounds 3a–3f.
Measurements were taken at a scan rate of 100 mV sꢀ1
The aromatic boronic acids for the preparation of 4a–4f
were prepared according to the known procedures [8].
.
2.2. Device fabrication
Indium-tin oxide (ITO)-coated glass with a sheet resis-
tance of <50
X was used as the substrate. Pretreatment of
ITO includes a routine chemical cleaning using detergent
and alcohol in sequence, followed by oxygen plasma clean-
ing. Thermal evaporation of organic materials was carried
out using ULVAC Cryogenics at a chamber pressure of
10ꢀ6 Torr. Two types of devices were configured with
ITO/NPB/dye/BCP/AlQ3/LiF/Al (device A) and ITO/NPB/dye/
TPBI/LiF/Al (device B), in which NPB, BCP, AlQ3, TPBI, and
m-MTDATA are 4,40-bis[N-(1-naphthyl)-N-phenylami-
no]biphenyl, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthro-
line, tris(8-hydoxyquinolinato)aluminum, 2,20,200-(1,3,5-
benzenetriyl)tris(1-phenyl-1H-benzimidazole), and 4,40,400-
tris(phenyl-m-methylphenylamino)triphenylamine, respe-
ctively. Current–voltage and light–intensity measurements
were done on a Keithley 2400 source meter and a Newport
1835C optical meter equipped with a Newport 818-ST sil-
3-(20-Methyl-30-indoyl)-4-phenyl-N-benzylmaleimide
(3a). Physical data of 3a (yield 70%). 1H NMR (CD3COCD3,
400 MHz): d 2.25 (s, 3H), 4.83 (s, 2H), 6.82–6.86(m, 1H),
7.01–7.05 (m, 2H), 7.25–7.37 (m, 7H), 7.45–7.56 (m, 2H),
7.57–7.59 (m, 2H), 10.61 (s, 1H). 13C NMR (CD3COCD3,
125 MHz): d 13.37, 41.69, 102.63, 110.37, 120.07, 120.15,
121.84, 126.51, 127.55, 128.21, 128.54, 128.61, 129.05,
129.36, 130.28, 133.25, 134.04, 135.73, 136.94, 137.17,
170.63, 171.00. IR (KBr, cmꢀ1): 3396, 2362, 2342, 2091,
1771, 1760, 1748, 1734, 1697, 1650, 1637, 1492, 1474,
1458, 1431, 1399, 742, 694, 440. HRMS (m/z) calcd for
C26H20N2O2: 392.1525 (M+); found: 392.1526. LRMS (FAB,
m/z): 392.1 (M+, 100%), 307.2 (17), 154.1 (76), 136.2 (57),
107.2 (23).

606
Y.-S. Lee et al. / Organic Electronics 11 (2010) 604–612
3-(20-Methyl-30-indoyl)-4-(400-biphenyl)-N-benzylmalei-
3-(20-Methyl-30-indoyl)-4-(1-pyrenyl)-N-benzylmalei-
mide (3f). Physical data of 3f (yield 61%). 1H NMR
(CD3COCD3, 400 MHz): d 2.08 (s, 3H), 4.95 (s, 2H), 6.64–
6.68 (m, 1H), 6.85–6.89 (m, 1H), 7.12–7.16 (m, 2H), 7.30–
7.34 (m, 1H), 7.38–7.42 (m, 2H), 7.53–7.55 (m, 2H), 7.89
(d, J = 9.3 Hz, 1H), 8.00–8.03 (m, 2H), 8.09–8.13 (m, 1H),
8.14–8.20 (m, 3H), 8.25–8.27 (m, 2H), 10.40 (s, 1H). 13C
mide (3b). Physical data of 3b (yield 47%). 1H NMR
(CD3COCD3, 400 MHz): d 2.29 (s, 3H), 4.84 (s, 2H), 6.82–
6.86 (m, 1H), 7.01–7.07 (m, 2H), 7.26–7.30 (m, 1H), 7.34–
7.38 (m, 4H), 7.41–7.47 (m, 4H), 7.58–7.60 (m, 2H),
7.62–7.65 (m, 2H), 7.68–7.70 (m, 2H), 10.75 (s, 1H). 13C
NMR (CD3COCD3, 125 MHz):
d 12.84, 41.32, 102.42,
110.73, 119.60, 120.03, 121.36, 126.34, 126.66, 127.33,
127.65, 127.83, 128.47, 128.83, 129.82, 129.96, 132.88,
133.42, 136.41, 137.43, 137.80, 139.89, 141.06, 170.35,
170.85. IR (KBr, cmꢀ1): 3383, 3059, 3031, 2922, 2361,
2340, 1793, 1761, 1734, 1698, 1651, 1622, 1606, 1457,
1429, 1399, 746, 697, 452. HRMS (m/z) calcd for
C32H24N2O2: 469.1910 ([M+H]+); found: 469.1910. LRMS
(FAB, m/z): 469.1 ([M+H]+, 100%), 307.2 (17), 154.1 (36).
3-(20-Methyl-30-indoyl)-4-(1-naphthyl)-N-benzylmalei-
mide (3c). Physical data of 3c (yield 62%). 1H NMR (ace-
tone-d6, 400 MHz): d 2.02 (s, 3H), 4.90 (s, 2H), 6.73–6.77
(m, 1H), 6.92–6.96 (m, 1H), 7.13–7.22 (m, 3H), 7.30–7.32
(m, 1H), 7.36–7.40 (m, 3H), 7.48–7.51 (m, 3H), 7.57–7.59
(m, 1H), 7.76–7.78 (m, 1H), 7.84–7.92, (m, 2H), 10.43 (s,
1H). 13C NMR (acetone-d6, 125 MHz): d 12.74, 41.57,
119.59, 119.73, 121.23, 125.21, 125.72, 125.76, 125.82,
127.18, 127.38, 127.89, 128.27, 128.54, 128.73, 128.96,
129.40, 130.66, 133.63, 134.12, 135.95, 136.99, 137.50,
138.08, 170.43, 170.87. IR (KBr, cmꢀ1): 3419, 2361, 2340,
2090, 1793, 1760, 1748, 1733, 1698, 1649, 1635, 1490,
1458, 1429, 1398, 740, 698, 489. HRMS (m/z) calcd for
C30H22N2O2: 443.1760 ([M+H]+); found: 443.1753. LRMS
(FAB, m/z): 443.2 ([M+H]+, 100%), 154.1 (14).
NMR (CD3COCD3, 100 MHz): d 13.27, 42.11, 103.49,
111.00, 120.16, 120.24, 121.74, 124.29, 124.70, 124.91,
125.33, 125.66, 125.77, 126.18, 126.28, 127.35, 127.41,
127.57, 127.78, 128.38, 128.71, 128.89, 129.06, 130.68,
131.12, 131.85, 134.52, 136.31, 137.31, 137.66, 138.69,
171.003, 171.56. IR (KBr, cmꢀ1): 3394, 3050, 2360, 2340,
1794, 761, 1749, 1734, 1699, 1650, 1636, 1623, 1457,
1428, 1397, 741, 455. HRMS (m/z) calcd for C36H24N2O2:
517.1916 ([M+H]+); found: 517.1909. LRMS (FAB, m/z):
517.2 ([M+H]+, 100%), 391.2 (5), 307.1 (6), 289.1 (5),
154.1 (33), 136.1 (21).
2.5. General procedure for the synthesis of 4a–4f
Compound 3 (1.40 g, 3.6 mmol) was mixed with NaOH
(0.16 g, 4.0 mmol) in acetone (20 mL) under a nitrogen
atmosphere, while the solution turned gradually from red
to dark blue. To the mixture was added benzyl bromide
(0.64 mL, 5.4 mmol), then the solution was stirred with a
magnetic bar for 30 min, while the color tuned red. The
mixture was stirred for another 1 h at room temperature.
The reaction was quenched by the addition of water
(500 mL), then was filtered. The filtrate was concentrated
in vacuo, and the product was recrystallized from CH2Cl2
and hexane as red crystalline solids.
3-(20-Methyl-30-indoyl)-4-(9-anthryl)-N-benzylmalei-
mide (3d). Physical data of 3d (yield 50%). 1H NMR
(CD3COCD3, 400 MHz): d 2.02 (s, 3H), 4.96 (s, 2H), 6.57–
6.61 (m, 1H), 6.83–6.87 (m, 1H), 7.05–7.08 (m, 1H), 7.10–
7.13 (m, 1H), 7.30–7.39 (m, 4H), 7.39–7.45 (m, 3H), 7.52–
7.54 (m, 2H), 8.01–8.03 (m, 4H), 8.59 (s, 1H), 10.40 (s,
3-(20-Methyl-N-benzyl-30-indoyl)-4-(phenyl)-N-benzyl-
maleimide (4a). Physical data of 4a (yield 78%). 1H NMR
(CD2Cl2, 400 MHz): d 2.00 (s, 3H), 4.83 (s, 2H), 5.28 (s,
2H), 6.94–7.21 (m, 4H), 7.22–7.23 (m, 1H), 7.27–7.35 (m,
10H), 7.47–7.54 (m, 4H). 13C NMR (CD2Cl2, 100 MHz): d
12.17, 41.78, 46.75, 102.94, 109.57, 120.38, 120.45,
121.91, 125.95, 126.19, 127.50, 127.64, 128.30, 128.36,
128.63, 128.82, 129.13, 129.39, 130.47, 133.64, 134.26,
137.04, 137.07, 137.19, 138.32, 170.70, 170.99. IR (KBr,
cmꢀ1): 3032, 2940, 2361, 1762, 1701, 1621, 1467, 1454,
1399, 1350, 739, 432. HRMS (m/z) calcd for C33H26N2O2:
482.1994 (M+); found: 482.1986. LRMS (FAB, m/z): 482.2
(M+, 100%), 307.1 (20), 136.1 (40). Anal. calcd for
C33H26N2O2: C, 82.13; H, 5.43; N, 5.81, found: C, 82.01; H,
5.47; N, 5.93.
1H).13C NMR (CD3COCD3, 125 MHz):
d 12.78, 41.76,
102.97, 110.46, 119.46, 121.11, 125.22, 126.01, 126.10,
127.22, 127.37, 127.76, 128.57, 128.68, 129.99, 131.39,
133.70, 135.75, 137.59, 138.17, 170.29, 170.85. IR (KBr,
cmꢀ1): 3365, 3055, 2924, 2854, 2361, 2340, 1794, 1770,
1748, 1734, 1699, 1650, 1635, 1622, 1457, 1431, 1397,
734, 435. HRMS (m/z) calcd for C34H24N2O2: 493.1916
([M+H]+); found: 493.1908. LRMS (FAB, m/z): 493.1
([M+H]+, 72%), 154.1 (76).
3-(20-Methyl-30-indoyl)-4-(9-phenanthryl)-N-benzylma-
leimide (3e). Physical data of 3e (yield 62%). 1H NMR
(CD3COCD3, 400 MHz): d 2.92 (s, 3H), 4.92 (s, 2H), 6.71–
6.74 (m, 1H), 6.90–6.91 (m, 1H), 7.15–7.18 (m, 1H), 7.25–
7.27 (m, 1H), 7.30–7.32 (m, 2H), 7.38–7.40 (m, 2H),
7.51–7.58 (m, 4H), 7.59–7.67 (m, 1H), 7.86–7.90 (m, 2H),
7.94 (s, 1H), 8.76 (d, J = 8.4 Hz, 2H), 10.41 (s, 1H). 13C
3-(20-Methyl-N-benzyl-30-indoyl)-4-(400-biphenyl)-N-
benzylmaleimide (4b). Physical data of 4b (yield 75%). 1H
NMR (CDCl3, 400 MHz): d 2.12 (s, 3H), 4.84 (s, 2H), 5.31
(s, 2H), 6.97–7.04 (m, 3H), 7.10–7.13 (m, 1H), 7.20–7.21
(m, 1H), 7.26–7.30 (m, 1H), 7.33–7.35 (m, 4H), 7.37–7.41
(m, 3H), 7.43–7.46 (m, 2H), 7.50–7.52 (m, 6H), 7.62–7.64
(m, 2H). 13C NMR (CDCl3, 125 MHz): d 12.57, 41.94,
46.84, 103.18, 109.46, 120.60, 120.74, 122.08, 125.97,
126.88, 126.94, 127.69, 128.78, 128.87, 129.76, 133.15,
133.42, 136.77, 137.29, 138.18, 140.15, 141.51, 170.73,
171.01. IR (KBr, cmꢀ1): 3032, 2938, 2362, 2342, 1762,
1700, 1618, 1456, 1418, 1398, 1350, 740, 425. HRMS (m/
z) calcd for C39H30N2O2: 558.2307 (M+); found: 558.2310.
LRMS (FAB, m/z): 558.2 (M+, 100%), 154.1 (25), 105.2 (10).
NMR (CD3COCD3, 125 MHz):
d 12.95, 41.70, 103.16,
110.60, 119.67, 119.87, 121.31, 122.65, 122.91, 126.16,
126.71, 126.91, 127.45, 127.63, 127.99, 128.60, 129.04,
130.24, 130.37, 131.15, 134.08, 135.99, 137.28, 137.52,
138.26, 170.51, 170.94. IR (KBr, cmꢀ1): 3423, 2360, 2339,
1793, 1771, 1748, 1733, 1698, 1650, 1636, 1474, 1457,
1428, 1397, 740, 429. HRMS (m/z) calcd for C34H24N2O2:
493.1916 ([M+H]+); found: 493.1908. LRMS (FAB, m/z):
493.1 ([M+H]+, 100%), 391.2 (16), 107.2 (47).

Y.-S. Lee et al. / Organic Electronics 11 (2010) 604–612
607
3-(20-Methyl-N-benzyl-30-indoyl)-4-(100-naphthyl)-N-
benzylmaleimide (4c). Physical data of 4c (yield 76%). 1H
NMR (CDCl3, 400 MHz): d 1.63 (br, 3H), 4.93 (s, 2H), 5.08
(br, 2H), 6.54 (br, 2H), 7.04–7.15 (m, 7H), 7.30–7.40 (m,
4H), 7.46–7.60 (m, 6H), 7.53–7.81 (m, 2H). 13C NMR
(CDCl3, 125 MHz): d 11.99, 42.16, 46.44, 109.25, 120.16,
120.76, 121.97, 125.28, 125.48, 125.75, 125.91, 127.22,
127.74, 128.49, 128.68, 128.74, 128.88, 129.22, 129.82,
133.56, 136.39, 136.79, 136.91, 138.51, 170.71, 170.11.
IR (KBr, cmꢀ1): 3060, 2361, 1762, 1701, 1650, 1619,
1455, 1420, 1397, 1351, 738, 697, 442. HRMS (m/z) calcd
for C37H28N2O2: 533.2229 ([M+H]+); found:533.2238.
LRMS (FAB, m/z): 533.2 ([M+H]+, 22%), 307.2 (45),
136.1 (56). Anal. calcd for C37H28N2O2: C, 83.43; H,
5.30; N, 5.26, found: C, 82.87; H, 5.39; N, 4.93.
1455, 1396, 1350, 737, 431. HRMS (m/z) calcd for
C
43H30N2O2: 607.2386 ([M+H]+); found: 607.2386. LRMS
(FAB, m/z): 607.1 ([M+H]+, 100%), 154.1 (20), 136.2 (14).
Anal. calcd for C43H30N2O2: C, 85.12; H, 4.98; N, 4.62,
found: C, 84.91; H, 5.31; N, 4.54.
3. Results and discussion
3.1. Synthesis and characterization
The synthesis of the asymmetric indolylmaleimide
derivatives 4a–4f is illustrated in Scheme 1. 3-Bromo-4-
(20-methyl-30-indolyl)-N-benzylmaleimide (2) was synthe-
sized in a yield of 68% by a substitution reaction of 3,4-di-
bromo-N-benzylmaleimide (1) [1a] with one equivalent of
2-methylindole in the presence of CH3MgCl, and it was uti-
lized as the pivotal compound in the following coupling
reactions. The addition of the second substituent at C(4)
was performed by a Suzuki coupling reaction with aro-
matic boronic acids, i.e., phenylboronic acid, 4-biphenylbo-
ronic acid, naphthalene-1-boronic acid, anthracene-9-
boronic acid, phenathrene-9-boronic acid, and pyrene-1-
boronic acid, respectively, to yield 3a–3f in 47–70% yield.
In 1H NMR the hydrogens at methylene moieties appeared
as a singlet at d ꢁ 5.0 ppm, whereas in 13C field the car-
bonyl groups appeared at d ꢁ 170 ppm. The indolyl hydro-
gens in 3a–3f were masked by alkylation with benzyl
bromide in the presence of base. Compounds 4a–4f were
obtained as red color solids in yields of 75–84%. The addi-
tional substituents at C(4) of 4a–4f hindered the rotation
between the maleimide ring and the two aromatic substit-
uents, so that line broadening in 1H NMR spectra was ap-
peared at both the methyl and methylene positions of
indole. This phenomenon is more pronounced in 4c, 4e
and 4f, whereas the line broadening was weaker in 4d
due to the symmetrical geometry of anthracene group.
3-(20-Methyl-N-benzyl-30-indoyl)-4-(900-anthryl)-N-ben-
zyl maleimide (4d). Physical data of 4d (yield 76%). 1H NMR
(CDCl3, 500 MHz): d 1.60 (s, 3H), 4.98 (s, 2H), 5.01 (s, 2H),
6.34 (d, J = 8 Hz, 2H), 6.86–6.87 (m, 1H), 6.89–7.00 (m,
2H), 7.01–7.04 (m, 2H), 7.09–7.10 (m, 1H), 7.27–7.28 (m,
2H), 7.30–7.34 (m, 2H), 7.35–7.53 (m, 4H), 7.58 (d,
J = 8 Hz, 2 H), 7.77 (d, J = 9 Hz, 2H), 7.94 (d, J = 9 Hz, 2H),
8.41 (s, 1H). 13C NMR (CDCl3, 125 MHz): d 12.09, 42.30,
46.24, 109.17, 119.87, 120.73, 121.89, 124.23, 125.11,
125.19, 125.81, 128.74, 128.87, 129.29, 129.76, 131.31,
134.03, 136.36, 136.70, 136.87, 138.79, 139.60, 170.60,
170.97. IR (KBr, cmꢀ1): 3056, 2398, 2361, 2341, 1734,
1702, 1619, 1470, 1455, 1396, 1351, 732, 493. HRMS (m/
z) calcd for C41H30N2O2: 582.2307 (M+); found: 582.2300.
LRMS (FAB, m/z): 583.1 ([M+H]+, 100%), 391.3 (6), 289.1
(10), 136.2 (21). Anal. calcd for C41H30N2O2: C, 84.51; H,
5.19; N, 4.81, found: C, 84.07; H, 5.25; N, 4.93.
3-(20-Methyl-30-indoyl)-4-(9-phenanthryl)-N-benzylma-
leimide (4e). Physical data of 4e (yield 82%). 1H NMR
(CD3COCD3, 400 MHz): d 1.63 (br, 3H), 4.95 (s, 2H), 5.04
(br, 2H), 6.48 (br, 2H), 7.03–7.19 (m, 7H), 7.30–7.34 (m,
1H), 7.37–7.40 (m, 2H), 7.50–7.66 (m, 7H), 7.85–7.91 (m,
2H), 8.59–8.63 (m, 2H). 13C NMR (CD3COCD3, 125 MHz):
d 12.16, 42.24, 46.41, 109.34, 120.29, 120.88, 122.04,
122.54, 125.43, 126.55, 126.79, 127.67, 127.81, 128.68,
128.74, 128.94, 130.47, 130.62, 131.01, 133.91, 136.33,
136.59, 136.83, 136.93, 138.72, 170.77, 171.11. IR (KBr,
cmꢀ1): 3061, 3032, 2939, 2361, 2341, 1761, 1702, 1619,
1469, 1453, 1397, 1350, 740, 432. HRMS (m/z) calcd for
C41H30N2O2: 583.2386 ([M+H]+); found:583.2386. LRMS
(FAB, m/z): 583.2 ([M+H]+, 100%), 391.3 (4), 289.2 (4),
154.1 (21), 107.2 (6). Anal. calcd for C41H30N2O2: C,
84.51; H, 5.19; N, 4.81, found: C, 84.15; H, 5.24; N, 4.75.
3-(20-Methyl-N-benzyl-30-indoyl)-4-(1-pyrenyl)-N-ben-
zylmaleimide (4f). Physical data of 4f (yield 84%). 1H NMR
(CDCl3, 500 MHz): d 1.30 (br, 3H), 4.97 (s, 2H), 5.02 (br,
2H), 6.29 (br, 2H), 7.02–7.03 (m, 4H), 7.30–7.34 (m, 1H),
7.37–7.40 (m, 2H), 7.58–7.60 (m, 3H), 7.74 (br, 3H), 7.96–
8.01 (m, 3H), 8.05–8.07 (m, 3H), 8.14–8.20 (m, 2H). 13C
NMR (CDCl3, 125 MHz): d 12.01, 42.28, 46.39, 101.92,
103.88, 109.35, 119.61, 120.23, 120.89, 122.06, 124.42,
124.83, 124.98, 125.32, 125.60, 125.69, 126.08, 127.09,
127.29, 128.38, 138.78, 129.02, 130.09, 130.08, 130.59,
120.90, 131.11, 131.44, 131.86, 132.26, 134.23, 136.21,
136.56, 136.90, 137.03, 138.55, 170.87, 171.41. IR (KBr,
cmꢀ1): 3034, 2936, 2362, 2339, 1761, 1700, 1618, 1470,
3.2. Physical properties
Compounds 4a–4f exhibit three major absorption re-
gions in the range of 260–550 nm (Fig. 1). The absorptions
shorter than 300 nm are derived mainly from the local
*
transitions of both indole
*
p–p (major) and maleimide n–
p
(minor). The region between 300 and 400 nm comes
from the p–p transitions on the second aryl substituents,
*
while their exact positions depend on the nature of the aryl
group. The broad featureless band on the longest wave-
length side above 400 nm belongs to a charge transfer
transition along the dipole from indole toward maleimide,
which has been well documented in the literatures [4,9].
The emission spectra of all compounds 4a–4f overlap
heavily with each other (Fig. 1), which are essentially the
same as those of the symmetrical bis(indolyl)maleimide.
The lowest energy radiative transition is derived from a
charge-separated dipole directing from the indolyl nitro-
gen group toward the maleimide carbonyl. Such a charge
transfer (CT) transition normally exhibits a distinctive sol-
vatochromic effect (ESI, S1, S2 and S3). A typical example is
shown in Fig. 2, where an apparent blue shift can be seen in
the emission spectra of 4b along with the increasing of

608
Y.-S. Lee et al. / Organic Electronics 11 (2010) 604–612
N
O
N
O
N
N
1) CH3MgCl
O
O
Pd(PPh3)4
O
O
1) NaOH
N
O
O
R
arylbornic acid
2) 2-methyl indole
Br
R
2) PhCH2Br
Br
Br
N
H
N
H
3a-f
e)
4a-f
2
1
R = a)
b)
c)
d)
f)
Scheme 1. The synthetic sequence of compounds 4a–4f.
sample was heated again, yet a melting signal was not ob-
served in the second scan. Nevertheless, a small endother-
mic hump at 98 °C was detected, which can be assigned to
a glass-transition (Tg) temperature. The absence of Tm as
well as the presence of Tg revealed the amorphous nature
of solid films. All compounds 4a–4f exhibited analogous
glass-transition phenomena at considerable high tempera-
tures, i.e., 63–109 °C (Table 1). The compact and round
shape of these compounds is presumably responsible for
the high Tg values. Red fluorescence was observable not
only in solutions, but also in the solid state (ESI S5). This
property provides an unusual advantage for the fabrication
of LED devices in the form of neat films. In a common prac-
tice doping is usually necessary to avoid self-quenching in
order to separate individual chromophores apart from each
other.
Each compound showed two consecutive oxidative
waves in dichloromethane by cyclic voltammetry (CV).
The first wave at ca. 1.0 V corresponds to the oxidation of
the indolyl group and second one at ca. 1.15 V is due to
the aromatic substituent (Table 1). In the reductive sweep
a single wave was observed at about –1.45 V by the reduc-
tion of maleimide moiety. The lowest unoccupied molecu-
lar orbital (LUMO) could be estimated by these redox
potentials. The 0–0 transition energies were estimated at
the interception of the absorption and emission spectra.
The physical nature of compound 4c was further exam-
ined by semiempirical method AM1 implanted in Spartan
software. In an optimized geometry the two aryl substitu-
ents at C(3, 4) are twisted from each other, while each
maintains a dihedral angle of ca. 35° with respect to the
central plane of maleimide. The irregular geometrical
shape is favorable to the formation of amorphous solid
during film preparation. Therefore high glass-transition
temperatures (Tg) were detectable for all compounds (Ta-
ble 1). As for their electronic configurations, the electron
densities on the highest occupied molecular (HOMO) and
the second highest occupied molecular orbital (SHOMO)
are localized on the indolyl and naphthyl substituents,
respectively (Fig. 3). The lowest energy transition from
HOMO to LUMO (S1 state) induces a charge separation be-
tween the indole and maleimide moieties. Charge recombi-
nation from S1 to the ground state (S0) results in an
intensive red CT emission. It is interesting to find that
the energy of this emission is only slightly perturbed by
4a
4b
4c
4d
4e
4f
1.0
0.5
0.0
300
400
500
600
700
800
Wavelength (nm)
Fig. 1. Normalized absorption and emission spectra of compounds 4a–4f
in CH2Cl2. The emission spectra were obtained by irradiation at kmax of the
long wavelength absorption bands.
CH2Cl2
THF
EA
1.0
0.5
0.0
300
400
500
600
700
800
Wavelength (nm)
Fig. 2. Absorption and emission spectra of compound 4b in different
solvents.
solvent polarity. The red emissions with similar character-
istics are found in all compounds 4a–4f. A summary of
their spectroscopic parameters is listed in Table 1.
The thermal stability of 4a–4f was evaluated by differ-
ential scanning calorimetry (DSC). In a typical scan on a
crystalline sample of compound 4d (ESI S4), a sharp endo-
thermic peak was detected at 251 °C, corresponding to the
melting point (Tm) of the crystals. After cooling the same

Y.-S. Lee et al. / Organic Electronics 11 (2010) 604–612
609
Table 1
Physical data of compounds 4a–4f.
a
b
c
c
Abs (
e)
PL (Up)a
(nm (%))
E0-0
Eox
Ered
(V)
Band gapd
(eV)
HOMOe
(eV)
LUMOf
(eV)
Tg/Tm
(°C)
(nm (Mꢀ1cmꢀ1))
(eV)
(V)
4a
4b
4c
4d
4e
4f
462 (5.6 ꢂ 103)
470 (6.1 ꢂ 103)
460 (7.3 ꢂ 103)
448 (7.0 ꢂ 103)
459 (5.9 ꢂ 103)
433 (9.0 ꢂ 103)
630 (2.9)
634 (4.9)
620 (7.5)
627 (6.8)
617 (2.1)
634 (23.1)
2.28
2.22
2.28
2.26
2.22
2.22
1.01, 1.15
0.98, 1.12
0.99, 1.16
1.03, 1.18
1.01
ꢀ1.45
ꢀ1.44
ꢀ1.48
ꢀ1.44
ꢀ1.47
ꢀ1.44
2.46
2.42
2.47
2.47
2.48
2.43
ꢀ5.41
ꢀ5.38
ꢀ5.39
ꢀ5.47
ꢀ5.45
ꢀ5.39
ꢀ3.13
ꢀ3.16
ꢀ3.11
ꢀ3.21
ꢀ3.23
ꢀ3.17
63/158
81/222
83/225
98/251
104/277
109/193
0.99, 1.17
a
b
c
d
e
f
In CH2Cl2 at 293 K, quantum efficiency against Nile red (Up = 0.78) [6].
Taken at the crossing point of the absorption and emission spectra.
Fc/Fc+ at 0.35 V against Ag/AgNO3 as the internal reference.
Gap between Eox and Ered
.
Calculated as ꢀ(Eox + 4.40) [10].
Calculated as HOMO + band gap.
Fig. 3. Simulated frontier molecular orbitals of compound 4c. From left to right are LUMO, HOMO, and SHOMO, respectively. The lowest energy transition
from the first excited state, i.e., S1 corresponding to HOMO ? LUMO, is polarized from the indolyl substituent toward the central maleimide group. The
transition involved the naphthyl moiety is at higher energy, and will transmit its energy rapidly through internal conversion to S1. The simulation was
performed by semiempirical method AM1 implanted in Spartan software.
adjacent aryl group, so that it explains the high similarity
of PL spectra among all compounds 4a–4f.
case of device I, a highly intensive band at 400 nm is ob-
served, indicating holes have moved across the boundary
into the bulk of TPBI. In certain occasions such dual emis-
sions may be desired for the production of a white light
[2,11]. However, in the present case we sought to reduce
the hole leakage by inserting an additional layer of m-
MTDATA between ITO and NPB to slow down the move-
ment of holes. This new material is known to be capable
of forming a highly uniform amorphous film, while its high
HOMO level along with a moderate charge mobility can
smooth the injection of holes [12]. With an additional layer
of m-MTDATA, the quantum efficiency of device II was
slightly improved. It was also clear that the intensity of
600 nm band was reduced to about half that of I (Fig. 4).
The HT material TPBI in devices III–V was replaced by a
two-layer combination of BCP/AlQ3. In this design AlQ3
functioned as an electron injector and BCP a hole-blocker.
The film thickness of 4a in both III and IV was maintained
10 nm, while a layer of m-MTDATA was included in device
IV. In the EL spectra of III and IV, a minor shoulder was
found at peak maximum ca. 520 nm, which corresponded
to the emission of AlQ3. It was an evidence of a mild hole
leakage from the emitting layer to the medium of ET. The
improvement made by m-MTDATA in II and IV was some-
3.3. Device optimization
The electroluminescence of these materials was exam-
ined by the fabrication of LED devices. A typical lay-out
of device configuration was ITO/HT/dye/ET/LiF/Al, whereas
the hole transporter (HT) is either NPB or a combination of
m-MTDATA/NPB, and the electron transporter (ET) is either
TPBI or a pair of BCP/AlQ3. A few selected combinations
were compared in order to optimize the quantum yield
as well as the purity of the red emission color. The param-
eters of devices made with compound 4a in five trials I–V
are given in Table 2. The ET material used in devices I and II
was TPBI, while that in devices III–V was BCP/AlQ3. The
electroluminescence spectra of all devices are plotted in
Fig. 4. A noteworthy feature in spectra I–IV is the presence
of two emission bands, i.e., an extra peak appeared on the
high energy side deriving from the emission of ET. Nor-
mally in the presence of TPBI the migration of holes can
be blocked mostly at the film junction due to its excep-
tional low HOMO level (at ꢀ6.2 V). Therefore it functions
both as an ET as well as a hole-blocker. However in the

610
Y.-S. Lee et al. / Organic Electronics 11 (2010) 604–612
Table 2
Performance of electroluminescence devices made with 4a as the emitting dye. The configuration of devices are listed in the following, with the thickness of
films in nm embraced in the parentheses: I: ITO/NPB (40)/4a (10)/TPBI (40)/LiF (1)/Al (150); II: ITO/m-MTDATA (30)/NPB (20)/4a (30)/TPBI (40)/LiF (1)/Al (150);
III: ITO/NPB (40)/4a (10)/BCP (10)/AlQ3 (30)/LiF (1)/Al (150); IV: ITO/m-MTDATA (30)/NPB (20)/4a (10)/BCP (10)/AlQ3 (30)/LiF (1)/Al (150); V: ITO/m-MTDATA
(30)/NPB (20)/4a (30)/BCP (10)/AlQ3 (30)/LiF (1)/Al (150).
La
gex
(%)
glc
(cd/A)
gp
CIEe
(x, y)
b
d
Device type
Turn on
(V)
(cd/m2)
(lm/W)
I
II
III
IV
V
4.5
6.5
4.5
5.0
6.0
690, 3030 (16)
790, 2970 (22)
520, 3390 (14)
540, 3450 (16)
810, 3950 (18)
0.50
0.68
0.28
0.30
0.48
0.69
0.80
0.50
0.54
0.80
0.25
0.16
0.20
0.16
0.17
0.40, 0.29
0.44, 0.27
0.55, 0.43
0.57, 0.42
0.61, 0.39
a
b
c
Intensity at 100 mA/cm2, followed by the highest intensity at indicated voltage.
External quantum efficiency.
Luminance efficiency.
Power efficiency.
Commission Internationale de l’Eclairage 1931.
d
e
Device I
Device II
Device III
Device IV
Device V
Device VI
Device VII
Device VIII
Device IX
1.0
0.5
0.0
1.5
1.0
0.5
0.0
400
500
600
700
800
400
500
600
700
800
Wavelength
Wavelength
Fig. 4. EL spectra of 4a fabricated in device types I–V. Intensity is
normalized at 610 nm peak maximum.
Fig. 5. EL spectra of 4c fabricated in device types VI–IX. Intensity is
normalized at peak maxima.
what limited. Although it is able to promote the light effi-
ciency, however with an increased total thickness it also
resulted to a higher turn-on voltage as well as a lower
power efficiency comparing with those of I and II. A pure
red emission was finally achieved by increasing the film
thickness of 4a from 10 nm to 30 nm. The EL spectrum of
device IV, as shown in Fig. 4, exhibited a single emission
band deriving from the maleimide.
Devices made with dye 4c in different configurations
were further examined with an optimized thickness of
30 nm. The ET materials in devices VI, VII, and VIII were
composed of TPBI, BCP/AlQ3, and BCP/TPBI, respectively.
Their EL spectra are shown in Fig. 5, while each of them
exhibits a single major emission nearly identical to each
other. Hole leakage to ET is effectively blocked without
noticeable side band. The light efficiency of these three de-
vices was about 1%, with an optimal brightness in a range
of 6000 cd/m2 (Table 3). The CIE coordinate (Commission
Internationale de l’Eclairage 1931) was located in the prox-
imity of (0.6, 0.4), quite close to pure red color. The most
unique feather in the fabrication of these devices was the
use of neat films without doping. Devices made of neat
films are relatively easier to fabricate and better in color
stability especially under different operating conditions.
For doped devices another factor need to be considered is
the polarity of the host. For the charge transfer emission
of 4a–4f, the absorption maxima are quite sensitive to
the influence of solvent. To illustrate the effect, a special
device IX was made, in which dye 4c was co-deposited in
5% weight ratio with CBP as a host material. This doped de-
vice performed with a remarkable light efficiency 4.5%, and
an optimal brightness exceeding 1.2 ꢂ 104 cd/m2. How-
ever, its emission maximum was blue shifted ca. 44 nm
(from 615 to 571 nm) from those made of neat films as a
result of the lower polarity of CBP. The CIE coordinate of
device IX therefore was drifted to (0.47, 0.49), substantially
away from pure red.
As concluded from above, a device of either type VI or
type VII will be most suitable for producing a red emission,
while the thickness of emitting films was kept in 30 nm.
When the film is too thick, the turn-on voltage becomes
higher and power efficiency reduced; yet when it is too
thin, the color purity is lowered as a result of hole leakage.
To comply with the above limitations, device type VII was
selected for the fabrication using compounds 4b and 4d–
4f. The EL spectra of these devices are shown in Fig. 6,

Y.-S. Lee et al. / Organic Electronics 11 (2010) 604–612
611
Table 3
Performance of electroluminescence devices made with 4c as the emitting dye. The configuration of devices are listed in the following, with the thickness of
films in nm embraced in the parentheses: VI: ITO/NPB (40)/4c (30)/TPBI (40)/LiF (1)/Al (150); VII: ITO/NPB (40)/4c (30)/BCP (10)/AlQ3 (30)/LiF (1)/Al (150); VIII:
ITONPB (40)/4c (30)/BCP (10)/TPBI (30)/LiF (1)/Al (150); IX: ITO/NPB (40)/4c (5%): CBP (40)/TPBI (40)/LiF (1)/Al (150).
La
gex
(%)
glc
(cd/A)
gp
CIEe
(x, y)
b
d
Device type
Turn on
(V)
(cd/m2)
(lm/W)
VI
5.0
4.5
5.0
5.5
1110, 6040 (20)
930, 6250 (18)
950, 5330 (17)
4450, 12530 (22)
0.67
0.54
0.60
1.51
1.11
0.93
0.95
4.45
0.31
0.26
0.28
0.82
0.59, 0.41
0.58, 0.41
0.59, 0.40
0.47, 0.49
VII
VIII
IX
a
b
c
Intensity at 100 mA/cm2, followed by the highest intensity at indicated voltage.
External quantum efficiency.
Luminance efficiency.
Power efficiency.
Commission Internationale de l’Eclairage 1931.
d
e
and parameters of performance in Table 4. The emission
spectra of 4a–4e were found to be nearly identical, except
4f which displayed a mild ꢁ13 nm red shift with respect to
others. Nevertheless this mild shift is consistent with its
photoluminescence spectrum (Fig. 1).
and g–J (Fig. 8) plots. Most devices displayed quantum effi-
ciencies within the range of 0.4–0.6%, and maximal emis-
sion intensity 5500–6500 cd/m2. The high similarity of
these devices indicated that the variation of a C(4) substi-
tuent on maleimide does not influence their electrolumi-
The performance of devices of type VII fabricated for all
the materials 4a–4f can be depicted by both the L–V (Fig. 7)
8000
6000
4000
2000
0
104
103
102
101
4b
1.0
4d
4e
4f
4
6
8
10 12 14 16 18
Voltage (V)
4a
4b
4c
4d
4e
4f
0.5
0.0
4
6
8
10
12
14
16
18
Voltage (V)
400
500
600
700
800
Wavelength (nm)
Fig. 7. Plot of luminescent intensity (L) versus applied voltage (V) of
device VII using the neat film of compounds 4a–4f as the emitter. The
inset shows the plots of log(L) versus V, in which the turn voltages can be
better illustrated.
Fig. 6. EL spectra of device VIII using compounds 4b and 4d–4f as the
emitters. A slight red shift is observed in the spectrum of 4f.
Table 4
4a
4b
4c
4d
4e
4f
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Performance of electroluminescence devices employing compounds 4b and
4d–4f as emitting materials. The configurations of devices VI and VII are the
same as those in Table 3. The quantum efficiencies are recorded at 100 mA/
cm2.
La
gex
glc
gp
b
d
Dye
(device)
Turn on
(V)
(cd/m2)
(%)
(cd/A)
(lm/W)
4b (VII)
4d (VI)
(VII)
4e (VI)
(VII)
4.5
4.5
4.5
4.5
4.5
4.0
4.0
870, 5618 (17)
846, 6605 (17)
1052, 7794 (18)
696,5612 (17)
824, 5976 (17)
1068, 9750 (18)
1088, 7634 (17)
0.60
0.59
0.72
0.46
0.54
1.06
1.08
0.87
0.85
1.05
0.70
0.83
1.07
1.09
0.24
0.23
0.30
0.23
0.29
0.40
0.30
4f (VI)
(VII)
200
400
600
800
1000
a
Intensity at 100 mA/cm2, followed by the highest intensity at indi-
Current Density (mA/cm2)
cated voltage.
b
External quantum efficiency.
Luminance efficiency.
Power efficiency.
c
Fig. 8. External quantum efficiency (g) versus current density (J) plot of
device VII using the neat film of compounds 4a–4f as the emitter.
d

612
Y.-S. Lee et al. / Organic Electronics 11 (2010) 604–612
nescence significantly. Among all devices the one made
with 4f performed the best, which exhibited an optimal
1.2% quantum efficiency (Fig. 8), along with a maximal
brightness close to 10,000 cd/m2 (Fig. 7).
References
[1] (a) Y. Nishizuka, Nature 308 (1984) 693–698;
(b) S. Mahboobi, E. Eibler, M. Koller, S. Kumar KC, A. Popp, J. Org.
Chem. 64 (1999) 4697;
(c) S. Mahboobi, I. Dechant, H. Reindl, H. Pongratz, A. Popp, D.
Schollmeyer, J. Heterocycl. Chem. 37 (2000) 307.
[2] (a) T.S. Yeh, T.J. Chow, S.H. Tsai, C.W. Chiu, C.X. Zhao, Chem. Mater.
18 (2006) 832;;
4. Conclusions
In summary, asymmetric indolylmaleimide derivatives
with different aryl substituents at C(3) and C(4) were syn-
thesized and applied successfully to the fabrication of
OLED devices. These devices were made with neat films
of the dyes, and exhibited stable red emission with CIE
coordinate very close to pure red. This work has demon-
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ture design of red maleimide dyes. Other potential applica-
tions requires more complicated structures, such as ion
sensors and solar cell materials, may also become feasible
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Acknowledgements
This work was supported by Academia Sinica and the
National Science Council of the Republic of China.
Appendix A. Supplementary data
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Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.orgel.2009.
12.025.