Synthesis and properties of disk-shaped bipolar symmetric cyclic triindole derivative

Article abstract of DOI:10.3987/COM-08-S(F)22

The 1,3,4-oxadiazole-substituted donor-π-acceptor cyclic triindole, 5,10,15-tris(2-ethylhexyl)-2,7,12-tris-[4-(4-t-butylphenyl-1,3,4-oxadiazol-2-yl)-phenyl]-10,15-dihydro-5H-5,10,15-triazadiindeno[l,2-a;1′,2′-c]fluorene (3), showed an electroluminescence

Full text of DOI:10.3987/COM-08-S(F)22

HETEROCYCLES, Vol. 77, No. 1, 2009
595
HETEROCYCLES, Vol. 77, No. 1, 2009, pp. 595 - 602. © The Japan Institute of Heterocyclic Chemistry
Received, 27th May, 2008, Accepted, 2nd July, 2008, Published online, 3rd July, 2008.
DOI: 10.3987/COM-08-S(F)22
SYNTHESIS AND PROPERTIES OF DISK-SHAPED BIPOLAR
SYMMETRIC CYCLIC TRIINDOLE DERIVATIVE
Hidetaka Hiyoshi,a* Tetsuya Aoyama,b Tatsuo Wada,b Takaaki Sonoda,c and
Shuntaro Matakac
aPlanning Section, Development Department, Ihara Chemical Industry Co., Ltd.,
4-26 Ikenohata 1-chome, Taito-ku, Tokyo, 110-0008, Japan
bSupramolecular Science Laboratory, RIKEN (The Institute of Physical and
Chemical Research), 2-1 Hirosawa, Wako, Saitama, 351-0198, Japan
cInstitute for Materials Chemistry and Engineering, Kyushu University, 6-1
Kasuga-koh-en, Kasuga, Fukuoka, 816-8580, Japan
Abstract - The 1,3,4-oxadiazole-substituted donor-π-acceptor cyclic triindole,
5,10,15-tris(2-ethylhexyl)-2,7,12-tris-[4-(4-t-butylphenyl-1,3,4-oxadiazol-2-yl)-
phenyl]-10,15-dihydro-5H-5,10,15-triazadiindeno[1,2-a;1',2'-c]fluorene
(3),
showed an electroluminescence in a single-layer device which was fabricated by a
spin-coating process.
INTRODUCTION
Organic light emitting diodes (OLEDs) had seen increased to interest
in investigating the use of nitrogen containing hetero aromatic
compounds with either electron-donating or electron-withdrawing
abilities. Single-layer OLED devices fabricated by a wet process
have seen much attention due to a simple process and economical
profits, and the preparation of high performance materials suitable for
a wet process is essential for the development of OLEDs.1 Recently,
we prepared the donor-π-acceptor type triindole, 5,10,15-tris-
CN
N
NC
N
N
NC
1
(2-ethylhexyl)-2,7,12-cyano-10,15-dihydro-5H-5,10,15-triazadiindeno- Figure 1. Tricyano derivative 1
This paper is dedicated to Professor Emeritus Keiichiro Fukumoto on the occasion of his 75th birthday.

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HETEROCYCLES, Vol. 77, No. 1, 2009
[1,2-a;1',2'-c]fluorene2 (1) (Figure 1), which showed a relatively strong fluorescence due to intramolecular
charge transfer (ICT) between the symmetric cyclic triindole ring of a strongly electron-donating nature
and electron-withdrawing cyano groups at the terminal positions.3 A multi-layer electroluminescence
(EL) device with 1 as a light-emitting material was fabricated, however, this device did not show any
electroluminescence
from
triindole-layer.
The
device
exhibited
an
emission
from
tris-(8-hydroxyquinolinato)aluminum (Alq3). These results suggest that 1 does not have
electron-transporting and hole-blocking abilities. Therefore, 1 was considered unsuitable as an emitting
material for a single-layer EL device.
In this paper, we designed a new donor-π-acceptor type triindole 3, which combined the 1,3,4-oxadiazole
ring as an acceptor part and a symmetric triindole as a donor part. The 1,3,4-oxadiazole unit has been
widely used as an electron-transporting hole-blocking (ETHB) material.4
RESULTS AND DISCUSSION
For producing a single layer EL device, highly efficient blue light emitting materials are needed to make a
full-color EL display with a color filter. 5,10,15-Tris-(2-ethylhexyl)-2,7,12-tris-(4-cyanophenyl)-
10,15-dihydro-5H-5,10,15-triazadiindeno[1,2-a;1',2'-c]fluorene (2),3 which emitted a blue fluorescence
(λem = 480 nm) with a moderate relative quantum yield (φ = 0.47), was then selected and evaluated as an
emitting material for the OLEDs. However, 2 did not have any film forming ability based on the
differential scanning calorimetry (DSC).
R1
N
R1
R1=
N
CN
N
R1
2
Figure 2. Tricyanophenyl derivative 2
We designed a new triindole derivative 3 which might have good film morphology and show an
electroluminescence even in a single-layer device. For improvement of the film forming ability, bulky
alkyl groups were introduced to prevent aggregation in order to make allow the fabrication of the device

HETEROCYCLES, Vol. 77, No. 1, 2009
597
by a spin-coating process. We then prepared a new donor-π-acceptor type triindole 3 by combining the
1,3,4-oxadiazole rings as the acceptor and the symmetric cyclic triindole ring as the donor part (Figure 3)
by the Suzuki-Miyaura coupling reaction (Scheme 1 and 2).
R1
N
N
N
R1
R1=
N
O
N
tBu
R1
3
Figure 3. 1,3,4-Oxadiazolyl triindole derivative 3
A corresponding boronic acid was prepared by the reaction using bis(pinacolate)diborone under Miyaura
reaction conditions in the presence of a palladium (II) catalyst (Scheme 1).5 The reaction of
2-(4-t-butylphenyl)-5-(4-bromophenyl)-1,3,4-oxadiazole6 (4) with bis(pinacolate)diborone produced
2-(4-t-butylphenyl)-5-[4-(4,4,5,5-tetramethyl-1,3,2-dioxobororan-2-yl)phenyl]-1,3,4-oxadiazole (5) in
45% yield. (Scheme 1)
O
O
O
O
B B
N N
O
N N
O
O
tBu
B
tBu
Br
Pd(dppf)Cl2
AcOK
O
DMSO
4
5
Scheme 1
The desired 1,3,4-oxadiazolyl triindole 3 was produced by the reaction of 5,10,15-tris-(2-ethylhexyl)-
2,7,12-tribromo-10,15-dihydro-5H-5,10,15-triazadiindeno[1,2-a;1',2'-c]fluorene (6), which was perpared
by the cyclotrimerization reaction of N-(2-ethylhexyl)-5-bromoindolin-2-one,3 with 5 in refluxing toluene
in the presence of a palladium (0) catalyst in 55% yield.

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HETEROCYCLES, Vol. 77, No. 1, 2009
Br
N
Br
Pd(PPh3)4
N
3
5
N
sat. NaHCO3 aq.
toluene, reflux, 4 h
Br
6
Scheme 2
The absorption and fluorescence spectra of 1,3,4-oxadiazolyl triindole 3 are shown in Figure 4 and Table
1. The absorption spectrum of 3 exhibited two peaks, similarly to that of 3, around 310 and around 360
nm, respectively. These were assigned to the π → π* transitions (310 nm) and charge transfer transitions
(360 nm), respectively. Although the π-conjugation system of 3 is longer than that of 2, the absorption of
3 (360 nm) showed only a slight red-shift (9 nm). This suggests that the phenyl groups of the triindole
ring and the 1,3,4-oxadiazole ring were twisted around each other. The fluorescence of 3 was observed
around 490 nm in CH2Cl2 and the relative quantum yield of 3 was determined to be 0.53 using quinine
sulfate as the reference5 as shown in Table 1. Similar to the absorption spectrum, the fluorescence one of
3 showed a 10 nm red-shift. The relative quantum yield of 3 was the largest in the donor-π-acceptor type
triindole derivatives.3
1.6
3
3
2
1.4
1.2
1
2
0.8
0.6
0.4
0.2
0
350
400
450
500
550
600
650
270
320
370
420
470
wavelength (nm)
wavelength(nm)
Figure 4. UV/Vis spectra (right) and FL spectra (left) of triindole derivatives 2 (red) and 3 (black)

HETEROCYCLES, Vol. 77, No. 1, 2009
599
Table 1. Optical properties of 2 and 3
PL
φa
0.47
0.53
Compound λmax logε λem (nm)
λem (nm) solid
351
360
4.95
5.05
480
490
461
479
2
3
a Quinine sulfate (φ= 0.55) as a standard
The electrochemical properties of 3 was studied by cyclic voltammetry measurements in CH2Cl2 in the
presence of Bu4NPF6 as the supporting electrolyte using a glassy-carbon working electrode and Ag/AgCl
couple in electrolyte solution as the reference at room temperature (Figure 5). Both the oxidation and
reductive wave were observed. The oxidation potential (Eo) was evaluated at 0.51 V as a reversible
process, however, the reduction potential (Epc) around -1.45 V was irreversible. It was supposed that the
1,3,4-oxadiazole side chain increased the electron affinity of 3 as expected. The irreversible wave in the
reduction process was attributed to the poor stability of the radical anion.
20
10
0
-10
-20
-30
-40
-50
-60
-1.6
-1.1
-0.6
-0.1
0.4
potential (V)
Figure 5. Cyclic voltammogram of 3
The thermal property of 3 was investigated by differential scanning calorimetry (DSC). The glass
transition temperature (Tg) was observed at 95 ℃ in the first cooling run. No peaks due to crystallization
or melting were observed after the second heating and cooling process. These properties are favorable for
fabricating an OLED device with 3 by a solution process.
Preliminary evaluations of the electroluminescent properties of 3 are shown in Table 2. The device was
fabricated by a spin-coat method. A chloroform solution (3/CHCl3 = 15 mg/mL) of 3 was spin-coated on
ITO, then dried in an oven at 50 ℃ under vacuo for 15 h. A blue EL spectrum was observed around 490
nm, in a single-layer device (device A), though very weak. The maximum luminance of 56 cd/m2 at 17 V
was shown in device B. The EL peaks of these devices corresponded to the PL ones.

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HETEROCYCLES, Vol. 77, No. 1, 2009
Table 2. EL properties of devices using 3 as emitting layer
current density current efficiency
(mA/cm2)
Device applied voltage (V)
(cd/A)
EL (cd/m2)
2.1
A
B
23
17
190
560
0.0013
0.026
56.0
Device A: ITO/3 (100 nm)/Al
Device B: ITO/PEDOT:PSS (25 nm)/3 (100 nm)/LiF (0.5 nm)/Al
The HOMO energy level of 3 was determined to be -5.0 eV by ultraviolet photoelectron spectroscopy
(UPS). The LUMO energy level of 3 was estimated to be -2.1 eV by combining the HOMO energy level
together with the energy band gap obtained from the edge of the absorption spectrum (Figure 6). The
HOMO energy level of 3 was relatively close to the Fermi level of the anode. On the other hand, the
energy gap between the LUMO level of 3 and the cathode was large. Thus, holes were injected into the
1,3,4-oxadiazolyl triindole layer, while a sufficient injection of electrons was prevented.
2.1b)
3.5*
3
4.8*
ITO
PEDOT
:PSS
5.0a)
5.2*
LiF/Al
* work functional7)
a) HOMO b) LUMO
Figure 6. Energy diagram of device B
CONCLUSIONS
The 1,3,4-oxadiazole-substituted triindole 3 was prepared by the Suzuki-Miyaura coupling reaction.
Compound 3 had a good film-forming ability. Due to the donor-π-acceptor property, 3 exhibited
reversible oxidation potential (E0) and irreversible reduction potential (Epc). The 1,3,4-oxadiazolyl
triindole 3 showed an electroluminescence around 490 nm in the single-layer device fabricated by a
spin-coating process. Based on this investigation, the 1,3,4-oxadiazole-substituted symmetric cyclic
triindole 3 showed possibility as an EL material in a single-layer OLED device.

HETEROCYCLES, Vol. 77, No. 1, 2009
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EXPERIMENTALS
General
1
All melting points are uncorrected. The H-NMR spectra were determined in CDCl3 using a VARIAN
Mercury 300 spectrometer. Residual solvent protons were used as the internal standard and the chemical
shifts (δ) are given relative to tetramethylsilane (TMS). The coupling constants (J) are reported in hertz
(Hz). The elemental analyses were performed at the Elemental Analytical Center, Kyushu University. The
electron impact mass spectrometry (EI-MS) spectra were recorded by a JEOL JMS-70 mass spectrometer
at 70 eV using a direct inlet system. The fast atom bombardment mass spectrometry (FAB-MS) spectra
were recorded by a JEOL JMS-70 mass spectrometer with m-nitrobenzyl alcohol (NBA) as the matrix.
The UV-vis spectra were measured by a JASCO V-570 spectrophotometer in a 1.0 cm wide quartz cell
(0.01 mM). The fluorescence spectra were measured by a HITACHI F-4500 fluorescence
spectrophotometer. The cyclic voltammetry measurements were performed by a BAS model 600
electrochemical analyzer in deaerated dichloromethane solution (0.5 mM) containing tetrabutyl
ammonium hexafluorophosphate (0.1 M) as the supporting electrolyte at 298 K (100 mV s-1). The glassy
carbon working electrode was polished with a BAS polishing alumina suspension and rinsed with acetone
before use. The counter electrode was a platinum wire. The measured potentials were recorded with
respect to an Ag/AgCl (saturated KCl) reference electrode. The analytical TLC was carried out on a
silica-gel coated glass plate (Merck 60 F254). Column chromatography was carried out on silica gel
(KANTO 60N). The reagents were purchased from commercial suppliers and used without further
purification.
Preparation
of
2-(4-t-butylphenyl)-5-[4-(4,4,5,5-tetramethyl-1,3,2-dioxobroran-2-yl)phenyl]-
1,3,4-oxadiazole (5)
A mixture of potassium acetate (309 mg, 3.15 mmol), 2-(4-t-butylphenyl)-5-(4-bromophenyl)-1,3,4-
oxadiazole 4 (1.07 g, 3 mmol), bispinacolate diborone (798mg, 3.14mmol), [1,1-bis(diphenylphosphino)-
ferocene]dichloro palladium (II) dichloromethane complex (73.5 mg, 0.09 mmol) and dimethyl sulfoxide
(30 mL) was heated at 80 ℃ for 4 h. The reaction mixture was then cooled to rt, poured into water (300
mL) and extracted with EtOAc (200 mL). The organic layer was washed with brine (100 mL), dried over
anhydrous MgSO4, and evaporated in vacuo to dryness. The residue was separated by silica gel column
chromatography eluting with hexane/EtOAc to give 5 in 45% yield (540 mg).
1
White powder: mp 193.9-195.8 ℃: EI-MS; m/z 404 (M+): H-NMR;δ1.38 (s, 21H), 7.55 (d, J=8.7 Hz,
2H), 7.96 (d, J=8.7 Hz, 2H), 8.08 (d, J=8.7 Hz, 2H), 8.13(d, J=8.7 Hz, 2H).
Anal. Calcd for C24H29BN2O3: C, 71.30; H, 7.23; N, 6.93. Found: C, 70.80; H, 7.00; N, 7.07
Preparation of 5,10,15-tris-(2-ethylhexyl)-2,7,12-tris-[4-(4-t-buthylphenyl-1,3,4-oxadiazol-2-yl)-
phenyl]-10,15-dihydro-5H-5,10,15-triazadiindeno[1,2-a;1',2'-c]fluorene (3)

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HETEROCYCLES, Vol. 77, No. 1, 2009
To a mixture of 5,10,15-tris-(2-ethylhexyl)-2,7,12-tribromo-10,15-dihydro-5H-5,10,15-triazadiindeno-
[1,2-a;1',2'-c]fluorene 6 (200 mg, 0.218 mmol), tetrakis(triphenylphosphine)palladium(0) (40 mg, 5
mol%) in toluene (40 mL), and saturated aqueous NaHCO3 (20 mL) was added 5 (600 mg, 1.49 mmol) in
EtOH (5 mL) under a nitrogen atmosphere. The resulting mixture was heated at refluxing temperature for
4 h. The reaction mixture was poured into water (300 mL) and extracted with AcOEt (200 mL). The
organic layer was washed with brine (100 mL), dried over anhydrous MgSO4, and evaporated in vacuo to
dryness. The residue was separated by silica gel column chromatography eluting with hexane/EtOAc to
give 3 in 55% yield (180 mg).
1
Yellow powder: mp 289 ℃: FAB-MS(NBA, positive), m/z 1511([M+H]+): H-NMR;δ0.60-0.40 (m,
9H), 0.70-0.60 (m, 9H), 1.20-0.70 (m, 24H), 1.40 (s, 27H), 2.10-2.00 (m, 3H), 5.02 (d, J=6.6 Hz, 6H),
7.59 (d, J=8.4 Hz, 6H), 7.71 (d, J=8.7 Hz, 3H), 7.78 (d, J=8.4 Hz, 3H), 7.98 (d, J=8.4 Hz, 6H), 8.13 (d,
J=8.4 Hz, 6H), 8.31 (d, J=8.1 Hz, 6H), 8.54 (s, 3H).
Anal. Calcd for C102H111N9O3: C, 81.08; H, 7.40; N, 8.34. Found: C, 81.41; H, 7.37; N, 8.51
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