Novel formation of diimidazo[1,2-a:2′,1′-c]quinoxaline derivatives and their optical properties

Article abstract of DOI:10.1039/c1ob06002c

Reaction of 1,2-di(imidazolyl)benzene treated with n-BuLi proceeded to give diimidazo[1,2-a:2′,1′-c]quinoxaline in the presence of iodine or Pd(PPh₃)₄. Blue fluorescence was observed from 3,10-diarylated diimidazoquinoxalines with hi

Full text of DOI:10.1039/c1ob06002c

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Novel formation of diimidazo[1,2-a:2¢,1¢-c]quinoxaline derivatives and their
optical properties†
Shoji Matsumoto,*a Erdenebolor Batmunkh,a Motohiro Akazome,a Yoshiyuki Takatab and Michiko Tamanob
Received 22nd June 2011, Accepted 1st July 2011
DOI: 10.1039/c1ob06002c
Reaction of 1,2-di(imidazolyl)benzene treated with n-BuLi
proceeded to give diimidazo[1,2-a:2¢,1¢-c]quinoxaline in the
presence of iodine or Pd(PPh3)4. Blue fluorescence was
observed from 3,10-diarylated diimidazoquinoxalines with
high quantum yield. They were also applied to organic light-
emitting devices as emitters, in which the diphenyl derivative
emits a nearly pure blue light.
Heteroaromatics containing nitrogen atoms are extensively
utilized in the field of biological and functional materials.
In view of the example of optoelectronics, 3-(4-biphenyl)-
4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ)1 and 1,3,5-
tris(1-phenylbenz[b]imidazol-2-yl)benzene (TPBI) are used as
electron-transporting materials in organic electroluminescent de-
Scheme 1 Structural representations of 1, 2, 3, and 4.
vices. Focused on imidazole and benz[b]imidazole compounds,
their iridium, europium, and boron complexes have been in-
the novel formation of 3 and arylated 4, and their optical and
vestigated as luminescent materials for organic light-emitting
electroluminescent properties. The coupling reaction between the
diodes (OLEDs).2–4 While there have been many studies on the
two imidazole rings of 1,2-di(imidazolyl)benzene is achieved to
luminescence characteristics of the imidazole derivatives,5 there
give 3, which can be utilized for further functionalization. We also
reveal the optical properties of 4 that emit blue fluorescence with
high quantum yields.
For the efficient preparation of the diimidazo[1,2-a:2¢,1¢-
have been few reports on the electroluminescent properties of non-
chelated imidazole derivatives.6 We have been investigating the
optical properties of 2,2¢-bipyrrole derivatives7 and have found
that dipyrrolo- and diindolo[1,2-a:2¢,1¢-c]quinoxalines (1 and 2,
c]quinoxaline structure, we tried an intramolecular coupling reac-
respectively) exhibit fluorescence with a good quantum yield (UF
tion between the two imidazole rings of 1,2-di(imidazolyl)benzene
using iodine, a procedure which was used in the synthesis of 1
~ 0.55).7a When the pyrrole moieties of 1 are replaced by imidazole
moieties, it is expected that the fluorescence properties of the re-
and 2.7a 1,2-Di(imidazolyl)benzene was obtained in 87% yield
sulting diimidazo[1,2-a:2¢,1¢-c]quinoxaline (3) change significantly
by a Cu2O-catalysed coupling reaction (Scheme 2).9 When the
(Scheme 1). To the best of our knowledge, there has been only one
reaction of 1,2-di(imidazolyl)benzene with iodine was conducted
report published on the construction of a diimidazo[1,2-a:2¢,1¢-
in CH3CN at 80 ◦C, an unidentified product with low solubility in
c]quinoxaline structure by Ebrahimlo et al.8 They obtained a 20%
the solution was obtained. Therefore, we examined the reaction of
yield of the diimidazoquinoxaline derivative by the reaction of
imidazole anions with iodine, which led to the iodination reaction
2,3-dichloroquinoxaline with a specific isoxazolone derivative. The
at one imidazole ring followed by a substitution reaction by attack-
literature did not deal with its optical properties. Herein, we report
ing the other imidazole ring. When 1,2-di(imidazolyl)benzene was
treated with one molar amount of iodine after anion formation
aDepartment of Applied Chemistry and Biotechnology, Graduate School
using two equivalents of n-BuLi, the desired reaction proceeded to
give diimidazo[1,2-a:2¢,1¢-c]quinoxaline (3) in 71% yield at 45 ◦C
of Engineering, Chiba University, 1-33 Yayoicho, Inageku, Chiba, 263-
8522, Japan. E-mail: smatsumo@faculty.chiba-u.jp; Fax: +81-43-290-3401;
(Scheme 2, Method A). After further experiments, it was found
Tel: +81-43-290-3369
bTOYO INK SC HOLDINGS, CO., LTD., 27 Wadai, Tsukuba City, Ibaraki,
300-4247, Japan
that in this reaction, iodine could be replaced by a catalytic
amount of Pd(PPh3)4, also obtaining 3 in 71% yield (Method B).
Although the reaction mechanism is not clear, two equivalents of
n-BuLiwere necessary since the reaction withone molar equivalent
of n-BuLi resulted in recovery of the starting material. From
these investigations, we obtained 3 in 62% yield (in two steps)
† Electronic supplementary information (ESI) available: Experimental
procedures and characterization for all the new compounds, fabrication
and characterization methods for OLEDs, energy level diagram of EL
device, and the EL spectra of 4e with nondoped emitting layer for various
applied voltages. See DOI: 10.1039/c1ob06002c
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Scheme 2 Formation of 3 and 4.
from commercially available 1,2-dibromobenzene and imidazole.
Furthermore, we perceived that owing to the structural simplicity
of 3, the introduction of various substituents was possible. In fact,
the introduction of aryl substituents at the 3 and 10 positions of
3 was achieved to give 4 in moderate yield using a Pd-catalysed
coupling reaction according to the procedure reported by Miura
et al.10
The absorption and fluorescence spectra of 3 and 4 were
measured in THF (Fig. 1 and 2), and their optical properties
are summarized in Table 1. The absorption maximum (lmax) of 3
was observed at 309 nm, which is a shorter wavelength than that
of 1. This shift can be rationalized by investigations using density
functional theory (DFT) calculations with B3LYP/6-31+G level.
The orbital shapes of the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) of
3 resemble those of 1 (Fig. 3). The energy level of 3 and 1 is
significantly affected the HOMO because the orbital of HOMO
largely lies on the pyrrole and imidazole groups of 1 and 3,
respectively. Therefore, the difference between the energy level of
the HOMO of 3 and that of 1 is larger than the difference between
the energy levels of the LUMO, resulting in the observed blue shift
Fig. 1 UV-Vis absorption spectra of 3, and 4a–4f in THF (3.0 ¥ 10-5 M).
of the absorption peak. Blue shift of the emission peak of 3 (lem
=
367 nm), in comparison with that of 1, was also observed. The
Table 1 Optical properties of 3 and 4
Compound
l
max(nm)a [e (M-1 cm-1)]
l
em(nm)b [UF]
CIE (x, y)
3
309 [10,400]
307.5d [4,800]
335 [11,500]
336 [14,300]
321 [12,800]
335 [14,100]
343 [14,200]
366 [16,800]
367 [0.72]c
367e [0.70]c
428 [0.82]f
434 [0.76]f
415 [0.47]f
441 [0.67]f
432 [0.93]f
454 [0.68]f
---
---
Fig. 2 Fluorescence spectra of 3, and 4a–4f in THF (3.0 ¥ 10-7 M). Excited
at lmax
.
4a
4b
4c
4d
4e
4f
0.142, 0.133
0.143, 0.155
0.142, 0.144
0.144, 0.220
0.140, 0.106
0.156, 0.342
fluorescence quantum yield (UF) increased from 0.43 (1) to 0.72
(3). Furthermore, by changing a solvent from THF to CH3CN,
the emission peak of 3 was not affected, whereas a solvent effect
was observed in the case of 1 (R H). Thus, we found that the
various properties of 3 could be changed simply by replacing the
pyrrole moieties of 1 with the imidazole moieties.
1
321 [10,600]
320d [10,700]
416 [0.43]f
434e [0.17]c
---
---
a Measured in THF (3.0 ¥ 10-5 M). b Measured in THF (3.0 ¥ 10-7 M).
Excited at lmax
c Determined by p-terphenyl (UF = 0 87, excited at 265
nm) as a standard. d Measured in CH3CN (3.0 ¥ 10-5 M). e Measured in
f Determined by quinine sulfate
The absorption and emission spectra of 4a bearing two phenyl
rings at the 3 and 10 positions of 3 exhibited a red shift. lem was
reached in a blue region (428 nm), and UF was slightly increased
in comparison with that of 3. When methyl or methoxy groups
were introduced at the para position in the phenyl rings of 4a,
.
CH3CN (3.0 ¥ 10-7 M). Excited at lmax
.
(UF = 0.55, excited at 366 nm) as a standard.
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Table 2 Performance of EL devices containing compounds 4a and 4ea
Deviceb
Compound
lEL (nm)
Voc (V)
Lmax (cd m-2)
PE (lm W-1)
CE (cd A-1)
EQE (%)
CIE (x, y)
A
A
B
B
4a
4e
4a
4e
420
445
437
438
12
6
5
123 [15]
320 [11.5]
4,116 [11]
292 [9]
0.12 [12.5]
0.55 [5]
0.55 [5]
0.42 [12.5]
0.91 [5.5]
0.88 [5]
0.03c
0.96c
1.12c
0.06c
0.161, 0.132
0.150, 0.111
0.150, 0.104
0.202, 0.226
5
0.08 [4.5]
0.11 [4.5]
a Voc = turn-on voltage, Lmax = maximum luminance, PE = powere efficiency, CE = current efficiency. EQE = external quantum efficiency. Values in brackets
show the applied voltage (V). b Device A: ITO (150 nm)/PEDOT : PSS (40 nm)/4 : PVK : PBD (3 : 65 : 32) (70 nm)/TPBI (30 nm)/LiF (1 nm)/Al (150
nm). The hole-transporting and emitting layers were prepared by the spin-coating method. Device B: ITO (150 nm)/a-NPD (40 nm)/4 (30 nm)/TPBI
(30 nm)/LiF (1 nm)/Al (200 nm). Each layers were fabricated by the vacuum deposition method. c Estimated at 0.4 mA.
Fig. 4 Photographs of the emitted light from 4a, 4b, 4e, and 4f.
441 nm, UF = 0.33; 4e: lem = 430 nm, UF = 0.30) (Fig. 5).
Thus, they can give the light-emission in the aggregated state.
The performance characteristics of devices using 4a and 4e are
summarized in Table 2. First, we examined Device A fabricated
by a spin-coating method to produce an emitting layer with
the following configuration: ITO (150 nm)/PEDOT : PSS (40
nm)/4 : PVK : PBD (3 : 65 : 32)13 (70 nm)/TPBI (30 nm)/LiF
(1 nm)/Al (150 nm) (ITO: indium tin oxide; PEDOT : PSS:
poly(ethylenedioxythiophene) : poly(stylenesulfonate);
PVK:
poly(N-vinylcarbazole); PBD: 2-(4-biphenyl)-5-(4-tert-
butylphenyl)-1,3,4-oxadiazole). While the electroluminescent
peak (lEL) of 4a was at 420 nm (Fig. 5), a heterogeneity in
the light-emitting layer was observed. However, the device
containing 4e gave fine luminance at 437 nm in the blue region
(CIE coordinates: x = 0.150 and y = 0.111) without any surface
irregularity. The maximum luminance of the electroluminescent
(EL) device containing 4e was 320 cd m-2 at 11.5 V.
Fig. 3 Orbitals and energies of (a) LUMO and (b) HOMO of 1 and 3
calculated by DFT/B3LYP/6-31+G.
producing 4b and 4d, respectively, their emission peaks shifted
to a longer wavelength than that of 4a. Electron-withdrawing
substituents such as trifluoromethyl (4e) and formyl groups (4f)
also gave red shift of absorption and emission peaks. 4c bearing
a methyl group at the ortho position of the phenyl rings will have
little p-conjugation between the parent diimidazoquinoxaline and
the aryl substituents owing to steric hindrance. The absorption
and emission peaks of 4c were observed to have a blue shift and
to decrease UF (0.47) lower than that of 3 and other 4. Thus,
conjugation with the substituted aryl groups is to some extent
important in maintaining a high fluorescence quantum yield.
The fluorescence emissions of 4a–4f are exhibited in the
light-blue to blue region (Fig. 4). In OLEDs, blue fluorescent
materials can serve either as blue light sources11 or as host
materials for lower energy fluorescent or phosphorescent
dyes.12 Because the fluorescence of 4a and 4e is almost in the
pure blue region according to their Commission International
Fig. 5 Fluorescence spectra in solid state (green line) and EL spectra
(blue line from device A (spin-coating method), and red line from device
B (vacuum deposition method)) of (a) 4a and (b) 4e.
Recently, OLED devices with an nondoped emitting layer have
also attracted considerable attention for device fabrication because
the doping method requires complicated evaporation techniques
with precise process control.14 Therefore, we also examined the
performance of Device B, which was nondoped and prepared by
´
de l’Eclairage (CIE) chromaticity coordinates (Table 1), we
carried out preliminary investigations of the possibility of using
them as emitters in OLEDs. Both of 4a and 4e showed the
fluorescence even in solid state with moderate UF (4a: lem
=
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the vacuum deposition method, with the following configuration:
ITO (150 nm)/a-NPD (40 nm)/4 (30 nm)/TPBI (30 nm)/LiF
(1 nm)/Al (200 nm) (a-NPD: 4,4¢-bis((1-naphthyl)phenylamino)-
1,1¢-diphenyl). 4a exhibited a nearly pure blue EL spectrum with
CIE coordinates of x = 0.150 and y = 0.104, which are close to
the standard blue values of the National Television Standards
Committee (NTSC) (x = 0.14, y = 0.08). The device using 4a
demonstrated good performance, with a maximum luminance
of 4,116 cd m-2 and a current efficiency of 0.88 cd A-1. Its
external quantum efficiency was 1.12% at 0.4 mA. However, the
EL spectrum of 4e exhibited an unintentional color change when
the applied voltage was increased (Fig. S2†).
In summary, diimidazo[1,2-a:2¢,1¢-c]quinoxaline derivatives
were synthesized, and their optical properties were investi-
gated. The new methods to construct a diimidazo[1,2-a:2¢,1¢-
c]quinoxaline skeleton were achieved by the reaction of an-
ionic 1,2-di(imidazolyl)benzene with iodine or a Pd-catalyst with
good yields. Enhancement of the fluorescent quantum yield was
achieved by changing the pyrrole moieties of 1 to imidazole moi-
eties. Blue fluorescence with a high quantum yield was obtained
from4 bearing aryl groups at the 3 and 10 positions of 3. We also
showed that 4 have the potential to be used as efficient emitting
materials in OLEDs. Our investigations to further improve the EL
properties and on the use of diimidazoquinoxalines as fluorescence
sources are under way.
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