A novel class of emitting amorphous molecular materials as bipolar radical formants: 2-{4-[Bis(4-methylphenyl)amino]phenyl}-5-(dimesitylboryl)thiophene and 2-{4-[Bis(9,9-dimethylfluorenyl)amino]phenyl}-5-(dimesitylboryl)thiophene [9]

Full text of DOI:10.1021/ja0023332

J. Am. Chem. Soc. 2000, 122, 11021-11022
11021
A Novel Class of Emitting Amorphous Molecular
Materials as Bipolar Radical Formants:
We present here synthesis and characterization of a novel class
of high-performance blue-green and green emitting amorphous
molecular materials, 2-{4-[bis(4-methylphenyl)amino]phenyl}-
2
5
2
5
-{4-[Bis(4-methylphenyl)amino]phenyl}-
5
-(dimesitylboryl)thiophene (PhAMB-1T) and 2-{4-[bis(9,9-
dimethylfluorenyl)amino]phenyl}-5-(dimesitylboryl)thiophene
FlAMB-1T). To the best of our knowledge, these materials are
-(dimesitylboryl)thiophene and
-{4-[Bis(9,9-dimethylfluorenyl)amino]phenyl}-
-(dimesitylboryl)thiophene
(
the first definite examples having the desired bipolar radical
formant character that allows both stable cation and anion radicals,
intense fluorescent characteristics, and the capability to form stable
amorphous glasses with high glass-transition temperatures (T s)
g
and uniform thin films by vacuum deposition.
Yasuhiko Shirota,* Motoi Kinoshita, Tetsuya Noda,
Kenji Okumoto, and Takahiro Ohara
Department of Applied Chemistry
Faculty of Engineering, Osaka UniVersity
Yamadaoka, Suita, Osaka 565-0871, Japan
These novel emitting amorphous molecular materials are
different in molecular architecture from the hitherto reported
emitting materials containing heteroaromatic rings such as oxa-
ReceiVed June 28, 2000
6
7
diazole or triazole. The incorporation of the triphenylamine and
dimesitylboryl moieties, which are π-conjugated through the
thiophene ring, is intended to provide both electron-donating and
Amorphous molecular materials have demonstrated their suit-
ability and versatility for use in organic light-emitting diodes
1
(
OLEDs). In this work, we focused our attention to the design
-accepting properties, respectively, and to facilitate formation of
and synthesis of a new and better generation of emitting materials.
In any type of OLEDs consisting of a single emitting layer or of
multilayers using additional charge-transport layers, the emitting
layer functions as the recombination center for the holes and
electrons injected from the anode and cathode. The generated
electronically excited-state molecule of the emitting layer either
emits luminescence or serves as a host that transfers its excitation
energy to a luminescent dopant. Therefore, materials for use in
the emitting layer should meet the requirements of energy level
matching for charge carrier injection and acceptance of both holes
and electrons, and hence should desirably possess bipolar character
to permit the formation of both stable cation and anion radicals.
In addition, the emitting materials should form homogeneous thin
films and exhibit intense fluorescence. The fulfillment of these
material requirements is expected to lead to enhanced performance
and operational stability of OLEDs.
amorphous glasses due to their nonplanar molecular structures.
Scheme 1
3
Tris(8-quinolinolato)aluminum (Alq ) has been most widely
2
used as a green emitter or a host material for luminescent dopants.
It undergoes reversible reduction, but its anodic oxidation is
3
irreversible. The instability of the Alq
3
cation radical leads to
the long-term degradation of Alq
3
-based OLEDs, causing a
significant decrease in the luminescence efficiency.4 Other
emitting materials, including only scarce examples of materials
with bipolar character, e.g., compounds having an 1,3,4-oxadiazole
moiety, have also been reported.5 However, redox properties
of most of the reported materials for the emitting layer have not
been studied in any detail, some materials undergoing irreversible
oxidation or reduction. Ruthenium(II) complexes have bipolar
PhAMB-1T and FlAMB-1T were synthesized by the reaction
of dimesitylboron fluoride with lithiated N,N-bis(4-methylphenyl)-
-(2-thienyl)aniline or N,N-bis(9,9-dimethylfluorenyl)-4-(2-thienyl)-
4
-10
aniline in tetrahydrofuran (THF) under nitrogen atmosphere in
ca. 50% yield. The synthetic procedure of FlAMB-1T is shown
in Scheme 1. These compounds were purified by silica gel column
chromatography, followed by recrystallization from solution, and
identified by various spectroscopic methods, mass spectrometry,
and elemental analysis.1
1,12
PhAMB-1T and FlAMB-1T display intense blue-green and
green fluorescence, respectively. The electronic absorption and
fluorescence spectra of PhAMB-1T and FlAMB-1T in a THF
(
6) (a) Hamada, Y.; Adachi, C.; Tetsuo, T.; Saito, S. Jpn. J., Appl. Phys.
1
9
1
992, 31, 1812. (b) Tamoto, N.; Adachi, C.; Nagai, K. Chem. Mater. 1997,
, 1077. (d) Antoniadis, H.; Inbasekaran, M.; Woo, E. P. Appl. Phys. Lett.
998, 73, 3055.
(7) Kido, J.; Kimura, M.; Nagai, K. Chem. Lett., 1996, 47.
9
character, but do not form uniform thin films by themselves.
(8) Salbeck, J.; Yu, N.; Bauer, J.; Weiss o¨ rtel, F.; Bestgen, H. Synth. Met.
997, 91, 209.
1
*
To whom correspondence should be addressed. Phone: +81-6-6879-
(9) (a) Lyons, C. H.; Abbas, E. D.; Lee, J.-K.; Rubner, M. F. J. Am. Chem.
7
364. Fax: +81-6-6879-7367. E-mail: shirota@ap.chem.eng.osaka-u.ac.jp.
Soc., 1998, 120, 12100. (b) Gong, X.; Ng, P. K.; Chan, W. K. AdV. Mater.
1998, 10, 1337. (c) Handy, E. S.; Pal, A. J.; Rubner, M. F. J. Am. Chem.
Soc., 1999, 121, 3525.
(
1) Shirota, Y. J. Mater. Chem. 2000, 10, 1 and references therein.
2) (a) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (b)
(
Tang, C. W.; VanSlyke, S. A.; Chen, C. H. J. Appl. Phys. 1989, 65, 3610.
(10) (a) Noda, T.; Ogawa, H.; Noma, N.; Shirota, Y. AdV. Mater. 1997, 9,
(
3) Papadimitrakopoulos, F.; Zhang, X.-M.; Thomsen III, D. L.; Higginson,
K. A. Chem. Mater. 1996, 8, 1363.
4) Popovic, Z. D.; Aziz, H.; Hu, N.-X.; Hor, A.-M.; Xu, G. Synth. Met.
000, 111-112, 229.
720. (b) Noda, T.; Ogawa, H.; Shirota, Y. AdV. Mater. 1999, 11, 283.
+
1
3
(11) PhAMB-1T: MS: m/e 603 (M ). H NMR (600 MHz, CDCl ) δ
(
(ppm): 7.48 (2H, d, J ) 8.5), 7.38 (1H, d, J ) 3.7), 7.34 (1H, d, J ) 3.7),
7.07 (4H, d, J ) 8.3), 7.00 (4H, d, J ) 8.3), 6.96 (2H, d, J ) 8.5), 6.82 (4H,
2
(5) (a) Hamada, Y.; Sano, T.; Fujita, M.; Fujii, T.; Nishio, Y.; Shibata, K.
s), 2.31 (6H, s), 2.30 (6H, s), 2.17 (12H, s). Anal. Calcd for C42H42BNS: C,
Jpn. J. Appl. Phys. 1993, 32, L514. (b) Hamada, Y.; Sano, T.; Fujii, H.; Nishio,
Y.; Takahashi, H.; Shibata, K. Jpn. J. Appl. Phys. 1996, 35, L1339.
83.57; H, 7.01; N, 2.32, B, 1.79; S, 5.31. Found: C, 83.44; H, 7.14; N, 2.36;
S, 5.30.
1
0.1021/ja0023332 CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/21/2000

11022 J. Am. Chem. Soc., Vol. 122, No. 44, 2000
Communications to the Editor
of PhAMB-1T and FlAMB-1T showed only a broad halo in their
X-ray diffraction patterns. The glassy state of these materials is
very stable, no crystallization being observed even on heating
g
above their T s.
Triple-layer OLEDs using 4,4′,4′′-tris[3-methylphenyl(phenyl)-
amino]triphenylamine (m-MTDATA)13 as a hole-transport layer,
3
PhAMB-1T or FlAMB-1T as an emitting layer, and Alq or 5,5′-
bis(dimesitylboryl)-2,2′-bithiophene (BMB-2T)14 as an electron-
transport layer were fabricated by sequential vacuum deposition
of organic materials onto an indium-tin-oxide (ITO)-coated glass
-
1
-5
substrate at a deposition rate of 2-3 Å s at 10 Torr, followed
by vacuum deposition of an alloy of magnesium and silver
(
volume ratio: ca. 10:1) onto the electron-transport layer. The
fabricated devices, ITO/m-MTDATA(300 Å)/PhAMB-1T(200 Å)/
Alq (300 Å)/MgAg (device a) and ITO/m-MTDATA(300 Å)/
3
FlAMB-1T(200 Å)/BMB-2T(300 Å)/MgAg (device b), emitted
bright blue-green and green light, respectively, when a positive
voltage was applied to the ITO electrode. The electroluminescence
spectra of the two devices were in good agreement with the
photoluminescence spectra of the films of PhAMB-1T and
FlAMB-1T, respectively. These results indicate that the electro-
luminescence originates from the excited singlet state of PhAMB-
Figure 1. Cyclic voltammograms of PhAMB-1T (solid line) and FlAMB-
-
3
-3
1
T (dashed line) (1.0 × 10 mol dm ) in tetrahydrofuran containing
-3
tetra-n-butylammonium perchlorate (0.1 mol dm ). Cyclic voltammetry
was carried out using a platinum disk (1.6 mm in diameter) and a platinum
wire as the working and counter electrodes, respectively, and Ag/AgNO
3
-3
(
0.01 mol dm in acetonitrile) as the reference electrode. Scan rate: 500
-
1
mV s
.
1T for device a and FlAMB-1T for device b, generated by the
solution show bands with maxima (λmax) at 423 (log ꢀ ) 4.6)
and 515 nm (fluorescence quantum yield φ ) 0.58) and with
max at 435 (log ꢀ ) 4.6) and 537 nm (φ ) 0.50), respectively.
recombination of holes and electrons injected from the ITO and
MgAg electrodes into PhAMB-1T or FlAMB-1T by a stepwise
process via the hole- and electron-transport layers, respectively.
The emission started at a driving voltage of 3 V for both
devices. The devices exhibited high performance with a maximum
f
λ
f
These bands are understood in terms of intramolecular charge
transfer from the electron-donating triphenylamine or bifluorenyl-
amine moiety to the electron-accepting dimesitylboryl moiety.
PhAMB-1T and FlAMB-1T possess the desired bipolar char-
acter, generating stable cation and anion radicals. Figure 1 shows
cyclic voltammograms of PhAMB-1T and FlAMB-1T. Both
compounds showed reversible redox properties, exhibiting one
anodic and cathodic wave on oxidation and reduction, respec-
tively, together with the corresponding cathodic and anodic waves.
The half-wave oxidation and reduction potentials were 0.62 and
-
2
luminance of 12500 cd m at a driving voltage of 16 V, a
-
1
luminous efficiency of 1.1 lm W , and an external quantum
-
2
efficiency of 0.8% at a luminance of 300 cd m for device a
-
2
and a maximum luminance of 23700 cd m at a driving voltage
-
1
of 16 V, a luminous efficiency of 1.3 lm W , and an external
-
2
quantum efficiency of 1.0% at a luminance of 300 cd m for
device b. The maximum luminance of device b in particular is
among the highest values compared with those reported for
OLEDs in the absence of luminescent dopants.
+
-3
-
2.13 V vs Ag/Ag (0.01 mol dm ) for PhAMB-1T and 0.58
+
-3
and -2.01 V vs Ag/Ag (0.01 mol dm ) for FlAMB-1T. When
The present study shows that the synthesized new materials,
PhAMB-1T and FlAMB-1T, function as excellent blue-green and
green emitting materials for OLEDs. They are also expected to
serve as host materials for luminescent dopants. In addition, these
bipolar emitting materials are expected to find a variety of
applications, e.g. materials for organic light-emitting electro-
chemical cells and nonlinear optics.
In summary, a novel class of high-performance blue-green and
green emitting amorphous molecular materials with the desired
bipolar radical formant character, PhAMB-1T and FlAMB-1T,
have been created. They have proven to function as excellent
emitting materials for OLEDs. The present study presents a new
guideline for the molecular design of emitting amorphous
molecular materials, paving the way for the development of further
new emitting amorphous molecular materials for various kinds
of application.
the scan was repeated in the region between -2.4 and 0.8 V vs
+
-3
Ag/Ag (0.01 mol dm ), the same trace was obtained.
The new materials readily form stable amorphous glasses.
While FlAMB-1T was obtained as polycrystals by recrystallization
from THF/ethanol, PhAMB-1T was obtained as amorphous
powders despite attempted recrystallization from solution. Dif-
ferential scanning calorimetry performed on a crystalline sample
of FlAMB-1T showed that upon heating, it melted at 274 °C to
give an isotropic liquid. When the isotropic liquid was cooled on
standing in air, it formed a transparent, stable amorphous glass
via a supercooled liquid state. When the glass sample was again
heated, glass transition took place at 124 °C. Likewise, the
g
PhAMB-1T glass exhibited a T of 84 °C. The formation of the
glassy state was also evidenced by X-ray diffraction and polarizing
microscopy. While the crystalline sample of FlAMB-1T showed
sharp peaks characteristic of the crystal, the amorphous glasses
(
12) FlAMB-1T: MS: m/e 807 (M+). 1H NMR (600 MHz, THF-d
8
) δ
JA0023332
(
ppm): 7.69-7.61 (6H, m), 7.53 (1H, d, J ) 4.0), 7.40 (2H, d, J ) 7.3), 7.37
2H, d, J ) 3.7), 7.30 (2H, d, J ) 2.1), 7.26 (2H, t, J ) 7.3), 7.21 (2H, t, J
(
(13) (a) Shirota, Y.; Kobata, T.; Noma, N. Chem. Lett. 1989, 1145. (b)
Shirota, Y.; Kuwabara, Y.; Inada, H.; Wakimoto, T.; Nakada, H.; Yonemoto,
Y.; Kawami, S.; Imai, K. Appl. Phys. Lett. 1994, 65, 807.
)
7.3), 7.15 (2H, d, J ) 8.6), 7.08 (2H, d, J ) 8.2), 6.82 (4H, s), 2.27 (6H,
s), 2.15 (12H, s), 1.40 (12H, s). Anal. Calcd for C58
H54BNS: C, 86.22; H,
6
.74; N, 1.73; B, 1.34; S, 3.97. Found: C, 86.27; H, 6.99; N, 1.73; S, 3.85.
(14) Noda, T.; Shirota, Y. J. Am. Chem. Soc. 1998, 120, 9714.