C-9 fluorenyl substituted anthracenes: A promising new family of blue luminescent materials

Article abstract of DOI:10.1021/ol101553a

Syntheses, optical, and electrochemical properties of novel C-9 fluorenyl substituted anthracenes linked by a tetrahedral sp³-hybridized carbon atom are reported for blue light emitting materials. Remarkably, an unoptimized organic light-emitting diode based on 1-fold fluorene-functionalized anthracene 3 exhibits a radiance of 4100 cd/m² at 12 V and a maximum EL efficiency of 1.36 cd/A with color purity CIE x, y (0.157, 0.082), which is very close to the National Television System Committee standard blue.

Full text of DOI:10.1021/ol101553a

ORGANIC
LETTERS
2
010
C-9 Fluorenyl Substituted Anthracenes:
A Promising New Family of Blue
Luminescent Materials
Vol. 12, No. 17
3
874-3877
†
Jing Wang, Wen Wan,* Haizhen Jiang, Yan Gao, Xueyin Jiang,
,†
†
†
‡
‡
§
Huaping Lin, Weiming Zhao, and Jian Hao*
,†,|
Department of Chemistry, Shanghai UniVersity, Shanghai, China, School of Materials,
Shanghai UniVersity, Shanghai, China, Litewell Technology Inc., Dongguan,
Guangdong, China, and Key Laboratory of Organofluorine Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai, China
wanwen@staff.shu.edu.cn; jhao@staff. shu.edu.cn
Received July 7, 2010
ABSTRACT
3
Syntheses, optical, and electrochemical properties of novel C-9 fluorenyl substituted anthracenes linked by a tetrahedral sp -hybridized carbon
atom are reported for blue light emitting materials. Remarkably, an unoptimized organic light-emitting diode based on 1-fold fluorene-functionalized
2
anthracene 3 exhibits a radiance of 4100 cd/m at 12 V and a maximum EL efficiency of 1.36 cd/A with color purity CIE x, y (0.157, 0.082),
which is very close to the National Television System Committee standard blue.
4
Since the breakthrough discovery by Tang and VanSlyke in
organic light emitting diodes (OLEDs), significant progress
derivatives in recent years. In order to avoid the formation
of polycrystalline films and improve thermal stability, bulky
groups are often introduced to the fluorene moieties to obtain
the desired spiro materials due to its specific steric configura-
1
has been made because of their potential applications in full-
2
color large displays. Developing higher efficiency and purer
5
blue light emitting materials is still a challenge due to the
tions. Terfluorenes, oligomeric fluorenes, spirofluorene-
linked anthracenes, and the difuorene-indenofuorene com-
3
intrinsic wide band gap irrespective of the type of materials.
Because of their high solution and solid-state photolumi-
nescence (PL) quantum yields, research into blue-emitting
materials has been focused on fluorene and anthracene
(
3) (a) Cocherel, N.; Poriel, C.; Vignau, L.; Bergamini, J. F.; Rault-
Berthelot, J. Org. Lett. 2010, 12, 452–455. (b) Goel, A.; Chaurasia, S.;
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Venkatakrishnan, P.; Natarajan, P.; Huang, D. F.; Chow, T. J. J. Am. Chem.
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†
Department of Chemistry, Shanghai University.
School of Materials, Shanghai University.
Litewell Technology Inc.
Chinese Academy of Sciences.
‡
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A. P.; Kaminsky, W.; Jenekhe, S. A. AdV. Funct. Mater. 2007, 17, 863–
874. (c) Moorthy, J. N.; Natarajan, P.; Venkatakrishnan, P.; Huang, D. F.;
Chow, T. J. Org. Lett. 2007, 9, 5215. (d) Gebeyehu, D.; Walzer, K.; He,
G.; Pfeiffer, M.; Leo, K.; Brandt, J.; Gerhard, A.; Stoessel, P.; Vestweber,
H. Synth. Met. 2005, 148, 205–211.
§
|
(
(
1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913–915.
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Thompson, M. E. IEEE Trans. Electron DeVices 1997, 44, 1188–1203. (b)
Miyata, S.; Nalwa, H. S. Organic Electroluminescent Materials and DeVices;
Gordon and Breach Publishers: New York, 1997. (c) Wong, T. K. S. In
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American Scientific Publishers: Stevenson Ranch, CA, 2008; Vol. 2, pp
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4
13-472.
10.1021/ol101553a  2010 American Chemical Society
Published on Web 08/10/2010

pounds are generally regarded as the most promising
candidates for blue OLEDs.
center. This radical coupling of fluorene and anthracene was
examined by using a variety of metals, such as Zn, Fe, Cu,
6
To date, the anthracene moiety, which exhibits a highly
efficient fluorescence property, with derivatives that are one
of the best blue host materials, is often directly introduced
to fluorene Via aryl-aryl coupling. However, the introduction
of anthracene into the C-9 position of fluorene, in which the
anthracene and fluorene moieties are connected through a
2 4 2 4
Sm, in different solvents (CS , CCl , Et O, DMSO, NH Cl/
MeOH) in order to improve the yields of the desired
compounds 1 and 2. It was found that the zinc-mediated
2
radical reaction in CS solvent provided a much better yield
8
than other reaction conditions. In order to increase the
electrochemical stabilities of 1-fold fluorene-functionalized
anthracene, the methylation of 1 was applied Via deproto-
nation at the active C-9 position by n-BuLi and followed by
an electrophilic substitution reaction with methyl idoide. The
C-9 methylated 1-fold substituted derivative 3 was expected
to exhibit stronger rigidity due to the existence of a steric
effect of the methyl group.
3
tetrahedral sp -hybridized carbon atom, has not been dis-
closed. The C-9 introduction of a bulky anthracene moiety
is expected not only to increase molecular rigidity but also
to effectively hinder close packing and intermolecular
interactions, so that the tendency for molecules to crystallize
should be reduced.
In this paper, we wish to report our design and preparation
of two kinds of new chromophores, which contain one or
two tetrahedral centers in 1-fold or 2-fold fluorene-func-
tionalized anthracene derivatives. The tetrahedral nature of
The lithium-bromine exchange of 9,10-dibromoan-
thracene with n-BuLi at low temperature (-78 °C), followed
by quenching with fluorenone derivatives, provided the
difluorenols in moderate yields. Further alkylations with
1-bromohexane yielded the 2-fold substituted chromophores
4 and 5, in which two tetrahedral centers were constructed
simultaneously.
A single crystal of compound 2 (CCDC 765306) was
cultivated from pentane and analyzed by X-ray crystal-
lography to establish the pattern of substitution and its
structure. As shown in Figure 1, the bulky fluorene moiety
3
the sp -hybridized carbon atom (at the C-9 position of the
fluorene moiety) connects the conjugated moieties of fluorene
and anthracene through a σ-bond, which in turn serves as a
conjugation interrupt. Thus, the desired electronic and optical
properties of the corresponding chromophores combining
both fluorene and anthracene’s architectural specificities are
7
expected to be preserved. The photophysical, electrochemi-
cal, and electroluminescent characteristics of these chro-
mophores in unoptimized OLED devices are also initially
evaluated.
Scheme 1. Synthesis of C-9 Fluorenyl Substituted Anthracenes
Figure 1. (a) Molecular view of X-ray structure of 2 (left). (b)
Top view from the structure (right).
is nearly perpendicular relative to the anthracene ring, while
two tert-butyl groups are at the 2,6-positions of anthracene.
This conformation effectively releases the steric interactions
between the anthracene and fluorene cores and prevents
interchromophore interactions. Thus, the introduction of the
tetrahedral sp -hybridized carbon atom center should break
the π-stacking and prevent the excimer formation which
As shown in Scheme 1, 9-bromofluorene was introduced
into the 9-position of anthracene directly under the promotion
3
2
of activated zinc powder in CS solvent, affording the 1-fold
causes the concentration quenching of fluorescence in a solid
fluorene-functionalized derivatives 1 and 2 with a tetrahedral
9
state.
UV-vis absorption, photoluminescence, thermal, and
electrochemical properties of these compounds are sum-
marized in Table 1. The absorption spectra of the 1- and
(
6) (a) Poriel, C.; Rault-Berthelot, J.; Barriere, F.; Slawin, A. M. Z. Org.
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H. C.; Wu, C. C.; Wong, K. T. AdV. Mater. 2005, 17, 992–996. (e) Tao,
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J. Org. Chem. 2001, 3719–3729. (c) Li, C. J. Chem. ReV. 1993, 93, 2023–
2035.
S.; Peng, Z.; Zhang, X.; Wang, P.; Lee, C. S.; Lee, S. T. AdV. Funct. Mater.
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005, 15, 1716–1721.
(
7) (a) Shen, W. J.; Dodda, R.; Wu, C. C.; Wu, F. I.; Liu, T. H.; Chen,
H. H.; Chen, C. H.; Shu, C. F. Chem. Mater. 2004, 16, 930–934. (b) Chiang,
C. L.; Shu, C. F. Chem. Mater. 2002, 14, 682–687. (c) Wu, F. I.; Dodda,
R.; Reddy, D. S.; Shu, C. F. J. Mater. Chem. 2002, 12, 2893–2897.
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265, 765–768.
Org. Lett., Vol. 12, No. 17, 2010
3875

The C-9 methyl substituted 1-fold functionalized derivative
was found to show higher photoluminescence quantum
efficiency than the other 1-fold functionalized derivatives 1
and 2.
The thermal properties of the 1- and 2-fold functionalized
derivatives were investigated by differential scanning calo-
rimetry (DSC) under a continuous nitrogen purge. The 1-fold
functionalized derivatives 1 and 2 showed an endothermic
peak at 223 and 256 °C, respectively, which were much
higher than in the case of the 2-fold functionalized derivatives
3
Table 1. Optical, Thermal, and Electrical Properties of
Compounds 1-5
λ
max, Abs
λ
max, PL
λ
max, PL HOMO LUMO QE
T
m
a
a
b
c
d
Compd (nm)
(nm)
(nm)
(eV)
(eV)
(φ) (°C)
1
2
3
4
5
261
264
263
273
273
390, 418
409, 434
387, 408
446
457
435
443
463
451
5.62
5.56
5.61
5.43
5.42
2.54 0.47 223
2.64 0.59 256
2.52 0.67 165
2.56 0.90 166
2.56 0.45 181
c
433
4
and 5. However, the melting point of compound 3
a
b
Measured in CH
2 2
Cl .
Measured in thin film solid. Quantum
d
decreased to 167 °C. It is clear that the methyl group at the
C-9 position has a significant influence on the thermal
stabilities of the 1-fold functionalized derivatives.
effeciency. Melting point.
The electrochemical behaviors of 1-5 were also evaluated
by cyclic voltammetry experiments at ambient temperature
in CH Cl (0.1 M n-Bu NPF as a supporting electrolyte).
2 2 4 6
2
-fold fluorene-substituted anthracenes showed the charac-
teristic vibronic patterns in the range of 330 to 400 nm
shown in Figure 2a), indicating the existence of a π-π*
(
All the compounds showed good reversibility of the oxidation
potential. The values of the HOMOs for 1-5, which were
calculated from HOMO (eV) ) -[Eonsetox (vs SCE) + 4.4]
based on an SCE energy level of 4.4 eV relative to the
vacuum and the Ferrocene/Ferrocenium couple as the internal
standard, were 5.42-5.62 eV. From the HOMO values and
the optical band gap energy available from the UV-vis
spectra, the energies of the LUMO for 1-5 were determined
to be in the range of 2.54-2.64 eV.
Simple OLED devices as structures of ITO/MoO
NPB (20 nm)/emitting material (30 nm)/Bphen (20 nm)/LiF
0.7 nm)/Al were fabricated from compounds 1-5 by a
thermal evaporation technique, where N,N′-bis(naphthalene-
-yl)-N,N′-bis(phenyl)benzidine (NPB) acted as a hole
x
(4 nm)/
(
1
transporting layer, compounds 1-5 as an emitter, 4,7-
dipheny1-l,10-phenanthroline (Bphen) as an electron-trans-
porting layer, and LiF:Al as a composite cathode. The
performance characteristics (EL maximum, brightness, lu-
minance efficiency, and CIE coordinates) of these devices
are summarized in Table 2. As can be seen, the emission
Table 2. EL Performance of the Synthesized Compounds 1-5
ELmax
(nm)
Brightness
(cd/m )
Efficiency
(cd/A)
CIE
(x, y)
Figure 2
.
UV-vis absorption and photoluminescence emission
2
Compd
spectra of compounds 1-5: (a) in solution; (b) in thin film.
1
2
3
4
5
464
420
440
448
448
678
796
4100
75
0.42
0.41
1.34
0.096
0.062
(0.163, 0.198)
(0.167, 0.072)
(0.157, 0.082)
(0.204, 0.195)
(0.219, 0.240)
transition derived from the substituted anthracene backbone.
All the emission maxima in CH
derivatives 1, 2, and 3 lay in the region between 380 and
40 nm. A ca. 20-40 nm red shift in the thin film occurs
shown in Figure 2b) as compared to those in the solution
2 2
Cl for 1-fold functionalized
21
4
(
was almost fixed in the sky-blue region for compound 1 with
CIE coordinates (x ) 0.164, y ) 0.232), 4 (0.204, 0.195), 5
(0.219, 0.240) and in the deep blue region for 2 (0.167, 0.072)
and 3 (0.157, 0.082). The EL spectra of compounds 1, 3,
and 5 from the fabricated devices exhibited maxima close
to those observed in their PL (solid) spectra. The peak
intensity of the EL spectrum for 3 was at 440 nm, and the
full width at half-maximum was about 52 nm. The color is
pure blue (x ) 0.157, y ) 0.082) in CIE chromaticity
state. It is noteworthy that the absorption and emission
maxima shift toward the longer wavelength region by ca.
1
0-20 nm upon introduction of 2-fold functionalized an-
thracenes, as compared to those of 1-fold functionalized
anthracenes. The photoluminescence quantum yields (φ) for
1
-5 were determined in cyclohexane using quinine sulfate
as a standard. The efficiencies of both 1- and 2-fold
functionalized anthracenes were in the range of 0.45-0.90.
3876
Org. Lett., Vol. 12, No. 17, 2010

coordinates, which is quite close to the standard blue value
of the National Television Standards Committee (NTSC)
(
0.14, 0.08). Figure 3 shows the EL spectra for 3 measured
Figure 4. Current density (I)-voltage (V)-luminance character-
istics of the device of 3. Insert shows its L-I (luminance efficiency
vs current density) characteristics.
Figure 3. EL spectra for 3 at different applied currents.
at different applied currents. No obvious spectrum shifts were
observed. These high color purities of blue emissions confirm
that the noncoplanar molecular structures of the 1-fold
functionalized derivatives can effectively hinder intermo-
lecular aggregation.
The performance characteristics of devices fabricated for
1-fold substituted derivatives 1, 2, and 3 were far more
superior to those of 2-fold derivatives 4 and 5.
In conclusion, we have described here the first preparation
of 1- and 2-fold fluorene-functionalized anthracene deriva-
The device made from compound 1 exhibited a maximum
2
luminance of 678 cd/m at 18 V and a maximum efficiency
3
tives, in which a novel rigid linkage of a tetrahedral sp -
of 0.42 cd/A at 7.0 V with a turn-on voltage of 7.4 V. The
hybridized carbon atom between fluorene and anthracene
moieties (at the C-9 position of the fluorene) is presented.
The tetrahedral nature of the C-9 carbon atom is found to
preserve the electrical and optical properties of the fluorene
and anthracene units. Our initial results from the unoptimized
devices indicate that the 1-fold functionalized anthracenes
device of 2 had a lower turn-on voltage of about 5.2 V, with
2
a maximum brightness of 796 cd/m at a driving voltage of
1
6 V and a maximum efficiency of 0.41 cd/A at 9.0 V. To
our surprise, the device of 3 exhibited a much higher
2
luminance of 4100 cd/m at a lower voltage of 12 V with a
lower turn-on voltage of 4.3 V. The maximum efficiency
reached up to 1.36 cd/A. Figure 4 shows the current density
1
, 2, and 3 are novel blue light emitters. Especially, the
introduction of a methyl group into the C-9 position of the
fluorene moiety has been demonstrated to strongly enhance
the 1-fold based OLED properties. This work, which appears
as the first report of a nondoped OLED based on the C-9
fluorenyl substituted anthracenes, may pave the way to the
development of this novel class of materials.
(
3
I)-voltage (V)-luminance characteristics of the device of
, and the insert is its L-I (luminance efficiency vs current
density) characteristics. It is worthwhile to note that the
overall performances for the 3 based device are among the
best ever reported for high-efficiency, nondoped deep-blue
OLEDs with a CIE coordinate of y < 0.1, although they are
still in the minority in the literature. These unexpected
results reveal that the modification at the C-9 position of
fluorene-functionalized anthracene not only strengthens the
rigidity of the molecules but also significantly improves the
electroluminance.
1
0
Acknowledgment. This work was financially supported
by the National Natural Science Foundation of China (No.
20772079), Science and Technology of Shanghai Municipal
Committee (07ZR14040, 07JC14020, 08JC1409900), and
Shanghai Municipal Education Commission.
However, the devices fabricated from 2-fold substituted
derivatives 4 and 5 yielded a much poorer blue brightness.
Supporting Information Available: Experimental pro-
cedures, spectroscopic characterizations of all new com-
pounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
(
10) (a) Cocherel, N.; Poriel, C.; Vignau, L.; Bergamini, J.; Rault-
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2
143–2145.
OL101553A
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3877