6-N,N-diphenylaminobenzofuran-derived pyran containing fluorescent dyes: A new class of high-brightness red-light-emitting dopants for OLED

Article abstract of DOI:10.1021/ol060803c

Dye-doped organic light-emitting diode of ITO/α-NPB (70 nm)/Bebq ₂-1 (7 nm)/BCP (5 nm)/Bebq₂ (33 nm)/LiF (1 nm)/AI (150 nm) shows red electroluminescence with the efficiency of 2.9 cd/A at 100 cd/m ² and maximum brightness

Full text of DOI:10.1021/ol060803c

ORGANIC
LETTERS
2006
Vol. 8, No. 12
2623-2626
6-N,N-Diphenylaminobenzofuran-Derived
Pyran Containing Fluorescent Dyes:
A New Class of High-Brightness
Red-Light-Emitting Dopants for OLED
Man-kit Leung,†,‡ Chin-Chuan Chang,*,† Meng-Hsiu Wu,§ Kai-Hsiang Chuang,§
|
|
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Jiun-Haw Lee,§ Shwu-Ju Shieh, Shien-Chang Lin, and Chi-Feng Chiu
Department of Chemistry, Institute of Polymer Science and Engineering, and Graduate
Institute of Electrooptical Engineering and Department of Electrical Engineering,
National Taiwan UniVersity, Taipei 106, Taiwan, and RiTdisplay Corporation,
Hsin Chu Industrial Park 30316, Taiwan
mkleung@ntu.edu.tw
Received April 3, 2006
ABSTRACT
Dye-doped organic light-emitting diode of ITO/r-NPB (70 nm)/Bebq2-1 (7 nm)/BCP (5 nm)/Bebq2 (33 nm)/LiF (1 nm)/Al (150 nm) shows red
electroluminescence with the efficiency of 2.9 cd/A at 100 cd/m2 and maximum brightness of 62000 cd/m2. The physical organic aspects of the
current-induced fluorescent quenching effect are discussed.
Since Tang and VanSlyke1 reported a highly efficient organic
light-emitting diode (OLED) with high brightness output in
1987, extensive research has been undertaken.2 Red light is
one of the three necessary elements for a full-color display.
Although there has been great progress on the development
of novel red fluorescent3 and phosphorescent4 materials, the
family of pyran-containing fluorescent (PCF) dyes is still
an important class of red dopants for OLED.5
In a typical dye-doped OLED of ITO/R-NPB/Alq3-dopant/
LiF/Al,6 the PCF dyes are usually doped into Alq3. Energy
transfer from excited Alq3 to the PCF dopant leads to red
electroluminescence (EL). To avoid the self-quenching effect,
the dopant concentration is usually limited at low levels,
around 2%. In some cases, the energy transfer process is
incomplete, leading the OLED to red-orange emission. To
improve the performance of the OLED, Tang and co-workers
modified the pyran core to form DCJTB.7 The presence of
the alkyl substituents on DCJTB prevents it from molecular
† Department of Chemistry, National Taiwan University.
‡ Institute of Polymer Science and Engineering, National Taiwan
University.
§ Graduate Institute of Electrooptical Engineering and Department of
Electrical Engineering, National Taiwan University.
(4) (a) Rayabarapu, D. K.; Paulose, B. M. J. S.; Duan, J.-P.; Cheng,
C.-H. AdV. Mater. 2005, 17, 349-353. (b) Kavitha, J.; Chang, S.-Y.; Chi,
Y.; Yu, J.-K.; Hu, Y.-H.; Chou, P.-T.; Peng, S.-M.; Lee, G.-H.; Tao, Y.-
T.; Chien, C.-H.; Carty, A. J. AdV. Funct. Mater. 2005, 15, 223-229. (c)
Tung, Y.-L.; Wu, P.-C.; Liu, C.-S.; Chi, Y.; Yu, J.-K.; Hu, Y.-H.; Chou,
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| RiTdisplay Corporation.
(1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913-915.
(2) (a) Hatwar, T. K.; Spindler, J. P.; Ricks, M. L.; Young, R. H.;
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10.1021/ol060803c CCC: $33.50
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aggregation in the solid state and the self-quenching problem
is therefore reduced. However, another problem of current-
induced fluorescent quenching (CIFQ) effect was observed.8
The EL quantum efficiency (QE) of the OLED significantly
drops when the applied current-density increases. The loss
of QE is attributed to the formation of DCJTB+ that quenches
the emission from the excited DCJTB.9 Therefore, we decide
to develop novel red PCF dyes to solve the problem.
Scheme 1. Schematic Diagram for the Synthesis of 1
The CIFQ effect of the cationic intermediates is governed
by at least two factors: (1) the absorption spectrum and (2)
the steady-state concentration of the cationic intermediates
in the matrix. The PCF dopants are bipolar compounds that
contain an dialkylaminoaryl group linked to an (pyran-4-
ylidene)malononitrile component through an olefinic bridge.
In our design, a 6-N,N-(diphenylamino)benzofuran unit is
coupled with a (2-tert-butyl-6-vinylpyran-4-ylidene)malono-
nitrile unit to give 1. Triarylamines and furans are good hole-
transport materials.10 We wonder if replacement of the
aliphatic amino group by the diphenylaminobenzofuran unit
would lead to any positive response on the CIFQ effect and
improve the device performance.
accompanied by several unidentified products. Fortunately,
the PCC oxidation in pyridine proceeded to give 7 in 31%
yield. Condensation of 7 with 8 gave the desired 1.
Compound 1 shows a reversible oxidation wave at E1/2
)
0.51 V against ferrocene standard in the cyclic voltammetry
(CV) (Figure 1). The E1/2 is about 0.25 V higher than that
The synthesis of 1 was summarized in Scheme 1. N-
Phenylation of m-anisidine was performed under Ullman
conditions,11 followed by BBr3 demethylation to give 2.12
Hydroformylation under Vilsmeier conditions led to 3.
Conversion of 3 to benzofuran 4 was performed using the
Yoo procedure through intermediate 5, followed by LAH
reduction to give 6.13 However, oxidation of 6 to 7 is
unexpectedly difficult. Both Swern oxidation or simple PCC
oxidation were unsuccessful, leading to 7 in very low yield,
(5) (a) Jung, B.-J.; Lee, J.-I.; Chu, H. Y.; Do, L.-M.; Lee, J.; Shim, H.-
K. J. Mater. Chem. 2005, 15, 2470-2475. (b) Li, J.; Liu, D.; Hong, Z.;
Tong, S.; Wang, P.; Ma, C.; Lengyel, O.; Lee, C.-S.; Kwong, H.-L.; Lee,
S. Chem. Mater. 2003, 15, 1486-1490. (c) Lin, X. Q.; Chen, B. J.; Zhang,
X. H.; Lee, C. S.; Kwong, H. L.; Lee, S. T. Chem. Mater. 2001, 13, 456-
458. (d) Zhang, X. H.; Chen, B. J.; Lin, X. Q.; Wong, O. Y.; Lee, C. S.;
Kwong, H. L.; Lee, S. T.; Wu, S. K. Chem. Mater. 2001, 13, 1565-1569.
(e) Chen, C. H.; Tang, C. W.; Shi, J.; Klubek, K. P. Thin Solid Films 2000,
363, 327-331. (f) Hamada, Y.; Kanno, H.; Tsujioka, T.; Takahashi, H.;
Usuki, T. Appl. Phys. Lett. 1999, 75, 1682-1684. (g) Buloviæ, V.;
Shoustikov, A.; Baldo, M. A.; Bose, E.; Kozlov, V. G.; Thompson, M. E.;
Forrest, S. R. Chem. Phys. Lett. 1998, 287, 455-460.
Figure 1. CV plots of (a) DCJTB and (b) 1 in CH2Cl2 with Bu4-
NClO4 (0.1 M) as supporting electrolyte. Scan rate: 100 mV/s.
of DCJTB, indicating that 1 has a lower lying estimated
HOMO of -5.31 eV.14 More important is the fact that
electrochemical dimerization or polymerization was not
observed in the CV experiments. This result suggests that
the radical cation 1+ is electrochemically more stable in
comparison to other triarylamine compounds.15
(6) Tang, C. W.; VanSlyke, S. A.; Chen, C. H. J. Appl. Phys. 1989, 65,
3610-3616.
(7) (a) Chen, C. H.; Klubek, K. P.; Shi, J. US Patent 5908581, 1999. (b)
Chen, C. H.; Tang, C. W.; Shi, J.; Klubek, P. Macromol. Symp. 1998, 125,
49-58.
(8) Young, R. H.; Tang, C. W.; Marchetti, A. P. Appl. Phys. Lett. 2002,
80, 874-876.
(9) Haskins, T.; Chowdhury, A.; Young, R. H.; Lenhard, J. R.; Marchetti,
A. P.; Rothberg, L. Chem. Mater. 2004, 16, 4675-4680.
(10) (a) Zhang, L.-Z.; Chen, C.-W.; Lee,C.-F. Wu, C.-C.; Luh, T.-Y. Chem.
Commun. 2002, 2336-2337. (b) Shirota, Y. J. Mater. Chem. 2000, 10, 1.
(11) Gauthier, S.; Fa`echet, J. M. J. Synthesis 1987, 383-385.
(12) Chen, C.-T.; Chiang, C.-L.; Lin, Y.-C.; Chan, L.-H.; Huang, C.-H.;
Tsai, Z.-W.; Chen, C.-T. Org. Lett. 2003, 5, 1261-1264.
(13) Yoo, S.-e.; Lee, S.-H.; Kim, S.-K.; Lee, S.-H. Bioorg. Med. Chem.
1997, 5, 445-459.
Figure 2 shows overlay plots of the UV-vis absorption
and emission spectra of 1, and the emission spectra of
16
common host materials of Bebq2, Alq3, and Gaq3. Com-
pound 1 shows an absorption band peaked at 500 nm with red
emission peaked at 624 nm in CH2Cl2. The fluorescence QE
is 0.65. Although the spectral overlap of 1 with all three hosts
(14) Calculated according to the reported HOMO of 5.11 eV for DCJTB.
For reference, see. Qiao, J.; Qiu, Y.; Wang, L.; Duan, L.; Yang, L.; Zhang,
D. Appl. Phys. Lett. 2002, 81, 4913-4915.
2624
Org. Lett., Vol. 8, No. 12, 2006

layer. Noteworthy to mention that the CIFQ effect is minimal
in this device (Figure 3a). The device efficiency remains
almost constant with increasing current density.
Figure 2. (a) UV-vis absorption and (b) PL spectra of 1, and PL
spectra of (c) Bebq2, (d) Alq3, and (e) Gaq3.
is significantly large, almost perfect spectral match with Bebq2
makes them a good couple in the energy-transfer process.17
Red OLEDs with both a conventional multilayer structure
and a double heterostructure (DH) have been fabricated. To
compare the performance of 1 against previous literature data,
we adapted Alq3 as the electron transport (ET) material to
study. OLEDs with a multilayer structure of ITO/R-NPB (60
nm)/Alq3-1 (20 nm)/Alq3 (40 nm)/LiF (1 nm)/Al (150 nm)
were fabricated.6 At low dopant concentration of 0.5% of 1,
the EL spectrum showed broad co-emission ranging from
450 to 700 nm, spanning the emission of Alq3 and 1. When
the doping concentrations were gradually adjusted from 0.5
to 5%, significant shifts of the EL λmax from 580 to 650 nm
with the OLED efficiency ranged from 0.095 to 0.065 cd/A
at 100 mA/cm2 were observed (Table 1). The EL contribution
Figure 3. Plots of the EL efficiency vs current density, with various
thicknesses of BCP: (a) 0 nm (2), (b) 2 nm (b), and (c) 20 nm
(9).
To elucidate the CIFQ mechanisms, we subjected 1 to a
DH OLED of ITO/R-NPB (60 nm)/Alq3-1 (20 nm)/BCP (20
nm)/Alq3 (20 nm)/LiF (1 nm)/Al (150 nm).18 In the device,
a hole-blocking layer of BCP (20 nm) was introduced to
confine the excitions within the recombination zone.18 The
efficiency of the OLED dramatically increased (Table 2) due
Table 2. EL Performance of ITO/R-NPB (60 nm)/Alq3-1 (20
nm)/BCP (20 nm)/Alq3 (20 nm)/LiF (1 nm)/Al (150 nm)
brightnessa turn-on
[1] (%) EL (nm) (λmax
)
CIE
cd (A)
(cd/m2)
voltageb
Table 1. EL Performance of ITO/R-NPB (60 nm)/Alq3-1 (20
nm)/Alq3 (40 nm)/LiF (1 nm)/Al (150 nm)
0.5
2
5
600
648
0.52, 0.45 0.55
0.64, 0.35 0.20
0.03
3000
1000
500
8.5
9.0
11.0
EL (nm)
brightnessa turn-on
[1] (%)
(λmax
)
CIE
cd (A)
(cd/m2)
voltageb
a At 100 mA/cm2. b V at 100 cd/m2.
0.5
2
5
580
630
650
0.46, 0.48 0.085
0.61, 0.38 0.095
0.63, 0.35 0.065
2700
1500
1700
6.5
5.0
4.5
to higher possibility of charge recombination and excition
formation.
a At 100 mA/cm2. b V at 100 cd/m2.
On the other hand, perhaps the presence of the hole-
blocking layer enhances the steady-state concentration of 1+
in the emission zone, the CIFQ effect is significant in these
cases (Figure 3c). We managed to record the absorption
spectrum of 1+ (Figure 4) by spectral electrochemistry.19 The
spectral overlap of 1+ with the emission spectrum of 1, shown
as the shadow area, suggests that 1+ could be an effective
quencher for the S1 state of 1.
When the thickness of the BCP layer was reduced from
20 to 2 nm, the BCP layer was no longer thick enough to
completely block-up the hole migration process. Two distinct
emission bands were observed from the EL emission. The
from Alq3 gradually decreased. When the doping concentra-
tion was increased to 5%, the CIE coordinates of the EL
shifted to (0.63, 0.35). The overall EL efficiency is lower
than that of DCJTB. We attribute this to poor energy transfer
from Alq3 to 1 due to their poorer spectral match. When the
dopant concentration of 1 increased, the device turn-on
voltage dropped significantly. This observation suggests that
the presence of 1 facilitates hole injection into the emission
(15) (a) Chou, M.-Y.; Leung, M.-k.; Su, Y. O.; Chiang, C. L.; Lin, C.-C.;
Liu, J.-H.; Kuo, C.-K.; Mou, C.-Y. Chem. Mater. 2004, 16, 654-661. (b)
Leung, M.-k.; Chou, M.-Y.; Su, Y. O.; Chiang, C. L.; Chen, H.-L.; Yang,
C. F.; Yang, C.-C.; Lin, C.-C.; Chen, H.-T. Org. Lett. 2003, 5, 839-842.
(16) (a) Chen, B. J.; Lin, X. Q.; Cheng, L. F.; Lee, C. S.; Gambling, W.
A. Lee, S. T. J. Phys. D 2001, 34, 30 (b) Hamada, Y.; Sano, T.; Shibata
K.; Kuroki, K. Jpn. J. Appl. Phys. 1995, 34, L824-L826.
(17) Turro N. J. Modern Molecular Photochemistry; University Sci-
ence: Sausalito, CA, 1991.
(18) Xie, Z. Y.; Hung, L. S.; Lee, S. T. Appl. Phys. Lett. 2001, 79, 1048-
1050.
Org. Lett., Vol. 8, No. 12, 2006
2625

The EL spectral properties of the Bebq2-based OLED
varies when the dopant concentration increases (Figure 5).
Figure 4. (a) UV-vis-NIR absorption spectrum of 1+ and (b)
PL spectrum of 1.
Figure 5. Dopant concentration dependent EL spectra: (a) 0.5%,
(b) 1%, (c) 2% of 1 in Bebq2.
band peaked at 510 nm is attributed to the emission of Alq3
in the ET layer while the other band at 635 nm is assigned to
the EL of 1 in the emission zone. These results suggest that
the holes could partly escape from the emission zone,
penetrating the thin BCP barrier, and entering into the Alq3
zone for recombination. The steady-state concentration of 1+
in the emission zone would therefore maintain at relatively
lower level and the CIFQ phenomenon is again minimal
(Figure 3b).
The thickness of the emission zone is also an important fac-
tor for the EL efficiency. In our study, devices of ITO/R-NPB
(70 nm)/Alq3-1 (0.2%, x nm)/BCP (5 nm)/Alq3 (40 - x nm)/
LiF (1 nm)/Al (150 nm) with three thicknesses of x ) 7, 10,
and 15 nm for the emission layer were fabricated. Their EL
efficiencies were recorded as 1.1, 0.75, and 0.6 cd/A at 100
mA/cm2, respectively. The EL efficiency increased when the
thickness of the emission layer decreased, indicating that the
emission zone is confined in a very narrow region, around
only 7 nm. This value is in good agreement with the previous
conclusion of optimal thickness of 7 nm for DCM study.18
The spectral properties of the host materials are important
factors for the energy transfer process (Table 3). When Gaq3
When [1] was set at 0.5%, a shoulder of green EL emission
at around 500 nm was observed, indicating the energy trans-
fer process is incomplete. The proportion of the green emission
significantly reduced when the dopant concentration of 1 was
increased to 1%. The OLED exhibited good efficiency of 2.9
cd/A at 100 cd/m2 with the highest brightness of 62000 cd/
m2. When 1 was doped at 2%, the green emission from Bebq2
was almost completely suppressed. The EL efficiency of 1.4
cd/A at 100 cd/m2 was obtained with the highest brightness
of 18000 cd/m2. Linear relationship between the brightness
and the applied current density suggests small CIFQ effect
on these OLEDs. The operating lifetimes are longer in
comparison to other Bebq2 devices in the literature.20
The small CIFQ effect could be attributed to the following
factors: (a) The estimated HOMO of 1 (-5.31 eV) is only
0.19 eV higher than that of for Bebq2 (-5.5 eV),16b indicating
that the steady-state concentration of 1+ in Bebq2 should be
lower than DCJTB+ (estimated HOMO of DCJTB: -5.11
eV)14 in Alq3 (-5.6 eV).16b (b) High electron mobility of
16b
Bebq2 also leads to effective exciton formation in the
emission zone that limit the steady-state concentration of 1+
at low levels.
Table 3. EL Performance of the OLED with Different
Host/ETL Compounds in ITO/R-NPB (70 nm)/Host-1 (0.5%, 7
nm)/BCP (5 nm)/ETL (33 nm)/LiF (1 nm)/Al (150 nm)
In summary, 1 represents a novel class of red EL dopants.
Due to the large Stokes shift of 1, Bebq2 can be used as an
effective host for 1 in the EL device, which is uncommon
for other PCF dopants.
host/ETL
efficiencya
maximum brightnessb
Gaq3
Alq3
Bebq2
0.75
1.0
1.8
12500
15000
27500
Acknowledgment. We thank the National Science Coun-
cil and Ministry of Education of Taiwan for partial financial
support.
Supporting Information Available: 1H and 13C NMR
spectra of 1-7 and their preparation procedures and figures.
This material is available free of charge via the Internet at
http://pubs.acs.org.
a cd/A at 100 cd/m2. b cd/m2.
was used, the EL efficiency dropped presumably due to the
poor spectral overlap. When Bebq2 was adopted, the maxi-
mum EL efficiency was boosted up to 1.8 cd/A with the
maximum brightness of 27500 cd/m2. The efficiency and the
CIE coordinates maintained with increasing current density.
The higher efficiency of the OLED may be attributed to the
effective energy transfer from the excitions in Bebq2 to 1.
OL060803C
(19) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamen-
tals and Applications; Wiley: New York, 2001.
(20) Lee, J.-H.; Lin, T.-C.; Liao, C.-C.; Yang, F. H. Proc. SPIE Int.
Soc. Opt. Eng. 2005, 5632, 220-225.
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