New benzo[b]furans as electroluminescent materials for emitting blue light

Article abstract of DOI:10.1021/ol050196d

(Chemical Equation Presented) New functionalized mono- and bis-benzo[b]furan derivatives were synthesized and developed as blue-light emitting materials. They possessed a CN, CHO, CH=CHPh, CH=CPh₂, or CH=CHCOOH group at the C4-position. Two ben

Full text of DOI:10.1021/ol050196d

ORGANIC
LETTERS
2005
Vol. 7, No. 8
1545-1548
New Benzo[b]furans as
Electroluminescent Materials for
Emitting Blue Light
Jih Ru Hwu,*,†,‡ Kao-Shuh Chuang,† Shih Hsien Chuang,† and Shwu-Chen Tsay§
Organosilicon and Synthesis Laboratory, Department of Chemistry, National Tsing
Hua UniVersity, Hsinchu 30013, Taiwan, R.O.C., Center for Nano-Science and
Technology, UniVersity System of Taiwan, R.O.C., and Well-being Biochemical
Corporation, Taipei, Taiwan 10468, R.O.C.
jrhwu@mx.nthu.edu.tw
Received January 31, 2005
ABSTRACT
New functionalized mono- and bis-benzo[b]furan derivatives were synthesized and developed as blue-light emitting materials. They possessed
a CN, CHO, CH CHPh, CH CPh2, or CH CHCOOH group at the C4-position. Two benzo[b]furan nuclei in bis-benzo[b]furan derivatives were
d
d
d
connected by a divinylbenzene bridge. With good volatility and thermal stability, bis-benzo[b]furan 7a was fabricated as a device. It emitted
blue light with brightness 53430 cd/m2 (at 15.5 V) and high maximum external quantum efficiency 3.75% (at 11 V).
New organic light-emitting devices (OLEDs) are in great
demand by modern electronic industries.1 They can be used
for the manufacture of flat panel display and portable
electronic products.2 By modification of the molecular
structure, organic materials can emit the desired color with
low-drive voltage.3,4 In contrast, difficulties often arise in
the fabrication of inorganic materials as efficient light-
emitting devices.5
styrylarylenes,7 polyphenyls,8 perylenes,9 benzofurans,10 in-
doles,11 oxadiazoles,12 thiophenes, etc.13 Due to critical
requirements on physical and optical properties for fabrica-
tion, most of the existing devices leave much room for
(3) For recent development, see: (a) Mu¨llen, K.; Wegner, G. Electronic
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1513-1515.
Nowadays, compounds with blue luminescence are highly
desirable because of their wide applicability.6 The existing
organic materials with blue light-emitting capability include
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† National Tsing Hua University.
‡ University System of Taiwan.
§ Well-being Biochemical Corporation.
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Lett. 1995, 67, 3853-3855.
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409. (b) Hung, L. S.; Chen, C. H. Mater. Sci. Eng. 2002, R39, 143-222.
(c) Kolosov, D.; Adamovich, V.; Djurovich, P.; Thompson, M. E.; Adachi,
C. J. Am. Chem. Soc. 2002, 124, 9945-9954.
(2) Shinar, J. Organic Light-Emitting DeVices A SurVey; Springer: New
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10.1021/ol050196d CCC: $30.25
© 2005 American Chemical Society
Published on Web 03/17/2005

improvement. Herein we report our recent development of
functionalized benzo[b]furans,13 especially bis-benzo[b]furan
derivatives, as competent new materials for OLEDs.
For the synthesis of functionalized benzo[b]furans 3, we
first treated 4-formyl-2-hydroxy-3-iodoanisole14 (1a) with a
phenylacetylene15 (2, 2.0 equiv) in the presence of (PPh3)2-
PdCl2 (0.060 equiv), CuI (0.060 equiv), and triethylamine
(3.0 equiv) in DMF at 110 °C (Scheme 1).16 The desired
The formyl group in benzo[b]furans 3a,b provided a
chance to elongate their π-system by addition of another
functionality, including a styryl, â,â-diphenylvinyl, and car-
boxyvinyl group. Attachment of these auxochromes at the
C4-position would enable us to tune the emitting light with
bathochromic and hypsochromic shifts. Therefore, we con-
densed aldehydes 3a,b with a benzylphosphonate (i.e., 4a
or 4b)17 in the presence of NaH in THF to give vinylbenzo-
[b]furans 5a-d in 51-80% yields (Scheme 1). Moreover,
the Knoevenagel reaction was applied to aldehyde 3b and
manolic acid (1.0 equiv) in the presence of pyridine and
piperidine to produce 5e in 70% yield. Finally, reaction of
aldehydes 3a,b with bisphosphonate 6 produced the desired
bis-benzo[b]furan derivatives 7a,b in 70-72% yields (see
Scheme 2).
Scheme 1
Scheme 2
Structures of the new compounds 3b,c, 5b-e, and 7a,b
1
were fully characterized by H NMR, 13C NMR, IR, UV,
photoluminescence (PL), and mass spectroscopic methods.
For example, bis-benzo[b]furan 7a exhibited one character-
1
istic singlet at 6.85 ppm in its H NMR spectrum for the
C3-protons. Two doublets with J ) 8.4 Hz appeared at 7.40
and 7.91 ppm for the C5- and C6-protons; another two
doublets with J ) 16.2 Hz appeared at 7.18 and 7.42 ppm
for the vinylic protons. In its 13C NMR spectrum, the
resonance occurred at 157.09 and 125.70 ppm for the C2-
and C3-carbons, respectively, at 128.34 and 122.26 ppm for
the two vinylic carbons. In its IR spectrum, one medium
absorption band appeared at 1514 cm-1 for the OCdC
stretching vibration in the furan moiety.
4-formylbenzo[b]furans 3a,b were generated as solids in 60-
63% yield. Second, we obtained 4-cyanobenzo[b]furan 3c
in 50% yield from 4-cyano-2-hydroxy-3-iodoanisole (1b) and
(3,4-dimethoxyphenyl)acetylene (2b).15
(11) Lin, T. S. U.S. Patent 0 001 967, 2004.
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We unambiguously confirmed the molecular framework
of a benzo[b]furan by using single-crystal X-ray diffraction
analysis (see Figure 1). Among our synthesized benzo[b]-
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1990, 10, 2775-2790.
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1546
Org. Lett., Vol. 7, No. 8, 2005

Table 1. UV and Photoluminescent Properties of
Benzo[b]furans 3, 5, and 7
benzo[b]-
UV λmax
,
PL λmax
,
PL
a
entry
1
furan
substituent
CHO
nm (ꢀ)
nm
ΦPL
3a
3b
3c
5a
5b
5c
5d
5e
7a
7b
332 (13800)
263 (17310)
352 (19640)
282 (19310)
330 (25530)
272 (10020)
314 (26200)
282 (23200)
342 (29880)
310 (25240)
336 (38360)
294 (35120)
338 (36320)
300 (12600)
342 (8970)
406
457
393
426
425
450
462
483
463
470
0.03
0.36
0.45
0.12
0.23
0.01
0.01
0.21
0.43
0.46
2
3
CHO
CN
4
CHdCHPh
CHdCHPh
CHdCPh2
CHdCPh2
CHdCHCOOH
C6H4CHdCH
C6H4CHdCH
5
6
7
8
276 (15630)
398 (63030)
296 (41220)
400 (62020)
320 (42610)
9
10
a The photoluminescence quantum yields in CH2Cl2 were measured in
comparison with anthracene in ethanol (0.27).19 The excitation wavelength
was fixed at 340 nm.
Figure 1. ORTEP diagram of diphenylvinylbenzo[b]furan 5c
obtained by X-ray analysis.
furans, 5c was the only compound that can be obtained in
single-crystal form. Its triclinic crystals possessed space
group P1 with a ) 11.0613(2) Å, b ) 11.2060(2) Å, c )
18.367(3) Å, R ) 103.796(3)°, â ) 98.545(3)°, and γ )
90.565(3)°. The dihedral angle was 12° between the benzo-
furan nucleus and the C2-benzene moiety. Compounds with
a nonplanar structure generally have less tendency to pack
into crystal lattice and, hence, favor amorphous morphol-
ogy.18 Our X-ray data indicate that the aryl moiety attached
at the C2-position was not coplanar with the benzofuran
nucleus. This would lead to amorphism, which facilitates
device fabrication by use of various 2′-substituted benzo[b]-
furans.
being made as a device, compound 7a was found stable at
3.6, 7.0, 11, and 15.5 V. Nevertheless, degradation in this
OLED appeared in the form of a decrease in device
photoluminescence after the voltage reached 17.0 V.
We measured the HOMO energy of the materials with high
ΦPL by cyclic voltammetry (CV) with ferrocene (4.8 eV) as
the reference. The LUMO energy was calculated from the
HOMO and the lowest energy absorption edge of the UV/
Vis absorption spectra.21 Their first oxidation potential (Eox),
HOMO energy, LUMO energy, and band gaps are shown
in Table 2. The conjugation degrees of the aromatic π-system
Among monobenzo[b]furans 3a-c and 5a-e, cyano-
containing derivative 3c exhibited hypsochromic shift and
appealing photoluminescence (PL) quantum yield (ΦPL, entry
3 in Table 1). It is due to conjugation of the aromatic
π-system therein to an electron-withdrawing cyano group.3
Thus the cyano group can be used to tune the electronic and
the optical properties of benzo[b]furans. On the other hand,
bis-benzo[b]furans 7a,b showed significant bathochromic
shift (entries 9 and 10) with high ΦPL values (0.43-0.46).
Thermal properties of benzo[b]furans were analyzed by
differential scanning calorimetry (DSC) and thermal gravi-
metric analysis (TGA) methods. Our results indicate that the
bis-benzo[b]furans were more stable than mono-benzo[b]-
furans. The glass transition temperature (Tg), melting tem-
perature, and onset decomposition temperature of bis-
benzo[b]furan 7a were 83, 282-283, and 320 °C, individually.
Generally morphological instabilities of organic layers do
not seem to be a dominant factor in intrinsic degradation of
OLEDs, especially for operating conditions at room tem-
perature.20 The Tg value of 7a (83 °C) is higher than the
general organic materials used for OLEDs (g60 °C). While
Table 2. Electrochemical Data of Mono- and
Bis-benzo[b]furan Derivatives
energy
benzo[b]-
furan
Eox(V) vs HOMO LUMO
gap
entry
substituent Ag/AgCl
(eV)
(eV)
(eV)
1
2
3
4
5
3b
3c
5b
7a
7b
CHO
1.50
1.56
1.17
0.98
0.89
5.90
5.72
5.37
5.26
5.06
2.76
2.37
2.36
2.48
2.32
3.14
3.35
3.01
2.78
2.74
CN
CHdCHPh
C6H4CHdCH
C6H4CHdCH
can influence the energy gap. Our results indicate that the
energy gaps between HOMO and LUMO were significantly
lower for bis-benzofurans (entries 4 and 5 of Table 2) than
the mono-benzofurans (entries 1-3).
Among our synthesized benzo[b]furans, we selected 7a
for the device fabrication because of its higher PL quantum
(19) Scaiano, J. C. CRC Handbook of Organic Photochemistry; CRC:
Boca Raton, 1989.
(20) Aziz, H.; Popovic, Z. D. Chem. Mater. 2004, 16, 4522-4532.
(21) Anderson, J. D.; McDonald, E. M.; Lee, P. A.; Anderson, M. L.;
Ritchie, E. L.; Hall, H. K.; Hopkins, T.; Mash, E. A.; Wang, J.; Padias, A.;
Thayumanavan, S.; Barlow, S.; Marder, S. R.; Jabbour, G. E.; Shaheen, S.;
Kippelen, B.; Peyghambarian, N.; Wightman, R. M.; Armstrong, N. R. J.
Am. Chem. Soc. 1998, 120, 9646-9655.
(18) Shirota, Y. J. Mater. Chem. 2000, 10, 1-25.
Org. Lett., Vol. 7, No. 8, 2005
1547

yield, better thermal stability, and sublimation capability.
Moreover, the HOMO energy measured for 7a was 5.26 eV;
thus, hole injection from the NPB (5.40 eV) anode to 7a
should be feasible. The HOMO energy for TPBI is 6.2 eV;
its application as a hole-blocking layer would not favor holes
to enter the TPBI layer. As a result, recombination and
emission can take place in the layer of 7a. On the basis of
this design, we made our device by ITO/NPB (40 nm)/7a
(4.0%) in ADN (30 nm)/TPBI (10 nm)/Alq3 (30 nm)/Mg-
Ag (50 nm)/Ag (10 nm). The ADN functioned as a host
material layer and the TPBI as the hole blocking material.
The ITO, NPB, ADN, TPBI, Alq3, Mg-Ag represent indium
tin oxide, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl,
9,10-di(2-naphthyl)anthracene, 1,3,5-tris(N-phenylbenzim-
idizol-2-yl)benzene, tris(8-hydroxyquinoline)aluminum, and
magnesium-silver alloy (∼10:1) individually. The NPB and
Alq3 were used as the hole and electron-transporting material,
respectively.
In conclusion, a series of new mono- and bis-benzo[b]-
furan derivatives were synthesized successfully, which
possessed various substituents including CHO, CN, CHd
CHPh, CHdCPh2, and CHdCHCOOH groups at the C4-
position. Several compounds exhibited appealing photolu-
minescence quantum yields and thermal stability. A device
containing bis-benzo[b]furan (i.e., ITO/NPB/7a) was fabri-
cated in the ADN/TPBI/Alq3/Mg-Ag/Ag structure, which
emitted light in the blue region with high maximum external
quantum efficiency and great brightness. Accordingly, bis-
benzo[b]furans possess great potential as highly efficient blue
OLED materials. The key factors to our success include the
following: (1) Choosing benzo[b]furan as the nucleus with
an aryl group attached at the C2-position. This design leads
to nonplanarity of the molecules, which provides amorphous
morphology. (2) Elongating the π-system therein while
holding geometric symmetry for the target molecules (i.e.,
bis-benzo[b]furans). Subsequently, the light of the desired
wavelength is emitted brightly. (3) Controlling molecular size
and weight of the benzo[b]furan in its dimeric form. Thus,
the thermal stability and volatility of the target molecules
allow them to be easily fabricated as an OLED device.
The device of 7a had turn-on voltage of 3.6 V; its
brightness reached 303 cd/m2 at 1.0 mA and 6.0 V as well
as 1298 cd/m2 at 2.48 mA and 7.0 V. The initial color of
our device was blue-green (x ) 0.15 and y ) 0.25) in the
Commission Internationale de L’Eclairage (CIE) chromaticity
coordinates. The observed maximum brightness was 53430
cd/m2 at 85.5 mA and 15.5 V. The external maximum
quantum and power efficiency for the 7a device were 3.75%
and 2.03 lm/W, which were achieved at 11 V (314 mA/
cm2, 22418 cd/m2). To the best of our knowledge, the device
of 7a showed the greatest brightness and the maximum power
efficiency in comparison with all reported compounds in the
family of benzofurans.10
Acknowledgment. We thank Well-being Biochemical
Corp. for financial support.
Supporting Information Available: Experimental pro-
cedures, spectral data, and physical properties for new benzo-
[b]furans 3b,c, 5b-e, and 7a,b as well as fabrication of
OLEDs and their measurement. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL050196D
1548
Org. Lett., Vol. 7, No. 8, 2005