Synthesis and properties of 2,3,6,7-tetraarylbenzo[1,2-b:4,5-b′] difurans as hole-transporting material

Article abstract of DOI:10.1021/ja074365w

A new zinc-based annulation method produced regioselectively a variety of 2,3,6,7-tetraarylbenzo[1,2-b:4,5-b′]difurans in good yields. The organic electroluminescent devices using these tetraarylbenzodifurans as hole-transporting materials showed better p

Full text of DOI:10.1021/ja074365w

Published on Web 09/12/2007
Synthesis and Properties of 2,3,6,7-Tetraarylbenzo[1,2-b:4,5-b′]difurans as
Hole-Transporting Material
Hayato Tsuji,*,† Chikahiko Mitsui,† Laurean Ilies,† Yoshiharu Sato,‡ and Eiichi Nakamura*,†,‡
Department of Chemistry, School of Science, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan,
and Nakamura Functional Carbon Cluster Project, ERATO, Japan Science and Technology Agency, Department of
Chemistry, School of Science, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Received June 15, 2007; E-mail: tsuji@chem.s.u-tokyo.ac.jp; nakamura@chem.s.u-tokyo.ac.jp
Scheme 1 a
Arylamine derivatives have been used as the hole-transporting
material (HTM) in multilayer organic light-emitting diodes (OLEDs)
for 20 years.1,2 R-NPD,3 which is a diarylamino-substituted biphe-
nyl, is the current standard,4 giving us the impression that the
arylamino group is indispensable for high-performance HTMs.
Indeed, there have been scarcely any attempts to find new HTMs
that do not contain any arylamino groups.5 We describe herein a
new method for the synthesis of a series of 2,3,6,7-tetraarylbenzo-
[1,2-b:4,5-b′]difurans (BDFs), and report that they function as
HTMs in layered OLEDs. The BDF skeleton by itself serves as an
excellent HTM, and improvement of the physical properties can
be achieved by suitable functionalization.
The synthesis of BDFs has been known for a century, but none
of these compounds has found any practical use perhaps because
of very limited synthetic availability: limited scope and low yield.6,7
The key chemical finding that we report herein is a new synthetic
route shown in Scheme 1.
It is based on a recently reported zinc-mediated annulation
a
Yields for 3-6 are based on the analytically pure products obtained
strategy that allows modular construction of the benzofuran moiety
and hence shows considerable flexibility toward the substituents
and functionality.8 The synthesis starts with the 100% regioiso-
merically pure 2,5-dibromohydroquinone protected by tetrahydro-
pyran (THP). The Sonogashira-coupling reaction9 with phenylacet-
ylene followed by deprotection afforded 1. Double cyclization8b
of the substrate 1 afforded the desired 3,7-dizinciobenzodifuran 2
as a thermally stable intermediate in quantitative yield (determined
by protonation of 2). This intermediate permits introduction of the
substituents into 3,7-positions. The Negishi cross-coupling reaction10
of the dizincio intermediate 2 with an aryl halide was achieved in
the presence of a catalytic amount of palladium(0) and tri-tert-
butylphosphine11 to obtain analytically pure 2,3,6,7-tetraarylbenzo-
[1,2-b:4,5-b′]difurans 3-6 in good yields. The use of a toluene/
N-methylpyrrolidinone (NMP) mixture as solvent was found to be
beneficial to improve the yields.12 The two BDFs 3 and 4 lack any
functional groups other than the BDF core, while 5 and 6 contain
arylamino groups. The products were purified by chromatography
and repeated sublimation for the use in the physical measurements.
To evaluate the device properties, we employed the standard
OLED configuration, that is, ITO (indium tin oxide) (145 nm)/
PEDOT:PSS (poly(ethylenedioxy)thiophene:polystyrene sulfonate)13
(70 nm)/hole-transport layer (HTL) (45 nm)/Alq3 (tris(8-hydrox-
by sublimation.
yquinolinato)aluminum) (60 nm)/Liq (8-hydroxyquinolinato lithium)
(1 nm)/Al (80 nm), where HTL is either one of the BDF derivatives
3-6 or R-NPD deposited under vacuum. The performance of six
devices was evaluated for three standard criteria, driving voltage
(V1000), luminance efficiency (η1000), and current efficiency (L/J1000
)
as summarized in Table 1. The device F at the bottom is a reference
standard that lacks the HTL, and expectedly exhibited the lowest
performance by every criterion. Most interestingly, the OLED
devices A and B containing the non-arylamino BDFs 3 and 4,
respectively, displayed excellent device performance. Thus, the
device A generated electroluminescence of 1000 cd/m2 at the
driving voltage V1000 (5.1 V), which is slightly lower than the one
required in the R-NPD-based device E (5.2 V). Moreover, the
luminance efficiency η1000 and current efficiency L/J1000 of A (2.6
lm/W and 4.1 cd/A) at a luminance of 1000 cd/m2 were significantly
higher than those of E (2.0 lm/W and 3.6 cd/A). For the device B,
the current efficiency L/J1000 was noticeably improved (4.9 cd/A).
The results obtained for the devices C and D based on the
arylamino BDFs 5 and 6, respectively, indicated that the introduc-
tion of proper substituents can improve the overall performance of
the BDF, demonstrating that the synergetic effect of the BDF core
† The University of Tokyo.
‡ Nakamura Functional Carbon Cluster Project, ERATO.
9
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J. AM. CHEM. SOC. 2007, 129, 11902-11903
10.1021/ja074365w CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Performance of OLED Composed of either BDF 3-6 or
R-NPD as the Hole-Transport Layer (HTL)a
groups contribute to raise the Tg of the BDF compounds. Thus, the
Tg values of the arylamino BDFs 5 and 6 (124 and 135 °C,
respectively) are much higher than those of 3 and 4, which in turn
are comparable to that of R-NPD (96 °C).
b
c
d
V1000
[V]
η1000
L/J1000
device
HTL
[lm/W]
[cd/A]
In conclusion, we have developed an efficient and versatile
synthesis of BDF compounds and demonstrated that BDFs function
as efficient HTMs. The high performance of the compounds is
primarily due to the BDF core itself, which is in sharp contrast to
the fact that the biphenyl scaffold in R-NPD alone does not function
as a HTM. We also found that there is a synergetic effect of the
BDF core and the substituents as demonstrated by the arylamine-
substitued BDFs. We can therefore expect that the BDF molecule
will serve as a useful new molecular scaffold on which multiple
functional groups can be attached to obtain new properties.
A
B
C
D
E
F
3
4
5
6
5.1
6.3
5.3
5.0
5.2
5.8
2.6
2.6
2.2
2.7
2.0
1.6
4.1
4.9
4.3
4.4
3.6
3.1
R-NPD
nonee
a All performance data were collected at a luminance of 1000 cd/m2.
b Driving voltage. c Luminance efficiency. d Current efficiency. e No HTL.
The thicknesses of other layers are the same as the other devices.
Table 2. Properties of the BDFs 3-6 and R-NPD in the Solid
State
Acknowledgment. This research was supported by KAKENHI
provided by MEXT/JSPS (for E.N., Grant No. 18105004).
c
compound
IPa [eV]
µ
b [cm2 /Vs]
Tg
[
°
C]
3
4
5
6
5.75
5.56
5.53
5.47
5.43
6.4 × 10-4
8.0 × 10-5
2.8 × 10-3
5.6 × 10-4
3.6 × 10-4
92
90
Supporting Information Available: Detailed experimental pro-
cedures and properties of the compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
124
135
96
R-NPD
a Ionization potential measured by PYS for vacuum-deposited thin films.
b Hole mobility measured by the TOF method using vacuum-deposited films
at room temperature at an electric field of 2.5 × 105 V/cm. c Glass transition
temperature measured by DSC.
References
(1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913-915.
(2) (a) Sato, Y. In Electroluminescence I; Mueller, G., Ed.; Academic Press:
San Diego, CA, 2000; Vol. 64, pp 209-254. (b) Sato, Y.; Ichinosawa,
S.; Kanai, H. IEEE J. Select. Top. Quantum Electron. 1998, 4, 40-48.
(3) Van Slyke, S. A.; Chen, C. H.; Tang, C. W. Appl. Phys. Lett. 1996, 69,
2160-2162.
(4) (a) Shirota, Y.; Kageyama, H. Chem. ReV. 2007, 107, 953-1010. (b)
Shirota, Y. In Organic Light Emitting DeVices. Synthesis, Properties and
Applications; Mu¨llen, K., Scherf, U., Eds.; Wiley-VCH: Weinheim,
Germany, 2006; pp 245-263. (c) Adachi, C.; Tsutsui, T. In Organic Light-
Emitting DeVices. A SurVey; Shinar, J., Ed.; Springer: New York, 2004;
pp 43-69.
(5) (a) Zhang, L.-Z.; Chen, C.-W.; Lee, C.-C.; Luh, T.-Y. Chem. Commun.
2002, 2336-2337. (b) Wu, C.-C.; Hung, W.-Y.; Liu, T.-L.; Zhang, L.-
Z.; Luh, T.-Y. J. Appl. Phys. 2003, 93, 5465-5471. (c) Okumoto, K.;
Shirota, Y. Jpn. Kokai Tokkyo Koho 2003, 261473.
(6) (a) Domschke, G. Chem. Ber. 1966, 99, 930-993. (b) Dischendorfer, O.;
Limontschew, W. Monatsh. Chem. 1949, 80, 58-69. (c) Japp, F. R.;
Meldrum, A. N. J. Chem. Soc. 1899, 75, 1035-1042.
(7) The reaction reported afforded a sizable amount of regioisomeric 1,2,7,8-
tetraarylbenzo[1,2-b:4,3-b′]difuran as a byproduct: Le Fevre, R. J. W.;
Taylor, C. R.; Whittem, R. N. J. Chem. Soc. 1948, 1992.
(8) (a) Nakamura, M.; Ilies, L.; Otsubo, S.; Nakamura, E. Angew. Chem. Int.
Ed. 2006, 45, 944-947. (b) Nakamura, M.; Ilies, L.; Otsubo, S.;
Nakamura, E. Org. Lett. 2006, 8, 2803-2805.
(9) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 50,
4467-4470.
(10) (a) Negishi, E.; King, A. O.; Okukado, N. J. Org. Chem. 1977, 42, 1821-
1823. (b) Negishi, E.; Takahashi, T.; King, A. O. Org. Synth. 1985, 66,
67.
and the substituents which effectively extend the π-conjugated
system. Thus, the device D displayed the highest overall perfor-
mance.
By taking the device F, which lacks HTL, as a reference standard,
we can evaluate the relative performance of the HTMs. Thus,
R-NPD increased ∆η1000 and ∆L/J1000 values14 by 0.4 lm/W and
0.5 cd/A, respectively, while the increases caused by the use of
BDFs were twice to three times as much, that is, 0.6-1.1 lm/W
and 1.2-1.8 cd/A, respectively. These data highlight the much
higher improvement of the OLED performance of the BDF-based
device compared with R-NPD.
To gain insight into the high performance of the BDF-based
devices, basic physical properties of the BDFs 3-6 and R-NPD
were measured in the solid state (Table 2). The ionization potentials
(IPs) of thin films15 of 3-6 were evaluated by photoemission yield
spectroscopy (PYS).16,17 The IP value of 3 shown in the first column
is particularly high (5.75 eV) among the BDFs examined (5.47-
5.56 eV), and it is much higher than that of R-NPD (5.43 eV). The
high IP value of 3 must still be within the operating range of the
device (because the device showed high performance), and suggests
a potential advantage of BDFs over the low IP of arylamine-based
HTMs that may be more prone to oxidative degradation.18
Carrier-transporting properties of these compounds were mea-
sured by the time-of-flight (TOF) technique using films (thick-
ness: 4.6-8.3 µm) at room temperature at an electric field of 2.5
× 105 V/cm. The hole drift mobility of the BDFs 5 was 2.8 ×
10-3 cm2/Vs, 1 order of magnitude higher than that of R-NPD (3.6
× 10-4 cm2/Vs). The BDFs 3 and 6 displayed higher and 4
displayed lower mobilities than R-NPD.
(11) Yamamoto, T.; Nishiyama, M.; Koie, Y. Tetrahedron Lett. 1998, 39,
2367-2370.
(12) Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 2719-2724.
(13) Groenendaal, L.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R.
AdV. Mater. 2000, 12, 481-494.
(14) ∆η1000 ) (η1000 of either A-E) - (η1000 of device F); ∆L/J1000 ) (L/J1000
of either A-E) - (L/J1000 of device F).
(15) The BDFs formed stable homogeneous thin films.
(16) Honda, M.; Kanai, K.; Komatsu, K.; Ouchi, Y.; Ishii, H.; Seki, K. Mol.
Cryst. Liq. Cryst. 2006, 455, 219-225.
(17) Cyclic voltammetry measurement in CH2Cl2 indicated the reversibility
of the first oxidation of the BDFs 3-6. See Supporting Information.
(18) For stable p-type organic materials with high IPs, see: (a) Wu, Y.; Li,
Y.; Gardner, S.; Ong, B. S. J. Am. Chem. Soc. 2005, 127, 614-618. (b)
Heeney, M.; Bailey, C.; Genevicius, K.; Shkunov, M.; Sparrowe, D.;
Tierney, S.; McClulloch, I. J. Am. Chem. Soc. 2005, 127, 1078-1079.
The glass-transition temperature (Tg) obtained by differential
scanning calorimetry (DSC, Table 2) suggested that the arylamino
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