Perfluorinated oligo(p-Phenylene)s: Efficient n-type semiconductors for organic light-emitting diodes [18]

Full text of DOI:10.1021/ja002309o

10240
J. Am. Chem. Soc. 2000, 122, 10240-10241
Chart 1
Perfluorinated Oligo(p-Phenylene)s: Efficient n-Type
Semiconductors for Organic Light-Emitting Diodes
Sophie B. Heidenhain,† Youichi Sakamoto,†
Toshiyasu Suzuki,*,† Atsushi Miura,‡ Hisayoshi Fujikawa,‡
Tomohiko Mori,‡ Shizuo Tokito,‡ and Yasunori Taga‡
Institute for Molecular Science
Myodaiji, Okazaki 444-8585, Japan
TOYOTA Central Research and DeVelopment
Laboratories Inc.
Nagakute, Aichi 480-1192, Japan
ReceiVed June 27, 2000
Organic light-emitting diodes (OLEDs) and field-effect transis-
tors (FETs) based on π-conjugated oligomers have been exten-
sively studied as molecular thin-film devices.1 Organic n-type
semiconductors (electron-transport materials) with low electron-
injection barriers and high electron mobilities are required for
highly efficient OLEDs2 and n-type FETs.3 Radical anions of an
n-type semiconductor have to be generated easily at the interface
with a metal electrode (electron injection), and electrons must
move fast in the layer (electron mobility). Compared with organic
p-type semiconductors (hole-transport materials), organic n-type
semiconductors for practical use are few and rather difficult to
develop. In the last paper, we reported that perfluorinated
phenylene dendrimers (C60F42 and C132F90) function as the
electron-transport layer of OLEDs.4 A perfluorinated phenylene
with longer para-conjugation and higher electron affinity exhibited
better electron-transport capability. Perfluoro-1,3,5-tris(p-terphe-
nyl)benzene, which can be viewed as a perfluoro-p-quaterphenyl
derivative, showed the maximum luminance of 2860 cd/m2 at
24.4 V. To develop efficient organic n-type semiconductors and
improve the device performance, we decided to prepare perflu-
orinated phenylene oligomers with even longer para-conjugation.
In this work, we report the synthesis of perfluorinated oligo(p-
phenylene)s (PF-nPs: n ) 5-8)5 and the application for the
electron-transport layer of OLEDs.6 The use of higher PF-nPs
dramatically improved the electron injection into the emission
layer. Two PF-6P derivatives containing trifluoromethyl and
perfluoro-2-naphthyl groups were also prepared, and the latter is
Scheme 1
† Institute for Molecular Science.
‡ TOYOTA Central R&D Labs.
(1) Electronic Materials: The Oligomer Approach; Mu¨llen, K., Wegner,
G., Eds.; Wiley-VCH: Weinheim, 1998.
(2) For a recent review, see: Friend, R. H.; Gymer, R. W.; Holmes, A. B.;
Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos,
D. A.; Bre´das, J. L.; Lo¨gdlund, M.; Salaneck, W. R. Nature 1999, 397, 121-
128.
(3) (a) Horowitz, G.; Kouki, F.; Spearman, P.; Fichou, D.; Nogues, C.;
Pan, X.; Garnier, F. AdV. Mater. 1996, 8, 242-244. (b) Haddon, R. C.; Perel,
A. S.; Morris, R. C.; Palstra, T. T. M.; Hebard, A. F.; Fleming, R. M. Appl.
Phys. Lett. 1995, 67, 121-123. (c) Laquindanum, J. G.; Katz, H. E.;
Dodabalapur, A.; Lovinger, A. J. J. Am. Chem. Soc. 1996, 118, 11331-11332.
(d) Bao, Z.; Lovinger, A. J.; Brown J. J. Am. Chem. Soc. 1998, 120, 207-
208. (e) Scho¨n, J. H.; Berg, S.; Kloc, C.; Batlogg, B. Science 2000, 287, 1022-
1023. (f) Katz, H. E.; Lovinger, A. J.; Johnson, J.; Kloc, C.; Siegrist, T.; Li,
W.; Lin, Y.-Y.; Dodabalapur, A. Nature 2000, 404, 478-481.
(4) Sakamoto, Y.; Suzuki, T.; Miura, A.; Fujikawa, H.; Tokito, S.; Taga,
Y. J. Am. Chem. Soc. 2000, 122, 1832-1833.
(5) (a) The reaction of decafluorobiphenyl with excess C6F5MgBr in THF
gave a mixture of PF-nPs. PF-3P and -4P have been isolated and character-
ized. Maruo, K.; Wada, Y.; Yanagida, S. Bull. Chem. Soc. Jpn. 1992, 65,
3439-3449. (b) Treatment of Cp2Zr(C6F5)2 with excess C6F6 in THF provided
a mixture containing up to PF-13P, which was determined by LC-TOF mass
spectrometry: Edelbach, B. L.; Kraft, B. M.; Jones, W. D. J. Am. Chem. Soc.
1999, 121, 10327-10331.
(6) Partially fluorinated oligo(p-phenylene)s such as 2′,2′′,3′,3′′,4,4′′′,5′,5′′,
6′,6′′-decafluoro-p-quaterphenyl (C24H8F10) for OLEDs: Winkler, B.; Megh-
dadi, F.; Tasch, S.; Mu¨llner, R.; Resel, R.; Saf, R.; Leising, G.; Stelzer, F.
Opt. Mater. 1998, 9, 159-162.
a better electron transporter than conventional electron-transport
materials such as tris(8-quinolinolato)aluminum (Alq3).
We synthesized PF-5P to -7P by the organocopper cross-
coupling method, which has been successfully applied to perflu-
orinated phenylene dendrimers.4 As shown in Scheme 1, 1,4-
dibromotetrafluorobenzene (3) was allowed to react with excess
2,3,5,6-tetrafluorophenylcopper (4) in a THF/dioxane/toluene
mixture at 80 °C for 48 h to give 5 in 47%.7 Bromination of 5
yielded perfluoro-4,4′′-dibromo-p-terphenyl (6) in 33%. A similar
reaction of 4,4′-dibromooctafluorobiphenyl (7) with 4 followed
by bromination afforded perfluoro-4,4′′′-dibromo-p-quaterphenyl
(9). Treatment of excess pentafluorophenylcopper (10) with 6 and
9 produced perfluoro-p-quinquephenyl (PF-5P: C30F22; MW )
778) and perfluoro-p-sexiphenyl (PF-6P: C36F26; MW ) 926)
in 54 and 45%, respectively. Perfluoro-p-septiphenyl (PF-7P:
C42F30; MW ) 1074) was obtained in 54% by using nonafluoro-
4-biphenylcopper (11) and 6. Two PF-6P derivatives 1 (C38F30;
MW ) 1026) and 2 (C44F30; MW ) 1098) were prepared by the
reactions of 9 with 4-trifluoromethyltetrafluorophenylcopper (12)
(7) See Supporting Information for the experimental details.
10.1021/ja002309o CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/03/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 41, 2000 10241
Figure 1. Luminance-voltage characteristics of the OLEDs as a function
of the electron-transport layer.
Figure 2. Current-voltage characteristics of the OLEDs as a function
of the electron-transport layer.
and heptafluoro-2-naphthylcopper (13) in 60 and 49%, respec-
tively. The Ullmann coupling of perfluoro-4-bromo-p-quaterphe-
nyl (14), which was obtained by the reaction of 7 with 11 in
51%, gave perfluoro-p-octiphenyl (PF-8P: C48F34; MW ) 1222)
in 73%.8
luminance and current density decreased slightly compared to PF-
6P. A perfluoro-2-naphthyl group turned out to be an excellent
building block for constructing n-type semiconductors.12 The
luminance and current density of 2 are higher than those of Alq3
above 4.5 V. The luminance at 10.0 V reached 19970 cd/m2.
The above results (PF-5P < PF-6P ) PF-7P ) PF-8P) are
contrary to our expectation because the low LUMO energy of a
higher PF-nP should facilitate the electron-injection from a
cathode to an electron-transport layer (PF-5P < PF-6P < PF-
7P < PF-8P).13 This might indicate that the rate of electron
injection is not affected by the LUMO energy any more if its
level is low enough.14 We assume that electron mobilities are
not very different in these molecules because solid-state morphol-
ogy is expected to be similar. Therefore, the electron mobility in
the layer rather than the electron injection at the interface could
be responsible for determining the current density of PF-6P to
-8P.15
Perfluorinated oligomers were purified by train sublimation9
and used for characterization. They are colorless solids and
insoluble in common organic solvents.10 The structures were
determined by MALDI-TOF mass spectrometries and elemental
analyses.7 The differential scanning calorimetry (DSC) measure-
ments indicated that the samples purified by train sublimation
are highly crystalline solids: The melting endotherm of each
oligomer (PF-5P, 306; PF-6P, 352; PF-7P, 383; PF-8P, 410; 1,
350; 2, 352 °C) is accompanied by a broad peak due to
sublimation, which occurs simultaneously. No glass transitions
were observed on the second heating.
OLEDs were made on indium-tin oxide (ITO)-coated glass
substrates by high-vacuum thermal evaporation (5 × 10-7 Torr)
of TPTE (a tetramer of triphenylamine) as the hole-transport layer
(60 nm), Alq3 as the emission layer (40 nm), a perfluorinated
oligomer as the electron-transport layer (20 nm), LiF as the
electron-injection layer (0.5 nm), and aluminum as the cathode
(160 nm).7,11 For comparison, the device with Alq3 as both the
emission and electron-transport layers (60 nm) was also fabricated.
When a negative voltage was applied to Al, a green emission
due to Alq3 was observed. Figures 1 and 2 show luminance-
voltage and current-voltage characteristics of these OLEDs,
respectively. We found that the electron-transport capabilities of
perfluorinated oligo(p-phenylene)s are excellent compared with
perfluorinated phenylene dendrimers.4 The maximum luminance
of PF-5P is 5540 cd/m2 at 17.0 V, which is much better than
that of perfluoro-1,3,5-tris(p-terphenyl)benzene. When PF-5P was
replaced by PF-6P, the device performance was dramatically
improved. The device with PF-6P showed the maximum lumi-
nance of 12150 cd/m2 at 13.7 V. Interestingly, the luminance-
voltage and current-voltage characteristics of PF-7P and -8P
are almost identical to those of PF-6P. Substitution of a fluorine
atom with a trifluoromethyl group would change solid-state
morphology and electron-transport behavior. In the case of 1, the
In conclusion, we have synthesized perfluorinated oligo(p-
phenylene)s up to p-octiphenyl and shown that they are efficient
n-type semiconductors. Because PF-nPs have high electron
mobilities, n-type FETs with these materials are quite interesting.
Fabrication of FETs is currently underway and will be reported
elsewhere.16
Acknowledgment. We thank S. Makita for measuring MALDI-TOF
mass spectra. This work was supported by Japan Society for the Promotion
of Science (Grant-in-Aid for Scientific Research C-11640594 and
B-12554027).
Supporting Information Available: Experimental details and MALDI-
TOF mass spectra of PF-5P to -8P (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
JA002309O
(12) Naphthalene derivatives with high electron mobilities, see ref 3c.
(13) The PM3 molecular orbital calculations predict that the LUMO energy
level of PF-nP would become lower in the order PF-5P (-2.36 eV) > PF-
6P (-2.39 eV) > PF-7P (-2.41 eV) > PF-8P (-2.42 eV).
(14) This kind of saturation was also observed for perfluorinated oligo-
(2,6-naphthalene)s. Trimer (C30F20) and tetramer (C40F26) showed the same
luminance-voltage and current-voltage characteristics. Sakamoto, Y., un-
published work, 2000.
(8) Treatment of 9 with excess 11 produced only perfluoro-4-bromo-p-
sexiphenyl, which is insoluble in a THF/dioxane/toluene mixture.
(9) Wagner, H. J.; Loutfy, R. O.; Hsiao, C.-K. J. Mater. Sci. 1982, 17,
2781-2791.
(15) The electron mobility of PF-6P, determined by the time-of-flight
technique, is 2.0 × 10-3 cm2 V-1 s-1 at 9.4 × 105 V cm-1. This value is
much higher than that of Alq3 under the same conditions (1.3 × 10-5 cm2
V-1 s-1 at 1.0 × 106 V cm-1). Therefore, another possibility is that the low
electron mobility of Alq3 is responsible for the saturation of current density.
Fujikawa, H., unpublished work, 2000.
(10) PF-5P is slightly soluble in hot toluene.
(11) The thin films of the perfluorinated oligomers are transparent and
probably amorphous.
(16) Takada, M.; Tada, H., unpublished work, 2000.