Synthesis, characterization, and electron-transport property of perfluorinated phenylene dendrimers [16]

Full text of DOI:10.1021/ja994083z

1832
J. Am. Chem. Soc. 2000, 122, 1832-1833
Chart 1
Synthesis, Characterization, and Electron-Transport
Property of Perfluorinated Phenylene Dendrimers
Youichi Sakamoto,† Toshiyasu Suzuki,*,† Atsushi Miura,‡
Hisayoshi Fujikawa,‡ 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 NoVember 22, 1999
Recently, π-conjugated oligomers such as oligophenylenes and
oligothiophenes have received considerable attention as electronic
materials.1 Thin films of these low molecular weight compounds
can be obtained by sublimation under high vacuum and used for
electronic devices such as organic field-effect transistors and
organic light-emitting diodes (OLEDs).2 For OLEDs, each organic
layer (hole-transport, emission, and electron-transport layers) has
to be amorphous, and crystallization during the device operation
results in shorter lifetime. Unlike linear oligomers, dendrimers
have a branched structure and are expected to favor an amorphous
morphology.3 We have been interested in developing new
electron-transport materials4 for OLEDs and designed perfluori-
nated phenylene dendrimers5 because of (1) their low-lying
LUMOs and HOMOs, which are important for electron injection
and hole blocking, respectively, (2) relatively low sublimation
temperature, which makes it possible to deposit high molecular
weight compounds with high glass transition temperatures, and
(3) thermal and chemical stability due to strong C-F bonds.6
However, there have been few perfluorinated phenylene com-
pounds,7 and their general synthetic methods have not been
developed. We report herein the repetitive synthesis of perflu-
orinated phenylene dendrimers and their thermal properties.
OLEDs have been fabricated to investigate electron-transport
properties of these new materials.
Scheme 1
The cross-coupling reaction between two different fluorinated
phenyl groups is the most important step for preparing perflu-
orinated dendrimers. We found that organocopper chemistry gave
the most satisfactory results: pentafluorophenylcopper (C6F5Cu)8
was allowed to react with 1,3,5-tribromo-2,4,6-trifluorobenzene
(6) to give 19 in 81%.10 Then, this procedure was applied to
perfluorinated phenylene dendrimer 2 (C60F42; MW ) 1518) as
† 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.
shown in Scheme 1. Trifluorophenylcopper 7, prepared from the
corresponding Grignard reagent (C6H2F3MgBr) and copper(I)
bromide without isolation, was allowed to react with 6 in a THF/
dioxane/toluene mixture at 80 °C for 24 h to afford compound 8
in 69%. Bromination of 8 gave hexabromide 9 in 79%. Again,
the coupling reaction of 9 and C6F5Cu yielded 2 in 85%.11,12
Similarly, dendrimer 3 (C132F90; MW ) 3295), the higher
generation of dendrimer 2, was prepared from compound 9 by
repeating cross-coupling and bromination as depicted in Scheme
1. We also synthesized two C60F42 isomers 4 and 5 to see
structure-property relationships. Compounds 2-5 were purified
(2) For recent reviews, see: (a) 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. (b) Sheats, J. R.; Antoniadis, H.; Hueschen, M.; Leonard, W.; Miller, J.;
Moon, R.; Roitman, D.; Stocking A. Science 1996, 273, 884-888.
(3) (a) Dendrimers; Vo¨gtle, F., Ed.; Topics in Current Chemistry No. 197;
Springer-Verlag: Berlin, 1998. (b) Carbon Rich Compounds I; Meijere, A.
de, Ed.; Topics in Current Chemistry No. 196; Springer-Verlag: Berlin, 1998.
(4) (a) Strukelj, M.; Papadimitrakopoulos, F.; Miller, T. M.; Rothberg, L.
J. Science 1995, 267, 1969-1972. (b) Tamao, K.; Uchida, M.; Izumizawa,
T.; Furukawa, K.; Yamaguchi, S. J. Am. Chem. Soc. 1996, 118, 11974-11975.
(c) Noda, T.; Shirota, Y. J. Am. Chem. Soc. 1998, 120, 9714-9715 and
references therein.
(5) Miller et al. reported partially fluorinated phenylene dendrimers. Miller
T. M.; Neenan, T. X.; Zayas, R.; Bair, H. E. J. Am. Chem. Soc. 1992, 114,
1018-1025.
(9) Pozdnyakovich, Yu. V.; Shteingarts, V. D. Zh. Org. Khim. 1977, 13,
1911-1917.
(6) (a) Chemistry of Organic Fluorine Compounds II: A Critical ReView;
Hudlicky´, M., Pavlath, A. E., Eds.; ACS Monograph 187; American Chemical
Society: Washington, DC, 1995. (b) Organofluorine Chemistry: Principle
and Commercial Applications; Banks, R. E., Smart, B. E., Tatlow, J. C., Eds.;
Topics in Applied Chemistry; Plenum Press: New York, 1994.
(7) For example, perfluoro-3,3′,5,5′-tetrakis(phenyl)-1,1′-biphenyl: Kobrina,
L. S.; Salenko, V. L.; Yakobson, G. G. J. Fluorine Chem. 1976, 8, 193-207.
(8) Cairncross, A.; Sheppard, W. A.; Wonchoba, E. Organic Syntheses;
Wiley & Sons: New York, 1988; Collect. Vol. VI, pp 875-882.
(10) Attempts to use the Stille reaction for the synthesis of perfluorinated
phenylenes have been unsuccessful: the reaction of C6F5SnMe3 and 6 in the
presence of Pd(0) or Pd(II) catalysts gave 1 in 5%. (a) Farina, V.;
Krishnamurthy, V.; Scott, W. J. The Stille Reaction; Wiley & Sons: New
York, 1998. (b) Deacon, G. B.; Gatehouse B. M.; Nelson-Reed, K. T. J.
Organomet. Chem. 1989, 359, 267-283.
(11) See Supporting Information for the experimental details.
(12) The reaction of perfluoro-3,5-bis(phenyl)phenylcopper and 6 gave a
complex mixture.
10.1021/ja994083z CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/15/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 8, 2000 1833
To understand the above results, we performed the electro-
chemical measurements on compounds 2-5.11 In THF, all
compounds showed irreversible electroreductions by cyclic vol-
tammetry (CV). The reduction peaks shift more positive in the
order 3 (-2.66 V), 2 (-2.56 V), 4 (-2.31 V), and 5 (-2.24 V
vs Fc/Fc+).17 This order is consistent with the number of para-
conjugated benzene rings in these compounds: 2 and 3 (biphenyl)
< 4 (p-terphenyl) < 5 (p-quaterphenyl). When the LUMO energy
level of the electron-transport material becomes lower (in this
case, from 3 to 5), the electron injection from the metal layer to
the electron-transport layer should be easier. It has been pointed
out that there are no correlations between reduction potentials
and electron-transport capabilities: a good electron acceptor is
not always a good electron-transport material.4a This indicates
that chemical interactions between an organic molecule and a
metal should be taken into account.18 In our case, however, a
F-Li+ bond would be much weaker than an O-Li+ bond or a
N-Li+ bond in the case of O- or N-containing electron-transport
materials. Therefore, the reduction potentials correlate well with
the electron-injection barriers and the performance of the devices.
In conclusion, we have shown the general synthetic method
for perfluorinated phenylene compounds with relatively high
molecular weights. Contrary to our expectation, perfluorinated
phenylenes 4 and 5 gave more stable amorphous films and showed
better electron-transport capabilities compared with dendrimers
2 and 3. These findings should be useful in designing new
amorphous materials for electronic devices. We are currently
measuring electron mobilities19 for compounds 2-5 to obtain
further information about their electron-transport properties and
preparing perfluorinated phenylenes with longer para-conjugated
units.20
Figure 1. Luminance-voltage characteristics for the OLEDs, ITO/TPTE
(60 nm)/Alq3(40 nm)/2-5(20 nm)/LiF(0.5 nm)/Al(160 nm).
by train sublimation13 and used for characterization. The structures
of 2-5 were confirmed by 19F NMR, EI-MS, and elemental
analyses. All perfluorinated phenylenes are colorless solids and
soluble in CHCl3, THF, and aromatic solvents such as toluene.
The differential scanning calorimetry (DSC) measurements
indicated that dendrimers 2 and 3 purified by train sublimation
are highly crystalline solids (mp ) 277 and 426 °C, respec-
tively).11 In the case of 2, the melting endotherm shifted from
277 to 243 °C on the second heating, indicating the formation of
a different crystalline phase. A glass transition was observed at
125 °C, only when the melt of 2 was rapidly cooled by liquid
nitrogen. Compound 4 showed a melting endotherm at 313 °C
on the first heating, and a glass transition at 133 °C was observed
on the second heating. Although compound 5 is conformationally
more rigid compared with 2 and 4, it favors an amorphous
phase: only a glass transition at 135 °C was observed.
OLEDs were made on indium-tin-oxide (ITO) coated glass
substrates by high-vacuum thermal evaporation (5 × 10-7 Torr)
of TPTE14 (a tetramer of triphenylamine) as the hole-transport
layer (60 nm), tris(8-quinolinolato)aluminum (Alq3) as the emis-
sion layer (40 nm), 2-5 (20 nm), LiF15 (0.5 nm) as the electron-
injection layer, and Aluminum (160 nm) as the cathode.11 When
a negative voltage was applied to Al, a green emission due to
Alq3 was observed for each device. This indicates that perflu-
orinated phenylenes 2-5 work as the electron-transport layer.
Figure 1 shows the luminance-voltage characteristics for these
OLEDs. The performance of the devices is improved in the order
3 < 2 < 4 < 5.16 The maximum luminance of the device with 5
is 2860 cd/m2 at 24.4 V.
Acknowledgment. The authors at IMS thank Japan Society for the
Promotion of Science for Grant-in-Aid for Scientific Research (No.
11640594).
Supporting Information Available: Experimental details for 2-5
and OLEDs, 19F NMR and mass spectra of 2 and 3, and DSC curves and
CVs of 2-5 (PDF). This materials is available free of charge via the
Internet at http://pubs.acs.org.
JA994083Z
(16) After a few days, a decrease of luminance was observed for the device
with 3 because of crystallization.
(17) The reduction peak of Alq3 is -2.36 V under the same conditions.
(18) For example, it was suggested that Alq3 forms a stable anion radical
salt with Li. In the facial isomer, Li+ is strongly coordinated by the three
oxygen atoms of Alq3. Curioni, A.; Andreoni, W. J. Am. Chem. Soc. 1999,
121, 8216-8220 and references therein.
(19) (a) Tokuhisa, H.; Era, M.; Tsutsui, T.; Saito, S. Appl. Phys. Lett. 1995,
66, 3433-3435. (b) Redecker, M.; Bradley, D. D. C.; Jandke, M.; Strohriegl,
P. Appl. Phys. Lett. 1999, 75, 109-111.
(20) Very recently, we found that the use of perfluoro-p-sexiphenyl (C36F26)
as an electron-transport layer dramatically improved the device performance.
The maximum luminance reached 12200 cd/m2 at 13.7 V. Further experiments
are being carried out and will be reported elsewhere. Heidenhain, S.; Sakamoto,
Y.; Miura, A.; Fujikawa, H. Unpublished work, 1999.
(13) Wagner, H. J.; Loutfy, R. O.; Hsiao, C.-K. J. Mater. Sci. 1982, 17,
2781-2791.
(14) Tanaka, H.; Tokito, S.; Taga, Y.; Okada, A. J. Chem. Soc., Chem.
Commun. 1996, 2175-2176.
(15) (a) Hung, L. S.; Tang, C. W.; Mason, M. G. Appl. Phys. Lett. 1997,
70, 152-154. (b) Mori, T.; Fujikawa, H.; Tokito, S.; Taga, Y. Appl. Phys.
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