Synthesis of Octasubstituted Cyclooctatetraenes
J. Am. Chem. Soc., Vol. 122, No. 31, 2000 7481
OLEDs is aluminum tris(8-hydroxyquinoline), Alq3.1,9 Other
metal complexes have been reported for use as ETL materials
as well; however, their efficiencies and stabilities are typically
worse than those of Alq3-based OLEDs.10 Heterocyclic com-
pounds, such as oxidiazoles and triazolzes, have been the most
thoroughly studied organic ETL materials.11 ETL materials with
large energy gaps are very desirable, since they could be used
to fabricate OLEDs with a wide range of emission colors,
including blue. Alq3 and its analogues can be used for green to
red OLEDs, but their energy gaps are too small to make them
useful for blue OLEDs. While organic ETLs have been reported
which can be used to fabricate blue OLEDs,1 the efficiencies
of these devices and their lifetimes are often poor. The goal of
the work reported here is to develop new ETL materials, which
can be used in blue OLEDs.
1,12
We have prepared a number of different octasubstituted
cyclooctatetraenes (COTs) and investigated them as organic
electron transporting materials in OLEDs. Previously reported
synthetic routes to substituted COT derivatives typically give
Figure 1. Possible structures of tetraaryl-tetraarylkynyl-cyclooctatet-
raene derivatives (Ar ) arene). Acronyms used throughout the paper:
Ar ) phenyl (COT-H); Ar ) p-tolyl (COT-Me); Ar ) 2-thiophenyl
1
4,18,15,22
low yield.
In this paper we report novel, high-yield
(COT-th); Ar ) p-anisole (COT-OMe).
syntheses of both tetraaryl-tetraarylethynyl-COTs and octaaryl-
COTs. These COTs form highly stable glasses and act as
efficient electron transporting materials in organic LEDs. The
energy gaps for the COT derivatives studied here are sufficiently
wide (>3 eV) to make it possible to fabricate blue emitting
OLEDs with these materials.
Results and Discussion
Synthesis and Characterization of Octasubstituted Cy-
clooctatetraenes. Transition metal catalyzed cyclotrimerization
of acetylenes to yield benzene derivatives is well-known.13 On
the other hand, the transition metal catalyzed cyclotetrameriza-
tion of acetylenes to yield cyclooctatetraene derivatives is less
common. Reppe reported that acetylene itself could be tetramer-
ized by nickel catalysts to yield cyclooctatetraene (COT).14
However, nickel catalysts do not work well for substituted
(8) (a) O’Brien, D. F.; Burrows, P. E.; Forrest, S. R.; Koene, B. E.; Loy,
D. E.; Thompson, M. E. AdV. Mater. 1998, 10, 1108. (b) Koene, B. E.;
Loy, D. E.; Thompson, M. E. Chem. Mater. 1998, 10, 2235. (c) Adachi,
C.; Tsutsui, T.; Saito, S. Optoelectron. DeV. Technol. 1991, 6, 25. (d)
VanSlyke, S. A.; Chen, C. H.; Tang, C. W. Appl. Phys. Lett. 1996, 69,
2
160. (e) Shirota, Y. J. Mater. Chem. 2000, 10, 1.
9) Schmidt, M. L.; Anderson, N. R.; Armstrong, J. Appl. Phys. 1995,
8, 5619. Saito, S.; Tsutsui, T.; Era, M.; Takada, N.; Adachi, C.; Hamada,
15
acetylenes.
(
We have previously reported that a ruthenium catalysts will
7
1
6
Y.; Wakimoto, T. Proc. SPIE 1993, 1910, 212. Tang, C. W.; VanSlyke, S.
A.; Chen, C. H. J. Appl. Phys. 1989, 65, 3610. Curioni, A.; Andreoni, W.
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copolymerize benzophenone and dialkynes. When diphenyl-
butadiyne is treated with the Ru catalyst [(Ph3P)3Ru(CO)H2
17
activated with a stoichiometric amount of styrene], a cyclooc-
(10) Kido, J.; Hayase, H.; Hongawa, K.; Nagai, K.; Okuyama, K. Appl.
tatetraene, COT-H, is formed. Ru-catalyzed cyclotetrameriza-
tions of di-p-tolylbutadiyne, di-p-methoxyphenylbutadiyne, and
di(2-thienyl)butdiyne have also been carried out to produce the
corresponding COT derivatives, abbreviated COT-Me, COT-
OMe, and COT-th, respectively. This reaction is extremely
efficient and selective. COT-H is formed in 86% yield after
purification by chromatography and recrystallization. The mo-
lecular weight of the tetramer was established by mass
spectrometry. The major fragmentation pathway of the parent
cation radical is loss of diphenylbutadiyne to give a trimer cation
radical. Most of the octasubstituted COTs form stable glasses,
with high glass transition temperatures, e.g., Tg values for COT-
H, COT-Me, and COT-OMe are 177, 214, and 194 °C,
respectively. They are thermally stable and do not undergo loss
of weight by TGA below 310 °C.
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(
1
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994, 43, 2450.
(13) Berthelot, M. Ann. 1866, 139, 273.
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Schr o¨ der, G. Cyclooctatetraenes; Verlag-Chemie: Weinheim, Germany,
965.
1
(
(
15) Leto, J. R.; Leto, M. F. J. Am. Chem. Soc. 1961, 83, 2944.
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995, 28, 5686.
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Organic Chemistry; Harper and Row: New York, 1976.
19) Anet, F. A. L.; Bourn, A. J. R.; Lin, Y. S. J. Am. Chem. Soc. 1964,
6, 3576.
(
Assuming the aryl groups and the other carbons of the starting
diphenylbutadiyne maintain their initial connectivity, there are
four possible isomeric cyclooctatetraenes which can be formed,
labeled I-IV in Figure 1. Cyclooctatetraenes are not planar but
instead have been shown to be tub-shaped molecules, which
1
(
(
8
(
(
20) Freedman, H. H. J. Am. Chem. Soc. 1961, 83, 2195.
21) Huang, L.; Aulwurn, U. R.; Heinemann, F. W.; Kisch, H. Eur. J.
1
8,19
are highly fluxional at room temperature.
On the basis of a
Inorg. Chem. 1998, 1951.
22) (a) Braye, E. H.; Hubel, W. J. Am. Chem. Soc. 1961, 86, 4725. (b)
Hoberg, H.; Frolich, C. Angew. Chem. 1980, 92, 131. (c) Frolich, C.; Hoberg,
H. J. Organomet. Chem. 1981, 201, 131. (d) Eisch, J. J.; Piotrowski, A.
M.; Systems, N. Z. Naturforsch. B, Anorg. Chem., Org. Chem. 1985, 40B,
tub conformation for the COT ring (ignoring the phenyl ring
conformations), I and II each have D2 symmetry, III has C2
symmetry, while isomer IV has C1 symmetry. Therefore, the
(
1
3
number of acetylenic resonances in the C NMR for each
isomer is expected to be two, two, four, and eight for isomers
6
24. (e) Hoberg, H.; Richter, W. J. Organomet. Chem. 1980, 195, 355. (f)
Hoberg, H.; Richter, W. J. Organomet. Chem. 1980, 195, 347. (g) Eisch,
J. J.; Galle, J. E.; Aradi, A. A.; Beleslawski, M. P. 1986, 312, 399. (h)
Calderazzo, F.; Marchetti, F.; Pampaloni, G.; Hiller, W.; Antropiusova, H.;
Mach, K. Chem. Ber. 1989, 122, 2229.
13
I, II, III, and IV, respectively. In fact, the C NMR spectra of
COT-H has eight distinct acetylenic carbon resonances. This
is consistent with the unsymmetrical isomer IV. Similarly, eight