Facile synthesis of multifullerene-OPE hybrids via in situ ethynylation

Article abstract of DOI:10.1021/ol049447t

A series of fullerene-terminated oligo(phenylene ethynylene)s (OPEs) (1a-d and 2) have been synthesized and further characterized by cyclic voltammetry (CV) and UV-vis spectroscopy. The key step in the syntheses is an effective one-pot reaction that allow

Full text of DOI:10.1021/ol049447t

ORGANIC
LETTERS
2004
Vol. 6, No. 13
2129-2132
Facile Synthesis of Multifullerene-OPE
Hybrids via in Situ Ethynylation
Yasuhiro Shirai, Yuming Zhao, Long Cheng, and James M. Tour*
Department of Chemistry and Center for Nanoscale Science and Technology,
Rice UniVersity, MS-222, 6100 Main St., Houston, Texas 77005
tour@rice.edu
Received March 24, 2004
ABSTRACT
A series of fullerene-terminated oligo(phenylene ethynylene)s (OPEs) (1a−d and 2) have been synthesized and further characterized by cyclic
voltammetry (CV) and UV−vis spectroscopy. The key step in the syntheses is an effective one-pot reaction that allows the attachment of C
60
to multiple terminal alkynes.
Buckminster [60]fullerene is a useful building block for
making various molecular devices and nanoarchitectures due
to its unique three-dimensional electron delocalization and
small diameter (ca. 0.7 nm). So far, most well-defined
multiple-fullerene-containing molecules are derived from
methanofullerenes,1 fulleropyrrolidines,2 fullerene-acenes,3
and ethynylated fullerenes.4 Except for the dumbbell-type
fullerenes,4 there are few examples of multifullerene com-
pounds in which fullerene molecules are directly attached
via an acetylenic bond in a linear geometry. Lately, integrat-
ing fullerenes as terminal groups in conjugated oligomeric
systems has gained attention; however, to our knowledge,
the compounds made are limited to primarily mono- or bis-
(fullerene) derivatives, and in most cases, the fullerenes are
linked to the oligomers through more than one sp3 carbon
and in a nonlinear fashion.5 In our research, we extended
(2) (a) Zhang, S.; Gan, L.; Huang, C.; Lu, M.; Pan, J.; He, X. J. Org.
Chem. 2002, 67, 883. (b) de Lucas, A. I.; Mart´ın, N.; Sa´nchez, L.; Seoane,
C. Tetrahedron Lett. 1996, 37, 9391. (c) Sun, Y.; Drovetskaya, T.; Bolskar,
R. D.; Bau, R.; Boyd, P. D. W.; Reed, C. A. J. Org. Chem. 1997, 62, 3642.
(d) Segura, J. L.; Priego, E. M.; Mart´ın, N.; Luo, C.; Guldi, D. M. Org.
Lett. 2000, 2, 4021. (e) Mamane, V.; Riant, O. Tetrahedron 2001, 57, 2555.
(f) Higashida, S.; Imahori, H.; Kaneda, T.; Sakata, Y. Chem. Lett. 1998,
605.
(3) (a) Miller, G. P.; Briggs, J.; Mack, J.; Lord, P. A.; Olmstead, M. M.;
Balch, A. L. Org. Lett. 2003, 5, 4199. (b) Miller, G. P.; Briggs, J. Org.
Lett. 2003, 5, 4203.
(4) (a) Timmerman, P.; Witschel, L. E.; Diederich, F.; Boudon, C.;
Gisselbrecht, J.-P.; Gross, M. HelV. Chim. Acta 1996, 79, 6. (b) Komatsu,
K.; Takimoto, N.; Murata, Y.; Wan, T. S. M.; Wong, T. Tetrahedron Lett.
1996, 37, 6153. (c) Tanaka, T.; Komatsu, K. J. Chem. Soc., Perkin Trans.
1 1999, 1671.
(1) (a) Nierengarten, J.-F.; Schall, C.; Nicoud, J.-F. Angew. Chem., Int.
Ed. 1998, 37, 1934. (b) Armaroli, N.; Boudon, C.; Felder, D.; Gisselbrecht,
J.-P.; Gross, M.; Marconi, G.; Nicoud, J.-F.; Nierengarten, J.-F.; Vicinelli,
V. Angew. Chem., Int. Ed. 1999, 38, 3730. (c) Gonza´lez, S.; Mart´ın, N.;
Guldi, D. M. J. Org. Chem. 2003, 68, 779. (d) Diekers, M.; Luo, C.; Guldi,
D. M.; Hirsch, A. Chem. Eur. J. 2002, 8, 979. (e) Kessinger, R.; Thilgen,
C.; Mordasini, T.; Diederich, F. HelV. Chim. Acta 2000, 83, 3069. (f) Knol,
J.; Hummelen, J. C. J. Am. Chem. Soc. 2000, 122, 3226. (g) Nierengarten,
J.-F.; Herrmann, A.; Tykwinski, R. R.; Ru¨ttimann, M.; Diederich, F.;
Boudon, C.; Gisselbrecht, J.-P.; Gross, M. HelV. Chim. Acta 1997, 80, 293.
10.1021/ol049447t CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/28/2004

Scheme 1
Scheme 2
the “fullerene dumbbells” with rigid conjugated oligo-
(phenylene ethylene) (OPE) molecular wires, where only one
sp3 carbon is attached to the OPE. The significance of
synthesizing 1 and 2 is not only for their interesting
architecture but also the production of new candidates for
organic photovoltaic devices,6 third-order nonlinear optical
materials,7 and molecular electronics.8 Ethynylated fullerenes
are generally prepared via a method of lithium acetylide
addition as first reported by Komatsu et al.9 This ethynylation
procedure has wide applicability, and a number of acetylene
derivatives of C60 have been successfully prepared.4,10 None-
theless, as the original report claimed,9 the yield of this type
of reaction is highly dependent on experimental conditions.
In our numerous attempts to make multifullerene-OPE
derivatives using the above procedure, the reactions gave
very low yields of desired product. To overcome this
synthetic difficulty, we devised a reproducible and effective
approach, namely, an in situ-generated lithium acetylide
addition in THF (Scheme 1). Interestingly, in our hands only
lithium hexamethyldisilazide (LHMDS) has been found to
be efficacious in generating multifullerene species. Many
other bases, including n-BuLi, lithium diisopropylamide
(LDA), and lithium 2,2,6,6-tetramethylpiperidide (LTMP),
either failed to yield products or resulted in diminutive yields.
The reasons for the success of LHMDS are mechanistically
unclear; however, LHMDS may first react with the fullerene
to form a somewhat more soluble and reactive intermediate.11
A series of the C60-terminated phenylene ethynylene oligo-
mers 1a-d have been successfully prepared based on this
newly developed methodology (Scheme 1). A general
procedure is to treat a well-sonicated mixture of alkyne and
excess C60 (ca. 2 equiv per terminal alkyne) with LHMDS
(ca. 4 equiv) in dry THF and under a nitrogen atmosphere.
An indication that the reaction is proceeding is the formation
of a deep green-black color. In general, the reaction proceeds
smoothly at ambient temperature and is completed within 1
h. Afterward, it is quenched with excess trifluoroacetic acid
(TFA) to give a brownish slurry. The crude product is then
purified through a silica flash column. In our work, the
terminal OPEs were obtained from deprotection of their silyl-
protected OPE precursors followed by silica gel purification
for 3a and 4. The deprotected 3b-d were pure enough to
use without further purification. Our procedure has also been
successfully applied to the synthesis of an octupolar tris-
(fullerene)-OPE hybrid 2 (Scheme 2). Surprisingly, the yield
of 2 was similar to the yields for the bis(fullerene)-OPE
hybrids 1a-d. The preparation of compounds containing
higher numbers of fullerene species is presently underway
in our laboratories.
(5) (a) Gu, T.; Nierengarten, J.-F. Tetrahedron Lett. 2001, 42, 3175. (b)
Guldi, D. M.; Luo, C.; Swartz, A.; Go´mez, R.; Segura, J. L.; Mart´ın, N.;
Brabec, C.; Sariciftci, N. S. J. Org. Chem. 2002, 67, 1141. (c) van Hal, P.
A.; Knol, J.; Langeveld-Voss, B. M. W.; Meskers, S. C. J.; Hummelen, J.
C.; Janssen, R. A. J. J. Phys. Chem. A 2000, 104, 5974. (d) Obara, Y.;
Takimiya, K.; Aso, Y.; Otsubo, T. Tetrahedron Lett. 2001, 42, 6877. (e)
Ikemoto, J.; Takimiya, K.; Aso, Y.; Otsubo, T.; Fujitsuka, M.; Ito, O. Org.
Lett. 2002, 4, 309. (f) Eckert, J.-F.; Nicoud, J.-F.; Nierengarten, J.-F.; Liu,
S.-G.; Echegoyen, L.; Barigelletti, F.; Armaroli, N.; Ouali, L.; Krasnikov,
V.; Hadziioannou, G. J. Am. Chem. Soc. 2000, 122, 7467. (g) Peeters, E.;
van Hal, P. A.; Knol, J.; Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C.;
Janssen, R. A. J. J. Phys. Chem. B 2000, 104, 10174. (h) Liu, S.-G.;
Martineau, C.; Raimundo, J.-M.; Roncali, J.; Echegoyen, L. Chem. Commun.
2001, 913.
The OPE precursors 3a-d and 4 were prepared by
Sonogashira coupling via an iterative synthetic approach as
illustrated in Scheme 3. Hydroquinone was alkylated with
(6) (a) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Ahn, T.
K.; Kim, S. K.; Kim, D.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 9129.
(b) Hirayama, D.; Takimiya, K.; Aso, Y.; Otsubo, T.; Hasobe, T.; Yamada,
H.; Imahori, H.; Fukuzumi, S.; Sakata, Y. J. Am. Chem. Soc. 2002, 124,
532.
(7) (a) Hamasaki, R.; Ito, M.; Lamrani, M.; Mitsuishi, M.; Miyashita,
T.; Yamamoto, Y. J. Mater. Chem. 2003, 13, 21. (b) Lamrani, M.; Hamasaki,
R.; Mitsuishi, M.; Miyashita, T.; Yamamoto, Y. Chem. Commun. 2000,
1595.
(8) (a) Tour, J. M. Molecular Electronics: Commercial Insights,
Chemistry, DeVices, Architecture and Programming; World Scientific: River
Edge, NJ, 2003. (b) Bunz, U. H. F. Chem. ReV. 2000, 100, 1605-1644.
(9) Komatsu, K.; Murata, Y.; Takimoto, N.; Mori, S.; Sugita, N.; Wan,
T. S. M. J. Org. Chem. 1994, 59, 6101.
(10) (a) Okamura, H.; Murata, Y.; Minoda, M.; Komatsu, K.; Miyamoto,
T.; Wan, T. S. M. J. Org. Chem. 1996, 61, 8500. (b) Fujiwara, K.; Murata,
Y.; Wan, T. S. M.; Komatsu, K. Tetrahedron 1998, 54, 2049. (c) Draper,
S. M.; Delamesiere, M.; Champeil, E.; Twamley, B.; Byrne, J. J.; Long, C.
J. Organomet. Chem. 1999, 589, 157. (d) Komatsu, K.; Fujiwara, K.;
Murata, Y.; Braun, T. J. Chem. Soc., Perkin Trans. 1 1999, 2963. (e) Murata,
Y.; Ito, M.; Komatsu, K. J. Mater. Chem. 2002, 12, 2009. (f) Kunieda, R.;
Fujitsuka, M.; Ito, O.; Ito, M.; Murata, Y.; Komatsu, K. J. Phys. Chem. B
2002, 106, 7193.
(11) In a blank test, in which LHMDS was added into a solution of
fullerene in THF, we observed a color change (black green) similar to that
which occurred in normal fullerene ethynylation reactions. However, so
far, efforts to identify the intermediate have not been successful.
2130
Org. Lett., Vol. 6, No. 13, 2004

Scheme 3 a
a Pd/Cu ) PdCl2(PPh3)2, CuI.
1-iododecane in the presence of KOH in refluxing EtOH to
give 1,4-bis(decyloxy)benzene, which was then iodinated to
yield the building block 5 in an overall yield of 66%.12 Cross-
coupling of 5 with only 0.7 equiv of trimethylsilylacetylene
(TMSA) afforded the monoiodide 6. Then, cross-coupling
of 6 with triisopropylsilylacetylene (TIPSA) gave monomer
7, which was selectively deprotected using K2CO3 to afford
monomer 8. Compound 8 was further cross-coupled with 6
to give 3b, which was then treated with K2CO3, affording
OPE dimer 9.
With all these building blocks in hand, the synthesis of
OPE oligomers was readily achieved through a convergent
coupling strategy. Diiodide 5 was cross-coupled with excess
TMSA under Sonogashira conditions, followed by depro-
tection with K2CO3 yielding OPE monomer 3a in 85% yield.
In the same manner, cross-coupling of 5 with 8 and of 5
with 9 afforded OPE trimer 3c and pentamer 3d, respectively.
Finally, dimer 9 was coupled with triiodobenzene to give
the trigonal OPE 10, which was deproteced with TBAF to
afford 4.
The electrochemical behavior of these multifullerene-OPE
hybrids has been characterized by cyclic voltammetry (CV).
Typical CVs for 1a and 2 in comparison with pristine C60
are shown in Figure 1. The most pronounced feature of
Figure 1 is the close resemblance of CV curves for the
mutilfullerene-OPEs to C60. Although it is possible for there
to be electronic communication between fullerene moieties,13
only three couples of reversible redox waves are observed
in the accessible potential window of the solvent under the
present conditions for all cases (Supporting Information).
These results suggest that electronic interactions between
fullerene cages in the CV time scale are absent or very weak,
a conclusion that is in line with the earlier report on
acetylene-connected dumbbell-type fullerenes.4 Detailed re-
dox potential data of 1a-d and 2 are summarized in Table
1. The three redox waves (I-III) observed for 1a-d and 2
all shift toward more negative potentials relative to pristine
(13) (a) Balch, A. L.; Costa, D. A.; Fawcett, W. R.; Winkler, K. J. Phys.
Chem. 1996, 100, 4823. (b) Dragoe, N.; Shimotani, H.; Wang, J.; Iwaya,
M.; de Bettencourt-Dias, A.; Balch, A. L.; Kitazawa, K. J. Am. Chem. Soc.
2001, 123, 1294. (c) Lee, K.; Song, H.; Kim, B.; Park, J. T.; Park, S.; Choi,
M.-G. J. Am. Chem. Soc. 2002, 124, 2872. (d) Lee, G.; Cho, Y.-J.; Park,
B. K.; Lee, K.; Park, J. T. J. Am. Chem. Soc. 2003, 125, 13920.
(12) Zhou, Q.; Carroll, P. J.; Swager, T. M. J. Org. Chem. 1994, 59,
1294.
Org. Lett., Vol. 6, No. 13, 2004
2131

Figure 1. Cyclic voltammograms of C60, 1a, and 2 in 0.1 M
TBABF4/o-dichlorobenzene at room temperature. The working
electrode was glassy carbon, and the counter electrode was a Pt
wire; the nonaqueous reference electrode was Ag|AgNO3. The
formal potential of 2 mM ferrocene in 0.1 M TBABF4 and
acetonitrile was 70 mV vs this Ag|AgNO3 reference electrode. Scan
rate was 10 mV/s.
Figure 2. Electronic absorption spectra of 1a-d and 2 as measured
in o-dichlorobenzene.
nounced in the absorption curves of 1c and 1d because the
strong absorption of OPEs predominate in this region. The
analytical data confirm that there is no significant electronic
communication or charge-transfer interaction between the
fullerenes and OPEs occurring in the ground state, which is
in agreement with our CV data as well as results in the report
on monofullerene-OPE hybrids.5a Additionally, fluorescence
quenching was detected under a UV lamp for all the
fullerene-OPE hybrids compared to their strongly fluorescent
OPE precursors. This observation suggests that stronger
electronic interactions are taking place between the fullerene
cages and OPE backbones in the photoexcited state.14
In conclusion, an efficient and practical synthetic method
for directly attaching C60 to multiple terminal alkynes has
been developed using an in situ acetylide addition reaction
with LHMDS as the base. This method has enabled us to
synthesize a series of multifullerene-OPE hybrid molecules
with satisfactory yields. Spectroscopic and electrochemical
studies on these molecules did not show detectable electronic
communication between the OPE and fullerene moieties in
the ground state.
C60. This is probably because of the inductive effect from
the electron-withdrawing OPE moiety and/or the decrease
of π-delocalization on the fullerene cage due to the introduc-
tion of two sp3 carbon atoms.9 The formal potentials of the
first redox wave I cathodically shift with the increasing chain
length of the OPEs.
The electronic absorption properties of these multifullerne-
OPEs were studied by UV-vis spectroscopy. In the UV-
vis spectra (Figure 2), a bathochromic shift of the lowest
energy absorption maxima (λmax) from 1a to 1d is observed,
which is due to the increasing π-conjugation path in OPEs.
The same degree of shift of λmax is observed in the corre-
sponding OPE precursors; data that appear to rule out elec-
tronic communication between fullerenes and OPE back-
bones. A characteristic absorption of C60 derivatives centered
around ca. 430 nm10a is observed as in the UV-vis profiles
of 1a and 1b, whereas such a feature becomes less pro-
Acknowledgment. We thank the Welch Foundation in
Texas, the Defense Advanced Research Projects Agency, the
Office of Naval Research, and the Penn state MRSEC funded
by National Science Foundation (NSF) for financial support.
The NSF, CHEM 0075728, provided partial funding for the
400 MHz NMR. We thank Dr. I. Chester of FAR Research,
Inc., for providing trimethylsilylacetylene. Y. Zhao thanks
the NSERC Canada for providing a postdoctoral fellowship.
Table 1. Results of Cyclic Voltammetry
formal potentiala, E1/2
compd
I
II
III
C60
1a
1b
1c
1d
2
-0.832
-0.901
-0.905
-0.906
-0.909
-0.918
-1.227
-1.281
-1.288
-1.286
-1.292
-1.304
-1.691
-1.813
-1.814
-1.813
-1.814
-1.831
Supporting Information Available: Experimental details
along with spectroscopic data for all new molecules synthe-
sized and CVs of compounds 1b-d. This material is
available free of charge via the Internet at http://pubs.acs.org.
a Formal potentials are calculated as averages of oxidation and reduction
peak potentials. Potentials are given in volts vs a nonaqueous reference
electrode, Ag|AgNO3. The formal potential of 2 mM ferrocene in 0.1 M
TBABF4 and acetonitrile is 70 mV vs this Ag|AgNO3 reference electrode.
OL049447T
(14) Guldi, D. M.; Prato, M. Acc. Chem. Res. 2000, 33, 695.
2132
Org. Lett., Vol. 6, No. 13, 2004