Synthesis of fullerene-oligophenyleneethynylene hybrids

Article abstract of DOI:10.1016/S0040-4039(01)00384-7

Disymmetrically substituted oligophenyleneethynylene (OPE) derivatives have been prepared by a new iterative method and attached to C₆₀ by a 1,3-dipolar cycloaddition of the azomethine ylides generated in situ from the corresponding OPE aldehyd

Full text of DOI:10.1016/S0040-4039(01)00384-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 3175–3178
Synthesis of fullerene–oligophenyleneethynylene hybrids
Tao Gu and Jean-Franc¸ois Nierengarten*
Groupe des Mate´riaux Organiques, Institut de Physique et Chimie des Mate´riaux de Strasbourg,
Universite´ Louis Pasteur et CNRS, 23 rue du Loess, 67037 Strasbourg Cedex, France
Received 3 March 2001; revised 5 March 2001; accepted 6 March 2001
Abstract—Disymmetrically substituted oligophenyleneethynylene (OPE) derivatives have been prepared by a new iterative method
and attached to C60 by a 1,3-dipolar cycloaddition of the azomethine ylides generated in situ from the corresponding OPE
aldehydes and sarcosine. © 2001 Published by Elsevier Science Ltd.
Following the observation of photoinduced electron
transfer from conjugated polymers to C60,1 a novel
approach to the preparation of photovoltaic devices has
been proposed by Heeger, Wudl and co-workers.2 It is
based on an interpenetrating blend of donor (conju-
gated polymer) and acceptor (C60) sandwiched between
two asymmetric contacts (two metals with different
workfunctions). The performance of this type of device
is very sensitive to the morphology of the film.2 Ideally,
an acceptor species should be within the exciton diffu-
sion range from any donor species and vice versa.
However, the donor and acceptor molecules are usually
chemically incompatible and tend to undergo uncon-
trolled macrophase separation. In order to avoid any
problems arising from bad contacts at the junction, we
have recently proposed a new concept: the bicontinuous
network can be obtained by chemically linking a hole-
conducting oligophenylenevinylene moiety to an elec-
tron-conducting fullerene subunit.3,4 The hybrid
compound has been incorporated in a solar cell and a
photocurrent was effectively obtained.3 Even if the
efficiencies of the molecular photovoltaic systems are
still low, further improvements could be expected by
the utilization of new fullerene derivatives. In addition,
it is important to highlight that the behavior of a
unique molecule in a photovoltaic cell and the study of
its electronic properties allows us to obtain easily struc-
ture/activity relationships for a better understanding of
the photovoltaic system.4,5 As part of this research, we
now report the synthesis of C60 derivatives bearing
oligo-para-phenyleneethynylene (OPE) substituents for
future photovoltaic applications. The disymmetrically
substituted OPE precursors have been prepared by a
new iterative approach,6,7 using successive Corey–Fuchs
dibromoolefination,8 and treatment with an excess of
LDA followed by a metal-catalyzed cross-coupling
reaction of the resulting terminal alkyne.
The synthesis of the functionalized OPE derivatives is
depicted in Scheme 1. Reaction of hydroquinone with
1-bromooctane in DMF at 80°C in the presence of
K2CO3, followed by iodination of the resulting 1,4-
dioctyloxybenzene, yielded 1 in an overall yield of 57%.
Compound 2, the key building block to this synthesis,
was obtained in 85% yield by treatment of 1 with
n-BuLi (1 equiv.) in Et2O at 0°C followed by quenching
with DMF. Compound 2 was then subjected to a
Pd-catalyzed cross-coupling reaction9 with (triisopropyl-
silyl)acetylene to give alkynylated 3 in 79% yield. Sub-
sequent treatment with CBr4/PPh3/Zn under the
conditions described by Corey–Fuchs8 yielded dibromo-
olefine 4 in a quantitative yield. Elimination of HBr
and halogen–metal exchange was best achieved with an
excess of LDA10 in THF at −78°C and the resulting
anion was quenched with NH4Cl to give mono-pro-
tected bisalkyne 5 in 91% yield. The OPE dimer 6 was
then obtained in 69% yield by Pd-catalyzed cross-cou-
pling between 2 and 5. Dibromoolefination according
to Corey–Fuchs provided the desired intermediate
which after treatment with an excess of LDA in THF at
−78°C and quenching with NH4Cl afforded 7. Finally,
compound 7 was subjected to a Pd-catalyzed cross-cou-
pling reaction with 2 to yield the OPE trimer 8. This
new iterative methodology for the synthesis of disym-
metrically substituted OPE derivatives appears to be an
interesting alternative to the approaches reported by
the groups of Tour7a and Godt.7b On the one hand,
compared to the strategy based on the trimethylsilyl
and the 3,3-diethyltriazene functions as complementary
* Corresponding author. Fax: 33 388 10 72 46; e-mail: niereng@
ipcms.u-strasbg.fr
0040-4039/01/$ - see front matter © 2001 Published by Elsevier Science Ltd.
PII: S0040-4039(01)00384-7

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T. Gu, J.-F. Nierengarten / Tetrahedron Letters 42 (2001) 3175–3178
OH
OH
OR
OR
OR
OR
I
CHO
CHO
i, ii
iii
iv
(i-Pr)3Si
I
I
OR
OR
1 R = C8H17
2 R = C8H17
3 R = C8H17
OR
CHO
OR
OR
OR
H
OR
OR
OR
OR
Br
Br
v
vi
vii
OR
6 R = C8H17
(i-Pr)3Si
(i-Pr)3Si
OR
OR
(i-Pr)3Si
4 R = C8H17
5 R = C8H17
OR
CHO
H
viii, ix
OR
OR
OR
x
OR
OR
OR
7 R = C8H17
8 R = C8H17
(i-Pr)3Si
(i-Pr)3Si
Scheme 1. Reagents and conditions: (i) 1-bromooctane, K2CO3, DMF, 80°C, 24 h (90%); (ii) I2, KIO3, H2SO4, CH3CO2H, 80°C,
12 h (63%); (iii) n-BuLi (1 equiv.), Et2O, 0°C, 15 min., then DMF, 0°C to rt, 2 h (85%); (iv) (triisopropylsilyl)acetylene,
PdCl2(PPh3)2, CuI, Et3N, rt, 24 h (79%); (v) CBr4, PPh3, Zn, CH2Cl2, 0°C to rt, 12 h (99%); (vi) LDA, THF, −78°C, then aqueous
NH4Cl (91%); (vii) 2, PdCl2(PPh3)2, CuI, Et3N, rt, 24 h (69%); (viii) CBr4, PPh3, Zn, CH2Cl2, 0°C to rt, 12 h (99%); (ix) LDA,
THF, −78°C, then aqueous NH4Cl (92%); (x) 2, PdCl2(PPh3)2, CuI, Et3N, rt, 24 h (77%).
protecting groups for terminal alkyne and aryl iodine,7a
respectively, it avoids the use of large amounts of rather
volatile carcinogenic methyl iodide. On the other hand,
compared to the strategy based on the bromine–iodine
selectivity of the Pd-catalyzed alkyne–aryl coupling7b
which is not always completely iodo-selective, it pre-
vents the formation of undesirable symmetric
byproducts.
The preparation of the fullerene–OPE hybrids 9 and 10
is depicted in Scheme 2. The functionalization of C60 is
based on the 1,3-dipolar cycloaddition of the azome-
thine ylide generated in situ from the corresponding
aldehyde and N-methylglycine.11 In a typical proce-
dure, a solution of the OPE aldehyde 6 (350 mg), C60
(308 mg) and N-methylglycine (208 mg) in toluene (300
ml) was refluxed under argon for 16 h. After cooling,
6
Me
N
ii
i
RO
RO
OH
RO
RO
OR
OR
9 R = C8H17
(i-Pr)3Si
OR
(i-Pr)3Si
OR
11 R = C8H17
8
Me
ii
i
N
RO
RO
OH
RO
RO
OR
10 R = C8H17
OR
RO
RO
OR
OR
(i-Pr)3Si
OR
(i-Pr)3Si
OR
12 R = C8H17
Scheme 2. Reagents and conditions: (i) C60, N-methylglycine, toluene, D, 16 h (9: 36%; 10: 40%); (ii) DIBAL-H, THF, 0°C, 2 h
(11: 99%; 12: 92%).

T. Gu, J.-F. Nierengarten / Tetrahedron Letters 42 (2001) 3175–3178
3177
the resulting solution was evaporated to dryness and
column chromatography (SiO2, toluene/hexane 4:1)
yielded 9 (230 mg, 36%). Compound 10 was obtained
under similar conditions from 8, C60 and sarcosine. In
addition, compounds 11 and 12, which were used as
reference compounds for the absorption and emission
properties of the corresponding C60-OPE derivatives
were prepared by DIBAL-H reduction of 6 and 8,
respectively.
applications. Incorporation of compounds 9 and 10 in
devices is now under investigation for solar energy
conversion in collaboration with the Hadziioannou
group (Groningen, The Netherlands).
Acknowledgements
The C60-OPE derivatives 9 and 10 were characterized
This research was supported by ECODEV and the
French Ministry of Research (ACI Jeunes Chercheurs).
T.G. thanks the Direction de la Recherche of the
French Ministry of Research for a post-doctoral
fellowship.
1
by H and 13C NMR, FAB-MS and elemental analy-
ses.12 The UV–vis spectrum of both 9 and 10 corre-
sponds to the sum of the spectra of their two
components indicating that there are no significant
ground state interactions between the two chro-
mophores. As depicted in Fig. 1, the absorption spec-
trum of 9 recorded in CH2Cl2 shows the characteristic
bands of a fulleropyrrolidine derivative at 430 and 702
nm as well as the diagnostic OPE band at 365 nm. The
UV–vis spectrum of the higher homologue 10 is similar
but due to the increased length of the p-conjugated
system, the absorption maximum of the OPE moiety is
shifted to 392 nm. Preliminary luminescence measure-
ments show a strong quenching of the OPE fluores-
cence by the fullerene moiety in both 9 and 10
indicating the occurrence of intramolecular photo-
induced processes. Detailed photophysical studies are
currently under investigation in collaboration with the
Armaroli group (Bologna, Italy) and special emphasis
is placed on the detection of photo-induced and long-
lived charge-separated states.
References
1. Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F.
Science 1992, 258, 1474–1476.
2. Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A.
J. Science 1995, 270, 1789–1791.
3. Nierengarten, J.-F.; Eckert, J.-F.; Nicoud, J.-F.; Ouali,
L.; Krasnikov, V.; Hadziioannou, G. Chem. Commun.
1999, 617–618.
4. 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, 7467–7479.
5. 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, 10174–10190.
6. For reviews on p-conjugated systems, see: (a) Tour, J. M.
Chem. Rev. 1996, 96, 537–554; (b) Mu¨llen, K.; Wegner,
G. Electronic Materials: The Oligomer Approach; Wiley–
VCH: Weinheim, 1998; (c) Martin, R. E.; Diederich, F.
Angew. Chem., Int. Ed. 1999, 38, 1351–1377; (d) Bunz, U.
H. F. Chem. Rev. 2000, 100, 1605–1644; (e) Tour, J. M.
Acc. Chem. Res. 2000, 33, 791–804.
A new iterative approach for the synthesis of disymmet-
rically substituted OPE derivatives has been developed.
This enables us to prepare building blocks for the
construction of C60-OPE derivatives which present all
the characteristic features required for photovoltaic
7. (a) Schumm, J. S.; Pearson, D. L.; Tour, J. M. Angew.
Chem., Int. Ed. Engl. 1994, 33, 1360–1363; (b) Ziener, U.;
Godt, A. J. Org. Chem. 1997, 62, 6137–6143.
8. Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 13,
3769–3772.
9. (a) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Nagi-
hara, N. Synthesis 1980, 00, 627–630; (b) Metal-catalyzed
Cross-coupling Reactions; Diederich, F.; Stang, P. J.,
Eds.; Wiley–VCH: Weinheim, 1998.
10. (a) Nierengarten, J.-F.; Schreiber, M.; Diederich, F.;
Gramlich, V. New J. Chem. 1996, 20, 1273–1284; (b)
Solladie´, N.; Gross, M. Tetrahedron Lett. 1999, 40, 3359–
3362.
11. Prato, M.; Maggini, M. Acc. Chem. Res. 1998, 31, 519–
526.
12. Spectroscopic data for 9: brown solid (mp 124°C); 1H
NMR (200 MHz, CDCl3): 7.58 (s, 1H), 7.03 (s, 1H), 6.93
(s, 1H), 6.92 (s, 1H), 5.54 (s, 1H), 4.98 (d, J 9.5 Hz, 1H),
4.32 (d, J 9.5 Hz, 1H), 4.18–3.67 (m, 8H), 2.83 (s, 3H),
1.86–1.73 (m, 8H), 1.58–1.28 (m, 40H), 1.14 (s, 21 H),
0.88–0.84 (m, 12H); 13C NMR (50 MHz, CDCl3): 156.57,
154.97, 154.27, 154.03, 154.00, 153.71, 153.12, 151.43,
Figure 1. Absorption spectra of 9 (full line) and 11 (dashed
line). Inset: fluorescence spectra of optically matched solu-
tions of 9 (full line) and 11 (dashed line); uexc=365 nm. All
the experiments were carried out in CH2Cl2 solutions at 298
K.

3178
T. Gu, J.-F. Nierengarten / Tetrahedron Letters 42 (2001) 3175–3178
147.25, 146.69, 146.57, 146.22, 146.18, 146.14, 146.06,
1.28 (m, 60H), 1.15 (s, 21H), 0.89–0.85 (m, 18H); 13C
NMR (50 MHz, CDCl3): 156.50, 154.92, 154.28, 153.96,
153.61, 153.39, 153.15, 151.41, 147.18, 146.61, 146.53,
146.11, 146.06, 146.02, 145.84, 145.62, 145.46, 145.33,
145.20, 145.12, 145.01, 144.50, 144.44, 144.34,
144.24, 142.94, 142.90, 142.57, 142.52, 142.44, 142.19,
142.14, 142.03, 141.98, 141.85, 141.63, 141.53, 140.08,
140.05, 139.60, 139.35, 136.33, 136.17, 135.99, 134.49,
127.12, 117.87, 117.19, 117.08, 117.05, 116.38, 116.22,
116.16, 114.75, 114.35, 114.24, 114.11, 113.86, 113.38,
102.99, 96.37, 91.57, 91.53, 91.34, 90.42, 76.45, 75.59,
75.54, 69.94, 69.77, 69.57, 69.14, 69.11, 68.54, 40.08,
31.85, 31.82, 29.44, 29.39, 29.34, 29.26, 26.17, 25.98,
25.93, 22.70, 22.64, 18.67, 14.17, 14.10, 14.05, 11.33;
FAB-MS: 2003.9 ([M+H]+, calcd for C144H136NO6Si:
2003.01), 1282.9 ([M−C60]+, calcd for C84H135NO6Si:
1282.01), 720.0 (C60+, calcd for C60: 720.00). Anal.
calcd for C144H135NO6Si: C, 86.32; H, 6.79; found: C,
86.58; H, 6.87.
146.02, 145.90, 145.68, 145.52, 145.41, 145.25, 145.19,
145.07, 144.56, 144.50, 144.42, 144.29, 143.00, 142.95,
142.63, 142.59, 142.51, 142.23, 142.20, 142.09, 142.04,
141.91, 141.69, 141.58, 140.13, 140.08, 139.65, 139.39,
136.39, 136.20, 136.07, 134.53, 127.12, 117.77, 116.41,
116.14, 114.72, 114.32, 113.75, 113.33, 102.99, 96.35,
91.33, 90.42, 76.48, 75.59, 69.97, 69.73, 69.19, 69.13,
68.58, 40.13, 31.88, 31.85, 31.82, 29.50, 29.45, 29.37,
29.28, 26.18, 26.04, 26.01, 25.91, 22.74, 22.67, 18.68,
14.18, 14.12, 14.09, 11.34; FAB-MS: 1646.7 ([M+H]+,
calcd for C120H100NO4Si: 1646.74), 925.8 ([M−C60]+,
calcd for C60H99NO4Si: 925.73), 720.2 (C60+, calcd for
C60: 720.00). Anal. calcd for C120H99NO4Si: C, 87.50;
H, 6.06; found: C, 87.69; H, 6.19. For 10: brown solid.
1
(mp 108°C); H NMR (200 MHz, CDCl3): 7.59 (s, 1H),
7.04 (s, 1H), 6.99 (s, 2H), 6.93 (s, 2H), 5.55 (s, 1H),
4.98 (d, J 9.5 Hz, 1H), 4.32 (d, J 9.5 Hz, 1H), 4.15–
3.72 (m, 12H), 2.84 (s, 3H), 1.86–1.71 (m, 12H), 1.57–
.
.