Synthesis of fullerene-substituted oligo(phenylenebutadiyndiyl)

Article abstract of DOI:10.1016/j.tetlet.2006.03.142

A diethynylbenzene monomer substituted with two fullerene moieties has been prepared, and its oligomerization performed in the presence of phenylacetylene as end-capping reagent afforded the corresponding end-capped mono- and di-mer with an oligo(phenylenebutadiyndiyl) (OPB) conjugated backbone substituted with two and four fullerene subunits, respectively.

Full text of DOI:10.1016/j.tetlet.2006.03.142

Tetrahedron Letters 47 (2006) 3715–3718
Synthesis of fullerene-substituted oligo(phenylenebutadiyndiyl)
Juan Luis Delgado de la Cruz, Uwe Hahn and Jean-Franc¸ois Nierengarten*
`
`
´
Groupe de Chimie des Fullerenes et des Systemes Conjugues, Laboratoire de Chimie de Coordination du CNRS,
205 route de Narbonne, 31077 Toulouse Cedex 4, France
Received 21 February 2006; revised 18 March 2006; accepted 21 March 2006
Available online 12 April 2006
Abstract—A diethynylbenzene monomer substituted with two fullerene moieties has been prepared, and its oligomerization per-
formed in the presence of phenylacetylene as end-capping reagent afforded the corresponding end-capped mono- and di-mer with
an oligo(phenylenebutadiyndiyl) (OPB) conjugated backbone substituted with two and four fullerene subunits, respectively.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Following the preparation of the first photovoltaic
devices from C60-oligophenylenevinylene conjugates,1
considerable research efforts have been devoted to the
synthesis and the study of C60 derivatives substituted
with p-conjugated oligomers.2 The excited state proper-
ties of such hybrid systems have been extensively inves-
tigated, thus allowing a good understanding of the
intramolecular photoinduced energy and electron trans-
fer events occurring in these donor–acceptor dyads.3
Several covalently linked fullerene-(p-conjugated oligo-
mer) ensembles have also been tested as the active layer
in solar cells.4 Importantly, the behavior of a unique
molecule in a photovoltaic device allows to establish eas-
ily structure/activity relationships for a better under-
standing of the photovoltaic system.5 However, even if
the energy conversion efficiency obtained with some of
these organic photodiodes is quite promising,4 it has
to be improved dramatically for a commercial use. This
is clearly an important challenge for materials scientists
in general and for organic chemists in particular as new
hybrid compounds with a stronger absorption in the vis-
ible range and a better stability toward light are needed.
chemical reactivity of fullerene derivatives limiting the
range of reactions that can be used for the oligomeriza-
tion of such monomers. In addition, due to the low sol-
ubility of C60 derivatives, only a few examples of
conjugated backbones bearing multiple C60 units have
been reported so far.7 In this letter, we now report the
preparation of diethynylbenzene derivative 1 substituted
with two fullerene moieties. Oligomerization reactions
of monomer 1 have been performed in the presence of
an end-capping reagent in order to control the oligomer
length. The end-capped mono- and di-mer thus obtained
possess an oligo(phenylenebutadiyndiyl) (OPB) back-
bone and are substituted with two and four fullerene
groups, respectively.
RO
O
O
RO
O
O
H
O
O
O
O
As far as the synthesis of fullerene-(p-conjugated oligo-
mer) conjugates is concerned, almost all of them have
been prepared by the direct functionalization of C60 with
already constructed conjugated oligomers.2 In contrast,
the preparation of fullerene-substituted monomers and
their subsequent oligomerization has been considered
to a lesser degree.6 This is mainly associated with the
O
H
O
OR
O
1 R = C8H17
O
OR
The synthesis of fullerene-substituted monomer 1 is
based on the use of the fullerene carboxylic acid building
blocks developed in our group.8 To this end, we have
prepared a 1,4-diethynylbenzene derivative bearing two
*
Corresponding author. Tel.: +33 564 33 31 51; e-mail: jfnierengarten@
lcc-toulouse.fr
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.03.142

3716
J. L. D. de la Cruz et al. / Tetrahedron Letters 47 (2006) 3715–3718
hydroxy groups allowing for the attachment of two ful-
lerene carboxylic acid units via an esterification reaction.
The preparation of this diol is depicted in Scheme 1.
Reaction of 2 with triisopropylsilylacetylene under
Sonogashira conditions afforded 3 in 60% yield. LiAlH4
reduction followed by treatment of the resulting 4 with
tetra-n-butylammonium fluoride (TBAF) yielded 5.
Cu-catalyzed conditions9 with phenylacetylene as
stopper. In a typical procedure, Cu(OAc)2 (39 mg, 0.19
mmol), pyridine (0.06 ml, 0.74 mmol) and 1,8-diazabi-
cyclo[5.4.0]undec-7-ene (DBU) (0.01 ml, 0.066 mmol) were
added to a solution of 1 (129 mg, 0.049 mmol) and phen-
ylacetylene (50.52 mg, 0.49 mmol) in CH2Cl2 (100 mL).
The resulting mixture was stirred for 4 h at room tem-
perature and filtered over a pad of silica. Column chro-
matography (SiO2, CH2Cl2/hexane 7:3) followed by gel
permeation chromatography (Biorad, Biobeads SX-1,
CH2Cl2) gave 7 (54 mg, 39%) and 8 (18 mg, 7%). The
yield of 7 and 8 was highly dependant of the monomer
to end-capping reagent molar ratio. When a large excess
of phenylacetylene (10 equiv) was used, end-capped
monomer 7 and end-capped dimer 8 were obtained in
reasonable yields. In contrast, when 1 and phenylacetyl-
ene were used in a 1:1 molar ratio, 7 and 8 were isolated
in low yields (3% and 1%, respectively). It can also be
noted that higher oligomers were formed during these
reactions but they could not be separated into pure
compounds nor properly characterized due to their poor
solubility in common organic solvents.
Reaction of diol 5 with carboxylic acid 6 under esteri-
fication conditions using dicyclohexylcarbodiimide
(DCC) and 4-dimethylaminopyridine (DMAP) afforded
monomer 1 in 66% yield (Scheme 2). The oligomeriza-
tion reactions of monomer 1 were performed under
Si(i-Pr)3
OHC Br
OHC
a)
CHO
H
Br
CHO
2
3
(i-Pr)3Si
Si(i-Pr)3
HO
HO
Oligomers 7 and 8 are well soluble in common organic
solvents such as CH2Cl2, CHCl3, or THF and complete
spectroscopic characterization was easily achieved.10
c)
b)
OH
OH
1
The H NMR spectra of 1, 7 and 8 recorded in CDCl3
4
5
(i-Pr)3Si
H
are depicted in Figure 1. The spectrum of 1 shows the
characteristic features of the Cs symmetrical methano-
fullerene substituent as well as the signals of the central
diethynylphenyl core. In particular, three sets of signals
are observed at d 5.0–5.5 ppm for the three different CH2
Scheme 1. Reagents and conditions: (a) triisopropylsilylacetylene,
PdCl2(PPh3)2, CuI, Et3N, THF, 65 ꢁC, 4 d (60%); (b) LiAlH4, THF,
0 ꢁC to room temperature, 3 h (91%); (c) TBAF, THF, room
temperature, 3 h (82%).
RO
OR
O
O
RO
O
O
OH
O
O
O
O
O
RO
RO
RO
OR
OR
O
6 R = C8H17
O
O
O
O
O
O
O
a)
O
O
7 R = C8H17
b) or c)
O
O
O
1
O
+
O
O
O
O
O
8 R = C8H17
O
O
O
O
O
O
O
RO
O
O
O
O
OR
O
O
RO
RO
OR
OR
Scheme 2. Reagents and conditions: (a) 5, DCC, DMAP, CH2Cl2, room temperature, 5d (66%); (b) Cu(OAc)2, pyridine, DBU, phenylacetylene (10
equiv), room temperature, 4 h (7: 39%; 8: 7%); (c) Cu(OAc)2, pyridine, DBU, phenylacetylene (1 equiv), room temperature, 4 h (7: 3%, 8: 1%).

J. L. D. de la Cruz et al. / Tetrahedron Letters 47 (2006) 3715–3718
3717
1
Figure 1. H NMR spectra (CDCl3, 300 MHz) of 1, 7 and 8 (*: CH2Cl2 impurity).
groups (HA, HB and HC, see Figure 1) and a singlet is
absorption is clearly red-shifted when going from 7 to 8
as a result of the longer conjugated pathway in full
agreement with the proposed structures. Finally, preli-
minary luminescence measurements reveal no emission
from the OPB core in 7 or 8, indicating a strong quench-
ing of the OPB fluorescence by the fullerene moiety in
both 7 and 8 suggesting the occurrence of intramolecu-
lar photo-induced processes. Detailed photophysical
studies are currently under investigation and special
emphasis is placed on the detection of long-lived
charge-separated states.
revealed at d 3.47 ppm for the terminal alkyne protons.
1
The H NMR spectra of both end-capped oligomers 7
and 8 are in full agreement with their centrosymmetric
structures and revealed no signals around d 3.5 ppm cor-
responding to unreacted terminal alkyne functions. In
both cases, the characteristic signals of the monomer
units as well as those of the terminal phenyl groups
are clearly distinguished. The number of monomer units
in each oligomer is confirmed by the terminal phenyl to
benzylic CH2 proton ratio deduced from the integration
1
of the H NMR spectra.
In conclusion, a new approach for the synthesis of ful-
lerene-(p-conjugated oligomer) conjugates has been
developed. This enables us to prepare fullerene-rich
OPB derivatives, which present all the characteristic
features required for photovoltaic applications. Further
efforts are currently under way to prepare related
derivatives with longer OPB backbones.
The UV/vis spectra of CH2Cl2 solutions of 1, 7 and 8 are
shown in Figure 2. In the UV region, monomer 1 dis-
plays two intense bands typical of methanofullerene
derivatives,11 whereas in the visible region, the spectrum
is much less intense with two bands at 426 and 689 nm.
An analogous pattern is observed in UV/vis spectra of 7
and 8 with additional bands in the 350–420 nm region
resulting from the OPB conjugated backbone. The latter
Acknowledgement
This research was supported by the CNRS and by post-
doctoral fellowships from the EU (HPRN-CT-2002-
00171, FAMOUS) to J.L.D.C. and from the Deutscher
Akademischer Austausch Dienst (DAAD) to U.H.
2
1
References and notes
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500
600
λ/nm
700
0
300
400
500
λ/nm
600
700
Figure 2. Absorption spectra of 5 · 10À6 M CH2Cl2 solutions
(black), 7 (red) and 8 (blue).
1
2. For reviews on fullerene-(p-conjugated oligomer) dyads,
´
see: Segura, J. L.; Martın, N.; Guldi, D. M. Chem. Soc.

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J. L. D. de la Cruz et al. / Tetrahedron Letters 47 (2006) 3715–3718
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10. Compound 1: H NMR (CDCl3, 300 MHz): d = 0.87 (t,
1
J = 7 Hz, 12H), 1.19–1.44 (m, 40H), 1.74 (m, 8H), 3.47 (s,
2H), 3.89 (t, J = 7 Hz, 8H), 5.02 (s, 4H), 5.37 (s, 4H), 5.45
(s, 4H), 6.39 (t, J = 2 Hz, 2H), 6.59 (d, J = 2 Hz, 4H), 7.56
(s, 2H); 13C NMR (CDCl3, 75 MHz): d = 14.2, 22.7, 26.2,
29.3, 29.4, 31.85, 51.24, 62.5, 64.7, 68.1, 69.14, 71.2, 79.8,
85.2, 101.65, 107.3, 122.2, 132.5, 136.6,137.4, 138.5, 139.7,
140.8, 140.9, 141.8, 141.9, 142.15, 142.2, 142.9, 143.0,
143.8, 143.85, 144.4, 144.5, 144.6, 144.65, 144.9, 145.0,
145.1, 145.15, 145.2, 145.26, 163.0, 166.2; IR (KBr):
m = 3296 („C–H), 2197 (C„C), 1748 (C@O) cmÀ1; UV/
vis (CH2Cl2): kmax (e) = 259 (213,000), 327 (62,000), 426
(5100), 686 (200). Compound 7: 1H NMR (CDCl3,
300 MHz): d = 0.87 (t, J = 7 Hz, 12 H), 1.19–1.44 (m,
40H), 1.73 (m, 8H), 3.88 (t, J = 7 Hz, 8H), 5.02 (s, 4H),
5.39 (s, 4H), 5.45 (s, 4H), 6.39 (t, J = 2 Hz, 2H), 6.59 (d,
J = 2 Hz, 4H), 7.30–7.38 (m, 6H), 7.50–7.53 (m, 4H), 7.60
(s, 2H); 13C NMR (CDCl3, 75 MHz): d = 14.2, 22.7,
26.15, 29.3, 29.4, 31.85, 51.2, 62.6, 64.6, 68.2, 69.1, 71.1,
73.6, 81.9, 85.0, 101.6, 107.3, 128.5, 128.8, 131.0, 132.7,
136.6, 138.6, 139.7, 140.8, 140.9, 141.6, 141.8, 141.85,
142.1, 142.2, 142.9, 143.0, 143.8, 144.4, 144.5, 145.0, 145.1,
145.2, 160.5, 163.0, 166.0; IR (KBr): m = 2208 (C„C),
1750 (C@O); UV/vis (CH2Cl2): kmax (e) = 258 (224,000),
327 (91,000), 340 (sh) (85,500), 364 (sh) (63,000), 426
(6000), 686 (300). Compound 8: 1H NMR (CDCl3,
300 MHz): d = 0.87 (t, J = 7 Hz, 24H), 1.26–1.41 (m,
40H), 1.73 (m, 16H), 3.88 (t, J = 7 Hz, 16H), 5.05 (s, 8H)
5.38 (s, 8H), 5.45 (s, 8H), 6.38 (br s, 4H), 6.58 (br s, 8H),
7.32–7.40 (m, 6H), 7.50–7.56 (m, 4H), 7.59 (s, 4H); 13C
NMR (CDCl3, 75 MHz): d = 14.2, 22.7, 26.2, 29.3, 29.4,
29.7, 31.9, 62.5, 64.8, 68.2, 69.2, 71.1, 79.8, 85.2, 101.6,
107.3, 128.54, 128.84, 131.0, 132.7, 136.6, 138.6, 139.7,
140.8, 140.9, 141.6, 141.8, 141.85, 142.1, 142.2, 142.9,
143.0, 143.8, 144.4, 144.5, 144.6, 144.9, 145.0, 145.1, 145.2,
160.5, 163.0, 166.2; IR (KBr): m = 2208 (C„C), 1748
(C@O); UV/vis (CH2Cl2): kmax(e) = 258 (474,000), 327
(174,000), 368 (sh) (95,900), 401 (sh) (48,100), 425
(18,400), 686 (750).
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