Tetrafullerene conjugates for all-organic photovoltaics

Article abstract of DOI:10.1021/jo702740d

(Figure Presented) The synthesis of two new tetrafullerene nanoconjugates in which four C₆₀ units are covalently connected through different π-conjugated oligomers (oligo(p-phenylene ethynylene) and oligo(p-phenylene vinylene)) is described. Th

Full text of DOI:10.1021/jo702740d

Tetrafullerene Conjugates for All-Organic Photovoltaics
Gustavo Ferna´ndez,† Luis Sa´nchez,† Dirk Veldman,‡ Martijn M. Wienk,‡ Carmen Atienza,§
Dirk M. Guldi,*,§ Rene´ A. J. Janssen,*,‡ and Nazario Mart´ın*,†
Departamento de Qu´ımica Orga´nica, Facultad de Ciencias Qu´ımicas, UniVersidad Complutense de
Madrid, Ciudad UniVersitaria s/n, 28040-Madrid, Spain, Molecular Materials and Nanosystems,
EindhoVen UniVersity of Technology, 5600 MB EindhoVen, The Netherlands, and Department of Chemistry
and Pharmacy and Interdisciplinary Center for Molecular Materials, UniVersita¨t Erlangen-Nu¨rnberg,
Egerlandstrasse 3, 91058 Erlangen, Germany
nazmar@quim.ucm.es; r.a.j.janssen@tue.nl; dirk.guldi@chemie.uni-erlangen.de
ReceiVed January 8, 2008
The synthesis of two new tetrafullerene nanoconjugates in which four C60 units are covalently connected
through different π-conjugated oligomers (oligo(p-phenylene ethynylene) and oligo(p-phenylene vinylene))
is described. The photovoltaic (PV) response of these C60-based conjugates was evaluated by using them
as the only active material in organic solar cells, showing a low PV performance. Photophysical studies
in solution demonstrated a very fast (∼10 ps) deactivation of the singlet excited state of the central core
unit to produce both charge-separated species (i.e., C60•--oligomer+•-(C60)3 and C60 centered singlet
excited states). The charge-separated state recombines partly to the C60 centered singlet state that undergoes
subsequent intersystem crossing. Photophysical studies carried out in films support these data, exhibiting
long-lived triplet excited states. For both tetrafullerene arrays, the low yield of long-lived charge carriers
in thin films accounts for the limited PV response. On the contrary, utilizing the oligo(p-phenylene
vinylene) centered precursor aldehyde as an electron donor and antennae unit and mixing with the well-
known C60 derivative PCBM, the photophysical studies in films show the formation of long-lived charges.
The PV devices constructed from these mixtures showed a relatively high photocurrent of 2 mA cm-2.
The sharp contrast between the nanoconjugates and the physical blends tentatively was attributed to
improved charge dissociation and the collection of more favorable energy levels in the blends as a result
of partial aggregation of both of the components.
Introduction
clean energy sources for the independent operation of such
devices.2,3 The introduction of the bulk heterojunction concept
Miniaturization processes requiring us to attain smaller and
smaller devices and machines has prompted the development
of organic electronics.1 Organic photovoltaics (PV) play an
important role in this field since they can act as efficient and
(1) (a) Special Issue on Organic Electronics, Chem. Mater. 2004, 16,
4381-4846. (b) Wassel, R. A.; Gorman, C. B. Angew. Chem., Int. Ed. 2004,
43, 5120-5123. (c) Carroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed.
2002, 41, 4378-4400.
(2) For recent reviews on PV cells, see: (a) Brabec, C. J.; Sariciftci, N.
S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15-26. (b) Coakley, K.
M.; McGehee, M. D. Chem. Mater. 2004, 16, 4533-4542. (c) Gu¨nes, S.;
Neugebauer, H. Chem. ReV. 2007, 107, 1324-1338.
† Universidad Complutense de Madrid.
‡ Eindhoven University of Technology.
§ Universita¨t Erlangen-Nu¨rnberg.
10.1021/jo702740d CCC: $40.75 © 2008 American Chemical Society
Published on Web 03/22/2008
J. Org. Chem. 2008, 73, 3189-3196
3189

Ferna´ndez et al.
for PV has provided a major boost in power conversion
efficiencies, currently reaching values of 5% or more.3 In those
bulk heterojunction devices, the photoactive layer typically
consists of a mixture of a π-conjugated semiconducting polymer,
acting as p-type material, and a soluble C60 derivative as the
n-type material. A particular active strategy toward new PV
designs is the molecular heterojunction.4 Its foremost feature is
incorporation in the photoactive layer of the device of C60-based
conjugates in which different electron donor moieties have been
attached covalently to the fullerene cage. The close proximity
of the photo- and electroactive units in such conjugates generally
provides highly efficient photoinduced charge generation and
can be tailored to exhibit long radical ion pair lifetimes.
fullerene blends, where the fullerene content is often as high as
80% by weight.8
Hence, in the search for new suitable C60-based conjugates
to construct efficient molecular-based PV devices, materials that
possess a relatively high fullerene content seem to be favorable.
In this regard, we recently reported the synthesis and PV
application of a light-harvesting tetrafullerene nanoconjugate
in which four C60 units were covalently linked to a single OPE-
type π-conjugated oligomer. Photophysical studies carried out
in solution and in thin films demonstrated, however, that the
observed low device performance was due to the absence of an
efficient photoinduced electron-transfer process between the
OPE central core and the peripheral C60 units. The low tendency
for photoinduced charge generation in these tetrafullerene
conjugates was caused by a relatively high oxidation potential
for the OPE oligomer. Nevertheless, the combination of this
new conjugate as an electron acceptor with poly(3-hexylth-
iophene) (P3HT) as a donor led to devices that exhibited external
quantum efficiencies of 15%.9
Molecular heterojunction devices have been created by using
a variety of C60-based conjugates as the only photoactive
material. First examples used oligo(p-phenylene vinylene)s
(OPVs) of different lengths as donor molecules.5,6 More recently,
donor moieties such as oligothiophene, oligo(thienylene vi-
nylene), and oligo(p-phenylene ethynylene) (OPE) π-conjugated
oligomers, porphyrins, and π-extended tetrathiafulvalene deriva-
tives have been applied in the fabrication of PV devices.7 One
of the drawbacks associated with most donor-C60 conjugates
reported to date is the relatively low content of fullerene in the
final material, as compared to the more efficient polymer-
Herein, we take our first tetrafullerene prototype one step
further to synthesize new tetrafullerene hybrids (6 and 7) whose
redox and light-absorbing features were modulated finely by
the introduction of thiophene rings on the periphery of the
central core of the OPE or OPV trimers. Photophysical
investigations, in solution and in film, were performed, and the
corresponding all-organic solar cells were fabricated using these
new conjugates as the only photoactive material and were
compared to those using physical mixtures of the precursor
aldehydes (5a and 5b) with [C60]PCBM.10
(3) (a) Rowell, M. W.; Topinka, M. A.; McGehee, M. D.; Prall, H.-J.;
Dennler, G.; Sariciftci, N. S.; Hu, L.; Gruner, G. Appl. Phys. Lett. 2006,
88, 233506/1-233506/3. (b) Sivula, K.; Luscombe, C. K.; Thompson, B.
C.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2006, 128, 13988-13989. (c) Ma,
W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. AdV. Funct. Mater. 2005,
15, 1617-1622. (d) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.;
Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864-868. (e) Riedel, I.; von
Hauff, E.; Parisi, J.; Mart´ın, N.; Giacalone, F.; Dyakonov, V. AdV. Funct.
Mater. 2005, 15, 1979-1987. (f) Svensson, M.; Zhang, F.; Veenstra, S.
C.; Verhees, W. J. H.; Hummelen, J. C.; Kroon, J. M.; Ingana¨s, O.;
Andersson, M. R. AdV. Mater. 2003, 15, 988-991. (g) Wienk, M. M.;
Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hummelen, J. C.; van Hal, P.
A.; Janssen, R. A. J. Angew. Chem., Int. Ed. 2003, 42, 3371-3375. (h)
Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. AdV. Funct. Mater. 2003,
13, 1-4.
Results and Discussion
Synthesis. Target arrays 6 and 7 were obtained by following
a divergent multistep synthetic procedure using the previously
described diformyl functionalized oligomers 1a and 1b.11 The
introduction of peripheral thiophene rings gives rise to new
fluorophores and enhances the light-absorbing and redox
properties of the final systems.12 The peripheral functionalization
toward the precursor aldehydes (5a,b) began with the corre-
sponding bis(1,1-dibromovinylbenzene) derivatives (2a,b), which
(4) For recent reviews on molecular heterojunction, see: (a) Roncali, J.
Chem. Soc. ReV. 2005, 34, 483-495. (b) Segura, J. L.; Mart´ın, N.; Guldi,
D. M. Chem. Soc. ReV. 2005, 34, 31-47. (c) Nierengarten, J.-F. New J.
Chem. 2004, 28, 1177-1191. (d) Nierengarten, J.-F. Sol. Energy Mater.
Sol. Cells 2004, 83, 187-199. (e) Figueira-Duarte, T. M.; Ge´gout, A.;
Nierengarten, J.-F. Chem. Commun. (Cambridge, U.K.) 2007, 109-119.
(5) (a) Nierengarten, J.-F.; Eckert, J.-F.; Nicoud, J.-F.; Ouali, L.;
Krasnikov, V.; Hadziioannou, G. Chem. Commun. (Cambridge, U.K.) 1999,
617-618. (b) 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.
(8) (a) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science
(Washington, DC, U.S.) 1995, 270, 1789-1791. (b) Shaheen, S. E.; Brabec,
C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. C. Appl.
Phys. Lett. 2001, 78, 841-843. (c) van Duren, J. K. J.; Yang, X. N.; Loos,
J.; Bulle-Lieuwma, C. W. T.; Sieval, A. B.; Hummelen, J. C.; Janssen, R.
A. J. AdV. Funct. Mater. 2004, 14, 425-434.
(9) Atienza, C. M.; Ferna´ndez, G.; Sa´nchez, L.; Mart´ın, N.; Sa´ Dantas,
I.; Wienk, M. M.; Janssen, R. A. J.; Rahman, G. M. A.; Guldi, D. M. Chem.
Commun. (Cambridge, U.K.) 2006, 514-516.
(10) Power conversion efficiencies of around 1% were recently reported
by Roncali et al. for bulk heterojunction devices prepared from PCBM and
different triphenylamine derivatives: (a) Roquet, S.; Cravino, A.; Leriche,
P.; Ale´veˆque, O.; Fre`re, P.; Roncali, J. J. Am. Chem. Soc. 2006, 128, 3459-
3466. (b) Cravino, A.; Roquet, S.; Leriche, P.; Ale´veˆque, O.; Fre`re, P.;
Roncali, J. Chem. Commun. (Cambridge, U.K.) 2006, 1416-1418.
(11) For the synthesis of the OPE-type trimer, see: Atienza, C.; Insuasty,
B.; Seoane, C.; Mart´ın, N.; Ramey, J.; Rahman, G. M. A.; Guldi, D. M. J.
Mater. Chem. 2005, 15, 124-132. For the synthesis of the OPV-type trimer,
see: Giacalone, F.; Segura, J. L.; Mart´ın, N.; Guldi, D. M. J. Am. Chem.
Soc. 2004, 126, 5340-5341.
(12) The substitution pattern, conjugation, and electronic structure of
functionalized phenylacetylene and phenylvinylene structures are known
to exhibit remarkable optical and electronic properties because of their
multiple conjugated pathways. For recent examples, see: (a) Marsden, J.
A.; Miller, J. J.; Shirtcliff, L. D.; Haley, M. M. J. Am. Chem. Soc. 2005,
128, 2464-2476. (b) Hwang, G. T.; Son, H. S.; Ku, J. K.; Kim, B. H. J.
Am. Chem. Soc. 2003, 125, 11241-11248.
(6) 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.
(7) For oligo(phenylene ethynylene)-C60 dyads, see: Gu, T.; Tsamouras,
D.; Melzer, C.; Krasnikov, V.; Gisselbrecht, J.-P.; Gross, M.; Hadziioannou,
G.; Nierengarten, J.-F. ChemPhysChem 2002, 3, 124-127. For oligoth-
iophene-C60 dyads, see: (a) Negishi, N.; Yamada, K.; Takimiya, K.; Aso,
Y.; Otsubo, O.; Tetsuo, Y. Chem. Lett. 2003, 32, 404-405. (b) Negishi,
N.; Takimiya, K.; Otsubo, T.; Harima, Y.; Aso, Y. Synth. Met. 2005, 152,
125-128. (c) Narutaki, M.; Takimiya, K.; Otsubo, T.; Harima, Y.; Zhang,
H.; Araki, Y.; Ito, O. J. Org. Chem. 2006, 71, 1761-1768. For oligo-
(thienylene vinylene)-C60 hybrids, see: (a) 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, 1141-1152. (b) Negishi, N.; Yamada, K.; Takimiya,
K.; Aso, Y.; Otsubo, T.; Harima, Y. Chem. Lett. 2003, 32, 404-405. (c)
Handa, S.; Giacalone, F.; Haque, S. A., Palomares, E.; Mart´ın, N.; Durrant,
J. R. Chem.sEur. J. 2005, 11, 7440-7447. For C60-porphyrin conjugates,
see: Imahori, H.; Fukuzumi, S. AdV. Funct. Mater. 2004, 14, 525-536.
For C60-tetrathiafulvalene triads, see: (a) Sa´nchez, L.; Sierra, M.; Mart´ın,
N.; Guldi, D. M.; Wienk, M. M.; Janssen, R. A. J. Org. Lett. 2005, 7, 1691-
1694. (b) Kanato, H.; Narutaki, M.; Takimiya, K.; Otsubo, T.; Harima, Y.
Chem. Lett. 2006, 35, 668-669.
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Tetrafullerene Conjugates for PhotoVoltaics
SCHEME 1. Synthesis of Target Tetrafullerene Arrays 6 and 7
were readily obtained by reacting 1a or 1b with CBr4 and PPh3
following Corey and Fuch’s method.13 A Sonogashira cross-
coupling reaction between 2 and (trimethylsilyl)acetylene led
to the trimethylsilyl (TMS) derivatives 3a,b.14 In situ depro-
tection of their TMS groups with KF followed by a further 4-fold
Sonogashira reaction with 2-bromo-5-formylthiophene (4) yielded
the light-harvesting tetra-aldehydes 5a,b as reddish solids. The
last step toward the synthesis of target tetrafullerene arrays 6
and 7 was a 4-fold Prato 1,3-dipolar cycloaddition between the
azomethine ylidesgenerated in situ by reacting N-octylglycine
and the corresponding aldehydesand one of the double bonds
of the C60. All the compounds reported herein were fully
characterized through a wide variety of spectroscopic techniques
(see Experimental Section and Supporting Information).
The 1H NMR spectra of 6 and 7 show only one set of
resonances for each of the isochronous groups of protons. The
resonances at δ ∼5.4, 5.1, and 4.2 and also at δ ∼4.1
corresponding to the pyrrolidine rings and to the -O-CH2-
groups, respectively, are clear examples. These findings support
the high symmetry of the final conjugates and, on the other
hand, the lack of byproducts such as fullerene bis-adducts. At
FIGURE 1. Absorption spectra of tetrafullerene conjugates 6 and 7
and precursors 3a, 5a, and 5b (inset) in CHCl3 at 298 K.
reached by the introduction of the peripheral thiophene rings
in 5 in comparison to their TMS precursors 3 resulted in a
significant bathochromic shift of the π-π* absorption band into
the visible region of the solar spectrum (inset of Figure 1).
Especially interesting is the case of tetra-aldehyde 5b, in which
the absorption maximum was observed at 492 nm (ꢀ ) 110 000
M-1 cm-1).
The decoration of the oligomeric core with fulleropyrrolidine
moieties in hybrids 6 and 7 resulted in a slight reduction of the
conjugation length as evidenced from both small hypsochromic
shift and lower extinction coefficients in comparison to the
precursor aldehydes (Figure 1). Additionally, both arrays 6 and
7 show an appreciable absorption in the UV region and a long
tail covering the entire visible region up to ∼700 nm.
Cyclic Voltammetry (CV). The redox characteristics of
aldehydes 5a,b and conjugates 6 and 7 were examined by CV
1
the same time, the absence of the signals at δ ∼9.9 in the H
NMR spectra and ν ∼1660 cm-1 in the FTIR spectra, diagnostic
of the aldehyde functionality, rules out the presence of
conjugates endowed with a lesser number of C60 units. A
comparative HPLC analysis of 6 and 7 supports this theory (see
Figure S1).
Optical Properties. The light-absorbing features of all the
compounds described in this study were investigated by UV-
vis spectroscopy in chloroform. The increased conjugation
(13) Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 13, 3769-3772.
(14) Diederich, F.; Stang, P. J. Metal-Catalyzed Cross-Coupling Reac-
tions; Wiley-VCH: Weinheim, Germany, 1998.
J. Org. Chem, Vol. 73, No. 8, 2008 3191

Ferna´ndez et al.
FIGURE 2. CV spectra of tetrafullerene array 7 in ODCB/MeCN (4:
1), at room temperature and at 100 mVs-1
.
FIGURE 3. Room-temperature fluorescence spectra of the tet-
rafullerene conjugates 6 (purple) and 7 (green) together with those of
the precursor aldehydes 5a (blue) and 5b (red) in THF recorded with
solutions that exhibit optical absorptions at the 442 nm excitation
wavelength of 0.2. The inset shows the fluorescence spectra of 6
(purple) and 7 (green) in the wavelength range of 650-800 nm.
TABLE 1. Redox Potential Values for Tetrafullerene Hybrids 6
and 7 and Their Precursors 5a,b
compda E1
E2
E3
E1
E2
E2
E4
red
ox
ox
ox
red
red
red
5a
5b
6
7
C60
+910 +1130 +1170 -1410b
+860 +1110 +1560 -1430b
+810 +1160 +1640 -950
+760 +1020 +1440 -900
-760
-1330 -1890
-1290 -1820
-1180 -1650 -2130
presence of only one set of reduction waves points to the lack
of interaction among the four C60 units present in the molecule.
As expected, the saturation of one of the double bonds of the
fullerene units cathodically shifts those values as compared to
pristine C60 under the same experimental conditions.15 All the
reduction waves are accompanied by side waves that could be
attributed to the adsorption of the measured compound over
the electrode surface (see Figure 2).16
Photophysical Measurements in Solution. The excited-state
deactivation of the tetra-aldehydes 5a and 5b and conjugates 6
and 7 was examined by steady-state and time-resolved fluores-
cence spectroscopy in THF at room temperature. Considering
the strongly fluorescing features of π-conjugated oligomers, the
fluorescence of 5a,b as reference systems was tested first. In
accordance with the optical absorption, the fluorescence spectra
of the tetra-aldehydes recorded with excitation at 442 nm show
that the fluorescence maximum for 5a (i.e., 545 nm) is blue-
shifted as compared to that for 5b (i.e., 604 nm). The
fluorescence quantum yields of 5a and 5b are 0.31 and 0.14,
respectively. The reduction in fluorescence quantum yield
mainly is caused by an increased nonradiative decay as can be
inferred from the fluorescence lifetimes for 5a (i.e., 1.08 ns)
and 5b (i.e., 0.63 ns).
First insights into electron donor-acceptor interactions were
gathered by comparing the fluorescence of 6 and 7 with that of
5a and 5b upon excitation at 442 nm, where the absorption was
mainly that of the central core. Here, a significant quenching
of the oligomeric fluorescence is observed (see Figure 3).
Fluorescence quantum yields are 2 × 10-3 for both conjugates
6 and 7. The fluorescence is dual, that is, besides the oligomer
relevant features in the 400-700 nm region, we see in the 650-
800 nm range additional features similar to those seen for the
fullerene fluorescence in N-methylfulleropyrrolidines.19 If we
compare the fluorescence quantum yields of the core unit to
those of the corresponding aldehydes, we find that the quenching
is about 2 orders of magnitude, implying that deactivation of
the singlet state of the central chromophore occurs with a rate
a Experimental conditions: mV vs Ag/AgNO3, ODCB/CH3CN 4:1 as
solvent, GCE as working electrode, Pt as counter-electrode, Bu4NClO4 (0.1
.
M) as supporting electrolyte, and scan rate of 100 mV s-1 b Corresponding
to the aldehyde functionality.
and differential pulse voltammetry (DPV) techniques in ODCB/
MeCN (4:1) at room temperature (see Figure 2 for hybrid 7
and Table 1).
Concerning the electrochemical oxidation, the tetrafullerene
hybrids (6 and 7) show three irreversible waves corresponding
to the central π-conjugated oligomers. The first and second
waves are quite weak, whereas the last one is very intense
(Figure 2).17 These findings are in agreement with the redox
behavior observed for the precursor aldehydes 5.18 The incor-
poration of double bonds in the central oligomeric moiety in
5b and 7 induces an anodic shift of the oxidation process in
comparison with the OPE-type examples 5a and 6 (see Table
1). These data confirm that the OPV spacer causes a lower
oxidation potential in comparison to the OPE spacer and, hence,
features stronger electron donating properties and a lower band
gap.
The cyclic voltammograms of both hybrids 6 and 7 feature
both oxidation and reduction waves that account for the
amphoteric redox character of these samples. Regarding the
electrochemical reduction, three quasi-reversible waves corre-
sponding to the reduction of the fullerene cages appear. The
(15) It is well-established that functionalization of the fullerene cage
causes a rise of the LUMO energy level of the organofullerenes produced:
Echegoyen, L.; Echegoyen, L. E. Acc. Chem. Res. 1998, 31, 593-601.
(16) Bard, A. J.; Faulkner, L. R. Electrochemical Methods. Fundamentals
and Applications; John Wiley and Sons: New York, 2001.
(17) Unambiguous assignment of these weak waves to oxidative
processes corresponding to the central π-conjugated oligomer was achieved
using DPV measurements.
(18) Oxidation waves of π-conjugated oligomers sometimes appear as
broad and difficult to assign waves in CV. For recent examples, see: (a)
Zen, A.; Bilge, A.; Galbrecht, F.; Alle, R.; Meerholz, K.; Grenzer, J.; Neher,
D.; Scherf, U.; Farrell, T. J. Am. Chem. Soc. 2006, 128, 3914-3915. (b)
Narutaki, M.; Takimiya, K.; Otsubo, T.; Harima, Y.; Zhang, H.; Araki, Y.;
Ito, O. J. Org. Chem. 2006, 71, 1761-1768. (c) Zhao, Y.; Shirai, Y.;
Slepkov, A. D.; Cheng, L.; Alemany, L. B.; Sasaki, T.; Hegmann, F. A.;
Tour, J. M. Chem.sEur. J. 2005, 11, 3643-3658.
(19) Atienza, C.; Mart´ın, N.; Wielopolski, M.; Haworth, N.; Clark, T.;
Guldi, D. M. Chem. Commun. (Cambridge, U.K.) 2006, 3202-3204.
3192 J. Org. Chem., Vol. 73, No. 8, 2008

Tetrafullerene Conjugates for PhotoVoltaics
FIGURE 4. (a) Selected differential absorption spectra (visible and near-IR) obtained upon femtosecond photoexcitation (490 nm) of 5b in nitrogen
saturated THF solutions with several time delays between 0 and 1500 ps at room temperature and (b) time-absorption profiles of the spectra at
570, 625, and 1000 nm, monitoring the decay of the 5b singlet excited state.
FIGURE 5. (a) Selected differential absorption spectra (visible and near-IR) obtained upon femtosecond photoexcitation (490 nm) of 7 in nitrogen
saturated THF solutions with several time delays between 0 and 1500 ps at room temperature and (b) time-absorption profiles of the spectra at
475, 800, and 1000 nm, monitoring the formation and decay of the radical ion pair state.
constant that is 100 times faster than in the corresponding
aldehydes and hence occurs in the range of 10 ps. The quantum
yields of the fullerene part of the fluorescence (i.e., 5 × 10-4
and 3 × 10-4 for 6 and 7, respectively) are lower than found
for N-methylfulleropyrrolidine (MP-C60) (i.e., 6.0 × 10-4) but
show nevertheless that, eventually, a large fraction (50-80%)
of the absorbed photons in 6 and 7 end up producing the C60
singlet excited state. Hence, in solution, an energy-transfer
process takes place after photoexcitation of the cores of hybrids
6 and 7.
To elucidate the intramolecular electron donor-acceptor
interactions, we carried out time-resolved transient absorption
measurements, following pulsed photoexcitation, for the starting
aldehydes 5a and 5b as well as the corresponding tetrafullerene
nanoconjugates 6 and 7. Representative femtosecond time-
resolved absorption spectra for 5b dissolved in THF, taken after
a 150 fs laser pulse at 490 nm, are displayed in Figure 4. The
corresponding spectra for 5a are shown as Figure S2. The
differential spectrum recorded immediately after the laser pulse
was characterized by bleaching of the ground-state absorption
at 485 nm as well as a broad absorption between 530 and 1200
nm. These spectral attributes are indicative of the 5b singlet
excited state, which decays slowly (4.9 × 108 s-1) to the
energetically lower-lying triplet excited state predominantly via
intersystem crossing. The triplet excited state features absorption
maxima between 630 and 830 nm.20 Similar, although 2 times
faster, is the decay of the 5a singlet excited state (9.8 × 108
s-1) forming the corresponding triplet manifold.21
time delays after a 490 nm femtosecond laser pulse, and these
can be compared to what was seen for the 5a and 5b references.
At early times (i.e., 5-10 ps), these are practically identical to
those of the references, disclosing strong bleaching at 485 nm
and a broad absorption between 530 and 1200 nm, which is
attributed to absorption of the singlet excited state of the
conjugated core. Since energy transfer to C60 is known to
proceed at time scales of 200 fs or less,22 this broad transient
band between 430 and 1200 nm also can be partly due to the
singlet excited-state absorption of C60, which is known to occur
between 750 and 1150 nm, with a maximum at 960 nm.23
At a delay time of 100 ps (Figure 5), a new transition that
covers most of the visible range, and centered at 800 nm, grows
in, accompanied by another absorption in the near-IR range at
about 1000 nm. We ascribe the former to the π-radical cation
of the conjugated core, while the latter band belongs to the
fullerene π-radical anion.24 Support for the π-radical cation
assignment came from pulse radiolysis studies following the
one-electron oxidation of 5a and 5b.
The spectral changes for 5b are exemplified in Figure 6 and
reflect a good agreement with the transient absorption changes
(21) We note that, especially for 5b, the lifetime of the singlet excited
state inferred from transient absorption and fluorescence spectroscopy is
different. Since the fluorescence can be measured very sensitively over
several orders of magnitude and over a longer time period, these values
are likely more accurate.
(22) (a) van Hal, P. A.; Janssen, R. A. J.; Lanzani, G.; Cerullo, G.;
Zavellani-Rossi, M.; De Silvestri, S. Phys. ReV. B: Condens. Matter Mater.
Phys. 2001, 64, 75206/1-75206/7. (b) van Hal, P. A.; Janssen, R. A. J.;
Lanzani, G.; Cerullo, G.; Zavellani-Rossi, M.; De Silvestri, S. Chem. Phys.
Lett. 2001, 345, 33-38.
(23) Watanabe, A.; Ito, O.; Watanabe, M.; Saito, H.; Koishi, M. Chem.
Commun. (Cambridge, U.K.) 1996, 117-118.
(24) Guldi, D. M.; Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem.
Soc. 1997, 119, 974-980.
The transient absorption changes of tetrafullerene 6 (Figure
S3) and 7 (Figure 5) were also recorded in THF with several
(20) Peeters, E.; Marcos, A.; Meskers, S. C. J.; Janssen, R. A. J. J. Chem.
Phys. 2000, 112, 9445-9454.
J. Org. Chem, Vol. 73, No. 8, 2008 3193

Ferna´ndez et al.
FIGURE 6. Differential absorption changes (visible) following pulse
radiolytic oxidation of 5b in oxygenated dichloromethane with •OOCH2-
•
Cl or OOCHCl2 radicals.
FIGURE 7. Near steady-state photoinduced absorption spectra of drop-
cast films of 6 and 7 and blends of 5a and 5b with PCBM on quartz.
The samples were kept at 80 K and photoexcited at 488 nm with a 50
mW laser, modulated with 275 Hz.
TABLE 2. Device Properties, Short Circuit Current Density (JSC),
Open Circuit Voltage (VOC), Fill Factor (FF), Estimated Efficiency
(η), and Film Thickness of PV Devices with Device Structure
Glass/ITO/PEDOT:PSS/Active Layer/LiF/Al
active layer (ratio) JSC (mA cm-2
)
VOC (V)
FF
ηa
d (nm)
cm-2) being 6 times higher than that of 6. The low current is
consistent with the rapid charge recombination process described
in the previous section. The difference between the two
nanoconjugates can be explained partially by the fact that the
absorption of 7 extends further into the visible region, and hence,
more photons are absorbed. Additionally, 7 features a stronger
electron donor capability than 6, which may impact charge
generation and device performance positively. Also, in the
blends with PCBM, the device containing the OPV donor
(aldehyde 5b) performs much better than that with the OPE
donor (aldehyde 5a), the former having an appreciable JSC value
of 2.04 mA cm-2, resulting in an estimated light to electrical
power conversion efficiency of 0.21%. The external quantum
efficiency of this cell amounts to 15-20% in the absorption
band (350-550 nm). Using the absorbance A ) 0.15 of the 50
nm film in this range, 0.5 (1-10-2A) is an upper limit for the
fraction of absorbed photons, resulting in an internal quantum
efficiency of about 30-40%. Considering the rapid recombina-
tion rate and modest behavior of nanoconjugate 7, the good
performance of the 5b/PCBM blend indicates the importance
of phase separation. Apparently, the larger phase separation that
is likely present in the blend as compared to the well-defined
nanoconjugates is a requirement for the generation of free charge
carriers. Phase separation enables charges to be separated over
longer distances, reducing their probability to recombine.
Nevertheless, also in the 5b/PCBM blend, the majority (60-
70%) of charge carriers that are formed recombine before they
can be collected. Apart from fast geminate recombination, a
low hole mobility in these compounds is most probably
responsible for the observed recombination and is consistent
with the low fill factors.
6
7
0.04
0.27
0.10
2.04
0.42
0.21
0.12
0.40
0.28 0.005
0.25 0.015
0.25 0.003
0.26 0.214
64
37
67
50
5a/PCBM (1:4)
5b/PCBM (1:4)
a Efficiency (η) was not corrected for emission profile of tungsten test
lamp.
upon photoexcitation of 7, revealing a broad absorption with a
maximum at 800 nm. In accordance with these results, we
propose that in 6 and 7, electron transfer from the core to the
electron accepting fullerene creates the radical ion pair state.
The transfer reaction is responsible for the fast deactivation of
the photoexcited chromophores. Analyzing the absorption time
profiles, we derived rate constants of 3.0 × 1010 s-1 either for
6 or for 7 for the intramolecular charge separation.
The radical ion pair state is, however, short-lived and decays
with rate constants of 4.6 × 108 and 9.7 × 108 s-1 for 6 and 7,
respectively. Products of these charge recombination processes
in both nanoarrays (6 and 7) are partly the singlet fullerene
excited state, as evidenced by the fluorescence quantum yields
that are 50-80% of the N-methylfulleropyrrolidine (MP-C60)
used as the reference. This singlet excited state decays pre-
dominantly via intersystem crossing to the fullerene triplet as
evidenced by the characteristic triplet-triplet absorption at 700
nm, -which is seen to decay on the microsecond time scale
(Figure S4).
PV Devices. Four types of PV devices were prepared
comprising different photoactive layers, that is, nanoconjugate
6, nanoconjugate 7, and blends of aldehydes 5a and 5b with
[C60]PCBM. The blends with PCBM contain the same ratio as
the conjugates: one aldehyde molecule per four PCBM
molecules. All devices gave a modest PV response (Table 2
and Figure S5), exhibiting low fill factors (FF ) 0.25-0.30)
and open circuit voltages (VOC ) 0.1-0.4 V). The relation
between VOC and oxidation potentials in these cells is not linear.
This is mainly due to fact that the best performance was for
thin layers (Table 2), in which VOC not only was determined
by the energy levels of the molecules but also by the balance
between photocurrent and leakage current. The main differences
in device performance between the studied active layers stem
from the short circuit current density (JSC). For both nanocon-
jugates, JSC is well below 1 mA cm-2, with that of 7 (0.27 mA
Photophysical Measurements in Thin Films. Near steady-
state photoinduced absorption (PIA) spectra on thin films can
give further insight into the processes that occur upon photo-
excitation of the photoactive layers. The PIA spectra (Figure
7) of the films incorporating the OPE chromophore (5a/PCBM
and 6) are very similar to the spectra of 6 in THF (Figure S3)
at long delay times. The main contribution to the absorption is
that of the fullerene triplet state, having a maximum at 720 nm
(1.72 eV) and a lifetime of 50 µs, typical for that of the C60
excited triplet state. Apparently, the photoexcitation ends up at
the C60 excited triplet state, such as in the THF solution (see
Figure S4). This is consistent with the low photocurrent observed
3194 J. Org. Chem., Vol. 73, No. 8, 2008

Tetrafullerene Conjugates for PhotoVoltaics
in the corresponding PV devices. The spectra of 7 and the blend
of 5b with PCBM are more difficult to interpret (Figure 7).
The spectrum of the blend has a broad absorption from 600 to
1200 nm with a peak at 960 nm and a shoulder at 700 nm. In
addition, there is a weak absorption in the infrared portion of
the spectrum (1750 nm). We attribute the absorptions to a
superposition of the absorption of the radical cation of 5bs
with one broad band similar to that in Figure 6 and a maximum
at 800 nm and one less intense band at low energy (1750 nm)s
and the radical anion of PCBM at around 1000 nm. The main
feature of the spectrum of 7 is the absorption at 840 nm, about
100 nm red-shifted from the triplet absorption spectrum of the
aldehyde in solution (Figure 4). To explain the strong 840 nm
absorption and the concurrent absence of a band above 1300
nm in the PIA spectrum of 7, we assign it to the triplet state of
the conjugated core of 7. Previously, it was shown that for linear
dialkoxy-OPVs, the triplet-state energy of an OPV segment
endowed with three double bonds and four phenyl rings is equal
to the triplet-state energy of MP-C60 (1.5 eV) and lower for
longer derivatives.25 Therefore, the triplet energy may be
transferred from the fullerene back to the oligomer, an effect
that has been observed previously for other systems.26 This
explanation is in good agreement with the lower photocurrent
found for 7 as compared to the 5b/PCBM blend. The blends
for which the triplet state is not extensively populatedsshowed
a 20-fold better performance than nanoconjugate 7. The different
behavior of 7 and the blend of 5b and PCBM can be rationalized
assuming that phase separation is not only beneficial for charge
transport but also affects the energy levels of electrons and holes
in such a way that it favors charge separation and prevents
recombination. If the holes and electrons cannot be transported
from the molecular heterojunction at which they were created,
recombination to the ground state or to a suitably positioned
triplet state is inevitable. The chance for the photogenerated
hole hopping into a donor domain is much larger for the 5b/
PCBM blend than for nanoconjugate 7.
devices is limited. When the same chromophores OPE and OPV
aldehydes 5a and 5b are physically mixed with PCBM as an
acceptor, the photophysical processes change partially. For the
OPV aldehyde, we now observe the formation of long-lived
charges in the PIA experiment and at the same time are able to
collect a reasonably high photocurrent of 2 mA cm-2. These
changes between the nanoconjugates and the physical blends
are tentatively attributed to an improved charge separation after
charge generation as a result of partial aggregation of either or
both of the components that enables charges to dissociate from
the initial location where they were generated. Hence, the size
of the donor and acceptor domains that can be created is an
important parameter for the design of new covalently linked
donor-acceptor systems, such as block copolymers, for PV
applications.
Experimental Section
Compound 2b. To a solution of CBr4 (5.04 g, 15.22 mmol) and
PPh3 (7.98 g, 30.44 mmol) in dry CH2Cl2 (60 mL) was added at
once 1b (2.05 g, 3.80 mmol). The resulting mixture was stirred at
room temperature for 2 h, washed with water, and dried over
MgSO4. Upon filtration, the solvent was evaporated, and the crude
product was purified by column chromatography [SiO2, hexane/
dichloromethane (1:1)] to afford 2b (3.2 g, 98%) as an orange solid.
1H NMR (CDCl3, 300 MHz): δ 7.54 (m, 12 H), 7.15-7.09 (m,
4H), 4.07 (t, J ) 6.4 Hz, 4H), 1.89 (m, 4H), 1.57 (m, 4H), 1.46-
1.33 (m, 8H), 0.94 (t, J ) 6.8 Hz, 6H). 13C NMR (CDCl3, 75 MHz)
δ 151.6, 138.6, 136.9, 134.5, 129.1, 128.4, 127.2, 126.8, 124.8,
110.9, 89.4, 69.9, 32.0, 29.8, 26.3, 23.0, 14.4. FTIR (KBr): ν 3042,
2982, 1660, 1282, 1218, 832, 810 cm-1. UV-vis (CHCl3): λmax
(ꢀ) 344 (27 500), 413 (40 000) nm. ESI-MS m/z: 847.1 [M + H+].
Compound 3a. To a solution of 2a (300 mg, 0.35 mmol) in
Et3N (15 mL), Pd(PPh3)2Cl2 (49.7 mg, 0.07 mmol), CuI (7 mg,
0.035 mmol), and (trimethylsilyl)acetylene (0.4 mL, 2.8 mmol) were
added. The resulting mixture was stirred for 2 h, the solvent was
evaporated to dryness, and the crude product was purified by
column chromatography [SiO2, hexane/dichloromethane (3:1)] to
afford 3a (270 mg, 84%) as a yellow solid. 1H NMR (CDCl3, 300
MHz): δ 7.70 (dd, J ) 8.4 Hz, 8H), 7.07 (s, 2H), 7.02 (s, 2H),
4.04 (t, J ) 6.4 Hz, 4H), 1.91-1.81 (m, 4H), 1.60-1.53 (m, 4H),
1.40-1.25 (m, 8H), 0.93-0.88 (t, 6H), 0.29 (s, 18H), 0.25 (s, 18H).
13C NMR (CDCl3, 75 MHz): δ 153.9, 144.4, 135.4, 132.7, 131.6,
131.4, 129.1, 124.4, 117.0, 114.2, 104.2, 103.9, 102.3, 101.8, 95.2,
94.3, 88.1, 69.8, 32.1, 29.8, 25.8, 22.7, 14.4, 0.0, -0.25. FTIR (KBr)
ν 2956, 2937, 2148, 1629, 1598, 1514, 1250, 1218, 889, 844, 760,
698, 626, 540 cm-1. UV-vis (CHCl3): λmax (ꢀ) 352 (46 000), 407
(70 000) nm. ESI-MS m/z: 915.5 [M + H+].
Conclusion
We synthesized and characterized two new hybrids compris-
ing conjugated OPE and OPV segments and four dangling
fullerenes and investigated their behavior as molecular hetero-
junctions with transient optical spectroscopy and by preparing
PV devices. In solution, these nanoconjugates exhibit a very
fast (∼10 ps) deactivation of the singlet excited state of the
core unit that produces a mixture of a charge-separated state,
involving a radical cation on the π-conjugated oligomer and
the fullerene anion at one of the termini, and a local fullerene
singlet excited state. Formation of the charge-separated state is
more pronounced for 7, whose central core presents a stronger
donor character. Subsequently, the charge-separated state re-
combines to the C60 singlet state and then produces, via
intersystem crossing, the C60 triplet state. In the solid state, the
final outcome is slightly different for 7, where the preferred
long-lived excited state is a triplet OPV core. As both nano-
hybrids 6 and 7 do not form long-lived charge carriers in thin
films, the photocurrent observed in the corresponding PV
Compound 3b. To a solution of 2b (1 g, 1.176 mmol) in Et3N
(30 mL), Pd(PPh3)2Cl2 (206 mg, 0.294 mmol), CuI (22 mg, 0.1176
mmol), and (trimethylsilyl)acetylene (1.32 mL, 9.4 mmol) were
added. The resulting mixture was stirred for 10 h, the solvent was
evaporated to dryness, and the crude product was purified by
column chromatography [SiO2, hexane/dichloromethane (5:1)] to
1
afford 3b (460 mg, 43%) as a orangish solid. H NMR (CDCl3,
300 MHz) δ 7.91 (d, J ) 8.46 Hz, 4H), 7.57 (s, 1H), 7.51 (s, 1H),
7.50 (d, J ) 8.3 Hz, 4H), 7.17 (s, 1H), 7.13 (m, 3H), 7.08 (s, 2H),
4.07 (t, J ) 6.3 Hz, 4H), 1.89 (m, 4H), 1.56 (m, 4H), 1.40 (m,
8H), 0.93 (t, J ) 6.8 Hz, 6H), 0.28 (s, 18H), 0.24 (s, 18H). 13C
NMR (CDCl3, 75 MHz) δ 151.6, 145.24, 139.4, 134.9, 130.0, 128.6,
127.3, 126.6, 110.9, 104.7, 102.8, 102.3, 102.1, 94.0, 69.9, 32.0,
29.8, 26.3, 23.0, 14.4, 0.3, 0.07. FTIR (KBr) ν 3026, 2970, 2923,
1602, 1216, 756, 667 cm-1. UV-vis (CHCl3) λmax (ꢀ) 350 (16 000),
438 (29 000) nm. ESI-MS m/z: 916.6 [M + H+].
(25) van Hal, P. A.; Beckers, E. H. A.; Apperloo, J. J.; Janssen, R. A. J.
Chem. Phys. Lett. 2000, 348, 403-408.
(26) (a) 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,
5974-5988. (b) Schu¨ppel, R.; Uhrich, C.; Pfeiffer, M.; Leo, K.; Brier, E.;
Reinold, E.; Ba¨uerle, P. ChemPhysChem 2007, 8, 1497-1503.
Compound 5a. To a solution of 3a (320 mg, 0.355 mmol) and
5-bromothiophene-2-carboxaldehyde (0.42 mL, 3.52 mmol) in Et3N/
MeOH (3:1) (20 mL) under Ar atmosphere, KF (164 mg, 2.83
J. Org. Chem, Vol. 73, No. 8, 2008 3195

Ferna´ndez et al.
mmol), Pd(PPh3)2Cl2 (50 mg, 0.07 mmol), and CuI (6.8 mg, 0.035
mmol) were added. The resulting solution was stirred overnight at
room temperature. The solvent was evaporated to dryness, and the
crude product was purified by column chromatography [SiO2,
chloroform/methanol (20:1)] to afford 5a (120 mg, 32%) as an
orange solid. 1H NMR (CDCl3, 300 MHz) δ 9.92 (s, 2H), 9.90 (s,
2H), 7.74 (dd, J ) 8.4 Hz, 8H), 7.72 (dd, J ) 3.9 Hz, 4H), 7.39
(dd, J ) 3.9 Hz, 4H), 7.28 (s, 2H), 7.05 (s, 2H), 4.06 (t, J ) 6.4
Hz, 4H), 1.92-1.83 (m, 4H), 1.65-1.52 (m, 4H), 1.43-1.26 (m,
8H), 0.89 (t, J ) 6.8 Hz, 6H). 13C NMR (DMSO-d6, 75 MHz) δ
185.1, 154.1, 144.3, 139.8, 138.9, 138.6, 137.5, 137.0, 135.5, 134.9,
134.8, 134.7, 132.9, 132.4, 132.3, 131.4, 131.3, 130.9, 130.3, 129.1,
128.7, 80.2, 80.0, 79.8, 61.5, 55.7, 32.1, 29.8, 29.5, 26.3, 26.1, 22.9,
14.8. FTIR (KBr) ν 2922, 2852, 2200, 1662, 1598, 1456, 1217,
777, 756, 669 cm-1. UV-vis (CHCl3) λmax (ꢀ) 360 (83 400), 443
(88 500) nm. ESI-MS m/z: 1066.2 [M + H+].
145.15, 145.12, 145.07, 145.02, 144.9, 144.6, 144.55, 144.52, 144.3,
144.2, 144.14, 144.10, 143.9, 143.8, 143.4, 143.0,142.8, 142.5,
142.43, 142.40, 142.2, 142.1, 142.0, 141.98, 141.92, 141.88, 141.83,
141.7, 141.6, 141.9, 141.4, 140.0, 139.8, 139.6, 137.0, 136.4, 135.7,
135.3, 135.0, 132.1, 131.8, 131.7, 131.4, 128.79, 128.73, 127.9,
127.8, 127.77, 127.70, 124.3, 124.2, 123.9, 123.7, 116.5, 78.0, 69.3,
69.2, 68.3, 66.8, 32.05, 32.01, 31.6, 29.7, 29.48, 29.40, 28.4, 27.6,
25.8, 22.91, 22.88, 22.83, 14.3, 14.1. FTIR (KBr) ν 2920, 2850,
2187, 1596, 1463, 1210, 758, 526 cm-1. UV-vis (CHCl3) λmax (ꢀ)
322 (147 000), 426 (60 000) nm. MALDI-TOF MS m/z: 4446.4
[M+].
Compound 7. A solution of 5b (170 mg, 0.16 mmol), N-
octylglycine (172 mg, 0.95 mmol), and C60 (914 mg, 1.27 mmol)
in chlorobenzene (50 mL) was heated under reflux overnight. The
solvent was evaporated to dryness, and the crude product was
purified by column chromatography [SiO2, CS2, chloroform] to
afford 7 (150 mg, 21%) as a black solid. 1H NMR (D2O, 300 MHz)
δ 7.92 (br d, 4H), 7.54-7.05 (m, 20H, Ar), 5.45 (br s, 4H), 5.18
(br s, 4H), 4.22 (br s, 4H), 4.10 (br t, 4H), 3.54-3.08 (m, 34H),
2.73-0.99 (m, 60H). 13C NMR (D2O, 75 MHz) δ 156.2, 155.5,
154.3, 154.2, 153.39, 153.32, 153.27, 153.22, 153.1, 151.6, 147.8,
147.29, 147.25, 146.94, 146.91, 146.84, 146.80, 146.78, 146.73,
146.65, 146.62, 146.5, 146.4, 146.3, 146.28, 146.23, 146.1, 146.04,
146.02, 146.00, 145.84, 145.80, 145.7, 145.3, 145.27, 145.24, 145.1,
144.9, 144.1, 143.7, 143.6, 143.5, 143.4, 143.3, 143.1, 142.8, 142.7,
142.6, 142.5, 142.4, 142.3, 142.25, 142.22, 140.8, 140.6, 140.45,
140.43, 137.7, 137.29, 137.2, 136.9, 136.4, 136.1, 135.2, 132.7,
132.2, 131.8, 130.2, 128.7, 128.5, 128.3, 127.4, 127.2, 125.4, 125.2,
124.8, 110.6, 102.7, 97.1, 95.2, 94.2, 92.9, 89.8, 89.2, 83.8, 78.9,
77.7, 77.2, 77.0, 76.9, 71.1, 69.6, 69.0, 68.5, 67.6, 55.7, 54.4, 33.1,
33.0, 32.7, 30.8, 30.5, 29.5, 28.6, 27.1, 24.1, 24.0, 15.4, 15.3. FTIR
(KBr) ν 2920, 2850, 2180, 1595, 1463, 1178, 1114, 810, 769, 758,
526 cm-1. UV-vis (CHCl3) λmax (ꢀ) 255 (257 000), 325 (92 500),
467 (40 000) nm. MALDI-TOF MS m/z: 4450.6 [M+].
Compound 5b. To a solution of 3b (430 mg, 0.467 mmol) and
5-bromothiophene-2-carboxaldehyde (0.55 mL, 4.64 mmol) in Et3N/
MeOH (3:1) (40 mL) under Ar atmosphere, KF (217 mg, 3.74
mmol), Pd(PPh3)2Cl2 (65 mg, 0.093 mmol), and CuI (9 mg, 0.047
mmol) were added. The resulting solution was stirred overnight at
room temperature. The solvent was evaporated to dryness, and the
crude product was purified by column chromatography [SiO2,
1
chloroform] to afford 5b (270 mg, 54%) as a dark-red solid. H
NMR (CDCl3, 300 MHz) δ 9.92 (s, 2H), 9.89 (s, 2H), 7.90 (d, J )
8.4 Hz, 4H), 7.72 (dd, J ) 3.9 Hz, 4H), 7.61-7.57 (m, 6H), 7.39
(dd, J ) 3.8 Hz, 4H), 7.29 (s, 2H), 7.22 (s, 1H), 7.17-7.15 (m,
3H), 4.10 (t, J ) 6.4 Hz, 4H), 1.91 (m, 4H), 1.57 (m, 4H), 1.41
(m, 8H), 0.93 (br t, J ) 6.9 Hz, 6H). 13C NMR (CDCl3, 75 MHz)
δ 183.0, 182.8, 151.7, 146.4, 145.2, 144.6, 144.2, 140.5, 137.4,
136.4, 134.9, 134.8, 134.7, 134.4, 133.5, 133.3, 132.6, 132.1, 131.4,
130.7, 130.3, 128.8, 128.7, 128.6, 128.5, 128.4, 127.3, 127.1, 126.9,
125.8, 111.06, 100.8, 97.0, 94.9, 88.3, 82.3, 69.9, 32.0, 30.1, 29.8,
26.3, 23.0, 14.4. FTIR (KBr) ν 3053, 3022, 2925, 2854, 2194, 1668,
1595, 1425, 1220, 1197, 808, 721, 694, 669, 578, 542 cm-1. UV-
vis (CHCl3) λmax (ꢀ) 336 (94 600), 492 (110 000) nm. ESI-MS
m/z: 1070.3 [M + H+].
Acknowledgment. Financial support from the MCyT of
Spain and Comunidad de Madrid (Projects BQU2002-00855
and P-PPQ-000225-0505), the EU (RTN network “WONDER-
FULL” and “CASSIUS CLAYS”, and NAIMO Integrated
Project NMP4-CT-2004-500355), SFB 583, DFG(GU 517/4-
1), FCI, and the Office of Basic Energy Sciences of the U.S.
Department of Energy (NDRL-5000) is gratefully acknowl-
edged. G.F. thanks MEC for a research grant. Anne-Sophie
Dupain is thanked for help with the preparation of PV devices.
Compound 6. A solution of 5a (100 mg, 0.09 mmol), N-
octylglycine (101 mg, 0.56 mmol), and C60 (541 mg, 0.75 mmol)
in chlorobenzene (50 mL) was heated under reflux overnight. The
solvent was evaporated to dryness, and the crude product was
purified by column chromatography [SiO2, CS2, chloroform] to
1
afford 6 (150 mg, 36%) as a black solid. H NMR (CDCl3, 300
MHz) δ 7.67 (dd, J ) 8.6 Hz, 8H), 7.35-7.20 (m, 8H), 7.08 (s,
2H), 6.95 (s, 2H), 5.38 (br s, 4 H), 5.12 (d, J ) 9.4 Hz, 4H), 4.12
(d, J ) 9.4 Hz, 4H), 4.02-3.98 (br t, J ) 6.4 Hz, 4H), 3.5-3.36
(m, 30 H), 2.72-0.78 (m, 60 H). 13C NMR (CDCl3, 125 MHz) δ
55.7, 153.7, 153.6, 153.5, 153.4, 153.3, 152.7, 152.7, 152.63,
152.60, 147.1, 146.56, 146.50, 146.2, 146.1, 146.0, 145.99, 145.92,
145.8, 145.7, 145.6, 145.58, 145.51, 145.39, 145.31, 145.2, 145.18,
Supporting Information Available: General methods, ad-
ditional Figures S1-4, and copy of 1H and 13C NMR spectra. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
JO702740D
3196 J. Org. Chem., Vol. 73, No. 8, 2008