Synthesis and photoresponse of a fullerene-bis(styryl)benzene dyad

Article abstract of DOI:10.1039/b517933e

As part of our investigations into single-component organic solar cells, a symmetrical bis(styryl)benzene-fullerene dyad has been synthesized. An oxyethylene macrocyclic spacer, attached to meta-positions of side phenyl groups of bis(styryl)benzene, was l

Full text of DOI:10.1039/b517933e

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/njc | New Journal of Chemistry
Synthesis and photoresponse of a fullerene–bis(styryl)benzene dyadw
Nadia Camaioni,*b Graziano Fabbrini,a Enzo Menna,a Michele Maggini,*a
Giovanni Ridolfib and Alberto Zanellib
Received (in Montpellier, France) 16th December 2005, Accepted 24th January 2006
First published as an Advance Article on the web 9th February 2006
DOI: 10.1039/b517933e
As part of our investigations into single-component organic solar cells, a symmetrical
bis(styryl)benzene–fullerene dyad has been synthesized. An oxyethylene macrocyclic spacer,
attached to meta-positions of side phenyl groups of bis(styryl)benzene, was linked to [60]fullerene
through a synthesis based on the nucleophilic Bingel cyclopropanation of the fullerenes. The dyad
1
structure was confirmed by H, 13C and HR-FAB mass spectra. The UV-Vis spectrum of the
dyad is the superimposition of those of appropriate model systems, indicating that ground-state
electronic interactions between the constituent chromophores, in solution, are negligible, in line
also with the electrochemical results. Molecular modelling studies and NMR data suggest that the
bis(styryl)benzene moves back and forth at room temperature from one side of the fullerene to
the other. Interestingly, the overall power conversion efficiency (Z) calculated for solar cells made
of the bis(styryl)benzene–fullerene dyad was 0.022% under a white-light irradiation power of
86 mW cmꢀ2. This is, to our knowledge, the highest value reported, under high-level irradiation
conditions, for solar cells based on dyads in which a fullerene is tethered to a phenylenevinylene
structure.
backbone (p-cable). The covalent grafting of the active com-
1. Introduction
ponents of the solar cell would prevent, at least in principle,
either extensive phase segregation10 or unwanted ground state
Plastic solar cells, made of an interpenetrating, bicontinuous
network1 of an electron donor conjugated polymer (D) and an
electronic interactions between D and A chromophores. Both
electropolymerized11 and solution processable11b,12 conjugated
electron acceptor functionalized fullerene (A) have shown
potential to harness solar energy effectively2, although low
polymers with pendant fullerenes have been prepared and, in a
efficiency still limits their viability for commercial uses.3
few cases, tested in photovoltaic devices. Power conversion
However, following the fast growth of research in organic
efficiency values of about 0.1%, under white light illumination
materials for solar energy conversion,4 key bottleneck factors
of 100 mW cmꢀ2 intensity, were reported for a methanofuller-
to high-efficiency, such as photon losses, exciton losses or
ene covalently linked to a copolymer made of poly(p-pheny-
lene)vinylene and poly(p-phenylene)ethynylene.12a The
charge carrier mobility,3 especially that of the holes in donor
polymers,5 have become clear. Efforts are therefore made to
optimization of the fullerene mole fraction remains, most
minimize these losses through the realization of nano-struc-
likely, the major challenge for the development of efficient
tured heterojunctions6 by controlling, for instance, the self-
double cable, fullerene-based solar cells.9 It has been reported
organization of the D/A components.7 Currently, the best
that a fullerene content as high as 80 wt% would ensure best
conjugated polymer-based solar cells, reported in the litera-
performance for poly[2-methoxy-5-(30,70-dimethyloctyloxy)-
ture, are those obtained by mixing poly(3-hexyl)thiophene and
1,4-phenylene]vinylene (MDMO-PPV)–fullerene derivative
solar cells.13 More recently, it has been found that a 1 : 1
a soluble functionalized fullerene that gave a power conversion
efficiency approaching 5% under standard test conditions.7c
wt : wt fullerene–polymer ratio in the active layer yielded
Alternative approaches to fullerene–polymer blends have
balanced electron and hole transport in cells where poly
been actively pursued as well by several groups.8 In the so-
(3-hexylthiophene) was the donor.7b To our knowledge, no
called double cable approach,9 for instance, functionalized
soluble double cable materials, having such a large concentra-
fullerenes (n-cable) are tethered to a conjugated polymer
tion of fullerene, have been reported yet.
Other promising approaches to plastic solar cells followed
a
`
Dipartimento di Scienze Chimiche and CNR-ITM, Universita di
the considerable development of the chemistry of the full-
erenes.14 They include the use of molecular dyads and triads
made of a functionalized fullerene covalently linked to oligo-
meric, conjugated architectures that showed power conversion
efficiencies ranging from about 10ꢀ2% to our reported 0.37%,
under 80 mW cmꢀ2 white light illumination, for a fullerene–
azothiophene dyad.15 In this connection, the first example
of a single-component solar cell based on a fullerene–
Padova, via Marzolo 1, 35131 Padova, Italy. E-mail:
michele.maggini@unipd.it; Fax: þ39-049-8275792;
Tel: þ39-049-8275662
b
`
Istituto per la Sintesi Organica e la Fotoreattivita (CNR-ISOF), via
P. Gobetti 101, 40129 Bologna, Italy. E-mail: camaioni@isof.cnr.it;
Fax: þ39-051-6399844; Tel: þ39-051-6399779
w Electronic supplementary information (ESI) available: HPLC ana-
lysis, 1H NMR, 13C NMR and FAB mass spectra of dyad 1. See DOI:
10.1039/b517933e
ꢁc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006
New J. Chem., 2006, 30, 335–342 | 335

View Article Online
bis(styryl)benzene conjugate was reported in 1999 by J.-F.
Nierengarten and co-workers.16 Recently, accounts have been
published17 in which sections are devoted to some aspects of
this subject.
Instrumentation. Thin layer chromatography (TLC) and
column chromatography were performed using a Polygram
SilG/UV254 (TLC plates) and silica gel MN 60 (70–230 mesh)
by Macherey-Nagel. 1H (250.1 MHz) and 13C (62.9 MHz)
NMR spectra were recorded on a Bruker AC-F 250 spectro-
meter. ESI mass spectra were taken on a Thermo Finnigan
AQA LC/MS: ꢀ4 kV spray voltage; ꢀ10 V capillary voltage;
180 1C capillary temperature; nitrogen as nebulizing gas. The
samples were dissolved in methanol containing 1% trifluor-
oacetic acid. Exact mass determination was provided by the
Washington University Mass Spectrometry Resource at the
Washington University Center for Biomedical and Bioorganic
Mass Spectrometry. A matrix of 3-nitrobenzyl alcohol was
employed for the measurements. IR spectra were recorded on
a Perkin-Elmer FT-IR model 1720X. The UV-Vis spectra were
recorded on a Perkin-Elmer Lambda 16 spectrophotometer.
The emission spectra were recorded on a Perkin-Elmer LS 55
spectrofluorimeter. HPLC analysis of derivative 1 was per-
formed with a Phenomenex Luna column (250 ꢂ 4.6 mm,
SiO2, 5 m) using toluene–ethyl acetate–isopropanol solvent
mixtures: eluent A (toluene–ethyl acetate 8 : 2), eluent B
(toluene–isopropanol 9 : 1). Linear gradient elution: 100% A
for 15 min - 75% A (within 35 min). Flow = 1 ml minꢀ1. In
the mentioned HPLC conditions, compound 1 is eluted at
16 min (Fig. S1w).
Several groups reported the synthesis and photophysical
properties of triad ensembles.18 Also, a wide variety of other
donor-linked fullerenes have been proposed in which the
donor moiety is, for instance, a functionalized oligo(phenyle-
ne)vinylene,16,19 a Zn-phthalocyanine,20 a dihexyloxybenzene-
thiophene structure,21 a functionalized oligo(phenylene)ethy-
nylene22 or an azoaromatic dye.15,23 It is worth mentioning
that a general phenomenon in fullerene-based D–A dyads is
the competition between energy and electron transfer.18a,19a,24
This issue can be tackled by designing D–A molecular systems
characterized by a branching ratio between the two processes
markedly in favor of the charge transfer, which is the essential
prerequisite for photovoltaic applications.20,25 Both theory
and experiment indicate that donor–acceptor distance26 and
orientation play a pivotal role in controlling the photoinduced
electron transfer and consequently the efficiency of charge
separation and the lifetime of the charge separated state. To
this end, the groups of R. A. J. Janssen, J. Roncali and co-
workers developed a set of complementary dyads, where the b-
position of terminal thiophene rings, in a tetrathiophene
moiety, was connected to a functionalized fullerene in a single
or double fashion.27 The macrocyclic thiophene–fullerene con-
jugate showed rate constants for energy and charge transfer
that were more than one order of magnitude larger than those
observed for the flexible system. Schuster and co-workers28
investigated the topological control of the intramolecular elec-
tron transfer in a symmetrical parachute-shaped porphyrin–
fullerene dyad. Molecular modelling suggested that the
porphyrin moves over the side of the fullerene sphere, bringing
the two in close proximity. The efficient and rapid quenching
of the porphyrin luminescence, followed by the formation of
either the fullerene excited triplet state or the charge separated
state, revealed that the above mentioned conformation facil-
itates through-space interactions between D and A. Guldi and
co-workers29 studied the effect of orientation of a Zn-porphyr-
in and a fullerene moiety on electron transfer rates: p–p
interactions was confirmed to be the crucial parameter that
controls rates, efficiency and mechanism of electron transfer
from the porphyrin to the fullerene.
3-{2-[2-(2-Hydroxyethoxy)ethoxy]ethoxy}-4-methoxybenzal-
dehyde 2. A mixture of 3-hydroxy-4-methoxybenzaldehyde
(2.0 g, 13.1 mmol) and 2-[2-(2-methoxy)ethoxy]ethyl-p-tolue-
nesulfonate (4.0 g, 13.1 mmol) in a suspension of dry K2CO3
(5.44 g, 39.4 mmol) in acetone was stirred for 12 h at reflux
temperature, and monitored by TLC (eluent: ethyl acetate–
hexane 9 : 1, Rf (2) = 0.15). The mixture was then concen-
trated under reduced pressure and the crude product, treated
with 30 ml of CHCl3, washed with 1 M NaOH. The product
was purified by flash column chromatography (SiO2, ethyl
acetate–petroleum ether 9 : 1 then ethyl acetate–ethanol 9 : 1)
affording 3.32 g (89%) of a yellowish oil. IR (nujol) n 2932,
2876, 1685, 1596, 1587, 1514, 1438, 1268, 1137, 1072, 1022
cmꢀ1. 1H-NMR (CDCl3, 250 MHz) d (ppm): 9.60 (s, 1H), 7.26
–7.22 (m, 2H), 6.78–6.74 (d, 1H), 4.01 (t, 2H), 3.70 (s, 3H),
3.69 (t, 2H), 3.51–3.45 (m, 6H), 3.38 (t, 2H), 3.17 (s, 1H). 13C-
NMR (CDCl3, 62.9 MHz) d (ppm) 190.43, 154.37, 148.24,
129.38, 126.35, 110.52, 110.33, 72.10, 70.25, 69.77, 68.86,
67.86, 61.03, 55.52. ESI-MS (C14H20O6) m/z 307 [M þ Na]1.
In this paper we report the synthesis, electrochemistry and
photovoltaic properties of a fullerene–bis(styryl)benzene dyad
1 in which the flexible macrocyclic oxyethylene spacer might
allow the two chromophores to arrange in a conformation
where the reciprocal orientation and distance are the most
favorable for the electron transfer process.
3-(2-{2-[2-(tert-Butyldimethylsilanyloxy)ethoxy]ethoxy}ethoxy)
-4-methoxybenzaldehyde 3. To a solution of benzaldehyde-
alcohol 2 (3.0 g, 10.5 mmol) and imidazole (0.78 g, 11.5 mmol)
in dry CH2Cl2 (40 ml), a solution of tert-butyldimethylsi-
lylchloride (1.74 g, 11.5 mmol) in dry CH2Cl2 (10 ml) was
added dropwise over a period of 15 min, and the reaction was
monitored by TLC (eluent: hexane–ethyl acetate 1 : 9, Rf (3) =
0.95). When the addition was complete, the mixture was
stirred for 5 hours at room temperature, then the solvent
was removed under reduced pressure. The crude product was
purified by flash column chromatography (SiO2, ethyl acetate–
petroleum ether 7 : 3) yielding 3.64 g (82%) of 3 as a colorless
oil. IR (nujol) n 2953, 2929, 2858, 1689, 1597, 1587, 1513,
2. Experimental
Syntheses
Materials. All reagents for the synthesis, including dry
solvents, were purchased from Aldrich and used without
further purification. Bis(phosphonium chloride) 4 was pre-
pared as reported in the literature.30
ꢁc
This journal is the Royal Society of Chemistry the Centre National de la Recherche Scientifique 2006
336 | New J. Chem., 2006, 30, 335–342

View Article Online
1
1436, 1269, 1253, 1164, 1139, 1108, 837, 779 cmꢀ1. H-NMR
dry CH2Cl2 were added dropwise over a period of 1 h under a
nitrogen atmosphere. When the addition was complete, the
mixture was stirred at room temperature for 4 h, then the
solvent was removed in vacuo (TLC, eluent: ethyl acetate–
hexane 9 : 1, Rf (6) = 0.5). The residue was purified by flash
column chromatography (eluent: ethyl acetate–petroleum
ether 8 : 2) to give 39 mg (36%) of product 7 as a yellow solid.
IR (KBr) n 3419, 3053, 3003, 2925, 1516, 1266, 1229, 1211,
1138, 1105, 1062, 1043 cmꢀ1. UV-Vis (CH2Cl2) l 398, 342 nm.
1H-NMR (250 MHz, CDCl3) d (ppm): 7.43–7.42 (m, 2H), 7.35
–7.29 (d, 2H, 3JHH = 16.5 Hz), 7.10 (s, 2H), 7.08–7.02 (d, 2H,
3JHH = 16.5 Hz), 7.08–7.07 (m, 2H), 6.87–6.83 (m, 2H), 4.39
(t, 4H), 4.21 (t, 4H), 3.92 (s, 6H), 3.88 (s, 6H), 3.88–3.84 (m,
12H), 3.67–3.64 (m, 4H), 3.30 (s, 2H). 13C-NMR (62.9 MHz,
CDCl3) d (ppm): 166.44, 151.29, 149.54, 148.21, 131.07,
128.63, 126.38, 121.56, 120.71, 113.33, 111.51, 109.00, 68.92,
68.75, 64.31, 56.27, 55.91. ESI-MS (C41H50O14) m/z 766 [M]1.
(250 MHz, CDCl3) d (ppm) 9.78 (s, 1H), 7.43–7.39 (m, 2H),
6.94–6.91 (d, 1H), 4.19 (t, 2H), 3.89 (s, 3H), 3.87 (t, 2H), 3.71–
3.64 (m, 6H), 3.51 (t, 2H), 0.84 (s, 9H), 0.01 (s, 6H). 13C-NMR
(62.9 MHz, CDCl3) d (ppm) 190.69, 154.80, 148.78, 129.85,
126.66, 110.97, 110.61, 72.56, 70.76, 70.62, 69.31, 68.33, 62.58,
55.95, 25.78, 18.22, ꢀ5.40. ESI-MS (C20H34SiO6) m/z 421 [M
þ Na]1.
Bis(styryl)benzene 6. To a solution of 2.0 g (5.02 mmol) of
benzaldehyde 3 and 1.73 g (2.28 mmol) of 2,5-methoxy-1,4-
methyltriphenylphosphonium chloride 4 in 15 ml of absolute
ethanol, 9.50 ml of lithium tert-butoxide solution (1.0 M in
ethanol) were added through a syringe under a nitrogen
atmosphere. The solution was stirred for 12 hours at room
temperature (TLC, eluent: hexane–ethyl acetate 7 : 3; broad
spot with a retention factor of about 0.5 for the presence of a
mixture of isomers) then the solvent was removed in vacuo.
The residue was passed through a pad of SiO2 (eluent:
petroleum ether–ethyl acetate 8 : 2) and concentrated under
reduced pressure. Conversion of the mixture to the most stable
trans-trans isomer 5 was accomplished with iodine in refluxing
toluene over 4 hours. Derivative 5 was used for the deprotec-
tion step without further purification. (An analytical sample of
compound 5 can be obtained through flash chromatography
purification of crude 5 over SiO2; eluent: petroleum ether–
ethyl acetate 8 : 2). IR (nujol) n 2928, 2856, 1512, 1265, 1138,
1106, 1042 cmꢀ1. 1H-NMR (250 MHz, CDCl3) d (ppm) 7.35–
7.28 (d, 2H, 3JHH = 16.5 Hz), 7.16–7.07 (m, 4H), 7.11 (s, 2H),
7.07–7.01 (d, 2H, 3JHH = 16.5 Hz), 6.87–6.83 (d, 2H), 4.26 (t,
4H), 3.95 (t, 4H), 3.92 (s, 6H), 3.87 (s, 6H), 3.81–3.70 (m,
12H), 3.59–3.55 (m, 4H), 0.88 (s, 9H), 0.06 (s, 6H). 13C-NMR
(62.9 MHz, CDCl3) d (ppm) 151.25, 149.34, 148.29, 132.94,
130.96, 129.60, 126.32, 121.28, 120.44, 111.70, 111.34, 108.81,
72.60, 70.81, 69.59, 68.42, 66.59, 62.63, 56.21, 55.88, 25.84,
18.27, 5.35. ESI-MS (C50H78Si2O12) m/z 949 [M þ Na]1.) To
crude derivative 5, dissolved in 10 ml of dry THF, tetrabuty-
lammonium fluoride (0.66 ml, 2.2 mmol) was added under
nitrogen. The resulting mixture was stirred for 5 hours at room
temperature (TLC, eluent: ethyl acetate–ethanol 9 : 1, Rf (6) =
0.4), then the solvent was evaporated under reduced pressure.
The residue was purified by flash column chromatography
(eluent: ethyl acetate) affording 500 mg of unprotected
bis(styryl)benzene 6 (33% overall yield calculated from 3) as
yellow crystals. IR (KBr) n 3417, 3366, 2924, 2856, 1517, 1499,
Macrocyclic methanofullerene 1. To a solution of bis(styr-
yl)benzene 7 (39 mg, 0.51 mmol), C60 (44 mg, 0.61 mmol) and
iodine (14 mg, 0.61 mmol) in toluene (30 ml), a solution of
DBU (11 mL, 0.72 mmol) in 5 ml of toluene was added
dropwise over a period of 15 minutes. When the addition
was complete, the mixture was stirred at room temperature for
4 hours, then the solvent was removed under reduced pressure
(TLC, eluent: toluene–ethanol 95 : 5, Rf (1) = 0.55). The
product was purified by flash column chromatography (eluent:
toluene then toluene–2-PrOH 9 : 1), affording 32 mg (42%) of
pure 1 as a brownish solid compound. IR (KBr) n 3453, 3441,
2944, 2929, 2868, 1511, 1265, 1231, 1208, 1137, 1114, 1043,
1031, 963, 527 cmꢀ1. UV-Vis (CH2Cl2) l (e) 229 (92879), 256
(111632), 326 (46782 Mꢀ1 cmꢀ1) nm. 1H-NMR (250 MHz,
3
CDCl3) d (ppm): 7.39 (m, 2H), 7.33–7.26 (d, 2H, JHH = 15
Hz), 7.09 (s, 2H), 7.05–6.99 (d, 2H, JHH = 15 Hz), 7.05 (m,
3
2H), 6.85–6.81 (m, 2H), 4.57 (t, 4H), 4.36 (t, 4H), 3.95 (s, 6H),
3.88 (s, 6H), 3.84–3.79 (m, 12H), 3.64 (m, 4H). 13C-NMR (62.9
MHz, CDCl3) d (ppm): 163.5, 151.3, 149.4, 148.2, 145.2, 145.1,
145.0, 144.9, 144.8, 144.6, 144.4, 144.3, 143.7, 142.9, 142.8,
142.7, 142.0, 141.6, 140.7, 138.8, 131.1, 128.8, 126.5, 121.7,
120.6, 113.4, 111.5, 109.0, 71.2, 70.9, 70.6, 70.4, 68.9, 65.9,
56.3, 55.9, 52.4. HR-FAB MS m/z: 1485.2966 (theoretical
mass: m/z: 1485.3123; ꢀ10.6 ppm).
Cyclic voltammetry experiments
Cyclic voltammetries were performed at room temperature in
a three compartment glass electrochemical cell under Ar
pressure by using a potentiostat/galvanostat AMEL model
5000. The working electrode was a Pt sphere (area 0.13 cm2),
the reference electrode was a saturated calomel electrode
(SCE) and the auxiliary electrode was a Pt wire. The support
electrolyte was 0.1 M (C4H9)4NClO4 (Fluka, puriss. crystal-
lized from CH2Cl2 and vacuum dried) in CH2Cl2 (Merck,
Uvasol, distilled over P2O5 and stored under Ar pressure);
before use, the solution was stored for 24 hours over 3 A
molecular sieves (Merck, activated for 4 hours at 380 1C).
1465, 1444, 1410, 1268, 1235, 1211, 1138, 1042, 1020 cmꢀ1. 1H-
3
NMR (250 MHz, CDCl3) d (ppm): 7.34–7.28 (d, 2H, JHH
=
16.5 Hz), 7.16–7.07 (m, 4H), 7.10 (s, 2H), 7.07–7.00 (d, 2H,
3JHH = 16.5 Hz), 6.87–6.84 (d, 2H), 4.26 (t, 4H), 3.93 (t, 4H),
3.92 (s, 6H), 3.88 (s, 6H), 3.79–3.71 (m, 12H), 3.64–3.60 (t,
4H). 13C-NMR (62.9 MHz, CDCl3) d (ppm): 151.32, 149.25,
148.23, 131.05, 128.55, 126.41, 121.38, 120.55, 111.70, 111.20,
108.93, 72.63, 70.83, 70.31, 69.68, 68.36, 61.78, 56.32, 55.93.
ESI-MS (C38H50O12) m/z: 721 [M þ Na]1.
Macrocyclic bis(styryl)benzene 7. To a stirred suspension of
NaHCO3 (26 mg, 0.31 mmol) in CH2Cl2 (20 ml), a solution of
derivative 6 (100 mg, 0.14 mmol) in 10 ml of CH2Cl2 and a
solution of malonyl dichloride (30 mL, 0.31 mmol) in 10 ml of
Photovoltaic devices
The photoactive layers were spun at 2000–3000 rpm from
chloroform or chlorobenzene solutions (15 g lꢀ1) onto indium
ꢁc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006
New J. Chem., 2006, 30, 335–342 | 337

View Article Online
tin oxide (ITO)/glass substrates previously coated with a layer
of polystyrene sulfonic acid doped poly(ethylene-dioxythio-
phene) (PEDOT : PSS, from Bayer AG). The film thicknesses
(around 100 nm) were determined using a profilometer Alpha
Step 200 (Tencor Instruments). The structure of the photo-
voltaic devices was completed by subliming an aluminium top
electrode in vacuo, the active area of the devices was 3.14 mm2.
Except for the cathode deposition, each step of the device
fabrication was performed in ambient conditions. The elec-
trical characterization of the devices was performed with a
Keithley 2400 source measure unit, under a dynamic vacuum
of 2 ꢂ 10ꢀ4 mbar, in a home made chamber. Junctions were
illuminated through the ITO side with a 300 W Xe arc lamp.
The intensity of the incident light (86 mW cmꢀ2) was measured
with an Oriel thermopile. No correction was made for light
reflection. The optical bench was equipped with a water filter
to cut off IR radiation.
coupling reaction with bis(phosphonium chloride) 4, followed
by isomerization of the crude mixture with iodine in refluxing
toluene to afford the trans-trans bis(styryl)benzene 5. The
configuration of the double bonds in 5 was established via
3
NMR spectroscopy, through the measurement of the JHH of
the olefin protons which was found to be around 16 Hz, in line
with literature data for olefin trans protons.31 Removal of the
TBDMS groups from 5 with tetrabutylammonium fluoride
gave deprotected bis(alcohol) 6 in 33% overall yield calculated
from 3.
Compound 6, in the presence of malonyl dichloride and
sodium bicarbonate in CH2Cl2, afforded the product of
macrocyclization 7 in 36% isolated yield. Functionalization
of C60 with the cyclic bis(styryl)benzene-malonate derivative 7
was based on the nucleophilic Bingel cyclopropanation of the
fullerenes.32 The reaction of C60 and 7 in the presence of 1,8-
diazabicyclo[5.4.0]undec-7-ene and I2 in toluene at room
temperature afforded methanofullerene 1 in 42% isolated
yield. The presence of the oxyethylene groups confers to
compound 1 a good solubility in some organic solvents, such
as CHCl3, tetrahydrofuran, chlorobenzene or toluene. HPLC
analysis was employed to assess the purity of derivative 1 (Fig.
S1w) whereas NMR spectroscopy and exact mass determina-
tion were used to validate its proposed molecular structure
(Fig. S2–S3w). The symmetry of compound 1 is indicated by its
NMR spectra. The 13C spectrum shows 28 signals (12 signals
are attributed to the bis(styryl)benzene moiety and 16 to the
fullerene sphere) between d 165 and 105, as well as 9 signals in
the aliphatic region. The two sp3 fullerene carbons resonate at
d 71.2, together with the adjacent spiro carbon at d 52.4. The
structure of 1 is confirmed by HR-FAB mass spectrometry
with the molecular ion peak at m/z = 1485.2966 (theoretical
mass: m/z = 1485.3123).
3. Results and discussion
The synthesis of the macrocyclic methanofullerene 1 is illu-
strated in Scheme 1. It began with compound 2, which
was prepared in 89% yield through a Williamson reaction
between 3-hydroxy-4-methoxybenzaldehyde and 2-[2-(2-
methoxy)ethoxy]ethyl-p-toluenesulfonate in the presence of
K2CO3. Treatment of 2 with tert-butyldimethylsilylchloride
(TBDMSCl) and imidazole in CH2Cl2 afforded the protected
alcohol-aldehyde 3 in 82% yield. This was used for the Wittig
The geometry of the fullerene dyad 1 was optimized using
an ONIOM multilayer model, as implemented in the GAUS-
SIAN 03 program package.33 The structure of 1 was parti-
tioned into two layers. The bis(styryl)benzene moiety was
treated at the DFT level of theory, using the Becke three-
parameter Lee–Yang–Parr (B3LYP) exchange correlation po-
tential, through which electron correlation effects are included,
with a 3-21G* basis set. The methanofullerene with the
oxyethylene chains was considered as the outer layer and
described by a computationally less demanding, semi-empiri-
cal method (AM1). The mentioned calculations showed that
the bis(styryl)benzene moiety in 1 retains its planar and
conjugated molecular structure, with no major distortions
induced by the macrocyclic structure (Fig. 1). As a conse-
quence, no meaningful perturbation of its electronic properties
is expected because of the covalent linkage to the acceptor
moiety.
It is interesting to note that molecular modelling studies on
dyad 1 revealed that, in its lowest-energy conformation, the
bis(styryl)benzene moiety does not stay above but lies on a
side of the fullerene sphere, in a minimum energy conforma-
tion that brings the two chromophores in close proximity [Fig.
Scheme 1 Synthesis of fullerene–bis(styryl)benzene dyad 1: (a) 2-[2-
(2-methoxy)ethoxy]ethyl-p-toluenesulfonate, K2CO3, acetone, reflux,
12 h, 89%; (b) TBDMSCl, imidazole, CH2Cl2, 25 1C, 5 h, 82%; (c) 2,5-
methoxy-1,4-methyltriphenylphosphonium chloride 4, EtOH, t-BuO-
Li, 25 1C, 5 h; (d) Bu4NF, THF, 25 1C, 4 h, 33% calculated from 3; (e)
malonyl dichloride, NaHCO3, CH2Cl2, 1 h, 36%; (f) C60, 1,8-diazabi-
cyclo[5.4.0]undec-7-ene, I2, toluene, 25 1C, 3 h, 42%.
1
1(b)]. However, both H and 13C the NMR spectra of 1 are
consistent with a C2v symmetrical structure, thus suggesting
that the bis(styryl)benzene moves back and forth at room
temperature from one side of the fullerene to the other. This
ꢁc
This journal is the Royal Society of Chemistry the Centre National de la Recherche Scientifique 2006
338 | New J. Chem., 2006, 30, 335–342

View Article Online
Fig. 3 Fluorescence spectra of the macrocyclic dyad 1 and of the
bis(styryl)benzene precursor 7 in CH2Cl2 at 25 1C.
bis(styryl)benzene and fullerene chromophores, indicating a
negligible interaction between the two components in the
ground state. This is in agreement with the voltammetric
results (vide infra).
Emission measurements with dyad 1 and model 7 were
carried out in CH2Cl2 at 25 1C. Excitation of 7 (lexc = 400
nm) produces a strong emission band with a maximum at
around 445 nm (Fig. 3). A solution of dyad 1, with equal
absorbance at lexc = 400 nm, shows a remarkable quenching
of the previously mentioned 445 nm emission.
The voltammogram of 1 (Fig. 4) shows three reduction
waves at negative potentials (marked with 1 to 3 in Fig. 4), that
are due to the methanofullerene moiety, and two oxidation
waves (4 and 5) in the region of positive potentials.
Fig. 1 (a) ONIOM(B3LYP/3-21G*:AM1) optimized geometry of
fullerene dyad 1 (high level represented in ball and stick style; low
level AM1 in tube style. (b) Top view (space-filling) of the fullerene
dyad 1 represented in (a).
The voltammogram of 7 does not show reduction waves up
to the electrolyte breakdown at negative potentials but it
shows two oxidation waves, close to those observed for dyad
1, in the region of positive potentials. The voltammetric waves
phenomenon has been already documented for a porphyrin–
methanofullerene dyad.28
The absorption spectrum of dyad 1 in dichloromethane
(Fig. 2) is the superposition of the absorption features of the
Fig. 2 Absorption spectra of the macrocyclic dyad 1 and of the
bis(styryl)benzene precursor 7 in CH2Cl2. The spectrum of a 100 nm
thick film of 1, onto glass/ITO substrate, is also shown (F) for
comparison.
Fig. 4 Cyclic voltammograms at 20 mV sꢀ1 of 1 (dashed line) and 7
(full line) 0.5 nM in CH2Cl2–(C4H9)4 N ClO4 0.1 M.
ꢁc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006
New J. Chem., 2006, 30, 335–342 | 339

View Article Online
Table 1 Redox potentials (V vs. SCE) from cyclic voltammetries at
20 mV sꢀ1 of 0.5 mM solutions in CH2Cl2–(C4H9)4NClO4 0.1 M
1
2
1
2
1
2
1
2
1
2
Compound
E (1)
E (2)
E (3)
E (4)
E (5)
1
7
a
ꢀ0.59
ꢀ0.93
ꢀ1.40
þ0.83
þ0.84
þ1.03a
þ1.02
—
—
—
Maximum potential of the oxidation wave 5 [Eo(5)].
1 and 4 appear reversible, in fact the differences between the
maximum potential of the oxidation waves [Eo(i), where i is
the marker of the wave, in this case i = 1 and i = 4] and the
minimum potential of the related reduction waves [Er(i), in this
case Er(1) and Er(4)] are very close to 59 mV. Waves 2 and 3
appear quasi-reversible, likely because of follow-up reactions
associated to electrochemically-induced opening of the cyclo-
propane ring, as typically observed in malonate-fullerenes.34
1
Table 1 summarizes the redox potentials E (i) = [Eo(i) ꢀ
2
Er(i)]/2 that are in reasonable agreement with the values
recorded at 0.1 V sꢀ1 for pristine fullerene in the same
electrolyte,35 if the small influence of the derivatization of 1
on the redox potentials of the methanofullerene moiety is
taken into account.36 In fact, the functionalization of full-
erene, leading to methanofullerene C61H2, introduces a poten-
tial shift estimated at 120 mV, but larger potentials shift are
expected if stronger electron donors are attached.37 Further-
1
2
1
2
more, within the experimental uncertainty, E (4) and E (5) of 1
are equal to those of 7, confirming that electronic interactions
between methanofullerene and bis(styryl)benzene moieties are
negligible, as already reported for oligophenylenevinylene–
fulleropyrrolidine dyads.19b,19f
Dyad 1 has excellent film forming properties, an imperative
requisite for high performance photovoltaic devices, when
deposited from chlorinated solutions. The surface morphology
of films of dyad 1, spun onto ITO/PEDOT : PSS substrates
from chloroform or chlorobenzene solutions, was investigated
by means of scanning force microscopy and the images (not
shown here) revealed a fairly smooth surface, as already
observed for films of other fullerene dyads.23
Fig. 5 Current–voltage curves of an ITO/PEDOT : PSS/1/Al cell in
the dark (a) and under 86 mW cmꢀ2 white light irradiation (b).
Photovoltaic parameters: jsc = 9.3 ꢂ 10ꢀ5 A cmꢀ2; Voc = 0.68 V;
FF = 0.30; Z = 0.022%. The active layer was spun from CHCl3.
Room temperature.
Photovoltaic cells were realized, using the ITO/PED-
OT : PSS/1/Al configuration, to test the photoresponse of the
dyad. The devices were prepared at ambient conditions, apart
from the cathode deposition, and tested in dynamic vacuum
(2 ꢂ 10ꢀ4 mbar). Junctions with good rectification properties
were obtained. A typical current–voltage curve in the dark is
reported in Fig. 5(a). Under white-light irradiation of intensity
86 mW cmꢀ2 they gave current–voltage curves as shown in
Conclusion
In summary, we have found that a macrocyclic bis(styryl)ben-
zene-linked fullerene works as an active component in solar
cells. The photovoltaic power conversion efficiency, far from
being attractive to the market, represents nevertheless the
highest value reported at solar irradiation levels for cells based
on dyads in which a fullerene is tethered to a phenyleneviny-
lene structure. In particular, the limited conformational free-
dom between the fullerene and bis(styryl)benzene units in 1, if
compared to other reported oligo(phenylene)vinylene–fuller-
ene dyads, could play a favourable role in the transport
properties of the solar cell. It is worth underlining that the
intrinsic fullerene/bis(styryl)benzene moiety weight ratio of 1.8
in dyad 1 is significantly lower than the optimized fullerene/
polymer weight ratio of 4, presently used in the most efficient
MDMO-PPV/fullerene derivative solar cells.6 This might be a
Fig. 5(b), with a short-circuit current (jsc) of 9.3 ꢂ 10ꢀ5
A
cmꢀ2, an open-circuit voltage (Voc) of 0.68 V and a fill factor
(FF) of 0.30. The values for Voc and FF are respectively the
highest and among the highest reported for OPV–fullerene
dyads. The overall white-light conversion efficiency (Z) calcu-
lated for the devices was 0.022%. This is, to our knowledge,
the highest value reported in the literature, for cells based on
dyads which contain a fullerene tethered to an oligo(phenyle-
ne)vinylene, obtained under high level irradiation conditions
(not lower than 80 mW cmꢀ2). Solar cells whose active layer
was spun from chlorobenzene exhibited the same performance
of those spun from chloroform.
ꢁc
This journal is the Royal Society of Chemistry the Centre National de la Recherche Scientifique 2006
340 | New J. Chem., 2006, 30, 335–342

View Article Online
factor limiting the intrinsic efficiency of devices based on dyad
1 compared to that of the corresponding fullerene–PPVs
blends, although a competition between energy transfer and
electron transfer cannot be ruled out. Furthermore, because of
the limited length of the bis(styryl)benzene structure, the
absorption spectrum of 1 (Fig. 2) does not cover the wave-
length range of the corresponding polymer, leading to a lower
photon collection and hence to a lower overall power conver-
sion efficiency. Further improvement could be expected by the
utilization of new fullerene derivatives with a wider absorption
spectrum in the visible range.
14 Fullerenes: Chemistry, Physics, and Technology, ed. K. M. Kadish
and R. S. Ruoff, Wiley, New York, 2000.
15 M. Maggini, G. Possamai, E. Menna, G. Scorrano, N. Camaioni,
G. Ridolfi, G. Casalbore-Miceli, L. Franco, M. Ruzzi and C.
Corvaja, Chem. Commun., 2002, 2028.
16 J.-F. Nierengarten, J. F. Eckert, J. F. Nicoud, L. Ouali, V.
Krasnikov and G. Hadziioannou, Chem. Commun., 1999, 617.
´
17 (a) J. L. Segura, N. Martın and D. M. Guldi, Chem. Soc. Rev.,
2005, 34, 31; (b) J. Roncali, Chem. Soc. Rev., 2005, 34, 483; (c) J.-F.
Nierengarten, Sol. Energy Mater. Sol. Cells, 2004, 83, 187.
18 (a) P. A. van Hal, J. Knol, B. M. W. Langveld-Voss, S. C. J.
Meskers, J. C. Hummelen and R. A. J. Janssen, J. Phys. Chem. A,
2000, 104, 5974; (b) J. L. Segura and N. Martın, Tetrahedron Lett.,
´
1999, 40, 3239; (c) J. J. Aperloo, C. Martineau, P. A. van Hal, J.
Roncali and R. A. J. Janssen, J. Phys. Chem. A, 2002, 106, 21.
19 (a) E. Peeters, P. A. van Hal, J. Knol, C. J. Brabec, N. S. Sariciftci,
J. C. Hummelen and R. A. J. Janssen, J. Phys. Chem. B, 2000, 104,
10174; (b) J.-F. Eckert, J.-F. Nicoud, J.-F. Nierengarten, S. G. Liu,
L. Echegoyen, F. Barigelletti, N. Armaroli, L. Ouali, V. Krasnikov
and G. Hadziioannou, J. Am. Chem. Soc., 2000, 122, 7467; (c) T.
Gu, D. Tsamouras, C. Melzer, V. Krasnikov, J.-P. Gisselbrecht,
M. Gross, G. Hadziioannou and J.-F. Nierengarten, Chem-
PhysChem, 2002, 3, 124; (d) N. Armaroli, G. Accorsi, J.-P.
Gisselbrecht, M. Gross, V. Krasnikov, D. Tsamouras, G. Had-
ziioannou, M. J. Gomez-Escalonilla, F. Langa and J.-F. Eckert, J.
Mater. Chem., 2002, 12, 2077; (e) J.-F. Nierengarten, N. Armaroli,
G. Accorsi, Y. Rio and J.-F. Eckert, Chem.–Eur. J., 2003, 9, 34; (f)
N. Armaroli, F. Barigelletti, P. Ceroni, J.-F. Eckert, J.-F. Nicoud
and J.-F. Nierengarten, Chem. Commun., 2000, 599; (g) J. L.
Acknowledgements
Funding from MIUR (GR/No. PRIN2004035502,
RBAU017S8R, RBNE01P4JF) and from ITM-CNR is grate-
fully acknowledged. We thank M. D’Este for his help in
carrying out the fluorescence experiments.
References
1 (a) G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger,
Science, 1995, 270, 1789; (b) N. S. Sariciftci, L. Smilowitz, A. J.
Heeger and F. Wudl, Science, 1992, 258, 1474.
2 (a) C. J. Brabec, N. S. Sariciftci and J. C. Hummelen, Adv. Funct.
Mater., 2001, 11, 15–26; (b) H. Hoppe and N. S. Sariciftci, J.
Mater. Res., 2004, 19, 1924; (c) H. Spanggaard and F. C. Krebs,
Sol. Energy Mater. Sol. Cells, 2004, 83, 125.
3 C. J. Brabec, Sol. Energy Mater. Sol. Cells, 2004, 83, 273.
4 (a) Organic Photovoltaics Concepts and Realization, ed. C. J.
Brabec, V. Dyakonov, J. Parisi, N. S. Sariciftci, Springer, Berlin,
2003, vol. 60; (b) K. M. Coakley and M. D. McGehee, Chem.
Mater., 2004, 16, 4533; (c) S. Sun, Z. Fan, Y. Wang and J.
Haliburton, J. Mater. Sci., 2005, 40, 1429.
Segura, R. Gomez, N. Martı
Guldi, Chem. Commun., 2001, 707; (h) D. M. Guldi, C. Luo, A.
Swartz, J. L. Segura, R. Gomez and N. Martın, J. Am. Chem. Soc.,
2002, 124, 10875.
´
n, C. Luo, A. Swartz and D. M.
´
20 M. A. Loi, P. Denk, H. Hoppe, H. Neugebauer, C. Winder, D.
Meissner, D. Brabec, N. S. Sariciftci, A. Gouloumis, P. Vazquez
and T. Torres, J. Mater. Chem., 2003, 13, 700.
21 D. M. Guldi, C. P. Luo, A. Swartz, R. Gomez, J. L. Segura, N.
Martı
1141.
22 (a) T. Gu and J.-F. Nierengarten, Tetrahedron Lett., 2001, 42,
3175; (b) A. Atienza, B. Insuasty, C. Seoane, N. Martın, J. Ramey
and D. M. Guldi, J. Mater. Chem., 2005, 15, 124.
´
n, C. Brabec and N. S. Sariciftci, J. Org. Chem., 2002, 67,
´
5 H. Hoppe, T. Glatzel, M. Niggermann, A. Hinsch, M. Ch. Lunx-
Steiner and N. S. Sariciftci, Nano Lett., 2005, 5, 269.
23 G. Possamai, S. Marcuz, M. Maggini, E. Menna, L. Franco, M.
Ruzzi, S. Ceola, C. Corvaja, G. Ridolfi, A. Geri, N. Camaioni, D.
M. Guldi, R. Sens and T. Gessner, Chem.–Eur. J., 2005, 11, 5765.
24 I. B. Martini, B. Ma, T. Da Ros, R. Helgeson, F. Wudl and B. J.
Schwartz, Chem. Phys. Lett., 2000, 327, 253.
6 S. E. Shaheen, C. J. Brabec, N. S. Sariciftci, F. Padinger, T.
Fromherz and J. C. Hummelen, Appl. Phys. Lett., 2001, 78, 841.
7 (a) C.-H. Huang, N. D. McClenaghan, A. Kuhn, J. W. Hofstraat
and D. M. Bassani, Org. Lett., 2005, 7, 3409; (b) G. Li, V.
Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery and Y. Yang,
Nat. Mater., 2005, 4, 864; (c) W. Ma, C. Yang, X. Gong, K. Lee
and A. J. Heeger, Adv. Funct. Mater., 2005, 15, 1617.
8 (a) Thin Solid Films, 2004, 451–452: special issue on thin solid films
and nano-structured materials for photovoltaics (E-MRS 2003
Spring Conference - Symposium D); (b) Sol. Energy Mater. Sol.
Cells, 2004, 83: special issue on the development of organic and
polymer photovoltaics; (c) Appl. Phys. A: Mater. Sci. Process.,
2004, 79: special issue on organic photovoltaics.
25 J. L. Segura, F. Giacalone, R. Gomez, N. Martın, D. M. Guldi, C.
´ ´
Luo, A. Swartz, I. Riedel, D. Chirvase, J. Parisi, V. Dyakonov, N.
S. Sariciftci and F. Padinger, Mater. Sci. Eng., C, 2005, 25, 835.
26 (a) D. M. Guldi, F. Giacalone, G. De la Torre, J. L. Segura and N.
Martı
´
J. Orduna, J. Garı
n, Chem.–Eur. J., 2005, 11, 7199; (b) E. Allard, J. Cousseau,
n, H. X. Luo, Y. Araki and O. Ito, Phys. Chem.
Chem. Phys., 2002, 4, 5944.
´
27 P. Van Hal, E. H. A. Beckers, S. C. J. Meskers, R. A. J. Janssen, B.
Jousselme, P. Blanchard and J. Roncali, Chem.–Eur. J., 2002, 8,
5415, and references cited therein.
28 D. J. Schuster, P. Cheng, P. D. Jarowski, D. M. Guldi, C. Luo, L.
Echegoyen, S. Pyo, A. R. Holzwarth, S. E. Braslavsky, R. M.
Williams and G. Klihm, J. Am. Chem. Soc., 2004, 126, 7257.
29 D. M. Guldi, C. Luo, M. Prato, A. Troisi, F. Zerbetto, M.
Scheloske, E. Dietel, W. Bauer and A. Hirsch, J. Am. Chem.
Soc., 2001, 123, 9166.
30 (a) L. I. Belen’kii, Y. B. Vol’kenshtein and I. B. Karmanova, Russ.
Chem. Rev., 1977, 46, 891; (b) S. Nakatsuji, K. Matsuda, Y.
Uesugi, K. Nakashima, S. Akiyama, G. Katzer and W. J. Fabian,
J. Chem. Soc., Perkin Trans. 2, 1991, 861.
9 (a) A. Cravino and N. S. Sariciftci, J. Mater. Chem., 2002, 12,
1931; (b) A. Cravino and N. S. Sariciftci, Nat. Mater., 2003, 2, 360.
10 X. N. Yang, J. K. J. van Duren, R. A. J. Janssen, M. A. J. Michels
and J. Loos, Macromolecules, 2004, 37, 2151.
11 (a) T. Benicori, E. Brenna, F. Sannicolo, L. Trimarco, G. Zotti and
´
P. Sozzoni, Angew. Chem., Int. Ed. Engl., 1996, 35, 648; (b) J. P.
Ferraris, A. Yassar, D. C. Loveday and M. Hmyene, Opt. Mater.
(Amsterdam), 1998, 9, 34; (c) A. Cravino, G. Zerza, M. Maggini,
S. Bucella, M. Svensson, M. R. Andersson, H. Neugebauer and N.
S. Sariciftci, Chem. Commun., 2000, 2487; (d) B. Jousselme, P.
Blanchard, E. Levillain, R. de Bettignies and J. Roncali, Macro-
molecules, 2003, 36, 3020.
31 H. Gunther, NMR Spectroscopy. An Introduction, J. Wiley and
¨
Sons, Chichester, 1980.
12 (a) A. Marcos Ramos, M. T. Rispens, J. K. J. van Duren, J. C.
Hummelen and R. A. J. Janssen, J. Am. Chem. Soc., 2001, 123,
6714; (b) F. L. Zhang, M. Svennson, M. R. Andersson, M.
Maggini, S. Bucella, E. Menna and O. Inganas, Adv. Mater.,
2001, 13, 1871.
32 (a) C. Bingel, Chem. Ber., 1993, 126, 1957; (b) J.-F. Nierengarten,
W. Gramlich, F. Cardullo and F. Diederich, Angew. Chem., Int.
Ed. Engl., 1996, 35, 2101.
33 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N.
Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi,
13 J. K. J. van Duren, X. N. Yang, J. Loos, C. W. T. Bulle-Lieuwma,
A. B. Sieval, J. C. Hummelen and R. A. J. Janssen, Adv. Funct.
Mater., 2004, 14, 425.
ꢁc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006
New J. Chem., 2006, 30, 335–342 | 341

View Article Online
V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R.
Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O.
Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J.
B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J.
Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J.
J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M.
C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghava-
chari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S.
Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P.
Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A.
Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M.
W. Gill, B. G. Johnson, W. Chen, M. W. Wong, C. Gonzalez and
J. A. Pople, GAUSSIAN 03 (Revision C.02), Gaussian, Inc.,
Wallingford, CT, 2004.
34 M. A. Herranz, F. Diederich and L. Echegoyen, Eur. J. Org.
Chem., 2004, 2299.
35 D. Dubois, G. Moninot, W. Kutner, M. T. Jones and K. M.
Kadish, J. Phys. Chem., 1992, 96, 7137.
36 L. Echegoyen and L. E. Echegoyen, Acc. Chem. Res., 1998, 31, 593.
37 J. Chistumoff, and A. J. Bard, in Organic Conductive Molecules and
Polymers, ed. H. S. Nalwa, John Wiley & Sons, Chichester, 1997,
vol. 1, ch. 7, p. 379.
ꢁc
This journal is the Royal Society of Chemistry the Centre National de la Recherche Scientifique 2006
342 | New J. Chem., 2006, 30, 335–342