Synthesis and electronic properties of fullerene derivatives substituted with oligophenylenevinylen-ferrocene conjugates

Article abstract of DOI:10.1039/b711030h

C₆₀-bridge-Fc arrays (C₆₀-Fc, C₆₀-PV-Fc, C₆₀-2PV-Fc, C₆₀-3PV-Fc), bearing a fulleropyrrolidine and a ferrocene (Fc) unit connected via an oligophenylenevinylene (OPV) bridge have been prepared. Suitable dyads (PV-Fc, 2PV-Fc, 3PV-Fc) to be used as references for the study of the electronic properties of the largest arrays have been also synthesized. The electrochemical properties of all the dyad and triad multicomponent arrays have been studied by cyclic voltammetry, evidencing a rich and complex electrochemical pattern due to the presence of several electroactive moieties. The first reduction is always assigned to the fullerene moiety and the first oxidation is centred on the Fc group, making the triad systems suitable candidates for photoinduced electron transfer via the interposed bridge. Photophysical studies evidence a complete quenching of the fluorescence of organic conjugated moieties in PV-Fc, 2PV-Fc, 3PV-Fc, possibly via energy transfer to the Fc unit. In the more complex C₆₀/Fc arrays the quenching of the C₆₀ moieties is ultrafast in CH ₂Cl₂ solution and most likely attributable to electron transfer via the OPV wire. In toluene, the dynamic process of singlet and triplet fullerene quenching can be traced via time resolved fluorescence and transient absorption spectroscopy and the values of the rate constants are smaller with increasing donor-acceptor distance. Definitive assignment of the intercomponent quenching mechanism between the fullerene and the ferrocene moiety (energy or electron transfer) can hardly be obtained. Several repeated attempts to detect the radical anion fingerprint of fulleropyrrolidine with transient absorption failed, even in bimolecular quenching experiments. This supports the view that distance-dependent C₆₀ → Fc singlet-triplet and triplet-triplet energy transfer may compete successfully with the desired charge separation step. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b711030h

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Synthesis and electronic properties of fullerene derivatives substituted
with oligophenylenevinylen–ferrocene conjugates
a
b
b
a
´
Teresa M. Figueira-Duarte, Yannick Rio, Andrea Listorti, Beatrice Delavaux-Nicot,
Michel Holler,c Filippo Marchioni,d Paola Ceroni,*d Nicola Armaroli*b and
Jean-Franc¸
ois Nierengarten*a
Received (in Montpellier, France) 18th July 2007, Accepted 7th August 2007
First published as an Advance Article on the web 22nd August 2007
DOI: 10.1039/b711030h
C60-bridge-Fc arrays (C60-Fc, C60-PV-Fc, C60-2PV-Fc, C60-3PV-Fc), bearing a fulleropyrrolidine
and a ferrocene (Fc) unit connected via an oligophenylenevinylene (OPV) bridge have been
prepared. Suitable dyads (PV-Fc, 2PV-Fc, 3PV-Fc) to be used as references for the study of the
electronic properties of the largest arrays have been also synthesized. The electrochemical
properties of all the dyad and triad multicomponent arrays have been studied by cyclic
voltammetry, evidencing a rich and complex electrochemical pattern due to the presence of
several electroactive moieties. The first reduction is always assigned to the fullerene moiety and
the first oxidation is centred on the Fc group, making the triad systems suitable candidates for
photoinduced electron transfer via the interposed bridge. Photophysical studies evidence a
complete quenching of the fluorescence of organic conjugated moieties in PV-Fc, 2PV-Fc, 3PV-
Fc, possibly via energy transfer to the Fc unit. In the more complex C60/Fc arrays the quenching
of the C60 moieties is ultrafast in CH2Cl2 solution and most likely attributable to electron transfer
via the OPV wire. In toluene, the dynamic process of singlet and triplet fullerene quenching can
be traced via time resolved fluorescence and transient absorption spectroscopy and the values of
the rate constants are smaller with increasing donor–acceptor distance. Definitive assignment of
the intercomponent quenching mechanism between the fullerene and the ferrocene moiety (energy
or electron transfer) can hardly be obtained. Several repeated attempts to detect the radical anion
fingerprint of fulleropyrrolidine with transient absorption failed, even in bimolecular quenching
experiments. This supports the view that distance-dependent C60 - Fc singlet–triplet and
triplet–triplet energy transfer may compete successfully with the desired charge separation step.
layer in organic photovoltaic cells.5 More elaborated systems
in which an additional donor subunit has been connected to
the C60–(p-conjugated oligomer) system have also been de-
scribed.6–8 For example, a series of structurally well-defined
arrays that incorporate a p-extended tetrathiafulvalene (ex-
TTF) as electron donor and C60 as electron acceptor, linked by
oligophenylenevinylene (OPV) moieties of different lengths
has been prepared.6 The electrochemical studies reveal a lack
of significant electronic communication between the donor
(ex-TTF) and the acceptor (C60) moieties through the
p-conjugated oligomer. However, photoinduced electron
Introduction
In recent years, the use of C60 as an electron and/or energy
acceptor in photochemical molecular devices has been inten-
sively investigated.1 As part of this research, the combination
of the carbon sphere with p-conjugated oligomers for the
construction of donor–fullerene arrays is of particular inter-
est.2 Such hybrid systems have effectively shown excited state
interactions making them excellent candidates for fundamen-
tal photophysical studies.3 In addition, covalently linked full-
erene–(p-conjugated oligomer) derivatives offer interesting
perspectives for optical limiting or photodynamic therapy
applications.4 Furthermore, they can also be used as the active
transfer leading to the radical pair ex-TTF+-OPV-C60 has
ꢀ
been evidenced in all these compounds and the charge-recom-
bination dynamics indicates a very low influence of the
oligomer length on the electron transfer rate. These findings
clearly reveal the nanowire behaviour of the OPV units.9
A variety of C60-bridge-Fc triads has been also prepared
and characterized photochemically. Several bridge units have
been exploited such as olefins,10 ethers,11 oligothiophenes,12
OPV dendrons,13 and even mixed bridges.12,14 An extensively
investigated class of photoactive arrays with C60 and Fc is also
that involving a photoactive porphyrin moiety as central
bridge. In these systems the central porphyrin chromophore
acts as primary light absorbing unit and undergoes electron
a
`
`
´
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. E-mail:
jfnierengarten@lcc-toulouse.fr
b Molecular Photoscience Group, Istituto per la Sintesi Organica e la
`
Fotoreattivita, Consiglio Nazionale delle Ricerche, via Gobetti 101,
40129 Bologna, Italy. E-mail: nicola.armaroli@isof.cnr.it
´
Laboratoire de Physico-Chimie Bioinorganique, Universite Louis
Pasteur et CNRS (UMR 7509), Ecole Europeenne de Chimie,
`
Polymeres et Materiaux (ECPM), 25 rue Becquerel, 67087
Strasbourg Cedex 2, France. E-mail: mholler@chimie.u-strasbg.fr
`
Dipartimento di Chimica ‘‘G. Ciamician’’, Universita di Bologna, via
Selmi 2, I-40126 Bologna, Italy. E-mail: paola.ceroni@unibo.it
c
´
´
d
ꢁc
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transfer to the C60 fragment.15 Eventually charge shift from
the Fc to the porphyrin moiety leads to long-lived charge
separated species, but typically with low yield due to a
competitive energy transfer deactivation to the low lying Fc
triplet.16
in five steps from p-bromobenzaldehyde as previously re-
ported.17 Reaction of 1 with ferrocenecarboxaldehyde under
Wadsworth–Emmons conditions gave PV-Fc in 96% yield.
Subsequent treatment with CF3CO2H afforded 2. It is worth
nothing that this deprotection step must be carried out under
oxygen free conditions to prevent decomposition, resulting
most probably from the easy oxidation of the Fc unit under
acidic conditions. Reaction of 2 with phosphonate 1 in the
presence of t-BuOK afforded compound 2PV-Fc with 99%
yield. Treatment with CF3CO2H in CH2Cl2/H2O under oxy-
gen free conditions then gave 3. The synthetic approach to
prepare compounds C60-PV-Fc and C60-2PV-Fc relies upon
the 1,3-dipolar cycloaddition of the azomethine ylides gener-
ated in situ from the corresponding aldehydes and N-benzyl-
glycine derivative 4.18 Reaction of aldehydes 2 and 3 with 4
and C60 in o-dichlorobenzene (ODCB) at 100 1C gave the
corresponding pyrrolidinofullerene derivatives C60-PV-Fc and
C60-2PV-Fc in 40 to 33% isolated yield, respectively, after
column chromatography on silica gel. Compound C60-Fc was
prepared under similar conditions from C60, 4 and ferrocene-
carboxaldehyde. Owing to the presence of the 3,5-didodecyl-
oxybenzyl subunit, the pyrrolidinofullerenes C60-Fc, C60-PV-
Fc and C60-2PV-Fc are well soluble in common organic
solvents such as CH2Cl2, CHCl3, or THF and complete
spectroscopic characterization was easily achieved. Their
structure was further confirmed by mass spectrometry. In
all the cases, the expected molecular ion peak was clearly
observed.
In this paper we report the preparation and the electronic
properties of four novel C60-bridge-Fc arrays (C60-Fc, C60-PV-
Fc, C60-2PV-Fc, C60-3PV-Fc), bearing a fulleropyrrolidine
and a ferrocene (Fc) unit connected via a oligophenyleneviny-
lene bridge, in an attempt to demonstrate the occurrence of a
Fc - fullerene photoinduced electron transfer.8 Indeed the
terminal Fc unit promotes ultrafast quenching of the fullerene
moiety in CH2Cl2 for all of the multicomponent arrays, likely
attributable to efficient photoinduced electron transfer via the
OPV wire. By contrast, in less polar toluene, fullerene singlet
and triplet quenching can be resolved and the rate constants
show a substantial distance dependence. Combining the pre-
sent experimental findings on C60-Fc, C60-PV-Fc, C60-2PV-Fc,
C60-3PV-Fc and some suitably designed reference systems with
recent literature data, electron and energy transfer quenching
pathways between C60 and Fc are discussed.
Results and discussion
Synthesis
The preparation of compounds C60-Fc, C60-PV-Fc and
C60-2PV-Fc is shown in Scheme 1. Compound 1 was obtained
The length of the OPV-Fc system was further increased
(Scheme 2). In this case, phosphonate 519 with two alkyl chain
was used in order to prevent any solubility problems. Reaction
of aldehyde 3 with phosphonate 5 in the presence of t-BuOK
in THF afforded 3PV-Fc in 50% yield. Subsequent treatment
with CF3CO2H in CH2Cl2/H2O gave aldehyde 6 in a moderate
yield (37%) which after reaction with C60 and 4 in ODCB at
100 1C gave C60-3PV-Fc in 26% yield.
Scheme 1 Reagents and conditions: (a) ferrocenecarboxaldehyde,
t-BuOK, THF, 0 1C to rt, 2 h, 96%; (b) CF3CO2H, H2O, CH2Cl2,
rt, 5 h, 80%; (c) 1, t-BuOK, THF, 0 1C to rt, 2 h, 99%; (d) CF3CO2H,
H2O, CH2Cl2, rt, 5 h, 70%; (e) ferrocenecarboxaldehyde, C60, ODCB,
D, 12 h, 32%; (f) 4, C60, ODCB, 100 1C, 20 h, 40%; (g) 4, C60, ODCB,
100 1C, 20 h, 33%.
Scheme 2 Reagents and conditions: (a) 3, t-BuOK, THF, 0 1C to rt,
2 h, 50%; (b) CF3CO2H, H2O, CH2Cl2, rt, 5 h, 37%; (c) 4, C60
ODCB, 100 1C, 20 h, 26%.
,
ꢁc
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metry in THF/TBAPF6 solutions. The results are presented in
Table 1.
A very rich and complex electrochemical pattern has been
evidenced for these compounds. Indeed, multiple redox sites
are present, namely C60, OPV, Fc and dialkyloxybenzene. The
comparison with proper model compounds allows the assign-
ment of the electron transfer processes to the different redox
sites, as depicted in Fig. 3. In all the fullerene derivatives, we
can observe five reversible one-electron reduction processes
typical of the pyrrolidinofullerene core (solid squares in
Fig. 3). In the case of C60-3PV-Fc the last process is bielec-
tronic and one of the two reduction is assigned to the fullerene
moiety. The presence of the electron withdrawing dialkyl-
oxybenzene group on the nitrogen atom of the pyrrolidino-
fullerene moiety causes a very small positive shift of the E1/2
values, as evidenced by the comparison of N-methylpyrrolidi-
nofullerene (FP) with 7. In the case of all the compounds
containing C60 and Fc, the first E1/2 value are very similar
(Table 1 and Fig. 3).
Fig. 1 Compounds C60-Fc, C60-PV-Fc, C60-2PV-Fc and C60-3PV-Fc.
The characterization of compound C60-3PV-Fc was found
more difficult when compared to that of the shorter analogues
C60-PV-Fc and C60-2PV-Fc. Indeed, two diastereoisomers (A
and B) can exist in principle for compound C60-3PV-Fc (Fig. 1
and 2). However, the 1H and 13C NMR spectra (CDCl3, room
temperature) reveal the existence of a single C1 symmetric
compound. In addition, 1H-NMR spectra recorded at tem-
peratures from ꢀ50 to 120 1C are all similar and reveal no
dynamic exchange between A and B. Therefore only one of the
two possible conformers exists for C60-3PV-Fc. As already
observed for related ortho-substituted-phenyl pyrrolidinoful-
lerenes, 2D-NOESY experiments revealed that C60-3PV-Fc
adopts a conformation in which the unsubstituted ortho posi-
tion is located atop the C60 (conformer A).18 Therefore, the
reaction of 6 with C60 is diastereoselective, affording only one
isomer.
In the cathodic region, besides the fullerene-based reduc-
tions, reversible one-electron OPV centred reductions are also
observed for C60-PV-Fc, C60-2PV-Fc, and C60-3PV-Fc (open
triangles in Fig. 3). In particular, peak V of C60-PV-Fc, peaks
V and VI of C60-2PV-Fc have been assigned to the OPV
moieties, on the basis of the close similarity to the reduction
potentials of the corresponding model compounds. In parti-
cular, in both these fullerene derivatives a negative shift of the
OPV reduction potentials has been observed, compared to the
model OPV compounds (PV-Fc and 2PV-Fc, respectively).
This behaviour is expected on the basis of the electrostatic
repulsion with the negative charges on the fullerene cage. In
the case of C60-3PV-Fc (a representative cyclic voltammetric
curve is shown in Fig. 4), the assignment of the redox processes
is less straightforward: we tentatively assign peaks IV, VI and
VII (bielectronic process visible only at 218 K) to the OPV
reductions on the basis of the good correlation of the other
peaks with those of FP. It is worth noticing that the second
OPV reduction of 3PV-Fc is not chemically reversible, con-
trary to the corresponding peak VI of C60-3PV-Fc. This result
may explain the unexpected positive shift observed only for the
second OPV reduction in the triad compared to the model
3PV-Fc. Upon increasing the length of the OPV chain (C60-
PV-Fc o C60-2PV-Fc o C60-3PV-Fc), the first reduction
process of the OPV moiety occurs at less negative potentials,
as expected by the extension of the p orbital conjugated
system.20 For compound C60-3PV-Fc, the observed positive
Electrochemical properties
The electrochemical properties of C60-Fc, C60-PV-Fc,
C60-2PV-Fc, C60-3PV-Fc, PV-Fc, 2PV-Fc, 3PV-Fc, and
fullerene model compound 7 were studied by cyclic voltam-
Fig. 2 Schematic representation of the two possible atropisomers of
compound C60-3PV-Fc.
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Table 1 Half-wave potentials (E1/2 in V, vs. SCE) in THF/TBAPF6 solutions at 298 K and, in parenthesis, at 218 K
A
I
II
III
IV
V
VI
VII
Fc
FP
+0.46
ꢀ0.47
ꢀ1.04
ꢀ1.68
ꢀ2.15
ꢀ2.96
(ꢀ0.44)
ꢀ0.44
(ꢀ0.99)
ꢀ1.00
(ꢀ1.60)
ꢀ1.61
(ꢀ2.10)
ꢀ2.07
(ꢀ2.83)
ꢀ2.76
7
(ꢀ0.41)
ꢀ0.45
(ꢀ0.96)
ꢀ1.02
(ꢀ1.60)
ꢀ1.63
(ꢀ2.07)
ꢀ2.09
(ꢀ2.83)
ꢀ2.8a
C60-Fc
C60-PV-Fc
PV-Fc
+0.67
(+0.60)
+0.58
(+0.52)
+0.58
(+0.51)
+0.57
(+0.51)
+0.57
(+0.51)
+0.56
(+0.51)
+0.57
(+0.51)
(ꢀ0.44)
ꢀ0.47
(ꢀ1.00)
ꢀ1.03
(ꢀ1.63)
ꢀ1.64
(ꢀ2.14)
ꢀ2.10
(ꢀ2.92)
ꢀ2.45
ꢀ2.8a
(ꢀ0.42)
ꢀ2.34
(ꢀ0.97)
(ꢀ1.60)
(ꢀ2.07)
(ꢀ2.44)
(ꢀ2.87)
(ꢀ2.36)
ꢀ0.45
C60-2PV-Fc
2PV-Fc
C60-3PV-Fc
3PV-Fc
a
ꢀ1.02
(ꢀ0.98)
—
(ꢀ2.29)
ꢀ1.06
(ꢀ0.99)
ꢀ2.39
(ꢀ2.39)
ꢀ1.62
ꢀ2.01
ꢀ2.09
ꢀ2.31
—
(ꢀ2.91)
(ꢀ0.43)
ꢀ1.99
(ꢀ1.62)
(ꢀ2.01)
(ꢀ2.15)
(ꢀ2.41)
(ꢀ2.03)
ꢀ0.48
ꢀ1.67
(ꢀ1.65)
ꢀ2.79b
(ꢀ2.95)b
ꢀ1.86
ꢀ2.04
ꢀ2.16
—
(ꢀ2.98)bc
(ꢀ0.42)
ꢀ1.75
(ꢀ1.81)
(ꢀ1.98)
(ꢀ2.17)
(ꢀ1.73)
Electrochemical process superimposed to solvent discharge. Chemically irreversible process; Epa value at 0.2 V sꢀ1
.
Two-electron transfer
b
c
process.
shift of the first and second OPV reductions compared to C60-
2PV-Fc is not only due to the extension of the p system, but
also to the presence of two electron-withdrawing –OC8H17
substituents on one end of the OPV moiety.
acceptor pyrrolidinofullerene core and a positive shift of the
Fc oxidation is observed (DE = 100 mV), together with a
decrease of the heterogeneous electron transfer rate. Simula-
tion of the voltammetric curves in the range of scan rates
In the anodic region, only the one-electron oxidation of the
Fc moiety can be observed, since the dialkyloxybenzene one is
outside the available potential window. In a medium offering a
wider anodic potential region such as CH2Cl2/TBAPF6, a
between 0.1 and 2 V sꢀ1 yielded the following values: ksh
=
0.007 cmꢀ1 sꢀ1 and a = 0.7 much smaller than that reported
for pristine ferrocene.22 On the contrary, no significant per-
turbation of the fullerene-centred reductions has been ob-
served. This electrochemical behaviour would suggest that
the HOMO orbital is localized not only on the Fc, but also
partially on the fullerene core, conversely the LUMO is mainly
localized on the fullerene.
chemically irreversible electron transfer process with Epa
B
+1.5 V (vs. SCE) at 0.2 V sꢀ1 has been attributed to the
dialkyloxybenzene oxidation.21 The constancy of the E1/2
values corresponding to the ferrocene oxidation in compounds
C60-PV-Fc, C60-2PV-Fc, and C60-3PV-Fc compared to
the corresponding model compounds (PV-Fc, 2PV-Fc, and
3PV-Fc, respectively) and to pristine Fc demonstrates that the
ferrocene oxidation process is not affected by the presence of
the other redox sites. On the other hand, in the case of C60-Fc,
the ferrocene moiety is directly connected to the electron-
In summary, for all the investigated C60-OPV-Fc com-
pounds the first reduction is due to the fullerene moiety and
the first oxidation is centred on the Fc group.
Photophysical properties
The absorption spectrum of the C60 model-compound 7 is
virtually identical to similar pyrrolidinofullerene derivatives
Fig. 3 Diagram showing the half-wave potential values of the
Fig. 4 Cyclic voltammogram of compound C60-3PV-Fc in THF/
TBAPF6 at scan rate of 0.2 V sꢀ1 and T = 298 K.
investigated compounds in THF/TBAPF6 at 218 K.
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Fig. 5 Absorption spectra (CH2Cl2) of C60-2PV-Fc (light gray), 7
(black), 2PV-Fc (gray) and the spectral trace obtained by summing the
two model compounds 7 and 2PV-Fc (black dashed).
Fig. 6 Absorption spectra (CH2Cl2) of C60-Fc (black), C60-PV-Fc
(gray), C60-2PV-Fc (light gray), and C60-3PV-Fc (black dashed).
except C60-3PV-Fc, where it is masked by the band of the
OPV fragment.
reported in the literature with the two characteristic peaks at
255 nm and 430 nm.23 Pristine Fc in CH2Cl2 solution exhibits
a weak absorption band centred at 443 nm (e = 100 Mꢀ1
cmꢀ1), also in line with literature data.24
The absorption spectra of the fullerene hybrid systems were
compared to the spectra obtained by summing the absorption
spectra of the C60-model compound 7 with those of the
corresponding Fc-OPV model systems (for C60-2PV-Fc, see
Fig. 5). Excellent match between experimental and arithme-
tical traces is found for C60-3PV-Fc. With shorter spacers the
matching is progressively lost and the largest difference is
found for C60-Fc in the 400–500 nm region. This trend
suggests the presence of interactions between the C60 and the
Fc moiety, possibly mediated by the short OPV molecular
wire,9,29 progressively weaker with longer distance. This ex-
planation is supported by the electrochemical data (Table 1)
which show a substantial interaction between the two moieties
in C60-Fc.
The ferrocene reference compounds with the spacer
attached (PV-Fc, 2PV-Fc, 3PV-Fc) show the characteristic
absorption band of the phenylenevinylene fragment, progres-
sively red-shifted with the increasing conjugation: PV-Fc
(lmax = 312 nm, e = 23 600 Mꢀ1 cmꢀ1), 2PV-Fc (lmax
=
=
356 nm, e = 53 200 Mꢀ1 cmꢀ1, Fig. 5), 3PV-Fc (lmax
398 nm, e = 71 000 Mꢀ1 cmꢀ1). The potentially strong
fluorescence of the organic conjugated moiety in 2PV-Fc and
3PV-Fc25 is completely quenched by the ferrocene unit.
In principle, the spectral overlap between the fluorescence
profiles of OPV (peaked around 380–400 nm)25 and the weak
lowest absorption band of the Fc acceptor attributed to d–d
spin allowed transitions (ca. 2.7 eV, i.e. 460 nm)24 makes it
Whatever the C60-OPV-Fc array, it is possible to excite
selectively the C60 moiety at l 4 530 nm. In contrast, selective
excitation of the weakly absorbing24 ferrocene moiety is
always precluded. The occurrence of photoinduced processes
in the triads has been probed via excitation of the fulleropyr-
rolidine moiety at 610 nm in dichloromethane and toluene
solution. The fullerene fluorescence spectra of the C60-OPV-Fc
systems compared to the C60 model-compound 7 in CH2Cl2
are displayed in Fig. 7 (top panel).
possible a singlet energy transfer via Forster mechanism.
¨
Eventually, the lowest triplet electronic state of Fc, which
was recently located as low as 1.16 eV26 (to be compared to ca.
1.75 eV of previous assignments),27 might act as final energy
sink. Unfortunately excited electronic levels of ferrocenes are
known to be rather elusive from the spectroscopic point of
view and their energy content has been the object of debate for
decades in the photochemical community.24,26,27 These ultra-
short-lived levels do not display significant absorption or
emission signatures and only indirect proof for their genera-
tion can be obtained. Quenching by electron transfer following
excitation of the OPV moieties above 3.0 eV cannot be
discarded for 2PV-Fc and 3PV-Fc which have, respectively,
a thermodynamic driving force for charge separation of about
2.6 and 2.3 eV, as obtainable28 by the one-electron oxidation
and reduction potentials in Table 1.
The fluorescence quantum yield and singlet lifetimes of 7 are
typical for pyrrolidinofullerene derivatives in CH2Cl2 (f =
3.0 ꢂ 10ꢀ4, t = 1.3 ns). The fullerene emission is completely
suppressed for C60-Fc, C60-PV-Fc and reduced by 90% for
C60-2PV-Fc, and C60-3PV-Fc. The lifetime of the quenched
singlet state (o300 ps) is below our experimental resolution in
all cases. However, from the steady state luminescence (Fig. 7)
and according to eqn (1), a rate constant for the quenching
process can be estimated for the two longest arrays C60-2PV-
Fc and C60-3PV-Fc, k = 7 ꢂ 109 sꢀ1
The UV-visible absorption spectra of the four fullerene
multicomponent arrays C60-Fc, C60-PV-Fc, C60-2PV-Fc,
and C60-3PV-Fc in CH2Cl2 are depicted in Fig. 6. In the
UV region there is overlap between fullerene and OPV
absorption features, whereas in the Vis spectral window
(l 4 450 nm) only the fullerene moiety absorbs, with a
minor contribution of Fc until 530 nm. The characteristic
fulleropyrrolidine peak at 430 nm is recorded in all cases
ꢀ
ꢁ
Fref
F
ꢀ 1
kEnT
¼
ð1Þ
tref
In eqn (1), F and Fref are the fulleropyrrolidine fluorescence
quantum yields of the multicomponent arrays and the
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but still well detectable fullerene fluorescence is recorded for
C60-2PV-Fc and C60-3PV-Fc and the corresponding lifetimes
are progressively shorter, indicating a dynamic quenching of
the fullerene singlet level in toluene: 7 (1.6 ns), C60-3PV-Fc
(1.1 ns), C60-2PV-Fc (0.6 ns), C60-PV-Fc and C60-Fc
(o0.3 ns).
The observed fullerene singlet quenching process in the two
investigated solvents might be due to photoinduced electron
transfer leading to Fc+-OPV-C60ꢀ, in analogy with previously
reported fullerene–ferrocene systems with aliphatic,10 ether,11
or oligothiophene12 linkers. Such charge separated states,
according to electrochemical data, are placed at 1.12
(C60-Fc), 1.05 (C60-PV-Fc), 1.02 (C60-2PV-Fc) and 1.04 eV
(C60-3PV-Fc). Repeated attempts to evidence the formation of
the charge separated state in both CH2Cl2 and toluene
through the C60 radical anion fingerprint in the NIR spectral
region (900–1600), down to a time resolution of 10 ns, turned
out to be unsuccessful, suggesting an ultrafast charge recom-
bination process favoured by the good electronic coupling
offered by the OPV wire.9,29 On a shorter time scale (35 ps) the
available spectral window of our apparatus is 400–900 nm and
the C60 anion cannot be traced. No evidence of charge
separated products was previously obtained with ferroce-
ne–fullerene donor–acceptor partners connected via olefinic
chains.10 On the contrary, relatively longer lived charge sepa-
rated states were detected with ether (hundreds of ns)11 and
oligothiophene (up to 50 ns)12 spacers.
We wish to emphasize that the lack of experimental evidence
for electron transfer from the fullerene singlet might also be
due to an energy transfer quenching process. Fullerene -
ferrocene singlet (Forster) energy transfer is ruled out since
¨
endoergonic, whereas the singlet to triplet process via Dexter
mechanism might occur (for the triplet energy of Fc, see
above), as recently proposed in porphyrin–Fc systems where
porphyrin fluorescence quenching is observed.16
Fig. 7 Fullerene fluorescence spectra of 7 (black), C60-Fc (gray
dashed), C60-PV-Fc (black dashed), C60-2PV-Fc (light gray), and
C
60-3PV-Fc (gray) in CH2Cl2 (top) and toluene (bottom) with
In an attempt to evidence electron transfer quenching, we
also investigated the fate of the residual fullerene triplet of
C60-2PV-Fc and C60-3PV-Fc in toluene solution. In this
solvent, for some C60-bridge-Fc systems, relatively long-lived
CS states have been detected, because the charge recombina-
tion process is located in a Marcus-inverted region regime with
longer-lived CS states compared to polar solvents.14 Indeed,
for C60-2PV-Fc and C60-3PV-Fc, the residual fullerene triplet
state evidenced in toluene air equilibrated solution via singlet-
oxygen sensitization (Fig. 7, bottom panel, left inset), is
substantially quenched in oxygen free solutions, where triplet
lifetimes of 0.23 and 0.62 ms are found, to be compared to
15.7 ms for the reference system 7 (Fig. 8). Again, no evidence
for electron transfer was found. The same result was obtained
through bimolecular quenching studies between 7 and 2PV-Fc,
providing again no evidence for electron transfer products.31
These results strongly suggest that quenching of the full-
erene triplet by the ferrocene moiety occurs via triplet energy
transfer with a rate constant of 4.3 ꢂ 106 and 1.5 ꢂ 106 msꢀ1
for, respectively, C60-2PV-Fc and C60-3PV-Fc. Notably, the
poor (if any) photoinduced electron transfer efficiency between
ferrocene derivatives and the triplet of C60 has been high-
lighted recently by Araki et al. even in polar benzonitrile,26
and our results are in line with these findings.
lexc = 610 nm. Inset (top and bottom left): sensitized singlet oxygen
luminescence at lexc = 610 nm. Inset (bottom right): fluorescence
decay at lexc = 635 nm.
reference compound 7, respectively, and tref is the singlet
lifetime of the latter.
Photoexcitation of the four multicomponent systems in
CH2Cl2 does not generate fullerene triplet, as inferred by the
absence of the characteristic sensitized singlet oxygen lumines-
cence peaked at 1268 nm,30 which is observed for the reference
molecule 7. This indicates that the reactive excited state species
is the fullerene singlet, in line with several examples of
ferrocene–fullerene arrays reported to date.11,14
Spectral data in CH2Cl2 suggest that there is some (small)
difference in the extent of the fulleropyrrolidine quenching
between the two longest (90% yield) and the two shortest
(100% yield) arrays. This prompted us to make the same
experiment in toluene, in order to test if a less polar environ-
ment could better resolve the behaviour of the four hybrids.
Fulleropyrrolidine fluorescence spectra in toluene are reported
in Fig. 7, bottom panel. As observed in CH2Cl2, fullerene
fluorescence is almost completely quenched for the shortest
systems C60-Fc and C60-PV-Fc. On the contrary, a quenched,
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combination mediated by the efficient OPV molecular wire9
and in line with analogous triads with olefine short bridges.10
In toluene, instead, fullerene singlet and triplet quenching
process are slower and can be traced via time resolved lumi-
nescence or transient absorption spectroscopy. Both singlet
and triplet quenching show distance dependence along the
series, differently from previous reports which demonstrate an
excellent wire-like behaviour of OPV bridges with distance
independent electron transfer rates up to 28 A.9 This suggests
that fullerene - ferrocene singlet–triplet and triplet–triplet
energy transfer might be the operative quenching mechanism
in apolar toluene solvent, although direct spectroscopic evi-
dence for these processes is not obtainable. However, repeated
attempts to detect transient electron transfer products (even
via bimolecular quenching, which turned out to be successful
with a variety of donors) failed, and no traces of the full-
eropyrrolidine radical anion have been found. The present
results along with the recent findings of Araki
et al.,26 suggest some caution in choosing Fc units as electron
donors to promote efficient charge separation in multicompo-
nent arrays, also due to its optical elusiveness as a triplet or a
radical cation.32 Notably, very recent results have also evi-
denced the tendency of Fc to undergo energy transfer, thus
leading to a substantial abatement of the long-distance charge
separation in a Fc-porphyrin-C60 array.16
Fig. 8 Transient absorption spectra of 7 (black), C60-2PV-Fc (light
gray), and C60-3PV-Fc (gray) in oxygen free toluene solution imme-
diately after the laser pulse (full line) and after 1.5 ms (dashed line).
Inset: absorbance decays at 710 nm of oxygen free (top) and
air-equilibrated samples. lexc = 532 nm.
Table 2 Quenching rate constants of the singlet and triplet states of
the fullerene moieties in toluene solution (oxygen-free for triplet
measurements) calculated as kq = 1/t ꢀ 1/t0. t is the quenched
lifetime and t0 is the singlet and triplet lifetimes of the reference
fullerene molecule 7 (1.6 ns and 15.7 ms, respectively)
kqa, singlet/sꢀ1
kqb, triplet/sꢀ1
Experimental
C60-Fc
46 ꢂ 1010
46 ꢂ 1010
1.0 ꢂ 109
2.8 ꢂ 108
—
—
C60-PV-Fc
C60-2PV-Fc
C60-3PV-Fc
General methods
4.3 ꢂ 106
1.5 ꢂ 106
Reagents and solvents were purchased as reagent grade and
used without further purification. THF was distilled over
sodium benzophenone ketyl. Compounds 1,17 4,18 and 519
were prepared according to previously reported procedures.
All reactions were performed in standard glassware under an
inert Ar atmosphere. Evaporation and concentration were
done at water aspirator pressure and drying in vacuo at 10ꢀ2
Torr. Column chromatography: silica gel 60 (230–400 mesh,
0.040–0.063 mm) was purchased from E. Merck. Thin layer
chromatography (TLC) was performed on glass sheets coated
with silica gel 60 F254 purchased from E. Merck, visualization
by UV light. NMR spectra were recorded on a Bruker AC 200
(200 MHz) or a Bruker AM 400 (400 MHz) with solvent peaks
as reference. FAB-mass spectra (MS) were obtained on a ZA
HF instrument with 4-nitrobenzyl alcohol as matrix. Elemen-
tal analyses were performed by the analytical service at the
Institut Charles Sadron, Strasbourg.
a
Determined via fluorescence intensity quenching or lifetimes.
b
Determined via transient absorption measurements.
It is worth noticing that, in toluene solution, there is a 3.5-
fold and a 3-fold decrease in the rate of the quenching of the
fullerene singlet and triplet states, respectively, between C60-
2PV-Fc and C60-3PV-Fc (Table 2). It has been demonstrated
that electron transfer through OPV bridges identical to those
here reported, has no D–A distance dependence up to four
phenylene bridges,9 thus a different mechanism (i.e. energy
transfer) could be responsible for the photoinduced process in
toluene. On the contrary, the much weaker distance effects in
more polar CH2Cl2 might be an indication of an electron
transfer process through the OPV wire.
Conclusion
Compound PV-Fc
Four novel multicomponent architectures C60-Fc, C60-PV-Fc,
C60-2PV-Fc, C60-3PV-Fc along with suitable reference com-
pounds for spectroscopic and electrochemical characterization
have been prepared. The target systems are made of
fulleropyrrolidine (C60) and ferrocene (Fc) units directly con-
nected (C60-Fc) or separated through an oligophenyleneviny-
lene (OPV) bridge of increasing length. Photophysical studies
show a different behaviour in CH2Cl2 and toluene. In the
former virtually complete quenching of the fullerene singlet
excited states in all the multicomponent arrays is observed,
tentatively attributed to ultrafast charge separation and re-
t-BuOK (0.33 g, 2.9 mmol) was added to a degassed solution
of ferrocenecarboxaldehyde (0.57 g, 2.6 mmol) and 1 (1.0 g,
2.9 mmol) in dry THF (50 mL) at 0 1C under argon atmo-
sphere. The mixture was stirred for 2 h and filtered over celite.
The filtrate was evaporated to dryness and was taken up in
CH2Cl2. The organic layer was washed with H2O, dried over
MgSO4, filtered and evaporated to dryness. Column chroma-
tography (SiO2, hexane/CH2Cl2 3 : 2) yielded PV-Fc (1.02 g,
96%) as an orange powder (mp 204 1C). 1H NMR (300 MHz,
CDCl3): 7.45 (m, 4 H), 6.79 (AB, J = 17 Hz, 2 H), 5.39
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(s, 1 H), 4.46 (t, J = 2 Hz, 2 H), 4.28 (t, J = 2 Hz, 2 H), 4.13
(s, 5 H), 3.72 (AB, J = 11 Hz, 4 H), 1.31 (s, 3 H), 0.81 (s, 3 H).
13C NMR (75 MHz, CDCl3): 138.32, 127.27, 126.35, 126.04,
125.75, 101.58, 77.65, 69.71, 69.49, 67.10, 30.23, 23.05, 21.90.
Anal. calcd for C24H26O2Fe: C 71.65, H 6.51. Found: C 71.52,
H 6.54%. UV/Vis (CH2Cl2): 266 (19 000), 311 (30 400), 456
(1700).
Compound 3PV-Fc
t-BuOK (0.1 g, 0.84 mmol) was added to a degassed solution
of 3 (0.3 g, 0.7 mmol) and 5 (0.5 g, 0.84 mmol) in dry THF (150
mL) at 0 1C under argon. The mixture was stirred for 2 h and
filtered under celite. The filtrate was evaporated to dryness and
was taken up in CH2Cl2. The organic layer was washed with
H2O, dried over MgSO4, filtered and evaporated to dryness.
Column chromatography (SiO2, hexane/CH2Cl2 3 : 2) yielded
1
3PV-Fc (0.3 g, 50%) as a red glassy product. H NMR (300
Compound 2
MHz, CDCl3): 7.54–7.40 (m, 8 H), 7.19 (s, 2 H), 7.10 (m, 4 H),
6.78 (AB, J = 17 Hz, 2 H), 5.74 (s, 1 H), 4.50 (s, 2 H), 4.32 (s, 2
H), 4.16 (s, 5 H), 4.02 (m, 4 H), 3.71 (AB, J = 11 Hz, 4 H),
1.84 (m, 4 H), 1.33-1.26 (m, 23 H), 0.88 (m, 9 H), 0.80 (s, 3 H).
13C NMR (75 MHz, CDCl3): 151.12, 150.29, 137.21, 136.53,
135.89, 128.61, 128.08, 127.74, 127.32, 126.99, 126.83, 126.79,
126.69, 126.05, 125.66, 123.53, 111.49, 110.59, 97.04, 83.41,
77.85, 69.56, 69.34, 69.23, 69.11, 66.87, 31.80, 30.26, 29.66,
29.45, 29.41, 29.37, 29.33, 29.29, 26.25, 26.09, 23.20, 22.66,
21.85, 14.10. UV/Vis (CH2Cl2): 398 (71 000).
A 1 : 1 mixture of CF3CO2H/H2O (30 mL) was added to a
degassed solution of PV-Fc (1.02 g, 2.53 mmol) in CH2Cl2 (15
mL) under argon. The mixture was stirred at room tempera-
ture for 5 h under argon atmosphere. The reaction mixture
was then washed with H2O (until pH was near neutrality),
dried over MgSO4, filtered and evaporated to dryness. Column
chromatography (SiO2, CH2Cl2/hexane 1 : 1) yielded 2 (0.64 g,
80%) as a dark red powder. IR (neat): 1691 (CQO), 2733,
1
2822 (C–H). H NMR (300 MHz, CDCl3): 9.98 (s, 1 H), 7.83
(d, J = 8 Hz, 2H), 7.56 (d, J = 8 Hz, 2H), 6.91 (AB, J = 17
Hz, 2 H), 4.51 (t, J = 2 Hz, 2 H), 4.36 (t, J = 2 Hz, 2 H), 4.16
(s, 5 H). 13C NMR (75 MHz, CDCl3): 191.57, 144.00, 134.56,
131.47, 130.30, 126.02, 124.51, 82.18, 69.72, 69.33, 67.31.
Anal. calcd for C19H16OFe: C 72.18, H 5.10. Found: C
71.99, H 5.18%.
Compound 6
A 1 : 1 mixture of CF3CO2H/H2O (10 mL) was added to a
degassed solution of 3PV-Fc (0.24 g, 0.28 mmol) in CH2Cl2
(20 mL) under argon. The mixture was stirred at room
temperature for 5 h under argon atmosphere. The reaction
mixture was then washed with H2O (until pH was near
neutrality), dried over MgSO4, filtered and evaporated to
dryness. Column chromatography (SiO2, CH2Cl2/hexane 1 :
1) yielded 6 (0.08 g, 37%) as a dark red powder that was used
in the next step as received. 1H NMR (300 MHz, CDCl3):
10.38 (s, 1 H), 7.32 (m, 8H), 7.05 (AB, J = 16 Hz, 2H), 7.03 (s,
4 H), 6.72 (AB, J = 17 Hz, 2 H), 4.40 (s, 2 H), 4.22 (s, 2 H),
4.07 (s, 5 H), 4.01 (t, J = 6 Hz, 2 H), 3.95 (t, J = 6 Hz, 2 H),
1.78 (m, 4 H), 1.43–1.19 (m, 20 H), 0.83 (m, 6 H).
Compound 2PV-Fc
t-BuOK (0.25 g, 2.2 mmol) was added to a degassed solution
of 2 (0.64 g, 2.0 mmol) and 1 (0.76 g, 2.2 mmol) in dry THF (50
mL) at 0 1C under argon. The mixture was stirred for 2 h and
filtered under celite. The filtrate was evaporated to dryness and
was taken up in CH2Cl2. The organic layer was washed with
H2O, dried over MgSO4, filtered and evaporated to dryness.
Column chromatography (SiO2, hexane/CH2Cl2 3 : 2) yielded
1
2PV-Fc (1.02 g, 99%) as an orange powder. H NMR (300
Compound 7
MHz, CDCl3): 7.55–7.41 (m, 8 H), 7.11 (s, 2 H), 6.80 (AB, J =
17 Hz, 2 H), 5.41 (s, 1 H), 4.48 (t, J = 2 Hz, 2 H), 4.30 (t, J = 2
Hz, 2 H), 4.14 (s, 5 H), 3.73 (AB, J = 11 Hz, 4 H), 1.32 (s, 3
H), 0.82 (s, 3 H). Anal. calcd for C32H32O2Fe: C 76.19, H 6.39.
Found: C 76.36, H 6.47%. UV/Vis (CH2Cl2): 231 (25 700), 356
(55 200).
A mixture of formaldehyde (0.08 g, 0.1 mmol), C60 (0.07 g, 0.1
mmol) and 4 (0.11 g, 0.2 mmol) in ODCB (25 mL) was
refluxed for 12 h under argon. After cooling, the resulting
solution was evaporated to dryness and column chromatogra-
phy (SiO2, hexane/CH2Cl2 4 : 1) gave 7 (39%) as a brown
1
glassy product. H NMR (300 MHz, CDCl3): 6.88 (d, J = 2
Hz, 2 H), 6.48 (t, J = 2 Hz, 1 H), 4.45 (s, 4 H), 4.25 (s, 2 H),
4.03 (t, J = 6 Hz, 4 H), 1.82 (m, 4 H), 1.44 (m, 36 H), 0.88 (t, J
= 7 Hz, 6 H). 13C NMR (75 MHz, CDCl3): 160.53, 155.03,
147.26, 146.20, 146.09, 146.02, 145.65, 145.37, 145.24, 144.52,
143.06, 142.58, 142.21, 142.03, 141.83, 140.17, 140.10, 136.22,
107.04, 100.54, 70.78, 68.16. 67.31, 58.61, 31.92, 29.71, 29.58,
29.55, 29.47, 29.36, 29.33, 26.13, 22.69, 14.14. Anal. calcd for
C93H59O2N: C 91.37, H 4.86, N 1.15. Found: C 90.99, H 5.17,
N 0.91%.
Compound 3
A 1 : 1 mixture of CF3CO2H/H2O (30 mL) was added to a
degassed solution of 2PV-Fc (1.0 g, 1.98 mmol) in CH2Cl2
(100 mL) under argon. The mixture was stirred at room
temperature for 5 h under argon atmosphere. The reaction
mixture was then washed with H2O (until pH was near
neutrality), dried over MgSO4, filtered and evaporated to
dryness. Column chromatography (SiO2, CH2Cl2/hexane 1 : 1)
yielded 3 (0.58 g, 70%) as a dark red powder. IR (neat):
1690 (CQO), 2719 (C–H). 1H NMR (300 MHz, CDCl3): 10.00
(s, 1 H), 7.77 (AB, J = 8 Hz, 4 H), 7.49 (AB, J = 8 Hz, 4 H),
7.21 (AB, J = 16 Hz, 2 H), 6.82 (AB, J = 16 Hz, 2 H), 4.50 (t,
J = 2 Hz, 2 H), 4.33 (t, J = 2 Hz, 2 H), 4.16 (s, 5 H). Anal.
calcd for C27H22OFe: C 77.52, H 5.30. Found: C 76.74, H
5.45%.
Compound C60-Fc
A mixture of ferrocenecarboxaldehyde (0.119 g, 0.55 mmol),
C60 (0.4 g, 0.55 mmol) and 4 (0.59 g, 1.11 mmol) in ODCB
(100 mL) was refluxed for 12 h. After cooling to room
temperature, the resulting solution was evaporated to dryness
and column chromatography (SiO2, hexane/CH2Cl2 4 : 1) gave
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C60-Fc (0.25 g, 32%) as a brown glassy product. 1H NMR (300
MHz, CDCl3): 7.04 (d, J = 2 Hz, 2 H), 6.51 (t, J = 2 Hz, 1 H),
6.17 (d, J = 14 Hz, 1 H), 5.18 (s, 1 H), 4.86 (d, J = 9 Hz, 1 H),
4.65 (s, 1 H), 4.57 (s, 1 H), 4.31 (s, 5 H), 4.26 (t, J = 2 Hz, 2 H),
4.13 (d, J = 10 Hz, 1 H), 4.07 (m, 4 H), 3.79 (d, J = 14 Hz, 1
(s, 2 H), 6.84 (d, J = 2 Hz, 2 H), 6.79 (AB, J = 17 Hz, 2 H),
6.49 (t, J = 2 Hz, 1 H), 5.22 (s, 1 H), 4.93 (d, J = 9 Hz, 1 H),
4.52 (d, J = 13 Hz, 1 H), 4.47 (t, J = 2 Hz, 2 H), 4.29 (t, J = 2
Hz, 2 H), 4.19 (d, J = 10 Hz, 1 H), 4.14 (s, 5 H), 4.04 (t, J = 6
Hz, 4 H), 3.64 (d, J = 13 Hz, 1 H), 1.84 (m, 4 H), 1.43 (m, 36
H), 0.88 (t, J = 7 Hz, 6 H). 13C NMR (75 MHz, CDCl3):
160.57, 156.48, 154.08, 153.47, 153.37, 147.31, 146.83, 146.46,
146.30, 146.27, 146.22, 146.18, 146.14, 146.10, 145.95, 145.74,
145.53, 145.32, 145.26, 145.15, 144.71, 144.62, 144.42, 144.38,
143.15, 142.98, 142.68, 142.55, 142.33, 142.25, 142.15, 142.10,
142.07, 142.02, 141.92, 141.85, 141.65, 141.55, 140.16, 140.11,
139.95, 139.90, 139.57, 137.70, 137.37, 136.84, 136.44, 136.28,
136.00, 135.77, 129.83, 129.78, 128.92, 127.62, 127.16, 126.88,
126.14, 125.80, 107.32, 100.30, 80.99, 69.44, 69.31, 68.77,
68.23, 66.98, 56.62, 31.94, 29.72, 29.67, 29.51, 29.38, 26.17,
22.71, 14.15. Anal. calcd for C119H79O2NFe.H2O: C 87.75, H
5.01, N 0.86. Found: C 87.75, H 5.03, N 1.03%. FAB-MS:
calcd. for C119H79O2NFe 1610.79; found 1610.4 [M]+. UV/Vis
(CH2Cl2): 229 (154 600), 255 (168 000), 319 (859 00), 431
(7500), 703 (750).
H), 1.84 (m, 4 H), 1.44 (m, 36 H), 0.89 (t, J = 7 Hz, 6 H). 13
C
NMR (75 MHz, CDCl3): 160.80, 156.46, 154.29, 153.97,
153.30, 147.61, 147.29, 147.25, 147.19, 146.53, 146.29,
146.25, 146.21, 146.12, 146.05, 145.92, 145.75, 145.56,
145.49, 145.25, 145.21, 145.15, 144.70, 144.65, 144.43,
144.41, 143.09, 142.99, 142.67, 142.61, 142.56, 142.24,
142.20, 142.17, 142.14, 142.07, 142.04, 141.88, 141.77,
141.59, 141.44, 140.16, 140.02, 139.47, 138.86, 136.32,
136.26, 136.05, 135.80, 106.39, 100.21, 86.58, 78.08, 75.47,
69.38, 68.45, 68.29, 68.23, 67.68, 67.46, 67.22, 57.65, 31.94,
29.72, 29.66, 29.50, 29.38, 26.17, 22.71, 14.15. Anal. calcd for
C
103H68O2NFe: C 87.89, H 4.87, N 1.00. Found: C 87.30, H
4.77, N 1.04. FAB-MS: calcd. for C103H68O2NFe 1406.46;
found 1406.5 [M]+. UV/Vis (CH2Cl2): 255 (133 700), 306
(42 260), 430 (4300), 704 (360).
Compound C60-PV-Fc
Compound C60-3PV-Fc
A mixture of 2 (0.105 g, 0.33 mmol), C60 (0.24 g, 0.33 mmol)
and 4 (0.35 g, 0.66 mmol) in ODCB (60 mL) was heated at
100 1C for 20 h under argon. After cooling, the resulting
solution was evaporated to dryness and column chromatogra-
phy (SiO2, hexane/CH2Cl2 4 : 1) gave C60-PV-Fc (0.2 g, 40%)
as a brown glassy product. 1H NMR (300 MHz, CDCl3): 7.84
(br s, 2 H), 7.50 (d, J = 8 Hz, 2 H), 6.84 (d, J = 2 Hz, 2 H),
6.80 (AB, J = 17 Hz, 2 H), 6.49 (t, J = 2 Hz, 1 H), 5.20 (s, 1
H), 4.92 (d, J = 9 Hz, 1 H), 4.51 (d, J = 13 Hz, 1 H), 4.45 (t, J
= 2 Hz, 2 H), 4.28 (t, J = 2 Hz, 2 H), 4.17 (d, J = 10 Hz, 1 H),
4.14 (s, 5 H), 4.04 (t, J = 6 Hz, 4 H), 3.62 (d, J = 14 Hz, 1 H),
1.84 (m, 4 H), 1.44 (m, 36 H), 0.88 (t, J = 7 Hz, 6 H). 13C
NMR (75 MHz, CDCl3): 160.11, 156.07, 153.68, 153.12,
153.03, 146.87, 146.42, 146.03, 145.90, 145.86, 145.77,
145.73, 145.69, 145.65, 145.51, 145.48, 145.30, 145.12,
145.08, 144.89, 144.84, 144.79, 144.70, 144.27, 144.18,
143.98, 143.94, 142.71, 142.54, 142.23, 142.14, 142.11,
141.89, 141.81, 141.70, 141.66, 141.57, 141.55, 141.51,
141.40, 141.21, 141.11, 139.72, 139.66, 139.59, 139.47,
139.11, 137.69, 136.41, 136.00, 135.53, 135.32, 134.94,
129.35, 127.16, 125.72, 125.21, 106.84, 99.83, 82.81, 80.65,
68.78, 68.65, 68.34, 67.77, 66.50, 66.46, 66.11, 56.17, 31.49,
29.27, 29.22, 29.06, 28.94, 25.72, 22.26, 13.71. Anal. calcd for
A mixture of 6 (0.08 g, 0.1 mmol), C60 (0.07 g, 0.1 mmol) and 4
(0.11 g, 0.2 mmol) in ODCB (25 mL) was heated at 100 1C for
20 h under argon. After cooling, the resulting solution was
evaporated to dryness and column chromatography (SiO2,
hexane/CH2Cl2 4 : 1) gave C60-3PV-Fc (0.05 g, 26%) as a
brown glassy product. 1H NMR (300 MHz, CDCl3): 7.75
(s, 1 H), 7.49 (s, 4 H), 7.45 (d, J = 3 Hz, 1 H), 7.38 (d, J =
6 Hz, 4 H), 7.15 (s, 1 H), 7.14 (d, J = 12 Hz, 1 H), 7.09 (d, J =
2 Hz, 2 H), 6.83 (d, J = 2 Hz, 2 H), 6.74 (AB, J = 17 Hz, 2 H),
6.48 (t, J = 2 Hz, 1 H), 5.82 (s, 1 H), 4.91 (d, J = 7 Hz, 1 H),
4.56 (s, 2 H), 4.52 (d, J = 11 Hz, 1 H), 4.37 (s, 2 H), 4.25 (d, J
= 7 Hz, 1 H), 4.20 (s, 5 H), 4.09 (m, 3 H), 4.02 (t, J = 5 Hz, 4
H), 3.80 (m, 1 H), 3.70 (d, J = 10 Hz, 1 H), 1.82 (m, 6 H), 1.66
(m, 2 H), 1.48–1.26 (m, 56 H), 0.88 (t, J = 5 Hz, 12 H). 13C
NMR (75 MHz, CDCl3): 160.53, 156.98, 155.07, 154.48,
153.88, 152.06, 150.96, 147.25, 146.79, 146.70, 146.27,
146.19, 146.14, 146.07, 146.03, 145.90, 145.66, 145.53,
145.24, 145.19, 145.10, 145.05, 144.57, 144.52, 144.42,
144.33, 142.98, 142.94, 142.61, 142.52, 142.29, 142.23,
142.13, 142.04, 141.90, 141.74, 141.67, 141.64, 141.61,
140.12, 140.04, 139.99, 139.66, 139.45, 137.22, 136.92,
136.54, 136.27, 136.20, 136.17, 136.11, 134.70, 128.77,
127.99, 127.92, 127.05, 126.95, 126.78, 126.60, 126.43,
126.33, 125.72, 123.39, 114.76, 109.53, 107.14, 100.27, 76.10,
73.11, 70.23, 69.97, 69.60, 68.96, 68.64, 68.10, 67.19, 66.26,
56.40, 31.89, 29.53, 29.68, 29.63, 29.48, 29.34, 26.89, 26.27,
26.15, 26.09, 22.74, 22.67, 14.18, 14.10. Anal. calcd for
C
111H73O2NFe: C 88.37, H 4.88, N 0.93. Found: C 87.91, H
4.90, N 1.11%. FAB-MS: calcd. for C111H73O2NFe 1508.65;
found 1508.3 [M]+. UV/Vis (CH2Cl2): 229 (154 600), 255
(168 000), 319 (85 900), 431 (7500), 703 (750).
C143H117O4NFe.CH2Cl2: C 84.29, H 5.80, N 0.68. Found: C
84.49, H 6.44, N 0.92%. FAB-MS: 1247.6 (30, [M-C60]+,
calcd. for C83H117O4NFe 1247.83), 1968.6 (50, [MH]+, calcd.
for C144H117O4NFe 1968.8).
Compound C60-2PV-Fc
A mixture of 3 (0.23 g, 0.55 mmol), C60 (0.4 g, 0.55 mmol) and
4 (0.59 g, 1.11 mmol) in ODCB (100 mL) was heated at 100 1C
for 20 h under argon. After cooling, the resulting solution was
evaporated to dryness and column chromatography (SiO2,
hexane/CH2Cl2 4 : 1) gave C60-2PV-Fc (0.3 g, 33%) as a
Electrochemistry
The electrochemical experiments were carried out in argon-
purged THF (Aldrich) or CH2Cl2 (Romil Hi-Dryt) solutions
at 298, 218 K with an EcoChemie Autolab 30 multipurpose
1
brown glassy product. H NMR (300 MHz, CDCl3): 7.90 (br
s, 2 H), 7.61 (d, J = 8 Hz, 2 H), 7.44 (AB, J = 8 Hz, 4 H), 7.12
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instrument interfaced to a personal computer. THF was
distilled according to a literature procedure.33 In the cyclic
voltammetry (CV) the working electrode was a glassy carbon
electrode (0.08 cm2, Amel) or a Pt disk ultramicroelectrode
(r = 25 mm). In all cases, the counter electrode was a Pt spiral,
separated from the bulk solution with a fine glass frit, and a
silver wire was employed as a quasi-reference electrode
(AgQRE). The potentials reported are referred to SCE by
measuring the AgQRE potential with respect to ferrocene. The
concentration of the compounds examined was of the order of
5 ꢂ 10ꢀ4 M; 0.05 M tetrabuthylammonium hexafluorophos-
phate (TBAPF6) was added as supporting electrolytes. Cyclic
voltammograms were obtained with scan rates in the range
0.05–20 V sꢀ1. The CV simulations were carried out by the
DIGISIM program.
3 (a) D. M. Guldi and N. Martin, J. Mater. Chem., 2002, 12,
1978; (b) N. Armaroli and G. Accorsi, in Fullerenes.
Principles and Applications, ed. F. Langa and J.-F.
Nierengarten, The Royal Society of Chemistry, Cambridge, UK,
2007, p. 79.
´
4 T. M. Figueira-Duarte, J. Clifford, V. Amendola, A. Gegout, J.
Olivier, F. Cardinali, M. Meneghetti, N. Armaroli and J.-F.
Nierengarten, Chem. Commun., 2006, 2054.
5 For reviews, see: (a) J.-F. Nierengarten, Sol. Energy Mater. Sol.
Cells, 2004, 83, 187; (b) J.-F. Nierengarten, New J. Chem., 2004, 28,
1177; (c) J. L. Segura, N. Martin and D. M. Guldi, Chem. Soc.
Rev., 2005, 34, 31.
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Chem. Soc., 2004, 126, 5340; (b) F. Giacalone, J. L. Segura,
N. Martin, J. Ramey and D. M. Guldi, Chem.–Eur. J., 2005, 11,
4819.
7 G. de la Torre, F. Giacalone, J. L. Segura, N. Martin and D. M.
Guldi, Chem.–Eur. J., 2005, 11, 1267.
8 (a) H. Kanato, K. Takimiya, T. Otsubo, Y. Aso, T. Nakamura, Y.
Araki and O. Ito, J. Org. Chem., 2004, 69, 7183; (b) T. Nakamura,
H. Kanato, Y. Araki, O. Ito, K. Takimiya, T. Otsubo and Y. Aso,
J. Phys. Chem. A, 2006, 110, 3471.
Photophysics
The photophysical investigations were carried out in CH2Cl2
and toluene (Carlo Erba, spectrofluorimetric grade). Absorp-
tion spectra were recorded with a Perkin-Elmer l40 spectro-
photometer. Emission spectra were obtained with an
Edinburgh FLS920 spectrometer (continuous 450 W Xe
lamp), equipped with a Peltier-cooled Hamamatsu R928
photomultiplier tube (185–850 nm) or a Hamamatsu R5509-
72 supercooled photomultiplier tube (193 K, 800–1700 nm
range). Emission quantum yields were determined according
to the approach described by Demas and Crosby34 using
[Os(phen)3]2+ (F = 0.005) as standard.35 Emission lifetimes
were determined with the time correlated single photon count-
ing technique using an Edinburgh FLS920 spectrometer
equipped with a laser diode head as excitation source
(1 MHz repetition rate, lexc = 407 or 635 nm, 200 ps time
resolution upon deconvolution) and an Hamamatsu R928
PMT as detector. Transient absorption spectra in the nano-
second-microsecond time domain were obtained by using the
nanosecond flash photolysis apparatus described previously.36
Experimental uncertainties are estimated to be 8% for lifetime
determinations, 20% for emission quantum yields, 10% for
relative emission intensities in the NIR, 1 nm and 5 nm for
absorption and emission peaks, respectively.
9 (a) H. D. Sikes, J. F. Smalley, S. P. Dudek, A. R. Cook, M. D.
Newton, C. E. D. Chidsey and S. W. Feldberg, Science, 2001, 291,
1519; (b) W. B. Davis, M. A. Ratner and M. R. Wasielewski, J.
Am. Chem. Soc., 2001, 123, 7877.
10 D. M. Guldi, M. Maggini, G. Scorrano and M. Prato, J. Am.
Chem. Soc., 1997, 119, 974.
11 S. Campidelli, E. Vazquez, D. Milic, M. Prato, J. Barbera, D. M.
Guldi, M. Marcaccio, P. Paolucci, F. Paolucci and R. Deschenaux,
J. Mater. Chem., 2004, 14, 1266.
12 T. Nakamura, H. Kanato, Y. Araki, O. Ito, K. Takimiya, T.
Otsubo and Y. Aso, J. Phys. Chem. A, 2006, 110, 3471.
13 L. Perez, J. C. Garcia-Martinez, E. Diez-Barra, P. Atienzar, H.
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Garcia, J. Rodriguez-Lopez and F. Langa, Chem.–Eur. J., 2006,
12, 5149.
14 (a) M. Even, B. Heinrich, D. Guillon, D. M. Guldi, M. Prato and
R. Deschenaux, Chem.–Eur. J., 2001, 7, 2595; (b) M. Fujitsuka, N.
Tsuboya, R. Hamasaki, O. Ito, S. Onodera, M. Ito and Y.
Yamamoto, J. Phys. Chem. A, 2003, 107, 1452.
15 (a) H. Imahori, K. Tamaki, Y. Araki, Y. Sekiguchi, O. Ito,
Y. Sakata and S. Fukuzumi, J. Am. Chem. Soc., 2002, 124, 5165;
(b) H. Imahori, Y. Sekiguchi, Y. Kashiwagi, T. Sato, Y. Araki, O.
Ito, H. Yamada and S. Fukuzumi, Chem.–Eur. J., 2004, 10, 3184;
(c) H. Imahori, Org. Biomol. Chem., 2004, 2, 1425; (d) F. D’Souza,
R. Chitta, S. Gadde, D. M. S. Islam, A. L. Schumacher,
M. E. Zandler, Y. Araki and O. Ito, J. Phys. Chem. B, 2006,
110, 25240.
16 M. U. Winters, E. Dahlstedt, H. E. Blades, C. J. Wilson, M. J.
Frampton, H. L. Anderson and B. Albinsson, J. Am. Chem. Soc.,
2007, 129, 4291.
17 N. Armaroli, G. Accorsi, J. N. Clifford, J.-F. Eckert and J.-F.
Nierengarten, Chem.–Asian J., 2006, 1, 564.
Acknowledgements
18 F. Ajamaa, T. M. Figueira Duarte, C. Bourgogne, M. Holler,
P. W. Fowler and J.-F. Nierengarten, Eur. J. Org. Chem., 2005,
3766.
19 (a) M. J. Gomez-Escalonilla, F. Langa, J.-M. Rueff, L. Oswald and
J.-F. Nierengarten, Tetrahedron Lett., 2002, 43, 7507; (b) F. Langa,
M. J. Gomez-Escalonilla, J. M. Rueff, T. M. Figueira Duarte, J.-F.
Nierengarten, V. Palermo, P. Samori, Y. Rio, G. Accorsi and N.
Armaroli, Chem.–Eur. J., 2005, 11, 4405.
This research was supported by the CNR (Commessa
PM.P04.010 ‘‘Materiali Avanzati per la Conversione di En-
ergia Luminosa’’, MACOL), MIUR (PRIN, ‘‘Sistemi supra-
molecolari per la conversione dell’energia luminosa’’), the
CNRS (UPR 8241) and the EU (contract HPRN-CT-2002-
00171, FAMOUS).
20 (a) R. Schenk, H. Gregorius, K. Meerholz, J. Heinze and K.
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Gregorius, K. Mullen and J. Heinze, Adv. Mater., 1994, 6, 671; (c)
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T. Gu, P. Ceroni, G. Marconi, N. Armaroli and J.-F. Nierengar-
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