Synthesis of fullerohelicates and fine tuning of the photoinduced processes by changing the number of addends on the fullerene subunits

Article abstract of DOI:10.1016/j.tet.2005.07.126

Two fullerene-substituted m-phenylene-bis-phenanthroline ligands have been prepared. The synthesis of the first derivative (L1) is based on an esterification reaction between a C_s symmetrical cis-2 fullerene bis-adduct bearing a carboxylic acid

Full text of DOI:10.1016/j.tet.2005.07.126

Tetrahedron 62 (2006) 2060–2073
Synthesis of fullerohelicates and fine tuning of the photoinduced
processes by changing the number of addends
on the fullerene subunits
Michel Holler,a Franc¸ois Cardinali,a Hind Mamlouk,a Jean-Franc¸ois Nierengarten,a,b,
*
Jean-Paul Gisselbrecht,c Maurice Gross,c Yannick Rio,d Francesco Barigellettid
and Nicola Armarolid,
*
a
` ` ´ ´ ` ´
Groupe de Chimie des Fullerenes et des Systemes Conjugues, Ecole Europeenne de Chimie, Polymeres et Materiaux,
´
Universite Louis Pasteur et CNRS, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France
`
b
` ´
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
c
´
Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide, Universite Louis Pasteur et CNRS,
4 rue Blaise Pascal, 67070 Strasbourg Cedex, France
d
`
Istituto per la Sintesi Organica e la Fotoreattivita, Molecular Photoscience Group, Consiglio Nazionale delle Ricerche,
via Gobetti 101, 40129 Bologna, Italy
Received 10 June 2005; revised 5 July 2005; accepted 8 July 2005
Available online 2 December 2005
Abstract—Two fullerene-substituted m-phenylene-bis-phenanthroline ligands have been prepared. The synthesis of the first derivative (L1)
is based on an esterification reaction between a Cs symmetrical cis-2 fullerene bis-adduct bearing a carboxylic acid function and a bis-
phenanthroline alcohol (5). The second ligand (L2) has been obtained by reaction of a bis-phenanthroline malonate (9) and C60 under Bingel
conditions. The copper(I) complexes of L1 and L2 have been prepared by treatment with a slight excess of Cu(CH3CN)4BF4. NMR
spectroscopy and mass spectrometry analysis have unambiguously shown that these complexes are bis-copper(I) helicates substituted with
two fullerene moieties. The photophysical properties of the copper(I) complexes Cu2(L1)2 and Cu2(L2)2 have been investigated. In both
systems photoinduced electron transfer from the central metal-complexed unit to the external fullerenes may occur, in principle, by excitation
of both moieties. However, this is found to be the case only for the methanofullerene system Cu2(L2)2. Unexpectedly, for Cu2(L1)2,
photoexcitation of the peripheral carbon spheres is followed by regular internal deactivation. Possible reasons for this behavior are examined
in light of current theories for photoinduced energy and electron transfer.
q 2006 Elsevier Ltd. All rights reserved.
1. Introduction
for the synthesis of topologically non-trivial compounds
such as molecular knots16,17 and doubly-interlocked
[2]catenanes.18 In recent years, this field has moved
towards the creation of functional systems with increased
attention to potential applications. For example, it has been
shown by Ziessel and co-workers that a helicate core can
behave as a mesogenic unit, thus providing an original
chiral scaffold for the preparation of new liquid crystals.19
As part of this research, we have recently shown that
helicates can be incorporated in photoactive supramolecular
functional devices.20 Specifically, a bis-copper(I) helicate
substituted with two fullerene moieties has been prepared.
Photophysical measurements have revealed that photo-
induced electron transfer occurs from the photoexcited
Cu(I)-complexed unit to the C60 acceptor thus showing that
such helicate-acceptor arrays are potentially interesting
candidates for the preparation of solar cells. In this paper,
Transition metals have been widely used for the construc-
tion of spectacular two- and three-dimensional supra-
molecular structures.1–10 Among these self-assembled
systems, polymetallic helical coordination compounds
have generated significant research efforts in the last two
decades.11–13 Such supramolecular arrays, named helicates
by Lehn,14 have been intensively studied to understand
metal-directed self-assembly processes.15 Furthermore, the
structural features of helicates have been beautifully
exploited by Sauvage and Dietrich-Buchecker as templates
Keywords: Copper(I) complexes; Electron transfer; Fullerene; Phenanthro-
line ligands; Photophysical properties.
Corresponding authors. Tel.: C33 561 33 31 00; fax: C33 561 55 30 03
(J.-F.N.); Tel.: C39 051 639 9820; fax: C39 051 639 9844 (N.A.);
e-mail addresses: jfnierengarten@lcc-toulouse.fr; armaroli@isof.cnr.it
*
0040–4020/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.07.126

M. Holler et al. / Tetrahedron 62 (2006) 2060–2073
2061
we now give a full account on the synthesis and the
electronic properties of these fullerene-substituted helicates,
that is, fullerohelicates.
alcohol 5 (Scheme 2). To this end, compound 6 was prepared
first. This Cs symmetrical cis-2 fullerene bis-adduct was
obtained in ten steps according to a previously reported
procedure.24 Treatment of t-butyl ester 6 with CF3CO2H in
CH2Cl2 gavethe corresponding carboxylicacidderivative7 in
aquantitativeyield.Subsequentreactionwithbenzylicalcohol
5 under esterification conditions using N,N0-dicyclohexyl-
carbodiimide (DCC), 4-dimethylaminopyridine (DMAP) and
1-hydroxybenzotriazole (HOBt) led to ligand L1 in 24%yield.
2. Results and discussion
2.1. Preparation of the bis-phenanthroline ligands
In the design of these compounds, we have selected a
m-phenylene-bis-phenanthroline core known to form a
double-stranded helicate upon complexation to cop-
per(I).21 This ligand was actually functionalized with a
hydroxy group to allow the attachment of the two
fullerene subunits at the end of the synthesis. The
synthesis of the bis-phenanthroline precursor is depicted
in Scheme 1. Reaction of 3,5-dibromobenzylic alcohol
(1) with triisopropylsilyl chloride (TIPSCl) in DMF in
the presence of imidazole gave the protected alcohol 2 in
The synthesis of the second C60-substituted bis-phenanthro-
line ligand relies upon Bingel type chemistry25 and is shown
in Scheme 3. DCC-mediated esterification of benzylic
alcohol 5 with carboxylic acid 824 yielded malonate 9.
The reaction of C60 with compound 9, iodine and
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) under Bingel
conditions then gave methanofullerene L2 in 22% yield.
Ligands L1 and L2 were characterized by 1H- and 13C NMR,
UV–vis and IR spectroscopies. The 1H NMR spectra of both
compounds show the signals of the m-phenylene-bis-
arylphenanthroline core. Effectively, both spectra are
characterized by three sets of AB quartets in the aromatic
region in a typical pattern for a disymmetrically 2,
9-disubstituted-1,10-phenanthroline derivative, an AA’XX’
system for the aromatic protons of the 4-didodecyloxy-
phenyl unit and an A2X system for the aromatic protons of
the central phenyl ring. In the case of compound L2, the
1H NMR spectrum reveals also two singlets at d 5.96 and
5.48 ppm corresponding to the resonances of the two
different benzylic CH2 groups, an A2X system for the aro-
matic protons of the 3,5-dioctyloxyphenyl moiety as well as
the diagnostic signals of the alkyloxy groups. The 1H NMR
spectrum of L1 shows all the characteristic features of the
Cs symmetrical 1,3-phenylenebis(methylene)-tethered full-
erene cis-2 bis-adduct substituent.26 Effectively, in addition
to the signals arising from the 3,5-dioctyloxyphenyl
moieties, an AB quartet and a singlet are observed for the
diastereotopic benzylic CH2 groups and an AX2 system is
revealed for the aromatic protons of the 1,3,5-trisubstituted
bridging phenyl ring. Finally, the 13C NMR spectra of L1
and L2 are typical of Cs symmetric fullerene derivatives in
full accordance with the proposed molecular structures.
t
83% yield. Treatment of 2 with an excess of BuLi at
K78 8C followed successively by reaction of the
resulting organolithium derivative with mono-substituted
phenanthroline 322 (0.5 equiv), hydrolysis and rearomati-
zation with MnO2 afforded 4 in 31% yield. This
moderate yield is mainly associated to the difficulties
encountered during the purification of compound 4.
Indeed, ligand 4 has a high affinity for lithium23 and
even by washing extensively the crude material with
water, an important part of the fractions of 4 obtained
upon column chromatography on SiO2 are polluted with
traces of Li complexes of 4. Finally, treatment of TIPS-
protected derivative 4 with tetra-n-butylammonium
fluoride (TBAF) in THF at 0 8C then gave compound 5
in 88% yield.
In addition, methanofullerene 11 used as reference compound
for the photophysical studies was prepared in two steps from
malonic acid 8 (Scheme 4). DCC-mediated esterification of 8
with EtOH afforded 10 in 73% yield. Subsequent treatment
with C60 in the presence of I2 and DBU yielded compound 11
in 31% yield.
2.2. Preparation of the copper(I) helicates
The copper(I) complexes Cu2(4)2, Cu2(L1)2 and Cu2(L2)2
(Fig. 1) were obtained by treatment of the corresponding
ligands with a slight excess of Cu(CH3CN)4BF4 in CH2Cl2/
CH3CN at room temperature.
Cu2(4)2 was thus formed in a good yield; however, due to
difficulties encountered during its purification, the isolated
yield was quite low (22%). Actually, a part of the product
stuck all along the SiO2 (or Al2O3) column (this could be
easily observed because of the compound’s dark red color)
Scheme 1. Preparation of bis-phenanthroline 5.
The preparation of the first C60-substituted bis-phenanthroline
ligand (L1) is based on an esterification reaction between a
fullerene derivative bearing a carboxylic acid function and

2062
M. Holler et al. / Tetrahedron 62 (2006) 2060–2073
Scheme 2. Preparation of ligand L1.
Scheme 3. Preparation of ligand L2.
and could not be eluted any more. In order to prevent this
problem, the copper(I) complexes obtained from ligands L1
and L2 were not purified by column chromatography on
SiO2 or Al2O3. Indeed, we found that gel permeation
chromatography (Biorads, Biobeads SX-1, CH2Cl2) was an
efficient tool to obtain both compounds in a pure form
without significant loss of material. Cu2(L1)2 and Cu2(L2)2
were thus isolated in 75 and 52% yield, respectively.
1
The H NMR spectra of ligand 4 and complex Cu2(4)2 are
depicted in Figure 2. Upon complexation to copper(I),
dramatic changes are observed for the chemical shift of
the signals corresponding to the protons belonging to the
bis-phenanthroline moiety due to the stacking of the
aromatic subunits in the helicate. As a result of the ring
current effect of the aromatic units of one m-phenylene-bis-
arylphenanthroline moiety on the second ligand in
Scheme 4. Preparation of model compound 11.

M. Holler et al. / Tetrahedron 62 (2006) 2060–2073
2063
Figure 1. Helicates Cu2(4)2, Cu2(L1)2 and Cu2(L2)2.
1
the complex, the signals of the aromatic protons are shielded
by about 0.2–2.0 ppm in Cu2(4)2 when compared to the
corresponding signals in ligand 4. The shielding effect is
particularly dramatic for protons Hb, Hc, Hd, H3 and H4 as
one could expect by a simple inspection of the calculated
structure of Cu2(4)2 (Fig. 2). Importantly, the H NMR
spectrum provide clear evidence for the formation of a
chiral complex, that is, a helical system. In particular, the
methylenic protons that were enantiotopic in ligand 4 are
not equivalent any longer in complex Cu2(4)2 and give rise
4
5
3
b
6
a
7
8
c
d
N
N
N
N
A
TIPSO
H25C12
O
O
B
A
5/6
c
d
c
8
B
b
7
4 3
6/8
a
A
B
*
d
b
7
4
5
3
A/A’
B/B’
*
a
10
9
8
7
6
5
4
3
(ppm)
Figure 2. (A) 1H NMR spectrum (CDCl3, 300 MHz, *: solvent peak) of ligand 4. (B) 1H NMR spectrum (CDCl3, 300 MHz, *: solvent peak) of helicate
Cu2(4)2. Top: calculated structure of Cu2(4)2 (the four dodecyloxy groups have been replaced by methoxy units for clarity).

2064
M. Holler et al. / Tetrahedron 62 (2006) 2060–2073
Figure 3. 1H NMR spectrum (CDCl3, 400 MHz) of fullerohelicate Cu2(L1)2. The six AB quartets corresponding to the resonances of the six different
diastereotopic CH2 groups provide definitive evidence for the helical structure. Unambiguous assignment was achieved on the basis of 2D-COSY and NOESY
spectra recorded at room temperature in CDCl3.
to more complicated sets of signals. For example, the
Table 1. Reduction and oxidation characteristics of 6, 11, Cu2(4)2,
Cu2(L1)2 and Cu2(L2)2 observed by CV on a glassy C electrode in
CH2Cl2C0.1 M Bu4NPF6
benzylic protons of the CH2OTIPS unit are observed as
a singlet at d 5.25 ppm in 4 while they appear as an AB
quartet at d 4.77 ppm in the corresponding copper(I)
complex. Indeed, these protons are enantiotopic in 4 and
diastereotopic in the chiral complex Cu2(4)2.
a
Compound
Oxidation
Reduction
E2
E1
E1
E2
6
11
Cu2(4)2
Cu2(L1)2
Cu2(L2)2
K1.07 (45)b
K1.03 (80)
C0.46 (75) C0.35 (100) K1.83 (95)
K1.45b,c
K1.41 (80)
K2.23c
As observed for Cu2(4)2, dramatic changes in chemical shift
are seen for the resonances of the aromatic protons of the
bis-phenanthroline subunit in the 1H NMR spectra of
Cu2(L1)2 and Cu2(L2)2 when compared to the correspond-
ing signals in the spectrum of L1 and L2. Owing to the
chirality of both Cu2(L1)2 and Cu2(L2)2, the 2 equiv
fullerene moieties have lost their plane of symmetry. For
this reason, some of the protons that were equivalent in the
ligands are not equivalent any longer in the complex. This is
illustrated with the 1H NMR spectra of Cu2(L1)2 (Fig. 3). In
particular, the six AB quartets corresponding to the
resonances of the six different diastereotopic CH2 groups
provide definitive evidence for the helical structure. The
structure of Cu2(L1)2 was also confirmed by mass
spectrometry. The FAB-MS of Cu2(L1)2 is characterized
by a singly charged peak at m/z 5734.0 and a doubly charged
ion peak at m/z 2824.7, which can be assigned to Cu2(1)2
after loss of one and two tetrafluoroborate counteranions,
respectively.
C0.57 (75) C0.37 (75)
C0.61 (75) C0.41 (80)
K1.07 (100)d K1.44c
K0.96 (80)d
K1.42 (90)d
a Values for (EpaCEpc)/2 (V vs Fc/FcC) and DEpc (mV, in parenthesis) at a
scan rate of 0.1 V sK1
b From Ref. 29.
.
c Peak potential values at a scan rate of 0.1 V sK1, irreversible process.
d Dielectronic process.
2.3. Electrochemical properties
The electrochemical properties of 6, 11, Cu2(4)2, Cu2(L1)2
and Cu2(L2)2 were investigated by cyclic voltammetry
(CV) in CH2Cl2 C0.1 M nBu4NPF6 solutions. The results
are summarized in Table 1. As a typical example, the cyclic
voltammogram recorded for Cu2(L1)2 is depicted in
Figure 4.
Figure 4. Cyclic voltammetry of fullerohelicate Cu2(L1)2 on a glassy
carbon electrode at vZ0.1 V sK1 in CH2Cl2C0.1 M Bu4NPF6.

M. Holler et al. / Tetrahedron 62 (2006) 2060–2073
2065
Figure 5. Left: absorption spectra of 6 (grey full line), Cu2(4)2 (black dashed line) and Cu2(L1)2 (black full line) in CH2Cl2. Right: absorption spectra of 11
(grey full line), Cu2(4)2 (black dashed line) and Cu2(L2)2 (black full line) in CH2Cl2.
As already reported for related copper(I) helicates,20,27,28
a
As far as the reduction potentials are concerned, the two
fullerene subunits behave as independent redox centers in
both Cu2(L1)2 and Cu2(L2)2. Compounds 11 and Cu2(L2)2
show a similar behavior totally consistent with previously
reported data for methanofullerenes.29,30 Fullerene cis-2
bis-adducts Cu2(L1)2 and 6 essentially retain the cathodic
electrochemical pattern of methanofullerenes but the
reduction potentials are shifted to more negative values
when compared to those of Cu2(L2)2 and 11. Indeed, this
small shift results from the saturation of an additional
double bond on the C60 surfaces, which causes a partial loss
of ‘conjugation’ when going from mono- to bis-adducts.
strong electronic interaction between both copper atoms in
Cu2(4)2, Cu2(L1)2 and Cu2(L2)2 is evidenced by the
electrochemical measurements. Effectively, the anodic
region of their cyclic voltammograms is characterized by
two distinct waves for the Cu-centered oxidations. In other
words, the second oxidation process occurs at a higher
potential, indicating the influence of the first Cu(II) center,
which renders oxidation of the second metal more difficult.
Comparison of the oxidation potentials of Cu2(L1)2 and
Cu2(L2)2 with those of the corresponding model compound
Cu2(4)2 shows a small but evident trend. The two oxidation
potentials of both Cu2(L1)2 and Cu2(L2)2 are slightly
shifted to more positive values by 20–110 and 60–150 mV,
respectively. These shifts could be a consequence of small
electronic interactions between the dinuclear copper(I) core
and the C60 units, resulting in a more difficult oxidation for
the metallic centers. However, the photophysical studies
revealed no particular electronic interactions between the
fullerene moieties and the helicate core (see below). In
addition, the different subunits are separated by rather long
spacers. Therefore, it appears more reasonable to ascribe the
observed potential shift to solvation effects resulting from
the presence of the surrounding apolar fullerene groups.
2.4. Photophysical properties
Reference compounds. The photophysical properties of
methanofullerenes and bis-methanofullerenes have already
been investigated in detail.29,31,32 They exhibit a broad
absorption throughout the UV–vis region and a weak
fluorescence centered at about 700 nm. For the sake of
comparison with the corresponding helicates the absorption
and fluorescence spectra of 6 and 11 in CH2Cl2 are depicted
in Figures 5 and 6, whereas in Table 2 fluorescence quantum
yields and singlet lifetimes are reported.
Figure 6. (a) Emission spectra of 6 (grey full line), Cu2(4)2 (black dashed line) and Cu2(L1)2 (black full line) in CH2Cl2. (b) Emission spectra of 11 (grey full
line), Cu2(4)2 (black dashed line) and Cu2(L2)2 (black full line) in CH2Cl2. Inset: sensitized singlet oxygen luminescence spectra of reference fullerenes (grey)
and multicomponent arrays (black). In all cases lexZ540 (a) and 500 nm (b) where light partitioning Cu/C60 is 1/1. AbsorbanceZ0.30, uncorrected spectra.

2066
M. Holler et al. / Tetrahedron 62 (2006) 2060–2073
Table 2. Photophysical properties in CH2Cl2 at 298 K
C60 fluorescence MLCT band
t (ns) f!104 lmax
t (ns)
Fullerohelicate Cu2(L1)2. The electronic absorption spec-
trum of the bismethanofullerene Cu2(L1)2 in CH2Cl2 is
reported in Figure 5 and corresponds well to the sum of
the absorption spectra of its component units (6 and
Cu2(4)2). This is in contrast with what was observed for a
Cu(I)–bisphenanthroline complex sandwiched between two
fullerenes, where the absorption spectrum indicates elec-
lmax
(nm)a
f!104
(nm)a
Cu2(4)2
6
—
—
—
707 (741) !0.2,
130
6.9
702 (704) 1.4
3.0
3.0
4.5
0.7
—
—
—
—
—
—
—
d
—
—
—
—
tronic interactions among the subunits.46 In Cu2(L1)2
a
Cu2(L1)2 702 (704)b 1.4
11
Cu2(L2)2 702 (704)
698 (698) 1.4
c
spacer is inserted between the inorganic core and the
organic units. Thus, although the system is relatively
flexible, the interchromophoric distance is long enough to
prevent electronic interactions observable in the absorption
spectra.
a In brackets are reported the emission maxima from spectra corrected for
the photomultiplier response.
b The uncorrected emission spectrum published earlier (see Ref. 20) was
obtained with a different, near-IR sensitive photomultiplier tube.
c Non detectable due to signal weakness.
d Only the short-lived component (t!0.2 ns) is detected.
Since excitation cannot be selectively addressed to a specific
subunit of Cu2(L1)2, we chose to excite at 540 nm where
the light partitioning among the Cu(I) chromophores and
carbon spheres in Cu2(L1)2 is identical.46 Under these
conditions the quenching of the long-lived MLCT lumines-
cence band is complete and only a fluorescence band
attributable to the C60 moiety is detected, which exhibits the
same lifetime and a 50% intensity decrease relative to the
reference molecule 6 (Table 2 and Fig. 6). A 50% decrease
of the fullerene triplet yield is also measured, by means of
the intensity of the sensitized singlet oxygen emission band
(Fig. 6).47 From the above results it is possible to conclude
that (i) the metal complexed core of Cu2(L1)2 undergoes
photoinduced quenching not attributable to energy transfer
because no sensitization of the fullerene singlet and triplet
states occur; (ii) following light excitation, the fullerene
moieties of Cu2(1)2 behave in the same way as the reference
molecule 6.20
The absorption and luminescence properties of Cu2(4)2, to
be used as reference for the helicate core, are in line with
analogous Cu(I) complexes of 2,9-diphenyl-phenanthroline
ligands and the typical metal-to-ligand-charge-transfer
(MLCT) emission band around 700 nm is recorded.33–37
Notably, the luminescence decay trace is biexponential with
t1!0.5 ns and t2Z130 ns (oxygen free CH2Cl2, Fig. 7).
Since excitation is addressed selectively to the low energy
side of the MLCT bands (lexcZ637 nm), one can exclude
excitation of the free phenanthroline ligand or its protonated
form.38–40
The formation of the long-lived MLCT emitting state in
copper(I)–phenanthrolines is preceded by extensive struc-
tural rearrangements, namely (i) flattening distortion of the
initial pseudo-tetrahedral Franck-Condon geometry and (ii)
expansion of the coordination number.41,42 These processes
occur in the tens of picosecond timescale.43 The short-lived
component observed for Cu2(4)2, whose lifetime is shorter
than our instrumental resolution (200 ps), might be
associated with excited states that have not yet undergone
structural rearrangements. The long-lived component is
attributed to ‘relaxed’ excited states and stems from 1MLCT
and 3MLCT levels in thermal equilibrium and separated by
Fullerohelicate Cu2(L2)2. The absorption spectrum of
Cu2(L2)2 corresponds to the sum of the spectra of its
component units (Cu2(4)2C11!2), indicating negligible
electronic interactions among the organic and the inorganic
fragments. The most important difference that characterizes
the methanofullerene Cu2(L2)2 versus the bis-methano-
fullerene Cu2(L1)2 helicate is the fact that in the former, at
any excitation wavelengths, both the MLCT emission and
the fullerene fluorescence are dramatically quenched
(Fig. 6). Interestingly the only signal detected in the lifetime
an energy gap of about 1500 cmK1 44,45
.
Figure 7. Left: time profile of luminescence of 6 (grey), Cu2(4)2 (black) and Cu2(L1)2 (light grey) in CH2Cl2. Right: time profile of luminescence of 11 (grey),
Cu2(4)2 (black) and Cu2(L2)2 (light grey) in CH2Cl2.

M. Holler et al. / Tetrahedron 62 (2006) 2060–2073
2067
3
experiments of Cu2(L2)2 is the ultrashort-lived component
of the MLCT emission (Fig. 7, right). This indicates that the
intercomponent process related to the quenching of the
MLCT emission band is slower than the molecular
rearrangements, which follow photoexcitation (see above).
A negligible amount of fullerene triplet is evidenced by the
sensitized singlet oxygen experiments (Fig. 6) and transient
absorption spectra.
(fullerene singlet) or CuMLCT–F (3MLCT, Cu-complexed
moiety) are both moderately exergonic (DG0ZK0.31 and
K0.15 eV, respectively). In both cases electron transfer can
be reasonably located in the normal region of the Marcus
parabola;48 thus, in principle, the fullerene singlet should
3
promote a faster electron transfer than MLCT due to the
larger reaction exoergonicity. However, the experimental
trend is the opposite. As discussed previously with a similar
system46 we attribute this ‘asymmetric’ behavior (i.e.,
electron transfer originated only upon excitation of the
central core, despite similar thermodynamics when
the peripheral C60’s are stimulated) to the different nature
of the involved excited states, that is, charge transfer versus
localized. In particular, formation of a metal-to-ligand-
charge-transfer state, which occurs prior to Cu/C60
electron transfer, lowers both external and internal
reorganization energies related to the process and abates
the kinetic activation barrier.46 The external contribution is
related to the solvent repolarization around the newly
formed charged species while the internal reorganization
energy has to do with the molecular internal rearrangements
that accompany electron transfer processes. These events,
which facilitates electron transfer, occur to some extent
when the lowest MLCT state(s) of Cu(I)–phenanthrolines
are formed, whereas do no take place upon formation of a
localized pp* state (i.e., C60 singlet). In addition to these
arguments it must also be emphasized that the formation of
a MLCT state prior to the final charge separation may
contribute to yield a better electronic interaction between
the two chromophores since it causes redistribution of
negative charge on the phenyl–phenanthroline ligand,50 that
is, in the direction of the attached C60 electron acceptor,
according to the helicate structures. This makes the donor–
acceptor distance effectively smaller, likely favoring the
reaction kinetics. Similar electronic effects on the rate of
photoinduced electron transfer have been recently observed
for organic51 and hybrid52 donor–acceptor arrays.
Energy diagrams and rationale. The energy level diagrams
reporting the lowest electronic levels of Cu2(L1)2 and
Cu2(L2)2 are depicted in Figure 8; they facilitate the
rationalization of the above described light induced
processes, which are different for the two helicates.
3
The energy level of the MLCT state (1.59 eV) and of the
fullerene singlet were determined by 77 K emission spectra,
whereas the 1MLCT state is positioned taking into account a
3
gap of about 1500 cmK1 relative to MLCT.35 The energy
of the fullerene lowest triplets cannot be determined
experimentally due to the lack of phosphorescence emission
and was estimated by theoretical calculations.29,32 Finally,
the charge separated states are estimated from the one-
electron redox potential,48 without considering coulombic
interactions. This approximation is acceptable since no
electronic interactions are evidenced in the ground state for
these helicates and, in addition, the coulombic contribution
has been shown to be negligible in fullerene multicomponent
systems.49 Apart from the different location of the fullerene
triplet states, which probably do not actively participate in
photoinduced processes and are likely to be positioned with
scarce precision from theoretical methods a relevant, albeit
small, difference in the diagrams of the two helicates seems to
be the energy of the charge separated states.
In the case of the bis-methano fullerene system Cu2(L1)2
quenching by electron transfer of the MLCT state is the
3
only possibility, because no sensitization of the fullerene
levels is observed, ruling out the possibility of energy
transfer.20 By contrast, direct excitation of the fullerene
moieties ends up into unmodified deactivation of the carbon
sphere relative to the model compounds.
In the case of the methanofullerene system Cu2(L2)2 the
quenching of the MLCT state may occur, in principle,
3
via electron transfer to the lower lying charge separated
state (DG0ZK0.22 eV) or via energy transfer to full-
erene triplet level (DG0 ca. K0.10 eV), followed by
charge separation (DG0 ca. K0.12 eV). All these
processes occur at a faster rate than the resolution of
From the energy diagram in Figure 8 it is evident, for
Cu2(L1)2, that electron transfer from either Cu–1F
Figure 8. Energy level diagrams of Cu2(L1)2 (left) and Cu2(L2)2 (right) with the related photoinduced processes upon excitation of both chromophores. In the
shorthand notations, where Cu–F stands for the fullerohelicates Cu2(L1)2 or Cu2(L2)2, only one fullerene (F) unit is indicated because under our experimental
conditions, that is, light intensity, only one C60 unit can be excited.

2068
M. Holler et al. / Tetrahedron 62 (2006) 2060–2073
our flash photolysis apparatus (20 ns) and, whatever the
deactivation path, the final sink of photoinduced
processes is the charge separated level, since spectro-
scopic fingerprints of fullerene singlet and triplet levels
are not found in Cu2(L2)2 either via fluorescence or
transient absorption spectroscopy.
of Eq. 1:
ꢀ
ꢁ
Fref
F
K1
kEnT
Z
(1)
tref
where F and Fref are the C60 fluorescence quantum yield
of the Cu2(L2)2 helicate and of the reference compound
11, whereas tref is the singlet lifetime of the latter.
A more interesting issue is the quenching of the
fullerene singlet level, which is observed for the methanoful-
lerene system Cu2(L2)2 but not for the bismethanofullerene
array Cu2(L1)2. At this stage it is important to point out that
this result confirms a trend observed earlier in Cu(I)–
phenanthroline/C60 arrays having topologies ranging from
rotaxanes,32 to dendrimers,29 to sandwich-type triads,46 in
which the metal complexed unit is always the central core of
the molecular architecture. In fact we have found previously
that quenching of the fullerene moiety occurs in a rotaxane
having methanofullerenes as terminal units, whereas in
bismethanofullerene dendrimers and sandwich-type
complexes the carbon sphere is never quenched. In other
words, considering all the systems investigated so far, in
Cu(I)–phenanthroline/C60 arraysthefullereneunitsarealways
quenched by the metal-complexed unit when methano-
fullerenes are involved whereas quenching is never found
for bismethanofullerene arrays.
Using the available photophysical data, it is possible to
calculate the C60/Cu(I)–phenanthroline energy transfer
53
¨
rate according to the dipole–dipole Forster mechanism,
which turns out to be 4!108 sK1, that is, one order of
magnitude smaller than the experimental value. Thus, this
mechanism can be ruled out. Model calculations to estimate
the energy transfer rate according to the electron exchange
model (Dexter-type mechanism) are not easily amenable.
However, on the basis of experimental data it is possible to
evaluate the overlap integral according to Dexter, which
turns out to be virtually identical for Cu2(L1)2 and
Cu2(L2)2. Thus, given the completely different behavior
of the two helicates, it is unlikely that energy transfer plays a
major role in the quenching of the fullerene singlet, pointing
to an electron transfer process.
The most relevant difference in the photophysical para-
meters of the reference methano- and bismethanofullerenes
6 and 11 is the shape of the corrected emission spectra (see
Table 2 and Fig. 9). Having this in mind we carried out a
spectral analysis of such profiles in order to get more insight
into the tendency of the two functionalized carbon spheres
to undergo electron transfer from the lowest singlet excited
states. In particular, the analysis provides the internal
reorganization energy for deactivation of the fluorescent
level in the two cases. The results are collected in Table 3 and
illustrated in Figure 9 (see Section 4 for more details). As one
can see, along the vibrational mode corresponding to a typical
These results do not depend upon the separation of the
two chromophores, which in both cases has been
˚
estimated to be 8–12 A, based on molecular modeling.
The basic factor that seems to affect the final outcome is
the structure of the fullerene, that is, methano versus
bismethanofullerene.
From the energy diagram of Figure 8 one can derive that
in Cu2(L2)2 the quenching of the fullerene singlet can
3
occur in principle (i) via energy transfer to MLCT level
C–C vibration (Zum in the range 1350–1400 cmK1 54,55
)
or (ii) via electron transfer to yield the Cu(I)–phenanthro-
line/C60 charge separated state. From the residual
fullerene fluorescence (about 15% compared to the
reference molecule 11), one can estimate an upper limit
for the quenching rate constant of 4!109 sK1 by means
the fluorescent state of the bismethanofullerene 6 is more
distorted than that of the methanofullerene 11 (the displace-
ment parameter Sm amounts to 1.79 and 0.79, respectively).
This results in different values for the internal reorganization
Figure 9. Vibronic analysis of the corrected fluorescence profiles (experimental points, squares; fits, full lines) for methanofullerene 11 (a) and
bismethanofullerene 6 (b); see text for further details.

M. Holler et al. / Tetrahedron 62 (2006) 2060–2073
2069
Table 3. Data from vibronic analysis of the fluorescence spectraa
kinetic barrier on the way to the complete Cu(I)–
phenanthroline/C60 charge separation. A vibronic analysis
of the fluorescence spectra of the functionalized C60’s 6 and
11, sheds light into the different ability of methano- and
bismethanofullerenes to undergo electron transfer in arrays
containing Cu(I)–phenanthrolines. Both kinetic and thermo-
dynamic factors (related to internal reorganization energies
and reduction potentials) are likely to disfavor electron
transfer regardless of the fullerene singlet level to the metal
complex in the case of bismethanofullerenes, regardless the
molecular architecture.
E0 (cmK1
)
Zum (cmK1 b
)
li (eV)c
b
Sm
Compound
11
6
14330
14350
0.79
1.79
1380
1350
0.14
0.30
a According to Eq. 3 (see Section 4), CH2Cl2, 298 K; excitation performed
at 430 nm.
b Parameters for the high frequency mode (corresponding to aromatic C–C
vibrations);54,55,57,58 the low frequency mode, Zul was 500 cmK1 yielding
SlZ0.68 and 0.78 for 11 and 6, respectively, and Dyꢀ1=2Z680 and
930 cmK1, respectively.
c Internal reorganization energy along the prominent high frequency mode,
liZSm!Zum, see text.
energy for deactivation of the fluorescent level in the two
cases, liZ0.30 and 0.14 eV, for 6 and 11, respectively. As a
consequence, the nuclear factor (NF) for the electron transfer,
Eq. 2, turns out to be 0.29 and 0.05, respectively, as evaluated
by assuming lZloCli, with a solvent reorganization energy
loZ0.5 eV in both cases.56
4. Experimental
4.1. General
Reagents and solvents were purchased as reagent grade and
used without further purification. THF was distilled over
sodium benzophenone ketyl. Compounds 3,17 6,47 747 and
847 were prepared as previously reported. 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 10K2 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. UV–vis spectra were
recorded on a Hitachi U-3000 spectrophotometer. NMR
spectra were recorded on a Bruker AC 300 or a Bruker AM
400 with solvent peaks as reference. FAB-mass spectra
(m/z; % relative intensity) were taken on a ZA HF instru-
ment with 4-nitrobenzyl alcohol as matrix. Elemental
analyses were performed by the analytical service at the
Institut Charles Sadron, Strasbourg.
ꢂ
ꢃ
ðl CDG0Þ2
4lkBT
NF Z exp K
(2)
These results suggest that the singlet excited states of
methanofullerenesare moreprone to undergo electrontransfer
than those of bismethanofullerenes, thanks to the associated
smaller internal reorganization energy. In addition methano-
fullerenes are slightly easier to reduce than bismethanofuller-
enes (Table 1), also giving a thermodynamic advantage for
electron transfer in multicomponent arrays containing the
monofunctionalized derivative. The combined effect of the
above discussed kinetic and thermodynamic factors can
explain the different and somehow unexpected trend in
photoprocesses of multicomponent arrays containing Cu(I)–
phenanthrolines linked to methanofullerenes versus bismetha-
nofullerenes, which has been invariably found in a variety of
molecular architectures.29,32,46
4.1.1. Compound 2. To a solution of 3,5-dibromobenzylic
alcohol 1 (6.11 g, 23 mmol) in DMF (40 mL) at 0 8C was
added TIPSCl (6.17 mL, 28 mmol) and imidazole (3.75 g,
55 mmol). After 24 h the solvent was removed under
vacuum and the residue dissolved in ether. The organic
layer was washed with water, brine, dried over MgSO4 and
filtered. After removal of the solvent under reduced
pressure, purification by column chromatography (SiO2,
CH2Cl2/hexane 1:9), afforded compound 2 (8.0 g, 19 mmol)
as a colorless oil in an 83% yield. 1H NMR (CDCl3,
200 MHz): d 1.10 (m, 21H), 4.78 (s, 2H), 7.43 (s, 2H), 7.54
(s, 1H).
3. Conclusions
Bisphenanthroline ligands bearing a fullerene subunit have
been prepared. The corresponding copper(I) complexes
have been obtained by treatment with a cuprous salt. NMR
spectroscopy and mass spectrometry analysis have unam-
biguously shown that these complexes are bis-copper(I)
helicate substituted with two fullerene moieties. The
fullerohelicates Cu2(L1)2 and Cu2(L2)2 do not exhibit
ground state electronic interactions but undergo light
induced intercomponent processes upon light excitation.
In both systems photoinduced electron transfer from the
central metal-complexed unit to the external fullerenes may
occur, in principle, by excitation of both moieties. However,
this is found to be the case for only the methanofullerene
system Cu2(L2)2. Unexpectedly, for Cu2(L1)2, photoexci-
tation of the peripheral carbon spheres is followed by
regular internal deactivation. This ‘asymmetric’ behavior
has been interpreted on the basis of the different character
of the involved excited states, that is, charge transfer
(Cu–phenanthroline excitation), versus, localized (C60
excitation). In the former case the impact on external and
internal reorganization energy contributes to reduce the
4.1.2. Compound 4. A solution of tBuLi (6.1 mL,
10.4 mmol) in pentane was added to a solution of 2 (1.0 g,
2.4 mmol) in THF (4 mL) under argon at K78 8C. After 1 h,
the temperature was allowed to raise to 0 8C, and then
the mixture was cooled again to K78 8C. This mixture was
added to a solution of 3 (1.98 g, 4.5 mmol) in THF (10 mL)
under argon at 0 8C. After 2 h at 0 8C, the solution was
allowed to raise slowly to room temperature and then water
was added. The solvent was removed under reduced
pressure. The residue was dissolved in CH2Cl2 and washed
with water. The organic layer was dried over MgSO4 (10 g),
and MnO2 (30 g, 0.35 mol) was added in three times. After
30 min, the suspension was filtered over Celite and washed

2070
M. Holler et al. / Tetrahedron 62 (2006) 2060–2073
with CH2Cl2 (1 L). The solvent was removed and the residue
purified by column chromatography (SiO2, CH2Cl2/MeOH
99.8 0.2). Compound 4 (0.84 g, 0.73 mmol) was obtained as
a yellow solid in a 31% yield. 1H NMR (CDCl3, 300 MHz):
d 0.91 (t, JZ7 Hz, 6H), 1.27 (s, 57H), 1.87 (m, 4H), 4.03 (t,
JZ7 Hz, 4H), 5.25 (s, 2H), 7.02 (d, JZ9 Hz, 4H), 7.71 (AB,
JZ9 Hz, 4H), 8.02 (d, JZ8.5 Hz, 2H), 8.18 (d, JZ8.5 Hz,
2H), 8.32 (d, JZ8.5 Hz, 2H), 8.44 (d, JZ8.5 Hz, 2H), 8.49
(d, JZ9 Hz, 4H), 8.71 (broad s, 2H), 9.73 (broad s, 1H).
(s, 1H). 13C NMR (CDCl3, 75 MHz): d 14.10, 22.65, 22.67,
26.06, 26.12, 29.22, 29.24, 29.35, 29.49, 29.65, 29.69,
31.79, 31.91, 48.96, 65.57, 66.81, 67.01, 67.31, 68.03,
68.08, 68.61, 70.52, 101.56, 107.03, 112.59, 114.58, 116.07,
119.50, 119.85, 125.49, 126.27, 127.05, 127.44, 128.16,
128.44, 128.99, 129.23, 131.61, 134.37, 135.71, 136.04,
136.22, 136.49, 136.62, 136.83, 136.99, 137.73, 138.28,
139.97, 140.13, 141.00, 141.07, 142.21, 142.68, 143.10,
143.52, 143.66, 143.89, 144.10, 144.25, 144.52, 144.88,
144.92, 145.10, 145.28, 145.53, 145.64, 145.69, 145.42,
145.80, 146.00, 147.26, 147.41, 148.57, 155.48, 156.42,
157.80, 160.33, 160.55, 162.44, 162.51, 168.54.
4.1.3. Cu2(4)2 complex. Copper(I) salt Cu(CH3CN)4BF4
(44 mg, 0.14 mmol) and 4 (160 mg, 0.14 mmol) were
dissolved in CH2Cl2 (20 mL). After 2 h the solvent was
removed under reduced pressure and the residue purified by
column chromatography (SiO2, CH2Cl2/MeOH 99:1 in
presence of NaBF4 and then Al2O3, CH2Cl2/MeOH 99:1).
The Cu2(4)2 complex (40 mg, 0.015 mmol) was obtained as
a glassy red solid in a 22% yield. 1H NMR (CDCl3,
300 MHz): d 0.87 (t, JZ7 Hz, 12H), 1.32 (m, 114H), 1. 55
(m, 8H), 3.20 (m, 4H), 3.34 (m, 4H), 4.77 (AB, JZ14 Hz,
4H), 5.69 (d, JZ8.5 Hz, 8H), 6.50 (d, JZ8.5 Hz, 4H), 6.86
(broad s, 4H), 7.01 (d, JZ8.5 Hz, 8H), 7.57 (d, JZ8.5 Hz,
4H), 7.95 (d, JZ8.5 Hz, 4H), 8.05 (d, JZ8.5 Hz, 4H), 8.07
(d, JZ8.5 Hz, 4H), 8.32 (d, JZ8 Hz, 4H), 9.53 (broad s,
2H). Elemental analysis calculated for C152H192O6N8Si2-
Cu2B2F8 C 70.65%, H 7.49%, N 4.34%, found C 70.84%, H
7.70%, N 4.01%.
4.1.6. Compound Cu2(L1)2. Copper(I) salt Cu(CH3CN)4BF4
(13 mg, 0.036 mmol) and compound L1 (100 mg,
0.036 mmol) were dissolved in a mixture of CH2Cl2/
acetonitrile (10 mL). After 3 h the solvents were removed
under reduced pressure and the residue dissolved in CH2Cl2.
The organic layer was washed with water and dried over
MgSO4. After removal of the solvent under reduced
pressure the residue was purified by gel permeation
chromatography (Biorad, Biobeads SX-1, CH2Cl2).
Cu2(L1)2 was obtained (79 mg, 0.014 mmol) as a red
glassy product in a 75% yield. UV–vis (CH2Cl2): 437 (850),
313 (10,150), 258 (25,600). IR (KBr): 1748 (C]O).
1H NMR (CDCl3, 400 MHz): d 0.90 (m, 36H), 1.31 (m,
152H), 1.57 (m, 8H), 1.72 (m, 16H), 3.29 (m, 4H), 3.43 (m,
4H), 3.85 (m, 16H), 5.02 (AB, JZ12.5 Hz, 4H), 5.27 (AB,
JZ9 Hz, 4H), 5.23 (d, JZ10 Hz, 2H), 5.24 (d, JZ10 Hz,
2H), 5.32 (AB, JZ9 Hz, 4H), 5.33 (AB, JZ9 Hz, 4H), 5.75
(d, JZ8.5 Hz, 8H), 5.76 (d, JZ13.5 Hz, 2H), 5.88 (d,
JZ13.5 Hz, 2H), 6.36 (m, 4H), 6.48 (m, 8H), 6.73 (d,
JZ8.5 Hz, 4H), 7.00 (s, 4H), 7.08 (d, JZ8.5 Hz, 8H), 7.09
(s, 4H), 7.11 (s, 2H), 7.63 (d, JZ9 Hz, 4H), 7.77 (d,
JZ8.5 Hz, 4H), 7.83 (d, JZ8.5 Hz, 4H), 7.93 (d,
JZ8.5 Hz, 4H), 8.32 (d, JZ9 Hz, 4H), 9.61 (s, 2H).
13C NMR (CDCl3, 75 MHz): d 14.12, 14.15, 17.32, 21.83,
22.68, 22.71, 26.05, 26.10, 29.00, 29.26, 29.40, 29.62, 29.71,
31.83, 31.95, 49.06, 66.84, 67.84, 68.08, 68.78, 70.61,
101.66, 107.18, 112.62, 121.57, 124.42, 125.67, 126.70,
126.93, 127.36, 127.63, 129.02, 130.24, 135.74, 136.04,
136.52, 137.16, 137.63, 138.88, 139.88, 141.08, 142.25,
142.49, 142.73, 143.17, 143.59, 144.15, 144.54, 144.95,
145.23, 145.56, 145.70, 145.92, 147.47, 148.68, 151.05,
152.49, 156.59, 157.98, 159.80, 160.37, 162.53, 168.70.
FAB-MS: 5734.0 ([MKBF4]C), 2824.7 ([MK2 BF4]2C).
4.1.4. Compound 5. A 1 M TBAF solution (0.7 mL,
0.7 mmol) in THF was added to a solution of 4 (0.675 g,
0.6 mmol) in THF (20 mL) at 0 8C. After 30 min at the same
temperature, an aqueous solution of NH4Cl (30 mL) was
added. The THF was removed under reduced pressure and
the aqueous layer extracted with CH2Cl2. The organic layer
was dried over MgSO4, filtered and the solvent removed
under reduced pressure. The residue was purified by column
chromatography (SiO2, CH2Cl2/MeOH 98:2) and afforded
compound 5 (0.583 g, 0.6 mmol) as a yellow solid in an
88% yield. 1H NMR (CDCl3, 300 MHz): d 0.91 (t, JZ7 Hz,
6H), 1.37 (m, 36H), 1. 83 (m, 4H), 3.96 (t, JZ7 Hz, 4H),
4.84 (s, 2H), 7.01 (d, JZ9 Hz, 4H), 7.56 (AB, JZ9 Hz, 4H),
7.98 (d, JZ8.5 Hz, 2H), 8.02 (d, JZ8.5 Hz, 2H), 8.12
(d, JZ8.5 Hz, 2H), 8.23 (d, JZ8.5 Hz, 2H), 8.41 (d,
JZ9 Hz, 4H), 8.45 (d, JZ1.5 Hz, 2H), 9.44 (broad s, 1H).
4.1.5. Compound L1. DCC (32 mg, 0.16 mmol) and
DMAP (8 mg, 0.07 mmol) were added to a solution of
alcohol 5 (150 mg, 0.15 mmol) and acid 7 (282 mg,
0.16 mmol) in CH2Cl2 stabilized in amylene (50 mL) at
0 8C under argon. After 1 h, the cooling bath was removed
and a catalytic amount of HOBt was added. The mixture
was allowed to slowly warm to room temperature. After
40 h the mixture was filtered and the solvent removed under
reduced. Purification by column chromatography (SiO2,
CH2Cl2/MeOH 99.3:0.7) afforded L1 (100 mg, 0.04 mmol)
as a red glassy product in a 24% yield. IR (KBr): 1750
(C]O). 1H NMR (CDCl3, 300 MHz): d 0.89 (m, 18H), 1.27
(m, 76H), 1.67 (m, 8H), 1.83 (m, 4H), 3.82 (t, JZ6.5 Hz,
8H), 4.03 (t, JZ6.5 Hz, 4H), 4.83 (s, 2H), 4.93 (d,
JZ13.5 Hz, 2H), 5.25 (s, 4H), 5.63 (d, JZ13.5 Hz, 2H),
5.64 (s, 2H), 6.33 (t, JZ2 Hz, 2H), 6.43 (d, JZ2 Hz, 4H),
6.78 (s, 2H), 7.02 (s, 4H), 7.04 (s, 1H), 7.81 (s, 4H), 8.18
(AB, JZ8.5 Hz, 4H), 8.43 (m, 8H), 8.70 (s, 2H), 9.73
4.1.7. Compound 9. DCC (50 mg, 0.24 mmol) and DMAP
(10 mg, 0.08 mmol) were added to a solution of alcohol 5
(200 mg, 0.20 mmol) and malonic acid 8 (137 mg,
0.24 mmol) in CH2Cl2 stabilized with amylene (40 mL) at
0 8C under argon. After 1 h, the cooling bath was removed and
a catalytic amount of HOBt was added. The mixture was
allowed to slowly warm to room temperature. After 40 h the
mixture was filtered and the solvent removed under reduced
pressure. Column chromatography (SiO2, CH2Cl2/MeOH
99.9:0.1) afforded compound 9 (300 mg, 0.20 mol) partially
purified as a glassy brown solid. 1H NMR (CDCl3, 300 MHz):
d 0.89(m, 12H), 1.29(m, 72H), 1.86(m, 8H), 3.84(t, JZ7 Hz,
4H), 3.92 (t, JZ7 Hz, 4H), 4.60 (s, 2H), 5.13 (s, 2H), 5.60 (s,
2H), 6.36 (s, 1H), 6.47 (d, JZ2 Hz, 2H), 7.06 (d, JZ9 Hz,
4H), 7.79 (s, 4H), 8.10 (d, JZ8.5 Hz, 2H), 8.27 (d, JZ8.5 Hz,

M. Holler et al. / Tetrahedron 62 (2006) 2060–2073
2071
2H), 8.37 (d, JZ8.5 Hz, 2H), 8.45 (d, JZ8.5 Hz, 2H), 8.49
(d, JZ9 Hz, 4H), 8.70 (s, 2H), 9.76 (s, 1H).
ethanol (10 mL, 170 mmol) and malonic acid 8 (1.78 g,
3.17 mmol) in CH2Cl2 stabilized with amylene (40 mL) at
0 8C under argon. After 1 h, the cooling bath was removed
and the mixture allowed to slowly warm to room
temperature. After 40 h the mixture was filtered and the
solvent removed under reduced pressure. Purification by
column chromatography (SiO2, CH2Cl2/MeOH 99.5:0.5)
afforded compound 10 (1.36 g, 2.31 mmol) as a colorless oil
4.1.8. Compound L2. DBU (0.25 mL, 1.63 mmol) was
added to a solution of C60 (146 mg, 0.20 mmol), compound
9 (311 mg, 0.20 mmol) and iodine (155 mg, 0.61 mmol) in
toluene (150 mL) under argon. After 20 h the mixture was
filtered on SiO2 (CH2Cl2 then CH2Cl2/MeOH 95:5) and the
solvent was removed under reduced pressure. Purification
by column chromatography (SiO2, CH2Cl2/MeOH 99.8:0.2)
afforded L2 (100 mg, 0.04 mmol) as a red glassy product in
a 22% yield. UV–vis (CH2Cl2): 478 (1800), 426 (2900), 323
(86,600), 260 (170,700). IR (KBr): 1746 (C]O). 1H NMR
(CDCl3, 300 MHz): d 0.89 (m, 12H), 1.26 (m, 68H), 1.53
(m, 4H), 1.65 (m, 4H), 1.86 (m, 4H), 3.75 (t, JZ6.5 Hz,
4H), 4.07 (t, JZ6.5 Hz, 4H), 5.48 (s, 2H), 5.96 (s, 2H), 6.30
(broad s, 1H), 6.45 (d, JZ2 Hz, 2H), 7.12 (d, JZ9 Hz, 4H),
7.79 (s, 4H), 8.17 (AB, JZ8 Hz, 4H), 8.40 (AB, JZ8 Hz,
4H), 8.51 (d, JZ9 Hz, 4H), 8.86 (s, 2H), 9.71 (s, 1H).
13C NMR (CDCl3, 75 MHz): d 14.11, 22.67, 22.69, 26.08,
26.15, 29.25, 29.34, 29.37, 29.43, 29.53, 29.63, 29.66,
29.71, 31.90, 31.91, 52.00, 67.99, 68.12, 68.78, 71.44,
101.60, 106.55, 114.70, 119.33, 119.72, 125.37, 126.29,
126.83, 127.42, 128.17, 128.92, 129.08, 131.82, 135.95,
136.62, 136.73, 136.82, 138.83, 138.99, 140.11, 140.49,
140.59, 141.67, 141.98, 142.20, 142.65, 142.72, 143.40,
143.62, 143.96, 144.26, 144.43, 144.65, 144.82, 144.87,
144.94, 145.00, 145.03, 146.01, 155.35, 156.50, 160.33,
160.57, 163.40, 163.54.
1
in a 73% yield. H NMR (CDCl3, 200 MHz): d 0.89 (t,
JZ7 Hz, 6.5H), 1.28 (m, 39H), 1.76 (m, 4H), 3.41 (s, 2H),
3.91 (t, JZ6 Hz, 4H), 4.19 (q, JZ7 Hz, 2H), 5.09 (s, 2H),
6.40 (broad s, 1H), 6.47 (broad s, 2H).
4.1.11. Compound 11. DBU (0.26 mL, 1.66 mmol) was
added to a solution of C60 (300 mg, 0.42 mmol), compound
10 (246 mg, 0.42 mmol) and iodine (317 mg, 1.25 mmol) in
toluene (300 mL) under argon. After 20 h the mixture was
filtered on SiO2 (CH2Cl2 then CH2Cl2/MeOH 95:5) and the
solvent was removed under reduced pressure. Purification
by column chromatography (SiO2, CH2Cl2) afforded 11
(173 mg, 1.3 mmol) as a red glassy product in a 31% yield.
1H NMR (CDCl3, 300 MHz): d 0.90 (t, JZ7 Hz, 6H), 1.28
(m, 39H), 1.74 (m, 4H), 3.92 (t, JZ6.5 Hz, 4H), 4.54 (q,
JZ7 Hz, 2H), 5.45 (s, 2H), 6.43 (t, JZ2 Hz, 1H), 6.62 (d,
JZ2 Hz, 2H). 13C NMR (CDCl3, 75 MHz): d 14.13, 22.70,
26.13, 26.90, 29.29, 29.37, 29.45, 29.63, 29.66, 29.71,
30.19, 31.93, 43.47, 52.08, 63.51, 68.17, 68.90, 71.57,
101.70, 107.26, 136.61, 138.61, 139.47, 140.88, 140.93,
141.88, 141.94, 142.19, 142.21, 142.94, 143.01, 143.84,
143.89, 144.48, 144.56, 144.61, 144.69, 144.88, 145.04,
145.07, 145.16, 145.17, 145.22, 145.25, 160.54, 163.47.
4.1.9. Compound Cu2(L2)2. Copper(I) salt Cu(CH3CN)4BF4
(15 mg, 0.048 mmol) and compound L2 (90 mg, 0.04 mmol)
were dissolved in a mixture of CH2Cl2/acetonitrile (20 mL).
After 1 h the solvents were removed under reduced pressure
and the residue dissolved in CH2Cl2. The organic layer was
washed with water and dried over MgSO4. After removal of
the solvent under reduced pressure the residue was purified
by gel permeation chromatography (Biorad, Biobeads SX-1,
CH2Cl2). Cu2(L2)2 was obtained (50 mg, 0.01 mmol) as a
red glassy product in a 52% yield. IR (KBr): 1747 (C]O).
1H NMR (CDCl3, 300 MHz): d 0.89 (m, 24H), 1.27
(m, 144H), 1.52 (m, 8H), 1.73 (m, 8H), 3.23 (m, 4H),
3.37 (m, 4H), 3.89 (t, JZ6.5 Hz, 8H), 5.65 (AB, JZ12 Hz,
4H), 5.67 (AB, JZ12 Hz, 4H), 5.78 (d, JZ9 Hz, 8H), 6.39
(broad s, 2H), 6.48 (d, JZ8.5 Hz, 4H), 6.70 (d, JZ2 Hz,
4H), 7.10 (broad s, 4H), 7.20 (d, JZ9 Hz, 8H), 7.67 (d,
JZ8.5 Hz, 4H), 7.74 (d, JZ8.5, 4H), 7.78 (d, JZ8.5 Hz,
4H), 7.91 (d, JZ8.5 Hz, 4H), 8.50 (d, JZ8.5 Hz, 4H),
9.62 (s, 2H). 13C NMR (CDCl3, 75 MHz): d 14.13, 14.16,
22.69, 22.73, 26.07, 26.16, 29.05, 29.30, 29.37, 29.41,
29.47, 29.65, 29.70, 29.74, 31.92, 31.96, 52.64, 67.82,
68.20, 69.64, 71.47, 71.68, 101.98, 107.76, 112.58, 121.30,
124.66, 125.50, 126.83, 127.45, 127.63, 129.29, 130.18,
135.33, 136.01, 136.54, 136.92, 137.64, 137.82, 138.26,
139.86, 140.36, 140.80, 141.12, 141.75, 141.85, 142.08,
142.22, 142.29, 142.39, 143.08, 143.13, 143.24, 143.77,
143.85, 143.97, 144.08, 144.40, 144.49, 144.69, 144.84,
144.92, 145.08, 145.28, 145.37, 145.48, 152.38, 156.58,
159.90, 160.54, 163.56, 163.83. MALDI-TOF MS: 4711.5
([MKBF4]C).
4.2. Electrochemistry
The electrochemical studies were carried out in CH2Cl2
(Fluka-spectroscopic grade used without further purification)
containing 0.1 M Bu4NPF6 (Merck-electrochemical grade)
as supporting electrolyte. A classical three electrode cell
was connected to a computerized electrochemical device
Autolab (Eco Chemie B.V. Utrecht, Holland) driven by a
GPSE software running on a personal computer. The
working electrode was a glassy carbon disk (3 mm in
diameter), the auxiliary electrode a platinum wire and the
reference electrode an aqueous Ag/AgCl reference
electrode. All potentials are given versus Fc/FcC used as
internal standard.
4.3. Photophysical measurements
Absorption spectra were recorded with a Perkin-Elmer l40
spectrophotometer. 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 for singlet oxygen luminescence
spectra). Corrected spectra were obtained via a calibration
curve supplied with the instrument. Emission lifetimes were
determined with the time correlated single photon counting
technique using the Edinburgh FLS920 spectrometer
equipped with a laser diode head as excitation source
(1 MHz repetition rate, lexcZ407 or 637 nm, 200 ps time
4.1.10. Compound 10. DCC (0.97 g, 4.75 mmol) and
DMAP (0.193 g, 1.58 mmol) were added to a solution of

2072
M. Holler et al. / Tetrahedron 62 (2006) 2060–2073
2. Marquis-Rigault, A.; Dupont-Gervais, A.; Van Dorsselaer, A.;
Lehn, J.-M. Chem. Eur. J. 1996, 2, 1395–1398.
resolution upon deconvolution) and the above mentioned
Hamamatsu R928 PMT as detector. Nanosecond transient
absorption spectroscopy was carried out with an apparatus
described earlier.46
3. Fujita, M. Chem. Soc. Rev. 1998, 27, 417–425.
4. Fujita, M. Acc. Chem. Res. 1999, 32, 53–61.
5. Fujita, M.; Tominaga, M.; Hori, A.; Therrien, B. Acc. Chem.
Res. 2005, 38, 369–378.
4.4. Spectral analysis
6. Stang, P. J.; Olenyuk, B. Acc. Chem. Res. 1997, 30, 502–518.
7. Stang, P. J. Chem. Eur. J. 1998, 4, 19–27.
The vibronic band profiles of the corrected luminescence
spectra, I(E), on an energy scale (E, cmK1) were fitted by
using Eq. 3, allowing a two-mode analysis of the Frank–
Condon envelop.54,55
8. Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. Rev. 2000, 100,
853–907.
9. Seidel, S. R.; Stang, P. J. Acc. Chem. Res. 2002, 35, 972–983.
10. Schmittel, M.; Kalsani, V. Top. Curr. Chem. 2005, 245, 1–53.
11. Constable, E. C. Tetrahedron 1992, 48, 10013.
12. Piguet, C.; Bernardinelli, G.; Hopfgartner, G. Chem. Rev.
1997, 97, 2005–2062.
ꢄ
ꢅ
3
E0 KmZum KlZul Smm Sll
X X
IðEÞ Z
E0
m! l!
m
l
13. Albrecht, M. Chem. Rev. 2001, 101, 3457–3497.
14. Lehn, J.-M.; Rigault, A.; Siegel, J.; Harrowfield, J.; Chevrier,
B.; Moras, D. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 2565.
15. Dietrich-Buchecker, C. O.; Sauvage, J. P. Angew. Chem., Int.
Ed. 1989, 28, 189.
ꢂ
ꢄ
ꢅ ꢃ
2
EKE0 CmZum ClZul
Dyꢀ1=2
!exp K4ðln 2Þ
(3)
with and SjZlj/Zuj (jZm and l)
16. Dietrich-Buchecker, C. O.; Nierengarten, J. F.; Sauvage, J. P.;
Armaroli, N.; Balzani, V.; De Cola, L. J. Am. Chem. Soc. 1993,
115, 11237–11244.
In this equation, E0 is the energy of the 0–0 transition (the
energy gap between the 0–0 vibrational levels in the excited
and ground states), m and l are vibrational quantum numbers
for high-frequency and low-frequency modes, Zum and Zul
(in practice upper limits m and lZ5 are employed), Dyꢀ1=2 is
the width at half maximum of the vibronic band, l and S are
the reorganization energy and the displacement parameter
along those modes, and kB is the Boltzmann constant. High
values for Sm (typically, in the range 0.7–1 and more)57
indicate that the excited state is significantly distorted along
the concerned vibrational mode because of electronic
localization effects. On the contrary, when the exited state
undergoes extended electronic delocalization, low Sm values
are obtained (typically, in the range 0.2–0.6)58 indicating
that the electronic curve for the excited level is not much
displaced relative to that for the ground state.
17. Dietrich-Buchecker, C. O.; Sauvage, J. P.; Armaroli, N.;
Ceroni, P.; Balzani, V. Angew. Chem. Int., Ed. 1996, 35,
1119–1121.
18. Nierengarten, J. F.; Dietrich-Buchecker, C. O.; Sauvage, J. P.
J. Am. Chem. Soc. 1994, 116, 375–376.
19. El-ghayoury, A.; Douce, L.; Skoulios, A.; Ziessel, R. Angew.
Chem., Int. Ed. 1998, 37, 2205–2208.
20. Cardinali, F.; Mamlouk, H.; Rio, Y.; Armaroli, N.;
Nierengarten, J. F. Chem. Commun. 2004, 1582–1583.
21. Dietrich-Buchecker, C. O.; Sauvage, J. P.; De Cian, A.;
Fischer, J. Chem. Commun. 1994, 2231.
22. Armaroli, N.; Accorsi, G.; Gisselbrecht, J. P.; Gross, M.;
Eckert, J. F.; Nierengarten, J. F. New J. Chem. 2003, 27,
1470–1478.
23. Dietrich-Buchecker, C. O.; Hwang, N. G.; Sauvage, J. P. New
J. Chem. 1999, 23, 911–914.
24. Felder, D.; Nava, M. G.; Carreon, M. D.; Eckert, J. F.;
Luccisano, M.; Schall, C.; Masson, P.; Gallani, J. L.; Heinrich,
B.; Guillon, D.; Nierengarten, J. F. Helv. Chim. Acta 2002, 85,
288–319.
Acknowledgements
This work was supported by the CNRS, the French Ministry
of Research (ACI Jeunes Chercheurs to J.-F.N.) the CNR
(commessa PM-P03-ISTM-C4/PM-P03-ISOF-M5, Compo-
nenti Molecolari e Supramolecolari o Macromolecolari con
`
Proprieta Fotoniche ed Optoelettroniche) and the EU for the
RTN contract no HPRN-CT-2002-00171 (FAMOUS). F.C.
thanks the Region Alsace and the CNRS for a PhD
fellowship. We further thank M. Schmitt for high field
NMR spectra and J.-M. Strub for MS measurements.
25. Bingel, C. Chem. Ber. 1993, 126, 1957.
26. Zhang, S.; Rio, Y.; Cardinali, F.; Bourgogne, C.; Gallani, J. L.;
Nierengarten, J. F. J. Org. Chem. 2003, 68, 9787–9797.
27. El-Ghayouy, A.; Harriman, A.; De Cian, A.; Fischer, J.;
Ziessel, R. J. Am. Chem. Soc. 1998, 120, 9973–9974.
28. Gisselbrecht, J. P.; Gross, M.; Lehn, J. M.; Sauvage, J. P.;
Ziessel, R.; Piccini Leopardi, C.; Anieta, J. M.; Germain, G.;
Van Meersche, M. Nouv. J. Chimie 1984, 8, 661–667.
29. Armaroli, N.; Boudon, C.; Felder, D.; Gisselbrecht, J. P.;
Gross, M.; Marconi, G.; Nicoud, J. F.; Nierengarten, J. F.;
Vicinelli, V. Angew. Chem., Int. Ed. 1999, 38, 3730–3733.
30. Felder, D.; Nierengarten, H.; Gisselbrecht, J. P.; Boudon, C.;
Leize, E.; Nicoud, J. F.; Gross, M.; Van Dorsselaer, A.;
Nierengarten, J. F. New J. Chem. 2000, 24, 687–695.
31. Bensasson, R. V.; Bienvenue, E.; Fabre, C.; Janot, J. M.; Land,
E. J.; Leach, S.; Leboulaire, V.; Rassat, A.; Roux, S.; Seta, P.
Chem. Eur. J. 1998, 4, 270–278.
´
References and notes
1. Lehn, J. M. Supramolecular chemistry: concepts and
perspectives: a personal account built upon the George Fisher
Baker non-resident lectureship in chemistry at Cornell
University and the Lezione Lincee, Accademia nazionaleei
i.e. nazionale dei Lincei, Roma; VCH: Weinheim, New York,
1995.
32. Armaroli, N.; Diederich, F.; Dietrich-Buchecker, C. O.;
Flamigni, L.; Marconi, G.; Nierengarten, J. F.; Sauvage, J. P.
Chem. Eur. J. 1998, 4, 406–416.

M. Holler et al. / Tetrahedron 62 (2006) 2060–2073
2073
33. Armaroli, N. Chem. Soc. Rev. 2001, 30, 113–124.
34. Scaltrito, D. V.; Thompson, D. W.; O’Callaghan, J. A.; Meyer,
G. J. Coord. Chem. Rev. 2000, 208, 243–266.
35. Miller, M. T.; Gantzel, P. K.; Karpishin, T. B. Inorg. Chem.
1999, 38, 3414–3422.
46. Rio, Y.; Enderlin, G.; Bourgogne, C.; Nierengarten, J. F.;
Gisselbrecht, J. P.; Gross, M.; Accorsi, G.; Armaroli, N. Inorg.
Chem. 2003, 42, 8783–8793.
47. Armaroli, N.; Accorsi, G.; Felder, D.; Nierengarten, J. F.
Chem. Eur. J. 2002, 8, 2314–2323.
36. McMillin, D. R.; McNett, K. M. Chem. Rev. 1998, 98,
1201–1219.
48. Balzani, V.; Scandola, F. Supramolecular Photochemistry;
Ellis Horwood: Chichester, UK, 1991.
37. Gushurst, A. K. I.; McMillin, D. R.; Dietrich-Buchecker,
C. O.; Sauvage, J. P. Inorg. Chem. 1989, 28,
4070–4072.
49. Armaroli, N.; Accorsi, G.; Song, F. Y.; Palkar, A.; Echegoyen,
L.; Bonifazi, D.; Diederich, F. ChemPhysChem 2005, 6,
732–743.
38. Armaroli, N.; De Cola, L.; Balzani, V.; Sauvage, J. P.;
Dietrich-Buchecker, C. O.; Kern, J. M. J. Chem. Soc., Faraday
Trans. 1992, 88, 553–556.
50. Gordon, C. K.; McGarvey, J. J. Inorg. Chem. 1991, 30,
2986–2989.
51. Lukas, A. S.; Zhao, Y. Y.; Miller, S. E.; Wasielewski, M. R.
J. Phys. Chem. B 2002, 106, 1299–1306.
39. Armaroli, N.; Balzani, V.; De Cola, L.; Hemmert, C.; Sauvage,
J. P. New J. Chem. 1994, 18, 775–782.
52. Flamigni, L.; Marconi, G.; Dixon, I. M.; Collin, J. P.; Sauvage,
J. P. J. Phys. Chem. B 2002, 106, 6663–6671.
53. Eckert, J. F.; Nicoud, J. F.; Nierengarten, J. F.; Liu, S. G.;
Echegoyen, L.; Barigelletti, F.; Armaroli, N.; Ouali, L.;
Krasnikov, V.; Hadziioannou, G. J. Am. Chem. Soc. 2000,
122, 7467–7479.
40. Armaroli, N.; Ceroni, P.; Balzani, V.; Kern, J. M.; Sauvage,
J. P.; Weidmann, J. L. J. Chem. Soc., Faraday Trans. 1997, 93,
4145–4150.
41. Chen, L. X.; Shaw, G. B.; Novozhilova, I.; Liu, T.; Jennings,
G.; Attenkofer, K.; Meyer, G. J.; Coppens, P. J. Am. Chem.
Soc. 2003, 125, 7022–7034.
54. Barqawi, K. R.; Murtaza, Z.; Meyer, T. J. J. Phys. Chem. 1991,
95, 47–50.
42. Chen, L. X.; Jennings, G.; Liu, T.; Gosztola, D. J.; Hessler,
J. P.; Scaltrito, D. V.; Meyer, G. J. J. Am. Chem. Soc. 2002,
124, 10861–10867.
55. Claude, J. P.; Meyer, T. J. J. Phys. Chem. 1995, 99, 51–54.
56. Peeters, E.; van Hal, P. A.; Knol, J.; Brabec, C. J.; Sariciftci,
N. S.; Hummelen, J. C.; Janssen, R. A. J. J. Phys. Chem. B
2000, 104, 10174–10190.
43. Gunaratne, T.; Rodgers, M. A. J.; Felder, D.; Nierengarten,
J. F.; Accorsi, G.; Armaroli, N. Chem. Commun. 2003,
3010–3011.
57. Goze, C.; Chambron, J. C.; Heitz, V.; Pomeranc, D.; Salom-
Roig, X. J.; Sauvage, J. P.; Morales, A. F.; Barigelletti, F. Eur.
J. Inorg. Chem. 2003, 3752–3758.
¨
58. Hammarstrom, L.; Barigelletti, F.; Flamigni, L.; Armaroli, N.;
Sour, A.; Collin, J. P.; Sauvage, J. P. J. Am. Chem. Soc. 1996,
118, 11972–11973.
44. Kirchhoff, J. R.; Gamache, R. E.; Blaskie, M. W.; Del Paggio,
A. A.; Lengel, R. K.; McMillin, D. R. Inorg. Chem. 1983, 22,
2380–2384.
45. Felder, D.; Nierengarten, J. F.; Barigelletti, F.; Ventura, B.;
Armaroli, N. J. Am. Chem. Soc. 2001, 123, 6291–6299.