diastereotopic benzylic CH2 groups and an AX2 system is revealed
for the aromatic protons of the 1,3,5-trisubstituted bridging phenyl
ring. The spectrum is also characterized by three sets of signals in
a typical pattern for a disymmetrically 2,9-disubstituted-1,10-phe-
nanthroline, an A2X system for the aromatic protons of the bridging
phenyl unit and an AAAXXA system for the aromatic protons of the
two 4-dodecyloxyphenyl rings (Fig. S1†). Upon complexation to
copper( ), dramatic changes are observed for the chemical shift of
I
the signals corresponding to the protons belonging to the bis-
phenanthroline moiety due to the stacking of the aromatic subunits
in the helicate (Fig. S2†). Importantly, the 1H-NMR also provided
clear evidence for the formation of a chiral complex, i.e. a helical
system. In particular, the two equivalent fullerene cis-2 bis-adduct
moieties have lost their plane of symmetry. For this reason, the
protons that were enantiotopic in ligand 1 are not equivalent any
longer in complex Cu2(1)2 and give rise to a more complicated set
of signals (Fig. S3†). Finally, the structure of Cu2(1)2 was
confirmed by mass spectrometry. The FAB-MS of Cu2(1)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, re-
spectively.
Fig. 2 Energy level diagram for Cu2(1)2 and photoinduced processes upon
excitation of both chromophores (Cu and F denote a metal complex and a
fullerene moiety, respectively). The excited state energies are determined as
in ref. 10.
lifetime (Fig. 1). A 50% decrease of the fullerene triplet yield is
measured, by means of the intensity of the sensitized singlet oxygen
emission band (Fig. 1).10 From these results it is possible to
conclude that (i) the metal complexed core undergoes photoinduced
quenching not attributable to energy transfer because no sensitiza-
tion 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 6. The energy level diagram in Fig. 2 reports the
relevant electronic excited levels of the two types of chromophores.
The electronic absorption spectrum of Cu2(1)2 in CH2Cl2 is
reported in Fig. 1 and well corresponds to the sum of the absorption
spectra of its component units (6 and Cu2(4)2). This is in contrast
with what recently observed for a Cu( )-bisphenanthroline complex
I
sandwiched between two fullerenes, where the absorption spectrum
reveals electronic interactions among the subunits.10 In the present
case a spacer is located between the inorganic core and the organic
units. Thus, although the system is relatively flexible, the
interchromophoric distance is kept long enough to prevent
significant electronic interactions. The absorption and lumines-
cence properties of fullerene 6 have been already reported10 and
In principle electron transfer may occur from both the Cu( ) and the
I
C60 moiety, but only the former is effective in promoting charge
separation. This is related to the CT nature of the corresponding
excited state that facilitates the electron transfer process from the
kinetic point of view.10 The localized and short-lived 1pp* level of
C
60 is comparably less effective in promoting the same process and
regular internal deactivation takes place prior to photoinduced
electron transfer.
those of Cu2(4)2 are in line with analogous Cu( ) complexes of
I
2,9-diphenylphenanthroline ligands.11 6 exhibits the characteristic
weak fluorescence band of C60 fullerenes (lmax = 770 nm, t = 1.5
ns, and Fem = 3.0 3 1024).10 In Cu2(4)2 an emission band
attributed to the deactivation of a thermally equilibrated manifold
of the lowest singlet and triplet metal-to-ligand charge-transfer
states (1MLCT and 3MLCT), is detected (lmax = 720 nm, t = 130
ns and Fem = 5.1 3 1024 in oxygen-free CH2Cl2 solution).
Selective excitation of a specific component in Cu2(1)2 is not
possible (Fig. 1). We chose to excite at 540 nm where the light
In conclusion a novel fullerohelicate architecture has been
synthesized in which two chromophores are potentially able to
trigger electron transfer. However only for the inorganic unit the
process is competitive toward intrinsic deactivation. Further
synthetic efforts are now under way to prepare optically active
fullerohelicates to obtain new systems with original chiroptical
properties.
Work supported by CNR, CNRS, EU (RTN Contract HPRN-CT-
2002-00171), French Ministry of Research (ACI Jeunes Cher-
cheurs to J-.F.N.). This paper is dedicated to Dr C. O. Dietrich-
Buchecker in recognition of her seminal work in this field and for
her support and enthusiasm in the early stage of our career.
partitioning among the Cu( ) chromophores and carbon spheres in
I
Cu2(1)2 is 1 : 1.10 Under these conditions the quenching of the
metal-complexed core is complete, and only the C60 fluorescence
band (50% intensity relative to 6) is detected with unchanged
Notes and references
1 For reviews, see: E. C. Constable, Tetrahedron, 1992, 48, 10013; C.
Piguet, G. Bernardinelli and G. Hopfgartner, Chem. Rev., 1997, 97,
2005; M. Albrecht, Chem. Rev., 2001, 101, 3457.
2 J.-M. Lehn, A. Rigault, J. Siegel, J. Harrowfield, B. Chevrier and D.
Moras, Proc. Natl. Acad. Sci. USA, 1987, 84, 2565.
3 C. O. Dietrich-Buchecker and J.-P. Sauvage, Angew. Chem., Int. Ed.
Engl., 1989, 28, 189.
4 J.-F. Nierengarten, C. O. Dietrich-Buchecker and J.-P. Sauvage, J. Am.
Chem. Soc., 1994, 116, 375.
5 A. El-Ghayouri, L. Douce, R. Ziessel and A. Skoulios, Angew. Chem.,
Int. Ed., 1998, 37, 2205.
6 N. Armaroli, Chem. Soc. Rev., 2001, 30, 113.
7 C. O. Dietrich-Buchecker, J.-P. Sauvage, A. De Cian and J. Fischer,
Chem. Commun., 1994, 2231.
8 N. Armaroli, G. Accorsi, J.-P. Gisselbrecht, M. Gross, J.-F. Eckert and
J.-F. Nierengarten, New J. Chem., 2003, 27, 1470.
9 J.-F. Nierengarten, D. Felder and J.-F. Nicoud, Tetrahedron Lett., 1999,
40, 269.
10 Y. Rio, G. Enderlin, C. Bourgogne, J.-F. Nierengarten, J.-P. Gissel-
brecht, M. Gross, G. Accorsi and N. Armaroli, Inorg. Chem., 2003, 42,
8783.
Fig. 1 Absorption spectrum of Cu2(1)2 (blue), Cu2(4)2 (red), and 6 (green).
Left-hand side inset: uncorrected luminescence spectra in CH2Cl2 including
the sensitized singlet oxygen luminescence peak at 1270 nm. CH2Cl2
solution, lexc = 540 nm, A = 0.700 for all samples. Right-hand side inset:
emission decays of Cu2(1)2 (blue) and 6 (green) at lexc = 465 nm and lem
> 700 nm; black trace: laser decay.
11 D. Felder, J.-F. Nierengarten, F. Barigelletti, B. Ventura and N.
Armaroli, J. Am. Chem. Soc., 2001, 123, 6291.
C h e m . C o m m u n . , 2 0 0 4 , 1 5 8 2 – 1 5 8 3
1583