Functionalization of [60]fullerene with new light-collecting oligophenylenevinylene-terminated dendritic wedges

Article abstract of DOI:10.1016/S0040-4039(01)02083-4

New oligophenylenevinylene-terminated phenylenevinylene dendrons have been prepared and attached to C₆₀ by a 1,3-dipolar cycloaddition of the azomethine ylides generated in situ from the dendritic aldehydes and N-methylglycine. Preliminary phot

Full text of DOI:10.1016/S0040-4039(01)02083-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 65–68
Functionalization of [60]fullerene with new light-collecting
oligophenylenevinylene-terminated dendritic wedges
Gianluca Accorsi,a Nicola Armaroli,a,* Jean-Franc¸ois Eckertb and Jean-Franc¸ois Nierengartenb,*
aIstituto di Fotochimica e Radiazioni d’Alta Energia del CNR, via Gobetti 101, I-40129 Bologna, Italy
bGroupe des Mate´riaux Organiques, Institut de Physique et Chimie des Mate´riaux de Strasbourg,
Universite´ Louis Pasteur et CNRS, 23 rue du Loess, F-67037 Strasbourg Cedex, France
Received 17 October 2001; revised 2 November 2001; accepted 5 November 2001
Abstract—New oligophenylenevinylene-terminated phenylenevinylene dendrons have been prepared and attached to C60 by a
1,3-dipolar cycloaddition of the azomethine ylides generated in situ from the dendritic aldehydes and N-methylglycine.
Preliminary photophysical investigations of the resulting fullerodendrimers have revealed interesting light-harvesting properties.
© 2001 Published by Elsevier Science Ltd.
The synthesis and the study of dendrimer-based light-
harvesting structures have attracted increased attention
in the past decade.1 In such systems, an array of
peripheral chromophores is able to transfer the collected
energy to the central core of the dendrimer thus mimick-
ing the natural light-harvesting complex where antenna
molecules collect sunlight and channel the absorbed
energy to a single reaction center. The fullerene C60 is an
attractive functional core for the preparation of light-
harvesting dendrimers. Effectively, its first singlet and
triplet excited-states are relatively low in energy and
photoinduced energy transfer events have been evidenced
in fullerene-based dyads.2–4 In particular, photophysical
investigations of some fulleropyrrolidine derivatives sub-
stituted with oligophenylenevinylene (OPV) moieties
revealed a very efficient singlet–singlet OPV“C60 photo-
induced energy-transfer.3,4 Based on this observation,
dendrimers with a fullerene core and peripheral OPV
subunits appear as potentially interesting systems with
light-harvesting properties. In this paper, we report the
synthesis of such fullerene derivatives and show that the
C60 core in fullerodendrimers 1–3 is able to act as a
terminal receptor of the excitation energy.
Emmons conditions followed by treatment with
CF3CO2H gave the OPV tetramer 7. The functionaliza-
tion of C60 was based on the 1,3-dipolar cycloaddition6
of the azomethine ylide generated in situ from 7. The
reaction of C60 with 7 in the presence of an excess of
N-methylglycine in refluxing toluene gave compound 1
in 46% yield. In addition, compound 8, which was used
as the reference compound for the absorption and
emission properties, was prepared by LiAlH4 reduction
of 7.
The preparation of bisphosphonate 10, the key building
block for the preparation of the OPV-terminated
phenylenevinylene dendritic wedges,7,8 is shown in
Scheme 2. It was obtained in two steps from diol 9.9
Bromination using CBr4/PPh3 followed by treatment of
the resulting bisbromide with P(OEt)3 under Arbuzov
conditions yielded 10 in an overall 66% yield. Reaction
of 10 with aldehyde 6 in the presence of t-BuOK afforded
compound 11 with 91% yield. Deprotection (CF3CO2H/
CH2Cl2/H2O) and subsequent treatment of the resulting
aldehyde with C60 in the presence of N-methylglycine
gave 2. The next generation dendron was obtained in 90%
yield by reaction of bisphosphonate 10 with the aldehyde
resulting from the deprotection of 11. Subsequent depro-
tection with CF3CO2H in CH2Cl2/H2O yielded the corre-
sponding aldehyde, which after reaction with C60 and
N-methylglycine in refluxing toluene gave 3 in 25% yield.
The preparation of compound 1 is depicted in Scheme
1. Compound 4 was obtained in two steps from p-bro-
mobenzaldehyde as previously reported.4 Reduction with
LiAlH4, bromination (CBr4/PPh3) and treatment with
P(OEt)3 afforded phosphonate 5 in an overall yield of
64%. Reaction of 5 with aldehyde 65 under Wadsworth–
1
All the new compounds have been characterized by H
and 13C NMR, and elemental analyses. In addition, the
structures of fullerodendrimers 1–3 have also been con-
firmed by mass spectrometry.10
* Corresponding author. Fax: 33 388 10 72 46; e-mail: niereng@
ipcms.u-strasbg.fr
0040-4039/02/$ - see front matter © 2001 Published by Elsevier Science Ltd.
PII: S0040-4039(01)02083-4

66
G. Accorsi et al. / Tetrahedron Letters 43 (2002) 65–68
RO
RO
RO
RO
OR
OR
Me
N
1 R = C12H25
RO
RO
OR
RO
RO
RO
Me
N
Me
N
RO
2 R = C12H25
RO
RO
3 R = C12H25
RO
RO
OR
OR
OR
RO
The electronic absorption spectra of the model com-
pounds 8, 11, and 12 in CH2Cl2 solutions are reported
in Fig. 1. Interestingly, the umax is similar for the three
compounds showing that the meta-substitution does
not extend the conjugation in 11 and 12. Indeed, com-
pound 11 can be regarded as an assembly of two OPV
tetramers sharing a phenyl ring. Similarly, the highest
generation compound can be viewed as a combination
of four OPV tetramers. The three molecules also exhibit
intense fluorescence bands with umax at ca. 468 nm (Fig.
1). The fluorescence emission quantum yields (in the
range 0.66–0.77) and excited lifetimes (1.1–1.3 ns) are
similar along the series.
O
O
O
O
PO(OEt)2
i, ii, iii
CHO
5
4
RO
RO
RO
CHO
6 R = C12H25
iv, v
RO
RO
RO
The absorption spectra of the fullerodendrimers 1–3 in
CH2Cl2 are reported in Fig. 2 and present the spectral
features of both component units. However, exact
matching with the spectral profiles calculated by sum-
ming the absorption spectra of fulleropyrrolidine and
the pertinent OPV fragment is never obtained, in line
with previous reports.4 This is particularly evident for 3
and might suggest substantial intramolecular ground
state interaction between the fullerene core and the
large aromatic network of 3.11 Upon excitation at the
OPV band maximum, dramatic quenching of OPV
fluorescence is observed for all fullerodendrimers. This
is attributable to an OPV“C60 singlet–singlet energy
transfer process,3,4 as inferred by excitation spectra
X
7 X = CHO, R = C12H25
vi
vii
8 X = CH2OH, R = C12H25
Scheme 1. Reagents and conditions: (i) LiAlH4, THF, 0°C
(90%); (ii) PPh3, CBr4, THF, rt (84%); (iii) P(OEt)3, 150°C
(85%); (iv) 5, t-BuOK, THF, 0°C to rt (88%); (v) CF3CO2H,
CH2Cl2, H2O, rt (96%); (vi) LiAlH4, THF, 0°C (92%); (vii)
C60, N-methylglycine, toluene, D (46%).

G. Accorsi et al. / Tetrahedron Letters 43 (2002) 65–68
67
RO
OR
OR
PO(OEt)2
PO(OEt)2
OH
OH
O
O
O
O
i, ii
9
10
RO
RO
OR
RO
RO
RO
O
O
O
iv, v
iii
6
O
11 R = C12H25
12 R = C12H25
RO
RO
RO
RO
RO
iv, vi
vii, viii
OR
2
3
OR
RO
OR
Scheme 2. Reagents and conditions: (i) PPh3, CBr4, THF, rt (75%); (ii) P(OEt)3, 150°C (88%); (iii) 10, t-BuOK, THF, 0°C to rt
(91%); (iv) CF3CO2H, CH2Cl2, H2O, rt (90%); (v) 10, t-BuOK, THF, 0°C to rt (85%); (vi) C60, N-methylglycine, toluene, D (18%);
(vii) CF3CO2H, CH2Cl2, H2O, rt (90%); (viii) C60, N-methylglycine, toluene, D (25%).
Figure 1. Absorption (left) and fluorescence (right, uexc=314
nm O.D.=0.1) of compounds 8 (black), 11 (red) and 12
(blue). All the spectra have been recorded at 298 K in CH2Cl2
solutions.
Figure 2. Absorption (left) and fluorescence (right, uexc=500
nm O.D.=0.2) of 1 (black), 2 (red) and 3 (blue); the former
are multiplied by a factor of 20 above 450 nm. All the spectra
have been recorded at 298 K in CH2Cl2 solutions.
Finally, it must also be pointed out that in CH2Cl2,
selective excitation of the fullerene moiety at u>500 nm
does not evidence any fullerene fluorescence quenching
for 1. On the contrary, a progressive fluorescence yield
decrease is observed for the larger OPV-C60 hybrids (Fig.
2). This suggests the occurrence of OPV“C60 photoin-
duced electron transfer,12 to an increasing extent, along
the series. Detailed photophysical investigations are
currently in progress in order to elucidate the nature of
recorded at uem=730 nm. Simple comparison of the
molar absorptivities at the excitation wavelength (394
nm, corresponding to OPV band maxima) allows quan-
tification of the antenna effect obtained by increasing the
generation number: 95,800 for 1, 134,800 for 2 and
255,100 for 3. When compared to fulleropyrrolidine for
which the molar absorptivity is ca. 7600 at 394 nm, its
functionalization with the OPV-terminated dendrons
enables for the harvesting of increasing amounts of light.

68
G. Accorsi et al. / Tetrahedron Letters 43 (2002) 65–68
all the photoinduced processes occurring in fulleroden-
drimers 1–3.
7. For examples of phenylenevinylene dendrimers, see: (a)
Deb, S. K.; Maddux, T. M.; Yu, L. J. Am. Chem. Soc.
1997, 119, 9079–9080; (b) Meier, H.; Lehmann, M.;
Angew. Chem., Int. Ed. 1998, 37, 643–645; (c) Meier, H.;
Lehmann, M.; Kolb, U. Chem. Eur. J. 2000, 6, 2462–
2469.
8. For fullerene derivatives substituted with phenyl-
enevinylene dendrons, see: (a) Langa, F.; Gomez-
Escalonilla, M. J.; Diez-Barra, E.; Garcia-Martinez, J. C.;
de la Hoz, A.; Rodriguez-Lopez, J.; Gonzalez-Cortes, A.;
Lopez-Arza, V. Tetrahedron Lett. 2001, 42, 3435–3438;
(b) Segura, J. L.; Gomez, R.; Martin, N.; Luo, C.;
Swartz, A.; Guldi D. M. Chem. Commun. 2001, 707–708.
9. Compound 9 was prepared in four steps from 1,3,5-tri-
bromobenzene according to a previously reported proce-
dure, see: Nierengarten, J.-F.; Schall, C.; Nicoud, J.-F.
Angew. Chem., Int. Ed. 1998, 37, 1934–1937.
In conclusion, an efficient synthetic methodology for
preparation of OPV-terminated phenylenevinylene den-
dritic wedges has been developed. This enables us to
prepare building blocks for the construction of new
fullerodendrimers with light-harvesting properties. The
further functionalization of these fullerodendrimers
with a suitable electron donor is now underway to
produce a multicomponent artificial photosynthetic sys-
tem in which the photoinduced energy transfer to the
C60 core is followed by electron transfer.
Acknowledgements
10. Selected spectroscopic data for 1: 1H NMR (200 MHz,
25°C, CDCl3): 7.79 (broad d, J=8.5 Hz, 2H), 7.59 (d,
J=8.5 Hz, 2H), 7.50 (s, 8H), 7.13 (AB, J=16.5 Hz, 4H),
7.01 (AB, J=16.5 Hz, 2H), 6.72 (s, 2H), 5.01 (d, J=10
Hz, 1H), 4.95 (s, 1H), 4.28 (d, J=10 Hz, 1H), 4.03 (t,
J=6.5 Hz, 4H), 3.98 (t, J=6.5 Hz, 2H), 2.83 (s, 3H), 1.83
(m, 6H), 1.50–1.20 (m, 54H), 0.89 (t, J=6.5 Hz, 9H);
FAB-MS: 1712.6 (MH+); anal. calcd for C129H101NO3: C,
90.44; H, 5.94; N, 0.82; found C, 90.47; H, 6.13; N, 0.82.
This research was supported by the French Ministry of
Research (ACI Jeunes Chercheurs), a fellowship from
the DGA to J.-F.E. and a fellowship from the MIUR
(Legge 95/95) to G.A. Financial support from the joint
CNR-CNRS project ‘Supramolecular Fullerene Sys-
tems as Materials for Solar Energy Conversion’ is also
acknowledged. We further thank L. Oswald for techni-
cal help and M. Schmitt for recording the high field
NMR spectra.
1
For 2: H NMR (400 MHz, 25°C, CDCl3): 7.64 (s, 1H),
7.50 (m, 18H), 7.18 (s, 4H), 7.13 (s, 4H), 7.01 (AB,
J=16.5 Hz, 4H), 6.73 (s, 4H), 5.05 (d, J=10 Hz, 1H),
4.99 (s, 1H), 4.32 (d, J=10 Hz, 1H), 4.04 (t, J=6.5 Hz,
8H), 3.98 (t, J=6.5 Hz, 4H), 2.89 (s, 3H), 1.83 (m, 12H),
1.50–1.20 (m, 108H), 0.89 (t, J=6.5 Hz, 18H); FAB-MS:
2572.5 (M+); anal. calcd for C189H191NO6·H2O: C, 87.63;
H, 7.51; N, 0.54; found C, 87.69; H, 7.59; N, 0.56. For 3:
1H NMR (400 MHz, 100°C, C2D2Cl4): 8.02 (s, 2H), 7.77
(s, 1H), 7.50 (m, 38H), 7.34 (AB, J=16.5 Hz, 4H), 7.24
(AB, J=16.5 Hz, 8H), 7.18 (s, 8H), 7.03 (AB, J=16.5
Hz, 8H), 6.78 (s, 8H), 5.14 (d, J=10 Hz, 1H), 5.12 (s,
1H), 4.42 (d, J=10 Hz, 1H), 4.09 (t, J=6.5 Hz, 16H),
4.06 (t, J=6.5 Hz, 8H), 2.83 (s, 3H), 1.83 (m, 24H),
1.50–1.20 (m, 216H), 0.89 (t, J=6.5 Hz, 36H); MALDI-
TOF-MS: 4498 (M+); anal. calcd for C325H383NO12·
CHCl3: C, 84.84; H, 8.39; N, 0.30; found C, 85.09; H,
8.53; N, 0.29.
References
1. (a) Balzani, V.; Campagna, S.; Denti, G.; Juris, A.;
Serroni, S.; Venturi, M. Acc. Chem. Res. 1998, 31, 26–34;
(b) Fisher, M.; Vo¨gtle, F. Angew. Chem., Int. Ed. 1999,
38, 884–905; (c) Adronov, A.; Fre´chet, J. M. J. Chem.
Commun. 2000, 1701–1710; (d) Balzani, V.; Ceroni, P.;
Juris, A.; Venturi, M; Campagna, S.; Puntoriero, F.;
Serroni, S. Coord. Chem. Rev. 2001, 219–221, 545–572.
2. Guldi, D. M.; Torres-Garcia, G.; Mattay, J. J. Phys.
Chem. A 1998, 102, 9679–9685.
3. Armaroli, N.; Barigelletti, F.; Ceroni, P.; Eckert, J. F.;
Nicoud, J. F.; Nierengarten, J. F. Chem. Commun. 2000,
599–600.
4. 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.
11. (a) Seshadri, R.; Rao, C. N. R.; Pal, H.; Mukherjee, T.;
Mittal, J. P. Chem. Phys. Lett. 1993, 205, 395–398; (b)
Rath, M. C.; Pal, H.; Mukherjee, T. J. Phys. Chem. A
1999, 103, 4993–5002.
12. 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.
5. Aldehyde 6 was prepared as previously reported, see:
Eckert, J.-F.; Nicoud, J.-F.; Guillon, D.; Nierengarten,
J.-F. Tetrahedron Lett. 2000, 41, 6411–6414.
6. Prato, M.; Maggini, M. Acc. Chem. Res. 1998, 31, 519–
526.