An amphiphilic C60 derivative with a tris(2,2′-bipyridine) ruthenium(II) polar head group: Synthesis and incorporation in Langmuir films

Article abstract of DOI:10.1016/j.tetlet.2005.03.035

An amphiphilic C₆₀ derivative with a tris(2,2′-bipyridine) ruthenium(II) polar head group has been prepared. The Langmuir film of this compound has been characterized by its surface pressure versus molecular area (Π/A) isotherm and Brewster angle microscopy (BAM) observations.

Full text of DOI:10.1016/j.tetlet.2005.03.035

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 2969–2972
An amphiphilic C60 derivative with a
tris(2,20-bipyridine)ruthenium(II) polar head group:
synthesis and incorporation in Langmuir films
Franc¸ois Cardinali,a Jean-Louis Gallani,b Stefano Schergna,c
Michele Magginic,* and Jean-Franc¸ois Nierengartena,*
a
` ` ´ ´ ` ´
Groupe de Chimie des Fullerenes et des Systemes Conjugues, Ecole Europeenne de Chimie, Polymere et Materiaux,
´
Universite Louis Pasteur et CNRS, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France
b
´ ´
Groupe des Materiaux Organiques, Institut de Physique et Chimie des Materiaux de Strasbourg, Universite Louis Pasteur et CNRS,
´
23 rue du Loess, B.P. 43, 67034 Strasbourg Cedex, France
cDipartimento di Scienze Chimiche, Universita’ di Padova, Via Marzolo 1, 35131 Padova, Italy
Received 1 February 2005; revised 25 February 2005; accepted 7 March 2005
Abstract—An amphiphilic C60 derivative with a tris(2,20-bipyridine)ruthenium(II) polar head group has been prepared. The Lang-
muir film of this compound has been characterized by its surface pressure versus molecular area (P/A) isotherm and Brewster angle
microscopy (BAM) observations.
Ó 2005 Elsevier Ltd. All rights reserved.
In the last decade many covalently linked fullerene-
based photoactive molecular devices have been pre-
pared.1 They have proven to be of great importance to
gain insight into the dynamics and mechanism of intra-
molecular photoinduced energy and electron transfer
processes.1 Hybrid systems combining the fullerene
sphere with coordination compounds of d6 (RuII,
ReI),2 or d10 (CuI)3 metal ions are of particular interest
since the long lived metal-to-ligand charge transfer
(MLCT) excited state of such complexes has a marked
reducing character and is capable of starting up photo-
induced electron transfer to the fullerene subunit.2,3
Therefore such compounds appear as good candidates
for the preparation of photovoltaic devices. Indeed,
solar cells prepared from fullerene-substituted tris-
(2,20-bipyridine)ruthenium(II) derivatives have shown
promising energy conversion efficiencies.4 Since the
preparation of photovoltaic devices requires the effi-
cient incorporation of the photoactive compound into
thin films, we became interested in the synthesis of
new fullerene–ruthenium(II) conjugates allowing
controlled self-assembly on surfaces. In this letter, we
now report the preparation of amphiphilic C60 deriva-
tive 1 with a tris(2,20-bipyridine)ruthenium(II) polar
head group (Fig. 1) and its incorporation in Langmuir
films. It is worth noticing that the molecular design of
Keywords: Fullerene; Ruthenium complexes; Langmuir films.
*
Figure 1. Amphiphilic C60 derivative 1 with a tris(2,20-bipyridine)-
ruthenium(II) polar head group.
Corresponding authors. Tel.: +33 390 24 2645; fax: +33 390 24
2706; e-mail: jfnierengarten@chimie.u-strasbg.fr
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.03.035

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F. Cardinali et al. / Tetrahedron Letters 46 (2005) 2969–2972
1 is based on the structure of the amphiphilic fullerene
derivatives we have developed in recent years.5 In partic-
ular, we have demonstrated that the encapsulation of
the fullerene sphere in a cyclic addend surrounded by
at least four long alkyl chains is an efficient strategy to
prevent the irreversible aggregation resulting from the
strong fullerene–fullerene interactions usually observed
for amphiphilic C60 derivatives at the air–water inter-
face. Thus, these amphiphilic derivatives typically show
good spreading characteristics and a reversible behavior
upon successive compression/expansion cycles.6
substituted with a 1,3-phenylenebis(methylene)-tethered
bis-malonate was prepared by an esterification reaction
between diol 6 and a malonic acid monoester derivative.
The synthesis of intermediate 6 is depicted in Scheme 1.
Compounds 39 and 510 were prepared from 2 and 4,
respectively, as previously described in the literature.
Reaction of bromide 3 with phenol 5 in refluxing ace-
tone in the presence of K2CO3 afforded compound 6
in 42% yield.
Reaction of diol 6 with malonic acid monoester 75c
under esterification conditions using N,N0-dicyclohexyl-
carbodiimide (DCC) and 4-dimethylaminopyridine
(DMAP) in CH2Cl2 gave bis-malonate 8 in 91% yield
(Scheme 2). Treatment of 8 with C60, I2, and 1,8-diaza-
bicyclo[5.4.0]undec-7-ene (DBU) in toluene at room
temperature afforded the desired cyclization product 9
in 49% yield. The structure and purity of all new com-
pounds were confirmed by NMR and elemental analy-
The fullerene-substituted 2,20-bipyridine ligand used for
the preparation of the ruthenium(II) complex 1 was ob-
tained by taking advantage of the versatile regioselective
reaction developed in the group of Diederich,7 which led
to C60 bis-adducts by a cyclization reaction at the C60
sphere with bis-malonates in a double Bingel8 cyclo-
propanation. To this end, a 2,20-bipyridine derivative
1
sis.11 In particular, the H NMR spectrum of 9 shows
all the characteristic features of the Cs symmetrical
1,3-phenylenebis(methylene)-tethered fullerene cis-2
bis-adduct subunit.12 Effectively, two AB quartets are
observed for the two sets of diastereotopic benzylic
CH2 groups and an AX2 system is revealed for the aro-
matic protons of the 1,3,5-trisubstituted bridging phenyl
ring.
The Ru(II) complex 1 was then prepared from cis-di-
chloro-bis(2,20-bipyridine)ruthenium(II) [Ru(bipy)2Cl2]
and ligand 9 under standard conditions.3d In a typical
procedure,
a
mixture of [Ru(bipy)2Cl2] (61 mg,
0.12 mmol) and AgBF4 (70 mg, 0.36 mmol) in acetone
(30 mL) was refluxed for 3 h. After cooling and filtra-
tion, the solvent was removed and the mixture taken
up in DMF (30 mL). Compound 9 (250 mg, 0.12 mmol)
was then added and the resulting mixture refluxed for
3 h. After cooling, the crude product was precipitated
as its PF6 salt by addition of a methanolic solution of
NH4PF6. The brown solid was filtered, washed with
water, MeOH, and Et2O. Column chromatography
(SiO2, CH2Cl2 containing 5% MeOH) followed by
recrystallization from CH2Cl2/hexane yielded 1 (48 mg,
Scheme 1. Reagents and conditions: (i) (C6H5CO2)2O, NBS, CCl4
(30%); (ii) LiAlH4, THF, D (77%); (iii) K2CO3, acetone, D (42%).
Scheme 2. Reagents and conditions: (i) 6, DCC, DMAP, CH2Cl2, 0 °C to room temp (91%); (ii) C60, DBU, I2, toluene, room temp (49%);
(iii) [Ru(bipy)2Cl2], AgBF4, acetone, D; then filtration and evaporation; then 9, DMF, D (14%).

F. Cardinali et al. / Tetrahedron Letters 46 (2005) 2969–2972
2971
14%) as a dark red powder.13 The 1H NMR of complex
1 is consistent with the proposed structure. In addition
to the signals corresponding to the C60-substituted li-
gand, the resonances arising from the two unsubstituted
2,20-bipyridine ligands are clearly observed. The struc-
ture of 1 was also confirmed by mass spectrometry.
The MALDI-TOF-MS of 1 is characterized by a singly
charged peak at m/z 2700, which can be assigned to 1
after loss of one hexafluorophosphate counteranion.
The UV–vis spectrum of 1 in CH2Cl2 solution shows
the characteristic absorption features of the fullerene
units as well as the diagnostic MLCT band of the
tris(2,20-bipyridine)ruthenium(II) complex at 456 nm.
Interestingly, preliminary luminescence measurements
in CH2Cl2 solutions (kexc = 456 nm) show a strong
quenching of the tris(2,20-bipyridine)ruthenium(II) com-
plex emission by the fullerene moiety in 1, indicating the
occurrence of intramolecular photoinduced processes.
Detailed photophysical studies are currently under
investigation and will be reported in due time.
Figure 3. A successive compression/expansion cycle with a monolayer
of 1 showing the reversibility of the process. Top view (left) and side
view (right) of the calculated structure of 1 (molecular modeling
performed with Spartan) used to estimate the molecular area.
Langmuir films14 of compound 1 have been character-
ized by their surface pressure versus molecular area
(P/A) isotherms and Brewster angle microscopy
(BAM) observations. The P/A isotherm obtained with
1 is shown in Figure 2. The surface pressure rises around
which water can be seen. These domains smoothly weld
together when the molecular area goes below
2
˚
A ꢀ 200 A , and as long as the film does not enter the
collapse regime only defectless surfaces are observed.
This indicates clearly the formation of an homogeneous
monomolecular layer. When the molecular area reaches
2
˚
A ꢀ 210 A , and the final molecular area extrapolated at
2
A ꢀ 130 A (Pc ꢀ 30 mN mꢁ1), a change of slope ap-
˚
2
˚
zero surface pressure is A0 ꢀ 162 8 A , in good agree-
pears in the surface pressure curve indicating a greater
compressibility of the film. At the same time, defects
can be seen in the BAM pictures confirming that the film
is collapsing at this point.
ment with the value estimated by molecular modeling
(Fig. 3). These films show excellent reversibility in suc-
cessive compression–expansion cycles as long as the P
is kept below the collapse pressure Pc ꢀ 30 mN mꢁ1
(Fig. 3). The latter observation clearly indicates that
the four alkyl chains are capable of efficiently preventing
the aggregation due to fullerene–fullerene interactions in
good agreement with our previous findings on related
amphiphilic fullerene derivatives.5
In conclusion, we have shown that C60 derivative 1 with
a tris(2,20-bipyridine)ruthenium(II) polar head group is
a suitable amphiphilic derivative for the preparation of
stable Langmuir films. We are currently investigating
the physical properties of the Langmuir–Blodgett films
made from this molecule, and more specifically their
behavior in photoelectrochemical cells for solar energy
conversion.
BAM observations reveal the good quality of the films.
As shown in Figure 2, the film obtained from 1 is non-
continuous at large molecular areas, with holes through
Acknowledgements
This research was supported by a fellowship from the
´
CNRS and the Region Alsace to F.C. We further thank
L. Oswald for technical help and M. Schmitt for record-
ing the high field NMR spectra. M.M. wish to thank
MIUR (contracts 2004035502 and RBAU017S8R) for
financial support.
References and notes
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2
2
(P = 0 0.1 mN mꢁ1
)
˚
˚
and 200 A
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2972
F. Cardinali et al. / Tetrahedron Letters 46 (2005) 2969–2972
Chem. Rev. 1998, 98, 2527–2547; (f) Imahori, H.; Sakata, Y.
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11. Compound 9: 1H NMR (CDCl3, 400 MHz): d = 0.89 (t,
J = 7 Hz, 12H), 1.26 (m, 72H), 1.72 (m, 8H), 2.47 (s, 3H),
3.86 (t, J = 7 Hz, 8H), 5.05 (d, J = 13 Hz, 2H), 5.23 (s,
2H), 5.30 (AB, J = 12 Hz, 4H), 5.77 (d, J = 13 Hz, 2H),
6.37 (t, J = 2 Hz, 2H), 6.48 (d, J = 2 Hz, 4H), 6.88 (d,
J = 1 Hz, 2H), 7.15 (t, J = 1 Hz, 1H), 7.18 (dd, J = 5 and
1 Hz, 1H), 7.43 (dd, J = 5 and 1 Hz, 1H), 8.29 (d,
J = 1 Hz, 1H), 8.49 (d, J = 1 Hz, 1H), 8.57 (d, J = 5 Hz,
1H), 8.72 (d, J = 5 Hz, 1H); 13C NMR (CDCl3, 50 MHz):
d = 14.1, 21.2, 22.7, 26.1, 29.2, 29.3, 29.4, 29.6, 31.9, 48.9,
66.8, 67.2, 68.1, 68.5, 68.7, 70.5, 76.4, 101.6, 107.1, 112.7,
116.0, 118.9, 121.4, 122.0, 124.9, 134.3, 135.7, 136.1, 137.8,
138.3, 140.0, 141.0, 141.1, 142.2, 142.7, 142.8, 143.1, 143.6,
143.7, 143.9, 144.1, 144.3, 144.5, 144.9, 145.0, 145.1, 145.3,
145.5, 145.7, 146.0, 146.7, 147.3, 147.4, 148.2, 148.5, 149.0,
149.5, 155.5, 156.6, 158.3, 160.3, 162.5, 162.6; Anal. Calcd
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C 82.48, H 6.10, N 1.28.
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13. Compound 1: 1H NMR (CDCl3, 400 MHz): d = 0.89 (t,
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3.84 (t, J = 7 Hz, 8H), 5.15 (d, J = 14 Hz, 2H), 5.29 (br s,
2H), 5.31 (AB, J = 12 Hz, 2H), 5.32 (AB, J = 12 Hz, 2H),
5.69 (d, J = 14 Hz, 2H), 6.34 (br s, 2H), 6.45 (d, J = 2 Hz,
4H), 6.87 (d, J = 10 Hz, 2H), 7.17 (s, 1H), 7.24 (d,
J = 6 Hz, 1H), 7.49 (m, 6H), 7.64 (d, J = 6 Hz, 1H), 7.74
(m, 4H), 7.93 (m, 4H), 8.34 (m, 5H), 8.50 (s, 1H); 13C
NMR (CDCl3, 50 MHz): d = 14.1, 21.2, 22.6, 26.1, 29.2,
29.3, 29.4, 29.6, 31.9, 48.9, 53.4, 66.7, 67.3, 68.0, 68.8,
70.6, 101.6, 107.2, 112.5, 115.1, 121.7, 124.0, 125.4, 128.2,
129.2, 134.3, 135.6, 135.9, 136.4, 137.6, 137.8, 138.9, 139.8,
141.0, 141.1, 141.9, 142.6, 143.1, 143.4, 143.6, 144.8, 144.1,
144.5, 144.8, 144.9, 145.2, 145.5, 145.6, 145.7, 145.9, 147.3,
147.4, 148.7, 149.2, 150.8, 151.3, 155.9, 156.4, 156.5, 156.6,
157.8, 160.3, 162.5; UV/vis (CH2Cl2) kmax (e): 456
(19,600), 286 (136,000), 250 (126,00); MALDI-TOF-MS:
m/z: 2700 ([MꢁPF6]+ calcd for RuC168H144O13N6PF6:
2699.95).
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spreading in the order of 50 lL drops of a 0.005 M
solution of 1 in CHCl3 with a gas-chromatography
syringe; for a detailed description of the setup used for
the preparation and the studies of the Langmuir films, see
Ref. 5c.