Liquid-crystalline fullerodendrimers which display columnar phases

Article abstract of DOI:10.1021/ol0603920

The title compounds were synthesized by applying the 1,3-dipolar cycloaddition reaction of aldehyde-based poly(benzyl ether) dendrimers and sarcosine (N-methylglycine) to [60]fullerene (C₆₀). The dendritic building blocks used to functionalize

Full text of DOI:10.1021/ol0603920

ORGANIC
LETTERS
2006
Vol. 8, No. 9
1851-1854
Liquid-Crystalline Fullerodendrimers
which Display Columnar Phases
Julie Lenoble,† Natacha Maringa,† Ste´phane Campidelli,† Bertrand Donnio,‡
Daniel Guillon,*,‡ and Robert Deschenaux*,†
Institut de Chimie, UniVersite´ de Neuchaˆtel, AVenue de BelleVaux 51, Case Postale
158, 2009 Neuchaˆtel, Switzerland, and Groupe des Mate´riaux Organiques, Institut de
Physique et Chimie des Mate´riaux de Strasbourg, UMR 7504 (CNRS-ULP), 23 rue du
Loess, BP 43, 67034 Strasbourg Cedex 2, France
robert.deschenaux@unine.ch
Received February 15, 2006
ABSTRACT
The title compounds were synthesized by applying the 1,3-dipolar cycloaddition reaction of aldehyde-based poly(benzyl ether) dendrimers and
sarcosine (N-methylglycine) to [60]fullerene (C60). The dendritic building blocks used to functionalize C60 displayed cubic and hexagonal
columnar phases. The fullerene derivatives showed rectangular columnar phases of c2mm symmetry.
The design of supramolecular [60]fullerene (C60) assemblies,
in which the [60]fullerene-containing molecular units are
organized in a specific and controllable manner, is an elegant
and appealing way to exploit the physical properties of C60
in materials science (e.g., photoactive dyads and triads,1
photovoltaic devices2). Grafting dendrimers onto C60 (ful-
lerodendrimers) allowed this goal to be reached, as demon-
strated by the formation of ordered and stable Langmuir and
Langmuir-Blodgett films3 and micelles.4 Fullerene-contain-
ing thermotropic liquid crystals5-10 are promising chemical
components for the development of nanotechnologies by the
“bottom-up” approach (e.g., photoactive supramolecular
switches5c,h and supramolecular solar cell devices5e). Func-
tionalization of C60 with liquid-crystalline poly(arylester)
dendrimers containing mesomorphic groups at the periphery
by applying either the Bingel reaction11 (from malonate
(2) (a) 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. (b) Imahori, H.;
Fukuzumi, S. AdV. Funct. Mater. 2004, 14, 525. (c) van Duren, J. K. J.;
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Egbe, D. A. M.; Mu¨hlbacher, D.; Sariciftci, N. S. J. Mater. Chem. 2004,
14, 3462. (e) Winder, C.; Sariciftci, N. S. J. Mater. Chem. 2004, 14, 1077.
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† Universite´ de Neuchaˆtel.
‡ Institut de Physique et Chimie des Mate´riaux de Strasbourg.
(1) (a) Beckers, E. H. A.; van Hal, P. A.; Dhanabalan, A.; Meskers, S.
C. J.; Knol, J.; Hummelen, J. C.; Janssen, R. A. J. J. Phys. Chem. A 2003,
107, 6218. (b) Atienza, C.; Insuasty, B.; Seoane, C.; Mart´ın, N.; Ramey,
J.; Rahman, G. M. A.; Guldi, D. M. J. Mater. Chem. 2005, 15, 124. (c)
Sa´nchez, L.; Herranz, M. AÄ .; Mart´ın, N. J. Mater. Chem. 2005, 15, 1409.
(d) de la Torre, G.; Giacalone, F.; Segura, J. L.; Mart´ın, N.; Guldi, D. M.
Chem.sEur. J. 2005, 11, 1267.
10.1021/ol0603920 CCC: $33.50
© 2006 American Chemical Society
Published on Web 04/08/2006

derivatives yielding methanofullerenes5b,d,f,g,12) or the 1,3-
dipolar cycloaddition reaction13 (from aldehyde derivatives
yielding fulleropyrrolidines5e,f,h,j) led to a rich collection of
macromolecular liquid crystals, which display smectic A,
nematic, and chiral nematic (cholesteric) phases. In such
systems, C60 is embedded in the dendrimer. As a conse-
quence, the supramolecular organization within the liquid
crystal state depends mostly on the nature of the mesomor-
phic units.
Second-generation poly(benzyl ether) dendron (4-3,4,5-3,5)-
12G2-CO2H 3 (Scheme 1), the carboxylic acid function of
which allows structural modifications at the focal point, was
selected as a first candidate.14b Compound 3 shows a cubic
phase [Cr 35 Cub(Im3hm) 196 I],14b the corresponding
methylester derivative a hexagonal columnar phase [Cr -19
Scheme 1. Synthesis of Fulleropyrrolidines 1 and 2
Fullerene-containing liquid crystals, which display colum-
nar phases, are of interest for electronic and optoelectronic
applications (e.g., one-dimensional electron transportation).
The synthetic approach we have developed to obtain
fullerene-containing liquid crystals (see above) can be
adapted to prepare such compounds. Therefore, we envisaged
that functionalization of C60 with liquid-crystalline dendrim-
ers, which exhibit columnar mesomorphism, should lead to
fullerenes which also show columnar mesomorphism. To
functionalize C60, we decided to use the poly(benzyl ether)
dendrimers developed by Percec14 (prepared by a convergent
synthesis15) since these compounds give rise to cylindrical
and spherical supramolecular dendrimers that subsequently
self-organize into columnar and cubic lattices, respectively.
(3) (a) Cardullo, F.; Diederich, F.; Echegoyen, L.; Habicher, T.;
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Va´zquez, E.; Milic, D.; Prato, M.; Barbera´, J.; Guldi, D. M.; Marcaccio,
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D.; Nicoud, J.-F.; Nierengarten, J.-F. Chem.sEur. J. 2000, 6, 3501. (b)
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1852
Org. Lett., Vol. 8, No. 9, 2006

Colh(p6mm) 71 I], and the corresponding methyl alcohol
derivative hexagonal columnar and cubic phases [Cr 57 Colh-
(p6mm) 87 Cub(Pm3hn) 101 I].14b
Table 1. Phase Transition Temperatures and Enthalpy Changes
of 1, 2, and 6-9
Columnar mesomorphism was already observed in two
particular cases. First, attachment of five aromatic groups
around one pentagon of C60 yielded conical molecules, the
stacking of which gave rise to columnar phases.8 Second,
self-assembled columns were obtained by mixing two
nonmesomorphic compounds, a fullerene derivative, and a
disk-like molecule; the C60 units were located between the
columns.10 Our concept is a more general approach, owing
to the great number of dendrimers that can be used, and
should allow the design of fullerene-containing liquid crystals
with tailor-made mesomorphic properties.
compound Tga (°C)
transitionb
Tc (°C) ∆H (kJ/mol)
(∼64)e
6
(I f Cub)d
Cr f I
Cr f Colh
Colh f I
Cub f Colh
Colh f I
Cr f Colh
(Colh f Cub)d
Colh f I
66
57
2.8
6.1
7.6
6.3
6.8
6.8
3.8
7.7
3.9
7.2
7
88
84
98
55
(44)f
88
80
74
8
9
37
g
1
2
Colr f I
Colr f I
46
The synthesis of target fulleropyrrolidines 1 and 2 is
presented in Scheme 1. Condensation of 3 with 4 or 5 in the
presence of N,N′-dicyclohexylcarbodiimide (DCC), 4-(dim-
ethylamino)pyridinium toluene-p-sulfonate (DPTS), and
4-pyrrolidinopyridine (4-PPy) led to alcohol intermediates
6 and 7, which were oxidized to give aldehydes 8 and 9.
Addition of the latter with N-methylglycine (sarcosine) to
C60 gave 1 and 2.
a Tg ) glass transition temperature determined during the second heating.
b Cr ) semicrystalline solid, Colh ) hexagonal columnar phase of p6mm
symmetry, Colr ) rectangular columnar phase of c2mm symmetry, Cub )
cubic phase, I ) isotropic liquid. c Temperatures are given as the onset
values taken from the second heating run. d Monotropic transition. e De-
termined by POM. f Temperature is given as the onset value taken from
the first cooling run. g Not detected.
as (110), (111), and (200) in a P lattice or as (200), (211),
and (220) in an I lattice, were observed. As for 8 and 9, the
XRD patterns exhibited only two orders of diffraction in the
cubic phase in the ratio x3 and x4, the x2 reflection being
too weak to be measured. The XRD patterns of the hexagonal
columnar phases shown by 7, 8, and 9 presented only one
signal in the small angle region. On the basis of the optical
textures observed for these compounds, which are typical
for columnar phases (an illustrative example is given in
Figure 1), this reflection was indexed as the (10) reflection
of a two-dimensional hexagonal lattice. As already found
for similar dendrons, the cores of the columns or of the
micelles for the cubic phases result from interactions between
the polar central parts of the dendrons, the alkyl chains
Compounds 6-9 were purified by column chromatogra-
phy. Fulleropyrrolidines 1 and 2 were purified by column
chromatography and precipitated from MeOH. The structure
and purity of all compounds were confirmed by NMR
spectroscopy, mass spectrometry, GPC (all compounds were
found to be monodisperse), and elemental analysis. The UV-
vis spectra of 1 and 2 were in agreement with the fullero-
pyrrolidine structure.
The thermal and liquid-crystalline properties of 1, 2, and
6-9 were investigated by polarized optical microscopy
(POM), differential scanning calorimetry (DSC), and X-ray
diffraction (XRD). The phase transition temperatures are
reported in Table 1, and the XRD patterns and structural
parameters of the phases are in the Supporting Information.
The columnar and cubic phases could be identified by
POM from the observation of typical textures (columnar
phase, pseudo-focal conic texture; cubic phase, isotropic
(black) texture). On heating, compounds 7 and 9 gave, from
50 to 60 °C, a series of endothermic and exothermic peaks
(DSC) revealing melting and crystallization processes. The
compounds with the longer flexible spacer cleared at lower
temperatures than the materials containing the shorter spacer.
The cubic nature of the phases displayed by 6, 8, and 9
was confirmed by XRD. For 6, 3 orders of diffraction in the
square spacing ratio x2, x3, and x4, which can be indexed
(14) (a) Percec, V.; Ahn, C.-H.; Cho, W.-D.; Jamieson, A. M.; Kim, J.;
Leman, T.; Schmidt, M.; Gerle, M.; Mo¨ller, M.; Prokhorova, S. A.; Sheiko,
S. S.; Cheng, S. Z. D.; Zhang, A.; Ungar, G.; Yeardley, D. J. P. J. Am.
Chem. Soc. 1998, 120, 8619. (b) Percec, V.; Cho, W.-D.; Ungar, G.;
Yeardley, D. J. P. J. Am. Chem. Soc. 2001, 123, 1302. (c) Percec, V.;
Glodde, M.; Bera, T. K.; Miura, Y.; Shiyanovskaya, I.; Singer, K. D.;
Balagurusamy, V. S. K.; Heiney, P. A.; Schnell, I.; Rapp, A.; Spiess, H.-
W.; Hudson, S. D.; Duan, H. Nature 2002, 419, 384. (d) Ungar, G.; Liu,
Y.; Zeng, X.; Percec, V.; Cho, W.-D. Science 2003, 299, 1208. (e) Percec,
V.; Imam, M. R.; Bera, T. K.; Balagurusamy, V. S. K.; Peterca, M.; Heiney,
P. A. Angew. Chem., Int. Ed. 2005, 44, 4739. (f) Percec, V.; Dulcey, A. E.;
Peterca, M.; Ilies, M.; Sienkowska, M. J.; Heiney, P. A. J. Am. Chem. Soc.
2005, 127, 17902.
Figure 1. Thermal polarized optical micrograph of the hexagonal
columnar texture (pseudo-focal conic and homeotropic textures)
displayed by 9 at 87 °C.
(15) Grayson, S. M.; Fre´chet, J. M. J. Chem. ReV. 2001, 101, 3819.
Org. Lett., Vol. 8, No. 9, 2006
1853

arranged according to a hexagonal close compact packing
(hful ) (x3/2) × φ, φ being the diameter of C60, i.e., ∼10
Å). To understand the molecular organization of 1 and 2 in
the columnar phases, the number of molecules included in a
slice of 8.7 Å thickness was calculated. From the values of
the lattice parameters and the estimated molecular volumes
(4550 and 4700 Å3 for 1 and 2, respectively), this number
turned out to be about 10 for each compound. A close
hexagonal packing of the C60 units was then simulated
(Figure 2): bundles of 10 dendrons are superposed one over
the other to form an elliptic columnar core, the shape of
which is in agreement with the c2mm symmetry. The
dendritic moieties are arranged around this elliptic core in
such a way that they fill the intercolumnar space. The
supramolecular organization of 1 and 2 within the columns
and the texture of the columnar phase displayed by 1 are
shown in the Supporting Information.
The results reported here (columnar phases) and those we
have already published (smectic A, nematic, and chiral
nematic phases) prove that covalent functionalization of C60
with liquid-crystalline dendrimers is a valuable concept for
the design of fullerene-containing liquid crystals which
display tailor-made mesomorphic properties.
Figure 2. Postulated supramolecular organization of 1 and 2 within
the rectangular columnar phases.
constituting the aliphatic medium between the columns or
the micelles.
Acknowledgment. R.D. thanks The Swiss National
Science Foundation (Grant 200020-103424) for financial
support. B.D. and D.G. thank Mr. N. Beyer for his help with
some of the illustrations.
For fullerodendrimers 1 and 2, the XRD diffraction
patterns registered within the temperature range of the
mesophases displayed six sharp reflections, which were
unambiguously indexed as the (11), (20), (02), (22), (40),
and (13) reflections of a two-dimensional rectangular lattice
of c2mm symmetry (for 1, a ) 128.6 Å and b ) 86.0 Å; for
2, a ) 129.6 Å and b ) 89.4 Å). Two broad reflections
located at hch ) 4.6 Å and hful ∼ 8.7 Å are present in the
wide angle region. The first reflection is related to the molten
aliphatic terminal chains of the dendrons, and the second
one was attributed to interactions between the C60 units
Supporting Information Available: Synthetic procedures
1
and analytical data of all new compounds and copies of H
NMR spectra for 1 and 2; XRD data of the mesophases,
organization of 1 and 2 within the columns, and texture
displayed by 1 in the columnar phase. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL0603920
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Org. Lett., Vol. 8, No. 9, 2006