Self-assembly made durable: water-repellent materials formed by cross-linking fullerene derivatives

Article abstract of DOI:10.1002/anie.200900106

(Figure Presented) Fullerence flakes: A diacetylene-functionalized fullerence derivative self-organizers into flakelike microparticles (see picture). Both the diacetylene and C₆₀ moieties can be effectively cross-linked, which leads to supramol

Full text of DOI:10.1002/anie.200900106

Communications
DOI: 10.1002/anie.200900106
Fullerenes
Self-Assembly Made Durable: Water-Repellent Materials Formed by
Cross-Linking Fullerene Derivatives**
Jiaobing Wang, Yanfei Shen, Stefanie Kessel, Paulo Fernandes, Kaname Yoshida, Shiki Yagai,
Dirk G. Kurth, Helmuth Mꢀhwald, and Takashi Nakanishi*
The fine-tuning of the interactions between p-conjugated
molecules can enable the development of supramolecular
materials that have attractive properties.[1] However, the low
strength and stability of these materials, which are organized
by weak noncovalent interactions, impose limitations for
certain applications. Most self-organized materials suffer
from structural changes when exposed to organic solvents,
high temperatures, or mechanical stress. Accordingly, it is
crucially important to harness the sophisticated functional
properties found in noncovalent assemblies while imparting
them with the structural durability offered by covalent
chemistry. To address this problem, covalent cross-linking of
self-organized structures has been developed and has
received much attention. For instance, click chemistry,[2]
diene metathesis,[3] and disulfide bond formation[4] have
been successfully utilized to stabilize various structures such
as gel fibers, self-assembled nanotubes, and host frame-
works.[5] Subtle control of the reaction conditions is required
to maintain the structure and morphology and to simulta-
neously achieve efficient covalent cross-linking, which are the
key factors that determine the final performance of the
supramolecular materials in practical applications.
The fullerene C60 possesses unique physicochemical
properties,[6] and is a promising candidate for the construction
of versatile supramolecular architectures.[7–10] We recently
demonstrated that fullerene derivatives bearing three satu-
rated aliphatic chains (such as 2) self-assemble into unique
supramolecular nano- or microscopic architectures by p–p
and van der Waals interactions. By tuning the boundary
conditions of self-assembly, well-defined one-, two-, or three-
dimensional objects with various macroscopic morphologies
can be observed.[11] This strategy demonstrates the feasibility
of constructing supramolecular materials through the combi-
nation of appropriate molecular design and self-assembly
algorithms. However, these materials are structurally fragile
because of the weak intermolecular forces, which is a
disadvantage in technologies that require high durability.
Herein, we present a new design based on a fullerene
derivative (1, Scheme 1) that contains diacetylene (DA)
functional groups, which are well-known photo-cross-link-
ers,[12] in long aliphatic chains. This derivative self-assembles
into flakelike microparticles with a bilayer structure at the
molecular level. Irradiation with ultraviolet light leads to
effective cross-linking of both DA and C60 moieties, while
maintaining their nano- and macroscopic organization. This
process gives rise to a material with remarkable resistivity to
[*] Dr. J. Wang, Dr. Y. Shen, Dr. S. Kessel, Dr. P. Fernandes,
Prof. H. Mꢀhwald, Dr. T. Nakanishi
Department of Interfaces
Max Planck Institute of Colloids and Interfaces
Research Campus Golm, Potsdam 14424 (Germany)
Fax: (+49)331-567-9202
E-mail: nakanishi.takashi@nims.go.jp
Homepage:
http://www.nims.go.jp/macromol/nakanishi_eng/index.html
Dr. S. Yagai, Dr. T. Nakanishi
PRESTO, Japan Science and Technology Agency (JST, Japan)
Prof. D. G. Kurth, Dr. T. Nakanishi
Organic Nanomaterials Center
National Institute for Materials Science (Japan)
Dr. K. Yoshida
Institute for Chemical Research, Kyoto University (Japan)
Dr. S. Yagai
Department of Applied Chemistry and Biotechnology
Graduate School of Engineering, Chiba University (Japan)
Dr. P. Fernandes
Physical Chemistry II, Bayreuth University (Germany)
Prof. D. G. Kurth
Chemische Technologie der Materialsynthese
Universitꢁt Wꢂrzburg (Germany)
[**] This work was supported in part by a Grand-in-Aid from the Ministry
of Education, Sciences, Sports, and Culture (Japan), and PRESTO,
JST (Japan) (T.N.). We thank Dr. M. Takeuchi (NIMS) and T. Sievers
(MPI) for helpful discussions.
Scheme 1. a) Molecular structures of fullerene derivatives modified
with diacetylene (1) and saturated aliphatic chains (2) employed in
this study and b) a schematic representation of the photo-cross-linking
process in the bilayer structural subunit of 1.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200900106.
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solvent, heat, and mechanical stress. Moreover, the surface of
the cross-linked flakelike objects combines a water-repellent
nature with durability.
(ca. 4.5 nm), we propose that the molecular arrangement in
the assembly is a bilayer structure with interdigitated and/or
disordered alkyl chains.
The fullerene derivative 1 was obtained in five steps[13,14]
starting from 1-hexadecyne (see Scheme S1 in the Supporting
Information) and unambiguously characterized by 1H and
13C NMR, FTIR, and UV/Vis spectroscopy, and MALDI-
TOF mass spectrometry. Self-organized objects were pre-
pared from 1 as follows: A solution of 1 (2.2 mg, 1 mmol) in a
mixture of THF/MeOH (3:2 v/v, 1 mL) at 508C was cooled to
208C and stored at À148C for 24 h to afford a dark-yellow
precipitate in quantitative yield. MeOH (3 mL) was added to
ensure complete formation of the precipitate, which was
stable in solution at room temperature. Flakelike micro-
particles of compound 2, which was used as a reference, were
prepared according to our previously published procedure[11c]
by heating a solution of 2 (1 mm) in 1,4-dioxane to 708C
followed by cooling to room temperature. The self-assembled
particles were removed from the dispersions and spread onto
a water surface to form a compact and uniform particle layer
at the air–water interface, which was then transferred onto
various substrates (Si, quartz, or glass) and dried under
vacuum for further studies.[15]
The field-emission scanning electron microscopy (SEM,
Figure 1a) image shows that 1 reproducibly self-assembles
into flakelike particles around 1 mm in size with crumpled
petal-shaped structures around 100 nm in thickness. High-
resolution cryogenic transmission electron microscopy (HR-
cryo-TEM) studies at the edge of the particle reveal a multi-
lamellar structure (Figure 1b). Fast Fourier transform (FFT)
analysis (Figure 1b, inset) indicates a lamellar periodicity of
(5.7 Æ 0.1) nm. In addition, the appearance of third-order
spots in the FFT indicated that the lamellar organization is
highly periodic. By taking into account the dimensions of 1
The multilamellar arrangement suggested from HR-cryo-
TEM studies is consistent with the XRD analysis of 1. As
shown in Figure 1c, the XRD pattern of the bulk precipitate
of 1 measured at 258C displays reflections of the (001) to
(009) planes, which indicate a long-range ordered lamellar
structure. The interlayer distance obtained from the XRD
analysis is 6.0 nm, which is slightly larger than that obtained
from TEM analysis and may result from different exper-
imental conditions such as temperature or pressure. More-
over, the two broad halos that appear at approximately 10.38
and 19.48 can be attributed to the in-plane average distance
between neighboring C60 molecules (ca. 8.7 nm) and the
average distance between molten alkyl chains (ca. 0.46 nm,
see below), respectively. The UV/Vis spectrum of the
particles of 1 at 258C shows three bands with absorption
maxima at 333, 270, and 256 nm. When compared to the
absorption maxima of a solution of 1 in n-hexane, these bands
are red-shifted by 17, 16, and 2 nm, respectively, which
indicates p–p interactions between the neighboring C60
molecules in the assembled objects (see Figure S2 in the
Supporting Information).[11] The FTIR spectrum of the
particles of 1 at 258C displays asymmetric and symmetric
methylene stretching bands at 2924 and 2852 cmÀ1, respec-
tively, which suggests that the alkyl chains are not in a
crystalline state.[16] This result is in agreement with differential
scanning calorimetry (DSC) data of the bulk precipitate of 1.
A broad shoulder around 158C and a peak centered at
29.18C, which correspond to the pretransition and the solid–
liquid crystalline transition of the alkyl chain, are observed
with DH and DS values of 55.4 kJmolÀ1 and 180 JmolÀ1 KÀ1,
respectively, for the later peak (see Figure S9a in the
Supporting Information). In addition, because of the ortho-
rhombic subcell structure of the oligomethylene units, the
methylene scissoring mode shows a broadly split band at
around 1463 cmÀ1 in the FTIR spectrum, which indicates that
parts of the alkyl chains could be interdigitated (Figure S3 in
the Supporting Information).[17] These results confirm that the
assembled particle of 1 is composed of bilayer structural
subunits with noncrystalline and partially interdigitated alkyl
chains (Scheme 1b).
Photoinduced cross-linking studies on the flakelike par-
ticles with cross-linkable DA and fullerene moieties were
carried out. Photoreactions of the DA moieties were studied
by FTIR spectroscopy (Figure 2). Upon irradiation at 258C
with a 150 W low-pressure Hg lamp, the IR absorption bands
at 2256 and 2191 cmÀ1, assigned to the asymmetric vibrational
mode of the DA monomer, gradually decreased. This is
accompanied by the appearance of a new band centered at
2212 cmÀ1, which is assigned to the asymmetric vibrational
[12a,c]
ꢀ
mode of the C C bond in the poly(diacetylenes) (PDs).
Figure 1. a) SEM image of the self-organized particles of 1 prepared
from THF/MeOH solution. The inset shows an enlarged particle
image (scale bar=500 nm). b) HR-cryo-TEM image of the edge of one
particle. The inset shows the corresponding FFT analysis showing the
1st, 2nd, and 3rd order spots. The periodicity of the lamellae is
5.7 nm. c) X-ray diffraction pattern of the self-organized particles of 1
at 258C. The corresponding d-spacing value is 6.0 nm.
To unambiguously assign the spectral changes, an IR inves-
tigation of the precursor molecule of 1 (Scheme S1 in the
Supporting Information) containing the DA moieties was
carried out. UV irradiation caused a similar trend in the FTIR
spectra (Figure S4 in the Supporting Information). The bands
at 2256 and 2179 cmÀ1 gradually disappeared, and a new band
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Figure 2. FTIR spectra of the assembled particle of 1 upon UV
irradiation from 0 to 11 h under an Ar atmosphere at 258C.
Figure 3. SEM images of the cross-linked particles assembled from 1
a) before and b) after exposure to chloroform for 12 h. The last two
images show photoirradiated particles assembled from 2 c) before and
d) after rinsing with chloroform for 5 s. The inset in (c) shows an
enlarged image of particle 2 (scale bar=3 mm).
centered at 2215 cmÀ1 appeared. Photopolymerization of DA
groups results in an absorption band at around 550 nm if PDs
with long conjugation lengths are produced.[12b] This is clearly
observed for the precursor DA molecule (Figure S5 in the
Supporting Information), but not for the self-assembled
species of 1, which suggests that photo-cross-linking in the
latter produces PDs of varying length and conformation.[12e]
The moderate conjugation may be caused by imperfect DA
alignment in the assembly of 1, which hinders polymerization
to yield PDs with extended conjugation. The absence of an
isosbestic point in the IR spectra (Figure 2) indicates that
more than two species are involved in the cross-linking
reaction. It is noteworthy that UV irradiation results in
photoaddition reactions between neighboring C60 moieties
(Scheme 1b), which is indicated by the decreased and
broadened characteristic absorption of C60 (Figure S6 in the
Supporting Information) as well as the disappearance of the
pentagonal pinch mode of C60 at 1467 cmÀ1 in the Raman
spectra (see Figure S7 in the Supporting Information).[18]
Although the detailed assignments of the dimerized or
polymerized C60 structures need further careful analysis, the
lamellar organizations in the assemblies are maintained even
after the photo-cross-linking (see Figure S8 in the Supporting
Information).
The cross-linked flakelike particles of 1 display a remark-
able resistivity to organic solvents. As observed by SEM
(Figure 3a,b), the cross-linked particles maintain their shape
after immersion in chloroform for 12 hours. In sharp contrast,
non-cross-linked particles are completely dissolved by the
same solvent within seconds. Moreover, a control experiment
employing self-organized globular particles prepared from 2
(without the DA group) indicated that cross-linking of DA
moieties is indispensible for the achievement of a robust
material. As shown in Figure 3c,d, when exposing the photo-
irradiated particles of 2 to chloroform for five seconds, most
of the particles disassembled and dissolved in the solvent. The
sparsely preserved residue might be caused by the photo-
addition reaction between the neighboring C60 moieties.
In addition to the solvent tolerance, the thermal stability
and stiffness of the cross-linked particles of 1 are remarkably
enhanced. Before cross-linking, the DSC thermogram (see
Figure S9a in the Supporting Information) of the particles
displays two endothermic peaks that are attributed to the
phase transition from the solid to the mesomorphic phase
(29.18C) and from the mesomorphic phase to the isotropic
phase (140.38C). On the other hand, a thermogram of the
cross-linked particles (see Figure S9b in the Supporting
Information) shows only small endothermic peaks with less
than 5% intensity of the original phase transition temper-
atures, which correspond to remaining monomer. At the same
time, a broad peak around 518C appears, which is assigned to
the glass transition of the alkyl chains in the cross-linked
species. Although the melting point of 1 is 140.38C, no visible
changes in the organized structures were seen in the SEM
images, even when the cross-linked particles were heated at
1508C for up to 12 h (see Figure S10a in the Supporting
Information).
AFM force spectroscopy allows the determination the
mechanical properties of a material.[19] Analysis of the data of
force versus deformation can provide information about
stiffness and adhesion (typical force versus deformation
curves (approach and retract) of a sample before and after
UV treatment are shown in Figure S11a,b in the Supporting
Information). Based on several measurements (see the
Supporting Information), UV irradiation of 1 resulted in a
remarkable increase of the determined stiffness of about 25-
fold (from below (0.54 Æ 0.48) NmÀ1 to (14.5 Æ 1.7) NmÀ1)
and a decrease in the adhesion (from 425 nN to 140 nN). The
results clearly reveal that UV irradiation of the flakelike
particles of 1 results in a robust assembly and a less adhesive
glassy material.
For practical applications, covalent cross-linking of self-
organized supramolecular materials opens a path for the
development of functional materials with improved durabil-
ity. In the present study, we demonstrate the potential of using
cross-linked particles formed from DA-functionalized full-
erene derivatives as a robust water-repellent surface. The
nature of water repellency is important in daily life and in
industry for self-cleaning coatings and anti-wetting agents,[20]
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which mimic the surface properties of the Lotus leaf.[21]
According to the Cassie model, the microscopic roughness
on the particle surface accommodates tiny pockets, which
entrap air and enhance the surface hydrophobicity.[22] Contact
angle (CA) measurements showed that a surface that was
fully covered with the flakelike (fractal-shaped) particles of 1
indeed exhibits high water repellency, with a CA of around
146.28 (Figure 4a). This value is significantly larger than the
corresponding CA of 102.18 of a flat surface made by spin-
coating with non-assembled 1 (Figure S12a in the Supporting
THF/methanol mixture. Photoirradiation effectively cross-
links both the diacetylene and C60 moieties, which leads to
supramolecular materials with significantly enhanced stiffness
(> 25-fold) and remarkable resistivity to various organic
solvents, acidic or basic aqueous media, as well as to heating
up to 2008C. The potential of this strategy is illustrated by the
fabrication of a highly durable water-repellent surface that
satisfies some of the requirements for water-repellent materi-
als, such as facile and substrate independent production, high
durability to different ambient or harsh conditions, as well as
enhanced mechanical stiffness. The design of molecular
building blocks that self-assemble into well-defined hierarch-
ical architectures following topochemical cross-linking is
expected to be an important research direction in supra-
molecular materials science, for instance, towards coating
technologies in micro- or nano-electromechanical systems
(MEMS/NEMS). Further investigations into the photoelec-
tronic performances that originate from the rich p conjuga-
tion in these assemblies are underway.
Received: January 7, 2009
Published online: February 10, 2009
Keywords: cross-linking · fullerenes · polydiacetylenes ·
.
superhydrophobicity · supramolecular chemistry
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