Supramolecular and core-shell materials from self-assembled fibers

Article abstract of DOI:10.1039/b917279c

The present study reports on the formation of supramolecular fibers from a novel cyclen-based ligand and metal salts. In particular, the fibers were shown to stabilize supramolecular porous materials of low density. It was also demonstrated that these fibers could be functionalized by radical polymer growth on their surface. Such new supramolecular fibers constitute a simple and tunable starting material for the synthesis of 1D core-shell objects. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b917279c

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Supramolecular and core–shell materials from self-assembled fibersw
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Louis Moreau,* Alexia Balland-Longeau, Philippe Mazabraud, Alain Duchene and
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Jerome Thibonnet*
Received (in Cambridge, UK) 21st August 2009, Accepted 11th December 2009
First published as an Advance Article on the web 13th January 2010
DOI: 10.1039/b917279c
The present study reports on the formation of supramolecular
fibers from a novel cyclen-based ligand and metal salts. In
particular, the fibers were shown to stabilize supramolecular
porous materials of low density. It was also demonstrated that
these fibers could be functionalized by radical polymer growth on
their surface. Such new supramolecular fibers constitute a simple
and tunable starting material for the synthesis of 1D core–shell
objects.
under vacuum. The TSC was thus obtained as a white powder
at a 60% yield and was subsequently characterized by nuclear
magnetic resonance (NMR), mass spectroscopy and by X-ray
crystallography.w4
In the presence of metal salts such as cupric chloride, zinc
chloride, nickel(II) chloride or cobalt(II) chloride, and together
with a polar solvent such as an aliphatic alcohol, TSC
self-assembled to become a gel. Such gels can also be obtained
starting from a total mass concentration of 1%. For instance,
1.85 mL of a cupric chloride solution (0.02 M) in 96% ethanol
was added to 0.15 mL of a solution of TSC in THF (0.31 M),
preheated at 50 1C. The medium was stirred for several
seconds and then stored at room temperature and the formation
of a fibrous gel (Fig. 1a) was observed after a few minutes. In
order to dry this gel, it was first immersed in cyclohexane,
and then when all the solvents had become exchanged with
cyclohexane, the gel was frozen and freeze-dried.
Organic micro- and nano-fibers are the key to numerous
innovative materials, both for academic research and industrial
development. Considerable advances have been made in the
technology concerning 1D objects in recent years and these
objects are the starting materials for many applications in the
fields of optoelectronics, sensors, filtration, medicine and
catalysis.1 A powerful approach to enlarging the application
field of 1D objects involves functionalizing their surfaces, for
example by polymer growth.2 This approach, known as a
core–shell or core sheath method, is well known in the field
of polymer fibers developed by electrospinning. However,
when starting from 1D supramolecular objects, using such
a core–shell approach is less common, despite the fact
that supramolecular fibers offer many advantages in terms
of composition, nanostructure, shape (ribbon, wire, helix,
network, etc.), physico-chemical properties and ease of access
(self-assembly).3
This method provided supramolecular objects that were
robust enough to be unmolded and handled despite their
low density. Fig. 1b shows a photograph of these objects.
Moreover, scanning electron microscopy (SEM) observations
of the objects showed that they consisted of an interlaced fiber
network (Fig. 1c and 1d).
The metal presence and homogenous repartition as fibers
constitutive elements was confirmed by STEM (scanning
transmission electron microscopy) analysis. As we can see in
Fig. 2, the elements chlorine (2b) and copper (2c) are present
and uniformly distributed inside all the fibers.
The study presented here reports on the formation of a
supramolecular fiber from a new cyclen-based ligand and
metal salts, and demonstrated that these supramolecular fibers
can be functionalized by radical polymers grown on their
surfaces.
The fibers were in the form of ribbons, with thicknesses of a
few tens of nanometres and widths ranging from 0.1 to
2 micrometres. However, all fibers were very long, i.e., in the
millimetre range.
The new ligand, that we have called ‘‘tetrastyrylcyclen’’
(TSC, 2), was synthesized in a single step, starting from cyclen
(1) and 4-vinylbenzyl chloride. The synthesis pathway is
presented in Scheme 1. In most cases, cyclen, 4-vinylbenzyl
chloride and triethylamine were refluxed in the presence of
acetonitrile and dichloromethane. After 24 h of reflux, the
TSC that had precipitated in the medium was isolated by
filtration, washed with acetonitrile and methanol, and dried
Isolated fibers were obtained by dispersion of the gel in a
polar solvent such as an aliphatic alcohol. The fibers were then
isolated by filtrating the suspension. Pure fibers were obtained
a CEA, DAM, Le Ripault, F-37260 Monts, France.
E-mail: Moreaulouis@aol.com; Fax: +33 247345168;
Tel: +33 24734 4243
b
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Laboratoire de Physicochimie des Materiaux et des Biomolecules,
EA 4244, Faculte´ des Sciences de Tours, Parc de Grandmont,
37200 Tours, France. E-mail: jerome.thibonnet@univ-tours.fr;
Fax: +33 24736 7041; Tel: +33 24736 7359
w Electronic supplementary information (ESI) available: X-ray crys-
tallography of TSC and TSC.Cu. CCDC 702134 & 744758. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b917279c
Scheme 1 Synthesis scheme of TSC from cyclen and 4-vinylbenzyl
chloride.
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Scheme 2 Chemical structures of the monomers used for polymer
growth on the fibers: divinylbenzene (DVB) (3) and fluorescein
methacrylate (4).
When this gel was heated for 5 h at 70 1C, a solid was obtained
which could then be dried easily under vacuum, without
mechanical deformation, providing a robust material capable
of being unmolded and machined. SEM observations of this
material showed that it consisted of fibers covered with small
points of growing polymer.
The fibers were covered with polymer to greater or lesser
degrees according to the quantity of DVB used. The same
experiment, but this time mixing the DVB and the AIBN
with already formed fibers, also provided fibers coated with
polymer. Similar results were obtained with various vinyl
monomers and other metal-based supramolecular fibers.
For example, Fig. 3 shows fibers consisting of TSC and zinc
chloride, onto which a fluorescent dye monomer (i.e., fluorescein
methacrylate) was polymerized in diluted medium (Scheme 2).
As in the case of DVB, SEM demonstrated that the fibers were
coated with polymer after polymerization. Optical fluorescence
microscopy showed green fluorescent fibers, which clearly
confirmed that the dye had polymerized on the surface.
These experiments reveal that the new supramolecular fibers
can be functionalized by radical polymerization of vinyl
monomers on the fibers surface. The fiber surface probably
presents reactive vinyl bonds which permit the growth
of polymer chains. In fact, in the case of polymerization of
Fig. 1 Development of supramolecular materials from TSC in the
presence of cupric chloride: (a) photograph of the supramolecular gel;
(b) photograph of the supramolecular object obtained after drying the
gel; (c) and (d) SEM images of the supramolecular object (scale
bars are 20 mm and 0.5 mm, respectively); (e) photograph of dried
supramolecular object obtained after radical polymerization in the
presence of DVB; (f) and (g) SEM images of the dried supramolecular
object (scale bars are 20 mm and 0.5 mm, respectively).
Fig. 2 (a) TEM BF image of fibers obtained from TSC and cupric
chloride. (b) STEM image of chlorine. (c) STEM image of copper.
only when the TSC/metal salt ratio = 1. Furthermore, high
resolution ESI-HRMS analysis of the fibers demonstrated
that the cation was TSC.CuCl+ (C44H52N4CuCl+,
m/z = 734.3171) in the case of cupric chloride-based fibers
and TSC.ZnCl+ (C44H52N4ZnCl+, m/z = 737.3148) when
the fibers were based on zinc chloride. These values confirmed
that the ligand/metal ratio was 1/1 in the fibers, which
corresponded to the stoichiometry generally observed in
copper- and zinc-based cyclen derivative complexes.5 The
structure of the complex with TSC and CuCl2 was confirmed
by X-ray crystallography.w
Fig. 3 (a) SEM micrograph of fibers obtained from TSC and zinc
chloride (scale bar = 1 mm). (b) SEM micrograph of fibers based on
TSC and zinc chloride after radical polymerization in the presence of
fluorescein methacrylate (scale bar = 1 mm). Optical micrographs of
the same fibers under (c) blue light (l = 485 nm, scale bar = 100 mm)
and (d) white light (scale bar = 100 mm).
Production of a gel under the same conditions, but in the
presence of a polymerizable monomer such as divinylbenzene
(DVB, Scheme 2) and AIBN, provided an identical material.
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non-reticulating monomers, the polymer chains obtained on
the fiber surface were shown to be non-soluble in organic
solvents such as THF. This suggests a covalent link between
the fibers and the polymers.
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2 A. Greiner and J. H. Wendorff, Angew. Chem., Int. Ed., 2007, 46,
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3 L. F. Lindoy and I. M. Atkinson, Self-Assembly in Supramolecular
Systems, RSC, Cambridge, 2000; J.-M. Lehn, Nobel Lecture,
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In addition, this supramolecular/covalent hybrid strategy,
resulting in self-assembled materials hardened by polymerization,
is an efficient way to obtain metal-doped materials of low
density (as low as to 15.10–3 mg/cc) with good mechanical
properties compatible with micromachining. The simple
self-assembly of TSC/metal yielded materials with high metal
atom levels (0.97%). In fact, TSC contains precisely 100 atoms
and the metal two ions. The growth of polymers on the fibers
constituting the materials may make it possible to vary the
levels of contributing metal by the addition of other elements
(carbon, hydrogen, or other metals) according to the
monomer used.
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W. Gong and C. J. Brinker, Nature, 1998, 394, 256; L. Sanchez,
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4 Cyclen (2 g, 1 eq, 11.6 mM), 25 mL of anhydrous acetonitrile, 25 mL
of anhydrous dichloromethane, 4-vinylbenzyl chloride (12.4 g, 7 eq,
81.3 mM) and triethylamine (17.6 g, 15 eq, 170 mM) were charged in
a round bottom flask under argon atmosphere. The medium was
stirred and heated at reflux for 24 h. The reaction was stopped,
cooled at room temperature and the medium filtered. The crude
solid was washed with 20 mL of acetonitrile and three times with
50 mL of methanol. The solid was dried under vacuum and 4.21 g of
pure product was isolated as a white powder. 1H NMR (CDCl3) d
7.33 (m, 16H, aromatic), 6.73 (dd, 4H, J = 10.8/17.6 Hz, vinyl), 5.75
(d, 4H, J = 10.8 Hz, vinyl), 5.24 (d, 4H, J = 17.6 Hz, vinyl), 3.44
(s, 8H, CH2-Ph), 2.71 (s, 16H, CH2-cyclen). 13C NMR (CDCl3)
d 139.84, 136.77, 135.85, 129.06, 125.89, 112.99, 59.78, 53.12. ESI(+)-
HRMS calcd for C44H53N+ [M+H]+: 637.4265; found: 637.4265.
5 D. Kong, X. Huang and Y. Xie, Inorg. Chim. Acta, 2002, 340, 133;
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The latter characteristics are very promising for the fabrication
of laser target components intended for inertial confinement
fusion (ICF) experiments.6
In terms of application, the fibers obtained by self assembly
starting from various metals constitute a novel, simple and
tunable means of developing organic core–shell materials.
This research was supported by the Laser MegaJoule (LMJ)
´
Program and especially by the Target Technologies Project.
The authors wish to thank their colleagues of CEA Le Ripault
for their helpful collaborations: S. Lambert for Scanning
Transmission Electron Microscopy (STEM) images, J-C.
Birolleau for carrying out elemental analysis and P. Palmas
for NMR spectra. Many thanks expressed also to M. Giorgi
6 W. P. Steckle Jr and A. Nobile Jr, Fusion Sci. Technol., 2003, 43,
301; N. G. Borisenko, A. E. Bugrov, I. N. Burdonskiy,
I. K. Fasakhov, V. V. Gavrilov, A. Yu. Goltsov, A. I. Gromov,
A. M. Khalenkov, N. G. Kovalskii, Yu. A. Merkuliev,
V. M. Petryakov, M. V. Putilin, G. M. Yankovskii and
E. V. Zhuzhukalo, Laser Part. Beams, 2008, 1; C. Croix,
A. Balland-Longeau, H. Allouchi, M. Giorgi, A. Duchene and
J. Thibonnet, J. Organomet. Chem., 2005, 690, 4835; L. Moreau,
C. Levassort, B. Blondel, C. De Nonancourt, C. Croix, J. Thibonnet
and A. Balland-Longeau, Laser Part. Beams, 2009, 27, 537;
A. Balland-Longeau, L. Moreau, J. Thibonnet and E. Velasquez,
FR0758126.
for X-ray Crystallography (Spectropole, Universite
´
Marseille)
and C. Bressac (IRBI, Universite Tours) for optical micro-
´
graphs of the fibers.
Notes and references
1 J. Venugopal, S. Low, A. T. Choon and S. Ramakrishna1,
J. Biomed. Mater. Res., Part B, 2008, 84b, 34; L. Zang, Y. Che
and J. S. Moore, Acc. Chem. Res., 2008, 41, 1596; K. M. Sawicka
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L. Ye, P. Stenlund and I. S. Chronakis, Chem. Commun., 2008,
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This journal is The Royal Society of Chemistry 2010
1466 | Chem. Commun., 2010, 46, 1464–1466