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Insulated Nanowire Bundles through Consecutive Template Synthesis.

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Highlights
Host/Guest Polymerization
Insulated Nanowire Bundles through Consecutive
Template Synthesis
Stefan Spange*
Keywords:
host–guest systems · polymerization · sol–gel
processes · template synthesis · zeolites
Since
the discovery of hexagonal
MCM-41[1] and related silicate materials
with a defined pore radius,[2] hardly a
week goes by when a new application or
specification of these materials is not
reported. The search engine Google
currently finds more than 4000 entries
on MCM-41 on the Internet.
It is extraordinary how the scientific
fantasy and experimental art of chemists
is pointing to new targets and directions
for materials science in the area of
organic–inorganic hybrid materials. An
example is the targeted introduction of
organic functional groups into the interior of the MCM-41 channels.[3] The
palette currently extends from photoswitchable morphology control, through
cis–trans isomerization of stilbene units
covalently incorporated into the wall of
MCM-41,[4] and the directed energy
transfer between internally immobilized
porphyrin units,[5] to photoinduced reversible liberation of guest molecules
from coumarin-modified MCM-41.[6]
The development of new generations
of solid catalysts immobilized in MCM41 channels has expanded into a research area of its own as shown by the
exponential growth in the number of
original scientific publications and patents.[7–10]
A particularly important challenge is
the formation of molecular wires of
electrically and photochemically active
[*] Prof. Dr. S. Spange
Institut fr Chemie
Technische Universit"t Chemnitz
Strasse der Nationen 62
09111 Chemnitz (Germany)
Fax: (+ 49) 371-531-1642
E-mail: stefan.spange@chemie.
tu-chemnitz.de
4430
compounds.[11a] To be able to generate
molecular wires as desired and with
defined lengths to be placed between
two molecular or supramolecular poles,
they must be completely insulated externally to avoid short circuits owing to
faulty contacts or electrical interference.[11b] The materials from which these
wires are made, doped polypyrrole,
polythiophene, and polyacetylene, are
extremely sensitive to oxidation so insulation is also necessary for chemical
reasons.[12, 13] The encapsulation of conducting polymers can be carried out
using various strategies.[11]
A possible solution to this insulation
problem is given by a new and amazingly simple but elegant method. A
procedure was recently published in
two papers which appeared almost simultaneously.[14, 15] To better understand
these concepts, the three basic procedures for encapsulating conducting polymers in mesoporous materials are presented briefly here.
A) The already synthesized polymer
is encapsulated using organic vinyl monomers in a sol–gel process or by crosslinking copolymerization. Such procedures, which were developed for various
types of organic templates,[16, 17] are however, not applicable or only have limited
applicability for mesoporous materials,
as conducting polymers are usually very
poorly soluble and are chemically extremely sensitive when doped, that is,
they react readily with oxygen and
water. To what extent such wires can
be moved over insulator surfaces, arranged between poles and then insulated through chemical processes and fixed
is still a challenge. For semiconducting
polymers, however, template procedures could be used.[18]
B) The guest polymer is produced by
polymerization in the insulating
host.[19–26] There has been much research
activity in recent years in the area of
host–guest polymerization, which has
included more than just the synthesis
of molecular wires of polypyrrole or
polythiophene in inorganic hosts. In
particular the pioneering work of Bein
and co-workers should be mentioned in
this context.[20] Inspired by this work,
various synthesis procedures have been
developed to generate polymers directly
in inorganic hosts, such as HY-zeolites
or MCM-41, by using various polymerization techniques. Vinyl monomers can
be polymerized using radicals,[20] with
immobilized transition-metal complexes,[21–23] or cationically as the organic
guest in an inorganic host.[24] The polymerization of aniline, pyrrole, or thiophene is initiated in the inorganic host
using redox catalysts, preferably FeCl3
or immobilized Cu2+ ions.[20, 25, 26]
C) The inorganic insulating layer
and the potentially electrically conducting polymer are constructed simultaneously. A first very promising experiment
was carried out by Aida et al. two years
ago.[27] Substituted diacetylenes were
incorporated covalently into MCM-41
and converted into polyenynes by thermally induced polymerization. Polyenynes are often insoluble and difficult
to characterize.[27] Furthermore polyenynes are only suitable as nanowires
under certain conditions because of
energetically unfavorable band gaps.
However, important pioneering conceptual work was possible with these inorganic–organic hybrid materials.
Route C is particularly promising, as
the solubility problems of electrically
conducting polymers and the slow ad-
DOI: 10.1002/anie.200301654
Angew. Chem. Int. Ed. 2003, 42, 4430 –4432
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angewandte
Chemie
sorption of the guest monomer into the
inorganic host are circumvented. The
latter effect often leads to a broad
molecular-mass distribution of the encapsulated polymer or to blockage of
the nanopore entries of the host,[24, 25] as
polymer fractions are formed initially in
the entry area of the nanopores as a
result of catalyst diffusion.
A prerequisite for the synthesis of
defect-free polymer chains, that is, molecular wires with no breaks, is the
regular and synergistic formation of
the channel with monofunctional monomer building blocks in the hybrid material. The simultaneous formation of the
MCM-41 framework and the linear,
immobilized, guest-monomer components in a balanced stoichiometric ratio
has been realized and verified by two
research groups almost simultaneously.
Aida's group used a template with a
terminal N-functionalized pyrrole group
as the polymerizable monomer along
with a non-polymerizable cotemplate
(usually
dodecyltrimethylammonium
bromide) for the known MCM-41
framework synthesis using tetramethoxylsilane (TMOS) in the sol–gel process.[14] After MCM-41 formation the
post-functionalization of the immobilized pyrrole unit to a one-dimensional
(1D) polypyrrole wire is effected by
FeCl3 (Figure 1). A slight modification
of the procedure allows the construction
of two-dimensional (2D) polypyrrole
structures. For example, if di(dodecyl)dimethylammonium bromide is used as
a cotemplate then morphology control
and formation of lamellar silicate films
are achieved.
Li, Fuhrhop et al. had almost the
same idea for the 1D polymer structure.[15] They used terminal thiophen-3yl-substituted trimethylammonium salts
as the monomer template (Figure 2). As
in the previous example, the redox
catalyst FeCl3 was used to initiate the
polymerization of the immobilized thiophene units giving a polythiophene
guest polymer. The channel diameter is
exactly 3 nm. After the inorganic matrix
has been dissolved in HF the encapsulated polymers can be readily investigated because of the solubilizing properties of the side chains (see also ref. [18].
For the characterization of polymers in
MCM-41 see ref. [24c]). The defect-free
polythiophene chains with lengths of
Angew. Chem. Int. Ed. 2003, 42, 4430 –4432
Figure 1. Template synthesis of a polypyrrole/MCM-41 nanocomposite using 1-[12-(pyrrol-1-yl)dodecyl]trimethylammonium or 1-[10-(pyrrol-1-yl)decyltrimethylammonium bromide (1) with
tetramethoxysilane in a sol–gel process and subsequent polymerization with FeCl3.[14]
Figure 2. Scheme showing the production of polythiophene chains in mesoporous MCM-41.
The template monomer trimethyl[11-thiophen-3-yl-undecyl)ammonium bromide (2) is first synthesized, then inserted into the MCM-41 (Figure 1), and finally the polymerization is initiated by
FeCl3. The polymer chains are liberated by dissolving the inorganic host in HF. Replacing Br by
PF6 ions leads to extension and association of the polymer chains.[15]
100 nm (corresponding to ca. 330 thiophene units) were thus prepared.
It is particularly pleasing to see how
the nature of the counterion controls the
conformation of the C11-chains with
terminal trimethylammonium groups
bonded to 2,2’-linked thiophene units.
Replacing the solvating bromide by the
non-nucleophilic PF6 ion leads to alignment of the chains (see Figure 2). Thus
the method of Li and Fuhrhop[15] also
offers the possibility, by using the MCM41 template effect, of obtaining watersoluble polymers which are potentially
electrically conducting, with defined
molecular masses on a gram scale. As a
result, by use of template co-monomers
and the current synthesis routes employing MCM-41, materials scientists have a
www.angewandte.org
very promising tool available for immobilizing membranes or porous films with
enclosed bundles of wires on conducting
catalysts.[28] These examples will be able
to be used for preliminary experiments
on linking nano building blocks and
molecular wires. A further possibility is
the use of templates functionalized at
both ends which contain both the organic monomer unit and a trialkoxysilane group as the potential inorganic
component. Although the problem of
precisely adjusting the reaction conditions still exists, with this method it may
be possible to produce a conducting
molecular wire with an insulating silicate layer.[28] These ideas and the possibilities offered by the synthetic procedures certainly bring the possibility of
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4431
Highlights
formation of molecular wires made up
of linearly arranged monomers nearer.
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