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SEMICONDUCTING CONJUGATED POLYMERS
During the past three decades
electrically conducting or semiconducting conjugated
polymers have attracted an overwhelming interest in
laboratories around the globe. These materials may
combine the processability and outstanding mechanical
characteristics of polymers with the readily-tailored
electrical, optical and magnetic properties of
functional organic molecules. In particular the
potential use of these materials in light-emitting
diodes, field-effect transistors, photovoltaic cells,
and other opto-electronic devices has motivated the
development of synthesis and processing methods of
conjugated polymer materials with unique properties. The
electronic characteristics of these materials are
primarily governed by the nature of the molecular
conjugation, but intermolecular interactions also exert
a significant influence on the macroscopic materials
properties. |
PPEs
Among a variety of materials
platforms, poly(arylene ethynylene) (PAE)
derivatives have attracted the attention of a
growing number of research groups. Hundreds of
different PAEs have been reported to date, and
during the last ten years this family of
conjugated polymers has established itself as an
important class of materials with interesting
optical and electronic properties. The
spectacular progress made on many frontiers has
propelled PAEs into the scientific mainstream,
and many technologically relevant applications
that utilize these polymers have been spurred.
Our group has a specific interest in the
structure-property relationships of poly(p-phenylene
ethynylene)s (PPE)s and for many years we have
studied and used this family of conjugated
polymers.
Many of our PPE activities are summarized in the recently
published monograph
Poly(arylene ethynylene)s - From Synthesis to
Applications; Advances in Polymer Science Series Vol.
177; Weder, C., Ed.; Springer, Heidelberg, 2005.
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Conjugated Polymer Networks
The charge-carrier mobility
of conjugated polymers is usually limited by
disorder effects, which prevent efficient
inter-chain coupling. Rapid charge-transport,
however, is important for the full exploitation
of organic semiconductors in field-effect
transistors, photovoltaic cells and other
applications. Our approach to solve this problem
is based on the synthesis of well-defined
conjugated polymer networks. We demonstrated
that the intrinsic problem of synthesizing and
processing these by default insoluble and
non-melting cross-linked materials can readily
be overcome by two different general approaches.
In collaboration with
Dr. Kenneth Singer (Physics Department CASE) we
showed that polymer networks display
significantly enhanced charge-carrier mobility.
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Structure of covalently cross-linked conjugated
PPE.
One concept for the
preparation of tractable CP networks relies on
synthesizing covalently cross-linked conjugated
polymers in the form of aqueous micro- and
mini-emulsions, which can easily be processed by
standard techniques. This approach exploits that
some of the metal-catalyzed cross-coupling
reactions employed for the synthesis of
conjugated macromolecules - such as the
Sonogashira coupling used by us for the
preparation of PPEs - are tolerant to the
presence of water. We began to expand the
experimental program to other selected classes
of conjugated polymers as well as to
hyperbranched conjugated macromolecules.
Preliminary studies suggest that the latter may
be extremely useful for sensor applications that
rely on the principle of amplified fluorescence
quenching.
Pictures of cross-linked
conjugated PPE milli- (left), micro- (center),
and nanoparticles (right). |
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Another possibility for the synthesis of conjugated network
structures is the use of non-covalent interactions. Recent
work of our group focused on the synthesis and investigation
of organometallic networks, which are formed through
coordination bonds between ligand sites comprised in the
organic semiconductor and metallic cross-links. These
metallopolymers are accessible either via ligand-exchange
reactions or, alternatively, through the polymerization of
pre-fabricated ligand-metal complexes.
Much of our work on
Conjugated Polymer Networks is summarized in a recently
published review article:
Weder, C.; Conjugated Polymer Networks; Chem. Comm.
2005, 5378-5389.
Other Selected Recent
Publications:
Kokil, A.; Yao, P.;
Weder, C.; Organometallic Networks Based on
2,2'-Bipyridine-Containing Poly(p-phenylene ethynylene)s; Macromolecules 2005,
38, 3800-3807.
Hittinger, E.; Kokil, A.; Weder, C.; Synthesis and
Characterization of Cross-Linked Conjugated Polymer Milli-,
Micro-, and Nanoparticles; Angew. Chem. Int. Ed.
2004, 43, 1808-1811. (Angew. Chem. 2004,
116, 1844-1847).
Kokil, A.; Shiyanovskaya,
I.; Singer, K.D.; Weder, C.;
Charge Transport in p-Conjugated
Organometallic Polymer Networks; J. Am. Chem. Soc.
2002, 124, 9978-9979.
Metallosupramolecular Conjugated Polymers
In collaboration with
Prof. Stuart Rowan (Macromolecular Science
and Engineering CASE), we began to expand our
activities to linear
metallosupramolecular
conjugated polymers based on metal-ligand
interactions. Of interest to us are the
fundamental aspects of charge transport through
the ligand-metal portion and the design of
conjugated polymers which combine ease of
processing and good mechanical properties. We
are also interested to explore the nature of
supramolecular architectures that can be
obtained by using self-assembly schemes,
supramolecular organic-inorganic hybrid polymers,
and the use of organometallic
conjugated systems
in sensor applications.
Selected Recent
Publications:
Knapton, D.;
Rowan, S.J.; Weder, C.; Synthesis and Properties of Metallo-Supramolecular Poly(p-phenylene ethynylene)s;
Macromolecules 2006, 39, 651-657.
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