Conjugated polymers are often poorly soluble in conventional organic solvents.
This makes processing from solution and deposition of thin films, as required for
electronic devices, difficult [92–100]. Attachment of solubilizing alkyl groups is
then indispensable – as long as they do not obstruct the extended π-conjugation.
Such substituents may be considered as diluting the electronic effect, but often
play an important role in controlling the packing and self-assembly. An alternative
way of dealing with intractable polymers is the use of a precursor synthesis.
Thereby, a soluble precursor polymer is synthesized, cast into a film, and
transformed by heat or irradiation into the supposedly insoluble conjugated target
polymer. A well-known case is poly(phenylenevinylene) synthesis [74, 101, 102],
where the vinylene unit is established by a 1,2-elimination. A special kind of
precursor route will later be introduced for the synthesis of graphene nanoribbons.
It is clear, however, that the perfection of the final transformation is decisive for the
quality of the material and its performance in a device [103].
Polymer synthesis is thus determined both by the method applied and the
envisaged target structure. We now focus on target materials that have played an
important role in the development of polymer synthetic methods and that have
seen a remarkable renaissance in the service of chemical graphene synthesis.
3 Ribbon (or Ladder) Polymers
Special emphasis is paid to a modular approach in which functionalized benzene
moieties serve as building blocks for chain-type (1D), disc-type (2D), and
dendrimer-type (3D) graphene derivatives (Fig. 1) [104, 105]. The critical question,
when taking on graphene as a challenge for polymer synthesis, is whether any of
these polyphenylenes can serve as precursors for graphenes and how the necessary
chemical transformation to flat graphene sheets can be accomplished.
It is interesting, indeed, to first present three cases taken from the mid-1990s in
which our group focused on the synthesis of so-called ribbon or ladder polymers.
These macromolecules have paved the way to graphene nanoribbons, although at
that time we did not refer to them as GNRs.
Linear poly(para-phenylene)s 1 are prototypical conjugated polymers [106–108].
Electronically, they are characterized by a wide band gap and have played an
important role as blue emitters in light-emitting diodes [109–112]. Dieter Schlu ¨ter
and Gerhard Wegner [79, 113–116] introduced the Suzuki coupling [49, 117] for
the synthesis of poly(dialkylphenylene)s, marking a milestone for the synthesis
of conjugated polymers (Scheme 1). However, the alkyl substituents required for
solubilization cause significant torsion about the inter-ring bonds and thus hamper
extended π-conjugation [118, 119].
Therefore, it was our idea to proceed from linear to ladder-type polyphenylene
structures by bridging neighboring benzene rings via methylene groups. This would
not only enforce a planarization of the whole π-system, but also provide extra
carbons to attach solubilizing alkyl chains without compromising the conjugation.
Graphene as a Target for Polymer Synthesis
65
This makes processing from solution and deposition of thin films, as required for
electronic devices, difficult [92–100]. Attachment of solubilizing alkyl groups is
then indispensable – as long as they do not obstruct the extended π-conjugation.
Such substituents may be considered as diluting the electronic effect, but often
play an important role in controlling the packing and self-assembly. An alternative
way of dealing with intractable polymers is the use of a precursor synthesis.
Thereby, a soluble precursor polymer is synthesized, cast into a film, and
transformed by heat or irradiation into the supposedly insoluble conjugated target
polymer. A well-known case is poly(phenylenevinylene) synthesis [74, 101, 102],
where the vinylene unit is established by a 1,2-elimination. A special kind of
precursor route will later be introduced for the synthesis of graphene nanoribbons.
It is clear, however, that the perfection of the final transformation is decisive for the
quality of the material and its performance in a device [103].
Polymer synthesis is thus determined both by the method applied and the
envisaged target structure. We now focus on target materials that have played an
important role in the development of polymer synthetic methods and that have
seen a remarkable renaissance in the service of chemical graphene synthesis.
3 Ribbon (or Ladder) Polymers
Special emphasis is paid to a modular approach in which functionalized benzene
moieties serve as building blocks for chain-type (1D), disc-type (2D), and
dendrimer-type (3D) graphene derivatives (Fig. 1) [104, 105]. The critical question,
when taking on graphene as a challenge for polymer synthesis, is whether any of
these polyphenylenes can serve as precursors for graphenes and how the necessary
chemical transformation to flat graphene sheets can be accomplished.
It is interesting, indeed, to first present three cases taken from the mid-1990s in
which our group focused on the synthesis of so-called ribbon or ladder polymers.
These macromolecules have paved the way to graphene nanoribbons, although at
that time we did not refer to them as GNRs.
Linear poly(para-phenylene)s 1 are prototypical conjugated polymers [106–108].
Electronically, they are characterized by a wide band gap and have played an
important role as blue emitters in light-emitting diodes [109–112]. Dieter Schlu ¨ter
and Gerhard Wegner [79, 113–116] introduced the Suzuki coupling [49, 117] for
the synthesis of poly(dialkylphenylene)s, marking a milestone for the synthesis
of conjugated polymers (Scheme 1). However, the alkyl substituents required for
solubilization cause significant torsion about the inter-ring bonds and thus hamper
extended π-conjugation [118, 119].
Therefore, it was our idea to proceed from linear to ladder-type polyphenylene
structures by bridging neighboring benzene rings via methylene groups. This would
not only enforce a planarization of the whole π-system, but also provide extra
carbons to attach solubilizing alkyl chains without compromising the conjugation.
Graphene as a Target for Polymer Synthesis
65