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NO 2
o -C 6 H 4 Cl 2
175 ºC
122
175
10-20%
Scheme 46 Extension of an armchair edge via Diels-Alder reaction [ 111 ]
Diels-Alder reaction to form bay regions on PAHs [ 111 ]. They also showed that
pure acetylene effects the transformation, although the reaction is not as efficient
(Scheme 46 )[ 112 ].
Unfortunately, this strategy does not allow for the infinite growth of large
graphene sheets from smaller PAHs fragments. For a successful reaction a bay
region at the periphery must be present. While these are always regenerated in the
case of carbon nanotubes, this is not true for 2D graphene-type structures.
The use of bifunctional dienophiles, such as parabenzoquinone, has been
demonstrated as an efficient way to join PAHs to larger graphene-type structures
[ 113 ].
4 Surface Assisted Methods for the Synthesis
of Graphene-Type Structures
Solubility is the issue inhibiting the preparation of larger and larger PAHs from
becoming more and more graphene like. For application purposes, e.g., in the area
of molecular electronics, the final products have to be transferred to a surface to be
able to address the compounds, ideally individually. Therefore, a preparation
method directly on a surface would be highly useful [ 114 ]. Additionally, it would
offer new possibilities in synthesis. Solubility would no longer be an issue. As
described above, most transformations require a metal as catalyst or as mediator in
stoichiometric amounts. This function can also be fulfilled by a metal surface.
Furthermore, the structure of the surface can be utilized to enhance selectivity in
the synthesis process. Modern microscope techniques also allow the direct obser-
vation of individual molecules on surfaces, allowing an easy in situ monitoring of
the assembly.
Ten years ago the group of M¨ llen prepared PAHs and graphite islands,
representing graphene-type structures, via surface-induced reaction of small
molecules [ 115 ]. Although the reaction was unselective, the study demonstrated
the general feasibility of such an approach (Scheme 47 )[ 11 , 14 ].
In collaboration with the group of Cai and Ruffieux, M ¨ llen took the strategy a
step further by employing a diiodobisanthracene derivative as a precursor for the
formation of nano-ribbons [ 116 , 117 ]. First, an Ullmann-type coupling on the
surface connects the individual building blocks, which will then be dehydrogenated
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