Strategies in Organic Synthesis for Condensed Arenes, Coronene, and Graphene - Polyarenes - page 131

Fig. 3 Different edges of
graphene molecules

Zig-zag edge

Armchair edge

and 67 (Scheme 17 , (1) and (2)) or the alkyne 70 and 75 (Scheme 17 , (3) and (4)) or
both (Scheme 17 , (5)), PAHs with different edge types have been prepared.
Introduction of different phenanthrenyl units into the precursor 75 allowed modifi-
cation of the edge from fully armchair to partially zig-zag. Phenanthrene alkyne
derivatives 75 were used for the PAHs with one zig-zag-edge, pyrene cyclopenta-
dienones 79 for the two zig-zag edged PAHs. The synthesis was made using alkyl
substituted building blocks to ensure solubility, thus facilitating the characterization
and analysis of their properties. The properties of such zig-zag-edge molecules are
enhanced over PAHs with an armchair edge obtained by the usual strategy.

2.3 Cyclotrimerization

Alkyne cyclotrimerization is an elegant strategy for the synthesis of symmetrically
substituted HBCs [ 45 ]. Similar to the Diels-Alder strategies described in the
previous chapter, the product of the cycloaddition, the polyphenylene, can be
cyclodehydrogenated to provide the final graphene-type structure. A general exam-
ple is shown in Scheme 18 .
With this method, a methoxy substituted polyphenylene precursor was
synthesized. The molecule was adsorbed on a copper surface and the cyclodehy-
drogenation was thermally induced directly on the surface [ 46 ]. The synthesis of
substituted HBCs permits tuning of the properties in this class of compounds, for
example in a columnar supramolecular assembly [ 47 ]. The influence of substitution
at the periphery with either oligoether (water soluble) or alkyl chains with different
degrees of branching and different lengths has been studied [ 48 ].
The tuning of properties by synthesizing HBCs with functional groups also
allows new applications. One example is the cyclotrimerization of double
substituted tolane 85 (methylaryl and methoxyaryl protected arylamine), which
yielded both possible isomers 86 and 87 in statistical yields (Scheme 19 ). These