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M. Gille et al.
LETTER
tuned, for example, by doping graphene with other atoms reactive groups suitable for the synthesis of graphene rib-
and/or the use of graphene ribbons of defined width and bons.
edge structures. Graphene ribbons are made using either
The synthetic route to compounds 1a–d is summarized in
Scheme 2. In a first step a twofold Knoevenagel conden-
template-mediated growth4 or longitudinal unzipping of
carbon nanotubes.5 None of these methods can provide
sation generates the cyclopentadienone13 core that reacts
uniform edge structures or exact tunable widths, and prop-
as the diene in the subsequent Diels–Alder reaction with a
erties of functionalized graphene nanoribbons are so far
(di)halogenated tolane derivative. Immediate aromatiza-
treated mainly theoretically.6 Therefore, a bottom-up ap-
tion by CO extrusion yields the corresponding hexa-
proach with preorganized graphene monomers yielding
defect-free graphene ribbons with a well-defined and (if
needed) functionalized edge structure is required. For this
phenylbenzene derivatives 4a–d.14 In the case of 4c,d the
missing phenyl rings are introduced in an intermediate
step via a Suzuki coupling to yield 5c,d, which can be pla-
narized, as well as 4a,b to yield the desired HBC mono-
purpose we envisioned to use Ullmann-type on-surface
polymerization on gold surfaces.7,8 In the following we
mers 1a–d using the Scholl protocol with anhydrous
describe a modular concept for the synthesis of potential
monomers for graphene nanoribbons.
FeCl3 as oxidizing agent.15,16
By applying our recently developed sequential growth
If one thinks of dibromo-substituted hexabenzocoronene
approach17 it should be possible to prepare graphene-type
(HBC) as basic monomer unit, providing 42 carbon atoms
networks on gold surfaces. An appropriate HBC monomer
arranged already in the right way, one faces the problem
would provide two iodo substituents for the initial 1D po-
of the formation of wires with gaps that need to be filled
lymerization and four bromo substituents for the consecu-
in order to generate ribbons (Scheme 1, a). Adding two
tive 2D cross-linking step (Scheme 3). Therefore the HBC
derivative 9 was synthesized to deliver first information
more benzene units per monomer can solve this problem.
We therefore decided to use trapezoidal monomers
about the difficulties of this particular concept (Scheme
(Scheme 1, b).
4).
The underlying chemistry for the synthesis of these mole-
To conclude, we accomplished the synthesis of possible
cules has been pioneered mainly by Scholl et al. as early
monomers for graphene nanoribbons, wires, as well as
as in 19129 and Dilthey et al. in the 1930s10 and was redis-
graphene-type networks with predefined graphene struc-
covered and optimized by the Müllen group in the begin-
ture in order to minimize the number of structural defects.
ning of the 1990s.12 Herein, we describe the synthesis of
Currently, we – in collaboration with surface physicists –
new derivatives (Figure 1) with original geometries and
test these monomers in on-surface polymerization reac-
Br
Br
I
I
1a
1b
Br
Br
Br
Br
1c
1d
Figure 1 Synthesized monomers: monomers 1a,b for graphene wires and trapezoidal monomers 1c,d for graphene nanoribbons
Synlett 2013, 24, 259–263
© Georg Thieme Verlag Stuttgart · New York
Gille, Marie
