reactive dienes and dienophiles there remain troublesome issues such as
solubilizing the rigid ribbon structures and, again, of transforming the precursors
into the target polymers by dehydrogenation of cyclohexadiene and by deoxygenation of oxa-bridged cyclohexadiene repeat units [147]. Interestingly, graphene
oxide (see Sect. 8), a product of graphite oxidation, is nowadays used as an
intermediate in graphite exfoliation and believed to incorporate, among many other
oxygen-containing functions, epoxy groups. After processing, graphene oxide must
be reduced back to graphene, a process that is normally far from complete.
Similarly, neither the deoxygenation nor the dehydrogenation of the ribbon
precursor polymers 16a are believed to proceed quantitatively, and the process is
further complicated by the anticipated chemical instability of the target ribbon
structure.
4 Polycyclic Aromatic Hydrocarbons as Nanographenes:
The Precursor Route
Before dreaming up improved ways to synthesize graphene nanoribbons and their
precursors, another synthetic concept must be introduced, and that is cyclodehydrogenation of non-planar oligo- and polyphenylenes towards graphene-like benzenoid
polycycles. This reaction has played an important role in our search for graphenes
and, like the above transformations, has served as a key tool in fabricating 2D
polymers.
In 1995, our attention was attracted to the synthesis of larger and larger polycyclic
aromatic hydrocarbons (PAHs). These π-systems, originally pioneered by Erich Clar
[148, 149], have long been investigated as test cases for spectroscopy and molecular
orbital theory, as carbon-containing constituents of interstellar space [150, 151] and,
more recently, as organic semiconductors in electronic devices [103]. It was only a
decade after our synthesis of giant PAHs that they became considered as minisubunits of graphenes and the term “nanographenes” was coined to define graphene
structures smaller than 100 nm in size [152]. Our starting point was the high-yield
cyclodehydrogenation of the propeller-shaped hexaphenylbenzene 23a toward hexaperi-hexabenzocoronene 23 upon treatment with oxidants such as iron(III)chloride in
dichloromethane at room temperature [153] (Scheme 6).
While appearing conceptually simple, the mechanism of this “graphitization”
is quite complex. Two neighboring phenyl groups undergo an electrocyclic ring
closure after arenium cation or radical cation formation followed by the elimination
of protons [154, 155]. The stepwise flattening of all phenyl units can proceed in
different ways, where the radical cation mechanism is less favorable for larger
systems [156].
In our hands, the “superbenzene” (C42) 23 was the starting member of a whole
new PAH family of varying sizes and symmetries [157, 158] (Fig. 4). A key structural
modification relates to the nature of the periphery. Although we have in Fig. 4
70
K. Mu ¨llen
solubilizing the rigid ribbon structures and, again, of transforming the precursors
into the target polymers by dehydrogenation of cyclohexadiene and by deoxygenation of oxa-bridged cyclohexadiene repeat units [147]. Interestingly, graphene
oxide (see Sect. 8), a product of graphite oxidation, is nowadays used as an
intermediate in graphite exfoliation and believed to incorporate, among many other
oxygen-containing functions, epoxy groups. After processing, graphene oxide must
be reduced back to graphene, a process that is normally far from complete.
Similarly, neither the deoxygenation nor the dehydrogenation of the ribbon
precursor polymers 16a are believed to proceed quantitatively, and the process is
further complicated by the anticipated chemical instability of the target ribbon
structure.
4 Polycyclic Aromatic Hydrocarbons as Nanographenes:
The Precursor Route
Before dreaming up improved ways to synthesize graphene nanoribbons and their
precursors, another synthetic concept must be introduced, and that is cyclodehydrogenation of non-planar oligo- and polyphenylenes towards graphene-like benzenoid
polycycles. This reaction has played an important role in our search for graphenes
and, like the above transformations, has served as a key tool in fabricating 2D
polymers.
In 1995, our attention was attracted to the synthesis of larger and larger polycyclic
aromatic hydrocarbons (PAHs). These π-systems, originally pioneered by Erich Clar
[148, 149], have long been investigated as test cases for spectroscopy and molecular
orbital theory, as carbon-containing constituents of interstellar space [150, 151] and,
more recently, as organic semiconductors in electronic devices [103]. It was only a
decade after our synthesis of giant PAHs that they became considered as minisubunits of graphenes and the term “nanographenes” was coined to define graphene
structures smaller than 100 nm in size [152]. Our starting point was the high-yield
cyclodehydrogenation of the propeller-shaped hexaphenylbenzene 23a toward hexaperi-hexabenzocoronene 23 upon treatment with oxidants such as iron(III)chloride in
dichloromethane at room temperature [153] (Scheme 6).
While appearing conceptually simple, the mechanism of this “graphitization”
is quite complex. Two neighboring phenyl groups undergo an electrocyclic ring
closure after arenium cation or radical cation formation followed by the elimination
of protons [154, 155]. The stepwise flattening of all phenyl units can proceed in
different ways, where the radical cation mechanism is less favorable for larger
systems [156].
In our hands, the “superbenzene” (C42) 23 was the starting member of a whole
new PAH family of varying sizes and symmetries [157, 158] (Fig. 4). A key structural
modification relates to the nature of the periphery. Although we have in Fig. 4
70
K. Mu ¨llen