In a number of papers, our group has proven the high degree of structural
perfection of these dendrimers, even for molecular weights in excess of 1 MDa
[192–194]. Key roles were played by Martin Baumgarten, Andreas Herrmann, and
Tanja Weil. Surprisingly, these giant dendrimers are soluble in organic solvents and,
in many cases, the solubility actually increases with molecular weight. Due to their
shape-persistence (note that back-bending of the dendron arms is not possible), these
dendrimers can serve as molecularly defined, functional nanoparticles [182, 195].
This stands in marked contrast to the situation prevailing in dendrimers made from
conformationally flexible repeat units [182, 183]. Indeed, the semirigid character
of the PPD scaffolds leads to a perfect nanosite definition of functional groups
[190] such as chromophores [196, 197], catalysts [190, 198, 199], or electrolytes
[200] at the core, in the scaffold, or on the rim of the dendrimers [201]. Although
these aspects are beyond the scope of the present text, it should be mentioned
that polyphenylene dendrimers, due to their unique structure and functionalization,
have served, for example, as light harvesting complexes [133, 202], receptors for
gas sensing [203, 204], and carrier systems for crossing the blood–brain barrier
[205, 206].
6 Graphene Nanoribbons: The “Solution” Approach
In turning now to the synthesis of GNRs, the design of the precursor polyphenylene
structures is the first and decisive step [207]. In this realm, we have synthesized
a series of polyphenylenes, mostly using transition metal-catalyzed aryl–aryl
coupling mechanisms (i.e., Suzuki and Yamamoto), with varying aspect ratios,
and we have investigated their dehydrogenation [208]. Figure 7 presents only
one example. As learned from the oligomeric model cases, the accessibility of the
monomeric starting compounds, the ease of their polymerization by aryl–aryl
coupling, and the perfection of the final cyclodehydrogenation are the key criteria
in judging the chemical pathways to GNRs.
Also, the molecular weights and thus the lengths of the final GNRs are critical
parameters in subsequent physical experiments [208, 209]. These include, for
example, the attachment of electrodes for charge transport measurements. A key
problem concerns solubility and solution processability. Not unexpectedly from
our experience with polyphenylene dendrimers, many precursor polymers were
soluble in organic solvents, even without additional alkyl substitution, and could be
well characterized. However, the oxidative dehydrogenation led to flat, completely
insoluble materials and this, of course, also hampered a determination of the
structural perfection of the graphenic structure [210]. Three questions had thus to
be dealt with: (i) how could we keep the synthesized GNRs soluble, (ii) how could
we synthesize non-planar polyphenylene precursors with high molecular weights,
and (iii) how could we control the widths to tune the resulting band gaps? [16]
A repetitive Diels–Alder cycloaddition between the “double” diene 34b and
the “double” dienophile (AA+BB) afforded high molecular weights of precursor
76
K. Mu ¨llen
perfection of these dendrimers, even for molecular weights in excess of 1 MDa
[192–194]. Key roles were played by Martin Baumgarten, Andreas Herrmann, and
Tanja Weil. Surprisingly, these giant dendrimers are soluble in organic solvents and,
in many cases, the solubility actually increases with molecular weight. Due to their
shape-persistence (note that back-bending of the dendron arms is not possible), these
dendrimers can serve as molecularly defined, functional nanoparticles [182, 195].
This stands in marked contrast to the situation prevailing in dendrimers made from
conformationally flexible repeat units [182, 183]. Indeed, the semirigid character
of the PPD scaffolds leads to a perfect nanosite definition of functional groups
[190] such as chromophores [196, 197], catalysts [190, 198, 199], or electrolytes
[200] at the core, in the scaffold, or on the rim of the dendrimers [201]. Although
these aspects are beyond the scope of the present text, it should be mentioned
that polyphenylene dendrimers, due to their unique structure and functionalization,
have served, for example, as light harvesting complexes [133, 202], receptors for
gas sensing [203, 204], and carrier systems for crossing the blood–brain barrier
[205, 206].
6 Graphene Nanoribbons: The “Solution” Approach
In turning now to the synthesis of GNRs, the design of the precursor polyphenylene
structures is the first and decisive step [207]. In this realm, we have synthesized
a series of polyphenylenes, mostly using transition metal-catalyzed aryl–aryl
coupling mechanisms (i.e., Suzuki and Yamamoto), with varying aspect ratios,
and we have investigated their dehydrogenation [208]. Figure 7 presents only
one example. As learned from the oligomeric model cases, the accessibility of the
monomeric starting compounds, the ease of their polymerization by aryl–aryl
coupling, and the perfection of the final cyclodehydrogenation are the key criteria
in judging the chemical pathways to GNRs.
Also, the molecular weights and thus the lengths of the final GNRs are critical
parameters in subsequent physical experiments [208, 209]. These include, for
example, the attachment of electrodes for charge transport measurements. A key
problem concerns solubility and solution processability. Not unexpectedly from
our experience with polyphenylene dendrimers, many precursor polymers were
soluble in organic solvents, even without additional alkyl substitution, and could be
well characterized. However, the oxidative dehydrogenation led to flat, completely
insoluble materials and this, of course, also hampered a determination of the
structural perfection of the graphenic structure [210]. Three questions had thus to
be dealt with: (i) how could we keep the synthesized GNRs soluble, (ii) how could
we synthesize non-planar polyphenylene precursors with high molecular weights,
and (iii) how could we control the widths to tune the resulting band gaps? [16]
A repetitive Diels–Alder cycloaddition between the “double” diene 34b and
the “double” dienophile (AA+BB) afforded high molecular weights of precursor
76
K. Mu ¨llen