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the surface leading to a number of new properties, that also give rise to new
applications in materials science and biology-related applications. The fascination
of polyphenylene dendrimers is thus not only based on their structure and dynamics
but also on their synthetic variability.
The three-dimensional (3-D) structures of dendrimers have gained tremendous
interest, since the early work of de Gennes and Hervet [19], who developed a model of
increasing density at the surface, caused by the exponential growth of the dendrimers
with the number of generations. However, such ideal structures with functional groups
remaining on the surface when introduced on the last sphere could not be found at the
desired location in flexible dendritic structures such as Fr
echet-type polyaryl-ether
dendrimers [20]. Surprisingly, in such flexible cases end groups have been found
throughout the entire dendrimer volume, leading to homogenous density distribu-
tion [9]. This raised tremendous interest in stiffening the dendrimer generations, in
order to prevent backfolding to the interior.
Prior to the introduction of a new concept of polyphenylene dendrimer synthesis,
which allowed easy extension to higher generations in a divergent method, only a
few examples of relatively stable shape persistent dendrimers are known in the
literature. Hart introduced nanometer-sized dendrimers in which benzene units
wereboundtoeachotherviatwo
, Scheme 5.1) [21]. These dendrimers,
based on extended iptycenes, although extremely stiff and shape persistent, did not
allow any rotational movement, but could not be applied for attachment of
additional functional groups at desired positions by demand. Another example of
rigid and shape-persistent dendrimers based on branched triangulenes, such as
s
bonds (
1
,
was published by DeMeijere and coworkers [22], but further applications were
hampered since these dendrimers neither could be extended to higher generations
nor could other functional groups be introduced. Moore et al. [23] presented
dendrimers such as
2
constructed from phenylacetylene units (Scheme 5.1). Miller
and coworkers [24] contributed to some polyphenylene dendrimers as
3
4
. Both these
kinds of dendrimers
were synthesized by the convergent method, because the
metal catalyzed coupling reactions led to side products and incomplete reactions
rendering a divergent synthesis not practicable. Therefore, a new strategy based on
noncatalytic Diels-Alder cycloaddition reaction was developed [25], allowing the
divergent growth and synthesis of higher generations where one example
3
and
4
is
depicted in Scheme 5.1. Furthermore, it is immediately clear from the structures in
Scheme 5.1, that manymore phenylene units can be incorporated in
5
5
compared to
3
and
4
. Additionally, it turned out that dendrimers
3
and
4
, based on 1,3,5-substituted
benzene rings, led to conformational isomers.
In general, the synthesis of dendrimers is performed by two routes either (i) the
expanding divergent synthesis with stepwise growth of generations starting from a
core or (ii) the convergent synthesis of large dendrons finally coupled to a core [13].
The latter approach is, however, often limited to lower generations of dendrimers
such as G2 and G3 caused by the steric hindrance of accessing the core properly.
For
, it was shown nicely that both approaches toward G2 yielded the same
products in slightly different yields, but due to the many phenylene units and steric
hindrance it was not possible to go beyond the second generation via the convergent
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