Polyphenylene Nanostructures - Cluster for Molecular Chemistry

1764 Chemical Reviews, 1999, Vol. 99, No. 7 Berresheim et al.
Scheme 30 Scheme 31
tives with suitable ethynyl compounds was again
used. The enormous thermal and chemical stability
of the polyphenylene structure allowed the use of a
reaction temperature that guaranteed, even by the
exponential increase in functional groups per molecule
resulting from the generation-by-generation
build up, complete reaction of all ethynyl groups with
an overall yield of >85%.
Müllen et al. were recently also able to synthesize
their dendrimers using the convergent method starting
with the deprotection of 4,4′-di(triisopropyl)ethynylbenzil
(123a), a tetraphenylcyclopentadienone
precursor. 180 The product was successfully treated
with unsubstituted tetraphenylcyclopentadienone
(80a) followed by 1,3-diphenylacetone (125) to form
the cyclopentadienone 126 (Scheme 32) which can be
regarded as one branch of a convergent dendrimer.
This method is useful in the rapid synthesis of
several dendrimers and in the formation of further
hyperbranched polyphenylenes of greater size and
with fewer growth imperfections. 180
A selective generation-by-generation build up in
the scope of the divergent dendrimer synthesis
described above was obtained using the difference in
Diels-Alder activity of the free ethynyl groups and
those protected by triisopropylsilyl groups. Thus, the
activation of the dienophile units takes place in an
initial reaction step by the cleavage of the silyl
protecting groups. The construction procedure based
706a
on the [4+2]-cycloaddition of tetraphenylcyclopentadienone
derivatives, which are once again equipped
with ethynyl functions protected with triisopropylsilyl
groups, then follows. 20,175,176,190,191
It is a key feature of the polyphenylene dendrimer
synthesis that the [4+2]-cycloaddition is not influenced
by further substituents on the tetraphenylcyclopentadienones,
as long as the latter survive the
reaction conditions. 175,176 Therefore, it is possible to
introduce a variety of functional groups in the final
generation using disubstituted tetraphenylcyclopentadienones
such as 127 (Chart 5). These functional
groups are exclusively located on the surface of the
dendrimer. 175,176 This approach provided dendrimers
carrying, for example, methylthiofunctions, and it
was possible to deposit the dendrimers by physisorption
from solution onto gold surfaces. 176 Visualization
by atomic force microscopy (AFM) indeed revealed
the existence of single species corresponding to the
expected dendrimer size; however, particle aggregates
were also observed. 177 This seems to be due to
the relatively weak interaction of the dendrimers
with the surface. The synthesis of thiol-functionalized