Chemistry Reference
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the back bending of arms. This creates a high degree of shape persistencewith,
however, sufficient solubility in organic solvents.
(iii) The method of synthesis via repetitive Diels-Alder cycloaddition ensures
a high structural perfection, which is superior to that in other dendrimers.
The most convincing proof for this unique character is the synthesis of
monodisperse defect-free, three-dimensional polymers with molecular
weights in excess of half a million Dalton.
(iv) It is a characteristic feature of the polyphenylene dendrimers that in the course
of their synthesis a large variety of functional groups can be site-selectively
placed in the core, in the scaffold, or on the surface of the dendrimers. It is this
property that qualifies the PPDs as unique functional nanoparticles with
perfect nanosite definition of their functional groups.
(v) An illustrating example of complex functional nanoparticles is a whole family
of functional multichromophores, which can serve for example in light-
harvesting systems, single photon emitters, and bioassays.
(vi) Polyphenylene dendrimers, due to their molecular design create a lot of empty
space in their interior, giving rise to unique host-guest interactions, which can
be best utilized in sensor fabrication.
(vii) The ability of carrying a payload of organic molecules is tightly connected
with solubility and mobility of dendrimers in organic or aqueous media or
across a membrane. There, the controlled surface modification, for example,
by grafting reactions under formation of polyelectrolyte shells is highly
relevant. Here again, the structural perfection and functional versatility of
PPDs are of utmost importance.
These few points outlined above do not define the end of the polyphenylene dendrimer
story, but rather open numerous opportunities for further work, some of which are be
mentioned below:
(i) While it is possible to build electrolyte functions into the interior or onto the
surface of a strongly hydrophobic polyphenylene dendrimer, it is challeng-
ing to have one single cationic or anionic function in the dendrimer. This is
already the case in the Ru
2
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-centered PPDs. The idea is to surround a single
charge by rigid, unpolar encapsulation, which would create totally new
questions for the chemistry and physics of ion pairs.
(ii) Decorating PPDs with multiple cationic charges leads to opportunities of
making polyelectrolyte-polyelectrolyte complexes for example with DNA
molecules. The resulting question of DNA transfection into cells strongly
suggests a comparison between surface functionalized dendrimers and other
viral and nonviral DNA carriers.
(iii) The endcapping of surface functionalized dendrimers allows the simulta-
neous introduction of functional groups which would otherwise avoid each
other, such as polar and unpolar or alkyl and perfluoro alkyl moieties. This
