Chemistry Reference
In-Depth Information
SCHEME 5.10
First- and second-generation polyphenylene dendrimers with benzophenone
cores.
Alkaline metal reduction of the encapsulated pyrene core was carried out to afford the
corresponding pyrene radical anions, for which hampered electron transfer to the core
was observed with increasing dendrimer generation. This served as further proof of
the site isolation due to the polyphenylene shell. But these charging experiments also
demonstrated that for device applications such as organic light-emitting diodes
(OLEDs), one should not go to higher dendrimer generations beyond G3, since the
charge recombination may be hindered. High quantum efficiency yield in solution
(Qf
>
0.92) was already achieved by applying a second-generation dendrimer shell.
Even a gram-scale synthesis of this new materials based on second-generation
dendrimers for device applications became possible.
Only a very few reports have tackled the question of synthetic transformation of the
dendrimer core. For testing the core accessibilities toward charging activities and
postsynthetic transformations, first- and second-generation PPDs were synthesized
starting from benzophenone [57]. In order to obtain high density and thereby a strong
shielding by polyphenylenes around the benzophenone core (Scheme 5.10), a large
number of starting points for the dendrimer growth was necessary, which was
achieved applying the tetraethynylated building block
.
Variation of the size and the density of the surrounding dendrimer shell resulted in
different isolation of the cores. The dense-shielded dendrimer
32
34
was obtained from
. Upon
modeling, it was directly shown how well the keto function is protected by the
surrounding phenylene units.
To investigate the reactivity of the core upon shielding, chemical reaction of the
core with aryl-/alkyllithium and Grignard reagents, as well as the alkali-metal
reduction of the encapsulated benzophenone corewere performed. The phenyllithium
reaction took place easily with the first-generation dendrimer
the core
32
using the A
4
B CP unit
7
for second-generation dendrimer
34
35
to achieve a
functional phenylated hydroxyl methyl group
36
. For the second-generation den-
drimer
very prolonged reaction times (3 days of reflux) were needed to find some
incomplete hydroxylation, while larger nucleophiles as biphenyl lithium did not react
34
