Chemistry Reference
In-Depth Information
HPB
for 1,2,3,4,5,6-hexakis(4-ethynylphenyl)benzene) was necessary to, later on,
distinguish the polyphenylene dendrimer generations (G1-G7) on different cores.
The typical divergent reaction sequence is shown for 1,3,5-triethynylbenzene
12
yielding the first-generation dendrimer
B-G1
(
16
) upon coupling with
8
or the
. All
TIPS groups could be removed with fluoride anions yielding the first-generation
functionalized dendrimer
protected triisopropylethynyl functionalized dendrimer
17
upon reaction with
6
18
, which was then endcapped with
8
resulting in a second-
generation dendrimer (B-G2)
19
(Scheme 5.4). Alternatively
18
could be further
extended with
up to the fourth generation.
In addition the convergent route has been shown to be practicable for achieving
these polyphenylene dendrimers by extending the cyclopentadienones to the next
higher generation dendrons. However, to prevent self-coupling, this approach started
from the 4,4
0
-diethynylbenzil
6
for growth reactions via
20
21
that became available after deprotection of
9
and
contained the two dienophile groups for Diels-Alder reaction with
8
yielding the
dendronized benzil
22
for Knoevenagel condensation. Thereby the second-generation
dendron
23
was available for coupling with one of the aforementioned cores
12-15
(Scheme 5.5).
The divergent and the convergent approach resulted in the same monodisperse
products in comparable yields. The convergent approach provided a fast access to
second-generation dendrimers, but could not be used to synthesize higher generation
PPDs with the relatively small cores considered here so far. A functionalized second-
generation dendron is an extremely powerful tool for desymmetrization, as it is shown
later [28-30].
Also the other cores
were transformed into the first-, second-, and higher-
generation dendrimers, in order to study their influence on the shape and rigidity of
the resulting dendrimers. Luckily, for three polyphenylene dendrimers of the first
generation with different cores, single crystals could also be grown large enough for
X-ray structural investigation [31] (Figure 5.1).
For the first-generation dendrimers geometry optimizations, even on a semi-
empirical level, were well able to reproduce the shape and distances compared to
their X-ray structure. The calculated distances (radii) derived from semiempir-
ical AM1 calculations (which are well-suited for analyzing large and twisted
p
13-15
-systems) between the center of the core and the outside protons, for instance
were within 0.02 nm, and only slightly worse for the molecular mechanics
calculations.
As nicely seen in Figure 5.2, the variation of the cores causes the dendritic branches
to grow in certain directions as a dumbbell shape for
13
based
Biph-G2
and more flat
globular shape for
.
The shape persistence of the newly derived polyphenylene dendrimers has further
been investigated for different generations by various techniques. Advanced solid-
state nuclear magnetic resonance (NMR) experiments with rapid magic angle
spinning were applied for gaining more insight into the dynamics of these molecules.
Fast processes in the megahertz regime were shown to be restricted to fast vibrations
of terminal phenyl rings, while the phenyl rings in the core and scaffolds displayed
very limited dynamics with restricted movements [32,33] Comparing
14
based
TPM-G2
Biph-G2
and
