Polyphenylene Nanostructures - Cluster for Molecular Chemistry

1766 Chemical Reviews, 1999, Vol. 99, No. 7 Berresheim et al.
Figure 3. Three-dimensional ball-and-stick models of the
first four generations of a polyphenylene dendrimer built
from the biphenylene core 118 and the AB2 building block
106.
the fourth-generation dendrimer 122 in common
organic solvents. 200 Solid-state proton NMR measurements
are underway to obtain information on the
dynamic behavior of these molecules. 202,203
The above-mentioned solubility permits that the
particle size may be determined using light scatter-
ing, 178 and it is quite remarkable that hydrodynamic
radii thus obtained fall into the range of those
predicted using the computer-generated, ball-andstick
molecular models. 176,190
The 3-dimensional structure of the dendrimers can
be determined by the choice of the core. 176,192,200 If the
tetrahedral tetraethynyltetraphenylmethane 130 is
used in place of the biphenyl derivative 118, even the
second generation 132 of the resulting dendrimer
shows a more pronounced spherical shape than that
of the comparable dendritic polyphenylene 120, although
both are calculated to possess a maximum
extension of approximately 3.7 or 3.8 nm. As the
hydrodynamic radii determined for the dendrimer
based on 118 correspond very well to those calculated,
these values are assumed to be reliable. 177 The
formation of 132 takes place in an analogous fashion
to the dendrimer synthesis shown in Scheme 31; a
comparative portrayal of each of the second generations
120 and 132, respectively, is shown in Figure
4.
Although the tetrahedral or the dumb-bell shape
of the core is still reflected in the geometry- and
energy-optimized surface of the dendrimer, according
to force-field calculations using the program Cerius2,
considerable differences appear in the spatial distribution
of the individual repeat units. These differences
are found even when using the same branching
units. Therefore, they can only be explained by the
geometry of the central building block. Further
variations in the ethynyl-substituted starting molecule,
176 as can be seen in Figure 4, provide an
impression of the various shapes of the dendrimers
that are easily accessible through the application of
different cores in a sort of “building block kit”. In both
of these further cases, a change in the overall space
filling can be observed through the markedly different
geometry of the core. The calculated diameter of
135 and 137 amounts to approximately 3.5 and 5.0
nm, respectively. 200 Moreover, in the case of 137, the
number of phenylene rings is approximately twice as
high as that in 135. As the available space is different
by a factor of approximately two, both should have a
similar density of phenylene rings.
Of all the above cores, 1,3,5-triethynylbenzene
(133) 176 permits the best comparison of the Miller and
Neenan dendrimers with the Müllen dendrimers.
While the third generation 117 of the dendrimer
synthesized by Miller and Neenan using aryl-aryl
coupling is composed of only 22 phenylene units, 187,188
the second generation 135 (shown in Scheme 33) of
the Müllen dendrimer prepared by divergent Diels-
Alder synthesis possesses 46 phenylene rings, more
than twice as many. The astonishingly rapid growth
of dendrimer 135 is advantageous in comparison with
the fact that only one additional phenylene unit is
obtained through a transition-metal-catalyzed coupling
of an aryl-aryl bond, whereas an additional five
phenylene groups are obtained by a Diels-Alder
reaction with tetraphenylcyclopentadienone. Considering
the length of one repeat unit to be approximately
0.4 nm, this would mean that the third
generation of the Miller and Neenan dendrimer
possesses a maximum diameter of about 2.2-2.3 nm