chemical reactions, including dissociation and solvolysis, one is interested in weak
anion–cation interactions [20]. A well-known case is metallocene-catalyzed olefin
polymerization, where chain growth occurs at a zirconocenium cation. The necessary counteranion should only weakly interact with the extremely sensitive catalyst
center and this is why methylaluminoxanes (MAO) as anionic species without
nucleophilic power have become so important. Huge excesses of MAO are often
applied and this has prompted the use of tetraphenylborate as stoichiometrically
applied counterions. It is clear that our dendritic borates are much larger, giving the
possibility to leave the positively charged catalyst as unperturbed as possible during
polyolefin synthesis.
2.4 PPDs as Light-Harvesting, Light-Emitting,
and Photoswitchable Multichromophores
Rylene dyes can be tuned so that absorption and emission cover the whole spectrum of
visible light and extend far into the near infrared. It is then logical to incorporate these
dyes at the core, in the scaffold, or on the rim of a PPD [26–28]. In that way, one can
precisely control the steric and electronic conditions for a Fo ¨rster resonant energy
transfer and for light-induced electron transfer. The resulting dendrimers are unique
multichromophoric light-harvesting systems [29–32] and have become important
objects, even for single molecule spectroscopy [33–35]. Using, for example, a
perylenediimide chromophore as a core, one obtains nanosized emitters with constant
fluorescence wavelength, but increasing diameter. These dyes have served as probes
for studying the dynamics of solid polymers by defocused widefield imaging. Another
good case can be made for using PPDs as active components of light-emitting diodes
(LEDs). A well-known troublesome problem is to match the concentration of holes
and electrons and to keep the recombination zone away from the electrode. This is
normally accomplished by complex multilayer devices. The dendrimers, in a sense,
also represent a layer-by-layer design, but now with molecular precision. More
specifically, we have synthesized PPDs with a triphenylamine-based hole-capturing
layer on the surface [36], an inner layer of triphenylenes where excitons are formed
upon charge recombination, and a central core of pyrene that removes the excitons
from the area of charge recombination by a rapid energy transfer (Fig. 8) [37]. The
light is finally emitted from the inner core. The advantages of this design in terms of
device stability and efficiency define a topic of future research.
An additional challenge appears when multiple azobenzenes are introduced into
otherwise rigid scaffolds and PPDs thus become stimulus-responsive, molecularly
defined nanoparticles [38, 39]. One might argue that incorporation of azobenzenes
into polymers or polymer nanoparticles is a well-known concept for changing
structures (Fig. 9).
When combined with the rigid PPDs, however, this seemingly conventional
method proves of special value. Only two examples will be mentioned. Lightinduced transition from the trans- to the cis-isomer of the dendronized borate anion
128
K. Binder et al.
anion–cation interactions [20]. A well-known case is metallocene-catalyzed olefin
polymerization, where chain growth occurs at a zirconocenium cation. The necessary counteranion should only weakly interact with the extremely sensitive catalyst
center and this is why methylaluminoxanes (MAO) as anionic species without
nucleophilic power have become so important. Huge excesses of MAO are often
applied and this has prompted the use of tetraphenylborate as stoichiometrically
applied counterions. It is clear that our dendritic borates are much larger, giving the
possibility to leave the positively charged catalyst as unperturbed as possible during
polyolefin synthesis.
2.4 PPDs as Light-Harvesting, Light-Emitting,
and Photoswitchable Multichromophores
Rylene dyes can be tuned so that absorption and emission cover the whole spectrum of
visible light and extend far into the near infrared. It is then logical to incorporate these
dyes at the core, in the scaffold, or on the rim of a PPD [26–28]. In that way, one can
precisely control the steric and electronic conditions for a Fo ¨rster resonant energy
transfer and for light-induced electron transfer. The resulting dendrimers are unique
multichromophoric light-harvesting systems [29–32] and have become important
objects, even for single molecule spectroscopy [33–35]. Using, for example, a
perylenediimide chromophore as a core, one obtains nanosized emitters with constant
fluorescence wavelength, but increasing diameter. These dyes have served as probes
for studying the dynamics of solid polymers by defocused widefield imaging. Another
good case can be made for using PPDs as active components of light-emitting diodes
(LEDs). A well-known troublesome problem is to match the concentration of holes
and electrons and to keep the recombination zone away from the electrode. This is
normally accomplished by complex multilayer devices. The dendrimers, in a sense,
also represent a layer-by-layer design, but now with molecular precision. More
specifically, we have synthesized PPDs with a triphenylamine-based hole-capturing
layer on the surface [36], an inner layer of triphenylenes where excitons are formed
upon charge recombination, and a central core of pyrene that removes the excitons
from the area of charge recombination by a rapid energy transfer (Fig. 8) [37]. The
light is finally emitted from the inner core. The advantages of this design in terms of
device stability and efficiency define a topic of future research.
An additional challenge appears when multiple azobenzenes are introduced into
otherwise rigid scaffolds and PPDs thus become stimulus-responsive, molecularly
defined nanoparticles [38, 39]. One might argue that incorporation of azobenzenes
into polymers or polymer nanoparticles is a well-known concept for changing
structures (Fig. 9).
When combined with the rigid PPDs, however, this seemingly conventional
method proves of special value. Only two examples will be mentioned. Lightinduced transition from the trans- to the cis-isomer of the dendronized borate anion
128
K. Binder et al.