Chemistry Reference
In-Depth Information
Using the LB technique, it has been possible to transfer the Langmuir films made
from
and to obtain multilayers on solid substrates. In accordance with previous
observations, transfer is more efficient upon increasing the size of the polar-head
group. The excellent quality of the LB films prepared from these amphiphilic
dendrimers is deduced from the plot of their UV-vis absorbance as a function of
the layer number that results in straight lines, indicating that the films obtained from
21
21
-
23
is
the broadening of the absorption in the filmwhen compared to the solution. The latter
observation is indicative of fullerene-fullerene interactions resulting from contacts of
carbon spheres among neighboring layers. Remarkably, a clear evolution can be seen
by going from
-
23
grow regularly. The main feature of the UV-vis spectrum of the LB film of
21
21
to
23
: the broadening of the absorption spectrum seen for
21
is
almost vanished for
23
are close to the ones recorded in CH
2
Cl
2
solutions suggesting limited fullerene--
fullerene interactions within the LB film. This observation is in full agreement with
the more compact structures proposed for
22
and
23
. Actually, the UV-vis spectra of LB films of
22
and
22
and
23
at the air-water interface when
compared to
have
revealed a conformational change in the dendritic structure with the size of the polar-
head group. Due to a better anchoring onto the water surface, the compounds with the
largest polar-head groups adopt a more compact structure and the dendritic branches
are forced towrap the fullerene core. This model is nicely confirmed by the amount of
fullerene-fullerene interactions within the LB films deduced from their absorption
spectra. On the one hand, the results obtained in this systematic study show some of
the fundamental architectural requirements for obtaining stable Langmuir films with
amphiphilic dendrimers. On the other hand, it is worth noting that the fullerene
chromophores are almost completely isolated from external contacts by the dendritic
structure thus paving the way toward ordered thin film of isolated functional
molecular units. This appears to be an important finding for future nanotechnological
applications, in particular for data storage at a molecular level.
Although the incorporation of fullerenes in thin ordered films has been extensively
studied during the past few years, liquid crystal ordering of such materials has been
probed to a much lesser degree. Indeed, the fullerene sphere is a quite large molecular
unit and the preparation of fullerene-containing liquid crystals appears to be difficult.
Deschenaux and coworkers [41] have shown that the functionalization of C
60
with a
malonic ester bearing two mesogenic cholesterol subunits resulted in a fullerene
derivative with liquid-crystalline properties. However, the mesomorphic behavior of
this fullerene derivative is limited in comparison with that of the corresponding
malonic ester precursor due to the presence of the C
60
core that acts as a bulky spacer
between the two mesogenic units. The same group has shown that a dendritic addend
exhibits similar mesogenic properties as those of the corresponding fullerene-
functionalized dendrimer
21
. In summary, the Langmuir studies of fullerodendrimers
21
-
23
is
buried in the middle of the dendritic structure and thus prevents unfavorable effects of
the C
60
unit such as aggregation or steric hindering. Therefore, incorporation of
fullerenes into well ordered structures can be easily achieved.
The concept developed to elaborate fullerene-containing thermotropic liquid
crystals, that is, grafting dendritic mesogenic subunits onto the C
60
core, was
24
[42] (Figure 6.12). In this case, the C
60
core of
24
