Chemistry Reference
In-Depth Information
Figure 11.45 Structures of amphiphilic monodendrons containing a hydrophilic azobenzene
focal group.
in the all-azobenzene dendrimer was attributed to greater aggregation in the
Langmuir-Blodgett films, which inhibit E
!
Z isomerization. Another approach
to enhance the extent and reversibility of photoisomerization focused on creating
amphiphiles with a cross-sectional mismatch between a hydrophilic azobenzene
focal group and a large dendritic hydrophobic region (Fig. 11.45; Peleshanko et al.
2002; Genson et al. 2005).
This cross-sectional mismatch between the small anchor and the large dendritic
component modified the intermolecular packing in the Langmuir-Blodgett mono-
layer and film so that sufficient mobility was present near the azobenzene group.
This mobility greatly facilitated efficient molecular reorganization in response to
E
!
Z photoisomerization.
11.5.4. Fullerene Monolayers
The preparation of thin films of Buckminsterfullerene (C
60
) is severely hampered
by their tendency to aggregate at the air-water surface and form ill-defined films
of three-dimensional aggregates at the air-water surface (Mirkin and Caldwell
1996). The attachment of a hydrophilic head group to the fullerene significantly
improves the spreading ability by providing an attractive interaction with the water
phase. However, successive compression-expansion cycles exhibit hysteresis due
to the occurrence of strong fullerene-fullerene interactions that induce irreversible
aggregation upon compression of the monolayer. Many of these problems were cir-
cumvented by encapsulation of the fullerene within amphiphilic glycodendrons to
suppress self-association by sterically isolating the C
60
molecules (Fig. 11.46a;
