Chemistry Reference
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but also larger areas than predicted from close-packed hexagonally ordered chains.
The nonlinear behavior of series b is a consequence of the connectivity of the
alkyl chains via a singe carbon atom precluding normal paraffin-like packing. This
inefficient packing increases the molecular area taken up by the amphiphiles and
greatly reduces the stability of the monolayers.
11.5.3. Photoresponsive Monolayers
Amphiphilic dendrimers functionalized with azobenzene groups have also been
exploited to produce photoresponsive films. Generally, E
!
Z photoisomerization
is often suppressed in Langmuir-Blodgett films because of aggregation of the azo-
benzene chromophores, as evidenced by peak broadening in the UV visible (UV-
vis) spectrum (Nishiyama and Fujihira 1988; Yabe et al. 1988). Meijer demonstrated
that this self-association can be hampered by incorporating the azobenzene chromo-
phore at the periphery of the PPI dendrimer in the presence of varying ratios of pal-
mitolyl groups to reduce the tendency of the azobenzenes to pack. Although
dendrimers having either 1:1 or 1:0 palmitoyl/azobenzene ratios exhibited UV-vis
spectra indicating monomeric azobenzene groups, their behavior at the air-water
interface was quite different. The dendrimers orient themselves with the polar dendri-
mer core flattened to optimize the surface area anchored in the water phase and
the alkyl-azobenzene groups extended perpendicular to the surface (Fig. 11.44;
Weener and Meijer 2000; Su et al. 2007).
Because of some aggregation (H aggregation) of the azobenzene chromophores,
the collapse pressure increased with higher azobenzene/palmitoyl ratios. However,
only the palmitoyl functionalized dendrimer exhibited a change in surface area
upon irradiation with 365-nm light. The lack of photoinduced surface area changes
Figure 11.44 (Left) Structures of photoresponsive poly(propylene imine) dendrimers.
(Right) The isomerization behavior of palmitoyl-functionalized amphiphilic dendrimers at
the water surface.
