Chemistry Reference
In-Depth Information
d
n
1
r
3
n
g
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7
Figure 3.9
Molecular structures of compounds 10-12.
3
.
Figure 3.10
Illustration of gel-solution transition based on the photodimerization
and dissociation of 13 under UV-light irradiation and heating.
azobenzene segment. The gel-solution transition for the corresponding orga-
nogels can be reversibly tuned by light irradiation as a result of trans-cis iso-
merization of the azobenzene unit. Later, Koumura and coworkers reported
15
another azobenzene-based compound 11 with two urethane moieties linked to
two cholesteryl ester units. After photoirradiation, the gel-solution transition
can also occur. In these systems, it usually takes a long time to complete the
phase transition. However, Kim and coworkers reported
16
a gelator 12 con-
sisting of an azobenzene dendron that could undergo a rapid gel-solution
transition. The collapse of the gel occurred in 2.0 min upon UV-light irradi-
ation and gel regeneration was achieved by subsequent irradiation of visible
light in just 5.0 s.
Photodimerization of anthracene may occur when intermolecular arrange-
ments are appropriate. Shinkai and coworkers reported
17
the binary gelator 13
composed of alkylammonium and anthracene-9-carboxylate. The gels formed
in cyclohexane were converted (Figure 3.10) to the solutions after UV-light
irradiation because of the generation of photodimers. The gel phase can be
reformed by heating the solution and further cooling. This finding suggests that
the photoinduced monomer-dimer transformation eventually destroys the gel
superstructure. However, the monomer-dimer conversions occur with side
products and as a result multiple cycles of photoinduced gel-solution phase
transitions cannot be achieved.
