Chemistry Reference
In-Depth Information
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Figure 3.13
Illustration of the gel-to-gel transformation of 17 after heating, cooling,
UV and visible-light irradiations.
Reprinted with permission from ref. 23. Copyright (2010) John Wiley &
Sons, Inc.
reformed again after further UV irradiation. This light-triggered gel-solution
transition is due to the fact that merocyanine moiety has a strong tendency to
form p-p stacking in comparison with closed form.
Some of us have reported
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a spiropyan-functionalized dendron 17
(Figure 3.12) that can form organogels in toluene and benzene (Figure 3.13).
Absorption spectral studies clearly demonstrate that the photochromic reaction
of the spiropyran unit in 17 can take place in both solution and gel phase.
However, after UV-light irradiation, the gel phase is not destroyed, but the
yellow gel is transformed into the purple-blue gel. It is very interesting to find
that the purple-blue gel exhibits relatively strong red fluorescence. Further
visible-light irradiation of the purple-blue gel leads to the yellow gel again,
which is nonfluorescent.
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3.3 Chemical Stimuli-Responsive Molecular Gels
A number of molecular gels responding to chemical stimuli have been disclosed in
recent years. These chemical stimuli-responsive gels can be categorized as follows:
- pH-responsive gels;
- anion-responsive gels;
- metal-ion-responsive gels;
- redox-responsive gels;
- gels responding to neutral chemical species.
Furthermore, the combination of several different stimuli may lead to mul-
tiresponsive gels. In the following, we will present and discuss examples of each
of these chemical stimuli-responsive gels.
3.3.1 pH-Responsive Gels
The pH-sensitive materials are interesting for applications in drug delivery and
biomedical systems. As such, pH-responsive gels, which can be generated by
