Chemistry Reference
In-Depth Information
d
n
1
r
3
n
g
|
7
Figure 3.19
Response of gels of 30 in ethyl acetate to solutions of Ag
1
and Li
1
.
Reproduced from ref. 38.
Apart from tuning rheological properties, other physical properties of gels
can also be modulated after addition of metal ions. Liu and coworkers re-
ported
39
an amphiphilic gelator 31 (Figure 3.18) entailing both Schiff base and
L
-glutamide moieties. LMWG 31 can form thermotropic gels in many organic
solvents. Interestingly, the introduction of Cu
21
ions causes a chiral twist in the
nanofiber, whilst the addition of Mg
21
ions leads to a significant enhancement
of the fluorescence of the gel. More importantly, the presence of Mg
21
enables
the gel to be able to exhibit chiral recognition towards chiral molecules such as
tartaric acid.
3
.
3.3.4 Redox-Responsive Gels
The redox stimulus is important for the construction of the electromechanical
soft materials such as artificial muscles and electrorheological fluids. In this
respect, redox-responsive gels have attracted wide interest. Functional groups
that can undergo reversible oxidation-reduction reactions have been in-
corporated into LMWGs to render the gel responding to redox reactions.
It is known that tetrathiafulvalene (TTF) and its derivatives can be reversibly
transformed into the respective radical cation (TTF
1
) and dication (TTF
21
)
states by either chemical or electrochemical redox reactions. Conversion of
TTF group to either TTF
1
or TTF
21
would largely modulate the interaction
of adjacent TTF units, therefore, the gel may be destroyed. By taking advan-
tage of this unique feature, we and other groups have reported
40-43
the TTF-
based gelators and the resulting gels that show responsiveness to redox
reactions. For instance, we reported
44
TTF-derived LMWG 32 (Figure 3.20),
which can gel (Figure 3.21) several solvents including cyclohexane and 1,2-
dichloroethane. Either addition of Fe
31
or application of an oxidation po-
tential to the gel can lead to gel-solution transition. Interestingly, the gel state
can be restored by electrochemical reduction, followed by heating and cooling
(Figure 3.21). Alternatively, the gel phase of LMWG 32 can also be modulated
by reaction with tetracyanoquinodimethane (TCNQ), which is a strong elec-
tron acceptor. Addition of TCNQ to the gel of 32 from 1,2-dichloroethane
results in the destruction of the gel state, but the reaction of 32 with TCNQ in
cyclohexane leads to a dark-green gel. This may be attributed to the fact that
1,2-dichloroethane is more polar than cyclohexane and as a result, the TTF unit
in LMWG 32 interacts more strongly with TCNQ in 1,2-dichloroethane.
