conformations and similar chemical states. The relative photoresponsive chromophores mainly include azobenzene, stilbene and butadienes [46–49].
2.2.1.1 Azobenzenes
Azobenzene derivatives have been investigated for the design of photoresponsive
gels for several decades. The trans-azobenzene exists an absorbance at near 360 nm
originating from the p–p* state, and the cis-azobenzene generates a decreasing of
p–p* band, while increasing of the n–p* band at near 290 and 460 nm, respectively, when the trans–cis reaction occurs by UV radiation. The reverse cis–trans
reaction is realized by visible light radiation or just keeping under dark condition at
room temperature for several hours [50–52].
As for azobenzene derived gelators, Shinkai and his co-workers have made
significant contributions. The gelation properties of two types of gelators 6–11 with
cholesterol groups having the natural (S) or inverted (R) configuration have been
studied (Scheme 2.2). Among them, the gelators 7 and 8 with p-alkoxy azobenzene
moiety show better gelation ability, and gelate non-polar solvents or polar [53–55]
to “read-out” the sol–gel transition and the supramolecular chirality by CD spectra
and SEM images [56, 57].
Trans–cis isomerization may affect the structure or stability of gels. Given that
most azobenzene-based gelators strongly favour a trans configuration under
ambient condition, conversion to cis configuration will alter the shape of a molecule
and the interactions of self-assembly. Such changes will induce dissolution of the
gel and cause gel–sol transition, which is generally reversible. Liu and co-workers
demonstrated that the gelator 12 shows a positive CD activity and an inverted CD
signal in polar solvents and non-polar solvents, respectively (Fig. 2.7). Detailed
chiroptical studies and semiempirical quantum mechanical calculations reveal that
the difference in azobenzene moiety interaction plays a vital role under the relevant
conditions. In polar solvents, the gelator forms H-type self-assembly aggregates
with strong p–p interaction of azobenzene moiety. While the gelator self-assembled
into the same H-type aggregate with an interdigitated stacking mode through weak
p–p interaction and strong hydrogen bonding in non-polar solvents. The gels both
show responsive to heat and photoirradiation stimulus [58].
Yagai and co-workers have studied the cooperative gelation properties of the
azobenzene functionalized melamine (13) and the complementary H-bonding barbiturate (B1) or cyanurate (C1) derivatives to fabricate photoresponsive rosette
assemblies (Fig. 2.8). The experiment results show that a rosette containing barbiturate (B1) cannot hierarchically organize into higher-order columnar aggregates
in aliphatic solvents due to the steric bulk of tridodecyloxyphenyl substituent.
However, the sterically non-demanding N-dodecylcyanurate C1 forms gels with
hierarchically organized elongated columnar fibrous aggregates through p–p
stacking and hydrogen bonding interaction in cyclohexane. Dynamic light scattering (DLS) and UV–Vis spectra studies revealed that the dissociation and the
18
2 Supramolecular Gels
2.2.1.1 Azobenzenes
Azobenzene derivatives have been investigated for the design of photoresponsive
gels for several decades. The trans-azobenzene exists an absorbance at near 360 nm
originating from the p–p* state, and the cis-azobenzene generates a decreasing of
p–p* band, while increasing of the n–p* band at near 290 and 460 nm, respectively, when the trans–cis reaction occurs by UV radiation. The reverse cis–trans
reaction is realized by visible light radiation or just keeping under dark condition at
room temperature for several hours [50–52].
As for azobenzene derived gelators, Shinkai and his co-workers have made
significant contributions. The gelation properties of two types of gelators 6–11 with
cholesterol groups having the natural (S) or inverted (R) configuration have been
studied (Scheme 2.2). Among them, the gelators 7 and 8 with p-alkoxy azobenzene
moiety show better gelation ability, and gelate non-polar solvents or polar [53–55]
to “read-out” the sol–gel transition and the supramolecular chirality by CD spectra
and SEM images [56, 57].
Trans–cis isomerization may affect the structure or stability of gels. Given that
most azobenzene-based gelators strongly favour a trans configuration under
ambient condition, conversion to cis configuration will alter the shape of a molecule
and the interactions of self-assembly. Such changes will induce dissolution of the
gel and cause gel–sol transition, which is generally reversible. Liu and co-workers
demonstrated that the gelator 12 shows a positive CD activity and an inverted CD
signal in polar solvents and non-polar solvents, respectively (Fig. 2.7). Detailed
chiroptical studies and semiempirical quantum mechanical calculations reveal that
the difference in azobenzene moiety interaction plays a vital role under the relevant
conditions. In polar solvents, the gelator forms H-type self-assembly aggregates
with strong p–p interaction of azobenzene moiety. While the gelator self-assembled
into the same H-type aggregate with an interdigitated stacking mode through weak
p–p interaction and strong hydrogen bonding in non-polar solvents. The gels both
show responsive to heat and photoirradiation stimulus [58].
Yagai and co-workers have studied the cooperative gelation properties of the
azobenzene functionalized melamine (13) and the complementary H-bonding barbiturate (B1) or cyanurate (C1) derivatives to fabricate photoresponsive rosette
assemblies (Fig. 2.8). The experiment results show that a rosette containing barbiturate (B1) cannot hierarchically organize into higher-order columnar aggregates
in aliphatic solvents due to the steric bulk of tridodecyloxyphenyl substituent.
However, the sterically non-demanding N-dodecylcyanurate C1 forms gels with
hierarchically organized elongated columnar fibrous aggregates through p–p
stacking and hydrogen bonding interaction in cyclohexane. Dynamic light scattering (DLS) and UV–Vis spectra studies revealed that the dissociation and the
18
2 Supramolecular Gels