unique mechanically linked character and the presence of uncomplexed host and
guest moieties foreshadow the use of such systems for their responses to external
stimuli with the added benefit of providing molecular recognition sites useful as
delivery vehicles. Use of other host–guest motifs to form the semi-rotaxane A 2 B
monomers is possible and complementary systems with higher binding constants
will enable efficient syntheses of high molecular weight, mechanically linked hyperbranched polymers.
Liu et al. (2016) [30] successfully constructed a crown ether-based supramolecular gel with a three-dimensional network structure via a hierarchical induced
assembly strategy. Therein, the coordination linkage of bis(terpyridyl)dibenzo-24crown-8 (1) with Zn
2+ produces the 1/Zn
2+ supramolecular polymer as the primary
assembly. Then, the anthryl-dibenzylammonium guest 2 was grafted to the 1/Zn
2+
supramolecular polymer via the noncovalent association of dibenzylammonium
moieties on guests with the dibenzo-24-crown-8 rings on supramolecular polymer.
Furthermore, the supramolecular polymer with anthryl grafts underwent a photoinduced secondary assembly to produce the supramolecular gel, which could be
reversibly disassembled by a thermoinduced dissociation of anthracene dimers.
These photo/thermo-induced hierarchical assembly/disassembly behaviors will provide a potential way to construct degradable hierarchical supramolecular assemblies
employing environment-friendly external stimuli, such as light and heat, as controlling method.
16.5 Photoresponsive Supramolecular Polymers Based on
Calixarene
Calixarenes are macrocyclic molecules composed of 2- and 6-positions of methylene-linked phenolic units and are a class of widely studied organic supramolecular
hosts. Because it is easy to be modified, its possibilities for development
are unlimited. The encapsulation ability of the unmodified calixarene cavity is not
as good as the crown ether, cyclodextrin, and cucurbituril mentioned above, so
extensive chemical modification of calixarene is necessary to achieve sufficient
inclusion.
Sulfonated calix[n]arenes ( p-sulfonatocalix [n] arenes, SCnAs, n = 4–8, Fig. 13)
were first reported by Shinkai et al. (1984) [31], which is a class of outstanding
water-soluble calixarene derivatives. The calixarene derivative has a strong
binding ability to a guest in aqueous solution. Advantages that sulfonated calix[n]
arenes possess as a host are as follows [32]: first, it can be easily obtained by direct
sulfonation of the upper edge of the calixarene in a relatively satisfactory yield;
second, it has high water solubility. Compared with organic solvents, it can be more
effectively encapsulated with the guest by hydrophobic and π-π stacking in water;
third, the upper edge of the sulfonic acid group provides a connection point as
a π-rich electron cavity. It acts as a complementary combination; therefore, it has
strong binding ability and high selectivity with a series of organic cations. Finally, it
is biocompatible.
16 Photoresponsive Supramolecular Polymers Based on Host-Guest Interactions
467
guest moieties foreshadow the use of such systems for their responses to external
stimuli with the added benefit of providing molecular recognition sites useful as
delivery vehicles. Use of other host–guest motifs to form the semi-rotaxane A 2 B
monomers is possible and complementary systems with higher binding constants
will enable efficient syntheses of high molecular weight, mechanically linked hyperbranched polymers.
Liu et al. (2016) [30] successfully constructed a crown ether-based supramolecular gel with a three-dimensional network structure via a hierarchical induced
assembly strategy. Therein, the coordination linkage of bis(terpyridyl)dibenzo-24crown-8 (1) with Zn
2+ produces the 1/Zn
2+ supramolecular polymer as the primary
assembly. Then, the anthryl-dibenzylammonium guest 2 was grafted to the 1/Zn
2+
supramolecular polymer via the noncovalent association of dibenzylammonium
moieties on guests with the dibenzo-24-crown-8 rings on supramolecular polymer.
Furthermore, the supramolecular polymer with anthryl grafts underwent a photoinduced secondary assembly to produce the supramolecular gel, which could be
reversibly disassembled by a thermoinduced dissociation of anthracene dimers.
These photo/thermo-induced hierarchical assembly/disassembly behaviors will provide a potential way to construct degradable hierarchical supramolecular assemblies
employing environment-friendly external stimuli, such as light and heat, as controlling method.
16.5 Photoresponsive Supramolecular Polymers Based on
Calixarene
Calixarenes are macrocyclic molecules composed of 2- and 6-positions of methylene-linked phenolic units and are a class of widely studied organic supramolecular
hosts. Because it is easy to be modified, its possibilities for development
are unlimited. The encapsulation ability of the unmodified calixarene cavity is not
as good as the crown ether, cyclodextrin, and cucurbituril mentioned above, so
extensive chemical modification of calixarene is necessary to achieve sufficient
inclusion.
Sulfonated calix[n]arenes ( p-sulfonatocalix [n] arenes, SCnAs, n = 4–8, Fig. 13)
were first reported by Shinkai et al. (1984) [31], which is a class of outstanding
water-soluble calixarene derivatives. The calixarene derivative has a strong
binding ability to a guest in aqueous solution. Advantages that sulfonated calix[n]
arenes possess as a host are as follows [32]: first, it can be easily obtained by direct
sulfonation of the upper edge of the calixarene in a relatively satisfactory yield;
second, it has high water solubility. Compared with organic solvents, it can be more
effectively encapsulated with the guest by hydrophobic and π-π stacking in water;
third, the upper edge of the sulfonic acid group provides a connection point as
a π-rich electron cavity. It acts as a complementary combination; therefore, it has
strong binding ability and high selectivity with a series of organic cations. Finally, it
is biocompatible.
16 Photoresponsive Supramolecular Polymers Based on Host-Guest Interactions
467