Exploring macrocycles in functional supramolecular gels: from stimuli responsiveness to systems chemistry

CONSPECTUS: Supramolecular gels are ideal candidates for soft, stimuli-responsive materials, because they combine the elastic behavior of solids with the microviscous properties of fluids. The dynamic networks of fibers in supramolecular gels are reminiscent of the cytoskeleton of a cell and provide scaffolds to implement function. When gels are made responsive to stimuli, these mechanical properties can be controlled. Gel-sol transitions also open opportunities to immobilize molecules inside the gel's cavities and to release them on demand. To establish selective responsiveness, suitable recognition sites are required influencing the properties of the fiber network depending on the presence of the stimulus. Supramolecular gels are expected to be stimuli-responsive per se, for example, to temperature, mechanical stress, or an environment that is competitive with the noncovalent interactions connecting the low-molecular weight gelators. Nevertheless, the opportunities for controlling the mechanical properties are rather limited, if one merely relies on interfering with these interactions. It would be much more promising to equip the gel with additional receptor sites that offer selectivity for a broader variety of chemical stimuli. Macrocycles often exhibit a distinct host-guest chemistry and thus are excellent candidates for this purpose. A broad variety of macrocycles differing with respect to structure, topology, solubility, or biocompatibility have been incorporated in gels and endow gels with responsiveness and function. Macrocycles can have different roles: They offer rather rigid scaffolds for the construction of structurally well-defined gelator molecules. Furthermore, their host-guest interactions can be integral to gel formation, if these interactions are required to build the gel fibers. Finally, macrocycles can also be functional groups with which gelators are equipped that would also form gels in the absence of the macrocycle. Here, the macrocycle can be used as a binding site to allow additional stimuli control. To combine different stimuli for triggering gel-sol transitions certainly expands the options for establishing stimuli responsiveness. If, for example, an agent trapped inside the gel is only liberated when two different stimuli are present simultaneously, its release can be controlled with much higher precision and selectivity compared with a gel that responds to one stimulus only. In this Account, the recent progress in the construction of functional macrocycle-containing supramolecular gels is summarized. First, recent strategies to engineer responsiveness into macrocycle-containing gels are discussed. Next, different functions are presented including applications as responsive reaction media, for controlled drug-delivery or tissue engineering, and as self-healing materials. Finally, we highlight the recent progress in designing macrocycle-containing supramolecular gel materials exhibiting complex behavior. This field is part of systems chemistry and still in its infancy but appears to be one of the most promising routes to smart responsive materials.