Photoresponsive Pseudopolyrotaxane Hydrogels Based on Competition of Host–Guest Interactions

Abstract

Reversibility is a basic and crucial feature of supramolecular systems. In designing and fabricating new supramolecular materials, the realization of reversibility is particularly important as it could enable these substances to be superior to conventional materials. An excellent example is an elastomer reported recently, made of small functional molecules which form both long chains and cross-linkers through intermolecular multiple hydrogen bonding. When broken or cut, the elastomer can be simply repaired by just bringing together the fractured surfaces and allowing the recovery of the hydrogen bonding. There are plenty of reports on supramolecular assemblies of polymers showing reversible responses to environmental changes; however, these are mostly based on the inherent stimuli-sensitive properties of the building blocks, such as the temperature-induced coil–globule transition of poly(N-isopropyl acrylamide) (PNIPAM) and pH-induced protonation of poly(vinyl pyridine), rather than the reversibility of the supramolecular interactions. Therefore, taking full advantage of the reversibility of the noncovalent interactions to construct supramolecular materials is still a big challenge. This has drawn increasing interest in recent years, and has led to a series of promising results. For example, some hydrophobically modified water-soluble polymers realized sol–gel transition as a result of the reversible interactions of cyclodextrin and alkyl chains. The assembly and disassembly of polymeric vesicles can be controlled by photosensitive interactions between cyclodextrin and azobenzene compounds. The self-assembly of polyethylene glycol (PEG) and a-cyclodextrins (a-CDs) to form linear pseudopolyrotaxane (PPR), with PEG as the axis and a-CDs as threaded rings, was reported by Harada et al. Since the ability of PPR to form physical hydrogels was first reported in 1994, the system has been extensively studied as the hydrogels show very promising uses as biomedicine materials. There is now good understanding of the formation mechanisms and structures of such PPR hydrogels, but little has been explored concerning their dissociation and reassembly, though increasing temperature and shearing could make the hydrogel turn to a sol. Herein, we demonstrate a facile photocontrollable supramolecular route to realize the disassembly and reassembly of the PPR hydrogels. The addition of a photoresponsive compound containing an azobenzene moiety to the PPR hydrogel was found to be effective in converting the hydrogels to transparent solutions. By subsequent alternation of UVand visible irradiation, reversible sol-to-gel and gel-to-sol transitions were observed. Thus, the widely investigated PEG/aCD PPR hydrogel is proved to be “active” in supramolecular chemistry, and the reversible nature of supramolecular materials is fully realized. In our study, the PPR hydrogel (Figure 1a) was prepared by using PEG10K (molecular weight 10000) and a-CD in water as described in reference [7c]. The concentrations of