An amphiphilic azobenzene derivative as a crosslinker in the construction of smart supramacromolecular hydrogels

The reversible photoisomerization of azobenzene (Azo) has been utilized to construct supramacromolecular hydrogels (SMHGs) in which the structure can be controlled by light. However, most of these examples have been realized only through grafting Azo functional group onto the macromolecular chains, and low-molecular-weight Azo derivatives are relatively scarce. Here, a novel amphiphilic azobenzene derivative (APA) has been designed and synthesized, which can be used as a low-molecular-weight crosslinker to construct SMHGs via host-guest interactions between Azo and β-cyclodextrin (β-CD). APA, with two hydrophobic ends and a hydrophilic segment, can self-assemble to spherical particles in the mixed solvent of THF and H2O, and disassemble upon UV light irradiation. When APA is mixed with the solution of hyaluronic acid modified by β-CD (HA-CD), hydrogels can form at different mass fractions of HA-CD. Moreover, when the molar ratio of APA and β-CD is 0.5, the resultant hydrogel has the highest G' value. Benefiting from the host-guest interactions between Azo and β-CD, as well as the intermolecular hydrogen bonds of hyaluronic acid, hydrogels undergo both photo- and thermo-responsive sol-gel transition. The release of Rhodamine B from the hydrogel, could be controlled by UV light irradiation or high temperature. The work presented here demonstrates a simple strategy to construct photo-responsive SMHGs by Azo-derived low-molecular-weight crosslinkers.