Macrocycle/ferrocene complexation-mediated smart supramolecular gel materials and their applications

As a guest molecule, ferrocene (Fc) forms host-guest complexes with various types of macrocyclic hosts, exhibiting satisfactory binding affinity. By oxidizing Fc to the positively charged ferrocenium ion (Fc⁺), the binding constant between Fc and the macrocyclic host can be significantly decreased or remarkably increased. Fc-based redox-responsive host-guest complexation not only serves as a cross-linking driving force to construct "smart" supramolecular gel materials but also acts as a modulator to regulate the swelling-shrinking properties of Fc-based hydrogels. The expansion-contraction of Fc-based supramolecular hydrogels regulated by redox stimuli can be applied in controlling the release of loaded drugs and in sensing redox reactions inside the hydrogels. Furthermore, the distinct colors displayed by Fc and Fc⁺ can be utilized to fabricate environment-responsive gel materials whose colors can be regulated by redox reactions. These materials have potential applications in the field of smart windows. In this review, the construction methods and applications of smart supramolecular gel materials based on different redox-responsive macrocycle/Fc host-guest interactions are summarized. Additionally, the prospects and limitations of Fc-based smart supramolecular gel materials are also discussed. This review is expected to inspire further refinement of these methods and applications, driving the development of more smart gel materials tailored for diverse applications.