Red-light-responsive cucurbit[8]uril based host–guest interaction enabling the construction of a photoswitchable supramolecular polymeric hydrogel

Abstract

The development of an aqueous phase host–guest interaction system with combined features of responding to light within the biooptical window, high binding constant and low cytotoxcity is promising to offer a new efficient tool for constructing appealing photoresponsive functional supramolecular assemblies with potential bio-application. This paper presents the construction of a novel red-light-responsive host–guest interaction system based on cucurbit[8]uril (CB[8]) and water soluble azobenzene (AB) derived guests. Upon alternate red light (650 nm) and blue light (420 nm) irradiation, the reversible Z/E photoisomerization of the azo guest could be triggered to enable the system to exhibit distinct photoswitchable host–guest complexation behavior. While the E-isomeric guest could be encapsulated by CB[8] to form a head-to-tail 2 : 2 stoichiometric complex with a high binding affinity (K_a up to 10²⁰ M⁻³), the Z-isomeric guest could only form a 1 : 1 stoichiometric complex with CB[8]. By introducing such a photoswitchable host–guest binding motif as a supramolecular crosslinker, a polymeric hydrogel featuring red light-controlled gel–sol transformation could be fabricated. In addition, both the molecular building block and the resulting supramolecular crosslinking polymer could reveal low cytotoxicity. These advantages greatly benefit such a host–guest system to be potentially applied in the fabrication of red-light-responsive biofunctional supramolecular assemblies and materials.