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Supramolecular Hydrogels with Tunable Swelling by Host Complexation with Cyclobis(paraquat-p-phenylene)

Controlling the swelling properties of hydrogels is of primary importance for many applications ranging from actuators and valves to tissue engineering and drug delivery. Herein, we report the use of cyclobis(paraquat-p-phenylene) (CBPQT4+,4X–) as a versatile host to fine-tune the swelling behavior of 1,5-dialkyloxynaphthalene guest containing poly(N,N-dimethylacrylamide) hydrogels (NaphtGelz) through supramolecular host–guest complexation. While the equilibrium swelling of NaphtGelz in water decreases with increasing amount of hydrophobic naphthalene groups, the opposite behavior is observed with superabsorbing behavior (up to 180 times their initial dry mass) upon immersion in aqueous solutions containing the macrocyclic CBPQT4+,4X– due to formation of tetracationic host–guest complexes. In this case, the swelling amplitude could be conveniently programmed either by variation of the naphthalene content of the hydrogels or by controlling the stoichiometry of the host–guest binding events. Furthermore, by modifying the nature of the counterions (X = Cl–, Br–, and I–) of the tetracationic CBPQT4+ macrocyclic host, the swelling of the hydrogels could be tuned in line with Pearson’s absolute hardness scale of X–. The swelling behavior of these supramolecular hydrogels could be successfully described by a theoretical model, taking into account the hydrophobic association of the naphthalene groups and their screening by host–guest complexation. Finally, addition of sodium dodecyl sulfate as a surfactant to the supramolecularly swollen hydrogels led to a large decrease in hydrogel size due to dissociation of the host–guest complexes and the formation of CBPQT4+,4DS– within the hydrogel.