Photoprinting and expansion-induced erasure with supramolecular hydrogels crosslinked by pseudorotaxanation

A series of cyclodextrin dimers (6-NDI-CDs) that were bridged by 1,4,5,8-naphthalenediimide (NDI) were synthesized. Photoirradiation of 6-NDI-CDs in an aqueous solution led to a color change from almost colorless to brown due to the formation of NDI anions, which could be quenched by oxygen to recover neutral NDI, accompanied by fading of the brown color. A tetraammonium axle was introduced into the sidechain of polyacrylamide through free-radical co-polymerization, and a supramolecular hydrogel was generated when mixing the co-polymer with a 6-NDI-CD and cucurbit[6]uril, as a result of pseudorotaxanation-based crosslinking. The supramolecular hydrogel thus obtained exhibited excellent ductility, could be stretched approximately up to 20 times its original size without breaking and could restore to its original size after releasing the pulling force, demonstrating that pseudorotaxane-based non-covalent crosslinking provided significant dynamic properties with excellent energy dissipation ability. Meanwhile, the existence of covalent chemical bonds in the polymeric material also guaranteed its mechanical strength, and excellent robustness and stability were given with Young's modulus up to 1.01 MPa, toughness up to 3.80 MJ m(-3), and maximum stress up to 1.12 MPa. The 6-NDI-CD-based supramolecular polymer hydrogel retained the characteristics of photoinduced color change, and the NDI anion in the hydrogel showed better resistance to oxygen fading, but the fading could be drastically accelerated by stretching the gel back and forth to realize erasure.