The recognition afforded by macrocycle hosts in binding a suite of different guests enables routes to rationally design soft materials from the molecular scale and afford specific and tunable functionality. Certain macrocyclic host–guest chemistries achieve among the highest affinity interactions ever observed, offering a non-covalent approach to enable recognition and bond formation in complex environments. One particular macrocycle, cucurbit[7]uril (CB[7]), affords binding affinities (Keq) to an array of guests extending over ~10 orders of magnitude. Using CB[7], we have probed the parameter of host–guest affinity in the design of assorted supramolecular hydrogels. The affinity of crosslinking interactions in this platform enables a molecular-scale approach to control the crosslink exchange dynamics of the material, translates to tunable release of macromolecular payloads, and dictates the rate of cell infiltration and material clearance in vivo. In building on these responsive soft materials, photo-responsive guest chemistries are also explored for external and reversible control of the affinity and dynamics of supramolecular recognition. Using a larger macrocycle, CB[8], ternary complex formation then enables dynamic crosslinking of macromers using the soluble macrocycle bound to two guests simultaneously. The CB[8] macrocycle furthermore catalyzes reversible light-mediated cycloaddition of certain guests, offering an external stimulus for reversible in situ conversion between soft materials dynamic supramolecular and covalent elastic crosslinking. Finally, hydrogel states are also possible through transiently upgrading guest binding affinity, enabling temporal control over material formation and crosslinking dynamics.

Tunable & dynamic supramolecular crosslinking via host–guest recognition

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ACS Spring 2023

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