Dual Photo- and pH-Responsive Foldamer Metallogels

Foldamers have been widely used as responsive motifs within discrete supramolecular systems on account of their programmable stimuli-sensitivity. However, translation of this functionality into bulk matter and toward the creation of smart materials, such as switchable metallogels, remains largely unexplored. Further, characterization of local structure in metallogel networks is inherently challenging because of their semisolid nature, precluding the use of solution phase NMR spectroscopy. We report a dual light- and pH-responsive foldamer metallogel, comprised of aromatic oligoamide foldamers coordinated to palladium(II) centers. We show reversible gel-sol-gel behavior using UV illumination and subsequent heat treatment. Exposure to aqueous acid also induces a gel-sol transition. We determine the local structure of the gel using magic angle spinning (MAS) dynamic nuclear polarization (DNP) NMR spectroscopy. We thus identify square planar 4-fold coordination of Pd (II) by terminal pyridines in the foldamer as the primary structural motif comprising the gel backbone. Finally, successful exchange of the organic solvent with water, without compromising the gel structure, leads to the first example of a metallo-foldamer hydrogel.