Stimuli-Responsive Hierarchical Structuring Controls Survival and Programs Hydrogelation of Supramolecular...

Hierarchical organization is a central strategy by which biological systems stabilize fragile assemblies and regulate function in hostile environments. Translating this principle to synthetic polymer systems in water remains challenging, as noncovalent interactions can readily disrupt supramolecular polymerization. Herein, we report on aqueous supramolecular metallofibers formed by Zn(II) Salphen complexes functionalized with poly(N-isopropylacrylamide) (PNIPAM), which undergo reversible, multi-stimuli-responsive hierarchical structuring. Individual nanofibers form spontaneously in dilute aqueous solution via supramolecular copolymerization with water molecules and reversibly organize into micron-sized bundles in response to temperature, ionic strength, and solvent composition. This higher-order organization kinetically stabilizes the assemblies under conditions of dilution, low pH, and chemical degradation, where both monomers and isolated nanofibers rapidly disassemble. Hierarchical structuring further modulates photophysical properties, enables selective sorting and survival of responsive over non-responsive fibers in mixed systems, and translates microscopic robustness into macroscopic hydrogelation at unusually low concentrations. Leveraging the intrinsic photothermal response of the Zn(II) Salphen chromophore, light irradiation drives an energy-dependent, out-of-equilibrium bundled state that persists only while light energy is supplied, thereby controlling chemical stability in acidic environments. Collectively, this work establishes hierarchical supramolecular structuring in water as a general strategy to program stability, selectivity, and energy-dependent function in synthetic soft materials.