Nanosheets and Hydrogels of Self-assembled 2-nm Metal-Organic Cages with Electrostatic interaction

We report a hydrogel formation process from 2-nm emissive, low molecular-weight metal-organic cages at low concentrations (>15 mg/mL) based on counterion-mediated attraction, π-π/hydrophobic interactions as well as σ-π interactions. Experiments and all-atom simulations confirm that with additional of small electrolytes, the cages in aqueous solution first self-assemble into 2D nanosheets via counterion-mediated attraction due to their unique molecular structure and charge distribution, as well as σ-π interactions. The stiff nanosheets are difficult to bend into 3-D hollow, spherical blackberry type structures, as observed in many other macroions systems. Instead, they stay in solution and their very large excluded volumes lead to gelation at low (~1.5 wt%) MOC concentrations, with additional helps from hydrophobic and partially π-π interactions similar to the gelation of graphene oxides.