Nanosheets and Metallo-Hydrogel Formed by 2-nm Metal-Organic Cages based on Electrostatic Interaction*

Macroionic solutions, including metal-organic cages, would form the single layered hollow sperical blackberry-type structure based on counterion-mediated attraction. Here, we observed 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 and π-π/hydrophobic interactions. With addition of small electrolytes, the cages in aqueous solution will follow the counterion-mediated attraction and self-assemble into 2D nanosheets, like normal macroionic solutions. However, the unique molecular structure and the charge distribution of the cages make the bending of nanosheets into spheres difficult, leading to stable, standing alone MOC nanosheets in solution and their very large excluded volumes lead to gelation at very low (~1.5 wt%) cage concentrations, with the helps from hydrophobic and partially π-π interactions similar to the gelation of graphene oxides.