The structural versatility available to POCs, both dynamic and kinetically stable, enables their design for binding a wide range of guests in solution, including anions 131 (of particular note, cage 56 exhibited attomolar affinity (Ka ~10 17 M -1 ) for Cl - ) 114 and cations 132 as well as neutral molecules such as saccharides, 133 small aromatics, 134 polycyclic aromatic hydrocarbons 108d,110 and fullerenes. Such host-guest chemistry can be exploited for sensing and separations, 136 and the cavity space can be engineered to possess endohedral catalytic sites. In the solid-state, many POCs have been explored as porous materials for applications involving the uptake and separation of gases. 91