3 nm-Scale Molecular Switching between Fluorescent Coordination Capsule and Nonfluorescent Cage

A 3 nm-sized cage-shaped Hg₆L₄ complex with large openings was quantitatively formed from trismonodentate ligand (L) and Hg²⁺ ions in a self-assembled manner. The cage complex shows reversible interconversion with a capsule-shaped Hg₆L₈ complex in response to the ratio of ligand to Hg²⁺. This structural switching is coupled with reversible ON/OFF switching of fluorescence between fluorescent Hg₆L₈ capsule and nonfluorescent Hg₆L₄ cage.