Open versus Closed Polyaromatic Nanocavity: Enhanced Host Abilities toward Large Dyes and Pigments

Host functions of polyaromatic nanocavities were revealed by using an M₂L₄ molecular cage and capsule. On the basis of the previously reported M₂L₄ capsule with a closed polyaromatic cavity, a new M₂L₄ cage (as a mixture of the isomers) was prepared by the quantitative assembly of two metal ions and four desymmetrized bispyridine ligands with a single polyaromatic panel. The obtained, open nanocavity of the cage exhibited enhanced binding abilities toward large dyes and pigments in water. For example, two molecules of coumarin dyes were bound in the nanocavity and showed strong whitish emission (up to Φ_F=34 %). Furthermore, metallopigments, the sizes of which are larger than the inner cavities of the cage and capsule, were bound only in the open polyaromatic nanocavity of the cage to give water-soluble 1:1 host–guest complexes.