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Introduction of atropisomeric axes into a bent bispyridine ligand leads to the quantitative formation of a complex mixture of atropisomeric M 2 L 4 cages upon treatment with metal ions. Whereas the isomer ratio of the obtained cage mixture, consisting of up to 42 isomers, is insensitive to temperature and solvent, the quantitative convergence from the mixture to a single isomer is accomplished upon encapsulation of a large spherical guest, namely fullerene C 60 . The observed isomerization with other guests depends largely on their size and shape ( e.g. , <10 and 82% convergence with planar triphenylene and bowl-shaped corannulene guests, respectively). Besides the unusual guest-induced convergence, the present cage mixture displays the strongest guest emission ( Φ F = 68%) among previously reported M n L m cages and capsules, upon encapsulation of a BODIPY dye in water., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Host functions of polyaromatic nanocavities were revealed by using an M 2 L 4 molecular cage and capsule. On the basis of the previously reported M 2 L 4 capsule with a closed polyaromatic cavity, a new M 2 L 4 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., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)