Cavity Constriction and Chiralization of Polyaromatic Subnanotubes via Post-assembly Ligand/Metal Modification

Late-stage modification of coordination-driven tubular and cage-like structures is a promising method to tune and alter their cavity functions yet remains difficult owing to the intrinsic instability of metal-ligand bonds against the modification conditions. Here we report polyaromatic tubes with a subnanometer-sized cavity, composed of three metal hinges and three polyaromatic ligands, both of which are modifiable in a post-assembly fashion. The new subnanotube, prepared from PtCl₂ salts and the anthracene-embedded bispyridine ligands, is reversibly converted into a constricted subnanotube by the <i>symmetrical</i> modification of the <i>ligands</i> with O₂. Using an analogous aqueous tube, one molecule of rod-like coumarin dyes is bound by the subnanocavity in water, and their emission properties are largely enhanced (up to 4-fold) through the ligand modification. On the other hand, the <i>asymmetrical</i> modification of the <i>metal hinges</i> with chiral ammonium groups generates a cryptochiral subnanotube while retaining the polyaromatic cavity. The resultant tube also binds the coumarin dye in water to induce its chiroptical properties as strong CD and CPL activities (up to |<i>g</i>_abs| = 2 × 10^-3 and |<i>g</i>_lum| = 3 × 10^-3). Two-directional post-assembly multi-modifications are demonstrated unprecedentedly, using polyaromatic subnanotubes, for the development of multi-functional host-guest systems.