Strong Co-Ion Effect via Cation−π Interaction on the Self-Assembly of Metal–Organic Cationic Macrocycles

The predesigned metal–organic macrocycle Zn3QDB3(NO3)4 (Zn-QDB) was observed to self-assemble into a hollow, spherical, single-layered “blackberry”-type structure. The self-assembly behaviors of the Zn-QDB are significantly influenced by additional small ions. Specifically, the cations exhibit strong co-ion effects on the interaction between cationic macrocycles which are different from the previously reported co-ion effects of simple anions on anionic polyoxometalates. This unusual phenomenon is due to the unique cation−π interaction between small cations and electron-rich cavity of Zn-QDB, as confirmed by UV–vis, 1H NMR, and fluorescence spectra. The variation of hydrodynamic radius (Rh) of assemblies with the changes of solution ionic strength and the type of cations reveals the competition between counterion-mediated attraction and cation−π interaction during the self-assembly process. Furthermore, the cooperativity of cation−π interaction and π–π stacking play a vital role in enhancing the stability ...