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 (R_h) 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 of the supramolecular structure.