With sizes ranging from 1 to 10 nm, macroions demonstrate attractive and unique solution behaviors which cannot be described either by Derjaguin-Landau-Verwey-Overbeek (DLVO) theory or Debye-Hückel theory. With charges carried by macroions, most of them show great solubility in polar solvent, and electrostatic interaction possesses a significant role in their solution behaviors. As single molecules, the association of counterions to macroions, based on electrostatic interaction, could show spatial predilection due to variable constructive units within macroions and the large size disparity between macroions and counterions. With moderate charges, many of macroions are capable to spontaneously self-assemble into single-layer hollow spherical blackberry-like structures via counterion-mediated attraction, which can be accurately controlled by pH, solvent polarity, and electrolyte concentration. Based on the complex structures of macroions, various interactions, such as hydrophobic interaction, hydrogen bonding, and sigma -π interactions, can be involved to compete or cooperate with electrostatic interaction to regulate their solution behaviors and achieve multiple self-assembled structures. Furthermore, with the larger supramolecular structures formed by macroions, phase transitions, such as sol-gel transition, take place. Therefore, exploring the solution behaviors of macroions at their single-molecular, self-assembled states, and consequent phase transition would be an efficient method to understand weak non-covalent interactions in the system, which are difficult to evaluate via traditional tools.
In this work, the structural well-defined metal-organic cages (MOCs) and uranyl peroxide molecular clusters serve as great models for understanding the solution behaviors of macroions either at single-molecular state or during their self-assembly processes. A series of MOCs with aggregation-induced emission (AIE) luminophores are designed and prepared. The change in fluorescence emission offers direct visualization of microenvironment changing around AIE luminophores based on effect from macroions, counterions and solvent molecules, which provides further understanding on their solution behaviors. Multiple self-assembled structures, e.g., nanosheets and blackberry-type structures, are observed, regulated by electrostatic interactions, sigma-π interactions, anion-π interactions and so on. Their optical properties and resulting assemblies in the systems mentioned above are characterized by static light scattering (SLS), dynamic light scattering (DLS), transmission electron microscopy (TEM), high-resolution scanning electron microscopy (HRSEM), small-amplitude oscillatory shear (SAOS), isothermal titration calorimetry (ITC), conductivity, fluorescence spectroscopy and UV-vis absorption, etc.