Probing Counterion Distribution Around Cationic Metal–Organic Cages by Fluorescence, Analytical Ultracentrifugation, and Molecular Dynamics

Abstract

We report a convenient fluorescence-based approach to probe the counterion association around soluble, cationic metal–organic cages (MOCs) in aqueous solution. Two MOCs incorporating tetra-(4-pyridylphenyl) ethylene (TPPE), an aggregation-induced emission (AIE) chromophore, were employed to monitor the spatial distribution and binding of counter-anions across a range of ionic strengths. Alterations in the fluorescence response of the TPPE groups sensitively respond to the changes in their local environment, providing a probe of counterion location and distribution. The results suggest a stepwise counter-anion association mechanism around MOCs: first around the Pt vertices dominated by electrostatic interactions at low counterion-to-MOC ratios, then interacting with the TPPE ligands at higher ratios. Analytical ultracentrifugation (AUC) results and all-atom molecular dynamics simulations are in good agreement with this proposed two-stage association mechanism and suggest distinct spatial preferences of different anions. This combined experimental–computational strategy provides a broadly applicable method for elucidating counterion distributions and dynamics around charged nanoscale species in solution.