Acid-/Base- and Site-Hindrance-Induced Cage Transformation and Hierarchical Self-Assembly of Regular Nanoaggregates

Abstract

Regulating the topology redistribution of coordination cages through acid-/base-responsive ligand conversion remains challenging, especially when cage topology is further directed by substituent steric effects and spatial orientation. Herein, we report a supramolecular cage system showing substituent-directed topology redistribution and fluorescence modulation under acid/base regulation. Acid- and base-induced ring-closing and ring-opening reactions provide a covalent structural switch in bisimidazolium ligands, and the resulting ligands self-assemble with Zn2+ ions into cage assemblies assigned to cuboctahedral, octahedral, square-prismatic, and triangular-prismatic cage architectures. The steric hindrance and spatial orientation of the substituents modulate the apparent cage topology distribution, while coordination-induced cage rigidity and substituent-dependent local steric restriction jointly give rise to fluorescence modulation with quantum yields varying by more than two orders of magnitude. Furthermore, the hierarchical self-assembly of cuboctahedral cages into hexagonal nanoaggregates or lyotropic nanostructures was observed by TEM, depending on the electrostatics and geometry-induced packing forces. This study demonstrates how substituent-directed ligand design couples acid-/base-responsive cage topology redistribution with optical and aggregation behaviors.