A Tetrameric Cobalt(II) Assembly Integrating Aggregation-Induced Emission and Weak Antiferromagnetic Coupling

Abstract

A supramolecularly rigidified tetrameric cobalt(II) assembly, [Co₄T₈(H₂O)₂] (T-Co), where T = Tropolonate ion, has been constructed using tropolonate ligands to investigate the interplay between aggregation-controlled photophysics and magnetic exchange interactions in multinuclear 3d systems. Single-crystal analysis reveals a distorted octahedral Co₄ framework interconnected through oxygen-bridged pathways and reinforced by cooperative O–H···O hydrogen bonding, π–π stacking, and C–H···π interactions, producing an extended caterpillar-like supramolecular arrangement. The restricted structural dynamics imposed by this densely packed architecture give rise to pronounced aggregation-induced emission (AIE) behavior, in which progressive aggregation in water-rich media enhances fluorescence by suppressing nonradiative relaxation channels. Dynamic light scattering measurements correlate the emissive enhancement with aggregate growth, confirming aggregation-governed excited-state modulation. Magnetic studies reveal weak antiferromagnetic exchange between high-spin Co(II) centers, accompanied by pronounced single-ion magnetic anisotropy arising from spin–orbit coupling and rapid magnetization dynamics, although slow magnetic relaxation characteristic of robust single-molecule magnets is not observed. The present study demonstrates how supramolecular organization within a polynuclear cobalt framework simultaneously dictates excited-state confinement and magnetic communication, providing a viable strategy for engineering multifunctional luminescent magnetic materials through cooperative structural rigidification.