Cage-on-Cage Evolution toward Rigid Metallo-Organic Giant for Enhanced Fluorescence

Metallo-cages with distinctive cavities and intricate molecular frameworks have found extensive applications in diverse fields. However, it is still challenging to construct larger frameworks with high rigidity and complexity via facile methods and simple ligands, which is crucial for further applications of these alluring architectures. Herein, we propose a cage-on-cage assembly based on the development of a judiciously designed dendritic ligand that enables the bottom-up growth of a series of C_2v-symmetric multitopic ligands (LA, LB, and LC). Through systematic increase of branches on the ligand, the assemblies were precisely controlled, yielding a series of supramolecular architectures with hierarchically increased complexity from a C_2h-symmetric chairlike [Zn₂LA₂], to a C_3v-symmetric bowl-shaped [Zn₆LB₃], and ultimately into a T_d-symmetric giant tetrahedral cage [Zn₃₆LC₁₂]. Remarkably, the [Zn₃₆LC₁₂] metallo-cage achieved an unprecedented molecular weight of 51 kDa and an outer ball diameter of 6.7 nm, representing the largest single-component ligand-based molecular tetrahedron reported to date in terms of both molecular weight and physical dimensions. Owing to the high rigidity of the single-component framework, the [Zn₃₆LC₁₂] exhibited a superior fluorescence quantum yield of 60.3%. This research introduces an innovative hierarchical strategy for the architecture of single-component metallo-organic cages, which provides a promising candidate for the development of luminescent materials in advanced optoelectronics.