The first family of designable fortress-like alkyltin-oxo molecular cages: Ligand-regulated structural evolution and optical limiting effect

The development of organometal-oxo molecular cages remains challenging due to the difficulties associated with constructing novel structures through conventional bottom-up self-assembly methods. In this study, we present a rationally designed approach to construct the first family of novel fortress-like alkyl-Sn₁₂ molecular cages with size of ~ 0.8 × 0.7 nm. Utilizing an open hollow framework as a structural model, we employed a ligand regulation strategy to modify the skeleton and successfully create closed alkyltin-oxo molecular cages. Unlike previously reported football-shaped cages that solely feature alkyl or phenyl groups, our fortress-like alkyl-Sn₁₂ molecular cages are functionalized with various targeted π-conjugated bifunctional O/N ligands. This tunable functionalization allows us to explore the relationship between structure and nonlinear optical limiting (OL) properties at the nanoscale. The OL properties of these cages are influenced by the electron-donating or -withdrawing abilities of the ligands and the distance between adjacent cages (d(cages)). Additionally, the heavy atom substitution effect plays a significant role in the nonlinear OL response. Notably, the CTGU-SnC-9 cage exhibits the best nonlinear OL performance, attributed to its electron-donating groups and the large d(cages) value, outperforming both reported tin-oxo clusters and many other metal-oxo clusters/cages. This work provides new insights into the innovative construction and modulation of optical properties in organometal-oxo molecular cages.