Single-Atom Control of Aromaticity and Excited-State Dynamics in Endohedral Zirconium-Antimony Zintl Clusters.

Three-dimensional (3D) aromaticity provides a powerful framework for understanding the stability and electronic structures of superatomic clusters, yet how such aromaticity evolves during atom-by-atom structural growth remains poorly understood. Herein, we report two endohedral zirconium-antimony Zintl clusters, [Zr@Sb] and [Zr@Sb], that offer a rare one-atom comparison of nuclearity-dependent aromaticity and photodynamics. Structural and bonding analyses reveal that [Zr@Sb] behaves as an integrated Sb framework rather than three discrete Sb fragments coordinated to Zr. Adaptive natural density partitioning (AdNDP) and magnetic-response analyses establish a closed-shell SP superatomic configuration, giving rise to pronounced spherical all-metal aromaticity. In contrast, incorporation of one additional Sb atom reorganizes the framework into a cage-like [Zr@Sb] cluster, disrupts superatomic shell closure, and produces an SP configuration with strongly attenuated spherical aromaticity. Femtosecond transient absorption (fs-TA) spectroscopy further reveals that the aromatic [Zr@Sb] cluster exhibits markedly longer-lived excited-state species than [Zr@Sb]. These findings establish a direct structure-aromaticity-dynamics relationship, demonstrating that single-atom control of cluster nuclearity can regulate not only ground-state aromatic stabilization but also excited-state robustness in all-metal superatoms.