Incorporation of the ⁹⁹TcO₄^- Anion within the Ag₂₄(C≡CBu)₂₀⁴⁺ Cluster Unveiling the Unique Shell-to-Core Charge...

We present the first example of an ⁹⁹TcO₄^- anion entrapped within the cavity of a silver cluster, revealing an unprecedented photoinduced charge transfer phenomenon. [Ag₂₄(C≡CBu)₂₀(⁹⁹TcO₄)]·(BF₄)₃ (denoted as TcO@Ag) was successfully synthesized and structurally characterized. Single-crystal X-ray diffraction and Raman spectroscopy reveal that the tetrahedral structure of the ⁹⁹TcO₄^- anion sustains significant symmetry breaking with weakened Tc-O bond strength under confinement within the Ag₂₄(C≡CBu)₂₀⁴⁺ cluster. Notably, TcO@Ag exhibits a broadband electronic absorption spectrum in the visible region, which was absent for the other ⁹⁹TcO₄^--containing compounds. Density functional theory calculations elucidate that host-guest electrostatic interactions result in an electron polarization effect between the ⁹⁹TcO₄^- anion core and the Ag₂₄ cationic shell. The emergence of an absorption band in TcO@Ag is rationalized by intermolecular charge transfer from the Ag₂₄ electronic states to the lowest unoccupied molecular orbitals of ⁹⁹TcO₄^- instead of the intramolecular electron transition observed in other ⁹⁹TcO₄^--containing compounds.