Incorporation of the ⁹⁹TcO₄^– Anion within the Ag₂₄(CC^tBu)₂₀⁴⁺ Cluster Unveiling the Unique Shell-to-Core Charge Transfer

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C^tBu)₂₀(⁹⁹TcO₄)]·(BF₄)₃ (denoted as ⁹⁹TcO₄^–@Ag₂₄) 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C^tBu)₂₀⁴⁺ cluster. Notably, ⁹⁹TcO₄^–@Ag₂₄ 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 ⁹⁹TcO₄^–@Ag₂₄ 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.