Incorporation of the 99TcO4– Anion within the Ag24(C≡CtBu)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≡CtBu)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≡CtBu)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.