Incorporation of the 99 TcO 4 - Anion within the Ag 24 (C≡C t Bu) 20 4+ Cluster Unveiling the Unique Shell-to-Core Charge Transfer.

We present the first example of an 99 TcO 4 - anion entrapped within the cavity of a silver cluster, revealing an unprecedented photoinduced charge transfer phenomenon. [Ag 24 (C≡C t Bu) 20 ( 99 TcO 4 )]·(BF 4 ) 3 (denoted as 99 TcO 4 - @Ag 24 ) was successfully synthesized and structurally characterized. Single-crystal X-ray diffraction and Raman spectroscopy reveal that the tetrahedral structure of the 99 TcO 4 - anion sustains significant symmetry breaking with weakened Tc-O bond strength under confinement within the Ag 24 (C≡C t Bu) 20 4+ cluster. Notably, 99 TcO 4 - @Ag 24 exhibits a broadband electronic absorption spectrum in the visible region, which was absent for the other 99 TcO 4 - -containing compounds. Density functional theory calculations elucidate that host-guest electrostatic interactions result in an electron polarization effect between the 99 TcO 4 - anion core and the Ag 24 cationic shell. The emergence of an absorption band in 99 TcO 4 - @Ag 24 is rationalized by intermolecular charge transfer from the Ag 24 electronic states to the lowest unoccupied molecular orbitals of 99 TcO 4 - instead of the intramolecular electron transition observed in other 99 TcO 4 - -containing compounds.