Ultrafast Relaxation Dynamics of Luminescent Rod-Shaped, Silver-Doped Ag_<i>x</i>Au_{25–<i>x</i>} Clusters

The luminescent ligand protected metal clusters have attracted considerable attentions while the origin of the emission still remains elusive. As recently reported in our previous work, the rod-shaped Au₂₅ cluster possesses a low photoluminescence quantum yield (QY = 0.1%), whereas substituting silver atoms for central gold atom in the rod-shaped Au₂₅ cluster can drastically enhance the photoluminescence with high quantum yield (QY = 40.1%). To explore the enhancement mechanism of fluorescence, femtosecond transient absorption spectroscopy is performed to determine the electronic structure and ultrafast relaxation dynamics of the highly luminescent silver-doped Ag_<i>x</i>Au_{25–<i>x</i>} cluster by comparing the excited state dynamics of doped and undoped Au₂₅ rod cluster, it is found that the excited state relaxation in Ag_<i>x</i>Au_{25–<i>x</i>} is proceeded with an ultrafast (∼0.58 ps) internal conversion and a subsequent nuclear relaxation (∼20.7 ps), followed by slow (7.4 μs) decay back to the ground state. Meanwhile, the observed nuclear relaxation is much faster in Ag_<i>x</i>Au_{25–<i>x</i>} (∼20.7 ps) compared to that in undoped Au₂₅ rod (∼52 ps). We conclude that it is the central Ag atom that stabilizes the charges on LUMO orbital and enhances the rigidity of Ag_<i>x</i>Au_{25–<i>x</i>} cluster that leads to strong fluorescence. Meanwhile, coherent oscillations around ∼0.8 THz were observed in both clusters, indicating the symmetry preservation from Au cluster to Ag alloying Au clusters. The present results provide new insights for the structure-related excited state behaviors of luminescent ligand protected Ag alloying Au clusters