Ultrafast Relaxation Dynamics of Luminescent Rod-Shaped, SilverDoped AgxAu25-x 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 Au-25 cluster possesses a low photoluminescence quantum yield (QY = 0.1%), whereas substituting silver atoms for central gold atom in the rod-shaped Au-25 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 AgxAu25 cluster by comparing the excited state dynamics of doped and undoped Au-25 rod cluster, it is found that the excited state relaxation in AgxAuls, is proceeded with an ultrafast ps) internal conversion and a subsequent nuclear relaxation (-20.7 ps), followed by slow (7.4 ms) decay back to the ground state. Meanwhile, the observed nuclear relaxation is much faster in AgxAu25. ('-.'20.7 ps) compared to that in undoped Au-25 rod (,52 ps). We conclude that it is the central Ag atom that stabilizes the charges on LUMO orbital and enhances the rigidity of Agx.Au-25, cluster that leads to strong fluorescence. Meanwhile, coherent oscillations around similar to 08 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.