Ultrafast Electron Dynamics in Thiolate-Protected Plasmonic Gold Clusters: Size and Ligand Effect

The influence of passivating ligands on electron-phonon relaxation dynamics of the smallest-sized gold clusters was studied using ultrafast transient absorption spectroscopy and theoretical modeling. The electron dynamics in Au₂₇₉, Au₃₂₉, and Au₃₂₉ passivated with 4-tert-butylbenzene thiol (TBBT), phenylethane thiol (SC₂Ph), and hexane thiol (SC₆), respectively, were investigated. These clusters were chosen as they are the smallest gold clusters reported till-date to show plasmonic behavior. Ultrafast transient absorption measurements were also carried out on Au_∼1400 (SC₆) and Au_∼2000 (SC₆) to understand the influence of the size on electron-phonon relaxation with the same passivating ligand. The study has revealed interesting aspects on the role of ligands on electron-phonon relaxation dynamics wherein the aromatic passivating ligands, SC₂Ph and TBBT, have shown smaller power dependence and broader surface plasmon bleach, indicating dampened plasmon resonance, whereas the cluster with aliphatic passivating ligands has behaved similarly to regular plasmonic gold nanoparticles. To model the effect of the ligand on the plasmonic properties of the investigated samples, free electron density correction factor of each one was calculated using the three-layered Mie theory, and the results show that SC₆ interacts least with core-gold, whereas TBBT and SC₂Ph have a greater effect on the surface electronic conductivity that is attributed to π-interaction of the ligand with gold. The results also shed light on unusual electron-phonon relaxation and smaller slope observed for Au₃₂₉ (SC₂Ph) that is ascribed to surface gold-πinteractions creating a hybrid state. In contrast, extended π-interaction is probably the reason for the plasmonic nature observed in Au₂₇₉ (TBBT), even though its size is smaller when compared to Au₃₂₉. In addition, the results also have shown that the electron-phonon coupling increases with an increase in the size of the cluster, and theoretical modeling has shown increased electron conductivity for larger plasmonic gold clusters.