Photodynamics studies of ligand-protected gold nanoclusters by using ultrafast transient infrared spectroscopy

Highly monodisperse samples of three ligand-protected gold nanoclusters Au102(pMBA)44, Au144(SC2H4Ph)60, and a cluster tentatively identified as Au130(pMBA)50, were characterized by UV/vis and infrared spectroscopy, and their photodynamics was studied by transient absorption spectroscopy. The dynamics study for each cluster was performed by electronically exciting the cluster with a pump pulse in the visible or near infrared region and by monitoring the transient absorption of vibrational modes of the ligands with a mid-IR probe pulse. The photodynamics studies were used to determine the molecular or metallic behavior of the cluster, and also to gain important size dependent information about the relaxation processes and energy states involved in them. A drastic difference between the relaxation dynamics is observed between Au102(pMBA)44 and Au130(pMBA)50 clusters when compared to Au144(SC2H4Ph)60, which clearly shows that the two smaller clusters show molecular behavior, while the Au144 species shows relaxation typical for metallic species. Based on this, the transition region between molecular and metallic behavior for gold can be narrowed to occur between 130 and 144 gold atoms. By combining the experimental results with energies of the electronic states of the clusters obtained from DFT calculations, the observed photodynamics for the molecular clusters could be explained. For both clusters, the re- laxation was shown to involve relaxation via triplet states, and also revealed a vital role of the relative energies of different energy states in singlet and triplet manifolds. Vibrational cooling was also found to have a significant role in the relaxation, since the excess vibrational energy in the hot system was found to facilitate the observed relaxation processes.