Electrochemical and Optical Spectroscopic Probing of Transition‐Sized Au130(SR)50 Nanoclusters

Ultrasmall metal nanoclusters (NCs) exhibit a quantized conduction band, hence, a distinct HOMO‐LUMO gap (Eg). Such a quantized electronic structure gives rise to multiple discrete peaks in the optical absorption spectrum of the NCs. As the size grows to 130 gold atoms (Au130 protected by ligands), electrical charging and optical behaviors seem to show certain metal‐like features (hence, transition‐sizes). To probe such behaviors, especially the potential ligand effect, we have devised the synthesis of Au130 NCs protected by phenylethanethiolate and naphthalenethiolate, respectively, with the former having a nonconjugated separation between the aromatic molecular group and the metal core while the latter being in direct bonding. A careful comparison of these two Au130 nanoclusters with the earlier reported analogues is carried out, including the structurally characterized Au130(pMBT)50 and the aqueous counterpart. While all of these nanoclusters possess the same 80 free electron counts in the core, some notable differences in electrochemical and optical properties are found, which are attributed to the ligand effects. The obtained insights may stimulate further interest in the transition‐sized nanoclusters and also promote their applications in optics, energy conversion, and biomedicine. The size‐induced transition from nonmetallic to metallic state in metal nanoclusters has long been of fundamental interest. The Au130(SR)50 nanocluster is within the transition‐size regime, in which excitons start to bundle up due to the build‐up of electronic screening, but detailed analyses reveal an ultrasmall bandgap (~0.2 eV) and still nonmetallic behavior. The ligand‐caused differences on the absorption spectrum, bandgap, charging energy, and ultrafast electron dynamics are further discussed.