Enhanced Chiroptical Activity in Glutathione-Protected Bimetallic (AuAg)18 Nanoclusters with Almost Intact Core–Shell Configuration

We here focus on the chiroptical response of a glutathione (GS)-protected bimetallic (AuAg)18 nanocluster containing comparable amounts of Au and Ag atoms. X-ray photoelectron spectroscopy (XPS) as well as the electrophoretic pattern allow us to determine the chemical composition of the nanocluster as Au8.5Ag9.5(SG)14. Chiroptical activity of the Au8.5Ag9.5 nanocluster is enhanced as compared to that of the corresponding monometallic Au counterpart, whereas that of bimetallic (AuAg)18 nanocluster with the atomic ratio of Au/Ag = 4.0 (Au14.4Ag3.6) decreases. Since thiolate-protected Au18 nanocluster has recently been revealed to consist of a face-fused Au9 bioctahedral kernel protected by one Au4(SR)5 tetramer, one Au2(SR)3 dimer, and three Au(SR)2 monomers, we use density functional theory (DFT) calculations to predict the lowest-energy structure of a model cluster compound Au9Ag9(SH)14 on the basis of this geometry, yielding a Agcore–Austaple intact core–shell configuration. This structure is in good agreement with the experimental XPS data of Au8.5Ag9.5(SG)14. Additionally, calculations give an overall increase in the chiroptical response as compared to the monometallic Au18 compound when nine Ag atoms are incorporated in the 18-metal-atom bimetallic nanocluster. From a structural viewpoint, in reality, a limited number of possible configurational isomers with similar (but slightly higher) energies may lower the response magnitude, but the overwhelming presence of the nanocluster with almost intact core–shell configuration prevents positional fluctuation of Ag atoms in the cluster, suppressing the averaging of circular dichroism (CD) responses with positive and negative signs of different (possible) geometrical isomers, which should be conclusively responsible for enhancing the chiroptical activity in this bimetallic nanocluster system with heavy Ag doping.