Enantioseparation and chiral induction in Ag nanoclusters with intrinsic chirality.

The optical activity of a metal nanocluster (NC) is induced either by an asymmetric arrangement of constituents or by a dissymmetric field of a chiral ligand layer. Herein, we unveil the origin of chirality in Ag NCs, which is attributed to the intrinsically chiral atomic arrangement. The X-ray crystal structure of a Ag(BDT)(TPP) NC (BDT: 1,3-benzenedithiol; TPP: triphenylphosphine) manifested the presence of intrinsic chirality in the outer shell capping the icosahedral achiral Ag core. The enantiomers of the Ag(BDT)(TPP) NC are separated by high-performance liquid chromatography (HPLC) using a chiral column for the first time, showing mirror-image circular dichroism (CD) spectra. The CD spectra are reproduced by time-dependent density functional theory (TDDFT) calculations based on enantiomeric Ag models with achiral 1,3-propanedithiolate ligands. The mechanism of chiral induction in the synthesis of Ag(DHLA) (DHLA: α-dihydrolipoic acid) NCs with a chiral ligand system is further discussed with the aid of DFT calculations. The use of the enantiomeric DHLA ligand preferentially leads to a one-handed atomic arrangement which is more stable than the opposite one, inducing the enantiomeric excess in the population of intrinsically chiral Ag NCs with CD activity.