A comparison of the chiral counterion, solvent, and ligand used to induce a chiroptical response from Au25− nanoclusters

Abstract

A 25-atom gold nanocluster capped with an achiral thiolate exhibits no chiroptical signals in circular dichroism (CD) measurements. Herein, we report a systematic study on the effects of the chiral environment on the CD response from the Au₂₅ metal core. We found that Au₂₅(SC₂H₄Ph)₁₈⁻TOA⁺ dissolved in a chiral solvent did not give rise to a CD response, nor did Au₂₅(SC₂H₄Ph)₁₈⁻ when associated with a chiral counterion (e.g., (−)-N-dodecyl-N-methylephedrinium, DME⁺). Both scenarios imply that the interaction of the chiral counterion (or chiral solvent molecules) with the achiral Au₂₅(SC₂H₄Ph)₁₈⁻ nanocluster is not strong enough to induce CD signals from the metal core. In contrast, when the metal core is capped with chiral ligands (i.e., Au₂₅(SCH₂C*H(NH₂)CH₂Ph)₁₈), strong CD signals in the visible wavelength range were observed. Thus, the induction of CD signals by surface chiral ligands is much stronger than that by the external chiral environment (including the chiral solvent or counterion). This work reveals some further insight into the origin of the chiroptical response of the Au nanoclusters. These chiral nanoclusters hold potential for practical applications in bioconjugation, sensing, and chiral catalysis.