Asymmetric Transformation of Monolayer-Protected Gold Nanoclusters via Chiral Phase Transfer

Phase transfer of aqueous racemic penicillamine-protected gold nanoclusters into chloroform is achieved by hydrophobizing the anionic nanocluster surface with a chiral ephedrinium cation (that is, chiral phase transfer), yielding asymmetric transformation or symmetry breaking of the optically inactive gold nanoclusters: The phase-transferred nanoclusters display appreciable optical activity in the metal-based electronic transition region. The ephedrinium cation contains chiral substituents with R,S absolute configuration, so that nanocluster diastereomers also can be produced via the phase transfer of enantiopure (S)- or (R)-penicillamine-protected gold nanoclusters with the chiral cations. Upon phase transfer, absorption spectra of the gold nanoclusters are almost invariant, whereas their circular dichroism (CD) signals exhibit a significant change in the metal-based electronic transition region. On the basis of the optical/chiroptical responses of the asymmetrically transformed gold nanoclusters as well as model quantum chemical calculations, the induced optical activity is most likely due to the ligand dissymmetric field brought about by the surface stereostructures.