Chiral functionalization of optically inactive monolayer-protected silver nanoclusters by chiral ligand-exchange reactions

We report the ligand-exchange reaction between the optically inactive racemic penicillamine monolayer on a silver nanocluster surface and enantiopure D- or L-penicillamine dissolved in solution. Emergence of the identical band positions in the gel electrophoresis separation assures the presence of size-invariant silver nanoclusters (1.05 and 1.30 nm in core diameter) during the ligand-exchange reaction and allows us to further examine the optical/chiroptical properties of these nanoclusters. Consequently, chiral functionalization of the achiral silver nanoclusters has been demonstrated, yielding large Cotton effects in metal-based electronic transitions with an almost mirror-image relationship between the enantiomeric compounds. The ligand-exchange experiments as well as the normal syntheses of the silver nanoclusters revealed that their absorption profiles and anisotropy factors were strongly dependent on the enantiomeric purity (or enantiomeric excess) of surface chiral penicillamine, so that (several-fold) larger chiroptical responses of the silver nanoclusters as compared to those of the analogous gold clusters with a comparable size could be induced by the metal core deformation or rearrangement along with a universally influential vicinal contribution from the chiral ligand field.