Synthesis and chiroptical study of D/L-penicillamine-capped silver nanoclusters

We report the synthesis and chiroptical properties of silver nanocluster enantiomers. The surface of the silver nanoclusters is covered with L/D-penicillamine or their racemate. The clusters are separated by gel electrophoresis according to their charge and size into well-defined compounds. The mean core diameter of each fractioned component varies from 1 to 3 nm. The separated compounds of silver nanoclusters covered with L- or D-form thiol show strong optical activity or circular dichroism with opposite sign (mirror image relationship) in the metal-based electronic transitions (obtained anisotropy factor is on the order of 1 x 10(-3) to 1 x 10(-5)), suggesting that the nanoclusters have well-defined stereostructures as common chiral molecules do. With a decrease in the mean cluster diameter, the anisotropy factor gradually increases at first, but a steep rise is observed when the diameter becomes smaller than similar to 1.5 nm. The chiroptical response of the silver nanoclusters is consequently several-fold larger than that of the gold nanoclusters having the same ligand. The possibilities of a dissymmetric field model as well as an inherently chiral core model are discussed as the origin of the intense optical activity.