Chiral chemistry in metal nanoclusters : Fundamentals and applications

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We found large optical activity in gold nanoclusters (<2nm) protected by a pair of penicillamine enantiomers. The observed optical activity was attributed to a dissymmetric field induced by the chiral ligands. We have also succeeded in synthesizing a pair of optical isomers of silver nanoclusters. Their chiroptical response was several-fold larger than that of the analogous gold nanoclusters with comparable core size. The dissymmetric field effect from chiral ligands should be a universal origin for their optical activity, but its enhancement observed in a small size region of silver might be attributed to additional contributions from a chiral silver core. Next, the VCD spectra of penicillamine-protected silver nanoclusters have been studied experimentally, and analyzed theoretically. The spectral assignment suggested that the solvent effect was necessary to account for an agreement between the experimental and theoretical VCD spectra, resulting in the P_N-type conformation as the most probable isomer. Furthermore, we have achieved asymmetric transformation or symmetry breaking of the optically inactive gold and silver nanoclusters using chiral phase-transfer or boronic-acid chemistry.