Postsynthetic asymmetric transformation of monolayer-protected metal nanoclusters and their chiroptical responses

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We have intensively studied on postsynthetic asymmetric transformation of optically inactive monolayer-protected metal nanoclusters. We found that;(1) Gold nanoclusters protected by achiral mercaptophenylboronic acid were successfully synthesized. Addition of D- or L-fructose brought about appreciable circular dichroism (CD) signals with an opposite sign (mirror-image relationship) in metal-based electronic transitions. Simultaneous deconvolution analysis of electronic absorption and magnetic circular dichroism (MCD) spectra of the pristine nanoclusters gave quantitative inter- pretation on the complicated CD responses:(2) Reaction of the outermost amino groups on D-/L-penicillamine-protected gold nanoclusters with ethyl isocyanate modified the chiral surface structure. Ligand size (length) was extended through additional reactions of the carbamoylated amino groups. Their optical and chiroptical responses were overall similar to each other, but the ligand size had a distinct influence on the chiroptical response:(3) Cross-linking chemistry of optically-inactive gold nanoclusters by chiral dithiol with two stereogenic centers (L-dithiothreitol) was demonstrated. The large chiroptical activity can be due to surface intrinsic handedness caused by a cyclic cross-linking with dithiol molecules.