Emergence of Large Chiroptical Responses by Ligand Exchange Cross-Linking of Monolayer-Protected Gold Clusters with Chiral Dithiol

We here present a study of cross-linking chemistry of optically inactive monothiol-protected gold clusters by chiral bidentate dithiol with two stereogenic centers, (2R,3R)-1,4-dimercapto-2,3-butanediol (L-dithiothreitol; L-DTT), and explore the impacts of the cross-linking on their chiroptical responses. The pristine protective ligand is racemic penicillamine (rac-Pen), and the products of the ligand exchange reactions include clusters containing both rac-Pen and L-DTT (partial exchange). Electrophoresis using polyacrylamide gel with a very low gel concentration (3%) can make the products separable into two components, each of which has the similar mean core diameter of 0.78 and 0.83 nm, so the difference in the relative mobility is mainly ascribed to the size of the cluster assembly. In addition, very large optical activity with the maximum anisotropy factors of about 1.0 x 10(-3) is found for the assemblies. In comparison with chiral 1,3-dithiol protection incapable of cross linking between gold clusters, we propose that the observed optical activity is due to surface intrinsic handedness caused by a cyclic cross linking with at least two L-DTT molecules.