Giant gold-glutathione cluster compounds: Intense optical activity in metal-based transitions

A series of giant metal-cluster compounds, each composed of a gold core and a glutathione (GSH) adsorbate layer, have been prepared from Au(I)SG polymers and separated by gel electrophoresis, using methods reported in a recent Letter [J. Phys. Chem. B 1998, 102, 10643-6]. Identification of the separated compounds by core mass is accomplished through laser desorption mass spectrometry of matrix-diluted films. Three principal compounds have core masses of ca. 4.3, 5.6, and 8.2 kDa tin the range of similar to 20-40 Au atoms), and show structured optical absorption spectra with clear optical absorption onsets near 1.7, 1.3, and 1.0 eV, respectively. Each of these shows unusually strong chiroptical activity in the metal-based electronic transitions across the near-infrared, visible, and near-ultraviolet regions, whereas neither the crude (unseparated) mixture nor its higher molecular weight components possess such strong optical activity. The location and strength of the optical activity suggest a metal electronic structure that is highly sensitive to the chiral environment imposed by the glutathione adsorbate groups, if indeed the gold core is not inherently chiral. Mechanisms that could account for the observed optical activity are discussed, but the mere presence of strong chiroptical effects in this metallic-cluster system places these novel compounds in a special class of molecular substances. Previously known giant metal-cluster compounds land nanocrystals) have not been reported to exhibit significant optical activity. With these results, there emerges a rather complete picture of the evolution of optical and electronic properties of thiol-based gold cluster compounds (or self-assembled monolayer passivated gold nanocrystals) from 20 to 1000 Au atoms (0.7-3.2 MI core diameter).