Chirality Induction in Bulk Gold and Silver

Abstract

In a recent series of papers we introduced a new family of materials, namely, organically doped metals. Both small molecules and polymers were used for the preparation of these new composites, which exhibit properties that are both metallic and organic. A typical example is silver doped with the acidic polymer Nafion (Nafion@Ag), rendering this metal with an unorthodox acidic property that was indeed applied to some classical organic catalytic reactions. Entrapment within the metal is a distinctly different process from adsorption. For example, water-soluble molecules, which are easily washed away from the metal surface when adsorbed on it, do not readily leach out of the composites when they are washed with water. The intimate entanglement of the organic component between the aggregated metallic nanograins was proven by various methods, of which thermogravimetric analysis–differential scanning calorimetry (TGA–DSC) was particularly informative. Although most of our efforts have concentrated on doped silver, proving the generality of the developed methodology has also been proven by preparing organic composites with copper and gold. A particularly interesting challenge has been to dope metals with chiral molecules, in an attempt to induce chiral properties in the metal. Here we report that this goal has been achieved. Specifically, we present experimental evidence for consistent chiral asymmetry in the photoemission of electrons from gold and silver embedded with chiral biomolecules (L-glutathione, L-quinine, and Dand L-tryptophan). Thus, counter-clockwise circularly polarized light (ccw-CPL) and clockwise CPL (cw-CPL) induce different photoelectron emission yields from the doped metal sample, thus proving different diastereomeric interactions between light and the metal electrons. The experimental methodology, which has already been applied successfully to chiral monolayers on gold surfaces, is described in detail below. While the silver composites have been explored in detail in past studies, the gold composites are new, and therefore some background experiments with achiral dopants (Congo red (CR) and thionine (Th)) in gold were also carried out. It is relevant to mention some other studies in which metals and chirality were placed in the same context, including the chiral structure of metal nanoclusters with organic adsorbates, the helicity of metal nanowires, the adsorption of chiral molecules on the surface of metals, and the formation of chiral “2D” surfaces by cutting metal crystals along high-Miller-index planes. Many of these studies were carried out with metallic gold; the induction of chirality in metallic silver has been much less explored, although there is a recent example of induction of chirality in silver nanoparticles through the adsorption of DNA. Chirality is detected, amongst other methods, by circular dichroism (CD) spectroscopy, which requires good optical transparency. However, the compressed discs of dopant@metal are devoid of this property. Therefore, the idea behind the present study was to look for possible electron circular dichroism in photoelectrons emitted from the metals. Any successful detection of chirality involves different diastereomeric interactions between the enantiomers of a chiral specimen and a chiral probe, or between a chiral specimen and the enantiomers of the chiral probe. In our experiment, the chiral specimens are chiral-molecule@metal, and the chiral probe is circularly polarized UV light (ccw-CPL and cw-CPL). The details of the experiment are given below. Entrapment of Molecules in Gold: Because one of the novel aspects of this report is the extension of the homogeneous entrapment-in-metal methodology to gold, we first performed some comparative experiments to those reported for achiral CR and Th entrapped in Ag at a ratio of 1:100. X-ray diffraction (XRD) analysis (Table 1; carried out with a Philips automated powder diffractometer equipped with a PW1830 C O M M U N IC A IO N