Helical Nanostructure of Achiral Silver p-Tolylacetylide Molecules

Silver p-tolylacetylide is an achiral molecule; however, its nanostructure has been found to consist of twisted nanoribbons. The twisted ribbon is a helicoid that combines translation and perpendicular rotation along the ribbon axis. A helix, a typical chiral structure, can be created by the aggregation of achiral molecules, and the recrystallization conditions control the twist of the nanoribbons. Therefore, the recrystallization controls the chirality. 1. Introduction There are many known helical structures of various sizes. The winding staircase is a typical helical architectural feature on the macroscale. On the other hand, various important molecules in biology are microscopic helices, such as the DNA double helix and the alpha helix in proteins. Recent development in nanotechnology has produced helical species on the nanoscale. The aggregation of molecules can form nano-helical structures in supramolecular chemistry [1–5]. Helices coiled in the right-handed or left-handed direction can be formed, depending on the chirality of the constituent molecules. It is reasonable to assume that the chirality of a protein or structure comes from the chirality of the component amino acids (L-stereoisomers). Of course, a helix with the opposite direction of rotation will result from the use of amino acids of the opposite chirality (D-stereoisomers) [6]. We have studied nanostructures of metal acetylide molecules [7, 8] and applied their nanostructures to gas sensors, catalysis, and so on [9–11]. In this study, we succeeded in producing a helical nanostructure from an achiral molecule of silver acetylide. The molecular structure of silver p-tolylacetylide is shown in Figure 1. The molecule contains no asymmetric carbon atoms and has mirror symmetry. Although silver p-tolylacetylide is an achiral molecule, the aggregated crystal is a twisted nanoribbon structure, a helicoid. The mechanism for aggregation from the achiral molecules to the chiral helical nanostructure has not been fully understood; however, primary results have been presented in order to reveal the creation of chirality with scanning electron microscopy (SEM) and powder X-ray diffraction measurements. Figure 1: Molecular structure of silver p-tolylacetylide. 2. Experiment Silver p-tolylacetylide was prepared by the reaction of silver(I) nitrate and 4-ethynyltoluene in the presence of triethylamine in an acetonitrile solvent. Since the crude product cannot be dissolved in most solvents, trimethylphosphine was added in a dichloromethane solution. The trimethylphosphine complex of silver