Two polymorphic forms of 10,12-pentacosadiyn-1-ol molecular layers on graphite transferred from an aqueous surface

Abstract

Molecular layers of the diacetylene (DA) 10,12-pentacosadiyn-1-ol (PCDYol) were transferred from an aqueous surface onto a highly oriented pyrolitic graphite (HOPG) substrate. Two polymorphic forms, having herringbone and parallel molecular arrangements, were observed by scanning tunneling microscopy (STM) in air. In contrast to DA molecules having the COOH end group, where only the parallel arrangement is realized, the herringbone and parallel polymorphic forms arise from the flexibility of hydrogen bonds between the OH end groups of PCDYol, as observed for alkanols. The area ratio for these arrangements on the HOPG surface depended on the density of the molecules on the water surface and the sample preparation temperature. By one-dimensional (1D) contrast modulation (moiré pattern), the point-on-line coincidence between the lattice points of the PCDYol layers and the HOPG surface was determined for the herringbone and parallel arrangements. Because it promotes stability of the alkane, the epitaxy is primarily controlled by the coincidence of the molecular zigzag skeleton in the parallel arrangement with the benzenoid rings of graphite. In contrast, the herringbone arrangement was lacking in this coincidence with the graphite structure. Therefore, as is evident from the fluctuation of the orientation of molecular arrays (lamella axes), the herringbone arrangement is relatively unstable on the HOPG substrate. Consequently, we conclude that the herringbone arrangement is quasi-stable while the parallel arrangement is stable.