Supramolecular chiral films

The self-assembly of racemic and enantiopure Heptahelicene, a helically shaped polyaromatic hydrocarbon (C30H18), on single-crystal surfaces was studied at temperatures between 150 K and 1000 K by means of surface sensitive methods like scanning tunneling microscopy (STM), temperature programmed desorption (TPD), low-energy electron diffraction (LEED), time-of-flight secondary mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy and diffraction (XPS, XPD), and Auger electron spectroscopy (AES). On Ni(111), Ni(100) and Cu(111), the molecule remains intact up to 450 K. Above that temperature it decomposes in several steps into carbon and hydrogen, the latter desorbing subsequently as H-2. The adsorption of racemic Heptahelicene on Cu(111) leads to a two-dimensional enantiomeric separation into at least 20 nm wide homochiral domains. In the first monolayer, the adsorbate-substrate complex has a geometry in which the molecule is oriented with three terminal rings parallel to the surface. After adsorption of enantiopure Heptahelicene onto the stepped Cu(332) surface, an azimuthal alignment of the molecular spirals is observed, creating a single-phase orientational order. X-ray absorption studies (NEXAFS) using synchrotron radiation show for the molecule in the saturated monolayer on Ni(100) a tilted geometry.