Nanoscale Materials Science Laboratory, Empa, Swiss Laboratories for Materials Science and Technology, Ueberlandstrasse 129, CH-8600 Dübendorf (Switzerland).
Department of Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zürich (Switzerland).
Angew Chem Int Ed Engl. 2015 Nov 23;54(48):14422-6. doi: 10.1002/anie.201507590. Epub 2015 Oct 6.
Although crystallization is the most important method for the separation of enantiomers of chiral molecules in the chemical industry, the chiral recognition involved in this process is poorly understood at the molecular level. We report on the initial steps in the formation of layered racemate crystals from a racemic mixture, as observed by STM at submolecular resolution. Grown on a copper single-crystal surface, the chiral hydrocarbon heptahelicene formed chiral racemic lattice structures within the first layer. In the second layer, enantiomerically pure domains were observed, underneath which the first layer contained exclusively the other enantiomer. Hence, the system changed from a 2D racemate into a 3D racemate with enantiomerically pure layers after exceeding monolayer-saturation coverage. A chiral bias in form of a small enantiomeric excess suppressed the crystallization of one double-layer enantiomorph so that the pure minor enantiomer crystallized only in the second layer.
虽然结晶是化学工业中分离手性分子对映异构体最重要的方法，但在分子水平上，该过程中涉及的手性识别仍未得到很好的理解。我们通过 STM 在亚分子分辨率下观察到，从外消旋混合物中形成层状外消旋晶体的初始步骤。在手性铜单晶表面生长的七螺旋芳烃在第一层内形成手性外消旋晶格结构。在第二层中，观察到对映体纯的畴，在其下，第一层仅包含另一种对映体。因此，该体系在超过单层饱和覆盖后，从二维外消旋体转变为具有对映体纯层的三维外消旋体。手性偏差以小的对映体过量的形式抑制了一种双层对映体的结晶，使得纯的次要对映体仅在第二层结晶。
