Nanoscale Materials Science, Empa, Swiss Federal Laboratories for Materials Science and Technology , Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.
Department of Chemistry, University of Zurich , CH-8057 Zürich, Switzerland.
ACS Nano. 2017 Jan 24;11(1):865-871. doi: 10.1021/acsnano.6b07424. Epub 2017 Jan 11.
Gaining insight into molecular recognition at the molecular level, in particular, during nucleation of crystallites, is challenging and calls for studying well-defined model systems. Investigated by means of submolecular resolution scanning tunneling microscopy and theoretical molecular modeling, we report chiral recognition phenomena in the 2D crystallization of the helical chiral aromatic hydrocarbon pentahelicene on a Cu(111) surface. Homochiral, van der Waals bonded dimers constitute building blocks for self-assembly but form heterochiral as well as homochiral long-range-ordered structures. 2D racemate crystals, built up by homochiral dimers of both enantiomers, are observed at coverages close to a full monolayer. As soon as the coverage leads to second-layer nucleation, the dense racemate phase in the first layer disappears and a homochiral dimer conglomerate phase of lower 2D density appears. Our results show that, at the onset of second-layer nucleation, a local change of enantiomeric composition in the first layer occurs, causing the transition from a 2D racemate to a 2D conglomerate.
深入了解分子水平上的分子识别，特别是在微晶成核过程中，是一项具有挑战性的工作，需要研究定义明确的模型体系。通过亚分子分辨率扫描隧道显微镜和理论分子建模，我们报告了在螺旋手性芳烃 pentahelicene 在 Cu(111)表面的二维结晶过程中手性识别现象。同手性、范德华键合二聚体构成自组装的构建块，但也形成异手性和同手性长程有序结构。在接近单层满覆盖度的情况下，观察到由两种对映体的同手性二聚体构建的二维外消旋晶体。一旦覆盖度导致第二层成核，第一层中密集的外消旋相就会消失，而出现二维密度较低的同手性二聚体聚集体相。我们的结果表明，在第二层成核开始时，第一层中对映体组成发生局部变化，导致从二维外消旋体向二维聚集体的转变。
