Department of Chemistry, Tufts University, Medford, Massachusetts 02155, USA.
Thomas Young Centre, London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom.
J Chem Phys. 2018 Jul 21;149(3):034703. doi: 10.1063/1.5035500.
The delicate balance between hydrogen bonding and van der Waals interactions determines the stability, structure, and chirality of many molecular and supramolecular aggregates weakly adsorbed on solid surfaces. Yet the inherent complexity of these systems makes their experimental study at the molecular level very challenging. In this quest, small alcohols adsorbed on metal surfaces have become a useful model system to gain fundamental insight into the interplay of such molecule-surface and molecule-molecule interactions. Here, through a combination of scanning tunneling microscopy and density functional theory, we compare and contrast the adsorption and self-assembly of a range of small alcohols from methanol to butanol on Au(111). We find that longer chained alcohols prefer to form zigzag chains held together by extended hydrogen bonded networks between adjacent molecules. When alcohols bind to a metal surface datively via one of the two lone electron pairs of the oxygen atom, they become chiral. Therefore, the chain structures are formed by a hydrogen-bonded network between adjacent molecules with alternating adsorbed chirality. These chain structures accommodate longer alkyl tails through larger unit cells, while the position of the hydroxyl group within the alcohol molecule can produce denser unit cells that maximize intermolecular interactions. Interestingly, when intrinsic chirality is introduced into the molecule as in the case of 2-butanol, the assembly changes completely and square packing structures with chiral pockets are observed. This is rationalized by the fact that the intrinsic chirality of the molecule directs the chirality of the adsorbed hydroxyl group meaning that heterochiral chain structures cannot form. Overall this study provides a general framework for understanding the effect of simple alcohol molecular adstructures on hydrogen bonded aggregates and paves the way for rationalizing 2D chiral supramolecular assembly.
氢键和范德华相互作用之间的微妙平衡决定了许多在固体表面上弱吸附的分子和超分子聚集体的稳定性、结构和手性。然而，这些系统的固有复杂性使得它们在分子水平上的实验研究非常具有挑战性。在这一探索中，吸附在金属表面上的小分子醇已成为获得对这些分子-表面和分子-分子相互作用相互作用的基本理解的有用模型体系。在这里，我们通过扫描隧道显微镜和密度泛函理论的结合，比较和对比了一系列从小甲醇到正丁醇的小分子醇在 Au(111)上的吸附和自组装。我们发现，长链醇更喜欢形成由相邻分子之间扩展的氢键网络结合在一起的锯齿链。当醇通过氧原子的两个孤对电子中的一个与金属表面发生 dative 键合时，它们会变成手性的。因此，链结构是由相邻分子之间的氢键网络形成的，其中交替吸附着手性。这些链结构通过较大的单元晶格来容纳较长的烷基链，而醇分子中羟基的位置可以产生更密集的单元晶格，从而最大化分子间相互作用。有趣的是，当分子中引入内在手性时，如在 2-丁醇的情况下，组装完全改变，观察到具有手性口袋的正方形堆积结构。这可以通过以下事实来解释：分子的内在手性指导吸附的羟基的手性，这意味着不能形成异手性链结构。总的来说，这项研究为理解简单醇分子吸附结构对氢键聚集体的影响提供了一个通用框架，并为合理设计二维手性超分子组装铺平了道路。
