Division of Molecular and Nanomaterials, Department of Chemistry and INPAC-Institute of Nanoscale Physics and Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, P.O. Box 2404, 3001 Leuven, Belgium.
Chemistry. 2010 Dec 27;16(48):14447-58. doi: 10.1002/chem.201001653.
The structures of the self-assembled monolayers of various 4-alkoxybenzoic acids physisorbed at the liquid-solid interface were established by employing scanning tunnelling microscopy (STM). This study has been essentially undertaken to explore the competitive influence of van der Waals and hydrogen-bonding interactions on the process of two-dimensional self-assembly. These acid derivatives form hydrogen-bonded dimers as expected; however, the dimers organise themselves in the form of relatively complex lamellae. The characteristic feature of these lamellae is the presence of regular discommensurations or kinks along the lamella propagation direction. The formation of kinked lamellae is discussed in light of the registry mechanism of the alkyl chains with the underlying graphite substrate. The location of the kinks along a lamella depends on the number (odd or even) of carbon atoms in the alkyl chain. This result indicates that concerted van der Waals interactions of the alkyl chain units introduce the odd/even chain-length effect on the surface-assembled supramolecular patterns. The odd/even effects are retained even upon complexation with a hydrogen-bond acceptor. However, as the solvent is changed from 1-phenyloctane to 1-octanoic acid, the kinked lamellae as well as the odd/even effects disappear. This solvent-induced convergence of supramolecular patterns is attained by means of co-crystallisation of octanoic acid molecules in the 2D crystal lattice, which is evident from high-resolution STM images. The solvent co-adsorption phenomenon is discussed in terms of competing van der Waals and hydrogen-bonding interactions.
通过扫描隧道显微镜（STM）建立了各种 4-烷氧基苯甲酸在固-液界面上的自组装单分子层的结构。这项研究的主要目的是探索范德华力和氢键相互作用对二维自组装过程的竞争影响。这些酸衍生物形成预期的氢键二聚体；然而，二聚体以相对复杂的薄片形式组织自己。这些薄片的特征是在薄片扩展方向上存在规则的不调和或扭结。扭结薄片的形成是根据烷基链与基底石墨的登记机制来讨论的。扭结的位置取决于烷基链中的碳原子数（奇数或偶数）。这一结果表明，烷基链单元的协同范德华相互作用对表面组装的超分子图案引入了奇数/偶数链长效应。即使与氢键受体络合，奇数/偶数效应也会保留。然而，当溶剂从 1-苯基辛烷变为 1-辛酸时，扭结薄片以及奇数/偶数效应消失。这种溶剂诱导的超分子图案收敛是通过在二维晶格中形成辛酸分子的共结晶来实现的，这从高分辨率 STM 图像中可以明显看出。溶剂共吸附现象是根据竞争的范德华力和氢键相互作用来讨论的。
