Department of Chemistry , Seoul National University , 1 Gwanak-ro, Gwanak-gu, Seoul 08826 , Korea.
Department of Chemistry , Indiana University , 800 E. Kirkwood Avenue , Bloomington , Indiana 47405 , United States.
J Am Chem Soc. 2018 Apr 4;140(13):4726-4735. doi: 10.1021/jacs.8b01805. Epub 2018 Mar 21.
Small molecule self-assembly at surfaces offers an efficient route to highly ordered organic films that can be programmed for a variety of chemical and electronic applications. The success of these materials depends on the ability to program intermolecular interactions to guide precise structural ordering. Toward this objective, we have designed and synthesized a series of bis(triazolo)benzene-based π-conjugated molecules. Our synthesis exploits a last-stage C-C cross-coupling reaction to close up zigzag-shaped linear precursors to cyclized products, so that direct side-by-side comparisons can be made for their structure-dependent self-assembly behavior at surfaces and response to external stimuli. Indeed, scanning tunneling microscopy (STM) analysis revealed distinct differences as the conformational flexibility of the molecular backbone and the chemical structure of the peripheral groups are varied. Specifically, alkyl chains adsorb and form interdigitated structures, whereas oligo ethylene glycol (OEG) chains remain desorbed and thus shift self-assembly to more densely packed π-conjugated cores. While the macrocycles self-assemble immediately and spontaneously, their linear precursors exhibit slower self-assembly kinetics, which could be attributed to the difference in the degree of conformational freedom. We also found that perturbation by the STM tip and the addition of cosolutes profoundly impacted the kinetics of self-assembly and surface patterning. This highly unusual behavior highlights the importance of noncovalent interactions that are inherently weak in solution but can be made strong for symmetric and conformationally restricted molecules confined within 2D surfaces.
小分子在表面的自组装为高度有序的有机薄膜提供了一种有效的途径，这些薄膜可以通过编程应用于各种化学和电子应用。这些材料的成功取决于能够对分子间相互作用进行编程以指导精确的结构有序化的能力。为了实现这一目标，我们设计并合成了一系列基于双（三唑）苯的π-共轭分子。我们的合成利用最后阶段的 C-C 交叉偶联反应来封闭锯齿形线性前体，得到环状产物，从而可以对其在表面的结构依赖性自组装行为以及对外界刺激的响应进行直接的并排比较。事实上，扫描隧道显微镜（STM）分析显示，随着分子主链的构象灵活性和外围基团的化学结构的变化，存在明显的差异。具体而言，烷基链吸附并形成交错结构，而聚乙二醇（OEG）链仍然解吸，从而将自组装转移到更密集的π-共轭核。虽然大环可以立即自发地自组装，但它们的线性前体表现出较慢的自组装动力学，这可能归因于构象自由度的差异。我们还发现，STM 针尖的扰动和共溶剂的添加对自组装和表面图案化的动力学有深远的影响。这种非常不寻常的行为突出了非共价相互作用的重要性，这些相互作用在溶液中本来很弱，但对于对称和构象受限的分子在二维表面内被限制时，可以变得很强。
