How Surface Bonding and Repulsive Interactions Cause Phase Transformations: Ordering of a Prototype Macrocyclic Compound on Ag(111)
How Surface Bonding and Repulsive Interactions Cause Phase Transformations: Ordering of a Prototype Macrocyclic Compound on Ag(111)
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DOI：
10.1021/nn305487c
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发表时间：
2013-04-01
期刊：
影响因子：
17.1
通讯作者：
Barth, Johannes V.
中科院分区：
文献类型：
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作者：
Bischoff, Felix;Seufert, Knud;Barth, Johannes V.
我们研究了游离碱卟啉（2H-P）（所有卟啉的母体化合物）在光滑贵金属载体上的表面键合和排序。我们的多技术研究揭示了令人惊讶的丰富和复杂的行为，包括分子内质子转换、排斥性分子间相互作用和密度驱动的相变。对于小浓度，使用低温扫描隧道显微镜进行的分子水平观察清楚地显示了与所使用的 Ag(111) 表面直接接触的 2H-P 分子之间的排斥相互作用的作用，从而防止了岛的形成。分子覆盖率的增加导致平均分子间距离的持续减小，这与多个相变相关：系统从各向同性的类气体构型通过类流体相演变为晶体结构，最终让位于无序层。在这里，相当多的位点特异性分子底物相互作用，有利于 Ag(111) 晶格的桥位置上的排他性吸附，发挥着重要作用。因此，2H-P/Ag(111)层的2D组装由吸附能最大化与保留由排斥分子分子相互作用抵消的单个吸附位点之间的平衡决定。长程排斥与 2H-P/Ag(111) 界面处的电荷重新分布相关，包括部分填充最低未占据分子轨道，从而导致吸附物之间的长程静电相互作用。事实上，第二层中的 2H-P 分子仅与 Ag 基底弱电子耦合，没有表现出排斥行为，而是形成密集的岛。
We investigated the surface bonding and ordering of free-base porphine (2H-P), the parent compound of all porphyrins, on a smooth noble metal support. Our multitechnique investigation reveals a surprisingly rich and complex behavior, induding intramolecular proton switching, repulsive intermolecular interactions, and density-driven phase transformations. For small concentrations, molecular-level observations using low-temperature scanning tunneling microscopy dearly show the operation of repulsive interactions between 2H-P molecules in direct contact with the employed Ag(111) surface, preventing the formation of islands. An increase of the molecular coverage results in a continuous decrease of the average intermolecular distance, correlated with multiple phase transformations: the system evolves from an isotropic, gas-like configuration via a fluid-like phase to a crystalline structure, which finally gives way to a disordered layer. Herein, considerable site-specific molecule substrate interactions, favoring an exclusive adsorption on bridge positions of the Ag(111) lattice, play an important role. Accordingly, the 2D assembly of 2H-P/Ag(111) layers is dictated by the balance between adsorption energy maximization while retaining a single adsorption site counteracted by the repulsive molecule molecule interactions. The long-range repulsion is associated with a charge redistribution at the 2H-P/Ag(111) interface comprising a partial filling of the lowest unoccupied molecular orbital, resulting in long-range electrostatic interactions between the adsorbates. Indeed, 2H-P molecules in the second layer that are electronically only weakly coupled to the Ag substrate show no repulsive behavior, but form dense-packed islands.