Evolution of Metastable Clusters into Ordered Structures for 1,1′-Ferrocenedicarboxylic Acid on the Au(111) Surface
Evolution of Metastable Clusters into Ordered Structures for 1,1′-Ferrocenedicarboxylic Acid on the Au(111) Surface
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DOI：
10.1021/acs.jpcc.7b00996
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发表时间：
2017-03-23
影响因子：
3.7
通讯作者：
Kandel, S. Alex
中科院分区：
文献类型：
--
作者：
Brown, Ryan D.;Coman, Joseph M.;Kandel, S. Alex
通过一系列实验和电子结构计算，确定了在Au（111）衬底真空溶液沉积过程中形成的亚稳1,1′-二茂铁羧酸超分子结构，并观察了它们在温和退火条件下向更稳定物质的演化。电喷雾电离质谱测量确定哪些物质可能存在于快速蒸发的液滴中，这些实验发现六聚体在沉积过程中可以存在于溶液中，尽管是亚稳态物质。利用超高真空扫描隧道显微镜对溶液沉积后的分子簇进行了观察和分析，发现初始单层包含四种基本结构：有序二聚体域、倾斜二聚体行、方形四聚体和矩形手性六聚体。电子结构计算表明，手性六聚体由一个中心二聚体组成，由四个分子包围，与其他羧基形成分叉氢键，并与芳香环上的氢形成较弱的氢键。计算还表明，四聚体是由羧酸二聚体键组成的簇，每个环上的键相互垂直，这种构象比四个分子组成的簇上的两个二聚体稍微稳定一些。退火该表面。50℃加热1小时，会形成孤立的四聚体和有序的四聚体行，从而破坏端到端的二聚体结构域，只剩下很少的手性六聚体。在50℃下进一步退火，以及在65℃下退火，驱动系统形成手性二聚体结构域，以及其他一些较小的结构。在75℃下退火导致表观表面覆盖率急剧下降，并且大多数有序结构以大倾斜的二聚体行存在，无论是孤立的还是在有序域中。这种表面覆盖率的下降可能是由于分子分解、解吸或三维晶体结构的生长的某种组合。观察到多种形式的有序二聚体结构在退火后共存，表明1,1'-二茂铁羧酸的平衡构象是某种排列的有序二聚体，退火后出现的各种超分子结构表明该体系是在动力学控制的生长条件下进化的。
A series of experiments and electronic structure calculations were performed to identify metastable 1,1'-ferrocenedicarboxylic acid supramolecular structures formed during solution deposition in a vacuum on a Au(111) substrate, as well as to observe their evolution into more stable species under mild annealing conditions. Electrospray ionization mass spectrometry measurments were performed to determine which species are likely to be present in the rapidly evaporating droplet, and these experiments found that a hexamer can exist in solution during deposition, albeit as a metastable species. The molecular clusters present after solution deposition were observed and analyzed using ultrahigh-vacuum scanning tunneling microscopy, and the initial monolayer contains four basic classes of structures: ordered dimer domains, tilted dimer rows, square tetramers, and rectangular chiral hexamers. Electronic structure calculations indicate that the chiral hexamers consist of a central dimer surrounded by four molecules oriented to form birfurcated hydrogen bonds with other carboxylic acid groups and weaker hydrogen bonds with hydrogens from the aromatic rings. The calculations also indicated that the tetramers are clusters held together by carboxylic acid dimer bonds on each ring oriented perpendicular to each other, and that this conformation is slightly more stable than two dimers for a cluster of four molecules. Annealing this surface at. 50 degrees C for 1 h results in the formation of both isolated tetramers and ordered tetramer rows at the expense of the end-to-end dimer domains, with few chiral hexamers remaining. Further annealing at 50 degrees C, as well as annealing at 65 degrees C drives the system to form chiral dimer domains, as well as several other minor structures. Annealing at 75 degrees C resulted in a dramatic decrease in apparent surface coverage, and most ordered structures existed as large tilted dimer rows, whether isolated or in ordered domains. This drop in surface coverage is likely due to some combination of decomposition of the molecule, desorption, or the growth of three-dimensional crystal structures. The observed coexistence of many forms of ordered dimer structures after annealing, indicates that the equilbrium conformation of 1,1'-ferrocenedicarboxylic acid is some array of ordered dimers, and the variety of supramolecular structures present after annealing is an indicator that this system evolves under kinetically controlled growth conditions.