Cui Lihua, Miao Xinrui, Xu Li, Hu Yi, Deng Wenli
College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China.
Phys Chem Chem Phys. 2015 Feb 7;17(5):3627-36. doi: 10.1039/c4cp04773g. Epub 2015 Jan 2.
In this present work, a scanning tunneling microscope (STM) operated under ambient conditions was utilized to probe the self-assembly behavior of 2,7-bis-nonyloxy-9-fluorenone (F-OC9) at the liquid-solid (l/s) interface. On the highly oriented pyrolytic graphite (HOPG) surface, two-dimensional (2D) polymorphism with diversity of intermolecular dipole interactions induced by solvent was found. Solvents ranged from hydrophilic solvating properties with high polarity, such as viscous alkylated acids, to nonpolar alkylated aromatics and alkanes. 1-Octanol and dichloromethane were used to detect the assembly of F-OC9 at the gas-solid (g/s) interface. The opto-electronic properties of F-OC9 were determined by UV-vis and fluorescence spectroscopy in solution. Our results showed that there were tremendous solvent-dependent self-assemblies in 2D ordering for the surface-confined target molecules. When a homologous series of alkanoic acids ranging from heptanoic to nonanoic acid were employed as solvents, the self-assembled monolayer evolved from low-density coadsorbed linear lamellae to a semi-circle-like pattern at relatively high concentrations, which was proven to be the thermodynamic state as it was the sole phase observed at the g/s interface after the evaporation of solvent. Moreover, by increasing the chain length of the alkylated acids, the weight of the carboxylic group, also being the group responsible for the dielectric properties, diminished from heptanoic to nonanoic acid, which could make the easier/earlier appearance of a linear coadsorption effect. However, this was not the case for nonpolar 1-phenyloctane and n-tetradecane: no concentration effect was detected. It showed a strong tendency to aggregate to generate coexistence of separate domains of different phases due to the fast nucleation sites. Furthermore, thermodynamic calculations indicated that the stable structural coexistence of the fluorenone derivative was attributed to synergistic intermolecular dipole-dipole and van der Waals (vdWs) forces at l/s interface. It is believed that the results are of significance to the fields of solvent induced polymorphism assembly and surface science.
在本研究中，使用在环境条件下操作的扫描隧道显微镜（STM）来探测2,7-双壬氧基-9-芴酮（F-OC9）在液-固（l/s）界面的自组装行为。在高度取向热解石墨（HOPG）表面上，发现了由溶剂诱导的具有分子间偶极相互作用多样性的二维（2D）多晶型现象。溶剂范围从具有高极性的亲水性溶剂化性质的物质，如粘性烷基化酸，到非极性烷基化芳烃和烷烃。使用1-辛醇和二氯甲烷来检测F-OC9在气-固（g/s）界面的组装。F-OC9的光电性质通过溶液中的紫外-可见光谱和荧光光谱来测定。我们的结果表明，对于表面受限的目标分子，在二维有序排列中存在大量依赖于溶剂的自组装现象。当使用从庚酸到壬酸的一系列同系链烷酸作为溶剂时，自组装单分子层从低密度共吸附线性薄片演变为相对高浓度下的半圆形图案，这被证明是热力学状态，因为它是溶剂蒸发后在g/s界面观察到的唯一相。此外，通过增加烷基化酸的链长，负责介电性质的羧基的权重从庚酸到壬酸逐渐减小，这可能使得线性共吸附效应更容易/更早出现。然而，对于非极性的1-苯基辛烷和正十四烷并非如此：未检测到浓度效应。由于快速成核位点，它表现出强烈的聚集倾向，从而产生不同相的分离域共存。此外，热力学计算表明，芴酮衍生物的稳定结构共存归因于l/s界面处协同的分子间偶极-偶极和范德华（vdWs）力。相信这些结果对溶剂诱导多晶型组装和表面科学领域具有重要意义。
