Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan).
Chemistry. 2015 Sep 21;21(39):13569-76. doi: 10.1002/chem.201500707. Epub 2015 Jul 28.
An appropriate understanding of the process of self-assembly is of critical importance to tailor nanostructured order on 2D surfaces with functional molecules. Photochromic compounds are promising candidates for building blocks of advanced photoresponsive surfaces. To investigate the relationship between molecular structure and the mechanism of ordering formation, 2-thienyl-type diarylethenes with various lengths of alkyl side chains linked through an amide or ester group were synthesized. Their self-assemblies at a liquid/solid interface were investigated by scanning tunneling microscopy (STM). The concentration dependence of the surface coverage was analyzed by using a cooperative model for a 2D surface based on two characteristic parameters: the nucleation equilibrium constant (Kn) and the elongation equilibrium constant (Ke). The following conclusions can be drawn. 1) The concentration at which a stable 2D molecular ordering is observed by STM exponentially decreases with increasing length of the alkyl chain. 2) Compounds bearing amide groups have higher degrees of cooperativity in self-assembly on 2D surfaces (i.e., σ, which is defined as Kn/Ke) than compounds with ester groups. 3) The self-assembly process of the open-ring isomer of an ester derivative is close to isodesmic, whereas that of the closed-ring isomer is cooperative because of the difference in equilibrium constants for the nucleation step (i.e., Kn) between the two isomers.
适当理解自组装过程对于通过功能分子在 2D 表面上定制纳米结构有序性至关重要。光致变色化合物是构建先进光响应表面的有前途的候选材料。为了研究分子结构与有序形成机制之间的关系，合成了通过酰胺或酯基团连接的各种烷基侧链长度的 2-噻吩型二芳基乙烯。通过扫描隧道显微镜（STM）研究了它们在液/固界面处的自组装。通过基于两个特征参数（成核平衡常数（Kn）和伸长平衡常数（Ke））的二维表面的协同模型分析了表面覆盖率的浓度依赖性。可以得出以下结论。1）通过 STM 观察到稳定的 2D 分子有序的浓度随烷基链长度的增加呈指数下降。2）在 2D 表面上进行自组装时，具有酰胺基团的化合物比具有酯基团的化合物具有更高的协同性（即 σ，定义为 Kn/Ke）。3）酯衍生物的开环异构体的自组装过程接近等物质的，而闭环异构体的自组装过程是协同的，因为两种异构体的成核步骤（即 Kn）的平衡常数不同。
