Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.
Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.
J Am Chem Soc. 2020 Mar 11;142(10):4852-4861. doi: 10.1021/jacs.0c00043. Epub 2020 Feb 27.
A major challenge in synthetic polymers lies in understanding how primary monomer sequence affects materials properties. In this work, we show that charge transport in single molecule junctions of conjugated oligomers critically depends on the primary sequence of monomers. A series of sequence-defined oligomers ranging from two to seven units was synthesized by an iterative approach based on the van Leusen reaction, providing conjugated oligomers with backbones consisting of para-linked phenylenes connected to oxazole, imidazole, or nitro-substituted pyrrole. The charge transport properties of these materials were characterized using a scanning tunneling microscope-break junction (STM-BJ) technique, thereby enabling direct measurement of molecular conductance for sequence-defined dimers, trimers, pentamers, and a heptamer. Our results show that oligomers with specific monomer sequences exhibit unexpected and distinct charge transport pathways that enhance molecular conductance more than 10-fold. A systematic analysis using monomer substitution patterns established that sequence-defined pentamers containing imidazole or pyrrole groups in specific locations provide molecular attachment points on the backbone to the gold electrodes, thereby giving rise to multiple conductance pathways. These findings reveal the subtle but important role of molecular structure including steric hindrance and directionality of heterocycles in determining charge transport in these molecular junctions. This work brings new understanding for designing molecular electronic components.
合成聚合物面临的一个主要挑战是理解主要单体序列如何影响材料性能。在这项工作中，我们表明，在共轭寡聚物的单分子结中，电荷传输关键取决于单体的原始序列。通过基于范洛伊森反应的迭代方法合成了一系列从两个到七个单元的序列定义的寡聚物，为具有由连接到噁唑、咪唑或硝基取代吡咯的对苯撑组成的骨架的共轭寡聚物提供了单体。使用扫描隧道显微镜-断结（STM-BJ）技术来表征这些材料的电荷传输特性，从而能够直接测量序列定义的二聚体、三聚体、五聚体和七聚体的分子电导率。我们的结果表明，具有特定单体序列的寡聚物表现出意想不到的和独特的电荷传输途径，使分子电导率提高了 10 倍以上。通过使用单体取代模式进行系统分析，我们确定在特定位置含有咪唑或吡咯基团的序列定义的五聚体在主链上为金电极提供了分子附着点，从而产生了多种传导途径。这些发现揭示了分子结构的微妙但重要作用，包括杂环的空间位阻和方向性，在确定这些分子结中的电荷传输中起着重要作用。这项工作为设计分子电子元件提供了新的认识。
