State Key Laboratory of Applied Organic Chemistry (SKLAOC), College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, China.
Chemistry. 2011 Jul 18;17(30):8414-23. doi: 10.1002/chem.201003507. Epub 2011 Jun 7.
Understanding the effects of intermolecular interactions on the charge-transport properties of metal/molecule/metal junctions is an important step towards using individual molecules as building blocks for electronic devices. This work reports a systematic electron-transport investigation on a series of "core-shell"-structured oligo(phenylene ethynylene) (Gn-OPE) molecular wires. By using dendrimers of different generations as insulating "shells", the intermolecular π-π interactions between the OPE "cores" can be precisely controlled in single-component monolayers. Three techniques are used to evaluate the electron-transport properties of the Au/Gn-OPE/Au molecular junctions, including crossed-wire junction, scanning tunneling spectroscopy (STS), and scanning tunneling microscope (STM) break-junction techniques. The STM break-junction measurement reveals that the electron-transport pathways are strongly affected by the size of the side groups. When the side groups are small, electron transport could occur through three pathways, including through single-molecule junctions, double-molecule junctions, and molecular bridges between adjacent molecules formed by aromatic π-π coupling. The dendrimer shells effectively prohibit the π-π coupling effect, but at the same time, very large dendrimer side groups may hinder the formation of Au-S bonds. A first-generation dendrimer acts as an optimal shell that only allows electron transport through the single-molecule junction pathway, and forbids the other undesired pathways. It is demonstrated that the dendrimer-based core-shell strategy allows the single-molecule conductance to be probed in a homogenous monolayer without the influence of intermolecular π-π interactions.
了解分子间相互作用对金属/分子/金属结电荷输运性质的影响，是将单个分子用作电子器件构建块的重要步骤。本工作系统地研究了一系列“核壳”结构寡聚（苯乙炔）（Gn-OPE）分子线的电子输运性质。通过使用不同代数的树状大分子作为绝缘“壳”，可以在单一组分单层中精确控制 OPE“核”之间的分子间π-π相互作用。采用三种技术评估 Au/Gn-OPE/Au 分子结的电子输运性质，包括交叉线结、扫描隧道谱（STS）和扫描隧道显微镜（STM）断结技术。STM 断结测量表明，电子输运途径强烈受到侧基大小的影响。当侧基较小时，电子传输可以通过三种途径进行，包括通过单分子结、双分子结以及由芳香族 π-π 耦合形成的相邻分子之间的分子桥。树状大分子壳有效地阻止了 π-π 耦合效应，但同时，非常大的树状大分子侧基可能会阻碍 Au-S 键的形成。第一代树状大分子作为一种最佳的壳，只允许电子通过单分子结途径传输，而禁止其他不需要的途径。结果表明，基于树状大分子的核壳策略允许在没有分子间 π-π 相互作用影响的情况下，在同质单层中探测单分子电导。
