Starr Rachel L, Fu Tianren, Doud Evan A, Stone Ilana, Roy Xavier, Venkataraman Latha
J Am Chem Soc. 2020 Apr 15;142(15):7128-7133. doi: 10.1021/jacs.0c01466. Epub 2020 Apr 6.
Aryl halides are ubiquitous functional groups in organic chemistry, yet despite their obvious appeal as surface-binding linkers and as precursors for controlled graphene nanoribbon synthesis, they have seldom been used as such in molecular electronics. The confusion regarding the bonding of aryl iodides to Au electrodes is a case in point, with ambiguous reports of both dative Au-I and covalent Au-C contacts. Here we form single-molecule junctions with a series of oligophenylene molecular wires terminated asymmetrically with iodine and thiomethyl to show that the dative Au-I contact has a lower conductance than the covalent Au-C interaction, which we propose occurs via an oxidative addition reaction at the Au surface. Furthermore, we confirm the formation of the Au-C bond by measuring an analogous series of molecules prepared with the complex Au(PPh) in place of the iodide. Density functional theory-based transport calculations support our experimental observations that Au-C linkages have higher conductance than Au-I linkages. Finally, we demonstrate selective promotion of the Au-C bond formation by controlling the bias applied across the junction. In addition to establishing the different binding modes of aryl iodides, our results chart a path to actively controlling oxidative addition on an Au surface using an applied bias.
芳基卤化物是有机化学中普遍存在的官能团，然而，尽管它们作为表面结合连接体以及用于可控石墨烯纳米带合成的前体具有明显的吸引力，但在分子电子学中却很少如此使用。关于芳基碘化物与金电极键合的困惑就是一个典型例子，关于配位金 - 碘和共价金 - 碳接触都有模棱两可的报道。在这里，我们用一系列以碘和硫甲基不对称终止的寡聚亚苯基分子线形成单分子结，以表明配位金 - 碘接触的电导低于共价金 - 碳相互作用，我们认为后者是通过金表面的氧化加成反应发生的。此外，我们通过测量用配合物金（三苯基膦）代替碘化物制备的类似系列分子，证实了金 - 碳键的形成。基于密度泛函理论的输运计算支持了我们的实验观察结果，即金 - 碳连接的电导高于金 - 碘连接。最后，我们通过控制跨结施加的偏压，证明了金 - 碳键形成的选择性促进。除了确定芳基碘化物的不同结合模式外，我们的结果还描绘了一条利用施加偏压在金表面主动控制氧化加成的途径。
