Department of Chemistry, Columbia University, New York, New York 10027, USA.
J Am Chem Soc. 2013 Aug 14;135(32):11724-7. doi: 10.1021/ja4055367. Epub 2013 Aug 6.
We have measured the single-molecule conductance of a family of bithiophene derivatives terminated with methyl sulfide gold-binding linkers using a scanning tunneling microscope based break-junction technique. We find a broad distribution in the single-molecule conductance of bithiophene compared with that of a methyl sulfide terminated biphenyl. Using a combination of experiments and calculations, we show that this increased breadth in the conductance distribution is explained by the difference in 5-fold symmetry of thiophene rings as compared to the 6-fold symmetry of benzene rings. The reduced symmetry of thiophene rings results in a restriction on the torsion angle space available to these molecules when bound between two metal electrodes in a junction, causing each molecular junction to sample a different set of conformers in the conductance measurements. In contrast, the rotations of biphenyl are essentially unimpeded by junction binding, allowing each molecular junction to sample similar conformers. This work demonstrates that the conductance of bithiophene displays a strong dependence on the conformational fluctuations accessible within a given junction configuration, and that the symmetry of such small molecules can significantly influence their conductance behaviors.
我们使用基于扫描隧道显微镜的断键技术测量了一系列带有甲基硫醚金键接头的联噻吩衍生物的单分子电导。与甲基硫醚封端的联苯相比，我们发现联噻吩的单分子电导呈现出较宽的分布。通过实验和计算的结合，我们表明这种电导分布的增宽可以用噻吩环的五重对称性与苯环的六重对称性的差异来解释。噻吩环的对称性降低导致这些分子在结中两个金属电极之间结合时，其扭转角空间受到限制，从而导致每个分子结在电导测量中采样不同的构象。相比之下，联苯的旋转基本上不受结结合的阻碍，允许每个分子结采样类似的构象。这项工作表明，联噻吩的电导强烈依赖于给定结构型内可获得的构象涨落，并且这种小分子的对称性可以显著影响它们的电导行为。
