This Account provides an overview of our recent efforts to unravel charge transport characteristics of a metal–molecule–metal junction containing an individual π-conjugated molecule. The model system of our choice is an oligo(phenylene-ethynylene) consisting of three rings, in short OPE3, which represents a paradigmatic model system for molecular-scale electronics. Members of the OPE family are among the most studied in the field thanks to their simple and rigid structure, the possibility of chemically functionalizing them, and their clear transport characteristics. When investigating charge transport in molecular systems, two general directions can be distinguished: one in which assemblies composed of many molecules contacted in parallel are studied, while in the other a single molecule is investigated at a time. In the former approach, molecule–molecule interactions and ensemble-averaged quantities may play a role, thereby introducing broadening of spectral features and hindering the study of the behavior of individual molecules making it more difficult to deconvolute local and intrinsic molecular effects from collective ones. In contrast, single-molecule experiments directly probe individual molecular features and, when they are repeated many times, allow build up of a statistical representation of the changes introduced by, e.g., different junction configurations. Especially in recent years, experimental techniques have advanced such that now large sets of individual events can be measured and analyzed with statistical tools. To study individual single-molecule junctions, we use the break junction technique, in which two sharp movable electrodes are formed by breaking a thin metallic wire and used to contact a single or few molecules. By probing thousands of single-molecule junctions in different conditions, we show that their creation involves independent events justifying the statistical tools that are used. By combining room- and low-temperature data, we show that the dominant transport mechanism for electrons through the OPE3 molecule is off-resonant tunneling. The simplest model capturing transport details in this case is a single-level model characterized by three parameters: the level alignment of the frontier orbital with the Fermi energy of the leads and the electronic couplings to the leads. Variations in these parameters give a broad distribution (1 order of magnitude) in the observed conductance values, indicating that at the microscopic level both the hybridization with the metallic electrodes and the molecular electronic configuration can fluctuate. The low-temperature data show that these variations are due to abrupt changes in the configuration of the molecule in the junction leading to changes in either one of these parameters or both at the same time. The complementary information gained from different experiments is needed to build up a consistent and extended picture of the variability of molecular configurations, omnipresent in single-molecule studies. Knowledge of this variability can help one to better understand the behavior of molecules at the atomic level and at the metal–molecule interface in particular.

中文翻译：

---

通过单个共轭刚性分子的量子传输，机械断裂连接研究

该帐户概述了我们最近为揭示包含单个π共轭分子的金属-分子-金属结的电荷传输特性所做的努力。我们选择的模型系统是由三个环组成的低聚（亚苯基-亚乙炔基），简称OPE3，它代表分子规模电子学的范式模型系统。OPE系列的成员由于其简单而刚性的结构，对其进行化学功能化的可能性以及清晰的运输特性而成为该领域中研究最多的成员。在研究分子系统中的电荷传输时，可以区分两个大致方向：一个方向研究由许多平行接触的分子组成的组装体，而另一个方向同时研究单个分子。在前一种方法中，分子间的相互作用和整体平均量可能起一定作用，从而导致光谱特征的拓宽并阻碍了对单个分子行为的研究，从而使从局部分子效应和固有分子效应中解卷积变得更加困难。相反，单分子实验直接探测单个分子特征，并且当它们重复多次时，可以建立由例如不同的连接构型引起的变化的统计表示。尤其是近年来，实验技术得到了发展，现在可以使用统计工具测量和分析大量的单个事件。为了研究单个单分子连接，我们使用了断裂连接技术，其中两个尖锐的可动电极通过折断细金属线形成，并用于接触单个或几个分子。通过在不同条件下探测成千上万的单分子连接，我们表明它们的创建涉及独立的事件，这些事件证明了所使用的统计工具的合理性。通过组合室温和低温数据，我们表明电子通过OPE3分子的主要传输机制是非共振隧穿。在这种情况下，最简单的捕获运输细节的模型是具有三个参数的单层模型：边界轨道的水平对准与导线的费米能量以及与导线的电子耦合。这些参数的变化会在观察到的电导值中产生较宽的分布（1个数量级），表明在微观水平上与金属电极的杂交和分子电子构型都可能发生波动。低温数据表明，这些变化是由于接合处分子构型的突然变化导致这些参数之一或同时发生变化。需要从不同实验中获得的补充信息来建立一致且扩展的单分子研究中不存在的分子构型变异性图。了解这种可变性可以帮助人们更好地理解分子在原子级，特别是在金属-分子界面上的行为。低温数据表明，这些变化是由于接合处分子构型的突然变化导致这些参数之一或同时发生变化。需要从不同实验中获得的补充信息来建立一致且扩展的单分子研究中不存在的分子构型变异性图。了解这种可变性可以帮助人们更好地理解分子在原子级，特别是在金属-分子界面上的行为。低温数据表明，这些变化是由于接合处分子构型的突然变化导致这些参数之一或同时发生变化。需要从不同实验中获得的补充信息来建立一致且扩展的单分子研究中不存在的分子构型变异性图。了解这种可变性可以帮助人们更好地理解分子在原子级，特别是在金属-分子界面上的行为。需要从不同实验中获得的补充信息来建立一致且扩展的单分子研究中不存在的分子构型变异性图。了解这种可变性可以帮助人们更好地理解分子在原子级，特别是在金属-分子界面上的行为。需要从不同实验中获得的补充信息来建立一致且扩展的单分子研究中不存在的分子构型变异性图。了解这种可变性可以帮助人们更好地理解分子在原子级，特别是在金属-分子界面上的行为。