The rational design of single molecule electrical components requires a deep and predictive understanding of structure-function relationships. Here we explore the relationship between chemical substituents and the conductance of metal-single molecule-metal junctions, using functionalized oligophenylenevinylenes as a model system. Using a combination of mechanically controlled break-junction experiments and various levels of theory including non-equilibrium Green's functions, we demonstrate that the connection between gas-phase molecular electronic structure and in-junction molecular conductance is complicated by the involvement of multiple mutually correlated and opposing effects that contribute to energy level alignment in the junction. We propose that these opposing correlations represent powerful new "design principles," because their physical origins make them broadly applicable, and they are capable of predicting the direction and relative magnitude of observed conductance trends. In particular, we show that they are consistent with the observed conductance variability not just within our own experimental results, but also within disparate molecular series reported in literature and, crucially, with the trend in variability across these molecular series, which previous simple models fail to explain. The design principles introduced here can therefore aid in both screening and suggesting novel design strategies for maximizing conductance tunability in single-molecule systems.

Structure–function relationships constitute an important tool to investigate the fundamental principles of molecular electronics. Most commonly, this involves identifying a potentially important molecular structural element, followed by designing and synthesizing a set of related organic molecules, and finally interpretation of their experimental and/or computational quantum transport properties in the light of this structural element. Though this has been extremely powerful in many instances, we demonstrate here the common need for more nuanced relationships even for relatively simple structures, using both experimental and computational results for a series of stilbene derivatives as a case study. In particular, we show that the presence of multiple competing and subtle structural factors can combine in unexpected ways to control quantum transport in these molecules. Our results clarify the reasons for previous widely varying and often contradictory reports on charge transport in stilbene derivatives and highlight the need for refined multidimensional structure–property relationships in single-molecule electronics.

Most single-molecule transport experiments produce large and stochastic datasets containing a wide range of behaviors, presenting both a challenge to their analysis, but also an opportunity for discovering new physical insights. Recently, several unsupervised clustering algorithms have been developed to help extract and separate distinct features from single-molecule transport data. However, these clustering approaches are in general neither designed nor appropriate for identifying very rare features and behaviors, such as switching events, chemical reactions, or particular binding modes, which may nonetheless contain physically meaningful information. In this work we introduce a completely new analysis framework specifically designed for rare event detection in singlemolecule break junction data as a necessary component to enable such studies in the future. Our approach leverages the concept of correlations of breaking traces with their own history to robustly identify paths through distanceconductance space that correspond to reproducible rare behaviors. As both a demonstrative and important example, we focus on rare conductance plateaus for short molecules, which can be essentially invisible when examining raw data. We show that our grid-based correlation tools successfully and reproducibly locate these rare plateaus in real experimental datasets. This result provides useful insight into the nanoscopic junction environment, enables a broader variety of molecules to be considered in the future, and validates our new approach as a powerful tool for detecting rare yet meaningful behaviors in single molecule transport data.

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SUMMARY 35The synthesis of dimethyl-[ Slsulphide in high yield from the contact of hydrogen-[35s] sulphide and methanol over a Y A1203.1% Si02 catalyst is described.The procedure produces high yields of dimethyl-[35Sl sulphide of low specific activity compared to that of the hydrogen-[3%1 sulphide starting material. 35