UC Riverside

The scanning tunneling microscope break-junction technique is a robust, statistically powerful approach to integrating single molecules into electrical circuits and investigating their conductance behavior. Through this technique, insights into how individual molecules behave in molecular junctions are obtained which provide information about molecular behavior that might otherwise be overlooked in the bulk. Herein we report how fundamental organic principles such as conformation, conjugation, and resonance manifest at the scale of a single molecule and how these principles can be used to perturb charge transport in single molecule junctions.In the first chapter, we show how the weakest of the intramolecular forces, London dispersion forces, can be used to control single molecule conductance. We synthesize a series of thiomethyl-terminated oligo(dimethylsilmethylene)s that bear [CH2-Si(CH3)2]n repeat units, where all backbone dihedral states are sterically equivalent. In the absence of any steric preference for a specific conformation, London dispersion forces “staple” the freely rotating backbone into kinked conformations that serve to lower conductance of the molecule in the junction. These wires are shown to possess the record highest observed conductance decay per junction length and demonstrate how intramolecular London dispersion interactions can be used to control conductance. The second chapter uses rational molecular design to investigate the molecular conductance properties of the smallest sila-diamondoid, sila-adamantane (SiAd). We compare SiAd’s conductance properties against its bicyclic silicon analog Si[3.3.1] and non-cyclic silicon analog Si3 which contain the same number of atoms in their linear trisilane molecular backbones. We find that sila-adamantane conducts significantly lower as a result of accessible conductive pathways which possess destructive quantum interference (DQI). The third chapter discusses the synthesis and conductance measurement of molecular switches comprised of rhodamine dyes. By exploiting the rhodamine molecule’s pH responsive lactone-zwitterion equilibrium and the difference in conjugation between these two forms we synthesize a molecular switch with a 47x difference in conductance between its “on” and “off” forms which is substantially higher than other switches in its class. The final chapter outlines the components and common issues encountered in our homebuilt STM-BJ device and provide troubleshooting approaches aimed to be useful for a novice STM-BJ user.