Correlating Electron Transport and Molecular Structure in Organic Thin Films**

Understanding how electrons travel through organic matter is important in designing materials for organic microelectronics[1] and understanding biological electron-transport processes.[2, 3] Herein we describe a simple experimental procedure to measure rates of electron transport across organic thin films having a range of structures, compare the data for several types of films, and outline a theory appropriate for analyzing these rates. We use a junction that is particularly easy to assemble: M-SAM(1)SAM(2)-M' (Figure 1, aSAMo is a self-assembled monolayer).[4, 5] We have Figure 1. Schematic illustrations of junctions with structures JAg-SAM(1)//SAM(2)-Hg and JAu-SAM(1)-X~Y-SAM(2)-Hg (see text for the nomenclature). The photographic image (a) is that of a JAg-C16//C16-Hg junction; the scale bar represents 0.5 mm. To assemble JAg-SAM(1)//SAM(2)-Hg junctions (b), a SAM was formed on the surface of a thin evaporated film of silver; we normally used MˆAg, since silver gives highly ordered SAMs.[5] This electrode was covered with a solution of hexadecane containing 1 mm thiol. A small drop of Hg (5 mL) was expressed into a solution of hexadecanethiol (HDT) from a capillary connected to a mercury reservoir, and a SAM of HDTallowed to form on it.[5] The HDT-covered mercury drop (C16-Hg) was then brought into contact with the solid electrode using a micromanipulator. The area of interfacial contact was estimated by using a microscope. With each electrode connected to an electrometer (in two-electrode mode), we applied a potential and recorded the response, and then increased the potential in steps over a range of voltages to generate I ± V curves. Junctions of structure JAu-SAM(1)-X Y-SAM(2)-Hg (c) were made as described for JAg-SAM(1)//SAM(2)-Hg except that one electrode was a thin film of gold, and the thiols used had terminal groups that could react and form covalent bonds (Xˆ carboxylic anhydride, YˆH2N), or interact strongly but noncovalently (XˆCO2H and YˆHO2C, or XˆCO2H and YˆH2N) through hydrogen or ionic bonds.