Electron Transport and Redox Reactions in Solid-State Molecular Electronic Devices

A core concept in electrochemistry is activated electron transfer (ET) between an electrode, usually a conducting solid, and a redox system in the nearby solution. The vast literature on ET kinetics describes the importance of ET to chemical and biological processes, and the underlying phenomenon of coupling a chemical reaction to the flow of current is the basis of >$300 billion of annual gross national product. Chapter 1 in this volume describes ET in nanoscale systems, mainly at an interface between an electrode and an electrolyte solution. A widely studied example of ET kinetics of relevance to the current chapter deals with ET occurring through a self-assembled monolayer (SAM) to a redox molecule (e.g., ferrocene) bonded to the SAM at the solution interface,1–3 as shown in Figure 7.1a. Such experiments stimulated a large research effort to understand the relationship between ET from a solid to a redox system through a nonredox active SAM and the thoroughly investigated dependence of ET within molecules, such as occur in biological metabolism and photosynthesis. An important conclusion about ET at electrodes as well as between two molecules in solution is the fact that the electrode and redox center (or the two redox centers in solution) need not be in direct contact to transfer electrons. It is possible, and quite common, for electrons to transfer through a SAM or intervening spacer (even a vacuum) by quantum mechanical tunneling, as described in Chapters 1 and 6 and in Section 7.3. For ET at both electrodes in solution and between redox centers within molecules, the ET rate depends on the driving force in terms of free energy and on the composition of the intervening solution or molecular structure. In addition to tunneling, ET in such systems may occur by other mechanisms, such as redox exchange, superexchange, and a sequence of ETs between distinct redox centers.4 The current chapter deals with ET in solid-state devices, which lack a solution but may involve electron transport, activated redox reactions, and/or ion motion. Extensive past investigations of transport in organic semiconductors, conducting polymers, and redox polymers provide important CONTENTS