Electron transfer through molecules and assemblies at electrode surfaces.

Chemically modified electrodes were first introduced to the scope of electrochemistry by Anson, Bard, Murray, Saveant, and others about three decades ago in an effort to provide selectivity to highly sensitive electrode surfaces. While electrochemical techniques had high sensitivity because of the availability of accurate current measurement techniques, the lack of any differential selectivity of electrodes for analytes over impurities complicated the analysis. A functionalized electrode however would and does give the opportunity to chemically modify the surface of an electrode, providing the means to make the surface much more chemically selective. In addition to the numerous contributions to the field of electrochemical biosensors (for example, in the field of electrochemical biosensing, the lock and key enzyme substrate relationship can be exploited by immobilizing an enzyme on the electrode surface to analyze a solution of the substrate even in the presence of impurities; major contributions in this field are presented in a recent review by Bakker and Qin), chemically modified electrodes opened up the possibility for electrochemists to be able to investigate electron transfer on electrode surfaces while side-stepping most of the mass transport problems (note that counterion diffusion and solvent rearrangement still have to take place). Once reliable techniques for modifying electrode surfaces were established, molecules with well positioned redox centers and with tunable redox potentials could be placed on electrode surfaces to be able to systematically vary and observe the effects of important parameters such as distance, and chemical environment, among others. For example, the work by Chidsey et. al describes long-range electron transfer making use of exquisite control over the placement of the redox active site relative to the electrode surface. In more recent work, Amatore et. al investigated the effects of chemical environment on electron transfer by making mixed monolayers of electroinactive alkanes with compounds with well-defined redox centers. Molecular electronics can also be dated to similar times as chemically modified electrodes. Though it has been argued that molecular electronics dates back to the days of Mulliken and his proposals of charge transfer salts, the general consensus is that the popularization of the field dates back to the cornerstone paper by Aviram and Ratner in 1974 in which they proposed a molecular structure that should act as a diode when electron transport was measured across it. They designed the molecule based on a donor-bridgeacceptor model and calculated the electron transport with a semi empirical INDO approach. A number of experimental methods have been proposed to measure conductance through molecules and molecular assemblies since then including scanned probe techniques, mercury drop electrodes, electrical or mechanical break junctions, sandwich electrodes, and others. The common concept in all of these methods is to be able to “wire” the molecules between two electrodes (generally metallic, though semiconductors are also employed in some rare cases) and measure current as a function of an applied potential. A third electrode (gate) coupled through an electronically insulating dielectric is generally used to modulate the electrostatics around the active material, changing, in a deliberate fashion, its electronic energy levels. If the device is immersed in an electrolyte solution, the gate electrode takes on the role of the more traditional reference electrode with identical function. In nearly all efforts related to measuring conductance across molecules and molecular assemblies, the experiments have been based on making a chemically modified electrode to establish the first electrode-molecule contact. The second electrode is then either brought into contact temporarily (scanned probe) or permanently (crossbar, sandwich) or the single electrode is broken into two (break junctions) to measure those molecules that are statistically trapped across the junction. The over three decades experience in modifying electrodes’ electronic and physical properties puts electrochemistry at center stage for the molecular electronics efforts along with nanofabrication. The experimental efforts on molecular electronics were pushed forward by two separate events. First, the development of scanning tunneling microscopy (STM) by Binnig and Rohrer in 1981, and second, the ever shrinking micro* To whom correspondence should be addressed. E-mail: hda1@cornell.edu. Chem. Rev. 2008, 108, 2721–2736 2721