Surprisingly, a large number of important topics in electrochemistry is not covered by up-to-date monographs and series on the market, some topics are even not covered at all. The series Monographs in Electrochemistry fills this gap by publishing indepth monographs written by experienced and distinguished electrochemists, covering both theory and applications. The focus is set on existing as well as emerging methods for researchers, engineers, and practitioners active in the many and often interdisciplinary fields, where electrochemistry plays a key role. These fields will range -among others -from analytical and environmental sciences to sensors, materials sciences and biochemical research. Á ngela Molina • Joaquín González

Alkoxyamines are heat-labile molecules, widely used as in-situ source of nitroxides in polymer and materials sciences. Here we show that the one-electron oxidation of an alkoxyamine leads to a cation radical intermediate that even at room temperature rapidly fragments releasing a nitroxide and carbocation. Digital simulations of experimental voltammetry and current-time transients suggest the unimolecular decomposition which yields the "unmasked" nitroxide (TEMPO) is exceedingly rapid and irreversible. High-level quantum computations indicate the collapse of the alkoxyamine cation radical is likely to yield a neutral nitroxide radical and a secondary phenylethyl cation. However, this fragmentation is predicted to be slow and energetically very unfavorable. To attain qualitative agreement between the experimental kinetics and computational modelling for this fragmentation step the explicit electrostatic environment within the double layer must be accounted for. Single-molecule break-junction experiments in a scanning tunneling microscope using solvent of low dielectric (STM-BJ technique) corroborate the role played by electrostatics forces on the lysis of the alkoxyamine CON bond. This work highlights the electrostatic aspects played by charged species in a chemical step that follows an electrochemical reaction, defines the magnitude of this catalytic effect by looking at an independent electrical technique in non-electrolyte systems (STM-BJ), and suggests a redox on/off switch to guide the cleavage of alkoxyamines at an electrified interface.

Predicting or manipulating charge-transfer at semiconductor interfaces, from molecular electronics to energy conversion, relies on knowledge generated from a kinetic analysis of the electrode process, as provided by cyclic voltammetry. Scientists and engineers encountering non-ideal shapes and positions in voltammograms are inclined to reject these as flaws. Here we show that non-idealities of redox probes confined at silicon electrodes, namely full width at half maximum <90.6 mV and anti-thermodynamic inverted peak positions, can be reproduced and are not flawed data. These are the manifestation of electrostatic interactions between dynamic molecular charges and the semiconductor’s space-charge barrier. We highlight the interplay between dynamic charges and semiconductor by developing a model to decouple effects on barrier from changes to activities of surface-bound molecules. These findings have immediate general implications for a correct kinetic analysis of charge-transfer at semiconductors as well as aiding the study of electrostatics on chemical reactivity.

An analytical explicit expression for the current obtained with the well-known multipulse technique square wave voltammetry (SWV), corresponding to the reversible reduction/oxidation of multicenter redox molecules whose centers may or may not interact, has been given in Appendix. This equation is valid for spherical electrodes of any size, including planar electrodes (r 0 f ∞) and ultramicroelectrodes (r 0 f 0) as limit cases, and it also permits us to deduce the behavior of these processes at the limit situations corresponding to small and great square wave amplitudes. For the sake of simplicity, we have analyzed the behavior of bicenter molecules, ranging from noninteracting centers, for which the square wave current obtained is twice that corresponding to a single E mechanism, to strongly interacting centers which present two successive and well-separated signals of one electron each. The results obtained here are easily extendable to molecules with any number of redox centers. The theoretical predictions have been tested with two experimental systems, quinizarine in acetonitrile and pyrazine in aqueous acid media, and an excellent agreement between theory and experiments is found.

The design of devices whose functions span from sensing their environments, to convert light into electricity or to guide chemical reactivity at surfaces, often hinges around a correct and complete understanding of the factors at play when charges are transferred across an electrified solid/liquid interface. For semiconductor electrodes in particular, published values for charge transfer kinetic constants are scattered. Furthermore, received wisdom suggests slower charge transfer kinetics for semiconductor than for metal electrodes. We have used cyclic voltammetry of ferrocene-modified silicon photoanodes and photocathodes as the experimental model system, and described a systematic analysis to separate charge transfer kinetics from diode effects and interactions between adsorbed species. Our results suggest that literature values of charge transfer kinetic constants at semiconductor electrodes are likely to be an underestimate of their actual values. This is revealed by experiments and analytical models, showing that the description of the potential distribution across the semiconductor/monolayer/electrolyte interface has been largely oversimplified.