It is often thought that the ability to control reaction rates with an applied electrical potential gradient is unique to redox systems. However, recent theoretical studies suggest that oriented electric fields could affect the outcomes of a range of chemical reactions, regardless of whether a redox system is involved. This possibility arises because many formally covalent species can be stabilized via minor charge-separated resonance contributors. When an applied electric field is aligned in such a way as to electrostatically stabilize one of these minor forms, the degree of resonance increases, resulting in the overall stabilization of the molecule or transition state. This means that it should be possible to manipulate the kinetics and thermodynamics of non-redox processes using an external electric field, as long as the orientation of the approaching reactants with respect to the field stimulus can be controlled. Here, we provide experimental evidence that the formation of carbon-carbon bonds is accelerated by an electric field. We have designed a surface model system to probe the Diels-Alder reaction, and coupled it with a scanning tunnelling microscopy break-junction approach. This technique, performed at the single-molecule level, is perfectly suited to deliver an electric-field stimulus across approaching reactants. We find a fivefold increase in the frequency of formation of single-molecule junctions, resulting from the reaction that occurs when the electric field is present and aligned so as to favour electron flow from the dienophile to the diene. Our results are qualitatively consistent with those predicted by quantum-chemical calculations in a theoretical model of this system, and herald a new approach to chemical catalysis.

Static electricity is central to many day-to-day practical technologies, from separation methods in the recycling of plastics to transfer inks in photocopying, but the exploration of how electrostatics affects chemical bonding is still in its infancy. As shown in the Companion Tutorial, the presence of an appropriately-oriented electric field can enhance the resonance stabilization of transition states by lowering the energy of ionic contributors, and the effect that follows on reaction barriers can be dramatic. However, the electrostatic effects are strongly directional and harnessing them in practical experiments has proven elusive until recently. This tutorial outlines some of the experimental platforms through which we have sought to translate abstract theoretical concepts of electrostatic catalysis into practical chemical technologies. We move step-wise from the nano to the macro, using recent examples drawn from single-molecule STM experiments, surface chemistry and pH-switches in solution chemistry. The experiments discussed in the tutorial will educate the reader in some of the viable solutions to gain control of the orientation of reagents in that field; from pH-switchable bond-dissociations using charged functional groups to the use of surface chemistry and surface-probe techniques. All of these recent works provide proof-of-concept of electrostatic catalysis for specific sets of chemical reactions. They overturn the long-held assumption that static electricity can only affect rates and equilibrium position of redox reactions, but most importantly, they provide glimpses of the wide-ranging potential of external electric fields for controlling chemical reactivity and selectivity.

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.

There is considerable ongoing interest in understanding the electrical properties of single molecules both from a fundamental point of view and for potential applications in singlemolecule technologies. [1][2][3][4] An important goal in molecular electronics is the ability to switch, by means of electrochemical gating, the conductance through a single molecule and, in this context, the anthraquinone/hydroanthraquinone, AQ/H 2 AQ, redox couple has been proposed as a suitable candidate for study. [5] Indeed, calculations [6] predict that electrochemical gating of conductance in AQ-based molecular switches should be strong, with conductance on(H 2 AQ)/ off(AQ) ratios of several orders of magnitude. The switching mechanism is due to the presence of destructive quantum interference (QI) between various conductance channels in the cross-conjugated AQ, which is absent in the linearconjugated H 2 AQ, thereby resulting in lower conductance in AQ, compared to H 2 AQ. Recently, Fracasso et al. [7] have experimentally confirmed the operation of QI in bulk conductance studies of self-assembled monolayers (SAMs) of arylethynylene thiolates (aryl = anthracene, AQ, 9,10dihydroanthracene). [7] We now report the first experimental evidence for the operation of electrochemically controlled QI in a novel AQbased norbornylogous bridge tetrathiol, 5AQ5 (Scheme 1), from single-molecule conductance measurements using the scanning tunneling microscopy (STM) break junction technique. [8] We show that the AQ moiety in 5AQ5 can be electrochemically and reversibly switched in situ between the high-conducting H 2 AQ form and the low-conducting AQ system. Further, we demonstrate that the potential range of the conductance enhancement can be shifted using different pH values. This pH dependency of the AQ/H 2 AQ redox reaction constitutes an extra degree of freedom that can control single-molecule conductivity.A key design feature of 5AQ5 is the cementing of the AQ group into a rigid, structurally well-defined norbornylogous (NB) unit bearing two pairs of thiol groups at each end, thereby conferring additional stability to SAMs derived therefrom. The 19.8 length of 5AQ5 is much greater than the gate thickness, that is, the electrochemical double layer that relates to the diameter of the ions used in the electrolyte, thereby ensuring that the field screening effect due to the proximity of the source and drain electrodes is negligible. [9] Norbornylogous bridges have played pivotal roles in investigating many fundamental aspects of electron-transfer (ET) processes, [10,11] including those involving SAMs derived therefrom. [12][13][14][15][16] In particular, NB bridges are very efficient mediators of ET by the superexchange mechanism and it was hoped that the NB bridge would likewise facilitate coherent charge transport in 5AQ5, which is a prerequisite for QI to be operative. This issue was first investigated by determining the magnitude and distance dependence of the single-molecule conductivity in 5AQ5 and its longer cognate, 8AQ8. X-ray photoele...

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.