Observation of Electrochemically Controlled Quantum Interference in a Single Anthraquinone‐Based Norbornylogous Bridge Molecule

Abstract

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...

Exploiting the interplay of quantum interference and backbone rigidity on electronic transport in peptides: a step towards bio-inspired quantum interferometers

“…A similar correlation was previously found for β-strand linear/cyclic peptides attached to electrode(s) comprising a triazole linker. 27 Destructive quantum interference is always accompanied by a reduction in the rate of electron transfer, 41,42 which was observed with the cyclic peptide 9. Despite peptides 9 and 10 sharing a common β-strand conformation, the effects of destructive quantum interference were found to occur essentially in the cyclic peptide, through the heterogenous backbone and the additional tunneling pathway provided by the side-bridge constraint.…”

Single-Molecule Electronics: Chemical and Analytical Perspectives

“…Electrochemical gating allows precise and reproducible control of very strong gating fields in the molecular junction and switching of molecular wires between different chemical redox states. Examples of electrochemical gating of wired single-molecule bridges with the following redox centers include viologens (29,(108)(109)(110), pyrrolo-tetrathiafulvalene (pTTF) (107,111), catechol-type dithiol-terminated OPEs (112), perylene tetracarboxylic bisimides (113)(114)(115)(116), oligoanilines (117,118), anthraquinone (119,120), and benzodifuran (63). Figure 13 provides an example of the electrochemical gating of a pTTF molecular bridge.…”

Electrochemical Control of Single-Molecule Conductance by Fermi-Level Tuning and Conjugation Switching

“…[26][27][28][29][30][31] Destructive QI leads to very low conductance -much lower than anticipated from a simple "Lorentzian" model treating each molecular level as an independent transport channel. It occurs as a result of a (nearly) complete cancellation of transmission probability due to interference between different electron pathways, and is predicted to take place in organic molecules whenever the path connecting the left and right electrodes via the molecule is cross conjugated.…”