Side‐Group‐Mediated Mechanical Conductance Switching in Molecular Junctions

Abstract

A key target in molecular electronics has been molecules having switchable electrical properties. Switching between two electrical states has been demonstrated using such stimuli as light, electrochemical voltage, complexation and mechanical modulation. A classic example of the latter is the switching of 4,4'-bipyridine, leading to conductance modulation of around 1 order of magnitude. Here, we describe the use of side-group chemistry to control the properties of a single-molecule electromechanical switch, which can be cycled between two conductance states by repeated compression and elongation. While bulky alkyl substituents inhibit the switching behavior, π-conjugated side-groups reinstate it. DFT calculations show that weak interactions between aryl moieties and the metallic electrodes are responsible for the observed phenomenon. This represents a significant expansion of the single-molecule electronics "tool-box" for the design of junctions with electromechanical properties.

Side-Group Effect on Electron Transport of Single Molecular Junctions

“…The absence of high G value for BPY-N and BPY-2N might be caused by the steric hindrance of the nitro groups, which prevent the pyridine ring from directly binding to the Au electrode at an angle in the electrode-molecule interface. A similar phenomenon has been reported recently by Ismael and co-workers, they found that the BPY decorated with bulky alkyl side-groups reduces the molecule-electrode coupling strength in the high-conductance geometry, thus preventing the formation of high conductance geometry [ 38 ].…”

Hemilabile Ligands as Mechanosensitive Electrode Contacts for Molecular Electronics

“…where s is an energy shift of the orbital due to interactions with the electrodes,a nd G describes the strength of the coupling that causes abroadening of the orbital resonance,as shown in Figure 1a.I tc an be inferred from the above equations that i) the off-resonance transmission coefficient (and therefore the conductance of the molecular junction) is strongly dependent on the strength of the coupling to the electrodes,a nd ii)if G can be modulated reversibly and reproducibly in response to an external stimulus,t hen this would yield af unctional nanoelectronic device.W ef ocussed our efforts on the use of mechanical forces as the triggering stimuli, because these have been used to exert some degree of control on G by exploiting weak interactions between the metallic electrodes and an eighbouring p-system in a hapto coordination, for instance by using 4-pyridyl contact groups [18] or vicinal phenyl rings. [19][20][21] In the following,wedemonstrate that hemilabile ligands also satisfy the above requirements, with further advantages over the existing strategies lying in chemical control on G modulation, chemically-defined contact configurations,and improved sensitivity and amplitude of conductance changes.T he reason lies in an enhanced metalmolecule coupling in the bidentate contact compared with the monodentate configuration ( Figure 1c), with af ully reversible mono-to-bidentate transition and reproducible behaviour imparted by the hemilabile ligands.…”

“…It can be inferred from the above equations that i) the off‐resonance transmission coefficient (and therefore the conductance of the molecular junction) is strongly dependent on the strength of the coupling to the electrodes, and ii) if Γ can be modulated reversibly and reproducibly in response to an external stimulus, then this would yield a functional nanoelectronic device. We focussed our efforts on the use of mechanical forces as the triggering stimuli, because these have been used to exert some degree of control on Γ by exploiting weak interactions between the metallic electrodes and a neighbouring π‐system in a hapto coordination, for instance by using 4‐pyridyl contact groups or vicinal phenyl rings . In the following, we demonstrate that hemilabile ligands also satisfy the above requirements, with further advantages over the existing strategies lying in chemical control on Γ modulation, chemically‐defined contact configurations, and improved sensitivity and amplitude of conductance changes.…”

Functional molecular electronic devices through environmental control

“…1a. Firstly and most importantly, molecular engineering plays a key role in the device functionalization [12,13] because specially designed molecules are able to endow the intrinsic molecular properties, which are directly correlated to their electronic structures, into molecular electronic devices. Secondly and complementarily, interfacial engineering plays an important role in the electrical characteristics of molecular devices [14].…”