Stretching helical molecular springs: the peculiar evolution of electron transport in helicene junctions

TL;DR

This work addresses how mechanical manipulation of helicene-based molecular springs alters electron transport in single-molecule junctions. By integrating cryogenic break-junction experiments with ab initio DFT/NEGF transport calculations, it reveals a robust, nonmonotonic U-shaped conductance response driven by helix deformation and interface coupling. The mechanism involves a transition from zero to two destructive interferences within the HOMO–LUMO gap, governed by distance-dependent orientation of sulfur-anchoring orbitals and electrode geometry, elucidated through a four-level model. These insights advance the design of mechanoelectronic devices that exploit controlled quantum interference in flexible, helical molecular systems, with potential extensions to thermoelectric and spin-transport applications.

Abstract

Single-molecule junctions represent electromechanical systems at the edge of device miniaturization. Despite extensive studies on the interplay between mechanical manipulation and electron transport in molecular junctions, a thorough understanding of conducting molecular springs remains elusive. Here, we investigate the impact of mechanical elongation and compression on the electron transport and electronic structure of helicene-based spring-like single-molecule junctions, utilizing 2,2'-dithiol-[6]helicene and thioacetyl-[13]helicene molecules bridging two gold electrodes. We observe robust, reversible U-shaped conductance variations with interelectrode distance. Ab-initio electronic structure and quantum transport calculations reveal that this behavior stems from destructive quantum interference, induced mainly by modifications of the coupling at the metal-molecule interface as a peculiar outcome of the helical backbone deformation. These findings highlight the central role of the helical geometry in combination with contact properties in the electromechanical response of conducting molecular springs, offering insights for designing functional electromechanical devices that leverage similar mechanisms.

Figures (5)

• Figure 1: Schematics of the break junction setup and conductance measurements. (a) Schematic illustration of the break-junction setup and the chemical structure of the studied helicene molecules. (b) Schematic illustration of pull-push cycle measurements of a single helicene molecule in the break junction. (c) Examples for traces of conductance versus interelectrode displacement during elongation for bare Au, Au/[6]helicene, and Au/[13]helicene junctions in light blue, red, and cyan color, respectively. (d) Examples of conductance traces during the compression for bare Au, Au/[6]helicene, and Au/[13]helicene junctions in light blue, red, and cyan color, respectively. All shown conductance-displacement traces are taken during elongation and compression of junctions at 200 mV applied bias voltage. The conductance is determined by dividing the current by voltage, assuming linear response.
• Figure 2: Conductance-displacement density plot for molecular junctions during pull and push processes. Conductance-displacement density plot of (a) Au/[6]helicene and (b) Au/[13]helicene molecular junctions during pulling. (c,d) Same as panels (a) and (b) but for pushing.
• Figure 3: One-dimensional histogram of the U-shape displacement $d_\text{U}$ and percental relative upturn conductance change. (a) One-dimensional histogram of $d_\text{U}$, constructed from pull traces of Au/[6]helicene (I) and Au/[13]helicene (II) junctions. The inset in panel (a,I) illustrates the definitions of $d_\text{U}$, $G_\text{max}$ and $G_\text{min}$ for an example conductance trace exhibiting a U-shape. (b) The same as panel (a) but for push traces. (c) One-dimensional histogram of $\Delta G_\text{Nor}$, constructed from pull traces of Au/[6]helicene (I) and Au/[13]helicene (II) junctions. (d) The same as panel (b) but for push traces. The medians of $d_\text{U}$ and $\Delta G_\text{Nor}$ histograms are marked by dashed lines.
• Figure 4: Results of DFT-based quantum transport simulations for different helicene single-molecule junctions, namely (a) [6]helicene-TT, (b) [6]helicene-HH, (c) [13]helicene-TT and (d) [13]helicene-HH. The first row shows the simulated helicene junction at a selected electrode separation. The second row shows the total transmission $\tau(E_\text{F},d)$ and the transmission of the first and second eigenchannel, $\tau_{1}(E_\text{F},d)$ and $\tau_{2}(E_\text{F},d)$, as a function of the electrode displacement $d$ at $E_\text{F}$. The gray vertical line indicates the displacement at which the junction geometries are plotted in the first row. The third row shows the transmission as a function of energy and displacement, $\tau(E,d)$. Points, where the transmissions of first and second eigenchannels are similar ($\tau_{2}(E,d)/\tau_{1}(E,d)\ge 0.7$), are indicated by white dots in each two-dimensional transmission map. Horizontal dashed black lines indicate the Fermi energy $E_\text{F}$. Note that all transport properties are shown up to the rupture point, i.e. contacts break in the next pulling step of $0.1$ Å at the largest displacements.
• Figure 5: (a) Evolution of the energies of frontier levels HOMO-1, HOMO, LUMO, LUMO+1 as a function of electrode displacement. They are determined from DFT calculations of the [6]helicene-TT junction by removing all gold atoms and saturating terminal sulfur atoms at each side with a single hydrogen. Color-coded for each orbital is the change in Mulliken population on a sulfur atom. (b) wave functions of the four frontier molecular orbitals at displacement points $d=-1.0$, 1.0 and 4.0 Å, as obtained from DFT. For the HOMO, terminal sulfur atoms are encircled in green to show that the $\pi$-symmetric orbitals on these atoms change their orientation upon stretching. (c) Two-dimensional contour plot of the square of the absolute value of the retarded propagator, $|G^{(0),\text{r}}_\text{lr}|^2$, as a function of energy and electrode displacement for the four-level model. The energy levels are taken from DFT calculations, see panel (a), and other parameters such as residues and the broadening $\eta=0.06$ eV are chosen to mimic the DFT results of the [6]helicene-TT junction in figure \ref{['fig:DFT_transmission']}a. We normalize the squared propagator such that the maximum value inside the full energy-distance map equals 1. See section 3.5 of the Supporting Information for further discussion and choice of parameters.