Cross-conjugation and quantum interference: a general correlation?

Abstract

We discuss the relationship between the π-conjugation pattern, molecular length, and charge transport properties of molecular wires, both from an experimental and a theoretical viewpoint. Specifically, we focus on the role of quantum interference in the conductance properties of cross-conjugated molecules. For this, we compare experiments on two series of dithiolated wires. The first set we synthesized consists of three dithiolated oligo(phenylene ethynylene) (OPE) benchmark compounds with increasing length. The second series synthesized comprises three molecules with different π-conjugation patterns, but identical lengths, i.e. an anthracene (linear conjugation), an anthraquinone (cross-conjugation), and a dihydroanthracene (broken conjugation) derivative. To benchmark reliable trends, conductance experiments on these series have been performed by various techniques. Here, we compare data obtained by conductive-probe atomic force microscopy (CP-AFM) for self-assembled monolayers (SAMs) with single-molecule break junction and multi-molecule EGaIn data from other groups. For the benchmark OPE-series, we consistently find an exponential decay of the conductance with molecular length characterized by β = 0.37 ± 0.03 Å(-1) (CP-AFM). Remarkably, for the second series, we do not only find that the linearly conjugated anthracene-containing wire is the most conductive, but also that the cross-conjugated anthraquinone-containing wire is less conductive than the broken-conjugated derivative. We attribute the low conductance values for the cross-conjugated species to quantum interference effects. Moreover, by theoretical modeling, we show that destructive quantum interference is a robust feature for cross-conjugated structures and that the energy at which complete destructive interference occurs can be tuned by the choice of side group. The latter provides an outlook for future devices in this fascinating field connecting chemistry and physics.

Using coherence to enhance function in chemical and biophysical systems

“…For example, when a molecule spans two electrodes 20,21 transport junctions form, revealing striking differences in current according to the interference of the pathways by which electrons can be routed through a molecule 22,23 . As the electron tunnels through the molecule it traverses physical, structural pathways according to the amplitudes and energies of molecular orbitals [24][25][26] . Quantum interference between pathways through the π -system can prevail, despite competing pathways through the σ -bonds.…”

Structure dependent spin selectivity in electron transport through oligopeptides

“…Conductive probe atomic force microscopy (CP-AFM) was found to be a very powerful tool for nanoscale electrical characterization in general, and for molecular electronics in particular. [24][25][26][27] This technique enables the study of several parameters that affect the spin dependent electron transfer or the overall electron transfer, in a relatively simple manner. The same concept was applied in the past both with CP-AFM and STM for investigating length dependent [28][29][30][31][32] and force dependent conduction.…”

Two‐Terminal Molecular Memory through Reversible Switching of Quantum Interference Features in Tunneling Junctions

“…In its pristine state, a destructive QI feature is clearly present in the transmission spectrum near E f (see Figure S44 A) 45. In single‐molecule STM break‐junctions comprising TCNAQ , this QI feature manifests as a low conductance, comparable to that of AQ , which is known to exhibit strong destructive QI effects in both single‐molecule and SAM‐based junctions (see Figure S41) 6,46.…”