Understanding the Role of Parallel Pathways via In‐Situ Switching of Quantum Interference in Molecular Tunneling Junctions

Abstract

This study describes the modulation of tunneling probabilities in molecular junctions by switching one of two parallel intramolecular pathways. A linearly conjugated molecular wire provides a rigid framework that allows a second, cross‐conjugated pathway to be effectively switched on and off by protonation, affecting the total conductance of the junction. This approach works because a traversing electron interacts with the entire quantum‐mechanical circuit simultaneously; Kirchhoff's rules do not apply. We confirm this concept by comparing the conductances of a series of compounds with single or parallel pathways in large‐area junctions using EGaIn contacts and single‐molecule break junctions using gold contacts. We affect switching selectively in one of two parallel pathways by converting a cross‐conjugated carbonyl carbon into a trivalent carbocation, which replaces destructive quantum interference with a symmetrical resonance, causing an increase in transmission in the bias window.

A review of oligo(arylene ethynylene) derivatives in molecular junctions

“…62 Subsequent studies examined more complex OAEs. 63,[66][67][68][69] An older method for determining ensemble conductance that is rare in recent literature is a crossed-wire junction. 70 A SAM assembled on a Au wire is contacted by a second perpendicular Au wire, forming a junction containing ca.…”

“…131 Polycyclic systems have been widely studied, particularly as the central units of symmetric OAEs. Purely hydrocarbon units include biphenyl (9), 68,[135][136][137] fluorene (10), 68,[135][136][137][138] naphthalene (11), 125,136 anthracene (12), 125,136,139 anthanthrene (13) 117,140 and azulene (14); 141 example structures are shown in Fig. 11.…”

“…11. Heterocyclic derivatives include carbazole (15a), 137 dibenzothiophene (15b), 137 dibenzofuran (15c), 137 benzodifuran (16), 142,143 benzodithiophene (17), 66 bithiophene (18), 68 and (metallo-)porphyrins (19); [144][145][146] example structures are shown in Fig. 12.…”

Application of Micro/Nanofabrication Techniques to On‐Chip Molecular Electronics

“…From then on, more sophisticated structures have been produced, benefiting from the development of micro/nanofabrication techniques, such as plasma etching and electron beam lithography (EBL). In retrospect, the development of micro/nanofabrication on‐chip and measuring methods for molecular junctions significantly promote the functionalization and application of molecular electronics in the era of self‐assembled monolayer (SAM) molecular junctions [ 10,11 ] and single‐molecule devices. [ 12,13 ]…”

Control of Intramolecular Electron Transport Pathways by Varying Localized Electron Distribution

“…[1][2][3][4] In different electron transport pathways, the phase of transmission amplitudes is different, resulting in different quantum interference (QI) patterns to form various conductance states. [5][6][7][8] The QI effects can be regulated through structural modification, such as connection site variation [9][10][11] and side group substitution, [12][13][14][15] which offers the opportunity for the control of specific pathways.…”